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New angles on South Atlantic opening
Abstract
Existing models of relative motion between the South America and Africa plates significantly misrepresent the azimuth of Lower Cretaceous seafloor spreading in the South Atlantic Ocean. An improved model is derived from fits of fracture zones, magnetic reversal anomalies, and the edges of a rifted large igneous province at the Northeast Georgia Rise and Agulhas Plateau. An absolute date for this rifting event is not known but, by assuming least change in spreading rates during magnetic chron C34, rifting can be estimated to have occurred at ∼100 Ma. This modelling demands a plate divergence history that involves diachronous opening of the South Atlantic, consistent with published estimates of the ages of break-up from sedimentary basins on the South American and African passive margins. The diachronous opening lasts approximately 40 Myr, during which time it must be accommodated by significant intracontinental deformation. A reconstruction using the intra-C34 rotation also illustrates the earliest possibility for direct deep-water connection between the Central and South Atlantic Oceans. One further consequence of this model is that a total reconstruction derived from it and closure of the Central Atlantic between North America and Africa suggests that the Venezuelan and Gulf of Mexico basins may be conjugates formed during the earliest opening of the Central Atlantic, and not separate marginal basins.
... A suite of geophysical and geological observations indicates that seafloor spreading between South America and Africa began ca. 134 Myr ago (magnetic anomaly M13) in the southernmost part of the South Atlantic Ocean, and then propagated towards the Equatorial Atlantic Ocean (see e.g., Channell et al., 1995;Eagles, 2007). These observations include seafloor isochrons, flow lines, the kinematics of fracture zones, coherent continental and oceanic structures on either side of the ridge, radiometric dating of igneous rocks and interpolation between rotation parameters. ...
... EAGE BASILONE et al. et al., 1965;Eagles, 2007;Heine et al., 2013;Moulin et al., 2010;Nürnberg & Müller, 1991;Torsvik et al., 2009). The Equatorial Atlantic Ocean, including the BEM, is characterised by different orientations of extension respect to the Southern Atlantic and by the development of small rift basins along the Brazilian margin and its African conjugate (Heine et al., 2013). ...
Stratigraphy along the Brazilian Equatorial Margin is a crucial guide to the geo-dynamic history of rifting of Pangea and formation of the South Atlantic Ocean. Understanding the evolution of the Brazilian Equatorial Margin, which intersects the Saint Paul and Romanche Fracture Zones on the western margin of South Atlantic Ocean, is also key for reconstructing eustatic histories and natural resource exploration. In this study, we quantify the stratigraphic and subsidence histories of three sedimentary basins-Barreirinhas, Ceará, Potiguar-that sit within the margin. Stratigraphy was mapped using ca. 900-line-km of two-dimensional seismic data. Biostratigraphic and check-shot data from 23 wells drilled on the continental shelf, slope and in the distal parts of these basins were used to date and depth-convert stratigraphy. Check-shot data were also used to parameterise compaction. The mapped stratigraphy was backstripped to calculate subsidence histories for the basins. Subsidence curves were decompacted, water-loaded and corrected for palaeo-water depths using biostratigraphic data from well reports. The mapped stratigraphy of the Barreirinhas and Ceará Basins and theoretical subsidence curves indicate that stretching factors did not exceed 1.6. These values suggest that these basins can be regarded as failed rifts. In contrast , more distal stratigraphy mapped in the Potiguar Basin to the south indicates that it stretched by a factor of 5-6. Calculated subsidence histories indicate that this basin formed primarily because of Cretaceous rifting and Cretaceous to Recent post-rift thermal sag, with amplitudes governed by the amount of initial stretching. K E Y W O R D S backstrip, Brazilian Equatorial Margin, Cenozoic, Cretaceous, rift, sedimentary basin, subsidence
... Second, magnetic anomalies were used for dating the oceanic blocks, to identify the continent-ocean transition zone, the onset of oceanic crustal accretion, as well as the velocity of seafloor spreading Collier et al., 2017) (Fig. 3b). Over time, marine magnetic anomalies, combined with other geological information (fracture zones, marginal salt basins), provided the framework for the kinematic reconstructions of the break-up, the genesis of the Atlantic rifted margins and the geometry of seafloor spreading (Collette et al., 1984;Olivet et al., 1984;Klitgord and Schouten, 1986;Sahabi et al., 2004;Eagles, 2007;Labails et al., 2010;Moulin et al., 2010;Perez-Diaz and Eagles, 2014). ...
... Several studies have been published to improve the Bullard (1965) original opening model of the South Atlantic, by using other geological, geophysical, magnetic and gravity data, but until now the opening mechanism of the South Atlantic Ocean has remained a matter of debate (Le Pichon and Hayes;Dietz, 1973;Mascle and Sibuet, 1974;Smith and Briden, 1977;Sibuet and Mascle, 1978;Curie, 1984;Unternehr et al., 1988;Eagles, 2007;Moulin et al., 2010). During the opening . of the South Atlantic, several conjugate pairs of rift basins were formed, Argentina -Namibia BASINS, Santos, Campos -Espirito Santo -Angola basins and Bahia-Sergipe-Gabon BASINS. ...
Deep seismic data and plate kinematic reconstructions help understand the mechanisms of rifting and opening of new oceans, basic principles of plate tectonic cycles. In this study, available deep wide-angle seismic velocity models from the Atlantic margins are reviewed and plate reconstructions used to define conjugate model pairs. The main objective was to study the question of how magma-rich and magma-poor margins develop and the role of inheritance in the break-up. We also studied the question of the mechanism of formation and the origin of transform marginal plateaus, which are typically found at the border of two ocean basins of different ages and are mostly characterized by at least one volcanic phase during their formation. The results of the study include the comparison of crustal thickness, oceanic plate thickness and the influence of volcanism along the Atlantic margins. The conjugate profiles image different degrees of asymmetry of the Atlantic Margin rifts. Marginal plateaus might form when rifting stops at barriers leading to the accumulation of heat in the mantle and increased volcanism directly before or after halting of the rifting.
... In the Early Cretaceous, the first marine incursions are represented by the deposition of evaporites in many sedimentary basins of the Brazilian and African continental margins. Sedimentary rocks deposited above evaporites mark the moment of transition from a restricted to an open marine environment phase, which allowed for the connection between the Central Atlantic and the South Atlantic oceans (Azevedo, 2004;Eagles, 2007;Moulin et al., 2010). During the AlbianeCenomanian, the South Atlantic ocean already had open marine conditions, which was evidenced mainly by the diversity of marine fossils and extensive limestones that deposited in shallow marine carbonate shelfs (Melguen, 1978;Wagner and Pletsch, 1999;Mohriak et al., 2008;Beglinger et al., 2012). ...
... During the Cretaceous, many sedimentary basins of the Brazilian and African continental margins show evidence of marine incursions that were deposited above evaporites, as evidenced by numerous marine fossils and extensive biogenic limestone layers. Several studies that analyze these deposits mark them as the moment of transition from restricted to open marine environment conditions when the waters of the Central Atlantic and the South Atlantic oceans became connected (e.g., Azevedo, 2004;Eagles, 2007;Moulin et al., 2010). During the AlbianeCenomanian interval, the South Atlantic Ocean already had open ocean conditions, with extensive, shallow marine carbonate shelfs (Melguen, 1978;Mohriak et al., 2008;Beglinger et al., 2012;Wagner and Pletsch, 1999). ...
The Albian–Cenomanian was an interval of extreme warmth due to greenhouse climatic conditions, as well as significant changes in the paleogeography and paleoceanography of the oceans, which affected the evolution of marine ecosystems on a global scale. This study analyzed the calcareous nannofossil assemblages from 72 samples that were recovered from Site 364 (Kwanza Basin, Angola), which was drilled by the Deep Sea Drilling Project (DSDP) Leg 40. A total of 103 calcareous nannofossil species were recovered, enabling the identification of biozones (CC 8 to CC 10a; or BC 23 to UC 1b/UC 4a) allowing to assign an Albian–Cenomanian interval for the deposition of the studied section. The recovered assemblages have a subtropical-tropical affinity and are indicative of a surface water connection between the Central Atlantic and the South Atlantic oceans, that extends at least as far as offshore Angola. The area sedimentation was predominantly calcareous and pelagic during this interval, with the exception of the basal section, where Albian black shales interbedded with dolomitic and marly limestones were deposited. Throughout the section, high concentrations of TOC and sulfur and the presence of dissolution-susceptible species likely indicate low euxinic conditions for the Kwanza Basin. Dissolution is a significant process for Site 364 in the majority of the studied samples, although not to the point where species richness was severely impacted, as small amounts of dissolution-susceptible taxa were observed. Dissolution showed a marked decrease in intervals in which paleontological and geochemical data indicated an increase in surface water fertility/productivity.
... To facilitate comparison with other paleomagnetic poles in the 170-140 Ma time interval, we rotated our paleomagnetic poles into a South African reference frame. Due to uncertainty in the relative paleogeographic position of South America with respect to Africa, especially regarding the partitioning of intraplate deformation within both continents (Eagles, 2007;Nürnberg and Müller, 1991;Torsvik et al., 2009), multiple Euler rotations have been proposed to map between South American and South African coordinates (Lottes and Rowley, 1990;Nürnberg and Müller, 1991;König and Jokat, 2006;Eagles, 2007;Torsvik et al., 2008Torsvik et al., , 2009. To quantify the effect of the chosen South America-South Africa reconstruction on the resulting global APW path, we computed our poles in South African coordinates using the rotation parameters of both Lottes and Rowley (1990) and Torsvik et al. (2008), which are representative of models that incorporate low and significant levels of intra-plate displacements within South America, respectively (Table 3). ...
... To facilitate comparison with other paleomagnetic poles in the 170-140 Ma time interval, we rotated our paleomagnetic poles into a South African reference frame. Due to uncertainty in the relative paleogeographic position of South America with respect to Africa, especially regarding the partitioning of intraplate deformation within both continents (Eagles, 2007;Nürnberg and Müller, 1991;Torsvik et al., 2009), multiple Euler rotations have been proposed to map between South American and South African coordinates (Lottes and Rowley, 1990;Nürnberg and Müller, 1991;König and Jokat, 2006;Eagles, 2007;Torsvik et al., 2008Torsvik et al., , 2009. To quantify the effect of the chosen South America-South Africa reconstruction on the resulting global APW path, we computed our poles in South African coordinates using the rotation parameters of both Lottes and Rowley (1990) and Torsvik et al. (2008), which are representative of models that incorporate low and significant levels of intra-plate displacements within South America, respectively (Table 3). ...
True polar wander (TPW) is the reorientation of the crust-mantle system driven by the redistribution of masses in the mantle and on the Earth's surface. In the ideal case, characterization of TPW requires paleomagnetic constraints on the motion of all major plates and independent reconstructions of relative plate positions. While such complete datasets are absent for pre-Mesozoic TPW inferences due to the absence of oceanic plates, they are available for the Late Jurassic (165-145 Ma) “monster shift”, a ∼30° amplitude proposed TPW event. Here we perform paleomagnetic sampling and Ar-Ar geochronology on the La Negra volcanics of Northern Chile, producing two new paleomagnetic poles with ages 165.8 ± 1.8 Ma (1σ; 84.3°N 0.9°E; α95=7.6°; N=28) and 152.8 ± 0.8 Ma (84.5°N 256.4°E; α95=10.8°; N=18). By combining these data with other recently published results, we compute a net lithospheric rotation of 25.3° ± 7.3° (1σ) at a mean rate of 1.21° ± 0.35° My⁻¹ between 170 and 145 Ma with a peak rate of 1.46° ± 0.65° My⁻¹ between 160 and 145 Ma. These rates are consistent with inferences from the Pacific Plate, implying true whole lithosphere rotation. Given coherent motion involving the entire lithosphere, we conclude that the Earth underwent rapid TPW between approximately 165 and 145 Ma, potentially driven by the cessation of subduction along the western North American margin.
... The first full integration of the South Atlantic tectonic evolution, from a preopening position to the present, was proposed by Nürnberg and Müller (1991, Fig. 2.3). During the first decade of the 21st century, authors such as Eagles (2007) and Torsvik et al. (2009) introduced to the discussion, the importance of intracontinental deformation during the opening process. In parallel, many authors have focused on modeling the evolution of the opening of different segments of the South Atlantic (e.g. ...
... The most recent kinematic models were introduced by Heine et al. (2013) and Pérez-Díaz and Eagles(2014), and it is on that basis that this description of the evolution of the South Atlantic Ocean is presented. These new studies allow us to reconstruct the current geometry of the ocean combining processes of extension of the crust, generation of oceanic crust, displacement of course through transformations, and using less intracontinental deformations than those interpreted byEagles (2007) andTorsvik et al. (2009), especially for the southern portion of South America. ...
This work is focused on basin formation and evolution in the Argentinean South Atlantic Margin and the Mesozoic breakup of SW Gondwana. Rifting evolution was studied in the Malvinas and Colorado/Salado basins. Three superimposed rifting events were identified in the latter. The first rifting event is associated with the Late Triassic extensional reactivation of Late Paleozoic thrusts of the Ventania-Cape fold belt. A second and main rifting stage (Early-Middle Jurassic) is related to faults forming the main depocenters and intersecting the older structures. Finally, Early Cretaceous extension linked to the opening of the South Atlantic Ocean focused on the outer continental fringe and produced emplacement of SDRs. The rifting evolution of the Malvinas basin was seismically characterized. New zircon U-Pb ages constrain rifting in the Jurassic. A new model for Gondwana breakup is presented with focus on the evolution of the Mesozoic peri-Atlantic basins. The post-breakup evolution of the Argentinean South Atlantic margin was also studied via seismic interpretation and stratigraphic characterization. Three stages of drift evolution were identified. After the Hauterivian/Barremian breakup, the Cretaceous drift unit is conditioned by the thermal subsidence over the main depocenters. Only after the Maastrichtian-Danian regional transgression, the margin becomes a single continental platform. The Paleogene drift stage is characterized by subsidence and sedimentary input centered in the Salado area, while the Neogene drift stage is characterized by a cylindrical behavior and the remarkable influence of contour currents.
... In the Early Cretaceous, West Gondwana, a southern constituent of Pangaea, broke up to form South America and Africa with continuous spreading resulting in the sustained expansion of the South Atlantic Ocean ( Rabinowitz and Labrecque, 1979;Ben-Avraham et al. 1997Lawver et al., 1998Jokat et al., 2003;Eagles, 2007;Moulin et al., 2009;Lovecchio et al., 2018) (Fig. 4). The contemporaneous Paraná-Etendeka continental flood-basalt provinces in Brazil and Namibia, respectively, are frequently attributed to an Early Cretaceous Tristan da Cunha plume with the Walvis Ridge and Rio Grande Rise comprising plume tail magmatism (Morgan, 1981;Peate, 1997). ...
... Breakup is reasonably well understood but location and magnitude of continental intraplate deformation during rifting, particularly affecting South America, requires further work (see e.g. Eagles, 2007;Heine et al., 2013;Moulin et al., 2009;Torsvik et al., 2009). Barremian (Early Cretaceous; 133-129 Ma, and 134-130 Ma, respectively). ...
The breakup of Pangaea was accompanied by extensive, episodic, magmatic activity. Several Large Igneous Provinces (LIPs) formed, such as the Central Atlantic Magmatic Province (CAMP) and the North Atlantic Igneous Province (NAIP). Here, we review the chronology of Pangaea breakup and related large-scale magmatism. We review the Triassic formation of the Central Atlantic Ocean, the breakup between East and West Gondwana in the Middle Jurassic, the Early Cretaceous opening of the South Atlantic, the Cretaceous separation of India from Antarctica, and finally the formation of the North Atlantic in the Mesozoic-Cenozoic. We demonstrate that throughout the dispersal of Pangaea, major volcanism typically occurs distal from the locus of rift initiation and initial oceanic crust accretion. There is no location where extension propagates away from a newly formed LIP. Instead, LIPs are coincident with major lithosphere-scale shear movements, aborted rifts and splinters of continental crust rifted far out into the oceanic domain. These observations suggest that a fundamental reappraisal of the causes and consequences of breakup-related LIPs is in order.
... Shear orientations indicate that the positions of fault zones were controlled by prominent crustal weaknesses in equatorial Brazil, namely northeast-striking features associated with Proterozoic and Paleozoic basement structures (Mascle et al., 1988;Genik, 1992;Guiraud and Maurin, 1992). In addition, Moulin et al. (2010) and other authors (e.g., Nürnberg and Müller, 1991;Macdonald et al., 2003;König and Jokat, 2006;Eagles, 2007) model shows that opening of the equatorial Atlantic Ocean occurred during the Aptian-Albian and supports the hypothesis of a northward propagation of the South American deformation, as proposed by Eagles (2007). ...
... Shear orientations indicate that the positions of fault zones were controlled by prominent crustal weaknesses in equatorial Brazil, namely northeast-striking features associated with Proterozoic and Paleozoic basement structures (Mascle et al., 1988;Genik, 1992;Guiraud and Maurin, 1992). In addition, Moulin et al. (2010) and other authors (e.g., Nürnberg and Müller, 1991;Macdonald et al., 2003;König and Jokat, 2006;Eagles, 2007) model shows that opening of the equatorial Atlantic Ocean occurred during the Aptian-Albian and supports the hypothesis of a northward propagation of the South American deformation, as proposed by Eagles (2007). ...
