CENOZOIC EVOLUTION OF THE NORTHWESTERN BOUNDARY OF THE BASIN AND RANGE: GEOLOGIC CONSTRAINTS FROM THE WARNER RANGE AND SURPRISE VALLEY REGION

The Warner Range in northern California is bounded to the east by the Surprise Valley Fault, a major down-to-the-east normal fault that forms the active western boundary of the Basin and Range Province at this latitude. Preliminary mapping, geochronology, seismic reflection data, and drill core data from the Warner Range and Surprise Valley provide insight into the Cenozoic evolution of this boundary.

Geologic mapping at 1:24,000 documents a 3 km thick sequence of volcanic, volcaniclastic and sedimentary units in the range, dipping WSW at 10-35 degrees. Paleocurrent indicators in conglomerates near the base of the section suggest multiple sources from the SW, W, and SE. An exploratory geothermal well north of Lake City drilled through the range-bounding fault, revealing similar but stratigraphically deeper units. Based on this drill hole data, the thickness of the entire Tertiary section exceeds 5 km; the depth to pre-Tertiary basement is unknown.

Basalt flows and tuffaceous sediments exposed in eastern Surprise Valley dip 5-20 degrees WSW and are cut by a series of small-offset, east-dipping normal faults. Seismic reflection data across the valley indicate several kilometers of sediments may overlie these flows in the west, thinning rapidly to the east. The basalts range in age from 9 to 3 Ma and are similar in composition and age to the Devil's Garden Basalt on the western side of the Warner Range (Ian Carmichael, pers. comm., 2005). Although the basalt does not crop out at the top of the range, it is locally tilted with underlying units on its western flank; it is therefore possible (though not required) that Pliocene basalt flows were once continuous across the range and may predate significant faulting in the region.

These data, combined with geometric relationships in cross-section and U-Th/He ages of ~3 Ma from the base of the exposed section, imply that at least 8 km of slip has occurred on the Surprise Valley fault, possibly in the last 3-5 my. This value is significantly greater than the 3.6 km of slip proposed by previous workers, and represents an averaged slip rate of 1-3 mm/yr. This rate is consistent with slip rates suggested by Quaternary fault scarps and preliminary results from a USGS paleoseismic study of the Surprise Valley fault.