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We determine the paleoelevation of the northern Sierra Nevada (California) in the Oligocene based on hydrogen stable isotope compositions of meteoric water preserved within volcanic glass from ignimbrites sampled across the range. A 48‰ decrease in the isotopic composition of hydrated glass from ignimbrites located near paleosea level to ignimbrites 100 km to the east refl ects the effect of ancient high topography on precipitation. These data show that 31-28 Ma ago, the northern Sierra Nevada had a steep western gradient and elevations similar to the present. This study, placed in the context of other paleoaltimetry studies, suggests that the range was a high topographic feature throughout the Cenozoic and that the majority of uplift occurred in the Late Cretaceous to early Cenozoic, much earlier than some studies have proposed.

Records of past topography connect Earth's deep interior to the surface, reflecting the distribution of heat and mass, past crustal structure, and plate interactions. Many tectonic reconstructions of the North American Cordillera suggest the presence of an Altiplano-like plateau in the location of the modern Basin and Range, with conflicting timing and mechanisms for the onset of surface-lowering extension and orogen collapse. Here we show, through a paleotopographic profile, that from the Eocene to the Oligocene a high, broad orogen stretched across Nevada, with a distinct crest that divided a continuous westward-draining slope extending to central California from an internally drained eastern Nevada plateau. This paleo-orogen maintained demonstrably higher-than-modern elevations, reaching 3500 m in the late Oligocene. Despite the long-term high gravitational potential energy of the crust supporting this topography, surfacelowering extension did not occur until the transition to a transform margin changed the external kinematic framework of the system. Maximum surface lowering was spatially decoupled from brittle upper crustal extension, requiring a large component of mid-crustal flow.

Slab rollback processes alter the intraplate force balance and buoyancy of the overriding plate, driving surface uplift or extension. From ca. 55-24 Ma, Farallon slab rollback produced migrating volcanism and sedimentation across the western United States as stress on the North American plate transitioned from subduction-driven compression to widespread extension. Hypotheses regarding rollback-driven surface deformation differ widely in timing and magnitude. Here we combine hydrogen isotope ratios with high-resolution geochronology to show that a high-elevation plateau extended westward from the Sevier-Laramide fold-thrust belt across Utah and eastern Nevada prior to slab rollback. Quantitative paleoelevation estimates show that this plateau had obtained over 80% of peak paleoelevations by middle Eocene. Slab rollback, heating, and lithospheric delamination generated 400-600 m of Oligocene surface uplift. Concurrent extension limited overall uplift and rollback-induced mantle flow likely contributed to the propagation of upper crustal extension that formed the Basin and Range province.Plain Language Summary Changes in surface elevations are critical to understanding the formation and evolution of mountains and basins across ancient landscapes. Slab rollback-retreat and steepening of the downgoing tectonic plate during subduction-can transform Earth's surface by altering the stresses on the overriding plate. By measuring the ratios of hydrogen isotopes in ancient rainwater, preserved for millions of years in volcanic ash, we can estimate past elevations. Here we measure past elevations across the Basin and Range province of Utah and Nevada to reconstruct topography before and during slab rollback. Results show that a high elevation plateau extended westward from the western edge of the Rocky Mountains across Utah and eastern Nevada prior to slab rollback. Slab rollback generated a shortlived interval of 400-600 m of surface uplift, much less than has been previously estimated. Instead, subsequent processes resulting from rollback likely contributed to collapse of the high plateau and the widespread extensional faulting that created the characteristic topography of the modern Basin and Range province.

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Debate surrounds the origin, uplift, and evolution of the northern Sierra Nevada and western Basin and Range. The studies presented here integrate different scales of observation, from local paleovalley morphology, estimation of local slopes, and braided stream alluvial architecture, to regional assessments of sediment and volcanic provenance and paleoelevations across the proposed ancestral Sierra Nevada-Nevadaplano to gain a better understanding of early Cenozoic topography, morphology, and landscape evolution of the region, and to assess the possible tectonic and climatic drivers for that evolution. Results from sedimentologic analysis of Eocene fl uvial deposits show diachronous, localized paleovalley incision and braided stream aggradation in a system infl uenced by Eocene climate, eustasy, and Laramide tectonism, and suggest that previous estimates of the timing and amount of range uplift based on paleochannel gradients may be invalid. Overlying Oligocene ignimbrites deposited in the Sierra Nevada were geochemically and geochronologically correlated to sources in central Nevada, and results from this work show that ignimbrites traveled over 200 km from their source calderas across what is now the crest of the Sierra Nevada, and that in the Oligocene, no drainage divide existed between Nevada source calderas and sample locations 200 km west. Hydrated volcanic glass from these units was used as a proxy for the isotopic composition (δD) of Oligocene meteoric water, which refl ects the effect of ancient topography on precipitation. δD decreases from west to east across the Sierra Nevada by ~48‰, which is similar to the isotopic gradient of precipitation over the area today. δD across Nevada decreases at a signifi cantly lower gradient, refl ecting a signifi cant reduction in the rate of increase of paleoelevation with distance, and may refl ect a gradual increase in mean elevation from west to east or partially closed system hydrology. This multidisciplinary approach provides both a detailed reconstruction of the evolution of the ancestral Sierra Nevada drainage system from Eocene to Oligocene time and multiple lines of evidence to support the conclusion that the northern Sierra Nevada likely acted as the steep western fl ank of a gradually sloping high-elevation plateau ("Nevadaplano") in the Oligocene. Miocene to Holocene extension lowered elevations across what is now the Basin and Range, possibly associated with gravitational spreading of overthickened, magmatically and radiogenically heated crust.

Assessing temporal relationships between foreland and hinterland deformation in foldthrust belts is critical to understanding the dynamics of orogenic systems. In the western U.S. Cordillera, the central Nevada thrust belt (CNTB) has been interpreted as a hinterland component of the Sevier fold-thrust belt inUtah. However, imprecise timing constraints on CNTB deformation have hindered evaluation of space-time patterns of strain partitioning between these two thrust systems. To address this problem, new 1:24,000-scale geologic mapping and balanced cross sections are presented through the CNTB near Eureka, Nevada, in conjunction with industry drill-hole data, conodont age determinations, and 40 Ar/ 39 Ar and U-Pb ages from volcanic, intrusive, and sedimentary rocks.Our mapping redefi nes the fi rst-order structures and deformation geometry of the CNTB at the latitude of Eureka. Contractional structures include two north-striking, east-vergent thrust faults, the Prospect Mountain thrust and Moritz-Nager thrust, which are connected as the same fault in cross section, several north-striking map-scale folds, and a Cambrian over Silurian relationship observed in multiple drill holes, corresponding to repetition of ~2-2.5 km of stratigraphy, that defi nes the blind Ratto Canyon thrust. Two distinct sets of normal faults cut the contractional structures, and are overlapped by a regional late Eocene (ca. 37 Ma) subvolcanic unconformity. Retrodeformation of both sets of normal faults reveals the existence of the Eureka culmination, a 20-km-wide, 4.5-kmtall anticline with limb dips of 25°-35°, that can be traced for ~100 km north-south on the basis of Paleogene erosion levels. The culmination is interpreted as a fault-bend fold that formed from ~9 km of eastward displacement of the Ratto Canyon thrust sheet over a buried footwall ramp.The type exposure of the Early Cretaceous (Aptian) Newark Canyon Formation (NCF) is preserved on top of Mississippian, Pennsylvanian, and Permian rocks on the eastern limb of the Eureka culmination. We propose that the NCF was deposited in a piggy back basin on the eastern limb of the culmination as it grew, which is consistent with published east-directed paleocurrents and provenance data suggesting derivation from proximal late Paleozoic subcrop units. Syncontractional deposition of the NCF is used to defi ne the probable Aptian construction of the Eureka culmination and associated slip on the Ratto Canyon thrust at depth. After deposition, the NCF continued to be folded during late-stage growth of the culmination.Aptian deformation in the CNTB at Eureka postdated migration of the Sevier thrust front into Utah by at least ~10 m.y. and possibly as much as ~30 m.y., and therefore represents out-of-sequence hinterland deformation. CNTB deformation was coeval with emplacement of the Canyon Range thrust sheet in the type-Sevier thrust belt in western Utah, and may represent internal shortening of this orogen-scale thrust sheet that acted to promote further eastward translation.