The importance of inputs to the Nankai Trough subduction zone; integration of results from ODP and IODP drilling transects in the Shikoku Basin

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Author(s): Underwood, M.; Saito, S.; Henry, P.; Kanamatsu, T.
Integrated Ocean Drilling Program, Expedition 322 Scientific Team
Author Affiliation(s): Primary:
University of Missouri, Department of Geological Sciences, Columbia, MO, United States
Institute for Frontier Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Japan
Centre Européen de Recherche et d'Enseignement des Géosciences de l'Environnement, France
Volume Title: AGU 2011 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2011; American Geophysical Union 2011 fall meeting, San Francisco, CA, Dec. 5-9, 2011. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: Three ODP and IODP drilling transects have been completed across the Nankai Trough subduction zone: Ashizuri, Muroto, and Kumano. An essential component of each transect is the so-called "reference site" seaward of the trench (ODP Sites 1173 and 1177; IODP Sites C0011 and C0012). These Shikoku Basin deposits are unusually heterogeneous, which means that virtually all types of frictional, geotechnical, and hydrogeological properties (including pore pressure) vary in 3-D both landward and seaward of the subduction front. By integrating drilling results from the three transects, we recognize the following basin-wide highlights: (1) Boundaries between lithologic units are not time-correlative. As an example, the base of the upper Shikoku Basin unit (hemipelagic/pyroclastic facies) varies in age from 7.8 Ma (C0012) to 3.3 Ma (1173). (2) A basin-wide boundary caused by diagenesis of dispersed volcanic glass shifts its stratigraphic position, with host-sediment ages that range from 5.2 Ma (C0011) to 3.3 Ma (1173). This reaction is important because it affects both porosity and interstitial water geochemistry prior to subduction. (3) Units of siliciclastic and volcaniclastic turbidites occur at middle to lower depths of the stratigraphy, but they cannot be correlated from the west to the east side of the basin. Activity within those channelized and sheet-flow turbidite systems was sensitive to changes in detrital provenance, as well as autocyclic and allocyclic forcing. (4) Clay mineral assemblages changed consistently through time. Muds deposited during the early and middle Miocene are highly enriched in smectite (>45 wt-% of bulk mud), which maximizes volumetric fluid production as strata move down the plate interface and experience smectite dehydration. (5) Because of unusually high heat flow, smectite-to-illite diagenesis begins outboard of the subduction front along the Muroto transect. That local effect of S=>I diagenesis obscures the basin-wide temporal trends in clay mineralogy. (6) Profiles of interstitial water geochemistry differ fundamentally from site to site. The water chemistry is affected by varying combinations of in situ hydration reactions (e.g., dispersed volcanic glass alters to smectite), in situ dehydration reactions (e.g., smectite to illite), lateral flow through highly permeable turbidite sands, and diffusional exchange with fluids in the basalt basement. Chlorinity profiles show that strata in lower half of Site C0011 (on the flank of Kashinosaki Knoll) are not in hydrogeologic communication with Site C0012 (on the summit). (7) Most mudstones in the Shikoku Basin are overconsolidated. One anomalous interval of underconsolidated mudstone occurs at Site 1177 near the top of a sandy turbidite facies. The likely cause of this condition is excess pore pressure that builds up as fluids flow in the sands beneath a stratigraphic seal of mudstone. The overpressured interval probably influences the position of the decollement near Site 1177. (8) In all three transects, the frontal decollement follows a stratigraphic interval of hemipelagic mudstone (plus/minus volcanic ash) that ranges in age from about 4 Ma to 7 Ma. Reasons for the fault's lithostratigraphic preference remain cryptic.
Year of Publication: 2011
Research Program: IODP Integrated Ocean Drilling Program
ODP Ocean Drilling Program
Key Words: 07 Marine Geology and Oceanography; Cenozoic; Clastic rocks; Decollement; Deep-sea sedimentation; Depositional environment; Diagenesis; Faults; Geotraverses; Hemipelagic environment; IODP Site C0011; IODP Site C0012; Integrated Ocean Drilling Program; Kumano Basin; Leg 190; Lithofacies; Lithostratigraphy; Marine environment; Marine sedimentation; Messinian; Miocene; Mudstone; NanTroSEIZE; Nankai Trough; Neogene; North Pacific; Northwest Pacific; ODP Site 1173; ODP Site 1177; Ocean Drilling Program; Ocean floors; Pacific Ocean; Paleoenvironment; Permeability; Physical properties; Programs; Research; Sedimentary rocks; Sedimentation; Shikoku Basin; Subduction; Subduction zones; Tertiary; Three-dimensional models; Turbidite; Upper Miocene; Volcaniclastics; West Pacific
Coordinates: N324453 N324453 E1365501 E1365501
N324944 N324945 E1365256 E1365254
N321500 N321500 E1350200 E1350200
N313900 N314000 E1340100 E1340000
Record ID: 2018017278
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