Compositional and fluid pressure controls on the state of stress on the Nankai subduction thrust; a weak plate boundary

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doi: 10.1016/S0012-821X(03)00388-1
Author(s): Brown, K. M.; Kopf, A.; Underwood, M. B.; Weinberger, J. L.
Author Affiliation(s): Primary:
Scripps Institution of Oceanography, La Jolla, CA, United States
Other:
University of Missouri, United States
Volume Title: Earth and Planetary Science Letters
Source: Earth and Planetary Science Letters, 214(3-4), p.589-603. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0012-821X CODEN: EPSLA2
Note: In English. 61 refs.; illus., incl. sects., sketch map
Summary: We show that both fault mineralogy and regional excess fluid pressure contribute to low resolved shear stresses on the Nankai subduction plate boundary off southwest Japan. Ring and direct shear tests indicate that saturated clay minerals in the fault possess intrinsically low residual friction coefficients (µr) at stress levels between 1.0 and 40 MPa. The direct shear µr values for purified smectite are ∼0.14±0.02, for illite ∼0.25±0.01, and for chlorite 0.26±0.02 (for point load velocities of 0.0001 mm/s). These clay minerals dominate the Nankai subduction decollement zone. Illite (plus quartz) is mechanically important in the altered incoming Muroto section and the predicted decollement µr should lie between 0.2 and 0.32. This low residual strength, together with elevated fluid pressure, limits shear stresses to below ∼4 MPa within the frontal ∼50 km of the subduction system, consistent with the low wedge taper in this region. A higher wedge taper off the Ashizuri peninsula indicates basal shear stresses rise slightly along strike towards this region. Our analysis indicates lower fluid pressures must predominantly be responsible because only small second order along strike variations in µr are predicted to occur as a result of variations in smectite and total clay content. These variations should be further reduced at depth under the wedge as smectite is diagenetically altered to illite. However, our data suggest the low µr values of the clay-rich decollement still limit shear stresses to between ∼17 and 29 MPa within the frontal ∼50 km of the wedge, consistent with other estimates of plate boundary weakness. Indeed, we propose that it should be expected that subduction plate boundaries like Nankai will be weak because of the intrinsic presence of clay-rich faults and moderate fluid overpressures. Our data do not support the hypothesis that the smectite-to-illite reaction directly controls the onset of seismogenic behavior deep in the Nankai system because there is already a mechanical dominance of illite (rather than smectite) in the shallow decollement zone, and we find all the clay phases tend to velocity strengthen. However, temperature-activated clay diagenesis and dehydration may cause secondary changes in the fault properties and state of stress across the up-dip limit of the seismogenic zone. Abstract Copyright (2003) Elsevier, B.V.
Year of Publication: 2003
Research Program: DSDP Deep Sea Drilling Project
ODP Ocean Drilling Program
Key Words: 18 Geophysics, Solid-Earth; Ashizuri Peninsula; Asia; Clay mineralogy; Composition; DSDP Site 297; Decollement; Deep Sea Drilling Project; Diagenesis; Experimental studies; Far East; Faults; Fluid pressure; Friction; Instruments; Japan; Laboratory studies; Leg 190; Leg 31; Marine sediments; Nankai Trough; North Pacific; Northwest Pacific; ODP Site 1177; Ocean Drilling Program; Pacific Ocean; Plate boundaries; Plate tectonics; Sedimentary rocks; Sediments; Shear strength; Shikoku; Shikoku Basin; Standard materials; Strength; Stress; Subduction; Subduction zones; Thrust faults; West Pacific; X-ray diffraction data
Coordinates: N305222 N305222 E1340953 E1340953
N313900 N314000 E1340100 E1340000
Record ID: 2004082814
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands