THE SOUTH VIRGIN-WHITE HILLS DETACHMENT: THE CONTROLLING STRUCTURE OF THE EASTERN LAKE MEAD EXTENSIONAL DOMAIN, NEVADA AND ARIZONA

Three major low-angle normal faults in the eastern Lake Mead area, Nevada and Arizona, are segments of a regional, 55 km-long, detachment fault. This fault, the South Virgin-White Hills detachment (SVWHD), comprises the Lakeside Mine, Salt Spring (SSF), and the Cyclopic Mine fault (CMF) segments. Displacement on the SVWHD decreases from a maximum at the Gold Butte block in the north to a minimum at the Cyclopic Mine in the south. All three segments dip gently west and record top-west displacement. ⁴⁰Ar/³⁹Ar dates dates on variably tilted volcanic rocks in the upper plate of the Salt Spring segment constrain the principal phase of syntectonic sedimentation and tilting to between 15.2-14.6 Ma. The along-strike, southward decrease in displacement is accompanied by a change in type of fault rock from mylonite along the Lakeside mine fault (northern segment), to chlorite cataclasite along the SSF (central segment), to unconsolidated fault breccia along the CMF (southern segment). Differences in fault rock may reflect decreasing exhumation of footwall rocks as a result of decreased displacement to south.

Two structures associated with the SVWHD are worthy of note. (1) The Golden Rule Peak lineament, an E-trending alignment of structural and topographic features, appears to mark the northern limit of a series of transfer faults that collectively exhibit ~5 km of left-lateral displacement. The lineament itself may be a major transverse structure that accommodates differential displacement between the SSF and the CMF. (2) A 20-30 m-thick, low-temperature mylonite zone lies in the footwall of the SSF structurally below an extraordinary, 1-km thick zone of intensely and pervasively fractured and brittlely folded rock. Apatite fission-track thermochronology suggests that the SSF and the mylonite zone were active contemporaneously. The origin of the folded and fractured zone is unclear but it may reflect shearing of material during coeval motion along the SSF and the mylonite zone. The Grand Wash fault forms the present-day physiographic boundary between the Colorado Plateau and the Basin and Range provinces; however, based on greater amount of deformation and exhumation, we suggest that the SVWHD is the principal structure accommodating regional extension in the eastern Lake Mead extensional domain.