STRUCTURAL AND KINEMATIC ANALYSIS OF MIOCENE ROCKS, BLACK CANYON, ARIZONA AND NEVADA

A detailed structural study of seven major side canyons to Black Canyon of the Colorado River, part of a larger study of the hot springs below Hoover Dam, focuses on the kinematic histories of faults preserved in variably tilted Miocene volcanic and plutonic rocks. The rocks are cut by both high- and low-rake faults of various strikes, reflecting different fault generations. The dominant strike of these faults is northwest to north-northeast. Younger Mt. Davis volcanic rocks display fanning dips and faults lose displacement upward. Mostly west-dipping, low- to moderate-angle normal faults cut east-dipping strata. Near Hoover Dam, faults display widely varying strikes, dips, and rakes. In contrast, to the south older rocks (Patsy Mine volcanics) display dominantly northwest-striking, low- and high-rake faults while younger rocks preserve only northerly striking normal and oblique-slip faults.

Previously published paleomagnetic data indicate counter-clockwise rotation at the northern end of the study area. This is supported by the rotation of strikes of low- to moderate-angle faults as well as bedding attitudes near Hoover Dam. These faults pre-date rotation and are in turn cut by high-angle, dominantly northwest-striking, right-lateral faults.

For kinematic analysis of the faults, we used the techniques of Allmendinger et al. (1989) and Marrett and Allmendinger (1990). Analysis for the southern two-thirds of the study area yielded a sub-horizontal extension direction oriented approximately east-west (086°); extension directions for individual side canyons deviate by a maximum of 12°. Pseudo fault-plane solutions and linked Bingham axes of the fault data show strain compatibility between the low-rake and high-rake faults. When fault data are separated into low-rake and high-rake groups extension directions remain east-west and the combined shortening directions suggest constrictional strain. Fault-plane solutions from the northern part of the study area are complex, reflecting interplay between faulting and rotation. We suggest rotation is caused by shear induced on blocks bounded by sub-parallel northwest-striking, right-lateral strike-slip faults that are secondary structures to the northeast striking left-lateral Lake Mead fault system, which bounds the study area to the north.