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1 Departamento de Geologia, Centro de Investigacion Cientifica y de Educacion Superior de Ensenada, Ensenada, Baja California, Mexico
2 Department of Earth and Space Sciences, 3806 Geology Building, University of California, Los Angeles, California 90095-1567, USA
3 Department of Geological Sciences, 5500 Campanile Drive, San Diego State University, San Diego, California 92182-1020, USA
4 Department of Earth and Space Sciences, 3806 Geology Building, University of California, Los Angeles, California 90095-1567, USA
5 Department of Geosciences, Gould-Simpson Building #77, 1040 E 4th Street, University of Arizona, Tucson, Arizona 85721, USA
The Magdalena fan is an apparently beheaded submarine depocenter that has figured prominently in reconstructions of middle to late Miocene Pacific–North American plate interactions. The deposit accumulated rapidly at the base of the continental slope on top of newly formed oceanic crust of the Magdalena microplate from 14.5 to 13 Ma. Subduction of this crust ceased as the Pacific-Magdalena 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 American plates occurred along faults west of Baja California prior to the onset of dextral-transtensional shearing in the gulf ca. 6 Ma. We measured 1796 detrital zircon U-Pb ages from 65 samples in an effort to characterize the provenance of the fan, determine its source region, and define the cumulative dextral slip along faults offshore of southwestern Baja California. Zircons from the fan are dominantly 120–65 Ma with subordinate 15–35 Ma grains. Excellent matches to the fan can be obtained by mixing Magdalena shelf strata and/or adding detritus from the west-draining portion of the Los Cabos block. The same cannot be accomplished with zircons from the east-draining portion of the Los Cabos block and mainland Mexico. Our results favor a western Baja source region for the fan and suggest that cumulative dextral slip along faults west of Baja was <150 km, much less than previously believed. We propose that the fan was fed by erosional denudation of the Magdalena shelf produced by increased mantle buoyancy due to the ridge-trench juxtaposition. The fan's source was cut off when faults west of Baja California began to accommodate transtensional shearing and form rift basins that captured detritus that previously reached the trench.
Key Words: Magdalena fan • detrital zircon • U-Pb • Baja California • Mexico • Integrated Ocean Drilling Program • Gulf of California • Neogene
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