KINEMATICS OF THE EASTERN LAKE MEAD FAULT SYSTEM, NEVADA AND ARIZONA

NE-striking, left-oblique faults of the east part of the Lake Mead fault system (LMFS), NW Arizona and adjacent Nevada, are mechanically and kinematically linked to N-striking normal faults and minor NW-striking right-lateral faults. From north to south, the Hen Spring, Cabin Canyon-Front, Lime Ridge and Gold Butte faults are distributed in a broad 45 km N-S zone that was formed by left steps in the regional left-slip fault system. These left-oblique faults partitioned the broad zone into structural blocks. Kinematic data collected in the south Virgin Mountains part of the Lake Mead fault system indicate an overall oblique extensional or transtensional strain signature, with an east-west extension direction and the shortening direction partitioned between vertical for the extension-dominated structural blocks and south-directed for the strike-slip faults. The kinematic data also shows that left-lateral displacement at the block boundaries is transferred to normal displacement within the blocks via oblique-slip fault segments.

Geologic mapping shows that initiation of large displacement normal faults that linked to the left-oblique faults was preceded by early vertical uplift and bedding plane slip in the northern part of the south Virgin Mountains. The first-formed Gold Butte and Cabin Canyon-Front left-oblique faults created a step over geometry linked by the large normal faults. This geometry was later cut by an under step that developed between a normal fault splaying northward off of the eastward tip of the Lime Ridge fault and the Hungry Valley normal fault splaying southward off of the Front fault. A narrow basement-cored block, Whitney Ridge, was uplifted in the under step.

The oblique extension accommodated by left-lateral faults of the Lake Mead fault system in the Virgin Mountains was active about 17 to 14 Ma. Subsequent south-directed shortening may have caused vertical strain accumulation against the north side of the Virgin Mountains, which then forced westward-directed tectonic escape, first using the Hen Spring fault and subsequently the Hamblin Bay fault as deformation shifted to the western Lake Mead region.