Crooked Ridge and White Mesa in northeastern Arizona (southwestern United States) preserve, as inverted topography, a 57-km-long abandoned alluvial system near the present drainage divide between the Colorado, San Juan, and Little Colorado Rivers. The pathway of this paleoriver, flowing southwest toward eastern Grand Canyon, has led to provocative alternative models for its potential importance in carving Grand Canyon. The ~50-m-thick White Mesa alluvium is the only datable record of this paleoriver system. We present new ⁴⁰Ar/³⁹Ar sanidine dating that confirms a ca. 2 Ma maximum depositional age for White Mesa alluvium, supported by a large mode (n = 42) of dates from 2.06 to 1.76 Ma. Older grain modes show abundant 37–23 Ma grains mostly derived ultimately from the San Juan Mountains, as is also documented by rare volcanic and basement pebbles in the White Mesa alluvium. A tuff with an age of 1.07 ± 0.05 Ma is inset below, and hence provides a younger age bracket for the White Mesa alluvium. Newly dated remnant deposits on Black Mesa contain similar 37–23 Ma grains and exotic pebbles, plus a large mode (n = 71) of 9.052 ± 0.003 Ma sanidine. These deposits could be part of the White Mesa alluvium without any Pleistocene grains, but new detrital sanidine data from the upper Bidahochi Formation near Ganado, Arizona, have similar maximum depositional ages of 11.0–6.1 Ma and show similar 40–20 Ma San Juan Mountains–derived sanidine. Thus, we tentatively interpret the <9 Ma Black Mesa deposit to be a remnant of an 11–6 Ma Bidahochi alluvial system derived from the now-eroded southwestern fringe of the San Juan Mountains. This alluvial fringe is the probable source for reworking of 40–20 Ma detrital sanidine and exotic clasts into Oligocene Chuska Sandstone, Miocene Bidahochi Formation, and ultimately into the <2 Ma White Mesa alluvium. The <2 Ma age of the White Mesa alluvium does not support models that the Crooked Ridge paleoriver originated as a late Oligocene to Miocene San Juan River that ultimately carved across the Kaibab uplift. Instead, we interpret the Crooked Ridge paleoriver as a 1.9–1.1 Ma tributary to the Little Colorado River, analogous to modern-day Moenkopi Wash. We reject the “young sediment in old paleovalley” hypothesis based on mapping, stratigraphic, and geomorphic constraints. Deep exhumation and beheading by tributaries of the San Juan and Colorado Rivers caused the Crooked Ridge paleotributary to be abandoned between 1.9 and 1.1 Ma. Thermochronologic data also provide no evidence for, and pose substantial difficulties with, the hypothesis for an earlier (Oligocene–Miocene) Colorado–San Juan paleoriver system that flowed along the Crooked Ridge pathway and carved across the Kaibab uplift.

The Ruby Mountains–East Humboldt Range–Wood Hills–Pequop Mountains (REWP) metamorphic core complex, northeast Nevada, exposes a record of Mesozoic contraction and Cenozoic extension in the hinterland of the North American Cordillera. The timing, magnitude, and style of crustal thickening and succeeding crustal thinning have long been debated. The Pequop Mountains, comprising Neoproterozoic through Triassic strata, are the least deformed part of this composite metamorphic core complex, compared to the migmatitic and mylonitized ranges to the west, and provide the clearest field relationships for the Mesozoic–Cenozoic tectonic evolution. New field, structural, geochronologic, and thermochronological observations based on 1:24,000-scale geologic mapping of the northern Pequop Mountains provide insights into the multi-stage tectonic history of the REWP. Polyphase cooling and reheating of the middle-upper crust was tracked over the range of <100 °C to 450 °C via novel ⁴⁰Ar/³⁹Ar multi-diffusion domain modeling of muscovite and K-feldspar and apatite fission-track dating. Important new observations and interpretations include: (1) crosscutting field relationships show that most of the contractional deformation in this region occurred just prior to, or during, the Middle-Late Jurassic Elko orogeny (ca. 170–157 Ma), with negligible Cretaceous shortening; (2) temperature-depth data rule out deep burial of Paleozoic stratigraphy, thus refuting models that incorporate large cryptic overthrust sheets; (3) Jurassic, Cretaceous, and Eocene intrusions and associated thermal pulses metamorphosed the lower Paleozoic–Proterozoic rocks, and various thermochronometers record conductive cooling near original stratigraphic depths; (4) east-draining paleovalleys with ~1–1.5 km relief incised the region before ca. 41 Ma and were filled by 41–39.5 Ma volcanic rocks; and (5) low-angle normal faulting initiated after the Eocene, possibly as early as the late Oligocene, although basin-generating extension from high-angle normal faulting began in the middle Miocene. Observed Jurassic shortening is coeval with structures in the Luning-Fencemaker thrust belt to the west, and other strain documented across central-east Nevada and Utah, suggesting ~100 km Middle-Late Jurassic shortening across the Sierra Nevada retroarc. This phase of deformation correlates with terrane accretion in the Sierran forearc, increased North American–Farallon convergence rates, and enhanced Jurassic Sierran arc magmatism. Although spatially variable, the Cordilleran hinterland and the high plateau that developed across it (i.e., the hypothesized Nevadaplano) involved a dynamic pulsed evolution with significant phases of both Middle-Late Jurassic and Late Cretaceous contractional deformation. Collapse long postdated all of this contraction. This complex geologic history set the stage for the Carlin-type gold deposit at Long Canyon, located along the eastern flank of the Pequop Mountains, and may provide important clues for future exploration.

