Paleomagnetism of the Carboniferous–Permian Myall blocks, Tamworth Belt, southern New England Orogen: Permian counterclockwise rotations and Triassic clockwise rotation

Abstract

This paleomagnetic study on the Myall blocks of the southern New England Orogen complements studies on four other Tamworth Belt blocks aiming to unravel evolution of the Tamworth Belt and the Manning Orocline in particular. Focus is on ignimbritic successions that have partly interlinked stratigraphies across the five blocks and can retain primary magnetisations despite prevalent overprinting. This makes the Tamworth Belt prospective to oroclinal testing by comparing poles and pole paths for individual blocks across its curved structure. Paleomagnetic, rock magnetic and magnetic fabric results are detailed from an upper lower Carboniferous to lower Permian succession, sampled across forearc basin and arc sites (64) of the western Myall Block and forearc basin sites (16) of the eastern Myall Block. Predominantly thermal demagnetisations show a widely present low-temperature component, attributed to late Oligocene weathering, intermediate-temperature primary and overprint components confined to the Nerong Volcanics, and high-temperature primary and overprint components. Overprinting proved more prevalent in the Myall blocks than in other Tamworth Belt blocks, with five phases dated from mid-Carboniferous to mid-Triassic. Comparison of primary and overprint poles against Carboniferous, Permian and Triassic reference paths shows counterclockwise rotations ranging from 20–45° for the western Myall Block forearc basin, to 30° to 90° west to east across the southwestern Myall Block arc complex, to 85–110° for the eastern Myall Block forearc basin, occurring from the latest Carboniferous–early Permian Tablelands phase probably to the mid- to late Permian initial pulse of the Hunter-Bowen phase. The southwestern Myall Block also shows a mid-Triassic, or later, clockwise rotation ranging west to east from 50° to 110°.

KEY POINTS

1. Paleomagnetism of upper lower Carboniferous to lower Permian successions of the Myall blocks constrains evolution of the Manning Orocline.

2. The Myall blocks are heavily affected by overprints, with five phases dated from mid-Carboniferous to mid-Triassic.

3. Primary and overprint magnetisations show various counterclockwise and clockwise rotations of the western, eastern and southwestern parts of the Myall blocks.

4. Counterclockwise rotations occurred from latest Carboniferous–earliest Permian probably to mid- to late Permian and clockwise rotation during or after mid-Triassic.

Keywords:

• Manning Orocline
• Myall blocks
• oroclinal bending
• paleomagnetism
• rotation
• southern New England Orogen
• Tamworth Belt

Acknowledgements

Many thanks to the late John Roberts (UNSW) for generous access to his geological compilation maps, to Rob Barnes (GSNSW) for digital geological maps, to Andrew Cross (GA) for GIS assistance and to Peter Flood (UNE), Albert Brakel (GA), Brian Engel and Noreen Morris (UN) for regional guidance. Many thanks to Peter Percival (GA) for field assistance, to Uros Rokvic (GA), Peter Percival, David Edwards (ANU) and Xiang Zhao (ANU) for laboratory assistance, to John Giddings (GA) for data processing assistance, to Brian Harrold (ANU) for software assistance, to Helmut Michels (Max Planck Institute) for his DISLIN graphics platform, to Phil McFadden (GA) for his fold test program, to Cor Langereis (Utrecht University) for the ANIPO magnetic fabric program, to Rick Allmendinger (Cornell University) for his Stereonet program, to Richard Harrison (University of Cambridge) and Joshua Feinberg (University of Minnesota) for their FORCinel program, and to Brad Pillans, Andrew Roberts and David Heslop for access to the Black Mountain Paleomagnetic Laboratory (RSES/ANU). David Ellis, Richard Arculus, Patrick De Deckker and the late Bear McPhail are thanked for visiting fellowships at the former Geology Department/Earth and Marine Sciences Department (ANU), and Brian Kennett, Andrew Roberts, Ian Jackson, Stephen Eggins and Dorrit Jacob are thanked for visiting fellowships/visitorships at RSES/ANU. Many thanks to Fabio Speranza and an anonymous reviewer for helpful reviews.

Disclosure statement

No potential conflict of interest was reported by the author.

Additional information

Funding

Funding by the Australian National University Faculties Research Grants Scheme F02020 (2002, CK) is gratefully acknowledged.