Federico M. Dávila

Cenozoic provenance history of synorogenic conglomerates in western Argentina (Famatina belt): Implications for Central Andean foreland development

In Famatina, at the modern northern flat-slab segment of the Central Andean belt, synorogenicconglomeratefaciesprovideaunique opportunity to reconstruct paleogeography and tectonic setting. Stratigraphically constrained provenance analyses record a multi-event history of the Andean foreland and a complex pattern of exhumed uplands, which, in turn, controlled the depocenter history. Western and eastern sources can be differentiated. The western domain was composed of equigranular granitoids and lower and upper Paleozoic volcano-sedimentary and sedimentary units, whereas the eastern domain was mainly formed by porphyrytic granitoids and minor slightly metamorphosed lower Paleozoic clastic rocks. The surrounding regions expose medium- to high-grade metamorphic rocks, which are not recorded in any Cenozoic conglomerate of Famatina. Unroofing sequences can be deduced from the compositional trends. Conglomerate composition and paleocurrent data show input from intraba-sinal sources and important participation of basement since the beginning of the Neogene. We infer an early Miocene broken foreland during deposition of the Del Creston Formation.A significant source inversion, with clasts supplied from the eastern basement domain, is recorded ∼2000 m from the base, within the middle Miocene Del Abra Formation; reactivation of western granitic thrust sheets occurred by the late middle Miocene and late Miocene, as indicated by provenance studies of the Santo Domingo and El Durazno Formations. In contrast to simple asymmetric foreland models, provenance analysis documents early participation of crystalline rocks, suggesting early development of intrafore-land basement highs within the southern Central Andes. At a regional scale, basement thrusting and associated intermontane basin development were diachronous. This interpretation is supported by broad propagation of basement thrusting from the northern Sierras Pampeanas (and Famatina) toward the central and eastern Sierras Pampeanas between early-middle Miocene and late Miocene–Pliocene time. Early Miocene reconstructions of the Juan Fernandez Ridge position on the Central Andean margin suggest that during this interval, shallow subduction was likely unrelated to oceanic plateau subduction under the Sierras Pampeanas.

Exhumation history of the Andean broken foreland revisited

The Andean broken foreland in west-central Argentina, located 400–800 km from the Peru-Chile Trench, is associated with flat subduction linked to collision of the oceanic Juan Fernandez Ridge. While the conditions associated with flat subduction would be expected to produce increases in rock uplift and exhumation, where thrusting and dynamic forces work together, a prevalence of pre-Cenozoic apatite fission track (AFT) cooling ages suggests that there is no such link. The lack of Cenozoic cooling ages is at odds with structural reconstructions and basin studies along the foreland that show several kilometers of exhumation. This paradox can be reconciled by taking into account the thermal effects of flat subduction that removed the mantle wedge, and a significant source of heat flow into the crust. Reinterpretation of published AFT exhumation data (>320 ages) using more realistic lower geothermal gradient values allows for substantial exhumation and explains the lack of Cenozoic exhumation ages across the foreland region.

Unraveling 470 m.y. of shortening in the Central Andes and documentation of Type 0 superposed folding

In the Famatina Ranges, Central Andes of western Argentina, 470 m.y. of protracted shortening is recorded through episodic coaxial-coplanar fold amplification. Ordovician anticlines within the east-vergent thrust belt were refolded and tightened during later Paleozoic and Cenozoic shortening. Folding episodes are recorded in four major unconformities that can be measured on both flanks of one of these map-scale anticlines. Superposed folding (Type 0), interpreted from mapping and bedding plots, shows similar axial orientation in older and younger rocks throughout the different stratigraphic layers. Of the shortening, 30% is pre-Cenozoic and allows the recording of episodic deformation during continuous plate convergence at the Gondwana-South American margin.

El Grupo Cerro Morado (Ordovícico Medio) en el Famatina (28-29°S), Andes centrales del oeste argentino

Se propone el nombre de Grupo Cerro Morado en la region central del Famatina, ubicada en el antepais andino de Argentina, para denominar a un espeso conjunto volcanosedimentario (727 m) que suprayace en discordancia angular al Grupo Famatina (Ordovicico Temprano). Desde la base al techo esta formado por las volcanitas de la Formacion El Portillo (580 m, ex Formacion o Volcanitas Cerro Morado) y un registro mixto volcanosedimentario con intervalos marinos denominado Formacion La Escondida (147 m). Este registro extiende el rango de edades en la region al Ordovicico Medio y posiblemente mas joven. La alternancia de ciclos regresivostransgresivos marca la existencia de una actividad volcanica recurrente en la region del Famatina, propia de arcos magmaticos. Estos ciclos se interpretan como la respuesta flexural a episodios de volcanismo activo e intervalos de tranquilidad volcanica, durante los cuales se produjeron incursiones marinas en ambientes someros. Este comportamiento episodico habria influido generando fluctuaciones relativas del nivel del mar responsables del patron ciclico observado en el Famatina durante gran parte del Ordovicico. Este estudio permite establecer una estrecha vinculacion con el proceso de subduccion y acrecion de la Precordillera. En este contexto, la discordancia angular intraordovicica entre los grupos Cerro Morado y Famatina se relaciona con el evento de deformacion ocloyico

