Refining accretionary orogen models for the Tasmanides of eastern Australia

The well-known southwest-to-northeast younging of stratigraphy over a present-day cross strike distance of >1500 km in the southern Tasmanides of eastern Australia has been used to argue for models of accretionary orogenesis behind a continually eastwards-rolling paleo-Pacific plate. However, these accretionary models need modification, since the oldest (ca 530 Ma) outcrops of Cambrian supra-subduction zone rocks occur in the outboard New England Orogen, now ～900 km east of the next oldest (520–510 Ma) supra-subduction zone rocks. This is not consistent with simple, continuous easterly rollback. Instead, the southern Tasmanides contain an early history characterised by a westwards-migrating margin between ca 530 and ca 520 Ma, followed by rapid eastwards rollback of the paleo-Pacific plate from 520 to 502 Ma that opened a vast backarc basin ～2000 km across that has never been closed. From the Ordovician through to the end of the Carboniferous, the almost vertical stacking of continental margin arcs (within a hundred kilometres of each other) in the New England Orogen indicates a constant west-dipping plate boundary in a Gondwana reference frame. Although the actual position of the boundary is inferred to have undergone contraction-related advances and extension-related retreats, these movements are estimated to be ～250 km or less. Rollback in the early Permian was never completely reversed, so that late Permian–Triassic to Cretaceous arcs lie farther east, in the very eastern part of eastern Australia, with rifted fragments occurring in the Lord Howe Rise and in New Zealand. The northern Tasmanides are even more anomalous, since they missed out on the middle Cambrian plate boundary retreat seen in the south. As a result, their Cambrian-to-Devonian history is concentrated in a ～300 km wide strip immediately west of Precambrian cratonic Australia and above Precambrian basement. The presence in this narrow region of Ordovician to Carboniferous continental margin arcs and backarc basins also implies a virtually stationary plate boundary in a Gondwana frame of reference. This bipolar character of the Tasmanides suggests the presence of a segmented paleo-Pacific Plate, with major transform faults propagating into the Tasmanides as tear faults that were favourably oriented for the formation of local supra-subduction zone systems and for subsequent intraplate north–south shortening. In this interpretation of the Tasmanides, Lower–Middle Ordovician quartz-rich turbidites accumulated as submarine fan sequences, and do not represent multiple subduction complexes developed above subduction zones lying behind the plate boundary. Indeed, the Tasmanides are characterised by the general absence of material accreted from the paleo-Pacific plate and by the dominance of craton-derived, recycled sedimentary rocks.     

Narooma Terrane: Implications for the construction of the outboard part of the Lachlan Orogen

In its type area around Narooma, the Narooma Terrane in the Lachlan Orogen comprises the Wagonga Group, which consists of the Narooma Chert overlain by the argillaceous Bogolo Formation. Conodonts indicate that the lower, largely massive (ribbon chert) part of the Narooma Chert ranges in age from mid-Late Cambrian to Darriwilian-Gisbornian (late Middle to early Late Ordovician). The upper Narooma Chert consists of shale, containing Eastonian (Late Ordovician) graptolites, interbedded with chert. Where not deformed by later faulting, the boundary between the Narooma Chert and Bogolo Formation is gradational. At map scale, the Narooma Terrane consists of a stack of imbricate thrust slices caught between two thrust faults that juxtaposed the terrane against the coeval Adaminaby Superterrane in Early Silurian time. These slices are best defined where Narooma Chert is thrust over Bogolo Formation. The soles of such slices contain multiply foliated chert. Late extensional shear bands indicate a strike-slip component to the faulting. The Narooma Terrane, with chert overlain by muddy ooze, is interpreted to be an oceanic terrane that accumulated remote from land for ～50 million years. The upward increase in the terrigenous component at the top of the Wagonga Group (shale, argillite, siltstone and sandstone of the upper Narooma Chert and Bogolo Formation) records approach of the terrane to the Australian sector of the Gondwana margin. Blocks of chert, argillite and sandstone reflect extensional/strike-slip disruption of the terrane as it approached the transform trench along the Gondwana-proto-Pacific plate boundary. Blocks of basalt and basalt breccia represent detritus from a seamount that was also entering the trench. There is no evidence that the Narooma Terrane or the adjacent Adaminaby Group formed in an accretionary prism/ subduction complex.     

Growth and provenance of a Paleozoic subduction complex in the Broken River Province,Mossman Orogen: evidence from detrital zircon ages

A Paleozoic subduction complex dominates the Mossman Orogen developed at the northern extremity of the Tasmanides, eastern Australia. Its southern part, displayed in the Broken River Province, is characterised by dismembered ocean-plate stratigraphy in which turbidite-dominated packages and widespread tectonic mélange development are characteristic. The Broken River complex is characterised by formations with quartzose sandstone alternating with those largely formed of sandstone of more labile character. The two compositional groups are considered to reflect separate, age-significant sedimentary regimes, but their ages have hitherto been poorly constrained. With the use of 1082 concordant detrital zircon ages from 13 samples we provide age control for the complex and track its sedimentary provenance. Of quartzose units, the Tribute Hills Arenite and Pelican Range Formation are late Cambrian–Early Ordovician, and the Wairuna Formation is Middle to Late Ordovician, in age. The more labile units (Greenvale, Perry Creek and Kangaroo Hills formations) are collectively of late Silurian–mid-Devonian age. Development of the complex spanned some 130 Myr. Continent-derived sediment involved in accretion of much the complex, from mid-Ordovician to mid-Devonian, was largely sourced from a nearby magmatic arc of late Cambrian–Devonian age, now represented by granitoid plutons of the Macrossan and Pama igneous associations. An older far-field Pacific-Gondwana sediment source is characteristic of early-phase (late Cambrian–Early Ordovician) accretion, in common with sedimentary units of this age generally developed in the Tasmanides. We consider the complex to have grown largely by underplating that positioned younger components beneath those that are older, with out-of-sequence thrust interleaving of these components occurring late in the accretionary history. A Late Devonian contractional folding and cleavage development (Tabberabberan orogenesis) is uniformly expressed across the entire complex and reflects an abrupt change in plate engagement with imposition of a compressional stress regime.     

Financial statement as at 31 December 1969

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.     

Coeval basin formation,plutonism and metamorphism in the Northern Tasmanides: extensional Cambro-Ordovician tectonism of the Charters Towers Province

Abstract

The Charters Towers Province, of the northern Thomson Orogen, records conversion from a Neoproterozoic passive margin to a Cambrian active margin, as characteristic of the Tasmanides. The passive margin succession includes a thick metasedimentary unit derived from Mesoproterozoic rocks. The Cambrian active margin is represented by upper Cambrian–Lower Ordovician (500–460?Ma) basinal development (Seventy Mile Range Group), plutonism and metamorphism resulting from an enduring episode of arc–backarc crustal extension. Detrital zircon age spectra indicate that parts of the metamorphic basement of the Charters Towers Province (elements of the Argentine Metamorphics and Charters Towers Metamorphics) overlap in protolith age with the basal part of the Seventy Mile Range Group and thus were associated with extensional basin development. Detrital zircon age data from the extensional basin succession indicate it was derived from a far-field (Pacific-Gondwana) primary source. However, a young cluster (<510?Ma) is interpreted as reflecting a local igneous source related to active margin tectonism. Relict zircon in a tonalite phase of the Fat Hen Creek Complex suggests that active margin plutonism may have extended back to ca 530?Ma. Syntectonic plutonism in the western Charters Towers Province is dated at ca 485–480?Ma, close to timing of metamorphism (477–467?Ma) and plutonism more generally (508–455?Ma). The dominant structures in the metamorphic basement formed with gentle to subhorizontal dips and are inferred to have formed by extensional ductile deformation, while normal faulting developed at shallower depths, associated with heat advection by plutonism. Lower Silurian (Benambran) shortening, which affected metamorphic basement and extensional basin units, resulted in the dominant east–west-structural trends of the province. We consider that these trends reflect localised north–south shortening rather than rotation of the province as is consistent with the north–south paleogeographic alignment of extensional basin successions.

1. KEY POINTS

2. Northern Tasmanide transition from passive to active margin tectonic mode had occurred by ca 510?Ma, perhaps as early as ca 530?Ma.

3. Cambro-Ordovician active margin tectonism of the Charters Towers Province (northern Thomson Orogen) was characterised by crustal extension.

4. Crustal extension resulted in the development of coeval (500–460?Ma) basin fill, granitic plutonism and metamorphism with rock assemblages as exposed across the Charters Towers Province developed at a wide range of crustal levels and expressing heterogeneous exhumation.

5. Protoliths of metasedimentary assemblages of the Charters Towers Province include both Proterozoic passive margin successions and those emplaced as Cambrian extensional basin fill.

     

Geochemistry of Early–Middle Palaeozoic basalts in the Hodgkinson Province: a key to tectono-magmatic evolution of the Tasman Fold Belt System in northeastern Queensland, Australia

The Palaeozoic Hodgkinson Province in northeastern Queensland, Australia, is host to Late Ordovician to Devonian rock assemblages that contain tholeiitic to calc-alkaline basalts. These basalts occur as massive fault-bounded units interspersed with marine sedimentary rocks and limestones that are metamorphosed to lower greenschist facies in the Ordovician Mulgrave, Silurian Chillagoe and Devonian Hodgkinson formations, respectively. The petrogenetic and Sm–Nd isotope characteristics of these mafic volcanic rocks were investigated to constrain the tectonic setting in which they erupted. Major, trace and rare earth element analyses were carried out on samples from these formations and intrusive dolerites. The mafic rocks can be classified as basalts and basaltic andesites with distinct MORB characteristics. Furthermore, the basalts are characterized by a slight to moderate enrichment in Th and concomitant depletion in Nb, both of which become less pronounced with basalt evolution through time. These features are consistent with decreasing volcanic arc affinity of Silurian and Devonian MORB-type basalts in the Hodgkinson Province. Sm–Nd isotope characteristics of these basalts indicate a change in source region from dominantly sub-continental lithospheric mantle in the Silurian to asthenospheric input in the Devonian. Collectively, the geochemical and isotopic characteristics of the Hodgkinson Province basalts are interpreted to reflect deposition in an evolving back-arc basin setting. The onset of basin extension was initiated in the Silurian. Accelerated basin subsidence occurred throughout the Devonian and was halted by basin inversion in the Late Devonian. Basin evolution was controlled by an eastward stepping subduction zone outboard of the Australian Craton.     

Structural elements of the southern Thomson Orogen (Australian Tasmanides): a tale of megafolds

Abstract

Multi-scale, multi-method integration of geological constraints, with new interpretations of potential field data and seismic reflection data, has resulted in a comprehensive structural interpretation of the southern Thomson Orogen, eastern Australia. The interpretation reveals ～50 major faults and shear zones, many of which can be traced for several hundred kilometres. The interpretation suggests that the southern Thomson Orogen can be subdivided into several structural domains that can be distinguished by differences in: (i) spatial orientation, (ii) geographic distribution, and (iii) partly the timing of major faults, but also to varying degrees by (iv) the evolution and spatial orientation of other structural elements, such as folds, minor faults and fractures, (v) broader lithological trends, (vi) stratigraphy, and (vii) structural style. The two largest domains are the Western Structural Domain that contains numerous faults and shear zones, and the fold-dominated Eastern Structural Domain, which is more strongly affected by late- to post-Devonian thrusting than the Western Structural Domain. Notwithstanding their differences, the domains can be integrated into a coherent structural model for the southern Thomson Orogen, which suggests that the area represents a set of megafolds or oroclines, which may have formed during the Bindian Orogeny.     

