Extensional geometries in the northern Grant Range, east-central Nevada - Implications for oil occurrences in Railroad Valley

Tertiary heterogeneous extension in the northern Grant Range, Nevada, is manifested by a stacked array of curviplanar low-angle attenuation faults that formed concurrent with arching. Attenuation was controlled by lithologic character, structural depth, and geometry of the arch. Extension appears to be greater on the west side of the range than on the east. On the east side of the range, the stacked array of low-angle attenuation faults is subparallel to bedding and attenuation is distributed across many stacked fault zones; except at highest crustal levels, these faults are blind. On the west side of the range, the low-angle attenuation faults of the stacked array merge into a single, major down-to-the-west fault zone across which as much as 19,000 ft of strata are omitted. Arching of the fault array resulted in an extensional culmination. Cross sections incorporating seismic and drill-hole data suggest that the low-angle attenuation faults (particularly the major down-to-the-west attenuation fault) seen in the range extend into Railroad Valley on the west side of the range with no significant offset by high-angle normal faults. Thus, the topographic expression of Grant Range and Railroad Valley may be due to the synchronous arching and low-angle faulting. These data indicate thatmore » both petroleum source and reservoir rocks in Railroad Valley oil fields are located in relatively immature but extensively fractured rocks of the upper plate to the extensional ramp. Lower plate rocks are metamorphosed, illustrate ductile behavior, and lack significant porosity and permeability.« less