Rapid Miocene slip on the Snake Range-Deep Creek Range fault system, east-central Nevada

New fission-track data together with 1:24 000-scale geologic mapping and an alysis of Tertiary sedimentary deposits provide better constraints on the t ime and nature of motion along the Snake Range decollement, a classic Basin and Range metamorphic core complex detachment fault in east-central Nevada . Here, the fission-track method provides a particularly effective tool for dating faulting where bracketing or crosscutting relations are not availab le. These new data suggest that the Snake Range decollement forms part of a more extensive, 150-km-long north-south-trending fault system, the Snake R ange-Deep Creek Range fault system, This fault system extends along the eas tern flank of the northern and southern Snake Range, Kern Mountains, and De ep Creek Range, and accommodated at least 12-15 km of rapid slip in the Mio cene, ca, 17 Ma. This component of motion is distinctly younger (by about 1 5-20 m.y.) than an earlier episode of slip and extension across the region bracketed stratigraphically and geochronologically as late Eocene-early Oli gocene age. Apatite fission-track ages (n = 57) in most parts of the Snake Range and ad jacent ranges cluster at 17 Ma, indicating rapid cooling from >125 to <50 d egrees C during exhumation at that time. In the northern Snake Range, zirco n fission-track ages (n = 3) are essentially concordant with the apatite ag es, indicating very rapid cooling from >310 to <50 degrees C, Formation of at least part of the pervasive mylonitic fabrics in the northern Snake Rang e may have occurred during this Miocene time interval, very late rather tha n early in the extensional history of the region. Coarse fanglomerate and r ock-avalanche deposits in flanking Tertiary basins provide additional evide nce for major tectonism at this time. Comparison of the timing of events in the northern Snake Range to that along strike of the fault system indicate s that Miocene slip along the low-angle northern Snake Range decollement an d exhumation of extensive footwall mylonites were coeval with more typical Basin and Range high-angle rotational faulting in the Deep Creek Range and Kern Mountains to the north and in the southern Snake Range to the south. T his suggests that the two styles of faulting (low-angle detachment and high -angle rotational) can occur simultaneously along the length of a single no rmal fault system, Data from the northern Snake Range also underscore the i mportance of a vertical component of uplift of the range in Miocene time, l eading to the present domal geometry of the northern Snake Range decollemen t. When considered together with footwall deformational fabrics, the new data are most simply explained as the consequence of higher local geothermal gra dients and a shallower brittle-ductile transition zone along the northern S nake Range part of the fault system. It can be speculated that the Snake Ra nge metamorphic core complex represents the top of a stretching welt of hot ter, deeper level crust that rose during extension, This rising welt may ha ve been localized by the presence of previously thickened crust beneath the region and could have been triggered by increased regional magmatism and h eating accompanying rapid extension in Miocene time.