Paleomagnetic constraints on formation of the Manning Orocline, far-field effects of Pangea-B to -A transformation and breakup?

Dr Chris Klootwijk1

1Research School of Earth Sciences, ANU, , Australia

The Myall blocks of the southeastern Tamworth Belt (TB), southern New England Orogen (SNEO), have been studied for paleomagnetic control on evolution of the Manning Orocline, focussing on ignimbritic rocks which best retain primary magnetisations despite pervasive overprinting. Paleomagnetic, rockmagnetic and magnetic fabric results from upper lower Carboniferous to lower Permian successions cover 64 forearc basin, arc-fringe and arc sites from across the western Myall block and 16 forearc basin sites from the westernmost eastern Myall block. Thermal demagnetisations show a widely present low-temperature weathering overprint, intermediate-temperature primary and overprint components confined to the Nerong Volcanics (~340 Ma), and high-temperature primary and overprint components. Overprinting is far more prevalent in the Myall blocks than in other TB blocks studied, with five phases dated tentatively from mid Carboniferous to mid Triassic.

Myall blocks results are interpreted against the Carboniferous SNEO polepath based on Rocky Creek, Werrie, Rouchel and Gresford blocks data, against a Permian polepath for the Australian craton, and against a Triassic Indian polepath transferred to Australia within Gondwana. Comparison of the Myall blocks primary and overprint poles against these reference polepath segments shows counterclockwise rotations that increase from 20°- 45° for the main part of the western Myall block and from 30°-90° for its southwestern margin, to 80°-110° for the eastern Myall block. Rotations likely started with the Tablelands phase (~305 Ma, L2 loop on SNEO polepath) and ended during the late Permian (~270-260 Ma) initial pulse of the Hunter-Bowen phase of the Gondwanide Orogeny. The southwestern margin of the western Myall block also shows a mid Triassic, or later, clockwise rotation, increasing from ~50° near the Williams River Fault to ~110° for the Port Stephens arc complex.

Deformation of the SNEO is attributed customarily to changes in Paleopacific subduction, less so to changes in global plate movements as proposed herein. Comparison of the SNEO Carboniferous polepath with a Carboniferous polepath for the European Variscan massifs (Edel 2003; Edel et al. 2018) shows comparable mid to late Carboniferous (~330-305 Ma) segments, spanning large arcs (~130° between poles defining the L2 and L3 loops on the SNEO polepath; ~105° between the Cp and A1 poles on the Variscan polepath). Euler pole matching of these polepath segments, representing antipodal fringes of Gondwana, relocates “Armorica” as a northern spur of Gondwana in a Pangea-B configuration.

This novel, significant, finding opens avenues for interpretation of Australian mid Carboniferous to mid Triassic tectonics as primarily driven by the African, rather than Pacific, LLSVP-centred mantle upwelling, acknowledges the “Hercynian unconformity” as a late Carboniferous Gondwana-wide uplift reflecting heat accumulated beneath Pangea from the African upwelling, identifies the L3 loop (~305 Ma) on the SNEO polepath as the start of the Pangea-B to -A transformation displacing Gondwana westward along an equatorial shear, interprets the early to mid Permian Pangea-wide extension – including eastern Australia – as a reflection of that transformation by dispersing accumulated heat, and argues for development of the Manning Orocline as driven primarily by changes in movement of Gondwana rather than the Paleopacific.


Biography

Chris Klootwijk studied geology, geophysics and Indian paleomagnetism at Utrecht University, carried out further paleomagnetic studies on the India–Asia collision at ANU, IPGP and the University of Paris, and on Australia/Gondwana at BMR/AGSO. For the past two decades he has been offered refuge as a visitor at ANU.

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