The Stress Path of Post-Solidus Granite Within the Crystalline Core of the New England Appalachians

新英格兰阿巴拉契亚山脉结晶核心内后固相线花岗岩的应力路径

• 批准号: 0440233
• 负责人: Terry Engelder
• 金额: --
• 依托单位: Pennsylvania State Univ University Park
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-01-01 至 2008-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0440233&HistoricalAwards=false
• 关键词: Stress; Path; Post; Solidus; Granite

This study is developing a new approach for understanding the post-solidus stress-temperature-time paths of granite during exhumation of the core of mountain belts. The new approach is based on the interpretation of a suite of microstructures characteristic of brittle deformation in granite known as rift (direction of easiest splitting) and grain (a second direction of relatively easy splitting). The hypothesis is that post-solidus isobaric cooling and isothermal decompression lead to distinct brittle processes and that these brittle processes reflect different paths during the cooling and exhumation of post-solidus granite. At the solidus of granite, the state of stress is lithostatic where the horizontal and vertical stress components are equal. Isobaric cooling leads to a stress state where the minimum horizontal stress is less than the vertical stress and eventually the minimum horizontal stress is tensile. Tensile stresses act to drive simple mode I microcracks in the vertical orientation normal to the minimum horizontal stress. Isothermal decompression leads to a stress state where the maximum horizontal stress is greater than the vertical stress and all stress components remain compressive. A large horizontal stress can drive axial splitting cracks that are recognized as a complex network of coalescing microcracks. Thus, isobaric cooling and isothermal decompression can be distinguished by microcrack orientation and microcrack structure. Rift developed during isobaric cooling is vertical and a rift developed during isothermal decompression is horizontal. The hypothesis is being tested on a suite of granites collected from various localities in the New England Appalachian Mountains, where systematic patterns of rift cracks have been identified by previous workers. Microscopic measurements on oriented samples include line and area scanlines, orientation measurements, morphologic observations in thin section and with the scnanning electron microscope, and paleostress analysis.

本研究为认识造山带核部折返过程中花岗岩的后固相线应力-温度-时间路径提供了新的途径。这种新方法是基于对花岗岩脆性变形的一系列微观结构特征的解释，这些特征被称为裂缝（最容易分裂的方向）和晶粒（相对容易分裂的第二个方向）。假设固相线后等压冷却和等温减压导致不同的脆性过程，这些脆性过程反映了固相线后花岗岩冷却和折返过程中的不同路径。在花岗岩的固相线处，应力状态为岩石静应力，水平应力分量与垂直应力分量相等。等压冷却导致应力状态，其中最小水平应力小于垂直应力，并且最终最小水平应力是拉伸的。拉伸应力的作用是驱动简单的模式I微裂纹垂直于最小水平应力的垂直方向。等温减压导致应力状态，其中最大水平应力大于垂直应力，并且所有应力分量保持压缩。一个大的水平应力可以驱动轴向分裂裂纹，被认为是一个复杂的网络的聚结微裂纹。因此，等压冷却和等温减压可以通过微裂纹取向和微裂纹结构来区分。等压冷却过程中形成的裂谷是垂直的，等温减压过程中形成的裂谷是水平的。这一假设正在一组从新英格兰阿巴拉契亚山脉不同地方收集的花岗岩上进行测试，以前的工作者已经确定了裂谷裂缝的系统模式。定向样品的显微测量包括线扫描、面扫描、定向测量、薄片和扫描电镜形态观察以及古应力分析。