Refining accretionary orogen models for the Tasmanides of eastern Australia

The well-known southwest-to-northeast younging of stratigraphy over a present-day cross strike distance of >1500km in the southern Tasmanides of eastern Australia has been used to argue for models of accretionary orogenesis behind a continually eastwards-rolling paleo-Pacific plate. However, these accretionary models need modification, since the oldest (ca 530 Ma) outcrops of Cambrian supra-subduction zone rocks occur in the outboard New England Orogen, now approximate to 900km east of the next oldest (520510 Ma) supra-subduction zone rocks. This is not consistent with simple, continuous easterly rollback. Instead, the southern Tasmanides contain an early history characterised by a westwards-migrating margin between ca 530 and ca 520 Ma, followed by rapid eastwards rollback of the paleo-Pacific plate from 520 to 502 Ma that opened a vast backarc basin approximate to 2000km across that has never been closed. From the Ordovician through to the end of the Carboniferous, the almost vertical stacking of continental margin arcs (within a hundred kilometres of each other) in the New England Orogen indicates a constant west-dipping plate boundary in a Gondwana reference frame. Although the actual position of the boundary is inferred to have undergone contraction-related advances and extension-related retreats, these movements are estimated to be approximate to 250km or less. Rollback in the early Permian was never completely reversed, so that late PermianTriassic to Cretaceous arcs lie farther east, in the very eastern part of eastern Australia, with rifted fragments occurring in the Lord Howe Rise and in New Zealand. The northern Tasmanides are even more anomalous, since they missed out on the middle Cambrian plate boundary retreat seen in the south. As a result, their Cambrian-to-Devonian history is concentrated in a approximate to 300km wide strip immediately west of Precambrian cratonic Australia and above Precambrian basement. The presence in this narrow region of Ordovician to Carboniferous continental margin arcs and backarc basins also implies a virtually stationary plate boundary in a Gondwana frame of reference. This bipolar character of the Tasmanides suggests the presence of a segmented paleo-Pacific Plate, with major transform faults propagating into the Tasmanides as tear faults that were favourably oriented for the formation of local supra-subduction zone systems and for subsequent intraplate northsouth shortening. In this interpretation of the Tasmanides, LowerMiddle Ordovician quartz-rich turbidites accumulated as submarine fan sequences, and do not represent multiple subduction complexes developed above subduction zones lying behind the plate boundary. Indeed, the Tasmanides are characterised by the general absence of material accreted from the paleo-Pacific plate and by the dominance of craton-derived, recycled sedimentary rocks. ?????Tasmanides??????1500?????????-???????????????,?????????????????????????????????,???????????,????????????????(?530Ma)????????????,????????(520-510Ma)?????????900???,??????????????Tasmatides??????????????530?520Ma??,??????,???520?502Ma??,????????????,???2000???????????,???????????????????,??????????????????(??????????)??????????????????????????????????????????????????,??????????250??????????????????????,?????-?????????????????,??????????;???????Lord Howe ???????Tasmanides?????,??????????????????????,???????????????????????????,????????????300????????????????????????????????????????????????????????Tasmanides??????????????????????,??????????Tasmanides???????,?????????????????????????-??????Tasmanides???????,?-???????????????????,??????????????????????????????,Tasmanides?????,?????????????,?????????????????????