EVIDENCE FOR EXTENSIONAL TECTONICS IN THE TUMUT TROUGH, LACHLAN FOLD BELT, NSW

Rocks in the Brungle-Darbalara area of the Tumut Trough form two distinct domains: basement (mainly Bullawyarra Schist), of Cambrian-Ordovician age, and an Ordovician - Early Silurian sedimentary and volcanic cover sequence. These two domains are separated by a sharp discontinuity that marks an abrupt change in rock type, structure, metamorphic grade and deformation style. Cover sequences have undergone only one major penetrative deformation during the Late Silurian, involving sub-greenschist facies metamorphism and upright folding. In contrast, the basement also underwent at least two older deformations at greenschist facies and contains distinct high-strain zones subconcordant with the basement-cover contact. The high-strain zones, characterized by a ubiquitous south-southeast trending mineral lineation, record a discontinuous history of ductile followed by brittle behaviour, consistent with an extensional origin. The structural and metamorphic discontinuity separating basement from Silurian cover is characterized by widespread cataclasis and alteration and is interpreted as a major detachment fault associated with lithospheric extension and the development of the Tumut Trough in the Early Silurian. During the main period of movement on the detachment, which took place prior to intrusion of the Blacks Flat Diorite into the Bullawyarra Schist, mafic and serpentinized ultramafic rocks either were tectonically emplaced or intruded into the high strain zones. This preceded and accompanied extensional faulting of the cover and deposition of Silurian trough sediments and volcanics which unconformably overlie and onlap older units. The development of the Tumut Trough, in the Brungle-Darbalara area, bears many similarities with that of Cordilleran metamorphic core complexes. Such a model is consistent with environments suggested for the trough by previous workers. The south-southeast extension direction parallels the trough-bounding faults and implies an overall strike-slip tectonic setting. © 1990, Taylor & Francis Group, LLC.