The Paleogene Evolution of the Cordilleran Hinterland

Abstract:

Paleogene terrestrial and volcanic strata in the Cordilleran hinterland record changes in continental drainage, the dynamics of Farallon slab rollback, and the timing of the onset of surface-lowering extension. Sedimentology, geochronology, and stable isotope paleoaltimetry show that from the Eocene to the Oligocene, a high (up to 3.5 km), broad orogen stretched across Nevada, with a distinct crest that divided a continuous westward-draining slope extending to central California from an internally drained orogenic plateau in the east. Carbonate-rich lacustrine strata present across northeastern Nevada record prolonged Eocene continental drainage ponding and the transition from overfilled to balanced-fill lake conditions over time. Paleogene volcanic glass hydration waters across the Cordilleran hinterland at multiple latitudes show decreasing hydrogen isotope composition (δD_glass) values eastward from the paleo-coast, suggesting higher than modern elevations in the hinterland. Detrital zircon U-Pb-He double dating indicates that the eastern Nevada Elko basin received sediment from both local basement and back-arc plutonic sources, and from the Challis volcanic field, demonstrating that Eocene hinterland lakes in Nevada were fed in part by a south-flowing drainage from a higher elevation terrain in central Idaho.

From the Eocene into the Oligocene, sediment accumulation, volcanism, and unconformity migrated from northeast to southwest across Nevada, possibly in response to the migration of a regional topographic high caused by broad dynamic uplift and magmatism following Farallon slab rollback. Although rollback rearranged drainage networks across the Cordillera, δD_glass values show that surface uplift did not exceed 500-900 m. Oligocene provenance data document significant sediment input from proximal volcanics, increased erosion of local basement sources, and isolation from extra-regional volcanics. Despite the long-term high gravitational potential energy of the crust supporting high hinterland topography, surface-lowering extension did not occur until the transition to a transform margin changed the external kinematic framework of the Cordillera in the late Oligocene.