Data_Sheet_1_Fracture Shape and Orientation Contributions to P-Wave Velocity and Anisotropy of Alpine Fault Mylonites.PDF

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.

P波各向异性在阿尔卑斯断层糜棱岩化剪切带中具有重要意义。矿物和纹理诱导的各向异性在这些岩石中占据主导地位，但其存在亦因裂缝的出现而变得更加复杂。通过对出露的片麻岩、原糜棱岩、糜棱岩和超糜棱岩样品进行电子背散射衍射和同步辐射X射线微断层扫描（微CT）数据的采集，以量化矿物相、晶体优选取向、微裂缝和孔隙率。样品由石英、斜长石、云母和副矿物石榴石组成，孔隙率为3-5%。基于微CT数据，典型孔隙的纵横比约为5:2:1。为计算裂缝岩石的速度，比较了两种数值模型：二维波传播模型和微分有效介质模型（三维）。两种模型的结果均显示出相似的孔隙无裂缝快速和慢速速度，分别为6.5和5.5公里/秒。引入纵横比为5:2:1、最长轴与片理平行的5%裂缝后，这些速度分别降至6.3和5.0公里/秒。同时加入随机取向和片理平行裂缝，阻碍了裂缝体积增加所导致的各向异性提升。当80%的裂缝随机取向时，各向异性不再随孔隙率变化。模拟的二维和三维各向异性在相似裂缝纵横比下存在差异，分别为30%和15%，这种差异源于基本假设和局限性的影响。我们的数值结果解释了裂缝取向和形状对先前发表的基于现场和实验室研究的场和实验室研究的影响。通过这一数值研究，我们展示了以云母为主导的孔隙无裂缝P波各向异性与阿尔卑斯断层原岩和剪切带岩石的裂缝体积、形状和取向之间的关系。