R.J. Korsch

TEMORA 1: a new zircon standard for phanerozoic U-Pb geochronology

The role of the standard is critical to the derivation of reliable U–Pb zircon ages by micro-beam analysis. For maximum reliability, it is critically important that the utilised standard be homogeneous at all scales of analysis. It is equally important that the standard has been precisely and accurately dated by an independent technique. This study reports the emergence of a new zircon standard that meets those criteria, as demonstrated by Sensitive High Resolution Ion MicroProbe (SHRIMP), isotope dilution thermal ionisation mass-spectrometry (IDTIMS) and excimer laser ablation–inductively coupled plasma–mass-spectrometry (ELA–ICP–MS) documentation. The TEMORA 1 zircon standard derives from the Middledale Gabbroic Diorite, a high-level mafic stock within the Palaeozoic Lachlan Orogen of eastern Australia. Its 206Pb/238U IDTIMS age has been determined to be 416.75±0.24 Ma (95% confidence limits), based on measurement errors alone. Spike-calibration uncertainty limits the accuracy to 416.8±1.1 Ma for U–Pb intercomparisons between different laboratories that do not use a common spike.

The application of SHRIMP to Phanerozoic geochronology: a critical appraisal of four zircon standards

Derivation of Phanerozoic zircon 206Pb/238U ages by SHRIMP depends on calibration against an independently dated standard. The qualities of four different zircon standards (SL13, QGNG, AS3 and TEMORA 1) are assessed herein. Not all of these behave consistently on SHRIMP with respect to their ages as determined by IDTIMS. SL13, the most commonly used standard over the past decade and a half, is the most heterogeneous in Pb/U. In addition, when SL13 is used as the calibration standard, the varied ages resulting from that heterogeneity are generally younger than ages derived from the other three standards. AS3-calibrated ages are the oldest of the group. Only QGNG and TEMORA 1, when calibrated relative to each other, yield ages on SHRIMP that are consistent with their IDTIMS ages. Of these two, TEMORA 1 has the distinct advantage of producing consistent IDTIMS ages at high precision. Because of these factors and its availability, we recommend its use in geological studies where precise and accurate Pb/U zircon ages are imperative. Approximate conversion factors have been derived to improve quantitative inter-comparison between SHRIMP ages that have been calibrated against the different standards. These refinements significantly advance the role that SHRIMP can play in the numerical calibration of the Phanerozoic timescale.

Early Permian East Australian Rift System

The Early Permian to Middle Triassic Bowen and Gunnedah Basins in eastern Australia developed in response to a series of interplate and intraplate tectonic events that occurred to the east of the basin system. The initial event was extensional and stretched the continental crust to form part of the major Early Permian East Australian Rift System that occurred at least from far-north Queensland to southern New South Wales. The most important of the rift-related features, in a commercial sense, are a series of half-grabens that form the Denison Trough, now the site of several producing gasfields. The eastern part of the rift system commenced at about 305 Ma and was volcanic dominated. In contrast, the half-grabens in, and to the west of, the Bowen Basin were non-volcanic, and appear to have initiated significantly later, at about 285 Ma. These half-grabens are essentially north–south in length with an extension direction of approximately east-northeast. Mechanical extension appears to have ceased at about 280 Ma, when subsidence became driven by thermal relaxation of the lithosphere. The extension occurred in a backarc setting, in response to far-field stresses that propagated from the west-dipping subduction system at the convergent plate margin of East Gondwanaland that was located to the east of the East Australian Rift System.

Contractional structures and deformational events in the Bowen, Gunnedah and Surat Basins, eastern Australia

During the Permian and Triassic, eastern Australia was part of an active Gondwanaland convergent plate margin. The Bowen and Gunnedah Basins formed in a backarc setting, which was initially extensional, but switched to contractional in the mid-Permian, leading to the development of a major west-directed retroforeland thrust belt in the New England Orogen, and the formation of a major foreland basin phase to the west in the Bowen and Gunnedah Basins. The contractional deformational style is asymmetric, changing from the eastern side of the basins, adjacent to the thrust belt, to the western side of the basins which was not physically affected by the retrothrust belt. In the east, new thrusts are hard-linked to the growing thrust wedge further to the east, which propagated westwards and cannibalised the eastern part of the basin system. In the western part of the basin, however, the transmission of far-field compressional stresses led to the inversion of Early Permian extensional faults as thrusts, along with the development of new thrusts and backthrusts, which are not hard-linked to the retrothrust belt in the east. During the sustained period of rapid subsidence and sedimentation driven by thrust loading in the Bowen and Gunnedah Basins in the Late Permian to Late Triassic, there are several short periods of non-deposition and contraction. The contractional events were usually short-lived, less than a few million years each in duration, in an overall period of subsidence that lasted for ∼30–35 Ma. It is suggested that shallow to flat subduction over much of this period produced strong coupling across the plate boundary, which allowed the transmission of compressive far-field stresses well into the distal part of the foreland, possibly during times of global plate boundary reorganisation. A final contractional event in the early Late Cretaceous corresponds with the cessation of sedimentation in the Surat Basin, uplift and reactivation of earlier structures.

