The stratigraphy and deformation history of the Tumut region: Implications for the development of the Lachlan Fold Belt

The Tumut region forms the southern part of the Tumut Synclinorial Zone in the southeastern part of the Lachlan Fold Belt (LFB). The area contains two major stratigraphie packages: Ordovician to Early Silurian quartz-rich to quartz-intermediate flysch and volcanics (package A), and overlying fossiliferous Early/Late Silurian volcanics and flysch (package B). Rhyolite within the older package (A) yields a U-Pb zircon age of 428 ± 6 Ma. Both packages were meridionally folded during the Siluro-Devonian Bowning Orogeny following intrusion of the Gocup Granite at 411 ± 5 Ma. An earlier deformation, characterised by thrust faulting, E-W recumbent folding and later local coaxial upright folding, is present only in the older flysch and volcanics (package A). This earlier deformation is part of the Benambran Orogeny that affects the Ordovician metamorphics of the Wagga Metamorphic Belt and other parts of the LFB, and is constrained to about 425 Ma. Fold characteristics of this deformation are indicative of thin-skinned intraplate transpressional deformation rather than classical collisional tectonics as envisaged by some workers for the Benambran Orogeny here and elsewhere in the LFB. The composition and deformational history of the older flysch indicates that it is part of the Ordovician to Early Silurian Molong Volcanic Belt. Consequently, the older concept of an Early Silurian Tumut Trough incorporating both packages of rocks is rejected. Instead, it is proposed that up to 2500 m of Early/Late Silurian rocks formed in a pull-apart basin (the Tumut Basin). Thus the Silurian depositional history of the Tumut region, previously considered unique in the LFB, is little different from other basins of similar age throughout the LFB.