Geochronology and geochemistry of the Devonian Gumbardo Formation (Adavale Basin): evidence for cratonisation of the Central Thomson Orogen by the Early Devonian

Abstract

The Devonian subsurface Adavale Basin occupies a central position in the Paleozoic central Thomson Orogen of eastern Australia and records its tectonic setting during this time interval. Here, we have focussed on the basal volcanics of the Gumbardo Formation to clarify the tectonic setting of the basin. The approach has been to undertake stratigraphic logging, LA-ICP-MS U–Pb zircon geochronology and whole-rock geochemical analysis. The data indicate that basin initiation was rapid occurring at ca 401?Ma. The volcanic rocks are dominated by K-feldspar phyric rhyodacitic ignimbrites. The whole-rock geochemical data indicate little evidence for extensive fractional crystallisation, with the volcanic suite resembling the composition of the upper continental crust and exhibiting transitional I- to A-type tectonomagmatic affinities. One new U–Pb zircon age revealed an Early Ordovician emplacement age for a volcanic rock previously interpreted to be part of the Early Devonian Gumbardo Formation, and older basement age is consistent with seismic interpretations of uplifted basement in this region of the western Adavale Basin. Five ignimbrites dated from different stratigraphic levels within the formation yield similar emplacement ages with a pooled weighted age of 398.2?±?1.9?Ma (mean square weighted deviation?=?0.94, n?=?93). Significant zircon inheritance in the volcanic rocks records reworking of Ordovician and Silurian silicic igneous basement from the Thomson Orogen and provides insight into the crustal make-up of the Thomson Orogen. Collectively, the new data presented here suggest the Adavale Basin is a cover-type basin that developed on a stabilised Thomson Orogen after the major Bindian deformation event in the late Silurian.     

Continuation of the Ross–Delamerian Orogen: insights from eastern Australian detrital-zircon data

Abstract

Information on the Paleozoic tectonic evolution of the Tasmanides in eastern Australia is limited due to the presence of an extensive younger sedimentary cover. Based on the spatio-temporal distribution of deformation and magmatism, the Tasmanides have traditionally been subdivided into five domains (Delamerian, Thomson, Lachlan, Mossman and New England). To test the relationships between these domains, we compiled over 10000 published detrital-zircon ages from basement rocks of the Tasmanides. We split the dataset spatially to test postulated connectivity between domains, and temporally to isolate corresponding age populations that can highlight subtle variations between domains. Results show that the Delamerian and Thomson domains originated as a single orogenic belt that likely received detritus from an early Paleozoic continental-scale drainage system. We also recognise a remarkably similar pattern of Paleoproterozoic and Archean ages in the Delamerian, Thomson and New England domains. This similarity indicates that the late Paleozoic development of the New England domain involved recycling of rocks from the Delamerian–Thomson domain(s). These findings shed new light on the crustal architecture of eastern Australia, and the nature of Paleozoic drainage and sediment recycling in eastern Gondwana.     

Genesis of orogenic-gold deposits in the Broken River Province,northeast Queensland

We report results of metallogenic, structural, petrological, and fluid-inclusion studies that characterise the nature of gold mineralisation in the Amanda Bel Goldfield, the most significant gold producer in the Palaeozoic Broken River Province of northeastern Queensland, Australia. Gold–antimony–arsenic and gold–arsenic deposits in the Amanda Bel Goldfield occur along distinctive northeastern trends, suggesting a strong structural control for their development during several phases of deformation in the Devonian to Carboniferous. Field evidence, as well as petrographic, scanning electron microscope and fluid-inclusion analysis of mineralised samples, indicate the presence of two main stages of gold genesis. These are distinguished by the coarse grained versus invisible nature of gold particles and their association with particular sulfide phases. A third stage of gold deposition is attributed to introduction of antimony±gold-rich ore fluids. Fluid-inclusion studies record minimum trapping temperatures between 140 and 380°C, and salinities of up to 6.5 wt% NaCl equivalent for the two main gold-forming stages. Our analyses further indicate that mineralising solutions for the earlier of the two main gold-forming stages were slightly more saline, and that the ore-hosting veins formed at higher temperatures. The style of gold mineralisation in the Amanda Bel Goldfield is compatible with orogenic gold deposits that form primarily during compressional and transpressional deformation along convergent plate margins in accretionary and collisional orogens. The increased understanding gained from our studies on the origin and nature of the deposits aids predictive mineral discovery elsewhere in the Broken River Province, and also in analogous terranes throughout the Tasman Fold Belt System of eastern Australia.