Lithospheric deformation beneath the San Gabriel Mountains in the Southern California Transverse Ranges

High-resolution tomographic images from Los Angeles Region Seismic Experiment (LARSE) array and southern California Seismic Network (SCSN) teleseismic data suggest that the entire lithosphere below the San Gabriel Mountains and San Andreas fault in the Transverse Ranges has thickened in a narrow, vertical sheet. P wave travel time inversions of the combined data support the presence of the well-documented upper mantle high-velocity anomaly that extends ∼200 km into the mantle under the northernmost Los Angeles basin and Transverse Ranges, and is associated with mantle downwelling due to oblique convergence. We find that the high-velocity, high-density upper mantle anomaly comprises a 60–80 km wide sheet of mantle material that lies directly below a substantial crustal root in the San Gabriel Mountains. The velocity perturbations are as large as 3% in the anomaly, corresponding to a ∼2% density increase. The tomographic images suggest that deformation in the ductile lower crust and mantle lithosphere may be partially coupled mechanically and thermally if the thickening is occurring together in response to convergence and that it may be a local compressional feature. ; © 1999 American Geophysical Union. (Received December 3, 1998; revised March 17, 1999; accepted March 31, 1999) The work presented here was greatly strengthened by enlightening discussions with Paul M. Davis. The author wishes to thank Peter Bird, Ken Dueker, Rebecca Saltzer, the AE, and an anonymous referee in particular for valuable comments on the manuscript. LARSE was a joint effort involving scientists from the University of California at Los Angeles, the U.S. Geological Survey, the California Institute of Technology, and the University of Southern California. The LARSE experiment was made possible by the loan of IRIS PASSCAL seismometers and the support of its staff. Thanks go to Ken Dueker for providing the large compilation of Southern California Seismic Network P wave travel time residual data. Egill Hauksson and Bill Lutter were ...