Australia: The Land Where Time Began
Lachlan Fold Belt (Lachlan
A major part of the southeastern section of the Murray-Darling Basin (MDB) is comprised of the Lachlan Fold Belt (LFB), that stretches from central and southeastern New South Wales to central and eastern Victoria. It has been described as 'a composite orogenic belt composed of Early Cambrian to Early Devonian pre-cratonic complexes developed over mixed ensialic (microcontinents) and ensiamatic (oceanic or ophiolitic) basement' (Scheibner & Basden, 1996).
3 Major lithospheric assemblages make up the Lachlan Fold Belt (Coney, 1992). Early-Middle Cambrian greenstone belts, turbidite fan deposits from the Ordovician to Silurian, and from the Early-Middle Silurian to Late Devonian-Early Carboniferous, complex sedimentary, volcanic and plutonic associations.
Turbidite fan deposits, of quartzite-rich sands and muds extending across the entire Lachlan Fold Belt, that appear to be of Gondwanan origin (Coney, 1992), are of Silurian-Devonian age.
A number of stages of deformation have taken place in the Lachlan Fold Belt, most of it being affected by the Benambran Orogeny of the Late Ordovician to Early Silurian, causing the inversion of a back arc basin that resulted in the Wagga-Ormeo Zone, a collision thrust belt (Scheibner & Basden, 1996). The localised Bowning-Bindi deformation, that occurred in the Late Silurian to Early Devonian, followed on from the extension of the eastern Lachlan Fold belt of the Early to Middle Silurian.
In the Silurian and Devonian, extensive orogenic granites were emplaced (Chappell et al., 1988), representing a major transition of the Lachlan Fold Belt towards a mature continental margin orogen, linked with explosive volcanism occurring on a very large scale, and complex sedimentary associations (Coney, 1992). The development of the Lachlan Fold Belt was completed by post-orogenic granites of Carboniferous age.
Pre-cratonic development was terminated by the Tabberabberan orogeny of the Middle Devonian, which was followed by a diachronous onset of transitional volcanism and sedimentation. The Lachlan Fold Belt was converted into a neocraton by the Kanimblan orogeny, and subsequently, sedimentation in the Late Carboniferous to Holocene, has provided platformal character (Scheibner & Basden, 1996).
A proposed model of formation
The Lachlan Fold Belt, about 700 km wide, is part of the Tasman Orogen in eastern Australia, which in turn is a small part of an orogenic system that formed along the Pacific margin of Gondwana in the Palaeozoic, that was originally much longer. Research on this fold belt has revealed a complex history of deformation that included the formation of a number of thrust wedges, synchronous imbrication in opposite directions, magmatism that is believed to be related to subduction and deposition of syn-deformation sedimentary sequences (mostly melanges).
Most of the models proposed for the tectonic evolution of the Lachlan Fold Belt during the mid-Palaeozoic include 1 or more subduction zones that were active along the eastern margin of Gondwana from the late Ordovician to the Early Carboniferous. In these models the plate setting assumed for outboard of the active margin has often been compared to sectors of the western Pacific such as the Philippine, Mulucca and New Guinea sectors. 3 subduction zones were proposed in the latest model [at the time of writing], that operated in the Silurian-Devonian, relating the differences between the 3 major subprovinces of the Lachlan Fold Belt - eastern, central and western - to their different geographical and structural locations at the time the convergent margin system was evolving. The final accretion of the fold belt and to its morphology of the present, following the closure of the 2 oceanic basins between the subduction zones in the Devonian-Carboniferous.
A new model has been proposed by Braun & Pauselli (see link below) in which they propose that continuous subduction along a single subduction zone drove the entire deformation of the Lachlan Fold Belt during the Palaeozoic, from 460-360 Ma. They suggest a sequence of progressive advection of parts of the orogen above a subduction zone that was relatively fixed (in the frame of reference of cratonic Australia), a sequence of deformation, metamorphic and volcanic events taking place in this period of time.
According to their model, because of low viscosity decollement at the base of the turbidite sequence, it is not possible to deduce the exact location of the subduction zone from the nature and/or timing of structure recorded from the sedimentary cover, at least during the early stages of orogen development. They predict faulting (strain localisation) is controlled by lateral strength gradients, such as the contact between the turbidite sequence from the Ordovician and the Delamerian Orogen in the western part of the Lachlan Fold Belt.
Implications for evolution of the Lachlan Fold Belt
The authors suggest that a consequence of their single subduction model is that the mantle lithosphere beneath the central and western parts of the Lachlan Fold belt is not recycled during the development of the orogen. This is in agreement with a substantial body of evidence that beneath the western part of the Lachlan Fold Belt the basement is of Delamerian age, at least, though possibly older. Recently Os ages of a suite of spinel peridotite xenoliths across the Delamerian Orogen and the western Lachlan Fold Belt indicate the extension of early Proterozoic basement east to Mt Porndon, near the Avoca Fault.
The transition from cold, thick lithosphere, that is characteristic of Proterozoic terranes to hot, thin lithosphere characteristic of younger terranes, based on seismic velocity models of the Australian continent, does not align with the Tasman Line (the surface expression of the same boundary between the Proterozoic and Phanerozoic of Australia). A step in lithospheric thickness is indicated by most seismic models that is located further to the east.
In the Lachlan Fold Belt2 40Ar-39Ar dating (Turner,1996) of detrital micas that were dominantly from the flysch, of Ordovician age (Cas, 1983; Fergusson et al., 1989) revealed that they are from the Late Delamerian Orogeny and of very limited range, indicating that the terrain was being exhumed and eroded very rapidly through the 350oC-450oCAr-muscovite/biotite closure temperature (Turner et al., 1996).
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