Subsidence history and basin phases of the Bowen, Gunnedah and Surat Basins, eastern Australia

The Early Permian to Middle Triassic Bowen and Gunnedah Basins and the Early Jurassic to Early Cretaceous Surat Basin exhibit a complex subsidence history over a period of about 200 Ma. Backstripped tectonic subsidence curves, constructed by removing the effects of processes such as sediment loading, loading due to the water column and sediment compaction, allow the subsidence histories of the basin to be examined in terms of the tectonic drivers that caused the subsidence of the basins. In the Early Permian, rapid subsidence was driven by mechanical extension, forming a series of half-grabens along the western margin of the Bowen and Gunnedah Basins. Mechanical extension ceased at about 280 Ma, being replaced by a phase of passive thermal subsidence, resulting in more widespread, uniform sedimentation, with reduced tectonic subsidence rates. At the start of the Late Permian, the passive thermal subsidence phase was interrupted by the onset of lithospheric flexure during a foreland basin phase, driven by convergence and thrust loading to the east in the New England Orogen. Initially, dynamic loading, caused by viscous corner flow in the asthenospheric wedge above the west-dipping subducting plate, led to limited tectonic subsidence. Later in the Late Permian, the dynamic loading was overwhelmed by static loading, caused by the developing retroforeland thrust belt in New England, leading to very high rates of tectonic subsidence, and the development of a major retroforeland basin. Peneplanation in the Late Triassic was followed by sedimentation at the start of the Jurassic, forming the Surat Basin, where the tectonic subsidence can again be interpreted in terms of dynamically induced platform tilting. Subduction ceased at about 95 Ma, resulting in rapid uplift, due to the rebound of the lithosphere following either cessation of subduction, or it stepping well to the outboard of Australia.

Crustal architecture of the Capricorn Orogen, Western Australia and associated metallogeny

A 581 km vibroseis-source, deep seismic reflection survey was acquired through the Capricorn Orogen of Western Australia and, for the first time, provides an unprecedented view of the deep crustal architecture of the West Australian Craton. The survey has imaged three principal suture zones, as well as several other lithospheric-scale faults. The suture zones separate four seismically distinct tectonic blocks, which include the Pilbara Craton, the Bandee Seismic Province (a previously unrecognised tectonic block), the Glenburgh Terrane of the Gascoyne Province and the Narryer Terrane of the Yilgarn Craton. In the upper crust, the survey imaged numerous Proterozoic granite batholiths as well as the architecture of the Mesoproterozoic Edmund and Collier basins. These features were formed during the punctuated reworking of the craton by the reactivation of the major crustal structures. The location and setting of gold, base metal and rare earth element deposits across the orogen are closely linked to the major lithospheric-scale structures, highlighting their importance to fluid flow within mineral systems by the transport of fluid and energy direct from the mantle into the upper crust.

Crustal architecture of central Australia based on deep seismic reflection profiling

Abstract The crustal architecture of central Australia is interpreted from deep seismic reflection profiling conducted by the Australian Geological Survey Organisation in two surveys in 1985 and 1993. The seismic traverses, oriented normal to the main surface structures, ran north-south in central Australia, and crossed parts of the Arunta Block, Amadeus Basin, Musgrave Block and Officer Basin. The present crustal fabric was set in place by the end of the Mesoproterozoic (by about 1100 Ma). Reactivation of the structures took place in a continental intraplate setting mainly during the Middle-Late Palaeozoic Alice Springs Orogeny, but also during other orogenic events. In the Arunta Block. the crust is dominated by major north-dipping planar structures, interpreted as thick-skinned thrusts. Many of these thrusts cut deep into the crust, and at least one, the Redbank Thrust Zone, appears to cut and offset the Moho. In contrast, in the northern Musgrave Block, limited field mapping and teleseismic data suggest that the major crustal-scale planar structures are south-dipping. In the central to southern Amadeus Basin, deformation is essentially thin-skinned, with north-directed thrusting confined to the sedimentary succession. Thus, the deep seismic profiles in central Australia show a present day crustal architecture that is the response of the crust to Mesoproterozoic terrane amalgamation and to later reactivation by intraplate deformational events. Therefore. central Australia is a model for intraplate cratonic deformation that occurs in continental crust that is cold. thick and strong.

