Frontal Cordillera Research Articles

Petrological, geochemical, isotopic, and geochronological constraints for the Late Devonian–Early Carboniferous magmatism in SW Gondwana (27–32°LS): an example of geodynamic switching

We report a study integrating 13 new U–Pb LA-MC-ICP-MS zircon ages and Hf-isotope data from dated magmatic zircons together with complete petrological and whole-rock geochemistry data for the dated granitic rocks. Sample selection was strongly based on knowledge reported in previous investigations. Latest Devonian–Early Carboniferous granite samples were collected along a transect of ~ 900 km, from the inner continental region (present-day Eastern Sierras Pampeanas) to the magmatic arc (now Western Sierras Pampeanas and Frontal Cordillera). Based on these data together with ca. 100 published whole-rock geochemical analyses we conclude that Late Devonian–Early Carboniferous magmatism at this latitude represents continuous activity (ranging from 322 to 379 Ma) on the pre-Andean margin of SW Gondwana, although important whole-rock and isotopic compositional variations occurred through time and space. Combined whole-rock chemistry and isotope data reveal that peraluminous A-type magmatism started in the intracontinental region during the Late Devonian, with subsequent development of synchronous Carboniferous peraluminous and metaluminous A-type magmatism in the retro-arc region and calc-alkaline magmatism in the western paleomargin. We envisage that magmatic evolution was mainly controlled by episodic fluctuations in the angle of subduction of the oceanic plate (between flat-slab and normal subduction), supporting a geodynamic switching model. Subduction fluctuations were relatively fast (ca. 7 Ma) during the Late Devonian and Early Carboniferous, and the complete magmatic switch-off and switch-on process lasted for ~ 57 Ma. Hf TDM values of zircon (igneous and inherited) from some Carboniferous peraluminous A-type granites in the retro-arc suggest that Gondwana continental lithosphere formed during previous orogenies was partly the source of the Devonian–Carboniferous granitic magmas, thus precluding the generation of the parental magmas from exotic terranes.

Basin evolution during Cretaceous-Oligocene changes in sediment routing in the Eastern Precordillera, Argentina

The response of sedimentary basins to earliest onset of Andean contraction and lithospheric flexure in the southern Central Andes is debated and not well-resolved. The Upper Cretaceous to Oligocene strata of the Cuesta de Huaco anticline in the Argentine Precordillera record sedimentation, regional deformation, and climate patterns prior to the highly studied Oligocene-Miocene foreland basin phase. These deposits have recently been recognized as Cretaceous and Paleogene in age, prompting a re-evaluation of this depocenter as part of the early Andean system, prior to deposition of the aeolian foredeep sediments of the Oligocene Vallecito Formation. This work presents new data from the Argentine Precordillera fold-and-thrust belt at 30°S that sheds light on new reinterpretations of the timing of sedimentation for an important interval in Andean retroarc foreland basin history. We report the first Paleocene detrital radiometric ages from the Cuesta de Huaco ‘red strata’ of the pre-Oligocene Bermejo Basin. Detailed sedimentology and provenance data from the Cenomanian-Turonian Ciénaga del Río Huaco and Danian-Priabonian Puesto La Flecha formations reveal a Cenomanian-Turonian braided stream system that transitioned into a shallow freshwater lacustrine depocenter in Paleocene-Eocene time. During Late Cretaceous time, sediment in the braided river system was derived primarily from northeastern cratonic sources; during the Paleocene-Eocene, uplift and unroofing of the Andean arc and Frontal Cordillera resulted in an influx of western-derived sediment. We therefore suggest a revised timing of sedimentation for the transition to Andean retroarc foreland basin deposition.

Lithological and Hydrothermal Alteration Mapping of Epithermal, Porphyry and Tourmaline Breccia Districts in the Argentine Andes Using ASTER Imagery

The area of interest is located on the eastern flank of the Andean Cordillera, San Juan province, Argentina. The 3600 km2 area is characterized by Siluro-Devonian to Neogene sedimentary and igneous rocks and unconsolidated Quaternary sediments. Epithermal, porphyry-related, and magmatic-hydrothermal breccia-hosted ore deposits, common in this part of the Frontal Cordillera, are associated with various types of hydrothermal alteration assemblages. Kaolinite – alunite-rich argillic, quartz – illite-rich phyllic, epidote – chlorite – calcite-rich propylitic and silicic are the most common hydrothermal alteration assemblages in the study area. VNIR, SWIR and TIR ASTER data were used to characterize geological features on a portion of the Frontal Cordillera. Red-green-blue band combinations, band ratios, logical operations, mineral indices and principal component analysis were applied to successfully identify rock types and hydrothermal alteration zones in the study area. These techniques were used to enhance geological features to contrast different lithologies and zones with high concentrations of argillic, phyllic, propylitic alteration mineral assemblages and silicic altered rocks. Alteration minerals detected with portable short-wave infrared spectrometry in hand specimens confirmed the capability of ASTER to identify hydrothermal alteration assemblages. The results from field control areas confirmed the presence of those minerals in the areas classified by ASTER processing techniques and allowed mapping the same mineralogy where pixels had similar information. The current study proved ASTER processing techniques to be valuable mapping tools for geological reconnaissance of a large area of the Argentinean Frontal Cordillera, providing preliminary lithologic and hydrothermal alteration maps that are accurate as well as cost and time effective.

Geochemistry of heavy minerals and U–Pb detrital zircon geochronology in the Manantiales Basin: Implications for Frontal Cordillera uplift and foreland basin connectivity in the Andes of central Argentina

The Manantiales Foreland Basin, located at ~32°15′S in the Frontal Cordillera, Argentina, contains the sedimentary record of erosion of igneous basement and Miocene volcanic rocks exhumed during the Andean orogeny. U–Pb ages of detrital zircons from the basin fill succession (Chinches Formation) constrain the onset of deposition to ca. 22Ma (early Miocene). We present the first geochronological control (ca. 22Ma) from the lower part of the Manantiales Foreland Basin (Areniscas Chocolate member) on the basis of U–Pb ages of detrital zircons, shedding light about an older initiation of this foreland basin. The main heavy mineral association in the sedimentary basin, clinopyroxene+amphibole, indicates two events of volcanic supply, probably corresponding to the Farellones Formation in the Principal Cordillera, and the La Ramada Volcanic Complex and subvolcanic rocks similar to the La Laguna body in the Argentinean Frontal Cordillera. Detrital zircon geochronology indicates dominant sourcing from upper Paleozoic–lower Triassic Choiyoi Magmatic Province rocks. Initial sediment supply at ca. 22–19Ma was from the Cenozoic volcanic arc of the Principal Cordillera. At ca. 19Ma, exhumation of the Frontal Cordillera was registered by supply from the Choiyoi Magmatic Province, related to tectonic uplift of a western block (La Ramada–El Espinacito ranges) through a thrust of the La Ramada fold-and-thrust belt. At ca. 16Ma, supply from the Cenozoic volcanic arc recommenced, providing evidence for capture and erosion of the Principal Cordillera, which could be related to thrusting in normal sequence of the La Ramada fold-and-thrust belt. At ca. 10Ma, the La Ramada Volcanic Complex and rocks similar to the La Laguna subvolcanic body became a local source of sediment for the basin. The Manantiales Foreland Basin was linked to Neogene sedimentary basins in the Precordillera at its first part in its history, between ca. 22 and 19Ma and probably until ca. 12Ma, when the eastern Frontal Cordillera (Cordillera del Tigre) and Precordillera were uplifted, and the foreland basin was broken. Thus, the Frontal Cordillera was uplifted through two diachronous thick-skinned blocks from west (La Ramada–El Espinacito ranges) to east (Cordillera del Tigre). This result can explain the diverse ages for uplift deduced for the Frontal Cordillera in previous studies, where it has been considered rather as a singular N-S block. The source of late Mesoproterozoic detrital zircons registered in the Manantiales Basin is uncertain, but we infer recycling from Mesozoic sedimentary rocks of the Principal Cordillera.

Within plate seismicity analysis in the segment between the high Cordillera and the Precordillera of northern Mendoza (Southern Central Andes)

Crustal seismicity in northwestern Mendoza Province in Argentina, corresponding to the transition zone between the Chilean-Pampean flat subduction zone (26.5–33.5°S) and the Southern Central Andes normal subduction zone to the south, is studied in order to i) identify its relationship with the mapped structure, ii) determine deformational mechanisms and iii) constrain the geometry of the fold and thrust belt in the lower crust. Through this, we aim to determine which are the structures that contribute to Andean construction, east of the Frontal Cordillera in Argentina and at the western Principal Cordillera in Chile. Data from a temporary local seismic network are reprocessed in order to achieve a precise location of hypocenters and, whenever possible, to build focal mechanisms. Results are interpreted and compared with previous seismic studies and structural models. Analyzed seismicity is grouped around the eastern front of Frontal Cordillera, with hypocenters mainly at depths of 25–40 km. Contrastingly, earthquakes in the Principal Cordillera to the west are located at the axial Andean sector and Chilean slope, with depths shallower than 15 km. Obtained focal mechanisms indicate mainly strike-slip displacements, left lateral at Frontal Cordillera and right lateral at Principal Cordillera. Based on these observations, new possible structural models are proposed, where seismogenic sources could be either associated with inherited basement structures from the Cuyania-Chilenia suture; or correspond to deep-blind thrusts linked with a deeper-than-previously-assumed decollement that could be shared between Frontal Cordillera and western Precordillera. This deeper decollement would coincide in turn with the one determined from receiver function analysis for the eastern Sierras Pampeanas in previous works, potentially implying a common decollement all through the fold and thrust belt configuration. Apart from this, a new interpretation of seismogenic structures in Principal Cordillera near the Argentina–Chile boundary is provided.

High-resolution compositional analysis of a fluvial-fan succession: The Miocene infill of the Cacheuta Basin (central Argentinian foreland)

The Miocene infill of the Cacheuta Basin (central Argentinian foreland) comprises the Mariño and La Pilona formations, which record continental sedimentation related to the major phases of uplift of the Andean chain around 33° S over 1500m of stratigraphy. Sedimentological and stratigraphic evidence suggest the succession to represent progradation of a fluvial-fan sourced from the western, orogenic margin of the foreland basin. The integration of compositional data and sedimentological observations consent to disentangle the relative roles of allogenic factors over long time scales, and to separate them from system-scale autogenic dynamics. The geochemical, mineralogical and radiogenic-isotope composition of sandstones through the succession show a progressive change in the composition of magmatic source rocks from more primitive (basaltic-andesitic) to relatively more evolved (dacitic), testifying the uplift of the western part of the Principal Cordillera, followed by the sequential eastward advance of the thrust and volcanic fronts toward the foreland basin, in agreement with established chronologies of Andean structural development at these latitudes. Sandstones from the La Pilona Formation record compositional signatures suggesting the first stages of uplift of the Frontal Cordillera. The gradual changes in the compositional signal are attributed to changing weathering conditions related to climate and to source-rock changes, related to the Miocene tectonic evolution of the Andean range. However, results suggest that the influence of large-scale allogenic factors is partly blurred by the effects of autogenic fluctuating depositional processes, especially during deposition of the La Pilona Formation. The gradual transitions in provenance and weathering signals suggest that vertical architectural and facies changes through the succession are likely related to the long-term, progradational evolution of the fluvial-system, rather than to allogenic changes in basin accommodation or climate-related sediment supply.

Thermo-mechanical analysis of the Andean lithosphere over the Chilean-Pampean flat-slab region

Flat subduction segments are generally related to cold lithospheric zones, which suggest low Heat flow values and therefore a deeper Curie isotherm. In this study we perform a geophysical analysis by using high - resolution magnetic data, obtaining shallower Curie point depths through the Chilean-Pampean flat-slab zone. Then using temperatures from borehole data and Curie Point Depths, we performed a Heat Flow map which indicates the existence of mean to high thermal anomalies across the Chilean - Pampean flat-slab. These results were contrasted with Elastic Thickness map (Te) obtained from the Eigen-6C4 gravity model. Low Te values acquired indicate zones with weak mechanical behavior in correspondence to thermal anomalous zones. This spatial correlation suggests that the local high thermal anomalies are conditioning the mechanical behavior of the lithosphere in the study area and the locations of geothermal sources. Overall, our results indicate that the thermal structure of the flat-slab segment is more heterogeneous than previously thought. This has implications for evolution, maturity and fluid circulation in foreland basins of the Main and Frontal cordilleras and the Precordillera thrust belts. Finally, further studies will allow improving our database as well as extending our understanding of the thermal structure of the Chilean-Pampean flat-slab.

Late Cretaceous to Cenozoic deformation and exhumation of the Chilean Frontal Cordillera (28°–29°S), Central Andes

The Frontal Cordillera in northern Chile is located over the flat-slab subduction segment of the Central Andes. This tectonic province is characterized by a thick-skinned structural style showing evidence of tectonic inversion and basement-involved compressive structures. Field data, U-Pb geochronological and apatite fission track data were used to unravel partially the tectonic history of the area. Previous U-Pb ages of synorogenic deposits exposed on the flanks of basement-core anticlines indicate that Andean deformation started probably during Late Cretaceous with the tectonic inversion of Triassic and Jurassic half-grabens. New U-Pb ages of the synorogenic Quebrada Seca Formation suggest that this deformation continued during Paleocene (66-60 Ma) with the reverse faulting of pre-rift basement blocks. The analysis of new apatite fission-track data shows that a rapid and coeval cooling related to exhumation of the pre-rift basement blocks occurred during Eocene times. This exhumation event is interpreted for first time in the Chilean Frontal Cordillera and it could have occurred simultaneously with the propagation of basement-involved structures. The age of this exhumation event coincides with the Incaic orogenic phase, which is interpreted as the most important to the Central Andes in terms of shortening, uplift and exhumation.

Geochemistry of Permian to Triassic igneous rocks from northern Chile (28º-30º15’S): Implications on the dynamics of the proto-Andean margin.

Permian to Triassic igneous rocks cropping out in the Coastal and Frontal cordilleras in northern Chile between 28º00’S and 30º15’S have long been interpreted to represent products of magmatism related to an extensional tectonic setting, either as the result of crustal anatexis or asthenospheric mantle decompression melting, in a passive continental margin. Eighty-six samples of plutonic (61) and volcanic (25) rocks from this region are characterized petrographically and geochemically. They are Permian to Early Jurassic in age, but the majority of the studied rocks correspond to the Lower to Middle Triassic Chollay Plutonic Complex, the volumetrically most important unit in the area. The rock samples have features typical of magmas derived from flux-induced melting of a depleted mantle such as: broad range of petrographic composition with predominance of intermediate to acid members, highly porphyrytic volcanic rocks, magnetite as the Fe-Ti oxide mineral phase, enrichment in LILE over HFSE, marked depletion in Nb, Ta, Ti, and P and moderate to no negative Eu anomalies. Few of the studied rock samples (<10%) have alkaline signature and trace element contents representative of anorogenic magmatism. In this work, we propose that subduction of an oceanic plate beneath the South American continent is responsible for the evolution of the margin from the Permian to Early Jurassic, at the studied latitudes. A preliminary interpretation of the margin architecture of the Andean margin from the Permian to the Triassic would be that the Chanchoquin and Chollay plutonic complexes represent the roots of a magmatic arc developed from the Permian to the Middle or early Late Triassic, whereas the Guanaco Sonso and Pastos Blancos formations would be the shallower parts of such arc. The La Totora Formation and some volumetrically minor Upper Triassic intrusive units represent magmatic products with alkaline signatures, which developed immediately before the establishment of the magmatic arc in the present-day Coastal Cordillera (during the Rhaetian to Early Jurassic).

Uplift sequence of the Andes at 30°S: Insights from sedimentology and U/Pb dating of synorogenic deposits

The South Central Andes at 30°S represent a key area to understand the Andes geodynamics as it is in the middle of the flat slab segment and all the morphotectonic units of the Central Andes are well developed. This work is focused in the proximal synorogenic deposits of the Western Precordillera, in the La Tranca valley, in order to unravel the uplift sequence of this belt. Nine facies associations were recognized; most of them represent piedmont facies with local provenance from Precordillera and were deposited in the wedge-top depozone, as is expected for proximal sinorogenic deposits. However there are intercalations of transference fluvial systems, which show mixed provenance indicating that Permo-Triassic igneous rocks were already exposed to the west (Frontal Cordillera). There are also lacustrine deposits which are interpreted as the result of damming by fault activity at east of the studied basin.Finally, two maximum depositional ages at ca. 11 Ma and 8 Ma of these deposits indicate that the onset of uplift of the Precordillera at 30°S is little older than 11 Ma. These data change two previous ideas about the evolution of the Precordillera: its uplift at 30° S is younger than proposed by previous works and it is nearly synchronous along strike.

Roberto Caminos (1931-1997) un geólogo regional con base petrológica: desde macro- a microescala

Roberto Caminos (1931-1997), recibido en la Universidad de Buenos Aires, fue un notable geólogo regional. Su sólida base petrológica era apropiada para mapear terrenos ígneos y metamórficos. Relevó varias hojas geológicas, cuyo desarrollo comenzaban en el campo con lupa y finalizaban en el laboratorio con microscopio. Así combinaba escalas megascópicas con escalas meso- a microscópicas, método que le dio versatilidad en el mapeo de unidades ígneas y metamórficas. Los trabajos de campo duraban varios meses y se recogía mucha información. Sus contribuciones más importantes fueron en Sierras Pampeanas, Precordillera de La Rioja, Cordillera Frontal, Bloque de San Rafael, Macizo Norpatagónico y Cordillera Fueguina. Subdividió las Sierras Pampeanas Noroccidentales en dos fajas, occidental y oriental. La occidental la distinguió por la abundancia de metamorfitas de origen calcáreo y rocas máficas y ultramáficas. La oriental, en cambio, la identificó por la presencia de batolitos graníticos y metamorfitas con protolitos areno-arcillosos. Estas dos fajas servirían posteriormente para explicar la colisión de Cuyania con Gondwana. En Cordillera Frontal reasignó las unidades carboníferas tempranas al Carbonífero Tardío-Pérmico y reconoció la fase orogénica San Rafael. Además, describió en detalle la fase Huárpica, y al hacerlo caracterizó la finalización de la estructuración de la Cordillera Frontal. En el Macizo Norpatagónico subdividió las rocas magmáticas pérmicas en un complejo plutónico y en otro plutono-volcánico, separados por una discordancia. En isla de los Estados describió el metamorfismo de la Formación Lemaire, mencionando por primera vez estilpnomelano, indicador de metamorfismo dinámico y equivalente a la facies de prehnita-pumpellita.

