Pampia Terrane Research Articles

Combined geochronological U-Pb, Hf isotopes and trace element zircon studies from Piedra Alta Terrane, Rio de la Plata Craton (West Uruguay, South America): A geodynamic model

This work delves into the intricate Paleoproterozoic geological setting of Uruguay, focusing on the Rio de la Plata Craton (RPC) and its prominent constituent Piedra Alta Terrane (PAT). The PAT constitutes the core of the RPC, characterized as a complex geological entity that encompasses a granite-migmatite basement, metamorphic belts, granite intrusions, and Proterozoic magmatism. To the west, it interfaces with the Transbrasiliano-Kandi tectonic corridor, bounded by the Late-Neoproterozoic Pampia Terrane. The PAT s southeastern boundary extends to Sierra de Tandil and perhaps to Sierra de La Ventana, demarcating its limits with Phanerozoic rocks of the North Patagonian Massif. The Sarandí del Yí shear zone defines the eastern limit of the PAT. Despite varied interpretations of the Piedra Alta Terrane’s significance, it is important to note that consensus prevails regarding its four major associations: a granite-gneiss-migmatitic basement complex, three volcanosedimentary belts of low to medium metamorphic grade, diverse late igneous intrusions, and extensional Statherian magmatism (1.79 Ga), represented by tholeiitic gabbroic dikes. In this study, a novel tectonic model for the PAT evolution emerges, supported by an extensive zircon dataset comprising new U-Pb ages, Hf isotopes, and trace element geochemistry. The analytical core involves U-Pb analyses on zircon for dating igneous rocks and establishing the provenance and maximum age of deposition of metavolcanosedimentary sequences. Lu-Hf isotopic analyses on the same zircon crystals provide petrogenetic insights, revealing episodes of crustal growth and minor recycling. Trace element geochemistry and zircon εHf(t) data further accentuate these findings, hinting at mantle source enrichment due to subduction within an intraoceanic magmatic-arc arc setting.

The Ordovician of southern South America

Abstract Early Paleozoic rocks are widespread, superbly exposed, and reach several thousand metres thick in southern South America. A largely quadripartite geotectonic subdivision of this huge area encompasses: (1) intracratonic basins forming the sedimentary cover of the Amazonian craton (Brazilian collage); (2) a clastic platform surrounding the Amazonian craton and the Pampia Terrane (Sierras Subandinas and Cordillera Oriental); (3) subduction-related parautochthonous volcanic arcs and associated volcano-sedimentary basins (Puna–Famatina arc); and (4) crustal fragments accreted to the proto-Andean margin of Gondwana (e.g. Cuyania Terrane). In this context, disparity in the geodynamic histories, preserved record and geological knowledge are remarkable. Biostratigraphical frameworks allow the recognition of global chronostratigraphical Ordovician subdivisions with fairly good resolution in the Early and Middle Ordovician of the Precordillera and the Early Ordovician of the Cordillera Oriental of Argentina. In Sierras Subandinas and Cordillera Oriental of Bolivia the Ordovician statigraphy is almost complete, although these extensive regions are still poorly known. In addition, trilobite-rich assemblages from the Cordillera Oriental and brachiopod-rich ones from Precordillera and Famatina offer a remarkable template for dissecting regionally different scenarios underlying Ordovician diversifications. Overall, a more complete knowledge of this key area of Gondwana will certainly enhance our understanding of the global dynamics during the Ordovician.

Sandstone petrofacies, deformational events and the dynamic of the Valle Fértil Lineament during the late Paleozoic (Paganzo Basin, northwestern Argentina)

The behavior during the late Paleozoic of the Valle Fértil Lineament, a first-order structural element placed along the suture between Cuyania and Pampia Terranes in northwest Argentina, is analyzed. The study was undertaken following three different lines of research: 1) the analysis of compositional changes of Carboniferous and Permian sandstones, 2) the interpretation of sizeable soft-sediment deformation, and 3) the analysis of the stratigraphic relationships along and away from the Valle Fértil megashear zone. At the Guandacol Hill, three sandstones petrofacies were recognized in the Carboniferous and Permian Guandacol, Tupe and Patquía formations: quartzolithic (Q-L), quartzofeldspathic (Q-F), and lithic-volcanic (L-V). The vertical distribution of these petrofacies allows distinguishing four stratigraphic intervals. The first, which corresponds to the uppermost part of the Guandacol Formation and the lower Tupe Formation, displays a marked alternation between Q-L and Q-F petrofacies, indicating supply from the metamorphic basement of the local Umango-Maz Block (Q-L) and from granitoids of the Sierras Pampeanas System (Q-F). The second interval, corresponding to the upper Tupe Formation, is widely dominated by Q-F petrofacies owing to the progradation of arkosic wedges from the Sierras Pampeanas System and a limited contribution of the Umango-Maz Block. The third interval, the lower Patquía Formation, shows a sharp change in the composition of the sandstones since they are dominated by acidic and intermediate volcanic fragments (L-V petrofacies), suggesting the establishment of a volcanic field in the area. Finally, the fourth stratigraphic interval, the middle Patquía Formation, is again composed of the Q-F petrofacies indicating a new episode of progradation of arkosic wedges from the Sierras Pampeanas System. Evidence of tectonic activity during the deposition of the Guandacol Formation is also suggested by the presence of different scales of syn-sedimentary deformational structures, from small-scale soft-deformation to large-scale mass transport deposits. This deformational belt only occurs along the Valle Fértil Lineament and is not recorded in other localities of the Paganzo Basin located either to the west or east of the megashear zone. Regional analysis of the late Paleozoic stratigraphy provides additional information about tectonic instability. Firstly, while the contact between the Guandacol and Tupe Formations is concordant throughout the basin, some localities close to the Valle Fértil Lineament show an angular unconformable relationship. Secondly, in the Agua de la Peña creek, located just on the trace of the Valle Fértil Lineament, the Permian Talampaya Formation covers through an angular unconformity the Guandacol Formation, a relation not observed in other areas of the Paganzo Basin. Thirdly, the presence of Carboniferous magmatism, including volcanic rocks in the Guandacol Hill and a granitic intrusion in the Veladero Hill, has been only observed along the Valle Fértil Lineament.

Paleogeographic and tectonic evolution of the Pampia Terrane in the Cambrian: New paleomagnetic constraints

Paleomagnetic, magnetic fabric and rock magnetic studies were carried out in the late Middle to early Late Cambrian Campanario Formation exposed in NW Argentina. The study also presents preliminary results from the Early Cambrian metasediments of the Puncoviscana Formation and from Mojotoro intrusive. A new paleomagnetic pole, C2, was computed for the Campanario Formation (23.6°N, 346.5°E, A95, 7.0°, N: 11 sites) from the localities of Tilcara (T), Terma de Reyes (R) and El Perchel (EP), including previous data from Santa Victoria Oeste (M), in northernmost Argentina. A positive fold test was obtained for Tilcara locality while a positive regional tilt test was obtained for mean site directions for the four localities. This pole can be considered as the representative pole position for the Pampia terrane for the late Middle to Late Cambrian. Previous paleomagnetic data for the same formation from the Iruya-Matancillas locality show an in situ clockwise rotation of about 30° around a vertical axis when compared with the mean Pampia pole. The same is observed when comparing the early Ordovician pole for the Santa Rosita Formation at those localities with the recently obtained coeval pole of the La Pedrera Formation, indicating that local tectonic rotations, affected that area. These results suggest that previous paleomagnetic interpretations for these Late Cambrian and Early Ordovician rocks in the region as recording displacement of the Pampia terrane in Cambrian times are probably incorrect. Additionally, two virtual geomagnetic poles were computed for the Puncoviscana Formation (TP1: 34.5°N, 52.3°E, A95: 5.0°, n: 22 specimens and TP2: 38.3°N, 33.6°E, A95: 5.7°, n: 14 specimens) and one for the Mojotoro Intrusive (MO: 9.1°N, 345.6°E, A95: 16.4°, n: 11 specimens). These and previous Early Cambrian paleomagnetic data from the Pampia terrane differ from Gondwana coeval reference directions in inclination and/or declination, lacking a simple pattern.

Lithospheric structure of the Pampean flat slab region from double-difference tomography

We obtain earthquake locations and a detailed three-dimensional velocity model of the flat slab subduction zone in west-central Argentina (latitudes: 32-30°S and longitudes: 70-66°W) using a regional-scale double-difference tomography algorithm with earthquake data recorded by the SIEMBRA (2007–2009) and ESP (2008–2010) broadband seismic networks. In this region, the flat subduction of the Nazca Plate including the Juan Fernandez Ridge is spatially correlated with a volcanic gap and the basement-cored uplifts of the Sierras Pampeanas in the overriding South American Plate. Our results show the subducting Nazca Plate as a continuous band of mostly increased P-wave velocities coinciding with the Wadati-Benioff Zone. In the overriding South American Plate, the lithospheric mantle appears to be heterogeneous but mostly characterized by a ratio between P- and S-wave velocities (Vp/Vs) of 1.75–1.77, which is consistent with depleted peridotites. Two Vp/Vs anomalies deviate from this mantle with lower (1.70–1.73) and higher (1.78–1.82) Vp/Vs, which are interpreted as localized dry and hydrated regions, respectively. The lower Vp/Vs is consistent with an enrichment of 40–80% of orthopyroxene and the higher Vp/Vs with up to 5% mantle hydration. The size, orientation, and location of these seismic anomalies suggest the progressive eastward dehydration of the subducting slab and the presence of an east-dipping large-scale lithospheric suture, which is interpreted as evidence of an ancient subduction zone and also as a weak zone that facilitates the hydration of the upper plate. Our inversion results suggest a thicker South American crust in the Western Sierras Pampeanas and the partial eclogitization of the lower crust beneath that region where velocities match three types of eclogites at depths of 40–60 km. In the middle-to-upper crust, velocities are reduced in the Precordillera and Vp/Vs is higher in the Cuyania and Chilenia terranes (>1.75) than in the Pampia terrane (1.67–1.75). These observations are consistent with the presence of a thick carbonate sequence in the Precordillera, mafic-ultramafic rocks in Cuyania and Chilenia, and felsic rocks in Pampia. The higher variability in Vp/Vs and strong velocity changes at crustal depths within the Precordillera and the Cuyania Terrane agree with more complexity in crustal structure for these regions and reveal two mid-crustal discontinuities as well as the Chilenia-Precordillera suture zone. Finally, the relocated slab earthquakes refine the slab geometry and suggest that at depths of ~100 km, the flat slab segment is ~240 km wide and has a slight westward dip (~2°) before it resumes its descent into the mantle with a steep angle (~25°). The observation of a wider flat slab segment than the width of the Juan Fernandez Ridge offshore (~100 km) implies that there might be additional contributing factors for the flattening besides the subduction of the overthickened oceanic crust.

New insights on regional tectonics and basement composition beneath the eastern Sierras Pampeanas (Argentine back-arc region) from seismological and gravity data

The eastern Sierras Pampeanas (ESP) are the easternmost expression of a series of foreland uplifts in the Argentine back arc region (~30–34°S) and show spatial and temporal connections with the subduction of the Juan Fernández Ridge (JFR) under the South American plate. In order to get new insights on the mechanisms that control crustal regional tectonics, we computed teleseismic receiver functions (RF) and jointly invert them with Rayleigh-wave phase velocity dispersion curves. RFs allow resolving crustal thickness and intra crustal velocity variations with a good vertical resolution whereas surface wave information helps to constrain absolute seismic wave velocities.Our seismic images have been combined with crustal density modeling in order to further investigate if the shear wave velocity structure obtained from the RF-SW joint inversion could explain the observed gravity variations. Our results show a crustal thickness varying from 35–40 km (east) to 45–50 km (west) with a Moho step at ~66°W. This step regionally presents a NW-SE orientation and is parallel to the trace at the surface of the Valle Fértil–La Huerta (VFLH) lineament (Cuyania-Pampia boundary). Our images also reveal the presence of a high wave velocity (high density) lower crust west of this Moho step, beneath the eastern Sierras Pampeanas (at 66–67°W). This observation suggests the east-dipping extension at depth of the VFLH structure and the underthrusting of the Cuyania lower crust under the Pampia terrane along this structure. Finally, we evidenced localized low velocity zones located at about 10 km beneath late Cenozoic volcanic fields (Pocho, Morro). We believe that these low velocity zones correspond to old magma chambers associated to the recent, slab flattening-related volcanism in the ESP.

Crustal Structure of the Andean Foreland in Northern Argentina: Results From Data‐Integrative Three‐Dimensional Density Modeling

AbstractPrevious thermomechanical modeling studies indicated that variations in the temperature and strength of the crystalline crust might be responsible for the juxtaposition of domains with thin‐skinned and thick‐skinned crustal deformation along strike the foreland of the central Andes. However, there is no evidence supporting this hypothesis from data‐integrative models. We aim to derive the density structure of the lithosphere by means of integrated 3‐D density modeling, in order to provide a new basis for discussions of compositional variations within the crust and for future thermal and rheological modeling studies. Therefore, we utilize available geological and geophysical data to obtain a structural and density model of the uppermost 200 km of the Earth. The derived model is consistent with the observed Bouguer gravity field. Our results indicate that the crystalline crust in northern Argentina can be represented by a lighter upper crust (2,800 kg/m3) and a denser lower crust (3,100 kg/m3). We find new evidence for high bulk crustal densities >3,000 kg/m3 in the northern Pampia terrane. These could originate from subducted Puncoviscana wackes or pelites that ponded to the base of the crystalline crust in the late Proterozoic or indicate increasing bulk content of mafic material. The precise composition of the northern foreland crust, whether mafic or felsic, has significant implications for further thermomechanical models and the rheological behavior of the lithosphere. A detailed sensitivity analysis of the input parameters indicates that the model results are robust with respect to the given uncertainties of the input data.

Meso to Neoproterozoic layered mafic-ultramafic rocks from the Virorco back-arc intrusion, Argentina

