No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
New insights into Andean evolution: An introduction to contributions from the 6th ISAG symposium (Barcelona, 2005)
... Gansser [1973] divided the Andean chain into a Northern (∼12 • N to ∼5 • S), Central (∼5 • S to ∼37 • S), and Southern unit (∼37 • S to ∼55 • S). The Central unit is characterized by the Altiplano and Puna Plateaus, a magmatic arc (e.g., Western Cordillera) to the west, and a tectonically shortened Eastern Cordillera and foreland to the east [Sempere et al., 2008]. Most models of the orogenic history of the Andes have three main phases [Steinmann et al., 1929]: Peruvian during Late Cretaceous, Incaic in middle Eocene (or Oligocene, according to Sempere et al. [1990]), and Quechua since late Miocene. ...
... Most studies agree that prominent uplift of the mountain chain started in middle Eocene, reaching a peak in the Oligocene, with a second uplift period in late Miocene [Sempere et al., 1990[Sempere et al., , 2008. Middle Eocene and early Oligocene exhumation occurs in the Central Andes [Barnes et al., 2006;Ege et al., 2007;Gillis et al., 2006] . ...
... Significant topographic uplift is inferred since late Miocene from paleosol carbonates [Barke and Lamb, 2006;Ghosh et al., 2006;Schildgen et al., 2007;Thouret et al., 2007] and paleomagnetic data [Rousse et al., 2003]. Gregory-Wodzicki [2000] reached similar conclusions, although leaf morphology studies appear to underestimate paleoelevations [Sempere et al., 2008]. Figure 3 summarizes the tectonic and topographic evolution of the Andes. ...
The South Atlantic region displays (1) a topographic gradient across the basin, with Africa elevated relative to South America, (2) a bimodal spreading history with fast spreading rates in Late Cretaceous and Eo-Oligocene, and (3) episodic regional uplift events in the adjacent continents concentrated in Late Cretaceous and Oligocene. Here we show that these observations can be linked by dynamic processes within Earth's mantle, through temporal changes in asthenosphere flow beneath the region. The topographic gradient implies westward, pressure-driven mantle flow beneath the basin, while the rapid spreading rate changes, on order 10 million years, require significant decoupling of regional plate motion from the large-scale mantle buoyancy distribution through a mechanically weak asthenosphere. Andean topographic growth in late Miocene can explain the most recent South Atlantic spreading velocity reduction, arising from increased plate boundary forcing associated with the newly elevated topography. But this mechanism is unlikely to explain the Late Cretaceous/Tertiary spreading variations, as changes in Andean paleoelevation at the time are small. We propose an unsteady pressure-driven flow component in the asthenosphere beneath the South Atlantic region to explain the Late Cretaceous/Tertiary spreading rate variations. Temporal changes in mantle flow due to temporal changes in regional mantle pressure gradients imply a correlation of horizontal and vertical motions: we find that this prediction from our models agrees with geologic and geophysical observations of the South Atlantic region, including episodes of passive margin uplift, regional basin reactivation, and magmatic activity.
... Gansser [1973] divided the Andean chain into a Northern (∼12 • N to ∼5 • S), Central (∼5 • S to ∼37 • S), and Southern unit (∼37 • S to ∼55 • S). The Central unit is characterized by the Altiplano and Puna Plateaus, a magmatic arc (e.g., Western Cordillera) to the west, and a tectonically shortened Eastern Cordillera and foreland to the east [Sempere et al., 2008]. Most models of the orogenic history of the Andes have three main phases [Steinmann et al., 1929]: Peruvian during Late Cretaceous, Incaic in middle Eocene (or Oligocene, according to Sempere et al. [1990]), and Quechua since late Miocene. ...
... Most studies agree that prominent uplift of the mountain chain started in middle Eocene, reaching a peak in the Oligocene, with a second uplift period in late Miocene [Sempere et al., 1990[Sempere et al., , 2008. Middle Eocene and early Oligocene exhumation occurs in the Central Andes [Barnes et al., 2006;Ege et al., 2007;Gillis et al., 2006] . ...
... Significant topographic uplift is inferred since late Miocene from paleosol carbonates [Barke and Lamb, 2006;Ghosh et al., 2006;Schildgen et al., 2007;Thouret et al., 2007] and paleomagnetic data [Rousse et al., 2003]. Gregory-Wodzicki [2000] reached similar conclusions, although leaf morphology studies appear to underestimate paleoelevations [Sempere et al., 2008]. Figure 3 summarizes the tectonic and topographic evolution of the Andes. ...
Kinematic models for the opening of the South Atlantic ocean predict
strong variations in spreading rates over times of a few million years.
These short-term variations require a substantial decoupling of the
plate from the lower mantle, which in turn points to a low viscosity
asthenosphere acting as a lubricating layer. With the aid of a high
resolution tomographic model of the South Atlantic upper mantle, we are
able to image the spatial extent of this low viscosity layer below the
oceanic plate and to assess its significance in connection with the
spreading history of the South Atlantic ocean.
... The Central Andes are currently the world's largest mountain belt to have been built by subduction-related processes (Isacks, 1988;Sempere et al., 2008) but the detailed history of its evolution, in particular the interplay between tectonic and climatic effects, still remains poorly known. Generally, two major pulses of surface uplift are described. ...
... Generally, two major pulses of surface uplift are described. One during Oligocene to Early Miocene (Isacks, 1988;Allmendinger et al., 1997;Sempere et al., 2008) and a second one in the late Miocene (Schildgen et al., 2007;Thouret et al., 2007;Garzione et al., 2008;Sempere et al., 2008;Schildgen et al., 2009). Crustal thickening and related surface uplift are generally accepted to have started about Mid-Eocene to Late Oligocene time but was asynchronous along and across strike of the Andes (Isacks, 1988;Sanchez, 1999;Gregory-Wodzicki, 2000;Sempere et al., 2008;Mamani et al., 2010). ...
... Generally, two major pulses of surface uplift are described. One during Oligocene to Early Miocene (Isacks, 1988;Allmendinger et al., 1997;Sempere et al., 2008) and a second one in the late Miocene (Schildgen et al., 2007;Thouret et al., 2007;Garzione et al., 2008;Sempere et al., 2008;Schildgen et al., 2009). Crustal thickening and related surface uplift are generally accepted to have started about Mid-Eocene to Late Oligocene time but was asynchronous along and across strike of the Andes (Isacks, 1988;Sanchez, 1999;Gregory-Wodzicki, 2000;Sempere et al., 2008;Mamani et al., 2010). ...
A large sedimentary forearc basin developed in Cenozoic times between the present-day Coastal Cordillera and the Western Cordillera of the Central Andes, called Moquegua basin in southern Peru. The basin is filled by Moquegua Group deposits (~50 to 4Ma) comprising mostly siliciclastic mudstones, sandstones and conglomerates as well as volcanic intercalations. Several facies changes both, along orogenic strike and through time, are described and have led to subdivision into four sedimentary units (Moquegua A, B, C and D). In this paper we present a refined stratigraphic scheme of the Moquegua Group combined with the first provenance analysis of the Moquegua basin based on (i) semi-quantitative analysis of heavy mineral abundance, (ii) electron microprobe (EMP) and laser ablation (LA) ICP-MS analyses of single detrital amphibole and Fe–Ti oxide grains, and (iii) comparative analysis of the different potential source rocks to clearly identify the most likely sources. Results allow us to reconstruct sediment provenance and to relate changes of the erosion-sedimentation system in the Moquegua basin to the evolution of the Andean orogen. At ~50 to ~40Ma the Moquegua basin was close to sea level and fed by low energy rivers transporting mainly metamorphic basement and Jurassic-Cretaceous sedimentary detritus from local and distal sources. The latter might be as far as the present Eastern Cordillera. From ~35Ma on the distal sediment sources were cut off by the uplift of the Altiplano and Eastern Cordillera leading to higher energy fluvial systems and increasing importance of local sources, especially the relevant volcanic arcs. From 25Ma on volcanic arc rocks became the predominant sources for Moquegua Group sediments. The 10Ma time lag observed between the onset of uplift-induced facies and provenance changes (at ~35Ma) and the onset of intense magmatic activity (at ~25Ma) suggests that magmatic addition was not the main driver for crustal thickening and uplift in the Central Andes during latest Eocene to Oligocene time.
... The Central and Southern Andes have been built by tectonic and magmatic processes produced by oceanic subduction beneath western South America, in a non-collisional setting (e.g., Sempere et al., 2008). The ∼4000 km-long Central Andes form the largest and most mountainous segment of the Andes (Fig. 1), which is itself segmented into the northern Central Andes (5°30'S-∼ 13°S), Central Andean Orocline (CAO, ∼ 13°S-28°S), and southern Central Andes (28°S-37°S); the transition between the CAO and northern Central Andes is formed by the Abancay deflection, a peculiar segment where the Andean structural strike exhibits a significant rotation . ...
... Although a large amount of work has been performed in the Central Andes during the last decades, the chronology of the orogeny and related uplift is still debated (e.g., Sempere et al., 2008). The existing limited agreement between the many approaches used so far implies that more data are needed to complement current knowledge on the Andean uplift issue. ...
... The picture that emerges from these results is that high altitudes and crustal thickening remained essentially restricted to the Central Andean Orocline during most of the Miocene, and that an orogenic breakthrough occurred in the Late Miocene, allowing uplift and thus crustal thickening to rapidly propagate into central and northern Peru. This suggests that crustal thickening in the Central Andes developed in two steps (Sempere et al., 2008), one starting in the Late Oligocene and/or earliest Miocene, and the second in the Late Miocene, in agreement with recent thermochronologic and paleoaltimetric results from southern Peru and Bolivia (e.g., Ghosh et al., 2006;Gillis et al., 2006;Schildgen et al., 2007;Quade et al., 2007;Thouret et al., 2007). ...
Physical paleoaltimetric methods are increasingly used to estimate the amount and timing of surface uplift in orogens. Because the rise of mountains creates new ecosystems and triggers evolutionary changes, biological data may also be used to assess the development and timing of regional surface uplift. Here we apply this idea to the Peruvian Andes through a molecular phylogeographic and phylochronologic analysis of Globodera pallida, a potato parasite nematode that requires cool temperatures and thus thrives above 2.0–2.5 km in these tropical highlands. The Peruvian populations of this species exhibit a clear evolutionary pattern with deeper, more ancient lineages occurring in Andean southern Peru and shallower, younger lineages occurring progressively northwards. Genetically diverging G. pallida populations thus progressively colonized highland areas as these were expanding northwards, demonstrating that altitude in the Peruvian Andes was acquired longitudinally from south to north, i.e. in the direction of decreasing orogenic volume. This phylogeographic structure is recognized in other, independent highland biotaxa, and point to the Central Andean Orocline (CAO) as the region where high altitudes first emerged. Moreover, molecular clocks relative to Andean taxa, including the potato–tomato group, consistently estimate that altitudes high enough to induce biotic radiation were first acquired in the Early Miocene. After calibration by geological and biological tie-points and intervals, the phylogeny of G. pallida is used as a molecular clock, which estimates that the 2.0–2.5 km threshold elevation range was reached in the Early Miocene in southernmost Peru, in the Middle and Late Miocene in the Abancay segment (NW southern Peru), and from the latest Miocene in central and northern Peru. Although uncertainties attached to phylochronologic ages are significantly larger than those derived from geochronological methods, these results are fairly consistent with coeval geological phenomena along the Peruvian Andes. They strongly suggest that orogenic volume initially developed in the CAO during most of the Miocene until a breakthrough in the latest Miocene allowed the northward propagation of crustal thickening into central and northern Peru, possibly by ductile crustal flow from the CAO. Such a combined phylogeographic and phylochronologic approach to regional uplift opens perspectives to estimate the direction(s) and timing of acquisition of altitude over other Cenozoic orogens.
... The Central Andes are currently the world's largest mountain belt to have been built by subduction-related processes (Isacks, 1988;Sempere et al., 2008) but the detailed history of its evolution, in particular the interplay between tectonic and climatic effects, still remains poorly known. Generally, two major pulses of surface uplift are described. ...
... Generally, two major pulses of surface uplift are described. One during Oligocene to Early Miocene (Isacks, 1988;Allmendinger et al., 1997;Sempere et al., 2008) and a second one in the late Miocene (Schildgen et al., 2007;Thouret et al., 2007;Garzione et al., 2008;Sempere et al., 2008;Schildgen et al., 2009). Crustal thickening and related surface uplift are generally accepted to have started about Mid-Eocene to Late Oligocene time but was asynchronous along and across strike of the Andes (Isacks, 1988;Sanchez, 1999;Gregory-Wodzicki, 2000;Sempere et al., 2008;Mamani et al., 2010). ...
... Generally, two major pulses of surface uplift are described. One during Oligocene to Early Miocene (Isacks, 1988;Allmendinger et al., 1997;Sempere et al., 2008) and a second one in the late Miocene (Schildgen et al., 2007;Thouret et al., 2007;Garzione et al., 2008;Sempere et al., 2008;Schildgen et al., 2009). Crustal thickening and related surface uplift are generally accepted to have started about Mid-Eocene to Late Oligocene time but was asynchronous along and across strike of the Andes (Isacks, 1988;Sanchez, 1999;Gregory-Wodzicki, 2000;Sempere et al., 2008;Mamani et al., 2010). ...
During the Cenozoic the landscape at the western margin of South America
changed dramatically due to the tectonic evolution of the Andes (Isacks
1988, J Geophysical Res 93) and significant variations in climate
(Gregory-Wodzicki 2000, GSA Bulletin 112). At present day the climate in
the western Central Andes (N-Chile, S-Peru) is arid (rainfall average is
less than 100-200 mm/y) Climate has changed significantly through time,
for example, sediments on the Altiplano and the eastern Central Andes
indicate a period of increased precipitation at around 7-8 Ma (Gaupp et
al. 1999, PPP 151; Uba et al. 2007, Geology 35). The uplift of the Andes
started some 30-25 My ago (Isacks, 1988,). Siliciclastic sedimentation
along the western flank of the Central Andes started at 55 Ma and lasted
until recent time (Moquegua Group, Roperch et al. 2006, Tectonics 25).
This implies that a river system with sediment deposition was already
developed before the Andean uplift which occurred during deposition of
the Moquegua Group. The Moquegua group is composed of four units:
Moquegua A (55-45Ma), Moquegua B (45-30Ma), Moquegua C (30-15?Ma) and
Moquegua D (15?-0Ma) (Roperch et al. 2006). The sedimentary basin of
Moquegua has a complex internal structure and is composed of different
subbasins. We focus in this study respectively, from north to south, on
the Cuno Cuno section (cut by the Rio Ocoña), the Majes section
(cut by the Rio Majes) and the Moquegua section (cut by the Rio
Moquegua). Several facies and compositional changes of Moquegua Group
sediments, both along orogenic strike and through time, are already
described; however, it lacks a detailed provenance study to constrain
the tectonic and climatic controls on sediment generation, dispersal,
and accumulation. To do so sandstones from all Moquegua units from the
three different sections have been sampled. Because it is crucial to
know all the potential source rocks in some detail, the
Proterozoic-Paleozoic basement, the Jurassic-Cretaceous sediments and
the six different volcanic arcs (Chocolate, Toquepala, Tacaza,
Huaylillas, Barroso and the frontal arc respectively from the oldest to
the youngest) were sampled as well. The heavy mineral fraction has been
separated for each potential source rock and the sediment samples.
Single grain amphiboles and oxides have been used for geochemical
analysis. Major elements from single grain amphiboles and oxides have
been measured using the electron microprobe. The amount of trace
elements from single grain amphiboles has been obtained using LA-ICPMS.
Those two methods allow us to distinguish the different potential source
rocks from each other and to define the sediment provenance for each
member of the Moquegua Group, in each of the sections from N to S.
Further thermochronological investigations, such as zircon fission track
dating, will allow us to better constrain the uplift of the Central
Andes.
