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© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  29 May 2020

29 May 2020

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A revised version of this preprint is currently under review for the journal SE.

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

Martina Kirilova1,2, Virginia Toy1,2, Katrina Sauer1, François Renard3,4, Klaus Gessner5, Richard Wirth6, and Xianghui Xiao7,8 Martina Kirilova et al.
  • 1Department of Geology, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand
  • 2Institut für Geowissenschafte, Johannes Gutenberg Universität-Mainz, J. J. Becher Weg 21, D-55128, Mainz, Germany
  • 3Department of Geosciences, The Njord Center, University of Oslo, Oslo 0316, Norway
  • 4Université Grenoble Alpes, Université Savoie Mont Blanc, CNRS, IRD, IFSTTAR, ISTerre, BP53, 38041 Grenoble, France
  • 5Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, Australia
  • 6Helmholtz-Zentrum Potsdam, GFZ, Sektion 4.3, Telegrafenberg, 14473 Potsdam, Germany
  • 7Advanced Photon Source, Argonne National Laboratory, Lemont, IL 60439, USA
  • 8National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, NY 11973, USA

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.

Martina Kirilova et al.

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Martina Kirilova et al.

Martina Kirilova et al.


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Publications Copernicus
Short summary
Processes associated with open pores can change the physical properties of rocks and cause earthquakes. In borehole samples from the Alpine Fault zone, we show that many pores in these rocks were filled by weak materials that can slide easily. The amount of open spaces was thus reduced and fluids circulating within them built up high pressures. Both weak materials and high pressures within pores reduce the rock strength, thus the state of pores here can trigger the next Alpine Fault earthquake.
Processes associated with open pores can change the physical properties of rocks and cause...