VOLCANO-TECTONIC HISTORY OF THE NORTHERN WARNER RANGE IN NORTHEASTERN CALIFORNIA

The northwestern margin of the Basin and Range experienced a complex history of Tertiary volcanism and extension. Arc-volcanism produced Oligocene and mid-Miocene shield volcanoes that acted as barriers to later volcanic flows. Normal faulting occurred ca. 14-8 Ma. Late Miocene-Pliocene low-K, high-Al olivine tholeiites (LKOT) erupted ca. 8-3 Ma, filling topographic lows and obscuring the older volcanic history. All units were cut by normal faults ca. 3 Ma and extension continues today. The Warner Range (WR) exposes this entire sequence of events, providing an ideal location to study the geologic history of the margin.

New mapping in the Fort Bidwell quadrangle in the northern WR reveals volcanic rocks similar to those seen further south. At the base of the sequence is a series of interbedded mafic and silicic tuffs and basalt flows, with a total exposed thickness of ~550 m. The basalts contain ~1 cm euhedral or resorbed plagioclase phenocrysts and they have Ba and K₂O values similar to Oligocene basalts in the central WR. At the southern end of the new mapping, the sequence is 85% basalt and 15% tuff. The basalt thins to the north over 10 km to 15% of the sequence. Two late Miocene-Pliocene units overlie Oligocene units: diktytaxitic, low-K olivine tholeiite (LKOT) flows and lithic tuff. Unlike further south in the range, there are few mid-Miocene arc-volcanic rocks.

All units are cut by the Fandango Valley and Surprise Valley faults, suggesting that motion occurred primarily post-eruption of the closest dated unit, ~7.5 Ma. The ~13 km long Fandango Valley fault trends NW, parallel to a regional fabric whose origin is enigmatic, and does not appear to be active as it lacks fault scarps. In contrast, the NNE-trending Surprise Valley fault likely cuts the Fandango Valley fault, has visible fault scarps, and is likely active today. This new mapping does not reveal any new units, but the thickness of the late Miocene-Pliocene volcanic rocks here suggests that the northern Warner Range was a topographic low by the late Miocene. This may have been partially controlled in part by faulting, but pre-existing volcanic topography appears to be the primary factor influencing their distribution.