Australia-Laurasia convergence, Alice Springs Orogeny and tectonic extrusion of the Thomson Orogen

Chris Klootwijk

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

A) Red band shows Carboniferous palaeolatitudes for the New Guinean promontory of the Australian craton according to SNEO results (yellow squares, Klootwijk 2002, 2003, in prep.). Green squares show Devonian-Carboniferous palaeolatitudes for the Kazakhstan Orocline and Tuva terrane of the Central Asian Orogenic Belt. B) Red arrows indicate compression from Australia-Laurasia convergence during the Devonian-Carboniferous. Orange arrows indicate ductile flow of lower crust from the Larapintine Graben into mainly the Thomson and Lachlan Orogens. Major ENE-WSW fault zones guided up to 200 km upper crustal eastward displacement of the Thomson Orogen and the NNEO. The yellow compartments indicate at large the weaker, heated, crust of the Larapintine Graben and the weaker, originally oceanic, crust of the Tasman Orogenic System.

Most of the year has been spent on fine-tuning the concept by expressing it into figures and finalizing literature searches. The hypothesis has been presented at the Australian Earth Science Convention in Perth. Work is now geared towards publication. Current status of the concept summarizes as follows:

Palaeomagnetic results from the ignimbrite-rich Carboniferous succession of the Tamworth Belt, Southern New England Orogen (SNEO), show a northward excursion over more than 30° of latitude with an apex in middle-late Visean (Figure 1A). The excursion is identifiable also in limited palaeomagnetic data from the Australian craton and the Tasman Orogenic System (TOS) and may have started in the Early Devonian. By middle-late Visean, the promontory of the Australian craton in New Guinea, as part of Gondwanaland, reached 30° - 40°N, well within the latitudinal range of the Central Asian Orogenic Belt (CAOB). Devonian-Carboniferous convergence/collision of northeastern Gondwana (Australia) and southern Laurasia (CAOB) is thought the cause of contemporaneous, Variscan, tectonism in the CAOB and in Australia (Alice Springs Orogeny [ASO], Quilpie and Kanimblan Orogenies). Compressional deformation in Australia was largely confined behind the New Guinean promontory, between the Bintuni, Bonaparte and Ord Basins, Halls Creek Fault Zone and the Lasseter Shear Zone in the west and the Aure Trough, Queensland Basin and Bowen-Gunnedah-Sydney Fault Zone in the east.

Convergence-driven N-S compression, hot crust in the Larapintine Graben and a free oceanic boundary, constituted Variscan Australia-Asia conditions that were comparable to the Cenozoic India-Asia indentation/extrusion. Tectonic extrusion of ductile lower crust (and melt?) from the central Larapintine Graben caused eastward displacement of the Thomson Orogen and the Northern New England Orogen (NNEO) along the Diamantina River Lineament-Clarke River Fault Zone in the north and along the Darling River/Cobar-Inglewood Lineaments and Cato Fracture Zone in the south (Figure 1B). The buttress of the NNEO caused telescoping of an unpinned SNEO during Stephanian reversal of Gondwana's rotation.

Different tectonic grains (ASO, Quilpie, Kanimblan, kinkbanding) represent the integrated effects from convergence/collision on the brittle upper crust (direct N-S compression) and on the ductile, partially molten?, lower crust (hydraulic transmission, fanning out from N-S compression toward alignment with an E-W pressure gradient). A single N-S compressional event can thus lead to contemporaneous deformations with widely different tectonic grains, varying from N-S to E-W.

Seismic tomography shows continental-like velocities in the lower crust/upper mantle of the more internal TOS and E-W fanning of SV azimuthal anisotropy in support of the extrusion model. Large-scale negative magnetic anomalies in the Larapintine Graben and the TOS are likely to represent hematite-residing Kiaman reverse remanence in the lower and upper crust and may trace lower crustal flow throughout the TOS.