THE NORTHWESTERN BOUNDARY OF THE BASIN AND RANGE PROVINCE: A STRUCTURAL STUDY OF THE NORTHERN TERMINATION OF THE WARNER RANGE, CA

The Warner Range is a young, tectonically active range in the northwestern-most Basin and Range and is ideally located to elucidate how the extensional province dies out to the north. This study examines the tilts of Miocene basaltic lava flows which cap the range and relates those tilts to the fault systems on both the east and west flanks of the Warner Range. For most of the central portion of the range, the eastern flank is defined by an east-dipping, high-angle normal fault with a maximum offset of > 2 km. A slip rate of at least 1.6 mm/ yr along the east dipping system has created the Surprise Valley to the east, which has a fill of 1 – 2 km. At the latitude of maximum extension, tilts in the capping basaltic flows are ~20° W. The magnitude of these tilts decreases to ~10° W further north, and to the north of Fandango Valley, a major, northwest-trending accommodation structure which cuts across the entire Warner Range, the polarity of tilting is reversed, with the capping flows dipping ~10° E. This abrupt change in tilt direction as well as the topographic expression of faulting to the north of the Fandango Valley structure suggests that faults along the western flank of the range, not the eastern flank, accommodate most extension north of the Fandango Valley. The eastern range-bounding fault system dies out as a localized, discrete fault zone immediately to the north of the Fandango Valley structure, at the northern end of Surprise Valley (41°54'). Further north, the eastward tilts in the capping basalts decrease to sub-horizontal by latitude 42° 15' as the Warner Range dies out as a well-defined fault block. Although the Abert Rim and the South Warner Rim fault scarps can be traced northward from the western and eastern Warner Range fault systems, respectively, previous studies indicate that these structures accommodate relatively little extension. Thus, the Warner Range appears to be the northwestern-most expression of significant Basin and Range extension, with a very gradual transition to very low magnitude extension north of the Fandango Valley accommodation zone.