THE LITHOSPHERIC STRUCTURES OF THE SOUTHE RN CENTRAL ANDES BASED ON GEOPHYSICAL INTERPRETATIONS - A REVIEW

The authors present the results of seismic, gravimetric and magnetotel luric investigations in the southern Central Andes (20-26 degrees S) d uring the 80's. The structural data allow to deduce a strong asymmetry of the lithosphere of the Andean orogen. The crustal thickening in th e backarc is the result of an overthrust of the actual Eastern Cordill era over the Brazilian shield. Units of high seismic velocities in the upper crust (20-25 km depth) of the present Eastern Cordillera are in terpreted as overthrust lower crust. The Moho of the Brazilian shield can be traced up to the eastern border of the Altiplano at a depth of 70 km. In this region and in the magmatic are the Bouguer anomaly (-45 0 mGal) indicates a crustal thickness about 70 km. There are no Moho o bservations in this region;the seismic waves are strongly attenuated w ith a reduction of the seismic velocities. Beneath the magmatic are th e electric resistivities decrease to values of 0,5-1 Omega m at 20 km depth indicating a zone of partial melting. The continental crust of t he forearc has a thickness of 25-35 km without indications for a clear Moho. Between the subducting Nazca Plate and the continental crust th ere is a wedge of material with low seismic velocities and with low de nsities, typical values for crustal material, which are interpreted as a mixture of hydrated upper mantle relicts and material eroded from t he continental margin. The reduced electric resistivities indicate the presence of fluids in this zone.