Structural controls on the Surprise Valley fault system, northwestern margin of the Basin and Range, based on combined geologic, potential-field, and seismic reflection modeling

Surprise Valley of northeastern California is a major high-angle fault-bounded basin that marks the northwestern margin of the Basin and Range province. On the western margin of the valley, the east-dipping Surprise Valley Fault separates the valley from the Warner Range and accommodates at least 7 km of normal slip. On the eastern margin, a west-dipping normal fault has exhumed the Hays Canyon Range, but offset along this fault decreases significantly to the north. In between these two major faults, numerous Holocene fault scarps and hot springs suggest that the area is still actively extending. Previous workers have mapped several NW-SE-trending structures cutting across the valley that control the geothermal system, but the role of these structures in basin evolution and their relationship to the range-bounding faults is poorly understood. The Surprise Valley fault system is of particular interest given its location at the margin of the Basin and Range, its relative isolation from other major normal fault systems, and the recent offset along the faults. Our focus is on identifying and characterizing intra-basin and basin-bounding faults and structures and constraining the basin geometry through a combination of geological mapping, potential field modeling and seismic reflection modeling. We have collected nearly 800 new gravity stations, ~75 line-kilometers of truck-towed magnetometer data, and ~17 km of foot-traversed gradient magnetometer data within and surrounding Surprise Valley for potential field modeling. These data, combined with new 1:24,000-scale geologic mapping and a ~16 km vibrator-source reflection survey collected in the summer of 2004, confirm the importance of the NW-trending structures not only in the distribution of hot springs, but also the location and orientation of accommodation zones between N-S trending, antithetic normal fault systems. The NW trend of these structures, including the Lake City Fault Zone and Fandango Valley, parallels a prominent regional system of faults that includes the Eugene-Denio and Mt. McLoughlin fault zones to the north and the Likely Fault to the south. Field relationships imply that both N- S and NW-SE trending fault sets have been active since the late Tertiary. The widespread, NW-trending fabric, locally present in the Lake City fault zone may be accommodating strike-slip motion predicted in recently published GPS data.