Seismic reflection and gravity evidence for multiple phases of basin formation in Pahrump and Mesquite Valleys, Nevada and California

The Pahrump and Mesquite basins are aligned NW-SE along the general trace of the Pahrump Valley-State Line (PV-SL) fault system, presumably forming as pull-aparts due to dextral, transtensional motion along the fault system. These basins have also been inferred to be supradetachment basins that lie near the eastern breakaway of large-magnitude extension of the Death Valley region. We reprocessed approximately 225 line- km of industry seismic reflection data from these valleys to emphasize reflectors in the basin-fill and have combined these data with analysis of gravity data in order to investigate the Cenozoic tectonic evolution of these basins. Five lines are oriented NE-SW, transverse to both the PV-SL fault system and the axes of the deep basins, Pahrump and Mesquite, that are defined through the inversion of gravity observations. These five seismic sections are tied together by two NW-SE oriented lines, one of which is subparallel to the inferred trace of the PV-SL fault system. The seismic lines portray the complex geometry of the PV-SL fault system and provide evidence for multiple ages of faulting along structures that bound Pahrump Basin. Locally, thick sequences of pre-extensional Tertiary sedimentary rocks are cut by large offset, relatively high-angle normal faults that record a phase of extensional basin formation that preceded the present-day transtension. Subsequent Neogene dextral offset on the PV-SL fault system resulted in the formation of steep-sided basins, local arching and tectonic inversion, and the burial of earlier-formed normal faults with coarse clastic detritus. Gravity models that are constrained to match the basin architecture observed in the seismic lines require lateral variations in basin-fill and bedrock density, and they require the rocks of Black Butte, a topographic high that separates Pahrump and Mesquite Valleys, to be separated from underlying bedrock by low-density sedimentary deposits.