M. S.

Fold and fault structures in sub-greenschist strata of the most internal, Southern Appalachian foreland thrust sheet developed in progressive simple shear near the ductile-to-brittle transition. Heterogeneous simple shear (γ < 3) was localized in incompetent, anisotropic pelitic rocks (Rome Formation) along the base of a composite, 5-11 km. thick thrust sheet. Folds, which vary from upright and open to isoclinal and strongly overturned, developed during ductile shearing and display a correlation between tightness and axial surface orientation. Deformation culminated by movement along brecciated thrust zones which evolved progressively from zones of greatest ductile strain. This resulted in low angle truncation of fold axis trends and approximately coaxial refolding of earlier structures. PhyIlosiIicate recrystallization during ductile folding and cleavage formation records the thermal maximum (200-300+°C). Syntectonic veins indicate that fluid pressures (P_f) in excess of only the minimum compressive stress prevailed during ductile deformation. Increasing fluid pressures and/or decreasing confining pressures (P_c) eventually led to hydrofracturing and, possibly, fault initiation in pelitic strata. Fault rock morphology is largely lithology dependent; most breccias developed by refolding, cataclasis, and later dedolomitization of previously cleaved, veined, and boudinaged, anisotropic strata. Significant intracrystalline strain indicates conditions near the quasiplastic elastico-frictional transition during fault initiation. Changing physical conditions (decreasing temperature and increasing P_f : P_c) during deformation resulted in the transition from ductile folding to brittle thrusting. These changes may reflect erosional unroofing during uplift and thrust sheet emplacement. / M.S.