CONSTRAINTS ON THE UPPER-CRUSTAL STRUCTURE OF THE WESTERNMOST BASIN AND RANGE: PRELIMINARY RESULTS FROM REFLECTION SEISMOLOGY AND STRUCTURAL STUDIES IN THE SURPRISE VALLEY AND THE WARNER RANGE, CA

Reflection seismology and structural studies describe the upper-crustal structure of the Warner Range/Surprise Valley system in the northwestern-most Basin and Range. The Warner Range is bound on the east by east-dipping, high-angle normal fault of large offset (>2 km), and constitutes a young (<5 Ma), high relief (~1.6 km), mountain range isolated from the rest of the Basin and Range by volcanic plateaus in northwestern Nevada. This relatively unstudied tectonically active corner of the Basin and Range is characterized by significant Holocene slip on the range-bounding fault, hot-spring and geothermal exploration, and mud volcanoes developed from over-pressured strata in the basin. Reflection seismology and geologic cross-section constrain basin depth, velocity structure, and fault offset beneath Surprise Valley. The ~16 km vibrator-source reflection survey consisted of a 2D profile spanning Surprise Valley with 10-m source spacing and 40-m receiver spacing. Moderately tilted Tertiary volcanic and volcaniclastic strata, comprising the Warner Range and the hills east of Surprise Valley, suggest a valley fill thickness of 1-2 km and total fault slip of approximately 6 km. Additional faulting in the down-thrown block east of Surprise Valley includes a series of N-S trending normal faults with moderate displacement of ~ 300 m. The faulted sequence consists of a basal section of Oligocene andesitic lahars and volcaniclastic conglomerate, Miocene mafic lava flows, silicic ash flows, and a capping basalt flow that dips west 10-20°. The onset of faulting is constrained by the age of the youngest basalt flow on the west flank of the range which has been dated ~ 4 Ma (Ian Carmichael, 2004 pers, comm.) and is correlated eastward across the Warner Range to outcrops east of Surprise Valley. U-Th/He ages of ~ 3 Ma from apatite separated from granitic cobbles in Eocene conglomerate exposed at the base of the Warner Range are consistent with large amounts of post 4 Ma slip. Previous studies further south estimate faulting began after 14 Ma and report an average slip rate of .26mm/yr on the Surprise Valley Fault. Constraints provided by our study indicate a minimum slip rate of 1.6 mm/yr.