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  • 1
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    Vp/Vs ratio of incoming sediments off Cascadia subduction zone from analysis of controlled-source multicomponent OBS records (2020)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 125(6), (2020): e2019JB019239, doi:10.1029/2019JB019239.
    Description: P‐to‐S‐converted waves observed in controlled‐source multicomponent ocean bottom seismometer (OBS) records were used to derive the Vp/Vs structure of Cascadia Basin sediments. We used P‐to‐S waves converted at the basement to derive an empirical function describing the average Vp/Vs of Cascadia sediments as a function of sediment thickness. We derived one‐dimensional interval Vp/Vs functions from semblance velocity analysis of S‐converted intrasediment and basement reflections, which we used to define an empirical Vp/Vs versus burial depth compaction trend. We find that seaward from the Cascadia deformation front, Vp/Vs structure offshore northern Oregon and Washington shows little variability along strike, while the structure of incoming sediments offshore central Oregon is more heterogeneous and includes intermediate‐to‐deep sediment layers of anomalously elevated Vp/Vs. These zones with elevated Vp/Vs are likely due to elevated pore fluid pressures, although layers of high sand content intercalated within a more clayey sedimentary sequence, and/or a higher content of coarser‐grained clay minerals relative to finer‐grained smectite could be contributing factors. We find that the proto‐décollement offshore central Oregon develops within the incoming sediments at a low‐permeability boundary that traps fluids in a stratigraphic level where fluid overpressure exceeds 50% of the differential pressure between the hydrostatic pressure and the lithostatic pressure. Incoming sediments with the highest estimated fluid overpressures occur offshore central Oregon where deformation of the accretionary prism is seaward vergent. Conversely, landward vergence offshore northern Oregon and Washington correlates with more moderate pore pressures and laterally homogeneous Vp/Vs functions of Cascadia Basin sediments.
    Description: This research was funded by National Science Foundation (NSF) Grant OCE‐1657237 to J. P. C, OCE‐1657839 to A. F. A. and S. H., and OCE‐1657737 to S. M. C. Data used in this study were acquired with funding from NSF Grants OCE‐1029305 and OCE‐1249353. Data used in this research were provided by instruments from the Ocean Bottom Seismic Instrument Center (http://obsic.whoi.edu, formerly OBSIP), which is funded by the NSF. OBSIC/OBSIP data are archived at the IRIS Data Management Center (http://www.iris.edu) under network code X6 (https://doi.org/10.7914/SN/X6_2012). Data processing was conducted with Emerson‐Paradigm Software package Echos licensed to Woods Hole Oceanographic Institution under Paradigm Academic Software Program and MATLAB package SeismicLab of the University of Alberta, Canada (http://seismic-lab.physics.ualberta.ca), under GNU General Public License (MATLAB® is a registered trademark of MathWorks).
    Description: 2020-11-28
    Keywords: Vp/Vs ; sediments ; ocean bottom seismometer ; Juan de Fuca plate ; Cascadia
    Repository Name: Woods Hole Open Access Server
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  • 2
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    Seismic reflection imaging of the Juan de Fuca plate from ridge to trench : new constraints on the distribution of faulting and evolution of the crust prior to subduction (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 121 (2016): 1849–1872, doi:10.1002/2015JB012416.
    Description: We present prestack time-migrated multichannel seismic images along two cross-plate transects from the Juan de Fuca (JdF) Ridge to the Cascadia deformation front (DF) offshore Oregon and Washington from which we characterize crustal structure, distribution and extent of faults across the plate interior as the crust ages and near the DF in response to subduction bending. Within the plate interior, we observe numerous small offset faults in the sediment section beginning 50–70 km from the ridge axis with sparse fault plane reflections confined to the upper crust. Plate bending due to sediment loading and subduction initiates at ~120–150 km and ~65–80 km seaward of the DF, respectively, and is accompanied by increase in sediment fault offsets and enhancement of deeper fault plane reflectivity. Most bend faulting deformation occurs within 40 km from the DF; on the Oregon transect, bright fault plane reflections that extend through the crust and 6–7 km into the mantle are observed. If attributed to serpentinization, ~0.12–0.92 wt % water within the uppermost 6 km of the mantle is estimated. On the Washington transect, bending faults are confined to the sediment section and upper-middle crust. The regional difference in subduction bend-faulting and potential hydration of the JdF plate is inconsistent with the spatial distribution of intermediate-depth intraslab seismicity at Cascadia. A series of distinctive, ridgeward dipping (20°–40°) lower crustal reflections are imaged in ~6–8 Ma crust along both transects and are interpreted as ductile shear zones formed within the ridge's accretionary zone in response to temporal variations in mantle upwelling, possibly associated with previously recognized plate reorganizations at 8.5 Ma and 5.9 Ma.
