The Tasmanian Tyennan Domain–a structural synthesis and review with tectonic and dynamic implications for continental margin subduction and exhumation

Abstract

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.

KEY POINTS

1. The Tyennan Domain represents the subducted part of a microcontinent during a Cambrian arc–microcontinent collision outboard of the eastern Pacific margin of Gondwana.

2. The Tyennan Domain consists of stacked, recumbent and isoclinally folded slabs of allochthonous high-grade schist and low-grade pelite-quartzite, overlying parautochthonous low-grade dolomitic marl, quartzite and dolostone.

3. Isoclinal, asymmetric, macrofold ‘wave trains’, megasheath folds and a ‘leading edge’ fold nappe with ∼200 km length scale dominates the structure of the Tyennan Domain.

Keywords:

• Tyennan Domain
• Tasmania
• structural geology
• isoclinal macrofolds
• megasheath folds
• subduction
• exhumation
• high-P metamorphism
• Cambrian continental margin reconstruction
• orocline

Acknowledgements

This re-investigation in the Tyennan Domain of Tasmania has benefited significantly from:

1. Previous ARC supported research in the Otago Schist of New Zealand (1991–1994) and the subduction–exhumation tectonics of continental margin rocks exposed in the Oman Mountains (1994–2006), in particular an Australian Professorial Fellowship at the University of Melbourne (2002–2006). Discussions over this period with colleagues including Nick Woodward, Robert Gregory, David Foster, Richard Norris, Ben Goscombe, John Miller and Catherine Spaggiari are gratefully acknowledged.

2. The mapping and structural data collection by all previous workers in the Tyennan Domain over a period from the 1950's through to the present. This synthesis would not have been possible without their contribution.

3. The generosity of Ron Berry in providing field notes, transport direction data and sources, field photographs and photomicrographs.

4. The mapping and contributions of Mike Hall over a 60-year period, as well as providing field photographs and field data.

5. Bushwalker photographs of rocks and ridgelines, in particular from Grant Dixon, David Noble and Becca Lunnon (RockMonkeyAdventures).

6. The financial and logistical support from Mineral Resources Tasmania and the Department of State Growth.

7. Support by Parks and Wildlife Service Tasmania with WHA and PWS permits to access and data sample the World Heritage Area, including assistance from Jason Bradbury (Department of Natural Resources and Environment) in obtaining the permits for scientific research.

8. Rick Allmendinger and Nestor Cardozzo for the use of OSX Stereonet.

9. Detailed, thorough and insightful reviews by Ron Berry and Sebastien Meffre that helped significantly improved the manuscript.

10. This paper is published with the permission of the Director of Mines, Mineral Resources Tasmania, Department of State Growth.

11. The Royal Society of Tasmania is thanked for giving permission to reproduce figures 4 and 6b from Maclean and Bowen (1971) and figure 5 from Spry (1963a).

Data availability statement

The authors confirm that the data supporting the findings of this study are available within the article and in the Mineral Resources Tasmania Geological Survey papers of the authors (see References).

Disclosure statement

No potential conflict of interest was reported by the authors.