Alice Springs Orogeny Research Articles

Alice Springs age shear zones from the southeastern Reynolds Range, central Australia

The southeast Reynolds Range, central Australia, is cut by steep northwest‐trending shear zones that are up to hundreds of metres wide and several kilometres long. Amphibolite‐facies shear zones cut metapelites, while greenschist‐facies shear zones cut metagranites. Rb–Sr and 40Ar–39Ar data suggest that both sets of shear zones formed in the 400–300 Ma Alice Springs Orogeny, with the sheared granites yielding well‐constrained 40Ar–39Ar ages of ca 334 Ma. These data imply that the shear zones represent a distinct tectonic episode in this terrain, and were not formed during cooling from the ca 1.6 Ga regional metamorphism. A general correlation between regional metamorphic grade and the grade of Alice Springs structures implies a similar distribution of heat sources for the two events. This may be most consistent with both phases of metamorphism being caused by the burial of anomalously radiogenic heat‐producing granites. The sheared rocks commonly have undergone metasomatism implying that the shear zones were conduits of fluid flow during Alice Springs times.

Intraplate deformation in central Australia, the link between subsidence and fault reactivation

Central Australia has experienced two intraplate orogenic events involving significant north–south shortening: the late Neoproterozoic to Early Cambrian Petermann Orogeny and the Devonian to Carboniferous Alice Springs Orogeny. In each event pre-existing structures inherited from Mesoproterozoic terrain amalgamation were reactivated and basement rocks exhumed from beneath thick sedimentary successions accumulated in the Centralian Superbasin. The pattern of fault reactivation during these events shows a striking similarity to the pattern of subsidence in the overlying basin. Immediately prior to the Petermann Orogeny, the Centralian Superbasin was thickest in the vicinity of the Musgrave Block, the region in which deformation was subsequently localised. At the same time crustal-scale faults elsewhere in central Australia that were covered by a relatively thin sheet of sediment remained inactive despite being favourably oriented to accommodate the north–south shortening. Between the Petermann and Alice Springs Orogenies, subsidence patterns shifted, such that fault systems in the Arunta Block and also those in the southern Musgrave Block were buried by significant thicknesses of sediment, whereas the major structures that were exhumed during the Petermann Orogeny were not significantly buried. During the Alice Springs Orogeny reactivation once again occurred along the most deeply buried faults, even in the instances where those faults had remained inactive during the earlier Petermann Orogeny. Importantly the major Petermann-aged structures that were not buried during renewed subsidence remained inactive during the Alice Springs Orogeny. The record of reactivation implies that the presence of pre-existing crustal-scale faults alone was insufficient to localise deformation. Rather, fault reactivation appears to have required a priming process that modulated the strength of the lithosphere on a regional scale. The correspondence between the distribution of basement fault reactivation and subsidence patterns during both the Petermann and Alice Springs Orogenies implies a link between relatively thick sedimentation and long-term lithospheric weakening. We show that this link is compatible with the thermal effects of a thick sedimentary blanket. In the context of central Australia the mechanical impact of basin formation is likely to be enhanced by the presence of regionally elevated heat production in the Proterozoic basement.

Folded basement-cored tectonic wedges along the northern edge of the Amadeus Basin, Central Australia: evaluation of orogenic shortening

Intracontinental deformation at the northern margin of the Amadeus Basin, Central Australia, is characterised by the presence of basement-cored, downward-facing antiformal synclines. These structures were formed during the Devonian–Carboniferous Alice Springs Orogeny and in the past were explained as erosional relics of fold nappes indicating large horizontal crustal shortening within the Australian continent. This notion is in stark contrast with both the uncomplicated homoclinal structure that contains the `nappes' and with seismic data suggestive of a simple crustal structure formed during only modest intraplate shortening. Balanced and sequentially restored sections across the Razorback `nappe' and its vicinity show that such downward-facing synformal anticlines can be produced by emplacement of a basement cored tectonic wedge along shortcut thrusts in the footwall of a crustal-scale (Redbank) shear zone which initially formed during the Mesoproterozoic. The forelandward-propagating wedge proper is formed by basement and the lower two successions of the Amadeus Basin. Higher sedimentary successions accommodate shortening by hinterlandward backthrusting above a detachment horizon which is hosted in carbonates and evaporites of the Bitter Springs Formation. During the formation of the homocline which characterises much of the northern margin of the Amadeus Basin, the tectonic wedge was rotated through 90° and now forms a downward-facing antiformal syncline. The orogenic shortening indicated by this succession of structural events is about 19 km which is compatible with seismic data. The proposed model explains the existence of downward-facing synformal anticlines as folded basement-cored tectonic wedges; although they resemble parts of fold nappes, these folded tectonic wedges do not necessarily imply large amounts of crustal shortening.

