Structural Geology and Geochronology of Cordilleran Forearc Assemblages, Baja California and British Columbia

Abstract

Subduction complexes and ophiolites are primary geologic records of orogenesis, fromsubduction initiation to continental collision to orogenic collapse. However, the subducted rock record is often variably incomplete along convergent margins due to local removal of material from the upper plate via subduction erosion, periods of no net-accretion due to lack of sedimentation in the trench, strike-slip displacement of forearc material, and/or relatively shallow structural levels of exposure. In this dissertation, I present new findings from field and geochronologic investigations of subduction complex rocks conducted along two less-well studied portions of the Cordilleran margin in Baja California and southern British Columbia. Through new geologic mapping and field observations, I show that the structural architecture of these two regions are broadly similar and defined by low-grade to non-metamorphosed upper plates composed of arc/ophiolitic basement and overlying arc-derived marine siliciclastic forearc basin strata structurally overlying blueschist-bearing subduction complexes along variably attenuated but regionally extensive serpentinite-matrix mélange shear zones. 40Ar/39Ar, garnet Lu-Hf, and zircon U-Pb geochronology provide new age constraints for the accretion, metamorphism, and exhumation of the forearc assemblages in response to regional plate kinematic shifts and terrane accretion. These findings provide new challenges to current ideas around subduction initiation in the Cordillera and new opportunities for evaluating the geologic history and the thermomechanical evolution of Cordilleran subduction systems margin back through time.