Australia Nuna and Beyond (original) (raw)
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Tectonics
Four continental margin turbidite ± black shale terranes of the Lachlan Orogen in the southern Tasmanides of eastern Australia formed in two major systems along the east Gondwana margin and constrain the Ordovician assembly of this accretionary orogen. Key features are the dissimilar stratigraphies of the adjacent Bendigo and Melbourne terranes in the western system; the dissimilar stratigraphies of the adjacent Melbourne and Albury-Bega terranes that reflect juxtaposition of the two systems during the Middle Devonian, and the presence of the Albury-Bega Terrane both west and east of the Macquarie Arc in the eastern system that also includes the ocean floor Narooma Terrane and igneous ocean crust terrane(s). Repetition of the Albury-Bega Terrane either side of the arc requires either rifting or orogen-parallel, strike-slip duplication of a once contiguous package. Terrane interactions began in the earliest Gisbornian with early docking, uplift, deformation, and exchange of detritus....
The New England Orogen, the youngest subduction-related component in the Australian continent, records a prolonged history of west-dipping subduction from the Devonian to the Triassic. From midlate Permian (ca 265 Ma) to Upper Triassic (ca 235 Ma), the New England Orogen was subjected to pronounced contractional deformation (Hunter-Bowen Orogeny) and widespread I-type calc-alkaline magmatism. We obtained zircon U-Pb ages from seven granitic samples within the I-type magmatic system. The new SHRIMP U-Pb data ranging from 255 to 215 Ma, combined with previous geochronological data from the southern New England Orogen, suggest that magmatism during the Hunter-Bowen Orogeny was spatially distributed along a NNE-SSW belt that was likely associated with a west-dipping Andean-type subduction zone. In contrast, younger magmatism (235-215 Ma) is aligned along a N-S belt farther east. Assuming that magmatism was subduction-related and was spatially distributed parallel to the subduction zone, we interpret the spatio-temporal change in magmatism as an indicator for eastward arc migration, possibly in response to slab rollback. The trench position is not well constrained but was possibly located between the Australian continent and the Lord Howe Rise. We propose a model involving asymmetric slab rollback, possibly in response to pinning of the northern part of subduction zone by the Gympie Terrane accretion. Our model invokes that this phase of tectonic activity marked the transition from contractional deformation in the continental margin to extensional tectonics that represents the earliest phase of the Mesozoic rifting of eastern Australia.
Tectonic setting of cambrian rifting, volcanism and ophiolite formation in western tasmania
Tectonophysics, 1987
In western Tasmania, small Precambrian regions are surronnded by a ramifying system of troughs filled with Cambrian sedimentary and volcanic rocks, and ophiolite complexes. The volcanic associations include a rift-related olivine tholeiite association, dacite-rhyolite and andesite associations, and a low-Ti, high-Mg andesite-tholeiite opbiolite association, and may have formed during a long-lived period of crustal thinning, punctuated by episodes of crustal rupturing, magmatism, and small scale rifting. Such extensional tectonism could occur in an active continental margin associated with strike-slip faulting of regional scale, and the vokanic associations may together constitute an igneous assemblage characteristic of magmatism in a transcurrent tectonic regime within an active continental margin undergoing break-up. The western Tasmanian Cambrian palaeogeography and volcanism preserve a transitional stage between the Late Proterozoic Kanmantoo regime of sedimentary basins with little volcanism developed at the rifting margin of the Proterozoic craton, and the tectonic regime of the Palaeozoic Lachlan Fold Belt where the Cambrian volcanic rocks are dominated by island-arc associations and the rift-related olivine tholeiite association is absent. Eastern Australian lithosphere may have grown by the insertion of newly-formed igneous complexes within the stretched and rifted continental margin, as well as by the accretion of "terranes" and the addition of packets of subduction complexes which developed offshore. There are no rocks exposed in the Tasman Fold Belt of the mainland of eastern Australia (Fig. 1) that are known to be older than the Cambrian, and it has long been argued that the eastern Australian continental lithosphere could have developed by lateral accretion upon a Proterozoic cratonic margin. Following the appearance of the plate tectonics concept, this accretionary process was generally envisaged as one of growth and closure by subduction and sedimentary-infilling, of back-arc basins, accompanied by the welding to the continental margin of the remnants of volcanic arcs that had formed offshore. This process would produce a lower crust formed from packets of Palaeozoic subduction complex and oceanic lithosphere (e.g. Crook, 1980), presumably embedded in and resting on mantle rocks. An alternative to this process of lateral accretion, suggested by Rutland (1976) and others, is the proposal that the Palaeozoic fold belt rocks of eastern Australia rest on a substrate of Proterozoic crust. Evaluation of this idea is difficult because contacts between the eastern Australian Palaeozoic rocks and the Proterozoic cratonic rocks to their west are poorly exposed. However, circumstantial evidence for the presence of Proterozoic material in the fold belt comes from isotopic
