Mario Giménez

A crustal model based mainly on gravity data in the area between the Bermejo Basin and the Sierras de Valle Fértil, Argentina

Abstract The Sierras Pampeanas ranges of central-western Argentina (26°00′S to 33°15′S, 63°30′W to 68°30′W; an area about 450xa0km wide and 800xa0km long) consists of a series of uplifted basement blocks bounded by deep sedimentary basins of relatively low relief and covered by modern sediments. The basement blocks of the Sierras Pampeanas are bounded by longitudinal faults that originated during or were reactivated by the Andean Orogeny, then uplifted by reverse faults during the late Tertiary and the Quaternary. Utilizing primarily gravity data, interpreted with the aid of geological information and seismological and seismic data, we developed models consistent with: (1) seismic data on the Moho, and (2) a significant regional gradient of “g” from west to east. In order to be consistent with (1), we took into account the gravimetric effects of the subhorizontal Nazca plate, whereas to be consistent with (2), it was necessary to wedge the Bermejo Basin sediments under very dense positive masses within the upper crust underlying the Sierra de Valle Fertil. Two crustal models were developed by inversion of the Bouguer anomaly. These fully fit the Bouguer anomaly and the geological data and reveal two zones of high density that are interpreted as paleosutures.

Did Patagonia collide with Gondwana in the Late Paleozoic? Some insights from a multidisciplinary study of magmatic units of the North Patagonian Massif

The origin of Patagonia and its relations with the South American crustal blocks to the north have been a matter of debate for decades. We report results from a multidisciplinary study centered on Paleozoic granitoids exposed in the northeastern corner of the North Patagonian Massif. Microstructural and magnetofabric studies reveal two suites of granitoids. Late Carboniferous (?) granitoids (Yaminue Complex, Tardugno Granodiorite, Cabeza de Vaca leucogranite) were emplaced and subsequently deformed in a major NNE-SSW compressive stress regime that also provoked top-to-the-SW thrust deformation in shallow crustal levels. Gravity and geobarometric studies show that the same major deformation event has been recorded at different crustal levels. The age and type of deformation of this event recorded across the northern boundary of Patagonia strongly supports a Late Carboniferous – Early Permian frontal collision between Patagonia and Gondwana. This major deformation event ceased by 281 Ma when the Navarrete Plutonic Complex, which shows mainly magmatic fabrics, was emplaced under a far-field WNW-ESE stress regime. Crustal continuity between the North Patagonian Massif and the Pampia and Arequipa- Antofalla terranes is suggested by similar Late Paleoproterozoic crustal model ages, comparable detrital zircon ages in Early Paleozoic successions, the apparent continuity of an Early Ordovician continental magmatic arc and paleomagnetic data. Reconciliation of this evidence with the Late Paleozoic frontal collision is obtained in a tectonic model that suggests that the North Patagonian Massif is a parautochthonous crustal block.

Neogene Growth of the Patagonian Andes

After a Late Cretaceous to Paleocene stage of mountain building, the North Patagonian Andes were extensionally reactivated leading to a period of crustal attenuation. The result was the marine Traiguen Basin characterized by submarine volcanism and deep-marine sedimentation over a quasi-oceanic basement floor that spread between 27 and 22 Ma and closed by 20 Ma, age of syndeformational granitoids that cut the basin infill. As a result of basin closure, accretion of the Upper Triassic metamorphic Chonos Archipelago took place against the Chilean margin, overthrusting a stripe of high-density (mafic) rocks on the upper crust, traced by gravity data through the Chonos Archipielago. After this, contractional deformation had a rapid propagation between 19 and 14.8 Ma rebuilding the Patagonian Andes and producing a wide broken foreland zone. This rapid advance of the deformational front, registered in synorogenic sedimentation, was accompanied at the latitudes of the North Patagonian Andes by an expansion of the arc magmatism between 19 and 14 Ma, suggesting a change in the subduction geometry at that time. Then a sudden retraction of the contractional activity took place around 13.5–11.3 Ma, accompanied by a retraction of magmatism and an extensional reactivation of the Andean zone that controlled retroarc volcanism up to 7.3–(4.6?) Ma. This particular evolution is explained by a shallow subduction regime in the northernmost Patagonian Andes, probably facilitated by the presence of the North Patagonian massif lithospheric anchor that would have blocked drag basal forces creating low-pressure conditions for slab shallowing. Contrastingly, to the south, the accretion of the Chonos Archipelago explains rapid propagation of the deformation across the retroarc zone. These processes occurred at the time of rather orthogonal to the margin convergence between Nazca and South American plates after a long period of high oblique convergence. Finally, convergence deceleration in the last 10 My could have led to extensional relaxation of the orogen.

