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New studies of selected basins in the Miocene extensional belt of the northern Lake Mead domain, southern Nevada, suggest refinements on previous models for the early extensional history of the region. Critical data come from (1) the Longwell Ridges area, west of Overton Arm and within the Lake Mead fault system; (2) the Salt Spring Wash Basin, in the hanging wall of the South Virgin Mountains–White Hills detachment fault; and (3) previously studied subbasins of the South Virgin Mountains in the Gold Butte step-over region. Our model focuses on the early history of extension and involves analysis of the lower Horse Spring Formation and correlative strata. The basins and fault patterns suggest two stages of basin development related to two distinct faulting episodes, an early period of detachment faulting, followed by a switch to faulting mainly along the Lake Mead transtensional fault system while detachment faulting waned. Apatite fission-track ages suggest that the footwall block of the detachment fault began cooling at 18–17 Ma. The 18–17 Ma time period appears to be the age of the upper limestone of the Rainbow Gardens Member of the Horse Spring Formation, which is interpreted to be a pre-extensional unit deposited only north of Gold Butte block in the Gold Butte step-over basin, where facies patterns and slow rates of sedimentation make faulting uncertain. The first definite basin stage occurred ca. 16.5–15.5 Ma, during which there was slow to moderate faulting and basin subsidence in a contiguous basin along the South Virgin Mountains–White Hills detachment fault and in the Gold Butte step-over basin; the step-over basin had complex fluvial and lacustrine facies and was synchronous with landslides and debris flows in the basin in the hanging wall of the detachment fault. At ca. 15.5–14.5 Ma, there was a dramatic increase in sedimentation rate related to formation or increased activity on the Gold Butte fault, a change from lacustrine to widespread fluvial, playa, and local landslide facies in the step-over basin, and the peak of exhumation and faulting rates on the detachment fault. The simple early Gold Butte step-over basin broke up into numerous subbasins at ca. 15.5–14.5 Ma as initial faults of the Lake Mead fault system formed. From 14.5 to 14.0 Ma, a major change occurred from dominantly detachment faulting to dominantly transtensional (strike-slip + normal) faulting in the Lake Mead fault system as detachment faulting waned. At this time, the Lake Mead fault system began to propagate to the west, and activity on faults and in subbasins north of Gold Butte slowed or ceased, accompanied by major progradation of alluvial conglomerates over the step-over basin. The geometry of the South Virgin Mountains–White Hills detachment fault that dominated the early Lake Mead extension history fundamentally controlled patterns of faulting and magmatism throughout the rest of the extensional history, even as the detachment faulting itself slowed from 14 to 11 Ma, when it ceased to be active.

In a regional view, the detachment faulting in eastern Lake Mead is linked to and forms the northern end of the ca. 20–11 Ma northern Colorado River extension corridor. Similar to the rest of the corridor, faulting and exhumation peaked at 15 Ma, but at the north end of the corridor in eastern Lake Mead, detachment faulting changed rapidly to dominantly transtensional left-lateral faulting of the Lake Mead fault system. Eastern Lake Mead shows evidence for a spatial boundary between the southern and central Basin and Range that is best thought of as a northeast-southwest–trending feature located on numerous older tectonic boundaries. The area also records a temporal change from detachment to transtensional faulting characteristic of the central Basin and Range after 15 Ma.

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