VERTICAL AXIS ROTATION ALONG THE LAKE MEAD FAULT SYSTEM NEAR HOOVER DAM, ARIZONA-NEVADA

Geologic mapping of faulted volcanic-plutonic rocks in Black Canyon below Hoover Dam (HD), Arizona-Nevada, indicate local counterclockwise (CCW) vertical axis rotation related to the NE-striking, left-lateral Lake Mead fault system (LMFS) and a regional clockwise shift in the Neogene extension direction. Black Canyon exposes tilted Patsy Mine Volcanics (~18-15 Ma), intruded by ~14 Ma Boulder City pluton and overlain by Mount Davis Volcanics (~14 – 12 Ma) that are less tilted. We document an older NNW normal fault set cutting Patsy Mine that caused most of the stratal tilting and has a resolved 250^o azimuth extension direction. Within 3 km of the LMFS, the older set is rotated 30^o or more CCW about a vertical axis relative to the same set 8 km south. A younger fault set with a complex pattern of NNE-striking normal faults and NW – NE strike-slip faults cuts both volcanic units and has a ~290^o extension direction. This set is partly coeval with but also post-dates the rotation.

The data corroborate earlier studies. A paleostress study at HD by Angelier et al. (1985) documented a 60^o divergence in extension direction between older and younger events and suggested it resulted from either clockwise (CW) rotation in the stress field or CCW rotation of the rocks. A paleomagnetic study by Geissman et al. (1989) south of HD documented 35^o of post-tilting relative CCW rotation within 3 km of the LMFS. Ron et al. (1993) suggested both datasets could be explained by 30^o CCW block rotation related to the LMFS and 30^o CW rotation of the regional stress field. Studies further south of the LMFS, in likely un-rotated areas, document an early regional extension direction of ~260° suggesting the faults that record our early extension direction may have been rotated 10° CCW. If so, early effects of CCW rotation extend kms from the LMFS.

North of the LMFS in the River Mountains, a paleostress study by Faugure (1985) suggested similar older and younger events and CCW rotation. Rotation of the River Mountains is antithetic to CW rotation of Frenchman Mountain even further north, related to right-lateral movement on the NW-striking Las Vegas Valley shear zone (LVVSZ). The antithetic rotational strain accommodated westward extrusion of the Frenchman – River Mountains blocks between the LVVSZ and LMFS.