HR: 16:00h
AN: S14B-01    [Abstracts]
TI: Is the Chrystalls Beach Accretionary Melange a Fossil Subduction Channel Shear Zone?
AU: * Fagereng, A
EM: ake@geology.co.nz
AF: Department of Geology, University of Otago, PO Box 56, Dunedin, 9054, New Zealand
AU: Sibson, R H
EM: Rick.Sibson@otago.ac.nz
AF: Department of Geology, University of Otago, PO Box 56, Dunedin, 9054, New Zealand
AB: In the northeast of the Hikurangi subduction margin, a 1-2 km thick layer of high Vp/Vs, low Qp and distributed microseismicity is present along the subduction megathrust interface (Eberhart-Phillips & Chadwick, 2002). This zone is interpreted as a 'subduction channel' consisting of a fluid-saturated, highly sheared mixture of trench-fill sediments, which have been subducted below (or eroded from) the accretionary prism (von Huene & Scholl, 1991). Seismic style within subduction channels may vary from large megathrust ruptures to aseismic slip associated with microseismic activity. The factors controlling these variations in style are not well understood due to the inaccessible nature of active subduction thrust interfaces. The Chrystalls Beach Complex, SE Otago, New Zealand, is a possible analogue for the seismogenic zone of an active subduction channel. This complex comprises an intensely sheared mixture of chert, terrigenous mud and sand, minor volcanogenic sediments and pillow lavas. It has a 'block-in-matrix' mélange structure, where asymmetric, dismembered beds of sand-rich competent material are enclosed within a relatively incompetent, cleaved pelitic matrix. The rock assemblage has been progressively deformed in a top-to-the-north shear zone, and is pervaded by an anastomosing network of quartz/calcite shear- and extension veins, where individual veins can be traced for tens of metres. The presence of extension veins indicates episodes where the tensile overpressure condition (P_f > σ₃) was locally attained. Initially the sediments experienced compaction, volume loss and layer-parallel soft sediment shearing, developing a slaty cleavage and viscous S/C shear structures. The dense vein network developed during subsequent brittle deformation. The mineral assemblage (pumpellyite-chlorite to pumpellyite-actinolite), mica b₀ spacing and illite crystallinity indicate deformation in a high pressure - low temperature environment ( ~ 3-6 kbar, ~ 200-300°C). This P-T environment and structural character appear to match that inferred for microseismically active portions of the Hikurangi subduction channel. Slickenfibres coating shear veins in the Chrystalls Beach Complex formed by a 'crack-seal' mechanism, suggesting formation by episodic slip coupled to fluid pressure cycling and solution transfer. These veins may therefore record incremental slip associated with microearthquakes like those seen in highly overpressured regions of active subduction zones and other creeping fault segments. Observations from this fossil shear zone provide constraints for laboratory and theoretical models, highlighting the chaotic nature of natural faults. For example, shear veins in the complex are commonly localised along lithological contacts, while extension veins are concentrated in the more competent units. If the process forming these veins mirrors the triggering mechanism of subduction zone microseismicity, then the structure and composition of the subduction channel shear zone impose a significant control on deformation along the subduction thrust interface. In particular, heterogeneity in fluid pressure and the ratio of competent to incompetent material seem likely to be important sources of strength heterogeneity along the interplate megathrust.
DE: 7209 Earthquake dynamics (1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8010 Fractures and faults
DE: 8021 Melanges
DE: 8163 Rheology and friction of fault zones (8034)
SC: Seismology [S]
MN: 2007 Fall Meeting