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• 1. [Article] Stratigraphic correlation of seismoturbidites and the integration of sediment cores with 3.5 kHz chirp subbottom data in southern Cascadia

  In order to evaluate the shallow stratigraphy along the southern Cascadia abyssal margin and northern California abyssal plain, CHIRP subbottom profiles capable of imaging individual turbidite beds in ...

  Citation × Citation Citation Abstract Copy Title: Stratigraphic correlation of seismoturbidites and the integration of sediment cores with 3.5 kHz chirp subbottom data in southern Cascadia Author: Black, Bran In order to evaluate the shallow stratigraphy along the southern Cascadia abyssal margin and northern California abyssal plain, CHIRP subbottom profiles capable of imaging individual turbidite beds in the upper tens of meters of the subsurface were collected. Reflectors imaged with the 3.5 kHz CHIRP subbottom data represent turbidite beds within the first 20-30 m of seafloor sediments that have sufficient impedance contrast and are separated by at least 20 cm vertically. Correlation of subbottom reflectors along both margins, constrained by core data at numerous sites along strike, revealed that many of these reflectors were contiguous over distances of hundreds of kilometers in southern Cascadia. The placement of subbottom reflectors coincided strongly with the configuration of many of the turbidite beds identified in nearby cores. A number of thin, low density beds evident in the cores are not imaged in the subbottom data in the northern part of the study area near Rogue Canyon, but become more robust southward where some are imaged as reflectors in the subbottom data. The independently derived core and subbottom based correlations of beds in southern Cascadia were found to be in fairly close agreement, and collectively support the continuity of beds over significant distances of up to 200 km along strike. Robustness of the beds between Trinidad and Rogue Canyons does not appear to be related to proximity to canyon mouths, suggesting that these beds and their transport are not exclusively related to canyon sources. The combined core and reflection data and the continuity of beds support a seismogenic source for Holocene beds correlated over long distances owing to the broad areal extent and lack of strong linkage to canyon systems. Data from paleoseismic studies worldwide were compiled in an effort to parameterize the likely triggering distances associated with turbidity currents initiated by earthquakes on the CSZ and NSAF. These ranges are estimated at 60-120 km for the CSZ and 40-75 km for the NSAF. A similar approach using MMI values associated with earthquakes linked to seismoturbidites yielded a threshold estimate for northern California of ~ MMI 5.5. A region from between Eel Canyon and Mendocino Channel in the north to the area north of Noyo Canyon in the south was identified as having a higher likelihood for recording at least a portion past seismicity from both the NSAF and CSZ in local sediments. The high frequency of turbidite beds in southernmost Cascadia could be the result of Cascadia and NSAF seismicity combined, though some input from hyperpycnal flows cannot be excluded. Further study of sediments in this zone may improve our understanding of potential stress triggering interactions between the CSZ and NSAF. Title: Stratigraphic correlation of seismoturbidites and the integration of sediment cores with 3.5 kHz chirp subbottom data in southern Cascadia Author: Black, Bran In order to evaluate the shallow stratigraphy along the southern Cascadia abyssal margin and northern California abyssal plain, CHIRP subbottom profiles capable of imaging individual turbidite beds in the upper tens of meters of the subsurface were collected. Reflectors imaged with the 3.5 kHz CHIRP subbottom data represent turbidite beds within the first 20-30 m of seafloor sediments that have sufficient impedance contrast and are separated by at least 20 cm vertically. Correlation of subbottom reflectors along both margins, constrained by core data at numerous sites along strike, revealed that many of these reflectors were contiguous over distances of hundreds of kilometers in southern Cascadia. The placement of subbottom reflectors coincided strongly with the configuration of many of the turbidite beds identified in nearby cores. A number of thin, low density beds evident in the cores are not imaged in the subbottom data in the northern part of the study area near Rogue Canyon, but become more robust southward where some are imaged as reflectors in the subbottom data. The independently derived core and subbottom based correlations of beds in southern Cascadia were found to be in fairly close agreement, and collectively support the continuity of beds over significant distances of up to 200 km along strike. Robustness of the beds between Trinidad and Rogue Canyons does not appear to be related to proximity to canyon mouths, suggesting that these beds and their transport are not exclusively related to canyon sources. The combined core and reflection data and the continuity of beds support a seismogenic source for Holocene beds correlated over long distances owing to the broad areal extent and lack of strong linkage to canyon systems. Data from paleoseismic studies worldwide were compiled in an effort to parameterize the likely triggering distances associated with turbidity currents initiated by earthquakes on the CSZ and NSAF. These ranges are estimated at 60-120 km for the CSZ and 40-75 km for the NSAF. A similar approach using MMI values associated with earthquakes linked to seismoturbidites yielded a threshold estimate for northern California of ~ MMI 5.5. A region from between Eel Canyon and Mendocino Channel in the north to the area north of Noyo Canyon in the south was identified as having a higher likelihood for recording at least a portion past seismicity from both the NSAF and CSZ in local sediments. The high frequency of turbidite beds in southernmost Cascadia could be the result of Cascadia and NSAF seismicity combined, though some input from hyperpycnal flows cannot be excluded. Further study of sediments in this zone may improve our understanding of potential stress triggering interactions between the CSZ and NSAF. Close

