A recipe for microcontinent formation
Abstract
Accreted slivers of continental margins are common in the geologic record, but the processes that lead to their formation are poorly understood. We observe an association of plume-related microcontinent isolation and subsequent long-term asymmetries in oceanic crustal accretion based on four recent examples: the Seychelles in the Indian Ocean, Jan Mayen in the Norwegian-Greenland Sea, and the East Tasman Plateau and the Gilbert Seamount Complex in the Tasman Sea. These microcontinents formed by rerifting of a young continental margin (
... It has long been recognized that mid-ocean ridges or a few of the spreading segments of the ridge systems keep their proximity always close to the location of the mantle plumes due to ridge jumps, hotspot migrations, and/or absolute plate motions, leading to interactive processes between them. Further, it has been established that ridge-plume interactions may lead to detachment of microcontinents from the young continental margins (Müller et al. 2001). In this respect, Müller et al. (2001) and Talwani et al. (2016) have presented models of plume-related microcontinent formation such as Jan Mayen in the Norwegian-Greenland Sea, East Tasman Plateau, and the Gilbert Seamount Complex in the Tasman Sea, and Seychelles and parts of the Kerguelen plateau in the Indian Ocean. ...
... Further, it has been established that ridge-plume interactions may lead to detachment of microcontinents from the young continental margins (Müller et al. 2001). In this respect, Müller et al. (2001) and Talwani et al. (2016) have presented models of plume-related microcontinent formation such as Jan Mayen in the Norwegian-Greenland Sea, East Tasman Plateau, and the Gilbert Seamount Complex in the Tasman Sea, and Seychelles and parts of the Kerguelen plateau in the Indian Ocean. We discuss here specific case studies of interactions between the Kerguelen hotspot and Wharton spreading ridge segments in Indian Ocean (Fig. 3), and Azores hotspot and Mid-Atlantic Ridge segments in Atlantic Ocean (Fig. 4). ...
... Microcontinents represent isolated continental pieces stranding in oceanic lithosphere and are ubiquitous in both modern ocean basins and ancient orogens (Torsvik et al., 2013;Xiao et al., 2015). They have been widely studied to understand their formation (Müller et al., 2001;Whittaker et al., 2016;Molnar et al., 2018) and their accretion to convergent margins (Moresi et al., 2014;Gün et al., 2021). Microcontinents are generally weaker and more buoyant than the surrounding oceanic lithosphere (Molnar, 1988;Gün et al., 2021), with strong rheological contrasts but not too much lithospheric strength. ...
... Model M2 (Fig. 2B): Müller et al. (2001) suggested that the impingement of a mantle plume adjacent to an active spreading ridge could cause renewed continent rifting followed by microcontinent separation. In this scenario, the earlier active spreading ridge would be extinct in favor of the new seafloor spreading. ...
Subduction initiation is a pivotal process in plate tectonics. Models of subduction initiation include the collapse of passive margins, oceanic transform faults, inversion of oceanic core complexes, and ridge failure but have ignored the potential effects of continental crust relicts within the oceanic crust. In this paper, we explore the role of microcontinents on subduction initiation through two-dimensional thermo-mechanical numerical modeling. We consider three scenarios with variable ages of oceanic crust surrounding the microcon-tinent and parametrically examine the microcontinent characteristics (size, crustal thickness , thermal gradient, and rheology), oceanic plate age, and convergence rates. Results suggest that moderate-size (≥300 km) microcontinents can nucleate subduction initiation at the junction between continental and oceanic plates. A large part of the microcontinent would be dragged into the subduction zone, and the subsequent asthenosphere upwellings would incorporate part of the microcontinent. Our numerical models add a new hypothetical scenario for subduction initiation, especially for those places where a young and buoyant plate subducts beneath an older and denser oceanic plate. Moreover, they can explain the origin of exotic crust materials and ultrahigh-pressure minerals in supra-subduction zone ophiolites.
... In an opposite scenario where a switch in the tectonic regime can make break-up possible, the East African volcanic province will be an equivalent of LIP-Producer. Importantly, mantle plume impingement can produce not only (super)continent rupture, but also the separation of relatively small continental ribbons or microcontinents 90,91 . In particular, several continental fragments are known to have drifted away from northern Gondwana during the Paleozoic (the Avalonia terranes and the Cimmerian blocks) 92 and Mesozoic (the Apulian microcontinent) 93 , possibly due to the combined effect of slab pull by continuous subduction of paleo-oceans (from Proto-Tethys to Neo-Tethys) 94 beneath the active margin of opposing continents (from Laurentia to Laurasia) 95 and mantle plumes periodically arriving at the base of the African lithosphere 96 . ...
