Australia: The Land Where Time Began
The Delamerian Orogeny occurred possibly between about 514 million years ago and 500 million years ago, beginning near the end of the Cambrian and continuing into the Ordovician. The Flinders Ranges and the Mt Lofty Ranges are highly eroded remains of the large mountain range that resulted from the folding and faulting that occurred as the sedimentary deposits of the Adelaide geosyncline (now called the Adelaide Fold belt) buckled. There were also a number of volcanic intrusions, such as the granites of Victor Harbour, and in the eastern Mt Lofty Ranges.
Some parts of the Adelaide Geosyncline were not affected by the orogeny. In the northwest, the Stuart Shelf was not affected and in the north and northeast, in the Cooper Basin and the Pedirka Basin limestones and shales were deposited.
The zone of uplift and folding extended north through Adelaide, the seas over parts of the continent and the Gulf of Carpentaria retreating as the crust was uplifted and folded.
The Delamerian Orogeny continued into the Ordovician, a zone extending from western Tasmania to the north-northwest across western Victoria to north-central South Australia. A climate of high rainfall is indicated by the rapid rate of erosion of this raised crust.
The onset of uplift of the Delamerian Orogeny is marked by the Pantapinna Formation, dated to 515 Ma.
Timing and Duration3
According to the authors3 the transfer of stress to the outboard trailing edge of the Gondwana Supercontinent that was newly formed resulted in the Ross Orogeny in Antarctica and the Delamerian Orogeny in Australia. In the Early to Middle Cambrian tectonic reorganisation occurred following the completion of the Pan-African deformation and subduction that occurred along the sutures when the landmasses of eastern and western Gondwana was sutured together. Prior to this rocks in eastern Australia, of Neoproterozoic to Cambrian age, formed in a passive margin that recorded the dispersal of the fragments of Rodinia, as well as the opening of the proto-Pacific Ocean. The geochronology analysis carried out by the authors3 using U-Pb and Rb-Sr methods indicated that at about 514 ± 3 Ma the Adelaide Fold Belt, in the South Australian Domain of the Delamerian Orogen, contractional orogenesis began, continuing until 490 ± 3 Ma, being terminated by the uplift, cooling and extension associated with post-tectonic magmatism.
When new and published U-Pb and 40Ar-39Ar geochronology were integrated from the full Ross-Delamerian Belt they indicated the Delamerian Orogeny and Ross Orogeny both have a late magmatic and cooling terminal history that is synchronous, though the Ross Orogeny started its orogenic history of convergence at about 540 Ma, the Delamerian Orogeny beginning 25 My later. Eastern Australia was still in a state of extension, or transtension, in the Early Cambrian, with the Kanmantoo Basin opening and the associated magmatism that was largely mafic and anorogenic, the basin receiving sediment from the Ross Orogen that was already exposed to the South. At about 514 Ma the first occurrence of strain fabrics and magmatism related to subduction, that included boninite, granite, and andesite lavas, occurred simultaneously in New Zealand, Victoria, South Australia, New South Wales and Tasmania is suggested by the authors3 to imply that ridge-push forces transmitted by the beginning of subduction, that was west-dipping, drove the Delamerian Orogeny. At 490 Ma, when the new slab had reached the transition zone at a depth of 650 km, there may have been eastward slab rollback that resulted in the extension of upper plate and magmatism of anorogenic Basin and Range-style, in the Mt Reed Belt of South Australia and Tasmania. The westward motion of the Australian part of eastern Gondwana continued being accommodated in the late Early Cambrian by deformation and subduction along either the Mozambique Suture, or at the northern end of the South Prince Charles Mountains-Prydz Bay Suture, as is implied by the delayed initiation of subduction in the Australian section of the margin.
Results - see Source 3
Delamerian events - timing and duration
In the Kanmantoo Trough, in the Stansbury Basin, the earliest age of deep-water deposition of turbidites has been constrained to be mid-Early Cambrian by the age of zircons in a tuff horizon in the Normanville Group, that have been dated to 522 ± 2 Ma (Cooper et al., 1992; Jenkins et al., 2002). The initiation of basin inversion and contraction of the Delamerian has been defined by the age of 514 ± 4 Ma for the earliest syntectonic granite in the belt, the Rathjen Gneiss (Foden et al., 1999), resulting in the beginning of the Delamerian Orogeny following at most 8 ± 6 My of deposition of sediment. At various widely dispersed places around the belt, such as the Peake and Denison Ranges (Bungadillina monazite; Morrison and Foden, 1990) in the far north, 513 ± 0.8 Ma, quartz porphyry intrusions in the Glenelg Inlier, Victoria, 514 ± 3 Ma (SHRIMP U-Pb on zircon; Ireland et al., 2002) and the Heazlewood Tonalite, western Tasmania (510 ± 3 Ma, SHRIMP U-Pb on zircon; Turner et al., 1998). The deformation of the Delamerian ended at 492 ± 2 Ma, based on the dating by the authors3, definitely prior to the intrusion of felsic dikes and sills that occurred at 487 Ma. This puts it at the same age as in the Glenelg Zone in which the deformation termination is bracketed by the Wando tonalite (493 ± 8 Ma; SHRIMP U-Pb on zircon) and Loftus Creek granodiorite (491 ± 8 Ma; SHRIMP U-Pb on zircon; Ireland et al., 2002). According to the authors3 this was probably after the end of deformation at the western boundary of the orogen in southern Kangaroo Island, where they suggest it must have taken place after 504Ma (S-type granite that has been deformed at Stun'sail Boom River; Fanning, 1990); C.M.Fanning pers.com. to the authors3, 2004) and before 500 Ma (felsic dikes at Cape Gantheaume that are undeformed). As a result of the weakening, on a local scale, caused by thermal weakening near magmatic intrusions that have been emplaced into a prevailing stress field, leading to the late deformation stages being diachronous. In the Arkaroola-Mt Painter area in the north new Rb-Sr and Sm-Nd isochrons on deformed leucogranites are also in the range 506 ± 9 Ma and 496 ± 8 Ma, respectively, which indicates that Delamerian contraction ended no more than 499 ± 12 Ma, based on the Rb-Sr age of a pegmatite that was deformed (Elburg et al., 2003), suggesting the length of the Delamerian contraction could have continued for no more than 24 ± 5 My.
The beginning of the production of granite occurred synchronously with the start of deformation, the magmatic ages of the earliest syntectonic granites defining this timing, the Rathjen Gneiss being 514 ± 4 Ma (Foden et al., 1999). The interaction between melts from asthenospheric mantle that was decompressed and the sediments that filled the rift basin contemporaneously, as discussed elsewhere (Foden et al., 2002b), produced these granites, ranging from I- to S-type. A change to post-tectonic magmatism, that was bimodal and derived substantially from the mantle, marked the end of deformation at the close of the Delamerian (Turner et al., 1992; Turner and Foden, 1996; Foden et al., 2002a, 2002b), comprised of A-type granite and mafic intrusions. A renewed influx of hot mantle beneath the belt, implied by this magmatic phase that occurred at about 493-480 Ma in the Early Ordovician, the authors3 suggesting could possibly been associated with the combined effects of extension of the lithosphere and detachment of lithospheric mantle.
The proposed obduction onto passive margin basalt sediment sequences, of Early Cambrian age, in Tasmania, of ultramafic complexes in the earliest recognised Delamerian event (Berry & Crawford, 1988; Crawford & Berry, 1992), prior to the deposition of the Dundas Group in the late Middle Cambrian and the Late Cambrian that is comprised of marine turbidites that were partially Molassic. The timing of the Delamerian D1 event is constrained by the Adelaide Fold Belt. Widespread volcanism, that was intermediate to felsic, of the Mount Read Volcanics and the Tyndall Group were characteristic of the Middle to Late Cambrian in Tasmania, U-Pb SHRIMP ages (Perkins & Walsh, 1993) of these ranged from 503 ± 4 Ma for the Mount Charter Rhyolite to 494 ± 4 Ma for the Comstock Tuff. These ages are consistent with the deformation of the Tasmanian samples, that was limited and localised, and are synchronous with the late D2 and D3 events in the Adelaide Fold Belt.
Compared with the Ross Orogen, Antarctica
A good basis to compare the Delamerian Orogen with the Ross Orogen is the geochronology of the Authors3. A comprehensive geochronological picture of the Ross Orogen age, mainly based on U-Pb zircon dates of good quality, has been achieved by recent studies (Pankhurst et al., 1988; Goodge & Dallmeyer, 1992, 1996; Goodge et al., 1993a, 1993b; Millar & Storey, 1995; Encarnacion & Grunow, 1996; Goodge, 1997). At about 550 Ma there began a period of major volcanism and intrusion, that was intermediate to felsic, following on from the Beardmore Orogeny of the Neoproterozoic (Rowell et al., 1993; Encarnacion & Grunow, 1996), as indicated by data from North and South Victoria Land and the central part of the Transantarctic Mountains. The beginning of subduction and major reorganisation of tectonic plates have been suggested to be the trigger for the onset of the Ross Orogeny (Encarnacion & Grunow, 1996; Goodge, 1997). In localised basins there was sedimentation and the intrusion of granite, metamorphism and deformation that continued into the Late Cambrian. The formation of sedimentary basins and the generation of sediment that continued through this period have been pointed out as being direct results of Ross tectonism (Myrow et al., 2002). The Granite Harbour intrusives, that are undeformed, and pegmatic and volcanics, that are related, range between 505 Ma to 485 Ma. In the Thiel Mountains volcaniclastic rocks that have been dated to 500 Ma (Pankhurst et al., 1988) are also undeformed. The beginning of orogenic magmatism at about 540 Ma (Goodge et al., 1993a, 1993b), that continued for about 35 My, it is indicated by the results that the history of the Ross Orogen included active convergent or transpressional tectonism (Goodge et al., 1993a, 1993b). When compared with the Delamerian Orogen, the Australian end of the belt, it is clear that the Ross Orogen began much earlier than the Delamerian Orogen. At the time felsic magmatism, deformation and metamorphism began in the Adelaide Fold Belt, where the oldest granite is from the Cambrian, 514 Ma (Foden et al., 1999) it had been going for more than 25 My in the Ross Orogen. Over much of this period the formation of sedimentary basins and associated magmatism that was mantle derived took place, in particular, the Kanmantoo Group is at least 7 km thick, and as it was deposited in as little as about 8 My (Haines et al., 2001) it would have required a very high rate of sediment production. It has been demonstrated by zircon provenance studies that the Kanmantoo Group, that is rich in turbidite, contains a large population of zircons that have been dated to the Early Cambrian, as well as populations of Grenville and Pan-African age. To the west of the fold belt there are no obvious sources, as noted by Wombacher and Münker, 2000). The supply of sediment to the Kanmantoo Trough is implied by the current directions in basal turbidites to have been from the south (Haines et al., 2001). The data obtained by the authors3 very firmly support this conclusion, indicating that the sediments were derived from the part of the Ross-Delamerian Orogen to the south, a region that was undergoing tectonic shortening and rapid erosion prior to the initiation of basin inversion to the north at the Australian end.
A series of 40Ar-39Ar cooling ages have been obtained from the Geologists Range in the central part of the Transantarctic Mountains, and in northern Victoria Land, the Lanterman Range (Goodge & Dallmeyer, 1996). It is suggested by their evidence that there was contrasting behaviour in the more southerly part of the Ross Orogen compared to the part closest to Australia. Uplift and erosion began at about 539 ± 5 Ma that resulted in cooling in the central Transantarctic Mountains, the authors suggesting it could possibly account for the supply of sediment to the Kanmantoo Trough to the north. At about 95 Ma cooling below the temperature of muscovite closure occurred. The northern part of Victoria Land has cooling ages that are very similar to those of Delamerian Fold Belt, with ages in the range of 488-482 Ma from hornblende and muscovite, that implies cooling was rapid, 30oC/My, as well as rapid uplift, 1.2 mm/year, and erosion (Goodge & Dallmeyer, 1996).
The Cape Fold Belt of southernmost Africa, that has rapid exhumation, cooling and erosion (Armstrong et al., 1999), is an extension of the Ross Orogen. The Cape Granite, that is syntectonic, has been dated by the U-Pb SHRIMP zircon method, to 540 ± 4 Ma, and cooling ages for muscovite and biotite of 536 ± 1 Ma, by the 40Ar39Ar method.
The Delamerian Orogeny - Convergence
A conclusion from their results that the authors3 say is important is the common late histories, of both the Delamerian Orogeny and the Ross Orogeny, of late magmatism and cooling, both of which resulted from the transfer of stress to the trailing passive margin after the assembly of the Gondwana supercontinent. The subduction-related orogenic history of the Ross Orogeny began, particularly towards its southern end (Weddell Sea) at least 25 My prior to the deformation and subduction in Australia of the Delamerian Orogeny, and during this gap period the basin receiving the sediments from the Kanmantoo Group continued to extend. It has been proposed that, consistent with this conclusion, the Kanmantoo Trough was developed as a jog on the transcurrent fault, that was left-lateral E-W, to the south of the present southern margin of Australia, this fault separating the Australian Pacific margin sectors from Antarctica (Flöttmann et al., 1998). During the Middle to Late Cambrian the westward motion of the Australian part of eastern Gondwana was continually accommodated by subduction or deformation of its western margin, possibly in the Mozambique Suture or more likely in the Southern Prince Charles Mountains-Prydz Bay Suture, as indicated by the delay of subduction initiation during the opening of the Kanmantoo Trough in the Delamerian zone. According to the authors3 this is consistent with recent geochronological results in the Southern Prince Charles Mountains-Prydz Bay Suture (Collins and Pisarevsky, 2005). In the Leeuwin Complex, southwestern West Australia, an age of 522 ± 5 Ma for terminal mobile orogenesis very well supports this conclusion. As this complex is part of the Australian craton it predates the initiation of the Delamerian Orogeny, though it is synchronous with the Ross Orogeny.
