QUANTIFYING DEFORMATION ACROSS THE BASIN AND RANGE -- COLORADO PLATEAU TRANSITION OVER THE LAST ~12 MA

The 12-6 Ma Hualapai Limestone, deposited at the mouth of the Grand Canyon, occupies a critical location and time frame for understanding the evolution of the Colorado River. It overlies locally derived siliciclastic sediments and is in turn overlain by gravels that predate a 4.4 Ma basalt and contain far-traveled clasts, heralding the arrival of the integrated Colorado River drainage.

Several lines of evidence suggest that the Hualapai was deposited in a series of tectonically active, isolated sub-basins that coalesced near the end of deposition (~6 Ma). Measured sections reveal that the Hualapai Limestone in the Grand Wash Trough and Gregg Basin thickens eastward toward the Wheeler and Grand Wash faults, respectively. Individual bedsets can be seen thickening toward these structures as well. The relative abundance of lacustrine limestone and marsh facies vs. fluvial siliciclastic and mixed facies indicate excess accommodation space as well as sourcing of formation waters adjacent to the faults. The youngest sediments in all sections are lacustrine limestones, suggesting a widespread high stand at the end of Hualapai deposition.

The latest Hualapai sediments thus provide a marker for tracking relative vertical deformation across the region over the last ~6 Ma. A structure contour map derived from digital photogrammetry reveals over 500 m of relief. A broad syncline in the Grand Wash trough suggests that the Grand Wash fault may have experienced minor blind slip post Hualapai. Major faults that cut the upper surface are the Wheeler (WF) and Lost Basin Range (LBRF) faults. Hanging wall strata along the WF are down-dropped ~275 m with only minor folding, while the hanging wall of the LBRF is marked by a prominent down-warping of Hualapai strata toward the fault that ends at a fault-parallel syncline ~500 m from the present-day trace. 2D mechanical models illustrate how differences in fault geometry, specifically lower and upper tip depths, may have generated along-strike differences and we conclude that the LBRF may have once been deeply buried under Hualapai sediment. The integrated Colorado River thus flowed across a dynamic landscape with highly variable post-6 Ma apparent incision rates of 34 to 115 m/Ma that reflect fault-dampening of incision and differential uplift/subsidence across the CP- BR boundary faults.