Rocky Cape Group Research Articles

Multi-mineral geochronology reveals a protracted Mesoproterozoic tectonic and metallogenic link between southeastern Australia and Laurentia

Plate tectonic reconstructions place Laurentia and eastern Australia together during the Mesoproterozoic. Specifically, the Belt-Purcell Basin in the USA and Canada has been linked to the Rocky Cape Group in northwest Tasmania based on detrital zircon studies. The Belt-Purcell Basin is highly endowed in economic deposits of Cu, Pb, Zn, Ag and critical metals such as Co, formed during the Mesoproterozoic. The Rocky Cape Group of northwest Tasmania hosts several sediment-hosted Cu-Pb-Zn ± Co prospects, however they were previously thought to be Devonian in age. New laser ablation inductively-couple plasma mass spectroscopy (LA-ICP-MS) in situ U-Pb monazite, xenotime and apatite data, sensitive high resolution ion microprobe (SHRIMP) in-situ U-Pb monazite data, and LA-ICP-MS galena Pb isotope data were collected from these prospects. By using multiple minerals with different U–Pb closure temperatures, we have unravelled a complex and protracted history of thermal pulses in the Rocky Cape region associated with Cu-Pb-Zn ± Co mineralisation. These data indicate that primary sulphide mineralisation formed at c. 1350 Ma, with several resetting or remobilisation events recorded at c. 1250 Ma, c. 1100 Ma and c. 950 Ma. These ages are contemporaneous with previously published authigenic monazite and metamorphic monazite ages from the Rocky Cape Group. This suggests that mineralisation is broadly syn-sedimentary and related to the development of the basin into which the Rocky Cape Group was deposited. These new ages are similar to those of sediment-hosted ore deposits in the Belt-Purcell Basin in the USA and Canada, further supporting the proposed link between northwest Tasmania and North America during the Mesoproterozoic. The high metal endowment of age-equivalent rocks in the Belt-Purcell Basin, highlight the potential for new discoveries of base and critical metals in southeastern Australia.

Mesoproterozoic age for andalusite in the lower Rocky Cape Group, northwest Tasmania

New U–Th–Pb chemical data from both electron probe microanalysis and U–Pb laser ablation-inductively coupled plasma mass spectrometry on monazite inclusions in giant andalusite porphyroblasts indicate that the lowermost subdivision of the Rocky Cape Group in northwest Tasmania underwent metamorphism at ca 1100 Ma. The Proterozoic Rocky Cape Group is a ∼10 km-thick succession of quartz arenite, siltstone and pelite. These sediments were deposited between ca 1450 and ca 1010 Ma. Metamorphism within the Rocky Cape Group ranges from the prehnite–pumpellyite to upper greenschist facies, locally reaching garnet grade. In the lowermost formation of the Rocky Cape Group, andalusite-bearing mineral assemblages in the Pedder River Siltstone formed prior to or synchronously with monazite development. The andalusite-bearing mineral assemblage indicates relatively low-pressure conditions of <200 MPa, consistent with the stratigraphic thickness of the Rocky Cape Group (∼10 km). However, the presence of the intermediate-temperature biotite–andalusite mineral assemblages (500 °C) in the study area suggests the contribution of additional heat sources. Garnet porphyroblasts were observed ∼2 km away. Similar-age medium-grade metamorphism and magmatism at about 1100 Ma have been observed regionally in the South Tasman Rise and East Tasman Plateau, and in areas that are thought to have been contiguous with northwestern Tasmania such as Dronning Maud Land, central Arizona (Yavapai–Mazatzal) and Belt–Purcell Basin in the southwestern and northwestern Laurentian crustal sequences. The pressure and temperature relationships, mineral associations and regional data suggest additional heat flow was present through the lower part of the Rocky Cape Group. The higher geothermal gradient could be attributed to burial during basin development and deposition of the upper Rocky Cape Group or associated magmatism on the margin of Laurentia during assembly of Rodinia.

Revised stratigraphy and first geochronology of the Miocene submarine volcanic succession at Kennaook/Cape Grim, northwestern Tasmania

Kennaook/Cape Grim in far northwestern Tasmania, Australia, was a site of submarine intraplate basaltic volcanism during the Miocene. The succession is exceptionally well preserved and is dominated by pillow lava, massive lava and pillow fragment breccia. The total volume of volcanic products (1.1 km3) is relatively small on a global scale, but the preservation is world class. The oldest unit, the Woolnorth Tuff lies unconformably on the Neoproterozoic Rocky Cape Group. The first 40Ar/39Ar dating of the volcanic rocks reveals an early Miocene (24.5–23 Ma) age for the entire sequence. The Woolnorth Tuff is composed almost entirely of devitrified basaltic glass shards and olivine crystal fragments. The Woolnorth Tuff is overlain by the Slaughter Bluff Volcanic Breccia (23.12 ± 0.19 Ma) at Kennaook/Cape Grim. The Slaughter Bluff Volcanic Breccia is dominated by diffusely bedded pillow fragment breccia. North of Kennaook/Cape Grim, the Little Trefoil Basalt (ca 24 Ma) intrudes the Woolnorth Tuff. To the south of Kennaook/Cape Grim, the Woolnorth Tuff is overlain by the Studland Bay Basalts (23.73 ± 0.08 Ma) and intruded by a newly recognised unit, the Hippo Basalt (24.52 ± 0.12 Ma). The Studland Bay Basalts comprise mounds of basaltic pillow lavas followed by a succession of diffusely bedded, matrix-dominated, pillow fragment breccia and basaltic breccia. Detailed field mapping and sampling have revealed that the environment of deposition of all Kennaook/Cape Grim units was submarine and that they were emplaced in relatively rapid succession. KEY POINTS First 40Ar/39Ar geochronology for the Kennaook/Cape Grim volcanic succession. Little Trefoil Basalt has been re-interpreted from a subaerial extrusive unit to a submarine intrusive unit. A new intrusive unit, the Hippo Basalt, has been recognised. The stratigraphy of the Kennaook/Cape Grim volcanic succession has been revised.

Efficient regional scale 3D potential field geophysical modelling to redefine the geometry of granite bodies beneath prospective, geologically complex, northwest Tasmania

In this study, a regional model that defines the three-dimensional geometry of the subsurface geology beneath the complex, prospective northwestern Tasmania has been developed. This has been achieved using a series of potential field inversions constrained by surface geology, geological sections, seismic interpretations and a newly extended petrophysical dataset. Three major episodes of granitic magmatism are preserved in Tasmania: in the Neoproterozoic, Cambrian and Devonian. Granite bodies are hence considered important indicators of mineralization for explorers in an area of challenging vegetation, topography and cover sequences. Forward modelling and property-based inversions of the pre-existing geological model show that the previously interpreted subsurface geometry is not compatible with potential field data. Four sub-regions displayed a large discrepancy between calculated and observed data. This study redefines the subsurface geometries of these sub-regions through individual geometry inversions. The density and magnetic susceptibility ranges of units are further refined through property inversions. The modified geometry of the Devonian granites in the four sub-regions may be summarized as follows: 1) the Housetop Granite is relatively thin (≤5 km thickness), whereas 2) the Heemskirk and Meredith Granites are very thick and granite extends to a shallower depth between these bodies than previously interpreted. This region between plutons is thus a more prospective region than previously thought. 3) For the first time, an intrusive body underlying the eastern part of the Rocky Cape Group has been identified. Its petrophysical properties are similar to that of a granite, and its top is interpreted at a depth of >3 km. This interpreted low density (granitic) unit may be either Neoproterozoic or Devonian. 4) A new non-magnetic, low density Cambrian granite, with a minimum burial depth of 1 km, is also modelled in 3D, within the Mount Read Volcanics, in the south of the study area. Our approach, whereby sub-regions are identified for more detailed modelling, enables new constraints to be introduced in a computationally efficient way, and has general application to refining the geometry of key structures in prospective regions.

Rodinian devil in disguise: Correlation of 1.25–1.10 Ga strata between Tasmania and Grand Canyon