... The setting and geometry of the Andes vary substantially along their longitude. The largest sector is formed by the Central Andes; it ranges from 5°300′ S to 37° S and is, in turn, subdivided into three subsectors: Northern Central Andes, Central Andes Orocline, and Southern Central Andes [20]. The portion of the Andean Cordillera within Peruvian territory falls within the Northern Central Andes subsector and the northern part of the Central Andes Orocline subsector. ...
... The area investigated in this study ( Figure 1) is located around the transition from the Northern Central Andes to the Central Andes Orocline at the Nazca Ridge subduction zone. The area comprises (i) the Peruvian forearc system, subdivided into East and West Pisco basins [22]; (ii) the Western Cordillera, consisting of a western part with sequence stratigraphy dominated by the Coastal Batholiths [23] and an eastern part characterized by the Marañón fold and thrust belt; (iii) the Eastern Cordillera, an anticline structure characterized by tectonic shortening propagated during the Oligocene-Miocene tectonic inversion of the original Triassic-Jurassic rift system [20,24]; (iv) the Subandean Zone, including Camisea and Ene basins; and (v) the Fitzcarrald arch, an area overhead ~500 m above the adjacent basin [25], that marks the transition between the northern and southern parts of the Amazonian foreland basin [26]. Table 1. ...
The aseismic Nazca Ridge produces localized flat-slab subduction beneath the South American margin at latitudes 10° to 15° S. The geological evolution and the spatio-temporal pattern of deformation of the upper plate have been strongly influenced by the presence of the flat slab. In this study, we investigated the lithospheric thermal structure of this region by elaborating a 2D geothermal model along a section across the top of the Nazca Ridge, the Peru–Chile trench, the Andean Cordillera, and the Amazonian Basin, for a total length of 1000 km. For the sake of modelling, the crust of the overriding plate was subdivided into two parts, i.e., a sedimentary cover (including the entire lithostratigraphic sequence) and a crystalline basement. Applying an analytical methodology, we calculated geotherms and isotherms by setting (i) thickness, (ii) density, (iii) heat production, and (iv) thermal conductivity for each geological unit and considering (v) heat flux at the Moho, (vi) frictional heating produced by faults, and (vii) plate convergence rate. The resulting model could make a significant advance in our understanding of how flat slab geometry associated with the Nazca Ridge subduction affects the thermal structure and hence the tectonic evolution of the region.
... The timescale of mantle convection, on the order of ∼100 Myr (Bunge et al. 1998), is relatively long compared to those associated with other tectonic processes. It exceeds, for instance, the time inferred for building large mountain ranges, such as the Andes, which rose to their current, prominent elevation over the past ∼20 Myr (e.g., Ghosh et al. 2006, Sempere et al. 2008, Carrapa et al. 2014. ...
... Husson & Ricard (2004), Iaffaldano et al. (2006) and Husson et al. (2012) report tectonic forces of the current Andean topography amounting to ∼8 · 10 12 N/m on average. Uplift of the Andean mountain chain peaked in the Oligocene, with another prominent uplift period in the Late Miocene (Oncken et al. 2006, Sempere et al. 2008). This latest uplift has been used to argue for a correlation between the recent (Neogene) South Atlantic spreading reduction and growth of the high Andes (Iaffaldano & Bunge 2009). ...
Past and current plate motions are increasingly well mapped from high-temporal-resolution paleomagnetic and geodetic studies, revealing rapid variations that occur on short timescales relative to the time it takes for the large-scale structure associated with mantle buoyancy to evolve. The rates of change of plate velocities hold key information on the geodynamic, tectonic, and Earth's surface processes that may have caused them. Rapid plate motion changes thus provide us with a unique opportunity to quantify the forcing associated with these processes. Important mechanisms capable of inducing such rapid changes include evolving plate boundary forces, for example, those associated with slab sinking or orogeny along convergent margins, as well as temporal variations in pressure-driven flow within the asthenosphere that link plate velocity variations explicitly to changes in dynamic topography. Here, we focus on (a) findings from recent kinematic observations and (b) the quantitative framework that allows their geodynamic interpretation.
... Oroclinal zones include (1) a forearc zone, corresponding to the Pacific slope and offshore areas; (2) an arc zone, represented by the present chain, consisting of the Coastal Cordillera and the so-called "Western Cordillera" and "Altiplano"; and (3) a back-arc area, which includes the "Eastern Cordillera" and the sub-Andean zone underlain by foreland basins (Jaillard et al., 2002). Tectonic shortening, initiated at ca. 30 Ma, led to crustal thickening in the Central Andean orocline (Isacks, 1988;Gregory-Wodzicki, 2000;Garzione et al., 2008;Sempere et al., 2008). The main uplift of the mountain chain occurred between ca. 10 and 6 Ma (Garzione et al., 2006(Garzione et al., , 2007Hartley et al., 2007;Sempere et al., 2008). ...
... Tectonic shortening, initiated at ca. 30 Ma, led to crustal thickening in the Central Andean orocline (Isacks, 1988;Gregory-Wodzicki, 2000;Garzione et al., 2008;Sempere et al., 2008). The main uplift of the mountain chain occurred between ca. 10 and 6 Ma (Garzione et al., 2006(Garzione et al., , 2007Hartley et al., 2007;Sempere et al., 2008). ...
The Yanque nonsulfide Pb-Zn deposit (inferred resources 12.5 Mt @ 3.7% Pb and @ 3.5% Zn) is located in the Andahuaylas-Yauri ore province (Cuzco, southern Peru). The deposit occurs within a base metal mineralized district, centered on the medium-sized Dolores porphyry copper. A thorough geological, mineralogical and geochemical study has carried out in order to define: the relationships between the Dolores Cu-porphyry ore and the Yanque Zn-Pb polymetallic mineralization, and the characteristics of the economic nonsulfide concentrations.
Both sedimentary and igneous rocks constitute the backbone of the Yanque-Dolores area. The sedimentary lithologies belong to the Soraya, Mara and Ferrobamba Fms. (upper Jurassic-middle Cretaceous). The Yanque orebody is hosted by the Mara Fm., which prevailingly consists of a siliciclastic sedimentary breccia. The original sulfide mineralization consisted of galena, pyrite and sphalerite. The host rock has been affected by a strong hydrothermal alteration, characterized by prevailing sericite/illite, as in the typical porphyry-related phyllic-argillic alteration stage, and by minor kaolinite, dolomite and quartz. Minor element geochemistry, characterized by Sb, As, Mn, Ag and locally also by Cu, points to magmatic-hydrothermal related mineralizing fluids. The Pb isotopic compositions from Dolores and Yanque sulfides are similar, and are typical of the Tertiary magmatically-derived ores in this part of Peru. The hydrothermally altered rocks at Yanque have the same Pb isotopic compositions as the sulfides, thus confirming the hypothesis that the Yanque primary Zn-Pb mineralization may have been produced by hydrothermal circulation related to the emplacement of the Dolores Cu-porphyry, as it is the case of other porphyry Cu systems associated with polymetallic mineralization elsewhere. However, no simple genetic model for the mineralization involving just one fluid circulation episode is able to explain the data.
The Yanque economic nonsulfide ore association consists of sauconite, hemimorphite, smithsonite and cerussite, which result from the weathering and alteration of the original sulfide mineralization. Zinc is allocated mainly in sauconite (Zn-smectite), rather than in carbonates: a factor strictly related to the prevailing siliciclastic character of the host rock.
Distinctive features of the Yanque orebody are the comparable ore grades for both Pb and Zn (3.5% Zn and 3.7% Pb), and the inverse supergene chemical zoning. In fact, contrary to other supergene ores of this type, zinc prevails in the top zone of the Yanque deposit, whereas lead content increases with depth. Considering the different mobility of the two metals in solution, it may be assumed that most of the primary zinc that was the source for the Yanque nonsulfides was originally located far from the position occupied by the galena mineralization, whose remnants have been observed on site. Zinc sulfides may have been originally contained in the eroded rock volumes that surrounded the actual deposit: the zinc-rich solutions have possibly migrated through the siliciclastic Mara Fm. and precipitated the nonsulfide minerals by porosity filling and replacement processes. In this sense, the Yanque secondary Zn-Pb deposit could be considered as a special type of “Exotic” mineralization.
... Most studies favour the hypothesis that Andean uplift started in the Eocene or Oligocene, but many do recognize a subsequent resumption of uplift starting in the late Miocene (Garzione et al., 2008;Sempere et al., 2008;Hoorn et al., 2010). Thus, it is mostly considered that mountain building was attained in two steps, the first slowly developing from the mid-Eocene to the late Oligocene and early Miocene (including the uplift of the proto-Cordillera Oriental of Bolivia called the Yungas), and a later, apparently vigorous step starting at ∼10 to 9 Ma and possibly lingering into the Pliocene and/or Present (Garzione et al., 2008;Sempere et al., 2008). ...
... Most studies favour the hypothesis that Andean uplift started in the Eocene or Oligocene, but many do recognize a subsequent resumption of uplift starting in the late Miocene (Garzione et al., 2008;Sempere et al., 2008;Hoorn et al., 2010). Thus, it is mostly considered that mountain building was attained in two steps, the first slowly developing from the mid-Eocene to the late Oligocene and early Miocene (including the uplift of the proto-Cordillera Oriental of Bolivia called the Yungas), and a later, apparently vigorous step starting at ∼10 to 9 Ma and possibly lingering into the Pliocene and/or Present (Garzione et al., 2008;Sempere et al., 2008). Although, as already pointed out by Mulch et al. (2010), additional studies integrating evidence from a larger spatial and temporal sampling size are required to resolve whether the paleogeographic data indicate a rapid or rather a gradual uplift of the Andes toward the late Miocene. ...
The Andes, the world's longest mountain chain, harbours great taxonomic and ecological diversity. Despite their young age, the tropical Andes are highly diverse due to recent geological uplift. Speciation either followed the orogeny closely or occurred after the Andean uplift, as a result of subsequent climatic changes. Different scenarios have been proposed to explain the diversification of high Andean taxa. The Melanoplinae grasshopper Ponderacris Ronderos & Cigliano is endemic to the eastern slopes of the Andes of Peru and Bolivia, mostly distributed between 1000 and 4000 m above sea level. Diversification in several montane habitats of Bolivia and Peru allows tests via cladistic analysis of distinct possible geographic modes of speciation. Eight species are recognized, with three described here as new with revised diagnostic morphological characters provided: Ponderacris carlcarbonellisp.n.,P. chulumaniensissp.n. and P. amboroensissp.n. Cladistic analyses of 15 species (8 ingroup and 7 outgroup) and 38 morphological characters, under equal and implied weighting, confirm the monophyly of Ponderacris. Characters from the external morphology and colour pattern provided less phylogenetic information than did the male abdominal terminalia and phallic complex. Species distributed in the Peruvian Andes constituted a monophyletic group, whereas those from the Bolivian Andes formed a basal paraphyletic grade. Dispersal–vicariance analysis resulted in one ancestral distribution reconstruction indicating that the most recent common ancestor was distributed in the Lower Montane Yungas of Bolivia. Eleven dispersal and one vicariant events are postulated, with a South-to-North speciation pattern coincident with progressive Andean uplift. Vicariance could relate to fragmentation of montane forest during the dry intervals of the late Cenozoic. From the Bolivian area, ancestral Peruvian Ponderacris may have dispersed northward, coinciding with the rise of the Andes. Ten of 11 dispersal events occurred at terminal taxa and are likely to be recent. However, diversification of Ponderacris cannot be explained solely by the South-to-North speciation hypothesis, but may also include both vicariance and dispersal across barriers influenced by Pleistocene climatic cycles.
... The South American Andes extend over a north-south length of 8500 km, and are the world's longest mountain chain, with an elevation range from 700 to 7000 m asl. (Sempere et al., 2008). This region encompasses high diversity and endemicity of several species (Novillo and Ojeda, 2012;Ceballos and Ehrlich, 2006;Maestri and Patterson, 2016). ...
The Neotropical Region is one of the major biogeographic divisions for both plants and animals on Earth. It spans from central Mexico and the Caribbean islands to southern South America. The Neotropical Region contains the largest number of mammal species in the New World, the second-highest number of mammal species on the planet, and is the region with the largest number of endemic mammal families in the world. This article provides an overview of the current state of our knowledge regarding the mammals of the Neotropical Region, including relevant geological events, evolution, biogeographic patterns, and large-scale assessment of threats, conservation status, and prospects
(25) (PDF) Diversity and Conservation of Neotropical Mammals. Available from: https://www.researchgate.net/publication/373915078_Diversity_and_Conservation_of_Neotropical_Mammals [accessed Sep 30 2023].
... 01). They also suggest that, despite the Andes cordillera emerged around 60 Ma, the three major Colombian cordilleras differ in age and may have emerged in the last 8 Ma (Gregory-Wodzicki 2000; Sempere et al. 2008;Mamani et al. 2010). Given the morphology and high diversity in the Andean region, it is possible that the recent developments in the We noticed the presence of prolateral keels in some species of Schismatothele. ...
Seven new species of Schismatothele Karsch, 1879 (Araneae, Theraphosidae) are described, almost doubling the diversity of the genus: S. caeri sp. nov.; S. caiquetia sp. nov.; S. merida sp. nov.; S. moonenorum sp. nov.; S. quimbaya sp. nov.; S. timotocuica sp. nov. and S. wayana sp. nov. An identification key for all species of Schismatothele (except S. kastoni) is presented, as well as a complementary diagnosis for the genus. Also, a standardized nomenclature is proposed to describe the prolateral keels of male palpal bulbs of species of Schismatothele.
... Far field effects from plate boundary forces associated with the Andean margin [93] have been invoked to explain these events. But most studies agree that prominent uplift of the Andes started in the mid Eocene, reaching a peak in the Oligocene, with a second uplift period in Late Miocene [96]. It is not obvious how increased topographic loads from the Andes would induce faster South Atlantic spreading. ...
Histories of large-scale horizontal and vertical lithosphere motion hold important information on mantle convection. Here, we compare continent-scale hiatus maps as a proxy for mantle flow induced dynamic topography and plate motion variations in the Atlantic and Indo-Australian realms since the Upper Jurassic, finding they frequently correlate, except when plate boundary forces may play a significant role. This correlation agrees with descriptions of asthenosphere flow beneath tectonic plates in terms of Poiseuille/Couette flow, as it explicitly relates plate motion changes, induced by evolving basal shear forces, to non-isostatic vertical motion of the lithosphere. Our analysis reveals a timescale, on the order of a geological series, between the occurrence of continent-scale hiatus and plate motion changes. This is consistent with the presence of a weak upper mantle. It also shows a spatial scale for interregional hiatus, on the order of 2000–3000 km in diameter, which can be linked by fluid dynamic analysis to active upper mantle flow regions. Our results suggest future studies should pursue large-scale horizontal and vertical lithosphere motion in combination, to track the expressions of past mantle flow. Such studies would provide powerful constraints for adjoint-based geodynamic inverse models of past mantle convection.
... The Northern Andes contain sequences of oceanic terranes that accreted from Upper Jurassic to Eocene times (e.g., Braz et al., 2018;Kennan & Pindell, 2009;Spikings et al., 2015). This contrasts with the Central Andes, which lack evidence for Jurassic and younger terrane accretions (e.g., Sempere et al., 2008), instead containing sedimentary and volcanic formations interpreted as reflecting deposition in a convergent-margin setting since early Paleozoic times (Pepper et al., 2016). ...
