Evidence for cyclical fault zone sealing and strengthening, Alpine Fault, New Zealand

The Alpine Fault, a transpressional plate boundary fault between the Australia-Pacific plates, ruptures periodically ( 300yr) with large magnitude earthquakes (Mw 8) and is currently locked and nearing the end of its interseismic period. Using temporal and fine-scale spatial variations in carbonate deformation microstructures and geochemistry, this study investigates the influential role of calcite in sealing and restrengthening the Alpine Fault. Phase 1 of the Deep Fault Drilling Project (DFDP-1) at Gaunt Creek revealed a fault zone structure with 1m fault core and associated damage zone, overprinted by a zone of alteration (AZ) in the hanging wall, formed through enhanced fluid-rock interaction on a fault which currently has very low permeability. Carbonate is one of the primary authigenic minerals in this AZ, with mineralisation concentrated within fault core fractures. Through Electron Backscatter Diffraction, Cathodoluminescence (CL) and Secondary Ion Mass Spectrometry multiple episodes of fracture generation and mineralisation have been recognised. Early carbonate veins exhibit dull CL and high Fe concentrations; these generations have accommodated extensive deformation including intense mechanical e-twinning and dynamic recrystallization, indicating formation at temperatures of >250°C. Younger generations, attributed to more recent seismicity exhibit bright CL and greater Mn concentrations and lack deformation microstructures, supporting the absence of stable creep between rupture events on the central Alpine Fault. Variations in trace elements highlight fluctuations in ambient conditions/fluid source between generations of calcite veining. Variation within a single generation indicates multiple pulses of fluid were required to fully seal some voids. Older calcite generations and those closer to the principal slip zone contain greater amounts of Fe and Mg, while younger generations and recrystallized regions contain more Mn. The precipitation of secondary minerals plays an important role in fault sealing through the brittle seismogenic crust. Calcite is observed to dominate within the Alpine Fault core to depths of <4km, below which calcite and quartz are likely to co-precipitate in the fault core; however evidence of this is overprinted by multiple cycles of cataclasis.