According to the authors3 an obvious source of sediments, containing populations of zircons that are distinctive dated to the Early Cambrian and Grenvillian, for the Kanmantoo Trough, was provided by the deformation of the Ross Orogeny occurring before the Delamerian Orogeny (Ireland al., 1998). The same deformation has been suggested as a probable source of sediments for the Takaka Terrane of New Zealand. It has been suggested that these sediments were deposited to to west of the Devil River Volcanic Arc in a back arc basin, which led to the conclusion that sediments deposited in this back arc, that was sourced in the Ross Highlands, it could only been on the same plate and therefore above a subduction zone that dipped to the west, that was situated to the south of any Australian-Antarctic Fault that was postulated (Wombacher & Münker, 2000).
The authors3 have suggested that unlike the situation in the Ross Orogeny, along the Pacific margin of Australia the earliest subduction phase was to the east, and the Delamerian Orogeny was the result of collision and obduction of an outboard arc complex that was postulated on the Pacific Plate across the margin of Australia (Crawford & Berry, 1992; Münker & Crawford, 2000). The interpretation of the boninite ultramafic complexes of Tasmania, are the main base for this conclusion, that are of Early Cambrian age, originated in forearcs that had travelled great distances and were emplaced as allochthons by westward obduction of the Pacific Plate (Berry & Crawford, 1988; Crawford & Berry, 1992; Boger & Miller, 2004). According to the authors3 recent results, that include the results of their present study, suggest that this model should be called into question, in spite of its long acceptance. In the Victorian Glenelg Zone boninite-related magmas have been intruded into the attenuated continental margin (Kemp, 2003). The authors suggest that these boninite magmas are not allochthonous and it would require westward subduction for their generation. It is suggested that the boninites from Victoria are likely to be of the same age, 514 Ma, as those in Tasmania (Turner et al., 1998) and those in Tasmania and South Australia from the beginning of orogenesis in the Delamerian Orogeny (Foden et al., 1999, 2002a), the boninite/arc association in the Takaka/Devil River/Mount Benson Terrane in New Zealand (Münker, 2000; Münker & Crawford, 2000), and in the Victorian Glenelg Zone, the earliest granite formation. As the simultaneous appearance of magmatism of a subduction type and contractional deformation are all indicated by these ages, the authors3 infer that the age of subduction inception is implied by the synchronicity of these events. In the Takaka/Devil RiverArc and the continental margin of Australia in South Australia, Victoria and Tasmania volcanism began at the same time, it implied that the oceanic Takaka system must have been an offshore, synchronous (parallel) subduction system or an along-strike extension of the eastern Australian system.
According to the authors3 as the presence of synorogenic magmatism in the Adelaide Fold Belt, on what would have been the underthrusting plate, according to that interpretation, is difficult to reconcile with eastward subduction at 514 Ma. If eastward subduction had taken place in the Early Cambrian, in the period before the Delamerian Orogeny, the synchronicity with the extension of the Kanmantoo Trough in South Australia, with its E-MORB mafic magmatism (Foden et al., 2002a), is also difficult to explain. The authors3 conclude that neither accretion nor collision with offshore arcs, that were hypothetical, or continental fragments drove the Delamerian Orogeny, the driving mainly resulting from transfer of far-field (ridge-push?) stresses to the continental margin that was previously attenuated, at the beginning of subduction. The strain that resulted from this event was partitioned heterogeneously across the margin of the continent, eventually being focused in crust from prior rifts that were thermally weakened and attenuated.
The authors3 conclude that the Delamerian Orogeny is an orogenic event that was powered by the inception of subduction in this part of the margin of Gondwana, in response to changes in plate dynamics resulting from the completion of subduction and collision to the western side that produced Gondwana (Borger et al., 2001; Borger & Miller, 2004; Collins and Pisarevsky, 2005).
