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40Ar/39Ar detrital sanidine (DS) dating of river terraces provides new insights into the evolution and bedrock incision history of the San Juan River, a major tributary of the Colorado River, USA, at the million-year time scale. We dated terrace flights from the San Juan−Colorado River confluence to the San Juan Rocky Mountains. We report >5700 40Ar/ 39Ar dates on single DS grains from axial river facies within several meters above the straths of 30 individual terraces; these yielded ∼2.5% young (<2 Ma) grains that constrain maximum depositional ages (MDAs) and minimum incision rates. The most common young grains were from known caldera eruptions: 0.63 Ma grains derived from the Yellowstone Lava Creek B eruption, and 1.23 Ma and 1.62 Ma grains derived from two Jemez Mountains eruptions in New Mexico. Agreement of a DS-derived MDA age with a refined cosmogenic burial age from Bluff, Utah, indicates that the DS MDA closely approximates the true depositional age in some cases. In a given reach, terraces with ca. 0.6 Ma grains are commonly about half as high above the river as those with ca. 1.2 Ma grains, suggesting that the formation of the terrace flights likely tracks near-steady bedrock incision over the past 1.2 Ma. Longitudinal profile analysis of the San Juan River system shows variation in area-normalized along-stream gradients: a steeper (ksn = 150) reach near the confluence with the Colorado River, a shallower gradient (ksn = 70) in the central Colorado Plateau, and steeper (ksn = 150) channels in the upper Animas River basin. These reaches all show steady bedrock incision, but rates vary by >100 m/Ma, with 247 m/Ma at the San Juan−Colorado River confluence, 120−164 m/Ma across the core of the Colorado Plateau, and 263 m/Ma in the upper Animas River area of the San Juan Mountains. The combined dataset suggests that the San Juan River system is actively adjusting to base-level fall at the Colorado River confluence and to the uplift of the San Juan Mountains headwaters relative to the core of the Colorado Plateau. These fluvial adjustments are attributed to ongoing mantle-driven differential epeirogenic uplift that is shaping the San Juan River system as well as rivers and landscapes elsewhere in the western United States. 

The Cenozoic Colorado Plateau physiographic province overlies multiple Precambrian provinces. Its ∼2-km elevation rim surrounds an ∼1.6-km elevation core that is underlain by thicker crust and lithospheric mantle, with a sharp structural transition ∼100 km concentrically inboard of the physiographic boundary on all but its northeastern margin. The region was uplifted in three episodes: ∼70–50 Ma uplift above sea level driven by flat-slab subduction; ∼38–23 Ma uplift associated with voluminous regional magmatism and slab removal, and less than 20 Ma uplift associated with inboard propagation of basaltic magmatism that tracked convective erosion of the lithospheric core. Neogene uplift helped integrate the Colorado River from the Rockies at 11 Ma to the Gulf of California by ∼5 Ma. The sharp rim-to-core transition defined by geological and geophysical data sets suggests a young transient plateau that is uplifting as it shrinks to merge with surrounding regions of postorogenic extension. ▪ The Colorado Plateau's iconic landscapes were shaped during its 70-million-year, still-enigmatic, tectonic evolution characterized by uplift and erosion. ▪ Uplift of the Colorado Plateau from sea level took place in three episodes, the youngest of which has been ongoing for the past 20 million years. ▪ Tectonism across the Colorado Plateau's nearest plate margin (the base of the plate!) is driving uplift and volcanism and enhancing its rugged landscapes. ▪ The bowl-shaped Colorado Plateau province is defined by ongoing uplift and an inboard sweep of magmatism around its margins. ▪ The keel of the Colorado Plateau is being thinned as the North American plate moves southwest through the underlying asthenosphere. 

Abstract 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.