Myriam Patricia Martinez

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.

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.

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.

Geophysical characterization of the upper crust in the transitional zone between the Pampean flat slab and the normal subduction segment to the south (32–34°S): Andes of the Frontal Cordillera to the Sierras Pampeanas

Abstract The Nazca Plate subducting beneath the South American Plate has strongly influenced Cenozoic mountain growth in western Argentina and Chile sectors (32–34°S; 70–66°W). At these latitudes, the Pampean flat slab has induced the development of prominent mountain systems such as the Frontal Cordillera, the Precordillera, and the associated Sierras Pampeanas in the eastwards foreland region. Through a gravity study from the Frontal Cordillera to the Sierras Pampeanas region between 32 and 34°S, we delimit a series of geological structures that are accommodating shortening in the upper crust and others of regional and subsurface development, without any clearly defined mechanics of deformation. Additionally, through an isostatic residual anomaly map based on the Airy-Heiskanen local compensation model, we obtain a decompensative gravity anomaly map that highlights anomalous gravity sources emplaced in the upper crust, related to known geological structures. In particular, by applying the Tilt method which enhances the gravity anomalies, the NW-trending Tunuyan Lineament is depicted south of 33.4°S following previous proposals. Using the decompensative gravity anomaly, two profiles were modelled through the northern sector of the study area using deep seismic refraction lines, borehole data and geological information as constraints. These density models of the upper crust of this structurally complex area accurately represent basin geometries and basement topography and constitute a framework for future geological analysis.