David Baumont

Seismic detection and characterization of the Altiplano-Puna Magma Body, Central Andes

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 silicicmagmatism since 10 Maproducing 10major eruptive calderas and edifices, some aremultiple-eruption resurgent complexes as large as theYellowstone orLongValley caldera. SevenPASSCALbroadband 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 £ 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–Punamagma 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 andVs can generate a ‘‘splitPs’’ phase that, in addition to thePs phase from the bottomof a thin isotropic VLVL, produces an interference waveform that varies with backazimuth.We have forwardmodeled 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.

Strong crustal heterogeneity in the Bolivian Altiplano as suggested by attenuation of Lg waves

Using the seismological data of four temporary networks installed across the Andes of Bolivia and Chile between 1990 and 1995, we study the amplitudes of the Lg phase for 45 regional events located at crustal depths. This analysis reveals that Lg is strongly attenuated for most of the ray paths crossing the Altiplano independent of the azimuth of propagation. To map the anomalies and evaluate their frequency dependence, we conduct a regionalization of the crustal average Qs factor using a damped least squares inversion technique in the frequency range 0.6–5 Hz. The resulting maps delineate regions with contrasting values of apparent attenuation. The Altiplano is characterized by strong attenuation with an average Qs of 100 at 1 Hz, whereas the Cordilleras are regions of weaker attenuation. The frequency dependence of Qs is investigated in an attempt to separate the source of the apparent attenuation into scattering and anelastic absorption. Within the hypothesis of a weak frequency dependence of the intrinsic Q, we show that at 1 Hz, scattering plays a major part in the attenuation of Lg waves whereas at 4 Hz, both mechanisms are of equal importance. These results indicate that the attenuation of Lg waves in the Altiplano is due more to scattering by small-scale heterogeneities than to a high percentage (>1%) of partial melt involving a large part of the crust.