Fluid behavior during evolution of plate boundary fault from trench to seismogenic depths

Author(s): Ujiie, Kohtaro; Yamaguchi, Asuka; Kimura, Gaku; Hisamitsu, Toshio; Taira, Asahiko
Author Affiliation(s): Primary:
Japan Agency for Marine-Earth Science and Technology, Institute for Research on Earth Evolution, Japan
Other:
University of Tokyo, Japan
Volume Title: Coupled mechanical and fluid-pressure responses and fluid flow processes in the geosphere
Volume Author(s): Wang, Herbert F., prefacer
Source: Coupled mechanical and fluid-pressure responses and fluid flow processes in the geosphere, prefaced by Herbert F. Wang. Chigaku Zasshi = Journal of Geography, 115(3), p.353-366. Publisher: Tokyo Geographical Society, Tokyo, Japan. ISSN: 0022-135X CODEN: CGZAAL
Note: In Japanese with English summary. 34 refs.; illus., incl. sects., sketch map
Summary: Fluid behavior during the evolution of the plate boundary fault (pbf) from a trench to seismogenic depths is the central problem when evaluating the relationship between fluids and seismicity in subduction zones. Ocean Drilling Program Legs 190 and 196 at the toe region of the Nankai accretionary margin reveal that fluid-filled dilatant fractures and underconsolidated underthrust sediments lead to an elevated fluid pressure in and below the pbf, respectively. The pbf with elevated fluid pressure extends down-dip to ∼35 km, resulting in the absence of seismic behavior at shallow depths and mechanical decoupling between accreted and underthrust sediments. Underconsolidated underthrust sediments are primarily caused by rapid tectonic loading compared to the rate of fluid escape in underthrust sediments and secondarily by a low-permeability cap due to the compactively deformed pbf. Fluid-filled dilatant fractures represent the overconsolidate state within the pbf, which is caused by the generation of high fluid pressure after compactive deformations. The exhumed plate boundary rocks (i.e., tectonic melange) in the Shimanto accretionary complex indicate that the underthrust sediments became rocks due to dewatering, pressure solution, and other diagenetic reactions, thus acquiring elastic strength. The pbf in the upper part of the seismogenic depths was weak due to elevated fluid pressure; this facilitated the downward step of the pbf and the underplating of underthrust rocks. The pbf under low effective stress was unlikely to nucleate the instability; however the fluid-related repeated deformations, which probably reflect the seismic cycle in the subduction zone, could be recorded. The coseismic deformations were attributed to hydraulic implosion breccias, injection of ultracataclasite, and fluid inclusion stretching in the pbf. Implosion breccias suggest rapid depressurization associated with the passage of the rupture through dilational jog. Other deformations represent shear heating and fluidization along the narrow ultracataclasite layer, which could enhance the propagation of instability at the pbf in the upper parts of the seismogenic depths.
Year of Publication: 2006
Research Program: ODP Ocean Drilling Program
Key Words: 18 Geophysics, Solid-Earth; Asia; Deformation; Depth; Far East; Faults; Fluid dynamics; Japan; Leg 190; Leg 196; Loading; Nankai Trough; North Pacific; Northwest Pacific; Ocean Drilling Program; Pacific Ocean; Plate boundaries; Plate tectonics; Seismicity; Seismotectonics; Shimanto Belt; Subduction zones; Tectonics; West Pacific
Coordinates: N313000 N324500 E1351500 E1340000
N321400 N322200 E1350200 E1345600
Record ID: 2007116685
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