Search

Early Ordovician (Phase 1) magmatism in the Macquarie Arc was followed by a magmatic hiatus of ∼9 million years, between late Bendigonian and early Darriwilian (i.e. between ca 475 and ca 466 Ma). Resumption of magmatism in the Middle Ordovician produced Phase 2 rocks, recorded by three major rock suites: (i) medium-K calc-alkaline lavas in the Cargo block (Molong Volcanic Belt) have primitive Nd values (+6.9 to +7.8) and volcanic facies suggesting eruption in an intra-oceanic arc stratovolcano; lavas in the fault-bounded Parkes Volcanics in the Junee - Narromine Volcanic Belt are compositionally identical to those in the Cargo block, suggesting that similar Phase 2 Middle Ordovician arc-type lavas may underlie the Cowra Trough; (ii) medium- to high-K dioritic to monzodioritic intrusions in the Narromine and Cowal Igneous Complexes of the Junee - Narromine Volcanic Belt have ages that cluster in the 470 - 460 Ma interval, and intrude presumed Phase 1 lavas and volcaniclastics; and (iii) in all three main volcanic belts, Middle Ordovician lavas range from medium-K to dominantly high-K calc-alkaline compositions with a clear trend to shoshonitic compositions late in the Phase 2 magmatic episode. Phase 2 units in the Molong Volcanic Belt (lower Blayney, Byng and lower Fairbridge Volcanics) and Rockley - Gulgong Volcanic Belt (Rockley and lower Sofala Volcanics) are dominated by significantly more unfractionated high-MgO lava compositions than contemporaneous lavas in the Cargo block or Junee - Narromine Volcanic Belt, suggesting that rifting of the arc had occurred by this time, and that the main extensional zone lay along the eastern side of the Macquarie Arc. Identical compositions of unusual shoshonitic ultramafic lavas in the Byng Volcanics of the Molong Volcanic Belt and the Rockley Volcanics of the Rockley - Gulgong Volcanic Belt provide strong evidence that these volcanic belts were once contiguous and were disrupted during Silurian - Devonian opening of the Hill End Trough. Phase 3 magmatism in the Macquarie Arc is represented by a widespread but relatively small volume magmatic event, dominated by shallow intrusive rocks of the Copper Hill Suite, emplaced in the Eastonian - Bolindian, between 456 and 441 Ma. These distinctive porphyritic dacites and associated holocrystalline diorites and granodiorites show medium-K calc-alkaline compositions, and their emplacement was intimately linked to an episode of regional uplift, erosion and limestone deposition in the Junee - Narromine Volcanic Belt and western Molong Volcanic Belt. Phase 4 magmatism extended from late Eastonian or Bolindian until Early Silurian time, and was dominated by relatively evolved (compared with Phase 2 lavas) shoshonitic lavas until the end of the Bolindian and porphyries in the Early Silurian. Collision-related shut-down of the arc, and initiation of arc extension and dismemberment, occurred around 438 Ma in the latest Ordovician. Post-arc magmatism during the Early Silurian is represented by high-Th, high-Nb lavas of the shoshonitic Nash Hill Volcanics in the Junee - Narromine Volcanic Belt, and Alaskan-type zoned ultramafic intrusions of the Fifield complexes farther west. The latter were emplaced through deformed Ordovician turbidites of the Girilambone Group, and their radiogenic isotope signatures show significant crustal involvement. [ABSTRACT FROM AUTHOR]

The provenance of the upper Cambrian to Upper Ordovician sedimentary rocks of Tasmania and Waratah Bay in southern Victoria provides information about the complex and dynamic tectonic environment present during their deposition. This paper uses U–Pb detrital zircon data to constrain stratigraphic comparisons and tectonic reconstructions of these rock sequences. Multivariate statistics are used to investigate the similarity between the U–Pb ages and quantify the disparity among different samples from various locations. In western and central Tasmania, the Tyennan region supplied most detrital zircons during the late Cambrian and Early Ordovician. The overlying Middle Ordovician Pioneer Sandstone records a switch in provenance with zircons derived from the Mount Read Volcanics (MRV) mixed with zircons similar to those from continental-derived Paleozoic sedimentary rocks deposited throughout east Gondwana. The Middle to Upper Ordovician Gordon Group in western and central Tasmania lacks detrital zircons younger than 1.2 Ga, which indicates a return to a local provenance from Precambrian rocks. In southern Tasmania, the switch to zircons derived from the MRV and east Gondwana-like sources occurred earlier within the Cambrian Deadmans Bay Formation, which is dominated by the east Gondwana Paleozoic zircon age signature. In the East Tasmania Terrane, Ordovician sedimentary rocks from Lefroy have detrital zircon populations dominated by Neoproterozoic and earliest Paleozoic sources similar to the Ordovician sedimentary rocks in the Lachlan Orogen. In southern Victoria, the Bear Gully Chert from Waratah Bay exhibit both Tyennan and distal Gondwana detrital sources. The switching of detrital zircon sources in the west Tasmanian sedimentary sequences implies the docking of Tasmania with mainland Australia during the Cambrian Tyennan Orogeny. The arrival of the distal zircons into these basins occurred at different times in the different areas, reflecting a complex local topography and paleogeography. Paleozoic sedimentary rocks in Tasmanian exhibit multisource detrital U–Pb age signatures that change over time, implying tectonic activity during their deposition. Paleozoic sedimentary rocks in northeastern Tasmania show Gondwana-wide detrital signatures similar to Lachlan Orogen. The Ordovician Bear Gully Chert at Waratah Bay in southern Victoria shows mixed Tasmanian and distal Gondwana detrital populations. The change in detrital zircon signature in western Tasmania suggests that VanDieland docked with the Australian continent during the Cambrian Tyennan Orogeny. [ABSTRACT FROM AUTHOR]

The Ordovician Turquoise Bluff Slate in northeastern Tasmania is a 2 km-thick sequence of deep-marine siliceous black slates. It is dominated by meta-siltstones with bimodal grainsize distributions typical of turbidite TE-1 and TE-2 facies. The slates have high SiO2 indicating they are hemipelagites. The high Ba and V indicate they were deposited in an anoxic environment associated with high oceanic productivity. All these features are common in muddy turbidites. U-Th-Pb dating of detrital monazite and authigenic xenotime in the slates supports previous evidence that the dominant cleavage, in this unit, formed during the Benambran Orogeny. The whole-rock composition of the slates is similar to black slates in the Adaminaby Group, NSW. A review of Paleozoic whole-rock compositions from the Lachlan Orogen confirms they all have trace element contents similar to average Australian shale. However, there are subtle differences in composition. The Turquoise Bluff Slate and other Mathinna Supergroup rocks from the Eastern Tasmania Terrane have higher average Cr content than similar age turbidites from Victoria and NSW. This probably reflects a small contribution from Tasmania Cambrian ultramafic rocks in the provenance. If this were correct, northeastern Tasmania was closer to western Tasmania in the Paleozoic than other provinces of the Lachlan Orogen, southeastern Australia. Other subtle features of the whole-rock composition of Paleozoic sedimentary rocks from the Lachlan Orogen indicate it may be possible to recognise provincial variations in composition that will provide new constraints on tectonic models of southeastern Australia. [ABSTRACT FROM AUTHOR]

Nine Paleozoic sedimentary formations from western Tasmania and two from the Lachlan Orogen—one in northeast Tasmania and one in Waratah Bay in Victoria—contain detrital zircons that fall into major U–Pb age clusters at 2.0–1.4, 1.25–0.95 and 0.62–0.49 Ga. The zircon Th/U ratios and rare earth element (REE) patterns suggest a magmatic origin for detrital zircons in all these rock sequences. The REE geochemical signatures on bivariate discrimination diagrams indicate that most of the zircons originated in continental orogenic settings. The oldest group of zircons have a southwest Laurentian signature previously recognised from Proterozoic metasedimentary rocks in Tasmania. The 1.25–0.5 Ga zircons from all the samples have very similar εHfi values. They are not statistically different from those of typical Lachlan Orogen sandstones and resemble those in Rodinia and early Gondwana orogenic belts. The εHfi values of the Cambrian zircons within the Pioneer Sandstone are similar to the Mount Read Volcanics but cannot be distinguished on εHfi values from other sources in East Gondwana. The detrital zircon provenance of the Bear Gully Chert Bed at the base of the Digger Island Marlstone at Waratah Bay includes the west Tasmanian Proterozoic, the East Gondwana margin and the Macquarie Arc. This mixed provenance provides evidence for the late Cambrian docking of VanDieland with East Gondwana. New zircon Hf isotope data are reported for early Paleozoic sedimentary rocks in Tasmania and Waratah Bay, southern Victoria. The western Tasmania basement was the dominant source for Tasmanian sandstones and a minor source for the Lachlan Orogen sandstones on the northeast margin of VanDieland. The mixed provenance of the Bear Gully Chert supports a late Cambrian accretion of VanDieland to the East Gondwana margin. The youngest zircons in the Bear Gully Chert at Waratah Bay were potentially sourced from the Macquarie Arc. [ABSTRACT FROM AUTHOR]

Recumbent folding in eastern Tasmania affected turbidites containing Lower to Middle Ordovician (Bendigonian Be1 to Darriwilian Da3) fossils, but not stratigraphically overlying turbidites containing Silurian (Ludlow) graptolites, and is of a timing consistent with Ordovician to Silurian Benambran orogenesis on the Australian mainland. Two subsequent phases of upright folding post-date deposition of turbidites containing Devonian plant fossils but pre-date intrusion of Middle Devonian granitoids, and are of Tabberabberan age. A closely spaced disjunctive cleavage (S2), associated with the first phase of Tabberabberan folding, everywhere cuts a slaty cleavage (S1) associated with the earlier formed recumbent folds. However, refolding associated with development of S2 is not always clear in outcrop and it is proposed that coincident tectonic vergence between the two events has resulted in reactivation of recumbent D1 structures during the D2 event. The transition to rocks not affected by recumbent folding coincides with a marked change in sedimentology from shale- to sand-dominated successions. This contact does not outcrop but, from seismic data, appears to dip moderately to the east, and can only be explained as an unconformity. The current grouping of all pre-Middle Devonian turbidites in eastern Tasmania into the one Mathinna Group is misleading in that the turbidite sequence can be subdivided into two distinct sedimentary packages separated by an orogenic event. It is proposed that the Mathinna Group be given supergroup status and existing formations placed into two new groups: an older Early to Middle Ordovician Tippogoree Group and a younger Silurian to Devonian Panama Group. [ABSTRACT FROM AUTHOR]

Neoproterozoic and Cambrian tectonic and magmatic events in southeast Australia, Tasmania, New Zealand and the Transantarctic Mountains have been described as aspects of the Ross–Delamerian Orogeny. Summaries of the salient points suggest they are parts of the same active margin that developed along a paleo-Pacific rifted edge of the fragmented Rodinia supercontinent. Late Neoproterozoic to Cambrian subduction produced a major magmatic arc that cuts Neoproterozoic and early Cambrian rocks. The Ross–Delamerian Orogen includes two exotic blocks, the Beardmore and VanDieland microcontinents, which accreted in the early Cambrian and middle Cambrian respectively. Other possible exotic elements are the Bowers Terrane in northern Victoria Land and the Takaka Terrane in New Zealand, with both sharing a common middle to late Cambrian history. Compressive deformation occurred at poorly constrained intervals in the early Cambrian but is well constrained in the middle Cambrian. In Australia and Victoria Land of Antarctica, magma compositions changed in the late Cambrian reflecting a change to extension. Bi-modal volcanism and stratigraphy in the Pensacola Mountains of Antarctica are similarly interpreted. Subduction slowed and ceased towards the end of the Cambrian implying a change in relative motion along the Australian–Antarctica segment of the Gondwanan plate boundary. [ABSTRACT FROM AUTHOR]

The Upper Cambrian Owen Conglomerate of the West Coast Range, western Tasmania, comprises two upward-fining successions of coarse-grained siliciclastic rocks that exhibit a characteristic wedge-shaped fill controlled by the basin-margin fault system. Stratigraphy is defined by the informally named basal lower conglomerate member, middle sandstone member, middle conglomerate member and upper sandstone member. The lower conglomerate member has a gradational basal contact with underlying volcaniclastics of the Tyndall Group, while the upper sandstone member is largely conformable with overlying Gordon Group marine clastics and carbonates. The lower conglomerate member predominantly comprises high flow regime, coarse-grained, alluvial-slope channel successions, with prolonged channel bedload transport exhibited by the association of channel-scour structures with upward-fining packages of pebble, cobble and boulder conglomerate and sandstone, with abundant large-scale cross-beds derived from accretion in low-sinuosity, multiply active braided-channel complexes. While the dipslope of the basin is predominantly drained by west-directed palaeoflow, intrabasinal faulting in the southern region of the basin led to stream capture and the subsequent development of axial through drainage patterns in the lower conglomerate member. The middle sandstone member is characterised by continued sandy alluvial slope deposition in the southern half of the basin, with pronounced west-directed and local axial through drainage palaeoflow networks operating at the time. The middle sandstone member basin deepens considerably towards the north, where coarse-grained alluvial-slope deposits are replaced by coarse-grained turbidites of thick submarine-fan complexes. The middle conglomerate member comprises thickly bedded, coarse-grained pebble and cobble conglomerate, deposited by a high flow regime fluvial system that focused deposition into a northern basin depocentre. An influx of volcanic detritus entered the middle conglomerate member basin via spatially restricted footwall-derived fans on the western basin margin. Fluvial systems continued to operate during deposition of the upper sandstone member in the north of the basin, facilitated by multiply active, high flow regime channels, comprising thick, vertically stacked and upward-fining, coarse-grained conglomerate and sandstone deposits. The upper sandstone member in the south of the basin is characterised by extensive braid-delta and fine-grained nearshore deposits, with abundant bioturbation and pronounced bimodal palaeocurrent trends associated with tidal and nearshore reworking. An increase in base-level in the Middle Ordovician culminated in marine transgression and subsequent deposition of Gordon Group clastics and carbonates. [ABSTRACT FROM AUTHOR]

Seismic data from three separate experiments, a marine active source survey with land-based stations, and two teleseismic arrays deployed to record distant earthquakes, are combined in a joint inversion for the 3D seismic structure of the Tasmanian lithosphere. In total, travel-time information from nearly 14 000 source-receiver paths are used to constrain a detailed model of crustal velocity, Moho geometry and upper mantle velocity beneath the entire island. Synthetic reconstruction tests show good resolution beneath most of Tasmania with the exception of the southwest, where data coverage is sparse. The final model exhibits a number of well-constrained features that have important ramifications for the interpretation of Tasmanian tectonic history. The most prominent of these is a marked easterly transition from lower velocity crust to higher velocity crust which extends from the north coast, northeast of the Tamar River, down to the east coast. Other significant anomalies include elevated crustal velocities beneath the Mt Read Volcanics and Forth Metamorphic Complex; thickened crust beneath the Port Sorell and Badger Head Blocks in central northern Tasmania; and distinctly thinner, higher velocity crust beneath the Rocky Cape Block in northwest Tasmania. Combined with existing evidence from field mapping, potential-field surveys and geochemical data, the new results support the contention that east and west Tasmania were once passively joined as far back as the Ordovician, with the transition from lithosphere of Proterozoic continental origin to Phanerozoic oceanic origin occurring some 50 km east of the Tamar River; that the southeast margin of the Rocky Cape Block may have been a former site of subduction in the Cambrian; and that the Badger Head and Port Sorell Blocks were considerably shortened and thickened during the Cambrian Tyennan and Middle Devonian Tabberabberan Orogenies. [ABSTRACT FROM AUTHOR]

The Benambra Terrane of southeastern Australia is the eastern, allochthonous portion of the Lachlan Fold Belt with a distinctive Early Silurian to Early Devonian history. Its magmatic, metamorphic, structural, tectonic and stratigraphic histories are different from the adjacent, autochthonous Whitelaw Terrane and record prolonged orogen-parallel dextral displacement. Unlike the Whitelaw Terrane, parts of the proto-Benambra Terrane were affected by extensive Early Silurian plutonism associated with high T/low P metamorphism. The orogen-parallel movement (north–south) is in addition to a stronger component of east–west contraction. Three main orogenic pulses deformed the Victorian portion of the terrane. The earliest, the Benambran Orogeny, was the major cratonisation event in the Lachlan Fold Belt and caused amalgamation of the components that comprise the Benambra Terrane. It produced faults, tight folding and strong cleavage with both east–west and north–south components of compression. The Bindian (= Bowning) Orogeny, not seen in the Whitelaw Terrane, was the main period of southward tectonic transport in the Benambra Terrane. It was characterised by the development of large strike-slip faults that controlled the distribution of second-generation cleavage, acted as conduits for syntectonic granites and controlled the deformation of Upper Silurian sequences. Strike-slip and thrust faults form complex linked systems that show kinematic indicators consistent with overall southward tectonic transport. A large transform fault is inferred to have accommodated approximately 600 km of dextral strike-slip displacement between the Whitelaw and Benambra Terranes. The Benambran and Bindian Orogenies were each followed by periods of extension during which small to large basins formed and were filled by thick sequences of volcanics and sediments, partly or wholly marine. Some of the extension appears to have occurred along pre-existing fractures.... [ABSTRACT FROM AUTHOR]

