T. Flöttmann

Development of the early Paleozoic Pacific margin of Gondwana from detrital-zircon ages across the Delamerian orogen

Detrital-zircon age spectra have been determined for sedimentary rocks from the Delamerian orogen, southern Australia. In Neoproterozoic sedimentary rocks, patterns progressively change from Mesoproterozoic- to Neoproterozoic-dominated detritus and there are few zircons that are close to the depositional age. The base of the Cambrian Kanmantoo Group marks an abrupt change in provenance to detrital patterns dominated by Ross and Delamerian (600–500 Ma) and Grenvillean ages (1200–1000 Ma). These patterns are strikingly similar to those obtained from Lachlan fold belt sedimentary rocks, indicating that the sedimentation recorded in the Kanmantoo Group marks a change from deposition of sediments derived from the Australian cratons to those representative of the early Paleozoic Gondwana mudpile. If sedimentary rocks with zircon-provenance characteristics such as those of the Kanmantoo rocks extend under elements of the Lachlan fold belt, they would provide suitable protoliths for the S-type granites of southeastern Australia.

Source of the Lachlan fold belt flysch linked to convective removal of the lithospheric mantle and rapid exhumation of the Delamerian-Ross fold belt

During the Cambrian-Ordovician the paleo–Pacific margin of Gondwana underwent a rapid transition from a site of convergent deformation (Delamerian-Ross orogeny) to one of sedimentation during which the thick turbiditic sequences of the adjacent basal Lachlan fold belt were deposited. Laser-probe 40 Ar- 39 Ar data show that the synkinematic (523–486 Ma), mid-crustal I- and S-type granites and metamorphic country rocks of the Delamerian fold belt were rapidly cooled at 490–485 Ma, coincident with the intrusion of a suite of high-level, postkinematic A-type granites and gabbros (ca. 497–481 Ma). 40 Ar- 39 Ar data on detrital muscovites from the basal sections of the Lachlan fold belt provide information on exhumation rather than cooling but yield an age pattern (507–480 Ma) identical to that of the Delamerian fold belt, demonstrating that this was the source of the sediments. The combined data require that some 15 km of exhumation occurred very rapidly (∼ 5–15 mm ·yr −1 ), coincident (as implied by overlapping ages) with the cessation of convergent deformation and partial melting to form the postkinematic magmas. This scenario implies a causative link, which we infer to have been convective removal of lithospheric mantle following orogenic thickening. The model is analogous to the Tertiary uplift of the India-Asia orogen, which provided the sediment source for the Bengal fan. Our results suggest that similar processes were important in the evolution of mountain belts at least as far back as the early Phanerozoic.

Early Palaeozoic foreland thrusting and basin reactivation at the Palaeo-Pacific margin of the southeastern Australian Precambrian Craton: a reappraisal of the structural evolution of the Southern Adelaide Fold-Thrust Belt

Abstract Regional and detailed structural mapping, kinematic analysis and balancing and restoration of cross sections has lead to a re-interpretation of the Adelaide Fold Belt in South Australia. In this belt sedimentary rocks of the Late Proterozoic Adelaidean and Early Cambrian Normanville and Kanmantoo sequences were deposited during at least two episodes of subsidence related to major crustal attenuation. Contraction and crustal thickening accompanied by granitoid intrusions are related to the Cambro-Ordovician Delamerian orogeny. During this event both basins were reactivated (“inverted”) and the sedimentary rocks are now incorporated in a WNW-verging foreland fold and thrust belt at the margin of the Proterozoic southeast Australian craton. Maximal shortening of the orogen was around 55%. Within the Adelaidean basin, shortening is dominantly accommodated by major mylonitic shear zones and reverse faults. In the Cambrian Kanmantoo basin, several thrusts are demonstrably reactivated growth faults, across which thickness changes of Cambrian sedimentary rocks are revealed by balancing and restoration of cross sections. Owing to the steep easterly dips of these faults, in the western part of the Kanmantoo basin lateral shortening of up to 58% is largely accommodated by intense folding and fold axial-planar flattening strain. Further east, strain magnitudes wane and the overall shortening of around 30% is accommodated by both, discrete thrust zones and regional folds. The basin reactivation along the Australian part of Gondwanas palaeo-Pacific craton margin reflects the rapid transition from passive to active tectonism, which possibly is a consequence of the disintegration of the formerly conjugate margins of Gondwana and Laurentia.

Formation and reactivation of the Cambrian Kanmantoo Trough, SE Australia: implications for early Palaeozoic tectonics at eastern Gondwana’s plate margin

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.

Geochemical and geochronological constraints on the Glenelg River Complex, western Victoria

During the Cambro‐Ordovician the muddy sandstone and mudstone of the Glenelg River Complex underwent low grade (greenschist) metamorphism involving one principal phase of deformation (Di), which produced open style folds. K‐Ar total‐rock ages of well‐cleaved slate are 550–570 Ma suggesting a minimum Cambrian age of metamorphism. However, around syn‐metamorphic plutons the metasediment has reached upper amphibolite grade and shows at least three phases of deformation including isoclinal F2 folds. In these rocks the onset of partial melting and development of andalusite‐biotite‐muscovite‐feldspar‐garnet assemblages records pressures ∼500 MPa and temperatures reaching 600–700°C. K‐Ar biotite and total‐rock ages on schist and gneiss in this sequence fall in the range 480–500 Ma indicating the time of postmetamorphic closure and, therefore, a minimum Late Cambrian or Early Ordovician age for the metamorphism. Nd (depleted mantle) model ages on the metasediment are ∼ 1.5–2 Ga (at 516 Ma, ϵNd ‐11; 87Sr/86Sr 0.72...