Traditionally, the emplacement of the Large Igneous Provinces (LIPs) is considered to have caused continental break-up. However, this does not always seem to be the case, as illustrated by, for example, the Siberian Traps, one of the most voluminous flood basalt events in Earth history, which was not followed by lithospheric rupture. Moreover, the classical model of purely active (plume-induced) rifting and continental break-up often fails to do justice to widely varying tectonic impacts of Phanerozoic LIPs. Here, we show that the role of the LIPs in rupture of the lithosphere ranges from initial dominance (e.g., Deccan LIP) to activation (e.g., Central Atlantic Magmatic Province, CAMP) or alignment (e.g., Afar LIP). A special case is the North Atlantic Igneous Province (NAIP), formed due to the “re-awakening” of the Iceland plume by the lateral propagation of the spreading ridge and the simultaneous approach of the plume conduit to adjacent segments of the thinner overlying lithosphere. The proposed new classification of LIPs may provide useful guidance for future research, particularly with respect to some inherent limitations of the common paradigm of purely passive continental break-up and the assumption of a direct link between internal mantle dynamics and the timing of near-surface magmatism.
... The sedimentary record (Chlupáč, 1993) reveals that the Barrandian synform (the southern part of the TB) has remained closely above or below sea-level from the Cambrian into the Early Carboniferous, with no evidence of high topography and subsequent collapse. Following a recipe of Müller et al. (2001) for microcontinent formation in the Indian Ocean, Franke et al. (2019a) have suggested that the spreading ridge of the Galicia-Moldanubian Ocean jumped northwards, in the early Devonian, into the extended (i.e., mechanically weak) southern margin of an Armorican microcontinent, where it gave birth to the Saxo-Thuringian Ocean separating Bohemia from Thuringia. Isostasy explains why that extended Bohemian margin segment (the later TB) has never risen much above sea level -like the Seychelles-Mascarene-Réunion microcontinent (Ashwal et al., 2017;Torsvik et al., 2013). ...
Competing hypotheses attribute the regional loss of 1.2–1.0 Ga detrital zircon from the Cambrian Sauk Sequence in southwestern North America to differing tectonic controls on surface topography. We test three hypotheses with source‐to‐sink detrital zircon provenance analysis via tandem in situ and isotope dilution U–Pb geochronology paired with geochemical and Hf‐isotope tracers. Our data indicate that the lower‐to‐middle Sixtymile Formation in Grand Canyon was derived from ca. 1.1 Ga rocks of the Llano Uplift and the ca. 539–523 Ma Wichita igneous province, approximately 1400 km away. In contrast, new U–Pb geochronology links the upper Sixtymile and Tapeats formations to the 513–510 Ma Florida Mountains intrusive complex, southern New Mexico, and proximal 1.4 and 1.7 Ga basement approximately 650 km away. We attribute a regional provenance shift to plume–lithosphere interactions on the Iapetan margin, tectonism along ‘leaky’ intracratonic transverse fault zones and the rift‐to‐drift transition on the Cordilleran margin.
Despite discussions extending back almost 160 years, the means by which Madagascar's iconic land vertebrates arrived on the island remains the focus of active debate. Three options have been considered: vicariance, range expansion across land bridges, and dispersal over water. The first assumes that a group (clade/lineage) occupied the island when it was connected with the other Gondwana landmasses in the Mesozoic. Causeways to Africa do not exist today, but have been proposed by some researchers for various times in the Cenozoic. Over-water dispersal could be from rafting on floating vegetation (flotsam) or by swimming/drifting. A recent appraisal of the geological data supported the idea of vicariance, but found nothing to justify the notion of past causeways. Here we review the biological evidence for the mechanisms that explain the origins of 28 of Madagascar's land vertebrate clades [two other lineages (the geckos Geckolepis and Paragehyra) could not be included in the analysis due to phylogenetic uncertainties]. The podocnemid turtles and typhlopoid snakes are conspicuous for they appear to have arisen through a deep-time vicariance event. The two options for the remaining 26 (16 reptile, five land-bound-mammal, and five amphibian), which arrived between the latest Cretaceous and the present, are dispersal across land bridges or over water. As these would produce very different temporal influx patterns, we assembled and analysed published arrival times for each of the groups. For all, a 'colonisation interval' was generated that was bracketed by its 'stem-old' and 'crown-young' tree-node ages; in two instances, the ranges were refined using palaeontological data. The synthesis of these intervals for all clades, which we term a colonisation profile, has a distinctive shape that can be compared, statistically, to various models, including those that assume the arrivals were focused in time. The analysis leads us to reject the various land bridge models (which would show temporal concentrations) and instead supports the idea of dispersal over water (temporally random). Therefore, the biological evidence is now in agreement with the geological evidence, as well as the filtered taxonomic composition of the fauna, in supporting over-water dispersal as the mechanism that explains all but two of Madagascar's land-vertebrate groups.
