Geological history and structural evolution of the Warner Range and Surprise Valley, northwestern margin of the Basin and Range province [electronic resource]
Keywords: Geological and Environmental Sciences.

Along the northwestern margin of the Basin and Range province, mid-Miocene to Pliocene volcanic plateaus obscure much of the earlier history of this region. In the northeastern corner of California, however, slip on the relatively isolated Surprise Valley fault has resulted in the uplift of the Warner Range, providing unique insight into the tectonic and magmatic history of this region. In contrast to most of the very active western boundary, the northwestern margin of the Basin and Range remains sparsely mapped and studied. The geologic mapping, cross-sections, geochronology, geochemical analysis, and provenance studies of rock units present in the Warner Range, and geophysical modeling of the Surprise Valley presented here add detail to this understudied region. A history of subduction-related arc volcanism lasting from ~40 Ma to the mid-Miocene is exposed in the > 4 km of volcaniclastic sediments and volcanic rocks of the Warner Range. At the base of the range, ~40 Ma andesite lavas and debris flows mark a return to subduction-related arc volcanism associated with the ancestral Cascade arc. A thick sequence of latest Eocene to Oligocene volcaniclastic rocks thins from 1500 m to 200 m over 35 km of exposure. Predominantly volcanic clast compositions, detrital zircon ages, and paleocurrent indicators suggest that the sequence was deposited in an alluvial plain ~20 km NNE of a volcanic source area. This localized accumulation differs markedly from contemporaneous drainages to the south that transported material westward from central Nevada to the paleoshoreline. A series of Oligocene volcanic edifices built on this sequence, recording arc volcanism from 28--24 Ma. In the southern part of the range, these Oligocene volcanic rocks are unconformably overlain by mid-Miocene (16--14 Ma) mafic lava flows and tuffs, once thought to be the southern extension of the mid-Miocene flood basalts, but which are now shown to be locally derived and related to arc volcanism. Mid-Miocene rocks were never present in the northern part of the range, however, where Oligocene rocks are directly overlain by late Miocene (10--7 Ma) rhyolite flows. Extension and uplift appears to have occurred in two episodes, the first in the middle Miocene and the second initiating ~3 Ma, which continues to the present day. The range-bounding Surprise Valley fault currently dips at a moderate angle of ~35° to the east. Seismic velocity and potential field modeling of the basin identified numerous intra-basin faults, but it is not clear if these steeply-dipping faults cut and offset the Surprise Valley fault or sole into it. Geothermal springs occur primarily along intra-basin faults rather than the range-bounding fault, however, which may suggest that this is a system in transition to a new set of more favorably-oriented faults. The total magnitude of extension in the Warner Range region may be as much as 15%, the majority of which is accommodated along the Surprise Valley fault. While this is a relatively minor amount of extension, the Surprise Valley fault appears to have persisted as the westernmost boundary of Basin and Range extension since the mid-Miocene.