Locating the continuation of the ca. 1.30–1.00 Ga Grenville orogen on continents formerly adjacent to Laurentia is central to resolving the paleogeography of the supercontinent Rodinia. Here we emphasize a correlation of late Mesoproterozoic foreland basins that, prior to truncation by Neoproterozoic rift margins, may have extended west of Laurentia within Rodinia. We propose correlation of the Unkar Group (Grand Canyon, Arizona, USA) with the upper Rocky Cape Group (Tasmania, southeast Australia) based on their similar stratigraphy, 1.25–1.10 Ga depositional age, and detrital zircon U-Pb age distribution and Hf isotope composition. This correlation places Tasmania adjacent to southwest Laurentia in the late Mesoproterozoic, which supports a new paleogeographic model for Rodinia. In this model, Tasmania and crustal fragments of Laurentia comprising the South Tasman Rise and the Coats Land block form key links between the Grenville orogen in southwest Laurentia and the Maud orogen (East Antarctica). A 1.14–1.07 Ga connection between the combined Grenville-Maud orogen and the Musgrave orogen of central Australia is compatible with paleomagnetic data but requires ∼4000 km of relative motion between Australia-Antarctica and Laurentia prior to the final assembly of Rodinia at ca. 0.90 Ga. We hypothesize that the final assembly of Rodinia was achieved by dextral motion between the crust of Australian and Laurentian affinity along a plate boundary concealed beneath ice cover in East Antarctica.

Constraining the VanDieland microcontinent at the edge of East Gondwana, Australia

Using airborne magnetic and marine gravity data, the geological subdivisions of western Tasmania have been interpreted north across Bass Strait into Victoria. The three westernmost Tasmanian zones, the King Island, Rocky Cape and Burnie zones, are inferred to form the largely concealed Selwyn Block in Victoria. The Eastern Tasmania Zone correlates with the Victorian Tabberabbera Zone. Thus the Tasmanian Tamar Fracture Zone corresponds with the Victorian Governor Fault. The Victorian Ceres Gabbro is correlated with magnetic rocks west of King Island that are tentatively considered to be Neoproterozoic. Most of the Cambrian felsic volcanic rocks of the Tasmanian Mount Read Volcanics lie above the Burnie Zone, as do the similar rocks exposed in the Jamieson, Licola and Glen Creek windows in central Victoria. Reinterpretation of a Victorian deep seismic reflection line indicates Burnie Zone equivalent rocks were thrust south-west over Rocky Cape Zone equivalents.A link between western Tasmania and central Victoria is evident from Upper Devonian granites intruded into the Selwyn Block region. The eastern end of the Upper Devonian Cobaw Complex and the Warburton Granodiorite contain calcsilicate enclaves interpreted to be derived from a northern equivalent to the Smithton Basin. The Mount Disappointment Granodiorite has high Ni and Cr contents and pseudomorphs after orthopyroxenes, consistent with having been partly sourced from the underlying basaltic rocks like those on the eastern margin of the King Island Zone. The magnetic responses under the Strathbogie Complex, the Cerberean Caldera and the Lysterfield Granodiorite are attributed to metamorphism of part of an extension of the Smithton Basin, probably equivalents to the 580Ma Spinks Creek Volcanics. Quartzite cobbles in a Devonian conglomerate in the south-eastern Melbourne Zone may be derived from Rocky Cape Group equivalents. When integrated with the geological interpretation of Tasmania, we provide a stratotectonic map of the VanDieland micro-continent.

Mesoproterozoic Tasmania: Witness to the East Antarctica–Laurentia connection within Nuna: COMMENT