The Virorco layered mafic-ultramafic intrusion is part of a belt that extends over 100 km from NE to SW in the Eastern Sierras Pampeanas of San Luis, Argentina. The rocks of this belt carry a Fe-Cu-Ni sulphide mineralization in veins and as disseminated and massive ore. Platinum group minerals are associated with the sulphides.The Virorco intrusion exhibits modal, textural and cryptic layering. New results allow the characterization of six layered units (Modal Layered Unit, Pyroxenitic Macro-Layered Unit, Gabbroic Unit, Banded Unit, Hornblende Norite Unit and Gabbronorite Unit) present in three sectors of the intrusion (Eastern, Central and Western). The units from the Western Sector (Banded Unit, Hornblende Norite Unit and Gabbronorite Unit) and the Modal Layered Unit from the Eastern Sector belong to the Marginal Border Series of the intrusion. Meanwhile, the Central sector units (Pyroxenitic Macro-Layered Unit and Gabbroic Unit) are from the Layered Series. The presence of crescumulate texture (Modal Layered Unit) and colloform banding (Banded Unit) are evidences of “in situ” crystallization due to supercooling of a MgO-rich hydrated mafic magma, where cooling proceeded from the walls towards the interior of the magma chamber.In previous studies the mafic-ultramafic rocks have been considered to be Cambrian to Ordovician. Here we present a Sm-Nd whole rock isochron which shows that the formation age of these intrusions is 1002 ± 150 Ma and that the protolith age of the Pringles Metamorphic Complex metasedimentary rocks is 1289 ± 97 Ma. Our study also indicates that the San Luis mafic-ultramafic layered intrusives most probably formed in a back-arc tectonic setting, from an enriched sub-continental mantle, influenced by a subducting slab and/or crust injection into the Pampia Terrane prior to its collision with the Rio de la Plata Craton.

Modelo petrofísico del borde oriental de las sierras de Valle Fértil- La Huerta, Argentina a partir de datos sísmicos y petrológicos

This paper is a contribution to the knowledge of the crustal structure of the eastern flank of the Valle Fértil - La Huerta ranges (Western Sierras Pampeanas, San Juan, Argentina) at 31°S, in the Andean foreland region, where the Nazca plate is subducting horizontally at about 100 km depth. A 1D velocity model was constrained, combining petrographic and seismological observations from analysis of 19 igneous and metaigneous plutonic rocks belonging to the Famatinian (Ordovician) magmatic arc, which make up most of the crystalline basement of these ranges. Granitoid lithologies predominate in the northern region whereas mafic lithologies are more common to the south. The seismological analysis consisted of modeling teleseismic receiver functions near three seismological stations: LUNA, MAJA and CHUC, in places where those rocks are dominant. Thus, P and S seismic-wave velocities (Vp and Vs) and Poisson´s coefficient (ν), among other elastic parameters, were obtained. The seismic velocity model indicates an overthickened crust with an average thickness between 55 and 60 km, which matches with global average values (~41km); this agrees well with the hypothesis of partial eclogitization in the lower crust. The presence of two seismic velocity discontinuities at mid-crustal levels (12 and 28 km depths), likely associated to décollements, might be related to the accretion of the Cuyania terrane to the Pampia terrane. We obtained low P seismic-wave velocities (Vp ~5.8 km/s), Vp/Vs ratio (~1.70) in upper crust levels consistent with granitoid lithologies, as well as high P seismic-wave velocities (Vp ~6.76 km/s), Vp/Vs ratio (~1.78) in lower crust levels; these figures match with mafic lithologies of a more dense (~3.00 g/cm3), lower crust with respect to other back-arc Andean regions. Also, these values are consistent with the existence of mafic rocks composed of olivine, ortho- and clinopyroxene, which constitute the root of the Famatinian magmatic arc. These results indicate high-grade metamorphic conditions and depths corresponding to geophysical properties of middle to lower crust and correlate with the hypothesis of a dehydrated, cool and magnesium-enriched mantle located in the region between the subducted Nazca slab and the bottom of the Cuyania terrain crust.

Reactivation, inversion and basement faulting and thrusting in the Sierras Pampeanas of Córdoba (Argentina) during Andean flat-slab deformation

AbstractThe Sierras Pampeanas of Córdoba are the easternmost uplifted blocks caused by Andean foreland deformation, over 700 km from the Chile trench. This deformation started atc.340 Ma through basement faults, thrusts and reactivation of normal faults of the Cretaceous rift during the opening of the Atlantic Ocean. Other older faults, major oblique lineaments, were also reactivated. Thermochronological and geothermobarometric data indicate that some topographic relief could have been Palaeozoic–Mesozoic relicts and not only produced by the Andean orogeny. Faults are partially controlled by the early Cambrian S2metamorphic foliation, coincident with the curved fault traces at map scale. During Pliocene time, two deformation phases post-dating Miocene–Pliocene magmatism are recognized. Shallow seismicity data (c.25 km depth) indicate that the Sierras de Córdoba accommodate Quaternary displacement. Magnetotelluric studies detect the interface between the Pampia terrane and the Río de la Plata craton. The role of the oblique lineaments in the nucleation and development of the Tertiary faulting has been little considered; they could be correlated with an old pan-Gondwanan trend. During the Cretaceous period these lineaments worked in a transtensive way, producing the uplift of high-grade rocks and segmentation of the mountain chain favouring the diachronous uplift along the ranges. Recently, both the brittle–ductile transition atc.23 km depth and the crustal thickness have been determined by seismicity analysis. The oblique lineaments displace normally the Mohorovicic discontinuity. Main basement thrusts were probably rooted in the suture between the Pampia terrane and the Río de la Plata craton.

