PLIO-PLEISTOCENE DEFORMATION IN THE LAKE MEAD REGION AND UPLIFT OF THE COLORADO PLATEAU

The spatial and temporal distribution of Pliocene to Holocene Colorado River deposits form a primary dataset to better understand the evolution of a continental-scale river system and to quantify regional deformation. To this end, we have compiled information about the distribution of ancestral Colorado River deposits from geologic mapping and subsurface studies downstream from Grand Canyon. In particular we focus on the 4.5-3.5 Ma Bullhead Alluvium, a 200-300-m-thick aggradation package of primarily Colorado River sand and gravel. To investigate the vertical and longitudinal position of these and other deposits, we use a Python script to project outcrop locations and subsurface data onto a longitudinal valley profile. Downstream from western Lake Mead, ancestral Colorado River deposits are generally preserved to below river level, suggesting little if any bedrock incision there since ca. 4.5 Ma. In the Lake Mead region, new and previously dated ca. 4.5 Ma basalt flows indicate long-term bedrock incision rates between <20 and 60 m/Ma. Previously dated speleothems in western Grand Canyon have been used to suggest incision rates of 90-100 m/Ma over similar timeframes. The magnitude of post-4.5 Ma differential incision across Lake Mead is similar to the regional vertical offset of the ca. 4 Ma Bullhead Alluvium tread and the top of the ca. 6 Ma Hualapai Limestone across the Fortification and Wheeler fault systems. This suggests that differential incision is due to either differential uplift or subsidence across these faults. Regional subsurface data suggests that the base of the Bullhead Alluvium remains graded to Pliocene sea level. This negates subsidence of downstream blocks, and instead requires that upstream blocks are uplifted. These observations are in accordance with numerous studies that suggest ongoing or recent uplift of the Colorado Plateau. The faults that offset ancestral Colorado River deposits may be a surficial response to the deep-seated differential uplift. Together, these datasets suggest ~175 m of uplift over the last 2 Ma. An additional 225 m of uplift from 4 to 2 Ma is possible but less certain, due to unresolved uncertainties about the degree to which the Bullhead Alluvium filled western Grand Canyon, which could affect the bedrock incision significance of the speleothem data there.