Seismic Reflection and Magnetotelluric Imaging of Southwestern Dixie Valley Basin, Nevada

摘要

Seismic reflection and magnetotelluric (MT) data acquired in southern Dixie Valley provide a unique opportunity to evaluate stratigraphy and conductivity structure associated with the seismically active, east-southeast-dipping Dixie Valley fault and related structures. Based on analysis of 2-D seismic reflection profiles and lithological data from deep borehole 66-16 in southern Dixie Valley, we recognize four major subsurface lithologic units in Dixie Valley (youngest to oldest): (1) Late Cenozoic fluvial basin fill deposits (approximately 122 m to 183 m thick); (2) Older Basin Fill deposits (244 m to 305 m thick); (3) a Volcanic Tuff and Porphyry unit (about 360 m thick); and (4) a Lower Volcanic unit (about 1190 m thick) resting on crystalline basement (approximately 1933 m depth). All units are offset by one or more strands of the Dixie Valley fault. The 3-D MT data reveal a conductive anomaly or anomalies in southwestern Dixie Valley adjacent to the Stillwater Range front at a depth of about 2000 ft to 3600 ft (700 to 1300 m). Locally, the range-front conductive anomaly is spatially associated with the Elevenmile Canyon temperature anomaly at the southwestern margin of the study area. The anomaly is primarily located within the Volcanic Tuff and Porphyry and Lower Volcanic units in the hanging wall of the Dixie Valley fault, and appears be capped above by the Dixie Valley basin fill deposits. We interpret the conductive zone to be associated with the presence of fluids and/or hydrothermal alteration. A more laterally extensive conductive anomaly is present in the upper 2800 ft (850 m) of south-central Dixie Valley at the very northern end of the Navy seismic array and is spatially associated with the Pirouette Mountain high-temperature anomaly. Assuming that this MT anomaly is associated with geothermal fluids and/or alternation, and not simply conductive clays within the fluvial basin deposits, the source of the fluids may be a deeper reservoir north of the reflection array in Dixie Valley basin. If this is correct, the fluids may have migrated up the Louderback Mountains fault or other structures east of the Dixie Valley fault.