Observations of flexure indicate that the effective flexural rigidity of oceanic lithosphere is a function of the age of the lithosphere at the time of loading, and hence temperature. We have used a yield stress envelope model constrained by data from experimental rock mechanics to determine how the flexural parameters and rheologic properties of oceanic lithosphere are related. The results of our model for seamounts and oceanic island loads in the interior of plates suggest that following loading, rapid stress relaxation occurs as the plate 'thins' from its short-term to its long-term (more than 106yr) mechanical thickness. The mechanical thickness, which determines the effective flexural rigidity of the plate, is strongly dependent on temperature and weakly dependent on load size and duration (more than 1-10Ma). The results of our model for convergent plate boundaries suggest that change in the shape of the Outer Rise along an individual trench system may be due to variations in the horizonal load acting across the boundary (less than 1kbar). The model predicts a narrow zone of high strain accumulation seaward of a trench which is in agreement with variations in crustal velocities and seismicity patterns observed along some trench systems. -Authors
Large igneous provinces (LIPs) are a continuum of voluminous iron and magnesium rich rock emplacements which include continental flood basalts and associated intrusive rocks, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups, and ocean basin flood basalts. Such provinces do not originate at “normal” seafloor spreading centers. We compile all known in situ LIPs younger than 250 Ma and analyze dimensions, crustal structures, ages, and emplacement rates of representatives of the three major LIP categories: Ontong Java and Kerguelen-Broken Ridge oceanic plateaus, North Atlantic volcanic passive margins, and Deccan and Columbia River continental flood basalts. Crustal thicknesses range from 20 to 40 km, and the lower crust is characterized by high (7.0–7.6 km s−1) compressional wave velocities. Volumes and emplacement rates derived for the two giant oceanic plateaus, Ontong Java and Kerguelen, reveal short-lived pulses of increased global production; Ontong Java's rate of emplacement may have exceeded the contemporaneous global production rate of the entire mid-ocean ridge system. The major part of the North Atlantic volcanic province lies offshore and demonstrates that volcanic passive margins belong in the global LIP inventory. Deep crustal intrusive companions to continental flood volcanism represent volumetrically significant contributions to the crust. We envision a complex mantle circulation which must account for a variety of LIP sizes, the largest originating in the lower mantle and smaller ones developing in the upper mantle. This circulation coexists with convection associated with plate tectonics, a complicated thermal structure, and at least four distinct geochemical/isotopic reservoirs. LIPs episodically alter ocean basin, continental margin, and continental geometries and affect the chemistry and physics of the oceans and atmosphere with enormous potential environmental impact. Despite the importance of LIPs in studies of mantle dynamics and global environment, scarce age and deep crustal data necessitate intensified efforts in seismic imaging and scientific drilling in a range of such features.
The distribution of allochthonous versus parautochthonous carbonate
platforms, combined with the timing of initial continental-rift
magmatism versus the timing of subsequent rifting of the continental
margin, provides tectonostratigraphic evidence for the formation and
isolation of the oldest documented microcontinent, perhaps as a
consequence of the impingement of a mantle plume onto a cratonic margin
during the Paleoproterozoic. Initial rifting of the Archean nucleus of
North America in eastern Canada is constrained to have been diachronous
between 2.17 and 2.03 Ga. Renewed rifting of a segment of the
continental margin, 292 to 120 m.y. later, was accompanied by the
emplacement of ultramafic layered sills, the accumulation of komatiitic
and alkalic basalts, the deposition of banded iron formations and the
isolation of a microcontinent and its 1.93 Ga continental shelf
succession (subsequently accreted to the telescoped continental margin
during collisional orogenesis).
Over 70% of the North American Cordillera is made up of 'suspect terranes'. Many of these geological provinces are certainly allochthonous to the North American continent and seem to have been swept from far reaches of the Pacific Ocean before collision and accretion into the Cordilleran margin mostly in Mesozoic to early Cenozoic time.
The 50 000 km2 East Tasman Plateau has been regarded as a continental block on the basis of geophysical data, but this study is the first to prove its continental nature directly by sampling of basement rocks. Continental rocks recovered from four stations on its bounding scarps include Neoproterozoic orthogneiss, rhyolite, metasediments and metabasite, quartzite, quartz, quartzose sandstone and ferricrete. Geophysical and petrological evidence suggest that the plateau was adjacent to the South Tasman Rise until it was transported about 130km east-northeast (relative to Tasmania) during early formation of the Tasman Basin in the Late Cretaceous (95-83 Ma), as part of the breakup of East Gondwana. Stretching and sea-floor spreading formed the L'Atalante Depression between the plateau and the rise. Lord Howe Rise separated from the East Tasman Plateau at 75 Ma, as part of Tasman Basin spreading. The plateau supports Cascade Seamount, a Late Eocene guyot that is part of the trace of the Balleny mantle plume and consists of volcanic breccia, hyaloclastite and alkali olivine basalt. Seismic profiles suggest that the weight of the seamount caused subsidence of the central plateau, and up to 1500 m of sediment fill the resultant depression. Seismic profiles and some sedimentary evidence indicate that the depression is filled by 500-1000 m of Late Eocene and Early Oligocene volcaniclastic sediments, and 200-500 m of Late Oligocene and younger calcareous ooze and chalk.
