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Title: Thermal environment of the Southern Washington region of the Cascadia subduction zone

ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [2]
  1. School of Oceanography, University of Washington, Seattle Washington USA
  2. College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis Oregon USA
Publication Date:
Sponsoring Org.:
OSTI Identifier:
Grant/Contract Number:
FE0013998; 1458211; 1339635
Resource Type:
Journal Article: Publisher's Accepted Manuscript
Journal Name:
Journal of Geophysical Research. Solid Earth
Additional Journal Information:
Related Information: CHORUS Timestamp: 2017-10-24 17:45:44; Journal ID: ISSN 2169-9313
American Geophysical Union
Country of Publication:
United States

Citation Formats

Salmi, Marie S., Johnson, H. Paul, and Harris, Robert N. Thermal environment of the Southern Washington region of the Cascadia subduction zone. United States: N. p., 2017. Web. doi:10.1002/2016JB013839.
Salmi, Marie S., Johnson, H. Paul, & Harris, Robert N. Thermal environment of the Southern Washington region of the Cascadia subduction zone. United States. doi:10.1002/2016JB013839.
Salmi, Marie S., Johnson, H. Paul, and Harris, Robert N. 2017. "Thermal environment of the Southern Washington region of the Cascadia subduction zone". United States. doi:10.1002/2016JB013839.
title = {Thermal environment of the Southern Washington region of the Cascadia subduction zone},
author = {Salmi, Marie S. and Johnson, H. Paul and Harris, Robert N.},
abstractNote = {},
doi = {10.1002/2016JB013839},
journal = {Journal of Geophysical Research. Solid Earth},
number = ,
volume = ,
place = {United States},
year = 2017,
month = 8

Journal Article:
Free Publicly Available Full Text
This content will become publicly available on August 8, 2018
Publisher's Accepted Manuscript