I concur with the finding of Mulder et al. (2015) that the Rocky Cape Group was not sourced from Australia. However, the Rocky Cape Group is but one stratigraphic unit of the micro-continent of VanDieland (Cayley, 2011). For their proposed location for VanDieland to be valid, it must satisfy the constraints imposed by all of the units in the microcontinent, not just one from the period from ca. 1200–1450 Ma. VanDieland also includes the east South Tasman Rise (Cayley, 2011; Mulder et al., 2015). Rocks there include granites with U/Pb zircon ages of 1120 ± 9 Ma (Fioretti et al., 2005) and with U/Th/Pb monazite ages of 1051 ± 8 Ma that were metamorphosed at ca. 1015 Ma and 920–930 Ma (Berry et al., 2008). Mulder et al.’s figure 2 offers two VanDieland positions near the USA-Canadian border, but neither fits these data. While there is evidence of Grenville-age zircon growth in the Belt Supergroup and adjacent rocks (Doughty and Chamberlain, 2008), there is a dearth of similar-aged granites known north of the Grenville Orogen in ‘western’ Laurentia (modern position) and in Victoria Land in East Antarctica, and metamorphism in the region of the Belt Supergroup and the corresponding regions in Antarctica appears to have ceased at ca. 1000 Ma (May et al., 2013; Rocchi et al., 2015). Rather, locating the east South Tasman Rise outboard of the southwestern Grenville Orogen, as suggested by Fioretti et al. (2005) and Moore et al. (2015), provides a more coherent fit than either offered by Mulder et al. Mulder et al. dismiss the 1010 ± 45 Ma detrital zircon ages that Black et al. (2004) obtained from the Jacob Quartzite as resulting from Pb loss. Do they also dismiss results from the Wings Sandstone, another western Tasmanian marginal marine quartzite, where Black et al. (2004) obtained a population of 1022 ± 17 Ma (12 grains), and another as young as 914 ± 44 Ma? Moore et al. (2015, their figure 20) suggested that these come from the erosion of areas near the Grenville Orogen. How do Mulder et al. account for these younger populations? And where is the source for their ca. 1300 Ma upper Rocky Cape Group zircons? None is offered in their figure 4. Mulder et al. describe the Rocky Cape Group as a “marine shelfal package.” Similar modern and ancient packages lie on older cratonic basements (e.g., Jones et al., 2011). Black et al. (2010) found that the Paleozoic western Tasmanian granites had an excess of 1600 Ma zircons when compared to the Rocky Cape Group, implying a basement with excess zircons of that age. Two-stage Nd model ages on granites that intruded the Rocky Cape Group gave ages of 1530 and 1780 Ma, and all other granite model ages in western Tasmania are younger than 2100 Ma (Champion, 2013). Indeed, Mulder et al.’s Hf data suggest a highly juvenile zircon source, difficult to reconcile with their locations. The data imply that the basement under the Rocky Cape Group formed in the middle to late Paleoproterozoic, similar to that of the Cochise Block and the overlying quartzites in southern Arizona, and unlike the Archean to early Paleoproterozoic rocks likely to be outboard of the Belt Basin (Doe et al., 2013; Whitmeyer and Karlstrom, 2007). Leaving VanDieland unrotated, in the position suggested by Moore et al. (2015), would allow these correlations. How do Mulder et al. resolve these issues? Like Mulder et al., I consider that VanDieland was once located outboard of Laurentia. However, they have failed to recognize the significance of the Grenville-age orogenic and sedimentary events and the concealed late Paleoproterozoic basement, constraints outside of the Rocky Cape Group. When these are considered, a Nuna/Rodinia position for VanDieland close to the USA–Mexican border, with the South Tasman Rise aligned with and outboard of the Grenville Orogen, seems more likely than one rotated and up to 1500 km further north.

Mesoproterozoic Tasmania: Witness to the East Antarctica–Laurentia connection within Nuna

