Australia: The Land Where Time Began
Drifting - direction
change in the Middle Cretaceous and Middle Eocene
During the Middle Cretaceous, about 99 million years ago, a clockwise bend in the linear chains of volcanoes of the tropical Pacific occurred. At the same time, the azimuth of plate divergence in the eastern Indian Ocean changed in a clockwise direction. Prior to 99 million years ago, the Pacific Plate was being subducted beneath eastern Gondwana in a head-on collision, in a Chilean type of subduction. Between 99 and 43 million years ago this changed to a sinistral oblique Mariana-type strike slip and breakup by simple seafloor spreading between Australia and Antarctica, as well as by back-arc spreading in the southwest Pacific. An unconformity between volcaniclastic sediment below and quartzose sediment above, documents the separation of Australia from Antarctica in the Middle Cretaceous. The stati-tectonic change from the Innamincka Regime to the Potoroo Regime, formed the basis of modern Australia, with a "passive" margin mountain chain in the east and lowlands on a truly passive margin on the south.
The Emperor-Hawaiian bend, the bend that occurred at 43 Ma in the linear volcanic chains in the Pacific, and simultaneous changes in the Indian Ocean, coincide with a change in stress from the Pacific, Australasia being subjected to dextral transcurrence. Results of this included the Challenger Rift of New Zealand, the ininitial growth structure on the oilfield of the Gippsland Basin and oil-shale grabens in Queensland, and faulting between East and West Antarctica.
Hotspot lineaments and ages predicted by Duncan & Clague (1985) include prominent bends at 43 Ma and 100 Ma. The Emperor Seamounts-Hawaiian volcanic chain of the North Pacific indicate that at 43 million years ago the Pacific Plate swung 40o left (Dalrymple & Clague, 1976), and increased their drift rate from 70 to 93 mm/year. Volcanic chains in the tropical Pacific generated from hotspots now at Easter island, Sala y Gomey, and Macdonald Seamount in the Austral Indies, and spreading in the southwest Pacific, also demonstrate the bend at 43 Ma (Stock & Molnar, 1987).
The rate of movement of the Pacific Plate over the past 10 Ma (Lithgow-Bertelloni & Richards, 1998) is parallel to the youngest section of the hotspot trails, and almost orthogonal to those for the Australian Plate. The western Pacific and Australia swung 70o counterclockwise at 43 Ma, in the Middle Eocene, from the direction of movement between 45-43 Ma to the new direction from 43-25 Ma.
The bend that occurred at 99 Ma between the Pacific Mountains and the Line Islands, shows a swing to the right of 58 o, and a speed increase from 46 to 70 mm/year, as well as an increase in the amount of crust being produced globally. The western Pacific and Australia were moving towards each other, head-on, from 119-100 Ma. From 100-94 Ma, the Pacific swung clockwise by 65o, sideswiping the eastern margin of Australia.
Events at 43 Ma on the Indian, Australian and Antarctic Plates
The seafloor of the eastern Indian Ocean underwent a dramatic change at this time, the spreading ridge at A20=43 Ma was abandoned (Liu et al., 1983), the Indian Plate fusing with the Australian Plate to form the Indo-Australian Plate.
The spreading half-rate of the Southeast Indian Ocean Ridge (SEIR) changed from 6.5 mm/year at 46.3 million years ago to 22/year at 40.1 Ma.
Structures that developed at other places in the Middle and Late Eocene (time slice 2 of Langford et al., 1995), such as the St Vincents Basin and complementary Mt Lofty-Flinders Ranges, folds in the Eromanga Basin and Cooper Basin, the lignitic Hale Basin and Burt Basin (see Senior et al., 1995) valley sediment at Bungonia (Truswell & Owen, 1988) covered by basalt ("deep-leads"), as well as other parts of the Southeast Highlands and Tasmania, which, by the Middle Eocene had developed much of the terrain that is seen at the present. Rifts in northeast Queensland had also developed, including oil-shale half-grabens (Melver et al.., 1992). There was a marine transgression in the western part of the Gulf of Papua in Anchor Cay (Veevers, 1984).
