We investigate the crustal thickness under the Peninsular Ranges using P-to-S converted phases of teleseismic body waves recorded on a temporary broadband seismometer array and isolated by the receiver function method. Ps minus P times at sites west of a compositional boundary that separates the Peninsular Ranges batholith into east and west zones indicate a relatively flat, deep Moho. Ps minus P times at sites east of 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 western zone. In the eastern zone the crust thins rapidly from 35 km thick at the compositional boundary to 25 km at the edge of the Salton trough, a lateral distance of 30 km. The lack of correlation between topography and Moho depths suggests compensation via lateral density variations in the lower crust or upper mantle. We propose that the compositional boundary decouples the eastern and western portions of the batholith, and that the eastern portion has thinned in response to regional Miocene extension, or Salton trough rifting, or both. ¿ American Geophysical Union 1996