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

要了解地震破裂和造山的过程，需要对地球对外力和边界条件的反应有一个定量的了解。构造板块由三个主要机械层组成：上地壳、下地壳和上地幔。利用地质和地球物理工具的组合，可以直接研究上地壳。上地幔的成分相对简单，因此根据实验室变形实验预测其力学行为相对容易。因此，就形变而言，构造板块中最鲜为人知的部分是下地壳。约束下地壳流变学是构造分析中的首要问题，最近由美国国家科学基金会赞助的两个独立的研讨会突出了这一点。这一问题是一项正在进行的下地壳变形研究的重点，该研究利用了澳大利亚中部阿伦塔地块的特殊暴露：100平方公里的几乎连续的岩石露头，记录了活动构造区典型的下地壳高温。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变形，导致在折返之前发生麻粒岩相变质作用。量化下地壳变形的一个主要限制是典型的缺乏应变标志物。到目前为止的工作表明，下地壳的应变梯度是通过以下方式识别的：1)保存有初级结构的地区与没有保存原始结构的地区的非均质性长度尺度的比较；以及2)中尺度组构强度的确定，或面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，类似于更广泛认识的中地壳剪切带。这张图结合应变和显微构造分析，被用来确定一种弱矿物或岩石类型是否一致地局部化变形(“弱元素”概念)。主要矿物的变形机制受光和背散射电子岩相学、晶格择优取向的电子背散射衍射分析和透射电子显微镜的制约。Arunta地块暴露出的下地壳深度范围使人们能够对这一“弱元素”概念进行评估，以确定它是否具有较低的地壳形变的一般特征，或仅适用于特定的P-T条件。这笔补助金将在研究的每一年资助一名研究生和一名本科生现场助理。不同方法的集成使学生了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集成。认识到这一特定的研究项目有可能联合结构地质学和构造界一般不定期交流的要素，这项工作将作为2007年或2008年举行的侧重于大陆岩石圈流变学制约因素的讲习班或会议的基础。

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.