We analyze mantle structure under South America in the DETOX‐P1 seismic tomography model, a global‐scale, multifrequency inversion of teleseismic P waves. DETOX‐P1 inverts the most extensive data set of broadband, waveform‐based traveltime measurements to date, complemented by analyst‐picked traveltimes from the ISC‐EHB catalog. The mantle under South America is sampled by ∼665,000 cross‐correlation traveltimes measured on 529 South American broadband stations and on 5,389 stations elsewhere. By their locations, depths, and geometries, we distinguish four high‐velocity provinces under South America, interpreted as subducted lithosphere (“slabs”). The deepest (∼1,800–1,200 km depth) and shallowest (<600 km) slab provinces are observed beneath the Andean Cordillera near the continent’s northwest coast. At intermediate depths (1,200–900 km, 900–600 km), two slab provinces are observed farther east, under Brazil, Bolivia and Venezuela, with links to the Caribbean. We interpret the slabs relative to South America’s paleo‐position over time, exploring the hypothesis that slabs sank essentially vertically after widening by viscous deformation in the mantle transition zone. The shallowest slab province carries the geometric imprint of the continental margin and represents ocean‐beneath‐continent subduction during Cenozoic times. The deepest, farthest west slab complex formed under intra‐oceanic trenches during late Jurassic and Cretaceous times, far west of South America’s paleo‐position adjoined to Africa. The two intermediate slab complexes record the Cretaceous transition from westward intra‐oceanic subduction to eastward subduction beneath South America. This geophysical inference matches geologic records of the transition from Jura‐Cretaceous, extensional “intra‐arc” basins to basin inversion and onset of the modern Andean arc ∼85 Ma.
... Crustal thickening in the NW-trending region of the Bolivian Orocline has developed since~30 Ma (Picard et al. 2008;Sempere et al. 2008 and references therein). The onset of crustal thickening thus coincided with the widespread development of mafic magmatism during the Late Oligocene and earliest Miocene. ...
The Bolivian tin belt is a metallogenic province in the Eastern Cordillera of the Andes known for its Sn, W, Ag, and base metal deposits. Cassiterite, which is a major constituent in many magmatic-hydrothermal ore deposits from the Bolivian tin belt, can incorporate dozens of elements within its crystal lattice, making it a useful geological tracer mineral and also a potential host of critical elements. New U-Pb dating of cassiterite yields Late Triassic (Kellhuani deposit) and Late Oligocene to earliest Miocene (Viloco, Huanuni, and Llallagua deposits) ages. These ages confirm that Sn mineralization in the Bolivian tin belt occurred at least in two separate events during two major magmatic episodes apparently triggered by mantle upwelling, decompression melting, and basalt production promoting high heat flow into the overlying crust. The composition of studied hydrothermal cassiterite yields some geochemical trends that are attributed to its distance to the causative intrusion and/or level of emplacement. For example, cassiterite is generally enriched in Nb and Ta and yields higher Ti/Zr and Ti/Sc ratios in samples from xenothermal ore deposits located adjacent to intrusive complexes relative to shallow xenothermal and epithermal ore deposits. Therefore, these geochemical trends in cassiterite are useful tracers pointing to magmatic-hydrothermal centers. REE distribution in cassiterite was likely influenced by boiling processes, which resulted in tetrad-type irregularities. Cassiterite from the Bolivian tin belt is unattractive as a source for Nb (interquartile range [IQR] 4.84–0.037 ppm), Ta (IQR 0.0924–0.0126 ppm), and Ge (IQR 3.92–0.776 ppm). Some deposits, however, contain cassiterite relatively enriched in In (IQR 96.9–9.78 ppm, up to 1414 ppm) and Ga (IQR 92.1–3.03, up to 7437 ppm), that could constitute an attractive supplementary source for these elements in addition to sulfide minerals in the same deposits.
... La sedimentación de las secuencias Terciarias se produjo sobre el margen continental a partir del inicio de la subducción de la placa de Nazca por debajo de la placa Sudamérica. La tectónica compresiva produjo la orogenia Andina que se manifestó desde el Turoniano (Gil W., 2001) El levantamiento Andino se habría iniciado desde el Eoceno, luego se habría producido una subsecuente reactivación en el Mioceno Tardío y continúo durante el Plioceno hasta el presente (Sempere et al., 2008). ...
La cuenca Ucayali Sur desde los años 80 ha tomado especial interés por la presencia del campo de gas y condensado de Camisea. La exploración de hidrocarburos ha contribuido a la obtención de nueva información geológica y geofísica, la cual permitiría explicar la evolución estructural y su relación en la carga de hidrocarburos en los reservorios. El Lote 57 se encuentra al Sur de la cuenca de Ucayali, en la faja plegada zona sub andina. Las principales acumulaciones de gas y condensado se encuentran en reservorios clásticos del Pérmico y Cretácico involucrados en los anticlinales generados por fallas inversas. Generalmente se interpretaba que solo los corrimientos de bajo ángulo eran los responsables de formar las trampas anticlinales. Después de la interpretación de nueva información sísmica, de pozo, dataciones e información geoquímica podemos decir que también existió un control de las estructuras extensionales del Paleozoico sobre la deformación compresiva Andina. Así mismo, se estableció una correlación entre el pulso de expulsión de hidrocarburos y el tiempo de formación de las trampas estructurales. También, se identificó el límite de la zona deformada por tectónica de piel gruesa hacia el norte y la zona afectada únicamente por la tectónica de piel delgada hacia el Sur, controlado por la distribución estratigráfica de unidades del Paleozoico inferior que favorecerían la presencia de los niveles de despegue de los corrimientos de bajo ángulo.
... The timing and the processes involved in the creation of the Andean relief in this region are still debated. Sempere et al. (2008) propose a rapid uplift of about 2.5 km since the Late Miocene (11 to 6 Ma) in response to a large-scale mantle delamination. Armijo et al. (2015) propose that the topography was controlled by crustal thickening during the Paleogene (50 to 30 Ma) in response to the tectonic shortening of the Central Andes. ...
The central part of the Western Andes holds an exceptional concentration of giant paleolandslides involving very large volumes of rock material (v > km3). While those gravitational slope failures are interpreted consensually as an erosional response to the geodynamic activity of the Andes (relief formation and tectonic activity), the question of their triggering mechanisms remains enigmatic. To clarify the respective roles of climatic versus seismic forcing on the Andean landslides, new temporal constraints on paleo movements are essential. Here, we focus on one of those giant slope failures, the Aricota giant landslide that damned the Locumba valley in southern Peru. We conducted fieldwork, high-resolution DEM analysis and cosmogenic nuclide dating to decipher its development history and failure mechanisms. Our results point to the occurrence of two successive rockslide events. A giant failure mobilizing a rock volume of ca. 2 km3 first produced a dam at 17.9 ± 0.7 ka. Considering its height of ca. 600 m, the Aricota rockslide dam is one of the five largest landslide dams. At 12.1 ± 0.2 ka, a second event produced ca. 0.2 km3 of material, and the rock-avalanche debris spread out over the dam. As the chronology of those two events is pointing to the two main paleoclimatic pluvial periods in this region (Heinrich Stadial 1a and Younger Dryas), we favor the interpretation of a climatic forcing. At a regional scale, the concomitant aggradation of alluvial terraces and fan systems along the nearby valleys highlights higher regional erosion, sediments supply and mass-wasting events during those paleoprecipitation events and strengthens this conclusion.
... The South American Andes extend over a north-south length of 8500 km, and are the world's longest mountain chain, with an elevation range from 700 to 7000 m asl. . Within this region, three major geographical units can be identified: The Northern, the Central, and the Southern Andes (Sempere et al., 2008). Between the NA and CA, there is a transition zone termed "Amotape-Huancabamba". ...
South American alpine environments are home to 200 mammal species, among them 19% can be considered restricted or endemic to the region. Also, almost 20% of the species have conservation concerns, while 15% lack sufficient conservation data. Rodents, bats and carnivores represent most of the mammal species of the alpine region, whereas some of these species only just reach the high Andes. Some species can be considered emblematic of the alpine, such as the Andean Bear, Mountain tapir, the Andean subspecies of White tail deer, and the Andean cat, which encompass most conservation efforts.
... En este trabajo caracterizamos, por un lado, los patrones de diversidad de especies y endemismos de roedores en función de latitud, altitud y área en los Andes áridos centrales, tanto a nivel regional, entre los 8° y 42° S (Young et al., 2007;Sempere et al., 2008) como a nivel local (Provincia de Mendoza). Por otro lado, analizamos los factores topográficos, climáticos y espaciales involucrados en el modelado de estos patrones. ...
the Andes constitutes an extensive mountain range and a true laboratory for the study of the evolution of South American biota. In recognition to José Yepes, one of the pioneers in characterizing the distribution of mammals of Argentina we seek in this contribution to synthesize, from ecological biogeography, the knowledge we have about the diversity patterns of small mammals of the central Arid Andes (AcA). We characterize regional and local patterns of rodent richness and endemism as a function of latitude, altitude and area. At regional scale, we con- structed a species database using species range maps, meanwhile at the local scale, we conducted small mammal surveys using standardized techniques along 4 elevational transects between 32oS and 35oS. Among the results, we highlight, at regional level: a) a high number of endemisms (> 50%); b) richness increase with elevation, and c) endemic species – area relationship. Among the results at local level we stress: d) richness increases with el- evation, mean precipitation and topographic heterogeneity, and e) greater abundance at intermediate elevations. Biogeographic-ecological research allows us to depict patterns of composition and distribution of the central Andean biodiversity, and assess some of its causal factors. However, the long term conservation of the Andes re- quires not only solid biological / ecological knowledge but also conservation policies that integrate the biological, social and cultural dimensions of the Andean ecosystem. this approach seems the most appropriate alternative in the face of the different impacts induced by man and the climate change in the region.
... The distinctiveness of the Andean biomes probably also reflects the differential timing and impacts of the uplift of the Andes on each biome. Despite debate over the precise timing and rates of uplift (Sempere et al., 2008;Ehlers & Poulsen, 2009), it is well established that the Andes were still relatively low (400-2500 m palaeoelevations) c. 10 Ma (Barke & Lamb, 2006;Ghosh et al., 2006;Schildgen et al., 2007;Thouret et al., 2007;Garzione et al., 2008). Fossil evidence indicates that forests similar to the low-elevation SDTF were present in the central Andes at this time (Burnham, 1995;Burnham & Carranco, 2004), as well as on the Pacific coast of South America where dry prevailing climates pre-date the final, rapid phase of Andean uplift (Hartley, 2003;Houston & Hartley, 2003;Hartley et al., 2005). ...
Molecular phylogenies which include multiple accessions of species and near complete taxon sampling can be an important tool for estimating species diversity when used in combination with traditional morphology‐based taxonomy. Here we use a densely sampled plastid gene tree for a morphologically complex group within the legume genus Mimosa (sect. Batocaulon ser. Andinae) to improve estimates of species limits and diversity in the poorly known dry inter‐Andean valley of the Río Marañón, northern Peru. Based on the plastid gene tree, Mimosa ser. Andinae, which previously comprised four species, is re‐circumscribed to include six Andean dry‐forest species from northern Peru, Ecuador and southern Colombia, including the new species, M. jaenensis, described here. A further three candidate species are identified within the section based on high levels of sequence variation among accessions. With the additional species, the Marañón valley is now known to harbour nine narrowly restricted endemic species of Mimosa, a pattern of multiple congeneric endemics mirrored in many other plant genera as well as several animal groups. Our results, in combination with other published studies, suggest that overall species diversity in the Marañón has been significantly under‐estimated. Further work is needed to identify conservation priority areas in the Marañón in order to protect its unique flora.
... En este trabajo caracterizamos, por un lado, los patrones de diversidad de especies y endemismos de roedores en función de latitud, altitud y área en los Andes áridos centrales, tanto a nivel regional, entre los 8° y 42° S (Young et al., 2007;Sempere et al., 2008) como a nivel local (Provincia de Mendoza). Por otro lado, analizamos los factores topográficos, climáticos y espaciales involucrados en el modelado de estos patrones. ...
The Andes constitutes an extensive mountain range and a true laboratory for the study of the evolution of South American biota. In recognition to José Yepes, one of the pioneers in characterizing the distribution of mammals of Argentina we seek in this contribution to synthesize, from ecological biogeography, the knowledge we have about the diversity patterns of small mammals of the Central Arid Andes (ACA). We characterize regional and local patterns of rodent richness and endemism as a function of latitude, altitude and area. At regional scale, we constructed a species database using species range maps, meanwhile at the local scale, we conducted small mammal surveys using standardized techniques along 4 elevational transects between 32noS and 35noS. Among the results, we highlight, at regional level: a) a high number of endemisms ( > 50%); b) richness increase with elevation, and c) endemic species - area relationship. Among the results at local level we stress: d) richness increases with elevation, mean precipitation and topographic heterogeneity, and e) greater abundance at intermediate elevations. Biogeographic-ecological research allows us to depict patterns of composition and distribution of the central Andean biodiversity, and assess some of its causal factors. However, the long term conservation of the Andes requires not only solid biological / ecological knowledge but also conservation policies that integrate the biological, social and cultural dimensions of the Andean ecosystem. This approach seems the most appropriate alternative in the face of the different impacts induced by man and the climate change in the region. © 2018 Museo Argentino de Ciencias Naturales Bernardino Rivadavia.
... The Andes constitute the world's longest mountain system and harbour an outstanding taxonomic and ecological diversity (Young et al., 2002;Graham, 2009). The Central Andes in Peru and Bolivia comprise the Central Highlands geomorphological unit (about 4000 m in elevation), bounded by the Western and Eastern Cordilleras (Garzione et al., 2008;Sempere, Folguera & Gerbault, 2008;Gonzalez & Pfiffner, 2012). It is generally accepted that the uplift of the Central Andes was achieved in two steps Ehlers & Poulsen, 2009): the first slowly developing from the mid-Eocene and reaching a peak in the Late Oligocene and Early Miocene (c. ...
The Andes harbour an outstanding taxonomic and ecological diversity, for which several mechanisms promoting diversification, including ecological gradients and allopatric speciation, have been cited. The grasshopper genus Orotettix is an informative but challenging group to study diversification mechanisms because species in the genus are morphologically very similar, have low vagility and display local endemism over a complex topography in the Central Andes. We conducted several tests using ecological niche overlap and predictions of geographical distributions of Orotettix species on a phylogenetic framework to disentangle their speciation patterns. A multilocus molecular phylogeny was generated for Orotettix. Niche similarity tests were performed and the degree of niche overlap was estimated between species. Ecological niche models were generated to assess the realized ecological niche and potential distribution. The phylogenetic signal between the phylogenetic relatedness and niche overlap, and geographical and the environmental distances were analysed. Our findings suggest that speciation was not restricted to a single period and that species origins might have coincided with glacial-interglacial cycles of the Pleistocene. Given that we only found cases of niche conservatism for Orotettix, we infer that allopatric speciation had the primary role in its diversification. No significant phylogenetic signal was found, probably due to an island-like radiation process.
... from ancient crusts according to the extremely low ε Nd (t) values (−20 to −5) (Ducea and Barton, 2007). The crustal shortening and thickening since the mid-Oligocene (~30 Ma) (de Silva et al., 2006;de Silva and Gosnold, 2007;Garzione et al., 2008;Gregory-Wodzicki, 2000;Haschke et al., 2002;Sempere et al., 2008;Wörner et al., 1992) have produced the Altiplano-Puna Plateau in central Andes, where a Miocene to Holocene magmatic flare-up built. Available bulk Sr/Nd isotopic data (e.g., Haschke et al., 2002) suggest an increasing contribution of crustal mass to the magma since the crustal thickening. ...
The Permo-Triassic Jomda-Weixi-Yunxian continental margin arc belt in the southeastern Tibetan Plateau was formed by the subduction of the main Paleotethyan branch underneath the Cathaysian-affinity blocks along the Longmu Co-Shuanghu-Changning-Menglian suture. This belt consists of three segments with distinctive magmatism history. Available geochronological data revealed that the northern and southern segments of this belt comprise several volcanic successions intercalated with terrestrial clastics suggesting long-lived and pulsed magmatism. In contrast, the middle segment is made of a single volcanic pile of dacite/rhyolite and minor basalt with thickness of >1.6 km. Numerous granodiorite batholiths and a few diabase dykes intruded the volcanic pile. Detailed zircon U-Pb dating indicates that the volcanic rocks and intrusive rocks were formed during a short time period of ~7 million years (~244–251 Ma), defining a magmatic flare-up. Petrographic and geochemical data including Sr-Nd isotopics demonstrate that the felsic magmas were derived from ancient crust, while the basalt and diabase are sourced from the mantle wedge induced by subduction of the Paleo-Tethys. Structural studies reveal syn-subduction crustal shortening and thickening of the middle segment. The thickened crust likely has prevented the subduction-induced basaltic magma from eruption; they alternatively were accumulated in the bottom of the crust yielding a magma chamber. The chamber heated, softened, and melted the crust, forming felsic magmas. When the crust was weakened to a critical threshold, the felsic magmas rose up and erupted to the surface, resulting in the magmatic “flare-up”. This study suggests that syn-subduction shortening and thickening of the overriding crust seems to have played a significant role in the development of magmatic flare-ups along a continental margin arc-system. Other well-studied examples include the Mesozoic Serra Nevada and the late Miocene to Holocene Altiplano-Puna Plateau, central Andes.