Dynamics of deformation and sedimentation in the northern Sierras Pampeanas: An integrated study of the Neogene Fiambalá Basin, NW Argentina: Comment and Discussion

In a recent contribution to the Geological Society of America Bulletin , [Carrapa et al. (2008)][1] provided an interesting geochronological and thermochronological database for the Fiambala Basin, northern end of the Sierras Pampeanas, which, together with stratigraphic, sedimentological and

Depósitos sinorogénicos en el antepaís neopaleozoico del cinturón de Famatina, centro-oeste de Argentina: implicancias paleoambientales, paleogeografías y tectonosedimentarias

RESUMEN:En el cinturon de Famatina, ubicado entre las provincias geologicas argentinas de Sierras Pampeanas y Precordillera, se describe y analiza una sucesion de conglomerados de -400 m, considerada en trabajos previos como la seccion superior del intervalo postglacial neopaleozoico del Grupo Paganzo (Pensilvaniano tardio). En este trabajo estos conglomerados (Formacion Las Pircas nom.nov.), extensamente desarrollados en Famatina, son separados como una unidad litoestratigrafica diferente asociada a un episodio de deformacion, sobre la base de: 1. la presencia de una discordancia angular en su base, 2. un fuerte contraste litofacial con la unidad glacial subyacente y 3. su organizacion estratigrafica interna. El analisis de facies indica un predominio de depositos de abanicos aluviales proximales, que pasan en transicion hacia sistemas fluviales entrelazados y desarrollo local de depositos lacustres. La recurrencia de megacapas con bloques y conglomerados de composicion granitica indica una significativa exhumacion de basamento, sugiriendo actividad tectonica coetanea. Asimismo, los estudios de procedencia indican participacion de clastos de areniscas fluviales pensilvanianas y volcanitas acidas ordovicicas que actualmente se exponen en la sierra de Famatina. Esto es compatible con el analisis de paleocorrientes, que muestra una dispersion de los sedimentos hacia el oeste. La posicion de los altos topograficos de basamento al este del arco principal, junto con evidencias de plegamiento, sugieren una etapa de deformacion de zocalo dentro del antepais no descrita con anterioridad. Esto permite explicar la fuente principal de esta cuna clastica depositada de este a oeste parcialmente equivalente a la Formacion Tupe en el ambito de Precordillera. Este modelo de antepais puede ser comparado con el antepais fragmentado moderno de los Andes Centrales.ABSTRACT:In the Famatina belt, western Argentina, a -400 m thick conglomerate succession is extensively developed and has been previously considered as the uppermost section of a postglacial interval (late Pennsylvanian) of the Paganzo Group. Here we separate it as a different lithostratigraphic unit (Las Pircas Formation, nom. nov.) with a significant tectonic meaning based on: 1. the angular discordance at its base; 2. the strong lito facial contrast with the underlying glacial unit and 3. its internal stratigraphic organization. Facies analyses indicate topographically controlled proximal alluvial fans succeeded by braided fluvial systems, and local development of lacustrine deposits. Granite bearing-megaboulder beds and conglomerates indicate significant basement exhumation and tectonic unroofing. Clast composition also depicts early Pennsylvanian sandstones and Ordovician volcanic rocks mainly exposed in the central part of Famatina (at present). Paleocurrent analyses are compatible with a dispersal pattern mostly to the west. The position of basement topographic high further east from the main contemporaneous arc together with evidences of folding allow us to interpret previously unrecognized basement thrusting within the foreland. This may have been the main source for the westward coarse arkosic clastic wedge known in the Argentine Precordillera as the Tupe Formation and equivalents. This late Pennsylvanian-Early Permian foreland model can be compared with the modern broken foreland of the Central Andes.