The Brazilian Equatorial Margin (BEM) evolved in response to transform motion
between Brazil and Africa. In 2012, Petrobras drilled the Pecém well in the Mundaú
sub-basin (Ceará basin) of the BEM to record the first deep water oil discovery in the
region. This work investigates the deep-water evolution of the Mundaú sub-basin,
focusing on its structural and sedimentary evolution, as well as characterizing
petroleum systems in this new exploration frontier. For such purposes, post-stack
seismic reflection, borehole, and geochemical data were used. Three
tectonostratigraphic sequences representing syn-rift (Mundaú Formation), transitional
(Paracuru Formation) and drift strata (Ubarana Formation) were divided into seven
seismic units. Different tectonic domains were interpreted: proximal, distal and
Romanche Fracture Zone (RFZ). Typical structures of transform margins, such as
marginal ridges and marginal plateaus, were not identified in the Mundaú sub-basin.
Instead, the sub-basin was predominantly deformed by transtensional movements. The
Mundaú and Paracuru formations are mature within the oil window, while the Ubarana
Formation is immature. Main reservoir intervals consist of ~1 m thick intercalations of
sandstone between shales, siltstones and marls. The seal rocks are composed of
shales of the Ubarana Formation, while the hydrocarbon trap is related to an
unconformity and a normal fault. This work concludes that the Paracuru Formation is
the main source and reservoir in the deep-water Mundaú sub-basin, constituting the
Paracuru-Paracuru petroleum system. The results have significant implications on
petroleum exploration in the BEM by proposing a developed petroleum system in the
distal parts of NE Brazil.
... Plate breakup is generally believed to have occurred around 112 Ma (Rabinowitz and LaBrecque, 1979;Curie, 1984;Guiraud and Maurin, 1992;Davison, 1999;Karner et al., 2003;Mohriak et al., 2002;Moulin et al., 2005;Torsvik et al., 2009;Moulin et al., 2010;Araujo et al., 2023). Although magnetic anomaly stripes cannot determine the exact timing of oceanic crust formation due to the Cretaceous Magnetic Quiet Zone in the central segment of the South Atlantic (Mascle and Phillips, 1972;Larson and Ladd, 1973;Eagles, 2007;Torsvik et al., 2009;Moulin et al., 2010;Pérez-Díaz and Eagles, 2014;Bird and Hall, 2016). Since the Albian period, post-rift sediments have been primarily composed of marine limestone and shale deposits (Guardado et al., 2000;Davison, 2007;Winter et al., 2007). ...
Gravity inversion is a highly effective method for investigating regional geological structures, and this paper
proposes an optimization scheme for constrained three-dimensional (3D) gravity inversion to obtain a 3D density
model, utilizing prior geological and geophysics information. Specifically, the proposed method enhances deep
structural imaging resolution and minimizes false structures by progressively inverting deep and shallow-density
structures using long and short-wavelength signals of gravity anomaly with prior information. The scheme is
applied in the southeast passive continental margin of Brazil, and the results show that the density model is
consistent with the previous reflection and refraction seismic data. Moreover, the 3D density model reveals
several insights: (i) The Abimael Ridge (AR) and the S˜ao Paulo Plateau (SPP) exhibit thin crustal thickness (~5–7
km) indicative of proto-oceanic crust. The SPP and AR area crustal thinning may be related to an aborted
opposing rift propagator pair. (ii) The rifting modes of Santos and Campos Basins differ significantly. Campos
Basin exhibits a depth-dependent lithospheric stretching model with a relatively intact upper crust. In contrast,
Santos Basin shows a highly brittle upper crust that is partially thinned and, in some regions, even absent under
the far-field effect of spreading failed rifts, while the lower crust remains relatively intact. Moreover, the upper
crustal stretching factor is about five to ten higher than the lower crustal. Thus, the constrained 3D gravity
inversion scheme provides a new avenue for continental rifted margin geological structure studies.
... The Cretaceous is characterized by a global increase of seafloor spreading rates, leading to the progressive opening of South-Atlantic, proto-Indian and Austral oceans in southern hemisphere (Larson, 1991a(Larson, , 1991bLawver et al., 1991;Eagles, 2007;Moulin et al., 2010). ...
... The W-SW trending paleoflow in the fluvial systems points to potential sources in the Precambrian terranes to the east of the Araripe Basin (Fig. 14). The paleoflow change detected is associated with the Albian uplift of the continental margin after the Gondwana breakup (Rabinowitz and LaBrecque, 1979;Eagles, 2006;Assine, 2007;Moulin et al., 2010). ...
This study presents UPb dating of detrital zircons from sandstones of the Araripe basin (NE Brazil) assisting in the reconstruction of the Mesozoic paleogeography. In particular, we focus in the seaway connection between the Proto-Atlantic ocean and this intracontinental basin in northwest Gondwana.The new UPb age spectra of detrital zircon grains from the pre-Jurassic and Mesozoic clastic sequences of the Araripe Basin, along with paleocurrent, stratigraphic and structural data, demonstrate a significant change in provenance during the LaterLater Jurassic to mid-Cretaceous. Six distinct provenance patterns are identified in the sequences, which may reflect the rapid evolution of the paleogeographic scenarios during tectono-sedimentary history of the northwest Gondwana rift-related basins throughout the onset to the opening of the Atlantic Ocean. Here, we propose an alternative model for paleogeography of the northwest Gondwana. In the Later Jurassic, the main source of terrigenous clastics to the Araripe Basin was the Paleozoic-Triassic sedimentary rocks of the Parnaíba Basin, which formed a prominent barrier to separate the Araripe Basin from the Mesozoic units of the Parnaíba Basin. In the mid-Aptian, erosion led to the retraction of the Parnaíba Paleozoic sedimentary units to the west, as the Precambrian terranes from the Borborema Province, to the northwest, became the main source for the Araripe Basin. The rapid rifting evolution show that the Rio Salgado Belt, from NE region, became topographically depressed, potentially initiating a link between the Araripe and the Potiguar basins. During late-Aptian, the Araripe Basin probably connected to the Proto-Equatorial Atlantic Ocean from the northeast, through the Potiguar Basin and associated grabens on the north. The present paleogeographical reconstruction may explain the occurrence of warm water adapted Tethyan fauna in the Araripe Basin, more plausible than previous hypothesis of a connection to Proto-South Atlantic Ocean.
... West Gondwana rifting involved substantial intraplate deformation inside the supercontinent, with inherited crustal discontinuities reactivated during its breakup and early drifting (e.g., Genik, 1992;Eagles, 2007;Torsvik et al., 2009;Aslanian et al., 2009;Moulin et al., 2010;Seton et al., 2012;Chaboureau et al., 2013;Heine et al., 2013;Matos et al., 2021b). For example, Precambrian structures were reactivated during the rifting-controlled tectono-sedimentary evolution of Brazilian marginal basins (Ponte and Asmus, 1978;Chang et al., 1992;Cainelli and Mohriak, 1999;Milani et al., 2007). ...
Intraplate deformation during the Mesozoic breakup of West Gondwana predominantly occurred along major ancient lithospheric discontinuities. The Precambrian Patos Shear Zone (PASZ) is an inherited lithospheric discontinuity in the interior of northeastern Brazil which is closely linked to the evolution of the Araripe Basin. In this study, unlike previous interpretations of purely extensional tectonics, we conducted stratigraphic and structural field mapping as well as paleostress reconstruction to demonstrate the role of intraplate transtensional tectonics during the Araripe Basin evolution. Limited to the north by the PASZ and associated with sinistral reactivation, a set of normal to oblique NE–SW faults constituting a transtensional horsetail structure generated the initial basin geometry. Stratigraphic records of this rift phase comprise the Jurassic to Early Cretaceous Brejo Santo, Missão Velha, and Abaiara formations. Paleostress reconstruction showed a NE–SW/subhorizontal σ1 axis, a vertical σ2 axis, and a NW–SE/subhorizontal σ3 axis, resulting in a sinistral transtensional tectonic regime, which was mainly governed by intraplate processes that reflected the opening of the Brazilian East Margin. The subsequent Aptian post-rift early phase was characterized by NE–SW reverse faults and a succession of normal to oblique normal faults striking NW–SE to WNW–ESE. This fault arrangement generated a new NW–SE transtensional basin, which controlled the Barbalha Formation and modified and partially preserved the previous NE-SW transtensional basin. Paleostress reconstruction showed a NW–SE/subhorizontal σ1 axis and NE–SW/subhorizontal σ3 axis for strike-slip faults but NE–SW extension for normal faults. This is related to a second dextral reactivation of the PASZ following propagation of intraplate stress during the opening of the Brazilian Equatorial Margin. In summary, tectonic evolution of the Araripe Basin was strongly influenced by stress propagation from the Gondwana breakup into intraplate settings, with two distinct reactivations of the PASZ pre-existing basement structures which results in a paleostress inversion during basin evolution.
... Although different models exist for the absolute position of Gondwana [30] as well as the relative positioning of cratons can be done with small margins of error (Figure 4; [32]). The formation of Gondwana is often presented as a merger of East Gondwana (Antarctica, Australia, and India) with West Gondwana (those currently in Africa and South America). ...
The geodynamics of the Southwestern Nigeria Precambrian Basement Rocks were studied with aim of understanding the evolution of rocks globally. Magnetic carriers of Precambrian Basement rocks samples collected from 110 locations were prepared for rock magnetism, optical microscopy and Scanning Electron Microscopy (SEM). The Natural Remanent Magnetisation (NRM) of the remagnetised and unmagnetised rocks are strong (0.3–1.7 A/m -< 0.5 A/m) showed northwesterly direction with moderate inclination and weak NRM with westerly shallow direction respectively. Primary and secondary NRMs are carried by maghemite, and the remagnetised and unmagnetised rocks revealed a higher coercivity for alternating field demagnetisation (<20 mT – < 10 mT median destructive field). Optical microscopy revealed maghemite, poor titanomagnetite, titanomaghemite lamellae >30 pm and finer maghemite/magnetite grains finer than 10 pm. X-ray Diffratometry (XRD) and SEM results implied NW remanence in the remagnetised rocks reside in the fine poor-maghemite during the alteration of hornblende to actinolite while the coarse-grained maghemite in both rocks carries the W remanence of a thermoremanent magnetisation acquired in the Pan – African times. Global cold collision geodynamics resulted in the generation of ultra-high pressure metamorphic complexes and remagnetisation and True Polar Wander drifts of the paleomagnetic pole move towards the equator.
... The consequence of this model is the possibility of continental breakup by southward propagation of the oceanic ridge from the northern Espírito Santos and Campos basins towards the Santos Basin. The traditional view of uniform oceanic propagation from southern to the northern segments of the South Atlantic is described by Le Pichon and Hayes (1971), Sibuet and Mascle (1978), Rabinowitz andLabrecque, 1979, Pindell andDewey (1982), Fairhead and Okereke (1987), Unternehr et al. (1988), Nürnberg and Müller (1991), Lawver et al. (1999), Schettino and Scotese (2005), Eagles (2007) and Torsvik et al. (2009). In the model presented by Heine et al. (2013), a tight-fit reconstruction based on structural restoration of the conjugate South Atlantic margins and intraplate rifts was achieved, without the need for complex intracontinental shear zones., but through a multi-phase and multidirectional extension history. ...
The Western African Sub-Saharan continental margin includes a large region from Cameroon to South Africa comprising distinct margin types – magma-rich, magma-poor and transform margins. The integrated study of geological, seismic and potential field data allows the mapping of a structural framework subdivided in proximal, necking, distal, outer and oceanic domains, characterized by a set of comparable architectural elements and magmatism. These key domains display a strong and consistent continuity along the continental margin. Longitudinal transitional regions occur in a gradual way, but interactions between structural domains along transversal section may also occur, displaying spatial and temporal overprints. Because of this complexity, the outer domain that is the key feature in the magma-rich margin, tends to disappear northwards where the distal and proximal domains of the magma-poor segments become the most important domains of the margin. In the north of Gabon Basin, it is observed both the reappearance of the outer domain and the reworking of the distal domain by fracture zones. The basement of the distal domain within the magma-poor margin is interpreted as made by a hyperextended continental crust but with magmatic additions. Finally, the new proposed continent-ocean boundary (COB) in the region of the Namibe Basin (transitional zone between magma-rich and magma-poor margins) achieves a best fit in a reconstruction model with the limits recognized at the South America side. The results will imply in a drastic reduction on the gap proposed by other recent reconstruction models and also minimize the intraplate deformation required by some other models.
... It matches with our NNE-oriented extension of syn-rift phase I data (Figure 12b) once the continent rotation of ∼45° is considered (Moulin et al., 2010). As the Lower Cretaceous rift developed, the E-W-oriented σ3 was associated with the South Atlantic opening and continuously migrated toward the north (Eagles, 2007) up to the Rio do Peixe Basin, thus progressively becoming the dominant stretching direction in this stage ( Figure 12c). As the E-W extension occurred with higher velocities at the southern part of the South Atlantic, the South American Plate was forced to rotate clockwise (Heine et al., 2013), thus gradually increasing the contribution of the E-W-oriented extension in the Borborema Province, changing the stress field in the Rio do Peixe Basin and further reactivating the basin-boundary Portalegre and Malta faults (Figures 12a and 12c). ...
We constrained the tectonic evolution of the intracratonic Cretaceous Rio do Peixe Basin (RPB) in NE Brazil, combining structural and Anisotropy of Magnetic Susceptibility (AMS) data. We analyzed the structural features of four sites along two major faults bordering the basin, the NE‐striking Portalegre Fault and the E‐W‐striking Malta Fault. AMS data from 42 sites in the syn‐rift sandstone suggest two stretching directions driving the opening of the RPB. The early syn‐rift phase I resulted from N‐S to NNE‐SSW stretching direction with vertical σ1, producing normal fault displacement along the E‐W‐striking Malta Fault and right‐lateral transtension along the NE‐striking Portalegre Fault, Sítio Saguí and Lagoa do Forno faults. The syn‐rift phase II resulted from NW‐SE stretching direction with vertical σ1, causing normal displacement on NE‐striking major faults and left‐lateral transtension on E‐W‐striking major faults. Additionally, the NW‐SE extension was responsible for forming NE‐striking extensional faults and deformation bands in sedimentary units. The RPB developed due to the intraplate deformation of the Borborema Province during the early stage of the Pangea Breakup and recorded two stretching directions that gradually shifted from NNE‐SSW to NW‐SE as a consequence of the South America clockwise rotation. The extensional stress orthogonal to the main E‐W‐striking and NE‐striking Precambrian shear zones facilitated the opening and evolution of the RPB.
... The margin hosts two main basins, the Orange and Walvis basins. These basins contain continental and marine sediments up to 8 km thick that record the evolution of the margin from rift to oceanic spreading (Emery et al. 1975;Austin and Uchupi 1982;Gerrard and Smith 1982;Light et al. 1993;Brown 1995;Gladczenko et al. 1998;Bauer et al. 2000;Séranne and Anka 2005;Eagles, 2007;Torsvik et al., 2009). ...
This paper presents a semi‐quantitative analysis of gravity‐driven deformation along the Namibian margin using extensive 2D depth converted seismic data. The geometries, internal characters and distribution of gravity‐driven systems were investigated through regional and detailed seismic studies. The research shows that surficial slumps are typically ~50 m thick and are characterised by contorted seismic facies commonly occurring along the slopes of the margin. They commonly funnel and cluster within high relief areas such as canyons and pre‐existing landslide scars. These contrast with coherent slides that are up to 2 km thick which extend laterally along the margin for tens to hundreds of kilometres. Slides preferentially occur in the proximal part of the margin and are constrained within the main margin depocenters. Here, high sedimentation rates and loading promote the generation of distinct, weak, overpressured layers that favour initiation of sliding of relatively coherent sediment masses. This research also shows that one‐third in volume of the post‐rift sediments on the Namibian margin were affected by slides and slumps. This demonstrates that gravity‐driven deformation is a key geological process that can strongly modify the evolution of rifted passive margins.
... 2). Esta separação levou à individualização da Gronelândia e a uma reconfiguração do que é hoje o Golfo do México e o Mar das Caraíbas (Stanley, 1993) e, mais tarde, no Cretácico inferior (cerca dos 100 Ma), ao início do afastamento da América do Sul relativamente ao continente Africano (Smith e Briden, 1977;Smith, 1999;McLoughlin, 2001;Eagles, 2007). A Crista Médio-Atlântica, também conhecida por Dorsal ...
... 2). Esta separação levou à individualização da Gronelândia e a uma reconfiguração do que é hoje o Golfo do México e o Mar das Caraíbas (Stanley, 1993) e, mais tarde, no Cretácico inferior (cerca dos 100 Ma), ao início do afastamento da América do Sul relativamente ao continente Africano (Smith e Briden, 1977;Smith, 1999;McLoughlin, 2001;Eagles, 2007). A Crista Médio-Atlântica, também conhecida por Dorsal ...
ÁVILA S.P., R.S. RAMALHO, C.M. DA SILVA, M.E. JOHNSON, A. UCHMAN, B. BERNING, R. QUARTAU, P. MADEIRA, C.S. MELO, A.C. REBELO, L. BAPTISTA, S. ARRUDA, E. GONZÁLEZ, M.W. RASSER, A. HIPÓLITO, R. CORDEIRO, R. MEIRELES, V. RAPOSO, J. POMBO, R. CÂMARA, M.X. KIRBY, J. TITSCHACK, J.M. HABERMANN, R. VULLO, A. KROH, J.H. LIPPS, M. CACHÃO & J. MADEIRA, 2022. Os fósseis de Santa Maria (Açores). 2. Pedra-que-pica: uma história com 5 milhões de anos, 160 pp. OVGA, Lagoa.
