Abstract
The Altiplano-Puna Volcanic Complex (APVC) in the central Andes is
the product of an ignimbrite "flare-up" of world class proportions
(de Silva, 1989). The region has been the site of large-scale silicic
magmatism since 10 Ma, producing 10 major eruptive calderas and edifices,
some of which are multiple-eruption resurgent complexes as large
as the Yellowstone or Long Valley caldera. Seven PASSCAL broadband
seismic stations were operated in the Bolivian portion of the APVC
from October 1996 to September 1997 and recorded teleseismic earthquakes
and local intermediate-depth events in the subducting Nazca plate.
Both teleseismic and local receiver functions were used to delineate
the lateral extent of a regionally pervasive \~20-km-deep, very
low-velocity layer (VLVL) associated with the APVC. Data from several
stations that sample different parts of the northern APVC show large
amplitude Ps phases from a low-velocity layer with Vs h 1.0 km/s
and a thickness of \~1 km. We believe the crustal VLVL is a regional
sill-like magma body, named the Altiplano-Puna magma body (APMB),
and is associated with the source region of the Altiplano-Puna Volcanic
Complex ignimbrites (Chmielowski et al., 1999). Large-amplitude P-SH
conversions in both the teleseismic and local data appear to originate
from the top of the APMB. Using the programs of Levin and Park (1998),
we computed synthetic receiver functions for several models of simple
layered anisotropic media. Upper-crustal, tilted-axis anisotropy
involving both Vp and Vs can generate a "split Ps" phase that, in
addition to the Ps phase from the bottom of a thin isotropic VLVL,
produces an interference waveform that varies with backazimuth. We
have forward modeled such an interference pattern at one station
with an anisotropy of 15\%-20\\\\ that dips 45 deg within a 20-km-thick
upper crust. We develop a hypothesis that the crust above the "magma
body" is characterized by a strong, tilted-axis, hexagonally symmetric
anisotropy. We speculate that the anisotropy is due to aligned, fluid-filled
cracks induced by a "normal-faulting" extensional strain field associated
with the high elevations of the Andean Puna.
Links and resources
Tags