Folds formed at Barrow Island, in the Carnarvon Basin. During the later Cenozoic, a younger generation of folds formed in the Eromanga Basin and Cooper Basin (Evans, 1982) that are believed to have probably arisen from collision with the Sunderland Block, as well as terranes docking in New Guinea.
By 43 Ma, the Challenger Rift System, of the Middle Eocene to end-Oligocene, had rifted New Zealand (Kemp, 1986), impressed on convergent terrane of Early Cretaceous origin, except in places, especially Avoca (A), Canterbury, where sediment of Oligocene age above and reworked sediments from the Middle Eocene below, are boundaries of an angular unconformity (McLennan & Bradshaw, 1984).
In Marie Byrd Land, the Antarctic Plate vector was unchanged at 109o. This is consistent with the dextral oblique extension between East and West Antarctica that began at 45 Ma that had been postulated (Sutherland, 1995). This was linked with the uplift of the Transantarctic Mountains by Brink et al. (1997). Spreading south of the Tasman Sea continued very slowly until 43 Ma (chron 20), after which spreading began in the nearby Emerald Basin (Cande et al., 1998).
Effects of the 99 Ma direction change on Indian and Australian plates
The west of the continent
In the west, breakup with spreading of the seafloor at the northwest margin began about 156 Ma and on the western margin about 130 Ma. The initial spreading azimuth bend, fracture zones, from northwest to north, is 150 km northwest of DSDP 256. At an estimated spreading rate of 36 mm/year, this is equivalent to 4 million years. At the site of DSDP 256 the basalt was dated to 94.3 +/- 4 Ma, by K-Ar and converted to new constants (Davies et al., 1974). The oldest fossils Immediately above the basalt, indicate the eiffellithus turriseiffeli nanno zone. This has been placed in the latest Albian to mid-Cenomanian, 101-96 Ma, by Harland et al. (1990). According to Veevers (2000), the best seafloor age estimate is 96 Ma, and of the bend, an additional 4 Ma making it 100 Ma, which coincides with the age of 99 Ma for the transition from the Innamincka Regime to the Potoroo Regime.
The half-rate spreading velocity changed from 36 to 84 mm/year at the bend of the spreading azimuth from 310o to 360o.
Along a 1000 km length of the North West Shelf, renewed rifting and thinning of the lithosphere occurred at this time, as indicated by the rapid subsidence of the Malita Graben-Sahul Trough, as well as the Londonderry High and moderate subsidence of the Browse Basin. 99 Ma is also the time when mainly detrital sedimentation is replaced by carbonate deposition along the entire margin (Veevers, 1984). This has been interpreted as the beginning of thermohaline circulation in the main body of the Indian Ocean, following the clearing of the Exmouth Plateau by the drifting Indian continent. Veevers (2000) notes the precise coincidence in facies change at the identical time (Albian-Cenomanian, or Early-Late Cretaceous) in Europe, originally mapped by William Smith, the Greensand being the lower part and the White Chalk the upper part (Harland et al., 1990).
The south of the continent
In the south, the basal sediment in Jerboa-1, containing the Murospora florida spore-pollen zone (Moreton & Drexel, 1995) of the Late Jurassic (Oxfordian-Kimmeridgian, 160-150 Ma) indicates the beginning of rifting. This is indistinguishable from the start of spreading in the Argo Abyssal Plain at 156 Ma. In Jerboa-1, tectonic subsidence (Hegarty et al., 1988) accelerated during the Aptian, 120 - 112 Ma, and the Cenomanian, 99-93.5 Ma. Subsidence of 100 m/million years in the Early Cretaceous increased to 300 m/million years in the Late Cretaceous, in the Ceduna depocentre, on the southern side of the eponymous Potoroo-1 and the main rift boundary fault. During the Cenozoic, it changed to 20 m/million years (Veevers). Extrapolating the rate of spreading from the last anomaly, A34, about 83.00 Ma, at the edge of the Quiet Zone, to the boundary between the continent and the ocean (continent-ocean boundary=COB), indicates when drifting began. According to this estimation (Veevers & Eittriem, 1988; Tikku & Cande, 1999), the mid-ocean ridge between Australia and Antarctica began very slowly 99 Ma. It was modelled as a transform fault that links to a segment of ridge that lies to the south of the Pitt-Chatham Islands. From 99 Ma to 68.7 Ma, the half-spreading rate off the south is 5.0 mm/year (A31o), to 53.3 Ma it is 1.5 mm/year, to 46.3 Ma it is 6.5 mm/year (A21y). From 46.3 - 40.1 Ma (A18o) it is 10.0 mm/year, and from 40.1 Ma to the present it is 22 mm/year.
