The tectonic and geologic setting of eastern Oregon includes the volcanically active High Lava Plains (HLP) province and the accreted terrains of the Blue and Wallowa Mountains and is bounded by the Columbia River flood basalts to the north, Basin and Range extension to the south, the Cascade arc to the west, and stable North America to the east. Several models have been proposed to explain the tectonic evolution of eastern Oregon and, in particular, the voluminous volcanic activity in the HLP. but a consensus on which model fully describes the complex range of processes remains elusive. Measurements of the seismic anisotropy that results from active mantle flow beneath the region can provide a crucial test of such models. To constrain this anisotropy, here we present new SKS splitting results obtained at approximately 200 broadband seismic stations in eastern Oregon and the surrounding region. Data come from the USArray Transportable Array (TA) and two temporary experiments carried out in the HLP and in the Wallowa Mountains. Our splitting data set includes similar to 2900 individual splitting measurements from SKS phases recorded between 2006 and 2008. Stations in eastern Oregon exhibit significant shear wave splitting, with average delay times at individual stations between similar to 0.8 s and similar to 2.7 s. In the HLP, nearly all observed fast directions are approximately E-W, while to the north in the Blue and Wallowa Mountains there is more variability in the splitting patterns. The average delay time observed at stations located in the heart of the HILP province is similar to 2 s, well above the global average of similar to 1 s for continental regions. We infer from the large split times and homogeneous fast directions that there must be significant active flow in a roughly E-W direction in the asthenosphere beneath the HLP; this inferred flow field places a strong constraint on models that seek to explain the young tectonomagmatic activity in the region. In the Wallowa region, the anisotropic signature is more complicated and there may be a significant contribution from fossil fabrics in the crust or mantle lithosphere. (C) 2009 Elsevier B.V. All rights reserved.

The tectonic and geologic setting of eastern Oregon includes the volcanically active High Lava Plains (HLP) province and the accreted terrains of the Blue and Wallowa Mountains and is bounded by the Columbia River flood basalts to the north, Basin and Range extension to the south, the Cascade arc to the west, and stable North America to the east. Several models have been proposed to explain the tectonic evolution of eastern Oregon and, in particular, the voluminous volcanic activity in the HLP. but a consensus on which model fully describes the complex range of processes remains elusive. Measurements of the seismic anisotropy that results from active mantle flow beneath the region can provide a crucial test of such models. To constrain this anisotropy, here we present new SKS splitting results obtained at approximately 200 broadband seismic stations in eastern Oregon and the surrounding region. Data come from the USArray Transportable Array (TA) and two temporary experiments carried out in the HLP and in the Wallowa Mountains. Our splitting data set includes similar to 2900 individual splitting measurements from SKS phases recorded between 2006 and 2008. Stations in eastern Oregon exhibit significant shear wave splitting, with average delay times at individual stations between similar to 0.8 s and similar to 2.7 s. In the HLP, nearly all observed fast directions are approximately E-W, while to the north in the Blue and Wallowa Mountains there is more variability in the splitting patterns. The average delay time observed at stations located in the heart of the HILP province is similar to 2 s, well above the global average of similar to 1 s for continental regions. We infer from the large split times and homogeneous fast directions that there must be significant active flow in a roughly E-W direction in the asthenosphere beneath the HLP; this inferred flow field places a strong constraint on models that seek to explain the young tectonomagmatic activity in the region. In the Wallowa region, the anisotropic signature is more complicated and there may be a significant contribution from fossil fabrics in the crust or mantle lithosphere. (C) 2009 Elsevier B.V. All rights reserved.

Since the mid-Miocene, the northwestern United States has experienced extensive flood basalt volcanism, followed by the formation of two time-progressive tracks of silicic volcanism: the Yellowstone/Snake River Plains (YSRP) and the High Lava Plains (HLP). The YSRP track progresses towards the northeast, parallel to North American plate motion, and has therefore often been attributed to a deep mantle plume source. However, the HLP track progresses to the northwest over the same time frame in a direction not consistent with any regional plate motion. The causes of the mid-Miocene flood basalts and the tracks of the YSRP and HLP are a matter of ongoing debate. We present results of Rayleigh wave phase velocity inversions and inversions for 3-D shear wave velocity structure of the northwestern United States using data collected from the High Lava Plains seismic experiment and the EarthScope USArray Transportable Array (TA). The large number of stations used in these inversions allows us to show an unprecedented level of detail in the seismic velocity structures of this tectonically complex area. Our velocity images indicate that low S-wave velocities in the uppermost mantle do not well match the track of HLP volcanism. While at the surface the Newberry caldera appears to anchor the NW end of the HLP hotspot track, the seismic results show that it lies in a separate, north-south trending low velocity band just east of the Cascades that is distinct from the main HLP trace.

