DETACHMENT-CONTROLLED CONTINENTAL RUPTURE AND MULTIPLE PHASES OF SEAFLOOR SPREADING IN THE PESCADERO BASIN COMPLEX, SOUTHERN GULF OF CALIFORNIA

A dense network of seismic lines across the Pescadero basin complex (PBC) in the southern Gulf of California gives new insight into the processes and conditions that affect the rupture of continental lithosphere as well as those that control magma supply and accommodate plate separation across nascent seafloor spreading systems. The ruptured edge of continental lithosphere around the PBC is well defined by inward-facing topographic escarpments, abrupt changes in the seismic character of basement rocks and deep irregular reflections and diffractions produced by injections of dikes and sills. Only ~55 km of new oceanic lithosphere has been accreted across the PBC and thus continental rupture occurred ca. 1-2 Ma, which is significantly delayed compared to other nearby spreading segments like Guaymas and Alarcon. The modern plate margin in the PBC is defined by a series of narrow, intersecting, rhombohedral grabens, where the top of oceanic basement drops from 2,500 to as deep as 4,000 mbsl. This significant thinning of the ophiolitic crust is related to a decrease in upper mantle magma production and an increase in the amount of plate separation that is mechanically accommodated by faulting. Therefore, in the short time interval since continental rupture, the PBC records two distinct regimes of accretion of oceanic lithosphere. The highly-attenuated continental crust around the PBC contains zones of strongly-deformed foliated rocks that we interpret to represent cataclasite and mylonite that comprise the antiformally-arched footwall of a regional east-directed detachment system that extends NW through a series of domino fault blocks toward a breakaway zone along the Baja California coast and projects SE beneath a major rift basin on the Sinaloa margin.

Elevated heat flow associated with the earlier Alarcon rupture may have promoted the lower-crustal flow required to prolong detachment faulting in the adjacent PBC. Subsequent cooling due to the tectonic separation of the two regions led to continental rupture and the transition to magma-starved seafloor spreading in the PBC. The presence of regional detachment systems extending across the Gulf of California is not consistent with the heterogeneous concentration of extensional strain as proposed in the pull-apart hypothesis of continental rupture.