Delamerian-Ross Orogeny - Termination
It has been suggested by a number of lines of evidence that convergent deformation in the Delamerian Orogeny ceased abruptly at the close of the Cambrian, associated with rapid buoyant uplift and exhumation. The authors3 say they came to this conclusion based on 40Ar-39Ar and Rb-Sr ages obtained from detrital mica and 40Ar-39Ar cooling ages that indicate that these ages don't span the age of the orogen, clustering at the age the deformation terminated, about 503-490 Ma (Turner et al., 1996). Composition changes of felsic magmas mark this tectonic transition from syntectonic granites of I-S-type to magmatic suites that were bimodal mantle-derived, that include siliceous, potassic post-tectonic A-type granites and volcanics (Turner et al., 1992; Turner, 1996; Turner & Foden, 1996; Foden et al., 2002b). Syntectonic metamorphic and igneous complexes that the A-type granites and volcanics intruded at high crustal level and the exhumation during the terminal phase of the Delamerian Orogeny exposed by and eroded them. In the Lachlan Fold Belt 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-450oC Ar-muscovite/biotite closure temperature (Turner et al., 1996).
In the Adelaide Fold Belt and the Glenelg Zone the ages are mostly from the granites and gneisses. The ages cover a narrow range, 492-485 Ma, slightly younger than the age of the end of deformation reported in this article, U-Pb method with high closure temperatures of about 490 Ma at Reedy Creek. This post-tectonic igneous intrusion is indicated to have been emplaced into upper crust that was cool by 40Ar-39Ar age of hornblende (Turner, 1996) from the Marcollat granite, that was A-type, being indistinguishable from the U-Pb age from zircons.
In Tasmania preserved proximal molasse deposits are widespread that might record the terminal uplift of the Delamerian Orogeny, though in South Australia there are no known such deposits. They are present in the Denison Group, of latest Cambrian and Early Ordovician age, that are comprised of deposits rich in conglomerates, including the Jukes Conglomerate and the Owen Conglomerate, of terrestrial and shallow marine origin (Noll & Hall, 2003), that appear to have resulted from exhumation and erosion, that was rapid and sequential, of the Cambrian and the Ordovician basement. Eclogites are present among these basement rocks that have Rb-Sr mica ages of 485 ± 4 Ma (Raheim & Compston, 1977). It is indicated by this age and 40Ar-39Ar analysis of 489 ± 4 9 Ma on hornblende from the Mount Read Volcanics (Everard & Villa, 1994) that there was cooling that resulted from exhumation and erosion at the same time in Tasmania and in the Adelaide Fold Belt and the Glenelg Inlier.
It appears to be suggested by the coincidence of cooling ages from the fold belt and those from detrital micas deposited in flysch, that is mainly of Ordovician age, there may be a causal link between the uplift and erosion of the Delamerian Orogeny and the erosion that occurred at the termination of deformation (Turner,1996). According to the Authors3 the Delamerian Orogeny is strongly implied by their results to have been both caused and maintained by subduction, the evidence indicating that 24 My after the inception of convergent deformation the fold belt reverted to extension. This, coupled with evidence that as post-tectonic uplift occurs after the end of orogenic deformation, it is best interpreted as exhumation that is controlled by buoyancy. That this was associated with new mantle influx is indicated by an increase in mafic magmatism and a shift of initial εNd values of post-Delamerian A-type granites. It is pointed out by the authors3 that the 24 My of deformation that followed the apparent beginning of subduction is the same period of time required for a newly foundering slab, with a down-dip velocity of 2-3 cm/yr to reach a depth of 650 km, the depth of the discontinuity. It has been established that slabs that are newly subducting lose most of the negative buoyancy when they reach this transition zone in the middle mantle (Ranalli et al., 2000; Kincaid & Griffith, 2004). This would have ended the Delamerian Orogeny as at this depth slab rollback and the immediate transmission loss of the compressive stresses across the plate boundary to the margin of the continent that drive the plate. In tern this also severs viscous coupling in the subduction zone between the upper and lower plate, thus eliminating flexural drag and the result is the rapid exhumation of the upper plate.
The authors3 suggest the source of high-temperature post-tectonic magmatism, that includes the A-type granites in South Australia, and in Tasmania, the Tyndall Group and the Mount Read Volcanics, could be the hot asthenosphere that was mobilised by this rollback event.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|