EVOLUTION OF THE NORTHWESTERN MARGIN OF THE BASIN AND RANGE: INSIGHTS FROM NEW GEOLOGIC MAPPING IN NORTHEASTERN CALIFORNIA

Along the northwestern margin of the Basin and Range province, slip on the Surprise Valley Fault has resulted in the uplift of the Warner Range. New geologic mapping in the Warner Range and surrounding region documents a complex history of volcanism and extension from the Eocene to the present, recorded by volcanic rock units that display significant variability in their distribution, along-strike stratigraphic thickness, and their relationship to normal faults and pre-existing topography.

At the base of the range, a late Eocene to early Oligocene (34-31 Ma) volcaniclastic alluvial sandstone and conglomeratic sequence is exposed for about 25 km along strike, decreasing in thickness from 1500-200 m south to north. These rocks are overlain by a series of overlapping volcanic edifices that record Oligocene arc volcanism from 28–24 Ma. In the southern part of the range, Oligocene volcanic rocks are unconformably overlain by Miocene (19–12 Ma, predominantly 16–14 Ma) tuffs and locally-sourced mafic lava flows. Mid-Miocene rocks were never present in the northern part of the range, however, where the Oligocene rocks are directly overlain by late Miocene (10–7 Ma) rhyolite flows.

The modern profile and elevation of the Warner Range thus includes a large component of “original” topography. Although high elevations (<3000 m) in the central Warner Range (Warren Peak) correspond to the greatest stratigraphic throw on the Surprise Valley Fault, the southern Warner Range (Eagle Peak) sits at a similar elevation, despite significantly less throw on the fault there, due to substantial Miocene volcanic edifices in the footwall.

On the west side of the Warner Range, the ~11 Ma Alturas tuff and ~3-7 Ma basalt flows appear to have been deposited in pre-existing topography developed in part through motion along normal faults. These flat-lying units are also cut by faults, however, suggesting at least two episodes of faulting: one between 14 and 11 Ma and one after 3–4 Ma. Pliocene basalt flows are also present east of the Warner Range, where they dip 10-15° west and are cut by numerous normal faults. Despite the similarity of these basalt flows on either side of the range, new mapping indicates they never formed a continuous sheet and thus cannot be used to calculate the total offset and long-term slip rate along the Surprise Valley Fault.