Refining accretionary orogen models for the Tasmanides of eastern Australia

Abstract

The well-known southwest-to-northeast younging of stratigraphy over a present-day cross strike distance of >1500 km 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 ∼900 km east of the next oldest (520–510 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 ∼2000 km 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 ∼250 km or less. Rollback in the early Permian was never completely reversed, so that late Permian–Triassic 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 ∼300 km 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 north–south shortening. In this interpretation of the Tasmanides, Lower–Middle 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.

澳洲东部的Tasmanides南部走向距离1500多公里的地层在西南-东北方向上渐新。这一点广为人知，并用来支持这样的模式：加积造山运动导致古太平洋板块持续向东隆起。然而，这些加积模式还需要改进，因为最老的寒武纪超俯冲带岩石露头（约530Ma）出现于新英格兰造山带外侧，现在位于第二最老（520-510Ma）超俯冲带岩石之东约900公里处，这与简单的持续东向后隆不符合。Tasmatides南部具有这样一个早期历史：在约530和520Ma期间，边缘向西移动，接着在520至502Ma期间，古太平洋板块快速向东后隆，形成约2000公里宽的巨大的后弧盆地，至今从未闭合过。自奥陶纪直到石炭纪末期，新英格兰造山带的几乎垂直的大陆边缘弧（彼此相距不超过一百公里）标志着冈瓦纳范围之内的持续西倾的板块边界的存在。虽然边界的实际位置经历了收缩相关的前移和扩张相关的后退，这些位移距离估计为约250公里或更少。二叠纪早期的后隆从未复原过。因此，二叠纪晚期-三叠纪至白垩纪的岛弧位于向东更远处，澳大利亚东部的最东部；开裂碎块出现于Lord Howe 抬升和新西兰。Tasmanides北部更不寻常，那里缺乏出现于南部的中寒武世板块边界后退。结果，那里的寒武纪至泥盆纪历史集中于前寒武纪克拉通澳大利亚之东，前寒武纪基底之上的一片约300公里宽的窄地带。这个奥陶纪至石炭纪大陆边缘弧和后弧盆地的窄地带的存在还暗示着冈瓦纳参照框架内的完全稳定的板块边界。Tasmanides的这种两极性特征说明分段的古太平洋板块的存在，主要转换断层扩散进入Tasmanides而成为横推断层，其定向有助于当地超俯冲带系统的形成和其后的板块间南-北向缩短。对Tasmanides的解释是这样的，下-中奥陶统富石英浊积岩作为水下扇序列而堆积，并不代表形成于板块边界之后的俯冲带之上的多俯冲复合体。确凿而言，Tasmanides的特征包括，缺失来自古太平洋板块的物质，以及以来源于克拉通的再沉积的沉积岩占优势。

KEY WORDS:

• refining
• accretionary
• orogen
• models
• Tasmanides
• eastern Australia

ACKNOWLEDGEMENTS

Thanks to Cam Quinn for many long discussions about the Tasmanides. Many thanks to those Geological Survey staff, as well as those from other institutions, who have helped me wrestle with the Tasmanides over the years. Thanks to the Geospatial Group, especially Cheryl Hormann and Gary Colquhoun, for production of figures. In a wide-ranging paper such as this, I apologise in advance if I have omitted any key references. I thank Cam Quinn for reading an early version, and John Greenfield and Ian Percival for comments on a later manuscript. Reviews by John Dewey and Russell Korsch helped improve the final paper immeasurably. Russell's suggestions for more detailed descriptions and primary references were especially helpful, even if they made the paper a bit longer. This paper is published with permission of the Acting Director, Geological Survey of New South Wales, Division of Resources and Energy, NSW Department of Trade and Investment. This is a contribution to IGCP project 592.