EVIDENCE FOR MULTIPHASE BASIN FORMATION IN PAHRUMP VALLEY, NEVADA AND CALIFORNIA: REINTERPRETATION OF SEISMIC AND GRAVITY OBSERVATIONS

Pahrump Valley is underlain by two main basins that are thought to have formed as pull-aparts in a transtensional environment related to the Pahrump Valley-State Line (PV-SL) fault system. We reprocessed approximately 225 line-km of industry seismic reflection data to emphasize reflectors in the basin-fill, and from these data we interpret a phase of basin formation that preceded the present-day transtension. 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 inversion of gravity data. 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, provide evidence for multiple ages of faulting along structures that bound Pahrump basin, and portray a thick sequence of older Tertiary sedimentary rocks at depth. The seismic data are not tied to well control because the deepest water wells penetrate only the shallowest reflectors seen on the sections. Prior gravity analysis suggested the existence of a steep-sided horst below the PV-SL fault system within Pahrump basin, at odds with revised seismic images of a narrow and one-sided fault surface at-depth. 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 re-assessment of the outcrop of Black Butte as not being rooted to underlying bedrock. Stratigraphic and structural interpretation of these seismic data bear on potential interbasin groundwater flow and on the volume of aquifer material available for groundwater storage within the basins.