Geological Society of America Bulletin, 2015
The New England orogen of eastern Australia is characterized by tight orogenic curvatures (oroclines). Oroclinal bending commenced in the Early Permian during a period of extension that involved crustal melting, widespread emplacement of S-type granitoids, high-temperature metamorphism, exhumation of metamorphic complexes, extensional faulting, and development of rift basins. One of these basins is the Early Permian Nambucca block, which is situated in the "core" of the oroclinal structure, but its origin and time of deposition are poorly constrained. Here, we present new U-Pb ages of detrital zircons from the Nambucca block, which include age populations as young as 299 and 285 Ma, confirming the Early Permian deposition of the succession. Additional Devonian-Carboniferous and Precambrian ages indicate that detritus was mainly derived from the New England subduction complex and cratonic Gondwana. The range of ages suggests that the Nambucca Basin received detritus from both arc and continent and that deposition occurred in a back-arc setting. Given the coeval formation of the Nambucca Basin and the New England oroclines, we propose that this back-arc extensional basin was controlled by trench retreat, which resulted in "Mediterranean-style" orogenic curvatures along the plate boundary of eastern Gondwana. The recognition of a genetic link between oroclinal bending and back-arc extension may explain how accretionary orogens, such as the eastern Australian Tasmanides, were able to obtain an anomalous width without a substantial contribution of accreted exotic terranes. A similar mode of tectonism may have played an important role in other accretionary orogens.
Late Neoproterozoic to Early Mesozoic Sedimentary Rocks of the Tasmanides, Eastern Australia
Sediment Provenance
The Tasmanides in eastern Australia are the most widely exposed part of the East Gondwana Paleozoic active margin assemblage. Diverse sedimentary assemblages are abundant and include: (1) extensive quartz-rich turbidites and shallow marine to fluvial successions, (2) continental margin and island arc derived sedimentary successions with abundant volcanic lithic detritus, and (3) widespread deep-marine to subaerial successions formed from reworking of older rocks. Apart from island arcs such as the Devonian Gamilaroi-Calliope Arc, most of the Tasmanides sedimentary assemblages formed along or in close proximity to the Gondwana margin. We highlight the interplay and provenance switching between the development of igneous dominated detritus related to adjoining magmatic arcs, such as the Macquarie Arc, and interactions with Gondwana derived sedimentary successions. Paleocurrents and detrital zircon ages indicate periodic influxes of mainly quartz-rich sand derived from the East Gondwana margin and adjacent interior with a common Pacific-Gondwana detrital zircon age signature (600-500 Ma), especially in the Cambrian, Early to Middle Ordovician, and Middle Triassic. A major quartzose turbidite deposit formed in an oceanic setting was accreted to the northern New England accretionary complex in the Late Carboniferous. This contrasts with the bulk of the Devonian to Carboniferous succession of the southern New England Orogen, which is dominated by volcaniclastic input with minor Gondwana-derived detritus. Surprisingly, even the base of the intraoceanic Macquarie Arc shows detrital zircon ages indicative of Gondwana input. The prevalence of Gondwana clastic input into the Tasmanides shows that much of the orogenic belt developed in an active continental margin setting with limited strike-slip displacements, and apart from offshore island arcs, lacks exotic terranes.
Gondwana Research, 2011
Ocean-continent transform boundary Basement terranes Delamerian-Ross orogen Structural inheritance Detrital zircon ages Crustal architecture in formerly contiguous basement terranes in SE Australia, Tasmania and northern Victoria Land is a legacy of late Neoproterozoic-Cambrian subduction-related processes, culminating in formation of the Delamerian-Ross orogen. Structures of Delamerian-Ross age were subsequently reactivated during late Mesozoic-Cenozoic Gondwana breakup, strongly influencing the geometry of continental rifting and providing clues about the origins and configuration of the pre-existing basement structures. An oceancontinent transform boundary developed off western Tasmania follows the trace of an older Paleozoic strikeslip structure (Avoca-Sorell fault system) optimally oriented for reactivation during the final separation of Australia from Antarctica. This boundary cuts across rocks preserving an earlier record of arc-continent collision during the course of which continental crust was subducted to mantle depths and Cambrian maficultramafic island arc rocks were thrust westwards over late Neoproterozoic-Cambrian passive margin sequences. Collision was accompanied by development of a foreland basin into which 520-600 Ma arcderived detrital zircons were shed. Following a reversal in subduction polarity, and change to transcurrent motion along the Gondwana margin, Tasmania migrated northward along the proto-Avoca fault system before entering a subduction zone located along the Heathcote-Governor fault system, precipitating a second collision, south-vergent thrusting, and tectonic reworking of the already accreted Cambrian arc-forearc assemblages and underlying passive margin sequences.
