Journal of the Geological Society, London, Vol. 155, 1998, pp. 525–539. Printed in Great Britain Formation and reactivation of the Cambrian Kanmantoo Trough, SE Australia: implications for early Palaeozoic tectonics at eastern Gondwana’s plate margin T. FLÖTTMANN1, P. HAINES2, J. JAGO2, P. JAMES1, A. BELPERIO3,4 &J.GUM2 1Department of Geology and Geophysics, University of Adelaide, SA 5005, Australia (e-mail: tfl[email protected]) 2Department of Applied Geology, University of South Australia, The Levels, SA 5095, Australia 3Mines and Energy, South Australia, 191 Greenhill Road, Parkside, SA 5063, Australia 4Present address: Minotaur Gold, 1a Gladstone Street, Fullarton, SA 5063, Australia Abstract: The Kanmantoo Group is a thick and largely metamorphosed sedimentary succession that filled an isolated arcuate Cambrian basin (Kanmantoo Trough) which formed within continental Gondwana, and now lies on the southern margin of the present Australian continent. Kanmantoo Group sediments unconformably overlie Neoproterozoic and older Cambrian rocks. We consider that the geometry of the southern part of this trough was influenced by strike-slip movement along an intra-continental tear fault. To the north, the basin changes to a style dominated by orthogonal extension and eventually tapers and dies out normal to the tear fault. Balanced sections show that the kinematic style and strain distribution developed during early Palaeozoic inversion was controlled by the specific architecture of the Kanmantoo Trough. Early Palaeozoic tear faulting could have linked contrasting subduction polarities along the then contiguous palaeo-Pacific margin of Gondwana. The Kanmantoo Trough is considered to have formed at a passive margin related to east-directed subduction in what is now the Australian continent. In contrast, west-directed subduction formed an active margin at contiguous parts of current Antarctica. Kanmantoo Group sediments were derived from the south by erosion of a Grenvillean province mixed with sediments eroded from the emergent active margin of Gondwana. The inception, localization and sedimentation in the Kanmantoo Trough reflects a complex interaction of tectonic processes along the encroaching Ross–Delamerian Orogen. Keywords: Gondwana, Palaeozoic, sedimentary basins, tectonics, orogeny. The early Phanerozoic marks a period of fundamental re- southern Adelaide Fold–Thrust Belt, Neoproterozoic strata organization of the tectonic regime along the palaeo-Pacific are succeeded unconformably by platformal Early Cambrian margin of Gondwana. This margin records the shift from deposits of the Normanville Group. The latter is, in turn, Neoproterozoic to Cambrian upper plate extension and crustal overlain unconformably by a thick (c. 8 km) sequence of attenuation, to early Palaeozoic contraction and crustal thick- rapidly deposited, mostly turbiditic greywackes of the ening (Moores 1991; Dalziel 1991). Convergent tectonism is Kanmantoo Group. The combined succession was incorpor- related to subduction followed by accretion of arc/trench ated into the Delamerian Orogen prior to c. 510 Ma (Chen & systems during the Delamerian Orogeny in southeastern Liu 1996). Australia and the Ross Orogeny in Antarctica (Flöttmann The origin of the accommodation space for the Kanmantoo et al. 1993). The resulting orogenic zones (Delamerian and Group (referred to as the ‘Kanmantoo Trough’; a term which Ross Orogens) are believed to have been continuous prior to we retain here in a non-genetic sense), remains controversial, the Cretaceous–Tertiary separation of the Australian and but this issue is fundamental to the full understanding of Antarctic plates (Grindley & Davey 1982; Stump et al. 1986; the tectonic processes along the palaeo-Pacific margin of Findlay 1987; Preiss 1987; Flöttmann & Oliver 1994). The Gondwana. Although Clarke & Powell (1989) have suggested transition from an extensional to a convergent regime is best that the Kanmantoo Group is allochthonous, there are a exemplified in southeastern Australia, where a record from the number of locations where the base of the group is unequivo- Archaean–Palaeoproterozoic craton, through Neoproterozoic cally sedimentary, and thus it is clearly not a tectonically and Cambrian sedimentation, to subsequent orogenesis is accreted terrane. More recently Jago et al. (1994) have preserved in the region of the southern Adelaide Fold–Thrust interpreted this contact as a Type 1 sequence boundary. Belt, constituting the foreland portion of the Cambro- Daily et al. (1973) and von der Borch (1980) suggested that Ordovician Delamerian Orogen. To the east of the Delamerian the Kanmantoo Trough formed during rifting at the south- Orogen lies the Lachlan Fold Belt, which contains an extensive eastern margin of the Australian craton. Conversely, Turner Palaeozoic turbidite succession. et al. (1994) suggested that the Kanmantoo Group might have Deposition of Neoproterozoic sediments, in what is now the been deposited in a foreland basin related to an orogen Adelaide Fold–Thrust Belt, reflects repeated rift and sag encroaching from the east, as was previously speculated by phases which developed during and subsequent to the breakup Jenkins (1990), Coney et al. (1990) and Mancktelow (1990). of Rodinia and the formation of the formerly conjugate It has been speculated that the eastern margin of the continental margins of eastern Gondwana and Laurentia Kanmantoo Group is exposed as the Glenelg River Complex (Hoffman 1991; Moores 1991; Dalziel 1991, 1995). In the in western Victoria (VandenBerg 1978; Fig. 1). It remains 525 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/155/3/525/4886864/gsjgs.155.3.0525.pdf by guest on 28 September 2021 526 T. FLÖTTMANN ET AL. a90) bend to an essentially westerly trend, cross-cutting the grain of underlying Neoproterozoic to Palaeoproterozoic rocks (Fig. 1). Some workers have attempted to explain this bend of the basin margin by underlying wrench zones of various orientations and kinematics (for summary see Marshak & Flöttmann 1996). There is, however, no conclusive kinematic evidence for scenarios involving substantial wrench- ing. Other workers suggest that the strongly arcuate geometry simply reflects the shape of the original Kanmantoo Trough margin (e.g. Daily et al. 1973; Mancktelow 1990). Age spectra of detrital zircons from the Kanmantoo Group appear to be distinctly different from those of the exposed Archaean–Proterozoic basement, as well as those of under- lying Neoproterozoic and Early Cambrian sediments (Ireland et al. 1995, 1998; Foden 1996). There are significant simi- larities, however, with the detrital zircon age signature of the younger Lachlan Fold Belt sediments (Ireland et al. 1995, 1998). The formation of the Kanmantoo Trough and the nature and origin of its sedimentary fill poses several questions regarding the tectonics at the palaeo-Pacific margin during the early Phanerozoic. It appears unclear how the timing of sedimentation and the development of the Kanmantoo Trough fit into the known tectonic framework at this margin. There are no explanations as to why this succession was deposited in a seemingly confined accommodation space, and what con- trolled its localization and geometry. The controversies out- lined above highlight fundamental uncertainties surrounding the shift from extensional to contractional tectonics at the palaeo-Pacific margin, which in turn impinges on global tectonic models (Moores 1991; Dalziel 1991, 1995). Here we first summarize the stratigraphy and facies of the Kanmantoo Group and present palaeocurrent data which provides new insights into the source and dispersal of the sediments. Secondly, based on a structural study that inte- grates newly acquired seismic and aeromagnetic data we attempt to elucidate the pre-deformational geometry of the Kanmantoo Trough. Thirdly, we review the tectonics of the palaeo-Pacific margin of Antarctica. Finally, we present an integrated model for the formation of the Kanmantoo Trough which may provide a key to understanding the Fig 1. Main elements of the Delamerian Orogen in southeast early Phanerozoic tectonics at the palaeo-Pacific margin of Australia. Inset box shows location of Fig 4. solid line shows seismic Gondwana. survey of Fig. 8. Dashed line shows northeastern margin of Throughout the paper the use of time terms (e.g. Early, kanmantoo Trough based on compilation of aeromagnetic surveys Mid-Cambrian etc.) follows the latest Australian Phanerozoic (MESA 1996). Padthaway Ridge shows approximate line of outcrop time scale (Young & Laurie 1996). and subcrop of A-type granites. SVC, Mt Stavely Volcanic Complex; LFB, Lachlan Fold Belt. equivocal whether the Kanmantoo Group continues into the Geological setting, southeastern Australia Ross Orogen in Antarctica. In the Wilson Terrane of northern The Delamerian Orogen of southeastern Australia consists of Victoria Land, subduction related magmatism and regional four distinct domains. These are, from west to east, the metamorphism appears to have been ongoing during the Adelaide Fold–Thrust Belt, the Padthaway Ridge (largely period of deposition of the Kanmantoo Group in Australia concealed beneath the Cainozoic Murray Basin), the Glenelg (Black & Sheraton 1990; Adams 1986). Baillie (1983) has River Complex and the Mt Stavely Volcanic Complex (Fig. 1). speculated on the existence of an intra-continental fault system The Adelaide Fold–Thrust Belt is distinctly S-shaped. The which later roughly marked the line of the Cretaceous–Tertiary northern