Hydrostratigraphy as a control on subduction zone mechanics through its effects on drainage: an example from the Nankai Margin, SW Japan

Abstract

At many subduction zones, accretionary complexes form as sediments are offscraped from the subducting plate, and excess pore pressures commonly develop as low-permeability marine sediments undergo rapid tectonically driven loading. Mechanical models demonstrate that pore pressure controls the overall geometry of these systems by modifying shear strength both within the accretionary wedge and along its base. At the Nankai margin offshore SW Japan, the taper angle of the accretionary wedge varies markedly along-strike, from $4°along an eastern (Muroto) transect, to 8-10°along a western (Ashizuri) transect. Sediment stratigraphy on the subducting plate also varies: along the Ashizuri transect, the lowermost part of the section includes abundant sandy turbidites, whereas along the Muroto transect it is composed of monotonous hemipelagic mudstone. Here, I use a numerical model of fluid flow, together with laboratory measurements that constrain the bulk mudstone permeability, to quantitatively test the hypothesis that the turbidite-rich section along the Ashizuri transect allows drainage at the base of the accretionary complex, resulting in differences in mechanical strength sufficient to cause the differences in taper angle. My results demonstrate that if the turbidite-rich units are 2-100 times more permeable than the mudstone units, the variation in stratigraphy can indeed explain the observed taper angles. In contrast, permeability anisotropy within the turbidite-rich units has only a minor effect; anisotropy ratios of $1000:1 would be required to cause the differences in taper angle. Along the Ashizuri transect, simulated pore pressures result in a basal shear strength ranging from a few MPa at the trench to $20 MPa by 30 km arcward; along the Muroto transect shear strength is substantially lower, reaching only $5 MPa by 30 km. This work shows that lithostratigraphy can strongly influence the mechanical behavior of subduction zone faults, through its control on the distribution and magnitude of excess pore pressure.

The sub‐Himalayan fold‐thrust belt in the 1905 Kangra earthquake zone: A critical taper model perspective for seismic hazard analysis

“…For comparison with other orogenic critical tapers, both accretionary wedges and continental fold-andthrust belts, the l ratio commonly varies with depth, but it predominantly ranges between 0. 6 and 0.8 [e.g., Fertl, 1976;Hyndman et al, 1993;Lallemand et al, 1994;Moore et al, 1995;Adam and Reuther, 2000;Calderoni et al, 2009;Roure et al, 2010;Saffer, 2010]. These values are also confirmed by the results from analogue [e.g., Cobbold et al, 2001; Mourgues and Cobbold, 2006;Graveleau et al, 2012] and numerical [e.g., Saffer and Bekins, 2006] modeling.…”

“…[21] Also as concerns the fluid pressure ratio within the basal shear zone, l b , a comparison with other accretionary wedges shows it is commonly greater than l within the wedge [e.g., Saffer andBekins, 2002, 2006;Hayward et al, 2003]. According to numerical modeling and for the sake of simplicity, in our calculations we tested the l b varying from 0 to 50% greater than l. Figure 5.…”

Comparison of frictional strength and velocity dependence between fault zones in the Nankai accretionary complex

“…The lower friction of the samples from Site 1174 compared to that of the frontal thrust samples from Site C0007 is consistent with the higher taper angle of the outermost accretionary wedge along the Kumano transect (∼8–10°) than along the Muroto transect (∼4°) [e.g., Davis et al , 1983; Davis and von Huene , 1987; Dahlen , 1990; Kimura et al , 2007], assuming that the frontal thrust is similar in strength and composition to the décollement along the Kumano transect (which was not sampled). However, the differences in taper angle could also be explained by lower basal shear strength along the Muroto transect due to higher pore fluid pressures there, as suggested by several previous studies [ Kopf and Brown , 2003; Moore et al , 2005; Screaton et al , 2009; Skarbek and Saffer , 2009; Tobin and Saffer , 2009; Saffer , 2010], or by the influence of seamounts on the subducting plate that perturb the overriding accretionary wedge [ Park et al , 1999; Bangs et al , 2006]. Taken together, our data and the results of previous analyses suggest that differences in friction coefficient play a role in driving along‐strike differences in taper angle, although they are probably not the only factor.…”

Consolidation state of incoming sediments to the Nankai Trough subduction zone: Implications for sediment deformation and properties

“…Although our estimation of permeability downdip is simplified and the connectivity and thickness of the siliciclastic sandstones are unknown, the persistent higher permeability of the siliciclastic sandstones implies they may play an important role in drainage of the subduction system, as suggested for the toe of the NanTroSEIZE transect on the basis of drilling data [ Screaton et al ., ], and for other transects in the Nankai Trough from numerical modeling [ Saffer , ]. Additionally, pore water geochemical data, including freshening, document evidence for deeply sourced fluids at Site C0011 [Expedition 333 Scientists; 2012a].…”