Contorted Precambrian rock layers of the Tasmanian Wilderness World Heritage Area - Dr David Gray

Title: Contorted Precambrian rock layers of the Tasmanian Wilderness World Heritage Area— implications and significance

Abstract (Australian Journal of Earth Sciences, 2024, v. 71(2), p. 153): The Tyennan Domain of western Tasmania is a structurally emplaced, complexly deformed lithotectonic sheet (allochthon). It comprises stacked, isoclinally folded slabs of allochthonous high-grade schist including garnet schist and quartzite, amphibolite and rare eclogite (uppermost unit), overlying low-grade pelite, overlying low-grade quartzite and platy quartzite (middle unit), overlying parautochthonous low-grade dolomitic pelite, quartzite and dolostone (lowermost unit). It is a product of Cambrian continental margin subduction, rebound and exhumation coupled with ophiolite obduction. Tyennan rocks were transported to maximum depths of ~60–80 km sufficient to generate high-P metamorphism. Their deformation involved progressive shear, large shear strains and large shear displacements with a bulk west-over-east sense of shear related to the direction of subduction and buoyant exhumation. The Tyennan Domain shows a unique crustal architecture, including: (1) metamorphic sheets with 100 km lengths, 50 km widths and 5–10 km structural thicknesses, (2) a leading edge fold nappe with ~200 km axial surface trace length, (3) megasheath folds with 20–25 km dimensions, and (4) asymmetric folds within regional scale shear lozenges as macro-augen. It provides an unparalleled window beneath the now, largely eroded, obducted ophiolite sheet and provides structural insight into the mechanics of continental margin subduction–obduction. Deformation has been partitioned through the composite exhumed ‘slab’ with domains of isoclinal macrofolds transitioning into regional scale sheath folds bounded by zones of higher strain. The higher strain zones are characterised by transposition foliation, rootless mesoscopic isoclinal folds, small-scale sheath folds, overprinting crenulation cleavages and platy mylonitic to schistose zones in quartzite. The uppermost, high-grade part of the Tyennan allochthon is clearly a composite, ~1 km thick ‘sheet’ made up of a ‘welded’ amalgam of largely pelitic fragments that had been subducted to different depths, assembled and deformed during buoyant ascent to the current structural level.

Bio: David Gray graduated from the University of Newcastle in 1971 where he completed a BSc (Hons) degree including an Honours project along the eastern margin of the Sydney Basin. In 1972 he moved to Macquarie University where he obtained a PhD in 1976 and worked on the origin of crenulation cleavage in southeastern Australia. After a tutorship at the University of New South Wales (1975–1977) Dave then moved to the Virginia Polytechnic Institute & State University (Virginia Tech), Virginia, USA (1977–1983) where he was an Assistant / Associate Professor in Structural Geology. From there he moved to Monash University in 1983 (Lecturer to Senior Lecturer to Reader in Structural Geology). Dave then took up a fixed term ARC Research Professor position at Melbourne University (2002-2006). In 2007 Dave changed direction and became a full-time consultant in structural geology to industry and relocated to Tasmania in 2009. Since his retirement (2014) he has worked as a consultant to Mineral Resources Tasmania assisting with research and training in Structural Geology. Dave’s career research has aimed at examining processes of rock deformation in regions of collisional orogenesis. He has worked extensively around the planet including the Appalachians, Lachlan Orogen, Oman, southwest Africa, and New Zealand. Also, he has maintained a long-term interest in structural geology and tectonics of Tasmania and his work over the last 10 years (retirement occupation) has been resolving the intensely contorted Precambrian rocks of western Tasmania; the subject of tonight’s presentation. He has just jointly published (with M. J. Vicary and A. W. McNeill) a major synthesis on this topic in issue 2 of the Australian Journal of Earth Sciences (2024, available now in print and online) in addition to numerous Mineral Resources Tasmania papers.