Timing of Cenozoic volcanism and Basin and Range extension in northwestern Nevada: New constraints from the northern Pine Forest Range

Eocene-middle Miocene volcanic rocks in the northern Pine Forest Range, Nevada, are ideally situated for reconstructing the timing and style of volcanism and extensional faulting in the northwesternmost part of the Basin and Range province. A conformable sequence of Cenozoic volcanic and sedimentary strata in the northern Pine Forest Range dips similar to 30 degrees W, and 11 new 40Ar/39Ar ages from this sequence define 3 major episodes of volcanic activity. Pre-Tertiary basement and older (ca. 38 Ma) Tertiary intrusive rocks are overlain unconformably by Oligocene (ca. 30-23 Ma) basalt flows and dacitic to rhyolitic ash-flow tuffs interbedded with fine-grained tuffaceous sedimentary rocks. Oligocene rocks are overlain by similar to 550 m of ca. 17-16 Ma basalt flows equivalent to the Steens Basalt in southern Oregon, and basalt flows are capped by a thin 16.3 Ma ignimbrite that likely is correlative with either the Idaho Canyon Tuff or the Tuff of Oregon Canyon. The northern Pine Forest Range is bounded to the east by a major down-to-the-east normal fault that dips similar to 40 degrees E with well-developed fault striations indicative of dip-slip motion. This fault initiated at an angle of similar to 70 degrees and was rotated similar to 30 degrees during uplift of the range. A suite of 17 apatite fission-track ages from the Pine Forest footwall block demonstrates that exhumation, uplift, and slip on the range-bounding fault began ca. 12-11 Ma and continued until at least 7 Ma, with moderate slip since then. The Pine Forest Range did not undergo significant extension before or during peak Oligocene and Miocene voleanism, and similar geologic relationships in nearby ranges suggest that a larger region of northwestern Nevada was also little extended during this interval. Basin and Range faulting in northwestern Nevada appears to have begun no earlier than 12 Ma, making it distinctly younger than deformation in much of central and southern Nevada, where peak extension occurred in the middle Miocene or earlier.