Índice:
1. Localização geográfica
2. Enquadramento geodinâmico e paleoclimático
3. Enquadramento geológico dos Açores
4. História Geológica de Santa Maria
5. Os fósseis de Santa Maria
6. A jazida da “Pedra-que-pica”
7. A fauna da “Pedra-que-pica”
8. Reconstrução paleoambiental
9. Possíveis rotas de colonização das ilhas dos Açores
10. Os fósseis de Santa Maria à luz da legislação portuguesa
11. O potencial de visitação turística e de uso educacional da Pedra-que-pica
12. Agradecimentos
13. Fotografias dos participantes nos workshops “Paleontologia em Ilhas Atlânticas”
14. Bibliografia
15. Glossário
... Since the Triassic period, the overall tectonic regime along Pangea was extensional, which triggered the formation of the accommodation space of the Mesozoic basins and, finally, the supercontinent breakup (e. g., Vizán et al., 2017). In Gondwana, this extensional regime could be related with the collapse of the Godwanides (e.g., Kern et al., 2021;Ramos, 2008), the later opening of the Weddell Sea and the Rocas Verdes Basin during the Early-Middle Jurassic (e.g., Bastias et al., 2021;Calderón et al., 2007;Cao et al., 2022;Dalziel et al., 1974;Ferris et al., 2000;Ghidella et al., 2002;Grunow, 1993;Grunow et al., 1987;Jordan et al., 2017Jordan et al., , 2020König and Jokat, 2006;Riley et al., 2020;Suárez et al., 2019), and finally with the opening of the South Atlantic Ocean since the Early Cretaceous (e.g., Creer et al., 1972;Eagles, 2007;Nürnberg and Müller, 1991;Schult and Guerreiro, 1979;Torsvik et al., 2009). Furthermore, the regional crustal thinning and the associated uplift of the asthenosphere (e.g., Huang et al., 2019;Zhong et al., 2007), aided in the formation of Large Igneous Provinces such as the felsic Chon Aike Igneous Province (198-145 Ma;Kay et al., 1989), the mafic Paraná-Etendeka Magmatic Province (130-132 Ma; Renne et al., 1992) and the Central Atlantic Magmatic Province (190-202 Ma;May, 1971). ...
This work presents the analysis of paleomagnetic results obtained from four sampling areas of the Jurassic Bahía Laura Complex in the Deseado Massif and their implications on the regional deformation history during the breakup of Gondwana. Paleomagnetic data show cessation of tectonic block rotations about vertical axes around 160 Ma and a change from a transtensional to a mainly extensional tectonic regime. Two biotite samples yielded ⁴⁰Ar³⁹Ar radiometric ages: one from the eastern outcrops of the Chon Aike Formation (plateau age of 184.66 ± 0.55 Ma) and the other from the La Matilde Formation (plateau age of 157.40 ± 0.65 Ma), located at the central part of the Deseado Massif. Finally, an apparent polar wander path (APWP) was calculated for the Jurassic of South America (200 to 140 Ma). This APWP indicates that South America experienced a northward drift between 200 Ma and 170 Ma, a clockwise rotation (~10°) between 170 Ma and 160 Ma, and a westward drift between 160 Ma and 140 Ma. The 170–160 Ma rotation could has been triggered by the combined effects of the uncoupling of the Antarctic Peninsula, the high rates of subduction of the Phoenix plate beneath Patagonia, the opening of the Rocas Verdes basin and the Weddell Sea, and the cessation of the Tethys slab-pull. Furthermore, the calculated APWP for South America does not support a Jurassic massive true polar wander event.
... During this period, the Gondwana Supercontinent began to break-up into the South American and African continents, leading to the establishment of the South Atlantic Ocean (e.g., Poulsen et al., 2001;Moulin et al., 2010;Chaboureau et al., 2013). Current understanding about the Gondwana break-up suggests that the separation between South America and Africa started ~138-134 Ma (Channell et al., 1995;Jokat et al., 2003), gradually forming the South Atlantic basins until the clearing of continental-oceanic boundaries for the Equatorial Atlantic Gateway between ~110 Ma (Nürnberg and Müller, 1991;Jones et al., 1995;Eagles, 2007;Moulin et al., 2010;Heine et al., 2013) and 90 Ma (Fairhead et al., 2013;Heine et al., 2013;Granot and Dyment, 2015). This ultimately led to diachronous connections with other basins (e.g., Aslanian et al., 2009;Moulin et al., 2010;Chaboureau et al., 2013), including deep-water exchanges with the North Atlantic ~95-90 Ma (Tucholke and Vogt, 1979;Trabucho Alexandre et al., 2010;Friedrich et al., 2012;Ladant et al., 2020). ...
Early Cretaceous sedimentary basins on the South Atlantic continental margins of South America and Africa are closely related with the Gondwana break-up, and record coeval paleoceanographic, paleoclimatic, and biotic changes during the late Barremian–Albian. The Sergipe-Alagoas Basin of northeastern Brazil contains one of the most complete Lower Cretaceous stratigraphic successions of the South Atlantic Ocean, including sedimentary and paleontological records nearly absent in nearby basins on the African/Brazilian continental margins. Fossil-rich sedimentary successions from the Sergipe-Alagoas Basin have allowed to assess: (i) evolutionary stages of the early South Atlantic Ocean; (ii) paleobiogeography of marine incursions (Austral and/or North Atlantic/Tethyan influences); and (iii) local signatures of global-scale paleoceanographic, paleoclimatic, and biotic events. Here we provide an overview on fossil-based biostratigraphic and paleoenvironmental studies of the Sergipe-Alagoas Basin, from the first marine incursions in a continental setting, to fully open-marine conditions. Based on eight fossil groups (ammonites, calcareous nannofossils, echinoids, planktonic and benthonic foraminifera, ostracods, palynomorphs, and radiolarians) from 107 sites, we observed that the marine biota shows strong affinity with Tethyan Realm in the Aptian–Albian interval, and provincialism/endemism is likely tied to the early-stage development of the South Atlantic Ocean. Based on late Barremian–late Albian lithological, paleontological, and geochemical data, we suggest three Sergipe-Alagoas Paleoenvironmental Stages (SAPSs): (i) continental conditions with possible episodic marine incursions (SAPS-1); (ii) transitional system, from continental to restricted-marine conditions (SAPS-2); (iii) transition from restricted- to open-marine conditions (SAPS-3). These local-scale evolutionary stages are correlated with early phases of the South Atlantic Ocean and major late Early Cretaceous paleoceanographic and paleoclimatic events. Moreover, we provide new approaches on the response of low-latitude biota to changing ocean circulation patterns in a hot, ice-free “greenhouse” world.
... The geological evolution of the area is linked with the wider formation of the northern passive margin of West Africa. The margin developed through continental stretching in the Barremian-Aptian to continental separation and seafloor spreading in the late Aptian-early Albian (Lawrence et al. 2002(Lawrence et al. , 2017Brownfield and Charpentier 2006;Eagles 2007;Heine et al. 2013;Pérez-Díaz and Eagles 2014). In the region, the post-rift succession, which is made of predominantly marine shales and silts (Lawrence et al. 2002;Brownfield and Charpentier 2006), has been tectonically affected by the frontal thrusts of the Niger Delta (Fig. 3b). ...
This research seeks to test the hypothesis that rheology of submarine gravity-flows influences the planform geometry of associated fan lobe deposits. If this control exists, seismic geomorphology has the potential to predict the sedimentary characteristics of these deposits by analysing relatively simple seismic outputs such as time structure maps and amplitude maps. This predictive power could potentially aid identification of prospective reservoirs through automated processes of seismic interpretation.
In this work, Upper Cretaceous and Neogene deepwater reservoirs from East and West Africa with porosity in excess of 20% and multi-Darcy permeability were analysed. The overall external geometry of the reservoirs was described by integrating seismic amplitude extractions, structure and isochron maps derived from high quality 3D seismic data. The maps were generated from detailed interpretation of the top and base reservoir reflections. Core photos of the reservoirs were integrated into the seismic project and their examination allowed identification of dominant sedimentary facies, enabling a proposal for the classification of these deposits in terms of flow processes.
The research indicates confinement of flows exerts an obvious control on the shape of the deposits and resulted in sedimentary bodies with marked elongation at the seismic scale. However, the analysis of the reservoirs at the core scale indicated that these are dominated by massive, poorly sorted, medium- to coarse- grained sandstones, commonly with floating oversize clasts, as well as beds with disrupted laminae and sandy conglomerates. These characteristics may indicate ‘depositional freezing’ from high-density flows with yield strength, which may have also contributed to the limited lateral spreading of the deposits analysed.
The research also highlights the importance of the non-unique relationship between sedimentary processes and external geometries of resulting deposits at the seismic scale. This means that deposits with similar geometries may have markedly different sedimentary characteristics.
This work highlights the increasing importance of integrating detailed seismic geomorphology studies with accurate examination of core data to validate models of gravity flow processes and test their predictive value.
... Ocean (e.g., Nürnberg and Müller, 1991;Eagles, 2007;Torsvik et al., 2009;Moulin et al., 2010;Seton et al., 2012;Heine et al., 2013;Granot and Dyment, 2015). The stepwise opening of the South Atlantic Ocean resulted in diachronic deformation that led to the development of two very distinct margins around Brazil: the Eastern and Equatorial margins (Fig. 12.3;De Matos, 2000). ...
The Brazilian Equatorial Margin (BEM) is composed of several seamounts, banks, and guyots, mainly contouring the 1200-m isobath through all its northeast borders. The BEM is composed of R-T fracture zones associated with Riedel shear structures in the NW–SE direction where volcanic rocks, such as the São Luís Guyot and Belém Seamount, were emplaced. Moreover, important fractures are observed parallel to the Mesoceanic ridge (Romanche, Saint Paul, 4N, and Chain) with Romanche and Saint Paul being an active pair of transform faults configuring the largest directional offset in the Equatorial Atlantic with related volcanism in the E–W transform faults. The analyzed samples in this chapter are related to the Cenozoic onshore volcanism (Rio Grande do Norte Basalts, Fortaleza Alkaline Volcanic Province, Macau-Queimadas Alignment, and Fernando de Noronha Archipelago) and for three seamounts: Ceará and Belém Seamounts and São Luís Guyot with estimated ages younger than <38 Ma. They are composed of mugearite, and alkali basalts enriched in REEL and with a steep REE curve indicating residual garnet in the mantle source. BEM seamounts present a Dy/Yb ratio lower than the garnet-lherzolite melting curve, implying a larger contribution from the spinel-lherzolite mantle source in relation with a pure garnet-lherzolite mantle source. As noted in the Sr-Nd-Pb isotopic ratios diagrams, the Belém Seamount plots away from the Ceará Seamount and São Luís Guyot and the Rio Grande do Norte Basalts and the Fernando de Noronha basanites and nephelinites cluster, which may be indicative of a long-term enriched mantle source. As observed in almost all offshore Brazilian basalts, these ocean island basalts with EMI-EMII signatures in the South Atlantic Ocean may be related to the Brazilian Neoproterozoic continental lithosphere which was delaminated and contaminated a zone in the South Atlantic asthenosphere.
... The northern part of the basin corresponds to the Araçuaí Belt crust (Stanton et al., 2019), with structures oriented NNW-SSE. During Cretaceous, the extension direction at Campos Basin was approximately WNW-ESE (Szatmari and Moriak, 1995;Eagles, 2007) i.e., generally oblique to the main continental basement structural trends with varying degrees. The rifting resulted in structures mainly NE-SW oriented, with an important inflection to NNE northern of 21 0 S, probably related to a rheological variation associated with the transition from the Ribeira to Araçuaí terranes (Stanton et al., 2019). ...
... doi:10.1093/molbev/msab251 Additionally, the divergence between Metaperipatus from Chile and Opisthopatus þ Peripatopsis from South Africa was estimated at 154 Ma (95% HPD: 75-237 Ma), coinciding with the opening of the southern Atlantic Ocean ca.140-130 Ma (McLoughlin 2001;Eagles 2007). This parallels Gondwanan vicariant patterns seen in previous onychophoran studies ...
Onychophora (“velvet worms”) are charismatic soil invertebrates known for their status as a “living fossil”, their phylogenetic affiliation to arthropods, and their distinctive biogeographic patterns. However, several aspects of their internal phylogenetic relationships remain unresolved, limiting our understanding of the group’s evolutionary history, particularly with regard to changes in reproductive mode and dispersal ability. To address these gaps, we used RNA sequencing and phylogenomic analysis of transcriptomes to reconstruct evolutionary relationships and infer divergence times within the phylum. We recovered a fully resolved and well-supported phylogeny for the circum-Antarctic family Peripatopsidae, which retains signals of Gondwanan vicariance and showcases the evolutionary lability of reproductive mode in the family. Within the Neotropical clade of Peripatidae, though, we found that amino acid-translated sequence data masked nearly all phylogenetic signal, resulting in highly unstable and poorly supported relationships. Analyses using nucleotide sequence data were able to resolve many more relationships, though we still saw discordant phylogenetic signal between genes, probably indicative of a rapid, mid-Cretaceous radiation in the group. Finally, we hypothesize that the unique reproductive mode of placentotrophic viviparity found in all Neotropical peripatids may have facilitated the multiple inferred instances of over-water dispersal and establishment on oceanic islands.
... South to North: According to Nuernberg and Muller (1991), Eagles (2007); and Perez-Diaz and Eagles (2014), among many others, rift propagated northward from South Argentina. Late Jurassic/Early Cretaceous syn-rift sequences are recognized in the Colorado Basin, Argentina (Loegering et al., 2013). ...
The track of South Atlantic hotspots on oceanic crust is well known. However, the possible hotspot trails of Tristan-Gough (TG) and St. Helena (SH) plume heads on continental crust have not been clearly defined. The occurrence of a large igneous province in Northeast Brazil, previously named EQUAMP, is reviewed, enlarged in time and space, and renamed as the Borborema Large Igneous Province, or BOR-LIP (135-104 Ma). There is strong evidence of an active dual plume system during the South America breakup: The BOR-LIP, in Northeast Brazil, and the Paraná-Etendeka Magmatic Province – PEMP-LIP (135-131 Ma), in SE Brazil and SW Africa. As widely accepted, the post-breakup trajectories of the SH, TG, and Sierra Leone (SL) hotspot trails (on oceanic crust) illustrate how the South American and African plates drifted away from each other. Regarding the pre-breakup trajectories, coherent and robust evidence of the migration path of the TG and SH plume heads on continental crust within the Brazilian side of the margin is presented here. They are remarkable consistent with the northwestward movement of Africa and South America, during Early Cretaceous, when referred to a deeper lithospheric reference frame, not necessarily rooted in the core-mantle boundary. The migration paths on oceanic and continental crust of the SH and TG plumes, are consistent with an intense interplay between deep-sourced mantle plumes and the relative rotation between the South American and African plates, as illustrated by the non-linear TG hotspot trail within the extremely stretched continental crust of the Santos Basin. The expressive magmatism observed in the Santos Basin, stretched during the 130-114 Ma interval, is interpreted as a direct consequence of the change in the magma budget due to the arrival of the TG plume head beneath the basin, when decompression-driven may have released melting of plume material. Large volumes of CO2, sourced by the mantle and found in petroleum accumulations within the Santos Basin, may be also genetically related to the TG plume accelerating the exhumation processes in hyper-extended terrains.
... The initiation of regional shortening coincided with a change of absolute motion of the South American Plate, which became west directed ( Fig. 2; Maloney et al., 2013;Müller et al., 2016;Seton et al., 2012;Silver et al., 1998). Trenchward absolute motion of the overriding plate and an increase of convergence rates (Eagles, 2007;Maloney et al., 2013;Müller et al., 2016) were accompanied by slab shallowing manifested in the upper crust by the eastward expansion of the magmatic arc during the Late Cretaceous (Haller et al., 2010;Aragón et al., 2013). This change in slab dip favored a strong increase in coupling of the subducting plates and the overriding plate . ...
In cordilleran-type orogens, subduction geometry exerts a fundamental control on the tectonic behavior of the overriding plate. An integrated low-temperature, large thermochronological data set is used in this study to investigate the burial and exhumation history of the overriding plate in northern Patagonia (40°−45°S). Thermal inverse modeling allowed us to establish that a ∼2.5−4-km-thick section originally overlaid the Jurassic−Lower Cretaceous successions deposited in half-graben systems that are presently exposed in the foreland. Removal of the sedimentary cover started in the late Early Cretaceous. This was coeval with an increase of the convergence rate and a switch to a westward absolute motion of the South American Plate that was accompanied by shallowing of the subducting slab. Unroofing was probably further enhanced by Late Cretaceous to early Paleogene opening of a slab window beneath the overriding plate. Following a tectonically quiescent period, renewed exhumation occurred in the orogen during relatively fast Neogene plate convergence. However, even the highly sensitive apatite (U-Th)/He thermochronometer does not record any coeval cooling in the foreland. The comparison between Late Cretaceous and Neogene exhumation patterns provides clear evidence of the fundamental role played by inter-plate coupling associated with shallow slab configurations in controlling plate-scale deformation. Our results, besides highlighting for the first time how the whole northern Patagonia foreland was affected by an exhumation of several kilometers since the Late Cretaceous, provide unrivalled evidence of the link between deep geodynamic processes affecting the slab and the modes and timing of unroofing of different sectors of the overriding plate.
... The southern Atlantic Ocean between South America and Africa opened during the Cretaceous from south to north (Seton et al., 2012), with seafloor spreading commencing asynchronously along the rift, perhaps over a period of ∼40 Myr (Eagles, 2007). Volcanism was widespread along the opening margins (Franke, 2013), occurring on land from ∼138 to 120 Ma (Moulin et al., 2010). ...
