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The Lake Mead region of northwest Arizona and southeast Nevada contains exceptional exposures of extensional basins and associated normal and strike-slip faults of mainly Miocene age. The Salt Spring Wash Basin is located within the hanging wall of a major detachment fault in the northern White Hills in northwest Arizona, the South Virgin–White Hills detachment fault. The basin is the focus of a detailed basin analysis designed to investigate its three-dimensional structural and stratigraphic evolution in order to determine how a major reentrant in the detachment fault formed. Geochronology and apatite fission-track thermochronology from other studies constrain movement on this detachment fault system to ca. 18–11 Ma, while our study suggests faulting from ca. 16.5 to 11 Ma.

Salt Spring Wash Basin consists of variably tilted proximal rock avalanche and alluvial-fan deposits shed from uplifting hanging-wall and predominantly footwall blocks. The basinal strata were deformed during early to middle Miocene faulting on the detachment fault, normal faults, and a faulted rollover fold within the basin. New and existing 40Ar/39Ar ages on tilted volcanic tuffs and basalt lava flows within the basin strata constrain deposition of these deposits from 15.19 to 10.8 Ma. An apparent lag between the initiation of footwall uplift at 18–17 Ma (based on thermochronology) and basin subsidence at 16.5–16 Ma in the eastern Lake Mead region may be explained by the influences of preexisting paleotopography, or it may be an artifact of lack of exposure of the base of the basin. An early phase of faulting and basin sedimentation from 16.5–16 to 14.6 Ma generated the relief to produce a 500+-m-thick lower section of megabreccia (landslide) and conglomerate (debris flows). Salt Spring Wash Basin experienced relatively high sedimentation rates of 200–600 m/m.y. during its early history. A 14.64 Ma basalt lies at a facies change to 650 m of conglomerate of the middle sequence that was deposited in an alluvial-fan to braid-plain setting. Changes in basin geometry included the development of the reentrant in the northern Salt Spring Wash Basin with the rollover fold at its southern margin. The middle sequence records a significant decrease in sedimentation rates from hundreds of meters per million years to ~60–30 m/m.y., major facies changes, and decreased rate of uplift of footwall rocks. The upper sequence of the basin includes ca. 11–8 Ma basalts interbedded with conglomerate. The ca. 6 Ma lacustrine Hualapai Limestone caps the section and indicates a profound change in sedimentation. The history of the Salt Spring Wash Basin indicates that there was a step-over geometry in the detachment fault that was linked across the southern margin of the reentrant in the basin during deposition of the middle sequence.

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