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

Abstract

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 (μ_r) at stress levels between 1.0 and 40 MPa. The direct shear μ_r 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 décollement zone. Illite (plus quartz) is mechanically important in the altered incoming Muroto section and the predicted décollement μ_r 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 μ_r 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 μ_r values of the clay-rich décollement 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 décollement 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.

Introduction

We present the initial results of an ongoing experimental study that examines the role that clay minerals play in controlling the frictional properties of subduction plate boundary faults and how, together with regional fluid pressure variations, this controls the state of stress on the subduction thrust. We also address the role that clay diagenesis plays in the onset of seismogenic activity. We focus here on the Nankai Trough, southwest Japan, because it is representative of sediment-rich subduction zones (including Alaska and Cascadia) that are capable of generating very large earthquakes (M ∼8 and greater) [1], [2], [3], [4]. Two significant, seemingly contradictory attributes of these subduction zone faults are that they appear to be almost fully locked over much of their along-strike length during interseismic periods even though the faults appear to be extremely weak [5].

Accretionary wedge environments provide the opportunity to test more specific ideas as to how three-dimensional changes in parameters such as fault zone mineralogy, temperature, and fluid pressure (i.e. effective confining stress) can affect the stress state and seismogenic properties of plate boundaries. We chose the Nankai subduction zone as a study area, because it has a historical record of very large earthquakes that dates back to AD 684 [6], [7], [8]. The recurrence interval in the western Nankai region varies between 262 and 92 years, and the most recent ruptures were the 1944 Tonankai (M 8) and 1946 Nankaido (M 8.2) events offshore of the Kii and Muroto peninsulas [6], [9]. In addition, the nationwide permanent Global Positioning System network shows that the coastal forearc region is moving to the west-northwest at the full convergence rate of ∼4 cm/yr, which indicates the plate boundary is essentially fully coupled or locked during the interseismic period [10], [11]. However, the plate boundary fault also seems to be extremely weak if the maximum principal stress currently has a trench-parallel orientation and the margin-normal stress appears to be less than the vertical stress in the backstop region. For this to occur, numerical modeling suggests the maximum shear stress on the subduction thrust should not exceed ∼17 MPa even at the stronger down-dip end of the Nankai seismogenic zone [5].

Coring by the Deep Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP) has already revealed significant along-strike changes in the lithostratigraphy, heat flow, clay mineralogy, and diagenesis of the incoming Shikoku Basin section seaward of the Nankai Trough. Sediment composition at Nankai appears to be controlled by changing patterns of heat flow and thermally activated clay diagenesis [12], [13], [14]. These shallow, along-strike mineralogical changes mimic those within the fault zone with increasing depth, and thereby allow us to test whether such changes significantly influence the down-dip frictional properties of this plate boundary.

In this paper, we focus on experimentally determining the frictional strength of materials entering the subduction toe in order to independently evaluate physical reasons for the apparent weakness of the subduction fault at Nankai. We also address the issue of whether or not the smectite-to-illite reaction can lead to significant down-dip changes in fault strength. Smectite has certainly been recognized as a weak clay mineral, but it is not clear whether the transformation to illite causes substantial changes in friction coefficient. Our goal is to address how sediment composition impacts fault properties at effective normal stresses up to 40 MPa using both a ring shear and a newly developed direct shear apparatus. A combination of natural Nankai samples, individual mineral standards, and mixtures of smectite (Sm), illite (Il), chlorite (Ch), and quartz (Qtz) were used. Based on the experimental data we also address the potential role that regional pore pressure changes may play in determining the lateral changes in the overall fault strength at Nankai. In addition to arguments about the general state of stress, we are also able to offer insights on the debate over the potential diagenetic controls on the up-dip limit on seismogenic activity [2].