Abstract
P-wave anisotropy is significant in the mylonitic Alpine Fault shear zone. Mineral- and texture-induced anisotropy are dominant in these rocks but further complicated by the presence of fractures. Electron back-scattered diffraction and synchrotron X-ray microtomography (micro-CT) data are acquired on exhumed schist, protomylonite, mylonite, and ultramylonite samples to quantify mineral phases, crystal preferred orientations, microfractures, and porosity. The samples are composed of quartz, plagioclase, mica and accessory garnet, and contain 3–5% porosity. Based on the micro-CT data, the representative pore shape has an aspect ratio of 5:2:1. Two numerical models are compared to calculate the velocity of fractured rocks: a 2D wave propagation model, and a differential effective medium model (3D). The results from both models have comparable pore-free fast and slow velocities of 6.5 and 5.5 km/s, respectively. Introducing 5% fractures with 5:2:1 aspect ratio, oriented with the longest axes parallel to foliation decreases these velocities to 6.3 and 5.0 km/s, respectively. Adding both randomly oriented and foliation-parallel fractures hinders the anisotropy increase with fracture volume. The anisotropy becomes independent of porosity when 80% of fractures are randomly oriented. Modeled anisotropy in 2D and 3D are different for similar fracture aspect ratios, being 30 and 15%, respectively. This discrepancy is the result of the underlying assumptions and limitations. Our numerical results explain the effects that fracture orientations and shapes have on previously published field- and laboratory-based studies. Through this numerical study, we show how mica-dominated, pore-free P-wave anisotropy compares to that of fracture volume, shape and orientation for protolith and shear zone rocks of the Alpine Fault.
Highlights
The Alpine Fault is located along the West Coast of the South Island, New Zealand
We study the effect of fractures on P-wave velocity and anisotropy in two ways: (1) fractures are aligned to foliation and porosity volume is varied and (2) the porosity volume is constant, but the contribution of the fracture orientation is changed for different combinations of aligned and randomly oriented fractures
Electron Back-Scattered Diffraction (EBSD) data show that the shear-zone Alpine Fault rocks are composed of quartz, plagioclase, garnet and a considerable amount of non-indexed phase (Figure 3, Table 2), assumed to be biotite mica as the phyllosilicate minerals in the Alpine Fault mylonites are predominantly biotite (Toy et al, 2015)
Summary
The Alpine Fault is located along the West Coast of the South Island, New Zealand. It marks the transpressional plate boundary between the Australian and the Pacific Plates (Figure 1). The. Fracture Influence on P-Waves majority of the rocks on the Pacific Plate (hanging wall) are schist and fault rocks. Rocks sheared during the collision of the two plates form a series of fault rocks close to the principal slip zone consisting of fault gouge, cataclasite, and variable grades of mylonites which vary according to fault-perpendicular distance. The mylonites are derived from the Alpine Schist and developed foliation parallel and sub-parallel to the fault plane (Toy et al, 2008). The mylonites and schist are composed of quartz, plagioclase, biotite, muscovite, and accessory minerals such as chlorite, garnet, and calcite (Grapes and Watanabe, 1994; Little et al, 2002; Toy et al, 2015; Boulton et al, 2017)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
