Australia: The Land Where Time Began
The Palaeoproterozoic Era has been subdivided in the:
There is a lack of definitive evidence, in the form of belts of high pressure metamorphic rocks such as ophiolite remnants, and large-volume igneous suites with geochemical signatures indicating plate margins or arcs. Another characteristic of Australian Proterozoic terranes is the high concentration of elements that produce high heat, to a degree not common in Proterozoic provinces from around the world.
Three categories of tectonic models have been proposed for the Australian Proterozoic, interplate models, a hot-plate model, and models in which processes are analogues those of modern plate tectonics.
These are models in which vertical accretion, plating with mafic material, the crust being subsequently reworked during episodic granite emplacement. According to these models, small-scale convection in the upper mantle and mantle delamination were the driving force for intraplate orogenesis, leading to belts of intraplate mountains and basins.
In this model, lithospheric weakening and increased geothermal gradients during the Palaeoproterozoic, resulted from the heterogeneous distribution of radiogenic elements in the lower crust that produce heat. According to this theory, distal plate forces drove tectonism, with orogenic events and basin development occurring in regions of high heat production. Multiple episodes of magmatism that resulted in vertical redistribution of heat-producing elements from the lower to the upper crust led to the strengthening of the lithosphere of the Australian continent during the Proterozoic.
Processes analogous to modern plate tectonics
Myers et al. (1996) suggested that prior to their amalgamation during the collisional events that led to the formation of Rodinia, a supercontinent, the North Australian Craton (NAC), the West Australian Craton (WAC) and the South Australian Craton (SAC) evolved independently. There has been a growing consensus for models involving protracted subduction, accretion and reworking of the crust occurring along the southern margin of the Australian continent, and between 1800 Ma and 1600 Ma, continental back arc basin development in the continental interior.
During the Glenburg Orogeny, about 2000-1800 Ma, and the Capricorn Orogeny, about 1820-1780 Ma, the Yilgarn Craton and Pilbara Craton, and their marginal terranes, amalgamated to form the West Australian Craton. During the Barramundi Orogeny, about 1880-1840 Ma, the North Australian Craton amalgamated, the Kimberley Craton being added during the Halls Creek Orogeny, about 1850-1820 Ma. The last stage of the amalgamation of the supercontinent Columbia (Nuna), is recorded in these collisional events.
The site of tectonic activity moved to the southern margin of the continent about 1800 Ma with the evolution of a long-lived subduction system. Multiple cycles of terrane accretionary tectonics occurred during orogenic events in the Arunta Inlier and the Gawler Craton.
It is believed that accretion occurred during a number of orogenies. The West Australian Craton amalgamated with the North Australian Craton duirng the Yamba Orogeny and the Yapunku Orogeny, about 1820-1780 Ma. Accretion is also thought to have occurred during the amalgamation of the Archaean nucleus of the Gawler Craton, as well as East Antarctica, with the southern edge of the Australian continent, during the Strangways Orogeny and the Kimban Orogeny, about 1740-1690 Ma. More accretion is thought to have occurred when the Warumpi terrane accreted along the southern edge of the Arunta Inlier, during Warumpi and Ooldean event. In the interior of the North Australian Craton, major unconformities developed or basin inversion events occurred at the same time as each of the accretionary events. It appears basin formation occurred between accretionary events. It is believed there is a link between basin development and episodes of subduction rollback and the extension of lithosphere in the overriding plate.
At about 1620-1560 Ma, near the Palaeoproterozoic-Mesoproterozoic, there is good evidence of arc magmatism associated with subduction. Among the rocks are the Peter Suite, in the southern Gawler Craton, about 1620-1610 Ma, and the Birksgate Complex, protoliths, since reworked within the Musgrave Province.
Throughout central and eastern Australia, a major thermal and orogenic event punctuated arc magmatism. High-temperature metamorphism characterised this event, as does widespread magmatism, dominated by intraplate felsic (A-type), that is believed to define a continental hotspot, the track of which is orthogonal to what has been interpreted as the plate margin, as well as several episodes of crustal shortening. A hybrid tectonic model, combining aspects of intra-plate plume-driven models and plate margin models. The new model, a hybrid plume-modified orogenic model, has been suggested to include evidence for tectonic activity, whether subduction or plume-related.
Dr Pete Betts, Monash University, Australian Proterozoic Tectonics
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|