Discussion and Conclusions-Evolution of the Tasmanian Gateway | Table of Contents

An east-west rift along the southern margin of Australia and the northern margin of Antarctica and slow northwest-southeast separation may have begun as early as the Late Jurassic (Willcox and Stagg, 1990). This rift continued eastward in the Otway, Bass, and Gippsland Basins between Australia and Tasmania. Crustal thinning and subsidence occurred along the developing east-west rift, which was filled with thousands of meters of nonmarine Lower Cretaceous sediments. From the mid-Cretaceous (95 Ma) to the middle Eocene (43 Ma), Australia moved northwest relative to Antarctica at a slow rate of ~10 m/m.y. As the east-west rift widened into the Australo-Antarctic Gulf, the sea transgressed from the west depositing shallow-marine and deltaic Late Cretaceous to Eocene sediments. In the Late Cretaceous (75 Ma), the Tasman Sea began to form and rifting in the Bass Strait failed.

In the Tasmanian region, the TASZ terminated the east-west rift. The block east of the TASZ, consisting of southeast Australia, Lord Howe Rise, and Campbell Plateau, moved northwestward along it. Some crustal thinning occurred along the TASZ, with possible deposition of Early Cretaceous-age sediments. By the Late Cretaceous, the TASZ was subsiding steadily, marine transgression took place through the Gulf from the west, and shallow-marine deltaic sediments began to be deposited in the extreme eastern part of the Australo-Antarctic Gulf. This deposition continued into the Eocene.

In the Late Cretaceous (75 Ma), the Lord Howe Rise, Campbell Plateau, and the ETP all began to move away from Australia and Antarctica to the east-northeast (relative to Australia). At 65 Ma, in the latest Cretaceous, the ETP (with Site 1172) stopped moving with the other blocks, but a legacy of rifting before that movement was its thinned crust and the partially oceanic depression between the ETP and what became the STR. Site 1171 showed that strike-slip movement within the central and eastern STR (e.g., on the Balleny Fracture Zone) occurred during a short period in the late Paleocene, when spreading started south of the STR. This agrees with the plate tectonic reconstruction of Cande et al. (2000), which shows the east-west spreading extending from south of the STR and north of the Transantarctic Mountains (TAM) across to New Zealand from the late Paleocene. This is the period when uplift of the TAM has been considered to have begun (Fitzgerald, 1992). However, evidence from the 1999 Cape Roberts drilling, in the western Ross Sea, indicates that Victoria Land Basin first formed by ~3000 m of rapid subsidence east of the TAM at ~34 Ma, close to the Eocene/Oligocene boundary (Cape Roberts Science Team, 2000), suggesting that much of the uplift of TAM may have been synchronous.

This tectonic scenario for Antarctica seems to have been reflected in the structurally interconnected Tasmanian region. If late Paleocene uplift of the TAM did occur, this would have corresponded to the period of strike-slip movement, and associated vertical displacement, on the central and eastern STR. The end of this movement corresponds to the onset of spreading between the TAM and the STR, which separated the strike-slip faults within most of the STR from tectonic movement in Antarctica. However, the early Oligocene collapse of the continental margins everywhere in the Tasmanian region, documented in part by Leg 189 drilling, coincides precisely with the age of formation of the Victoria Land Basin on the conjugate Antarctic margin (~34 Ma). It would seem that there should be a causal relationship, but determining that relationship awaits postcruise research.

The Site 1168 region was part of the Tasmanian block throughout its Cretaceous and Cenozoic history and, within the depocenter caused by crustal thinning along the TASZ, the Sorell Basin. The Sorell Basin was fully separated from the conjugate eastern end of the Wilkes Land Basin in Antarctica once large quantities of oceanic crust began to form in the easternmost Australo Antarctic Gulf during the middle Eocene (43 Ma). In regard to the oldest seafloor spreading, Pyle et al. (1995) Ar-Ar dated the basalt at the base of DSDP Site 282 on nearby thinned continental crust at 60.7 Ma, but doubts remain about this date because of the extent of alteration of the basalt. Royer and Rollet (1997) suggest the presence of some oceanic crust as old as Chron 24 (55 Ma). Like the basins to the north, the Sorell Basin subsided throughout the Cretaceous and Cenozoic. This basin was filled with thick shallow-marine Upper Cretaceous to Eocene deltaic sediments derived largely from Tasmania. Strike-slip faulting, active into the Paleocene, cut the basin into a number of subbasins and ridges, between a pair of which Site 1168 is located. In the earliest Oligocene, the Sorell Basin subsided rapidly into deep water and the deposition of pelagic carbonate commenced, as also occurred to the northwest in the Otway Basin. In both cases, the subsidence appears to have been about hinge lines near the present coastlines. The cause of the initial rapid subsidence remains unclear, but it was a regional event. It may have resulted from the final clearance of southeast Australia (the southwestern tip of the STR) from Antarctica (Wilkes Land) near the Eocene/Oligocene boundary (Fig. 6). Site 1168 contains a remarkably continuous Eocene to Holocene sequence, but with a clear transition from siliciclastic deposition in the Eocene to pelagic carbonate deposition in the early Oligocene as the margin sank rapidly.

