NEOGENE TECTONIC EVOLUTION OF THE MAGDALENA SHELF AND SOUTHERN GULF OF CALIFORNIA: INSIGHTS FROM DETRITAL ZIRCON U-Pb AGES FROM THE MAGDALENA FAN AND ADJACENT AREAS

The Magdalena fan is an apparently beheaded submarine depocenter that has figured prominently in Middle to late Miocene reconstructions of Pacific-North American plate interactions. The deposit accumulated rapidly at the base of the continental slope atop newly formed oceanic crust of the Farallon plate from 14.5-13 Ma. Subduction of this crust ceased as the Pacific-Farallon spreading center encountered the trench. The widely accepted two-phase kinematic model for the formation of the Gulf of California holds that, ~300 km of dextral shear between the Pacific and North America plates occurred along faults west of Baja California prior to the onset of dextral-transtensional shearing in the Gulf at ~6 Ma. We measured over 1000 detrital zircon U-Pb ages in an effort to characterize the provenance of the fan, determine its source region, and constrain the cumulative dextral slip along faults offshore of southwestern Baja California. Our results indicate a western Baja source region for the fan and suggest that cumulative dextral slip along faults west of Baja was <150 km or far less than previously believed. Zircons from the fan are dominantly 115-65 Ma with subordinate 15-35 Ma grains. Excellent matches to the fan can be obtained by intermixing Magdalena shelf strata and/or adding detritus from the west-draining portion of Los Cabos block. The same cannot be accomplished with zircons from the east-draining portion of the Los Cabos block and mainland Mexico. We propose that the fan was fed by erosional denudation of the Magdalena shelf produced by the ridge-trench collision. The fan’s source shut off when faults west of Baja California began to accommodate transtensional shearing and form rift basins that captured detritus that previously reached the trench.