A MULTI-PROXY STUDY OF TOPOGRAPHY AND SEDIMENT PROVENANCE ACROSS THE EOCENE CORDILLERAN HINTERLAND

Eocene sedimentary and volcanic strata in the Cordilleran hinterland record changes in continental drainage systems, the timing of the onset of surface-lowering extension, and the thermal and geodynamic effects of Farallon slab rollback. Sedimentology, geochronology, and stable isotope paleoaltimetry show that extra-regional source terranes in what is now Idaho fed Eocene basins in eastern Nevada and western Montana across a high-elevation broad orogen. Carbonate-rich lacustrine strata of the Elko basin, present across northeastern Nevada, record prolonged Eocene continental drainage ponding to the west of the Sevier fold-thrust belt. Paleogene volcanic glass hydration waters across the Cordilleran hinterland at multiple latitudes (38-47°N) show decreasing hydrogen isotope (δD_glass) values eastward from the paleo-coast, suggesting higher than modern hinterland elevations. Detrital zircon U-Pb-He double dating indicates that the eastern Nevada Elko basin received sediment from the Challis volcanic field, as well as both local basement and back-arc plutonic sources. Challis volcanic zircon grain ages demonstrate 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. The internally drained fluvial-lacustrine Elko basin expanded in the middle Eocene, and lacustrine strata record a middle to late Eocene transition from overfilled to balanced-fill lake conditions, as documented by high-precision ⁴⁰Ar/³⁹Ar geochronology. Late Eocene and Oligocene provenance data from multiple basins west of the fold-thrust belt document significant sediment input from proximal volcanics, increased erosion of local basement sources, and isolation from extra-regional Challis volcanics. While slab rollback likely rearranged drainage networks across the Cordillera, δD_glass values show that surface uplift did not exceed 500-900 m.