In the centre of the Plate, at the site of Innamincka-1, 1000 km to the northeast, subsidence occurred at the rate of 9 m/million years, that continued in the Early Cretaceous. From 103-93.5 Ma subsidence increased to 28 m/ m.y., after the final regression in the area of the Winton Formation at 93.5 Ma indicates definitive uplift, that involved 0.5 km loss of section (Lowe-Young et al., 1996). Later in the Late Cretaceous very slow subsidence occurred intermittently, leading to the formation of the Mt Howie Sandstone, as well as sedimentation in the drainage that flowed toward the southwest (Veevers, 1984, p.219; Veevers, 1991, p. 8). Wopfner et al. (1974) interpreted the renewal, in the Middle Eovene, of epeirogenic from the facies of the Eyre Formation in the southwest of the Great Artesian Basin, that correlated with the lower N. asperus spore-pollen zone (Alley et al., 1996), and it was found that in far west New South Wales east to west compression occurred in the Middle Cretaceous and the Middle Eocene (Neef, 1998). The less entrenched drainage of the southwest of the continent was directed to the south and southeast. The anomalous vertical motion of Australia in the Late Cretaceous has been studied in the plate-dynamic models of Pacific subduction by Russell & Gurnis (1994) and Gurnis et al., 1996).
About 95 Ma, in the late Cenomanian, the continental margins in the east and southeast were uplifted, then rapidly denuded, losing 1-3 km of section (Worden et al., 1996). Zircons from the Anakie-Rubyvale alkali basalt pyroclastics have been dated to 110-102 Ma, which is coeval with the Whitsundays, and 90-83 Ma, that is coeval with breakup. Apatite fission track ages (AFTA) in the Surat Basin, and in the southern (Taroom Trough) and northern Bowen Basin, between 100 and 80 Ma up to 1.6 km of section was lost (Raza et al., 1995).
Tectonic subsidence in the Surat Basin accelerated about 112 Ma, from the beginning of the Albian. About 95 Ma the subsidence was reversed as a result of uplift that was brought about by contractional deformation, possibly "within an overall extensional regime, where ... the thermal bulge prior to seafloor spreading led to the formation of passive marginal mountains and the regional uplift" (Korch & Totterdell, 1996, p.316).
It is believed that 100-80 Ma, between 2 and 3 km of section were lost from the coastal section, and about 1 km, about 250 km inland, during regional uplift of the New England Orogen, and (Raza et al., 1996).
Thermal events have been summarised in the Sydney-Bowen Basin (Lackie & Schmidt, 1996). During the Cretaceous, 121-83 Ma, Normal Superchron, acquired a magnetic overprint, the intensity decreased away from the coastal section of the Sydney Basin, especially in the southeastern part. This has been interpreted as resulting from a heat pulse that produced low-temperature metamorphism, after which, there was uplift, erosion and cooling that was associated with the breakup, and the generation of the Tasman Sea. The Bathurst Granite, that is 100 km inland, has been dated to 95 +/- 10 Ma (Lackie & Smith, 1996). They have dated the Snowy Mountains to 90 +/- 10 Ma. South of Sydney, the coast has been dated to 80 to 90 Ma, which has been interpreted as being heated to temperatures above 100o C followed by cooling. They concluded that the on the Lachlan Fold Belt the eastern flank was heated and later, 95 +/- 10 Ma, lowered by more than 2 km by subsequent erosion during the development of a passive-margin mountain chain as the Lord Howe Rise and New Zealand broke away from Australia.
Ages 0f 100-80 Ma have been found further south in Victoria and Tasmania (Gleadow et al., 1996). A major episode of accelerated erosion in the Snowy Mountains has been suggested for 110-100 Ma (Kohn et al,. 1999). In the Bogong High Plains of Victoria an episode of fault activation occurred at 110-90 Ma (O'Sullivan et al., 1999). The same "critical moment", at 95 Ma, has been found for the Baralaba petroleum system in the central Bowen Basin and the northern Surat Basin (Korsch et al., 1996), that was found in the Eromanga Basin (Lowe-Young et al., 1996).
At 99 Ma the southern margin arose from a wide rift complex of Early Cretaceous age, as Australia broke away from Antarctica. It has been dated by the breakup unconformity in the Otway Basin to within the Phimopollenites pannosus palynozone of the latest Albian (Veevers, 1984). In the Ceduna depocentre, a rim basin 8 km thick had accumulated by 70 Ma that included a large volume of sediment from the Cenomanian (Veevers, 1984, p.224).
In the Late Permian to end-Jurassic, 260-144 Ma, a magmatic arc in New Caledonia was followed at 95 Ma by deformation and bimodal volcanics. Seafloor generated prior to the onset of regular seafloor spreading at 83 Ma (A34) in the Tasman Sea is indicated by ophiolites from 100-80 Ma (as in New Zealand) (Symonds et al., 1996).
Passive-margin mountains on the upper plate margin, that are buoyed up by underplating melts derived from the mantle, are separated by a detachment fault along the eastern Australian continent-ocean boundary (COB), from the Lord Howe Rise, as the conjugate highly extended lower plate margin. The process is driven by left-lateral shear from the Pacific (Lister & Etheridge, 1989), analogous with the Pacific borderland of North America (Jones & Veevers, 1983). It is thought that probably at about the position of the present coast, the drainage was divided into a long slope, in a southwestern direction, to the Ceduna depocentre, where 8 km of sediment accumulated in the Late Cretaceous, and with the slope to the east being short, to the low-lying areas of the eastern borderlands.
Wells in the Otway Basin, on the southeastern margin, register events associated with the southern margin, subsidence associated with rifting onshore, and offshore, subsidence associated with drift (Hegarty et al., 1988). The age of 99 Ma, Albian-Cenomanian age, for the "Top Strzelecki Unconformity", has been confirmed by Lowry & Longley (1991). In the Middle Eocene, right lateral movement in basement rocks began the growth en echelon folds, during a lacuna in Southeast Australia (Davidson, 1980). Seafloor spreading between Australia and the Lord Howe Rise-New Zealand off the eastern margin, continued from 83-52 Ma (Gaina et al., 1998).
In Marie Byrd Land and New Zealand, a convergent magmatic arc was followed from 99-95 Ma, by rift magmatism, also found in New Caledonia and New Guinea, with bimodal volcanics in the Early Cretaceous, and the Mt Victor granodiorite and basalt, at 97-91 Ma. The Whitsunday Volcanic Province, of bimodal within-plate magma, extended to the south, to the east of the Gippsland Basin, 120-105 Ma, where it provided a source for the coeval sedimentation that occurred between 130-105 Ma, in the Gippsland-Strzelecki Basins and the Otway Basin (Bryan et al., 1997). The within-plate magmatism of the Early Cretaceous tailed off at about 105 Ma. At 99 Ma, magmatism related to rifting that was occurring over wide areas, comprising silicic volcanics near Thurston Island, that have been dated to 105-90 Ma and 103-95 Ma in Marie Byrd Land, A-type granite (Weaver et al., 1994). Also at 6 sites in New Zealand, and on the Lord Howe Rise, a rhyolite. And also near the incipient continent-ocean boundary along the east coast of Tasmania and New South Wales, a group of 5.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|