Rio Grande Rise (RGR) and Walvis Ridge (WR) are South Atlantic large igneous provinces (LIPs), formed on the South American and African plates, respectively, mainly by volcanism from a hot spot erupting at the Mid‐Atlantic Ridge (MAR) during the Late Cretaceous. Both display morphologic complexities that imply their tectonic evolution is incompletely understood. We studied bathymetry, gravity, and vertical gravity gradient maps derived from satellite altimetry to trace faults providing indications of seafloor spreading directions and changes. We also examined magnetic anomalies for time constraint and reflection seismic data for structural information. Abyssal hill fabric and magnetic anomaly data indicate that the area between RGR and WR was anomalous between anomalies C34 (83.6 Ma) and C30 (66.4 Ma) owing to reorganization of a right‐lateral transform on the MAR. This event began ∼92 Ma as the transform shifted south to form multiple, short‐offset right‐lateral transforms, with the reorganization extending through anomaly C34 and ending before anomaly C30. Anomalous spacing of magnetic anomalies and discordant fault fabric indicate that a microplate formed with a core of Cretaceous Quiet Zone seafloor. As the MAR jumped eastward, this microplate was captured by the South American plate and now resides mostly in a basin between the main RGR plateau and a related ridge to the east (East Rio Grande Rise). The microplate is ringed by igneous massifs, implying a link with volcanism. The results presented here indicate that these two LIPs had a complex Late Cretaceous history that belies simple hot spot models.
... All VGPs are south poles on a Wulff projection. Eagles (2007) parameters. (Alva-Valdivia et al., 2003). ...
The Paraná Magmatic Province (PMP) in southern Brazil is divided into three sub-provinces historically characterized by their Ti content (low-Ti in the southern sub-province, high-Ti in the northern, and mixed in the central sub-province). Ar/Ar and U–Pb ages for the PMP usually places the whole volcanism in a brief interval of ∼135-133 Ma with younger ages to the north. The existing paleomagnetic data tend to show differences among the three areas, so far related to the age migration. New paleomagnetic data from 39 flows and sills from the Northern Paraná Magmatic Province were incorporated into the pre-existing database. A new calculated paleomagnetic pole based on all available data matches the poles for the southern sub-province and Etendeka, the African extension of the PMP. However, the recalculated CPMP pole using all available data is distinct from the other poles. The mean magnetic inclination for the rocks of central PMP is ∼5° lower than the inclination for the other areas, which may be caused by a N–S dipping of the central area.
... Opening of the central South Atlantic segment propagated northward from the Rio Grande Fracture Zone to the Ascension Fracture Zone Eagles, 2007;P erez-D ıaz & Eagles, 2014) and lasted about 30 Myr, from the Late Jurassic/Early Cretaceous to the Aptian . The study area encompasses three representative conjugate margins: Camamu/South Gabon, Campos/North Kwanza and North Santos/South Kwanza ( Figure 2). ...
Magma-poor rifted margins portray a wide range of tectonic styles. These differences
arise from the variability in key factors controlling continental rifting, such as the
initial lithospheric strength and extension velocity. The aim of this work is to use
thermomechanical numerical experiments to shed light on the processes controlling
the modes of continental extension and their impact on margin tectonic architecture
and nature of the continent-ocean transition (COT).
... Only one other global plate reorganization, during the mid-Cretaceous, is recognized to have occurred since the breakup of Pangea (Veevers, 2000;Matthews et al., 2012;Morra et al., 2013;Müller et al., 2016;Olierook et al., 2020). The mid-Cretaceous plate reorganization involved the cessation of longlived East Gondwana subduction (Laird and Bradshaw, 2004), major changes in the rates and directions of India-Antarctica and India-Australia seafloor spreading (Müller et al., 2016), kinks observed in seafloor fracture zones in the eastern Indian ocean (Johnson et al., 1980;Matthews et al., 2011), a slowdown in the absolute motion of Australia (Müller et al., 2016), and final separation of the equatorial Atlantic (Eagles, 2007). However, despite these various studies, several aspects of the mid-Cretaceous plate reorganization remain poorly understood. ...
Investigation of the ~2400-km-long Tan–Lu Fault Zone (TLFZ) in eastern China is the key to understanding how the Izanagi Plate in the western Pacific Basin and the East Asian continental margin responded to global plate reorganization during the mid-Cretaceous. We present new structural and geochronological data to show that the central segment of the NNE–SSW-striking TLFZ underwent a phase of sinistral transpression after the Early Cretaceous rifting. The resultant strike-slip structures are ductile shear belts in the south of the segment and brittle faults in the north. Quartz c-axis fabrics and other microstructures indicate deformation temperatures of 350–500 °C in different parts of the shear belts. The brittle faults were associated with the formation of NE–SW-trending folds and an angular unconformity between Lower and Upper Cretaceous volcanic or sedimentary rocks. Fault-slip data indicate that sinistral faulting was the result of NS compression. UPb dating constrains the timing of sinistral faulting between 97 and 82 Ma (early Late Cretaceous). Integration of these and existing data demonstrates that the entire TLFZ underwent sinistral displacement at the beginning of the Late Cretaceous, consistent with continental-scale NS compression in eastern China. Such compression in the overriding plate was caused by rapid oblique subduction of the Izanagi Plate and reflected global plate reorganization at this time. Both the changes in the kinematics of the Izanagi Plate and the resultant variation of stress states in the continental margin around the mid-Cretaceous are ascribed to this plate reorganization.
... The fragmentation of the supercontinent Gondwana commenced in the Early Cretaceous (ca. 136 Ma) as South America was sheared westward along the Falkland Agulhas Fracture Zone (Martin and Hartnady, 1986;Eagles, 2007), creating the steep and deep shelf break on South Africa's eastern seaboard that is now only ∼8 km wide. The unique morphology of the continental shelf plays an important role in Pondoland's climate and ecology, primarily by preventing the warm-water Agulhas Current from drifting too far from the coastline during the Pleistocene and Holocene (Winter and Martin, 1990;Peeters et al., 2004). ...
Coastal occupation and foraging during the last glacial maximum and early Holocene at Waterfall Bluff, eastern Pondoland, South Africa – Erratum - Erich C. Fisher, Hayley C. Cawthra, Irene Esteban, Antonieta Jerardino, Frank H. Neumann, Annette Oertle, Justin Pargeter, Rosaria B. Saktura, Katherine Szabó, Stephan Winkler, Irit Zohar
... The fragmentation of the supercontinent Gondwana commenced in the Early Cretaceous (ca. 136 Ma) as South America was sheared westward along the Falkland Agulhas Fracture Zone (Martin and Hartnady, 1986;Eagles, 2007), creating the steep and deep shelf break on South Africa's eastern seaboard that is now only ∼8 km wide. The unique morphology of the continental shelf plays an important role in Pondoland's climate and ecology, primarily by preventing the warm-water Agulhas Current from drifting too far from the coastline during the Pleistocene and Holocene (Winter and Martin, 1990;Peeters et al., 2004). ...
Waterfall Bluff is a rock shelter in eastern Pondoland, South Africa, adjacent to a narrow continental shelf that limited coastline movements across glacial/interglacial cycles. The archaeological deposits are characterized by well-preserved stratigra-phy, faunal, and botanical remains alongside abundant stone artifacts and other materials. A comprehensive dating protocol consisting of 5 optically stimulated luminescence ages and 51 accelerator mass spectrometry 14 C ages shows that the record of hunter-gatherer occupations at Waterfall Bluff persisted from the late Pleistocene to the Holocene, spanning the last glacial maximum and the transition from the Pleistocene to the Holocene. Here, we provide detailed descriptions about the sedimen-tary sequence, chronology, and characteristics of the archaeological deposits at Waterfall Bluff. Remains of marine mollusks and marine fish also show, for the first time, that coastal foraging was a component of some hunter-gatherer groups' subsistence practices during glacial phases in the late Pleistocene. The presence of marine fish and shellfish further demonstrates that hunter-gatherers selectively targeted coastal resources from intertidal and estuarine habitats. Our results therefore underscore the idea that Pondoland's coastline remained a stable and predictable point on the landscape over the last glacial/inter-glacial transition being well positioned for hunter-gatherers to access resources from the nearby coastline, narrow continental shelf, and inland areas.
... East Gondwana (Antarctica-India-Madagascar-Australia-New Zealand; Ali & Aitchison, 2008;Jokat, Boebel, König, & Meyer, 2003;McLoughlin, 2001). These landmasses continued breaking apart over the course of the Mesozoic, and while there has been debate over the precise timing (and in some cases, the order) of fragmentation, the commonly accepted pattern is as follows: South America separated from Africa starting ~150-120 Mya; India-Madagascar split from Antarctica-Australia-New Zealand ~130 Mya; New Zealand rifted away from Antarctica-Australia starting ~80 Mya; finally, Australia separated from Antarctica (and by very distant proxy, South America, which was connected to Antarctica via the Antarctic Peninsula) ~50-41 Mya, eventually resulting in the continental configuration seen today in the southern hemisphere (Ali & Aitchison, 2008;Eagles, 2007;Jokat et al., 2003;Sanmartín & Ronquist, 2004;Scotese, 2004;Upchurch, 2008;Wei, 2004). ...
Aim
We explored the extent to which Gondwanan vicariance contributed to the circum‐Antarctic distribution of the mite harvestman family Pettalidae, a group of small, dispersal‐limited arachnids whose phylogeny has been poorly resolved, precluding rigorous biogeographic hypothesis testing.
Location
Continental landmasses of former temperate Gondwana (Chile, South Africa, Sri Lanka, Australia and New Zealand).
Taxon
Pettalidae, Opiliones.
Methods
We generated transcriptomes for a phylogeny of 16 pettalids, spanning 9 genera. Data were analysed using maximum likelihood, Bayesian inference and coalescence methods. The phylogenetic position of the Sri Lankan genus Pettalus was further explored using quartet likelihood mapping and changes in gene likelihood scores. We also estimated divergence times and looked for signatures of extinction across Antarctica and central Australia using previously published phylogenies with near‐complete species sampling constrained to match our transcriptomic results. Finally, we estimated ancestral ranges and inferred instances of vicariance.
Results
We recovered a well‐supported topology with a division between taxa from landmasses that made up East Gondwana, and a grade of taxa from West Gondwana. Pettalus was resolved either as the sister group of the Queensland‐endemic Austropurcellia , or as the sister group to a larger clade from East Gondwana, though favouring Pettalus + Austropurcellia . Divergence times for multiple vicariance events coincided with Gondwana's breakup. Speciation–extinction analysis found one diversification process for the family: an initial burst of cladogenesis that slowed down through time.
Main Conclusions
Given that the order of cladogenesis corresponds to the order in which Gondwana fragmented, and the concurrent timing of vicariance and rifting, Gondwanan breakup explains major biogeographic patterns in Pettalidae. Some divergences predate initial rifting, but there is no evidence of trans ‐oceanic dispersal. The Sri Lanka–eastern Australia relationship makes sense in the light of large‐scale extinction across Antarctica and central Australia; however, we find no clear signatures of mass extinction.
... Direct land connection between Africa and South America was severed after the Late Cretaceous and by the middle Eocene the shortest distance between them exceeded 1000 km (Sclater et al. 1977;Scotese 2004;Eagles 2007). Hence, if land vertebrates migrated from one continent to the other after this period, some kind of transatlantic dispersal must have occurred (Hoffstetter 1972;De Oliveira et al. 2009). ...
Several teiid specimens (frontal, vertebra, maxillae) are described from the late Eocene of Europe (MP17, Phosphorites du Quercy). The results of phylogenetic analyses confirm that these European Eocene fossils belong to teiid lizards and more specifically to the subfamily Tupinambinae. So far, the Paleogene record of teiids is limited to South America and no occurrence of crown teiids is known in Europe. This disjunct distribution of teiids during the Eocene suggests transatlantic dispersal and this possibility is discussed. The presence of teiids in the European fossil record is brief (limited to standard level MP17). The circumstances that prevented the persistence of an invading clade in Europe are examined. Ecological (e.g. biotic interactions) and/or demographic (Allee effect) processes may have been involved.
... The plate reconstructions based on magnetic anomaly interpretation indicate an early inception of oceanic crust in the region offshore Argentina by the Late Jurassic/Early Cretaceous, followed by oceanic spreading in the Central South Atlantic during the Late Aptian-Early Albian, and culminating with the opening of the Equatorial South Atlantic by the Albian (Eagles 2007;Torsvik et al. 2009;Moulin et al. 2010;Heine et al. 2013). ...
The Brazilian continental margin includes several volcanic islands, submerged volcanic seamounts, and a unique non-volcanic archipelago located in a transform segment of the Equatorial South Atlantic. The mechanism of formation of these islands is related to post-breakup magmatic episodes dated as Late Cretaceous to Pleistocene. Diverse Late Cretaceous to Paleogene alkaline magmatic episodes are registered in southeast Brazil, resulting in igneous plugs onshore and volcanic structures offshore. The Abrolhos Volcanic Complex in eastern Brazil is characterized by several volcanic features on the continental shelf, including small islands that expose Paleogene sedimentary layers interbedded with volcanic sequences. The adjacent Vitória-Trindade Chain extends to oceanic crust forming basaltic to alkaline seamounts that outcrop at the Trindade Archipelago, the easternmost islands in Brazil with the youngest volcanic eruptions. The Fernando de Noronha lineament in northeast Brazil is characterized by Neogene alkaline igneous plugs. The small islets in the São Pedro—São Paulo archipelago, located near the mid-Atlantic ridge, are formed by exhumed mantle rocks related to compressional episodes a transform fault zone. The Rio Grande Rise in southern Brazil is characterized by shallow Paleogene seamounts and a large oceanic plateau probably related to subaerial spreading centers formed in the Late Cretaceous. Multiple mechanisms are responsible for the origin and evolution of the volcanic islands offshore Brazil in continental, transitional, and oceanic crust settings, including volcanic build-ups, leaking fracture zones, and hotspots. Some of the islands might be related to mantle plume activity, as indicated by comparisons with modern mantle plume analogues in the South Atlantic.
... The~800 km long South Coast of the Cape Floristic Region, the Cape South Coast, represents a tract of relatively low-relief but nonetheless dissected coastal plain bordered at its landward geographic limit by the Cape Fold Belt that rises up to 3000 m and at the seaward margin by the Indian Ocean (Fig. 1). The fragmentation of Gondwana in the South Coast region and the opening of the South Atlantic commenced in the Early Cretaceous (~136 Ma) as South America was sheared westward along the Agulhas-Falkland Fracture Zone (Martin and Hartnady, 1986;Eagles, 2007). This rifting was accompanied by extensive mafic igneous activity along the entire west coast of Southern Africa, from the Walvis Ridge in the north, to the Cape Peninsula in the south (Day, 1986;Trumbull et al., 2007). ...
The South African Cape South Coast is bordered by one of the broadest continental shelves in Africa. The Agulhas Bank, inshore shelf and presently exposed coastal plain make up the Palaeo-Agulhas Plain (PAP), though our area of study extends beyond this limit and as far inland as the first mountain belt. Quaternary sea levels have been significantly lower than at present for ∼90% of the Pleistocene, exposing a terrestrial ecosystem on what is now the submerged shelf. The presently drowned component makes up 94% of the total area of the PAP. Past work has hypothesised a contrast in character of this submerged landscape when compared to the subaerial environment. Here, we assimilate newly-acquired geophysical and geological datasets to produce geological- and soil maps from the Last Glacial Maximum on a scale of 1:750,000, covering an area of ∼55,000 km. Three broad geomorphic zones are defined, including the Western section from Cape Agulhas to Cape Infanta, the Central section from Cape Infanta to Knysna and the Eastern section extending eastward of Knysna. We demonstrate that Mesozoic sedimentary deposits crop out near the surface on this current-swept shelf and soils derived from siltstone and shale bedrock are prominent when the coast is up to 64 km distant from the modern shoreline at its maximum point. Beyond this, weathered limestone dominates the substrate sequences on the Agulhas Bank. We show that the submerged landscape was a unique terrestrial environment and that there is no exact modern-day analogue in the region other than a small (∼70 km²) area located at the edge of the Agulhas Plain near Cape Agulhas, and map major contrasts in the geological, topographic and edaphic nature of the landscape from the onshore to the offshore. The expansion of this plain is coupled with exaggerated floodplains, meandering shallowly incised rivers and wetlands. The submerged shelf is dominated by fertile soils compared to the dissected onshore belt, and extensive calcareous dunefields extending up to 10 km inland from their associated palaeoshorelines covered much of the emergent shelf. Sedimentary bedforms may have obstructed or slowed drainage as suggested by leached palaeosols and carbonate mixing observed in petrographic thin sections and grain mounts. The data show a low-relief “plains” landscape, which contrasts strongly to the topographically complex contemporary coastal foreland.
... This time period coincides with the timing of retrogression of the east-dipping subduction zone during exhumation, and compression at this time obducted blueschists and eclogites of the Raspas, Arquía, and Barragán complexes onto the South American margin. Compression may have occurred due to an increase in the convergence rates of the oceanic Caribbean Plate ) and South America, as a consequence of the opening of the South Atlantic, which started at ~120 Ma (Eagles, 2007). Detrital zircon fission track dates from the Oriente Basin of Ecuador ...
... Shaw and Cande, 1990) and range of applicability (e.g. Eagles, 2007;Pérez-Díaz and Eagles, 2014) of plate kinematic models. To enjoy these advantages, I implement an inversion scheme that was developed by Nankivell (1997) and is described by Eagles (2004) and Livermore et al. (2005). ...
Divergence of the Australian and East Antarctic plates is well understood from the late Jurassic onset of half graben development on the Australian continental shelf, and from post mid-Eocene (chron 20; 45 Ma) seafloor spreading isochrons further offshore. Relative plate motion between these times is less confidently interpretable from magnetic reversal anomalies landwards of isochron 20 and localised evidence for mid-to-late Cretaceous subsidence and growth strata from the continental shelf and rise south of Australia. A new test of this history examines it within the post-34y (84 Ma) Indian Ocean plate circuit, built using seafloor spreading data from the Wharton and central Indian Ocean basins. The Australian-Antarctic Jurassic-onset rift system is interpreted to have been abandoned before 34y, because motion in the circuit is inconsistent with an active plate boundary during 34y–26y (84–58 Ma). Starting 26y, the model depicts plate divergence distances and azimuths that, after 25y (57 Ma), can be independently confirmed by reassessment of the pre-chron 20 magnetic reversal anomaly pattern. Previous studies have identified evidence for mantle exhumation and focused magmatism in basement marginwards of these anomalies. These processes are not directly or confidently dated, but mantle exhumation is inconsistent with the circuit model's fast plate divergence at 26y–25y. Hence, plate motion during the immediate build-up to post-57 Ma seafloor spreading may have been accommodated by focused magmatism, whilst mantle exhumation may mark the conclusion of the Jurassic-onset rift phase during a slower pre-84 Ma period of plate divergence. Using the new model to make tectonic reconstructions results in a large overlap between Tasmania and Victoria Land that can be explained with reference to Eocene strike-slip faulting and transtension in recently- discovered subglacial basins of Wilkes and George V lands and Terre Adélie.
The early evolution of the South Atlantic Ocean following the Cretaceous break-up of Gondwana is extensively recorded in rift basins along the conjugate margins of Africa and Brazil. For the Brazil margin, divergent views of the source and pathway of the initial seawater incursion persist due to a paucity of recognized transitional sequences that document marine transgressive deposits over the continental interior. To address this, we conducted a high-resolution sedimentological and geochemical study through a core in the Campos Basin that encompasses the key lithologic switch from lacustrine carbonate to marine evaporite settings. Steroid lipid biomarkers, derived from marine algae, make a striking appearance in concert with a pronounced negative shift of 87Sr/86Sr ratios and coincident with the appearance of anhydrite. Importantly, the sulfur-sequestered biomarkers reveal a dynamic system where redox-stratified and anoxic conditions were amplified along with a deepening chemocline through the marine transition.
Argentina's offshore Colorado basin is a passive continental margin basin covering a vast area on the South America, yet remains underexplored today. Based on literature review, 2D seismic profiles interpretation and drilling data analysis, the tectonic evolution background, petroleum geological characteristics and exploration targets to be discovered in this basin are researched in this paper. It is concluded that the Colorado Basin is mainly influenced by the African and South America plates unzippering from south to north, which formed syn-rift grabens and half-grabens and continental-marine sediments with a maximum sediment thickness of more than 15 km. The tectonic and sedimentary characteristics of the basin control the conditions of hydrocarbon accumulation. The lower cretaceous Forting Formation is maily composed of lacustrine shale, which is a good potential source rock. Colorado Formation is mainly continental to shallow marine sandstone with high porosity and permeability, which has been proved to be oil-bearing reservoir. From the Eocene to the present, the basin deposited Elvira Formation, Barranca Final Formation and Pampa Group, the abyssal shale of which can be regional seal and cap rocks. Finally, the deep water and semi-deep water continental slope of the eastern Colorado Basin has good hydrocarbon accumulation conditions and certain exploration potential.
During the Early Cretaceous, several extensive carbonate shelves were developed in the sedimentary basins located on the Central Segment of the South Atlantic Ocean. These marine successions are characterized by mixed carbonate and siliciclastic sediments with a diverse fossil content, deposited during the late Aptian–Albian interval. The microbiofacies content of two continuous cores (SER-01 and SER-03), each approximately 200 m deep, drilled in the onshore region of the Sergipe–Alagoas Basin, north-eastern Brazil consists of mudstones, wackestones, packstones, grainstones, rudstones, marls, claystone, shales and sandstones in Core SER-01, whereas those from Core SER-03 are essentially fine-grained lithologies such as mudstones, wackestones, claystone and shales. Two low-frequency sedimentation cycles are identified, with SER-01 being characterized by restricted and shallow-marine facies that grade up in a transgressive trend, which is also present throughout Core SER-03. Six microfacies are recognized, indicating deposition in a high-energy shallow-marine environment with a sand-bar system, which later evolved into a low-energy deeper-marine environment (outer shelf). The microfossil content includes planktic and benthic foraminifera, which are dominant in both cores, as well as macrofossils such as echinoderms and molluscs. Cadosinids, green algae, microcrinoids and inoceramids occur locally in both cores. In addition, Core SER-01 is characterized by abundant microbial units. The fossil content shows palaeogeographic affinities with the Tethyan Realm. These records suggest water connections between the South Atlantic and the North Atlantic/West Tethys Sea since the late Aptian interval.
High purity fluorite veins (acid grade > 97 % CaF 2) are generally rare in a global context. The fluorite-(±quartz-calcite) veins in the Aukam Valley mining district in SW Namibia have not previously been studied but due to their excellent exposure they are an ideal natural laboratory to study such mineralization. These veins cross-cut Mesoproterozoic high-grade gneisses and granites (Namaqua-Natal Metamorphic Province) close to the unconformity with the overlying late-Neoproterozoic Nama Group sedimentary rock cover. Based on the regional context, their genesis has been assumed to be related to nearby fluorite-bearing pegmatites. However, the present contribution provides new microthermometry data from vein fluorite (with 25.0-25.6 wt% NaCl and T h of 270-300°C) that indicate precipitation from a mixed fluid with two chemically contrasting end-members for the early paragenetic fluorite stage. However, it was not possible to identify the chemical composition of the end-members. Based on the chemical composition of the fluid mixture and the regional geological context, we propose that these end-members are F-rich brines sourced from the Namaqua basement and Ca-rich limestone-derived formation fluids of the Nama Group. A fluorite-overgrowing quartz precipitation is likely the effect of post-mixing fluid cooling as the fluid inclusions hosted in quartz show lower homogenization temperatures (T h 230-260°C) with similar salinity. Locally, late-stage calcite is recognized with fluid inclusion compositions (0.2-2.7 wt% NaCl and T h of 60-92°C) different from those observed in the fluorite and quartz. With significantly lower homogenization temperatures, they likely represent precipitates formed during the shutdown and collapse of the hydrothermal system. The fluorite veins are hosted in post-Cambrian N-trending brittle structures that probably formed during the opening of the South Atlantic in late Mesozoic times. Interestingly, similar massive fluorite mineralization on both sides of the Atlantic Ocean is also likely associated with Pangea rifting and known from numerous mining districts operating on unconformity-related hydrothermal veins in Central and North America, northern Africa and Europe.
Le but de cette thèse est la détermination d’un champ de vitesse GNSS actuel de l’Afrique afin de caractériser sa cinématique et ses déformations actives. Différents modèles cinématiques, sismotectoniques ont montré que la plaque Afrique était subdivisée en deux grandes plaques Nubie et Somalie. La densification des données GNSS sur le Rift Est Africain et les blocs tectoniques orientaux connexes a en outre permis de séparer la plaque somalienne de 3 autres sous-plaques. Aujourd’hui, la densification des données géodésiques sur la plaque Nubie a permis de la subdiviser en 4 blocs tectoniques majeurs. Les cœurs de ces blocs sont respectivement formés des trois cratons principaux archéens et un méta-craton qui sont séparés les uns des autres par de ceintures orogéniques néo-protérozoïques et des monts-sous-marins. Le long de ces ceintures se sont formées des zones de cisaillement, des failles actives et du volcanisme actif constituant des lignes de déformation continues (LDC). Le long de LDC, les régimes déformation prédits par la géodésie sont cohérent avec les caractéristiques sismotectoniques de ces régions.
The margins of the North Atlantic Ocean, including the Newfoundland and Iberian Margins, present two distinct episodes of rifting: at Permian–Trias times and in the middle Cretaceous. In the South Atlantic Ocean, rifting occurred on the location of the Pan‐African suture, more than 450 Ma after its formation, and the two events are clearly dissociated. At first order, the geodynamic segmentation of the South and Equatorial Atlantic Oceans leads to the formation of different types of passive margins, showing a relationship between the regional geodynamic context and the structural architecture of passive margins. The Central Segment of the South Atlantic Ocean is characterized by sedimentary basins with pre‐ and syn‐break‐up magmatism, and the presence of an approximately 1–2 km‐thick salt layer in the so‐called continent‐ocean transition, overlying a mainly non‐marine sequence.
The Early Cretaceous geology of Northwestern (NW) Peru was characterized by emplacement of voluminous felsic volcanic rocks (e.g. the Oyotún Fm.), which have important implications for the understanding of Early Cretaceous crust-mantle interactions in the Central Andes. We present U-Pb-Hf isotopic compositions and whole-rock and trace elemental compositions of a series of Early Cretaceous volcanic rocks from the Western Cordillera and the Eastern Cordillera of the Central Andes. The volcanic rocks outcrop sporadically with limited thickness and uniform volcanic lithofacies in the western region, while the eastern region has distributions of larger scale volcanic rocks with more complex volcanic lithofacies. LA-ICP-MS zircon U-Pb dating defines the age of the Oyotún Fm. range from 138.8 Ma to 131.0 Ma in the Western Cordillera, and from 139.7 Ma to 132.3 Ma in the Eastern Cordillera. In situ Hf isotope analysis on dated zircon yield an εHf(t) range of +3.37 to +8.42 and a TDM2(Hf) range from 983 Ma to 650 Ma with a peak at ca. 840 Ma. The Early Cretaceous felsic magmas are predominantly dacites and rhyolites with minor components of basalt and andesite, and show relatively homogeneous Hf isotopic compositions of zircon in single samples. Both the εHf(t) of zircons and the magma temperature increase from the bottom to the top of the Early Cretaceous volcanic rocks in each studied area. All these facts imply an increasing contribution of contemporaneous underplated hot and dry mantle-derived magmas with time, both as a heat source inducing crustal melting and a source of material (melt) that variably mixed with the ancient continental crustal materials.
This contribution aims to present a proposal for the origin of the Cretaceous igneous alkaline rocks of SE Brazil. New studies with applications of petrographic, geochronological, structural, and geophysical tools establish a temporal and geodynamic correlation between the basaltic rocks of the Rio Grande Rise and alkaline rocks related to Mesozoic rifts of the SE Brazilian margin. Thus, a model of the generation of continental alkaline rocks is proposed as a result of the Rio Grande Rise formation, where the development of a meso-oceanic chain occurred as a result of mantle convection processes leading to spreading centers on the ocean floor at about 95–91 and 50 Ma, with subsequent volcanic activity until about 50 Ma. In this context, the extensional process consistent with the mantle geodynamics and the formation of meso-oceanic basalts resulted in the formation of mantle magmas whose intrusion through the continental crust resulted in alkaline magma (including crustal contamination) between 85 and 50 Ma. Interruption of the tentative opening of the oceanic crust in the region adjacent to the Rio Grande Rise was due to the obstruction of the South American Platform. This resulted in the abortion of the extensional process affecting the oceanic crust. In this sense, during the period of formation of oceanic basalts between 95 and 50 Ma, several continental intrusions are registered in the onshore region, a major magmatic event characterized by radiometric ages varying from 87 to 70 Ma. Between 70 and 60 Ma, the opening attempt continued with a new front of propagation and magmatic episodes resulting in the formation of an E–W chain of igneous rocks extending north of the Rio de Janeiro coastline, forming the Itatiaia-Cabo Frio lineament, with ages varying from 67 to 50 Ma. The consistency of ⁴⁰Ar/³⁹Ar ages of the continental alkaline rocks and the basaltic rocks of the Rio Grande Rise indicate that volcanism on the paleo-spreading center was coeval with an embryonic process of plate rupture and oceanic opening in regions adjacent to the continental margin. The process resulted from a mantle convection system that formed the oceanic crust at the Rio Grande Rise, simultaneous with the generation of continental alkaline magmas along igneous centers.
The change in plate kinematics, which may have affected slab dip, can deeply influence the interplate coupling and consequently the deformation of the overriding plate. The north-central Patagonia represents a unique region to study the impact of slab-dip changes as this area is characterized by alternating episodes of steep to flat-slab since the onset of subduction below the South American Plate. The alternation of the upper plate subsidence and exhumation events related with slab steepening and shallowing episodes, respectively, or with changes in kinematics has been widely studied in the recent years along the Andes. However, some topics remain highly debated, and especially the magnitude of the vertical movements through time and space. Low-temperature thermochronology systems, which are sensitive to events involving the upper first kilometers of the upper crust, represent an efficient approach for dating and quantifying significant episodes of burial and exhumation. This thesis investigates the thermal history of the north-central Patagonian broken foreland and the Cordillera. The latter area has been widely studied through bedrock thermochronology studies compared with data from the Patagonian foreland, where only few data are available. Thus, this work aims at improving our knowledge on the relationships between the surface cooling record and the geodynamic evolution.Three apatites thermochronometers (U-Pb, fission tracks and (U-Th-Sm)/He) were used on bedrocks, Mesozoic and Cenozoic sandstones as well as on modern sediments. All samples were collected from the inner part of the north-central Patagonian Cordillera to the distal part of the broken foreland along the Atlantic coast. Compilation of these diverse thermochronology datasets, integrated by inverse thermal modeling and provenance analysis, is discussed here and compared with the different geodynamic processes proposed for Patagonia (e.g. convergence changes, slab dip variations). The new thermochronology records indicate that main unroofing of Patagonia (~3 – 4 km), Cordillera and broken foreland included, occurred during a slab-shallowing episode from late Early Cretaceous to early Paleogene, after a Jurassic – Early Cretaceous heating episode related to a significant burial below a thick sedimentary cover. This regional exhumation phase was followed by a period between the late Eocene and the early Miocene of foreland subsidence associated with slab rollback, characterized by a steady-state post-shortening low exhumation rates. During this period of relative quiescence, detrital analysis point to the Cordillera as a primary contributor of sediments for the entire foreland with a local input from the broken foreland reliefs. Finally, Miocene substantial exhumation is recognized into the Cordillera after a change of convergence rates and obliquity, especially along a dextral strike-slip fault zone active along the Cordillera. However, this exhumation phase is not detected regionally in the broken foreland. Since no regional sedimentary burial or relevant exhumation events affected the north-central Patagonian foreland, the few Oligo-Miocene ages identified along the foreland have been ascribed to the coeval intraplate magmatic processes.
The Lower Congo basin's Albian-Early Cenomanian Pinda group is the most productive carbonate reservoir within the West African margin basins. However, the understanding of its diagenetic features is very limited. Core samples obtained from the Democratic Republic of Congo offshore section of the formation were petrographically analysed to image the sedimentary facies along with diagenetic processes and links to reservoir quality. The sedimentary facies obtained indicate a dominant lagoon sedimentation within the oolitic-siliciclastic ramp. Diagenetic processes detected include micritization, dolomitization, dissolution, cementation, and compaction. Diagenetic links to reservoir quality within the facies is established in the dominant occurrence of moldic pores which have been occluded either by calcite or dolomite cement, leaving three facies with poor reservoir quality. The facie exempted from this class possesses a fair reservoir quality linked to microporosity within its sandstone domain. These observations should serve as important considerations for future reservoir characterization studies within the Pinda group.
Muitos estudos têm sido realizados nas margens rifteadas no intuito de descrever sua
geometria e explicar o processo de seu desenvolvimento. No caso em que, ao longo de uma
mesma margem continental, ela transite entre distintos tipos - rica em magma, pobre em
magma e transformante, como é o verificado ao longo da margem continental do Oeste
Africano SubSaariano, objeto dessa pesquisa, várias questões fundamentais sobre a
configuração do arcabouço estrutural, o comportamento dos domínios estabelecidos, sua
relação com o embasamento adjacente e como se processaria e o que caracterizaria a transição
entre elas, voltam a emergir.
Northeast Georgia Rise is located on inferred oceanic crust that is considered Albian in age and to have formed during the separation of Africa and South America. Basalt overlying a weathered regolith was recovered at Site 698 and a basaltic substratum at other sites is inferred from the downhole variation in pore-water chemistry. The provenance of a 2-m-thick gravel bed containing abundant clasts of continental lithologies displaced into lower Oligocene ooze at Site 699 is an enigma. We infer that at least part of the Northeast Georgia Rise was formed at a spreading center by excessive volcanism. At least two episodes of deformation have subsequently modified the topography of the rise. -from Authors
The geometrical fit of the continents now separated by oceans has long been discussed in relation to continental drift. This paper describes fits made by numerical methods, with a `least squares' criterion of fit, for the continents around the Atlantic ocean. The best fit is found to be at the 500 fm. contour which lies on the steep part of the continental edge. The root-mean-square errors for fitting Africa to South America, Greenland to Europe and North America to Greenland and Europe are 30 to 90 km. These fits are thought not to be due to chance, though no reliable statistical criteria are available. The fit of the block assembled from South America and Africa to that formed from Europe, North America and Greenland is much poorer. The root-mean-square misfit is about 130 km. These geometrical fits are regarded as a preliminary to a comparison of the stratigraphy, structures, ages and palaeomagnetic results across the joins.
A new set of parameters for the total plate tectonic reconstruction of South America and south central Africa is presented: euler pole 46.75°N, 32.65°W; rotation angle 56.40°. This fit is constrained by at least three pre-drift tectonic features crossing from one continent to the other: (1) the geophysically defined eastern and western boundaries of the submarine Jurassic Outeniqua Basin (South Africa) and the Falkland Plateau Basin; (2) the Late Precambrian transcurrent fault and mylonite belts of Pernambuco (Brazil) and Foumban (West Africa); and (3) the Triassic northern tectonic front of the Cape Fold Belt and the major morphological feature on the Falkland Plateau with which it is closely lined up. Isotopic ages of Falkland Plateau gneisses correspond to Cape Pluton and Cape Fold Belt ages, suggesting their palaeoposition was within the realm of the Cape Fold Belt.
In addition, the bathymetrically and geophysically defined northeastern apex of the Falkland Plateau fits into the re-entrant angle defined on the South African margin by the steep southeast-facing sheared Agulhas margin and the southern face of the Tugela Cone. Simultaneously known Precambrian outcrops in northeastern Brazil and in the Gulf of Benin area of West Africa are juxtaposed rather than overlapped. Reconstructions producing a closer fit of these cratonic areas are considered untenable.
A Late Jurassic mantle plume may have generated hotspot tracks on the North American plate and the Yucatan Penninsula tectonic block as the Gulf of Mexico opened (ca. 150 Ma). The tracks are identified from deep basement structural highs that have been mapped by integrating seismic refraction and gravity data. They are associated with high-amplitude, distinctive gravity anomalies that provide the basis for a kinematic reconstruction that restores the western ends of the hotspot tracks with a 20j clockwise rotation of the Yucatan block or almost one-half the total rotation required to open the Gulf of Mexico Basin. The duration of track generation is estimated to have been about 8--10 m.y. or almost one-half the total time required to open the Gulf of Mexico Basin. Prior to this rotation, extension of continental crust over a 10--12-m.y. interval was the result of approximately 22j of counterclockwise rotation and crustal thinning. Autochthonous salt appears to be confined to the continental flanks of the hotspot tracks, confirming that salt was deposited during continental extension and not after ocean floor had begun to form. A prominent gravity anomaly along the western boundary of the basin is interpreted to be produced by a marginal ridge, which was created along the ocean-continent transform boundary as the basin opened. The eastern flank of this marginal ridge and the northernmost, easternmost, and southernmost terminations of the hotspot tracks are interpreted to coincide with the oceanic-continental crustal boundary in the basin.
Two contrasting end-members of passive continental margins, here referred to as wide and narrow margins, are present in the South Atlantic, although some margin segments are intermediate between these two styles. Narrow margins are characterized by a large bounding fault (>4 km throw) near the shelf edge; a sharp transition zone (10-20 km wide) from normal thickness to substantially thinned continental crust or oceanic crust; where the total width of extended continental crust is usually less than 100 km. Wide continental margins are characterized by a broad continental shelf and a wide zone (50-600 km) of thinned crust with an even distribution of faults. Wide margins usually have a more gentle continental shelf and slope. Narrow-narrow, wide-wide, and wide-narrow conjugate margin pairs exist in the South Atlantic and are suggested to be controlled mainly by the opening kinematics and the thermal structure of the lithosphere during rifting. Basement structure and strain rate appear to have exerted little control on the style of margin extension. Although basement architecture appears to control the trend of the initial rifts, the positions of fracture zones which separate narrow and wide segments, and the rate of lateral rift propagation.
2005.03 Synthetic apparent polar wander (APW) paths for North America, South America, Eurasia, India, Central Africa, Australia and Antarctica for the last 200 Myr are proposed. Computation of these APW paths is based upon the latest version (4.5a) of the Global Paleomagnetic Database (GPMDB), a revised global plate tectonic model since the Early Jurassic, and a new technique for generating smoothed APW paths. The smoothing technique includes the following steps: (1) pre-selection of palaeopoles, including pre-filtering parameters (number of sites, number of samples per site, 95 per cent confidence circle about mean direction, cleaning procedure, and time uncertainty); (2) generation of palaeolatitude and declination plots for a reference site on each continent that combines palaeopoles via a global plate tectonic circuit; (3) independent spline regression analyses of the palaeolatitude and declination plots; (4) removal of palaeolatitude or declination data that deviate by more than 10° from the regression curves (post-filtering process); (5) generation of synthetic APW paths from the resulting palaeolatitude and declination plots. These synthetic APW paths are then rotated into African coordinates to determine the best-fit APW path and a global palaeomagnetic reference frame. Four representative plate tectonic reconstructions and global plate velocity fields are presented for the three time intervals that correspond to globally synchronous changes in plate motion.
A Permo-Triassic reconstruction of western Pangea (North America, South
America, Africa) is proposed that is characterized by (1) definition of
the North Atlantic fit by matching of marginal offsets (fracture zones)
along the opposing margins, (2) a South Atlantic fit that is tighter
than the BuIlard fit and that is achieved by treating Africa as two
plates astride the Benue Trough and related structures during the
Cretaceous, (3) complete closure of the Proto-Atlantic Ocean between
North and South America, accomplished by placing the Yucatan block
between the Ouachita Mountains and Venezuela, (4) a proposed Hercynian
suture zone that separates zones of foreland thrusting from zones of
arc-related magmatic activity; to the northwest of this suture lie the
Chortis block and Mexico and most of North America, and to the southeast
lie South America, the Yucatan Block, Florida and Africa, and (5)
satisfaction of paleomagmatic data from North America, South America,
and Africa. Beginning with the proposed reconstruction, the relative
motion history of South America with respect of North America is defined
by using the finite difference method. Within the framework provided by
the proposed relative motion history, an evolutionary model for the
development of the Gulf of Mexico and Caribbean region is outlined in a
series of 13 plate boundary reconstructions at time intervals from the
Jurassic to the present. The model includes (1) formation of the Gulf of
Mexico by 140 Ma, (2) Pacific provenance of the Caribbean plate through
the North America-South America gap during Cretaceous time, (3)
Paleocene-Early Eocene back arc spreading origin for the Yucatan Basin,
whereby Cuba is the frontal arc and the Nicaragua Rise-Jamaica-Southern
Hispaniola is the remnant arc, and (4) 1200 km of post-Eocene cumulative
offset along both the Northern and Southern Caribbean Plate Boundary
Zones, allowing large-scale eastward migration of the Caribbean plate
with respect to the North and South American Plates.
Seventy sites of sills, flows and dikes from Northeastern Paraná Magmatic Province (PMP), were submitted to paleomagnetic, chemical and radiometric analyses. The rocks are high in TiO2 content, and similar in composition to the rocks from the northern region of PMP. The sills intrude mainly Paleozoic sediments, and can be subdivided into two domains; the northern being characterized by sills showing reversed polarities, and the southern essentially by sills of normal polarities. 40Ar/39Ar dating of three distinct sills gave plateau ages (129.9 ± 0.1, 130.3 ± 0.1 and 131.9 ± 0.4 Ma) that are similar to surface-outcropping flows of the Northern Paraná Basin, and the Ponta Grossa dikes. The new paleomagnetic data combined with existing data from the northern PMP allowed the calculation of a paleomagnetic pole at 71.4° E and 83.0° S (N = 92; α95=2.4°; k = 39). This pole is in good agreement with poles for central and southern PMP, which are slightly older than the northern PMP, as well as for the contemporaneous Central Alkaline Province (Paraguay) on the western side of PMP. In contrast, the coeval pole for the Ponta Grossa dikes (eastern border of PMP), however, is slightly displaced from that group of poles, suggesting that dikes in that area may have undergone some tectonic tilting.
The main structural and geomorphological features along the Amazon River are closely associated with Mesozoic and Cenozoic tectonic events.The Mesozoic tectonic setting is characterised by the Amazonas and Marajó Basins, two distinct extensional segments. The Amazonas Basin is formed by NNE–SSW normal faults, which control the emplacement of dolerite dykes and deposition of the sedimentary pile. In the more intense tectonic phase (mid-Late Cretaceous), the depocentres were filled with fluvial sequences associated with axial drainage systems, which diverge from the Lower Tapajós Arch. During the next subsidence phase, probably in the Early Tertiary, and under low rate extension, much of the drainage systems reversed, directing the paleo-Amazon River to flow eastwards. The Marajó Basin encompasses NW–SE normal faults and NE–SW strike-slip faults, with the latter running almost parallel to the extensional axes. The normal faults controlled the deposition of thick rift and post-rift sequences and the emplacement of dolerite dykes. During the evolution of the basin, the shoulder (Gurupá Arch) became distinct, having been modelled by drainage systems strongly controlled by the trend of the strike-slip faults. The Arari Lineament, which marks the northwest boundary of the Marajó Basin, has been working as a linkage corridor between the paleo and modern Amazon River with the Atlantic Ocean. The neotectonic evolution since the Miocene comprises two sets of structural and geomorphological features. The older set (Miocene–Pliocene) encompasses two NE-trending transpressive domains and one NW-trending transtensive domain, which are linked to E–W and NE–SW right-lateral strike-slip systems. The transpressive domains display aligned hills controlled by reverse faults and folds, and are separated by large plains associated with pull-apart basins along clockwise strike-slip systems (e.g. Tupinambarana Lineament). Many changes were introduced in the landscape by the transpressive and transtensive structures, such as the blockage of major rivers, which evolved to river-lakes, transgression of the sea over a large area in the Marajó region, and uplift of long and narrow blocks that are oblique to the trend of the main channel. The younger set (Pliocene–Holocene) refers to two triple-arm systems of rift/rift/strike-slip and strike-slip/strike-slip/rift types, and two large transtensive segments, which have controlled the orientation of the modern drainage patterns.
Closely spaced satellite altimeter profiles collected during the Geosat Geodetic Mission (∼6 km) and the ERS 1 Geodetic Phase (8 km) are easily converted to grids of vertical gravity gradient and gravity anomaly. The long-wavelength radial orbit error is suppressed below the noise level of the altimeter by taking the along-track derivative of each profile. Ascending and descending slope profiles aie then interpolated onto separate uniform grids. These four grids are combined to form comparable grids of east and north vertical deflection using an iteration scheme that interpolates data gaps with minimum curvature. The vertical gravity gradient is calculated directly from the derivatives of the vertical deflection grids, while Fourier analysis is required to construct gravity anomalies from the two vertical deflection grids. These techniques are applied to a combination of high-density data from the dense mapping phases of Geosat and ERS 1 along with lower-density but higher-accuracy profiles from their repeat orbit phases. A comparison with shipboard gravity data shows the accuracy of the satellite-derived gravity anomaly is about 4-7 mGal for random ship tracks. The accuracy improves to 3 mGal when the ship track follows a Geosat Exact Repeat Mission track line. These data provide the first view of the ocean floor structures in many remote areas of the Earth. Some applications include inertial navigation, prediction of seafloor depth, planning shipboard surveys, plate tectonics, isostasy of volcanoes and spreading ridges, and petroleum exploration.
The East Brazilian Rift system (Ebris) constitutes the northern segment of the South Atlantic rift system which developed during the Mesozoic breakup of South America and Africa. Following crustal separation in the Late Aptian, it evolved into a passive continental margin.Along the continental margin six basins are recognized, while three onshore basins form part of an aborted rift. Three continental syn-rift stratigraphic sequences are recognized, spanning Jurassic to Barremian times. The Jurassic (Syn-rift I) and Neocomian (Syn-rift II) phases were most active in the interior rift basins. During the Barremian (Syn-rift III), rift subsidence rates were twice as large as during the Neocomian (Syn-rift II), both in the interior rift and in the marginal rift segments, indicating that rift axis did not migrate from the interior to the marginal setting.Rift magmatism was centered on the southern ebris and peaked between 130 and 120 Ma during syn-rift phase II. Rift phase III was followed by a transitional marine, evaporitic megasequence of Aptian age, which directly overlies the rift unconformity and a marine drift megasequence which spans Albian to Recent times.During the Late Cretaceous, sedimentation rates responded to first-order eustatic sea-level fluctuations. Tertiary accelerated sedimentation rates can be related to local clastic supply which filled in spaces inherited from previous starved conditions. Between 60 and 40 Ma, post-rift magmatism, centered on the Abrolhos and Royal Charlotte banks, is probably related to development of a hot spot associated with the Vitória-Trindade Seamount Chain.Although crossing three distinct Precambrian tectono-thermal provinces, ranging from Archean through Late Proterozoic, rift structures follow a general NE trend, subparallel to the principal basement fabric. A NW-SE oriented stress field appears to be compatible with both Neocomian and Barremian phases of crustal extension.Profiles transverse to the rift axis indicate crustal stretching factors ranging between β = 2.16 and 2.88. In the shallow portions of the rift, surface extension and crustal thinning seem to be compatible; however, in the deep portions of the basins, this relationship could not be tested.Reinterpretation of refraction profiles, north and south of the Walvis-São Paulo Ridge transform, indicates that seafloor spreading, from M3 anomaly to Aptian off Pelotas Basin, was taken up by crustal extension in the São Paulo Plateau. Differences in stretching rates may have been accommodated by extension across the Ponta Grossa Arch.The Early Aptian syn-rift/post-rift transition in the ebris marginal basins does not coincide with the onset of the drift phase during the Early Albian. This apparent discrepancy may be explained by a change from distributed margin-wide extension to a focused mode of extension near the future continent/ocean boundary.
Mesozoic seafloor spreading anomalies M0—M10 have been identified in the Natal Valley between the south-eastern African margin and the Mozambique Ridge. These identifications confirm that seafloor spreading north and south of the Falkland Agulhas Fracture Zone (FAFZ) began simultaneously. The Natal Valley anomalies are offset ∼ 1300 km by the FAFZ from their equivalents in the southern Cape Basin. Their positions confirm that the Tugela Ridge marks the continent/ocean boundary (COB) in the Natal Valley. We have computed successive Falkland Plateau palaeo-positions using Natal Valley and Georgia Basin anomalies. These show that the offset in spreading ridges at the FAFZ remained ∼ 1300 km long from M10 to M0 time. By anomaly 34 time, the offset was ∼ 1270 km. Therefore no major ridge jumps had occurred by then.
Dating M0 as 108 Myr bp and anomaly 34 as 80 Myr bp, the average half-spreading rate immediately south of the FAFZ for the Cretaceous Normal Polarity Epoch is 4.2 cm yr⁻¹. Using this, we date: (a) the change in early pole of rotation at 105 Myr; (b) a reconstruction which juxtaposes salt boundaries in the Brazil and Angolan basins at 103.7 Myr; (c) final separation of the Falkland Plateau from southern Africa at 98.3 Myr; (d) the formation of the oceanic northern part of the Agulhas Plateau at 97.3–90.7 Myr.
Comparison of magnetic data with implied COB positions in the southernmost Cape and Argentine Basins, and the Georgia Basin suggests continental separation began 122–127 Myr bp with undeformed magnetic anomalies dating from 122 Myr (M10). These dates are consistent with micropalaeontological and sedimentological data around southern Africa.
Closely spaced satellite altimeter profiles collected during the Geosat Geodetic Mission (-6 km) and the ERS 1 Geodetic Phase (8 km) are easily converted to grids of vertical gravity gradient and gravity anomaly. The long-wavelength radial orbit error is suppressed below the noise level of the altimeter by taking the along-track derivative of each profile. Ascending and descending slope profiles are then interpolated onto separate uniform grids. These four grids are combined to form comparable grids of east and north vertical deflection using an iteration scheme that interpolates data gaps with minimum curvature. The vertical gravity gradient is calculated directly from the derivatives of the vertical deflection grids, while Fourier analysis is required to construct gravity anomalies from the two vertical deflection grids. These techniques are applied to a combination of high-density data from the dense mapping phases of Geosat and ERS 1 along with lower-density but higher-accuracy profiles from their repeat orbit phases. A comparison with shipboard gravity data shows the accuracy of the satellite-derived gravity anomaly is about 4-7 mGal for random ship tracks. The accuracy improves to 3 mGal when the ship track follows a Geosat Exact Repeat Mission track line. These data provide the first view of the ocean floor structures in many remote areas of the Earth. Some applications include inertial navigation, prediction of seafloor depth, planning shipboard surveys, plate tectonics, isostasy of volcanoes and spreading ridges, and petroleum exploration.
The data from a recent magnetic compilation by Verhoefet al. (1991) off west Africa were used in combination with data in the western Atlantic to review the Mesozoic plate kinematic evolution of the central North Atlantic. The magnetic profile data were analyzed to identify the M-series sea floor spreading anomalies on the African plate. Oceanic fracture zones were identified from magnetic anomalies and seismic and gravity measurements. The identified sea floor spreading anomalies on the African plate were combined with those on the North American plate to calculate reconstruction poles for this part of the central Atlantic. The total separation poles derived in this paper describe a smooth curve, suggesting that the motion of the pole through time was continuous. Although the new sea floor spreading history differs only slightly from the one presented by Klitgord and Schouten (1986), it predicts smoother flowlines. On the other hand, the sea floor spreading history as depicted by the flowlines for the eastern central Atlantic deviates substantially from that of Sundvik and Larson (1988). A revised spreading history is also presented for the Cretaceous Magnetic Quiet Zone, where large changes in spreading direction occurred, that can not be resolved when fitting magnetic isochrons only, but which are evident from fracture zone traces and directions of sea floor spreading topography.
An improved tectonic database for the South Atlantic has been compiled by combining magnetic anomaly, Geosat altimetry and onshore geologic data. We used this database to obtain a revised plate-kinematic model. Starting with a new fit-reconstruction for the continents around the South Atlantic, we present a high-resolution isochron map from Chron M4 to present.Fit reconstructions of South America and Africa that require rigid continental plates result in substantial misfits either in the southern South Atlantic or in the equatorial Atlantic. To achieve a fit without gaps, we assume a combination of complex rift and strike-slip movements: 1.(1) along the South American Parana-Chacos Basin deformation zone,2.(2) within marginal basins in South America (Salado, Colorado Basin) and3.(3) along the Benue Trough/Niger Rift system in Africa. These faults are presumed to have been active before or during the breakup of the continents.Our model describes a successive “unzipping” of rift zones starting in the southern South Atlantic. Between 150 Ma (Tithonian) and approximately 130 Ma (Hauterivian), rifting propagated to 38 °S, causing tectonic movements within the Colorado and Salado basins. Subsequently, between 130 Ma and Chron M4 (126.5 Ma), the tip of the South Atlantic rift moved to 28 °S, resulting in intracontinental deformation along the Parana-Chacos Basin deformation zone. Between Chron M4 and Chron MO (118.7 Ma) rifting propagated into the Benue Trough and Niger Rift, inducing rift and strike-slip motion. After Chron MO, the equatorial Atlantic began to open, while rifting and strike-slip motion still occurred in the Benue Trough and Niger Rift. Since Chron 34 (84 Ma), the opening of the South Atlantic is characterized by simple divergence of two rigid continental plates.
