Extension and volcanism: Tectonic development of the northwestern margin of the Basin and Range province in southern Oregon

The northwestern corner of the Basin and Range Province (NWBR) lies in southeast Oregon where extensive Late Cenozoic, mafic to bimodal basalt-rhyolite volcanism and extensional faults dominate a stark and arid landscape. Near Lake Abert, the Late Miocene volcanic section abuts Early Miocene, dominantly intermediate composition volcanoes at the Coleman Hills and Rabbit Hills. Such exposures are few in the NWBR and shed light on the volcanic and structural development of the region and significantly extend the known distribution of Early Miocene magmatism.; New 40Ar-39Ar ages indicate that magmatism occurred at the Coleman Hills between 21.4 and 22.3 Ma and at the Rabbit Hills between 20.2 and 23.4 Ma. Magmatism at the Coleman Hills initiated with an episode of silicic volcanism that was rapidly followed by the intrusion and emplacement of dacite and andesite dikes and lavas. Early Miocene rhyolite of the Coleman Hills exhibits high 87Sr/86Sr i relative to most Late Miocene rocks in the region (0.7051 vs. 0.7037) suggesting that crustal melting was important in their genesis. A notable lack of a Nb-trough, characteristic of subduction zone settings, and high 207Pb/204Pb, and 206Pb/204Pb suggests that these rocks were not generated within a volcanic arc setting and instead likely formed in a back-arc region that had been metasomatized by prior subduction. Crystal fractionation readily accounts for the generation of andesite and dacite compositions from basaltic andesite however a gap in the data between ∼66 to 70% SiO2, isotopic variability and disequilibrium textures indicate that crustal anatexis, magma mixing and/or crustal assimilation may also be an important process in the evolutionary history of the Coleman Hills.; Dike orientations within Early Miocene volcanoes often mimic the trend of faults that cut Late Miocene volcanic strata, suggesting that the faults may have utilized an existing structural fabric. Contact relations suggest as much as 500 m of paleo-topography existed near Lake Abert prior to deposition of the Middle Miocene Steens flood basalts at ∼16 Ma. Late Miocene structural development of the Abert Rim fault is chronologically linked to the formation and northward propagation of the Walker Lane transform fault system in Nevada between 9 and 3 Ma. At Lake Abert, bimodal basalt-rhyolite volcansim from 8.8 to 7.7 Ma occurred within a zone of active NW-striking faulting. Volcanism during this interval was likely related to the early development of the Abert Rim fault. On the basis of morphology and geochemical correlation, Juniper Mountain, Venator Buttes and Alkali Buttes are the source of 8.8 to 7.7 Ma volcanism north of 42°40' N latitude. Partial restorations demonstrate that the Abert Rim fault south of 42°40' N latitude had >250 m of stratigraphic separation prior to deposition of the Rattlesnake Tuff at 7 Ma. The topography north of latitude 42°40' N was gently sloping at 7 Ma, much like the High Lava Plains are today. Thus, faulting along the NNE-striking Abert Rim fault initiated between 7.7 and 7 Ma and after the formation of NW-striking fault systems.