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

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.