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  • Two upper Cenozoic depositional sequences of principally marine strata about 4,000 m thick overlie accreted basement terranes of the Central and Coastal belts of the Franciscan Complex in the onshore-offshore Eel River basin of northwestern California. The older depositional sequence is early to middle Miocene in age and represents slope basin and slope-blanket deposition, whereas the younger sequence, later Miocene to middle Pleistocene in age, consists largely of forearc basin deposits. Youthful tectonic activity related to Gorda-North American plate convergence indicates an active Cascadia subduction zone and strong partial coupling between these plates. Structures of the northeastern margin of themore » Eel River basin are principally north-northwest-trending, east-northeast-dipping thrust and reverse faults that form imbricate thrust fans. The Coastal belt fault, the early Tertiary accretionary suture between the Franciscan Central and Coastal belts, can be traced from Arcata Bay northward offshore to the southern Oregon border. It is tentatively extended farther northward based on aeromagnetic data to an offshore position west of Cape Blanco. Thereafter, it may coincide with the offshore Fulmar fault. The Cascadia subduction zone (CSZ) does not join the Mendocino transform fault at the commonly depicted offshore location of the Mendocino triple junction (MTJ). Instead, the CSZ extends southeastward around the southern Eel River basin and shoreward along Mendocino Canyon to join the Petrolia shear zone. Similarly, the Mendocino fault may extend shoreward via Mattole Canyon and join the Cooskie shear zone. These two shear zones intersect onshore north of the King Range, and the area of their intersection is the probable location of the MTJ.« less
  • Coastline deformation resulting from great shallow thrust earthquakes can provide information concerning the paleoseismicity of a subduction zone and thus information on the nature of potential seismicity. The Cascadia subduction zone is different from most other subduction zones in that it has been quiescent with respect to great earthquakes for at least the past 200 yr. The Washington-Oregon coastline also differs from most other coastlines associated with subduction zones in its lack of uplifted Holocene shoreline features and low overall rate of late Quaternary uplift (0.2-0.6 mm/yr). The uplift differences suggest that repeated great earthquakes have not occurred along themore » Cascadia subduction zone at least during the late Holocene. Alternatively, if the plate interface has generated earthquakes, the differences may be explained by longer recurrence intervals for great earthquakes, smaller magnitude earthquakes, or a mechanism that does not result in uplift of the coastline where expected.« less
  • Major volcanoes of the Southern Washington Cascades (SWC) include the large quaternary stratovolcanoes of Mount St. Helens (MSH) and Mount Adams (MA) and the Indian Heaven (IH) and Simcoe Mountain (SIM) volcanic fields. There are significant differences among these volcanic centers in terms of their composition and evolutionary history. The authors conclude that subducted fluids and sediments do not play an essential role in producing these magmas. Rather, they infer that they formed by variable degree melting of a mixed mantle source consisting mainly of heterogeneously distributed OIB and mid-ocean ridge basalt source domains. Relatively minor occurrences of high fieldmore » strength element (HFSE) depleted arclike basalts may reflect the presence of a small proportion of slab-metasomatized subarc mantle. The juxtaposition of such different mantle domains within the lithospheric mantle is viewed as a consequence of (1) tectonic mixing associated with accretion of oceanic and island arc terranes along the Pacific margin of North America prior to Neogene time, and possibly (2) a seaward jump in the locus of subduction at about 40 Ma. The Cascades arc is unusual in that the subducting oceanic plate is very young and hot. They suggest that slab dehydration outboard of the volcanic front resulted in a diminished role of aqueous fluids in generating or subsequently modifying SWC magmas compared to the situation at most convergent margins. Furthermore, with low fluid flux conditions, basalt generation is presumably triggered by other processes that increase the temperature of the mantle wedge (e.g., convective mantle flow, shear heating, etc.).« less
  • Large subduction earthquakes on the Cascadia subduction zone pose a potential seismic hazard. Very young oceanic lithosphere (10 million years old) is being subducted beneath North America at a rate of approximately 4 centimeters per year. The Cascadia subduction zone shares many characteristics with subduction zones in southern Chile, southwestern Japan, and Colombia, where comparably young oceanic lithosphere is also subducting. Very large subduction earthquakes, ranging in energy magnitude (M/sub w/) between 8 and 9.5, have occurred along these other subduction zones. If the Cascadia subduction zone is also storing elastic energy, a sequence of several great earthquakes (M/sub w/more » 8) or a giant earthquake (M/sub w/ 9) would be necessary to fill this 1200-kilometer gap. The nature of strong ground motions recorded during subduction earthquakes of M/sub w/ less than 8.2 is discussed. Strong ground motions from even larger earthquakes (M/sub w/ up to 9.5) are estimated by simple simulations. If large subduction earthquakes occur in the Pacific Northwest, relatively strong shaking can be expected over a large region. Such earthquakes may also be accompanied by large local tsunamis. 35 references, 6 figures.« less
  • Authigenic carbonates are intercalated with massive gas hydrates in sediments of the Cascadia margin. The deposits were recovered from the uppermost 50 cm of sediments on the southern summit of the Hydrate Ridge during the RV Sonne cruise SO110. Two carbonate lithologies that differ in chemistry, mineralogy, and fabric make up these deposits. Microcrystalline high-magnesium calcite (14 to 19 mol% MgCO{sub 3}) and aragonite are present in both semiconsolidated sediments and carbonate-cemented clasts. Aragonite occurs also as a pure phase without sediment impurities. It is formed by precipitation in cavities as botryoidal and isopachous aggregates within pure white, massive gasmore » hydrate. Variations in oxygen isotope values of the carbonates reflect the mineralogical composition and define two end members: a Mg-calcite with {delta}{sup 18}O = 4.86% PDB and an aragonite with {delta}{sup 18}O = 3.68% PDB. On the basis of the ambient bottom-water temperature and accepted equations for oxygen isotope fractionation, the authors show that the aragonite phase formed in equilibrium with its pore-water environment, and that the Mg-calcite appears to have precipitated from pore fluids enriched in {sup 18}O. Oxygen isotope enrichment probably originates from hydrate water released during gas-hydrate destabilization.« less