Since the mid-Miocene, the northwestern United States has experienced extensive flood basalt volcanism, followed by the formation of two time-progressive tracks of silicic volcanism: the Yellowstone/Snake River Plains (YSRP) and the High Lava Plains (HLP). The YSRP track progresses towards the northeast, parallel to North American plate motion, and has therefore often been attributed to a deep mantle plume source. However, the HLP track progresses to the northwest over the same time frame in a direction not consistent with any regional plate motion. The causes of the mid-Miocene flood basalts and the tracks of the YSRP and HLP are a matter of ongoing debate. We present results of Rayleigh wave phase velocity inversions and inversions for 3-D shear wave velocity structure of the northwestern United States using data collected from the High Lava Plains seismic experiment and the EarthScope USArray Transportable Array (TA). The large number of stations used in these inversions allows us to show an unprecedented level of detail in the seismic velocity structures of this tectonically complex area. Our velocity images indicate that low S-wave velocities in the uppermost mantle do not well match the track of HLP volcanism. While at the surface the Newberry caldera appears to anchor the NW end of the HLP hotspot track, the seismic results show that it lies in a separate, north-south trending low velocity band just east of the Cascades that is distinct from the main HLP trace.

We use data from the 118-station High Lava Plains (HLP) seismic experiment together with other regional broadband seismic data to image the 3D shear wave velocity structure in the Pacific Northwest using ambient noise tomography. This extensive data set allows us to resolve fine-scale crustal structures throughout the HLP area in greater detail than previous studies. Our results show 1) a high velocity cylinder in the crust and average velocities in the upper mantle beneath the Owyhee Plateau; 2) a mid-crustal high velocity anomaly along the Snake River Plain that also extends south into Nevada and Utah; 3) a low velocity anomaly directly beneath Yellowstone throughout the crust; and 4) low velocities beneath the HLP both in the crust and uppermost mantle, possibly indicating very thin or absent mantle lithosphere in the area. These features provide important constraints on possible models for Miocene to recent volcanism in the Pacific Northwest. Citation: Hanson-Hedgecock, S., L. S. Wagner, M. J. Fouch, and D. E. James (2012), Constraints on the causes of mid-Miocene volcanism in the Pacific Northwest US from ambient noise tomography, Geophys. Res. Lett., 39, L05301, doi:10.1029/2012GL051108.

We use data from the 118-station High Lava Plains (HLP) seismic experiment together with other regional broadband seismic data to image the 3D shear wave velocity structure in the Pacific Northwest using ambient noise tomography. This extensive data set allows us to resolve fine-scale crustal structures throughout the HLP area in greater detail than previous studies. Our results show 1) a high velocity cylinder in the crust and average velocities in the upper mantle beneath the Owyhee Plateau; 2) a mid-crustal high velocity anomaly along the Snake River Plain that also extends south into Nevada and Utah; 3) a low velocity anomaly directly beneath Yellowstone throughout the crust; and 4) low velocities beneath the HLP both in the crust and uppermost mantle, possibly indicating very thin or absent mantle lithosphere in the area. These features provide important constraints on possible models for Miocene to recent volcanism in the Pacific Northwest. Citation: Hanson-Hedgecock, S., L. S. Wagner, M. J. Fouch, and D. E. James (2012), Constraints on the causes of mid-Miocene volcanism in the Pacific Northwest US from ambient noise tomography, Geophys. Res. Lett., 39, L05301, doi:10.1029/2012GL051108.

The Pacific Northwest (PNW) has a complex tectonic history and over the past similar to 17 Ma has played host to several major episodes of intraplate volcanism. These events include the Steens/Columbia River flood basalts (CRB) and the striking spatiotemporal trends of the Yellowstone/Snake River Plain (Y/SRP) and High Lava Plains (HLP) regions. Several different models have been proposed to explain these features, which variously invoke the putative Yellowstone plume, rollback and steepening of the Cascadia slab, extensional processes in the lithosphere, or a combination of these. Here we integrate seismologic, geodynamic, geochemical, and petrologic results from the multidisciplinary HLP project and associated analyses of EarthScope USArray seismic data to propose a conceptual model for post-20 Ma mantle dynamics beneath the PNW and the relationships between mantle flow and surface tectonomagmatic activity. This model invokes rollback subduction as the main driver for mantle flow beneath the PNW beginning at similar to 20 Ma. A major pulse of upwelling due to slab rollback and upper plate extension and consequent melting produced the Steens/CRB volcanism, and continuing trench migration enabled mantle upwelling and hot, shallow melting beneath the HLP. An additional buoyant mantle upwelling is required to explain the Y/SRP volcanism, but subduction-related processes may well have played a primary role in controlling its timing and location, and this upwelling likely continues today in some form. This conceptual model makes predictions that are broadly consistent with seismic observations, geodynamic modeling experiments, and petrologic and geochemical constraints.

