CRUSTAL THICKNESS IN THE ANDES FROM TELESEISMICALLY RECORDED DEPTH-PHASE PRECURSORS

An important parameter in interpreting the evolution of continental orogens like the central Andes is an understanding of their modern crustal thickness distribution. This study builds on that of Zandt et al (1994) who calculated Central Andean crustal thicknesses of ~75-80 km near the Bolivia-Argentina-Chile border using earthquake depth-phase precursors (pmP). Although small in amplitude, the pmP phase can be clearly recognized on numerous single traces. Stacking seismograms from teleseismically located station arrays significantly reduces the signal to noise ratio allowing for more reliable identification of pmP. The method can be applied to any event exhibiting sufficient magnitude (> 6 Mw) and depth (>100km), so a framework of crustal thicknesses is generated for the Andes where suitable earthquakes occur. The results to date show that central Andean crustal thicknesses obtained with the pmP method are in good agreement with those from receiver functions. In particular, there is a high degree of correlation in the northern Puna and southern Altiplano region where crustal thicknesses have been reported on a one degree grid by Yuan et al (2002). Specifically, crustal thicknesses of 42-67 km under the northern-Puna and southern Altiplano determined by Yuan et al. (2002) compare with crustal thickness of 48 to 66 using the pmP method. Under the main Altiplano plateau, Yuan et al. (2002) obtained a crustal depth of 80 km crust, whereas the pmP method yielded 81 km. Further south, strong pmP reflections over the Chilean flatslab region in the Precordillera near 31°S 69°S yield crustal depths of 55-60 km in the Calingasta Valley that are again in good agreement with thicknesses from receiver functions determined by Fromm et al (2004) in their CHARGE experiment. We reveal new estimates of crustal thickness for Southern Peru of 55-70 km and also in the Southern Puna (27°S, 68.5°W) of 70km.