Australia: The Land Where Time Began
Central Australian Basins
There are many similarities of overall structure shared by the intracratonic basins of central Australian, from the Neoproterozoic to the Early Palaeozoic. In cross-section, most are asymmetric, with deep sub-basins that are connected by troughs along one margin and along the opposite margin, broad shallow platforms. In the Neoproterozoic, about 800 Ma, all the basins were initiated, and sedimentation ceased in most in the Late Palaeozoic as a result of the Alice Springs Orogeny, a major compressional event (Lindsay & Leven, 1996). Major thrust zones are paralleled by the basin margins nearest the deep sub-basins. Marked asymmetry is a typical feature of the architecture of foreland basins (DeCelles & Giles, 1996; Haddad ei al., 2001).
In the Ngalia, Amadeus and Officer Basins in central Australia deep seismic profiling has been carried out across their margins. The clearest images were obtained from the Officer Basin, showing the relationship between the basin and the underlying crust (Lindsay, 1995; Lindsay & Leven, 1996). At the northern margin of the basin a steeply dipping upturn is formed by the sedimentary deposits. A series of shallow thrust faults within the Musgrave Block, that are north-dipping, are present at depths of 30 km and towards the surface they extend for more than 40 km. The faults bifurcate about 17 km from the basin's erosional margin, and steepen slightly at depths of less than 9 km, intersecting the surface to the north of the margin of the basin.
It has been suggested that the basins of central Australia from the Neoproterozoic are possibly related in some way to supercontinent evolution (Lindsay et al., 1987), based on the similarity of depositional pattern and subsidence curves between the basins. It is suggested by the architecture of the crust of central Australia that in the Archaean and Palaeoproterozoic some components of the crustal mega-elements, such as the Yilgarn Craton and the Pilbara Craton, were assembled, it was not until the Mesoproterozoic that the crustal fabric was fully established, resulting from the assembly of Neoproterozoic Rodinia. North-dipping, thick-skinned thrust faults pervaded the crust that was formed during this period. The crust was thick, 40-50 km, and strong enough to support stress over geologically long time periods (Haddad et al., 2001). The substrate of crust was in place by 1.1 Ga, on which the basins of central Australia could evolve.
New evidence, such as deep seismic data, has become available, allowing better understanding of the origins of the central Australian basins (Lindsay & Leven, 1996). The data allowed the elimination of the possibility that the initiation of the basins had been by extensional tectonics as had been suggested earlier (Lindsay & Korsch, 1989). The default implication being that deep mantle processes, such as mantle plumes were involved (Zhao et al., 1994). According to Lindsay, there is increasing evidence of a supercontinent cycle (Worsley et al., 1984; Murphy & Nance, 1992; Duncan & Turcott, 1994; Veevers et al., 1997) in which landmasses aggregate into supercontinents and subsequently fragment, only to come together again in another supercontinent, driven by mantle convection on a large scale (Gurnis, 1988; Anderson, 1982; Kominz & Bond, 1991; Tackley, 2000).
Beginning about 1 Ga, evidence has been found for the assembly and fragmentation of a supercontinent in the Neoproterozoic, Rodinia (McMenamin & McMenamin, 1990), that has been broadly outlined (e.g. Bond et al., 1984; Lidsay et al., 1987; Dalziel, 1991,1992; Li et al, 1996), though it is not known in as much detail as Pangaea (cf Veevers, 1988, 1989). There is general consensus that supercontinents form above geoid lows, where mantle is downwelling, dispersing above geoid highs where the mantle is upwelling. It has been suggested to be a continuous cycle as the same forces are involved in the formation and the fragmentation of supercontinents (Condie, 1998). The convection of the mantle requires a downwelling point for an upwelling point to operate, as the upwelling and the downwelling are both parts of a convection cell, albeit on a vast scale, in the mantle. According to this proposal, the the next supercontinent begins to assemble above the downwelling, causing it to heat up once the crust has covered it, as heat can no longer escape as easily. When the temperature rises sufficiently the downwelling becomes an upwelling, which eventually causes the fragmentation of the crust of the supercontinent above it, a new downwelling forming elsewhere.
In the Officer Basin, study of the deep structure of the crust has found evidence supporting the existence of a mantle superplume (Lindsay, 1999b), suggesting a local origin for the sediments of Megasequence M1, regional uplift and peneplanation as the superplume rise generating the sediments. From about 900 Ma domed uplift of the continental crust and peneplanation occurred, the high ground producing large amounts of clastic materials (cf Dam et al., 1998). Widespread subsidence began about 800 Ma when thermal relaxation occurred as the plume declined (Zhao et al., 1994). There was a lag following the formation of the crust and the beginning of sedimentation in the basins (see Sun & Sheraton, 1992; Zhao et al., 1992). As thermal recovery began the sag basin formed and the eroded material was redistributed. Across central Australia, over an area of about 2.5 x 106 km2, uniform, slow subsidence began about 800 Ma (Lindsay & Leven, 1996). The region was named the Centralian Superbasin because of the broad scope of the early stages of subsidence of the basin (Walter et al., 1995). The sag basin that resulted from plume activity didn't last long (Dam et al., 1998). After a long hiatus, that varied locally from about 50-100 million years, a major compressional event, the Areyonga Movement, affected the region, isolating the intracratonic basins and forming a series of smaller foreland basins. A major thrust complex and uplifted intermediate basement blocks, such as Musgrave Block and the Arunta Province, delimited each basin. The asymmetry of the basins were established by this event, with initiation of a foreland setting and deposition of Megasequence M2 (Lindsay & Leven, 1996).
