Australia-Laurasia convergence, Alice Springs Orogeny and tectonic extrusion of the Thomson Orogen
Research School of Earth Sciences, Australian National
University, Canberra, ACT 0200, Australia
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
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.