Geochronology and provenance of the Late Paleozoic accretionary wedge and Gympie Terrane, New England Orogen, eastern Australia∗

In easternmost Australia, the New England Orogen contains a geological record dominated by subduction-related rocks, with plate convergence during the Late Devonian to Triassic being related to a west-dipping subduction system, assuming present-day orientation, at the boundary of eastern Gondwanaland and the Panthalassan Ocean. A well-preserved Late Paleozoic accretionary wedge contains deep-marine turbidites deposited as trench fill, plus infaulted slices of oceanic crust. The turbidites are mostly first-cycle, immature, quartz-poor, volcanic-derived sedimentary rocks, some of which contain detrital hornblende, along with less-common quartz-rich sandstones to the east. In this study, detrital zircons from sandstones in various tectonic blocks of the New England Orogen are dated by the U–Pb SHRIMP and LA-ICPMS techniques and detrital hornblendes by the Ar–Ar technique to constrain the age and provenance of sedimentary rocks in the accretionary wedge. All samples, except two quartz-rich sandstones from the northern Shoalwater Formation, have maximum depositional ages of 355–316 Ma, indicating that the accretionary wedge evolved over a period of at least 40 Ma, with principal sources from a contemporaneous active continental margin volcanic arc. Quartz-rich sandstones from the easternmost part of the accretionary wedge (Shoalwater Formation and eastern Beenleigh Block) contain a greater range of individual detrital zircon ages from Late Paleozoic to Archean (several individual grains >3000 Ma). These ages indicate that, although detritus from Carboniferous volcanic arc sources was involved, quartz-rich detritus mostly derived from the continental interior dominated the depocentres. We suggest that these quartz-rich sandstones accumulated from longitudinal transport along the trench, like the modern-day Barbados Ridge accretionary wedge, along with breaching of the marginal arc by streams draining the continental interior.

Mid-Cretaceous uplift and denudation of the Bowen and Surat Basins, eastern Australia: relationship to Tasman Sea rifting from apatite fission-track and vitrinite-reflectance data

Interpretation of apatite fission-track and vitrinite-reflectance data for samples from nine petroleum-exploration wells in the eastern part of the Bowen and Gunnedah Basins, eastern Australia, indicates that peak paleotemperatures were reached during the Early Cretaceous, through progressive exposure to higher temperatures due to increased depth of burial. The paleotemperatures were 28–58°C higher than at present. Cooling from the peak temperatures took place in the mid-Cretaceous, at some time during the interval 100–80 Ma, with the greatest amount of cooling occurring in the northern part of the study area. Paleogeothermal gradients were generally in the range 21–35°C/km, similar to present-day geothermal gradients in the region. The estimated maximum amount of denudation in the study area is ∼1.9 km, with a significant portion of the eroded succession being Jurassic to Early Cretaceous in age. The synchronicity between the times of cooling inferred from all the sampled wells, regardless of their location with respect to the fault system near the present eastern margin of the Bowen Basin, suggests that the uplift was widespread, rather than being localised by faults during the mid-Cretaceous. This can be correlated with uplift along much of the eastern margin of Gondwanaland at the same time, including all of eastern Australia, in New Zealand and in Marie Byrd Land, Antarctica. The onset of this mid-Cretaceous regional cooling and denudation coincided with a period of continental extension after the cessation of volcanism and subduction at about 95 Ma, and prior to the initiation of seafloor spreading at about 84 Ma and formation of the current passive margin.

Geodynamic significance of the boundary between the Thomson Orogen and the Lachlan Orogen, northwestern New South Wales and implications for Tasmanide tectonics

Interpretation of deep seismic reflection profiling, coupled with forward modelling of gravity and aeromagnetic data, new zircon U–Pb dating and the interpretation of the basement geology beneath the southern margin of the Eromanga Basin, has provided insights into the southern part of the underlying Thomson Orogen and its relationship with the Lachlan Orogen to the south. Our interpretations of these data suggest that the northern Lachlan and southern Thomson orogens had a shared history from the mid-Silurian to the Carboniferous. Major older differences, however, are suggested by the presence in the southern Thomson Orogen of: (i) a possible Neoproterozoic arc, (ii) latest Cambrian to earliest Ordovician turbidites, (iii) Late Ordovician turbidites, and (iv) geophysical evidence for thrusting of reflective ocean crust rocks high into the crust on a north-dipping detachment. The seismically imaged, north-dipping, crustal-scale Olepoloko Fault corresponds to the ‘surface expression’ of the Thomson–Lachlan boundary. We speculate that it reflects the partial reactivation and short-cutting of an older fault in the post-Devonian (?Carboniferous) and probably also in the latest Silurian and Early Devonian. Comparisons with the seismic architecture of the Lachlan Orogen immediately to the south, and with the central part of the Thomson Orogen ∼450 and 650 km to the north, suggest that the part of the Thomson Orogen west of the Quilpie Trough and the Nebine Ridge developed on inferred Neoproterozoic to Cambrian oceanic crust that floors the Barcoo Basin. This basin separated the continental margin at that time on the west from a sliver of continental crust preserved at Anakie on the east that was overlain by one or more, poorly dated, passive margin sedimentary ± volcanic sequences that predate a 500 Ma deformation. The southern margin of the Thomson Orogen also contains a sliver of old continental crust, sandwiched between the southern strike-slip margin of the Barcoo Basin to the north and the open proto-Pacific ocean to the south. It was locally the site of ca 580 Ma subduction, because seafloor spreading to the south lay oblique to the orogen margin. We suggest that the Thomson Orogen and Lachlan Orogen were amalgamated by the late Middle Ordovician, although the Thomson–Lachlan boundary remained a zone of weakness at least until the Triassic.