ABSTRACT The paleogeographic evolution of the western U.S. Great Basin from the Late Cretaceous to the Cenozoic is critical to understanding how the North American Cordillera at this latitude transitioned from Mesozoic shortening to Cenozoic extension. According to a widely applied model, Cenozoic extension was driven by collapse of elevated crust supported by crustal thicknesses that were potentially double the present ~30–35 km. This model is difficult to reconcile with more recent estimates of moderate regional extension (≤50%) and the discovery that most high-angle, Basin and Range faults slipped rapidly ca. 17 Ma, tens of millions of years after crustal thickening occurred. Here, we integrated new and existing geochronology and geologic mapping in the Elko area of northeast Nevada, one of the few places in the Great Basin with substantial exposures of Paleogene strata. We improved the age control for strata that have been targeted for studies of regional paleoelevation and paleoclimate across this critical time span. In addition, a regional compilation of the ages of material within a network of middle Cenozoic paleodrainages that developed across the Great Basin shows that the age of basal paleovalley fill decreases southward roughly synchronous with voluminous ignimbrite flareup volcanism that swept south across the region ca. 45–20 Ma. Integrating these data sets with the regional record of faulting, sedimentation, erosion, and magmatism, we suggest that volcanism was accompanied by an elevation increase that disrupted drainage systems and shifted the continental divide east into central Nevada from its Late Cretaceous location along the Sierra Nevada arc. The north-south Eocene–Oligocene drainage divide defined by mapping of paleovalleys may thus have evolved as a dynamic feature that propagated southward with magmatism. Despite some local faulting, the northern Great Basin became a vast, elevated volcanic tableland that persisted until dissection by Basin and Range faulting that began ca. 21–17 Ma. Based on this more detailed geologic framework, it is unlikely that Basin and Range extension was driven by Cretaceous crustal overthickening; rather, preexisting crustal structure was just one of several factors that that led to Basin and Range faulting after ca. 17 Ma—in addition to thermal weakening of the crust associated with Cenozoic magmatism, thermally supported elevation, and changing boundary conditions. Because these causal factors evolved long after crustal thickening ended, during final removal and fragmentation of the shallowly subducting Farallon slab, they are compatible with normal-thickness (~45–50 km) crust beneath the Great Basin prior to extension and do not require development of a strongly elevated, Altiplano-like region during Mesozoic shortening.

ABSTRACT Strata preserved within the Sevier foreland basin of North America contain a suite of lithologic variations influenced by hinterland tectonic processes. Using U-Pb detrital zircon geochronology, we compared provenance signals of Upper Jurassic and Lower Cretaceous strata from a west-to-east, foredeep-to-forebulge-to-backbulge depozone transect across the state of Wyoming and evaluated major tectonic mechanisms operating during the early evolution of the Sevier orogeny. Our data included new and compiled U-Pb detrital zircon ages ( n = 6013) from 50 localities that were integrated into a revised chronostratigraphic framework and subsidence history for the basin. At the onset of the Sevier orogeny, we found evidence for uplift and erosion of early Mesozoic and late Paleozoic strata within the nascent Sevier fold-and-thrust belt. This event occurred prior to the Aptian Stage of the Early Cretaceous and is recorded by the coordinated progradation of coarse-grained fluvial systems across the overfilled foreland basin. Continued emplacement of thrust loads in the hinterland generated accommodation in excess of sediment supply, a condition likely exacerbated by a relative reduction of siliciclastic sediment supply due to greater unroofing of Paleozoic carbonates during the Aptian and Albian Stages of the Early Cretaceous. This led to an underfilled condition characterized by widespread calcareous lacustrine deposition across much of the foredeep depozone and condensed stratigraphic intervals in the forebulge and backbulge depozones. During the late Albian–earliest Cenomanian, fluvial systems sourced in the Appalachians invaded the foreland basin from the east, followed by the rapid incursion of the Western Interior Seaway, driven by accelerated thrust emplacement, flexural subsidence, and potentially the onset of dynamic subsidence.

