Cordilleran Section - 97th Annual Meeting, and Pacific Section, American Association of Petroleum Geologists (April 9-11, 2001)

Paper No. 0
Presentation Time: 1:00 PM-5:00 PM


MCCARTHY, J. H.1, SCHWEICKERT, R. A.2, LAHREN, M. M.2, PATTERSON, M.2 and DINKELMAN, I.2, (1)U.S. Geol. Survey, Reno Field Office, University of Nevada, Reno, NV 89557, (2)Geol. Sci, Univ of Nevada, Reno, NV 89557,

On-land studies of Pleistocene to Holocene faults in the Lake Tahoe basin include geomorphic mapping, profiling of scarps, and soil gas studies. Soil gas samples have been collected at a depth of 70 cm along traverses across faults; sampling intervals were 10 m. Samples were analyzed onsite using a truck-mounted mass spectrometer. Here we report results from the Incline Village fault and the Golf Course fault, two normal faults within the North Tahoe-Incline Village fault zone. Steep scarps in unconsolidated glacial sediments and offsets in Holocene lake deposits indicate Holocene activity on these faults. The Incline Village fault has prominent, east-facing scarps in Tahoe outwash (about 1- to 11-m-high) and in Miocene volcanic rocks (up to 60 m). The highest scarp has slope angles of 170near the top, 280 in the central part, and 220 near the base. A prominent 50-m-wide CO2 soil gas anomaly occurs at the fault. Concentrations ~2-3x atmospheric (atm) occur on the upper part of the scarp, increase to 5x atm near the base, and peak at 8x atm 20 m into the hangingwall. The Golf Course fault has several small right-echelon steps and 2- to 14-m-high E-facing scarps in Tahoe outwash deposits. Slope angles of the highest scarp are about 170 in the upper part, 270 in the central part, and ~250 near the base. A prominent 60-m-wide CO2 anomaly at the fault has values near atmospheric along the upper part of the scarp, rising abruptly to 12x atm near the base, and reaching 15x atm 20 m into the hangingwall. O2 depletion coincides with peak CO2 values on the profiles. These results indicate that CO2 was emitted from depth; reactions of gasses with sulfides at depth may have consumed O2. These results are consistent with gasses streaming up along the faults and along fractures in the hanging walls. This technique shows great promise for evaluating faults mapped in Quaternary-Holocene deposits.