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Solid Earth An interactive open-access journal of the European Geosciences Union

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https://doi.org/10.5194/se-2017-112
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
10 Oct 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Solid Earth (SE).
Controls on fault zone structure and brittle fracturing in the foliated hanging-wall of the Alpine Fault
Jack N. Williams1, Virginia G. Toy1, Cécile Massiot2,3, David D. McNamara3,5, Steven A. F. Smith1, and Steven Mills4 1Department of Geology, University of Otago, PO Box 56, Dunedin 9054, New Zealand
2School of Geography, Environment, and Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington 6012, New Zealand
3GNS Science, PO Box 30 - 368, Lower Hutt 5040, New Zealand
4Department of Computer Science, University of Otago, PO Box 56, Dunedin 9054, New Zealand
5Department of Earth and Ocean Sciences, NUI Galway, University Road, Galway, Ireland
Abstract. The orientations and densities of fractures in the foliated hanging-wall of the Alpine Fault provide insights into the role of a mechanical anisotropy in upper crustal deformation, and the extent to which existing models of fault zone structure can be applied to active plate-boundary faults. Three datasets were used to quantify fracture damage at different distances from the Alpine Fault principal slip zones (PSZs): (1) X-ray computed tomography (CT) images of drill-core collected within 25 m of the PSZs during the first phase of the Deep Fault Drilling Project that were reoriented with respect to borehole televiewer images, (2) field measurements from creek sections at < 500 m from the PSZs, and (3) CT images of oriented drill-core collected during the Amethyst Hydro Project at distances of ~ 500–1400 m from the PSZs. Results show that within 160 m of the PSZs in foliated cataclasites and ultramylonites, gouge-filled fractures exhibit a wide range of orientations. At these distances, fractures are interpreted to form at high confining pressures and/or in rocks that have a weak mechanical anisotropy. Conversely, at distances greater than 160 m from the PSZs, fractures are typically open and subparallel to the mylonitic foliation or schistosity, implying that fracturing occurred at low confining pressures and/or in rocks that are mechanically anisotropic. Fracture density is similar across the ~ 500 m width of the hanging-wall datasets, indicating that the Alpine Fault does not have a typical “damage zone” defined by decreasing fracture density with distance. Instead, we conclude that the ~ 160 m-wide zone of intensive gouge-filled fractures provides the best estimate for the width of brittle fault-related damage. This estimate is similar to the 60–200 m wide Alpine Fault low-velocity zone detected through fault zone guided waves, indicating that a majority of its brittle damage occurs within its hanging-wall.

Citation: Williams, J. N., Toy, V. G., Massiot, C., McNamara, D. D., Smith, S. A. F., and Mills, S.: Controls on fault zone structure and brittle fracturing in the foliated hanging-wall of the Alpine Fault, Solid Earth Discuss., https://doi.org/10.5194/se-2017-112, in review, 2017.
Jack N. Williams et al.
Jack N. Williams et al.

Data sets

X-ray Computed Tomography and borehole televiewer images of the Alpine Fault's hanging-wall, New Zealand: Deep Fault Drilling Project phase 1 (DFDP-1) and Amethyst Hydro Project (AHP)
J. Williams, V. Toy, C. Massiot, and D. McNamara
https://doi.org/10.5880/ICDP.5052.004

Model code and software

Generating circumferential images of tomographic drill-core scans
S. Mills and J. Williams
https://doi.org/10.5880/ICDP.5052.005
Jack N. Williams et al.

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Short summary
We present new data on fracture orientations, fill and density around the Alpine Fault, a plate boundary fault on the South Island of New Zealand. Fractures < 160 m of the fault are filled and show a range of orientations, whilst fractures at greater distances (< 500 m) are open and parallel to the rock's mechanical anisotropy. We interpret the latter fracture set to reflect near-surface uplift, whilst the latter are potentially linked to deep-seated Alpine Fault seismicity.
We present new data on fracture orientations, fill and density around the Alpine Fault, a plate...
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