Early Permian tectonic evolution of the Last Chance thrust system: An example of induced subduction initiation along a plate boundary transform

Abstract

Abstract The late Paleozoic is an important precursor stage in the development of the Mesozoic Cordilleran subduction system along the western margin of North America, but the tectonic history remains ambiguous due to complex overprinting deformation and magmatism. Determining the driving mechanism of large magnitude Permian shortening in southwest Laurentia is critical to understanding the late Paleozoic transition from transform margin to subduction zone. We investigated the driving mechanism of the Permian Last Chance thrust system in east-central California to understand this transition prior to the development of the Mesozoic Cordilleran arc. Here, we present the results of new geological mapping, detrital zircon U-Pb geochronology, and a synthesis of regional tectonics to inform a kinematic model of the Last Chance thrust system and outline the Permian–Triassic tectonic evolution of the plate boundary during induced subduction initiation. The record of subduction initiation along an inferred late Paleozoic transform fault (the California-Coahuila transform) is preserved by (1) Permian arc magmatism, (2) the onset of volcaniclastic sedimentation, and (3) the development of a regional transpressional system in present-day east-central California. The evolution of this transpressional system and subduction zone is recorded by development of the Last Chance thrust system of the Death Valley region. Geological mapping in the Last Chance Range, northern Death Valley National Park, and the Inyo Mountains reveals the east-directed Last Chance thrust system was constructed by repetitive out-of-sequence deformation consistent with transpressional strain. The Last Chance thrust system accommodated a minimum of >75 km (60%) shortening, based on cross-section restorations guided by regional stratigraphic relationships and restoration of subsequent Mesozoic deformation. Our revised model of Jurassic extensional exhumation of the Snow Lake terrane argues the Last chance thrust was not reactivated during the Mesozoic. Large-magnitude shortening along the California-Coahuila transform accommodated a significant component of the convergent plate motion as the Panthalassan crust was thrust below the continental margin before initial slab sinking. Numerical models show the forces resisting subduction are greatest before initial slab sinking takes place, and compression is transmitted in board from the plate boundary. We argue the Last Chance thrust system developed in response to this compression. Early-middle Permian plutons and late Permian detrital zircons in coeval basins suggest subduction was well established by the early Permian. Collectively, the preservation of a thrust system, early arc magmatism, and syntectonic sedimentary basins, which are features typically destroyed by subduction magmatism and deformation, allow for the evaluation of subduction initiation mechanisms based on field observations.