Ductile-to-brittle transition in shear during thrust sheet emplacement,Southern Appalachian thrust belt |
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| Authors: | RG Gibson DR Gray |
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| Institution: | Department of Geological Sciences, Virginia Polytechnic Institute, Blacksburg, VA 24061, U.S.A.;Department of Earth Sciences, Monash University, Clayton, Victoria 3168, Australia |
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| Abstract: | Mesoscopic structures in anchimetamorphic (T = 200–300°C) strata of the Pulaski thrust sheet, Southern Appalachian thrust belt, developed in progressive, heterogeneous simple shear near the ductile-to-brittle transition. Shear (γ≤3) was localized in weak, anisotropic pelitic rocks (Rome Formation) along the base of this 5–11 km thick thrust sheet. Folds, which vary from upright and open to isoclinal and NW-facing, developed during ductile shearing and display a correlation between tightness and axial-surface dip. Movement along brecciated thrust zones, which evolved progressively from zones of greatest ductile strain, resulted in low-angle truncation of fold axis trends, coaxial refolding of earlier structures, and imbrication of the thrust sheet.Transient variations in fluid pressure (Pf) controlled the mechanical behavior of the thrust sheet. Systematic veins imply Pf >σ3 + T (T = tensile strength) during ductile deformation, whereas later non-systematic vein arrays in high strain zones record periods of nearly hydrostatic stress. Elevated Pf, which led to fracturing, dilation, and fault initiation, appears confined to pelitic zones within the Rome Formation. This, coupled with decreasing temperature, resulted in the transition from ductile folding to brittle thrusting. Changing physical conditions probably reflect erosional unroofing during uplift and late Paleozoic thrust sheet emplacement. |
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