Localization of Deformation in Lithologically Heterogeneous Lower Crust, Arunta Block, Central Australia

了解地震破裂和造山的过程需要定量了解地球如何对施加的力和边界条件作出反应。构造板块由三个主要的机械层组成：上地壳、下地壳和上地幔。利用地质和地球物理工具的结合，可以直接研究上地壳。上地幔的组成相对简单，因此通过实验室变形实验来预测上地幔的力学行为相对容易。因此，就构造板块的变形而言，人们最不了解的部分是下地壳。约束下地壳流变学是构造分析中的一级问题，最近由nsf赞助的两个独立研讨会强调了这一点。这个问题是正在进行的下地壳变形研究的焦点，该研究利用了澳大利亚中部Arunta地块的特殊暴露：100平方公里几乎连续的岩石露头记录了活跃构造区下地壳的典型高温。后者包括各种典型的下地壳岩石类型，在压力和温度条件下在一定深度下变形，导致麻粒岩相变质作用。对下地壳形变进行量化的一个主要限制是缺乏典型的应变标志。迄今为止的工作表明，下地壳的应变梯度是通过以下方法来识别的：1)在保存原生构造的地区与没有原生构造的地区，将非均质性的长度尺度进行比较；2)确定中尺度的织物强度，即叶理和线理的发育程度。这种方法允许划定高应变区。后者记录了下地壳的应变局部化，类似于更广泛认可的中地壳剪切带。这种映射与应变和微观结构分析相结合，用于确定弱矿物或岩石类型是否始终定位变形（“弱元素”概念）。主要矿物的变形机制受到光散射和背散射电子岩相学、电子背散射衍射分析和透射电镜的约束。Arunta地块暴露的下地壳深度范围允许对这种“弱元素”概念进行评估，以确定它是否通常表征下地壳变形，还是仅适用于特定的P-T条件。这项资助每年资助一名研究生和一名本科生实地助理。不同方法的整合使学生能够理解如何使用不同的工具来解决复杂的问题，并参与多学科数据的整合。认识到这一特殊研究项目的潜力，可以将不经常交流的构造地质学和构造学界的元素联合起来，这项工作将作为2007年或2008年举行的一个研讨会或会议的基础，该研讨会或会议的重点是大陆岩石圈流变学的限制。

Understanding the processes of earthquake rupture and mountain building requires a quantitative understanding of how the earth responds to applied forces and boundary conditions. The tectonic plates consist of three primary mechanical layers: upper crust, lower crust, and upper mantle. The upper crust can be directly studied using a combination of geological and geophysical tools. The upper mantle has a relatively simple composition, and thus it is relatively easy to predict its mechanical behavior based on laboratory deformation experiments. The most poorly understood part of a tectonic plate, with respect to deformation, is therefore the lower crust. Constraining lower crustal rheology is a first-order problem in tectonic analysis, highlighted by two separate, recent NSF-sponsored workshops. This problem is the focus of an ongoing study of lower crustal deformation that exploits the exceptional exposures in the Arunta block of central Australia: 100s of square kilometers of nearly continuous outcrop of rocks that record the high temperatures typical of the lower crust in active tectonic areas. The latter include a variety of typical lower crustal rock types, deformed at a range of depths at pressure and temperature conditions that resulted in granulite facies metamorphism prior to exhumation. A major limitation to quantifying deformation in the lower crust is the typical lack of strain markers. Work to date demonstrates that strain gradients in the lower crust are recognized by: 1) comparison of length scales of heterogeneities in areas in which primary structures are preserved with those in which they are absent; and 2) determination of meso-scale fabric intensity, or degree of development of foliation and lineation. This approach is allowng high strain zones to be delineated. The latter record strain localization in the lower crust, similar to more widely recognized shear zones in the middle crust. This mapping, combined with strain and microstructural analyses, are being used to determine whether or not a weak mineral or rock type consistently localizes deformation ('weak element' concept). Deformation mechanisms of major minerals are being constrained by light and back-scattered electron petrography, electron back-scattered diffraction analysis of crystal lattice preferred orientations, and transmission electron microscopy. The range in lower crustal depths exposed in the Arunta block is allowing an evaluation of this 'weak element' concept to determine whether it generally characterizes lower crustal deformation or is applicable only to specific P-T conditions.This grant is supporting a graduate student and an undergraduate field assistant for each year of the study. The integration of different approaches allows thee students to understand how different tools can be used to address a complex problem, and to participate in the integration of multi-disciplinary data. Recognizing the potential for this particular research project to unite elements of the structural geology and tectonics community that do not generally communicate regularly, this work will be used as a foundation for a workshop or conference focused on constraints on the rheology of continental lithosphere to be held in 2007 or 2008.