INSIGHT INTO THE EVOLUTION OF A MAJOR BASIN AND RANGE NORMAL FAULT THROUGH COMBINED GEOLOGICAL AND SEISMOLOGICAL INVESTIGATION

At the northwestern margin of the Basin and Range province, the Surprise Valley Fault (SVF) marks the transition from extension in northwestern Nevada to relatively unextended high volcanic plateaus in northeastern California and southern Oregon. The SVF is an east-dipping normal fault with at least 7 km of offset that separates the Warner Range from Surprise Valley. The SVF has been active for at least 8 my, and is therefore an ideal place to study the recent tectonic history of this margin. Additionally, the SVF is anomalously isolated from other significant Basin and Range faults by 90 km of flat-lying, unextended volcanic plateaus to the south and east, offering a unique opportunity to study the structural evolution of a significant-offset normal fault in relative isolation.

Our study combined 2D seismic reflection data from Surprise Valley with geologic mapping of the surrounding region, and was augmented by geothermal exploration, paleoseismological studies, and geochronological analyses. The reflection data delimit the relatively shallow present dip of the SVF, only 35°. Geologic mapping constrains footwall dips from 15-20 degrees in the range, with both footwall and hanging-wall dips flattening progressively away from the fault. Although the SVF is shallow (~35°) and planar to ~7 km depth, trenching across a strand of the fault exposed a steeply dipping fault plane at 65°. These relationships suggest that (1) like typical Basin and Range normal faults, the SVF may have initiated at 50-55° and rotated to its present low angle and (2) current motion along the range front may occur along more steeply dipping strands that either cross-cut or sole into the SVF, but (3) the SVF is not directly comparable to other domino-style examples of upper-crustal extension, given its isolation from other significant Basin and Range structures.

Neither geological nor geophysical data alone could have produced this insight. This study benefited through close collaboration of geologists and geophysicists at multiple institutions, cross-participation in field work and overlapping time in the field, and frequent communication to highlight and focus in on problem areas. Mapping and geochronological investigations are ongoing in this region to help constrain fault geometry and timing of extension.