The Lune River Quarry at Ida Bay, Tasmania exposes numerous palaeokarst features developed in the Ordovician Gordon Limestone. These palaeokarst features contain carbonate and siliciclastic deposits probably representing Late Devonian to early Late Carboniferous and Late Carboniferous karstification and sedimentation. Five facies of palaeokarst deposits are recognised, namely megabreccia, graded-bedded carbonate, laminated sandstone/siltstone, diamictite/quartz-lithic sandstone and coarse crystalline calcite. Pyrite, dolomite and sphalerite were emplaced in the palaeokarst deposits after the Carboniferous. These deposits are probably associated with a phase of hydrothermal cave development in Exit Cave, which adjoins the quarry. Pyrite weathering accounts for the abundance of gypsum speleothems and cave breakdown in Exit Cave. [ABSTRACT FROM AUTHOR]

Regional variations in illite crystallinity of the Ordovician-Devonian Mathinna Group of northeast Tasmania indicates that metamorphic grade is dominantly anchizonal (200–300°C, sub-greenschist facies) over an area of 80 km × 100 km. The b[sub 0] white mica variation (b[sub 0] 9.020 ± 0.008 Å) is characteristic of intermediate pressure Barrovian-type metamorphism. Peak metamorphism does not correlate with stratigraphic position and does not change across most of the large faults, indicating that peak metamorphism occurred after east-directed thrusting and as a result of crustal thickening. Most Devonian granites in northeast Tasmania intruded at much lower pressure than the peak metamorphic conditions, suggesting very rapid unroofing in the Middle Devonian. These results are very similar to the metamorphic characteristics reported from the Lachlan Fold Belt in Victoria. The metamorphic history is used to support a thin-skinned model for the deformation in a tectonic wedge that tapered to the east and was subsequently exhumed by backthrusting to the west. [ABSTRACT FROM AUTHOR]

Serpentinite/talc-matrix mélanges, bearing blocks of blueschist metavolcanics, occur within the Heathcote and Governor Fault Zones of the southern Lachlan Orogen. In the Heathcote Fault Zone, serpentinite-matrix mélange consists of blocks or small pods of boninite, andesite, ultramafic rocks, chert and volcanogenic sandstone variably metamorphosed to prehnite–pumpellyite, greenschist, or greenschist to blueschist facies. In the Governor Fault Zone, blueschist metavolcanics occur as blocks within serpentinite/talc matrix that is interleaved with prehnite–pumpellyite to greenschist facies, intermediate pressure slate and phyllite. Ar/Ar dating of white mica from slaty mud-matrix (broken formation) indicates that the main fabric development occurred at 446 ± 2 Ma. U–Pb (SHRIMP) dating of titanite from blueschists in the Governor Fault Zone indicates that metamorphism occurred at approximately 450 Ma, close to the time of mélange formation. Previously published, Ar/Ar dating of white mica from phyllite and biotite from metadiorite in the Heathcote Fault Zone suggest that blueschist metamorphism occurred at a similar time. These ages are supported by field relationships. Illite crystallinity and b0 data from white mica, and the preservation of blueschist blocks indicate that these fault zones maintained low temperatures both during and after intermediate- to high-pressure metamorphism. Occurrences of blueschists in the Arthur Lineament of the Tyennan (Delamerian) Orogen in Tasmania, and in the New England Orogen, have different ages, and in conjunction with the occurrences described here, suggest that subduction–accretion processes contributed significantly to the development of the Tasmanides from Cambrian through to Carboniferous times. [ABSTRACT FROM AUTHOR]

The Mount Lyell copper deposits are located in the middle Cambrian Mount Read volcanic belt of western Tasmania and consist of more than 24 separate copper-gold-silver orebodies. The dominant copper mineralisation style is disseminated pyrite-chalcopyrite subvertical pipes with subordinate chalcopyrite-bornite ± other copper phases, massive pyrite and base metal sulfides. A zonation in mineralisation style within the pipes is defined from chalcopyrite-magnetite at depth to chalcopyrite-pyrite at intermediate levels, to chalcopyrite-bornite at the shallowest level. Alteration is developed broadly symmetrically around the ore zones and zoned from quartz-chlorite-phengite ± biotite at depth to quartz-muscovite at intermediate levels, and a quartz-muscovite-pyrophyllite-zunyite assemblage at the shallowest levels. This is interpreted to be a result of a fluid that evolved from hot, reduced and neutral conditions at depth to cool, oxidised and acidic conditions at the shallowest level. The chalcopyrite-bornite deposits occur at the top of the hydrothermal system and are associated with intensely silicified rock and muscovite/pyrophyllite alteration. The close relationship of these deposits with the top of the pipes suggests they are part of a single mineralising event. Where the chalcopyrite-bornite deposits are juxtaposed with the Owen Group, rather than a simple chalcopyrite-bornite mineralogy, there are numerous other copper phases, which represent higher oxidation states and collectively suggest variable and fluctuating fluid conditions during deposition. It is proposed that these deposits are formed by an interaction of the reduced hydrothermal fluid with an oxidised fluid generated at very shallow levels within and during deposition of the Owen Group. Mineralisation within the middle Owen Group sandstones and clasts of altered rock within the middle and upper Owen Group sediments marks the end of the hydrothermal system. Around the entire edge of the Mt Lyell field, there is a variation in the white mica composition from proximal muscovite to distal phengite that represents the neutralisation of the hydrothermal fluid by fluid-wall rock interaction. [ABSTRACT FROM AUTHOR]

This study presents new data on the deformational and metamorphic history of previously unstudied Cambrian high-pressure metamorphic rocks exposed on the remote south coast of Tasmania. The Red Point Metamorphic Complex consists of two blocks of high-pressure, medium-grade metamorphic rocks including pelitic schist and minor garnet-bearing amphibolite, which are faulted against a sequence of low-grade phyllite and quartzite. The Red Point Metamorphic Complex records five phases of deformation, all of which except the first are expressed at a mesoscopic scale in both the medium- and low-grade rocks. Peak metamorphic conditions in the high-pressure epidote–amphibolite facies is recorded by medium-grade schist and amphibolite and was synchronous with the second major deformation event, which produced a pervasive schistosity and mesoscale isoclinal folds. The juxtaposition of the low- and medium-grade rocks is interpreted to have first occurred prior to the development of upright, opening folding associated with the third deformation. However, the present contacts between the two contrasting metamorphic sequences formed during widespread faulting and ductile-shear zone development associated with the fourth and fifth deformation events. The new data from the Red Point Metamorphic Complex provide insights into the structural and metamorphic history experienced by the medium-grade rocks of Tasmania during the Cambrian Tyennan Orogeny. This study demonstrates that Cambrian medium-grade metamorphic rocks are more widespread throughout Tasmania than previously realised, which represents an important step towards understanding the large-scale architecture of the Tyennan Orogen. [ABSTRACT FROM AUTHOR]

Jurassic dolerite is well exposed in fresh cuttings and quarries throughout Tasmania, and fault striations were found in a high proportion of these exposures in the south and west of the state. These fault striation provide a record of late Mesozoic faulting dominated by dextral west-northwest–east-southeast-striking faults and sinistral north–south-striking faults. This fault association indicates compression from the NNW. Indirect evidence suggests these structures formed during the Cenomanian (ca95 Ma) inversion in the Otway and Bass basins, and are probably part of the crustal shortening that formed the Transantarctic Mountains. [ABSTRACT FROM PUBLISHER]

The origin of the microcontinent VanDieland extends back to the late Paleoproterozoic, where it was positioned between East Antarctica and southwestern Laurentia, within the supercontinent Nuna and Rodinia. Paleo- to Mesoproterozoic events recorded in VanDieland have greater affinities with southwest Laurentia and East Antarctica, suggesting southern VanDieland was part of the Grenville Front, and the central Tasmanian part was adjacent to the Miller Range in the central Transantarctic Mountains. Late in the Neoproterozoic Rodinia break-up, VanDieland separated from East Antarctica and southwestern Laurentia, and moved north along the Terra Australis margin until its southern part was positioned next to the easternmost Robertson Bay Terrane of north Victoria Land. VanDieland comprises up to seven different crustal megaboudins or microcontinental ribbon terranes that likely had amalgamated by the end of the Cambrian; these ribbon terranes are bounded by major faults and suture zones. Some boundaries, such as the Arthur Metamorphic Complex, are well known. However, other boundaries, like the eastern edge of the Tyennan Zone, and the boundary between King Island and northwestern Tasmania, are more cryptic, as they are covered by younger geology or are under water. The boundaries are commonly defined by sedimentary and mafic volcanic infill that has been trapped between the crustal fragments. These rocks have previously been interpreted as allochthonous terranes but are more likely to represent inverted sections of attenuated transitional crust and back arc basin fill that formed along the eastern margin of the Gondwana plate during the Cambrian. This interpretation also provides an explanation for the previous tectonic analysis that suggests that Tasmania's mafic–ultramafic complexes were obducted westward onto older sequences and were subsequently transported southwards as other ribbons collided along the northeastern and western edges of the growing microcontinent, which existed in the overriding plate of a west-dipping subduction zone at the convergent margin between Gondwana and the proto-Pacific plate. [ABSTRACT FROM AUTHOR]

The Hellyer–Mt Charter region of western Tasmania includes three known and economically significant volcanic-hosted massive sulfide (VHMS) deposits. Thick vegetation and poor outcrop present a considerable challenge to ongoing detailed geological field mapping in this area. Numerous geophysical and soil geochemical datasets covering the Hellyer–Mt Charter region have been collected in recent years. These data provide a rich source of geological information that can assist in defining the spatial distribution of lithologies. The integration and analysis of many layers of data in order to derive meaningful geological interpretations is a non-trivial task; however, machine learning algorithms such as Random Forests and Self-Organising Maps offer geologists methods for indentifying patterns in high-dimensional (many layered) data. In this study, we validate an interpreted geological map of the Hellyer–Mt Charter region by employing Random Forests™ to classify geophysical and geochemical data into 21 discrete lithological units. Our comparison of Random Forests supervised classification predictions to the interpreted geological map highlights the efficacy of this algorithm to map complex geological terranes. Furthermore, Random Forests identifies new geological details regarding the spatial distributions of key lithologies within the economically important Que-Hellyer Volcanics (QHV). We then infer distinct but spatially contiguous sub-classes within footwall and hangingwall, basalts and andesites of the QHV using Self-Organising Maps, an unsupervised clustering algorithm. Insight into compositional variability within volcanic units is gained by visualising the spatial distributions of sub-classes and associated statistical distributions of key geochemical data. Compositional differences in volcanic units are interpreted to reflect contrasting primary composition and VHMS alteration styles. We conclude that combining supervised and unsupervised machine-learning algorithms provides a widely applicable, robust means, of analysing complex and disparate data for machine-assisted geological mapping in challenging terranes. [ABSTRACT FROM AUTHOR]

The metamorphic complexes of Tasmania formed during the Cambrian (ca 510 Ma) as a result of rapid compression in a subduction zone setting followed by rapid exhumation, which brought various fault-bounded metamorphic complexes back to the surface in less than 5 Ma. The two highest grade complexes, the Franklin Metamorphic Complex, and the Port Davey Metamorphic Complex, experienced initial growth of metamorphic garnets at ∼560°C, ∼0.56 GPa. However, their subsequent metamorphic histories diverge, with the FMC displaying a marked increase in pressure (to 1.4 GPa at peak P/T), while the PDMC shows only a slight increase in pressure (to ∼0.7 GPa). Both complexes show only a minor increase in temperature (∼100°C) between initial garnet growth and peak metamorphic conditions. Rapid exhumation of these complexes can be accounted for by a slab-breakoff model. However, the difference in peak pressure between these complexes requires either continued subduction of the FMC while the PDMC had already begun its return towards the surface or that the subduction zone geometry resulted in significantly different pressures occurring contemporaneously within portions of the channel, which are not far removed from one another. [ABSTRACT FROM AUTHOR]

Uncertainty in past and future anthropogenic carbon emissions obscures climate change modelling. Available allometrics are insufficient for regional-level accounting of old-growth, pre-logging carbon stocks. The project goal was to determine the aboveground carbon (biomass and necromass) for a typical old-growth Eucalyptus delegatensis-dominated mixed-forest in Tasmania. Allometrics were developed for aboveground biomass of Eucalyptus delegatensis and generic rainforest understorey species. A total of 207 eucalypts with DBH 0.21–4.5 m, and 897 rainforest understorey trees with DBH 0.01–2.52 m were measured across 7.7 ha. DBH frequency distribution of E. delegatensis showed at least two age cohorts and distinct positive skew, whereas its DBH carbon distribution showed distinct negative skew. Half of the eucalypt biomass was from trees with DBH > 2.4(0.1) m, and 16% with DBH ≥ 3.5 m (from ∼1.1 trees ha−1) – indicating the importance of allometrics for high DBH. Aboveground carbon was 622(180) Mg ha−1, with ∼20% from understorey and ∼25% from necromass. The carbon in aboveground biomass was above the median value for temperate forests. The long-term aboveground-carbon emissions from clearfelling the same forest type from 1999 to 2009 is likely to be 2.9(±1.3) Tg, depending on the growth and seral stages of the forest logged. [ABSTRACT FROM PUBLISHER]

In western Tasmania, near Melba Flats, a basalt within the Serpentine Hill Complex is overlain unconformably by Dundas Group sediments (Hodge Slate). About 32 m above the unconformity, a siltstone contains agnostoid and polymerid trilobites that indicate correlation with the middle Cambrian Ptychagnostus punctuosus Zone (Drumian Stage of unnamed Epoch 3 of the Cambrian). This provides an upper limit of about 505.2-504.5 Ma to the emplacement of ultramafic allochthons in western Tasmania. Other fossils present include dendroids, inarticulate brachiopods, hyolithids, helcionellids and sponges. The widespread agnostoid trilobite Ptychagnostus affinis (Brøgger 1878) and the holocephalinid trilobites Meneviella and Holocephalina are recorded for the first time in Tasmania. [ABSTRACT FROM AUTHOR]

A series of new Sensitive High-Resolution Ion MicroProbe (SHRIMP) U – Pb ages is presented for Palaeozoic (mainly Devonian and Carboniferous) granites from Tasmania. In virtually all instances the new ages are significantly older than previously determined Rb – Sr and K – Ar ages, even though the level of emplacement had been thought to be too shallow to allow loss of radiogenic daughter products. In two extreme cases, granite bodies at South West Cape and Elliott Bay that had previously yielded Carboniferous Rb – Sr and Early Devonian K – Ar ages, respectively, are now both shown to be Late Cambrian. In northeast Tasmania, granitic activity in the Blue Tier Batholith lasted for about 22 million years, with I-type magmas being followed by S-types only toward the end of that time. The exclusively I-type granites of the Scottsdale Batholith formed about 10 million years after the initiation of igneous activity in the Blue Tier Batholith, and were emplaced over a comparatively short time interval (4 – 5 million years). The new data confirm a previously held view, based on Rb – Sr analysis, that the economically important Lottah Granite crystallised roughly 9 million years later than the nearby Poimena Granite and, therefore, could not have been derived by magmatic fractionation of the latter. A regional deformation equated with the Tabberabberan Orogeny has been dated at about 390 Ma in northeastern Tasmania, based on the presence or absence of a northwest-trending foliation in the different granite bodies. The oldest granites occur in the northeast of Tasmania, with an irregular progression of ages to the west coast. A trend of this type could have arisen in an arc-free or arc-related environment. If the latter applies, either flat subduction or processes associated with the amalgamation of eastern and western basement terranes might be the controlling mechanism. Eastern Tasmania experienced a trend from mafic I-type to progressively more felsic, largely S-type igneous activity, but the trend for western Tasmania is not as obvious. The trend for eastern Tasmania is an exception to the general rule for the Lachlan Orogen, possibly signifying that the mid-crust was relatively cool when the first I-type granites were generated. Crustal thickening during the Tabberabberan Orogeny may have been a prerequisite for the generation of later, more felsic, S- and I-types. [ABSTRACT FROM AUTHOR]

A new model accounts for the complex structural geometries observed at the contact between the Middle to Upper Cambrian Mt Read Volcanics and the Upper Cambrian Owen Conglomerate at the Mt Lyell Mine, Queenstown, Tasmania. Re-examination of the structure has revealed that the Great Lyell Fault comprises a series of Middle Cambrian, Late Cambrian and Middle Devonian aged faults. The Great Lyell Fault comprises a collage of Middle Cambrian faults active during emplacement of the Mt Read Volcanics and Late Cambrian growth structures that controlled deposition of the Owen Conglomerate. These faults were reverse reactivated during initial Middle Devonian orogenesis. The D1 shortening event reverse reactivated the Mt Read Volcanics fault, with shortening manifest by reverse faulting and folding. The Owen Conglomerate basin-margin fault at the Mt Lyell mine site is interpreted to have been dissected by a (D1) reverse-reactivated Mt Read Volcanics fault, which also led to local overturning of the Owen Conglomerate succession. Structural geometries along the Owen Conglomerate – Mt Read Volcanics contact are further complicated by the competency contrast between the Owen Conglomerate and the Mt Read Volcanics, with the development of lobe and cusp folding of the contact during pervasive D2 deformation. Folding associated with the D2 shortening event refolded the Owen Conglomerate – Mt Read Volcanics contact and extensively faulted the Owen Conglomerate, leading to the complex outcrop patterns identified in the Southern Corridor of the mine site. The model accounts for the complex local geology, particularly in light of recent advances in understanding of the regional tectonic evolution, such as the identification of a Late Cambrian extension event in western Tasmania. [ABSTRACT FROM AUTHOR]

The Owen Conglomerate comprises coarse-grained siliciclastics that were deposited in response to Late Cambrian extension. The identification of normal faults that host thickened accumulations of siliciclastics is used here to support interpretation of syn-fill extension. Local mapping and section construction have identified a series of north-trending, en échelon , segmented normal faults that exhibit changes in along-strike polarity. The Late Cambrian faults are adjacent to sedimentary packages that define half-graben geometries, with an unconformity that defines basal contacts with underlying Mt Read Volcanics and onlap geometries onto the opposing basin margin. Faults that were active during deposition of the Owen Conglomerate were subsequently reversed during D1 Middle Devonian deformation, with reverse displacement controlling the development of inversion structures defined by north-trending fold structures. Pervasive northwest-trending D2 deformation extensively overprints earlier deformation features, and has led to the spectacular development of type 1 interference patterns that largely control outcrop distributions along the West Coast Range. Field evidence is documented in support for a simple structural history that accounts for geometries associated with Late Cambrian extension, prior to Middle Devonian inversion (D1) and subsequent shortening (D2). [ABSTRACT FROM AUTHOR]

Detrital zircons from 13 Late Mesoproterozoic to Early Neoproterozoic sandstones and two Palaeozoic sandstones from Tasmania were dated in order to improve constraints on depositional ages, to test correlation between Proterozoic inliers, and to characterise source regions. These include successions considered to be the oldest presently exposed in Tasmania. Typical features of the age distributions of the Proterozoic rocks are prominent data concentrations at 1800–1650 Ma and 1450–1400 Ma, and a minor spread of Archaean ages. Statistical testing of the similarity of the age profiles shows that wide-spread quartzarenaceous samples from the Detention Subgroup, Needles Quartzite and from the Tyennan region are strongly similar, consistent with broad correlation. Relatively large differences are seen between the Detention Subgroup and the conformable, stratigraphically higher Jacob Quartzite, which contains an additional spread of 1300–1000 Ma zircons suggestive of a Grenvillian source. Age profiles of the quartzarenites and quartzwacke turbidites (Oonah Formation and correlatives) cannot be readily differentiated. The Oonah Formation likewise includes samples with and without Grenvillian ages, and there is no 750 Ma zircon population that would be expected if the turbidites were genetically related to the Wickham Orogeny. The simplest interpretation is that the quartzarenites (Rocky Cape Group and correlatives) and the turbidites (Oonah Formation and correlates) are lateral equivalents, although a younger (post-Wickham Orogeny) age for the Oonah Formation cannot be discounted.A maximum age of ca 1000 Ma is inferred for the Oonah Formation, Rocky Cape Group and correlatives. A minimum age of ca 750 Ma is provided by the basal age of the overlying Togari Group and correlatives. In a metasediment from western King Island, the youngest detrital zircons ore ca 1350 Ma, allowing a pre-Grenvillian depositional age as suggested by previous dating of metamorphic monazite. However, the age profile of this sample is not dissimilar to the other Tasmanian successions that are inferred to be 1000–750 Ma. The Wings Sandstone, of southern Tasmania, contains an unusual profile dominated by Grenvillian ages, consistent with an allochthonous origin. Basement ages that broadly match the age spectra of the Tasmanian Proterozoic sediments are found in southwestern Laurentia, consistent with mutual proximity in Rodinia reconstructions. The Palaeozoic sandstones, from the turbiditic Mathinna Supergroup of northeastern Tasmania, have zircon age profiles typical of the Lachlan Fold Belt, with a predominant latest Neoproterozoic-Early Cambrian component and a lesser, broad Proterozoic data concentration at ca 1000 Ma. Western Tasmania was not a significant part of the source area for these rocks. [ABSTRACT FROM AUTHOR]

The Tasman Line, a much-discussed concept in the geology and tectonics of eastern Australia, has a long and chequered history of interpretation. This extends to current debates regarding the age and position of the Tasman Line in Gondwana–Rodinia reconstructions. We present constraints, from mapping, geochemistry and geophysics, on the interpretation of gravity and magnetic lineaments attributed to the Tasman Line in New South Wales, South Australia, Victoria and Tasmania. These pieces of evidence suggest a protracted and complex latest Neoproterozoic to Carboniferous geological history that produces a variety of geophysical responses, rather than a simple ‘Line’. We also find no evidence of Rodinian breakup age activity responsible for any of the anomalies. In light of these findings, our preference is that the Tasman Line concept be abandoned as misleading, especially with regard to models of Rodinia–Gondwana breakup, which must have occurred elsewhere, possibly well to the east. Instead, the rocks preserved in the westernmost part of the Tasmanides are consistent with previously proposed ‘Southwest Pacific’-style models for Neoproterozoic continental breakup, margin formation and reaccretion of continental fragments in the Early Palaeozoic. [ABSTRACT FROM AUTHOR]

The Arthur Lineament of northwestern Tasmania is a Cambrian (510 ± 10 Ma) high-strain metamorphic belt. In the south it is composed of metasedimentary and mafic meta-igneous lithologies of the ‘eastern’ Ahrberg Group, Bowry Formation and a high-strain part of the Oonah Formation. Regionally, the lineament separates the Rocky Cape Group correlates and ‘western’ Ahrberg Group to its west from the relatively low-strain parts of the Oonah Formation, and the correlated Burnie Formation, to its east. Early folding and thrusting caused emplacement of the allochthonous Bowry Formation, which is interpreted to occur as a fault-bound slice, towards the eastern margin of the parautochthonous ‘eastern’ Ahrberg Group metasediments. The early stages of formation of the Arthur Lineament involved two folding events. The first deformation (Ca D1 ) produced a schistose axial-planar fabric and isoclinal folds synchronous with thrusting. The second deformation (Ca D2 ) produced a coarser schistosity and tight to isoclinal folds. South-plunging, north–south stretching lineations, top to the south shear sense indicators, and south-verging, downward-facing folds in the Arthur Lineament suggest south-directed transport. Ca F1 and Ca F2 were rotated to a north–south trend in zones of high strain during the Ca D2 event. Ca D3 , later in the Cambrian, folded the earlier foliations in the Arthur Lineament and produced west-dipping steep thrusts, creating the linear expression of the structure. [ABSTRACT FROM AUTHOR]

