Regional scale folding in the Arthur Metamorphic Complex: structural constraints for the Keith River – Lyons River area, NW Tasmania

Cumming, G1, Jackman C.1, Everard, J. L.1 and Gray, D.2

 1 Geological Survey Branch – Mineral Resources Tasmania – Geological Survey Branch, Rosny Park, Australia, 2 Consultant Structural Geologist for Mineral Resources Tasmania – Geological Survey Branch, Rosny Park, Australia

New geological mapping in the Keith River-Lyons River area in NW Tasmania has provided insight into the structural framework of the northern Arthur Metamorphic Complex (AMC) in NW Tasmania. The AMC is flanked to the east by the Oonah Formation and to the west by the Rocky Cape Group. The main lithological units of the AMC share transitional metamorphic, interpreted low angle fault, and both conformable and unconformable contacts. At a regional scale a significant north-plunging synform, or a large north-tilted block, is contained within the high-strain core of the AMC. Five deformation episodes can be observed throughout the area at outcrop-scale. Three early deformational episodes are likely related to the Middle Cambrian Tyennan orogeny, manifested as early, high strain events which caused isoclinal folding and development of schistose axial planar fabric. A rotational shear component, apparent as shear bands, suggests north over south or sinistral transport. Subsequent D3 deformation within the AMC occurred during the later stage of the Tyennan Orogeny. This event folded and tightened the various stacked lithostratigraphic units to form non-cylindrical asymmetric folds. These were later subject to generally northwest-directed, potentially Devonian compression (D4) and tilting. At a regional scale, late-stage north-plunging folds are inferred along the highest strain zone of the northern AMC. This area is a locus for Mesozoic or early Cainozoic faults, and a half-graben also extends along this zone, which is also in the core of the AMC. A late stage (D5) folding event may be partly related to Devonian compression, although the timing and nature of this folding event is largely unclear.


Grace currently works as a geologist for the Geological Survey Branch at Mineral Resources Tasmania and has spent the last 8 years undertaking mapping work to compile numerous 1:25,000 geological maps of North West Tasmania. 

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