SLAB ROLLBACK AS A DRIVER OF THE EARLIEST EPISODE OF DISTRIBUTED EXTENSION IN THE NEVADAPLANO: A CASE STUDY FROM THE OLIGOCENE-EARLY MIOCENE GRANT RANGE DETACHMENT SYSTEM IN EASTERN NEVADA

In Nevada and Utah, the Cordilleran orogen underwent a protracted Cenozoic transition to an extensional setting. However, the geodynamic processes that controlled this transition are poorly understood, in part because the space-time patterns of extension are not known in many areas. Localities of pre-Neogene extension have the potential to elucidate the dynamics of the Cordilleran crust during the final stages of subduction. Here, we present data that constrain the timing of extension in the Grant Range in eastern Nevada, which is a core complex-like range that was deformed by a brittle, low-angle detachment fault system. We present temperature-time histories from eight granite samples exhumed by this fault system, which are constrained by muscovite ⁴⁰Ar/³⁹Ar multi-diffusion domain modeling and fission track and (U-Th)/He ages from zircon and apatite. These data demonstrate rapid cooling (20-35 °C/Myr) from 350-425 °C to 25-50 °C between ~30 Ma and ~17 Ma. The detachment system accommodated 24 km of extension (115%) and exhumed the granite samples from 7-9 km depths to the near-surface. Rapid Oligocene-early Miocene cooling is interpreted to date the duration of motion on the detachment system, and defines a long-term extension rate of 1.5-2.6 km/Myr. The Grant Range detachment system can be interpreted in the context of late Eocene-Oligocene extension of thickened Cordilleran crust during post-Laramide slab rollback, and was one of the most regionally-significant and highest-strain extensional fault systems active during this time. Upper-crustal extension, though variable in style and magnitude, was distributed across a large area of Nevada and Utah during the late Eocene-Oligocene. The timing of the Grant Range detachment system provides further support that the decrease in interplate coupling that accompanied slab rollback was likely the primary driver of the earliest episode of distributed extension within the Cordilleran plateau during the final stages of subduction. When combined with the well-documented phase of Neogene ‘Basin and Range’ extension that was triggered by Pacific-North American plate boundary reorganization, this illustrates that collapse of the Cordillera proceeded in distinct episodes that were initiated by changes in tectonic boundary conditions.