Mario Gimenez

Gravity characterization of the La Rioja Valley Basin, Argentina

TheLaRiojaValleyisasedimentarybasininthePampean Ranges, a region of basement uplifts and broad valleys in the Andean foreland. The present study advances understanding ofthegenesisofthebasinbyprovidingsubsurfacedataonits geometry and characteristics. Gravity and topographic data, including a newly completed gravity survey, were integrated tocreateacompleteBougueranomalymap.EulerandWerner deconvolution techniques sum with traditional potentialfieldsmethodsandsparseindependentdatafromseismicand magnetotelluric methods to test and constrain the subsurface geophysical characterization of the La Rioja Valley Basin. The residual anomaly, obtained through upward continuation, was inverted to obtain the best 3D gravity model of the La RiojaValley Basin, which shows the first-order geometry ofthebasin.Thisrevealsthatthebasinisasymmetric,withup to 6000 m of low-density basinfill in a depocenter located in thewest-centralsectorofthevalley.Second-orderfeaturesof thebasinincludeasoutheast-trendingbasementhigh,likelya fault zone that subdivides the basin, which has been revealed by a suite of analytical methods 3D model of depth to basement, a 2D inverse model, and Werner deconvolution solutions.

Influence of pre-Andean history over Cenozoic foreland deformation: Structural styles in the Malargüe fold-and-thrust belt at 35°S, Andes of Argentina

The Andes are the classic example of a subduction-related orogen. Segmentation of the orogenic belt is related to dynamics of the subduction zone and to upper plate thermomechanical properties. Understanding the controlling factors on deformation along the orogen requires studying cross sections at different latitudes and determining the respective roles of plate interactions, upper plate weakness zones, and crustal architecture. A newly constructed balanced cross section of the Argentinean Andes at 35°S, in the transition between a flat-slab and a normal subduction segment, shows tectonic inversion of Mesozoic normal faults and development of new thrusts during Andean shortening. Estimated shortening of 26.2 km, equivalent to 22% of the initial length, is lower than previous estimates obtained from partial cross sections using non-inversion structural models. Comparison of this estimate with crustal area balance constrained by geophysical data indicates that (1) crustal thickness was varied across the transect before Andean shortening, with a thick (∼45 km) crustal block to the west related to late Paleozoic orogeny, and a thinner block (∼32 km) in the east related to Mesozoic stretching; and (2) a structural model incorporating tectonic inversion is consistent with regional shortening and crustal thickness trends. Our results underscore the role of the inherited characteristics of the upper plate in subduction-related orogens, including preexisting faults and preorogenic crustal thickness variations.

A Comparative Analysis of Seismological and Gravimetric Crustal Thicknesses below the Andean Region with Flat Subduction of the Nazca Plate

A gravimetric study was carried out in a region of the Central Andean Range between and south latitudes and from and west longitudes. The seismological and gravimetrical Moho models were compared in a sector which coincides with the seismological stations of the CHARGE project. The comparison reveals discrepancies between the gravity Moho depths and those obtained from seismological investigations (CHARGE project), the latter giving deeper values than those resulting from the gravimetric inversion. These discrepancies are attenuated when the positive gravimetric effect of the Nazca plate is considered. Nonetheless, a small residuum of about 5 km remains beneath the Cuyania terrane region, to the east of the main Andean chain. This residuum could be gravimetrically justified if the existence of a high density or eclogitized portion of the lower crust is considered. This result differed from the interpretations from Project “CHARGE” which revealed that the entire inferior crust extending from the Precordillera to the occidental “Sierras Pampeanas” could be “eclogitized”. In this same sector, we calculated the effective elastic thickness (Te) of the crust. These results indicated an anomalous value of Te = 30 km below the Cuyania terrane. This is further conclusive evidence of the fact that the Cuyania terrane is allochthonous, for which also geological evidences exist.

New insights into the Andean crustal structure between 32° and 34°S from GOCE satellite gravity data and EGM2008 model

Abstract The subduction of the Nazca oceanic plate under the South American plate in the south-central Andes region is characterized by the oblique collision of the Juan Fernandez Ridge against the continental margin. The upper plate is characterized by a broken foreland, a thrust-and-fold belt and eastward migration of the volcanic arc promoted by the flattening of the slab. Topographic load, thermal state and plate rheology determine the isostatic state of the continental plate. We calculated the vertical gravity gradient from GOCE satellite data in order to delineate the main tectonic features related to density variations resulting from internal and external loads. Then, using the Bouguer anomaly, we calculated the crust–mantle discontinuity and the elastic thickness in the frame of the isostatic lithospheric flexure model applying the convolution method approach. The results obtained show substantial variations in the structure of the continental lithosphere related to variations in the subduction angle of the Nazca plate. These variations are reflected in the varying Moho depths and in the plate rigidity, presenting a distinct behaviour in the southern zone, where the oceanic plate subducts with an approximate ‘normal’ angle with respect to the northern zone of the study area where the flat slab occurs.

Geophysical Study of the Valle Fértil Lineament Between 28°45′S and 31°30′S: Boundary Between the Cuyania and Pampia Terranes

Abstract A gravimetric and magnetometric study was carried out in the north-eastern portion of the Cuyania terrane and adjacent Pampia terrane. Gravimetric models permitted to interpret the occurrence of dense materials at the suture zone between the latter terranes. Magnetometric models led to propose the existence of different susceptibilities on either side of the suture. The Curie temperature point depth, representing the lower boundary of the magnetised crust, was found to be located at 25 km, consistent with the lower limit of the brittle crust delineated by seismic data; this unusually thick portion of the crust is thought to release stress producing significant seismicity. Moho depths determined from seismic studies near western Sierras Pampeanas are significantly greater than those obtained from gravimetric crustal models. Considering mass and gravity changes originated by the flat-slab Nazca plate along Cuyania and western Pampia terranes, it is possible to reconcile Moho thickness obtained either by seismic or by gravity data. Thus, topography and crustal thickness are controlled not only by erosion and shortening but by upper mantle heterogeneities produced by: (a) the oceanic subducted Nazca plate with “normal slope” also including asthenospheric materials between both continental and oceanic lithospheres; (b) flat-slab subducted Nazca plate (as shown in this work) without significant asthenospheric materials between both lithospheres. These changes influence the relationship between topographic altitudes and crustal thickness in different ways, differing from the simple Airy system relationship and modifying the crustal scale shortening calculation. These changes are significantly enlarged in the study area. Future changes in Nazca Plate slope will produce changes in the isostatic balance.

Crustal structure and tectonic setting of the south central Andes from gravimetric analysis

Based on terrestrial gravity data, in this paper we prepared a map of Bouguer anomalies, which was filtered to separate shallow and deep gravity sources. Based on a density model and gravimetric inversion techniques, the discontinuous crust- mantle boundary and the top of crystalline basement were modeled. Subsequently, the equivalent elastic thickness (Te) was evaluated, considering information from the crust-mantle discontinuity and topographic load, finding high Te values in the eastern Andean foothills and west of the Velasco range. These results are consistent with the positive isostatic and residual Bouguer anomaly values, which suggest the presence of high-density rocks in the mid-to upper crust. In addition, petrographic and geochemical analysis conducted in surface outcrops suggest a mantle origin.

Density and Thermal Structure of the Southern Andes and Adjacent Foreland from 32° to 55°S Using Earth Gravity Field Models

GOCE satellite data and EGM2008 model are used to calculate the gravity anomaly and the vertical gravity gradient, both corrected by the topographic effect, in order to delineate main tectonic features related to density variations. In particular, using the Bouguer anomaly from GOCE, we calculated the crust–mantle discontinuity obtaining elastic thicknesses in the frame of the isostatic lithospheric flexure model applying the convolution method approach. Results show substantial variations in the density, compositional and thermal structure, and isostatic and flexural behavior of the continental lithosphere along the Southern Andes and adjacent foreland region.

Modelo gravimétrico en el borde noreste del macizo norpatagónico

We present a gravimetric study on the northeast edge of the North Patagonian massif in the province of Rio Negro, to ascertain the depth distribution of the different outcrop units in the study area, as well as the geometry of the Navarrete Plutonic Complex. The negative anomalies in the Bouguer residual map reproduce the lateral density contrast between what would be the bedrock of Cambrian age and the lithologic units of the Mesozoic age. The solutions of the analytical signal along with variations in the curve of the horizontal gradient of gravity, obtained along a profile across the outcrops of the Navarrete Plutonic Complex, show contacts between the intrusive body and the bedrock with different densities. Over this profile we obtained a 2D (dimensional) gravimetric forward model adjusted with the density data of the surface rocks. Above the outcrop of the Yaminue Complex there exists a gravimetric high which does not correspond to the density in this area. This is a new evidence that proves that it is a series of intrusions and thin subhorizontal tectonic slices and that its thickness is probably thin. On the other hand theNavarrete Pluton shows the greatest development in size and depth to the east of Treneta stream.

Evidences of a tectonic uplift and seismic hazard in south of the Pie de Palo Range, San Juan-Argentina

ABSTRACT In the Central Andean region of Argentina, we found gravimetric and geomorphological evidence of an uplifting of the crystalline basement of Pie de Palo range. Within this zone, we observed a positive gravimetric anomaly in the extreme South of Pie de Palo, extending towards the South of Pampean ranges. By means of the geophysical technics, it was possible to determine the magnitude and geometrical form of the anomalous body. The evidence of a tectonic uplifting is also clearly manifested in the LandSat images, by observing the displacement of the course of the San Juan River towards the South. The study region is one of the major cortical and lithospheric regions with seismic activity in the country, where three of the most devastating earthquakes occurred over the last 73 years. The results would indicate that this region will continue to be one of the major seismically generating potential, significantly implying seismic dangers. The seismic risk studies indicate that the greatest hazard zone is found between the Pre-Cordillera and the Pie de Palo Range. The highest maximum acceleration values (PGA) are 242, 393, and 543 gal for return periods of 72, 475, and 2475 years, respectively.

Tectonic Evolution of the Northern Patagonian Andes (40°S)

This chapter is focused on the tectonic evolution of the North Patagonian Andes comprised between 38° and 40°S. Field recognition of main structures allowed establishing a structural control for the main sedimentary packages that coexisted with Andean development. These structures affect Miocene strata at the eastern deformational front, indicating a last stage of development, although cooling ages suggest a Mid to Late Cretaceous exhumation of the Paleozoic basement exposed at the westernmost sector. Synextensional deposits of late Oligocene age imply an interruption of Andean constructional mechanisms at these latitudes. Finally, seismic tomographies at these latitudes show an area of relatively low seismic velocities in the orogenic front area, separated from the arc front zone. Computed elastic thicknesses from gravity data show a good correlation with these areas with abnormal heat flow associated with retroarc stretching and magmatic emplacement in the last 5–2 Ma.