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

澳大利亚中部阿伦塔地块岩性非均质下地壳变形局部化

• 批准号: 0440156
• 负责人: Laurel Goodwin
• 金额: --
• 依托单位: University of Wisconsin-Madison
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-01-01 至 2009-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0440156&HistoricalAwards=false
• 关键词: Localization; Deformation; Lithologically; Heterogeneous; Lower

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.

要了解地震破裂和造山运动的过程，就需要定量地了解地球对外力和边界条件的反应。 构造板块由三个主要的力学层组成：上地壳、下地壳和上地幔。 利用地质和地球物理工具相结合，可以直接研究上地壳。 上地幔具有相对简单的组成，因此，它是相对容易预测其力学行为的基础上，实验室变形实验。 因此，对于构造板块的变形，人们最不了解的部分就是下地壳。 约束下地壳流变学是构造分析中的一阶问题，最近由NSF主办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地块的异常暴露：100平方公里的几乎连续的岩石露头，记录了活跃构造区下地壳典型的高温。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范围内变形，导致剥露前发生麻粒岩相变质作用。定量下地壳变形的一个主要限制是缺乏应变标志。迄今为止的工作表明，下地壳的应变梯度是通过以下方式来识别的：1）比较原始构造保存区与不存在原始构造区的非均匀性长度尺度; 2）确定中尺度组构强度，或叶理和线理的发育程度。这一方法使得高应变区得以划定。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。这一绘图与应变和显微结构分析相结合，用于确定弱矿物或岩石类型是否始终局限于变形（“弱元素”概念）。主要矿物的变形机制受到光和背散射电子岩相学，电子背散射衍射分析的晶格择优取向，和透射电子显微镜。在阿龙塔地块暴露的地壳下部深度范围允许评估这种“弱元素”的概念，以确定它是否通常表征地壳下部变形或仅适用于特定的P-T条件。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。认识到这一特定研究项目有可能将结构地质学和构造学界通常不定期交流的元素联合起来，这项工作将用作将于2007年或2008年举行的重点讨论大陆岩石圈流变学限制的研讨会或会议的基础。