DATING DEFORMATION WITH TITANITE: AN EXAMPLE FROM CRETACEOUS SHEAR ZONES IN JOSHUA TREE NATIONAL PARK

The ability to directly date ductile deformation is significant for understanding numerous tectonic processes, including the spatiotemporal evolution of shear zones, the development of plate boundaries, and the feedbacks between brittle and crystal-plastic deformation. Titanite U-Pb geochronology is one of the best potential tools to date ductile deformation, as titanite is prevalent in crustal rocks, has a high closure temperature, responds to crystal-plastic deformation, and develops deformation microstructures. We present an integrated titanite U-Pb microstructural, geochronology, and geochemical dataset from Cretaceous shear zones in the Mojave arc segment of Joshua Tree National Park that record the timing of deformation. These cm- to 10s of cm-wide, anastomosing ductile shear zones are developed within the Late Jurassic (~155 Ma) Cottonwood Springs pluton and were active at ~75 Ma, as constrained by the crystallization age of apparently synkinematic aplitic dikes. Whereas Laramide contraction in the Cordilleran foreland was characterized by basement-involved thrusting and sparse magmatism, coeval deformation in eastern California overlapped with voluminous plutonism and may have involved contraction, extension, and transcurrent motion.

Titanite U-Pb dates from mylonites developed in the Jurassic Cottonwood Springs pluton range from ~150 to 70 Ma, spanning the crystallization age of the pluton and the inferred deformation age, respectively. The titanite textures in the mylonites include large, euhedral grains and polygonal aggregates with triple junction grain boundaries. Dates from the polygonal aggregates yield a single population (MSWD ~1) at ~73 Ma, suggesting that these titanite record dynamic recrystallization during Cretaceous shearing. Together, these data reveal the potential for microstructurally informed titanite U-Pb dates to reveal the timing of ductile deformation, particularly in plutonic shear zones.