Chronology of Chilean Frontal Cordillera building from geochronological, stratigraphic and geomorphological data insights from Miocene intramontane‐basin deposits

AbstractThe Chilean Frontal Cordillera, near 28°45′S, provides a remarkable example to explore the evolution of the Central Andes; this area provides conspicuous pediment surfaces and continental deposits, which allowed us to analyse the timing and propagation of deformation which controlled the Andes building during the Cenozoic using structural, geomorphological, sedimentological, stratigraphic and geochronological data. The study area is characterized by outcrops of the Cerro del Burro Gravels, a continental deposit which is surrounded by four morphostructural mountain systems. Based on a 46 Ma tuff affected by a syncline, which is sealed by a 44 Ma tuff, we recognized an Eocene fault activity that contributed to the uplift of the western and northern systems, which have remained inactive during the last 44 Ma. The deformed lithologies during the last pulse of activity of the western fault and the youngest lithology carved by pediment processes (21 Ma) indicate a pediment surface developed during the Late Eocene and Oligocene. This pediment extended below the Cerro del Burro Gravels associated to a base level which drained to the east. We also recognized Miocene fault activity that played a main role in the uplift of the eastern and southern systems. Geochronological, stratigraphic and geomorphological data suggest a first pulse of fault activity between 19 and 13 Ma, which interrupted the pedimentation processes, developed an intramontane depocenter, and forced the accumulation of the Laguna Grande Succession in an alluvial‐braided fluvial environment. After 13 Ma, an erosive event evidenced by the incision of valleys, resulted after the change in the extension and configuration of the hydric network.

Provenance of the Miocene Alto Tunuyán Basin (33°40′S, Argentina) and its implications for the evolution of the Andean Range: Insights from petrography and U–Pb LA–ICPMS zircon ages

The Alto Tunuyán Foreland Basin in western Argentina is located immediately south of the flat-slab segment of the Central Andes and its evolution is directly related to the propagation of structures to the east. Petrographic and geochronologic studies have been performed to determine the provenance of syntectonic sediments in the basin in order to establish their relationship to the Andean orogenic activity. The analysed detrital and igneous zircons in contrast with previous data, allow us to restrict the basin age between ca. 15 and 6Ma. Sandstones record two main contributions, one from andesitic volcanic rocks and the other from an acidic igneous source, the first probably corresponding to Miocene volcanic rocks from the Principal Cordillera (Farellones Formation) and the second to Permo–Triassic, acidic, igneous rocks from the Frontal Cordillera (Choiyoi Magmatic Province, CMP). Two secondary sources have been recorded, sedimentary and metamorphic; the first one is represented by Mesozoic rocks in the Principal Cordillera and the second by the Proterozoic/early Carboniferous Guarguaráz Complex (GC) in the Frontal Cordillera, respectively. Sandstones from the lower basin deposits (15–11Ma) register supply pulses from the Farellones Formation reflecting the unroofing of the Principal Cordillera by uplift pulses during the middle Miocene. Sandstones from the upper basin deposits (ca. 11–9Ma) record an increase in material derived from the CMP, reflecting important uplift of the Frontal Cordillera. A thick, ca. 9Ma old ignimbrite within the basin indicates an eruption in the Frontal Cordillera. Detrital zircons from the CMP have been detected also in the lower basin deposits, suggesting either recycling of Mesozoic deposits containing CMP zircons or an early paleorelief of the Frontal Cordillera. The good correlation between the age of the detrital zircons of the CMP and the GC in the lower basin deposits supports recycling of Mesozoic sedimentary deposits.

