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 (BHTV) 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. Conversley, 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.

The data provided here include CT scan 'core logs' for drill-core from both boreholes of the first phase of the Deep Fault Drilling Project (DFDP-1A and DFDP-1B) and from the Amethyst Hydro Project (AHP), the code to generate 'unrolled' CT images (which is to be run on imageJ), and an overview image of the integration of unrolled DFDP-1B CT images and BHTV images (DFDP-1B_BHTV-CT-Intergration.pdf). The header for the scan log images indicate 'core run-core section-upper depth-lower depth' for DFDP and 'borehole-core run-core section-upper depth-lower depth' for AHP boreholes. CT scan core logs cover the depth range 67.5-91.1 m in DFDP-1A drill-core and all of DFDP-1B drill-core. A classification of fracture type is given in Williams et al (2016). For DFDP-1 CT scan logs, title of each page labelled by: core run - core section - depth range. For AHP CT scan log, header of each page gives: borehole - core run - core section - depth. These are supplementary material to Williams et al. (submitted), in which a methodology for matching unrolled CT and BHTV images is given in Appendix A.