Mechanisms and Efficiency of Ductile Strain Localization Below Major Continental Strike-slip Faults: Numerical Experiments Incorporating Laboratory-derived Rheologies

本项目将研究大陆地壳主要走滑断层造成的长期变形和应变演化。特别是，该项目将使用数值模型来评估各种应变软化机制的效率，如热-机械耦合、晶粒尺寸减小和糜棱岩结构，并评估这些机制单独或联合促进或抑制应变局部化的程度，以响应长期断层滑动。这将使用有限元模型来完成，该模型将结合现实的地热、远场加载率和加载历史、深度相关成分以及从实验室实验推断的本构关系。模拟将研究在初始无应变韧性基底中永久剪切区可能发展的条件。本文将对具有韧性的下地壳和上地幔中偏应力的大小和分布进行评估，并对大陆岩石圈的长期强度作为温度状态、成分、变形速率、总位移和其他相关因素的函数进行推断。将用于模型预测的观测数据包括：暴露的中下地壳剪切带的粒度分布、岩石学和微观结构数据的偏应力推断、活动断裂带下的地震结构和各向异性，以及主要走滑断层引起的瞬态和长期变形的大地测量观测。本文将利用各自的数据和模型来验证以下假设：韧性基底中的分布粘弹性流动和局部剪切代表了不同成熟程度断裂带的端部行为，局部剪切普遍存在于高滑移率、高总偏移量（如板块边界）的断层上，而未成熟断层以扩散变形为主。应变在主要断裂以下的岩石圈韧性部分的局部化程度是大陆构造学中一个尚未解决的主要问题。人们提出了两类模型：一类假设下地壳和上地幔中存在广泛分布的粘性变形（“薄岩石圈”模型），另一类假设脆-韧性转变之下存在局部剪切延伸（“厚岩石圈”模型）。了解岩石圈剪切带的机制对于大陆构造学中的许多问题都是至关重要的，包括地壳和上地幔的长期强度，相对板块运动到发震断层的应力转移，以及最终的地震危险。地质和地球物理证据都支持“薄”和“厚”岩石圈模型，可能表明不同位置、构造环境、变形速率和总位移之间的变形风格差异。如果存在这种变异性，那么建立观测到的变形样式的主要控制因素和控制机制是有意义的。由实验确定的岩石延展性特性提供的长期变形的现实模型将对长期存在的争论产生影响，例如大陆内部的块状变形与扩散变形，大陆岩石圈的有效强度，以及大地震后瞬态变形的机制。

This project will investigate the long-term deformation and strain evolution due to major strike-slip faults in the continental crust. In particular, the project will use numerical models to evaluate the efficiency of various strain-softening mechanisms, such as thermo-mechanical coupling, grain-size reduction, and mylonitic fabric, and assess the degree to which these promote or inhibit strain localization, individually and in combination, in response to long-term fault slip. This will be accomplished using finite element models that will incorporate realistic geotherms, far-field loading rates and loading histories, depth-dependent compositions, and constitutive relationships inferred from laboratory experiments. The simulations will investigate conditions under which permanent shear zones may develop in an initially unstrained ductile substrate. The magnitude and distribution of deviatoric stresses in the ductile lower crust and upper mantle will be evaluated, and inferences made about the long-term strength of continental lithosphere as a function of temperature regime, composition, deformation rate, total displacement, and other relevant factors. Observables that will be brought to bear on the model predictions include grain size distributions from the exposed mid-to-lower crustal shear zones, inferences of deviatoric stress from petrologic and micro-structural data, seismic structure and anisotropy below active fault zones, and geodetic observations of transient and secular deformation due to major strike-slip faults. The respective data and models will be used to test the hypothesis that the distributed viscoelastic flow and localized shear in the ductile substrate represent end member behavior of fault zones with different degree of maturity, with localized shear prevalent on high-slip rate, high total offset (e.g. plate boundary) faults, and diffuse deformation dominating for immature faults.The degree to which strain is localized in the ductile part of the lithosphere below major faults is a major unresolved question in continental tectonics. Two classes of models have been proposed: one postulating a broadly distributed viscous deformation in the lower crust and upper mantle (the "thin lithosphere" model), and another one postulating extension of localized shear well below the brittle-ductile transition (the "thick lithosphere" model). Understanding the mechanics of lithospheric shear zones is essential for a number of problems in continental tectonics, including the long-term strength of the Earth's crust and upper mantle, stress transfer from the relative plate motion to seismogenic faults, and, ultimately, seismic hazards. Geological and geophysical evidence has been presented in support of both the "thin" and "thick" lithosphere models, possibly indicating differences in deformation styles between various locations, tectonic settings, deformation rates, and total displacements. If such variability exists, it is of interest to establish the main controlling factors and governing mechanisms on the observed deformation styles. Realistic models of long-term deformation informed by the experimentally determined ductile properties of rocks will bear on the long-standing debates such as the block-like versus diffuse deformation in the continental interiors, the effective strength of the continental lithosphere, and the mechanisms of transient deformation following large earthquakes.