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• 1. [Article] The structure and stratigraphy of the Virgin Valley-McGee Mountain area, Humboldt County, Nevada

  The Virgin Valley-McGee Mountain area is located in the northwest corner of Nevada in the northwestern part of the Basin and Range structural province. Rocks within the area consist predominantly of Miocene ...

  Citation × Citation Citation Abstract Copy Title: The structure and stratigraphy of the Virgin Valley-McGee Mountain area, Humboldt County, Nevada Author: Wendell, William Glen The Virgin Valley-McGee Mountain area is located in the northwest corner of Nevada in the northwestern part of the Basin and Range structural province. Rocks within the area consist predominantly of Miocene to Pleistocene volcanic rocks and volcanic derived sedimentary rocks. The volcanic rocks include rhyolite, rhyolite welded tuffs, and basalt. Sedimentary rocks are volcanic conglomerates, sandstones, siltstones, and mudstones with subordinate amounts of diatomite, lignite, opaline chert, and volcanic tuff. Unconsolidated deposits include Quaternary alluvium, landslide debris, and playa sediments. The entire sequence is 2200± feet thick. Rock units within the thesis area can be correlated with units of the same age throughout northeastern California, northwestern Nevada, central and southern Oregon, and western Idaho. The principal structural features within the area are the Virgin Valley syncline, Big Mountain-McGee Mountain anticline, and the tilted fault-block mountain that gives rise to the Big Mountain- McGee Mountain complex. The deformation that produced the broad, gentle folds and fault-block mountain were part of the Basin and Range orogeny. The episode of greatest faulting occurred during Pleistocene time, producing the present features of the Basin and Range structure within the thesis area and the surrounding region. Title: The structure and stratigraphy of the Virgin Valley-McGee Mountain area, Humboldt County, Nevada Author: Wendell, William Glen The Virgin Valley-McGee Mountain area is located in the northwest corner of Nevada in the northwestern part of the Basin and Range structural province. Rocks within the area consist predominantly of Miocene to Pleistocene volcanic rocks and volcanic derived sedimentary rocks. The volcanic rocks include rhyolite, rhyolite welded tuffs, and basalt. Sedimentary rocks are volcanic conglomerates, sandstones, siltstones, and mudstones with subordinate amounts of diatomite, lignite, opaline chert, and volcanic tuff. Unconsolidated deposits include Quaternary alluvium, landslide debris, and playa sediments. The entire sequence is 2200± feet thick. Rock units within the thesis area can be correlated with units of the same age throughout northeastern California, northwestern Nevada, central and southern Oregon, and western Idaho. The principal structural features within the area are the Virgin Valley syncline, Big Mountain-McGee Mountain anticline, and the tilted fault-block mountain that gives rise to the Big Mountain- McGee Mountain complex. The deformation that produced the broad, gentle folds and fault-block mountain were part of the Basin and Range orogeny. The episode of greatest faulting occurred during Pleistocene time, producing the present features of the Basin and Range structure within the thesis area and the surrounding region. Close

• 2. [Article] Two-stage opening of the northwestern basin and range in eastern Oregon : evidence from the Miocene Crane basin

  New data are presented that provide evidence for the onset of extensional deformation in the Northwestern Basin and Range within 1 million years after the eruption of the Steens Basalt at 16.5 Ma. New ...

