CONICET | Buscador de Institutos y Recursos Humanos

The study area is placed in the Chaco-Pampean Plain (Central-North Argentina), which is part of the distal plain of the central Andean piedmont originated by the uplift and erosion of the Andean Cordillera. A sedimentary aggradational process typifies this region and buries a large history related to Western Gondwana and Andean Orogeny. The Andean piedmont is composed of a collage of crustal blocks that were amalgamated by different orogeny and deformation belts. The Transbrasiliano lineament constitutes one of these continental belts and transversely intersects the South American Platform, from NNE to SSW. Northward, this lineament is well evidenced, while southward, in the distal Andean foreland, it remains unknown and is associated with the tectonic boundary between the Río de la Plata Craton (RPC) and the Pampean terrane (PT). This tectonic boundary is mostly unexposed in the Chaco-Pampean Plain. An east or west dipping subduction and a later collision between these terranes is still under debate. Deep geophysical studies are not abundant in this region. Few works have locally characterized the composition and structure of the crust and upper mantle beneath this extensive intracontinental plain. Magnetotelluric (MT), gravimetric and magnetic data have inferred the boundary between the RPC and the PT at the limit between Eastern Pampean Ranges and the Chaco-Pampean Plain. Furthermore, at regional scale, 3-D seismic tomographies and images of the seismic structure of the crust and the uppermost mantle have revealed the Moho depths and other lithospheric discontinuities. Here, we report the results of a MT survey along a W-E profile located at 27° S between 63º45? and 60º30? W, that characterized the geoelectric structure of the tectonic boundary between the RPC and the PT beneath the Chaco-Pampean Plain. The MT method provides an image of the electrical resistivity distribution of the Earth?s subsurface and constitutes an effective and complementary technique to determine lithospheric structures beneath thick sedimentary basins. So, MT data were processed and the dimensionality and distortion of the geoelectrical medium were analyzed. Finally, a 2-D inversion model was performed over a mesh with known geological constraints. The image distribution of the electrical resistivity at lithosphere scale shows a highly resistive crust (> 100,000 Ω m) in the east side of the profile, correlated with the RPC. In the west side of the profile, a less but still highly resistive crust (~ 12,000 Ω m) was correlated with the PT. Both highly resistive blocks are separated by an east dipping conductive anomaly (100-200 Ω m) correlated with the Transbrasiliano lineament. This conductive feature agrees with the east dipping subduction model and can be explained by the presence of graphite presumably lying in paleosutures of long-stable geological terranes. An oblique convergence between both terranes could develop a transpressional shear belt. Results provide a geoelectrical characterization at lithospheric scale beneath the Chaco-Pampean Plain and a new geophysical evidence of the tectonic boundary between the RPC and the PT, improving the knowledge of the Western Gondwana amalgamation.