... According to the DIVA based on the tree topology that included all the species of the genus, the ancestral area for the Romaleinae represented in this study appeared to be the Chacoan province. Considering that the split of the Romaleinae was estimated to have occurred during Late Cretaceous-early Palaeogene, the diversification of this group in the Chacoan coincided with a period of multiple geodynamic events, including the multiphased uplift of the Andes and the formation of Amazonian lowlands (Sempere et al., 2008;Hoorn et al., 2010). The ancestor of Zoniopoda may have been distributed in an area corresponding to the Chacoan and Cerrado provinces, within the Chacoan subregion (Morrone, 2014). ...
The open vegetation biomes, within the limits of the Chacoan subregion, occur along a diagonal in eastern South America covering a large range of environmental conditions. In order to contribute to the knowledge on the biodiversity of these open biomes, we analysed the phylogenetic relationships of the grasshopper genus Zoniopoda to the remaining South American Romaleinae, and examined the biogeographical patterns of diversification of the genus. The study is based on morphological and molecular ( COI and H3 ) evidence, including 12 species of Zoniopoda and 17 species of four tribes of South American Romaleinae. We describe a new species of Zoniopoda , and test its taxonomic placement within the group. Results of our phylogenetic analyses recovered Zoniopoda as a monophyletic group with high support values. According to the dispersion–vicariance analysis, the ancestor of Zoniopoda may have been distributed in an area corresponding to the Chacoan and Cerrado provinces. A vicariant event, that could be explained by the uplift of the Brazilian Plateau and the subsidence of the Chaco, is hypothesized to have occurred splitting the ancestral distribution of Zoniopoda , resulting in the independent evolution of the Tarsata group within the Cerrado and the Iheringi group in the Chacoan subregion.
This published work has been registered in ZooBank, http://zoobank.org/urn:lsid:zoobank.org:act:FCFB4C5D-1741-46F1-8E25-B37ED2B9D872 .
... Two broad scenarios have been summarized by Sempere et al. (2008) and Barnes and Ehlers (2009): (1) rapid uplift beginning in the late Miocene, with ~2.5 km of surface uplift occurring between 11 and 6 Ma in response to large-scale mantle delamination (Ghosh et al., 2006;Garzione et al., 2007Garzione et al., , 2008; Molnar and Garzione, 2007;Farias et al., 2008;Hoke and Garzione, 2008); or (2) gradual surface uplift beginning in the late Eocene, driven by crustal shortening and thickening in response to subduction of the Nazca plate (Victor et al., 2004;Barke and Lamb, 2006;Hartley et al., 2007;Barnes and Ehlers, 2009). ...
Based on an ⁴⁰Ar/³⁹Ar- and U/Pb-based chronostratigraphy of ignimbrite sheets and the geomorphological features of watersheds, river profiles and slope deposits in the Ocoña–Cotahuasi–Marán (OCM) and Colca valleys of southwest Peru, we reconstruct the valley incision history of the western Central Andes over the last c. 25 Myr. We further document the Pleistocene and Holocene evolution of deep valleys on the basis of 14 ¹⁰Be surface-exposure ages obtained on debris-avalanche deposits and river straths. The data suggest that uplift was gradual over the past 25 Myr, but accelerated after c. 9 Ma. Valley incision started around 11–9 Ma and accelerated between 5 and 4 Ma. Incision was followed by several pulses of valley cut-and-fill after 2.3 Ma. Evidence presented suggest that the post-5 Ma sequence of accelerated canyon incision probably resulted from a combination of drainage piracy from the Cordilleran drainage divide towards the Altiplano, accentuated flexural tilting of the Western Cordillera towards the SE, and increased rainfall on the Altiplano after late Miocene uplift of the Eastern Cordillera. The valley deepening and slope steepening driven by tectonic uplift gave rise to large occurrences of rockslope failure. The collapsed rock masses periodically obstructed the canyons, thus causing abrupt changes in local base levels and interfering with the steadiness of fluvial incision. As a result, channel aggradation has prevailed in the lower-gradient, U-shaped Pacific-rim canyons, whereas re-incision through landslide deposits has occurred more rapidly across the steeper V-shaped, upper valleys. Existing canyon knickpoints are currently arrested at the boundary between the plutonic bedrock and widespread outcrops of middle Miocene ignimbritic caprock, where groundwater sapping favouring rock collapse may be the dominant process driving headward erosion.
... This monophyletic genus (Cassild e et al. 2010(Cassild e et al. , 2012Penz et al. 2012) originated at ca. 32 Ma (Wahlberg et al. 2009) in the east side of the proto-Andes (Penz et al. (2012) and Blandin & Purser (2013)). The genus diversified into species living in lowland forests, low mountain forests and cloud forests, during a period of major and complex orogenic processes, high elevations being attained at different times in Central Andes and Northern Andes, as well as in western and eastern cordilleras (Garzione et al. 2008;Sempere et al. 2008;Eude et al. 2015). In order to discriminate between the Mio-Pliocene and Pleistocene diversification hypotheses, Andean-endemic cloud forest butterflies should provide ideal models, as they diversified within a single ecological belt that could develop only when cordilleras attained suitable elevations. ...
The monophyletic Morpho sulkowskyi butterfly group, endemic of Andean cloud forests, was studied to test the respective contributions of Mio-Pliocene intense uplift period and Pleistocene glacial cycles on Andean biodiversity. We sampled nine taxa covering the whole geographical range of the group. Two mitochondrial and two nuclear genes were analysed using a Bayesian method. We established a dated phylogeny of the group using a relaxed clock method and a wide-outgroup approach. To discriminate between two hypotheses, we used a biogeographical probabilistic method. Results suggest that the ancestor of the M. sulkowskyi group originated during the Middle–Late Miocene uplift of the Eastern Cordillera in northern Peru. Biogeographical inference suggests that the M. sulkowskyi and Morpho lympharis clades diverged in the northern Peruvian Andes. The subsequent divergences, from the Late Miocene to the Late Pliocene, should have resulted from a dispersal towards the Northern Andes (M. sulkowskyi clade), after the closure of the West Andean Portal separating the Central and Northern Andes, and a southwards dispersal along the Peruvian and Bolivian Eastern Cordilleras (M. lympharis clade). Only a few divergences occurred at the very end of the Pliocene or during the Pleistocene, a period when the more recent uplifts interfered with Pleistocene glacial cycles.
... Following the break-up of Rodinia, subduction along the North American Cordilleran margin initiated in the Middle-Late Devonian Davis 1972, 1975;Monger and Price 2002;Dickinson 2004Dickinson , 2009, whereas the western margin of South America preserves a record of almost continuous subduction since the Cambrian, with the inception of the Terra Australis orogen (Rapela et al. 1998a(Rapela et al. , 1998bPankhurst et al. 2000;Ramos and Aleman 2000;Cawood 2005; Chew et al. 2007; Collo et al. 2009). Despite having formed under one geodynamic regime, the Cordilleran orogen is segmented, i.e., features along-strike tectonic, structural, and morphological variations (Sempere et al. 2008;Ramos 2009). For the sake of the analysis in this paper, the Cordilleran margin is divided into eight sectors, from north (British Colombia) to south (Patagonia): (1) the Coast Ranges; (2) the Sierra Nevada; ...
Magmatic activity in continental arcs is known to vary in a non-steady-state manner, with the mechanisms driving magmatic activity being a matter of ongoing discussion. Of particular importance is the question of what extent episodic magmatism in continental arcs is governed by external factors (e.g., plate motions) and internal factors (e.g., feedback processes in the upper plate). To test existing models for magmatic episodicity, which are mostly based on temporally and spatially limited records, this study uses large data sets of geochronological, geochemical, and plate kinematic data to document the Paleozoic to Mesozoic development of the North and South American Cordilleras in eight transects from British Columbia to Patagonia. The temporal distribution of U/Pb bedrock and detrital zircon ages, used as a proxy for timing of magmatic accretion, shows that some minima and maxima of zircon abundance are nearly synchronous for thousands of kilometers along the arc. Some age patterns are characterized by a periodicity of 50-80 Ma, suggesting a cyclic controlling mechanism. Other magmatic lulls or flare-ups find no equivalents in adjacent sectors, indicating that either discrete events or variable lag times may also be important in governing magmatic activity in continental arcs. Magma composition in Mexico, the Peninsular Ranges, and the Sierra Nevada varies episodically and proportionally with the temporal record of arc activity. During flare-up events, there is an increase in Sm/Yb, indicating deeper melting, and a decrease in eNdb suggesting a higher degree of crustal assimilation. Geochemical scatter also increases during the initiation of flare-up events. Plate kinematic data provide a means of evaluating mantle heat input. The correlation between plate convergence rate and magmatic accretion varies for each sector, suggesting that different flare-ups or lulls likely reflect variable combinations of processes.
... The Andes constitute the world's longest mountain system and harbour an outstanding taxonomic and ecological diversity (Young et al., 2002;Graham, 2009). The Central Andes in Peru and Bolivia comprise the Central Highlands geomorphological unit (about 4000 m in elevation), bounded by the Western and Eastern Cordilleras (Garzione et al., 2008;Sempere, Folguera & Gerbault, 2008;Gonzalez & Pfiffner, 2012). It is generally accepted that the uplift of the Central Andes was achieved in two steps Ehlers & Poulsen, 2009): the first slowly developing from the mid-Eocene and reaching a peak in the Late Oligocene and Early Miocene (c. ...
The reciprocal illumination nature of integrative taxonomy through hypothesis testing, corroboration and revision is a powerful tool for species delimitation as more than one source has to support the hypothesis of a new species. In this study, we applied an integrative taxonomy approach combining molecular and morphological data sets with distributional patterns to examine the level of differentiation between and within the grasshopper Orotettix species. Orotettix was described based on five valid species distributed in the Andes of Peru. In our study, initially a molecular‐based hypothesis was postulated and tested against morphological data and geographical patterns of distribution. Results from molecular and morphological analyses showed agreement among the species delimitation in Orotettix, and were also consistent with the geographical distribution. The analyses allowed us to delimit five new species for the genus ( O. lunatus sp. nov., O . astreptos sp. nov., O. colcaensis sp. nov., O . paucartambensis sp. nov. and O . dichrous sp. nov.) from the Eastern and Western Cordilleras of Peru. We also provide critical knowledge on the phylogenetic relationships and distribution of the genus and conduct a revision of Orotettix. © 2015 The Linnean Society of London
... Andean topography is a system of mountain ranges resulting from an amalgamation of orogens and accreted terranes. Although the 9000-km-long Cordillera is situated along a continuous subduction zone, subduction is an unsteady and nonuniform process (Benavides-Cáceres, 1999;Sempere et al., 2008). The periodic subduction of seamounts, oceanic ridges, and oceanic plateaus has punctuated the tectonic and geomorphic response of upperplate topography by promoting shallow-angle subduction, although not in a systematic way (Skinner and Clayton, 2013). ...
Extensive tracts of low-gradient topography in steep mountain ranges, either forming rangetop plateaus or terraced pediments on range flanks, are widely distributed in mountain belts around the world. Before the advent of plate tectonics, such populations of planar landforms were interpreted as vestiges of a post-orogenic raised peneplain, i.e., a low-gradient land surface resulting from the decay, during long intervals of base-level stability, of a previous mountain range that was subsequently raised once again to great elevations—thus forming a new mountain range. This two-stage model has been challenged by theories that advocate continuity in tectonic processes and more gradual changes in base level, and thus expect a more immediate and proportionate response of geomorphic systems. Here we present a global survey of erosion surfaces in mountain ranges and put existing theories and empirical evidence into a broad perspective calling for further research into the rates and regimes of long-term mountain evolution. The resulting library of case studies provides opportunities for comparative analysis and helps to classify the landform mosaics that are likely to arise from the interplay between (i) crustal regimes, which at convergent plate margins need be neither uniform nor steady at all times; (ii) radiation-driven and gravity-driven geomorphic regimes, which are mainly determined by crustal boundary conditions and climate; and (iii) paleogeography, through which clues about base-level changes can be obtained. We examine intracratonic and plate-margin settings, with examples from thin-skinned fold belts, thick-skinned fold belts, island-arc and other subduction-related settings, and bivergent collisional orogens. Results reveal that the existence of erosion surfaces is not a simple function of geodynamic setting. Although some erosion surfaces are pre-orogenic, evidence about their predominantly post-orogenic age is supported by apatite fission-track and helium rock-cooling signatures, stratigraphic age-bracketing, stream channel gradient patterns, and other direct or indirect dating criteria. It follows that many portions of mountain belts undergo unsteady, nonuniform post-orogenic landscape evolution trajectories, with intermittent opportunities for relief reduction. The resulting erosion surfaces remain preserved as signatures of transient landscape evolution regimes. We find that (i) occurrences of planar topography form populations of discrete, insular landscape units, only some of which could be interpreted as fragments of a fluvially dissected, and/or tectonically fragmented, regional peneplain. (ii) The post-orogenic time required for achieving advanced stages of relief reduction is variable, ranging from 3 to 70 Ma. (iii) Partly depending on whether the adjacent sedimentary basins were over- or underfilled, some erosion surfaces may have been controlled by raised base levels and may thus have formed at high elevations; however, in many cases they were disconnected from marine base levels by rapid surface uplift, thus acquiring their elevated positions in recent time. In some cases, subcrustal processes such as asthenospheric anomalies, and/or lithospheric slab tear or breakoff, explain extremely rapid, regional post-orogenic uplift. (iv) Overall, the conditions for achieving surface preservation in steep and tectonically active terrain are predictable but also quite varied and contingent on context.
... There is considerable debate about the exact timing of the Andean uplift (Sempere et al., 2008;Ehlers & Poulsen, 2009), but it is clear that the northern Andes were still relatively low (400-2500 m) about 10 million years ago (Ma; Quade et al., 2007;Thouret et al., 2007). This implies a recent, rapid uplift in the past 10 million years. ...
Phylogenetic relationships of species in the papilionoid legume genus Dussia Krug & Urb. ex Taub. were investigated using nuclear ribosomal internal transcribed spacers and chloroplast trnL and trnD-T DNA sequences. Bayesian phylogenetic dating techniques suggest that: (1) the uplift of the Andean Mountains created a biogeographical barrier to migration but also contributed to speciation; (2) migration between South America and Central America occurred before the closure of the Panamanian isthmus, indicating that for Dussia the Pacific Ocean was less of a barrier than the Andes; and (3) the biogeographic affinities of species from the Chocó biogeographical region are with Central America.
... We further sub-divide the previously defined stratigraphic scheme of the Camaná Formation (Sempere et al., 2004), in order to (i) describe in detail the interactions between fluvial, deltaic, and marine sedimentation, and (ii) further constrain the depositional ages for the units and sub-units. This is then used to explain the relationship between varying sedimentary input and relative sea- (Sempere et al., 2008). The Pisco, Camaná/Mollendo, and Moquegua Basins are shown. ...