Research Article| September 01, 2015 Mesoproterozoic Tasmania: Witness to the East Antarctica–Laurentia connection within Nuna Jacob A. Mulder; Jacob A. Mulder * 1ARC Centre of Excellence in Ore Deposits (CODES), School of Physical Sciences, University of Tasmania, Private Bag 79, TAS 7001, Australia *E-mail: Jacob.Mulder@utas.edu.au Search for other works by this author on: GSW Google Scholar Jacqueline A. Halpin; Jacqueline A. Halpin 1ARC Centre of Excellence in Ore Deposits (CODES), School of Physical Sciences, University of Tasmania, Private Bag 79, TAS 7001, Australia Search for other works by this author on: GSW Google Scholar Nathan R. Daczko Nathan R. Daczko 2ARC Centre of Excellence for Core to Crust Fluid Systems and GEMOC, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia Search for other works by this author on: GSW Google Scholar Author and Article Information Jacob A. Mulder * 1ARC Centre of Excellence in Ore Deposits (CODES), School of Physical Sciences, University of Tasmania, Private Bag 79, TAS 7001, Australia Jacqueline A. Halpin 1ARC Centre of Excellence in Ore Deposits (CODES), School of Physical Sciences, University of Tasmania, Private Bag 79, TAS 7001, Australia Nathan R. Daczko 2ARC Centre of Excellence for Core to Crust Fluid Systems and GEMOC, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia *E-mail: Jacob.Mulder@utas.edu.au Publisher: Geological Society of America Received: 04 Apr 2015 Revision Received: 15 Jun 2015 Accepted: 16 Jun 2015 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2015 Geological Society of America Geology (2015) 43 (9): 759–762. https://doi.org/10.1130/G36850.1 Article history Received: 04 Apr 2015 Revision Received: 15 Jun 2015 Accepted: 16 Jun 2015 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jacob A. Mulder, Jacqueline A. Halpin, Nathan R. Daczko; Mesoproterozoic Tasmania: Witness to the East Antarctica–Laurentia connection within Nuna. Geology 2015;; 43 (9): 759–762. doi: https://doi.org/10.1130/G36850.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Most recent paleogeographic reconstructions of the supercontinent Nuna juxtapose the North Australian craton, Mawson continent (South Australia–East Antarctica), and Laurentia between 1.6 Ga and 1.3 Ga but differ in their relative positioning. The >10-km-thick siliciclastic Rocky Cape Group of Tasmania was deposited in an opening marine basin on the margin of East Antarctica during Nuna breakup. Based on a similar detrital zircon signature and depositional age, the Rocky Cape Group has been correlated with the upper Belt-Purcell Supergroup in Laurentia, thus representing a key tie point within Nuna. Here the detrital zircon age signature of Mesoproterozoic Rocky Cape Group quartzites is investigated by comparing new detrital zircon U-Pb-Hf isotopic data to an extensive compilation of zircon isotopic data from Australia, East Antarctica, and Laurentia. Our analysis demonstrates that the Rocky Cape Group is unlikely to have been sourced from any geological terrane exposed in present-day Australia. Instead, zircon U-Pb-Hf isotopic data from basement terranes in Laurentia and East Antarctica show striking similarities to the Rocky Cape Group detrital signature. Paleocurrent data indicate that the majority of sediment in the Rocky Cape Group was sourced from Laurentia, which was to the southeast (present-day coordinates) of Tasmania, supporting a SWEAT-like (southwest United States–East Antarctica) configuration for Nuna. We suggest that rifting left a thinned continental connection between East Antarctica and Laurentia onto which the lower-middle Rocky Cape Group was deposited between 1.45 and 1.30 Ga. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Authigenic monazite and detrital zircon dating from the Proterozoic Rocky Cape Group, Tasmania: Links to the Belt-Purcell Supergroup, North America

The oldest known rocks in Tasmania occur in the Proterozoic Rocky Cape Group, a ∼10km thick quartzarenite-siltstone-pelite-dominated succession, previously constrained to have been deposited between 1450Ma and 750Ma. The Rocky Cape Group contains the enigmatic fossil Horodyskia (‘string of beads’) and has the potential to place Tasmania within supercontinent reconstructions. Detrital zircon and authigenic monazite grains dated via U–Pb laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) analysis yield a deposition window between c. 1450Ma (youngest zircon populations) and c. 1330Ma (oldest authigenic monazite population) for the ∼9km thick lower-middle units (Pedder River Siltstone, Lagoon River Quartzite, Balfour Subgroup which hosts Horodyskia, Detention Subgroup). The upper units (∼1km) include the Irby Siltstone, which is younger than c. 1310Ma; this unit is likely separated from both the lower-middle units and the overlying c. <1010Ma Jacob Quartzite by disconformities. The lower-middle Rocky Cape Group is dominated by detrital zircon populations between 1600 and 1900Ma, with subordinate populations at c. 1450Ma and a spread of older grains mostly between 2300 and 2900Ma. The overlying Irby Siltstone has a bimodal detrital zircon distribution with a dominant peak at c. 1340Ma and a secondary peak at c. 1720Ma; no >1900Ma grains were identified.Authigenic monazite age distributions are complex, with multiple age domains within most samples. The common Pb corrected 206U/238Pb ages, defined by oldest grains in each sample, identify three statistically significant groups: (1) 1358–1292Ma (inclusive of two sigma errors) (Lagoon River Quartzite and Pedder River Siltstone), (2) 1283–1239Ma (Cowrie Siltstone and Balfour Subgroup), and (3) 1085±9Ma (Detention Subgroup). We suggest monazite was precipitated during episodic fluid flow events at these three stages in various parts of the basin. The original source for REE-bearing fluids could be detrital monazite, which is rarely preserved, and/or organic matter from the interbedded carbonaceous shales.The Rocky Cape Group has a shared provenance with the higher-grade metasediments (Surprise Bay and Fraser formations) of nearby King Island; the newly derived depositional ages also overlap and support the correlation of these rock associations. On the basis of current datasets, there are no obvious correlations that can be made with Mesoproterozoic basins preserved in mainland Australia. Instead, an overlap in the timing of deposition, similarities in detrital zircon signatures and analogous depositional environment suggests the c. 1.45–1.37Ga upper Belt-Purcell Supergroup (Missoula and Lemhi groups) of western North America constitutes a plausible correlation with the Tasmanian Mesoproterozoic succession. If the (unexposed) Palaeoproterozoic basement of Tasmania correlates with the Transantarctic Mountains region of East Antarctica as previously proposed, we suggest that the overlying Mesoproterozoic sequences were deposited during rifting of the supercontinent Nuna, between proto-Australia (including the Mawson craton of Antarctica) and Laurentia as predicted by the most recent palaeogeographic reconstructions. Both the Tasmanian and western Laurentian packages were affected by episodic post-depositional fluid flow events between c. 1.35 and 1.05Ga, possible thermotectonic imprints of the subsequent assembly of Rodinia.

