Polyphase deformation in the western Mount Isa Inlier, Australia: episodic or continuous deformation?

The polyphase deformation history of the Mount Novit Ranges, western Mount Isa Inlier, Australia, involved the development of four generations of N—S to NW-plunging folds, development of which alternated with movement on ∼N—S striking faults/shear zones with west-over-east displacement. The spatial relationships between successive structures and the distinct similarities in the kinematics, style, and asymmetry, are all consistent with continuous deformation during E—W to NE—SW shortening. Additional support for this hypothesis is based on the development of the three major fold generations, and the intervening faults and shear zones, during low-P, high-T metamorphism. Although the duration of this metamorphic event cannot be constrained, recent studies have linked low-P metamorphism to heat advection by migrating melts. Thermal modelling indicates that such metamorphic events will be short-lived in comparison to those associated with thermal relaxation of over-thickened crust. Low-P metamorphism is unlikely to persist through two or more separate tectonic events; therefore syn-metamorphic, polyphase deformation in the Mount Novit Ranges apparently formed during a single, continuous tectonic episode. tructural history differs from both the regional scheme and that of previous workers in this area. The main differences are: (i) no evidence was found for early thrust faulting prior to N—S folding; (ii) there are four generations of N—S to NW—SE folds, as opposed to two; and (iii) this study favors a single, consistent tectonic regime, in contrast to the interpretation of three discrete, isolated tectonic events.