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

本研究为了解山带岩心挖掘过程中花岗岩固后应力-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套微观结构特征的解释，即裂谷（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后花岗岩冷却和采掘过程中的不同路径。在花岗岩的固相处，水平和垂直应力分量相等，应力状态为静岩状态。等压冷却导致最小水平应力小于垂直应力的应力状态，最终最小水平应力为拉应力。拉应力作用于驱动垂直方向上与最小水平应力垂直的简单I型微裂纹。等温减压导致最大水平应力大于垂直应力，所有应力分量保持压缩的应力状态。一个大的水平应力可以驱动轴向劈裂裂纹，这被认为是一个复杂的微裂纹网络。因此，可以通过微裂纹取向和微裂纹结构来区分等压冷却和等温减压。等温冷却时发育的裂谷为垂直裂谷，等温减压时发育的裂谷为水平裂谷。这一假设正在新英格兰阿巴拉契亚山脉不同地区收集的一组花岗岩上进行测试，在那里，以前的工作人员已经确定了裂谷裂缝的系统模式。取向试样的显微测量包括线和面扫描线、取向测量、薄片和扫描电镜形貌观察以及古应力分析。

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.