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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 …