High-pressure/low-temperature metamorphism in the collision zone between the Chilenia and Cuyania microcontinents (western Precordillera, Argentina)

In central-western Argentina, an Early Paleozoic belt including mafic-ultramafic bodies, marine metasedimentary rocks and high-pressure rocks occur along the western margin of the Precordillera and in the Frontal Cordillera. First pressure-temperature estimates are presented here for low-grade rocks of the southern sector of this belt based on two metasedimentary and one metabasaltic sample from the Peñasco Formation. Peak metamorphic conditions resulted within the range of 345–395 °C and 7.0–9.3 kbar within the high-pressure greenschist facies. The corresponding low metamorphic gradient of 13 °C/km is comparable with subduction related geothermal gradients. Comparison between these results and data from other localities of the same collision zone (Guarguaraz and Colohuincul complexes) confirms a collision between Chilenia and the composite margin of western Gondwana and suggests a stronger crustal thickening in the south of the belt, causing exhumation of more deeply buried sequences. During the Early Paleozoic a long-lived marine sedimentation coupled with the intrusion of MORB-like basalts occurred along a stable margin before the collision event. This contrasts with the almost contemporaneous sedimentation registered during accretion in accretionary prism settings and additionally proves the development of a collision zone along western Precordillera and the eastern Frontal Cordillera as well as the existence of Chilenia as a separate microcontinent.

The role of changing geodynamics in the progressive contamination of Late Cretaceous to Late Miocene arc magmas in the southern Central Andes

The tectonic and geodynamic setting of the southern Central Andean convergent margin changed significantly between the Late Cretaceous and the Late Miocene, influencing magmatic activity and its geochemical composition. Here we investigate how these changes, which include changing slab-dip angle and convergence angles and rates, have influenced the contamination of the arc magmas with crustal material. Whole rock geochemical data for a suite of Late Cretaceous to Late Miocene arc rocks from the Pampean flat-slab segment (29–31 °S) of the southern Central Andes is presented alongside petrographic observations and high resolution age dating. In-situ U–Pb dating of magmatic zircon, combined with Ar–Ar dating of plagioclase, has led to an improved regional stratigraphy and provides an accurate temporal constraint for the geochemical data.A generally higher content of incompatible trace elements (e.g. Nb/Zr ratios from 0.019 to 0.083 and Nb/Yb from 1.5 to 16.4) is observed between the Late Cretaceous (~72Ma), when the southern Central Andean margin is suggested to have been in extension, and the Miocene when the thickness of the continental crust increased and the angle of the subducting Nazca plate shallowed. Trace and rare earth element compositions obtained for the Late Cretaceous to Late Eocene arc magmatic rocks from the Principal Cordillera of Chile, combined with a lack of zircon inheritance, suggest limited assimilation of the overlying continental crust by arc magmas derived from the mantle wedge. A general increase in incompatible, fluid-mobile/immobile (e.g., Ba/Nb) and fluid-immobile/immobile (e.g., Nb/Zr) trace element ratios is attributed to the influence of the subducting slab on the melt source region and/or the influx of asthenospheric mantle.The Late Oligocene (~26Ma) to Early Miocene (~17Ma), and Late Miocene (~6Ma) arc magmatic rocks present in the Frontal Cordillera show evidence for the bulk assimilation of the Permian–Triassic (P–T) basement, both on the basis of their trace and rare earth element compositions and the presence of P–T inherited zircon cores. Crustal reworking is also identified in the Argentinean Precordillera; Late Miocene (12–9Ma) arc magmatic rocks display distinct trace element signatures (specifically low Th, U and REE concentrations) and contain inherited zircon cores with Proterozoic and P–T ages, suggesting the assimilation of both the P–T basement and a Grenville-aged basement. We conclude that changing geodynamics play an important role in determining the geochemical evolution of magmatic rocks at convergent margins and should be given due consideration when evaluating the petrogenesis of arc magmas.

High-resolution images above the Pampean flat slab of Argentina (31–32°S) from local receiver functions: Implications on regional tectonics

In the flat slab region of the South Central Andes (∼31–32°S), geological observations suggest that the regional crustal structure is inherited from the accretion of different terranes during the Ordovician. These structures were later reactivated, first in extension during the Triassic and later in compression during the Andean uplift since the Miocene. Seismological observations confirmed that those fault structures extend to depth with décollement levels that accommodate current crustal shortening in the region. In order to get better insight on the regional tectonics we computed higher frequency receiver functions (RF) from local slab seismicity of intermediate ∼100 km depth. Using a common conversion point (CCP) stacking method we obtained cross sections showing high vertical resolution crustal structure at the transition between the Precordillera and the Frontal Cordillera. In addition we performed a joint inversion of our high frequency RFs with surface wave data from ambient noise tomography allowing us to constrain absolute seismic wave velocities. Our higher resolution images reveal more structural details down to a depth of 80 km and laterally over the flat slab in good agreement with previous studies. Our results help to better identify very shallow discontinuities in seismic velocities. Recent petrological analyses combined with our high-resolution RF structure correlates with a crustal mafic composition and partial eclogitization in the lower crust. We observe a shift in the crustal structure between the Precordillera (east) and the Frontal Cordillera (west). Regional seismicity and previously determined focal mechanisms superimposed over our images indicate this shifting is a thrust structure extending down to a depth of 40 km. Our results suggest the presence of a master fault between the Cuyania (Western Precordillera) and Chilenia (Frontal Cordillera) terranes that probably accommodates the crustal deformation in the Pampean flat slab region since the Late Ordovician.

