LARAMIDE UPLIFT OF THE SOUTHWEST COLORADO PLATEAU: GEOMORPHIC, STRATIGRAPHIC, PALEOMAGNETIC, AND (U-TH)/HE CONSTRAINTS

Laramide uplift of the SW margin of the Colorado Plateau created 1.2 km of relief eroded by NE-flowing streams that converge along the trend of the Hurricane fault. This relief is a minimum estimate of Laramide elevation, because the gradient of the master channel was reversed during mid Tertiary extension and plateau subsidence. The oldest arkosic sediments resting on the Laramide erosion surface are demonstrably younger than Cretaceous normal superchron C34 (83 Ma). The youngest preserved Laramide gravels contain nearly 50% volcanic clasts with apatite (U-Th)/He ages as young as 48 Ma. The vertical distribution of pebble types records progressive unroofing of Precambrian terranes as a decreasing ratio of quartzite to crystalline basement rocks, followed by an abrupt increase in late Cretaceous to Eocene volcanic clasts from < 5% to ~50% of total clasts. This records synorogenic Eocene volcanism with near synchronous incorporation of the resulting rocks into a continually evolving fluvial system. Otherwise, the youngest Eocene clasts would have been eroded first and would be more abundant near the base of the unroofing gravel sequence. Fossils 30m below the top of the truncated arkosic sediments, below the volcanic-rich horizon, suggest a late Paleocene to early Eocene age. Regionally, the Laramide arkose is deeply weathered, disconformably overlain by locally-derived conglomerates, and capped by latest Oligocene (24 Ma) to Miocene volcanic rocks. The prevolcanic disconformity represents the post-Laramide transition from a subtropical Eocene climate through the onset of Oligocene cooling and increased aridity. Recently acquired apatite U-Th/He data provide evidence that complements this uplift scenario. Cooling ages, derived from 36 apatite samples in a wide range of geologic locations and settings, show that regional Laramide unroofing occurred by 60 Ma in the western Grand Canyon with younger cooling ages, between 60 and 35 Ma, toward the SE along the Mogollon Rim. Unroofing of the plateau interior in a SW to NE direction occurred in both Laramide and mid Tertiary (28-16 Ma) episodes. Discrete erosion events following Basin and Range faulting along the western plateau are poorly constrained by a lack of datable horizons, but post-Laramide uplift on the order of 300 m cannot be excluded by current results.