SOIL GAS AND TOPOGRAPHIC PROFILES OF PLEISTOCENE TO HOLOCENE FAULTS IN THE LAKE TAHOE BASIN, NEVADA AND CALIFORNIA

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.