Magnetotelluric characterization through the Ambargasta-Sumampa Range: The connection between the northern and southern trace of the Río de La Plata Craton – Pampean Terrane tectonic boundary

The South American Platform was part of the Western Gondwana, a collage of plates of different ages assembled in late Neoproterozoic to Cambrian times. The Transbrasiliano Lineament, a continental shear belt that transversely intersects this platform from NE to SW, has its southern expression in the tectonic boundary between the Río de La Plata Craton and the Pampean Terrane. Magnetotelluric long-period data in a W–E profile (29°30′ S) that crosses the Ambargasta-Sumampa Range and the Chaco-Pampean Plain were obtained to connect information of this mostly inferred tectonic boundary. A 2-D inversion model shows the Chacoparanense basin, Río Dulce lineament, Ambargasta-Sumampa Range and Salina de Ambargasta in the upper crust. At mid-to-lower crust and 40 km to the east of the Ambargasta-Sumampa Range, a discontinuity (500–2000 Ω m) of 20-km-wide separates two highly resistive blocks, the Río de La Plata Craton (6000–20,000 Ω m) in the east, and the Pampean Terrane (5000–20,000 Ω m) in the west. This discontinuity represents the tectonic boundary between both cratons and could be explained by the presence of graphite. The geometry of the Pampean Terrane suggests an east-dipping paleo-subduction. Our results are consistent with gravimetric and seismicity data of the study area. A more conductive feature beneath the range and the tectonic boundary was associated with the NE–SW dextral transpressive system evidenced by the mylonitic belts exposed in the Eastern Pampean Ranges. This belt represents a conjugate of the mega-shear Transbrasiliano Lineament and could be explained by fluid–rock interaction by shearing during hundreds of years. The eastern border of the Ambargasta-Sumampa Range extends the trace of the Transbrasiliano Lineament. The electrical Moho depth (40 km to the west and 35 km to the east) was identified by a high electrical contrast between the crust and upper mantle. The upper mantle shows a resistive structure beneath the Río de La Plata Craton that could have been originated by stationary delamination by the presence of hydrated lithosphere.

Origin and age of ultramafic rocks and gabbros in the southern Puna of Argentina: an alleged Ordovician suture revisited

Ultramafic rocks and gabbros are exposed in the southern Puna (NW Argentina) in tectonic association with continental arc-related Ordovician (volcano) sedimentary successions and granitoids. The origin of this mafic rock suite has been debated for three decades as either representing an Ordovician terrane suture, primitive Ordovician arc-related rocks or relics of the pre-Ordovician basement in tectonic contact with the Ordovician retro-arc basin successions. We present the first U–Pb ages of primary and inherited zircon from gabbros of this mafic–ultramafic assemblage. LA-ICP-MS analyses on cores and rims of these zircon grains yielded a concordia age of 543.4 ± 7.2 Ma for the gabbroic rocks. Other analysed zircons have Mesoproterozoic, and Early Ediacaran core and rim ages indicating that the magmas also assimilated Meso- and Neoproterozoic crustal material prior to final crystallization. The mafic rocks witnessed higher metamorphic grade than associated Ordovician rocks, which are unmetamorphosed or only affected by anchimetamorphism. The gabbros are mostly tholeiitic and enriched in Zr, Th, as well as other incompatible elements and have eNd t=540Ma ranging from 1.3 to 7.4 with most of the values between 5 and 7. 147Sm/144Nd ratios show evidence of weak crustal contamination. The mafic rocks do not reveal any affinity to mid-ocean ridge basalts in their geochemistry but point instead to an emplacement in an active plate margin arc environment. Chromites from ultramafic rocks show typical Ti, Al, Cr#, Fe3+ abundances found in magmatic arc rocks. The formation of the gabbros and the associated ultramafic rocks in the southern Argentine Puna is related to the evolution of the margin of the Pampia terrane, including the Puncoviscana basin, during the Late Neoproterozoic and earliest Cambrian. In contrast to previous interpretations, the rocks predate the Ordovician evolution of the Central proto-Andean active margin. Consequently, interpretations assuming these rocks to represent an oceanic terrane suture of Ordovician age have to be dismissed as much as all palaeotectonic models that define Ordovician terranes in the Central Andes based on assumption that the ultramafic rocks and gabbros exposed in the southern Puna mark plate boundaries.