    Description: NSF Grant Numbers: OCE 1029411, 1029305
    Description: 2016-09-21
    Keywords: Cascadia Subduction Zone ; Juan de Fuca plate ; Multichannel seismic reflection imaging ; Subduction bending faulting ; Lower crustal dipping reflections ; Subducting plate hydration
    Repository Name: Woods Hole Open Access Server
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  • 3
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    Along-trench structural variations of the subducting Juan de Fuca Plate from multichannel seismic reflection imaging (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 123 (2018): 3122-3146, doi:10.1002/2017JB015059.
    Description: To characterize the along‐strike structural variations of the Juan de Fuca (JdF) Plate as it enters the Cascadia subduction zone, we present prestack time migrated multichannel seismic reflection images of the JdF Plate along a 400‐km‐long trench‐parallel transect extending from 44.3°N to 47.8°N. Beneath the 1.8–3.0‐km‐thick sediment cover, our data reveal basement topographic anomalies associated with a 1.2‐km‐high seamount and in the vicinity of propagator wakes (390–540‐m relief). Weak Moho reflections are imaged beneath the propagator wakes and coincide with reduced Vp in the lower crust and/or uppermost mantle. The inferred locations of propagator wakes in the downgoing plate collocate with some of the boundaries of episodic tremor and slip events. We propose that the structural and hydration heterogeneities associated with these features could lead to anomalous plate interface properties and contribute to episodic tremor and slip segmentation. Intracrustal reflections with apparent dips (20°–30°) consistent with subduction bending normal faults change near 45.8°N, from northward dipping reflections confined to the middle crust in the north to antithetic reflections through the crust in the south, coinciding with a Vp reduction in the lower crust. These observations indicate more extensive faulting deformation and associated hydration of the JdF Plate south of 45.8°N, which likely results from variations of slab dip and resistance to subduction across 46°N. Basement offsets and abrupt depth/amplitude changes in Moho reflections are imaged beneath the four major WNW trending strike‐slip faults that cross the Cascadia deformation front, providing strong evidence of a lower plate origin for these faults.
    Description: NSF Grant Numbers: OCE 1029411, 1029305
    Description: 2018-09-30
    Keywords: Cascadia subduction zone ; Juan de Fuca Plate ; Multichannel seismic reflection imaging ; Subduction bending faulting ; Propagator wakes ; Strike-slip faults
    Repository Name: Woods Hole Open Access Server
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  • 4
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    A 2-D tomographic model of the Juan de Fuca plate from accretion at axial seamount to subduction at the Cascadia margin from an active source ocean bottom seismometer survey (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 121 (2016): 5859–5879, doi:10.1002/2016JB013228.
    Description: We report results from a wide-angle controlled source seismic experiment across the Juan de Fuca plate designed to investigate the evolution of the plate from accretion at the Juan de Fuca ridge to subduction at the Cascadia margin. A two-dimensional velocity model of the crust and upper mantle is derived from a joint reflection-refraction traveltime inversion. To interpret our tomography results, we first generate a plausible baseline velocity model, assuming a plate cooling model and realistic oceanic lithologies. We then use an effective medium theory to infer from our tomography results the extent of porosity, alteration, and water content that would be required to explain the departure from the baseline model. In crust of ages 〉1 Ma and away from propagator wakes and regions of faulting due to plate bending, we obtain estimates of upper crustal hydration of 0.5–2.1 wt % and find mostly dry lower crust and upper mantle. In sections of the crust affected by propagator wakes we find upper estimates of upper crustal, lower crustal, and upper mantle hydration of 3.1, 0.8, and 1.8 wt %, respectively. At the Cascadia deformation front, we find that the amount of water stored at uppermost mantle levels in the downgoing JdF plate is very limited (〈0.3 wt %), with most of the water carried into the subduction zone being stored in the oceanic crust.
