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

作　　者：Siqi Liu Bo Zhang Jinjiang Zhang Jian Zhang Lei Guo Tao Wang Baoyou Hang Xiaorong Li 

机构地区：[1]Key Laboratory of Orogenic Belts and Crustal Evolution,School of Earth and Space Sciences,Peking University,Beijing 100871,China [2]Department of Earth Sciences,The University of Hong Kong,Hong Kong,China [3]Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China

出　　处：《Journal of Earth Science》2024年第3期769-785,共17页地球科学学刊（英文版）

基　　金：funded by the National Key Research and Development Program of China(No.2021YFA0716001);the National Science Foundation of China(No.42272245).

摘　　要：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 myl

关 键 词：amphibolitic mylonite MICROSTRUCTURES petrofabrics seismic property middle crust rheology tectonics. 

分 类 号：P31[天文地球—固体地球物理学]