We investigate the crustal thickness and Moho topography underlying the Peninsular Ranges near the U.S.-Mexico border with an east-west array of 9 broadband stations spanning the Peninsular Ranges batholith to record P-to-S converted phases generated at the Moho by teleseismic body waves. The converted phases are isolated by the teleseismic receiver function method. Signal enhancement is attained by a regularized time domain deconvolution technique that uses multiple events in the same source regions. Arrivals which we interpret as Ps (mantle P to crustal S converted phase) are identified at most of the stations. Based on this phase identification, the differential travel times (Ps minus P) at western sites are observed to be delayed relative to eastern sites. The largest variations in differential travel times occur across the compositional boundary that separates the Peninsular Ranges Batholith into east and west zones by geochemical, petrological and geophysical properties. 1D crustal velocity models under each site were constructed from 3D seismic tomography results and used to infer Moho depth from the Ps delays. The resultant Moho depth estimate varies laterally, shallowing from west to east, with a possible 5 km step coincident with the compositional boundary. East of the compositional boundary the Moho appears to shallow by 12 km vertical relief over 30 km lateral distance. We propose that the compositional boundary separates 25 to 35 km thick crust in the eastern batholith from 35 to 41 km thick crust in the western batholith.