Ductile strain in the footwall of a metamorphic core complex: A field example to test models for dynamics, timescales, and controls of mid-crustal flow.

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As is the case of many mountain belts, the western North American Cordillera underwent horizontal stretching that thinned the Earth?s crust. In the Pacific Northwest thinning began around 50 to 55 million years ago immediately following the end of mountain building, and was highly localized along faults and shear zones that commonly record tens of kilometers of extension. The manner in which the deeper levels of the crust respond to this localized stretching and thinning is presently debated. A currently popular model is that the deep crust is weak enough that it flows from unextended areas into the region of localized thinning due to the lessened gravitational stresses in the areas of thinning. This study will test this model and address the controls that determine whether or not this type of weak gravitationally induced flow occurs. The research conducted during this study will contribute to important societal outcomes by providing support for the training of graduate and undergraduate students in an important STEM (science, technology, engineering, and mathematics) discipline, thus contributing to development of a globally competitive workforce. The project will provide opportunity for undergraduate students to be involved in the research, including the completion of theses based on their research. The project also contains important outreach and educational activities through the University of Florida's "Florida Future Scientists" summer program for high school students, the graduate student will gain mentoring experience by assisting these "future scientists" in the universities analytical laboratories. Thus, the project contributes to increased scientific literacy and public engament with STEM. This project will use new and existing NSF-funded analytical facilities at the University of Florida. Funding for the project will therefore insure that these facilities are actively used for geological research and fully calibrated and operational for future NSF projects, including collaborations outside of the University of Florida. Continuous research and educational use of these instruments is critical for maintaining research infrastructure. Technical results of the research obtained during this study will be widely disseminated through presentations at professional geoscience meetings and the peer-reviewed scientific literature. The highly localized extension that characterizes metamorphic core complexes is thought to lead to lateral flow of the weak lower crust into the extending region in response to topographically induced pressure gradients brought about by localized exhumation. While there is significant support from indirect observations for this isostatically induced flow, direct field examples of this process in core complex footwalls are few. In the examples where this process has been inferred, the geochronology does not typically unambiguously demonstrate synchroneity of strain at the different levels. Furthermore, some of the often cited examples lack evidence for convergent flow, which is predicted from inflow. Numerical models of isostatically induced flow predict specific convergent structures that strongly resemble a small number of field examples. However, it is likely that there are controls in natural settings that are not able to be adequately addressed in numerical models. These differing characteristics likely lead to a spectrum of structures, the full range of which is yet to be identified in the field. In the Pioneer core complex (Idaho), we have identified a well-exposed abrupt vertical transition in lineation directions that may be interpreted as a zone of decoupled isostatically induced flow. Importantly, extensive suites of both deformed and undeformed dikes are exposed above and below the lineation transition. Uranium-lead geochronology applied to these dikes offers a rare opportunity to test whether these fabrics formed synchronously or sequentially. This geochronology integrated with detailed kinematic analysis, metamorphic Pressure-Temperature-time analysis, all tied to detailed mapping offers a unique opportunity to not only test the idea of isostatically induced flow, but to fully characterize the detailed geometries of the resulting structures, as well as the timescales and controls on flow.