In the external forearc of southern Peru (Arequipa region), the sedimentary facies and the stratigraphic architecture of the Cenozoic Camaná Formation are presented in the context of tectono-eustatic controls. The Camaná Formation is defined as ∼500 m thick coarse-grained deltaic complex that accumulated in a fault-bounded elongated depression extending from the Coastal Cordillera in the east to the offshore Mollendo Basin in the west and likely up to the Peruvian Trench. Based on the analysis of facies associations, we propose a refined stratigraphic scheme of the Camaná Basin fill. The Camaná Formation was formerly divided into the Camaná “A” and Camaná “B” units (CamA and CamB, respectively). We reinterpret the stratigraphic position and the timing of the CamA to CamB boundary, and define three sub-units for CamA, i.e. sub-units A1, A2, and A3. Each depositional unit shows individual stacking patterns, which are linked with particular shoreline trajectories through time.
... As a result, a landscape constituted by fault-blocks mountains and intermontane basins was developed in the Andean foreland, several hundred kilometers away from the trench (Cobbold et al., 2007;Cogné et al., 2013;Hain et al., 2011;Jordan and Allmendinger, 1986;Mon and Gutiérrez, 2009;Strecker et al., 2011). Usually, these segments of foreland deformation have been associated with a flat-slab subduction regime (e.g. the Sierras Pampeanas [Pampean Ranges] with a Pampean flat-slab subduction segment; Ramos and Folguera, 2009) that reactivated and/or inverted the pre-existing structures (Franzese et al., 2011;García Morabito et al., 2011;Ramos et al., 2002;Sempere et al., 2008). ...
The intraplate fault-block mountains and intermontane deposits of the Gastre Basin, which are recorded more than 550 km east of the Andean trench in central Patagonia, Argentina, are analyzed. The Gastre Basin is one of the largest Patagonian intermontane basins, limited by uplifted blocks strongly oblique to the Andean chain. It was originated by reverse faulting and inversion of pre-existing normal faults associated with a Mesozoic rift basin and defined by older crustal heterogeneities. The deformational event occurred
during the middle Miocene, related to a short contractional episode (16.1–14.86 Ma), probably in response to an eastward migration of the Andean fold and thrust belt. During Pliocene to Quaternary times, neither younger fault-block uplifts nor reconfigurations of the basin occurred. Similarities between the study area and other parts of the Patagonian foreland – such as the presence of Miocene reverse or inversion tectonics, as well as the accommodation of the Miocene sedimentary successions – suggest that the Gastre Basin is part of a major late early to middle Miocene broken foreland system (i.e. the Patagonian broken foreland) that exhumed discrete fault-block mountains and generated contemporary basins along more than 950 km parallel to the Andean trench (i.e. between 40°00′ and 48°00′ south latitude). Based on recent studies on the southern Andean Margin, this continental-scale contractional episode may be the result of a flat-slab subduction segment. Nevertheless, such a hypothesis is very difficult to support when analyzing such a large flat subduction segment along the entire Patagonian trench. This suggests the need to consider alternative flat-slab trigger mechanisms or other factors in the generation of broken foreland systems.
... Understanding of the geotectonic position of the source area of HP rocks and their P-T paths are essential for geodynamics of convergent plate tectonics (e.g. Brueckner, 1998;Brueckner and Medaris, 2000;Medaris et al., 2005;Oncken et al., 2007;Sempere et al., 2008;van Roermund, 2009;Schellart and Rawlinson, 2010;Sajeev et al., 2012). To define source material from which the rocks were derived conventional geochemical and isotope analyses are usually used (Carlson and Irving, 1994;Smith, 2003;Demidjuk et al., 2007). ...
This paper presents monomineral and multiphase inclusions in garnet from eclogites and clinopyroxenites, which form layers and boudins in garnet peridotites from two areas in the Moldanubian zone of the Bohemian Massif. The garnet peridotites occur in felsic granulites and reached UHP conditions prior to their granulite facies overprint. In addition to complex compositional zoning, garnets from hosting eclogites and clinopyroxenites preserve inclusions of hydrous phases and alkali silicate minerals including: amphiboles, chlorites, micas and feldspars. Amphibole, biotite and apatite inclusions in garnet have a high concentration of halogens; CO2 and sulfur are involved in carbonates and sulfide inclusions, respectively. The inclusion patterns and compositional zoning in garnet in combination with textural relations among minerals, suggest that the ultramafic and mafic bodies are derived from lithospheric mantle above the subduction zone and were transformed into garnet pyroxenites and eclogites in the subduction zone. Based on compositional, mineral and textural relations, all of these rocks along with the surrounding crustal material were overprinted by granulite facies metamorphism during their exhumation.
... Crustal growth and thickening result from combined magmatic and tectonic accretion as exemplified by the Paleozoic Caledonides and Hercinides (e.g. Collins, 2002;Draut et al., 2002) or the western Pacific in Taiwan and Japan (Lin, 2002;Malavieille and Trullenque, 2009), the Circum-Pacific Cordillera in North America (Monger et al., 1982;Wheeler and McFeely, 1991;Evenchick et al., 2005;Hollister and Andronicos, 2006) and South America (Allmendinger et al., 1997;Oncken et al., 2007;Sempere et al., 2008;DeCelles et al., 2009), the Himalayas (Gansser, 1964;Burg et al., 1984b;Petterson and Windley, 1985;Coward et al., 1986;Mattauer, 1986 ...
Convergent plate boundaries are characterized by the development of
crustal orogenic wedges and orogenic plateaus but also by gravitational
collapse of previously thickened crust leading to the opening of
intermontane and eventually oceanic back-arc basins. Foreland and
extensional sedimentary basins in the plate boundary region are filled
by the erosional products of the orogenic crust. Metamorphic rocks
forming orogenic crust attest to burial and exhumation under contrasted
geothermal gradients. These features portray the crustal orogenic cycle
and are first-order indicators of the thermal and mechanical evolution
of the crust within the plate boundary region. This evolution is
controlled by complex interactions among (i) the dynamic balance among
forces that arise from plate-tectonic, gravitational potential energy,
and buoyancy, (ii) the thermal balance between deformation-induced and
radioactive heat production and heat advection related to subduction,
orogenic deformation, and magma transfer, and (iii) the mass transfer
balance between uplift and erosion. To account for these geological
characteristics, a generic model, that integrates results from physical
modeling, is proposed for the thermal-mechanical evolution of crustal
orogenic belts and for its implication in controlling the transition
between the different phases of the orogenic cycle. In this model, the
transition from low to high geothermal gradient is associated with
increased heat production in the thickened crust owing to radioactive
decay and deformation. Partial melting and rheologic weakening of the
thermally mature thickened crust triggers gravity-driven lateral flow of
the lower crust and controls the transition from wedge to orogenic
plateau. Destruction of the orogenic crust is achieved in part by
erosion but mostly by gravitational collapse. The style of extension is
controlled by the rheology of the crust at the onset of gravitational
collapse and its evolution as the crust thins and cools. Gravitational
collapse is permitted by a modification of lithosphere dynamics in the
convergence zone and might eventually lead to opening of a new oceanic
basin if collapse is followed by thinning of the lithospheric mantle.
... The SCDA region was defined between 8° and 42°S (Young et al. 2007; Sempere et al. 2008), which covers an altitudinal range from 800 to 6000 m a.s.l. (Fig. 1). ...
The Andean mountain range has played an important role in the evolution of South American biota. However, there is little understanding of the patterns of species diversity across latitudinal and altitudinal gradients. In this paper, we examine the diversity of small mammals along the South Central Dry Andes (SCDA) within the framework of two contrasting hypotheses: (a) species richness decreases with increasing elevation and latitude; and (b) species richness peaks at altitudinal midpoints (mid-domain). We explore the composition of the species pool, the impact of species–area relationships and the Rapoport effect (i.e. size of geographic ranges) along latitudinal and elevational gradients. First, we constructed a database of SCDA small mammals. Then, species richness patterns were analysed through generalized models, and species–area relationships were assessed by log–log regressions; the curvilinear method (c = S/Az) was use to compute richness corrected by area size. Lastly, the Rapoport effect was evaluated using the midpoint method. Our results show: (1) a richness of 67 small mammals along the SCDA, of which 36 are endemic; (2) a hump-shaped pattern in species richness along elevation and latitudinal gradients; (3) a species–area relationship for both gradients; (4) endemic species corrected by area present a strong and positive relationship with elevation; (5) a Rapoport effect for the latitudinal ranges, but no effect across the elevational gradient; and (6) a major species turnover between 28° and 30° south latitude. This is the first study quantifying the diversity of small mammals encompassing the central Andean region. Overall, our macrogeographic analysis supports the previously postulated role of the Andes in the diversification of small mammals (i.e. in situ cladogenesis) and highlights some basic attributes (i.e. anatomy of geographic ranges; species–area relationships) when considering the consequences of climate change on biodiversity conservation of mountain ecosystems.
We present a petrographic and geochemical study of tourmaline from the Triassic Chacaltaya Sn-polymetallic district in the Cordillera Real of Bolivia. Tourmaline is associated with greisens, breccias, and veins, which occur around the Triassic Chacaltaya peraluminous granitic stock hosted by Silurian metasedimentary rocks. Three main petrographic types of hydrothermal tourmaline have been identified: pre-ore greisen-related (Tur-1), syn-ore breccia-related (Tur-2), and syn-ore vein-related (Tur-3). The three types of tourmaline belong to the alkali group and have Fe-rich compositions mostly close to the schorl end member. Overlapping Fe/(Fe + Mg) ratios suggest broadly similar compositions of the hydrothermal fluids during the deposition of tourmaline. The most notable differences in minor and trace element contents include relative enrichment in Zn and Li in Tur-1 and relative enrichment in Ca, Sc, V, Cr, Sr, Sn, Y, Cs, Be, and Zr in Tur-3, with Tur-2 showing intermediate compositions between those of Tur-1 and Tur-3. The progressive enrichment in Sn from Tur-1 (avg = 14 ppm) through Tur-2 (avg = 311 ppm) and Tur-3 (avg = 476 ppm) indicates an increase of Sn concentrations in the hydrothermal system coinciding with cassiterite deposition in breccias and veins. The transition from high Li and Zn contents in Tur-1 to elevated Ca, Sr, V, and Cr contents in Tur-3 is interpreted as reflecting interaction between a hydrothermal fluid of magmatic origin and the metasedimentary country rocks. Strong and relatively steady positive Eu anomalies in all tourmaline types suggest dominantly reduced hydrothermal conditions. In situ δ18O and δ11B analyses of greisen-related Tur-1 reveal crystallization in isotopic equilibrium with magmatic water derived from a peraluminous S-type granite. In contrast, higher δ18O values of breccia-related Tur-2 and vein-related Tur-3 indicate crystallization in isotopic equilibrium with a fluid of metamorphic origin or a magmatic fluid that variably interacted with the metasedimentary host rocks. Geochemical modeling reproduces interactions between a fluid of magmatic origin and the host metasedimentary rocks at moderate water/rock ratios between 0.1 and 0.5. We conclude that cassiterite mineralization in the Chacaltaya district was formed primarily through interaction between B-Sn–rich magmatic fluids and the metasedimentary country rocks.
The Vitória Seamount (VTS), distant ~300 km from the Brazilian coastline at latitude 20ºS, is the second closest offshore volcanic complex of the Vitória-Trindade Ridge (VTR) which corresponds to a ~1200 km long ridge of seamounts and islands composed of SiO2-undersaturated magmatic rocks commonly considered to be the volcanic track of the Trindade mantle plume in the South American Plate. Based on the first sample dredged from Vitória Seamount, new petrographic and electron microprobe analyses from its rock show an alkaline basalt with pseudotrachytic texture consisting of bytownite and salite phenocrysts, labradorite microliths, anhedral titanomagnetite, and a yellowish green pseudomorphic phase composed of MgO-Al2O3-SiO2-FeO. The fine-grained groundmass is mainly composed of strongly oriented lath-shaped labradorite microliths, opaque minerals, and vesicles filled by a yellowish green pseudomorphic phase. Whole-rock analyses of the Vitória Seamount rock reveal its SiO2 undersaturation (SiO2 ca. 40 wt.%; normative nepheline = 13.8), enrichment in Cr, Co, Ni, V and Sc, along with depletion in Zr, La and Nd contents compared to the other seamounts of the VTR. VTS show a strong enrichment in light-REE (La/SmN ca. 2.68) compared to heavy-REE (La/YbN = 20.79). Major and trace element evidence indicate that the melting of an enriched mantle source to generate the Vitória Seamount magma occurred dominantly in the garnet stability field. Trace element composition of VTS is consistent with ≤ 3% partial melting of the mantle source. Neodymium and Sr isotopic data suggest that the mantle source of the Vitória Seamount had been variably metasomatized by melts derived from enriched mantle component, which may have developed approximately 600 Ma, reconciling with the Brasiliano Orogeny, according to Nd age model. Modeling of the Nd-Sr isotope systematics points out that the primary melt was formed from an asthenospheric mantle (DMM – Depleted MORB [Mid-Ocean Ridge Basalts] Mantle) that underwent mixing with a continentally derived material (represented by EMI [Enriched Mantle I] component). This process can be explained by the mixing of melts from these mantle components during magma genesis.
With an area exceeding 25 000 km ² and volumes c. 5000 km ³ , south Peru hosts the Andes’ second largest Neogene ignimbrite field. We document the extent, stratigraphy and chronology of 12 ignimbrite sheets in the Río Ocoña–Cotahuasi–Marán and Colca deep canyons. Based on 74 ⁴⁰ Ar/ ³⁹ Ar and U/Pb age determinations, ignimbrite-forming episodes span 25 myr. Prior to 9 Ma, eight large-volume ignimbrites were produced every 2.4 myr. After 9 Ma, average lulls between small- to moderate-volume ignimbrites decreased to 0.85 myr. The refined volcanic stratigraphy reveals three main features. (1) Larger volume ignimbrites were emplaced by punctuated flare-ups between 25 and 9 Ma during uplift of the Western Cordillera. (2) Numerous smaller ignimbrites were emplaced after 9 Ma as the ignimbrite production rate decreased threefold. This decrease may be due to the declining crustal melting rate, decreasing plate convergence rate after 9 Ma, or more magma stagnation in the shallow crust, which promoted the growth of composite cones. (3) Growth of two volcanic arcs has added twice as much volume ( c. 53 km ³ Ma ⁻¹ ) to the Río Ocoña–Cotahuasi–Marán volcanic field than the ignimbrites after 2.27 Ma. Estimated linear arc magma output has, however, decreased twofold (0.15 – 0.08 km ³ km ⁻¹ Ma ⁻¹ ) from the Early Quaternary to the Pleistocene–Holocene.
Supplementary materials: Supplementary text, tables 1–3 and figures 1–4 are available at http://doi.org/10.6084/m9.figshare.c.3147100 .
Cordilleran-type batholiths are built by prolonged arc activity along active continental margins and provide detailed magmatic records of the subduction system evolution. They complement the stratigraphic record from the associated forearcs and backarcs. We performed in-situ U-Pb geochronology and Hf isotope measurements on zircon grains from a large set of plutonic rocks from the Coastal Batholith in southern Peru. This batholith emplaced into the Precambrian basement and the Mesozoic sedimentary cover. We identify two major periods of voluminous arc activity, during the Jurassic (200-175 Ma) and the Late Cretaceous-Paleocene (90-60 Ma). Jurassic arc magmatism mainly resulted in the emplacement of a dominantly mafic suite with εHf values ranging from -9.5 to +0.1. Published ages south of the Arequipa area suggest that the arc migrated southwestward out of the study area during the Middle Jurassic. After a magmatic gap of 85 Ma, arc activity abruptly resumed 90 Ma ago in Arequipa. Intrusive bodies emplaced into both basement and older Jurassic intrusions and strata. This activity culminated between 70 and 60 Ma with the emplacement of very large volumes of dominantly quartz-dioritic magmas. This last episode may be considered as a flare-up event, characterized by intense magmatic transfers into the crust and rapid relief creation. The Late Cretaceous-Paleocene initial εHf are shifted toward positive values (up to +3.3 and +2.6) compared to the Jurassic ones, indicating either a larger input of juvenile magmas, a lesser interaction with the ancient crust, or an increase of re-melting of young mantle-derived mafic lower crust. These magmatic fluxes with juvenile component are coeval with the onset of the crustal thickening at 90 Ma and represent a significant contribution to the formation of the continental crust in this area.
