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

A network of seismic profiles 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 multiple criteria including abrupt changes in the seismic character of basement rocks and only ~55 km of new oceanic lithosphere has been accreted across the PBC since 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 4,200 mbsl. This deflation of the oceanic crust is associated with more primitive MORB lavas, a decrease in magma production, and an increase in the amount of plate separation accommodated by faulting. The PBC is located within a NW-trending corridor of oblique extension defined by a series of east-directed normal faults that bound tilted domino fault blocks. Faults in this extensional corridor likely root into a regional ESE- directed detachment fault system and are laterally framed by a pair of NW-trending accommodation zones that project toward prominent rift segment boundaries along the Baja California coast. The highly attenuated continental crust around the PBC contains wavey discontinuous reflections that may represent mylonites in the antiformally-arched footwall of a low-angle fault zone that projects SE beneath the deep rift basins along the margin of Sinaloa. Elevated heat flow due to upwelling asthenosphere may have promoted the lower-crustal flow required to prolong detachment faulting. Subsequent cooling due to the westward migration of the plate margin relative to a fixed mantle reference frame led to continental rupture and the transition to magma-starved seafloor spreading.