Incrementally developed slickenfibers — Geological record of repeating low stress-drop seismic events?

An accretionary mélange of Triassic age ocean floor sediments exposed in the Chrystalls Beach Complex, South Island, New Zealand, comprises competent sandstone and chert phacoids set in a cleaved mudstone matrix, deformed in a continuous–discontinuous style at subgreenschist conditions. Deformation structures include a pervasive anastomosing fault–fracture mesh of multiple shearing surfaces, subparallel to cleavage, coated with incrementally developed quartz–calcite slickenfibers. Microstructural observations reveal slickenfiber growth by ‘crack-seal’ shear slip increments of 10–100 μm, with incremental slip transfer of the same order accommodated by opening of extension fractures that link en echelon slip surfaces. Individual slip surfaces can be traced for meters to tens of meters so that the ratio of average slip, u, to potential rupture length, L, predominantly lies within the range, 10− 6 < u/L < 10− 5, characteristic of microearthquakes obeying ‘constant stress-drop’ scaling with a low stress-drop Δτ ~ 30 kPa, typical of low frequency earthquakes. The host-rock assemblage, metamorphic environment, inference of near-lithostatic fluid overpressures, low stress-drop and mixed continuous–discontinuous shearing, resemble conditions and characteristics of low frequency earthquakes as identified within the seismic signals recorded during episodic tremor and slow slip events, at the downdip end of the seismogenic subduction thrust interface and within accretionary prisms.