Observed patterns of high-frequency seismic wave attenuation suggest that near 22¿S, the upper mantle structure beneath the central Andean plateau changes along strike to the south. Contrasting regions of high and low upper mantle seismic wave attenuation beneath the plateau are identified based on striking azimuthal variations in the character and frequency content of P and S waves propagting beneath the plateau to a portable seismic network deployed in Jujuy Province, Argenatina (24¿ S, 65¿ W). Ray paths from intermediate depth earthquakes located north and northwest transmit seismic waves with a higher frequency content than ray paths from earthquakes at similar depths and distances but located west and south of the network. The estimated apparent Q values fall into two categories: Qp>500 and Qs>350 for the high-Q paths, and Qp<350 and Qs<200 for the low-Q paths. In addition, Sn phases from regional crustal earthquakes in the Subandean foreland fold-thrust belt to the north propagate efficiently to the Jujuy network, while Sn is not observed from foreland earthquakes located at similar distances to the south of the network. These observations combined with data from previously reported wave propagation studies suggest that south of about 22¿ S, the upper mantle beneath the plateau and its adjacent foreland thrust belt is more highly attenuating than the upper mantle farther north.

Forward modeling of the Q measurements made at Jujuy indicates that the observations can be explained either by a variable thickness high-Q upper plate beneath the plateau, or by a thin, variable width, very low-Q zone in the asthenospheric wedge above the subducted slab. This lateral variation in upper mantle structure coincides with two physiographically distinct segments of the central Andean plateau and its adjacent foreland thrust belt to the east: the Bolivian Altiplano and Subandean ranges in the north and the Argentine Puna and Santa Barbara system in the south. We interpret the north-south change in upper mantle attenuation and the corresponding changes in physiography, topography, and tectonic style at the surface to be due to a mantle lid that is thicker beneath the Altiplano and the Subandean belt than beneath the Puna and the Santa Barbara ranges. ¿American Geophysical Union 1992