LATE MIOCENE EROSION OF TILTED FAULT BLOCKS IN THE GOLD BUTTE AREA AND DEPOSITION IN HALF-GRABEN BASINS, BASIN AND RANGE PROVINCE

The provenance and stratigraphic architecture of basin-filling middle to upper Miocene sediments around the Gold Butte area, southern Nevada, and adjacent highlands record the erosion of fault blocks that progressively tilted during Miocene extension. The large east-tilted South Virgin-White Hills block including the Gold Butte highlands was the primary provider of coarse detritus into adjacent half-graben basins on both sides. Voluminous, very coarse-grained sediments that were shed eastward down the back slope of this tilt block into the Grand Wash Trough suggest that there were large middle and late Miocene catchments on that side of the block, possibly inherited from a gentler dip slope early in the tilting history. As the block uplifted and tilted during slip on the west-dipping South Virgin-White Hills normal fault that bounds the west side of the block, coarse-grained debris eroded from the block was transported to the west, including rock-avalanche megabreccias shed while the fault was active. Longitudinal transport of coarse-grained sediment also occurred along the axes of basins on both sides of the block.

Progressive tilting of the highland block would have broadened its structural footwall on the west and narrowed its east-dipping back slope, but the drainage divide migrated by erosion and piracy. The modern divide ultimately created by these processes separates drainage roughly equally down the two sides of the widest part of the block.

The block continued in the late Miocene to shed clasts from distinctive sources into adjacent basins during and after the time when the bounding South Virgin-White Hills normal fault was active. Fault death at ~10 Ma followed rotation of this fault, and the along-strike Quail Spring fault, from initial dips > 55° to dips < 30°. This cessation of faulting led to a shift in locus of faulting eastward, to the steeper Wheeler fault system. Coarse sediment shed from the South Virgin-White Hills tilt block gradually declined as deformation waned, and limestone-rich sedimentation expanded onto the basin margins against the block. Where the rising sedimentary fills eventually bridged across the block and connected basins on either side, these bridge sites later served to focus sites for the Pliocene Colorado River when it developed across the area as an integrated regional stream.