LITHOSPHERIC THICKENING AND FOUNDERING: A COMPARISON OF THE CENTRAL AND SOUTH CENTRAL ANDES

We compare the central Andes with the south central Andes using seismic results based on two PASSCAL seismic deployments across the Andes at16-20°S and 30-36°S respectively. The central Andes have an active volcanic arc, high elevations with a central plateau, thick crust and a large amount of shortening in the backarc. The seismic velocities indicate an overall felsic quartz-rich crust with a heterogeneous upper mantle consistent with piecemeal removal of parts of the lithosphere. The northern part of the Altiplano and the Eastern Cordillera show low seismic velocities beneath the crust consistent with lithospheric removal. In contrast, seismic velocities beneath the central Altiplano are consistent with lithospheric mantle still attached to the crust. The large amount of shortening and the lack of a high velocity lower crust suggests that lithospheric material has been recycled into the mantle as the Brazilian craton has subducted beneath the Eastern Cordillera.

In the south central Andes the subducting Nazca slab has a subhorizontal geometry and extends inland over 300 km beneath the Sierras Pampeanas (SP) near 30°S but returns to a normal dip to the south at 33°S. Seismic studies indicate that the eastern SP have a crustal thickness of 35 km while the western SP crustal thickness increases to 55 km. The western SP have a high velocity lower crust that may be a higher density material that has not yet been removed. Seismic tomography shows the mantle lithosphere is also very heterogeneous with low seismic velocities beneath the volcanic arc region, high velocities directly below the Moho in the backarc, and anomalous mantle (low Vp/Vs ratio) directly above the flat slab. The along strike variations of the lithosphere are indications of different stages in the process of lithospheric removal and “felsification” of the crust.