Micro- and nano-porosity of the active Alpine Fault zone, New Zealand

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Abstract. Porosity reduction in rocks from a fault core can cause fluid overpressure, and consequently influence the recurrence time of earthquakes. We investigated the porosity distribution in the New Zealand's Alpine Fault core in samples recovered during the first phase of the Deep Fault Drilling Project (DFDP-1B) by using two-dimensional nanoscale and three-dimensional microscale imaging. Synchrotron X-ray microtomography-derived analyses of open pore spaces show total microscale porosities in the range of 0.1 to 0.24 %. These pores have mainly non-spherical, elongated, flat shapes and show subtle bipolar orientation. Transmission electron microscopy reveals that nanoscale pores ornament grain boundaries of the gouge material, especially clay minerals. Our data implies that: (i) the distribution of clay minerals controls the shape and orientation of the associated pores; (ii) porosity was reduced due to pressure solution processes; and (iii) mineral precipitation in fluid-filled pores can affect the mechanical behaviour of the Alpine Fault by decreasing the already critically low total porosity of the fault core, causing fluid overpressure, and/or introducing weak mineral phases, and thus lowering the overall fault frictional strength. We conclude that the current state of porosity in the Alpine Fault core is likely to play a key role in the initiation of the next fault rupture.

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