大陆中部地壳应变局部化与应变弱化

大陆岩石圈流变学研究是构造地质学学科发展的必然,也是发展板块构造理论、探索大陆板块内部变形与动力学演化的核心问题。大陆中部地壳是大陆岩石圈中一个具有特殊性的圈层,其主要成分以花岗质岩石为代表,位于岩石脆-韧性转变域。在中部地壳层次上,岩石既具有脆性变形特点,又具有韧性变形属性,而且常常表现出多种流变强度。研究成果显示,中部地壳岩石流变具有许多特殊性:1)应变局部化是中部地壳流动最为典型表现形式;2)存在大陆地壳多震层:多震与强震,显示出中部地壳既弱又强的流变学属性;3)液/岩反应强烈,流体相直接影响着岩石的流变性;4)在许多地区存在有地球物理异常体(低速高导体)。大陆中部地壳应变局部化是板块相互作用过程中地壳层次上应变积累与集中的重要表现。在宏观尺度、中小型尺度和微观尺度上都有着重要的构造特点。地壳岩石的应变弱化,是诱发应变局部化的主要机制。多种形式的水致弱化(包括液压致裂、反应弱化、水解弱化等)与结构弱化(包括细粒化、晶格取向、成分分带性等)对于应变局部化具有重要的贡献。大陆地壳岩石流变学、中部地壳弱化与应变局部化研究,是未来岩石圈流变学研究的重要方向。

Strain localization and strain weakening in the continental middle crust

Studies on the rheology of the continental lithosphere as the result of recent progress in the structural geology studies, forms the essential issue of developing plate tectonic theory, especially in the study of intraplate deformation and continental dynamics. The middle crust is of particular important in the deformation of the continental lithosphere. The domain is constituted mainly by granitic rocks and characterized by brittle-ductile transitional deformation. Rocks from the domain are deformed by both brittle and ductile deformation with various rheological strengths. It is shown that the middle crustal rocks have the following characteristics: 1) The middle crust flow is characterized by strain localization. 2) There are continental seismogenic zone in which most and strongest earthquakes are located, being indicative of a strong and weak middle crust. 3) Fluid/rock interactions are common, which directly influence the rheological properties of the rocks. 4) Geophysical investigations reveal the existence of low velocity and high conductivity layers in the middle crust. Strain localization is the most important characteristics of strain accumulation and concentration in the continental middle crust. It may occur at macro-, meso- and micro-scales. Strain softening of crustal rocks is the major triggering mechanism of strain localization. Different kinds of fluid-induced weakening mechanisms (e.g. hydraulic fracturing, reaction weakening and hydrolytic weakening etc.) and texture weakening (e.g. grain size reduction, preferred lattice orientation of crystals, banding of compositions) contributed significantly to strain localization. Rheology of the continental crustl rocks, middle crustal weakening and strain localization are key subjects for future studies of lithosphere rheology.