Mantle Influence on Andean and Pre-Andean Topography

Mantle convection can drive long-wavelength and low-amplitude topography, which can occur synchronously and superimposed to tectonics. The discrimination between these two topographic components, however, is difficult to assert. This is because there are still several uncertainties and debates in the geodynamic community, for example, the scales and rates of dynamic topography. Geological, geomorphological, geophysical measurements, and/or landscape analyses might assist to validate models. In this contribution, we provide new geological evidences along the Central and Patagonian Andes, which demonstrate that dynamic topography has been an important component on the South American landscape formation as well as in the ancient western Gondwana. Our examples in the Argentine Pampas show that dynamic topography is required to explain not only the basin subsidence but also the whole observed topography. We also suggest that the dynamic components in this region are much lower than numerical models (average dynamic subsidence rates of ~0.04 mm/yr—this work— which contrast with the ~0.1 mm/yr estimated in the US). We also propose two strategies to analyze ancient cases. The first requires of comparing a total elevation proxy, like the equilibrium lines (or ELA) in glaciated areas, with model topography derived from geochemical studies of mantle rocks. A second strategy was the analysis of the Triassic rifting evolution of western Argentina (post-rift sag deposits). Sag deposit thicknesses exceed 2 km, which do not correlate with the 100 m thick thermal calculated by rift subsidence modeling.

Cretaceous Orogeny and Marine Transgression in the Southern Central and Northern Patagonian Andes: Aftermath of a Large-Scale Flat-Subduction Event?

This review synthesizes the tectonomagmatic evolution of the southern Central and Northern Patagonian Andes between 35°30′S and 48° S with the aim to spotlight early contractional phases on Andean orogenic building and to analyze their potential driving processes. We examine early tectonic stages of the different fold and thrust belts that compose this Andean segment. Additionally, we study the magmatic arc behavior from a regional perspective as an indicator of potential past subduction configurations during critical tectonic stages of orogenic construction. This revision proposes the existence of a continuous large-scale flat-subduction with a similar size to the present-largest flat-slab setting on earth. This particular process would have initiated diachronically in late Early Cretaceous times and achieved full development in Late Cretaceous to earliest Paleocene, constructing a series of fold-thrust belts on the retroarc zone from 35°30′S to 48° S. Furthermore, dynamic subsidence focused at the edges of the slab flattening before re-steepening beneath the foreland zone may explain sudden paleogeographic changes in Maastrichtian–Danian times previously linked to continental tilting and orogenic loading during a high sea level global stage.

Basin Thermal Structure in the Chilean-Pampean Flat Subduction Zone

Flat-slab segments are considered refrigerated areas given that the asthenospheric wedge is forced to shift hundreds of kilometres away from the trench, and the flat and coupled subducting plate acts as a thermal insulator. Although lithospheric-scale thermal analysis based on numerical modelling and geophysical observations abound, studies on the thermal history of sedimentary basins are scarce. In this contribution, we present a temperature data compilation from more than 60 oil wells within the Chilean-Pampean flat-slab segment and the transitional zones to normal subduction to the north and south in the south-central Andes. The geothermal gradient data are correlated with basin-basal heat flow estimated from 1D modelling, Curie point depths derived from aeromagnetic surveys, and previous crustal and lithospheric thicknesses estimations. Their distribution evidences a quite good consistency and correlation from region-to-region. Our modelling demonstrates that sedimentation changes are not sufficient to explain the variations illustrated in the geothermal gradient map, and that basal heat flux variations are required to reproduce the reported values. According to our results, the coldest basins develop over the flat slab or cratonward regions, whereas the highest temperatures on areas where the slab plunges. This suggests that the flat-slab geometry as well as the lithospheric structure affects the thermal state within the upper crust and particularly the sedimentary basins. Further studies will allow improving our database as well as the knowledge about the radiogenic contribution of the lithosphere and the asthenospheric heat input to the basins basal heat flow.

The Cerro Morado Group (Middle Ordovician) in Famatina, Andean foreland, Argentina.

The new denomination of Cerro Morado Group is proposed for a thick volcanosedimentary succession (727 m) that unconformably overlies the Early Ordovician Famatina Group in the central region of the Sierra de Famatina, Andean foreland of western Argentina. From bottom to top a mainly volcanic unit, El Portillo Formation (580 m, former Cerro Morado Formation) is composed of acidic volcanic flows and succeeded by the volcanosedimentary La Escondida Formation (147 m). These records allow to expand the Ordovician record in Famatina to the Middle Ordovician and possibly younger. Alternations of regressivetransgressive cycles indicate recurrent volcanic activity, characteristic of volcanic arc settings. These cycles are interpreted as the flexural response to active volcanism followed by intervals of quiescence, where shallow marine environments interact with important volcaniclastic input. This episodic basin dynamics influenced relative sea level fluctuations and caused the cyclical pattern recognized throughout the Ordovician in Famatina. This study establishes a direct link to Ordovician subduction and later accretion of Precordillera. In this context, the intraordovician angular unconformity between the Cerro Morado Group and the Early Ordovician Famatina Group is related to the Ocloyic orogeny.