CRUSTAL THICKNESS VARIATIONS BENEATH THE PENINSULAR RANGES, SOUTHERN CALIFORNIA

We investigate the crustal thickness under the Peninsular Ranges using P-to-S converted phases of teleseismic body waves recorded on a tempo rary broadband seismometer array and isolated by the receiver function method. Ps minus P times at sites west of a compositional boundary th at separates the Peninsular Ranges batholith into east and west zones indicate a relatively hat, deep Moho. Ps minus P times at sites east o f the compositional boundary decrease eastward. Moho depth estimates ( made from the Ps delays and crustal velocities from seismic tomography ) indicate a relatively constant 36 to 41 km thick crust in the wester n zone. In the eastern zone the crust thins rapidly from 35 km thick a t the compositional boundary to 25 km at the edge of the Salton trough a lateral distance of 30 km. The lack of correlation between topograp hy and Moho depths suggests compensation via lateral density variation s in the lower crust or upper mantle. We propose that the compositiona l boundary decouples the eastern and western portions of the batholith and that the eastern portion has thinned in response to regional Mioc ene extension, or Salton trough sifting, or both.