VOLCANISM, STRIKE-SLIP FAULTING AND FAULT ROTATIONS NEAR HOOVER DAM, CLARK COUNTY, NEVADA

Knowledge of the structural geology near Black Canyon of the Colorado River lacks detail, yet is important to current studies attempting to constrain the local hydrogeology of hot springs. We present results of detailed (1:12,000 scale) geologic mapping of the south-central portion of the Boulder Beach quadrangle, Clark County, NV, a key area for understanding the geologic framework near Hoover Dam. We demonstrate a new local volcanic stratigraphy with units, from oldest to youngest, as follows: the Boulder City Pluton, dam conglomerate, Tuff of Hoover Dam, an extrusive basalt, and an extrusive intermediate rock. The dam conglomerate and Tuff of Hoover Dam likely were deposited discontinuously in structural and/or erosional depressions on the Boulder City Pluton. The basalt and intermediate rock may be lateral equivalents. Outcrop relationships and fault slip data suggest that southwest striking sinistral faults were active during and after deformation along southeast striking dextral faults. Dikes of intermediate to siliceous composition locally intrude faults and have been subsequently faulted. Dip-slip faults are rare. The informally named Hacienda fault shows a systematic change in strike from ~N60W in the southeast corner of the map area to ~N120W in the south central part of the map. We interpret this change in strike to reflect a counterclockwise rotation of the Hacienda fault by as much as 60° possibly related to interaction with a strand of the sinistral Lake Mead fault system to the north. A zone of alteration pervades the Boulder City Pluton in the center of the map area and appears to lessen in degree of alteration both to the north and to the south. The alteration signal is strongest along and to the north of the Hacienda fault, indicating this fault may act as a significant hydrological barrier.