Articles | Volume 9, issue 2
Solid Earth, 9, 469–489, 2018
Solid Earth, 9, 469–489, 2018

Research article 23 Apr 2018

Research article | 23 Apr 2018

Controls on fault zone structure and brittle fracturing in the foliated hanging wall of the Alpine Fault

Jack N. Williams1,a, Virginia G. Toy1, Cécile Massiot2,3, David D. McNamara3,4, Steven A. F. Smith1, and Steven Mills5 Jack N. Williams et al.
  • 1Department of Geology, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand
  • 2School of Geography, Environment, and Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6012, New Zealand
  • 3GNS Science, P.O. Box 30-368, Lower Hutt 5040, New Zealand
  • 4Department of Earth and Ocean Sciences, NUI Galway, University Road, Galway, Ireland
  • 5Department of Computer Science, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand
  • anow at: School of Earth and Ocean Sciences, Cardiff University, Cardiff, CF10 3AT, UK

Abstract. Three datasets are used to quantify fracture density, orientation, and fill in the foliated hanging wall of the Alpine Fault: (1) X-ray computed tomography (CT) images of drill core collected within 25 m of its principal slip zones (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 up to 500 m from the PSZs, and (3) CT images of oriented drill core collected during the Amethyst Hydro Project at distances of  ∼  0.7–2 km 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 have formed at relatively high confining pressures and/or in rocks that had a weak mechanical anisotropy. Conversely, at distances greater than 160 m from the PSZs, fractures are typically open and subparallel to the mylonitic or schistose foliation, implying that fracturing occurred at low confining pressures and/or in rocks that were mechanically anisotropic. Fracture density is similar across the  ∼  500 m width of the field transects. By combining our datasets with measurements of permeability and seismic velocity around the Alpine Fault, we further develop the hierarchical model for hanging-wall damage structure that was proposed by Townend et al. (2017). The wider zone of foliation-parallel fractures represents an outer damage zone that forms at shallow depths. The distinct < 160 m wide interval of widely oriented gouge-filled fractures constitutes an inner damage zone. This zone is interpreted to extend towards the base of the seismogenic crust given that its width is comparable to (1) the Alpine Fault low-velocity zone detected by fault zone guided waves and (2) damage zones reported from other exhumed large-displacement faults. In summary, a narrow zone of fracturing at the base of the Alpine Fault's hanging-wall seismogenic crust is anticipated to widen at shallow depths, which is consistent with fault zone flower structure models.

Short summary
We present new data on the orientation of fractures, their fill, and their 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 weakness. We interpret the latter fracture set to reflect near-surface processes, whilst the latter are potentially linked to deep-seated Alpine Fault seismicity.