Abstract
To investigate the mechanical properties and deformation patterns of megathrusts in subduction zones, we studied damage zone structures of the Nobeoka Thrust, an exhumed megasplay fault in the Kyushu Shimanto Belt, using drill cores and geophysical logging data obtained during the Nobeoka Thrust Drilling Project. The hanging wall, composed of a turbiditic sequence of phyllitic shales and sandstones, and the footwall, consisting of a melange of a shale matrix with sandstone and basaltic blocks, exhibit damage zones that include multiple sets of ‘brecciated zones’ intensively broken in the mudstone-rich intervals, sandwiched by ‘surrounding damage zones’ in the sandstone-rich intervals with cohesive faults and mineral veins. The fracture zones are thinner (2.7 to 5.5 m) in the sandstone-rich intervals and thicker in the shale-dominant intervals (2.3 to 18.6 m), which indicates a preference of coseismic slip and velocity-weakening in the former, and aseismic deformation in the latter. However, the surrounding damage zones observed in the current study are associated with an increase in resistivity, P-wave velocity, and density and a decrease in porosity, inferring densification and strain-hardening in the sandstone-rich intervals and strain-weakening in the mudstone-rich intervals. These observations indicate that the sandstone-rich damage zones may weaken in the short term but may strengthen in the geologically long term, contributing to a later stage of fault activity. In contrast, the mudstone-rich damage zones may strengthen in the short term but develop weak structures through longer time periods. The observed shear zone thickness in the hanging wall is thinner (2.3 to 18.6 m) compared to the footwall damage zones (12 to 39.9 m), possibly because faults in the hanging wall were concentrated and partitioned between the preexisting turbiditic sequence of alternating shale/sandstone-dominant intervals, whereas in the footwall, faults were more sporadically distributed throughout the sandstone block-in-matrix cataclasites. A splay fault may evolve and be characterized by physical property contrasts, the lithology dependence of deformation, and the variability of damage zone thickness due to a heterogeneous lithology distribution in the hanging wall and footwall. The deformation patterns observed in the Nobeoka Thrust provide insights to the strain-hardening/weakening behaviors of sediments along megathrusts over geological timescales.
Highlights
Shear localization in foliated, phyllosilicate-rich fault rocks is known to cause weakening in crustal fault zones (e.g., Stewart et al 2000; Imber et al 2001; Gueydan et al 2003; Collettini and Holdsworth 2004; Wibberley and Shimamoto 2005; Jefferies et al 2006)
Within the shale-dominant interval just above the hanging wall damage zone, structures resemble the dense cleavage development seen in sh1 and sh2 but are more disturbed due to increasing faults and fractures (Figure 3)
In our examination of the relationship between physical properties and faults, fractures, and mineral vein density in all of the surrounding damage zones at the Nobeoka Thrust, we found that resistivity and density have a negative correlation below a fracture density value of 20 per 1 m, indicating that deformation may have occurred in a strain-weakening manner (Figure 14)
Summary
Phyllosilicate-rich fault rocks is known to cause weakening in crustal fault zones (e.g., Stewart et al 2000; Imber et al 2001; Gueydan et al 2003; Collettini and Holdsworth 2004; Wibberley and Shimamoto 2005; Jefferies et al 2006). Various weakening mechanisms have been proposed including sliding and/or frictional-viscous flow in low-friction phyllosilicate gouges (e.g., Niemeijer and Spiers 2005; Boulton et al 2012), comminution of rock material and grain size reduction (e.g., De Bresser et al 2001), fault lubrication (e.g., Di Toro et al 2011), high pore fluid pressures (e.g., Smith et al 2008), fluid-enhanced reaction weakening (e.g., Wibberley and Shimamoto 2005), thermal pressurization (e.g., Brodsky and Kanamori 2001), and thermal melting (e.g., Leloup et al 1999). Due to the complicated structures of fault zones and the differences in mechanical strength contrast across the decollement, overriding wedge, and underthrust material, the development of phyllosilicaterich fault rocks may occur heterogeneously. The roles of foliated fault rocks in the process of strain localization and fault evolution in subduction zone settings are poorly understood
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