The rocks of the Seychelles can be divided into two age groups, namely Precambrian granites and younger (Cretaceous/Tertiary) intrusive rocks. The latter can be further subdivided into alkaline ring complexes (as found on the islands of Silhouette and North Island) and basic dykes (on Praslin, Felicité and Mahé islands). Evidence from offshore seismic work and drill holes suggests that Cretaceous/Tertiary magmatism occurred over the whole Seychelles Bank, producing both flood basalts and central volcanic complexes. The flood basalts extend at least as far south as 10°S/60°E.
The younger igneous rocks of the Seychelles show close similarities to the Deccan igneous rocks of India. Tholeiitic dykes from Praslin have previously been shown to resemble Bushe Formation tholeiites from the Deccan, and here we show that the Felicité Island dykes also resemble Bushe. We show also that the alkaline dykes of Mahé and North Island are chemically similar to the dykes at Murud on the west coast of India. Isotopically the Seychelles undersaturated rocks fall within the fields of the Deccan tholeiites.
In India, alkaline magmatism post-dates the tholeiitic magmatism; the age difference is of the order of 3 Ma. This is similar to the age difference between shield-building and rejuvenated-stage magmatism on Hawaiian volcanoes, which has been related to reactivation of the volcanoes by the passage of the Hawaiian Arch. We propose that the Deccan alkaline magmatism is a continental equivalent of oceanic rejuvenated-stage volcanism.
Two main tectonic phases were responsible for the formation of the Jan Mayen Ridge microcontinent: (1) the opening of the Norway Basin in late Palaeocene/early Eocene times, and (2) subsequent rifting within the Greenland margin by which complete separation was achieved in early Miocene times. During the first phase the eastern ridge flank developed as a volcanic passive margin. The initial break-up was associated with flexuring and the formation of sequences of eastward-dipping basalt flows, which are considered equivalent to similar features beneath the Vøring and Faeroe-Shetland marginal highs off Norway. Rifting along the Greenland margin during the second phase was accompanied by uplift, listric normal faulting and the formation of large extensional fault blocks. To the W and S of the ridge a flat volcanic marker of probable earliest Miocene age covers the subsided rift and masks the ocean-continent transition. It was formed by a volcanic event of large magnitude, either as submarine lava flows or as a sill complex.
Rift-flank uplifts and the breakup or postrift unconformity are characteristic features of many rifted, passive, or Atlantic-type continental margins, but are not predicted as primary features of simple lithospheric stretching models. The explanations that have been proposed for their origin remain controversial, either because they call upon special circumstances or because they are difficult to test. Plane-strain finite element models are used in this paper to explore the dynamics of lithospheric necking during rifting and rupture. The results agree with the conceptual interpretation that uplift of the rift boundaries to form flank mountains and uplift of the basin responsible for the breakup unconformity are related consequences of regional isostatic compensation of mass that is redistributed during the necking and rupture phases. Although the amplitudes of these uplifts depend on the model parameter values, the relations between and relative signs of these two effects appear to be fundamental. The explanation we propose may have been missed in recent studies because there has been a tendency to concentrate on kinematic stretching models, which assume local isostatic equilibrium through out the rifting process. Such an approach is predicated on the assumption that the lithosphere has an insignificant strength or flexural rigidity during extension, which is not true if our explanation is correct.
Despite the availability of numerous geophysical data, no convincing magnetic anomaly identification and structural map of a major part of the Arabian and eastern Somali Basins have been published to date. I propose a new interpretation of the available magnetic data based on the identification of ``tiny wiggles,'' i.e., singular second-order patterns of the magnetic signal. Only one fracture zone and a set of propagating rifts existed between the Owen Fracture Zone and the Chagos-Laccadive Ridge between anomalies 26 and 20. At anomalies 26-25, at least two eastward propagating rifts are identified, while at anomalies 24-20, seven westward propagating rifts are confidently recognized. The major consequence of this systematic ridge propagation is a tremendous regional asymmetry: between anomalies 26 and 25, about 65% of the crust formed at the Carlsberg Ridge was accreted to the African Plate, while at anomalies 24-20, more than 75% benefited to the Indian Plate. I suggest that these asymmetries result from the relative position of the Carlsberg Ridge and the nearby Deccan-Reunion hotspot, the ridge tending to remain near the hotspot.