• 2. [Article] A 3-D Tomographic Model of the P-wave Velocity Structure of the Central Cascadia Forearc

  Signals from the R/V Langseth’s tuned airgun array were recorded on an array of 33 EarthScope FlexArray seismometers deployed in the Oregon Coast Range from 43.5° to 45°N as part of the 2012 Ridge2Trench ...

  Citation × Citation Citation Abstract Copy Title: A 3-D Tomographic Model of the P-wave Velocity Structure of the Central Cascadia Forearc Author: Kenyon, Christopher Bradford Signals from the R/V Langseth’s tuned airgun array were recorded on an array of 33 EarthScope FlexArray seismometers deployed in the Oregon Coast Range from 43.5° to 45°N as part of the 2012 Ridge2Trench experiment to image the structure of the Juan de Fuca plate. This segment of the Juan de Fuca/North America plate boundary slipped in 2 moderate low-angle thrust earthquakes in 2004, and continuing seismicity has clustered around these events since that time. Previously acquired bathymetric, potential field, and seismic data indicate the presence of subducted seamounts in this region and hinted at large velocity variations of the overlying forearc crust immediately above the plate boundary. Approximately 18000 airgun shots along 4 profiles on the continental shelf and upper slope and a profile oblique to the coast that extends across the margin to the trench provided sources for this study. Data quality is excellent, with strong Pg and PmP arrivals observed at most stations for most shots. We present velocity models that merge the new data with constraints from onshore/offshore data acquired in 1989 and 1996 to improve the resolution and extend the velocity model for this segment of the Cascadia subduction zone, which appears to be a segment boundary in the paleoseismic record and currently exhibits anomalous locking behavior as indicated by geodetic data. Our approach to modeling involved starting with a 1D P-wave velocity model and using an inversion process to modify the model to beetter fit the observed first arrival times of the airgun shots. In a subsequent step, we used a 2D starting model that incorporated constraints from prior 2D experiments that extended farther seaward and landward of the 3D study region. Finally, a smoothed version of the model obtained from the 2D inversion was used as a starting model to see if the final model could be further refined. Our final preferred model used a 2D starting model and moderate smoothing. The RMS misfit of predicted travel times is reduced from 0.48s for the initial 2D model to 0.040s for the final preferred 3D model. This model includes interesting velocity structure including possible subducted seamounts on the subducting Juan de Fuca plate and a mafic dike related to Cape Perpetua volcanics within the upper plate. These results contribute to the broader goal of understanding the relationship between plate boundary behavior and crustal structure. Title: A 3-D Tomographic Model of the P-wave Velocity Structure of the Central Cascadia Forearc Author: Kenyon, Christopher Bradford Signals from the R/V Langseth’s tuned airgun array were recorded on an array of 33 EarthScope FlexArray seismometers deployed in the Oregon Coast Range from 43.5° to 45°N as part of the 2012 Ridge2Trench experiment to image the structure of the Juan de Fuca plate. This segment of the Juan de Fuca/North America plate boundary slipped in 2 moderate low-angle thrust earthquakes in 2004, and continuing seismicity has clustered around these events since that time. Previously acquired bathymetric, potential field, and seismic data indicate the presence of subducted seamounts in this region and hinted at large velocity variations of the overlying forearc crust immediately above the plate boundary. Approximately 18000 airgun shots along 4 profiles on the continental shelf and upper slope and a profile oblique to the coast that extends across the margin to the trench provided sources for this study. Data quality is excellent, with strong Pg and PmP arrivals observed at most stations for most shots. We present velocity models that merge the new data with constraints from onshore/offshore data acquired in 1989 and 1996 to improve the resolution and extend the velocity model for this segment of the Cascadia subduction zone, which appears to be a segment boundary in the paleoseismic record and currently exhibits anomalous locking behavior as indicated by geodetic data. Our approach to modeling involved starting with a 1D P-wave velocity model and using an inversion process to modify the model to beetter fit the observed first arrival times of the airgun shots. In a subsequent step, we used a 2D starting model that incorporated constraints from prior 2D experiments that extended farther seaward and landward of the 3D study region. Finally, a smoothed version of the model obtained from the 2D inversion was used as a starting model to see if the final model could be further refined. Our final preferred model used a 2D starting model and moderate smoothing. The RMS misfit of predicted travel times is reduced from 0.48s for the initial 2D model to 0.040s for the final preferred 3D model. This model includes interesting velocity structure including possible subducted seamounts on the subducting Juan de Fuca plate and a mafic dike related to Cape Perpetua volcanics within the upper plate. These results contribute to the broader goal of understanding the relationship between plate boundary behavior and crustal structure. Close

