Velocity variations in the uppermost mantle beneath the southern Sierra Nevada and Walker Lane
We model Pn waveforms from two earthquakes in the southwestern United States (Mammoth Lakes, California, and western Nevada) to determine a velocity model of the crustal and mantle structure beneath the southern Sierra Nevada and Walker Lane. We derive a one-dimensional velocity model that includes a smooth crust-mantle transition east of Death Valley and extending south into the eastern Mojave desert. West of Death Valley and toward the Sierra Nevada a low-velocity mantle (V_p = 7.6 km/s) directly below the crust indicates the lithosphere is absent. At the base of this low-velocity structure (at 75–100 km depth) the P wave velocity jumps discontinuously to V_p 8.0 km/s. The area of low velocity is bounded by the Garlock Fault to the south and the Sierra Nevada to the west, but we cannot resolve its northern extent. However, on the basis of teleseismic travel times we postulate that the anomaly terminates at about 38°N. The presence of a low-velocity, upper mantle anomaly in this area agrees with geochemical research on xenoliths from the southern Sierras and recent studies of receiver functions, refraction profiles, tomography, and gravity. However, the velocity discontinuity at 75–100 km is a new discovery and may represent the top of the once present, now unaccounted for and possibly sunken Sierra Nevada lithosphere.
Additional Information© 2003 by the American Geophysical Union. Received 26 September 2001; revised 12 February 2002; accepted 5 March 2003; published 2 July 2003. Article first published online: 2 Jul. 2003. We would like to thank Jascha Polet, Hiroo Kanamori, Egill Hauksson, Matt Pritchard, Jeroen Ritsema, and Mark Simons as well as two anonymous reviewers for their reviews. We would also like to thank Alisa Miller, Pete Savage, Nathan Niemi, Rowena Lohman, and Jason Saleeby for their helpful discussions, and TriNet and Berkeley Digital Seismological Network (BDSN) for seismic waveform data. This research was supported by the Defense Threat Reduction Agency under contract DSWA01-98-1-0010. Contribution 8827, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California.
Published - 2001JB001393.pdf