Controls on fault zone structure and brittle fracturing in the foliated hanging wall of the alpine fault
Williams, Jack N.
Toy, Virginia G.
McNamara, David D.
Smith, Steven A. F.
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Williams, Jack N. Toy, Virginia G.; Massiot, Cécile; McNamara, David D.; Smith, Steven A. F.; Mills, Steven (2018). Controls on fault zone structure and brittle fracturing in the foliated hanging wall of the alpine fault. Solid Earth 9 (2), 469-489
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 25m of its principal slip zones (PSZs) during the first phase of the Deep Fault Drilling Project that were reoriented with respect to borehole tele-viewer images, (2) field measurements from creek sections up to 500m from the PSZs, and (3) CT images of oriented drill core collected during the Amethyst Hydro Project at distances of similar to 0.7-2 km from the PSZs. Results show that within 160m of the PSZs in foliated cataclasites and ultra-mylonites, 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 160m 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 similar to 500m 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 &quot;outer damage zone&quot; that forms at shallow depths. The distinct &lt; 160m wide interval of widely oriented gouge-filled fractures constitutes an &quot;inner damage zone.&quot; 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.
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