Magnetotelluric evidence of the tectonic boundary between the Río de La Plata Craton and the Pampean terrane (Chaco-Pampean Plain, Argentina): The extension of the Transbrasiliano Lineament

Abstract The Transbrasiliano Lineament represents a continental shear belt that transversely intersects the South American Platform from NNE to SSW. Much evidence of this lineament exists to the north, but it remains uncharacterized in the distal Andean foreland to the south, where it is associated with the tectonic boundary between the Rio de la Plata Craton and the Pampean terrane. This tectonic boundary is mostly unexposed in the Chaco-Pampean Plain (Argentina). Debate continues about the existence of an east- or west-dipping subduction zone and a later collisional event between the terranes. Here, we report the results of a magnetotelluric survey along a W–E profile at 27° S between 63° 45′ and 60° 30′ W. To characterize the geoelectric structure, the magnetotelluric data were processed, the dimensionality and distortion of the geoelectrical medium were analyzed, and a 2-D inversion model was developed. The distribution of the resistivities at the lithospheric scale indicates a highly resistive crust (20,000 Ω m) on the east side of the profile that is correlated with the Rio de La Plata Craton. On the west side of the profile, a less resistive but still highly resistive crust (12,000 Ω m) is correlated with the Pampean terrane. The highly resistive blocks are separated by an east-dipping conductive anomaly (150–250 Ω m) that is correlated with the Transbrasiliano Lineament. This conductive feature is consistent with the east-dipping subduction model and can be explained by the presence of graphite in paleosutures in long-stable geological terranes. Oblique convergence between the terranes may have developed the transpressional shear belt. The results provide new geophysical evidence of the tectonic boundary between the Rio de La Plata Craton and the Pampean terrane beneath the Chaco-Pampean Plain that extends southward into the Transbrasiliano Lineament. The study also improves the knowledge of the amalgamation of Western Gondwana.

Crustal structure of the Eastern Sierras Pampeanas of Argentina using high frequency local receiver functions

The Eastern Sierras Pampeanas are basement cored outcrops uplifted in the Andean foreland where the easternmost segment of the Pampean flat slab segment starts dipping more steeply into the deeper mantle. These ranges of central Argentina known as the Sierras de Córdoba have an enriched-quartz composition and are bounded by a series of reverse faults. Different models have been suggested to represent the style of the thick-skinned deformation in this area. However the overall structure linking the exposed faults and terrane boundaries with their probable continuation at depth is unknown. In this paper we present images of the crustal structure beneath the Sierras de Córdoba using the common conversion point stacking method of high frequency local receiver functions recorded by the ESP broadband seismic array. The work consists of two transects located around 31°S and 32°S across the Sierras de Córdoba. The results show a consistent sharp Moho signal associated with a high contrast in seismic velocities in good agreement with the granitic character of the crust lying above the mafic upper mantle. The Moho morphology varies exhibiting thicknesses of 38km in the west to 35km in the east with a vertical shifting under the Sierra Chica. We relate this variable character to the presence of the boundary between the Pampia terrane and the Rio de La Plata craton. Our results for the intra-crustal structure indicate the presence of three discontinuities in the northern transect and at least two discontinuities in the southern transect. These discontinuities appear vertically displaced beneath the surface traces of the major range bounding faults providing evidence for the continuation to mid-crustal depths of the exposed reverse faults. Thus, the reverse faults seem to have displaced several horizontal intra-crustal interfaces. Finally in the lower crust we found a region which seems to be aseismic.

U–Pb (ID-TIMS) baddeleyite ages and paleomagnetism of 1.79 and 1.59 Ga tholeiitic dyke swarms, and position of the Rio de la Plata Craton within the Columbia supercontinent

The Tandilia Terrane (southernmost fringe of the Rio de la Plata Craton) is an igneous and metamorphic complex produced by an accretionary orogeny (2.25–2.02Ga). Calc-alkaline acidic dykes with E–W strike and a major shear zone with similar orientation are related with the late orogeny stage, as supported by field relations. In a previous study the acid dykes gave 40Ar–39Ar ages of 2007±24Ma to 2020±24Ma. A N and NW trending tholeiitic dyke swarm (Tandil swarm) is also present in the Tandilia Terrane. One sample from the NW-trending subset previously gave a U–Pb (ID-TIMS) baddeleyite age of 1588±11Ma. New precise U–Pb (ID-TIMS) baddeleyite dating of both N- and NW-trending Tandil dykes yielded crystallization ages of 1589±3Ma, 1588±3Ma and 1588±3Ma. Significantly older tholeiitic dykes known as the Florida swarm occur in the Northern Rio de la Plata Craton, for which a U–Pb (ID-TIMS) baddeleyite age of 1790±5Ma was previously reported. Consequently intermittent rifting (1.79, 1.59Ga) took place after tectonic stabilization of the late Paleoproterozoic lithosphere (proto-Rio de la Plata Craton). The available geochemical data for the 1.59Ga Tandil dykes define low- and high-TiO2 trends, although, only the low-TiO2 subgroup is firmly dated. Both the Tandil and Florida dykes have geochemical and Nd–Sr characteristics consistent with derivation from heterogeneous mantle sources that underwent metasomatic effects.The Tandil dykes may be linked with the 1.57±0.02Ga Capivarita anorthosite which occurs to the east of the northern part of the craton. Correlatives on other crustal blocks may include those in Baltica such as bimodal rock association (including the Breven-Hällefors and Åland-Åboland diabase dykes) and in the reconstructed Gawler Craton/NW Laurentia dolerites, bimodal magmatism and IOCG deposits. Contemporary within-plate bimodal associations are also present in the SW Amazonian Craton.Paleomagnetic data for the 1790Ma Florida dykes allows three possible reconstruction scenarios for the position of Rio de la Plata Craton: i) in the southern hemisphere linked to the Pampia terrane, and the Amazonia+Rio Apa landmass as close as they are in present day; ii) in the northern hemisphere, as a nearest neighbor to the reconstructed NW Laurentia and Gawler blocks (proto-Australia); or iii) in the northern hemisphere near the boundary of Amazonia (proto-Amazonian Craton) and Baltica. Positions 2 and 3 are consistent with all three criteria: paleomagnetic poles and proximity to matching ca. 1790Ma and 1590Ma mafic magmatism. Our data are consistent with the idea that the Columbia supercontinent started major break up at 1.59Ga.