The ‘string of beads’ fossil ( Horodyskia) in the mid-Proterozoic of Tasmania

Horodyskia has been found at a single Tasmanian locality, in the Cassiterite Creek Quartzite (ca. 1300–800 Ma), part of a thick Proterozoic, mildly deformed, low greenschist facies, marine shelfal siliciclastic succession known as the Rocky Cape Group. The rock hosting the fossils is thinly interbedded and interlaminated dark grey slaty shale and quartzose siltstone. The sharp-based, graded siltstone layers are interpreted as distal storm surge deposits on an outer marine shelf. The ‘strings of beads’ are mostly preserved at the base of the siltstone layers, in concave hyporelief (external moulds) on the soles of the event beds, and as convex epirelief (casts) on the tops of the underlying shale beds. The casts comprise shale identical to the underlying bed. The beads average 1.7 mm in diameter, and the gap between the borders of adjacent beads tends to be approximately equal to the bead diameter. Occasionally, the fossils are preserved within shale, as wholly flattened beads delineated by a subtle darkened halo. The Tasmanian ‘strings of beads’ have most of the morphological attributes of previously described Horodyskia, including regularity of size and spacing of beads in any one string, lack of branching, and in some instances, ‘haloes’ and casts with apical depressions. The strings on at least one bedding plane have a strong N–S preferred orientation of unknown origin. Tectonic deformation has resulted in 30% shortening in a SW–NE direction. The morphologic similarity, but differing mode of preservation of the Tasmanian Horodyskia to the two previously described Mesoproterozoic species is strong evidence for a biologic origin for the string of beads phenomenon. After morphological and morphometric comparisons with other species of Horodyskia, the Tasmanian specimens are assigned to Horodyskia williamsii.

Detailed teleseismic imaging of the crust and upper mantle beneath southeast Australia

Over the last decade, southeast Australia has been the focus of several teleseismic imaging studies that have mapped arrival time residual information from distant earthquakes as 3-D variations in seismic wavespeed in the upper mantle. Two of the most recent experiments are TIGGER and SEAL, which respectively involved the deployment of dense arrays of seismometers in northern Tasmania in 2002, and southern New South Wales and northern Victoria in 2004-2005. Tomographic imaging results from TIGGER show that the lithosphere beneath Tasmania is broadly characterized by (1) a marked transition from lower velocities in the west to higher velocities in the east. Significantly, the Tamar Fracture System does not overlie the narrow transition from lower to higher velocities; (2) an easterly dipping zone of relatively high velocity material beneath the Rocky Cape Group which may be related to remnant subduction of oceanic lithosphere. Tomographic imaging results from SEAL clearly reveal the west to east transition from Proterozoic to Palaeozoic lithosphere beneath the Murray Basin as a change from higher to lower velocities. Elevated velocities further east beneath the Wagga-Omeo Complex imply that the lithosphere beneath the central subprovince of the Lachlan Orogen is seismically distinct from that beneath the western subprovince. Our long term goal in southeast Australia is to combine the TIGGER and SEAL datasets with those from past, current and planned passive seismic arrays in order to construct a unified tomographic model of the entire region.

