New constraints on subduction and collision processes in the
Central Andes from P-to-S converted seismic phases

X. Yuan (Potsdam)

The Central Andes are the Earth's highest mountain belt related to 
ocean-continent collision. They were uplifted during the Cenozoic mostly 
due to thickening of the continental crust dominated by tectonic 
shortening. We employ P-to-S converted teleseismic waves from temporal
networks operated in the Central Andes between 1994 and 1997 to image 
deep structures. The Moho is imaged at 40-75 km, rising by 10-20 km 
from the Altiplano to the Puna. Relatively thin crust (50 km) beneath 
the high (> 4km) Puna plateau indicates that lithospheric mantle thinning 
significantly contributed to its uplift. The subducted oceanic crust 
is imaged down to 120 km depth where it becomes invisible to converted 
teleseismic waves, likely due to the completed gabbro-eclogite 
transformation, providing direct evidence for kinetically delayed 
metamorphic reactions in the subducting plate. Most of the intermediate 
depth seismicity stops at the same depth, suggesting a relation with 
this transformation. A 10-20 km thick intracrustal low velocity zone 
is evident below the entire Altiplano and Puna plateaux. We interpret 
this as a zone of ongoing metamorphism and partial melting that decouples 
upper crustal imbrication from lower crust thickening.