Abstract
Triaxial shear tests on mudstone samples from the Shimanto Belt and the Boso accretionary complexes (SW Japan and central Japan) were carried out. Pre-exhumation burial depths in the two paleo-accretionary prisms were up to 9,000 m and about 1,000 m for the Shimanto and the Boso samples, respectively. Three methods were applied: (1) pressure stepping tests at increasing confining pressures between 25 and 65 MPa and pore pressures between 20 and 52 MPa; (2) constant confining pressure tests at 55 and 65 MPa, with stepwise pore pressure decrease from 80% to 50% and 25%, and from 90% to 60% and 30% of the confining pressure; and (3) a cyclic loading test on one sample from Boso (19 cycles to 70-MPa differential stress). After some contraction due to pressurization in the first cycles, the sample showed tendencies to creep rather than to fracture. Effective shear parameters show that angles of internal friction between 30° and 50° are in part quite high in both sample subsets, and ranges of cohesion are between about 2 and 6 MPa (Boso) and 13 and >30 MPa (Shimanto). The mechanical results from these paleo-accretionary prisms are taken to constrain the shear parameters of rocks in the deeper parts of the present Nankai accretionary wedge and forearc. Static friction resembles results from experiments on a wide range of phyllosilicate-quartz-feldspar gouges and shows that the forearc is composed of relatively strong rock. Cohesion increase due to diagenesis and/or very low grade metamorphism is of overriding importance and probably permits stresses of up to 18 MPa to be transmitted to the updip end of the seismogenic zone at depth and 5 to 13 MPa to the backstop of the actively deforming frontal prism.
Highlights
Subduction zones are tectonically highly active regions on Earth and are known to generate large earthquakes by thrust faulting
Effective shear parameters were derived from suites of triaxial pressure stepping tests at increasing confining pressures
A cyclic loading test on one sample from Boso (18 cycles to 70MPa differential stress) showed that after some contraction due to pressurization in the first five cycles, the sample had tendencies to creep at constant stress rather than to fracture at maximum applied load. This may be typical for the Boso mudstones tested, as the other cores were not fractured during the experiments
Summary
Subduction zones are tectonically highly active regions on Earth and are known to generate large earthquakes by thrust faulting (for reviews on subduction zones, see von Huene and Scholl 1991; Saffer and Tobin 2011). Splay faulting plays a critical role in the building of accretionary wedges, and clay-rich sediments usually act as décollement horizon in the tectonic This means that the changing physical properties of clay-rich rocks, such as porosity, state of consolidation, mineral fabrics, and cementation, are instrumental in controlling seismicity (e.g., Scholz 1998; Moore and Saffer 2001) and determining the transition from stable frictional sliding behavior to unstable stick slip at convergent plate boundaries (e.g., Byrne et al 1988; Vrolijk 1990; Hyndman et al 1995; Oleskevich et al 1999; Moore and Saffer 2001; Kimura et al 2007). This lends justification to the approach to study samples of rocks, which have experienced a burial and diagenetic or metamorphic history comparable to those hosting the seismogenic zone at depth
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