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
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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 part of this belt is formed exclusively of folded fragmentation of Gondwana into the Australian and Antarctic Neoproterozoic strata of the Nackara Arc defined by the continents (Veevers & Li 1991). change in structural grain from westerly in the north, to The plan-view geometry of the exposed deformational northerly in the south. Contractional deformation appears to remnant of the Kanmantoo Trough has sparked repeated have been basement-detached along evaporite horizons low in controversy. At its northern end, the exposed Kanmantoo the Neoproterozoic section (Callanna Group: Preiss 1987; Trough terminates in a north-trending syncline. To the south, Marshak & Flöttmann 1996). The evaporitic succession is the western margin of the Kanmantoo Trough sweeps through absent further south where the S-shaped belt comprises
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Neoproterozoic rocks overlain by Cambrian strata of the suggested that this suite of rocks may be related to an Normanville and Kanmantoo Groups. This part of the belt is east-dipping, intra-oceanic subduction zone. Shortening dur- characterized by emergent basement-involved thrusting. A belt ing the Delamerian Orogeny is presumably related to an of Buchan-style metamorphism has formed around syn- arc–continent collision which also led to the west-directed kinematic Delamerian granitoids in the central parts of this obduction of ophiolites in the Glenelg River Complex. area (Dymoke & Sandiford 1992). Postorogenic andesites and dacites have been dated at Neoproterozoic and earliest Cambrian rocks record a pro- 501&9Mato495&5 Ma (Crawford et al. 1996). The bound- tracted period of non-marine, marine and glacigenic depo- ary with the younger Lachlan Fold Belt is now formed by the sition beginning at about 800 Ma (Preiss 1987, 1993). The Moyston fault zone (Ross Cayley, Melbourne, pers. comm. maximum age of the Kanmantoo Group is constrained by 1997; Gray et al. 1997). It is not known, however, whether the 526&4 Ma U–Pb zircon date from a tuff near the top of rocks affected by the Delamerian Orogeny extend beneath the the underlying Normanville Group (Cooper et al. 1992). Lachlan Fold Belt. Sandiford et al. (1992) suggest that the oldest syn-kinematic granites intruded the Kanmantoo Group at around 516 Ma, while Chen & Liu (1996) have dated post-kinematic dykes at c. 510 Ma. The Kanmantoo Group was thus deposited very Stratigraphic and sedimentological record of the rapidly, probably in less than 10 million years (for further Kanmantoo Trough discussion see Haines & Flöttmann 1998). The Kanmantoo Group was defined by Sprigg & Campana At the type section along the southern coast of Fleurieu (1953), who nominated a type section along the southern coast Peninsula (Fig. 1), the exposed Kanmantoo Group is estimated of Fleurieu Peninsula. The currently used stratigraphic nomen- to be of the order of c. 8 km thick (Haines et al. 1996). The clature is essentially that of Daily & Milnes (1971, 1973), who northern limit of preservation is in the axial region of the recognized eight formations at this locality. A reduced section north-trending Karinya Syncline (Fig. 1). In this area only is present to the north in the Karinya Syncline. The stratigra- the lower Kanmantoo Group is present and individual units phy of these areas, including estimated average thicknesses for show a notable thinning to the north, the entire group being the units, is presented in Fig. 2. only about 2 km or less in thickness. The original northern Although the Kanmantoo Group is often in fault contact limit of the Kanmantoo Group is poorly constrained, but if with older units, unequivocal sedimentary contacts are present it extended much beyond the current outcrop limit, these in a number of areas. In all such localities the base of the group deposits were probably relatively thin. Greater sediment is marked by the Carrickalinga Head Formation, which gen- thicknesses than those developed in the Karinya Syncline erally overlies the Heatherdale Shale of the Early Cambrian cannot be accommodated in the tight keel of that fold. Normanville Group. Jago et al. (1994) have interpreted this Seismic and gravity surveys image a steeply dipping, contact as a Type 1 sequence boundary. In places the contact west-trending northern Kanmantoo Trough margin on displays significant erosional relief and rare conglomerates Kangaroo Island (Belperio & Flint 1993; Flöttmann et al. may be found overlying it. Locally, on the eastern side of the 1995). In this area, the southern margin of the Kanmantoo Karinya Syncline, the contact appears to be gradational and Trough is concealed by the Southern Ocean. However, aero- basic tuffs, probably related to the Truro Volcanics of the magnetic surveys reveal probable Proterozoic basement Normanville Group, extend up into the basal Carrickalinga south of Kangaroo Island (Belperio et al. 1997), thus suggest- Head Formation (Gatehouse et al. 1993). Near the northern ing that the current southern outcrop limit might be close to end of the Karinya Syncline, the Carrickalinga Head the southern margin of the original Kanmantoo Trough. Thick Formation apparently lies directly on Neoproterozoic rocks, Mesozoic and Cenozoic sediments of the continental shelf although very poor outcrop precludes determining the exact conceal any possible western extension of the Kanmantoo nature of the contact. Internally the Kanmantoo Group can be Trough, west of exposures on Kangaroo Island. subdivided into lower and upper parts based on a regional In the Murray Basin/Padthaway Ridge province (Fig. 1), stratigraphic break. Palaeozoic rocks are largely concealed by Cenozoic sediments The lower Kanmantoo Group consists of the Carrickalinga but aeromagnetic trends are consistent with a continuation of Head Formation and overlying Backstairs Passage Formation. the Kanmantoo Group in this area (Brown et al. 1988; MESA The Carrickalinga Head Formation is a complex unit of 1996). Drillholes east of the outcropping Kanmantoo Group mineralogically immature sandstone (greywacke), mudstone (Rankin et al. 1991) intersect a belt of volcanics which contin- and minor carbonates mostly deposited under relatively deep ues into the A-type granites of the Padthaway Ridge (Turner water conditions although a shallower facies is present in the et al. 1992). north. Gatehouse et al. (1990) considered the sandstones in the The Glenelg River Complex (Fig. 1) consists of meta- lower part of the unit to represent sediment gravity flows, and sedimentary rocks which may be of Cambrian or Neoprotero- recognize a regional shallowing of the depositional environ- zoic age (VandenBerg 1978; Turner et al. 1993a). The Glenelg ment into the Backstairs Passage Formation, a kilometre-thick River Complex is intruded by c. 518 Ma syn-kinematic gran- marker unit displaying relative lateral and vertical constancy ites (Turner et al. 1993b). The metasedimentary rocks are of facies across a wide area. The Backstairs Passage Formation overthrust by ophiolites considered to be obducted relics of is dominated by relatively clean cross-bedded sandstone with Neoproterozoic oceanic crust (Turner et al. 1993b; cf. Gibson evidence of tidal influence. & Nihill 1992), but there is no evidence for a subduction The upper Kanmantoo Group consists of the Talisker complex in the Glenelg River Complex. Calc-Siltstone (and laterally equivalent Karinya Shale), the The poorly exposed Mt Stavely Volcanic Complex Tapanappa, Tunkalilla, Balquhidder and Petrel Cove For- (Crawford 1988; Fig. 1) consists of andesitic to dacitic volcan- mations, and the Middleton Sandstone (Fig. 2). The sharp ics and forearc-style boninites. Building on models developed erosional contact at the base of the Talisker Calc-Siltstone and for Tasmania (Crawford & Berry 1992), Crawford et al. (1996) Karinya Shale has been interpreted as a sequence boundary
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Fig 2. Cambrian stratigraphic sections and inferred correlations of Fleurieu Peninsula and Karinya Syncline.
