The role of attenuation in the formation of the Railroad Valley structural basin, east-central Nevada: Detachment control of petroleum reservoirs

The eastern boundary of Railroad Valley has long been considered a large displacement steep normal fault. By mapping a low-angle fault, called the White Pine detachment (WPD), from the ranges into the valley, we show that steep normal faulting was not a significant process in the formation of Railroad Valley. The WPD or its equivalent extends throughout the White Pine and Grant ranges, as shown by mapping of previous authors. Well data show that it is also present in Railroad Valley. Our structure-contour map of the WPD, based on surface and subsurface data, shows that the WPD dips uniformly from the ranges into Railroad Valley and is not displaced significantly by high-angle faulting. Even if high-angle faulting were assumed, the maximum displacement consistent with the data would be 2000 ft (610 m), much smaller than the total structural relief of at least 16,000 ft (4880 m) between the Grant Range and the adjacent valley. Structural elevations in the valley are low because the Paleozoic section has been greatly attenuated. A related contribution to structural relief is uplifting of the ranges caused by diapiric emplacement of plutons in response to tectonic unloading. The WPD is only one of many detachments in Railroad Valley. We propose that most or all of the oil reservoirs are in attenuated, brittle blocks between detachments. Seals are provided by detachments or by the discontinuous nature of associated fracturing. Attenuation, by juxtaposing hot infrastructure rocks with rocks that normally occur thousands of feet above the granitic and metamorphic basement, may have contributed to maturation of source rocks.