Siliciclastic Ordovician to Silurian units of the Argentine Precordillera: Constraints on provenance and tectonic setting in the proto-Andean margin of Gondwana

Abstract A combination of petrography, geochemistry, Sm–Nd, Pb–Pb and U–Pb detrital zircon dating provide constraints to the provenance of Middle Ordovician to Early Silurian clastic sequences of the Precordillera s.s. as a part of the Cuyania terrane in western-central Argentina. The uniformity shown by the provenance proxies indicate that there were no important changes in the provenance within these units. A dominant upper crustal component is evident as well as a subordinate mafic input. Nd isotopes indicate e Nd of −5.4, ƒ Sm/Nd −0.34 and T DM 1.57 Ga in average for all the units studied. Pb-isotope ratios display average values for 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 208 Pb/ 204 Pb of 19.37, 15.86 and 39.43 respectively. Detrital zircon dating further constraints the input as being dominantly of Mesoproterozoic age (51–83%), but with contributions from Neoproterozoic–Lower Cambrian (9–31%), Middle Cambrian to Ordovician grains (recorded up to 7%), and Palaeoproterozoic (2–7%) sources. The combination of the different provenance approaches applied indicates that the Mesoproterozoic Cuyania terrane basement, and partially the Pampia terrane were the main sources. The new information provided for the Ordovician to Silurian clastic sequences permitted enhanced the known geochemical and isotopic dataset to constrain the tectonic evolution of the Cuyania terrane. The interpretation of the obtained data supports that the terrane accretion to the proto-Andean margin of Gondwana probably occurred during Middle to early Upper Ordovician.

The Sierra Norte-Ambargasta batholith: Late Ediacaran–Early Cambrian magmatism associated with Pampean transpressional tectonics

The Sierra Norte-Ambargasta batholith is one of the largest plutonic expressions of the Pampean orogeny in western Argentina. A thorough petrographic, geochemical, isotopic (Sr and Nd) and geochronological (UePb SHRIMP) study is reported. The batholith comprises granitoid rocks that may be subdivided into those affected by Pampean D2 dextral shearing and mylonization and those emplaced after deformation had ceased; representative samples gave UePb zircon ages of 537 ±4 Ma and 530 ±4 Ma respectively. The earlier, dominant, groupwere derived largely frommetaluminous calc-alkaline subduction-related magmas, whereas the late granites are peraluminous. However, all have relatively high initial 87Sr/86Sr ratios (0.706 to at least 0.710, strongly negative eNdt values (-1.7 to -5.9) and, in some cases inherited 600 Ma and 970 Ma zircon, similar to the isotopic and zircon provenance seen in the metamorphic host rocks. A high degree of contamination of the magmas, possibly anatexis in the case of the post-mylonite granite, is related to emplacement during the latestage transpressional docking of the Pampean terrane against the Rio de la Plata craton. The absence of detrital zircon derived from the craton in either the Pampean metasedimentary host rocks or the batholith supports this collisional model for the Pampean orogen.

GOCE satellite derived gravity and gravity gradient corrected for topographic effect in the South Central Andes region

SUMMARY Global gravity field models, derived from satellite measurements integrated with terrestrial observations, provide a model of the Earth's gravity field with high spatial resolution and accuracy. The Earth Gravity Model EGM08, a spherical harmonic expansion of the geopotential up to degree and order 2159, has been used to calculate two functionals of the geopotential: the gravity anomaly and the vertical gravity gradient applied to the South Central Andes area. The satellite-only field of the highest resolution has been developed with the observations of satellite GOCE, up to degree and order 250. The topographic effect, a fundamental quantity for the downward continuation and validation of satellite gravity gradiometry data, was calculated from a digital elevation model which was converted into a set of tesseroids. This data is used to calculate the anomalous potential and vertical gravity gradient. In the Southern Central Andes region the geological structures are very complex, but not well resolved. The processing and interpreting of the gravity anomaly and vertical gradients allow the comparison with geological maps and known tectonic structures. Using this as a basis, a few features can be clearly depicted as the contact between Pacific oceanic crust and the Andean fold and thrust belt, the seamount chains over the Oceanic Nazca Plate, and the Famatinian and Pampean Ranges. Moreover the contact between the Rio de la Plata craton and the Pampia Terrain is of great interest, since it represents a boundary that has not been clearly defined until now. Another great lineament, the Valle Fertil-Desaguadero mega-lineament, an expression of the contact between Cuyania and Pampia terranes, can also be clearly depicted. The authors attempt to demonstrate that the new gravity fields can be used for identifying geological features, and therefore serve as useful innovative tools in geophysical exploration.

