Abstract.Receiver function analysis of 14 teleseismic events recorded by 6 temporary PASSCAL broadband stations within the Altiplano-Puna volcanic complex (APVC)shows a consistent -2 s negative-polarity P-to-S conversion for all stations for all available azimuths. Forward modeling of the largest amplitudes suggests that this conversion is produced by the top of a very low velocity zone at a depth of -19 km, with a Vs < 0.5 km/s and a thickness of 750-810 m. We interpret the characteristics of the low-velocity zone (low Vs, areal extent, and flatness) to be consistent with a sill-like magma body. On the basis of additional data from the German ANCORP experiment, the Altiplano-Puna magma body appears to underlie much of the APVC, and it may therefore be the largest known active continental crustal magma body.

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 multipleeruption 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 lowvelocity layer with Vs £ 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°within a 20-km-thick upper crust. We develop a hypothesis that the crust above the ''magma body'' is characterized by a strong, tiltedaxis, 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.

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Garnets with compositions between majorite and pyrope, Mj38, Mj48, Mj75 and Mj79 were synthesized at high pressures and temperatures in a 2000‐ton uniaxial split‐sphere apparatus (USSA‐2000) and investigated using high resolution synchrotron X‐ray powder diffraction and transmission electron microscopy. The results from both techniques are consistent with the tetragonal field for these garnets extending to a majorite composition just below Mj75. The cubic‐tetragonal structural phase transition in garnet along the majorite‐pyrope join is sensitive to both composition and temperature and is expected to result in anomalous behavior in elastic shear moduli. This phase transition may occur in the transition zone of the earth's mantle and will have important effects on the elastic and rheological properties of this region where these garnets are stable phases.