磁组构与构造变形

磁组构通常指磁化率各向异性,即AMS(Anisotropy of Magnetic Susceptibility),是一种重要的岩石组构,是弱变形沉积岩地区灵敏的应变指示计.近年来,AMS在造山带及前陆地区的广泛应用为构造变形研究提供了极大的帮助,同时提升了该方法的理论认识.本文在研读最新相关文献与著作的基础上,结合笔者及研究团队在龙门山地区获得的磁组构研究成果,综述了磁组构在沉积岩地区构造变形研究中的应用进展,并基于现有的研究认识对关键问题进行讨论,提出以下几点认识:(1)磁性矿物分析是AMS研究的关键,应结合多种岩石磁学实验及光学与电子显微构造研究手段展开详细的磁性矿物学分析;(2)磁化率椭球与应变椭球的对应主轴在绝大多数情况下相互平行,但在不同期次、不同种类复杂的磁性矿物组成,或者多期次构造变形的影响下,AMS与应变的关系相对复杂,应比对高场和低温AMS及非磁滞剩磁各向异性(AARM)测试结果,获得不同矿物的优选定向特征,并对获得的组构进行分期;(3)AMS可以揭示造山带及其前陆地区的构造演化历史,并且是分析断层相关褶皱的有限应变特征和变形机制的重要方法,同时也是厘定断裂带变形性状和期次及运动学分析的有效手段;(4)磁组构形成于成岩作用早期或构造变形的最早阶段,能很好地记录褶皱和逆冲作用之前的平行层缩短变形,因此可以揭示同沉积阶段的古构造应力方向.后期足够强烈的构造变形能局部改造或彻底掩盖先存AMS记录,构造流体有关的同构造期结晶矿物或先存矿物的重结晶导致的再定向被认为是其根本原因;(5)斜交磁线理是一种特殊的磁组构类型,反映了区域构造叠加或多期构造变形作用或隐伏斜向逆冲等可能的构造过程,有必要结合多方面的地质证据对其成因作出合理解释.

Magnetic fabric and structural deformation

Magnetic Fabric refers to Anisotropy of Magnetic Susceptibility (AMS) in general, it is an important petrofabric, and is a sensitive strain indicator in weakly deformed sedimentary region. Recently, AMS has been widely applied to orogenic belts and foreland area studies, providing large assists for structural deformation study and at the same time enhancing the theoretic comprehension of this method. Based on the study of the newest literatures related to AMS, combining with the achievements the author and his research team have gotten from the study of magnetic fabrics in Longmenshan area, this paper overviews research progresses related to the study of using AMS for structural deformation analysis in sedimentary area, meanwhile discusses some key issues based on current understandings and interpretations from previous researches, thus proposes the following ideas:(1) Magnetic mineralogy is an essential precondition in AMS study, detailed magnetic mineral analysis needs the combination of various rock magnetic tests and optical or electron microstructure techniques; (2) The corresponding principle axes of magnetic susceptibility ellipsoid and stain ellipsoid have parallel directions in most cases, but when influenced by complex mineral composition caused by minerals from different tectonic episodes and different kinds, or multi-stage structural deformation, the relation between AMS and strain becomes relatively complex, this requires comparison between the result of AMS with that of High-field and Low-temperature AMS and anisotropy of anhysteretic remanent magnetization test respectively to gain the preferred orientation of different kinds of minerals, and meanwhile stage classification of the obtained fabrics is needed; (3) AMS can be used to reveal the structural evolution history of orogenic belts and their foreland areas, and is an important method for the study of the strain features and deformation mechanism of fault related folds, it is at the same time an efficient tool for the determination of the features and periods of tectonic deformation as well for kinematic analysis in fault zones; (4) The magnetic fabrics occurred during pre-lithification stage or the earliest stage of structural deformation have a very good record of layer parallel shortening which happened before folding and thrusting, thus enable us to reveal the paleostress direction of synsedimentary stage. However, the early AMS record may be locally modified or completely covered by strong structural deformation of later stage, and the reorientation caused by syntectonic crystallized minerals or recrystallization related to tectonic fluid is considered to be the root cause; (5) Oblique magnetic lineation is an abnormal type for magnetic fabrics, it reflects various tectonic processes such as regional tectonic superposition or multi-stage structural deformation or blind oblique thrusting, it is necessary to make an reasonable explanation for its origin using geologic evidences from different aspects.