The age and tectonic setting of the Puncoviscana Formation in northwestern Argentina: An accretionary complex related to Early Cambrian closure of the Puncoviscana Ocean and accretion of the Arequipa-Antofalla block

Abstract TIMS and SHRIMP U–Pb zircon geochronology of selected parts of the Puncoviscana Formation suggest its deposition took place mainly during the Early Cambrian, coeval with 540–535 Ma calc-alkaline Pampean arc volcanism mainly preserved as tuff beds in the oldest identified parts of this unit. Syn- to post-tectonic plutons constrains the Tilcarian–Pampean orogeny to have occurred between ca. 530 Ma and deposition of the unconformably overlying Middle-Upper Cambrian Meson Group. Deposition of the Puncoviscana Formation continued after the onset of the Tilcarian–Pampean orogeny. We propose that the Puncoviscana Formation rocks older than 530 Ma were deposited in the arc-trench gap of the west-facing Pampean arc and/or the associated trench, whereas the rocks younger than 530 Ma were deposited in a syn-collision foreland basin. The Puncoviscana Formation rocks were progressively assembled into a west-younging accretionary complex, consistent with the style of deformation and low-grade metamorphism. The age of the syn-collision plutons (≤530 Ma) suggest the foredeep deposits record the transition from trench to foreland basin, due to arrival of the Arequipa-Antofalla block at the west-facing trench at ca. 530 Ma. Our geochronological and Pb-isotope investigations suggest that the Arequipa-Antofalla terrane was a coherent, ribbon-shaped crustal block that also included the western part of the Pampia terrane. A compilation of existing U–Pb zircon studies suggests that the Pampean arc extended along the length of the proto-Andean margin of West Gondwana, represented by the previously amalgamated Amazonia and Rio de La Plata cratons, and probably was initiated during the late Ediacaran after 600 Ma. Following earlier workers, we reaffirm that the Arequipa-Antofalla block was originally separating Laurentia and Amazonia in Rodinia. It probably rifted from Laurentia during the Ediacaran between 600 and 570 Ma, following an earlier departure of Amazonia (∼650 Ma?). The separation of Arequipa-Antofalla from Amazonia and Laurentia opened the Puncoviscana and Iapetus oceans respectively.

Identification and isotopic studies of early Cambrian magmatism (El Carancho Igneous Complex) at the boundary between Pampia terrane and the Río de la Plata craton, La Pampa province, Argentina

We have identified late Early Cambrian metaigneous rocks very poorly exposed at the Estancia El Carancho, in central La Pampa province, Argentina. They comprise calc–alkaline metadiorite and metagranite, and tholeiitic metapyroxenite and metagabbro. They are jointly referred to as the El Carancho Igneous Complex, and regarded to pertain to the Pampean magmatic arc and backarc, respectively. Titanite U–Pb SHRIMP dating of the metapyroxenite yielded 528±5Ma, and zircon U–Pb SHRIMP dating of the metadiorite yielded 520±1.4Ma. Hafnium isotope determinations on the dated zircons show 176Hf/177Hf ratios corresponding to positive εHf values from +7.18 to +9.37; Hf model ages of the Cambrian zircons yielded 884Ma. It is interpreted that the metadiorites of the Complex crystallized from an Early Neoproterozoic (Tonian) juvenile source. We argue that the inferred occurrence of juvenile Tonian magmatic rocks in the (otherwise, mostly Paleo-Mesoproterozoic) substratum of the southern Pampia terrane could indicate a zone of thinned basement possibly associated with the early stage of Rodinia's breakup. In addition, the studied segment of the Pampean magmatic arc is contaminated by also juvenile, Late Mesoproterozoic crust, as evidenced by the presence of xenocrystic cores of 1140–1194Ma – TDM-Hf 1720Ma and εHf values of +3.24 to +4.85 – in the Cambrian zircons, hence suggesting that the studied segment of the Pampean magmatic arc was intruded into juvenile Late Mesoproterozoic magmatic arc rocks. The El Carancho Igneous Complex would be located at the tectonic boundary between the Pampia terrane and the Río de la Plata craton. This boundary stands out in the aeromagnetic data as a change in the structural orientation about a roughly N-S line located approximately at 65° W and representing the suture zone between the Pampia terrane and the Río de la Plata craton. Our geotectonic model envisages westward dipping subduction of oceanic crust beneath the Pampia terrane; the El Carancho Igneous Complex would, therefore, have been originated on the Pampia side (upper plate) of the suture. Slivers of the arc- and backarc-type rocks would have been tectonically imbricated in the suture zone during the Pampean orogeny.