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

The Magdalena fan is an apparently beheaded submarine depocenter that has fi gured prominently in reconstructions of middle 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 kinematic model for the formation of the Gulf of California holds that ~300 km of dextral shear between the Pacifi c 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 defi ne 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.

Quantifying the forces needed for the rapid change of Pacific plate motion at 6Ma

“…The first phase from 12.3 to 6 Ma was accommodated west of Baja California with a total displacement of ca. 300-350 km (Fletcher et al, 2007). At 6.5-6.3 Ma a sudden reorganization of the PacificNorth American plateboundary took place and the residual slip was accommodated in the Gulf of California (Oskin et al, 2001;Oskin and Stock, 2003a, b).…”

“…12.3 Ma. This relative displacement was partitioned into two deformation belts on either side of Baja California and occurred in two distinct kinematic phases of plate margin shearing (Fletcher et al, 2007). The first phase from 12.3 to 6 Ma was accommodated west of Baja California with a total displacement of ca.…”

Estratigrafía de la margen occidental de la cuenca San José del Cabo, Baja California Sur

“…El origen de la cuenca ha sido relacionado con la apertura del golfo de California (McCloy, 1984; 2008; Piña-Arce, 2010), la cual inició con la apertura de un rift ortogonal transformándose en uno oblicuo (Fletcher et al, 2007).…”

“…La CSJC es parte de la Provincia Extensional del Golfo (Stock y Hodges, 1989;Martín-Barajas, 2000;Fletcher et al, 2007) y tiene una estructura de medio graben (Martínez-Gutiérrez y Sethi, 1997). Fletcher et al (2000) determinaron el inicio de la exhumación acelerada en el bloque de piso hace 12 -10 ma., marcando el inicio del fallamiento en la cuenca, el cual estos autores relacionaron con la activación de la FSJC.…”

Oblique rifting: the rule, not the exception

“…Prominent examples are the Main Ethiopian rift (Corti, 2008), the Levant rift system 10 including the Dead Sea rift (Mart et al, 2005), the Gulf of California rift (Atwater and Stock, 1998;Fletcher et al, 2007) and the Upper Rhine Graben (Bertrand et al, 2005). Structure and kinematics of past rift systems has been studied by surveying obliquely rifted margins (Fournier et al, 2004;Lizarralde et al, 2007;Klimke and Franke, 2016) and transform continental margins (Basile, 2015;Mericer de Lépinay et al, 2016).…”