and a local sandstone and conglomerate unit (Cooalinga easterly exposed unit on southern Fleurieu Peninsula, to Sandstone Member) has been recognized above the contact represent the youngest exposed formation of the Kanmantoo (Dyson et al. 1994, 1996). The Talisker Calc-Siltstone is Group. It is dominated by relatively clean, cross-bedded to essentially a phyllite with sandy, calcareous and pyritic inter- flat-laminated sandstone, similar to the Backstairs Passage beds, some of which (e.g. Nairne Pyrite Member) display Formation. Recent studies indicate that on Fleurieu Peninsula exceptional sulphide enrichment. In the Karinya Syncline, the it is always in thrust fault contact with units further west equivalent Karinya Shale is a carbonaceous and locally calcar- (Haines et al. 1996), and is also of higher metamorphic grade, eous black shale; both units were apparently deposited under suggesting the possibility that it represents a structural repeat relatively deep anoxic conditions. The bulk of the succeeding of older stratigraphy. The unit has also been mapped on Kanmantoo Group comprises fine- to medium-grained (minor Kangaroo Island (Belperio et al. 1997), but no contacts coarse and bimodal gravely horizons in the south) mineralogi- are exposed. cally immature sandstones (greywackes), interbedded with With the exception of the Backstairs Passage Formation and varying proportions of fine-grained clastic rocks, typically Middleton Sandstone, most of the Kanmantoo Group is metamorphosed to phyllites or schists. Interbedded pyritic typically flysch-like, with sharp-based immature sandstone units, commonly black pyritic shales, are common in many beds gradationally overlain by mudstone. Despite this resem- areas. Sandstone is dominant in the thick Tapanappa and blance a number of workers have noted the general absence of Balquhidder Formations, while fine-grained clastics are more classical turbidites within the sequence (e.g. Daily & Milnes abundant in the thinner intervening Tunkalilla Formation and 1973). The majority of sandstone beds are essentially massive succeeding Petrel Cove Formation. With the exception of a few and display little grading except at the top. We note, however, hundred metres of Tapanappa Formation in the axis of the that the features observed display the characteristics of sus- Karinya Syncline, exposure of these units is restricted to the tained high density turbidity flows as described by Kneller & southern part of the Kanmantoo Trough. Daily & Milnes Branney (1995). Interbeds of rare, relatively clean, cross- (1973) considered the Middleton Sandstone, the most bedded sandstones, suggest the possibility of occasional storm
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Fig 3. Palaeocurrent data. Cross bed data from Mills (1964) and Flint (1978) give mainly west to east transport in tidal facies of the Backstairs Passage Formation and Middleton Sandstone respectively. Sole mark measurements from this study are plotted as combined data from sediment gravity flows in the Carrickalinga Head, Tapanappa and Balquhidder formations from southern Fleurieu Peninsula. These indicate a predominantly south to north palaeoslope and transport direction for turbiditic facies in this area. The significance of the relationship between the ripple data of Boord (1985) and the sole marks data is discussed in the text.
wave or tidal current reworking in an environment with Peninsula to the north where similar rocks are exposed (Daily fluctuating water depth, but may also represent reworking by et al. 1980). deeper water currents. Apart from conglomeratic horizons associated with sequence boundaries, most sandstones in the Kanmantoo Group are fine to medium grained, although pebbly horizons are locally present in both the Tapanappa Palaeocurrents and Balquhidder Formations. The coarsest of these are Because of the varying degrees of metamorphism and struc- found in the south, where pebble to small cobble-sized clasts tural complexity, reliable palaeocurrent indicators are not occur in bimodally sorted conglomeratic sandstone beds. always preserved, and any measurements from those present Common clast types include quartzite, mudstone, gneiss need to be carefully corrected for tectonic dip and plunge. and marble (Daily & Milnes 1971). By comparison, the Previous studies have concentrated mainly on cross bedding in preserved Tapanappa Formation in the Karinya Syncline is such units as the Backstairs Passage Formation and Middleton notably fine-grained, rarely exceeding very fine sand Sandstone (Mills 1964; Flint 1978; Boord 1985). The results grain size. have generally indicated easterly to southeasterly directions of The northern part of Kangaroo Island (Fig. 1) contains current movement, which support previous interpretations exposures of an essentially un-metamorphosed Early to poten- that sediment was mainly derived from the Gawler Craton to tially Mid-Cambrian clastic sequence, the Kangaroo Island the west and dispersed into the Kanmantoo Trough essentially Group, which occupies a west-trending, narrow platform. This normal to the trough axis (e.g. Thomson 1976; Parker 1986). succession is generally considered as a thinner platformal The only previous palaeocurrent study involving the pre- equivalent to the Kanmantoo Group (e.g. Daily et al. 1980), dominant flysch-like facies of the Kanmantoo Group are although Jenkins (1990) and Haines & Flöttmann (1998) measurements from climbing ripples and small-scale crossbeds speculate that at least the upper part of the group may be in the Balquhidder and Petrel Cove Formations reported by younger. This dominantly clastic sequence occupies a west- Boord (1985). These also suggest an easterly to southeasterly trending, narrow platform. Although a single stratigraphic transport direction (Fig. 3). However, in confined turbidite section has been constructed for the Kangaroo Island Group basins it is common to find considerable variance (sometimes (e.g. Daily et al. 1980), faults separate many of the outcrops up to or exceeding 90)) between current directions indicated by and it is thus possible that several coeval successions may have erosive sole marks, such as flutes, and ripples higher in the been deposited in different sub-basins (Nedin 1995). Along the same bed (e.g. Kelling 1964; Kneller et al. 1991). In such cases north coast of Kangaroo Island, the upper part of the succes- sole marks are generally considered more reliable indicators of sion contains interbeds of conglomeratic rocks, clasts of which the palaeoslope and bulk sediment transport direction (Kneller include Early Cambrian fossiliferous limestones and gneisses et al. 1991). Primary erosive sole marks giving unequivocal reminiscent of Proterozoic basement. Deposition of the con- direction of flow are relatively rare in the Kanmantoo Group. glomerate units has been interpreted as a response to active However, we have now accumulated a number of such faulting which presumably uplifted a source area, e.g. Yorke measurements from the Carrickalinga Head, Tapanappa and
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Balquhidder Formations at several locations on Fleurieu lower Normanville Group and from the underlying Neopro- Peninsula, mainly scattered along the coastal type section terozoic sequence. While detrital zircons from the Neoprotero- (Fig. 3). These were all collected away from high strain zones zoic and lower Normanville Group support derivation from and were corrected for tectonic plunge. Although the data set adjacent Palaeo-Mesoproterozoic basement, the Kanmantoo of accurate measurements is relatively small the general trends Group gives a totally different and generally younger zircon obtained are supported by a significantly larger number of signature. The samples analysed by Ireland et al. (1995, 1998) observations from which accurate measurements have not give major peaks around 500–600 Ma and 1000–1200 Ma been possible. The combined measurements display an even (average c. 1100 Ma) with a scattering of older zircons, spread of about 90) with transport directions towards the similar to results obtained by these authors from the Lachlan northeast, varying to northwest. The mean vector is towards Fold Belt. Thus, if these samples are representative of the the north-northeast, approximately normal to the average of Kanmantoo Group as a whole, a significant contribution from the ripple data from Boord (1985). Our own observations of the Gawler Craton, Willyama Inlier or reworked Neoprotero- climbing ripples from the tops of flute-bearing beds confirm zoic sediments can be eliminated. This conclusion is consistent that this c. 90) relationship can occur within single thick beds. with the new palaeocurrent interpretation presented above. The average flute trend is essentially parallel to the apparent trough axis, the interpreted trough margin, and major fault trends in the Fleurieu Peninsula area. This is consistent with The structural framework of the Kanmantoo Trough the general observation that turbidity currents typically travel The Kanmantoo Trough area of the southern Adelaide along the axis of elongate confined basins. Kneller et al. (1991) Fold–Thrust Belt can be subdivided into different sectors that explain the common discrepancy between flute and ripple data show contrasting structural characteristics with respect to the in such circumstances as the effect of reflection of the flow principal fault systems active during extension and reacti- from basin margin slopes or internal obstacles (e.g. growth vation. The plan view structure of the Kanmantoo Trough is fault scarps) producing internal solitary waves normal to the given on Fig. 4. Transects of four key-areas that illustrate the reflecting surface. The existence of such growth faults in the progressively changing structural style are indicated on Figs Kanmantoo Trough was shown by Flöttmann et al. (1994). 5–7. They are, from south to north: Kangaroo Island (section Cross bedding in the Backstairs Passage Formation and A-A ), which encompasses the west-trending part of the Middleton Sandstone may be largely of tidal origin based on * Kanmantoo Trough; southern Fleurieu Peninsula (B-B ); the common presence of tidal bundles. In a confined basin the * central Fleurieu Peninsula (C-C ); and the Karinya Syncline palaeocurrent direction indicated by such crossbeds will reflect * area (D-D ). the paths by which tidal currents circulate within the basin (e.g. * Sztanó & de Boer 1995) and may have little or no relationship to the ultimate source location(s). Although the measurements by Mills (1964) give a mostly consistent west to east direction Kangaroo Island at one locality, scattered observations we have made elsewhere Kangaroo Island is characterized by two geologically distinct indicate substantial regional variation and thus may be provinces located to the north and south of a fundamental indicating complex tidal current pathways. We conclude from fault and shear zone (Flöttmann et al. 1995). The northern part the current state of Kanmantoo Trough palaeocurrent data of Kangaroo Island contains exposures of the essentially that the bulk sediment transport direction in most units un-metamorphosed Kangaroo Island Group. These rocks have was essentially from the south, although considerable been displaced northward by imbricate thrusts which merge redistribution took place when shallow conditions prevailed in into a basal detachment (Flöttmann et al. 1995; Figs 4, 5) the basin. above shallow cratonic basement at 2 km depth (drill hole data from Belperio & Flint 1993). Strain in the country rocks increases significantly at the southern edge of the platform, and section balancing requirements suggest minor basement Sediment provenance involvement in the thrusting (Flöttmann et al. 1995). Previous workers considered that the sediments of the The boundary between the northern and southern parts of Kanmantoo Group were largely sourced from uplifted local Kangaroo Island is marked by the steeply south dipping Archaean to Mesoproterozoic cratonic basement (Gawler Kangaroo Island shear (Flöttmann et al. 1995). The shear zone Craton and Willyama Inlier), as well as reworking of older is up to 1 km wide and oblique transpressional displacement Cambrian and Neoproterozoic sediments (e.g. Wopfner 1969; is directed towards the northwest. The southwest-trending Thomson 1969, 1976; Parker 1986). This notion was supported regional folds that are developed in the Kanmantoo Group by earlier palaeocurrent studies (see above). south of the shear zone indicate a comparable shortening Recently the problem of Kanmantoo Group provenance has vector. been investigated using isotopic and radiometric means. Depth-to-basement estimates, based on aeromagnetic data, Turner et al. (1993b) found that Kanmantoo Group sediments suggest a thickness of about 7 km for the basinal Kanmantoo exhibit a somewhat different Nd isotopic signature from the Group (Belperio & Flint 1993). The Kanmantoo Group is Neoproterozoic sediments of the Adelaide Fold–Thrust Belt regionally of greenschist metamorphic grade, but reaches (older model ages and lower åNd than Neoproterozoic sedi- amphibolite grade around syn-kinematic granitoids near the ments). Either these units have different source areas, or, as south coast of Kangaroo Island. Kanmantoo Group rocks are Turner et al. (1993b) suggested, the signature can be obtained not displaced onto the northern Kangaroo Island platform, by a mixing of Gawler Craton basement and reworked but strain within the Kanmantoo Group is accommodated by Neoproterozoic material. intense cleavage development at this margin. Here, the north- Ireland et al. (1995, 1998) and Foden (1996) have dated ern flanks of Kanmantoo Group anticlines are overturned and detrital zircons in sandstones from the Kanmantoo Group, develop an intense metre-scale deformation with a crenulation
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Fig 4. Geological map of southern Adelaide Fold–Thrust Belt. Arcuate outcrop pattern formed by southern Kanmantoo Trough, which tapers out to the north, A–A’-D–D’ give section lines of Figs 5–7.
geometry in which individual cleavage planes are coated by east of the exposed basement (Fig. 6a). In the southern central quartz veins indicating an elevated flux of metamorphic fluids. part of Fleurieu Peninsula, the eastern contact of these base- ment inliers is formed by an east-dipping fault system with reverse kinematics. This fault system juxtaposes younger Southern Fleurieu Peninsula Kanmantoo Group with basement that is now in the footwall Southern Fleurieu Peninsula shows the most continuous of that thrust system (Fig. 6a). That is, this fault also consti- outcrop of the Kanmantoo Group along its south coast type tutes a former growth fault (Fig. 6c) to allow for younger over section. Seismic imaging suggests that the western margin of older juxtaposition along a fault with reverse kinematics. the Kanmantoo Trough lies about 5 km offshore of the western edge of Fleurieu Peninsula beneath Gulf St Vincent (Flöttmann et al. 1998). The seismic images reveal a funda- Central Fleurieu Peninsula mental fault system at this location, west of which lies a thick The western part of central Fleurieu Peninsula displays and essentially undeformed sequence potentially deposited in a regional SSW-plunging, northwest-verging flexural slip folds, foreland basin to the Delamerian Orogeny (Flöttmann et al. with sub-vertical western limbs, followed eastward by an 1998). Towards the east, balanced section requirements suggest imbricate fan of Neoproterozoic strata. Here, discrete reverse that the Kanmantoo Group thickens across major reactivated faults and shear zones with a west-directed sense of shear growth faults (Flöttmann et al. 1994; Flöttmann & James juxtapose thrust packages of progressively older Neoprotero- 1997; Fig. 6a). During Delamerian compression, Kanmantoo zoic rocks with younger strata (Fig. 6b,c). The central part of Group rocks were regionally deformed by northwest verging central Fleurieu Peninsula contains a basement culmination. map-scale folds. Strain is mainly focused along the former Towards the north, a northward-widening wedge of Neopro- growth faults (Fig. 6a), which have been reactivated to pro- terozoic rocks occurs between the basement inliers and the duce highly strained shear zones with reverse kinematics. In Kanmantoo Group (Fig. 4). Here there is no evidence for a many cases, however, these faults still have normal net offset fault contact between Neoproterozoic and Normanville Group indicating that reverse displacement did not fully compensate rocks, and the Kanmantoo Group. The principal strain in this the preceding normal displacement (Flöttmann et al. 1994). part of the fold belt is accommodated along detachments and The most intensely strained part of this transect is the frontal 15–45) east-southeast dipping shear zones beneath the base- part where fault-bounded basement and overlying Neopro- ment inliers, as well as along isolated thrusts within Neo- terozoic rocks are translated onto the shelf margin. Only a thin proterozoic rocks east of the basement inliers (Fig. 6b). veneer of Kanmantoo Group is preserved west of the basement These relationships suggest that in the south-central part of culmination, the bulk of Kanmantoo Group rocks occurring Fleurieu Peninsula, Delamerian orogenic contraction mainly
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Kanmantoo Group removed by erosion is unknown, but structural considerations indicate that it is unlikely that thick- nesses comparable to those observed further south were ever present. Stratigraphic relationships and section balancing suggest that Kanmantoo Group deposition was governed by a west dipping normal fault along the eastern margin of the syncline (Fig. 7). The bulk of regional deformation in the thick pile of Neoproterozoic strata is accommodated by folding above a basal detachment, which crops out along the western margin of the fold belt.