Geochronology of early to middle Proterozoic fold belts in northern Australia: a review
Precambrian Research, 1988
This geochronological review integrates geologically relevant isotopic data from the several early to middle Proterozoic fold belts of northern Australia. A number of firm chronological benchmarks, largely established by means of U-Pb zircon studies, now provide a time framework within which to better quantify the tectonostratigraphic history of many of these terranes. The apparent, worldwide dearth of crustal events in the earliest Proterozoic, between 2500 and 2000 Ma, was broken in northern Australia with the advent of a major crust-forming episode between 1880 and 1850 Ma ago. These rocks may have evolved from an initial mantle or lower crustal melting and fractionation process about 400-300 Ma earlier, that is evidenced from Sm-Nd model ages. The extent, involvement, and/or degree of assimilation of any pre-existing Archaean crust is small, but whether or not it is negligible or zero is not satisfactorily known. The 1880-1850 Ma event, the Barramundi Orogeny, is well represented in northern Australia, both lithologically and analytically, and the rocks that pre-and postdate it have a similar tectonic setting over very wide areas of the continent. Two consistent and widespread elements of this tectonism are regional metamorphism and deformation of the first cycle of supracrustals (Barramundi Orogeny), and crystallization of I-type, K-rich felsic magmas closely following and, in some cases, overlapping with deformational processes. In the Halls Creek Inlier, the ages of this rapid deep crustal to supracrustal tectonic transition cannot be distinguished within the range 1860-1850 Ma. This major yet short-lived tectonism is mirrored in other fold belts such as Pine Creek, Tennant Creek, and Mount Isa, in which U-Pb zircon ages give slightly older constraints of 1885-1870 Ma, 1920-1870 Ma and 1885 ± 10 Ma, respectively. The ensuing felsic magmatism in these latter terranes continued for a few tens of millions of years, with the crystallization and emplacement of comagmatic volcanic/plutonic pairs throughout, in the interval 1870-1860 ]Via. In the Georgetown and Arunta Inliers, direct evidence of this otherwise ubiquitous event, if present, is masked by younger high-grade metamorphism. In the latter, however, metamorphic crustal components having approximate protolith ages of this order can be inferred from their evolved isotopic signatures. The geochronological and geochemical coherency of the extensive early Proterozoic, ~ 1870 Ma magmatism contrasts with that of post-1800 Ma magmatism, which is less extensive, generally anorogenic, commonly bimodal in character, and varies in age from one terrane to another. Several major sedimentary basins were being filled at this time. Major igneous events at 1790-1740 Ma, 1670 Ma and ~ 1500 Ma are recognized. Elongate, sublinear volcanic and plutonic belts, 1790-1740 Ma old, are present at Mount Isa. In high-grade granitic gneisses of the Arunta Inlier, a few U-Pb zircon ages and extrapolated Rb-Sr protolith ages are coincident at about 1760-1750 Ma, indicating that this event may be important in the development of the 'Division 2' sequences of that fold belt. The platformal Hart Dolerite and Oenpelli Dolerite, two of the largest mafic intrusions in the world, were emplaced at 1762 + 25 Ma and 1688 _+ 13 Ma. Younger felsic plutonism and volcanism gave rise to rapakivi-type granites in the Mount Isa, Tennant Creek and The Granites-Tanami Inliers at ~ 1670 Ma, a time close to that of the Aileron deformational/metamorphic event in central Australia. Further, at ~ 1650 Ma, there is growing evidence of an unexpectedly coherent hydrothermal episode that reset or disturbed many Rb-Sr ages, gave rise to pegmatites, and was possibly associated with mineralization in the Davenport and eastern Arunta Inliers. Discrete middle Proterozoic deformational events, between 1610 and 1470 Ma, punctuate the tectonostratigraphic history in the Arunta, Mount Isa and Georgetown fold belts where they are best documented.