MIOCENE OBLIQUE RIFTING OF THE SAN DIEGO TROUGH REGION, CALIFORNIA CONTINENTAL BORDERLAND

Northwest oblique rifting and rotation of the Western Transverse Ranges (WTR) crustal block created the Inner-Borderland Rift during Miocene development of the Pacific-North America plate boundary. The San Diego Trough region occupies the southern third of this segmented continental rift, where tilted crustal blocks and volcanic highland rift geometry is still preserved. Two major phases of oblique rifting are recognized. First, subducted lower crust and upper mantle rocks (Catalina Schist) were rapidly exhumed along the Thirtymile Bank and related detachment faults. San Onofre Breccia along the mainland coast and offshore islands records this initial rifting. Mid-Miocene volcanic flows, volcaniclastic and other sedimentary sequences covered the exposed detachment at the seafloor of the widening rift. A granodiorite pluton on Santa Catalina Island, ~19 Ma, records early volcanism within the rift, whereas widespread volcanic rocks, ~16-18 Ma, record a major magmatic episode. A volcanic archipelago developed along the rift axis defined by a suite of major calderas stretching from Santa Catalina Island through Catalina Basin to the Fortymile Bank and Navy Bank volcanic highlands. The Glendora and Conejo volcanic complexes of the WTR were part of this initial Miocene volcanic province, but subsequently transrotated away from the rift axis. A second extensional phase, possibly associated with later oblique rifting of the Outer Borderland block, disrupted the original detachment surfaces within the rift by high-angle faulting to deeper levels of detachment creating horst block ridges like Thirtymile Bank and adjacent half-graben basins like the San Diego Trough. The Thirtymile Bank detachment fault was further segmented by high-angle faulting near the coast as the Coronado Bank block was westward-tilted and separated from the mainland. Mid-Miocene and older rocks on Islas Los Coronados are tilted from 25° to 35° down to the west like correlative strata imaged within the upper Coronado Bank block. Basalt flows that bracket San Onofre Breccia sequences within the Rosarito Beach basin and derived from the Thirtymile Bank uplift, constrain timing of this later stage to post 16.1-15.5 Ma. As rifting ceased, the late Miocene San Diego Trough region was further sheared by major high-angle right-slip fault zones.