Concealed parts of the Kanmantoo Trough Offshore seismic evidence suggests that Kanmantoo Group rocks may continue for at least 160 km southeast from out- crops on southern Fleurieu Peninsula (Flöttmann & Cockshell 1996; Fig. 8). Interestingly, near the southeastern end of the seismic survey, the succession is dissected by west-dipping reflectors, which are interpreted to represent major reactivated growth faults similar to the faults that are associated with the western basin margin. As such, they may indicate the position of an eastern margin to the basin. As these faults are located west of the outcropping Glenelg River Complex onshore, the Glenelg River Complex may not necessarily represent the eastern continuation of the Kanmantoo Trough as previously proposed (VandenBerg 1978). If it is of similar age to the Kanmantoo Group, the sedimentary precursors of the Glenelg River Complex may therefore constitute an independent sub- basin east of the Kanmantoo Trough. At the southeastern end of the seismic survey, the Kanmantoo Trough is overlain by thick Cretaceous rocks of the Otway Basin (Fig. 8), the Fig 5. Structure of Kanmantoo Trough on Kangaroo Island. deposition of which is related to the separation of the (a) Deformed state. Note strongly downfaulted basin margin also delineated by seismic and gravity surveys (Belperio & Flint 1993). Australian and Antarctic continents. Note that northwest-directed Delamerian compression leads to The northeastern margin of the Kanmantoo Trough is not dextral transpression along northern Kanmantoo Trough margin. exposed, but can be inferred from regional aeromagnetic data Vertical scale at bottom left gives strain magnitude in the section (Brown et al. 1988; MESA 1996; Fig. 1). plane (X/Z) related to Delamerian deformation. Dots show individual strain measurements; strain increases towards basin margin. (b) Restored section. Symbols as in Fig 4. V=H. Geological setting of northern Victoria Land, Antarctica The Wilson Terrane of Northern Victoria Land (Fig. 9) reactivated the principal growth faults that controlled the is traditionally correlated with the inboard part of the deposition of the Kanmantoo Group. The basement inliers Delamerian Orogen (Oliver 1972; Grindley & Davey 1982; form the upthrust basin margin of the Kanmantoo Trough. Stump et al. 1986; Flöttmann et al. 1993). In both cases, Further north, however, more displacement was accommo- contraction has been similarly related to the accretion of dated along footwall shortcut thrusts and therefore no discrete outboard volcanic arc terranes (Flöttmann et al. 1993). thrust zone is developed at the base of the Kanmantoo or However, the orogenic infrastructure of the Wilson Terrane Normanville Groups. differs significantly from that of its correlative Delamerian In the eastern portion of central Fleurieu Peninsula, that counterpart. underwent syn- to late-kinematic regional metamorphism and The essential characteristics of the Wilson Terrane and the deformation, strain was accommodated by regional upright Ross Orogen are summarized below; for more comprehensive folding with pervasive axial planar fabrics (Fig. 4). Homoaxial descriptions appropriate references are included. Orogenic multiple deformation only occurs associated with amphibolite characteristics of the Wilson Terrane include a regional high- facies metamorphic aureoles around syn-kinematically grade metamorphic basement of upper amphibolite to granu- intruded granitoids (Jenkins & Sandiford 1992). lite facies (Castelli et al. 1991), with earliest metamorphic ages around 530 Ma (Adams 1986) and a metamorphic peak around 500 Ma (Borg et al. 1987). The orogenic imprint of Karinya Syncline the Wilson Terrane is related to continuous, west-directed, Towards its northern outcrop limit, the thickness of individual subduction processes at the eastern Antarctic plate margin stratigraphic units of the Kanmantoo Group are reduced or, (Kleinschmidt & Tessensohn 1987; Flöttmann & Kleinschmidt in the case of units younger than the Tapanappa Formation, 1991; Schüssler et al. 1993). West-directed subduction led to are completely absent (Fig. 2). The preserved thickness of the intrusion of S- and I-type granites (Borg et al. 1987). S-type the combined Normanville and Kanmantoo Groups in the granitic melts were formed from a sedimentary precursor Karinya Syncline is less than 2.5 km. The amount of upper with a significant contribution of zircons from a c. 1100 Ma
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Fig 6. Reactivation styles of the Kanmantoo Trough on Fleurieu Peninsula. (a) On southern Fleurieu Peninsula the principal displacement during basin reactivation is accommodated along footwall shortcut thrusts leading to the exhumation of basement inliers which are in fault contact with Kanmantoo Group rocks. (b) On central Fleurieu Peninsula principal shortening displacement is accommodated along reactivated growth faults that juxtapose Neoproterozoic rocks and basement; a stratigraphically undisturbed succession is exhumed in the hanging wall. (c) Pre-deformational setting of the Kanmantoo Trough in which sediment thickness increases across major growth faults. See caption of Fig. 5 for explanation of strain data below (a) and (b); see Fig. 4 for lithologies and section locations. V=H.
Further south along the Antarctic palaeo-Pacific margin, Goodge et al. (1993) report U–Pb zircon dates that constrain contractional deformation between 540 and 520 Ma. This is further substantiated by age constraints given by Rowell et al. (1993) and Encarnacion & Grunow (1996), who suggest, although based on local evidence, the presence of an active Neoproterozoic margin. Encarnaciaon & Grunow (1996) point out that convergence prior to 530 Ma was dominated by strike slip and/or shallow (amagmatic) subduction. Fig 7. Structure at the latitude of the Karinya Syncline, northern The outboard Bowers Terrane comprises a Cambrian vol- Kanmantoo Trough. Notice that Kanmantoo Trough is entirely canic arc (Weaver et al. 1984), overlain by Cambro-Ordovician deformed within the wedge of Neoproterozoic rocks. The sedimentary rocks (Laird & Bradshaw 1983). The formation of Kanmantoo Trough margin does not constitute a significant strain this volcanic arc is also related to west-directed subduction guide; strain is highest at frontal detachment. See caption of Fig. 5 at the eastern margin of the Bowers Terrane (Kleinschmidt for explanation of strain data; lithologies as in Fig. 4. V=H. & Tessensohn 1987; Dallmeyer & Wright 1992). During subduction–collision at the Lanterman suture, the central provenance (Black & Sheraton 1990). A belt of high pressure basement complex of the Wilson Terrane was divergently metamorphics (Kleinschmidt & Tessensohn 1987) occurs along detached along mylonitic thrust systems and displaced towards the Lanterman suture zone (Gibson & Wright 1985), which both the cratonic foreland and oceanward (Flöttmann & forms the boundary between the inboard Wilson Terrane and Kleinschmidt 1991). Dating of post-kinematic pegmatites that the accreted outboard Bowers Terrane. crosscut these thrust systems suggests that post-collisional
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Fig 8. Depth-converted line drawing of geological interpretation of seismic section DME 93-1 (location shown in Fig. 1). No vertical exaggeration.
Fig 9. Map of Northern Victoria Land, Antarctica. Notice the narrow orogenic belt formed by the inboard Wilson Terrane. Inset shows schematic cross section of subduction/accretion tectonics of the Ross Orogeny (adapted from Flo¨ttmann & Kleinschmidt 1991); bottom inset shows location.
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convergence lasted until c. 480 Ma (Schüssler & Henjes-Kunst 1994; unpublished data). This timing is supported by 40Ar– 39Ar ages of cleavage formation (500–460 Ma) in the eastern Bowers and the easterly adjacent Robertson Bay Terrane (Dallmeyer & Wright 1992). The sedimentary rocks of the outboard Robertson Bay Terrane comprise turbidites of Cambro-Ordovician age (Cooper et al. 1996) which are deformed by upright map-scale folds. The age of the protolith of the Wilson Terrane rocks is controversial. Enclaves of less metamorphosed metasedimentary rocks, such as the Rennick Schists in the Daniels Range (Babcock et al. 1986), for which Adams (1986) reports metamorphic ages of around 530 Ma, represent precursor rocks of the Wilson Terrane gneisses. For similar rocks in the Priestley Formation, Casnedi & Pertusati (1991) suggest similarities with the Kanmantoo Group on lithostratigraphic grounds, but Ribecai (1991) tentatively suggests a Neoproterozoic age for the same rocks based on palynomorphs. The Berg Mountains consist of greywackes (Berg Group) which are lithologically similar to the Fig 10. Palinspastic restoration of parts of the Kanmantoo Trough Kanmantoo Group. Preliminary dating of detrital zircons that crop out. Large arrow indicates trough-parallel palaeocurrents indicates some similarities to the age spectra of Kanmantoo transporting southerly derived sediment. Small arrows show Group detrital zircons (Ireland et al. 1995). However, the reflection of palaeocurrents perpendicular to basin margin, as Sm–Nd signature of the Berg Group as well as rocks from explained in the text. Legend same as Fig 4. McCain Bluff (Fig. 9), is more like that of Neoproterozoic strata of the Adelaide Fold–Thrust Belt than that of the ernmost end of the exposed Kanmantoo Trough, the Kanmantoo Group (Turner et al. 1993b). This suggests that strain distribution is controlled by an orogenic wedge of the Berg Group and the rocks at McCain Bluff received input Neoproterozoic strata overlain by thin Kanmantoo Group. from the adjacent craton (similar to the Neoproterozoic rocks Aeromagnetic data allows a tentative tracing of the eastern of South Australia), but also received input from a source boundary of the Kanmantoo Trough beneath Cainozoic cover similar to that of the Kanmantoo Group. of the Murray Basin. An interpretation of an offshore seismic section (Fig. 8) suggests that the eastern margin of the Kanmantoo Trough, south of the Murray Basin, may lie west of the Glenelg River Complex, where it is associated with Discussion and conclusions steeply west-dipping faults. Clearly the present-day shape of the outcropping Kanmantoo The structural and stratigraphic observations presented Trough is influenced by the original basin configuration and here preclude a continuous detachment at the base of the the present erosional level. The erosional level is, in turn, Kanmantoo Group. This rules out models that suggest an controlled by the amount of exhumation in different parts of allochthonous derivation of the Kanmantoo Group. Further- the Delamerian Orogen. However, key features of the craton- more, the present configuration of the basin margin has no ward margin and internal characteristics of the Kanmantoo geometric relationship with structures related to Delamerian Trough are reflected by strain concentrations related to basin shortening. Faults and displacement kinematics associated reactivation during the Delamerian Orogeny. with the reactivation of the Kanmantoo Trough rather reflect Over its entire length of more than 150 km, the Kangaroo systematic changes in response to reactivation of a strongly Island portion of the Kanmantoo Trough is bounded at its bent basin margin during the overall WNW-directed northern margin by steeply south dipping inverted growth compression of the Delamerian Orogeny. faults, across which balanced cross sections show a dramatic When restored to its original shape the outcropping thickening of the sedimentary section and hence deepening of Kanmantoo Trough is markedly triangular in plan view the underlying basement. Within the Kanmantoo Trough, (Fig. 10) and the basin is clearly narrower at its northern end. Delamerian orogenic contraction has produced a north- The southern part of the outcropping basin is significantly northeast trending structural grain that is oblique to the west wider, a notion further supported by the seismic data, trending basin margin. The steeply dipping basin margin acted which show continuation of the southern Kanmantoo Trough as a rigid buttress, along which strain was concentrated (Figs to the east. 5, 6) and displacement was resolved mainly by transpressional We suggest that the geometry of the southern part of the reverse slip with limited basement exhumation. North-directed Kanmantoo Trough was significantly influenced by strike-slip displacement on the northern Kangaroo Island platform is a displacement along an ESE-trending main wrench fault. This consequence of kinematic partitioning across this reactivated explains the east-west trend of the basin on Kangaroo Island, basin margin. On southern Fleurieu Peninsula, variations the steep dip and right-stepping jogs of the basin margin on of Delamerian strain magnitude are spatially related to the Kangaroo Island, and the lateral juxtaposition of the basinal geometry of the former basin margin. This is manifest by strain Kanmantoo Group rocks with the platformal rocks of the concentrations either in the hanging wall of inverted growth Kangaroo Island Group. Variation of depocentres within the faults, or along footwall shortcut thrusts. Kangaroo Island Group may also be related to a strike slip Between central Fleurieu Peninsula and the Karinya regime (C. Nedin pers. comm. 1996). The influence of this Syncline region, both the reactivated growth faults and strike slip regime wanes northward, where the Kanmantoo the basement-involved deformation disappear. At the north- Trough margin swings to a more northerly trend, giving the
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Kanmantoo Trough an overall triangular shape. During Delamerian orogenic shortening, the basin shape and north- ward waning depth of downfaulting of the Kanmantoo Trough into the underlying rocks, led to variations in strain magnitude and geometry along the margin. Intense strain concentrations are associated with the steeply dipping buttress of the southern strike-slip basin margin. Northwards, where normal faulting during formation of the basin margin is less intense, the distribution of strain related to Delamerian short- ening is increasingly distributed across the entire width of the fold-thrust belt and is not concentrated along the former margin of the Kanmantoo Trough. An early Palaeozoic tear fault (Fig. 10) with significant strike-slip movement has been repeatedly postulated between the Antarctic and Australian Gondwana fragments (e.g. Baillie 1985; Glen et al. 1992). Considering the framework of early Palaeozoic tectonics along the Australian and Antarctic palaeo-Pacific margin of Gondwana, strike-slip faulting could be an accommodation feature of contrasting subduction polarities at this formerly contiguous continental margin (Fig. 11a). Prior to the Cambro-Ordovician Delamerian Orogeny, the Australian margin was characterized by upper plate extension manifest by the deposition of the Neoproterozoic successions and the overlying Kanmantoo Group. If the Mt Stavely Volcanic Complex further to the east is related to east-dipping subduction, as interpreted by Crawford et al. (1996), we suggest a passive margin setting for the Australian segment of the palaeo-Pacific margin during the deposition of the Kanmantoo Group (Fig. 11a,b). In contrast, the correlative Antarctic margin in northern Victoria Land seemingly underwent protracted convergence related to west-directed subduction beneath the Antarctic continent and beneath the Bowers Terrane. Orogenic conver- gence occurred throughout the Cambrian and well into the middle of the Ordovician, exhuming the mid-crustal rocks of the Wilson Terrane of Northern Victoria Land. The Antarctic margin has clear hallmarks of an advancing subduction margin (Royden 1993; see Fig. 11c) characterized by significant upper plate contraction. In essence, the Australian and Antarctic segments of the palaeo-Pacific margin reflect different types of continental margins and record contrasting amounts of orogenic short- ening during the Cambro-Ordovician Ross–Delamerian Fig 11. (a) Contrasting tectonic settings along palaeo-Pacific margin Orogeny. This, in turn, could have resulted in orogen-normal during deposition of Kanmantoo Group leading to the formation of strike-slip displacement at the interface between these two a major intra-continental tear zone shown by dashed line. (b) segments of that margin. We suggest that the interaction Delamerian Orogen of Australia (AUS) with passive margin related between these contrasting orogenic regimes is reflected in to east-dipping subduction beneath Mt Stavely Volcanic Arc (V); (c) the localization, geometry and reactivation style of the Ross Orogen, Antarctica (ANT) showing advancing plate margin Kanmantoo Trough. A Recent sedimentary basin of similar with upper plate contraction leading to divergent Wilson and Exiles dimensions and geometry, and which is strongly influenced by thrusts (see Fig. 9). a strike-slip fault, is the Komandorsky Basin east of the Kamchatka peninsula (Baranov et al. 1991). The systematic variation of the shape of the Kanmantoo Considering the circumstances, these sequence boundaries Trough correlates broadly with systematic changes in the most likely reflect major tectonic events during the subsidence sedimentary record of the Kanmantoo Group. In the north, of the Kanmantoo Trough, rather than being of eustatic individual preserved formations of the Kanmantoo Group origin. thin, suggesting that the basin may have terminated not far A southerly source of the clastic wedge of the Kanmantoo to the north of the Karinya Syncline. In the south, both Group is suggested by the available palaeocurrent data the width of the Kanmantoo Trough, and the thickness of the obtained from sole marks on sediment gravity flows, although Kanmantoo Group increase where the sediments have on a local scale sediment movement was largely parallel to the the most proximal characteristics. Sedimentary rocks of the local orientation of the Kanmantoo Trough axis and major Kanmantoo Group comprise two major depositional suc- growth fault scarps. There was also a degree of axis-normal cessions each lying above a sequence boundary (Fig. 2). reflection of currents as flows waned. Coarse components of
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detrital material formed along and eroded off the encroaching active Antarctic margin of Gondwana. Such an origin may also apply to other greywacke-type rocks with similar detrital age spectra that were deposited later elsewhere in the circum- Pacific region (see Ireland et al. 1995). The sediments were probably transported trench-parallel towards the north along the palaeo-Pacific margin (Fig. 12), along which the Ross Orogeny also progressed northward through time. The for- mation of the Kanmantoo Trough is a manifestation of contrasting but mutually interacting tectonic processes along the then contiguous palaeo-Pacific margin of Gondwana.
