Ridge-trench interactions and the 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

Abstract

ABSTRACTThe Magdalena fan is an apparently beheaded submarine depocenter that has fi g-ured prominently in reconstructions of mid-dle to late Miocene Pacifi c–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 Pacifi c-Magdalena spreading center encountered the trench. The widely accepted two-phase kine-matic model for the formation of the Gulf of California holds that ~300 km of dextral shear between the Pacifi c and North Ameri-can plates occurred along faults west of Baja California prior to the onset of dextral-trans-tensional shearing in the gulf ca. 6 Ma. We measured 1796 detrital zircon U-Pb ages from 65 samples in an effort to character-ize the provenance of the fan, determine its source region, and defi ne the cumulative dextral slip along faults offshore of south-western Baja California. Zircons from the fan are dominantly 120–65 Ma with subor-dinate 15–35 Ma grains. Excellent matches to the fan can be obtained by mixing Magda-lena shelf strata and/or adding detritus from the west-draining portion of the Los Cabos block. The same cannot be accomplished with zir cons from the east-draining portion of the Los Cabos block and mainland Mex-ico. Our results favor a western Baja source region for the fan and suggest that cumula-tive 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 Califor-nia began to accommodate transtensional shearing and form rift basins that captured detritus that previously reached the trench.Keywords: