HR: 11:50h
AN: T22A-07    [Abstracts]
TI: Tectonic Setting of the Gravity Fault and Implications for Ground-Water Resources in the Death Valley Region, Nevada and California
AU: * Blakely, R J
EM: blakely@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025, United States
AU: Sweetkind, D S
EM: dsweetkind@usgs.gov
AF: U.S. Geological Survey, Denver Federal Center, Denver, CO 80225, United States
AU: Faunt, C C
EM: ccfaunt@usgs.gov
AF: U.S. Geological Survey, 4165 Spruance Rd, Suite 200, San Diego, CA 92101, United States
AU: Jansen, J R
EM: JJansen@ruekert-mielke.com
AF: Aquifer Science and Technology, W233 N2080 Ridgeview Parkway, Waukesha, WI 53188, United States
AU: McPhee, D K
EM: dmcphee
AF: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025, United States
AU: Morin, R L
EM: morin@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025, United States
AB: The Amargosa trough, extending south from Crater Flat basin to the California-Nevada state line, is believed to be a transtensional basin accommodated in part by strike-slip displacement on the northwest-striking State Line fault and normal displacement on the north-striking Gravity fault. The Gravity fault, lying along the eastern margin of the Amargosa trough, was first recognized in the 1970s on the basis of correlations between gravity anomalies and a prominent spring line in Amargosa Valley. The Gravity fault causes an inflection in water-table levels, similar to other (but not all) normal faults in the area. Pools along the spring line, some of which lie within Death Valley National Park and Ash Meadows Wildlife Refuge, include endemic species potentially threatened by increasing agricultural activities in Amargosa Valley immediately to the west, where water tables are declining. Most of the springs and pools lie east of the Gravity fault, however, and it is important to understand the role that the Gravity fault plays in controlling ground-water flow. We have conducted a variety of geophysical investigations at various scales to better understand the tectonic framework of the Amargosa Desert and support new ground-water-flow models. Much of our focus has been on the tectonic interplay of the State Line, Gravity, and other faults in the area using gravity, ground-magnetic, audiomagnetotelluric (AMT), and time-domain electromagnetic (TEM) surveys. With 1250 new gravity measurements from Ash Meadows and Stewart Valley, we have developed a revised three-dimensional crustal model of the Amargosa trough constrained by well information and geologic mapping. The model predicts approximately 2 km of vertical offset on the Gravity fault but also suggests a complex structural framework. The fault is conventionally seen as a simple, down-to-the-west normal fault juxtaposing permeable pre-Tertiary carbonate rocks to the east against less permeable Tertiary sediments to the west. The new gravity inversion indicates a more complex footwall: some springs, for example, are associated with a concealed ridge or horst, with a secondary basin lying to the east. Six ground-magnetic transects across the Gravity fault using a truck- towed magnetometer show a characteristic magnetic anomaly reflecting different magnetic properties in rocks east and west of the fault. Ground-magnetic measurements, interpreted in conjunction with existing aeromagnetic data, allow us to map the shallow aspects of the Gravity fault and other faults in Ash Meadows in detail. Three TEM transects across the Gravity fault showed no strong evidence of a faulted contact, although depth of penetration may have been insufficient to reach associated resistivity contrasts. An AMT transect, however, shows a narrow zone of high resistivity directly along the Gravity fault. Although other interpretations are possible, this resistivity anomaly may reflect carbonate-rich cementation along the fault plane, possibly contributing to its influence on ground-water flow.
DE: 0920 Gravity methods (1219)
DE: 1219 Gravity anomalies and Earth structure (0920, 7205, 7240)
DE: 1517 Magnetic anomalies: modeling and interpretation
DE: 1832 Groundwater transport
DE: 8109 Continental tectonics: extensional (0905)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting