VOLCANOTECTONIC DEVELOPMENT ALONG THE HIGH LAVA PLAINS - BASIN AND RANGE TRANSITION IN SOUTHERN OREGON: PRELIMINARY RESULTS OF A 2009 EDMAP PROJECT

Southern Oregon is an ideal natural laboratory for field-based studies of mid-Miocene and younger extensional tectonism and magmatism due to well-developed fault patterns and near continuous volcanism since ~16.6 Ma. Southern Oregon also hosts the transition between the Basin and Range and High Lava Plains provinces; a complex region with respect to magmatic and tectonic influences. These complexities are one focus of the multi-disciplinary High Lava Plains Project aimed at imaging the High Lava Plains crust and mantle on a regional scale to address issues of continental lithosphere formation and modification. In this context we have undertaken, with the support of the USGS EDMAP Program, a detailed mapping-based geologic study of the Hawks Valley-Lone Mountain area (HVLM) of south-central Oregon. Results from recently completed detailed mapping by a team that included one post-doctoral researcher, one Ph.D. student, and two undergraduate students are being integrated with the results of previous local and regional studies. Our observations and data indicate that the HVLM is characterized by a NW-striking structural valley that cuts an ~150 km² mid-Miocene, largely trachyte-rhyolite volcanic complex. Available radiometric ages and stratigraphic relationships indicate that the bulk of HVLM volcanism occurred during a <1 m.y. window synchronous with or shortly following eruption of the Steens basalt from fissures in the Steens-Pueblo Mountains located less than 20 km to the east. Local HVLM eruptions produced small volumes of Quaternary basalt that were erupted along and offset by reactivated NNE-striking structures. In contrast, NW-striking faults displace only the mid-Miocene silicic lavas and are in turn cut by NNE-striking structures. Additionally, there is a strong association of silicic vent locations and these structural trends. Numerous ~16 Ma rhyolite/trachyte vents are situated at or near the intersection of NNE- and NW-striking structures indicating that both structural trends were active and available for magma passage in the mid-Miocene We will present a working version of a 1:24,000 scale geologic map encompassing portions of the Hawks Mountain and Acty Mtn. NE Quadrangles, together with unit descriptions, geochemistry, chronology, and additional volcanotectonic interpretations.