Review of Precambrian-Palaeozoic relationships at the craton margins of southeastern Australia and adjacent Antarctica

Abstract Similarities in the evolution of late Proterozoic-early Palaeozoic mobile belts flanking the southeast Australian and adjacent Antarctic craton margins support the evidence provided by Archaean-early Proterozoic terranes for former juxtaposition of these two cratons in a Gondwanaland context. The mobile belts show an inversion from an extensional to a contractional regime during this evolution of the active palaeo-Pacific margin of Gondwana. Accretion of outboard terranes along the eastern margin of the mobile belts appears to be preceded by subduction in the Antarctic section whereas, in Australia, contraction of the mobile belt resulted in southeastward transpression at its eastern margin, following obduction of oceanic crust, prior to terrane accretion and collision. Further crustal extension expressed by the subsequent formation of basins in both continents led to lateral displacement of formerly contiguous geologic structures.

SEDIMENTARY THICKNESS VARIATIONS AND DEFORMATION INTENSITY DURING BASIN INVERSION IN THE FLINDERS RANGES, SOUTH AUSTRALIA

Abstract The central and northern parts of the Adelaide fold belt in the Flinders Ranges, South Australia, consist of a sequence of Neo-Proterozoic–Cambrian sediments overlying a Meso-Proterozoic basement complex, both of which were mildly deformed in an intracratonic setting during the ∼500 Ma Delamerian orogeny. The fold belt lies within a prominent heat flow anomaly (average heat flows of ∼90 mWm −2 ) reflecting extraordinary enrichments in heat producing elements in the Meso-Proterozoic basement, suggesting that anomalous thermal regimes may have been significant in localising Delamerian deformation. However, spatial variations in deformation intensity correlate more closely with variations in the thickness of the sedimentary sequence than with observed variations in heat flow, suggesting that the thickness of the sedimentary blanket plays a crucial role in localising Delamerian deformation during basin inversion. We use simple numerical models of lithospheric strength to investigate the potential role of sedimentary thickness variations on the distribution and style of deformation, focussing on the impact of a variable thickness sediment pile deposited above a ‘radioactive’ basement. We show that for thermal parameters appropriate to the Flinders Ranges, Moho temperatures may vary by ∼25–30°C for every additional kilometre of sediment. For a ‘Brace–Goetze’ lithospheric rheology, controlled by a combination of temperature-dependent creep processes and frictional sliding, the observed variations in thickness of the sedimentary pile are sufficient to cause dramatic reductions in the vertically-integrated strength of the lithosphere (by many orders of magnitude), thereby providing a plausible explanation for observed correlation between sediment thickness and deformation intensity during basin inversion.

Structural geometry of a thick-skinned fold-thrust belt termination: the Olary Block in the Adelaide Fold Belt, South Australia

The Olary Block comprises a set of Palaeoproterozoic to Mesoproterozoic basement inliers that were deformed together with the Neoproterozoic sedimentary cover of the Adelaide Geosyncline during the ca 500 Ma Cambro‐Ordovician Delamerian Orogeny. Balanced and restored structural sections across this region show shortening of less than 20%. These basement inliers represent the interface between a region of thick‐skinned deformation bordering the Curnamona Craton to the north and a region of thin‐skinned deformation to the south and west in the Nackara Arc. The basement inliers represent upthrust segments of the subsided basin margin with the sedimentary package thickening to the south and to the west. Earlier formed extensional faults provided the major strain guides during Delamerian shortening. An early phase of east‐west shortening is interpreted to be synchronous with dextral strike‐slip deformation along basement‐relay structures (e.g. Darling River lineament). During progressive shortening the tectonic transport direction rotated into a northwest to north direction, coeval with the onset of the main phase of thin‐skinned fold deformation in the adjacent Nackara Arc.

Rb/Sr dating of differentiated cleavage from the upper Adelaidean metasediments at Hallett Cove, southern Adelaide fold belt

In his discussion of our recent paper on dating of differentiated cleavage from Hallett Cove (Turner et al. 1994), Preiss argues that our interpretation of the Rb/Sr data from differentiated cleavages is inconsistent with regional structural correlations in the southern Adelaide fold belt. He also reviews the age data of lateto postkinematic granitoids; this is less pertinent to our arguments which concerns the early stages of deformation. In particular, Preiss argues that fabrics from the foreland region in the west, where the low grade rocks of Hallett Cove are exposed, can be correlated with fabrics in the more internal parts of the belt, some 50 km further east where higher grade rocks are exposed. As we pointed out in our discussion, and Preiss reiterates, independent isotopic data from intrusive granites constrain the ages of structures in the internal parts of the belt (51H85 Ma), but no such data exist in the external parts of the belt. We welcome the opportunity to discuss these issues further and will comment first on Preiss’s structural arguments, and secondly on his discussion of the geochronology.

Palaeozoic basins of southern South Australia: New insights into their structural history from regional seismic data

Interpretation of three recently recorded offshore seismic lines provides a regional picture of the geology from the Gawler Craton across the Stansbury and Troubridge Basins to the Otway Basin in South Australia. The 300 km transect crosses most of the Cambrian Stansbury Basin, which consists of a marginal platform in the west and the Kanmantoo Trough in the east. The Kanmantoo Trough is filled by an eastward deepening sedimentary prism formed by the Kanmantoo Group. The little‐deformed platformal Cambrian sedimentary rocks onlap the Gawler Craton and underlie the Gulf St Vincent. The eastern margin of the platform is separated from the western Kanmantoo Trough by a northeast‐trending zone of intense deformation. This transition zone comprises numerous southeast‐dipping faults which are correlated to faults and shear zones with reverse displacement that are mapped in outcrop on Kangaroo Island and Fleurieu Peninsula. These faults constitute contractionally reactivated former extensional faults, which cont...