Basin Geometry, Fault Offsets, and Influence of Pre-Existing Structure IN the Northern Colorado River Extensional Corridor and the Lake Mead REGION, Nevada and Arizona

The northern Colorado River extensional corridor and Lake Mead region are characterized by prominent gravity and magnetic anomalies that provide insight into the geometry of extensional basins, amount of vertical and strike-slip offset on faults that bound these basins, and composition of major basement blocks. Although large-magnitude extension throughout the corridor and major strike-slip faulting north of Lake Mead have highly disrupted many basins, most of the older basins (Middle to Late Miocene) are not reflected by prominent geophysical anomalies. Instead, the most conspicuous anomalies (e.g. gravity lows) generally correspond to the younger (Late Miocene to recent), structurally more coherent basins. Most of the geophysically expressed basins lie north of Lake Mead and are bounded by Quaternary normal and/or strike-slip fault zones.

Both Quaternary faults and geophysically conspicuous basins are largely absent south of Lake Mead, where the only prominent gravity low corresponds to a structurally intact basin filled primarily with halite along the less extended, eastern margin of the corridor. Relatively continuous northeast-trending magnetic anomalies south of Lake Mead, presumably sourced by Proterozoic basement rocks, suggest that strike-slip displacement is negligible on many of the major normal faults. In contrast, magnetic anomalies are smeared along the Lake Mead fault system and Las Vegas Valley shear zone. These magnetic basement blocks located at the northern end of the Colorado River extensional corridor may mark an old easterly-striking crustal flaw that extends 150 km east onto the Colorado Plateau. Offset magnetic anomalies suggest left-lateral displacement of 12-20 km for the Hamblin Bay fault zone, 12-15 km for the Lime Ridge fault, and 12 km on the Gold Butte fault. These values are comparable to or lower than published estimates based on geologic mapping.