The Ayabacas Formation, which crops out irregularly in the Andes of southern Peru, is a resedimented unit displaying spectacular deformation. It results from the submarine collapse, near the Turonian-Coniacian boundary (~90 Ma), of the carbonate platform that had developed during two transgressions, in the early to late Albian (~108.5 - ~102 Ma) and from the late Cenomanian to late Turonian (~95 - ~90 Ma). The collapse extended over more than 80 000 km2 and its deposits, which are locally lacking in the head, reached > 500 m in thickness in the toe. More than 10 000 km3 of sedimentary materials were displaced during a single event (at the scale of geological time). Its dimensions are comparable to those of recent giant slides and the Ayabacas collapse appears as the most extensive fossil submarine mass-wasting body currently known.
Deposits are organised from NE (head) to SW (toe) into six zones, on the basis of deformational facies and in relation with two important structural systems that were reactivated at the time of collapse. A seventh zone corresponds to the northeastern ‘stable' platform. In zones 1 to 3, deposits consist in a megabreccia, with elements of 10s to 100s of metres (limestone rafts and sheets, plastically folded, and less commonly rigid blocks deriving from Cretaceous or Paleozoic units) ‘floating' in a calcareous-siliciclastic mix of small clasts and red mudstones to siltstones. These materials were partly liquified and easily deformable and thus prone to ductile deformation: they acted as a sliding sole that greatly facilitated the downslope displacement of the larger elements. These zones are also characterised by deformation and brecciation at whichever scale. Zones 4 to 6, at the toe of the collapse, are exclusively calcareous and display stacked limestone masses that increase in size, due to the westward disappearance of the ductile sliding sole.
When compared with recent or fossil giant slides, the Ayabacas Formation appears as an atypical collapse because it occurred along an apparently stable backarc margin. The collapse occurred just prior to the rapid continentalization of the backarc basin of Peru, which have long been interpreted to mark the beginning of the Andean orogeny, and was one of the consequences of the significant changes that affected the Pacific mantle convection cell between ~91 and 70 Ma. Along the Peru margin, the conditions of subduction were abruptly modified starting ~91-89 Ma: decrease in slab subduction angle increased plate coupling and slab velocity, which dragged down and flexured the backarc lithosphere. This flexuration normal-faulted the backarc substratum, which triggered the giant collapse of its sedimentary cover.
Compositional variations of Central Andean subduction-related igneous rocks reflect the plate-tectonic evolution of this active continental margin through time and space. In order to address the effect on magmatism of changing subduction geometry and crustal evolution of the upper continental plate during the Andean orogeny, we compiled more than 1500 major- and trace-element data points, and 650 Sr-, 610 Nd-, and 570 Pb-isotopic analyses of Mesozoic-Cenozoic (190-0 Ma) magmatic rocks in southern Peru and northern Chile (Central Andean orocline), mostly from new data and the literature. This data set documents compositional variations of magmas since Jurassic time, with a focus on the Neogene period, when major crustal thickening developed and its influence on magma composition was most pronounced. We relate the observed variations in Sr/Y, La/Yb, La/Sm, Sm/Yb, and Dy/Yb ratios, as well as in Sr-, Nd-, and Pb-isotopic ratios, to the crustal structure and evolution of the Central Andean orocline. In particular, the evolution of Dy/Yb and Sm/Yb ratios, which track the presence of the higher-pressure minerals amphibole and garnet, respectively, in the lower crust, documents that crustal thickness has grown through time. Spatial variations in trace elements and isotopic ratios further suggest that crustal domains of distinct composition and age have influenced magma composition through some assimilation. The crustal input in Quaternary magmas is quantified to have been between 7% and 18% by simple two-components mixing. When comparing our geochemical data set to the geological record of uplift and crustal thickening, we observe a correlation between the composition of magmatic rocks and the progression of Andean orogeny. In particular, our results support the interpretation that major crustal thickening and uplift were initiated in the mid-Oligocene (30 Ma) and that crustal thickness has kept increasing until present day. Our data do not support delamination as a general cause for major late Miocene uplift in the Central Andes and instead favor continued crustal thickening.
Detailed Pb isotopic maps of the central Andes, based on 345 (163 previously published, 182 new) analyses of ores, volcanic rocks, and their host rocks, elucidate the gross structure of the basement and reveal that several isotopically distinct basement domains are juxtaposed in this region. The data clearly show that most of the Pb in central Andean igneous and ore samples is derived from the local basement, including Pb in ore deposits of the Bolivian tin belt. Some of the isotopic domain boundaries correspond to geologic structures and the residual gravity pattern, as well as to metallogenic boundaries such as the western edge of the Bolivian tin belt.
Three main tectonic periods are recognized between Kimmeridgian and Paleocene times in the Peruvian-Ecuadorian margin. The "Viru" period comprises a Kimmeridgian event probably equivalent to the Araucan phase of Argentina and Chile, a Tithonian phase related to terrane accretions and collision tectonics along the Ecuadorian margin and to a sudden extension along the north Peruvian margin, and a Berriasian event most probably originated by the incipient South Atlantic rifting. The "Mochica" period begins with tensional and volcanic precursor events (Late Aptian-earliest Albian). It continues with extensional effusions of coastal, back-arc or arc volcanic centres, which alternate with compressive crises (Early and Middle Albian). It ends with the accretion and deformation (thrusting ?) of the Albian volcanic arc or back-arc volcanic system (Late Albian-Early Middle Cenomanian). The "Peruvian" phase starts with a paleogeographic change probably triggered by the incipient coastal uplift (Turonian-Coniacian boundary), and continues in the Late Coniacian-Early Santonian with the initiation of northeastward overthrusts located in the southwestern boundary of the western Trough. It culminates in the latest Campanian, with the creation of intermontane basis which express the onset of the southwestern thrusts, and of foreland basins related to the onset of new overthrusts located in the northeastern boundary of the western Trough. An extensional regime was prbably dominant during the latest Jurassic and Early Cretaceous times, leading to formation of the main sedimentary basins, with an apparent quiescence of the subduction-related volcanic systems... (D'après résumé d'auteur)
Global Positioning System (GPS) measurements suggest the existence of a rigid Panama-Costa Rica microplate that is moving northward relative to the stable Caribbean plate. Northward motion of Central America relative to the Caribbean plate is independently suggested by the April 1991 Costa Rica earthquae, active folding in the North Panama deformed belt, and a south-dipping Wadati-Benioff zone beneath Panama. Panama may also be continuing to collide eastward with the northern Andes. Rapid subduction is occurring at the Middle America (72 mm/yr), Ecuador (70 mm/yr), and Colombia (50 mm/yr) trenches. The northern Andes are moving northeastward relative to stable South America. Preliminary GPS results also suggest Caribbean-North Andean convergence and an independent North Nazca plate.
About 6 Ma the Panama-Choco island ard collided with the northwestern margin of South America, eventually forming a land bridge between the Americas; closed the Pacific-Caribbean seaway, changing ocean circulation patterns and perhaps the world's climate; folded the East Panama deformed belt; and uplifted the Eastern Cordillera of Colombia. An interpretation of the paleo-Romeral suture in southern Colombia as a low-angle fault dipping to the west into the lower crust under the Cordillera Occidental is compatible with seismic velocity and gravity data. During the Late Cretaceous the Western Cordillera oceanic terrane was obducted eastward on the fault system over continental crust.
During the past few years, palaeontologists have agreed first that a land route was present between North and South America during the latest Cretaceous, and that this route still existed during the Palaeocene. Thus far, studies dealing with this problem have been based on tetrapod vertebrates only. In this study, data provided by tetrapods are listed and reevaluated. Moreover, information provided by fresh water fishes are taken into account. The terrestrial bridges which linked North and South America by latest Cretaceous and Palaeocene times probably comprised the Greater Antilles and the Aves Ridge which consisted of a magmatic submitted to uplift and deformation between North and South America at that time. Part of this arc began to collide with both North and South America during the Campanian, an age which corresponds to the beginning of the faunal interchanges. Another volcanic arc was present at that time on the southwestern margin of the Caribbean plate but it was very probably highly discontinuous and therefore could not act as a land bridge. -from English summary
The Neuquén Embayment, which developed along the eastern foothills of the southern Central Andes, has a complex history of intraplate deformation. The Paleozoic basement fabrics exerted a major influence in Mesozoic and Cenozoic deformation. The most important feature is an E-W-striking fault system that is related to a late Paleozoic fabric and is associated with the Huincul basement high, which truncates the basin. This fabric is interpreted as being the result of the accretion of the Patagonia terrane with Gondwana during the Early Permian. Two-dimensional (2D) and three-dimensional (3D) seismic coverage and subsurface information identify different sectors in the Neuquén Embayment that record alternating episodes of contraction and extension during the Jurassic and Cretaceous. The deformation history east of the thrust front of the Agrio fold-and-thrust belt is characterized by periods of (1) transpression and almost orthogonal contraction to the continental margin, (2) extension, and (3) relative quiescence, which alternates in different sectors. The earliest shortening occurred in the Early Jurassic when the main stress was oriented in the N-NW sector. The stress rotated to the northwest up to Valanginian times, when a more orthogonal orientation to the continental margin became dominant and prevailed after the Cenomanian. After a period of quiescence in the Neuquén Embayment associated with very oblique subduction during the Paleogene, the final contractional deformation took place in the late Miocene, with a west-east orientation of the main stress, and was followed by Pliocene extension. The changing stress patterns correlate with differences in convergence vectors between the Aluk, Farallon, and Nazca oceanic plates and the Gondwana or South American continental plates. The Aluk stage from the Jurassic to the Valanginian was characterized by tectonic inversion that is shown by shortening and right-lateral strike-slip structures that are concentrated in the Huincul system and more subtle deformation in the Chihuidos and Entre Lomas systems. The early Farallon stage was distinguished by reduced inversion and displacement in the Huincul system and a general retreat of deformation after the Valanginian. The change to late Farallon stage was characterized by a prominent tectonic inversion of the Entre Lomas system, which resulted from the inception of the formation of the Agrio fold-and-thrust belt in the retroarc area. This belt developed during most of the Late Cretaceous, when the embayment showed a general quiescence. The Nazca stage was characterized by the main episode of uplift, tectonic inversion of the older half-grabens, and important strike-slip faulting that was followed by local collapse of some structures during the Pliocene.
The Agrio fold-and-thrust belt is located between 37°S and 38°S latitude in the eastern part of the Neuquén Andes. The belt can be divided into a western inner sector and an eastern outer sector. The inner sector is characterized by a thick-skinned deformation style. The dominant structures are large anticlines produced by the inversion of half-grabens formed during the Triassic-Jurassic extension that initiated the Neuquén Basin. The outer sector is characterized by thin-skinned structures; recent studies have shown that these structures have been reactivated in a thick-skinned style. A long-standing question has been whether the deformation in this belt occurred in a continuous pulse or in two independent pulses. The analyses of synorogenic deposits, crosscutting relationships between magmatic rocks and sedimentary formations, and new single-crystal 40Ar/39Ar ages from volcanic rocks presented here indicate a minimum age of 102 Ma for the beginning of deformation in this belt and that deformation occurred in at least two pulses, one during the Lower to Middle Cretaceous, and a second one in the middle Miocene, with different degrees of propagation into the foreland.
Three large transcurrent systems, with related magmatism, are identified in southern Peru. A -40-80 km wide, N150-trending, dextral corridor (CECLLA =Cusco-Lagunillas-Laraqueri-Abaroa structural corridor) is characterized by thick Oligocene basic lavas and numerous intrusions. The other two systems are oriented -N12S130 and sinistral, and cartographically related to the CECLLA. Miocene to Quaternary basic magmas are associated to the northeastern system (SFUACC = Urcos-Ayaviri-Copacabana-Coniri faultsystem). In thesouthwestern region, the abundant Late Cretaceous-Paleocene arc magmatism seems related to the Incapuquio-EI Castillo fault system (SFIEC).
Abstract,: The first radiometric age for the Ncuqu6ii Croup, based on fission-track analysis made on ail ashflow
tuff at the base or Lhe IIuincul Fomacion. is reoorted iil this moor. The radiome!ric am offers a more
age of88 i; 3.9 My This value suggost Lhat he i!nincul Forination was deposited botwerii ail ago range ii.orri
84.1 to 91.9 My, chat is froin the biise .if the Turonian to the top of tho Sanlonian.
The American margins of the Caribbean comprise basins and accreted terranes recording
a polyphase tectonic history. Plate kinematic models and reconstructions back to the Jurassic
show that Mesozoic separation of the Americas produced passive margins that were overridden
diachronously from west to east by allochthonous Caribbean plate–related arc and oceanic
complexes. P-T-t and structural data, sedimentary provenance, and basin-subsidence studies
constrain this history. Caribbean lithosphere is Pacifi c-derived and was engulfed between the
Americas during their westward drift as the Atlantic Ocean opened. This began ca. 120 Ma
with development of a west-dipping Benioff zone between Central America and the northern
Andes, now marked by the Guatemalan and Cuban sutures in North America and by
the northern Colombian and Venezuelan “sutures” of South America, persisting today as the
Lesser Antilles subduction zone. Most Caribbean high-pressure metamorphic complexes originated
at this subduction zone, which probably formed by arc-polarity reversal at an earlier
west-facing Inter-American Arc and was probably caused by westward acceleration of the
Americas. The mainly 90 Ma Caribbean basalts were extruded onto preexisting Caribbean
crust ~30 m.y. later and are not causally linked to the reversal. The Great Caribbean Arc originated
at this trench and evolved up to the present, acquiring the shape of the preexisting Proto-
Caribbean Seaway. The uplift and cooling history of arc and forearc terranes, and history of
basin opening and subsidence, can be tied to stages of Caribbean plate motion in a coherent,
internally-consistent regional model that provides the basis for further studies.
We investigated the Neoproterozoic-early Paleozoic evolution of the Gondwanan margin of the north-central Andes by employing U-Pb zircon geochronology in the Eastern Cordilleras of Peru and Ecuador using a combination of laser-ablation-inductively coupled plasma-mass spectrometry detrital zircon analysis and dating of syn- and post-tectonic intrusive rocks by thermal ionization mass spectrometry and ion microprobe. The majority of detrital zircon samples exhibits prominent peaks in the ranges 0.45-0.65 Ga and 0.9-1.3 Ga, with minimal older detritus from the Amazonian craton. These data imply that the Famatinian-Pampean and Grenville (= Sunsas) orogenies were available to supply detritus to the Paleozoic sequences of the north-central Andes, and these orogenic belts are interpreted to be either buried underneath the present-day Andean chain or adjacent foreland sediments. There is evidence of a subduction-related magmatic belt (474-442 Ma) in the Eastern Cordillera of Peru and regional orogenic events that pre- and postdate this phase of magmatism. These are confirmed by ion-microprobe dating of zircon overgrowths from amphibolite-facies schists, which reveals metamorphic events at ca. 478 and ca. 312 Ma and refutes the previously assumed Neoproterozoic age for orogeny in the Peruvian Eastern Cordillera. The presence of an Ordovician magmatic and metamorphic belt in the north-central Andes demonstrates that Famatinian metamorphism and subduction-related magmatism were continuous from Patagonia through northern Argentina to Venezuela. The evolution of this extremely long Ordovician active margin on western Gondwana is very similar to the Taconic orogenic cycle of the eastern margin of Laurentia, and our findings support models that show these two active margins facing each other during the Ordovician.
