Subduction-related fault processes : ancient and active

Fagereng, Ake.

'Subduction channels', containing highly sheared, fluid-saturated, trenchfill sediments, are commonly present along subduction thrust interfaces. These shear zones accommodate fast plate boundary slip rates (1- 10 cm/yr) and exhibit high levels of seismicity, accomplishing displacement in a broad range of styles including standard earthquakes, non-volcanic tremor, slow slip and aseismic creep. The factors determining seismic style are not well understood, neither is partitioning of displacement between a thin, planar megathrust fault, and distributed brittle/ductile deformation within the tabular subduction channel shear zone. Mathematical models of the Hikurangi subduction margin, New Zealand, demonstrate only minor along-strike variations in thermal structure; temperature is therefore excluded as the dominant factor controlling observed 3D variations in seismic style along this margin. Variable effective stress, caused by heterogeneous permeability and variable fluid overpressuring, is proposed as an alternative factor that determines the spatial distribution of interseismic coupling. The Chrystalls Beach accretionary melange, in the Otago Schist accretioncollision assemblage, New Zealand, is analogous to an active subduction channel assemblage. It contains asymmetric competent lenses of sandstone, chert, and minor basalt enclosed in a relatively incompetent, cleaved mudstone matrix. This assemblage has been intensely sheared in a mixed continuous/ discontinuous style within a flat-lying, < 4 km thick, shear zone. Deformation occurred at T < 300°C and P < 550 MPa, conditions typically present along the seismogenic subduction thrust interface. Ductile fabrics such as folds, S/C-like structures, boudins and asymmetric phacoids formed by independent particulate flow and dissolution-precipitation lll creep. Brittle deformation is evident in an extensive fault-fracture mesh involving mutually cross-cutting subvertical extension veins and subhorizontal slickenfibre-coated shear surfaces. These veins exhibit 'crack-seal' extension and shear increments of< 100 J.Lm, possibly a record of episodic slip in microearthquakes or slower transient slip modes. Shear veins are mainly localised near lithological contacts, extension fractures commonly occur within competent lenses, and distributed ductile deformation is predominant where the ratio of competent to incompetent material is low. If deformation styles in the Chrystalls Beach melange reflect partitioning of aseismic and seismic slip, then the bulk rheology of the subduction channel shear zone imposes significant controls on slip style and length scales of deformation. 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. The fault-fracture mesh in the Chrystalls Beach melange formed under conditions where the hydrofracture criterion, Pt > a3 , was locally achieved. Subhorizontal slickenfibre-coated shear surfaces are oriented at a high angle ( < 85°) to inferred subvertical a 1 , and these 'dilational hydro-shears' required the presence of low-friction, low-cohesion, fluid overpressured planes for shear failure to occur. The vein network may represent a fossil analogue to an episbdic tremor and slow slip zone, where slow slip was recorded by slickenfibres developed by repeating low frequency earthquakes, and extension veins reflect episodic tremor generated by fluid redistribution. In both active and ancient subduction-related shear zones, fluid pressure cycling appears to be the predominant control (over shear stress cycling) on frictional failure, which primarily occurs on weak, fluid overpressured planes. Ductile deformation occurs in response to high brittle shear strength, or physical properties where other deformation mechanisms, such as dissolution- precipitation creep, are preferred. Thus distribution and containment of fluid overpressure is a critical factor determining deformation style in the subduction channel. Such may also be the case in brittle/ ductile shear zones in other tectonic settings.

xx, 604 p. : ill. (some col.), maps ; 30 cm.

2009Fagereng