Research was sponsored by the Australian Research Council (ARC) and the Department of Mines and Energy, South Australia (MESA). Over several years of work on this topic we benefited from discussions with S. Marshak, M. Sandiford, J. F. Foden, F. Tessensohn, R. J. F. Jenkins, W. V. Preiss, T. Redfield, P. C. Pertusati, F. Henjes-Kunst, C. Finn, U. Schuessler, S. P. Turner, S. Matzer, K. R. McClay, G. Kleinschmidt, J. Goodge, C. A. Ricci and R. L. Oliver. We enjoyed constructive reviews by A. J. Rowell and J. Bradshaw; A. J.. Maltman is thanked kindly for the editorial handling of the manuscript.
References Fig 12. Proposed tectonic setting along the palaeo-Pacific margin during the deposition of the Kanmantoo Group. Active convergence A, C.J. 1986. Age and ancestry of metamorphic rocks of the Daniels Range, along the Antarctic margin leads to formation of a magmatic arc USARP Mountains, Antarctica. In:S,E.(ed.)Geological investi- that provides Neoproterozoic–early Palaeozoic detrital components, gations in northern Victoria Land. American Geophysical Union, Antarctic which are mixed with eroded 1100 Ma Grenvillian crust (hatched Research Series, 46, 25–38. pattern) and transported trench-parallel towards the north. B, R.S., P, C.C., S,J.W.&A, C.J. 1986. Geology of the Daniels Range, North Victoria Land, Antarctica. In:S,E.(ed.) Formation of active inboard margin may have obliterated additional Geological investigations in northern Victoria Land. American Geophysical areas of Grenvillian crust. Active margin is dissected by right-lateral Union, Antarctic Research Series, 46, 1–24. tear faults separating domains of contrasting modes of plate B, P.W. 1985. A Palaeozoic suture in eastern Gondwanaland. Tectonics, 4, convergence. 653–660. B, B.V., S, N.I., M’,A.V.&M, E.L. 1991. The Komandorsky basin as a product of spreading behind a transform gravelly bimodal beds may have a local derivation from fault plate boundary. Tectonophysics, 199, 237–269. scarps or the basin margin. B,A.P.&F, R.B. 1993. The southeastern margin of the Gawler In the light of detrital zircon age spectra, the bulk of Craton. Australian Journal of Earth Sciences, 40, 423–426. Kanmantoo Group sands cannot be sourced from adjacent ——, P, W.V., F, M., G, C.G., G, J., H,J.& Archaean-Proterozoic cratons. These detrital components B, A. 1997. Tectonic and metallogenic framework of the Cambrian Stansbury Basin—Kanmantoo Trough. AGSO Journal of Australian require a significant input from sources of two ages: the older Geology and Geophysics, In press. around 1100 Ma (‘Grenvillian’) and the younger around B,L.P.&S, J.W. 1990. The influence of Precambrian source 600–500 Ma. The latter age may be associated with magmatic components on the U–Pb zircon age of a Palaeozoic granite from northern processes related to the active palaeo-Pacific margin. Uplift Victoria Land, Antarctica. Precambrian Research, 46, 275–293. that accompanied exhumation of the Wilson Terrane could B, R.A. 1985. Sedimentology of the Cambrian, upper Kanmantoo Group, southern Fleurieu Peninsula, South Australia. BSc Honours thesis, have provided a source for, at least some of the clastic University of Adelaide. sediments deposited in the Kanmantoo Trough. B, S.G., S, E., C, B.W., MC, M.T., W,D., The derivation of the older c. 1100 Ma detrital component is A,R.L.&H, J.R. 1987. Granitoids of northern not clear. Rocks of this age are described from Coates Land Victoria Land, Antarctica: implications of chemical and isotopic variations (Storey et al. 1994) and as xenocrysts in c. 500 Ma granites to regional crustal structure and tectonics. American Journal of Science, 287, 127–169. et al. from the Pensacola Mountains (van Schmus 1995). A B, C.M., T,D.H.&A, V. 1988. An interpretation of the Grenvillian provenance has also been suggested for the sedi- tectonostratigraphic framework of the Murray Basin region of southeast- mentary precursor of the Wilson Terrane gneisses (Black & ern Australia, based on an examination of airborne magnetic patterns. Sheraton 1990). Outcrops of Grenvillian rocks in Antarctica Tectonphysics, 154, 309–333. are over 3000 km away from Australia when the current C,R.&P, P.C. 1991. Geology of the Priestley Formation (northern Victoria Land, Antarctica) and its significance in the context of continents were joined. However, comparable distances the Early Paleozoic continental reconstruction. Memorie della Societa between source area and depositional realm are reported from Geologica Italiana, 46, 17–24. other active margin systems, such as the recent Sunda Trench C, D., L, B., O, G., R,P.&T, F. 1991. (e.g. Moore et al. 1982), or the remnant ocean basin of the Granulite facies rocks from the Wilson terrane (northern Victoria Land): ancient Ouachita Orogenic Belt (Ingersoll et al. 1995). We Campbell Glacier. Memorie della Societa Geologica Italiana, 46, 197–204. C, Y.D. & L, S.F. 1996. Precise U–Pb zircon dating of a post-D2 suggest that a significant part of the sediment comprising the meta-dolerite: constraints for rapid tectonic development of the southern Kanmantoo Group may have been derived as an erosional Adelaide Fold Belt during the Cambrian. Journal of the Geological Society, composite of Grenvillian basement remnants, mixed with London, 153, 83–90.
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C,G.L.&P, R. 1989. Basement cover interaction in the Adelaide ——, G,G.M.&K, G. 1993. Structural continuity of the Ross Foldbelt, South Australia: the development of an arcuate foldbelt. Tectono- and Delamerian orogens of Antarctica and Australia along the margin of physics, 158, 209–226. the paleo-Pacific. Geology, 21, 319–322. C, P.J., E, A., H, R., M,F.&W, D. 1990. The ——, H, P.W., C,C.D.&P, W.V. 1988. Re-assessment of regional tectonics of the Tasman orogenic system, eastern Australia. early Palaeozoic seismic stratigraphy of the Stansbury Basin (Gulf St Journal of Structural Geology, 12, 519–544. Vincent, South Australia). Australian Journal of Earth Science, In press. C, J.A., J, R.J.F., C,W.&W, I.S. 1992. Ion-probe ——, J, P.R., M, R., C, P., T, M., F,M., zircon dating of a mid-Early Cambrain tuff in South Australia. Journal of R,J.&M, S. 1995. The structure of Kangaroo Island the Geological Society, London, 149, 185–192. (South Australia): strain and kinematic partitioning during Delamerian C, R.A., J,J.B.&B, J.G. 1996. Cambrian trilobites from Northern basin and platform reactivation. Australian Journal of Earth Sciences, 42, Victoria Land, and their stratigraphic implications. New Zealand Journal of 35–49. Geology and Geophysics, 39, 363–387. ——, ——, R,J.&J, T. 1994. Early Palaeozoic foreland thrusting C, A.J. 1988. Cambrian. In:D,J.G.&F,J.A.(eds) and basin reactivation at the southeastern Paleo-Pacific margin of the Geology of Victoria. Victorian Division, Geological Society Australia, Australian Precambrian Craton: a reappraisal of the structural evolution of 37–62. the Southern Adelaide Fold-Thrust Belt. Tectonophysics, 234, 95–116. —— & B, R.F. 1992. tectonic implications of late Proterozoic–Early F, J.F. 1996. Provenance of Neoproterozoic and early Palaeozoic sediments Palaeozoic igneous rock associations in western Tasmania. Tectonophysics, east Australia: implications from Nd isotope and zircon studies. Geological 214, 37–56. Society of Australia, Abstracts, 41, 146. ——, D, A.G., B,L.P.&S-S, P.G. 1996. Enhancing the G, C.G., J, J.B., C,B.J.&MC, A.J. 1993. Early prospectivity of Victoria: Identification of Mount Read Volcanics correla- Cambrian volcanics from Red Creek, eastern Mt. Lofty Ranges, South tives in western Victoria. Geological Society of Australia Abstracts, 41, 100. Australia. Royal Society of South Australia, Transactions, 117, 57–66. D,B.&M, A.R. 1971. Stratigraphic notes on Lower Cambrian ——, —— & C, B.J. 1990. Sedimentology and stratigraphy of the fossiliferous metasediments between Campbell Creek and Tunkalilla Beach Carrickalinga Head Formation (low stand fan to high stand systems tract), in the type section of the Kanmantoo Group, Fleurieu Peninsula, South Kanmantoo Group, South Australia. In:J J.B. & M P.S. (eds) Australia. Transactions of the Royal Society of South Australia, 95, The evolution of a Late Precambrian–Early Palaeozoic rift complex: The 199–214. Adelaide Geosyncline. Geological Society of Australia, Special Publications, —— & —— 1973. Stratigraphy, structure and metamorphism of the Kanmantoo 16, 351–368. Group (Cambrian) in its type section east of Tunkalilla Beach, G,G.M.&N, D.N. 1992. Glenelg River Complex: western margin of South Australia. Transactions of the Royal Society of South Australia, 97, the Lachlan Fold Belt or extension of the Delamerian Orogen into western 213–251. Victoria? Tectonophysics, 214, 69–91. ——, J,J.B.&M, A.R. 1973. Large-scale horizontal displacement —— & W, T.O. 1985. Importance of thrust faulting in the tectonic within Australo-Antarctica in the Ordovician. Nature, Physical Science, development of northern Victoria Land, Antarctica. Nature, 315, 480–483. 244, 61–64. G, J.W., W,N.W.&H, V.L. 1993. Neoproterozoic-Cambrian ——, M,P.S.&R, B.R. 1980. Terrestrial-marine transition in the basement involved orogenesis within the Antarctic margin of Gondwana. Cambrian rocks of Kangaroo Island, South Australia. Sedimentology, 27, Geology, 21, 37–40. 379–399. G, R.A., S,E.&VB, A.H.M. 1992. Paleozoic intraplate D,R.D.&W, T.O. 1992. Diachronous cleavage development in escape tectonics in Gondwanaland and major strike-slip duplication in the the Robertson Bay Terrane, northern Victoria Land, Antarctica: Tectonic Lachlan orogen of southeastern Australia. Geology, 20, 795–798. implications. Tectonics, 11, 437–448. G, D.R., F,D.A.&B, M. 1997. Recognition and definition of D, I.W.D. 1991. Pacific margins of Laurentia and East Antarctica- orogenic events in the Lachlan Fold Belt. Australian Journal of Earth Australia as a conjugate rift pair: Evidence and implications for an Sciences, 44, 489–502. Eocambrian supercontinent. Geology, 19, 598–601. G, G.W. & D, F.J. 1982. The reconstruction of New Zealand, —— 1995. Earth before Pangea. Scientific American, 272, 58–63. Australia and Antarctica. In:C,C.(ed.)Antarctic Geoscience. D,P.&S, M. 1992. Phase relationships in Buchan Facies Series University of Wisconsin Press, Madison, 15–29. pelitic assemblages: Calculations with applications to andalusite-staurolite H,P.W.&F¨, T. 1998. The Delamerian Orogeny and potential parageneses in the Mount Lofty Ranges, South Australia. Contributions to foreland sedimentation: a review of age and stratigraphic constraints. Mineralogy and Petrology, 110, 121–132. Australian Journal of Earth Sciences, in press. D, I.A., G,C.G.&J, J.B. 1994. The significance of the ——, ——, G, J.C., J,J.B.&G, C.G. 1996. Integrated approach sequence boundary at the base of the Early Cambrian Talisker Calc- to the reinterpretation of the Cambrian Kanmantoo Group type section, siltstone and its relationship to mineralisation in the Kanmantoo Trough. South Australia. Geological Society of Australia Abstracts, 41, 177. Geological Society of Australia Abstracts, 37, 92–93. H, P.F. 1991. Did the breakout turn Gondwana inside-out? Science, 252, ——, —— & —— 1996. Sequence stratigraphy of the Talisker Calc-siltstone and 1409–1412. lateral equivalents in the Cambrian Kanmantoo Group, South Australia. I, R.V., G,S.A.&D, W.R. 1995. Remnant Ocean Quarterly Geological Notes, Geological Survey of South Australia, 129, Basins. In:B,J.&I,R.V.(eds)Tectonics of sedimentary 27–41. basins. Blackwell, Cambridge, Ma, 363–391. E,J.&G, A. 1996. Changing magmatic and tectonic styles I, T.R., F, C.M., F¨, T., W, S.D., B, along the paleo-Pacific margin of Gondwana and the onset of early J.D. & A, C. 1995. Continental structure on the Pacific margin of Paleozoic magmatism in Antarctica. Tectonics, 15, 1325–1341. Gondwana: detrital zircon ages from Lachlan Fold Belt, Kanmantoo F, R.H. 1987. A review of the problems important for interpretation of Group and Adelaide Fold Belt of Australia, and Victoria Land correlatives. the Cambro-Ordovician of northern Victoria Land (Antarctica), Tasmania 7th International Symposium on Antarctic Earth Sciences (VII ISAES), and New Zealand. In:MK,G.D.(ed.)Gondwana Six: Structure, Siena, Italy, 10–15 September 1995, Abstracts, 202. Tectonics and Geophysics. AGU, Geophysical Monographs, 40, 49–66. ——, F¨, T., F, C.M., G, G.M., P,, W.W. 1998. Development of the early-Paleozoic Pacific margin of Gondwana from F, D.J. 1978. Deep sea fan sedimentation of the Kanmantoo, Kangaroo detrital-zircon ages across the Delamerian orogen. Geology, 26, 243–246. Island. Royal Society of South Australia, Transactions, 102, 203–222. J, J.B., D,I.A.&G, C.G. 1994. The nature of the sequence F¨,T.&C, C.D. 1996. Palaeozoic basins of southern South boundary between the Normanville and the Kanmantoo Groups on Australia: new insight to their structural history from regional seismic data. Fleurieu Peninsula, South Australia. Australian Journal of Earth Sciences, Australian Journal of Earth Sciences, 43, 45–55. 41, 445–453. —— & J, P. 1997. Influence of basin architecture on the style of inversion J, R.J.F. 1990. The Adelaide Fold Belt: Tectonic reappraisal. In:J, and fold–thrust belt tectonics—the southern Adelaide Fold–Thrust Belt, J.B. & M, P.S. (eds) The evolution of a Late Precambrian–Early South Australia. Journal of Structural Geology, 19, 1093–1110. Palaeozoic Rift Complex: The Adelaide Geosyncline. Geological Society —— & K, G. 1991. Opposite thrust systems in northern Victoria Australia Special Publications, 16, 395–420. Land, Antarctica: Imprints of Gondwana’s Paleozoic accretion. Geology, ——&S, M. 1992. Observations on the tectonic evolution of the 19, 45–47. southern Adelaide Fold Belt. Tectonophysics, 214, 27–36. —— & O, R.L. 1994. Review of Precambrian-Palaeozoic relationships at K, G. 1964. The turbidite concept in Britain. In:B,A.H.& the craton margins of Australia and Antarctica. Precambrian Research, 69, B, A. (eds) Turbidites. Developments in Sedimentology, 3. Elsevier, 293–306. Amsterdam, 75–92.
Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/155/3/525/4886864/gsjgs.155.3.0525.pdf by guest on 28 September 2021 KANMANTOO TROUGH, SE AUSTRALIA 539
K,G.&T, F. 1987. Early Paleozoic westward directed S¨ ,U.&H-K, F. 1994. Petrographical and geochronological subduction at the Pacific margin of Antarctica. In:MK,G.D.(ed.) investigations on garnet–tourmaline pegmatite from Ringgold Knoll, Oates Gondwana Six: Structure, Tectonics and Geophysics. AGU, Geophysical Coast, Antarctica. Chemie der Erde, 54, 297–318. Monographs, 40, 89–106. ——, S, D.N.B. & R, N.W. 1993. Subduction related mafic to K,B.C.&B, M.J. 1995. Sustained high-density turbidity currents intermediate plutonism in the northwestern Wilson Terrane, North and the deposition of thick massive sands. Sedimentology, 42, 607–616. Victoria Land and Oates Coast, Antarctica. Geologisches Jahrbuch, E47, ——, E, D., MC,W.&M, R. 1991. Oblique reflection of 389–418. turbidity currents. Geology, 19, 250–252. S,R.C.&C, B. 1953. The age and facies of the Kanmantoo L,M.G.&B, J.D. 1983. New data on the lower Palaeozoic Bowers Group. Australian Journal of Science, 16, 12–14. Supergroup, Northern Victoria Land. In:O, R.L., J,P.R.& S, B.C., P,R.J.&J, A.C. 1994. Grenville province J,J.B.(eds)Antarctic Earth Science. Australian Academy of Science, within Antarctica: a test of the SWEAT Hypothesis. Journal of the Canberra, 123–126. Geological Society, London, 151, 1–4. M, N.S. 1990. The structure of the southern Adelaide Fold Belt, S, E., W, A.J.R. & B, S.G. 1986. Reconstruction of Australia South Australia. In:J,J.B.&M,P.S.(eds)The evolution of a Late and Antarctica: evidence from granites and recent mapping. Earth and Precambrian–Early Palaeozoic Rift Complex: The Adelaide Geosyncline. Planetary Science Letters, 79, 348–360. Geological Society of Australia Special Publications, 16, 369–395. M,S.&F¨, T. 1996. Structure and origin of the Fleurieu and S ´, O.& B , P.L. 1995. Basin dimensions and morphology as controls Nackara Arcs in the Adelaide fold-thrust belt, South Australia: salient and on amplification of tidal motions (the early Miocene North Hungarian Sedimentology, recess development in the Delamerian Orogen. Journal of Structural Bay). 42, 665–682. Geology, 18, 245–258. T, B.P. 1969. The Kanmantoo Group and Early Palaeozoic tectonics. In: MESA 1996. Total Magnetic Intensity map (1:2 000 000). Mines and Energy, P, L.W. (ed.) Handbook of South Australian Geology. Government South Australia Printer, Adelaide, 97–108. M, K.J. 1964. The structural petrology of an area east of Springton, South —— 1976. Kanmantoo Trough-regional geology and comments on mineralisa- Australia. PhD thesis, University of Adelaide. tion. In:K,C.L.(ed.)Economic geology of Australasia and Papua M, G.F., C,J.R.&E, F.J. 1982. Sedimentation in the Sunda New Guinea. The Australasian Institute of Mining and Metallurgy, Trench and forearc region. In:L,J.K.(ed.)Trench Forearc Geology. Melbourne, 555–560. Geological Society, London, Special Publications, 10, 245–258. T, S.P., A, C.J., F¨,T.&F, J.D. 1993a. Geochemical M, E.M. 1991. Southwest U.S.-East Antarctic (SWEAT) connection: A and geochronological constraints on the Glenelg River Complex, western hypothesis. Geology, 19, 425–428. Victoria. Australian Journal of Earth Sciences, 40, 275–292. N, C. 1995. The palaeontology and palaeoenvironment of the Early Cambrian ——, F, J., S,M.&B, D.R. 1993b. Sm-Nd isotopic evidence Emu Bay Shale, Kangaroo Island, South Australia. PhD thesis, University of for the provenance of sediments from the Adelaide Fold Belt and southeast Adelaide, Adelaide. Australia, with implications for crustal growth models. Geochimica et O, R.L. 1972. Some aspects of Antarctic-Australian geological relation- Cosmochimica Acta, 57, 1837–1856. ships. In:A,R.J.(ed.)Antarctic Geology and Geophysics. Universitets- ——, S, M., F¨,T.&F, J. 1994. Rb-Sr dating of forlaget, Oslo, 859–864. differentiated cleavage: an example from the Adelaidean metasediments at P, A.J. 1986. Tectonic development and metallogeny of the Kanmantoo Hallett Cove, southern Adelaide fold belt. Journal of Structural Geology, Trough in South Australia. Ore Geology Reviews, 1, 203–212. 16, 1233–1241. P, W.V. 1987. The Adelaide Geosyncline—late Proterozoic stratigraphy, ——, —— & F, J. 1992. Some geodynamic and compositional constraints sedimentation, palaeontology and tectonics. Geological Survey of South on ‘‘postorogenic’’ magmatism. Geology, 20, 931–934. Australia, Bulletin, 53, 1–438. VB, A.H.M. 1978. The Tasman Fold Belt System in Victoria. Tectono- —— 1993. Neoproterozoic. In:D, J.F., P,W.V.&P,A.J.(eds) physics, 48, 267–297. The Geology of South Australia. Geological Survey of South Australia, V S, W.R., MK, L.W., G,D.A.&R, A.J. 1995. Adelaide, 171–203 U/Pb Geochronology of parts of the Pensacola, Harold Byrd and Queen R, L.R., C,B.J.&F, M.G. 1991. Geology and mineral Maud Mountains, Antarctica. VII International Symposium on Antarctic potential beneath the Murray Basin, South Australia. South Australian Earth Sciences, 10–15 September 1995 Siena, Italy, Abstracts, 391 Department of Mines and Energy Report, 91/15, 64–69. V,J.J.&L, Z.X. 1991. Review of seafloor spreading around Australia. II R, C. 1991. Occurrence of the organic-walled microfossils from the Marine magnetic anomaly modelling. Australian Journal of Earth Sciences, Priestley Formation (Wilson Terrane, Northern Victoria Land, Antarctica) 38, 391–408. and the problem of their age: a preliminary report. Memorie della Societa Geologica Italiana, 46, 157–161. V B, C.C. 1980. Evolution of the late Proterozoic Adelaide Fold Belt, Australia; comparisons with post-Permian rifts and passive margins. R, A.J., R, M.N., D, E.M., W, E.T., V-S, Tectonophysics, 70, 115–134. W.R. & S, E.I. 1993. An active Neoproterozoic margin; evidence from the Skelton Glacier area, Transantarctic Mountains. Journal Geological W, S.D., B,J.D.&L, M.G. 1984. Geochemistry of Society, London, 150, 677–682. Cambrian Volcanics of the Bowers Supergroup and Implications for the R, L.H. 1993. The tectonic expression of slab pull at continental conver- early Paleozoic tectonic evolution of northern Victoria Land, Antarctica. gent boundaries. Tectonics, 12, 303–325. Earth and Planetary Science Letters, 68, 128–140. S, M., F, J.D., Z,S.&T, S.P. 1992. Granite genesis W, H. 1969. Palaeozoic Era. In:P,L.W.(ed.)Handbook of South and mechanisms of convergent orogenic belts with application to the Australian Geology. Government Printer, Adelaide, 84–132. Southern Adelaide Fold Belt. Transactions of the Royal Society of Y,G.C.&L, J.R. 1996. An Australian Phanerozoic Timescale. Oxford Edinburgh: Earth Sciences, 83, 83–93. University Press, Melbourne.
Received 17 June 1997; revised typescript accepted 19 September 1997. Scientific editing by Alex Maltman.
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