The spatial distribution of some major and trace element and isotopic characteristics of backarc Plio-Quaternary basaltic to high-Mg andesitic (51% to 58% SiO2) lavas in the southern Puna (24 degrees S to 27 degrees S) of the Central Andean Volcanic Zone (CVZ) reflect varying continental lithospheric thickness and the thermal state of the underlying mantle wedge and subducting plate. These lavas erupted from small cones and fissures associated with faults related to a change in the regional stress system in the southern Puna at approximate to 2 to 3 Ma. Three geochemical groups are recognized: (1) a relatively high volume intraplate group (high K; La/Ta ratio <25) that occurs over a thin continental lithosphere above a gap in the modern seismic zone and represents the highest percentage of mantle partial melt, (2) an intermediate volume, high-K calc-alkaline group (La/Ta ratio > 25) that occurs over intermediate thickness lithosphere on the margins of the seismic gap and behind the main CVZ and represents an intermediate percentage of mantle partial melt, and (3) a small-volume shoshonitic group (very high K) that occurs over relatively thick continental lithosphere in the northeast Puna and Altiplano and represents a very small percentage of mantle partial melt. Mantle-generated characteristics of these lavas are partially overprinted by mixing with melts of the overlying thickened crust as shown by the presence of quartz and feldspar xenocrysts, negative Eu anomalies (Eu/Eu* < 0.90; most < 0.80), and radiogenic Sr (> 0.7055) and Pb and nonradiogenic Nd (epsilon(Nd) < -0.4) isotopic ratios. Mixing calculations show that the lavas generally contain more than 20% to 25% crustal melt. The eruption of the intraplate group mafic lavas, the change in regional stress orientation, and the high elevation of the southern Puna are suggested to be the result of the late Pliocene mechanical delamination of a block (or blocks) of continental lithosphere (mantle and possibly lowermost crust), The loss of this lithosphere resulted in an influx of asthenosphere that caused heating of the subducting slab and yielded intraplate basic magmas that produced extensive melting at the base of the thickened crust. Heating of the subducting slab led to formation of the seismic gap and trenchward depletion of the slab component. Backarc calc-alkaline group lavas erupted on the margins of this delaminated block, whereas shoshonitic group lavas erupted over a zone of relatively thick nondelaminated lithosphere to the north.
A vast region of upper Paleozoic to Middle Jurassic (300-150 Ma) silicic magmatic rocks that erupted inboard of the Gondwana margin is a possible Phanerozoic analogue to the extensive Middle Proterozoic (1500-1350 Ma) silicic magmatic province that underlies much of the southern mid-continent of North America. Like the North American rocks, the Gondwana silicic magmas appear to be melts of crust that formed about 200-300 m.y. earlier. In the North American case, this older crust formed and was accreted to the continent during a major period of crustal formation (1700-1900 Ma), whereas in the Gondwana case, the crust that melted consisted mainly of magmatic are terranes accreted to the continental margin during the Paleozoic. In both cases, basic to intermediate magmatic rocks are extremely rare and magmatism is less abundant in regions that contain older (and previously melted) crust. The similarities between the North American and Gondwana silicic rocks suggest that both suites formed in extensional settings where basaltic magmas, ponded at the base of the preheated crust, caused extensive crustal melting that inhibited upward passage of the basalts. In both cases, silicic volcanism occurred after major assembly of a supercontinent by subduction and accretion processes, and before breakup of the supercontinent. By analogy with the polar wander curves for Gondwana, the granite-rhyolite provinces may have formed during a period of very slow motion of the supercontinents relative to the poles.
The tectonic setting of the Triassic Cuyo basin is analyzed on the basis of the sedimentary record, the regional structural framework, and the geochemistry of the Paramillos de Uspallata basalts. The study region is in the western Precordillera of the province of Mendoza, Argentina (69°13'W and 32°29'S). Overlap of Triassic rift basins in southern South America with the Permo-Triassic Choiyoi granite-rhyolite province suggests a genetic relation in an extensional regime. The Choiyoi rocks are interpreted as crustal melts associated with extensive basaltic
underplating during a period of relatively little motion of the Gondwana supercontinent. The Cuyo rift basin developed during the last stages of this silicic magmatism along suture zones of terranes accreted during the Paleozoic. At this time, faulting could occur and basalts were able to penetrate the cooling, refractory crust. Stratigraphic relationships and geochronologic and paleomagnetic data indicate a Middle Triassic (~235 Ma)
age for basalts emplaced during the synrift phase of active faulting in the Cuyo basin. Basalts are absent in the Late Triassic sag phase, which is dominated by generalized subsidence related to thermal decay and sedimentary loading. Later, the Middle Jurassic Andean arc developed to the west contemporaneous with generalized rifting associated with the early opening of the South Atlantic. Relatively, unaltered sills near Uspallata are composed of tholeiitic to slightly alkaline within-plate olivine basalts with moderate Ti02 (2 to 2.2) and K2 0 contents (1.0 to 1.2), FeO/MgO ratios near 1.35, moderately steep rare earth element (REE) patterns (La/Yb = 8 to 9, La = 15.6 to 17.5 ppm), and mantle-like isotopic ratios (eNd = 1.9, 87Sr/86Sr < 0.7040). Relatively low degrees of melting (4 to 5 percent) in the mantle are suggested, consistent with the comparatively narrow width of the Cuyo basin and eruption in the last stages of Choiyoi magmatism. Highly altered mafic rocks with younger K-Ar ages are interpreted as flows whose ages were reset by Tertiary intrusives.
The distribution and chemistry of late Oligocene to Recent Central Andean magmatic rocks and mineral deposits between 22°S and 33°S latitude reflect changes in the dip of the subducting Nazca plate and the thickness of the overlying lithospheric mantle and crust. Correlations of major magmatic and tectonic events at ca. 18 to 16 Ma, ca. 10 Ma, ca. 7 to 5 Ma, and ca. 2 Ma with previously proposed Andean-wide pulses support external causes for major events and regional geometric control on local style. Evolving magmatic and tectonic patterns indicate that the slab has shallowed beneath the modern Chilean flat-slab region (28° to 33°S), steepened beneath the modern northern Puna plateau (ca. 25° and 22° S), and remained in a transitional state beneath the intervening modern southern Puna. Shallowing in the Chilean flat-slab region is indicated by eastward migration of subduction related magmatism and deformation, termination of main arc andesitic volcanism by ca. 10 Ma, and the virtual cessation of volcanic activity by ca. 5 Ma. Shallowing was accompanied by crustal thickening, lithospheric thinning and hydration, and substantial loss of the asthenospheric wedge. Steepening of the slab below the northern Puna is indicated by widespread deformation and basin formation associated with virtual volcanic quiescence in the late Oligocene to middle Miocene, followed by westward contraction of the middle Miocene to Recent volcanic arc. A westward-shifting focus of giant late Miocene to Pliocene ignimbritic eruptions reflects massive melting caused by introduction of a thickening asthenospheric wedge above a steepening subduction zone, and below a thinned hydrated lithosphere. A contemporaneous eastward shift in the major zone of thrusting to the Subandean Belt can be explained by compressional collapse of the hot, ductile crust beneath the plateau. Lithospheric thickening accompanied deformation above the steepening slab. A persistent intermediate dip of the slab beneath the intervening southern Puna is supported by the lack of a volcanic gap, and by a transitional magmatic and tectonic history compared to that to the north and south. Extreme crustal thickening over the intermediately dipping slab resulted in instabilities in eclogitic lower crust that led to Pliocene continental lithospheric foundering (delamination). Evidence for delamination comes from Pliocene to Recent eruptions of the Cerro Galán ignimbritic center, a concentration of primitive mafic lavas associated with normal and strike-slip faults, high average regional elevation, and seismic evidence for a thin underlying lithosphere and an abnormally hot subducting slab. Temporal variations in mantle-derived mafic magma chemistry indicate Neogene mantle enrichment by introduction of crustal material during the subduction process. Within this framework, major Central Andean Neogene Au and Cu deposits in the greater El Teniente (ca. 32° to 34°S), greater El Indio (ca. 29° to31°S), and Maricunga (26° to 28°) belts formed as crustal thicknesses reached 45 to 50 km over the shallowing and cooling subduction zone. The general southward younging of these deposits reflects a southward pattern of crustal thickening. Emplacement of the deposits took place in the waning stages of arc volcanism as the arc front migrated eastward or extinguished. Mineralization occurred as geochemically-inferred hydrous hornblende-based residual mineral assemblages in equilibrium with erupted magmas dehydrated to yield high pressure garnet-bearing assemblages.
The geological record of the Western Andean Escarpment (WARP) reveals episodes of uplift, erosion, volcanism and sedimentation. The lithological sequence at 18°S comprises a thick pile of Azapa Conglomerates (25–19 Ma), an overlying series of widespread rhyodacitic Oxaya Ignimbrites (up to 900 m thick, ca. 19 Ma), which are in turn covered by a series of mafic andesite shield volcanoes. Between 19 and 12 Ma, the surface of the Oxaya Ignimbrites evolved into a large monocline on the western slope of the Andes. A giant antithetically rotated block (Oxaya Block, 80 km×20 km) formed on this slope at about 10–12 Ma and resulted in an easterly dip and a reversed drainage on the block's surface. Morphology, topography and stratigraphic observations argue for a gravitational cause of this rotation. A “secondary” gravitational collapse (50 km3), extending 25 km to the west occurred on the steep western front of the Oxaya Block. Alluvial and fluvial sediments (11–2.7 Ma) accumulated in a half graben to the east of the tilted block and were later thrust over by the rocks of the escarpment wall, indicating further shortening between 8 and 6 Ma. Flatlying Upper Miocene sediments (<5.5 Ma) and the 2.7 Ma Lauca–Peréz Ignimbrite have not been significantly shortened since 6 Ma, suggesting that recent uplift is at least partly caused by regional tilting of the Western Andean slope.
together with contributions by invited geoscientists The Central Andes, whose orogenic activity is so impressively documented by recent volcanism and and counterparts from other countries, during a workshop held in Berlin, 23-25 May 1990. A great earthquakes, have always attracted the attention of geoscientists. This interest became even more accen number of the papers presented at this workshop are tuated since, a quarter of a century ago, Plate included in this volume. While most of the chapters Tectonics became the basis for the New Global refer regionally to the segment of the southern Andes Tectonics concept, in which this huge mountain range mentioned above, others treat general aspects or deal was the most spectacular example of an active conti with Andean regions farther south, thus showing not only that the structures of this mountain range can be nental margin. Thus, in addition to the continuing research work by South American and foreign geo followed to more distant parts but also that there are scientists dedicated mostly to regional and economic significant structural variations along strike. problems, a great number of special research pro Like other books which originate from workshops grammes were initiated aiming at a better understand and are comprised of contributions from many ing of the processes acting at a convergent plate authors, also this one cannot give a complete and margin. well-balanced view of the scientific subject dealt In 1982, the earth science institutes of the Freie with, in this case the southern Central Andes.
Subduction erosion is commonly associated with strong interplate coupling and a consequent abrasion of the upper plate. Northern Chile is an often cited example of a strongly coupled erosional margin. Its crystalline basement is inferred to form a strong upper plate, the trench axis contains little detectable sediment, and the subducting lower plate has a high-relief horst-graben topography. With little water-rich sediment to reduce interplate friction, the high relief of an igneous ocean crust thrust beneath continental basement should generate high friction interplate abrasion. However, a prestack depth-migrated seismic record images slope debris that collects in a frontal prism. This debris, including ∼30% pore fluid, fills subducting grabens and is subsequently incorporated into an ∼1.5-km-thick interplate reflective layer. The subduction zone thrust passes through the upper part of this layer. Interplate seismicity and taper analyses indicate basal friction at levels that are common in sedimented convergent margins. The continued growth of lower plate grabens after subduction probably accommodates upper plate material, a process that erodes the upper plate. Erosion is aided by weakening of the upper plate rock framework beneath the continental slope. This erosion undermines the upper plate and tips it seaward thereby steepening the continental slope which induces midslope gravity tectonics. Despite sediment starvation, a frontal prism constructed of remolded slope debris elevates pore pressure to reduce interplate friction. Coeval erosion and prism building control the size of the frontal prism. Processes other than high friction abrasion best explain subduction erosion along northern Chile.
Geological framework. Geological setting. The Venezuela Andes or Mérida Andes (fig. 1) extend from the Colombian border in the SW to Barquisimeto in the NE, and constitute a basement uplift exceeding 5,000 m near Mérida (Pico Bolivar). This young chain is bordered to the W by the Maracaibo foredeep basin, and to the E by the Barinas-Apure foreland basin. The Boconó fault divides the Andean Belt in two parts along a NE-SW direction. This shows that the uplift of the Andes is contemporaneous with an oblique translation. In the study area, located on the northwestern flank near Maracaibo basin, three major structures are present : in the E, the N-S senestral strike slip Valera-Rio Momboy fault, in the S the E-W dextral strike slip Piñango fault and, in the center, the SW-NE striking Las Virtudes thrust verging toward NW. Lithologic and stratigraphic formations (fig. 4). The Las Virtudes Fault separates two different structural zones. In the SE, overthrust units are made of crystalline basement, Paleozoic substratum and preorogenic sedimentary formations (Cretaceous-Eocene). The foredeep flexural basin, located NW, is filled by synorogenic molasses (Neogene and Quaternary), largely developed within the Betijoque Fm. (Upper Miocene to Pliocene in age) which reaches a thickness of 5 000 m. Structure of the northwestern Andean flank. Las Virtudes Fault and its thrust slice zone. Near Las Virtudes village (fig. 5, 6-2), this thrust is systematically associated with a narrow overturned foredeep depobelt (Cretaceous to Neogene in age). These slices are unknown elsewhere in the Andean Chain and represent the terminal faulted part of the thrust drag. However, where this slice zone is missing (central and northeastern part of the study area), the Las Virtudes Fault is not clearly documented : its throw decreases rapidly and it is possible that the fault disappears northeastward. Andean unit. Near the main strike slip faults, NE trending SE verging reverse faults develop (fig. 6-5). In central and northeastern parts, the throw of the reverse faults increases toward the Valera Fault. It seems that reverse faults are horsetail of this major strike slip fault (fig. 5). Internal part of the northwestern Andean foredeep basin. The foredeep sedimentary formations generally dip toward the NW. Associated to the lack of some formations, tilted anticlines toward the SE are observed (fig. 6-3 and 6-7), and indicate the vicinity of décollement levels in the foredeep, located in Luna-Colon, Pauji and Betijoque Fm.. Seismic profiles show (fig. 7) that the major décollement level of the foredeep is located in La Luna and Colon Fms. [Audemard, 1991; De Toni and Kellogg, 1993 ; Colletta et al., 1997]. Crustal architecture and timing of the deformation. Several stages can be distinguished in the building of the Andes. Development of an intracutaneous thrust wedge. The first effects of the Andean phase during Miocene are the development of an intracutaneous thrust wedge [Price, 1986]. The lower flat is located in the basement and the upper one in Cretaceous formations. The transport direction is NW. The foredeep develops on the forelimb of this structure and collects detrital products coming from erosion of the first (oldest) reliefs. Décollements in the foredeep basin could be contemporaneous with this major overthrust. Their origin could be due to radius of curvature differences within the thick sedimentary formations (fig. 8). Las Virtudes Fault and backthrusting. Las Virtudes Fault is one of the last events of this part of the Andean Belt. During Plio-Pleistocene, the continental crust breaks with a dip of 35° SE. The Andean unit overthrusts the foredeep basin. Some of the foredeep décollements could still be active and form, together with Andean basement, a triangle zone. Las Virtudes Fault throw reaches 5 km between Las Virtudes and Monte Carmelo villages (fig. 8A). It decreases southwestwards and the back thrusts are probably younger. Northeastwards the throw decreases and eventually disappears (fig. 8B). In the same time the back thrust throws increase. Both seem to be contemporaneous. Conclusions. This structural model explains the basement occurrence in front of the Las Virtudes Fault on seismic profiles and allows to restore correctly the different northwestern flank structures of the Venezuela Andes. These structures can be explained by the conjugate movements of a NW verging intracutaneous thrust wedge and strike slip faults which create a SE verging triangular area (fig. 5). The Andean overthrust is transferred in the Falcon zone along the Valera fault. In the northeastern part of the Maracaibo block, the Valera and Boconó strike slip faults limit the Trujillo block (fig. 10) which moves towards the North during Neogene and Quaternary times.
