Modeling of highly anisotropic crust and application to the Altiplano‐Puna volcanic complex of the central Andes

Abstract

The Altiplano‐Puna volcanic complex (APVC) is located in the central Andes and covers an area over 50,000 km2. The style of volcanism is predominately caldera‐forming ignimbrite eruptions active between 10 and 1 Ma. Chmielowski et al. [1999] used teleseismic events, recorded from seven portable broadband seismic stations deployed in the APVC, to identify a 1‐km‐thick very low‐velocity zone at a depth of 19 km. Based on the correlation with surface volcanics, the extremely low shear velocities of 1 km/s or less, and the depth of the layer, the low‐velocity zone was interpreted to be a “magma body.” Using local events recorded during the same seismic deployment, we have improved the crustal model for the APVC and detected seismic anisotropy in the crust above the low‐velocity zone. Using a hexagonal symmetry anisotropy code, we computed synthetic receiver functions for many models of layered anisotropic media and found two different models consistent with the data. In both models, approximately 20–30% anisotropy is present in a 3‐km‐thick surface low‐velocity layer, and 15–20% anisotropy is present in the remaining crust above the buried low‐velocity zone. The first model has the 1‐km‐thick low‐velocity zone at 17 km, while the alternate model has the low‐velocity zone at 14‐km depth overlain by a 4‐km‐thick transition zone of intermediate velocities. This anisotropy may be due to the fracture system by which magma migrates to the surface from the midcrustal sill‐like magma body.