  Citation × Citation Citation Abstract Copy Title: Two-stage opening of the northwestern basin and range in eastern Oregon : evidence from the Miocene Crane basin Author: Milliard, Justin B. New data are presented that provide evidence for the onset of extensional deformation in the Northwestern Basin and Range within 1 million years after the eruption of the Steens Basalt at 16.5 Ma. New geologic mapping (1:24,000), stratigraphic sections, and ⁴⁰Ar/³⁹Ar dating of the Crane Basin rocks provide a control of the structural and depositional history of the basin. Extension-related horst and graben formation began at 15.5 Ma and continued until ~7 Ma. The Crane Basin, is one of a series of these half-grabens located to the east of the Harney Basin and Burns, Oregon on the footwall block of the Steens Mountain normal fault. Basin fill consists of volcaniclastic sedimentary rocks, tuffs, and lava flows. Key intra-basin marker beds include the 9.63 Ma Devine Canyon tuff and 11.5 Ma Visher Creek lava, in addition to a newly-discovered air fall ash with a ~12.5 Ma age, the 7.25 Ma Drinkwater basalt, and the 1.93 Ma Voltage basalt age. Crane Basin strata unconformably onlap tilted Steens Basalt. Angular unconformities exist in the Crane Basin strata, between the basal sedimentary rocks, overlying Devine Canyon tuff and capping Drinkwater basalt. Fault histories in the Crane Basin record similar rates of displacement from ~15.5 Ma to 9.63 Ma, divergent and heightened rates from 9.63 to 7.25 Ma and extremely slow rates after 7.25Ma. Together, these observations indicate that broad extensional deformation initiated following the eruption of the 16.5 Ma Steens Basalt and continuing until ~9.63 Ma (STAGE 1). After ~9.63 Ma deformation became heterogeneous and localized, migrating east to west, eventually shifting from the Crane Basin to the Harney Basin. The field relationships signal that the second deformational event was short-lived, ceased by ~7Ma (STAGE 2). Post 7.25 Ma the displacement on Crane Basin faults and the Harney Basin depositional record demonstrates that only minor fault deformation occurred. The findings of this study provide a previously undocumented resolution of the development stages of the margin of a continental extensional province. Moreover, these new data demonstrate a "temporal" link between magmatic and tectonic activity in Southeastern Oregon, suggesting that both deformation mechanisms jointly accommodated extensional strain at the margin of the developing Basin and Range Province. Title: Two-stage opening of the northwestern basin and range in eastern Oregon : evidence from the Miocene Crane basin Author: Milliard, Justin B. New data are presented that provide evidence for the onset of extensional deformation in the Northwestern Basin and Range within 1 million years after the eruption of the Steens Basalt at 16.5 Ma. New geologic mapping (1:24,000), stratigraphic sections, and ⁴⁰Ar/³⁹Ar dating of the Crane Basin rocks provide a control of the structural and depositional history of the basin. Extension-related horst and graben formation began at 15.5 Ma and continued until ~7 Ma. The Crane Basin, is one of a series of these half-grabens located to the east of the Harney Basin and Burns, Oregon on the footwall block of the Steens Mountain normal fault. Basin fill consists of volcaniclastic sedimentary rocks, tuffs, and lava flows. Key intra-basin marker beds include the 9.63 Ma Devine Canyon tuff and 11.5 Ma Visher Creek lava, in addition to a newly-discovered air fall ash with a ~12.5 Ma age, the 7.25 Ma Drinkwater basalt, and the 1.93 Ma Voltage basalt age. Crane Basin strata unconformably onlap tilted Steens Basalt. Angular unconformities exist in the Crane Basin strata, between the basal sedimentary rocks, overlying Devine Canyon tuff and capping Drinkwater basalt. Fault histories in the Crane Basin record similar rates of displacement from ~15.5 Ma to 9.63 Ma, divergent and heightened rates from 9.63 to 7.25 Ma and extremely slow rates after 7.25Ma. Together, these observations indicate that broad extensional deformation initiated following the eruption of the 16.5 Ma Steens Basalt and continuing until ~9.63 Ma (STAGE 1). After ~9.63 Ma deformation became heterogeneous and localized, migrating east to west, eventually shifting from the Crane Basin to the Harney Basin. The field relationships signal that the second deformational event was short-lived, ceased by ~7Ma (STAGE 2). Post 7.25 Ma the displacement on Crane Basin faults and the Harney Basin depositional record demonstrates that only minor fault deformation occurred. The findings of this study provide a previously undocumented resolution of the development stages of the margin of a continental extensional province. Moreover, these new data demonstrate a "temporal" link between magmatic and tectonic activity in Southeastern Oregon, suggesting that both deformation mechanisms jointly accommodated extensional strain at the margin of the developing Basin and Range Province. Close

• 3. [Article] 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 Miocene, mafic to bimodal basalt-rhyolite volcanism and extensional faults dominate a stark ...

  Citation × Citation Citation Abstract Copy Title: Extension and volcanism : tectonic development of the northwestern margin of the Basin and Range Province in southern Oregon Author: Scarberry, Kaleb C. The northwestern corner of the Basin and Range Province (NWBR) lies in southeast Oregon where extensive Late Miocene, 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/86Sri 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 important processes 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 volcanic 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 related to the initiation of faulting at Abert Rim fault. On the basis of morphology and geochemical correlation, Juniper Mountain and Venator Buttes are the source of silicic and mafic volcanism, respectively, between 8.8 to 7.7 Ma. Partial restorations demonstrate that the Abert Rim fault south of 42º40’ N latitude had >250 m of stratigraphic separation prior to deposition of the Rattlesnake Tuff at 7 Ma. The topography north of latitude 42º40’ N was gently sloping at 7 Ma, much like the High Lava Plains are today. Thus, faulting along the NNE-striking Abert Rim initiated between 7.7 and 7 Ma and after the formation of NW-striking fault systems. Title: Extension and volcanism : tectonic development of the northwestern margin of the Basin and Range Province in southern Oregon Author: Scarberry, Kaleb C. The northwestern corner of the Basin and Range Province (NWBR) lies in southeast Oregon where extensive Late Miocene, 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/86Sri 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 important processes 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 volcanic 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 related to the initiation of faulting at Abert Rim fault. On the basis of morphology and geochemical correlation, Juniper Mountain and Venator Buttes are the source of silicic and mafic volcanism, respectively, between 8.8 to 7.7 Ma. Partial restorations demonstrate that the Abert Rim fault south of 42º40’ N latitude had >250 m of stratigraphic separation prior to deposition of the Rattlesnake Tuff at 7 Ma. The topography north of latitude 42º40’ N was gently sloping at 7 Ma, much like the High Lava Plains are today. Thus, faulting along the NNE-striking Abert Rim initiated between 7.7 and 7 Ma and after the formation of NW-striking fault systems. Close