TECTONICALLY DRIVEN CHANGES IN PALEOTOPOGRAPHY AND DRAINAGE SYSTEM EVOLUTION IN THE CORDILLERAN HINTERLAND, WESTERN US

Geodynamic reconstructions of orogens depend on the spatial distribution and stability of paleotopography, which controls drainage system and sediment transport patterns. Here we integrate detrital zircon U-Pb provenance data with sedimentology and stable isotope paleoaltimetry to create a high-resolution synoptic reconstruction of the paleodrainages and paleotopography of the Cordilleran hinterland from ~45 to 20 Ma, during the transition from Laramide flat-slab subduction into extensional tectonics. In eastern Nevada, thick packages of gravel-bed braided river conglomerates are overlain by ~1000 m of deep lacustrine laminates, indicating increased accommodation and ponding of continental drainages in the Eocene. In central-western Nevada, much thinner early Oligocene conglomerates and sands, deposited by alluvial fans and gravel-bed braided rivers, are locally overlain by 5-20 m of freshwater lacustrine strata. Major grain age populations reflect a change in dominant sediment sources and a shift in the locus of deposition, coincident with the southwestward sweep of volcanism. Middle-late Eocene rivers flowed over 300 km west of the drainage divide to the paleoshoreline, while those east of the divide drained into a periodically closed intermontane lake system. Northeastern Nevada U-Pb provenance data show the dominance of multiple local and extra-regional volcanic sources and provide new maximum depositional ages. Central Nevada provenance data show increased erosion of basement terranes and isolation from extra-regional volcanics. Migrating regional construction of topographic barriers resulting from broad dynamic uplift associated with slab delamination likely drove basin formation and the migration of deposition. δD of volcanic glass indicate high elevations across what is now northern and central Nevada, reaching 3500 ⁺⁸⁰⁰/_-300 m by the late Oligocene, with a comparatively gradual western slope (0.8%-1.5% grade). This high topography was sustained from at least 45-23 Ma. The onset of extension and orogen collapse, facilitated by internal heat production and stored gravitational potential energy within the crust, was likely triggered by a change in the external kinematic framework – the delamination of the Farallon slab, which coupled the Sierra Nevada to the Colorado Plateau.