HR: 08:45h
AN: S41D-04    [Abstracts]
TI: Roles of Oceanic Intraplate Volcanism and Mantle CO$_{2}$ in the Seismicity of the Double Wadati-Benioff Seismic Zone of NE Japan: A New Proposed Mechanism for Lower-zone Seismicity
AU: * Kirby, S H
EM: skirby@usgs.gov
AF: U.S. Geological Survey MS977, 345 Middlefield Road, Menlo Park, CA 94061 United States
AU: Hasegawa, A
EM: hasegawa@aob.geophys.tohoku.ac.jp
AF: Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University, Faculty of Sciences, Sendai, 980-8578 Japan
AU: Umino, N
EM: umino@aob.geophys.tohoku.ac.jp
AF: Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University, Faculty of Sciences, Sendai, 980-8578 Japan
AU: Gamage, S
EM: shantha@aob.geophys.tohoku.ac.jp
AF: Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University, Faculty of Sciences, Sendai, 980-8578 Japan
AU: Suganomata, J
EM: suga@aob.geophys.tohoku.ac.jp
AF: Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University, Faculty of Sciences, Sendai, 980-8578 Japan
AU: Okada, T
EM: okada@aob.geophys.tohoku.ac.jp
AF: Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University, Faculty of Sciences, Sendai, 980-8578 Japan
AU: Uchida, N
EM: uchida@aob.geophys.tohoku.ac.jp
AF: Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University, Faculty of Sciences, Sendai, 980-8578 Japan
AB: Research in recent years has given insights into the occurrence of intraslab seismicity at intermediate depths as expressions of dehydration embrittlement in faults previously formed in oceanic lithosphere (Kirby et al., 1996). A special challenge exists to explain the lower zone of the double seismic zone by this process because the mechanism by which hydration occurs at depths of 30-40 km into the Pacific Plate is unclear. Based on recent seismological advances and theoretical developments at Tohoku University, we propose that CO2 released into the Pacific Plate by ascending mafic magmas associated with Cretaceous intraplate volcanism later facilitates embrittlement of the mantle when such structures are subducted. Wilshire and Kirby (Tectonophysics, 1989) earlier proposed that CO$_{2}$ facilitates mantle fracture and seismogenic faulting beneath Hawaii by such a mechanism. Extensive seismological and marine geological and geophysical evidence suggest that that widespread Cretaceous plume volcanism, perhaps associated with the mid-Cretaceous Superplume, led to widespread mid-plate magmatic activity and a broad region of CO$_{2}$-induced fracture and faulting in the Pacific Plate. After subsequent cooling and initial subduction, such supercritical CO$_{2}$ should become unstable, producing magnesite by reaction with depleted harzburgite (depleted peridotite). Magnesite has the same structure as calcite and is, by analogy, expected to be dramatically weaker than peridotite and we posit that thermal runaway by localized viscous dissipation can lead to CO$_{2}$ embrittlement in the slab by release of CO$_{2}$ or by shear melting. This model for lower-zone seismicity supports the Superplume hypothesis of Seno and Yamanaka (1996), but employs a different physical mechanism for faulting. This proposed structure of intraplate volcanics in subducting lithosphere is major source of pre-existing fault and mineralogical heterogeneity that can affect the distribution and focal mechanisms of upper-zone seismicity in NE Japan and elsewhere.
DE: 7230 Seismicity and seismotectonics
DE: 8100 TECTONOPHYSICS
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8150 Plate boundary--general (3040)
DE: 1236 Rheology of the lithosphere and mantle (8160)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting