Tasman Orogenic Research Articles

Structure of the Mount Isa Region Inferred from Gravity and Magnetic Anomalies

Most of the Mount Isa Inlier lies within the Mount Isa Geophysical Domain, 800 km long and 150 to 250 km wide. This domain is inferred to have formed later than crust to the east and west, and earlier than the Tasman Orogenic System to the south, and the crust to the north. Most major features of the domain parallel the arcuate NNE to NNW trending domain margins. The domain can be longitudinally subdivided using geophysical anomalies into six zones. The zone boundaries broadly correlate with the faults forming the boundaries of the major geological blocks of Blake and Stewart, and bound the extent of some stratigraphic units and batholiths. There are changes in rocks and structures within some zones, in particular at a 20° bend in the domain at 21.5?22°S. Rocks and structures similar to those of the inlier are shown to extend 300 km to the north, and to the south under cover. The western part of the inlier is found to belong in part to the Arunta and Murphy Geophysical Domains. The geophysical anomalies support the interpretation of most Mount Isa rocks forming in an intra-cratonic rift setting, but they show that the lower crustal structure differs from most other Middle Proterozoic mobile belts in that the rift truncates structures on both sides.

Geology and geochronology of the Balcooma area: Part of an early palaeozoic magmatic belt in North Queensland

The Balcooma Metavolcanics, in the eastern part of the mainly Proterozoic Georgetown Province, consist mostly of felsic metavolcanic and metasedimentary rocks. They were intruded by small microgranite and granite plutons, and porphyry dykes and sills before being deformed. Amphibolite facies metamorphism accompanied D1 and D2. The Balcooma Mylonite Zone separates the Balcooma Metavolcanics from the Proterozoic Einasleigh Metamorphics to the west, but the type of movement is uncertain. The Balcooma Metavolcanics have been regarded as probably of Early Palaeozoic age, because of similar lithologies and mineralization (e.g. the Balcooma Cu‐Zn‐Pb deposit) to the Cambrian to Ordovician Seventy Mile Range Group near Charters Towers, but a Proterozoic age was not ruled out. New U‐Pb isotopic data confirm the existence of Early Palaeozoic magmatism in the area. The data, obtained by ion‐microprobe analysis of zircons from metaporphyry related to the Balcooma Metavolcanics, yield ages of 471 ± 4 Ma (206Pb/238U), 478 ± 5 Ma (207Pb/235U), and 507 ± 22 Ma (207Pb/206Pb). The age of 471 ± 4 Ma (Middle Ordovician) is somewhat younger than that of the Seventy Mile Range Group, whereas the older age (507 ± 22 Ma) is more consistent with a correlation. Other metavolcanic rocks in the eastern Georgetown Province are also probably Early Palaeozoic in age. Unmetamorphosed Early and Late Ordovician mafic to felsic volcanic rocks are present in the western part of the Broken River Province, immediately east of the Georgetown Province. These, together with the Balcooma Metavolcanics and Seventy Mile Range Group, suggest the existence of a magmatic belt along the cratonic margin in the northern part of the Tasman Orogen in the Early Palaeozoic. A Rb‐Sr isochron age of 408 ± 6 Ma was obtained from metapelite in the Balcooma Metavolcanics, but its significance is uncertain.

An oblique subduction and transform faulting model for the evolution of the Broken River Province, northern Tasman Orogenic Zone

Formation of the Broken River Province, an important element of the northern Tasman Orogenic Zone, can be rationalized in terms of oblique subduction and strike‐slip faulting. Its two broadscale terranes, the smaller southwestern Graveyard Creek Subprovince and larger Camel Creek Subprovince, are regarded as Silurian‐Devonian forearc basinal, and subduction complex, assemblages, respectively. They are divided by the Gray Creek Fault, interpreted as a dextral strike‐slip structure of Siluro‐Devonian age. Part of the multiphase structural history of the Camel Creek Subprovince is ascribed to the partitioning of shortening and shear stress regimes known to characterize continental margins experiencing oblique‐slip subduction. Late Devonian (Famennian) crustal shortening is ascribed to a change in the direction of plate motion and an increase in its rate. A sheet of basement is thought to have been thrust eastwards to cover most of the forearc basinal assemblage with the southwestern part of the Burdekin River Fault developing as a tear structure at its southern margin. Development of the Clarke River Fault Zone, regarded as a transform offsetting the continental margin in a sinistral sense, is also inferred for Famennian time. Northeasterly trending folds characteristic of the Graveyard Creek Subprovince and recognized as a discrete fold phase in the southern Camel Creek Subprovince are considered to have been induced by shear adjacent to the Clarke River Fault Zone and to wedging of the continental margin sedimentary assemblage between basement blocks. Oblique subduction appears to have resumed in Early Carboniferous (Tournaisian) time. Two previously separate structures, the southwestern Burdekin River Fault and part of the older Gray Creek Fault, united to form a single dextral strike‐slip structure. Movement on this composite fault, the Burdekin River Fault of current nomenclature, sponsored the development of a deep Early Carboniferous pull‐apart molasse basin which retained stratigraphic and structural continuity with the preceding forearc basin. Extensive shallow Early Carboniferous molasse basins developed in response to vertical movements on the Gray Creek and Clarke River Fault Zones. With subsequent vertical movement coupled with erosion, the Burdekin River Fault has come to mark the eastern limit of overthrust basement.

Deep Seismic Profiling Across the Nebine Ridge, Surat Basin, Kumbarilla Ridge and Clarence Moreton Basin in Southern Queensland

In 1984, the Bureau of Mineral Resources (BMR) and the Geological Survey of Queensland recorded regional seismic reflection profiles of more than 800 km length from the eastern part of the Eromanga Basin to the Beenleigh Block east of the Clarence Moreton Basin (Johnstone et al. 1985; Wake-Dyster et al. 1985). Gravity measurements at 500 m intervals and an aeromagnetic survey at an altitude of 150 m were also undertaken. The main objectives of the work were as follows: to obtain seismological data that would assist in (i) understanding the evolution of the Surat and Clarence Moreton Basins, (ii) defining the role of the Tasman Orogenic Zone in the development of eastern Australia and (iii) providing regional ties between petroleum exploration leases. This paper provides a brief summary of the geological problems that have motivated the present work and emphasises results for the Nebine Ridge and Clarence Moreton Basin areas.

Geology of the Mt Windsor subprovince—a lower Palaeozoic volcano‐sedimentary terrane in the northern Tasman orogenic zone

The Mt Windsor Subprovince encompasses the dismembered remnants of a thick volcanic and sedimentary succession predominantly of Late Cambrian and Early Ordovician age located within the northern part of the Tasman Orogenic Zone. The succession is divided into four formations which together comprise the Seventy Mile Range Group. Its lower part, Puddler Creek Formation, comprises immature terrigenous clastic strata lacking in volcanics but hosting numerous penecontemporaneous dolerite sills and dykes. Its upper part, Mt Windsor Volcanics, Trooper Creek Formation and Rollston Range Formation, is dominated by acid and intermediate volcanics and volcaniclastics. The group has been dismembered, deformed and in part metamorphosed by emplacement of the Middle Ordovician Ravenswood Granodiorite Complex which is regarded as having been diapirically emplaced. Stratigraphic relationships and reconnaisance geochemistry suggest that the volcanics comprise a consanguineous series ranging from basaltic andesite to rhyoli...

Ordovician trimerellacean brachiopod shell beds

The large, thick-shelled, inarticulate brachiopod Eodinobolus forms many conspicuous deposits of shells in the Upper Ordovician limestones of central western New South Wales. Both in situ and reworked shell beds arc preserved at recurrent intervals through the successions, in similar facies of both transgressivc and regressive phases of deposition. In situ shell beds arc best developed in transgressivc sequences, with up to four generations of shells exhibited in the individual in situ beds. These monotypic and very low diversity shell beds are interpreted as having formed in marginal marine, quiet water conditions: (1) on the fringes of an offshore island (in part the Molong High of the Tasman Orogen), with the island still providing a fairly continuous supply of terrigenous material: and, (2) after submergence of the island, on the resulting terrigenous-free, major offshore Bahamas-like platform. This may imply that the shell beds developed in different salinity regimes. Possibly Eodinobolus was capable of tolerating a wider than normal range of salinity, from slightly brackish through normal marine, even to marginally hypersaline. However, in both settings, Eodinobolus, in its role as the dominant member of the respective pioneer community, colonized similar substrates in the low energy mud zone. This appears to suggest depositional environments most directly analogous to those of Palaeozoic virgianid pentamerides, and perhaps also comparable with some modern marginal marine oyster and mussel-bed occurrences. ?Ordovician, Brachiopoda, Eodinobolus, palaeoecology, facies, shell beds. New South Wales.

Big Bend Megafold or Broken River Triple Junction?

Abstract The Broken River Embayment is a major transverse structure in the Tasman Orogen in northern Queensland. Bell (1980) suggested that it is a hinge zone of an orocline, which he named the Big Bend Megafold. An alternative explanation, presented in this paper, involves a Silurian‐Devonian triple plate junction.

Structure and tectonic style of the Precambrian shields and platforms of northern Australia

The structure and tectonic style of Australia, to the north of the Musgrave Block and the southern Canning Basin and to the west of the Tasman Orogenic Province, are summarized. Northern Australia is largely occupied or underlain by the early Proterozoic North Australian Orogenic Province, which is bounded by younger mid-Proterozoic mobile belts of the Central Australian Orogenic Province along the eastern and southern margins. In the north, a basement of the Archaean West Australian Orogenic Province underlies the North Australian Orogenic Province. The strata of the North Australian Platform Cover were mildly to moderately deformed at the time when the mid-Proterozoic mobile belts were active. The late Proterozoic and Palaeozoic Central Australian Platform Cover developed over both the North and Central Australian Orogenic Provinces. Finally, the Mesozoic—Cainozoic Trans-Australian Platform Cover transgressed most of the region. The tectonic evolution of northern Australia can be clearly related to the times of cratonisation of its basement. A comparatively uniform pattern of major fractures, trending roughly northerly and northwest, was established throughout the region very early in its history. The subsequent evolution resulted from repeated reactivation of these fractures. Much of the structure may possibly be explained by a simple model in which a central block, roughly between the Kimberleys and Mount Isa, was displaced northwards relative to the blocks on either side and locally, the horizontal displacements were absorbed along east—west-trending zones of thrusting and folding, where the cover was crumpled against rigid blocks.

Porphyry-type copper-molybdenum mineralization belts in eastern Queensland, Australia

Four major periods of postorogenic porphyry-type Paleozoic and Mesozoic metallogenesis are recognized in eastern Queensland. The four periods, Siluro-Devonian, Permo-Carboniferous, Permo-Triassic, and Early Cretaceous, are widely distributed within, or immediately west of, the Paleozoic-early Mesozoic Tasman Orogenic Zone. The age of mineralization within the three younger periods is confined to discrete pulses with the ages of the deposits within each period decreasing with progression toward the east.Overall, the deposits are characterized by weakly developed potassic alteration assemblages, although widespread phyllic and propylitic assemblages are almost always present. Sulfide and alteration mineralogy is predominantly fracture controlled and usually exhibits rough zonation, commonly about a central core. Supergene enrichment is weakly developed or lacking and all deposits are at present economically submarginal. Depth of exposure increases with deposit age and from west to east.From east to west, the deposits exhibit characteristics of those found in island-arc, continental margin, and ensialic environments, with the majority exhibiting island-arc characteristics. However, with the exception of several deposits to the west, they were probably emplaced in a continental margin environment. This apparent inconsistency is thought to result from deposit emplacement into thin continental crust on the eastern edge of the developing Australian Plate.Within each period of metallogenesis, the deposits are confined to narrow, sometimes parallel, linear belts up to 25 km in width. These are either longitudinal or transverse in relation to the regional strike of the Tasman Orogenic Zone. Porphyry-type alteration, brecciation, and mineralization of calc-alkaline intrusives are restricted to the belts, rarely extending outside them. Diagonal intrabelt trends of mineralization and alteration are common, particularly at the intersection of transverse lineament zones and longitudinal belts. Mineralization within longitudinal porphyry-type belts is considered to be genetically related to either postulated subduction of westerly dipping oceanic plates during the Paleozoic and Mesozoic or to tension breaks on the flanks of structural highs, or to both. The location of deposits within these belts is influenced by transverse elements such as faults and major lineaments which are oriented east-northeast and east-west in southern and northern Queensland, respectively. Most of these elements are thought to be zones of structural weakness and appear to contain porphyry-type mineralization, especially in areas where previous copper mineralization has taken place. The intrabelt mineralization trends may result from shear stress in these zones during porphyry-type emplacement.

The eastern part of the Tasman Orogenic Zone

The eastern part of the Tasman Orogenic Zone (or Fold Belt System) comprises the Hodgkinson—Broken River Orogen (or Fold Belt) in the north and the New England Orogen (or Fold Belt) in the centre and south. The two orogens are separated by the northern part of the Thomson Orogen. The Hodgkinson—Broken River Orogen contains Ordovician to Early Carboniferous sequences of volcaniclastic flysch with subordinate shelf carbonate facies sediments. Two provinces are recognized, the Hodgkinson Province in the north and the Broken River Province in the south. Unlike the New England Orogen where no Precambrian is known, rocks of the Hodgkinson—Broken River Orogen were deposited immediately east of and in part on, Precambrian crust. The evolution of the New England Orogen spans the time range Silurian to Triassic. The orogen is orientated at an acute angle to the mainly older Thomson and Lachlan Orogens to the west, but the relationships between all three orogens are obscured by the Permian—Triassic Bowen and Sydney Basins and younger Mesozoic cover. Three provinces are recognized, the Yarrol Province in the north, the Gympie Province in the east and the New England Province in the south. Both the Yarrol and New England Provinces are divisible into two zones, western and eastern, that are now separated by major Alpine-type ultramafic belts. The western zones developed at least in part on early Palaeozoic continental crust. They comprise Late Silurian to Early Permian volcanic-arc deposits (both island-arc and terrestrial Andean types) and volcaniclastic sediments laid down on unstable continental shelves. The eastern zones probably developed on oceanic crust and comprise pelagic sediments, thick flysch sequences and ophiolite suite rocks of Silurian (or older?) to Early Permian age. The Gympie Province comprises Permian to Early Triassic volcanics and shallow marine and minor paralic sediments which are now separated from the Yarrol Province by a discontinuous serpentinite belt. In morphotectonic terms, a Pacific-type continental margin with a three-part arrangement of calcalkaline volcanic arc in the west, unstable volcaniclastic continental shelf in the centre and continental slope and oceanic basin in the east, appears to have existed in the New England Orogen and probably in the Hodgkinson—Broken River Orogen as well, through much of mid- to late Palaeozoic time. However, the easternmost part of the New England Orogen, the Gympie Province, does not fit this pattern since it lies east of deepwater flysch deposits of the Yarrol Province.

The Thomson Orogen of the Tasman Orogenic Zone

The Thomson Orogen forms the northwestern segment of the Tasman Orogenic Zone. It was a tectonically active area with several episodes of deposition, deformation and plutonism from Cambrian to Carboniferous time. Only the northeastern part of the orogen is exposed; the remainder is covered by gently folded Permian and Mesozoic sediments of the Galilee, Cooper and Great Artesian Basins. Information on the concealed Thomson Orogen is available from geophysical surveys and petroleum exploration wells which have penetrated the Permian and Mesozoic cover. The boundaries of the Thomson Orogen with other tectonic units are concealed, but discordant trends suggest that they are abrupt. To the west, the orogen is bordered by Proterozoic structural blocks which form basement west of the northeast-trending Diamantina River Lineament. The most appropriate boundary with the Lachlan and Kanmantoo Orogens to the south is an arcuate line marking a distinct change in the direction of gravity trends. The north-northwest orientation of the northern part of the New England Orogen to the east cuts strongly across the dominant northeast trend of the Thomson Orogen. The Thomson Orogen developed as a tectonic entity in latest Proterozoic or Early Cambrian time when the former northern extension of the Adelaide Orogen ∗ was truncated along the Muloorinna Ridge. Early Palaeozoic deposition was dominated by finegrained, quartz-rich clastic sediments. Cambrian carbonates accumulated in the southwest and a Cambro-Ordovician island arc was active in the north. Along the western margin of the orogen, sediments were probably laid down on downfaulted blocks of deformed Proterozoic rocks, with oceanic crust further to the east. A mid- to Late Ordovician orogeny which affected the whole of the Thomson Orogen marked the climax of its precratonic (orogenic) stage. The northeast structural trend of the orogen (parallel to its western boundary with the Precambrian craton) was imposed at this time and has controlled the orientation of later folding and faulting. Up to three generations of folding have been recognized and fine-grained metasediments exhibit a prominent slaty cleavage. Metamorphism was to the greenschist and amphibolite facies, the highest grade rocks being associated with synorogenic granodiorite batholiths in the north. Following deposition of Late Ordovician marine sediments at the eastern margin, emplacement of post-tectonic Late Silurian or Early Devonian batholiths ended the precratonic history of the Thomson Orogen. The subsequent transitional tectonic regime was characterized by deposition of Devonian to Early Carboniferous shallow marine and continental sediments including widespread red-beds and andesitic volcanics. The maximum marine transgression occurred in the early Middle Devonian. Localized folding affected the easternmost part of the Thomson Orogen at the end of Middle Devonian time and was followed by intrusion of Devono-Carboniferous granitic plutons. However, the terminal orogeny which deformed all Devonian to Early Carboniferous rocks of the orogen was of mid-Carboniferous age. It produced northeast-trending open folds and normal and high-angle reverse faults which are considered to reflect basement structures. The cratonization of the Thomson Orogen was completed with the emplacement of Late Carboniferous granites and the eruption of comagmatic volcanics in the northeast, permian and Mesozoic sediments accumulated in broad, relatively shallow down warps which covered most of the former orogen.

