RELATION BETWEEN EXTENSIONAL GEOMETRY OF THE NORTHERN GRANT RANGE ANDOIL OCCURRENCES IN RAILROAD VALLEY, EAST-CENTRAL NEVADA

In the northern Grant Range, heterogeneous Neogene extension was domin ated by synchronous arching and attenuation. Attenuation was accomplis hed along a stacked set of attenuation faults that formed at low angle s to bedding as the Paleozoic carbonate and Paleogene rocks arched abo ut a north-northwest axis. The style and amount of attenuation was con trolled by lithologic character and structural depth of rock units and by geometry of the arch. On the steeper west side of the Grant Range arch, the curviplanar low-angle attenuation faults converge into a sin gle shallowly west-dipping fault zone along which the stratigraphic ju xtaposition of Mississippian units over Middle Cambrian units and Late Cretaceous granite marks the zone of maximum attenuation. The distinc t geometry of the arched, westward-converging, low-angle fault array i s seen in windows into the deeper structure low on the west side of th e range. We conclude that arching and heterogeneous extension resulted from uplift of the Grant Range relative to the structural basin of Ra ilroad Valley to the west. This structural differentiation is expresse d as a complex zone of subparallel-to-bedding, shallow-dipping attenua tion faults rather than as a simple high-angle range-front fault. Seis mic and drill-hole data indicate that low-angle attenuation faults in the range extend into Railroad Valley and control the structure buried in the valley. Mississippian and Paleocene to Eocene petroleum source rocks and Devonian to Oligocene reservoir rocks in Railroad Valley oi l fields are in extensively fractured rocks of the upper plate to the major extensional fault system. Thus, relatively cold upper-plate rock s, immature with respect to hydrocarbon generation, were brought relat ively down into contact with hotter lower-plate rocks by Neogene atten uation faulting. Oil in Railroad Valley, which is sourced.from rocks a s young as Eocene, was probably generated by this juxtaposition during Neogene crustal attenuation, and subsequently migrated into upper-pla te fractured reservoirs.