Untitled Document
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.