Tectonic styles and crustal shortening of the Central Andes “Pampean” flat-slab segment in northern Chile (27–29°S)

The Andean orogenic belt, located in the Central Andes “Pampean flat-slab” segment in northern Chile (27–29°S), is composed of two major tectonic regions: the Coastal Cordillera and the Frontal Cordillera. To understand their internal tectonic styles, history of growth and the shortening absorbed by the upper crustal structure of this segment, we combined regional geological mapping data, new ages obtained from radiometric U–Pb dating, and a semibalanced and restored cross-section 225.18km in length. The results as shown in the previous Mesozoic extensional fault systems, established in northern Chile by the Gondwana breakup, have played a fundamental role in the orogenic buildup. The central structure is characterized by an asymmetric basin (Upper Cretaceous–Paleocene) confined by a doubly vergent fault system composed of inverted faults related to the edges of the Mesozoic Chañarcillo and Lautaro Basins. The U–Pb geochronological data obtained from synorogenic volcano-sedimentary deposits and the angular unconformities recorded between the Cenozoic geological units have revealed that the compressive deformation in this segment started at around ~80Ma by tectonic inversion in the eastern Coastal Cordillera and western Frontal Cordillera, however, the presence of Paleocene and Miocene synorogenic successions at the footwall of the basement reverse faults of the Frontal Cordillera suggests a migration of Andean deformation from the west to the east during the Paleocene–Miocene by propagation of ramps involving inherited basement highs. The pre-compression restoration makes it possible to estimate 40.94km of minimum shortening, concentrated by inversion anticlines and fault-controlled basement highs across the Frontal Cordillera.

Faulting and erosion in the Argentine Precordillera during changes in subduction regime: Reconciling bedrock cooling and detrital records

The Argentine Precordillera is an archetypal retroarc fold-and-thrust belt that records tectonics associated with changing subduction regimes. The interactions between exhumation and faulting in the Precordillera were investigated using apatite and zircon (U–Th–Sm)/He and apatite fission track thermochronometry from the Precordillera and adjacent geologic domains. Inverse modeling of thermal histories constrains eastward in-sequence rock cooling associated with deformation and erosion from 18 to 2 Ma across the Central Precordillera tracking thrusting during this time. The youngest AHe ages (5–2 Ma) and highest erosion rates are located in the eastern and western extremities of the Precordillera and indicate that recent denudation is concentrated at its structural boundaries. Moreover, synchronous rapid Pliocene cooling of the Frontal Cordillera, Eastern Precordillera, and Sierra del Valle Fértil was coeval with initiation of basement-involved faulting in the foreland. Detrital zircon U–Pb geochronology from the ca. 16–8.1 Ma Bermejo foreland basin strata suggests fluvial connectivity westward beyond the Frontal Cordillera to the Main Cordillera and Coast Range followed by an important shift in sediment provenance at ca. 10 Ma. At this time, we suggest that a substantial decrease in Permo-Triassic igneous sources in the Frontal Cordillera and concurrent increase in recycled zircons signatures of Paleozoic strata are best explained by uplift and erosion of the Precordillera during widening of the thrust-belt. Bedrock thermochronology and modeling indicate a 2–6 Myr lag time between faulting-related cooling in the hinterland and the detrital record of deformation in the foreland basin, suggesting that for tectonically active semi-arid settings, bedrock cooling may be more sensitive to onset of faulting. We suggest that high erosion rates in the Frontal Cordillera and Eastern Precordillera are associated with increased interplate coupling during shallowing of the subducting Nazca plate that may concentrate stress along weak structural boundaries of the Precordillera.

Geochemistry of Precordillera serpentinites, western Argentina: evidence for multistage hydrothermal alteration and tectonic implications for the Neoproterozoic–early Paleozoic.

