César R. Monaldi

TECTONIC SHORTENING AND CRUSTAL THICKNESS IN THE CENTRAL ANDES : HOW GOOD IS THE CORRELATION?

Exceptional thickening of continental crust beneath the Central Andes is believed to be mainly a result of tectonic shortening of the South American plate in Neogene time. This shortening has been estimated to have contributed as much as 70%–80% of the present crustal volume. A compilation of published shortening values and our own estimates based on balanced cross sections from the Central Andes between lat 3°S and 40°S suggest that 70%–80% is a maximum rather than an average value for this part of the Andes. Tectonic shortening and the crustal cross-section area are only loosely correlated. Variations in tectonic shortening are more abrupt than those of crustal areas, particularly near the northern and southern ends of the Altiplano-Puna high plateau, where thick crust is associated with relatively small amounts of shortening. Shortening there may account for no more than about 30% of the present crustal cross-section area. The processes that created the remaining crustal area are not clear, but are likely to involve poorly constrained pre-Neogene tectonic shortening, moderate magmatic additions to the crust, tectonic underplating of material derived from the forearc, and possible flow of ductile lower crust along strike.