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 Peru-Chile Margin from Global Gravity Field Derivatives

Deformation along the 3,500 km subduction Pacific margin of the Peru-Chile trench is partially controlled by ocean bathymetric heterogeneities and sediments. Oceanic highs (e.g. ridges, fracture zones, plateaus) influence deformation in the fore-arc zone where collision occurs, and control turbiditic flow dispersal and consequently the amount of sediments accreted at the frontal accretionary prism and subduction channel, compartmentalizing the trench into segments linked to seismic segmentation. Recent satellite missions (CHAMP, GRACE and GOCE) have introduced an extraordinary improvement in the reconstruction of the global gravity field. Earth gravity field models, mainly derived from satellite measurements, reflect mass inhomogeneities of the earth. This chapter focuses on the determination of mass heterogeneities over the oceanic plate and their relation to general distribution of sediments over the Peru-Chilean margin, seismic segmentation along the margin, and the relationship between trench sediment thickness and the variable Andean orogenic volume, by means of a gravimetric analysis. Using the gravity potential model EGM2008 and satellite GOCE data we calculated two functionals of the geopotential: the Bouguer anomaly and the vertical gravity gradient, both corrected for the topographic effect. The vertical gravity gradient field is of special interest as it highlights main geological features, and allows unraveling unknown structures that are concealed by sediments. From these, different features can be clearly depicted such as the contact between the Pacific oceanic crust and the South American plate, the Nazca Ridge, the Juan Fernandez Ridge and the Chile Rise, among others. The segmentation between a filled trench south of Juan Fernandez Ridge, a partially filled trench to the north up to the Copiapo Ridge, and a completely starved trench north of this latitude is depicted. Finally, the relationship between gravity derived fields, high oceanic features and seismic segmentation is discussed for the last megathrust earthquakes that affected this subductive plate boundary.

Multiple geophysical methods used to examine neotectonic structures in the western foothills of the Sierra de El Maitén (Argentina), North Patagonian Andes

A high-resolution geophysical study was carried out in a region of the retroarc of the Patagonian nAndes located on the western slope of the Sierra de El Maiten. This structure is characterized by an nimbricated west-vergent fault system developed in the orogenic front area of the North Patagonian nAndes that has uplifted. Oligocene volcanic rocks (Ventana Formation) affect Miocene to nQuaternary sediments. Even though neotectonic fault scarps are affecting Quaternary deposits in the nfoothills of this range, no direct observation of slip in Quaternary strata was determined. The main nobjective of this study is to determine geometry of recognized neotectonic structures, characterizing nthem by variations in magnetic susceptibility, density, and p-wave velocities. The combined application nof different geophysical methods has allowed the characterization of the bedrock geometry and nthe determination of neotectonic displacements along faults. The potential field model and its integration nwith a seismic profile show the accurate geometry of this tectonic zone, which is crucial for nseismogenic hazard analysis, in the area of northern Patagonia, a highly significant economic zone ndue to tourism with several towns (El Maiten, Esquel, and San Carlos de Bariloche) dispersed nthroughout the area of young tectonic activity.