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• 1. [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

• 2. [Article] Rhyolitic magmatism of the High Lava Plains and adjacent Northwest Basin and Range, Oregon : implications for the evolution of continental crust

  Understanding continental crust formation and modification is a fundamental and longstanding geologic problem. Influx of mantle-derived basaltic magma and partial melting of the crust are two ways to drive ...

  Citation × Citation Citation Abstract Copy Title: Rhyolitic magmatism of the High Lava Plains and adjacent Northwest Basin and Range, Oregon : implications for the evolution of continental crust Author: Ford, Mark T., 1973- Understanding continental crust formation and modification is a fundamental and longstanding geologic problem. Influx of mantle-derived basaltic magma and partial melting of the crust are two ways to drive crustal differentiation. This process results in a low density upper crust and denser, more refractory lower crust, creating significant and vastly different geochemical reservoirs over time. The High Lava Plains (HLP) and Northwestern Basin and Range (NWBR) in central and eastern Oregon provide an excellent example of intraplate volcanism where we can examine the beginnings of segregation of a relatively young, recently accreted crust. The origins of continental magmatism and its relationship to plate tectonics, especially away from the continental margins, are only slowly becoming revealed. The western United States is the most volcanically active part of North America during Cenozoic time, and this activity includes the enigmatic volcanism of the HLP and NWBR. Rhyolitic volcanism in the HLP and NWBR is age-progressive but in a direction that is nearly perpendicular to North American Plate motion. Despite being erupted through a similar crust and with a similar composition of mafic input, the HLP province is strongly bimodal (basalt-rhyolite) while the NWBR province exhibits a continuum of compositions. High silica rhyolites are commonplace in the HLP, with approximately a 1:1 ratio of rhyolite to basalt, even though the crust is comprised of mafic accreted terranes. Asthenospheric flow, mantle melting and crustal extension coupled with southwesterly North American plate motion explain the age-progressive volcanism of the HLP and NWBR. Differential asthenopheric counterflow and mantle upwelling created by the down-going Cascadia slab, coupled with transtensional stresses related to the rotation of the North American plate and Basin and Range extension, decreasing to the north, can produce the observed variations in rhyolite compositions and volumes in the two adjacent provinces. These differences are caused by fundamentally different petrogenetic processes that take place in the crust. In the HLP, an increase in mantle-derived magma flux into the lower crust has created low silica rhyolite via partial melt that separated, coalesced and rose buoyantly. This low silica rhyolite may erupt, solidify in the upper crust, or differentiate by fractional crystallization to produce high-iron, high-silica rhyolite containing an anhydrous phase assemblage. In the NWBR, a smaller flux of basaltic magma, coupled with greater transtension resulted in small crustal processing zones where fractional crystallization coupled with magma mixing and recharge created a wide range of compositions. Partial melting to form rhyolites was limited. These rhyolites have lower iron, and hydrous phases (biotite, amphibole) are common. These processes modify the crust in different ways, leaving a stratified crust in the HLP but a less modified crust in the NWBR. Recent geophysical and isotopic studies bear out these differences and allow for a unified, internally consistent model for both provinces, one that relies only on partial melt generation driven by current plate movements and do not require a mantle plume contribution. The bimodal volcanism of the HLP is a direct consequence of the processes that cause the gravitational differentiation of the continental crust into upper and lower units. The model for the HLP is generally applicable to other localities that have predominantly mafic crust and a similar balance of crustal transtension and mantle-derived basaltic flux. One such place is Iceland, which has strongly bimodal (basalt – rhyolite) volcanism. In areas where silicic crust has become substantially more mafic due to a high flux of intraplated basalts, such as in the bimodal Snake River Plain, the model is also applicable. Title: Rhyolitic magmatism of the High Lava Plains and adjacent Northwest Basin and Range, Oregon : implications for the evolution of continental crust Author: Ford, Mark T., 1973- Understanding continental crust formation and modification is a fundamental and longstanding geologic problem. Influx of mantle-derived basaltic magma and partial melting of the crust are two ways to drive crustal differentiation. This process results in a low density upper crust and denser, more refractory lower crust, creating significant and vastly different geochemical reservoirs over time. The High Lava Plains (HLP) and Northwestern Basin and Range (NWBR) in central and eastern Oregon provide an excellent example of intraplate volcanism where we can examine the beginnings of segregation of a relatively young, recently accreted crust. The origins of continental magmatism and its relationship to plate tectonics, especially away from the continental margins, are only slowly becoming revealed. The western United States is the most volcanically active part of North America during Cenozoic time, and this activity includes the enigmatic volcanism of the HLP and NWBR. Rhyolitic volcanism in the HLP and NWBR is age-progressive but in a direction that is nearly perpendicular to North American Plate motion. Despite being erupted through a similar crust and with a similar composition of mafic input, the HLP province is strongly bimodal (basalt-rhyolite) while the NWBR province exhibits a continuum of compositions. High silica rhyolites are commonplace in the HLP, with approximately a 1:1 ratio of rhyolite to basalt, even though the crust is comprised of mafic accreted terranes. Asthenospheric flow, mantle melting and crustal extension coupled with southwesterly North American plate motion explain the age-progressive volcanism of the HLP and NWBR. Differential asthenopheric counterflow and mantle upwelling created by the down-going Cascadia slab, coupled with transtensional stresses related to the rotation of the North American plate and Basin and Range extension, decreasing to the north, can produce the observed variations in rhyolite compositions and volumes in the two adjacent provinces. These differences are caused by fundamentally different petrogenetic processes that take place in the crust. In the HLP, an increase in mantle-derived magma flux into the lower crust has created low silica rhyolite via partial melt that separated, coalesced and rose buoyantly. This low silica rhyolite may erupt, solidify in the upper crust, or differentiate by fractional crystallization to produce high-iron, high-silica rhyolite containing an anhydrous phase assemblage. In the NWBR, a smaller flux of basaltic magma, coupled with greater transtension resulted in small crustal processing zones where fractional crystallization coupled with magma mixing and recharge created a wide range of compositions. Partial melting to form rhyolites was limited. These rhyolites have lower iron, and hydrous phases (biotite, amphibole) are common. These processes modify the crust in different ways, leaving a stratified crust in the HLP but a less modified crust in the NWBR. Recent geophysical and isotopic studies bear out these differences and allow for a unified, internally consistent model for both provinces, one that relies only on partial melt generation driven by current plate movements and do not require a mantle plume contribution. The bimodal volcanism of the HLP is a direct consequence of the processes that cause the gravitational differentiation of the continental crust into upper and lower units. The model for the HLP is generally applicable to other localities that have predominantly mafic crust and a similar balance of crustal transtension and mantle-derived basaltic flux. One such place is Iceland, which has strongly bimodal (basalt – rhyolite) volcanism. In areas where silicic crust has become substantially more mafic due to a high flux of intraplated basalts, such as in the bimodal Snake River Plain, the model is also applicable. Close

• 3. [Article] Shear development and overprinting in a back-arc basin, Klamath Falls, Oregon

  Celebration of Undergraduate Excellence (2013)

  Citation × Citation Citation Abstract Copy Title: Shear development and overprinting in a back-arc basin, Klamath Falls, Oregon Author: Meigs, Andrew, Waldien, Trevor Celebration of Undergraduate Excellence (2013) Title: Shear development and overprinting in a back-arc basin, Klamath Falls, Oregon Author: Meigs, Andrew, Waldien, Trevor Celebration of Undergraduate Excellence (2013) Close