The authors consider, correctly, that the soft-rock geology of southern Africa has had a low priority in teaching and publications. This has largely reflected a dominating interest in the economically important hard-rock metals and the accessibility of key Pre-Cambrian areas. However, a realization of the economic potential of continental margins, particularly for hydrocarbons, now combines with a very strong academic interest in continental margin evolution. Dingle et al. thus considered it opportune for a review and synthesis of the available data on the post-Palaeozoic development of this area which is key to many aspects of the development of both Gondwanaland and of continental margin evolution in general. The review is comprehensive – being adequately but not excessively detailed and is well written, with some 750 references. It is certainly a major advance on all previous texts, most of which either contain very little on this period or must now be considered to be out of date. The latter marks, to a large extent, the major stratigraphic advances largely stimulated by studies of the deep ocean sediments, but also the considerable advance in understanding of South African geology resulting from the work by the authors themselves. It thus marks a major advance in knowledge and is to be very strongly welcomed. The organization is basically stratigraphical, with successive periods being divided regionally. A palaeogeographic evaluation usually provides a summary of the more de-tailed discussions. Although the print is on the small size, reading is facilitated by use of double columns per page. The diagrams are clear and abundant, assisted by the large page size (28 X 20 cm). Although there are few photographs, this is no great loss as their inclusion would have considerably in-creased the cost. Clearly much of the contents must be considered as parochial to Southern Africa, but the documentation of the development of marginal basins, for example, is also of global interest. Similarly, the key position of Southern Africa as Gondwanaland disintegrated is of more than local importance. Occasionally, it is a pity that the authors did not stray slightly wider. There is, for example, an excellent discussion of the problems posed by structures associated with the Cape Folds, but insufficient discussion of its global relationships. This seems to reflect the authors laudable desire to provide a Southern African data base, and it is possibly un-fair, but complimentary, to have hoped for a somewhat more extensive coverage. The main criticism is that the authors have too deliberately concentrated on the soft-rock geology. They certainly discuss some of the igneous activity, but the kimberlites are only cursorily reviewed and the discussion of the Karoo basalts is almost entirely based on K. G. Cox's work (J. Geol. Soc., Lond., 128: 311-336, 1972). This is unfortunate, not be-cause of Cox's work, but because of the significance which attaches to the precise relationship between these eruptives and the stratigraphic development of the region. Such information is critical for the under-standing of the basic processes of continental rifting and hence development of all continental margins. Nonetheless, while there are some disappointments, these largely result from the interest generated by the excellent soft-rock studies, rather than being genuine omissions. The book certainly needs to be available in any geological library and will be a major reference source for some years to come. D. H. Tarling, Newcastle upon Tyne 1984.
The Argentine continental margin, which ranks among the most extensive in the world, has been the scene of intensive geologic and geophysical exploration during the last 15 years. The work was carried out by the Lamont-Doherty Geological Observatory of Columbia University, in collaboration with the Hydrographical Service of the Argentine Navy. This has led to very important contributions to our knowledge of the region (Ewing et al., 1963, 1971; Ludwig et al., 1965, 1968; Lonardi and Ewing, 1971), for through these studies the existence of several sedimentary basins has been found, some of which extend into the continental margin. During the last few years, thanks to this pioneering work, the Argentine government has offered several sectors of this region for petroleum exploration, which in turn has contributed to an increased knowledge of the offshore basins, through further seismic work and the drilling of exploration wells. Detailed sedimentary and palaeontologic studies have been carried out on material from these wells.
This paper begins by describing the regional setting to the basin. Thereafter the stratigraphy of its three sub-basins are considered in turn: the North Gabon sub-basin, the South Gabon sub-basin, and the Interior sub-basin, each of which has a different geologic history. Thereafter the basin evolution of the whole Gabon basin is considered together with oil and gas systems. -D.J.Sanderson
There are many key aspects to the geology and geophysics of the petroliferous basin systems of the eastern Gulf of Guinea, which extends from the Niger Delta to North Gabon. This article provides a background to the regional geology and focuses on the structural geology and the structural styles associated with the key tectonic episodes responsible for hydrocarbon trapping. This work results from several regional studies carried out by Exploration Consultants, based on public domain seismic and well data combined with the results of new analytical work.
The region has been divided into six geotectonic provinces (Figure 1):
Ceiba Field, offshore Equatorial Guinea approximately 75 km southwest of the port of Bata and 275 km south of the city of Malabo, was discovered with the drilling of Ceiba-1 in September 1999. The first appraisal well, Ceiba-2, was drilled in November 1999 approximately 1.6 km southwest of Ceiba-1. Both wells were based on interpretation of 2D seismic data acquired between 1984 and 1999. Open-hole logs, cores, pressure data, and a drill stem test of Ceiba-1 (maximum flow rate of 12 400 b/d) indicated that both wells contained Cretaceous sands of very high quality, capable of high rates of oil production.
It has been shown in the accompanying paper that the Sub-Andean foreland call be subdivided longitudinally into a number of tectonostratigraphic domains. To test the hypothesis that changes in palaeodepositional setting rely oil the presence of a series of transverse zones of structural accommodation, data have been digitally compiled from across the South American continent. Spatial and temporal geological relationships have been analysed and evaluated as a means of identifying the position of tectonostratigraphic domain boundaries (Structural accommodation zones). and patterns of subsidence and intraplate deformation. The results suggest that individually these structural accommodation zones represent a composite of deep crustal fractures which, oil a regional scale, unci-link to form a transcontinental belt or zone that call accommodate intraplate deformation during episodes A plate reorganization. Their strong spatial relationship with Mesozoic, intraplate, alkaline igneous activity suggests that they exerted all important control oil lithospheric melt siting during Gondwana breakup. These localized zones of high heat flow have important implications for source rock maturity in the interior, Phanerozoic intracratonic basins of South America. Oil the South Atlantic margin, the majority of these crustal lineaments correlate with failed arms of triple-junction rills and define the boundaries to tectonostratigraphic domains recognized along the South Atlantic Rift System.
North and South America diverged to approximately their present relative positions from Late Triassic? to Early Campanian (about 84 m.y. ago) time. This is the period during which the Gulf of Mexico and a Proto-Caribbean seaway were formed. Since the Campanian, only minor relative motion has occurred; from Early Campanian through to Middle Eocene times. South America diverged only another 200 km, and since the Middle Eocene, minor N-S convergence has occurred. These very minor post-Early Campanian motions have probably been accommodated by imperfect shear and compression along the Atlantic fracture zones to the east of the Lesser Antilles, and along the northern and southern borders of the Caribbean Plate. Accordingly, it is suggested that from Campanian time to the present, the relative motions between the North and South American plates have had only minor effects on the structural development of the Caribbean region.
The propagating rift model has resolved discrepancies that occur when rigid plate tectonic models are applied to the Pangea configuration of the continents, and furnish a number of guidelines for the process of continental breakup. Continents do not drift apart instantaneously in a rigid manner, but rather permit an ocean to form by means of a propagating rift. This rifting occurs in a manner resembling crack propagation, progressing in a direction that is normal to that of the regional least compressive stress. Hotspot tracks and other regions of weakened lithosphere act as stress guides for the rifting. The model also predicts that the continental-oceanic boundary is not an isochron, but rather becomes younger in the direction of rifting. Extension along the continental margin is a minimum at the initial rifting point, and reaches a minimum at the furthest extent of the rift.
The offshore region of Sierra Leone and Liberia has been underexplored on the continental shelf and unexplored in deepwater. However, a reconnaissance interpretation of new 2D seismic data, in conjunction with a review of the reservoir and geochemical information available to date, is positive about the hydrocarbon potential of the region. Three distinct prospective hydrocarbon basins are present—Sierra Leone Basin, Liberia Basin, and Harper Basin (Figure 1). Figure 1.
Location of Sierra Leone, Liberia, and Harper basins. Fault lines (blue) indicate major strike-slip fault systems.
Nine exploratory wells have been drilled and abandoned on the shelf, at the limits of the study area, considerably shoreward of the potential deepwater basins. In terms of exploration maturity, considering a well density of one well per 6700 km2, the area should be classified as “frontier.” However, the regional potential is far from unknown. The nine wells provide an abundance of key data, which contribute to the identification of several hydrocarbon systems.
The primary petroleum system contains at least three oil-prone, marine, and lacustrine source rock zones deposited during Lower Cretaceous time (Early Albian to Early Ceno-manian). A secondary petroleum system may have a Late Cenomanian to Turonian shale source rock. Trap types are numerous and relatively widespread, in particular those associated with Cretaceous syn-rift and transform related faulting and unconformities. In addition, traps on the slope and in the basin are ideally located for access to oil migration.
The basins developed in two phases, a syn-rift phase and a passive margin phase, both significantly overprinted by wrenching associated with Atlantic transform fault systems. The main rift phase, accompanied by continental to marginally marine sedimentation, took place from Lower Cretaceous Aptian to Middle Albian time. The passive margin and wrenching phase, initiated with seafloor spreading, began in Late Albian time and continues to the …
The Falkland Islands are surrounded by four major sedimentary basins: the Falkland Plateau Basin to the east, the South Falkland Basin to the south, the Malvinas Basin to the west, and the North Falkland Basin to the north. The four main basins appear to have formed initially as Triassic through earliest Cretaceous extensional rifts associated with the break-up of Gondwana. There is evidence of Cenozoic uplift in at least the North Falkland Basin. There is no evidence from offshore seismic and gravity-magnetic data to support interpretations that the Falkland Islands have rotated clockwise through up to 180° during Gondwana separation. With the exception of the South Falkland Basin all the major basins probably underwent initially, more or less east-west extension, and had a similar orientation to adjacent South American and western southern African basins.
During the Geophysical Measurements Across the Continental Margin of Namibia (MAMBA) experiments, offshore and onshore refraction and reflection seismic as well as magnetic data were collected. Together with the existing free-air gravity data, these were used to derive two crustal sections across the ocean-continent transition. The results show that the Early Cretaceous continental breakup and the separation of South Africa and South America were accompanied by excessive igneous activity offshore. Off Namibia we found a 150-200 km wide zone of igneous crust up to 25 km thick. The upper part of this zone consists of an extrusive section comprising three units of basaltic composition: two distinct wedges of seaward dipping reflectors (SDRs) separated by flat-lying volcanic flows. The inner wedge of SDRs can be modeled as the source of a long-wavelength magnetic anomaly that borders long parts of both South Atlantic margins (anomaly G). The crust underneath these extrusives is characterized by high-velocity and high-density material (average values 7 km s-1, 3×103kgm-3). Free-air gravity anomalies along both sides of the high-density crust are interpreted as edge effects resulting from juxtaposition with normal oceanic and continental crust on either side. We define the abrupt landward termination of this zone as the continent-ocean boundary, and consequently, the crust seaward is interpreted as exclusively igneous material and not intruded continental crust. Extrapolation of the interpreted geophysical features along the southwest African margin suggests a fast prograding narrow rift zone and sharp lithospheric rupture leading to the formation of a margin-parallel magmatic belt south of the Walvis Ridge. The influence of the Tristan da Cunha mantle plume may explain the widening of this thick igneous crust near the Walvis Ridge.
We present a plate kinematic evolution of the South Atlantic which is based largely on the determination of the equatorial fracture zone trends between the African and South American continental margins. Four main opening phases are dated by oceanic magnetic anomalies, notably MO, A34, and A13, and are correlated with volcanism and tectonic events on land around the South Atlantic Ocean. The Cera and Sierra Leone rises are probably of oceanic origin and were created 80 m.y. ago or later in their present-day positions with respect to South America and Africa.
A simplified model is proposed that can explain the evolution of
marginal offsets, ridges, and fracture zones. Submarine features of the
equatorial Atlantic margins are examined and are found to be in
reasonable agreement with the model. The results reaffirm the suggestion
that the South Atlantic opened in two stages beginning about 140 m.y.
and 80 m.y. ago. The pole of rotation describing the relative motion of
the South American and African plates changed radically when the
constraints imposed by adjacent continental blocks were relaxed.
More than 1,426 mi (2,300 km) of 48-trace, 12-fold seismic reflection profiles were used to examine the nature of the continent-ocean boundary off southwest Africa. Results are described. Intracontinental stretching and associated volcanism appear to have been important in the early history of the Cape Basin. Rifting and local dike intrusion may explain the presence of some lineated magnetic anomalies previously attributed to sea- floor spreading. Interpretation of the seismic data suggests that the initiation of normal spreading in the Cape Basin postdates by 4 to 9 m.y. the Valanginian age derived from prevailing plate tectonic reconstructions of the South Atlantic.-from Authors Univ of Texas at Austin, Inst for Geophysics, Galveston, Texas 77550, USA.
Evaluation of tertiary sequences of West Africa and Brazil typically requires large budgets and staffs to identify drilling targets. Since these tertiary sedimentary systems extend for hundreds of kilometers (Proc. Petrol. Geol. Deepwater Depositional Syst. (2001) 2; Marine Petrol. Geol. (1990) 94–122), well beyond the limits of individual 3D and many 2D seismic surveys, additional information is necessary to interpret the most favourable locations for detailed exploration. Potential field data are powerful but often underutilized assets in building an exploration framework for reducing costs and interpretation risks. Although drilling locations are normally based on 3D seismic interpretations, reservoir distribution is controlled by features mappable with other methods such as potential fields. Other factors crucial to petroleum maturation, migration and trap formation relate to deep-seated controls that are well imaged (and sometimes exclusively imaged) on potential field data. Our observations from potential field data help identify reservoir distribution; source pod locations, source maturity, possible migration pathways; and potential traps.This paper illustrates the power of combining both regional and basinal scale interpretations based primarily on gravity analyses with extensive knowledge of the underlying geology. We review data sets and methods employed and describe results of a regional overview based on special plate-tectonic reconstructions of the South Atlantic. Examples from basin-scale work offshore West Africa and Brazil are used to illustrate factors important to hydrocarbon exploration. We conclude with a look ahead to improvements in methodology and application.
Some Cretaceous reconstructions of East Antarctica, Africa, and Madagascar overlap the Mozambique ridge onto the margin of Dronning Maud Land, and others place Madagascar west of the Gunnerus ridge, into the Riiser–Larsen Sea. Our M10Ny (131.9 Ma), M0 (120.4 Ma), 96 Ma, and 34y (83 Ma) marine free-air gravity and topography reconstructions illustrate a solution to the overlap problem and also demonstrate that Madagascar separated from Antarctica east of the Gunnerus ridge. Our reconstructions chronicle the growth of the conjugate Mozambique and Riiser–Larsen Basins created at the Southwest Indian spreading ridge. Well defined spreading corridors, identified by conjugate fracture zones and Mesozoic magnetic anomaly identifications, extend from one basin into the other. We find that the eastern scarp of the Mozambique ridge aligns with the Astrid ridge, and there is no room for Madagascar west of the Gunnerus ridge. The overlap of the Mozambique ridge is solved by taking into account the motion of an independent Mozambique ridge microplate that existed from Chron M11 to circa M2.
Marine geophysical data from the southern Natal Valley and northern Transkei Basin, offshore southeast Africa, were used to study the structure of the crust and sedimentary cover in the area. The data includes seismic reflection, gravity and magnetics and provides information on the acoustic basement geometry (where available), features of the sedimentary cover and the basin's development. Previously mapped Mesozoic magnetic anomalies over a part of the basin are now recognized over wider areas of the basin. The ability to extend the correlation to the southeast within the Natal Valley further confirms an oceanic origin for this region and provides an opportunity to amplify the existing plate boundary reconstructions.The stratigraphic structure of the southern Natal Valley and the northern Transkei Basin reflects processes of the ocean crust formation and subsequent evolution. The highly variable relief of the acoustic basement may relate to the crust formation in the immediate vicinity of the continental transform margin. Renewed submarine seismicity and neotectonic activity in the area is probably related to the diffuse boundary between the Nubia and Somalia plates.2.5-D crustal models show that a 1.7–3.2-km-thick sediment sequence overlies a 6.3±1.2-km-thick normal oceanic crust in the deep southern Natal Valley and Transkei Basin. The oceanic crust in the study area is heterogeneous, made up of blocks of laterally varying remanent magnetization (0.5–3.5 A/m) and density (2850–2900 kg/m3). Strong modifications of accretionary processes near ridge/fracture zone intersections may be a reason of such heterogeneity.
