DIAPIRISM AND DETACHMENT ATTENUATION IN THE WHITE PINE AND HORSE RANGES, EAST-CENTRAL NEVADA; A NEW APPROACH TO SEGMENTATION OF MOUNTAIN RANGES

Detailed GPS-assisted mapping and geophysical investigation show that the horizontal separation (segmentation) between the White Pine and Horse Ranges is due to detachment attenuation in the Paleozoic suprastructure over offset domes within the metamorphic core. Previous explanations of the separation involving strike slip or transfer faults (Currant Summit fault of Moores et al, 1968 and Williams, 2000) are not supported by our field evidence.

Mapping of contacts to an accuracy of 1-3 m in a 1 by 3 km area reveals two detachment faults in ductile units, the regionally mapped White Pine detachment (WPD) in the Mississippian Chainman Shale (Francis and Walker, 2001) and the Currant Gap detachment (CGD) in the Cambrian Lincoln Peak Formation (Lacy, 2005). Both detachments are evidenced most importantly by outcrop patterns that follow topography, variable thinning of the ductile units, and locally intense brecciation and silicification. The CGD and WPD coalesce in the east end of our field area; seismic refraction data there show a low angle surface at about 10 m depth, with Chainman and melange above, and most likely Paleozoic carbonate below. Preliminary gravity data in the same area show no evidence of a steeply dipping interface; this is where the Currant Summit strike slip fault would have to be if it exists. Previous maps at lower scale show the Currant Summit fault mostly in alluvium; evidence for it is lacking in our GPS-guided mapping of Paleozoic outcrops. In the area of Currant Gap, where the CGD is exposed, isolated patches of Ordovician Eureka Quartzite overlie Cambrian Pole Canyon Formation. This occurs both north and south of the previously mapped trace of the Currant Summit fault, in contravention to the horizontal separation that that fault was meant to explain. Isotopic data in the same area indicate the absence of deep formation waters, which would have moved along a strike-slip fault (Holk, 2005).

Our mapping supports the model of Francis and Walker (2001) that ascribes separation of outcrops between the White Pine and Horse Ranges to offset diapiric domes in the infrastructure. Uneven diapirism may be responsible for other examples of segmentation, obviating many previously proposed block faults in these ranges and in the adjoining Railroad Valley. Additional coauthors are David W. Ferry and Kevin R. Gwinn.