• 3. [Article] Seismic Performance of Aging Prestressed Transmission Pole with Simulated Soil Foundation

  The Cascadia Subduction Zone has recently been identified as a source for an impending great earthquake. The potential of a large earthquake has created widespread interest into the resiliency of lifelines ...

  Citation × Citation Citation Abstract Copy Title: Seismic Performance of Aging Prestressed Transmission Pole with Simulated Soil Foundation Author: Argo, Maximo The Cascadia Subduction Zone has recently been identified as a source for an impending great earthquake. The potential of a large earthquake has created widespread interest into the resiliency of lifelines within the potentially affected areas. One critical lifeline is the electrical distribution and transmission systems which relies on legacy infrastructure components that were designed and constructed without regard to the seismic hazard. Of particular interest is the resiliency of prestressed concrete poles used for transmission towers within the region. Through material and structural testing of existing full scale specimens, moment capacities have been analyzed and compared to simulated earthquake moment demands. The structural testing allowed for an innovative solution to produce simulated site response through fabricating soil representative springs using steel plates. The soil simulated springs allowed for full scale testing with non-fixed base motions such as rotation and translation. After the conclusion of testing three different specimens, it was found that the poles remaining in the field are considerably vulnerable to site conditions but would structurally survive a subduction zone like earthquake. Title: Seismic Performance of Aging Prestressed Transmission Pole with Simulated Soil Foundation Author: Argo, Maximo The Cascadia Subduction Zone has recently been identified as a source for an impending great earthquake. The potential of a large earthquake has created widespread interest into the resiliency of lifelines within the potentially affected areas. One critical lifeline is the electrical distribution and transmission systems which relies on legacy infrastructure components that were designed and constructed without regard to the seismic hazard. Of particular interest is the resiliency of prestressed concrete poles used for transmission towers within the region. Through material and structural testing of existing full scale specimens, moment capacities have been analyzed and compared to simulated earthquake moment demands. The structural testing allowed for an innovative solution to produce simulated site response through fabricating soil representative springs using steel plates. The soil simulated springs allowed for full scale testing with non-fixed base motions such as rotation and translation. After the conclusion of testing three different specimens, it was found that the poles remaining in the field are considerably vulnerable to site conditions but would structurally survive a subduction zone like earthquake. Close

• 4. [Article] Automated detection and location of tectonic tremor along the entire Cascadia margin from 2005 to 2011

  This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/earth-and-planet...

