CRYPTIC THRUSTS AND NORMAL FAULTS IN THE SCHELL CREEK RANGE, NEVADA: STRUCTURAL ANALYSIS USING LASER RAMAN SPECTROSCOPY

Using Laser Raman Spectroscopy of Carbonaceous Material (LRCM), we have determined that there is a marked difference in peak temperature between late Proterozoic rocks on the eastern margin of the Schell Creek Range and the Paleozoic rocks in the rest of the range. The oldest rocks (the Precambrian McCoy Creek Group on the eastern margin) have peak temperatures exceeding 500°C, and Cambrian limestones just above the basal quartzite show peak temperatures of 300 ± ~20°C. Based on stratigraphic thickness, there should not be more than a 75°C jump between the Precambrian rocks and the Cambrian limestones, and yet we see a difference exceeding 200°C. There is little documented evidence for thrust duplication or major unconformities that could explain this difference. Detailed sampling for LRCM was conducted across the eastern margin with the aim of more accurately locating the jump and finding evidence for the nature of the discontinuity.

A comparable problem existed in the Snake Range, where unmetamorphosed Cambrian limestones rest on mylonitic basal quartzites and amphibolite facies McCoy Creek Group without apparent duplication or excision of stratigraphy (Miller and Gans, 1983). This is now interpreted as a result of excision of duplicated section by significant normal sense displacement on the Northern Snake Range Detachment (NSRD), which cuts out the evidence of thrust stacking (e.g. Gans et al 1985). Bartley and Wernicke (1984) proposed that the NSRD is continuous under the valley between the Snake and Schell Creek ranges and is exposed in the eastern Schell Creek Range. While we disagree with the cross section of Bartley and Wernicke in detail, the idea that the temperature jump can be explained by the continuation of the NSRD in the Schell Creek Range is an interesting hypothesis to test.