BORDERLAND

The borderland is broken up by right-slip faults into several northwest-trending blocks. Our cross section (Figure. 8.6) begins on the easternmost block, the "Catalina terrane" (Howell and others, 1985), bounded on the east by the Newport-Inglewood fault. The Catalina terrane is underlain, beneath patches of Tertiary volcanic rocks, by Franciscan assemblage, on the basis of outcrops on Santa Catalina Island (Platt, 1975, 1976; Jones and others, 1976) and submarine dredge and core samples (Vedder and others, 1974). The block west of the Catalina terrane, the "San Nicholas terrane" (Howell and others, 1985), is inferred to be underlain, beneath Cenozoic marine sedimentary rocks, by rocks similar to the Great Valley sequence and Coast Range ophiolite of central California, possibly in fault contact with Franciscan assemblage at depth (Vedder and others, 1974).

A reversed seismic-refraction profile just west of Santa Catalina Island indicates P-wave velocities of 5.8 km/s to 6-km depth and of 6.7 km/s to the Moho at about 24-km depth (Figure. 8.6A, Shor and Raitt, 1958). This velocity-depth section is similar to that for the Diablo Range of central California (see above), where Franciscan rocks are equated with the 5.8-km/s interval, and middle and lower crust of island arc(s) and (or) oceanic crust are equated with the 6.7-km/s interval. In this region, there is no clear evidence of landward movement of the Franciscan assemblage as a tectonic wedge, although such evidence may surface during future investigations. As in the Diablo Range, the lower crust must have been brought to its present 18-km thickness by (1) imbrication of slices of island-arc crust, (2) tectonic underplating of several thicknesses of oceanic crust, and (or) (3) magmatic underplating. Subduction continued beneath the borderland until sometime between 30 and 20 Ma (see Atwater, 1970), depending on the latitude to which the borderland is palinspastically restored.