ANOMALOUS CRUST OF THE BOLIVIAN ALTIPLANO, CENTRAL ANDES - CONSTRAINTS FROM BROAD-BAND REGIONAL SEISMIC WAVE-FORMS

A one-year deployment of broadband seismographs in the Bolivian Altipl ano recorded numerous intermediate-depth earthquakes at near-regional distances. We modeled the associated broadband waveforms of two earthq uakes to estimate an average crustal structure for the Altiplano. The resulting model is characterized by an anomalously low mean P velocity of 6.0 km/s, a low Poisson's ratio of 0.25, and a crustal thickness o f 65 km. The combination of the low mean velocity and low Poisson's ra tio can be explained only by a predominantly quartz-rich, felsic bulk composition. This constraint precludes significant volumes of magmatic addition from the mantle contributing to the great thickness of the A ltiplano crust, but is consistent with thickening by compressive short ening concentrated in a weak felsic layer.