Major tectonic events that affected the Australian plate reactivated deep-seated thrust faults, repeatedly renewing the subsidence within the basins (Walter et al., 1995; Lindsay & Leven, 1996). Although the superbasin lasted only a short time, being a response to regional thermal recovery after plume activity, but once the architecture of the crust was established, it allowed the polyphase central Australian basins to persist for almost 500 million years as a series of asymmetric basins that were independent, though related, that were thrust-controlled (Lindsay et al., 1987; Haddad et al., 2001). Within the basins the main depocentres developed as elongate troughs that paralleled the margins that were thrust-faulted, typical for foreland basins (Dickinson, 1974; Beaumont, 1981; Jordan, 1981). The morphology of the orogenic load determined their length (DeCelles & Giles, 1996) and the flexural rigidity of the lithosphere determined their width (Turcotte & Schubert, 1982; Watts, 1992; Haddad et al., 2001). Subsidence was not entirely synchronous within the foreland basins, that implied the central Australian basins evolved as discreet geological features after the superbasin was dismembered about 740-580 Ma, at the beginning of Megasequence M2 (Haddad et al., 2001; Lindsay & Leven, 1996).
According to the author (Source 1), formation of a supercontinent was a prerequisite for the formation of the central Australian basins, for the development of an extensive craton that was thick, and with a heat flow imbalance allowing superplume activity. According to Lindsay, the basins could not have formed without these evolutionary elements. During supercontinent assembly, the internal fabric that developed defined the morphology of the basins by controlling the reactivation of the thrust faults. Such basin development is tied to supercontinent cycle, though it is not unique. During the Early Palaeoproterozoic cycle, about 1.7 Ga (Lindsay, 1998; Lindsay & Brazier, 2000), similar basins developed, and in the Palaeozoic, the development of Pangaea (Veevers, 1988, 1989; Lindsay, 2000).
According to Lindsay, the central Australian basins that developed in the Neoproterozoic to Palaeozoic were a direct result of the formation and fragmentation of Rodinia. The basins persisted for nearly 500 million years, leaving a discontinuous, though comprehensive record of regional events in the form of supersequences, stacked megasequences. The basins are stacked polyphase basins on thick, strong, rigid crust (Lindsay & Korsch, 1989; Haddad et al., 2001).
Lindsay has listed 9 processes that were involved in the basins.
Several crustal mega-elements evolved from crustal nuclei that formed in the Archaean and Palaeoproterozoic (Shaw et al., 1996).
During the Mesoproterozoic the Australian mega-elements assembled, forming part of the supercontinent Rodinia.
A crustal architecture was produced by the process of the assembly, in the form of a pervading suite of thrust faults that reached almost to the base of the crust (Lindsay & Level, 1996. By 1.1 Ga the process had been completed, after which there was a hiatus of deposition that lasted about 200 million years.
Mantle instability and the formation of a superplume resulted from heat flow disequilibrium (cf Anderson, 1982; Gurnis, 1988).
A vast area of central Australia was uplifted, with peneplanation, as a result of the rising of the superplume, that produced large volumes of clastic sediment (Lindsay, 1999a).
About 800 Ma, regional subsidence and the accumulation of sediment resulted from thermal recovery after the superplume decayed. Thick, widespread basal sandstone resulted from the reworking of the clastic sediments that had been produced during the uplift (Lindsay, 1999a). The superbasin phase, primary sag, lasted for about 20 million years, ending about 780 Ma.
A reactivation of deep-seated thrust faults and basement block uplift, after a hiatus of about 50-100 million years, was triggered by the first in a series of extrinsic tectonic events, with the result that the superbasin was dismembered and a distinct foreland architecture developed that persisted for the life of the basins (Haddad et al., 2001). From this point on the basins of the present came into existence, the Centralian Superbasin ceasing to exist. These basins became distinct geological features, with many similarities in their architecture, and the subsequent evolution of each basin paralleling that of the others (Lindsay & Leven, 1996; Haddad et al., 2001).
The basin thrust margin uplift, that occurred during foreland basin development, are suggested by Lindsay to be related with the 2 known glaciations of the Neoproterozoic, the the interbasinal basement blocks that were uplifted possibly being the sites of nucleation of the ice sheets.
Lindsay suggests Rodinia probably broke up in the latest Proterozoic or earliest Cambrian, based on evidence of extension.
Sedimentation ceased, and the basin eventually closed about 290 Ma, as a result of the Alice Springs Orogeny, a major compressive event, that influenced much of central Australia (Jones, 1972, 1991).
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|