The pre-Carboniferous geology of Bass Strait and surrounds
2017-03-03T00:42:20Z (GMT) by
This thesis examines the Proterozoic and early Paleozoic connections between Victoria and Tasmania. Many different models have been suggested, and I conclude here that the Selwyn Block model is the most appropriate as it satisfies both the geological and geophysical data. The model proposed that the Proterozoic cratonic crust of western Tasmania continues north under Bass Strait and lies unconformably below the Melbourne Zone in central Victoria. The Selwyn Block is the northern end of the Proterozoic micro-continent, VanDieland, which also includes western Tasmania, the west South Tasman Rise and the East Tasman Plateau. In order to be able to extrapolate the major rock packages in Tasmania across Bass Strait, they first had to be determined in Tasmania. Seven Proterozoic zones were outlined—King Island, Rocky Cape, Burnie, Pedder, Tyennan, Sorell-Badger Head, and Glomar, with an eighth, Eastern Tasmania equivalent to the Paleozoic Tabberabbera Zone in eastern Victoria. Only the first three Proterozoic zones continue across Bass Strait, with the other four truncated either in Bass Strait or lying further south. Outcrops of rocks from the King Island and Burnie Zones are present in windows in Victoria but the Rocky Cape Zone is completely concealed. However, the presence in the mid-crust of the Rocky Cape and King Island zones can be seen in the enclaves and in the geochemical signatures of the Upper Devonian granites of central Victoria and in rare conglomerate clasts. VanDieland was initiated inside Nuna, between Laurentia and East Antarctica, at about 1.8 Ga. Much of the sedimentation seen in the Rocky Cape Zone is the erosional products of the Grenville Orogeny. As Rodinia broke up, VanDieland began to be extended at about 760 Ma, and this continued until final separation from Antarctica at about 570 Ma. After this, it drifted ‘north’ as micro-continental slivers along the Terra Australis margin until about 530 Ma. It then re-amalgamated in a closing back arc system within the greater Ross-Delamerian Orogeny, although it did not accrete onto Gondwana, remaining perhaps 200 to 300 km outboard. In the Early Devonian, as VanDieland got closer to Gondwana, a Banda Sea-style subduction system retreated southwards outboard of its eastern margin. This accreted VanDieland into Gondwana and gave rise to a crude, clockwise age distribution of the granites in Tasmania, from approximately 400 Ma on Flinders Island to 350 Ma on King Island.