Flow Properties of Moine Thrust Zone Mylonites in Northern Assynt, NW Scotland
Roth, Benjamin Louis
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Quartz-rich mylonites present along the Moine Thrust Zone are well suited for the application of various analytical techniques designed for investigating the flow processes by which rock deformation occurred. These analytical techniques were applied to a suite of samples from the footwall and hangingwall of the Moine thrust exposed along the Allt Pol aâ Mhadaich stream located in the northern part of the Assynt window. Vorticity analyses were performed to determine the relative contributions of pure and simple shear deformation within the penetratively deforming thrust sheets. Integration of vorticity data with 3D strain analyses demonstrated that sub-vertical shortening perpendicular to the flow plane, accompanied by thrust transport parallel extension, occurred during mylonitization, and was driven by emplacement of the overlying Moine nappe. Quartz c-axis fabrics in the mylonites are characterized by well-defined asymmetric Type-1 cross girdles in which internal and external skeletal asymmetries are indicative of a top-to-the-WNW shear sense, compatible with regional thrusting. These c-axis fabrics were also used to estimate deformation temperatures. Differential flow stresses associated with mylonitization were estimated from the grain size of dynamically recrystallized quartz. Deformation temperature and flow stress data were then incorporated into a dislocation creep flow law for quartz to estimate strain rates. Finally, along strike variation in these flow properties at the base of the Moine nappe to the north and south of the APM section were investigated and results from the APM section compared with previously published studies of mylonites exposed in eastern Assynt that occupy similar structural positions.
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