FLUID HISTORY OF THE LATE MIOCENE SILVER PEAK-LONE MOUNTAIN DETACHMENT FAULT, SOUTHERN NEVADA, USA

Detachment faults and metamorphic core complexes (MCC) often exhibit a progression of interaction with crustal fluids: early upwelling of hot mineralizing fluids, followed by downwelling cool meteoric fluids. The Silver Peak-Lone Mountain detachment fault (SPLM) in SW Nevada is unusual in exhibiting similar but coeval fluid variability. The SPLM was active from 11-5MY, with NW-directed extension accommodating the majority of 40-50 km of regional extension during that time.

MCC (e.g. Death Valley, DV) often exhibit strong footwall greenschist facies alteration (chlorite-epidote) near the detachment, generally interpreted to be evidence of hot upwelling metamorphic fluids. SPLM exhibits similar alteration in extensive dolomite-hosted talc deposits along its southern extent, with strong chloritization of upper plate rocks at the fault and infrared (IR) spectral indications of high fluid temperatures (elevated illite spectral maturity, ISM). The SPLM served primarily as a barrier to upflow at this location, although chloritic veins penetrate the upper plate, indicating mineralization < 5 MYA.

Intermediate temperature alteration along the SPLM is indicated in geothermal research borehole EM17-31, 30 km to the north, which penetrates the detachment at 850 m depth. ISM steadily increases (indicating higher temperature alteration) with depth, reflecting Mesozoic burial metamorphism. ISM abruptly decreases at the detachment, and smectites, zeolites and abundant iron oxides appear. These features suggest alteration by low temperature, downwelling, oxidizing fluids. The hydrosilicates occur in secondary pores, often in strongly silicified zones, conversely suggesting deposition by relatively dilute, upwelling warm fluids. In this case the SPLM appears to have been a conduit for horizontal flow, with meteoric fluids entering via younger normal faults dissecting the upper plate, again < 5 MYA.

Late-stage weakly consolidated gouges are found high on MCC at SPLM and DV. These gouges exhibit very low ISM relative to wall rocks, with abundant smectite and/or zeolite, consistent with low temperature alteration by surface fluids. Field IR spectroscopy identifies limited zones of elevated ISM, presumably reflecting upwelling zones for geothermal fluids.