Dehydration of incoming sediments at the Japan Trench

Author(s): Shimizu, Mayuko; Kameda, J.; Hamada, Y.; Tanikawa, W.; Kimura, G.
Author Affiliation(s): Primary:
Japan Atomic Energy Agency, Toki, Japan
Hokkaido University, Japan
Kochi Institute for Core Sample Research, Japan
University of Tokyo, Japan
Volume Title: AGU 2014 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2014; American Geophysical Union 2014 fall meeting, San Francisco, CA, Dec. 15-19, 2014. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: In the 2011 Tohoku-oki earthquake, the seismic fault slip propagated to the trench axis and caused an extremely large tsunami (Ide et al., 2011). Ductile deformation of unconsolidated sediments is commonly prominent in the aseismic shallow parts of the subduction zone. It is unknown how the seismic rupture reached the nearby trench axis. Based on the result of Deep Sea Drilling Project (DSDP) Leg 56 at site 436 (reference, 1977), it is expected that the subducting sediments at the Japan Trench mainly consist of vitric diatomaceous and radiolarian ooze with pelagic clay intervals. Opal and smectite in the pelagic sediments transform respectively into quartz and illite. Kinetic modeling demonstrated that these reactions will progress with active dehydration at 50-60 km horizontally away from the trench axis and with a temperature of 100-120°C. This region coincides with the plate-boundary marked by a prominent seismic reflector. It suggests that the main source of highly pressured fluids is the dehydration of pelagic sediments (Kimura et al., 2012). However, detailed dehydration processes are still unclear mainly due to lack of quantitative sediment composition data. Therefore, in this study, we examined whole rock composition including amorphous silica of the core samples recovered at site 436 as well as those from the Japan Trench by the IODP 343 Japan Trench Fast Drilling Project (JFAST). Analysis of amorphous silica at the drilling site of J-FAST documents that dehydration of the sediments is able to contribute to excess pressure at the shallow part of the megathrust if they underthrust as the same composition. At the drilling site of JFAST, a plate-boundary shear zone was identified around 820 mbsf (Chester et al., 2012). Our analysis showed that the shear zone is characterized by extremely high concentration of smectite (∼70 wt%). These results suggest that the abundant smectite may have possibly fostered localized rupture and slip during the earthquake, because smectite has low frictional coefficient. In our presentation, we will also show the results of the permeability and porosity measurements for the core samples recovered at site 436.
Year of Publication: 2014
Research Program: DSDP Deep Sea Drilling Project
IPOD International Phase of Ocean Drilling
Key Words: 07 Marine Geology and Oceanography; Cores; DSDP Site 436; Deep Sea Drilling Project; IPOD; Japan Trench; Leg 56; Marine sediments; North Pacific; Northwest Pacific; Pacific Ocean; Sediments; West Pacific
Coordinates: N395557 N395558 E1453329 E1453328
Record ID: 2015065145
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