ABSTRACT The Willard-Paris-Meade thrust sheet is the oldest and westernmost sheet to develop in the Wyoming salient of the Sevier fold-and-thrust belt. This dominant thrust sheet was transported ~60–70 km eastward, but despite its importance to the overall shortening budget of the Cordilleran orogenic system, the timing for onset and cessation of major fault slip is poorly constrained. Synorogenic strata associated with emplacement of the sheet thicken westward, reaching a total thickness >2 km near the thrust front, and are divided into five intervals: (1) lower and (2) upper parts of the Aptian(?) to Albian Gannett Group (Ephraim and Bechler Formations); (3) Upper Albian Smiths, Thomas Fork, and Cokeville Formations and correlative Bear River Formation; (4) uppermost Albian to Cenomanian Sage Junction Formation and partly correlative Aspen Shale; and (5) Upper Cenomanian to Turonian Frontier Formation. Integrated sedimentologic, detrital zircon provenance, and subsidence analyses of these foreland basin strata record a protracted history of thrust sheet emplacement. Detrital zircon U-Pb age spectra, conglomerate clast types, and sandstone petrography record an unroofing sequence, with progressive erosion of Lower Mesozoic to Upper Paleozoic to Lower Paleozoic bedrock levels, starting in the more western Paris sheet and continuing across the Meade and Willard sheets. Synorogenic strata also contain young grains derived from the Cordilleran magmatic arc that bracket maximum depositional ages of: 115.2 ± 1.8 Ma and 116.2 ± 2.3 Ma in the upper Ephraim Formation to 107.9 ± 2.4 Ma in the overlying Bechler Formation, which were deposited during early thrust slip; 101.6 ± 0.7 Ma, 101.4 ± 0.6 Ma, and 98.8 ± 0.4 Ma in the Cokeville, Sage Junction, and Aspen Formations, respectively, which were deposited during increased subsidence and thrust slip rates; and 95.6 ± 1.4 Ma in the Frontier Formation, which was deposited during late slip, eastward thrust propagation, and early uplift of the Wasatch anticlinorium. The lower Ephraim Formation was deposited during the Early Cretaceous Cordilleran magmatic lull and lacks young zircon grains, such that timing for thrust initiation is not well constrained, ranging from ca. 130 to 120 Ma. Regional variations in stratigraphic thickness and sedimentation rates, combined with flexural modeling, indicate that a foredeep began developing in front of the Paris thrust and then migrated eastward as the Meade and Willard thrusts propagated and overrode their own synorogenic strata, with increasing rates of thrust loading and subsidence from ca. 105 Ma to 95 Ma.

The Upper Cretaceous to Paleocene(?) Canaan Peak Formation of southwestern Utah consists of approximately 100 m of cobble conglomerate and subordinate sandstone. Coarse-grained lithofacies include massive to very crudely stratified pebble to cobble conglomerate (Gm) and trough cross-stratified conglomerate (Gt). Minor associated lithofacies include trough (St) and planar (Sp) cross-stratified sandstone and scour-fill sandstone (Ss). Gravel deposition occurred during high-discharge periods within a Scott/Donjek-type perennial braided fluvial system on longitudinal bars (Gm) and sinuous-crested transverse bars (Gt), and as a product of longitudinal bar-top and interbar channel scour filling (Gt). Sand accumulated under lower flow-velocity conditions through migration of interbar channel dunes and transverse bars (St/Sp) and development of scour and fill deposits (Ss). Clast imbrication and trough-axis orientation measurements indicate east to north-east paleoflow directions. Gravel-sized clasts are predominantly resistant lithologies including Upper Precambrian-Cambrian quartzite, fossiliferous Paleozoic chert, and some silicified middle Jurassic volcanic rocks; less resistant Paleozoic limestone clasts are only locally abundant. Canaan Peak Formation detritus was derived from erosion of highlands created by Cretaceous Sevier-style thrust-fault development to the west in southeastern Nevada and western Utah and distributed across an extensive gravel-dominated braidplain complex. However, it is not possible to determine whether Canaan Peak deposition records active uplift or is postorogenic. Sediment was transported eastward a minimum distance of 45 to 60 km and was subject to continuous high-energy reworking within the Canaan Peak fluvial system, resulting in destruction of all but the most stable clast lithologies.