A REGIONAL (>3000 KM2) DETACHMENT FAULT ROOTED IN AN UPPER PALEOZOIC ARGILLACEOUS DUCTILE UNIT: A STRATIGRAPHIC CONDUIT FOR ~200°C ORE-FORMING HYDROTHERMAL FLUIDS IN THE SHALLOW CRUST, GREAT BASIN, NEVADA

The White Pine detachment fault (WPD) in the White Pine Range, Grant Range, and adjacent Railroad Valley is stratigraphically controlled by the ductile Mississippian Chainman Shale (Walker et al., 1992; Francis and Walker, 2001). The WPD is a zone for long-distance migration of hydrothermal fluids (125° to 250°C), as evidenced by widespread silicification, gold mineralization, and petroleum reservoirs in Railroad Valley.

The WPD is different from a series of up to 14 “brittle” detachments mapped by other workers in lower Paleozoic carbonates of the Grant Range. The latter is a bottom-to-top system of detachment faults in which the lower faults rotated and younger faults incised the older ones; thus, they cut stratigraphy. The WPD also cuts down section but, in marked contrast, variably attenuated Chainman Shale is in widespread fault contact with lower plate rocks. Thus, the relative ductility of the Chainman controls the WPD, but a similar control does not exist for the brittle detachments in the Grant Range.

The Grant Range has a monotonous, almost 5,000 m thick Cambrian-Ordovician carbonate section, while the White Pine Range has the argillaceous Lincoln Peak Formation, about 1,000 m thick, within that interval, which is the controlling ductile unit for the Currant Gap detachment (CGD; Francis et al., 2021). Thus, attenuation in the White Pine Range is dominated by two ductile-rooted detachments in roughly the same stratigraphic section as that which contains up to 14 brittle detachments in the Grant Range; one of the latter was mapped by us as the WPD, and it cuts down through all of the older (deeper) detachments.

Prior to 17 Ma high temperature ductility in the thick carbonate section could have initiated a system of detachments; brittle attenuation could have continued past 17 Ma along thus established zones of weakness while attenuation also occurred along argillaceous, ductile units. Fanned dips in the Miocene-Pliocene Horse Camp formation suggest rotation of blocks along ductile-rooted detachments like the WPD and CGD.

The WPD dips uniformly from the ranges into Railroad Valley, as indicated by bore hole data, without a significant discontinuity from a basin-bounding steeply-dipping normal fault (Francis and Walker, 2001). We consider that most of the basin-and-range structural relief in this area is due to detachment attenuation.