VERTICAL AXIS ROTATION ALONG THE LAKE MEAD FAULT SYSTEM NEAR HOOVER DAM, ARIZONA-NEVADA
The data corroborate earlier studies. A paleostress study at HD by Angelier et al. (1985) documented a 60o 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 35o of post-tilting relative CCW rotation within 3 km of the LMFS. Ron et al. (1993) suggested both datasets could be explained by 30o CCW block rotation related to the LMFS and 30o 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.