The Pacific Northwest (PNW) has a complex tectonic history and over the past similar to 17 Ma has played host to several major episodes of intraplate volcanism. These events include the Steens/Columbia River flood basalts (CRB) and the striking spatiotemporal trends of the Yellowstone/Snake River Plain (Y/SRP) and High Lava Plains (HLP) regions. Several different models have been proposed to explain these features, which variously invoke the putative Yellowstone plume, rollback and steepening of the Cascadia slab, extensional processes in the lithosphere, or a combination of these. Here we integrate seismologic, geodynamic, geochemical, and petrologic results from the multidisciplinary HLP project and associated analyses of EarthScope USArray seismic data to propose a conceptual model for post-20 Ma mantle dynamics beneath the PNW and the relationships between mantle flow and surface tectonomagmatic activity. This model invokes rollback subduction as the main driver for mantle flow beneath the PNW beginning at similar to 20 Ma. A major pulse of upwelling due to slab rollback and upper plate extension and consequent melting produced the Steens/CRB volcanism, and continuing trench migration enabled mantle upwelling and hot, shallow melting beneath the HLP. An additional buoyant mantle upwelling is required to explain the Y/SRP volcanism, but subduction-related processes may well have played a primary role in controlling its timing and location, and this upwelling likely continues today in some form. This conceptual model makes predictions that are broadly consistent with seismic observations, geodynamic modeling experiments, and petrologic and geochemical constraints.

We perform a joint inversion of phase velocities from both earthquake and ambient noise induced Rayleigh waves to determine shear wave velocity structure in the crust and upper mantle beneath the Pacific Northwest. We focus particularly on the areas affected by mid-Miocene to present volcanic activity. The joint inversion, combined with the high density seismic network of the High Lava Plains seismic experiment and data from the EarthScope Transportable Array, provides outstanding resolution for this area. In Oregon, we find that the pattern of low velocities in the crust and uppermost mantle varies between the High Lava Plains physiographic province and the adjacent northwestern Basin and Range. These patterns may be due to the presence of the Brothers Fault Zone which separates the clockwise rotating northwest Basin and Range from the relatively undeformed areas further north. Further to the east, the Owyhee Plateau, Snake River Plain (SRP) and northeastern Basin and Range are characterized by high crustal velocities, though the depth extent of these fast wave speeds varies by province. Of particular interest is the mid-crustal high velocity sill, previously only identified within the SRP. We show this anomaly extends significantly further south into Utah and Nevada. We suggest that one possible explanation is lateral crustal extrusion due to the emplacement of the high density mafic mid-crustal sill structures within the SRP.

We perform a joint inversion of phase velocities from both earthquake and ambient noise induced Rayleigh waves to determine shear wave velocity structure in the crust and upper mantle beneath the Pacific Northwest. We focus particularly on the areas affected by mid-Miocene to present volcanic activity. The joint inversion, combined with the high density seismic network of the High Lava Plains seismic experiment and data from the EarthScope Transportable Array, provides outstanding resolution for this area. In Oregon, we find that the pattern of low velocities in the crust and uppermost mantle varies between the High Lava Plains physiographic province and the adjacent northwestern Basin and Range. These patterns may be due to the presence of the Brothers Fault Zone which separates the clockwise rotating northwest Basin and Range from the relatively undeformed areas further north. Further to the east, the Owyhee Plateau, Snake River Plain (SRP) and northeastern Basin and Range are characterized by high crustal velocities, though the depth extent of these fast wave speeds varies by province. Of particular interest is the mid-crustal high velocity sill, previously only identified within the SRP. We show this anomaly extends significantly further south into Utah and Nevada. We suggest that one possible explanation is lateral crustal extrusion due to the emplacement of the high density mafic mid-crustal sill structures within the SRP.