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

Jack N Williams,Cécile Massiot,Steven Mills,David D Mcnamara,Steven A F Smith,Virginia G Toy

doi:10.5194/se-9-469-2018

Abstract

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.

Highlights

Conceptual models of fault zone structure in the upper crust often invoke a relatively narrow “fault core” that accommodates most displacement, surrounded by a halo of heavily fractured rock termed the “damage zone” (Caine et al, 1996; Chester et al, 1993; Chester and Logan, 1986; Faulkner et al, 2010)
In the DFDP-1 computed tomography (CT) images, a total of 637 fractures were rotated into their true geographic orientation where they show a weak cluster about the orientation of the foliation and Alpine Fault principal slip zones (PSZs) at Gaunt Creek (015/43 E, Fig. 5a, Appendix B; Townend et al, 2013)
Fracture orientations and densities in the foliated hanging wall of the Alpine Fault’s central section were quantified in drill core from the DFDP-1, field transects in four creek sections, and drill core recovered from the Amethyst Hydro Project

Summary

Introduction

Conceptual models of fault zone structure in the upper crust often invoke a relatively narrow “fault core” that accommodates most displacement, surrounded by a halo of heavily fractured rock termed the “damage zone” (Caine et al, 1996; Chester et al, 1993; Chester and Logan, 1986; Faulkner et al, 2010). These models have been successfully applied in a variety of tectonic settings and for a wide range of fault displacements and exhumation depths Though damage zones are typically reported to be < 1 km wide (Faulkner et al, 2011; Savage and Brodsky, 2011), co-seismic ground shaking can modify fracture permeability many hundreds of kilometres away from the fault source (Cox et al, 2015; MuirWood and King, 1993; O’Brien et al, 2016)

Methods

Results

Discussion

Conclusion