COMPOSITION AND THICKNESS OF THE SOUTHERN ALTIPLANO CRUST, BOLIVIA

Slant stacking of broadband seismograms recorded in the western United States for two 1993 intermediate-depth earthquakes that occurred near the Bolivia-Argentina-Chile borders reveals small but clear precursor s to both the compressional (P) wave and shear (S) wave depth phases. We interpret and model these precursors as underside reflections from the thickened Andean Altiplano crust. We model the crustal structure o f the Altiplano with a grid-search technique to match the timing and a mplitudes of the depth phases and precursors in the waveforms of both the P wave and S wave. Our best-fit model has an average crustal veloc ity of 5.9-6.0 km/s, a crustal V(p)/V(s) of 1.6, a crustal thickness o f 75-80 km, and a high-velocity (V(p) = 8.4 km/s), high-V(p)/V(s) (1.9 ) mantle wedge. Assuming isotropy, the low V(p)/V(s) ratio of 1.6 for the crust corresponds to an anomalously low Poisson's ratio of 0.18. S uch a low value, in conjunction with the low average V(p) estimate, is consistent only with a felsic composition and high upper-crustal temp eratures. The finding of a thick felsic crust overlying a high-velocit y mantle supports models of Altiplano uplift due predominantly to crus tal shortening as opposed to mafic magmatic addition and is inconsiste nt with recent mantle delamination.