Microstructures, Fabrics, and Seismic Properties of Mylonitic Amphibolites: Implications for Strain Localization in a Thickening Anisotropic Middle Crust of the North China Craton

IF 4.1 2区 地球科学 Q1 GEOSCIENCES, MULTIDISCIPLINARY
Siqi Liu, Bo Zhang, Jinjiang Zhang, Jian Zhang, Lei Guo, Tao Wang, Baoyou Hang, Xiaorong Li
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Abstract

Strain localization processes in the continental crust generate faults and ductile shear zones over a broad range of scales affecting the long-term lithosphere deformation and the mechanical response of faults during the seismic cycle. Seismic anisotropy originated within the continental crust can be applied to deduce the kinematics and structures within orogens and is widely attributed to regionally aligned minerals, e. g., hornblende. However, naturally deformed rocks commonly show various structural layers (e.g., strain localization layers). It is necessary to reveal how both varying amphibole contents and fabrics in the structural layers of strain localization impact seismic property and its interpretations in terms of deformation. We present microstructures, petrofabrics, and calculate seismic properties of deformed amphibolite with the microstructures ranging from mylonite to ultramylonite. The transition from mylonite to ultramylonite is accompanied by a slight decrease of amphibole grain size, a disintegration of amphibole and plagioclase aggregates, and amphibole aspect ratio increase (from 1.68 to 2.23), concomitant with the precipitation of feldspar and/or quartz between amphibole grains. The intensities of amphibole crystallographic preferred orientations (CPOs) show a progressively increasing trend from mylonitic layers to homogeneous ultramylonitic layers, as indicated by the JAm index increasing from 1.9–4.0 for the mylonitic layers and 4.0–4.8 for the transition layer, to 5.1–6.9 for the ultramylonitic layers. The CPO patterns are nearly random for plagioclase and quartz. Polycrystalline amphibole aggregates in the amphibolitic mylonite deform by diffusion, mechanical rotation, and weak dislocation creep, and develop CPOs collectively. The polymineralic matrix (such as quartz and plagioclase) of the mylonite and the ultramylonite deform dominantly by dissolution-precipitation, combined with weak dislocation creep. The mean P and S wave velocities are estimated to be 6.3 and 3.5 km/s, respectively, for three layers of the mylonitic amphibolite. The respective maximum P and S anisotropies are 1.5%–6.4% and 1.8%–4.5% for the mylonite layers of the mylonitic amphibolite, and 6.0%–6.9% and 4.5%–5.0% for the transition layers; but for the ultramylonite layers, these values increase significantly to 8.0%–9.1% and 5.1%–6.0%, respectively. Furthermore, increasing strain (strain localization) generates significant variations in the geometry of the seismic anisotropy. This effect, coupled with the geographical orientations of structures in the Hengshan-Wutai-Fuping complex terrains, can generate substantial variations in the orientation and magnitude of seismic anisotropy for the continental crust as measured by the existing North China Geoscience Transect. Thickened amphibolitic layers by extensively folding or thrusting in the middle crust can explain the strong shear wave splitting and the tectonic boundary parallel fast shear wave polarization beneath the Hengshan-Wutai-Fuping complex terrains. Therefore, signals of seismic anisotropy varying with depth in the deforming continent crust need not deduce depth-varying kinematics or/and tectonic decoupling.

Mylonitic闪长岩的微结构、织构和地震特性:对华北克拉通增厚各向异性中壳应变定位的影响
大陆地壳中的应变局部化过程在广泛的尺度范围内产生断层和韧性剪切带,影响岩石圈的长期变形和地震周期中断层的机械响应。源于大陆地壳的地震各向异性可用于推断岩石圈内的运动学和结构,并被广泛归因于区域排列的矿物,如角闪石。然而,自然变形岩石通常显示出各种结构层(如应变定位层)。有必要揭示应变定位结构层中不同的闪石含量和结构如何影响地震特性及其对变形的解释。我们介绍了变形闪长岩的微观结构、岩石织物,并计算了其从麦饭石到超麦饭石的地震属性。从麦饭石到超基性岩的转变过程中,闪长石晶粒尺寸略有减小,闪长石和斜长石聚集体解体,闪长石长宽比增加(从 1.68 到 2.23),同时在闪长石晶粒之间析出长石和/或石英。闪石晶体学优选取向(CPOs)的强度呈现出从麦饭石层到均匀超麦饭石层逐渐增加的趋势,JAm 指数从麦饭石层的 1.9-4.0 和过渡层的 4.0-4.8 增加到超麦饭石层的 5.1-6.9。斜长石和石英的 CPO 模式几乎是随机的。闪长岩麦饭石中的多晶闪石聚集体通过扩散、机械旋转和弱位错蠕变发生变形,并共同形成 CPO。辉长岩和超辉长岩的多矿物基质(如石英和斜长石)主要通过溶解沉淀和弱位错蠕变发生变形。据估计,三层麦饭石闪长岩的平均 P 波和 S 波速度分别为 6.3 公里/秒和 3.5 公里/秒。麦饭石闪长岩层的最大 P 波和 S 波各向异性分别为 1.5%-6.4% 和 1.8%-4.5% ,过渡层的最大 P 波和 S 波各向异性分别为 6.0%-6.9% 和 4.5%-5.0% ;但对于超基性岩层,这些数值分别显著增加到 8.0%-9.1% 和 5.1%-6.0% 。此外,应变的增加(应变局部化)会使地震各向异性的几何形状发生显著变化。这一效应,加上衡山-五台-阜平复杂地形中构造的地理方位,可使现有华北地学横断面测量到的大陆地壳地震各向异性的方位和幅度产生巨大变化。中地壳广泛的褶皱或推覆作用使闪长岩层增厚,可以解释衡山-五台-阜平复合地形下的强剪切波分裂和构造边界平行快速剪切波极化。因此,变形大陆地壳中随深度变化的地震各向异性信号不一定能推导出随深度变化的运动学或/和构造解耦。
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来源期刊
Journal of Earth Science
Journal of Earth Science 地学-地球科学综合
CiteScore
5.50
自引率
12.10%
发文量
128
审稿时长
4.5 months
期刊介绍: Journal of Earth Science (previously known as Journal of China University of Geosciences), issued bimonthly through China University of Geosciences, covers all branches of geology and related technology in the exploration and utilization of earth resources. Founded in 1990 as the Journal of China University of Geosciences, this publication is expanding its breadth of coverage to an international scope. Coverage includes such topics as geology, petrology, mineralogy, ore deposit geology, tectonics, paleontology, stratigraphy, sedimentology, geochemistry, geophysics and environmental sciences. Articles published in recent issues include Tectonics in the Northwestern West Philippine Basin; Creep Damage Characteristics of Soft Rock under Disturbance Loads; Simplicial Indicator Kriging; Tephra Discovered in High Resolution Peat Sediment and Its Indication to Climatic Event. The journal offers discussion of new theories, methods and discoveries; reports on recent achievements in the geosciences; and timely reviews of selected subjects.
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