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Subduction channel shear zones, containing highly sheared, fluid-saturated trench-fill sediments subducted below (or eroded from) the accretionary prism, are common features of active convergent margins (von Huene & Scholl, 1991). Seismic style within subduction channels is highly variable and includes aseismic slip, episodic tremor, microearthquake swarms and megathrust ruptures, but the controlling factors of seismic style are not well understood. Temperature variations along subduction interfaces have been invoked as primary controls on the spatial distribution of seismic styles. Numerical modelling of the Hikurangi Subduction Interface, New Zealand, demonstrates only minor along-strike variations in thermal structure, excluding temperature as the major factor controlling observed 3D variations in seismic style along this margin.
Seismic style is likely affected by the bulk rheology of the subduction channel. The Chrystalls Beach Accretionary Mélange within the Otago Schist, New Zealand, is a possible analogue for an active subduction channel shear zone. Asymmetric phacoids of sandstone, chert and minor basalt are enclosed within a relatively incompetent, cleaved pelitic matrix. This assemblage has been intensely sheared in a mixed continuous/discontinuous style within a flat-lying, ∼4km thick, top-to-the-north shear zone where peak PT conditions reached 6-10kbar (∼25-40km) and 200-400°C. Initially, ductile deformation mechanisms predominated and sediments experienced compaction, volume loss and layer-parallel shear, developing a slaty cleavage and S/C shear structures. During and after a time-progressive transition from ductile to brittle failure a fault-fracture mesh filled by quartz/calcite veins developed, incorporating numerous anastomosing shear veins traceable for tens of metres at vein densities of 100m-1. 'Crack-seal' extension veins within this network, generally localised within dilational jogs, indicate that the tensile overpressure condition (Pfσ3) was locally attained. Slickenfibres within shear veins developed incrementally, suggesting formation by episodic slip (10?100m), possibly associated with microearthquakes like those seen in active creeping fault segments.
Failure style within the mélange depends on the distribution of competent material, with shear veins mainly localised near lithological contacts, extension fractures dominant within competent rock, and distributed ductile deformation predominating where the inclusion/matrix ratio is low. If the style of strain accommodation in the Chrystalls Beach Mélange reflects the partitioning of aseismic and seismic slip, then the structure and composition of the subduction channel shear zone impose significant controls on deformation mechanisms along the megathrust interface. In particular, heterogeneity in fluid pressure and the ratio of competent to incompetent material are likely to contribute significant strength heterogeneity (e.g. asperities) along interplate megathrusts.
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