Crustal thickening processes in the Central Andes and the different natures of the Moho-discontinuity

New seismic data from the Central Andes allow us to clarify the crustal str ucture of this mountain chain and to address the problem of crustal thicken ing, Evidence for the deep crustal root can be observed in both gravimetric and seismological data. Crustal structure and composition change significa ntly from east to west. In the eastern part of the backarc the Moho discont inuity is clearly recognisable, However only poor Moho arrivals are observe d by active seismic measurements beneath the Altiplano and the Western Cord illera where broad-band seismology data indicate such a discontinuity. In t he Precordillera, a pronounced discontinuity is detected at a depth of 70 k m. Along the coast, the oceanic Moho is developed at a depth of 40 km. Ther e are several processes which can change the petrological and petrophysical properties of the rocks forming the crust. Variations of the classical Moh o discontinuity are presented which do not correspond to the petrological c rust,mantle boundary. Tectonic shortening in the backarc is the dominant pr ocess contributing to at least 50-55% to the root formation along 21 degree s S. In the forearc and arc, hydration of the mantle wedge produced approxi mate to 15-20% of crustal thickening. Magmatic thickening and tectonic eros ion contributed only approximate to 5%. The other approximate to 25% is not yet explained. (C) 1999 Elsevier Science Ltd. All rights reserved.