HR: 14:10h
AN: S43D-03    [Abstracts]
TI: Incremental Growth of Quartz Slickenfibres on Shearing Surfaces in an Ancient Subduction Complex - Record of Paleomicroearthquakes?
AU: Remitti, F
EM: fremitti@unimore.it
AF: Dipartimento di Scienze della Terra, Universita di Modena e Reggio Emilia, Largo Sant'Eufemia, 19, Modena, 41100 Italy
AU: * Sibson, R H
EM: rick.sibson@stonebow.otago.ac.nz
AF: Department of Geology, University of Otago, P.O. Box 56, Dunedin, 9001 New Zealand
AB: An accretionary m$\'{e}$lange of Triassic age ocean floor sediments exposed at Chrystalls Beach in SE Otago, New Zealand is made up predominantly of competent sandstone and chert phacoids set in a cleaved mudstone matrix, all deformed in a subgreenschist environment (3 $<$ P $<$ 6 kbar; T $\sim$ 250$\pm$30 $\deg$C). Distributed brittle fracturing within the sandstone lenses contrasts with ductile deformation involving solution transfer accompanying cleavage development in the pelites. The more pelitic portions of the m$\'{e}$lange are also pervaded by an anastomosing cellular mesh comprising multiple shearing surfaces subparallel to cleavage coated with incrementally developed quartz (and locally calcite) slickenfibres. Sliding surfaces splay and amalgamate and are also commonly interlinked by sets of extension veins localised within dilational jogs. Microstructural observations reveal growth of the slickenfibres by `crack-seal' shearing increments of 10-100 microns, with incremental slip transfer of the same order across the dilational jogs accommodated by the opening of extension fractures that link en $\'{e}$chelon slip surfaces. Together with widespread extension fracturing in the more competent sandstones these structures suggest that the tensile overpressure condition (P$_{f}$ $>$ $\sigma$$_{3}$) was widely achieved during brittle deformation under near-lithostatic fluid overpressures. Individual slip surfaces can be traced for metres to tens of metres so that the ratio of slip, u, to potential rupture length, L, lies within the range, 10$^{-5}$ $<$ u/L $<$ 10$^{-4}$, characteristic of microearthquakes obeying `constant stress-drop' scaling with $\Delta$$\tau$ $\sim$ 3 MPa. For this scaling relationship, the slip increments are comparable to those expected for M0 to M-2 microearthquakes. The host-rock assemblage, metamorphic environment, inference of near-lithostatic fluid overpressures, and mixed continuous-discontinuous style of shearing, resemble conditions inferred for the active subduction channel shear zone associated with the Hikurangi subduction margin in the North Island, where clustered microseismicity is associated with creeping portions of the thrust interface.
DE: 8010 Fractures and faults
DE: 8030 Microstructures
DE: 8045 Role of fluids
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2004 AGU Fall Meeting