Alice Springs Orogeny

Australia: The Land Where Time Began

Alice Springs Orogeny

The Alice Springs Orogeny had its beginnings in the Late Ordovician, continuing during the Silurian and Devonian, and by the Carboniferous the folding of the sedimentary deposits of the central Australian basins had produced the mountainous terrain of the MacDonnell Ranges area. These ranges being the eroded remnants of the once high mountains produced by this folding between about 450 and 300 million years ago.

According to Chris Klootwijk, a northward excursion in the Early Carboniferous, of about 30^o, ending in the middle-late Visean, is indicated by palaeomagnetic results from the ignimbrite-rich Carboniferous succession of the Tamworth Belt, Southern New England Orogen (SNEO) in eastern Australia. The Australian Craton and the Tasman Orogenic System have also produced evidence, in the form of palaeomagnetic data, of the excursion that could possibly have begun in the Early Devonian. The promontory of the Australian Craton in central New Guinea had reached as far as 30-40^o N by the middle-late Visean, putting it well inside the range of the Central Asian Orogen Belt (CAOB) of what was then southern Laurasia. Australia was on the northeastern margin of Gondwana when that part of the supercontinent converged with, and then collided with the Central Asian Orogenic Belt in the Devonian-Carboniferous. This is proposed by Klootwijk to be the cause of variscan tectonism along the northern and southern margins of the Palaeopacific Ocean, as well as in the hinterland, that was contemporaneous with the convergence and collision, within the Central Asian Orogenic Belt and in Australia, the Alice Springs, Kanimblan and Quilpie Orogenies.

A "compression box", an area to which compressional deformation associated with convergence was essentially confined to in Australia, extending the the central New Guinean cratonic promontory, was bounded in the west by the Lasseter Shear Zone, and in the east by the future East Australian Rift System (Fig. 1A, source 1). The conditions of Variscan Australia-Asia were comparable to those of the Indian-Asian of the Cainozoic indentation and extrusion process, included north-south compression that was driven by convergence, in the Larapintine graben weak heated crust, and in the Tasman Orogenic System (TOS), oceanic basement (Fig. 1B, source 1), and the "free" oceanic boundary of the Palaeopacific (Royden et al., 2008; Burchfield et al., 2008).

According to Klootwijk (2000), the Thompson Orogen (mainly) and the Northern New England Orogen (NNEO) were displaced to the east by the tectonic extrusion of ductile lower crust and partial melt from the Larapintine graben that is believed to have possibly been associated with the slab rollback of subduction in the Palaeopacific. The Diamantina River Lineament-Clarke River Fault Zone guided upper crustal displacement in the north, and in the south it was the Darling River/Cobar-Inglewood Lineaments and the Caro Fracture Zone, as well as the Lake Blanche-Olepoloko Fault Zones and Lachlan Transverse Zone (Fig. 1B, source 1).

Klootwijk claims the tectonic extrusion hypothesis allows new ways of interpreting some aspects of the geological evolution of the Australian continent of the Late Palaeozoic.

'(i) Different tectonic grains of the Alice Springs Orogeny (E-W) and the Quilpie and Kanimblan Orogens (N-S) represent diverse effects of the Australia-Asia convergence/collision on the brittle upper crust and the ductile lower crust. Direct north-south compression through the upper crust led to the Alice Springs Orogeny of east-west tectonic grain, whereas the ductile lower crust hydraulic fanning out of north-south compression toward alignment with the east-west pressure gradient near the free boundary of the Paleopacific, led to the Quilpie and Kanimblan Orogenies of north-south tectonic grain;
(ii) East-west fanning out of the azimuthal anisotropy (Figure 1E) and the continental-like velocities in the lower crust/upper mantle of more internal parts of the TOS (figure 1C, 1D), indicated by surface wave tomography, traces lower crustal tectonic extrusion from the Larapintine Graben into the trough of the TOS.
(iii) Prominent negative magnetic anomalies in the Larapintine graben and the TOS (Figure 1H) may represent hematite-residing Kiaman reverse polarity remanence (Figure 1F) in lower and upper crust and outline lower crustal flow throughout the TOS;
(iv) Extent of lower crustal flow may be outlined further by mapping the extent of seismically highly reflective lower crust, assuming that its planar anisotropy represents horizontal ductile flow rather than vertical magmatic underplating;
(v) The widespread Namurian sedimentary lacune may represent thermal expansion resulting from lower crustal ductile flow, with denudation products transported from the elevated "compression box" into the non-elevated New England Orogen (NEO), and Western Australian basins;
(vi) Formation of the Kanimblan Highlands during the Late Carboniferous may reflect this thermal expansion process, with latest Carboniferous-Permian thermal relaxation leading to its demise;
(vii) Late Carboniferous heat flux may explain the fission track and low temperature isotopic disturbance record without the need for extensive burial and denudation;
(viii) A Stephanian change to clockwise rotation of Gondwana caused northward telescoping of the SNEO against the eastward displaced buttress of the NNEO, facilitated by structural detachment of the SNEO from the Lachlan Orogen along the northward progressing Eastern Australian Rift System, leading to the formation of the Texas-Coffs Harbour and Manning Oroclines.' (Klootwijk, 2000).