Abstract

New fission-track data together with 1:24 000-scale geologic mapping and analysis of Tertiary sedimentary deposits provide better constraints on the time and nature of motion along the Snake Range décollement, 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 available. These new data suggest that the Snake Range décollement forms part of a more extensive, 150-km-long north-south–trending fault system, the Snake Range–Deep Creek Range fault system. This fault system extends along the eastern flank of the northern and southern Snake Range, Kern Mountains, and Deep Creek Range, and accommodated at least 12–15 km of rapid slip in the Miocene, ca. 17 Ma. This component of motion is distinctly younger (by about 15–20 m.y.) than an earlier episode of slip and extension across the region bracketed stratigraphically and geochronologically as late Eocene–early Oligocene age.

Apatite fission-track ages (n = 57) in most parts of the Snake Range and adjacent ranges cluster at 17 Ma, indicating rapid cooling from >125 to <50 °C during exhumation at that time. In the northern Snake Range, zircon fission-track ages (n = 3) are essentially concordant with the apatite ages, indicating very rapid cooling from >310 to <50 °C. Formation of at least part of the pervasive mylonitic fabrics in the northern Snake Range may have occurred during this Miocene time interval, very late rather than early in the extensional history of the region. Coarse fanglomerate and rock-avalanche deposits in flanking Tertiary basins provide additional evidence 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 indicates that Miocene slip along the low-angle northern Snake Range décollement and 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. This suggests that the two styles of faulting (low-angle detachment and high-angle rotational) can occur simultaneously along the length of a single normal fault system. Data from the northern Snake Range also underscore the importance of a vertical component of uplift of the range in Miocene time, leading to the present domal geometry of the northern Snake Range décollement.

When considered together with footwall deformational fabrics, the new data are most simply explained as the consequence of higher local geothermal gradients and a shallower brittle-ductile transition zone along the northern Snake Range part of the fault system. It can be speculated that the Snake Range metamorphic core complex represents the top of a stretching welt of hotter, deeper level crust that rose during extension. This rising welt may have been localized by the presence of previously thickened crust beneath the region and could have been triggered by increased regional magmatism and heating accompanying rapid extension in Miocene time.

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