Geophysical characterization of the upper crust in the transitional zone between the Pampean flat slab and the normal subduction segment to the south (32– 348S): 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). Particularly, 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 eastward foreland region. Through a gravity study from the Frontal Cordillera to the Sierras Pampeanas region between 32-34° S, we delimit a series of geological structures that are accommodating shortening in the upper crust and others of regional and subsurface development, without a 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 and related with known geological structures. In particular, applying the Tilt method, that 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 modeled through the northern sector of the study area, using as constraints deep seismic refraction lines, borehole data and geological information. These are 39 density models of the upper crust of this structurally complex area that represent accurately basin 40 geometries and basement topography that constitute a framework for future geological analysis