The General Levalle basin forms a long, narrow, and deep Early Cretaceous intracratonic rift in southern Córdoba province, Argentina. It trends approximately north-south for more than 150 km (93 mi), ranges from 5 to 50 km (3 to 31 mi) wide, and is more than 6500 m (21,325 ft) deep. Below a prominent middle Cretaceous unconformity, steeply dipping normal faults bound tilted graben and halfgraben fault blocks. The lower rift-fill section, the General Levalle Formation (new formation name), is a Valanginian-Hauterivian siliciclastic and evaporite package more than 3200 m (10,500 ft) thick. It was deposited in an arid, restricted, rift basin that included a hydrologically closed saline lake. Nine lithology-based members represent one continuous cycle of deposition, with a lower coarse clastic sequence gradually fining upward into an evaporite member and then coarsening upward again to an upper sandstone. The uppermost rift-fill sequence, the Guardia Vieja Basalt (new formation name), is a series of Aptian basalt flows and sills more than 800 m (2625 ft) thick, with some thin red-bed intervals. Unstructured Upper Cretaceous to Pleistocene strata overlie the buried rift basin. Following an extensive exploration campaign, in 1995-1996 the first exploratory well in the basin tested a deep-seated anticline to 5179 m (16,991 ft), but encountered just one minor show. Reservoir-quality sandstone occurred only in the upper rift sandstone member, but this lacked adequate seals. Deeper sandstone beds were tightly cemented, and basin-center dark shale below the evaporite member was thin, surprisingly low in total organic carbon, and overmature for oil. Although additional geological, geophysical, and geochemical work could have improved predrill understanding and risk evaluation, in the end, only drilling the wildcat determined the actual subsurface situation. It is now evident that given the narrow, deep depocenter, unfavorable reservoir-seal relationships, and a paucity of source facies, an effective petroleum system probably never existed in the basin. © 2004. The American Association of Petroleum Geologists. All rights reserved.
A plate-tectonic model for the evolution of Middle America and the Gulf of Mexico-Caribbean Sea region is presented. The model, which is based upon the existence of the Mojave-Sonora megashear, incorporates into the Triassic Pangea reconstruction three microplates between N and S America, thus avoiding the overlap of the Bullard fit. These plates are the Yaqui, bounded on the N by the Mojave-Sonora megashear; the E and W Maya plates, bounded on the N by the Mexican volcanic zone and on the S by a predecessor of the Motagua fault zone; and the Chortis plate (parts of Guatemala and Honduras). -from Authors
Gravity and magnetic anomalies bordering the continental margins of the southern South Atlantic Ocean are compared, in detail, on conjugate sides of the ridge crest, and a model for the boundary between oceanic and continental basement is given. The area of study includes the predominantly sheared margins of the Agulhas-Falkland fracture zone and the rifted margins of Argentina and southern Africa south of the Rio Grande Rise and Walvis Ridge, respectely. These margins have associated with them, for the most part, linear magnetic anomalies that can be modeled as edge effect anomalies separating oceanic from continental basement. Coincident with the magnetic anomalies are gradients in the isostatic gravity anomaly. We have taken the location of these geophysical lineaments on the African margin and rotated them clockwise to fit the anomalies on the Argentine margin. This fit, which gives us a new pole of total closing for the South Atlantic Ocean, obviates, for the most part, the gaps and overlaps observed in other reconstructions. The improved fit thereby suggests rigid plate behavior and minimum stretching of continental crust during the early opening of the southern South Atlantic Ocean.
We present an inversion for plate kinematic that solves for finite rotation parameters using fracture zone (FZ) and magnetic anomaly location data jointly. We define misfit functions that incorporate properties unique to each data type; in particular, the FZ misfit function does not depend upon alignment of conjugate FZ traces in the same way as magnetic lineations under the finite rotations. This property is useful for FZ locations, in which the signals on conjugate sides of the ridge may include systematic differences, or where data from one side of the ridge are sparse or missing. Formal error bonds estimated for the pole parameters show that the magnetic and FZ data are complementary in their information content. Error bounds computed for the joint inversion are substantially smaller than for either the FZ or magnetics data used separately, indicating that simultaneous use of the data in an inversion is crucial. We apply this method to Seasat altimeter data and magnetic anomaly picks in the South Atlantic. We solve for finite poles corresponding to magnetic anomalies 5, 6, 8, 13, 21, 22, 25, 30, 33, 33r, and 34; these define a smooth path over the past 84 m.y. indicating that the plate motions have not been as erratic as found previously.
ABSTRACT Gondwana, with East Antarctica as its center, began to break up during Late Triassic to Early Jurassic time. Use of the satellite derived gravity map to approximate the ocean-continent boundary allows us to generate a much tighter fit for the reconstructed supercontinent then previously attempted. Major mantle plumes such as the Karoo-Ferrar Plume that first split Gondwana at about 182 Ma, the Paraná- Etendeka plume at 132 Ma that split South America and Africa, the Marion plume at 88 Ma that split Madagascar and India and finally the Reunion hotspot that split the Mascarene Plateau from India at 64 Ma, were all critical events in the break-up of Gondwana. Our tight-fit produces overlap between cratonic East Antarctica and the Limpopo Plain of Mozambique but there is no evidence that the crustal material underlying the Limpopo Plain pre-dates the break-up of Gondwana. Likewise Madagascar has been reconstructed so that it substantiallyoverlies coastal East Africa in the vicinity of the Anza Trough, an early Jurassic rift in Kenya. The western margin of the island of Madagascar may in fact be crustal material that is younger than the break-up. It may have been produced as a result of the Karoo mantle plume or some may have been the result of the Marion hotspot. Between South America and Africa there are three significant overlaps. Two of them are deltaic, and the third is the Abrolhos and Royal Charlotte banks which post-date Gondwanide,breakup by 80 to 100 million years. Keywords: Gondwana Break-up, Mesozoic Paleoreconstructions, Ocean-Continent Boundaries, Karoo-Ferrar Mantle Plume, Overlap Lawver, Gahagan, and Dalziel 3
The misfit problems raised by the pre-drift reconstructions of the South and Equatorial Atlantic compel us to resort to intraplate deformations. It is shown that acceptable deformations of the African plate in line with the Benue Trough and the Transafrican rifts and shear zones provide a far from satisfactory solution to the geometrical misfit problems in the South Atlantic. These problems can instead be alleviated by considering a degree of deformation of the South American plate along an intraplate boundary in line with the Parana Province, the Rio Grande Rise and the Walvis Ridge. In spite of the scarcity of field observations in this area of South America, we show that such a deformation cannot be ruled out, but it will have to be associated with deformations in the Equatorial Atlantic and in other areas of the African and South American plates.
The Guinean continental margin corresponds to a wide hinge zone which developed between the Jurassic Central Atlantic and the Cretaceous Equatorial Atlantic. The Guinea Marginal Plateau that characterizes the southern border of this margin is a wide triangular morphological feature bounded to the west by a typical rifted margin slope segment, while its southern edge corresponds to a narrow and morphologically complex continental slope probably resulting from intracontinental transcurrent motion during the Early Cretaceous.
Its key geographical position near the reconstructed centre of the Gondwana break-up between Antarctica, South America and Africa has brought attention to the Agulhas Plateau, an oceanic plateau in the southwest Indian Ocean, with regard to its crustal nature and origin. The majority of previous studies have suggested a dominantly continental origin. As part of the project SETARAP (Sedimentation and Tectonics of the Agulhas Ridge and Agulhas Plateau), we conducted an extensive seismic survey over the plateau with the aim of solving the questions about its crustal structure, origin and role in a plate tectonic reconstruction context. In addition to 1550 km of high-resolution seismic reflection profiles, we recorded deep-crustal large-offset and wide-angle reflection/refraction data from an ocean-bottom hydrophone (OBH) profile across the southern plateau. The reflection data show clear indications of numerous volcanic extrusion centres with a random distribution. We are able to date this phase of voluminous volcanism to Late Cretaceous time, a period when numerous other large igneous provinces formed. Traveltime inversion of the deep-crustal OBH records reveals an up to 25 km thick crust with velocities between 7.0 and 7.6 km s−1 for the lower 50–70 per cent of its crustal column. We do not find indications for continental affinity but rather a predominantly oceanic origin of the Agulhas Plateau, similar to that inferred for the Northern Kerguelen and Ontong–Java plateaus. In Late Cretaceous time, its main crustal growth was controlled by the proximity of spreading centres and by passage over the Bouvet hotspot at 80–100 Ma.
The geology of the wide shelves surrounding the South Atlantic is closely linked to the kinematics and history of the opening of the ocean. However, several wide sedimentary basins, which developed along the margins show peculiarities that are not yet understood in the context of the evolution of the South Atlantic.
The Colorado Basin, a wide sedimentary basin on the broad shelf of Argentina, extends in EW direction. The basin's evolution oblique or orthogonal to the continent–ocean boundary indicates that it is not a product of simple progressive extension and crustal thinning. In addition a basement high, paralleling the continental margin and separating the Colorado Basin from the deep-sea basin is a common interpretation. These findings are hardly in accordance with the idea that the Colorado Basin is an extensional basin that developed in conjunction with the early E–W opening phase of the South Atlantic in the Late Jurassic/Early Cretaceous. The composition, type, and structure of the basement, key points for the evaluation of the basins evolution, are widely speculative.
In this context multichannel seismic reflection data from the Argentine Shelf and a 665-km-long onshore–offshore refraction profile, running across the Colorado Basin onto the coast are discussed in combination with gravity data. The stratigraphy for the sedimentary successions was adopted from the literature and the reflection seismic marker horizons formed besides the interval velocities the input for the starting model for refraction seismic traveltime modelling. The modelling strategy was an iterative procedure between refraction seismic traveltime and gravity modelling. The preparation of the density models was coarsely orientated on published velocity–density relations. The modelling results are in favour of a continuation of the main onshore geological features beneath the sedimentary infill of the Colorado Basin. We interpret the basement along the line from west to east as offshore continuation of the Ventana Hills, the Claromecó depocentre, and of Palaeozoic to Middle Mesozoic rocks of the Patagonia terrane. In the deepest part of the pre-/ synrift graben within the Colorado Basin a volcanic/igneous intrusion was interpreted forming an injection into an extensional fault.
According to our interpretation most of the Colorado Basin developed in conjunction with an early opening phase of the South Atlantic (150–130 Ma) and thus represents a typical rift basin instead of an intracontinental sag basin. The origin of the oblique rift most probably resulted from extensional stress, acting either through or interfering with the prevailing Palaeozoic basement fabric, oriented NW–SE. Although there was certainly a strike-slip component in the basins evolution and it may be interpreted as pull-apart basin we suggest that the Colorado Basin represents a failed rift structure: The basin's floor is more or less flat across the shelf, shows a slow rise at the shelf break and deepens towards the deep-sea basin where it finally merges with the seaward-dipping reflector sequences, and the basin probably was affected by magmatic/volcanic intrusives/extrusives associated with the opening of the South Atlantic.
We confirm that a Malvinas Plate is required in the Agulhas Basin during the Late Cretaceous because: (1)oblique Mercator
plots of marine gravity show that fracture zones generated on the Agulhas rift, as well as the Agulhas Fracture Zone, do not
lie on small circles about the 33o-28y South America-Africa stage pole and were therefore not formed by South America-Africa
spreading, (2)the 33o-28y South America-Africa stage rotation does not bring 33o magnetic anomalies on the Malvinas Plate
into alignment with their conjugates on the African Plate, and (3)errors in the 33o-28y South America-Africa stage rotation
cannot account for the misalignment. We present improved Malvinas-Africa finite rotations determined by interpreting magnetic
anomaly data in light of fracture zones and extinct spreading rift segments (the Agulhas rift) that are clearly revealed in
satellite-derived marine gravity fields covering the Agulhas Basin. The tectonic history of the Malvinas Plate is chronicled
through gravity field reconstructions that use the improved Malvinas-Africa finite rotations and more recent South America-Africa
and Antarctica-Africa finite rotations. Newly-mapped triple junction traces on the Antarctic, South American, Malvinas, and
African Plates, combined with geometric and magnetic constraints observed in the reconstructions, enable us to investigate
the locations of the elusive western and southern boundaries of the Malvinas Plate.
The structure and evolution of Early Cretaceous rift basins in Western and Central Africa are described. Two stages of rift development and fracturing have been identified: (1) from Neocomian to Early Aptian roughly E-W and NW trending troughs (Upper Benue, N Cameroon, S Chad, Sudan etc.) opened in response to a submeridian extensional regime in Central Africa while in Western Africa the N-S trending transsaharian fault zone acted as a sinistral wrench; (2) from Middle Aptian to Late Albian large northwest trending troughs (E Niger, Sudan, Sirte, etc.) opened in response to a northeast extensional regime while the Central African fault zone (from Benue to Sudan) exhibited strike-slip movements, generating pull-apart basins.These rift and fracture systems delimit three large blocks within the African plate: a Western block, an Arabian-Nubian block and an Austral block. The Arabian-Nubian block tends to separate from the two other blocks, migrating towards the north during the first stage of basin development and then towards the NE during the second stage. The opening of the Atlantic Ocean was the dominant driving force for the Western and Austral blocks while the Arabian-Nubian block probably moved in response to the opening of the Indian Ocean and to the evolution of the Tethyan margin.
The geology of the Equatorial Atlantic is dominated by a broad east-west megashear belt where a cluster of large fracture zones offsets anomalously deep segments of the Mid-Atlantic Ridge (MAR). The origin and evolution of this megashear region may lie ultimately in an equatorial mantle thermal minimum. The notion of a mantle thermal minimum in the Equatorial Atlantic is supported by an equatorial minimum of zero-age topography, a maximum in mantle shear waves seismic velocity and a minimum in the degree of melting, indicated by the chemistry of MAR basalts and peridotites. This thermal minimum has probably been a stable feature since before the Cretaceous separation of Africa from South America; it caused a pre-opening equatorial continental lithosphere thicker and colder than normal. The Cretaceous Benue Trough in western Africa and the Amazon depression in South America are interpreted as morphostructural depressions created or rejuvenated by strike-slip, transpressional and transtensional tectonics ducing extension of the cold/thick equatorial lithosphere. The oceanic rift propagating northward from the South Atlantic impinged against the equatorial thicker, colder and, therefore, stronger than normal continental, lithosphere that consequently acted as a ‘locked zone’. This, and a low magmatic budget due to the cold upper mantle, caused a lower than normal rate of propagation of the oceanic rift into the equatorial belt, with diffuse deformation during mostly amagmatic extension. The thick/cold lithosphere prevented major Cretaceous igneous activity from the St. Helena plume. Eventually initial ‘weak’ isolated nuclei oceanic lithosphere were emplaced, separated by E-W continent/continent transforms. Opening occurred largely by strike-slip motion along these initial transforms. The consequences were that the Equatorial Atlantic opened prevalently along an E-W direction, in contrast to the N-S opening of the North and South Atlantic, and that sheared continental margins are particularly well developed in the Equatorial Atlantic. After further continental separation the cold equatorial mantle caused a low degree of melting (with Na-rich MORB and alkali basalt rather than normal MORB and with undepleted mantle peridotities), thin crust, depressed ridge segments and a prevalence of amagmatic extension. Similar conditions still exist today. Long transforms offsetting short ridge segments kept sea floor spreading unstable and dominated by transform tectonics, with transform migration, transpression, and transtension causing strong vertical motion, emersion and subsidence of lithospheric blocks, development of deep pull-apart basins, and preservation of relict slivers of old lithosphere (occasionally even of continental lithosphere) within younger crust. The equatorial transforms are caused ultimately by a long lived thermal minimum in the upper mantle and not vice versa; however, they then create second-order ‘rebound’ thermal effects that help maintain the thermal minimum in the upper mantle. It can be speculated that mantle thermal minima at the Earth's equator might be related to true polar wander triggered by subduction of dense masses into the mantle.
This study brings together existing and new gravity data to investigate the nature and cause of the Bouguer gravity field associated with the Cretaceous West African rift system in Nigeria and Cameroon. The new gravity measurements include data collected over the basement area between the Benue Trough and the Cameroon border and fill an important gap in the gravity coverage. After removal of the long wavelength negative anomaly from the observed gravity field the remaining positive gravity anomaly, associated with the rift system, is interpreted in terms of two and three dimensional crustal models. These models are constrained by crustal thicknesses derived from a seismic refraction study carried out across and to the south of the Yola rift. The results of simple three dimensional gravity modelling indicate the crust beneath the lower and middle Benue is approximately 20 km thick while beneath the Gongola rift the crust is approximately 25 km thick relative to a normal crustal thickness of 34 km away from the rift. Assuming the thinned crust associated with the rift system is the result of simple lithospheric stretching, then the maximum possible crustal extensions of 95, 65 and 55 km have taken place perpendicular to the Benue, Gongola and Yola rifts respectively. These crustal extensions do not necessarily reflect the total crustal movement affecting these rifts since the Benue and Yola rifts have been subjected to varying amounts of shear displacement during the Cretaceous and early Tertiary times.
We have identified a sequence of small rise-axis jumps on the Cocos—Nazca spreading center between 93° and 95.5°W. The locus of jumps has migrated 150 km west along the rise axis, away from the Galapagos Islands, during the last three million years, at an average rate of 50 mm/year. The linear increase in jump distance during this sequence of jumps has resulted in a change in regional azimuth of the rise axis from about 085° to 095°. We visualize this sequence of jumps as a new rift propagating through the Cocos plate, forming a new Cocos—Nazca spreading center. The region affected by these rise jumps appears to correlate with an area of exceptionally high-amplitude magnetic anomalies. The high-amplitude region seems to result from Fe-Ti-rich (FeTi) basalts of high remanent magnetization. We speculate that the development of the new accretion axis and concomitant rise jumps are related to the flow of FeTi basalt-producing asthenosphere away from the Galapagos hotspot. The snout of anomalous asthenosphere has remained nearly stationary, with respect to the Galapagos hotspot, during the last 3 m.y. A northwestward component of flow, reflecting the southward position of the plume center with respect to the spreading axis, might explain why the new spreading center is developing along a more northwesterly azimuth. The rise jumps have resulted in the sort of pattern of asymmetric accretion which is required to substantiate the hotspot hypothesis for the origin of the Cocos and Carnegie ridges. Several other puzzling platetectonic phenomena may be explained by the propagating rift model developed here.