We are confi dent that ocean drilling at intra-oceanic arcs, such as the IBM, is the way to answer the fundamental question of how continental crust is created; to test the hypothesis of whether continents themselves are fi rst born in the ocean; and to understand the role of subduction-factory processes in solid Earth evolution. The key objective of this drilling effort is to sample rocks that form the deeper part of the arc crust, which is feasible only by riser drilling with Chikyu. To implement these IODP expeditions, pre-expedition, non-drilling efforts will be required, including acquisition of multichannel seismic refl ection and wide-angle seismic data for better imaging of the crust and upper mantle, heat fl ow measurements for confi rming the deep drilling, and petrological/chronological examination of seafl oor and on-land samples.
The topographic data combined with information on structure, magmatism, seismicity, and paleomagnetism support a simple kinematic model for the late Cenozoic evolution of the central Andes. The model does not require collisional effects or enormous volumes of intrusive additions to the crust but instead calls upon plausible amounts of crustal shortening and lithospheric thinning. The model interrelates Andean uplift, a changing geometry of the subducted Nazca plate, and a changing outline (in map view) of the leading edge of the S American plate.-from Author
The Puca Group (Kimmeridgian?-Paleocene) of Bolivia recorded the external (distal) tectonic evolution of the Andean back-arc basin in these latitudes. Bolivia had been part of stable cratonic South America until Late Jurassic time, when it was captured by the Andean system during a large-scale extensional “Araucan”-age tectonic event. This episode seems to be related to the onset of large-scale extensional and transtensional conditions in northern Qüle and coastal Peru. In Bolivia, it led to initiation of the tectonically-controlled, highly fragmented Potosí basin filled with unfossiliferous continental siliciclastic deposits (mostly red beds), with relation to the reactivation of the major transversal Khenayani-Turuchipa paleostructural corridor. Extension was locally accompanied by basic volcanism, and created a tüted-block structure, with half-grabens showing topographic downwarps and uplifts upon which younger fine-grained strata onlapped. The oldest and most important extensional episode took place during deposition of the lowermost part of the Puca Group (Condo conglomerates). A younger minor extensional episode developed locally, possibly in Late Neocomian and/or Aptian time. Albian? time saw a large-scale onlap of brown to violet-red mudstones over the previous deposits and, locally, on the Paleozoic basement, indicating a relative change in the tectonic setting, marked by a slow and gentle widening of the sedimentation area without any small-scale extensional manifestations.
Since the start of the Mesozoic Era about 230 million years ago, large volumes of tholeiite basalt have been erupted through fissures on all of the continents. In several cases, the extrusion of these lavas occurred in conjunction with the break-up of the supercontinents of Gondwana and Pangea. Therefore, these basalt plateaus are now situated on the passive margins of the present continents on opposite sides of ocean basins that have opened between them. Typical examples of this phenomenon are the complementary basalts of Scotland and Greenland, as well as the basalt plateaus in Namibia (southwest Africa) and in the state of Paraná (Brazil). Fissures can also form in back-arc basins behind subduction zones located off the continental coast, causing basalt magma to form by decompression melting of lithospheric mantle. The standard model of flood-basalt petrogenesis developed by White and McKenzie (1989) and by McKenzie and Bickle (1988) is based on the conclusion that flood basalts are erupted thorough continental rifts caused by the interaction of asthenospheric plumes with the lithospheric mantle and with the overlying continental crust (Milanovskiy 1976). The arrival of an asthenospheric plume at the base of the continental lithosphere causes extension and rifting before large-scale decompression melting in the head of the plume can occur (Kent et al. 1992a).
The Cura Mallín basin is part of a chain of sedimentary basins that formed within the Andean volcanic arc between 33° and 43°S during the late Oligocene and early Miocene. Most previous studies of these basins have suggested that they are pull-apart-type basins, produced by strike-slip deformation of the Liquiñe-Ofqui fault zone and other structures, all of which are currently active. However, no direct evidence has been cited for a correlation between formation of the Oligocene-Miocene basins and concurrent strike-slip faulting. The Cura Mallín basin lies more than 100 km north of the modern Liquiñe-Ofqui fault zone and is one of the largest and best exposed of the Southern Andean Oligocene-Miocene basins, making it a promising study area for distinguishing between Oligocene-Miocene tectonic activity that produced the basin and subsequent tectonic activity. Stratigraphic and structural data presented here from the Cura Mallín basin and its surroundings include facies variations, stratal thickness patterns, internal and external structural features, 40Ar/39Ar radiometric ages, and apatite and zircon fission-track ages. Based on the distribution of sedimentary facies and their relation to geologic structures, we conclude that the Cura Mallín basin formed as a result of normal faulting, with little or no significant strike-slip deformation in the area. Due to the lack of supporting evidence for interpretations of the other Oligocene-Miocene basins as pull-apart basins, we suggest that the entire chain of Oligocene-Miocene sedimentary basins formed in response to extensional tectonics on the Southern Andean margin.
The Arequipa-Antofalla Basement, a Proterozoic crustal block exposed along the central Andean margin, provides a key to interpreting the pre-Andean history of South America. New U/Pb geochronology and whole-rock Pb and Nd isotope geochemistry from the Arequipa-Antofalla Basement refine the tectonic history and delineate three distinct crustal domains that decrease in age from north to south. The northern domain of southern Peru and western Bolivia contains juvenile Paleoproterozoic 2.02-1.79 Ga intrusions that were metamorphosed at 1.82-1.79 Ga. The Mesoproterozoic central domain in northernmost Chile contains a significant Mesoproterozoic juvenile component that incorporates Paleoproterozoic crust from the northern domain. Rock units from both the northern and central domains were metamorphosed between 1.20 and 0.94 Ga, with coeval magmatism occurring only in the central domain. The southern domain in northern Chile and northwestern Argentina comprises Ordovician rocks, derived from a mix of juvenile material and older crust. Similar Ordovician magmatism (476-440 Ma) also occurred in the northern and central domains followed by metamorphism at ca. 440 Ma. Based on this refined geologic and tectonic characterization of the Arequipa-Antofalla Basement and comparison with that of Amazonia, we conclude that: (1) the isolated exposures of the Arequipa-Antofalla Basement comprise a single basement block with multiple domains, (2) the Arequipa-Antofalla Basement was not derived from Amazonia, and (3) the Arequipa-Antofalla Basement accreted onto Amazonia during the 1.0 Ga Sunsas Orogeny.
Fission-track (FT) thermochronology has been applied to investigate the low-temperature cooling and denudation history of the Patagonian Andes along the southern part of the intra-arc transpressional Liquiñe-Ofqui fault zone between 42° and 46°S. The Liquiñe-Ofqui fault is shown to have been the focus of enhanced cooling and denudation initiated between ca. 16 and 10 Ma. Several fault blocks with different cooling histories are identified; these are separated by major oblique- or reverse-slip faults proposed to form the eastern part of a major (crustal-scale) dextral transpression zone. Local very fast rates of cooling and denudation between ca. 7 and 2 Ma were coeval with collision of the Chile Rise (an active mid-oceanic ridge) with the Peru-Chile Trench between ∼47° and 48°S. This location is close to the southern termination of the Liquiñe-Ofqui fault, implying that the collision of the ridge was a major force driving late Cenozoic transpression. The lack of significant cooling and denudation before ca. 16 Ma is indicative of pure strike-slip or transtensional movement along the Liquiñe-Ofqui fault before the collision of the ridge. Digital landscape analysis supports glacial and periglacial erosion as the main contributor to denudation since ca. 7 Ma, leading to restriction of topographic development. The combination of transpression-induced rock uplift and glacial erosion is shown to be very effective at causing localized denudation. Anomalously young FT ages along the Liquiñe-Ofqui fault are attributed to the existence of a late Cenozoic localized heat-flow anomaly along the fault.
The Jurassic to middle Eocene porphyry copper deposits and prospects exposed on the Pacific slopes of the central Andean Cordillera Occidental of southern Perú between latitudes 16°30′ and 18° S record a protracted, ca. 30-m.y. history of supergene processes that were fundamentally controlled by the evolving local geomorphologic environment, itself a response to successive regional tectonic events, including the late Eocene Incaic, the late Oligocene to earliest Miocene Aymará, and the middle to late Miocene Quechuan events. Weathering of the porphyry centers also overlapped temporally with the local resumption of arc volcanism in southern Perú at 25.5 Ma following a 27-m.y. amagmatic interval, and supergene processes were variously interrupted or terminated by ignimbrite blanketing, although in several locations supergene profiles were preserved by such cover. The landform chronology for the area surrounding the Cuajone, Quellaveco, and Toquepala deposits (ca. 17° S) is revised and extended northwestward through field mapping to the Cerro Verde-Santa Rosa district (ca. 16° 30′ S). The 40Ar- 39Ar incremental-heating dates of supergene alunite group minerals from the Angostura (38.1 and 38.8 Ma) and Posco (38.8 Ma) prospects and the Cerro Verde deposit (36.1-38.8 Ma) demonstrate that supergene processes were underway in the late Eocene beneath a subplanar topography resulting from uplift and erosion during the Incaic orogeny, now represented by a regional unconformity in the Cenozoic volcanic-sedimentary rock succession. Broadly contemporaneous supergene processes were probably active in the Cuajone-Quellaveco-Toquepala district. Slow erosion and the accumulation of clastic sediments through the tectonically quiescent early to mid-Oligocene are envisaged to have caused a rise in the water table and the widespread preservation of the Incaic supergene profiles. Aymará uplift subsequently led to the incision of the 23.8 to 24 Ma Altos de Camilaca and the 18.8 to 19.1 Ma Pampa Lagunas pediplains and their regional correlatives. The ensuing water-table lowering was associated with intense leaching and sulfide enrichment from the late Oligocene (24.4-28 Ma natroalunite at Cerro Verde, 26-27 Ma natroalunite at Santa Rosa, and 28.6 Ma jarosite at La Llave) to the early Miocene (23 Ma alunite and 21 Ma natroalunite at Cerro Verde, and 19.2 Ma jarosite at La Llave) and was plausibly responsible for much of the upgrading of the Cuajone and Toquepala deposits and thr Quellaveco prospect, which are intersected by both the Altos de Camilaca pediplain and erosional features representing upslope extensions of the Pampa Lagunas pediplain. The younger supergene profiles were widely superimposed on the remnants of those generated during the Incaic orogeny. Middle Miocene (≤14.2 Ma biotite age) Chuntacala Formation flows protected the Cuajone supergene profile from destruction by erosion, but at 13.0 Ma interrupted supergene processes at Quellaveco. Revision of volcanostratigraphic relationships in the latter area reveals that subsequent erosion of the Chuntacala Formation ignimbrites and part of the supergene profile took place prior to the deposition of a 10.1 Ma ash-flow tuff of the Asana Formation. Elsewhere, supergene activity persisted at the Cachuyito prospect through 11.4 Ma, and minor jarosite development occurred at least until 4.9 Ma both there and at Cerro Verde during and following the Multiple Pediment landform stage (ca. 7.9-15.0 Ma). The occurrence of relics of late Eocene alunite group minerals within considerably younger late Oligocene to late Miocene supergene alteration profiles suggests that the overall physiographic configuration of the Pacific piedmont of southern Perú remained remarkably consistent from the late Eocene to the middle Miocene. Moreover, the new age data confirm that, as in northern Chile, semiarid climatic conditions prevailed along much of the plate boundary from the mid-Eocene until the late Miocene or early Pliocene onset of hyperaridity. The local geomorp hologic and volcanic conditions in southern Perú, however, conspired to generate more complex supergene profiles with lower aggregate enrichment factors relative to the strongly enriched profiles in the late Eocene to early Oligocene porphyry copper belt of northern Chile, which underwent supergene upgrading over relatively brief periods.
The descent of the oceanic Nazca plate below the western margin of South America is responsible for the seismicity, tectonism and magmatism characteristic of Andean-type continental margins. Within this geological setting there is much debate concerning the petrogenesis of the calc-alkaline volcanic association. A thermal model for the descent of oceanic below continental lithosphere suggests that melting of subducted oceanic crust, the asthenospheric mantle wedge, and the lower crust might occur1, and hence calc-alkaline volcanic rocks erupted at the surface in such regions might be derived from one or more of these sources. In the Andes, for example, Pichler and Zeil2 have invoked partial melting of lower continental crust, while others3-5 envisage petrogenesis of andesites from sub-crustal mantle sources possibly in conjunction with crustal contamination6-9. We present here new O-isotope data for volcanic rocks from three contrasted volcanic settings in Ecuador, north Chile, and north-west Argentina. We then consider the significance of observed O-Sr isotope relationships, and attempt to evaluate the important factors in the petrogenesis of Andean andesitic lavas.
Scitation is the online home of leading journals and conference proceedings from AIP Publishing and AIP Member Societies
In the Tupiza area of southern Bolivia, Cenozoic strata are outstandingly exposed in five north-trending basins. Because they could yield important information on the local Andean tectonic development, these basins were independently studied by [Herail et al. (1996)][1], by a German group ([
Lithospheric delamination is the foundering of dense lithosphere into less dense asthenosphere. The causes for this density inversion are thermal, compositional, and due to phase changes. For delamination to occur in the specific, and probably common, case where lithospheric mantle is intrinsically less dense than underlying asthenosphere due to composition differences, a critical amount of shortening is required for the densifying effect of cooler temperature to counterbalance the effect of composition. Crustal thickening that results from shortening may result in a crustal root that, due to phase changes, becomes denser than the underlying mantle lithosphere and should delaminate with it: most of the negative buoyancy resides at the top of the mantle and the bottom of the crust. In most cases composition is not known well enough to calculate the driving energy of delamination from densities of equilibrium mineral assemblages in a lithospheric column Poorly known kinetics of phase changes contribute additional uncertainties. In all cases however, the effects of delamination under a region are readily recognizable: rapid uplift and stress change, and profound changes in crustal and mantle-derived magmatism (a reflection of changes in thermal and compositional structure). Characteristics of delamination magmatism are exhibited in the Southern Puna Plateau, central Andes. The consequences of delamination for theories of crustal and mantle evolution remain speculative, but could be important. Recognition of delamination-related magmas in older (including Archean) orogens may be the best way to recognize past delamination events, because the magmas are among the most indelible and least ambiguous of delamination indicators.
Evidence for a Miocene period of transient shallow subduction under the
Neuquén Basin in the Andean backarc, and an intermittent Upper Cretaceous to Holocene
frontal arc with a relatively stable magma source and arc-to-trench geometry
comes from new 40Ar/39Ar, major- and trace-element, and Sr, Pb, and Nd isotopic
data on magmatic rocks from a transect at ~36°–38°S. Older frontal arc magmas
include early Paleogene volcanic rocks erupted after a strong Upper Cretaceous contractional
deformation and mid-Eocene lavas erupted from arc centers displaced
slightly to the east. Following a gap of some 15 m.y., ca. 26–20 Ma mafic to acidic arclike
magmas erupted in the extensional Cura Mallín intra-arc basin, and alkali olivine
basalts with intraplate signatures erupted across the backarc. A major change followed
as ca. 20–15 Ma basaltic andesite–dacitic magmas with weak arc signatures and
11.7 Ma Cerro Negro andesites with stronger arc signatures erupted in the near to middle
backarc. They were followed by ca. 7.2–4.8 Ma high-K basaltic to dacitic hornblendebearing
magmas with arc-like high field strength element depletion that erupted in
the Sierra de Chachahuén, some 500 km east of the trench. The chemistry of these
Miocene rocks along with the regional deformational pattern support a transient
period of shallow subduction that began at ca. 20 Ma and climaxed near 5 Ma. The
subsequent widespread eruption of Pliocene to Pleistocene alkaline magmas with an
intraplate chemistry in the Payenia large igneous province signaled a thickening mantle
wedge above a steepening subduction zone. Apattern of decreasingly arc-like Plioceneto Holocene backarc lavas in the Tromen region culminated with the eruption of a
0.175 ± 0.025 Ma mafic andesite. The northwest-trending Cortaderas lineament,
which generally marks the southern limit of Neogene backarc magmatism, is considered
to mark the southern boundary of the transient shallow subduction zone.
