A retrodeformable cross section that integrates surface and subsurface data across the western Transverse Ranges, California, illustrates an actively developing fold and thrust belt that began forming at 2-3 Ma. High-level thrusts are interpreted to root in a mid-crustal detachment at 12-15 km depth, which coincides with the maximum depth of earthquakes. The cross section documents 53 km of convergence above the mid-crustal detachment; dividing this by the time since onset of deformation yields convergence rates of 17.6-26.5 mm/yr. The high-density lithospheric anomaly beneath the Transverse Ranges is related to subduction of lower crust and lithosphere below the mid-crustal detachment to balance the shallow crustal shortening. Thrust ramps coincide with zones of high seismicity in the Transverse Ranges; this suggests that ramp regions have the highest potential for compressive earthquake events: e.g., the recent Whittier Narrows earthquake of October 1, 1987 (M1 = 5.9), occurred along the eastern part of a ramp zone.

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