Serpentinites are a powerful tool to evaluate mantle composition and subsequent alteration processes during their tectonic emplacement. Exposures of this type of rocks can be found in the Argentine Precordillera (Cuyania terrane) and Frontal Cordillera, both located in central-western Argentina, within the Central Andes. In these regions are outcrops of a Neoproterozoic to Devonian mafic-ultramafic belt composed of serpentinites, metabasaltic dikes/sills and pillow lavas (with an E- to N-MORB geochemical signature) and mafic granulites, spatially associated with marine metasedimentary rocks. The serpentinite bodies consist of lizardite/chrysotile + brucite + magnetite, with scarce pentlandite and anhedral reddish-brown Cr-spinel (picotite, pleonaste and spinel sensu stricto ) as relict magmatic phases. The original peridotites were moderately-depleted harzburgites (ultramafic cumulates) with an intermediate chemical signature between a mid-ocean ridge and an arc-related ophiolite. Whole-rock REE patterns of serpentinites exhibit enriched REE patterns ((La/Yb) CN =13-59) regarding CI chondrite with positive Eu anomalies. These features are the result of an interaction between hydrothermal fluid and serpentinites, in which moderate temperature (350o-400oC), CO 2 -rich, mildly basic hydrothermal fluid was involved and was responsible for the addition of Ca, Sr and REE to serpentinites. The presence of listvenites (silica-carbonate rocks) in the serpentinite margins allow us to infer another fluid metasomatism, where low-temperatures (<250oC), highly-oxidized, highly-acid fluid lead to the precipitation of silica. The association of these metasomatized serpentinite bodies with neoproterozoic continental margin sucessions and MORB magmatism at the suture zone of the Cuyania and Chilenia terranes suggests the development of an oceanic basin between them during the Neoproterozoic-early Paleozoic.

Geodynamic controls on the contamination of Cenozoic arc magmas in the southern Central Andes: Insights from the O and Hf isotopic composition of zircon

Subduction zones, such as the Andean convergent margin of South America, are sites of active continental growth and crustal recycling. The composition of arc magmas, and therefore new continental crust, reflects variable contributions from mantle, crustal and subducted reservoirs. Temporal (Ma) and spatial (km) variations in these contributions to southern Central Andean arc magmas are investigated in relation to the changing plate geometry and geodynamic setting of the southern Central Andes (28–32°S) during the Cenozoic. The in-situ analysis of O and Hf isotopes in zircon, from both intrusive (granitoids) and extrusive (basaltic andesites to rhyolites) Late Cretaceous – Late Miocene arc magmatic rocks, combined with high resolution U–Pb dating, demonstrates distinct across-arc variations. Mantle-like δ18O(zircon) values (+5.4‰ to +5.7‰ (±0.4 (2σ))) and juvenile initial εHf(zircon) values (+8.3 (±0.8 (2σ)) to +10.0 (±0.9 (2σ))), combined with a lack of zircon inheritance suggests that the Late Cretaceous (∼73Ma) to Eocene (∼39Ma) granitoids emplaced in the Principal Cordillera of Chile formed from mantle-derived melts with very limited interaction with continental crustal material, therefore representing a sustained period of upper crustal growth. Late Eocene (∼36Ma) to Early Miocene (∼17Ma) volcanic arc rocks present in the Frontal Cordillera have ‘mantle-like’ δ18O(zircon) values (+4.8‰ (±0.2 (2σ) to +5.8‰ (±0.5 (2σ))), but less radiogenic initial εHf(zircon) values (+1.0 (±1.1 (2σ)) to +4.0 (±0.6 (2σ))) providing evidence for mixing of mantle-derived melts with the Late Paleozoic – Early Mesozoic basement (up to ∼20%). The assimilation of both Late Paleozoic – Early Mesozoic Andean crust and a Grenville-aged basement is required to produce the higher than ‘mantle-like’ δ18O(zircon) values (+5.5‰ (±0.6 (2σ) to +7.2‰ (±0.4 (2σ))) and unradiogenic, initial εHf(zircon) values (−3.9 (±1.0 (2σ)) to +1.6 (±4.4 (2σ))), obtained for the Late Oligocene (∼23Ma) to Late Miocene (∼9Ma) magmatic rocks located in the Argentinean Precordillera, and the Late Miocene (∼6Ma) volcanic rocks present in the Frontal Cordillera. The observed isotopic variability demonstrates that the assimilation of pre-existing continental crust, which varies in both age and composition over the Andean Cordillera, plays a dominant role in modifying the isotopic composition of Late Eocene to Late Miocene mantle-derived magmas, implying significant crustal recycling. The interaction of arc magmas with distinct basement terranes is controlled by the migration of the magmatic arc due to the changing geodynamic setting, as well as by the tectonic shortening and thickening of the Central Andean crust over the latter part of the Cenozoic.