According to Royer and Rollet (1997), Site 1170 was part of the Antarctic block (Wilkes Land) in the early Late Cretaceous, separated from Southeast Australia by the TASZ. After collision with the central STR block in the latest Cretaceous (66 Ma), the western STR block formed, on which Site 1170 is located. This block began to separate from Antarctica by strike-slip motion, with the TASZ transferring to its western side, and by the latest Paleocene, it was firmly welded to Southeast Australia. During the middle Eocene (43 Ma), the onset of fast spreading in the Australo-Antarctic Gulf increased the rate of movement along the TASZ to ~30 m/m.y. At the same time, fast spreading started south of the STR. Much of this block is covered by the Ninene Basin, which is cut by Cenozoic strike-slip faults into subbasins and ridges. The basin is filled by fairly thick Upper Cretaceous to Eocene siliciclastic sediments that are prograded and of shallow marine facies, at least in part. This western block was substantially thinned by tectonism and now lies much lower than the adjacent central block of the STR. Site 1170 is located only 10 km from the eastern side of the Ninene Basin, where the basin abuts against the north-south fault (and modern fault scarp) that marks the suture with the central block.

During the earliest Oligocene, Site 1170 and the Ninene Basin subsided rapidly into deep water, as at Site 1168. The initial fast subsidence is a regional event involving Southeast Australia and the Victoria Land Basin in Antarctica, at least. The final clearance of the western block from Antarctica near the Eocene/Oligocene boundary probably had an effect at Site 1170. The unusually thin upper Oligocene sequence may have been caused by increased current erosion caused by shallowing as the spreading axis, with its heat source, passed 100 km to the west (~26 Ma). Site 1170 contains a less complete Upper Eocene to Holocene sequence than Site 1168 because of generally greater current activity caused by its greater proximity to the opening of the Tasmanian Gateway between the STR and Antarctica and the associated Antarctic Circumpolar Current. The transition from Eocene siliciclastic sediments to pelagic carbonate, as the basin sank rapidly during the earliest Oligocene, was much sharper than at Site 1168. This more transitional change at Site 1168 was probably caused by the ongoing supply of siliciclastic sediments from the high hinterland of Tasmania.

During the Early Cretaceous, the southernmost Site 1171 was located on the east Gondwana block consisting of Australia, the Lord Howe Rise, and the Campbell Plateau across the strike-slip TASZ from Antarctica. Some Early Cretaceous-age sediments must have been deposited along the TASZ, but there is no evidence of them near the site. According to Royer and Rollet (1997), the central and eastern blocks of the STR separated from the Tasmanian block in the early Late Cretaceous (95-83 Ma). Site 1171 is located near the boundary between the two blocks, which jostled as they moved. As Australia moved northwest, the two blocks were "braked" by Antarctica until the latest Cretaceous (65.6 Ma), with a total stretching of 150 km between this and the Tasmanian block forming the South Tasman Saddle. Strike-slip basins and ridges developed within the two STR blocks, but there was only limited thinning of these cratonic blocks. Seismic profiles suggest that the basins were filled by prograded Upper Cretaceous and Paleocene sediments (probably shallow marine) derived from the subaerially exposed ridges and along the basins from higher areas to the north. In the Late Cretaceous, at 75 Ma, seafloor spreading started to the east with the Campbell Plateau and the ETP separating from the composite block and moving away east-northeast relative to Site 1171. The continental crust of the blocks was thick, so subsidence was limited.

The initiation of seafloor spreading to the south is open to interpretation, as profiles with magnetic anomalies are limited and the older anomalies are close together and questionable. Royer and Rollet (1997) reliably identify Anomaly 20 (45 Ma), questionably identify Anomaly 24 (55 Ma), and very doubtfully identify Anomaly 31 (67 Ma). Fast spreading clearly began in the middle Eocene (43 Ma). Pyle et al. (1995) Ar-Ar dated the basalt at the base of DSDP Site 280 on nearby oceanic crust at 64.2 Ma, but because of its alteration, doubts remain about this date. Further information on the breakup comes from Site 1171 itself, which lies in a small north-south basin bounded to the east by an arm of the Balleny Fracture Zone, the boundary between the central and eastern STR blocks. From this site, seismic and palynological information shows an unconformity between very broadly and gently folded sediments of Cretaceous to late Paleocene (58 Ma) age and onlapping and flat-lying sediments of early Eocene (54 Ma) age. This shows that much of the movement within continental crust along the Balleny Fracture Zone (which has been active in oceanic crust ever since) was confined to the latest Paleocene (~55 Ma). The end of this strike-slip faulting and associated folding agrees with the Anomaly 21 identification above, confirming that final separation south of the STR (from Wilkes Land) was in the late Paleocene.

During the earliest Oligocene, Site 1171 and the central STR block subsided into deep water, similar to Sites 1168 and 1170. The amount of subsidence was less than at those sites because of the greater crustal thickness. The likely cause of at least part of the early Oligocene subsidence (contemporaneously with that of the margins elsewhere in Southeast Australia and the Victoria Land Basin) is again the final clearance of the attached western STR block from Antarctica. Site 1171 contains a much thinner and less complete upper Eocene and Oligocene sequence than Sites 1168 and 1170 because of its immediate proximity to the opening of the Tasmanian Gateway between the STR and Antarctica and its exposure to the Antarctic Circumpolar Current thereafter. The transition from Eocene siliciclastic sediments to pelagic carbonate, as the basin sank rapidly in the earliest Oligocene, was more rapid than in the less current-swept and expanded sequences at Site 1168.

During the Early Cretaceous, the eastern Site 1172 was in the east Gondwana block consisting of Australia, the Lord Howe Rise, and the Campbell Plateau well away from the TASZ. Seismic data suggest that dipping and faulted Early Cretaceous-age sediments exist at depth. It is probable that a highly reflective sequence above these sediments consists of Late Cretaceous breakup volcanics, perhaps including the undated rhyolite dredged from the eastern scarp of the ETP (Exon et al., 1997a). According to Royer and Rollet (1997), breakup began to form the ETP during the Late Cretaceous (75-65 Ma), both to the plateau's west and east. The plateau moved east-northeast relative to Tasmania and the STR, initially with the continental block of the Lord Howe Rise and Campbell Plateau. However, this block soon separated, leaving the Tasman Sea oceanic crust to the east of the ETP. In the west, some oceanic crust formed between the ETP and the STR, forming L'Atalante Depression, and crustal thinning occurred between the ETP and Tasmania, eventually forming the East Tasman Saddle.

Although the ETP formed in the Late Cretaceous and might well have had a very different subsidence history from the other Tasmanian blocks drilled during Leg 189, this proved not to be the case, with a close similarity between the two Pacific Ocean Sites 1172 and 1171. At both sites, Paleocene and Eocene sediments are similar shallow-marine (neritic) siliciclastics, and at both sites there was rapid subsidence and a switch to deposition of pelagic carbonate beginning in the earliest Oligocene. The plateau had been planated by erosion and deposition during the latest Cretaceous and Paleogene, with siliciclastic sediment coming from the marginal granitic highs and perhaps from Tasmania, if the East Tasman Saddle subsided late. An additional feature of the ETP was the formation of a large hot spot volcano in its center during the late Eocene. According to Quilty (1997), calcareous microfossils show that the upper part of this 1500-m-high guyot, the Cascade Seamount, was in shallow-marine depths during the late Eocene. This clearly conflicts with the evidence of upper bathyal deposition of latest Eocene-age sediments at Site 1172 below the base of the seamount. An explanation awaits postcruise investigation.

A tabulation of vital information from ODP Leg 189 and DSDP Leg 29 sites (Kennett and Houtz, 1975) summarizes much of the above (Table 4). It shows that three breakup ages (75, 55, and 34 Ma) affected the various sites in various ways. The first breakup was at 75 Ma for the central and eastern Sites 1171, 281, 1172, and 283 and at 55 Ma for the western and southern Sites 282, 1168, and 280. The last breakup was at 34 Ma on the STR for Sites 1170, 281, and 1171. The subsidence since the Eocene is large on oceanic crust or thinned continental crust (Sites 282, 1168, 1170, 280, and 283) at 3100-3800 m. It is less on the thick continental crust of the central STR and the ETP (Sites 281, 1171, and 1172) at 1600-2500 m. The transition from siliciclastic to biogenic sedimentation is Eocene/Oligocene at most Leg 189 sites in basinal settings in shallow Eocene water depths, but it is Oligocene/Miocene in several Leg 29 sites on local highs in deeper Eocene water depths. This variation is probably caused in part by the position of the CCD relative to the sites during the Eocene to Miocene.