Constraints on the evolution of vertical deformation and Colorado River incision near eastern Lake Mead, Arizona, provided by quantitative structural mapping of the Hualapai Limestone

摘要

The 12a€“6 Ma Hualapai Limestone was deposited in a series of basins that lie in the path of the Colorado River directly west of the Colorado Plateau and has been deformed by an en-echelon normal fault pair (Wheeler and Lost Basin Range faults). Therefore, this rock unit represents an opportunity to study the sedimentological and structural setting over which the Colorado River first flowed after integration through western Grand Canyon and Lake Mead. In this study, we quantify the structural geometry of the Hualapai Limestone and separate the deformation into syn- and postdepositional episodes. Both the Wheeler and Lost Basin Range faults were active during Hualapai Limestone deposition, as shown by thickening of strata and fanning of time lines toward half-graben faults that bound the Hualapai subbasins. The structure is characterized by a prominent reverse-drag fold and broad, shallow syncline adjacent to the Lost Basin Range fault, and a small-magnitude reverse-drag fold and short-wavelength normal-drag fold adjacent to the Wheeler fault. We find a??450 m of throw between the footwall and hanging-wall Hualapai Limestone sections, suggesting faulting was ongoing after Hualapai Limestone deposition ceased and during Colorado River incision. To investigate a range of possible fault geometries that may have been responsible for Hualapai Limestone deformation, we compared our structural results against surface deflections calculated by a two-dimensional (2-D) geomechanical model. While nonunique, our results are consistent with a scenario in which the Wheeler fault was surface rupturing, or nearly surface rupturing throughout deposition of the Hualapai Limestone, but was inundated at ca. 6 Ma by coalescing paleolakes in Gregg and Grand Wash Basins as sedimentation kept pace with deformation. In contrast, we find evidence suggesting the Lost Basin Range fault was deeply buried by the Hualapai Limestone and likely propagated upward and laterally to break the surface sometime after 6 Ma. Therefore, we interpret the landscape over which the Colorado River first flowed to be of low relief within the terrain bounded by the Grand Wash Cliffs, the Hiller Mountains, and subtle topographic highs to the north and south of our field area. This original low-relief depositional surface was deflected into the structure exposed today by continuing deformation by the Wheeler and Lost Basin Range faults, allowing for calculation of apparent incision rates of the modern Colorado River drainage system that spatially vary between 33 and 42 m/m.y. in the hanging wall and between 108 and 115 m/m.y. in the footwall. Hanging-wall incision rate values are similar to, but faster than, a previously published point measurement, and footwall values are similar to measured incision rates in the western Grand Canyon, suggesting the Wheeler fault system may resolve as much as a??410 m of Colorado Plateau uplift in the last 6 m.y.