Columbia River province magmatism is now known to include abundant and widespread rhyolite centers even though the view that the earliest rhyolites erupted from the McDermitt Caldera and other nearby volcanic fields along the Oregon–Nevada state border has persisted. Our study covers little-studied or unknown rhyolite occurrences in eastern Oregon that show a much wider distribution of older centers. With our new data on distribution of rhyolite centers and ages along with literature data, we consider rhyolites spanning from 17.5 to 14.5 Ma of eastern Oregon, northern Nevada, and western Idaho to be a direct response to flood basalts of the Columbia River Basalt Group (CRBG) and collectively categorize them as Columbia River Rhyolites. The age distribution patterns of Columbia River Rhyolites have implications for the arrival, location, and dispersion of flood basalt magmas in the crust. We consider the period from 17.5 to 16.4 Ma to be the waxing phase of rhyolite activity and the period from 15.3 to 14.5 Ma to be the waning phase. The largest number of centers was active between 16.3–15.4 Ma. The existence of crustal CRBG magma reservoirs beneath rhyolites seems inevitable, and hence, rhyolites suggest the following. The locations of centers of the waxing phase imply the arrival of CRBG magmas across the distribution area of rhyolites and are thought to correspond to the thermal pulses of arriving Picture Gorge Basalt and Picture-Gorge-Basalt-like magmas of the Imnaha Basalt in the north and to those of Steens Basalt magmas in the south. The earlier main rhyolite activity phase corresponds with Grande Ronde Basalt and evolved Picture Gorge Basalt and Steens Basalt. The later main phase rhyolite activity slightly postdated these basalts but is contemporaneous with icelanditic magmas that evolved from flood basalts. Similarly, centers of the waning phase span the area distribution of earlier phases and are similarly contemporaneous with icelanditic magmas and with other local basalts. These data have a number of implications for long-held notions about flood basalt migration through time and the age-progressive Snake River Plain Yellowstone rhyolite trend. There is no age progression in rhyolite activity from south-to-north, and this places doubt on the postulated south-to-north progression in basalt activity, at least for main-phase CRBG lavas. Furthermore, we suggest that age-progressive rhyolite activity of the Snake River Plain–Yellowstone trend starts at ~12 Ma with activity at the Bruneau Jarbidge center, and early centers along the Oregon–Nevada border, such as McDermitt, belong to the early to main phase rhyolites identified here.

It has long been debated whether non-avian dinosaurs went extinct abruptly or gradually at the end-Cretaceous (66 million years ago), because their fossil record at this time is mostly limited to northern North America. We constrain a dinosaur-rich unit to the south, the Naashoibito Member in New Mexico, to the very latest Cretaceous (~66.4 to 66.0 million years), preserving some of the last-known non-avian dinosaurs. Ecological modeling shows that North American terrestrial vertebrates maintained high diversity and endemism in the latest Cretaceous and early Paleogene, with bioprovinces shaped by temperature and geography. This counters the notion of a low-diversity cross-continental fauna and suggests that dinosaurs were diverse and partitioned into regionally distinct assemblages during the final few hundred thousand years before the end-Cretaceous asteroid impact.