The Lost Basin Range in the eastern Lake Mead domain consists of Proterozoic rocks that bound the west side of the Grand Wash Trough. Exhumation of the Proterozoic rocks of the Lost Basin Range occurred from ca. 18 to 15 Ma based on seven apatite fission-track ages that range from 20 to 15 Ma. The Lost Basin Range fault lies along the west side of the Lost Basin Range and steps to the east to the southern end of the Wheeler fault, which then runs north for 60 km, where it joins 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. The intervening area of the fault step over, Gregg Basin, is interpreted as a relay ramp basin. New interpreted ages from stratigraphic units on the north and east sides of the Lost Basin Range integrated with existing structural data from the eastern Lake Mead domain reveal that faulting, sedimentation, and tilting of hanging-wall and footwall blocks along the southern Wheeler–Lost Basin Range fault system began by 15.3 Ma. Sedimentation continued until after 13 Ma along the southeastern Lost Basin Range, while the age of continuing sedimentation in Gregg Basin is poorly constrained. A paleocanyon in the footwall of the southern Wheeler fault filled with conglomerate and minor breccia between ca. 15.3 and ca. 14 Ma and then overtopped to the south to cover the Paleozoic rocks of south Wheeler Ridge. The Paleozoic strata of the south Wheeler Ridge area tilted east 20°–30° more than the Miocene strata that overlie them, and therefore this tilting occurred before ca. 14 Ma. Upward-decreasing (fanning) bedding attitudes in the overlapping Miocene conglomerate indicate that Paleozoic strata were being tilted along with the Miocene strata by ca. 14 Ma. Gentle (5° and less) east dips in the lower beds of the Hualapai Limestone above and east of the paleocanyon suggest that most tilting in the western Grand Wash Trough ceased by ca. 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 indicates that these faults were likely active at 11–7 Ma. The two faults appear to cut the Gregg Basin limestone, and therefore post–7 Ma fault activity at lower rates is likely.

The eastern Lake Mead region, to the north of the belt of metamorphic core complexes that define the Colorado River extensional corridor, underwent large-magnitude extension in the middle to late Miocene. We present two speculative new models for extension in this area that resolve several puzzling and paradoxical relations. These models are based on new field mapping and structural, geochronologic, and thermochronologic data from the northern White Hills, Lost Basin Range, and south Wheeler Ridge. The Meadview fault, a previously underappreciated structure, is an east-side-down normal fault that separates the northern Lost Basin Range to the west from south Wheeler Ridge to the east. Proterozoic crystalline rocks of the northern Lost Basin Range yielded an apatite fission-track (AFT) age of 15 Ma, whereas 2 km to the east, across the Meadview fault, crystalline rocks of south Wheeler Ridge yielded a 127 Ma AFT age. Similarly, at the south end of the Lost Basin Range, crystalline rocks with ca. 15 Ma AFT ages lie within 5 km of crystalline rocks of Garnet Mountain that yielded a 68 Ma AFT age across the Grand Wash fault. Neither of these relations can be explained by existing tilted crustal section or tilt-block models. In our “classic” metamorphic core complex model, the Grand Wash fault (breakaway), the Meadview fault, and the South Virgin–White Hills detachment represent different structural levels of a single, regional detachment that was active between ca. 16 and 11 Ma. The hanging wall of the detachment consists of rocks at south Wheeler Ridge, the Paleozoic ridges, and possibly part of the crystalline basement of the Gold Butte block, sedimentary and volcanic rocks in the hanging walls of the Salt Spring and Cyclopic Mine faults, and possibly stranded tilt blocks beneath the Grand Wash Trough supradetachment basin. The footwall, exhumed by subvertical simple shear and characterized by middle Miocene AFT ages, includes the central and western Gold Butte block, Hiller Mountains, and crystalline rocks of the White Hills and the Lost Basin Range. The east-dipping Meadview fault bounds the crystalline core on the east; the west-dipping South Virgin–White Hills detachment bounds the core on the west. Therefore, the Grand Wash fault represents the structurally highest part of the detachment, and the South Virgin–White Hills detachment represents the structurally deepest exposed part of the detachment. In the modified core complex model, the Grand Wash, Meadview, and South Virgin–White Hills detachment faults are separate structures, and the Grand Wash Trough is a “trailing-edge” basin bound on the east by the Grand Wash fault and on the west by the Meadview fault. The South Virgin–White Hills detachment is the main detachment along which extension was accommodated, and the Meadview fault is a major antithetic normal fault that facilitated exhumation of the core at the trailing edge of the detachment system.