| Abstract: | Five deformational “episodes” (D1-D5) during which folds and associated cleavages formed, and a later period of faulting dominated by strike-slip movements, comprise the structural sequence in the Nambucca slate belt. D1 structures are most widespread and involved greatest strain; D2-D4 structures have affected progressively smaller areas and indicate progressively smaller strains. Strong compressive stresses during D1 produced horizontal shortening and vertical extension, and the D2-D4 structures result from adjustments to this initial strain. Regional metamorphism accompanied D1, and D1 strain is greatest in the more highly metamorphosed rocks. Some granitic bodies were probably emplaced at this time, but most plutons rose only after folding had ceased. Orogenesis, as indicated by folding and regional metamorphism lasted less than 10 m.y., but faulting continued for at least another 30 m.y.The slate belt accumulated close to a consuming plate margin, but deformation commenced only after subduction ceased, with compressive stresses generated by coupling across the former plate boundary. The development of a wrench regime during D5, and its continued existence during a long period of faulting, suggests either that the consuming plate boundary was replaced by a transform fault, or that subduction stepped oceanward and underthrusting was obliquely directed. |
