The California Continental Borderland is key to understanding the Neogene tectonics of western North America, because the extension and exhumation of the inner Borderland involve the initiation of the San Andreas transform boundary, migration of the Mendocino and Rivera triple junctions, and rotation of the western Transverse Ranges. The extension and exhumation of the inner Borderland is believed to have been accommodated through metamorphic core complex-type exhumation. Two models for the extent of crustal deformation have been proposed. The models differ in that one predicts the presence of fossil oceanic crust at the base of the Borderland crust, whereas the other does not. Seismic investigations have yielded conflicting results that support both the existence of oceanic crust, west of the western Transverse Ranges and the absence of oceanic crust beneath the inner Borderland, south of Santa Catalina Island.
In this work, we re-interpret data from the 1990 PRAN offshore-onshore seismic wide-angle reflection experiment near Oceanside, California in hopes of resolving questions surrounding the different models for inner CCB exhumation. Six of the eight receiver-gathers from the PRAN transect show two wide-angle reflections that originate at depths of approximately 14 and 22 km beneath the inner Borderland. We interpret the upper reflector to be the top of an 8 km thick layer that has a P-wave velocity of 6.7 to 6.9 km/s and an average density of 3000 kg/m 3 . This layer thickens to 12 km beneath the Peninsular Ranges. The deeper reflector is from the Moho. We interpret this basal layer to be fossil oceanic crust, possibly thickened by magmatic underplating. The occurrence of a lower-crustal reflector beneath most of the outer and inner Borderland, west of the western Transverse Ranges and south of San Clemente Island, may support exhumation above a detachment that soles into the top of oceanic crust. On the other hand, absence of this reflector beneath the inner Borderland crust between San Clemente and Santa Catalina Island may suggest that the basal layer is locally broken or delaminated.
