LATE TERTIARY-QUATERNARY UPLIFT CONSTRAINTS FOR THE SOUTHWESTERN COLORADO PLATEAU

The Hualapai Plateau margin of the Colorado Plateau indicates that Laramide uplift followed by Basin and Range extension and erosion can account for most, if not all, of the current elevation and physiography of the southwestern plateau. The evidence is as follows: 1) Exhumed Laramide-age paleocanyons filled with Paleogene to Neogene sediments and Miocene volcanics preserve 1.5 km of regional paleorelief. 2) The northeast-trending Laramide paleovalleys are backtilted to the southwest by Miocene extension and document an actual reduction of initially higher Laramide elevations. 3) The western margin of the Hualapai Plateau is eroded back from existing fault or monocline traces into a highly irregular scarp, in contrast to the markedly straight Miocene scarp of the Grand Wash region. 4) The Grand Wash and Hurricane faults in the Hualapai Plateau region have no obvious geomorphic relief associated with their projected trends through late Neogene or Quaternary sediments. 5) An erosionally beveled Laramide pluton at the plateau margin is unconformably overlain by Miocene basalts. 6) Early Tertiary regional aggradation was superseded by incision following Miocene volcanism and sedimentation, as a result of the creation of Miocene structural relief. 7) A headwardly eroding Miocene canyon along the Laramide-age scarp that separates the Hualapai and Shivwits Plateaus (modern Colorado River course) accounts for the absence of deltaic facies in the Miocene Muddy Creek Formation.

Aside from subtle epeirogenic uplift or isostatic adjustments across the region, there is no convincing evidence that requires significant Neogene or Quaternary differential uplift along major faults of the Hualapai Plateau during late Miocene or Pliocene time. The tectonic history supports Farallon plate subduction as the prime cause of Cenozoic uplift, followed by extensional collapse of the Great Basin to create the modern relief difference and subsequent fluvial incision on the plateau. There is no clear evidence that structural offsets on major plateau boundary faults cannot be explained by down-to-the-west displacement, the result of Miocene extension of the terranes to the west of the plateau margin.