  Citation × Citation Citation Abstract Copy Title: Automated detection and location of tectonic tremor along the entire Cascadia margin from 2005 to 2011 Author: Brudzinski, M. R., Allen, R. M., Boyarko, D. C., Porritt, R. W., Tréhu, A. M. This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/earth-and-planetary-science-letters/ Title: Automated detection and location of tectonic tremor along the entire Cascadia margin from 2005 to 2011 Author: Brudzinski, M. R., Allen, R. M., Boyarko, D. C., Porritt, R. W., Tréhu, A. M. This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/earth-and-planetary-science-letters/ Close

• 5. [Article] PattonJasonR2014_envelope.pdf

  Citation × Citation Citation Abstract Copy Title: PattonJasonR2014_envelope.pdf Author: Patton, Jason Robert Title: PattonJasonR2014_envelope.pdf Author: Patton, Jason Robert Close

• 6. [Article] A sedimentary and geophysical investigation offshore of Sumatra : subduction zone earthquakes, ground motions, and submarine landslides

  Subduction zone earthquake and tsunami hazards affect tens of millions worldwide and the recurrence of these disasters can be evaluated with paleoseismic techniques, because earthquake cycles (and their ...

  Citation × Citation Citation Abstract Copy Title: A sedimentary and geophysical investigation offshore of Sumatra : subduction zone earthquakes, ground motions, and submarine landslides Author: Patton, Jason Robert Subduction zone earthquake and tsunami hazards affect tens of millions worldwide and the recurrence of these disasters can be evaluated with paleoseismic techniques, because earthquake cycles (and their supercycles) typically span many millennia. In these three chapters, I discuss a suite of analyses I use to evaluate the sedimentary record of earthquakes as sampled in cores collected by the authors on the R/V Roger Revelle in 2007 offshore Sumatra, cruise RR0705, aka KNOX05RR. In the first chapter I discuss the mid- to late-Holocene recurrence of earthquakes in the region of the 2004 Sumatra-Andaman subduction zone earthquake. I estimate that the early Holocene to historic recurrence of turbidity current triggering earthquakes in the region of the 2004 Sumatra-Andaman subduction zone earthquake to be 260 ± 160 calendar years. I identify the uppermost turbidite in fifteen cores to have been deposited as a result of strong ground shaking from this 2004 earthquake. In the second chapter I compare and contrast the physiographic control of seismoturbidite deposition along the continental margins of the Sumatra-Andaman and Cascadia subduction zones. Cascadia’s margin is dominated by glacial cycle constructed pathways which promote turbidity current flows for large distances. Sumatra margin pathways do not inherit analogous sedimentary systems, so turbidity currents and their deposits are more localized. Finally, I discuss slope stability and earthquake ground motions along the continental margin offshore the coast of Sumatra. In this third chapter I relate the potential for slope instability with core stratigraphy in slope basin cores along the Sumatra-Andaman subduction zone, both for generic earthquakes and for the 2004 earthquake. Using 2-D profile factor of safety analyses, I estimate that slopes become unstable when a seismic load of <0.2 g is imparted. Using infinite slope factor of safety analyses, I conclude that turbidity currents are triggered by earthquakes of at least magnitude M = 7. For the 2004 Sumatra-Andaman subduction zone earthquake, I find that the shaking intensity from ground motion does not explain the thickness of turbidites, which must be explained by other factors like site effects, site geomorphology, or source proximity. Title: A sedimentary and geophysical investigation offshore of Sumatra : subduction zone earthquakes, ground motions, and submarine landslides Author: Patton, Jason Robert Subduction zone earthquake and tsunami hazards affect tens of millions worldwide and the recurrence of these disasters can be evaluated with paleoseismic techniques, because earthquake cycles (and their supercycles) typically span many millennia. In these three chapters, I discuss a suite of analyses I use to evaluate the sedimentary record of earthquakes as sampled in cores collected by the authors on the R/V Roger Revelle in 2007 offshore Sumatra, cruise RR0705, aka KNOX05RR. In the first chapter I discuss the mid- to late-Holocene recurrence of earthquakes in the region of the 2004 Sumatra-Andaman subduction zone earthquake. I estimate that the early Holocene to historic recurrence of turbidity current triggering earthquakes in the region of the 2004 Sumatra-Andaman subduction zone earthquake to be 260 ± 160 calendar years. I identify the uppermost turbidite in fifteen cores to have been deposited as a result of strong ground shaking from this 2004 earthquake. In the second chapter I compare and contrast the physiographic control of seismoturbidite deposition along the continental margins of the Sumatra-Andaman and Cascadia subduction zones. Cascadia’s margin is dominated by glacial cycle constructed pathways which promote turbidity current flows for large distances. Sumatra margin pathways do not inherit analogous sedimentary systems, so turbidity currents and their deposits are more localized. Finally, I discuss slope stability and earthquake ground motions along the continental margin offshore the coast of Sumatra. In this third chapter I relate the potential for slope instability with core stratigraphy in slope basin cores along the Sumatra-Andaman subduction zone, both for generic earthquakes and for the 2004 earthquake. Using 2-D profile factor of safety analyses, I estimate that slopes become unstable when a seismic load of <0.2 g is imparted. Using infinite slope factor of safety analyses, I conclude that turbidity currents are triggered by earthquakes of at least magnitude M = 7. For the 2004 Sumatra-Andaman subduction zone earthquake, I find that the shaking intensity from ground motion does not explain the thickness of turbidites, which must be explained by other factors like site effects, site geomorphology, or source proximity. Close

• 7. [Article] Earthquake Relocation in Three-Dimensional Velocity Models for Central Cascadia Continental Margin

  2017

  Citation × Citation Citation Abstract Copy Title: Earthquake Relocation in Three-Dimensional Velocity Models for Central Cascadia Continental Margin Author: Ghorbani, Paria 2017 Title: Earthquake Relocation in Three-Dimensional Velocity Models for Central Cascadia Continental Margin Author: Ghorbani, Paria 2017 Close

• 8. [Article] Earthquake Relocation in Three-Dimensional Velocity Models for Central Cascadia Continental Margin

  We used data from Cascadia Initiative and Blanco Transform Fault Zone array of ocean bottom seismometers and hydrophone to look at the seismicity and specially depth of the Central Cascadia Continental ...

  Citation × Citation Citation Abstract Copy Title: Earthquake Relocation in Three-Dimensional Velocity Models for Central Cascadia Continental Margin Author: Ghorbani, Paria We used data from Cascadia Initiative and Blanco Transform Fault Zone array of ocean bottom seismometers and hydrophone to look at the seismicity and specially depth of the Central Cascadia Continental Margin events. Events were detected and located automatically with Antelope software package (Boulder Real Time Technologies). We looked at the seismicity in a 1-degree square region (44N-45N and 124W-125W) of the Cascadia Subduction Zone from 2012 to 2013. There were about 40 events in our catalog which was more than any other land network catalogs as we expected. Automatic earthquake location procedure sometimes associated with respectively large residuals for sharp arrivals for stations in strongly heterogeneous structure that associated with continental margin and subduction zones. These large residuals force us to remove them from location procedure because they cannot be associated in a simple velocity model. For smaller events that even can remove the event from the catalog because of small number of sufficient arrival in close stations. We selected 7 larger events (with local magnitudes greater than 2) which were located by other networks before to preform accurate depth analysis. We used and 2.5D model based on the position along one of ORWELL Lines (Gerdom et al., 2000) that are closed to the area (44.75N). We located hypocenters in resulting 3D model in NonLinLoc (Lomax et al., 2000). We used a very fine grid spacing of 500 m, different distance weighting and phase inclusion/omission strategies to locating hypocenters in NonlinLoc. In order to reduce the azimuthal gap, we combined the OBS data with land station data. Our earthquake relocation results in overall shift of 7 km to the west in seismicity pattern. Based on depth results 1 out of 7 events occurred in overriding plate. 5 events seem be occurred on plate interface and one event depth is located in the base of oceanic crust. Plate inter face in our model is located in about 15 km depth. In order to get better understanding of seismicity and including more reliable S arrivals in our analysis we have to adjust Vp/Vs ratio on continental block of the model and relocate all events on continental margin with grid-search methods that contains S arrival picks as well. Title: Earthquake Relocation in Three-Dimensional Velocity Models for Central Cascadia Continental Margin Author: Ghorbani, Paria We used data from Cascadia Initiative and Blanco Transform Fault Zone array of ocean bottom seismometers and hydrophone to look at the seismicity and specially depth of the Central Cascadia Continental Margin events. Events were detected and located automatically with Antelope software package (Boulder Real Time Technologies). We looked at the seismicity in a 1-degree square region (44N-45N and 124W-125W) of the Cascadia Subduction Zone from 2012 to 2013. There were about 40 events in our catalog which was more than any other land network catalogs as we expected. Automatic earthquake location procedure sometimes associated with respectively large residuals for sharp arrivals for stations in strongly heterogeneous structure that associated with continental margin and subduction zones. These large residuals force us to remove them from location procedure because they cannot be associated in a simple velocity model. For smaller events that even can remove the event from the catalog because of small number of sufficient arrival in close stations. We selected 7 larger events (with local magnitudes greater than 2) which were located by other networks before to preform accurate depth analysis. We used and 2.5D model based on the position along one of ORWELL Lines (Gerdom et al., 2000) that are closed to the area (44.75N). We located hypocenters in resulting 3D model in NonLinLoc (Lomax et al., 2000). We used a very fine grid spacing of 500 m, different distance weighting and phase inclusion/omission strategies to locating hypocenters in NonlinLoc. In order to reduce the azimuthal gap, we combined the OBS data with land station data. Our earthquake relocation results in overall shift of 7 km to the west in seismicity pattern. Based on depth results 1 out of 7 events occurred in overriding plate. 5 events seem be occurred on plate interface and one event depth is located in the base of oceanic crust. Plate inter face in our model is located in about 15 km depth. In order to get better understanding of seismicity and including more reliable S arrivals in our analysis we have to adjust Vp/Vs ratio on continental block of the model and relocate all events on continental margin with grid-search methods that contains S arrival picks as well. Close

• 9. [Article] Evolution and inheritance of Cascadia sub-arc mantle reservoirs

  Inheritance from pre-existing mantle domains and fluid and melt contributions from active subduction together produce the geochemical signatures of mantle-derived arc basalts. In this context, this work ...

  Citation × Citation Citation Abstract Copy Title: Evolution and inheritance of Cascadia sub-arc mantle reservoirs Author: Bromley, Sarah Ashley Inheritance from pre-existing mantle domains and fluid and melt contributions from active subduction together produce the geochemical signatures of mantle-derived arc basalts. In this context, this work evaluates the evolution of Cascadia mantle sources by documenting the isotopic and compositional characteristics of primitive basalts along a transect across the Eocene-Oligocene Proto-Cascadia (EOPC) arc at ~44.5-45.5° N. Primitive EOPC flows, dikes, and sills are exposed across a ~300 km transect that includes the Oregon Coast Range in the Cascadia forearc, the Western Cascades, flanking the modern arc, and the John Day and Eastern Clarno formations east of the Cascades. Like the modern arc, EOPC was built upon accreted terranes of western North America and within the Columbia embayment, which is lithosphere of oceanic affinity that crops out as the Siletzia terrane in the forearc and extends beneath the arc to the backarc. Potential mantle source reservoirs for EOPC magmas include contributions from mantle domains related to pre-existing underlying terranes, distinct North America lithosphere, and depleted Pacific-like upper mantle. In addition, the geochemical characteristics of EOPC magmas have likely been overprinted by subduction processes. Major, trace element, and isotopic data from the EOPC reveal a heterogeneous mantle source that was variably influenced by subduction processes. In the forearc, the high field strength (HFSE) enriched basalts of the Oregon Coast Range represent low degree partial melts of a relatively enriched mantle source. Despite this enriched character, there is little evidence in the trace element or isotopic data to suggest that the OCR (Oregon Coast Range) samples have been strongly influenced by either a crustal or subduction component. Their distinctive ²⁰⁶Pb/²⁰⁴Pb enrichment (as compared to ²⁰⁷Pb/²⁰⁴Pb) distinguishes the forearc magmas from the arc and backarc magmas and also from a hypothetical Cascadia subduction component. Forearc EOPC magmas share a mantle source with the accreted Siletzia terrane, as evidenced by their shared ²⁰⁶Pb/²⁰⁴Pb enrichment. At the apparent arc axis, the Western Cascades produced a diversity of primitive magmas that are, for the most part, interpreted to result from higher degrees of partial melting of a less enriched source. Fluid fluxing appears to have facilitated mantle melting beneath the Western Cascades. Additionally, the mantle beneath the arc may be slightly influenced by the ²⁰⁶Pb/²⁰⁴Pb enriched source underlying the forearc. Though this effect is difficult to resolve, the Western Cascades samples appear to be slightly more variable in ²⁰⁶Pb/²⁰⁴Pb (and generally higher in ²⁰⁶Pb/²⁰⁴Pb at a given ²⁰⁷Pb/²⁰⁴Pb) than their backarc counterparts, suggesting such an influence. Both trace element and isotopic data suggest a significant subduction contribution to EOPC backarc primitive magmas. The EOPC backarc magmas appear to have originated from a heterogeneous mantle and are variably influenced by a subduction component. Though this subduction influence could be an inherited feature, the affinity between John Day and Eastern Clarno magmas and a modern Cascadia sediment source suggest that this is at least a relatively young feature. Taken together, these results demonstrate the heterogeneity of source and process across the Eocene-Oligocene arc. Apparent subduction contribution increases from the geographic forearc to backarc, perhaps indicating a wider arc than is typically envisioned at the time. The distinct ²⁰⁶Pb/²⁰⁴Pb enrichment associated with the Siletzia mantle and apparent in the forearc appears to wane in influence to the east, having at most a minor influence on the Western Cascade arc magmas. As with the Siletzia mantle domain in EOPC arc and backarc, mantle domains associated with the Western Cascades and John Day/Eastern Clarno magmas do not persist in the High Cascades or High Lava Plains. Beneath and behind the arc, mantle reservoirs appear to have been largely replaced since the Eocene-Oligocene. However, the High Lava Plains basalts appear to carry a paleo-enrichment signature that may be an inherited feature from advected mantle. If this enriched mantle was advected from beneath North America, it may explain the observation that High Lava Plains basalts are more enriched to the east. Title: Evolution and inheritance of Cascadia sub-arc mantle reservoirs Author: Bromley, Sarah Ashley Inheritance from pre-existing mantle domains and fluid and melt contributions from active subduction together produce the geochemical signatures of mantle-derived arc basalts. In this context, this work evaluates the evolution of Cascadia mantle sources by documenting the isotopic and compositional characteristics of primitive basalts along a transect across the Eocene-Oligocene Proto-Cascadia (EOPC) arc at ~44.5-45.5° N. Primitive EOPC flows, dikes, and sills are exposed across a ~300 km transect that includes the Oregon Coast Range in the Cascadia forearc, the Western Cascades, flanking the modern arc, and the John Day and Eastern Clarno formations east of the Cascades. Like the modern arc, EOPC was built upon accreted terranes of western North America and within the Columbia embayment, which is lithosphere of oceanic affinity that crops out as the Siletzia terrane in the forearc and extends beneath the arc to the backarc. Potential mantle source reservoirs for EOPC magmas include contributions from mantle domains related to pre-existing underlying terranes, distinct North America lithosphere, and depleted Pacific-like upper mantle. In addition, the geochemical characteristics of EOPC magmas have likely been overprinted by subduction processes. Major, trace element, and isotopic data from the EOPC reveal a heterogeneous mantle source that was variably influenced by subduction processes. In the forearc, the high field strength (HFSE) enriched basalts of the Oregon Coast Range represent low degree partial melts of a relatively enriched mantle source. Despite this enriched character, there is little evidence in the trace element or isotopic data to suggest that the OCR (Oregon Coast Range) samples have been strongly influenced by either a crustal or subduction component. Their distinctive ²⁰⁶Pb/²⁰⁴Pb enrichment (as compared to ²⁰⁷Pb/²⁰⁴Pb) distinguishes the forearc magmas from the arc and backarc magmas and also from a hypothetical Cascadia subduction component. Forearc EOPC magmas share a mantle source with the accreted Siletzia terrane, as evidenced by their shared ²⁰⁶Pb/²⁰⁴Pb enrichment. At the apparent arc axis, the Western Cascades produced a diversity of primitive magmas that are, for the most part, interpreted to result from higher degrees of partial melting of a less enriched source. Fluid fluxing appears to have facilitated mantle melting beneath the Western Cascades. Additionally, the mantle beneath the arc may be slightly influenced by the ²⁰⁶Pb/²⁰⁴Pb enriched source underlying the forearc. Though this effect is difficult to resolve, the Western Cascades samples appear to be slightly more variable in ²⁰⁶Pb/²⁰⁴Pb (and generally higher in ²⁰⁶Pb/²⁰⁴Pb at a given ²⁰⁷Pb/²⁰⁴Pb) than their backarc counterparts, suggesting such an influence. Both trace element and isotopic data suggest a significant subduction contribution to EOPC backarc primitive magmas. The EOPC backarc magmas appear to have originated from a heterogeneous mantle and are variably influenced by a subduction component. Though this subduction influence could be an inherited feature, the affinity between John Day and Eastern Clarno magmas and a modern Cascadia sediment source suggest that this is at least a relatively young feature. Taken together, these results demonstrate the heterogeneity of source and process across the Eocene-Oligocene arc. Apparent subduction contribution increases from the geographic forearc to backarc, perhaps indicating a wider arc than is typically envisioned at the time. The distinct ²⁰⁶Pb/²⁰⁴Pb enrichment associated with the Siletzia mantle and apparent in the forearc appears to wane in influence to the east, having at most a minor influence on the Western Cascade arc magmas. As with the Siletzia mantle domain in EOPC arc and backarc, mantle domains associated with the Western Cascades and John Day/Eastern Clarno magmas do not persist in the High Cascades or High Lava Plains. Beneath and behind the arc, mantle reservoirs appear to have been largely replaced since the Eocene-Oligocene. However, the High Lava Plains basalts appear to carry a paleo-enrichment signature that may be an inherited feature from advected mantle. If this enriched mantle was advected from beneath North America, it may explain the observation that High Lava Plains basalts are more enriched to the east. Close

• 10. [Article] PattonJasonR2014.pdf

  Citation × Citation Citation Abstract Copy Title: PattonJasonR2014.pdf Author: Patton, Jason Robert Title: PattonJasonR2014.pdf Author: Patton, Jason Robert Close