The California Continental Borderland (CCB) is key to understanding the Neogene tectonics of western North America, because it lies near the point where the East Pacific Rise is thought to have first encountered the continental margin. Thus, the CCB marks the probable site of the initiation of the San Andreas transform boundary and the migration of the Mendocino and Rivera triple junctions. The inner CCB is believed to have formed by exhumation during oblique right-lateral transtensional motion of the outer CCB. Two modes have been proposed for the exhumation of the inner CCB, either through metamorphic core complex (MCC)-type exhumation above a crustal-scale detachment or magmatic upwelling within a slab gap or window. The models differ in that one predicts the presence of fossil oceanic crust at the base of the CCB 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 (EDGE line RU-10) and the absence of oceanic crust beneath the inner CCB, south of Santa Catalina Island (Los Angeles Regional Seismic Experiments [LARSE] lines 1 and 2). In this work, we re-interpret data from the 1991 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 CCB. We interpret the upper reflector to be the top of an 8 km thick layer that has a P-wave velocity of 6.7 - 6.9 km/s and an average density of 3000 kg/m3. This layer thickens to 12 km beneath the Peninsular Ranges. The deeper reflector is from the Moho. We interpret the 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 CCB, 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 CCB crust between San Clemente and Santa Catalina Island may suggest that the basal layer is locally broken or delaminated.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 0905 Continental structures (8109;
- 7205 Continental crust (1219);
- 9350 North America;
- 9605 Neogene