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 similar to 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. This structural 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.