Palaeomagnetism and the tectonic evolution of the Tasman Orogenic Zone

Abstract Palaeomagnetic data obtained from studies of rock formations in southeastern Australia are compared with data from the main Australian assemblage. Occurrences and distributions of some of the major serpentine and ultramafic belts in the Tasman Orogenic Zone are interpreted according to current plate theory as marking sites of ancient plate margins. A tectonic model based on the palaeomagnetic data with the aid of geological constraints is proposed. It describes the rotation of a small region in southeastern Australia, relative to the main assemblage, by about 90° since the Middle Silurian and 30° since the Early Devonian. The proposed small plate boundaries may be represented by the Great Serpentine Belt to the east and in part by the less well defined belt of serpentinites which outcrop from Kiandra to Nyngan on the western margin. It seems unlikely that the whole of the western serpentine belt was actively involved with accretion during the Siluro‐Devonian. The northern limit of the plate canno...

Australian palaeomagnetism and the Phanerozoic plate tectonics of eastern Gondwanaland

All palaeomagnetic investigations from the Phanerozoic of Australia are summarized and critically reviewed. Some smaller studies have been combined to produce more viable palaeomagnetic poles all of which are only considered if standard cleaning procedures were used. Analysis of the resulting data shows that during the Early Palaeozoic the pole paths for northern and central Australia are similar confirming the regions were a single unit during that time. However, these paths and the one derived from a limited region of southeast Australia approach each other from opposite directions and appear to converge during the Devonian. This observation is consistent with interpretations of the geology of the Tasman Orogenic Zone in terms of plate-tectonic models and with palaeomagnetic data from the Gondwanic continents. The presence of possible ancient plate margins bounding the region, from which most palaeomagnetic results from southeast Australia are derived, confirms that this region only became welded to the main Australian plate in Devonian times. Data for the Mesozoic of southeast Australia continue to be incompatible both with the generally accepted Australia—Antarctica relationship and with all other Gondwanic results. There appears to be no geological evidence in support of the large-scale relative motion inferred by the data and they remain a puzzling inconsistency. Cenozoic results, however, are entirely compatible with the northward motion of Australia away from Antarctica as inferred from sea-floor spreading. Comparisons with results from India suggest that the drift history of India prior to 75 m.y. ago involved movement from a location adjacent to Antarctica. It is proposed that the Wharton Basin was occupied by a northerly extension of Peninsular India which lay adjacent to western Australia. This larger Indian subcontinent broke away from both Antarctica and Australia about 140 m.y. ago.

Plate Tectonics and the Lower Palaeozoic of Tasmania

Solomon and Griffiths1 have applied a plate tectonics model to the Tasmanian part of the Tasman Orogenic Zone in the Lower Palaeozoic. Although this new interpretation of the Cambrian tectonics of Tasmania is timely and stimulating, we find that there are difficulties in fitting the facts to the model and that the evidence suggests that other explanations are possible if not, indeed, necessary.

Tectonic Evolution of the Tasman Orogenic Zone, Eastern Australia

The pattern of continental margin sedimentation in the Palaeozoic of eastern Australia probably did not involve a simple progressive eastward advance into the Pacific Ocean.