Lithospheric structure of Tasmania from a novel form of teleseismic tomography

In 2001 and 2002, a temporary array of 72 seismic recorders was deployed across northern Tasmania (SE Australia), with the aim of imaging the underlying crust and upper mantle using three‐dimensional (3‐D) teleseismic tomography. Using a recently developed adaptive stacking technique, 6520 relative P wave arrival time residuals have been picked from 101 distant earthquake records spanning a 5 month period. A novel iterative nonlinear tomographic procedure based on a subspace inversion scheme and the fast marching method, a grid‐based eikonal solver, is used to map the residual patterns as P wave velocity anomalies. The new scheme proves to be both fast and robust, making it well suited to large data sets and the reconstruction of complex anomalies. The resultant tomographic images of Tasmania exhibit significant lateral perturbations in P wave velocity structure (≤5%) from a 1‐D reference model. A marked transition from higher velocities in the east to lower velocities in the west strongly supports the idea that eastern Tasmania is underlain by dense rocks with an oceanic crustal affinity, contrasting with the continentally derived siliciclastic core of western Tasmania. Significantly, the Tamar Fracture System does not overlie the narrow transition from relatively fast to slow velocities, which suggests that it may be a near‐surface feature rather than a manifestation of deeper crustal‐scale suturing as previously thought. Farther west, an easterly dipping zone of relatively high velocity material beneath the Rocky Cape Group and Arthur Lineament may be related to remnant subduction of oceanic lithosphere associated with the mid‐Cambrian Delamerian Orogeny.

SHRIMP U–Pb detrital zircon ages from Proterozoic and Early Palaeozoic sandstones and their bearing on the early geological evolution of Tasmania

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 widespread 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 are 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.

Western Tasmanian Regional Minerals Program (part 2): airborne electromagnetic data – quality control and interpretation

Recent regional scale helicopter electromagnetic (HEM) data acquired on the west and northwest coasts of Tasmania have demonstrated the powerful geological mapping capabilities of the HEM technique. Quality control for the survey was provided by analysis of repeat data from daily test lines, and comparison with surface geophysical data. Data have been interpreted by conductivity-depth imaging, complemented by layered-earth inversion of selected lines. The maximum depth of investigation for the HEM survey has been determined to be around 150 m, based on both theoretical calculations and the results of conductivity-depth imaging. A preliminary interpretation of the HEM data is presented for the Balfour region. The HEM survey has clearly differentiated previously unmapped units within the Proterozoic Rocky Cape Group, and has defined several major structural trends within the survey area. A large number of bedrock conductors have been identified in the HEM data, particularly close to the historic copper mining centre of Balfour.

Structural history of the Arthur Lineament, northwest Tasmania: An analysis of critical outcrops

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 (CaD1) produced a schistose axial‐planar fabric and isoclinal folds synchronous with thrusting. The second deformation (CaD2) 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. CaF1 and CaF2 were rotated to a north‐south trend in zones of high strain during the CaD2 event. CaD3, 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.

Reconnaissance isotopic studies bearing on the tectonothermal history of Early Palaeozoic and Late Proterozoic sequences in western Tasmania

Reconnaissance isotopic dating (K‐Ar, Rb‐Sr, U‐Pb) of over 20 different units of Early Palaeozoic and Late Proterozoic age in western and northern Tasmania has assisted in clarifying some ages and relationships but has met with many problems. The Rb‐Sr systems in most Cambrian rocks have been re‐set or partially re‐set by post‐Cambrian tectonothermal events, particularly the mid‐Devonian Tabberabberan Orogeny, and U‐Pb zircon data show considerable scatter due to contamination by older zircon and to post‐crystallization Pb loss. K‐Ar dating, mainly of slates and schists, has been more successful in delineating major tectonothermal events, but spreads of ages due to incomplete re‐setting are again evident in some cases. The investigation has demonstrated some of the difficulties involved in dating in this complex region. Slate ages from the Late Proterozoic Burnie and Oonah Formations and Rocky Cape Group fall in the 630–690 Ma range, and confirm that the Penguin Orogeny was a Proterozoic event which, alth...

A revised stratigraphy for the precambrian in North-West Tasmania

Within the Proterozoic rocks of the Rocky Cape Geanticline in north-west Tasmania, three contrasting sedimentary assemblages are recognised. The Rocky Cape Group, which has been previously used very broadly, is redefined to include only one of these assemblages. The revised definition includes the orthoquartzite, siltstone and shale which form a continuous sequence in the Rocky Cape area. Some new formations are defined. The Smithton Dolomite, and related rocks, unconformably overlie the Rocky Cape Group to the west at Black River. The Burnie Formation is a thick sequence flanking the Rocky Cape Group to the east. The Rocky Cape Group and the Burnie Formation are separated by the Keith Metamorphics, which is a belt of low grade regional metamorphic greenschist, amphibolite and pelitic schist.