2-D High Resolution Seismic Imaging and Potential-Field Modeling of Small-Scale Intrabasin Faulting in Surprise Valley, California

Surprise Valley, located in the northeast corner of California, constitutes the westernmost basin of the Basin and Range Province (BRP) and acts as a transition zone between the unextended Cenozoic volcanic rocks of the Modoc Plateau to the west and the extended BRP to the east. Previous seismic experiments undertaken in Surprise Valley to examine the structural setting of a developing extensional basin imaged the Surprise Valley Fault (SVF), the large-scale structure that controls the basin formation. In this study, we image small-scale structures associated with a prominent N-S-trending magnetic anomaly in the basin; these small-scale structures have no surface expression, yet they presumably play a role in the active geothermal system in the valley and may lend insight into the development of the basin. We recorded a total of 198 shots fired between geophones on a fixed linear array spanning ~ 1 km using a Betsy Seisgun source. The source yielded excellent penetration depths of up to 400 m, presumably due to the ideal conditions of fine-grained lake deposits saturated almost immediately below the surface. We used a standard seismic processing sequence to create an unmigrated time section, but lack of velocity control at depth led to the creation of a suite of different migrated images to explore the effects of strong lateral velocity variations in the subsurface. . Images obtained by applying Kirchhoff pre-stack depth migration to the seismic data reveal what we interpret to be an east-dipping (~56°) normal fault offsetting tilted strata of probable Oligocene age with ~250 meters of vertical throw. In addition to the 1 km seismic line, detailed gravity and magnetic surveys were undertaken to fully image the structure(s) north and south of the seismic line. The magnetic survey was performed with a cesium vapor total-field magnetometer installed on our newly developed all-terrain vehicle (ATV) magnetometer system, which was designed for efficient surveying in desert environments. The ATV’s induced magnetic field is accounted for by a heading correction, similar to that used in aeromagnetic surveys, and the overall noise level of the system is ~ 4 nT. We collected ~ 300 km of magnetic data in 2 days, imaging a 400 nT N-S-trending magnetic high. Modeling of the potential-field data confirms the interpretation of the seismic data of a buried east-dipping normal fault. Future potential-field modeling will look at whether this tectonic model can be applied north of the seismic line where the magnetic anomaly broadens significantly or whether an intrusive body is necessary, signaling contemporaneous volcanic activity with faulting. The outcome of our study validates our strategy of rapid potential-field profiling over large areas to identify specific targets for more intensive and expensive seismic profiles, the interpretation of which can be validated by detailed potential-field modeling.