    Description: NSF Grant Numbers: OCE-1029305, OCE-1029411, OCE-1249353
    Description: 2017-02-14
    Keywords: Juan De Fuca plate ; Cascadia subduction zone ; Velocity model ; Subducting plate hydration ; Oceanic crust
    Repository Name: Woods Hole Open Access Server
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  • 5
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    Along-Trench Structural Variations of the Subducting Juan de Fuca Plate From Multichannel Seismic Reflection Imaging (2018)
    American Geophysical Union
    Details
    Publication Date: 2018-04-01
    Description: To characterize the along-strike structural variations of the Juan de Fuca (JdF) Plate as it enters the Cascadia subduction zone, we present prestack time migrated multichannel seismic reflection images of the JdF Plate along a 400-km-long trench-parallel transect extending from 44.3°N to 47.8°N. Beneath the 1.8–3.0-km-thick sediment cover, our data reveal basement topographic anomalies associated with a 1.2-km-high seamount and in the vicinity of propagator wakes (390–540-m relief). Weak Moho reflections are imaged beneath the propagator wakes and coincide with reduced Vp in the lower crust and/or uppermost mantle. The inferred locations of propagator wakes in the downgoing plate collocate with some of the boundaries of episodic tremor and slip events. We propose that the structural and hydration heterogeneities associated with these features could lead to anomalous plate interface properties and contribute to episodic tremor and slip segmentation. Intracrustal reflections with apparent dips (20°–30°) consistent with subduction bending normal faults change near 45.8°N, from northward dipping reflections confined to the middle crust in the north to antithetic reflections through the crust in the south, coinciding with a Vp reduction in the lower crust. These observations indicate more extensive faulting deformation and associated hydration of the JdF Plate south of 45.8°N, which likely results from variations of slab dip and resistance to subduction across 46°N. Basement offsets and abrupt depth/amplitude changes in Moho reflections are imaged beneath the four major WNW trending strike-slip faults that cross the Cascadia deformation front, providing strong evidence of a lower plate origin for these faults. ©2018. American Geophysical Union. All Rights Reserved.
    Print ISSN: 2169-9313
    Electronic ISSN: 2169-9356
    Topics: Geosciences , Physics
    Published by American Geophysical Union
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  • 6
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    Catalog of Offshore Seismicity in Cascadia: Insights Into the Regional Distribution of Microseismicity and its Relation to Subduction Processes (2018)
    American Geophysical Union
    Details
    Publication Date: 2018-01-01
    Description: We present a catalog of offshore seismicity generated from Cascadia Initiative (CI) ocean-bottom seismometer data. Earthquakes were detected within the CI data using a short-time-average/long-time-average trigger and located using 1-D velocity models developed from seismic reflection/refraction surveys. The catalog, which contains 271 earthquakes with magnitude 0.4–4.0 along the coasts of Vancouver Island, Washington, Oregon, and Northern California, spans all 4 years of the ocean bottom seismometer deployment and shows distinct along-strike variations in seismicity consistent with structural observations from recent active source seismic reflection/refraction studies. Seismicity is sparse off Vancouver Island and Washington (49°N–46°N) but increases off northern and central Oregon, corresponding to a roughened, more deformed subducting slab. Widespread earthquakes are observed at near-interface depths between 46°N and 45°N, though an increase in underthrust sediment thickness between 45°N and 43°N likely restricts seismicity to scattered asperities on the plate interface. South of 43°N, where both the overriding and subducting plates are severely deformed approaching the Mendocino triple junction, seismicity is abundant. We locate an additional 440 events in the Juan de Fuca plate seaward of the deformation front. The higher seismicity south of 46°N is consistent with more extensive intraplate deformation. Along with the complex stress field induced by the Mendocino triple junction, our observations imply that the smoothness and degree of hydration of the incoming plate, which are linked to the amount of underthrust sediment and extent of intraplate deformation, are major contributing factors to the distribution of microseismicity in the Cascadia subduction zone. ©2018. American Geophysical Union. All Rights Reserved.
    Print ISSN: 2169-9313
    Electronic ISSN: 2169-9356
    Topics: Geosciences , Physics
    Published by American Geophysical Union
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  • 7
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    Vp / Vs Ratio of Incoming Sediments Off Cascadia Subduction Zone From Analysis of Controlled‐Source Multicomponent OBS Records (2020)
    American Geophysical Union
    Details
    Publication Date: 2020-06-01
    Print ISSN: 2169-9313
    Electronic ISSN: 2169-9356
    Topics: Geosciences , Physics
    Published by American Geophysical Union
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  • 8
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    Post‐seafloor spreading magmatism and associated magmatic hydrothermal systems in the Xisha uplift region, northwestern South China Sea (2019)
    Wiley
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    Publication Date: 2019-02-08
    Description: Submarine magmatism and associated hydrothermal fluid flows has significant feedback influence on the petroleum geology of sedimentary basins. This study uses new seismic profiles and multibeam bathymetric data to examine the morphology and internal architecture of post-seafloor spreading magmatic structures, especially volcanoes of the Xisha uplift, in extensive detail. We discover for the first time hydrothermal systems derived from magmatism in the northwestern South China Sea. Numerous solitary volcanoes and volcanic groups occur in the Xisha uplift and produce distinct seismic reflections together with plutons. Sills and other localized amplitude anomalies were fed by extrusions/intrusions and associated fluid flow through fractures and sedimentary layers that may act as conduits for magma and fluid flows transport. Hydrothermal structures such as pipes and pockmarks mainly occur in the proximity of volcanoes or accompany volcanic groups. Pipes, pockmarks and localized amplitude anomalies mainly constitute the magmatic hydrothermal systems, which are probably driven by post-seafloor spreading volcanoes/plutons. The hydrothermal fluid flows released by magma degassing or/and related boiling of pore fluids/metamorphic dehydration reactions in sediments produced local overpressures, which drove upward flow of fluid or horizontal flow into the sediments or even seafloor. Results show that post-seafloor spreading magmatic activity is more intense during a 5.5 Ma event than one in 2.6 Ma, whereas the hydrothermal activities are more active during 2.6 Ma than in 5.5 Ma. Our analysis indicates that post-seafloor spreading magmatism may have a significant effect on hydrocarbon maturation and gas hydrate formation in the Xisha uplift and adjacent petroliferous basins. Consequently the study presented here improves our understanding of hydrocarbon exploration in the northwestern South China Sea. © 2019 The Authors. Basin Research © 2019 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists
    Print ISSN: 0950-091X
    Electronic ISSN: 1365-2117
    Topics: Geosciences
    Published by Wiley
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  • 9
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    Fine-scale segmentation of the crustal magma reservoir beneath the East Pacific Rise (2013)
    Springer Nature
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    Publication Date: 2013-09-15
    Description: The global mid-ocean ridge is segmented in its seafloor morphology and magmatic systems, but the origin of and relationships between this tectonic and magmatic segmentation are poorly understood. At fast-spreading ridges, tectonic segmentation is observed on a fine scale, but it is unclear whether this partitioning also occurs in the magmatic system. Fine-scale tectonic segmentation could have a deep origin, arising from the distribution of upwelling mantle melt, or a shallow origin, linked to offset intruding dikes from long, more continuous crustal reservoirs. Here we use seismic reflection data from the fast-spreading East Pacific Rise, between 820′ N and 1010′ N, which includes a unique area where two documented volcanic eruptions have occurred, to image the crustal magma bodies in high resolution. We find that the magma reservoirs form 5- to 15-km-long segments that coincide with the fine-scale tectonic segmentation at the seafloor and that three lens segments fed the recent eruptions. Transitions in composition, volume and morphology of erupted lavas coincide with disruptions in the lens that define magmatic segments. We conclude that eruptions at the East Pacific Rise are associated with the vertical ascent of magma from lenses that are mostly physically isolated, leading to the eruption of distinct lavas at the surface that coincide with fine-scale tectonic segmentation. © 2013 Macmillan Publishers Limited.
    Print ISSN: 1752-0894
    Electronic ISSN: 1752-0908
    Topics: Geosciences
    Published by Springer Nature
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  • 10
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    Links between sediment consolidation and Cascadia megathrust slip behaviour (2017)
    Springer Nature
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    Publication Date: 2017-11-20
    Description: At sediment-rich subduction zones, megathrust slip behaviour and forearc deformation are tightly linked to the physical properties and in situ stresses within underthrust and accreted sediments. Yet the role of sediment consolidation at the onset of subduction in controlling the downdip evolution and along-strike variation in megathrust fault properties and accretionary wedge structure is poorly known. Here we use controlled-source seismic data combined with ocean drilling data to constrain the sediment consolidation and in situ stress state near the deformation front of the Cascadia subduction zone. Offshore Washington where the megathrust is inferred to be strongly locked, we find over-consolidated sediments near the deformation front that are incorporated into a strong outer wedge, with little sediment subducted. These conditions are favourable for strain accumulation on the megathrust and potential earthquake rupture close to the trench. In contrast, offshore Central Oregon, a thick under-consolidated sediment sequence is subducting, and is probably associated with elevated pore fluid pressures on the megathrust in a region where reduced locking is inferred. Our results suggest that the consolidation state of the sediments near the deformation front is a key factor contributing to megathrust slip behaviour and its along-strike variation, and it may also have a significant role in the deformation style of the accretionary wedge. © 2017 The Author(s).
    Print ISSN: 1752-0894
    Electronic ISSN: 1752-0908
    Topics: Geosciences
    Published by Springer Nature
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