Landform evolution around the Mt Field Plateau has been strongly influenced by multiple stages of cold glacial climate. Only small glaciers were present during the late Last Glacial or Global Isotope Stage 2, but degraded moraines and the distribution of erratics indicate that ice cover was more extensive earlier when ice and meltwater invaded neighbouring karst areas and meltwater streams deposited gravel in caves. Weathering evidence suggests a significant glacial advance during Global Isotope Stage 4. Uranium-thorium dating of speleothems associated with gravels in proglacial caves suggests a major phase of gravel aggradation that post-dates Global Isotope Stage 5 and pre-dates Global Isotope Stage 2. [ABSTRACT FROM AUTHOR]

Blue amphibole minerals, including glaucophane, were formed in the Arthur Metamorphic Complex of northwest Tasmania during the early part (514–510 Ma) of the Tyennan Orogeny (514–490 Ma ~ Delamerian Orogeny). These blue amphiboles are now a rare relict phase in the predominantly greenschist facies Bowry Formation. The hosts range from an essentially prograde metadolerite to a substantially retrograde, mafic schist. Chemical parameters of the glaucophane indicate that it formed under minimum conditions of 350°C and 700 MPa, reflecting burial in a subducted wedge. Extrusion of the Bowry Formation from the wedge, with associated re-equilibration and deformation, commenced at 510 Ma (Early Cambrian). The formation was initially emplaced at a high crustal level, where it may have formed part of a ductile zone at the base of westward moving, upper crustal nappe of relatively unmetamorphosed Neoproterozoic to Lower Cambrian sedimentary, basaltic and ultramafic rocks. The Bowry Formation now occurs in the eastern boundary zone (Arthur Metamorphic Complex) of the Rocky Cape cratonic fragment. Other high-pressure metamorphic rocks of approximately the same age, but of deeper crustal origin, were emplaced at middle to upper crustal levels in the Tyennan and Forth regions, to the east of the Arthur Metamorphic Complex. These rocks were exposed by middle Middle Cambrian times (506 Ma) in domes that, along with the Rocky Cape craton, controlled the arcuate form of Middle to Late Cambrian sedimentary troughs. [ABSTRACT FROM AUTHOR]

Cambrian metamorphic complexes containing amphibolite- to eclogite-grade rocks are present throughout western and northwestern Tasmania. These complexes contain mostly quartz–albite–biotite schists, garnet–quartz–albite–biotite schists and mafic amphibolite lenses (up to 1 km long). The chemistry of these rocks is similar to unmetamorphosed, Late Neoproterozoic tholeiitic basalts and continental-derived siliciclastics. A few rocks in the metamorphic complexes have compositions that are transitional between the amphibolites and the schists, representing metamorphosed volcaniclastic rocks formed by mixing between mafic and siliciclastic sources. The rocks in these complexes were probably located on the edge of a thin Late Neoproterozoic passive margin that was partially subducted during a Cambrian arc–continent collision and uplifted during post-collisional crustal re-equilibration. The metamorphic condition, age and chemistry of both the schists and the amphibolites resemble those of metamorphic complexes in North Victoria Land in Antarctica. However, the structural setting of these complexes differs from those in Tasmania. Comparisons of the Tasmanian and North Victoria Land Cambrian structures and lithologies with those from more recent arc– continent collisions worldwide show that both are compatible with a model involving east-dipping subduction of a passive margin beneath an intraoceanic island arc. The differences between the two areas probably arise from differences in the geometry of the margins and the thickness of the passive-margin sediments. [ABSTRACT FROM AUTHOR]

Comments on the evolution of the Tasmanides in eastern Australia. Conference on the 15th Australian Geological Convention; Examination of the Arthur Lineament, enigmatic structure formed during the temporal equivalent of Delamerian Orogeny; Description of four mesoscopic structural events affecting Lower to Middle Devonian limestone.

A new tectonic model for Tasmania incorporates subduction at the boundary between eastern and western Tasmania. This model integrates thin- and thick-skinned tectonics, providing a mechanism for emplacement of allochthonous elements on to both eastern and western Tasmania as well as rapid burial, metamorphism and exhumation of high-pressure metamorphic rocks. The west Tamar region in northern Tasmania lies at the boundary between eastern and western Tasmania. Here, rocks in the Port Sorell Formation were metamorphosed at high pressures (700–1400 MPa) and temperatures (400–500°C), indicating subduction to depths of up to 30 km. The eastern boundary of the Port Sorell Formation with mafic–ultramafic rocks of the Andersons Creek Ultramafic Complex is hidden beneath allochthonous ?Mesoproterozoic turbidites of the Badger Head Group. At depth, this boundary coincides with the inferred boundary between eastern and western Tasmania, imaged in seismic data as a series of east-dipping reflections. The Andersons Creek Ultramafic Complex was previously thought of as allochthonous, based mainly on associations with other mafic–ultramafic complexes in western Tasmania. However, the base of the Andersons Creek Ultramafic Complex is not exposed and, given its position east of the boundary with western Tasmania, it is equally likely that it represents the exposed western edge of autochthonous eastern Tasmanian basement. A thin sliver of faulted and metamorphosed rock, including amphibolites, partially separates the Badger Head Group from the Andersons Creek Ultramafic Complex. Mafic rocks in this package match geochemically mafic rocks in the Port Sorell Formation. This match is consistent with two structural events in the Badger Head Group showing tectonic transport of the group from the west during Cambrian Delamerian orogenesis. Rather than being subducted, emplacement of the Badger Head Group onto the Andersons Creek Ultramafic Complex... [ABSTRACT FROM AUTHOR]