MIOCENE SEDIMENTATION NEAR THE LOST BASIN RANGE AND INITIATION OF WHEELER FAULT SYSTEM EASTERN LAKE MEAD, ARIZONA: 15 MA DISRUPTION OF THE FOOTWALL OF THE SOUTH VIRGIN – WHITE HILLS DETACHMENT FAULT

The Lost Basin Range in eastern Lake Mead consists of Proterozoic rocks along the west side of the Grand Wash trough that were exhumed from 18 – 15 Ma based on apatite fission track ages. New data support a major fault reorganization in eastern Lake Mead at ~15 Ma that cut the Proterozoic footwall of the South Virgin-White Hills detachment fault with high-angle normal faults (Wheeler fault system) and new subbasins that were active until ~11 Ma, had lesser activity 11 to 7 Ma, and slow faulting <7 Ma. The Lost Basin Range fault lies along the west side of Lost Basin Range and steps to the east to the south end of the Wheeler fault, which then runs 60 km north to join the Grand Wash fault. The geometry of the southern Wheeler/Lost Basin Range fault system is that of a relay ramp between two, west-dipping, high-angle normal faults following earlier workers. The Gregg basin lies in the step over and is interpreted as a relay ramp basin. New tuff ages and basin analysis from the north and east side of Lost Basin Range integrated with existing structural and age data from the area reveal that faulting, sedimentation and tilting of blocks along the southern Wheeler fault system began by 15.3 Ma. Sedimentation continued until after 13 Ma along the SE Lost Basin Range, while the age of continuing sedimentation in the Gregg basin is poorly constrained but likely after 11 Ma. A paleocanyon in the footwall of the southern Wheeler fault filled with conglomerate and breccia from the W from 15.3 - 14 Ma and then overtopped to the south to cover the Paleozoic rocks of south Wheeler Ridge. The Paleozoic – Miocene strata relations suggest 20° - 30° east tilt in Paleozoic strata before ca. 14 Ma. Upward fanning dips in Miocene conglomerate indicate ~20° east tilt with Paleozoic strata from 14-11 Ma. Gentle (<5°) east dips in lower Hualapai Limestone above the paleocanyon suggest that most tilting ceased by 11 Ma. The lower conglomerate of Gregg basin lies below, and interfingers with, the limestone of Gregg basin, which is undated but correlates with the 11 – 7 Ma Hualapai limestone in the adjacent Grand Wash trough. The syncline in upper Gregg basin strata is linked spatially to the Wheeler and Lost Basin Range faults, and the two faults appear to offset Gregg basin limestone modestly, and therefore lower rates of faulting occurred from 11-7 Ma and post-11 - 7 Ma fault activity is likely.