Phanerozoic reactivation along a fundamental Proterozoic crustal fault, the Darling River Lineament, Australia: constraints from apatite fission track thermochronology

The Darling River of eastern Australia flows along a relatively straight course for much of a ∼2000 km linear feature extending from South Australia to the offshore continental margin of Queensland. This crustal-scale feature, the Darling River Lineament (DRL), is considered to have originally been a part of the Tasman Line, a major Late Proterozoic fracture zone which was active during the breakup of the Rodinia supercontinent. Even though extensive Phanerozoic orogenic contraction has since affected the Darling River region, the present surface expression of the DRL indicates relatively recent reactivation. Time–temperature modelling of apatite fission track data from rocks straddling the southwestern portion of the DRL suggests that Phanerozoic cooling along this part of the lineament principally occurred during three major episodes. Each of these can be tentatively linked to tectonism at the time; Late Silurian to Middle Devonian (∼420–380 Ma) deformation during either the Lachlan Orogeny or the Alice Springs Orogeny, Late Permian to Early Triassic (∼260–240 Ma) deformation during the Hunter–Bowen Orogeny and Early to middle Tertiary (∼60–40 Ma) deformation in response to Pacific Plate rearrangement and north Australian collision in Papua New Guinea. Moreover, the results reveal that the magnitude of cooling differed significantly across the lineament during each episode, indicating that this major Late Proterozoic feature has been reactivated several times during the Phanerozoic.

Controls on the locus of intraplate deformation in central Australia

The locus of intraplate deformation in central Australia changed from the presently preserved southern margin of the Amadeus Basin during the late Neoproterozoic to early Phanerozoic Petermann Orogeny to the northern margin in the late Phanerozoic Alice Springs Orogeny. Immediately prior to each event the thickness of the Amadeus basin sediments varied from as little as 1.5 km in the central parts of the basin to as much as 7 km in the vicinity of the presently preserved margins of the basin with the locus of deformation mimicking the locus of maximum sedimentary thicknesses at the onset of each orogenic event. As such these orogenies represent extreme examples of basin inversion; a process observed in many intracratonic basins worldwide. We show that differential burial of the central Australian Proterozoic basement complexes beneath the Amadeus basin is capable of producing variations in Moho temperature of up to 30°C prior to the Petermann Orogeny and up to 110°C prior to the Alice Springs Orogeny. For a `Brace–Goetze' model of lithospheric rheology, the variations in Moho temperature equate to variations in effective lithospheric strain rate of 1–4 orders of magnitude, implying that variations in thickness of the sedimentary blanket may have played a primary role in localising Phanerozoic intraplate deformation in central Australia. An appealing aspect of this model lies in its corollary: that the removal of the sedimentary blanket during denudation of the orogen will be accompanied by dramatic cooling and strengthening of the lithosphere. This may provide a plausible explanation for the long-term persistence of the extraordinary gravity anomalies (∼150 mGal) developed during these central Australian intraplate orogenies.

The retrograde P–T–t path for low‐pressure granulites from the Reynolds Range, central Australia: petrological constraints and implications for low‐P/high‐T metamorphism

Several aspects of the petrogenesis of low‐pressure granulite facies rocks from the Reynolds Range (central Australia) are contentious, including: (a) the shape of the retrograde P–T –time path, and whether it is an artefact of repeated thermal events at different P–T conditions; (b) the type of regional metamorphism; and (c) the causes of metamorphism. Granulite facies rocks from the Reynolds Range Group experienced three major periods of mineralogical equilibration. Metapelitic rocks underwent dehydration‐melting reactions to form migmatites under peak M2 P–T conditions of c. 5.0–5.3 kbar and c. 750–800 °C. Metapsammitic rocks that did not melt during M2 show spectacular garnet–orthopyroxene intergrowths that developed at c. 3.5–3.7 kbar and c. 700–750 °C after penetrative regional deformation, but prior to amphibolite facies rehydration in discrete strike‐parallel zones. Rehydration occurred within the sillimanite stability field at P–T conditions close to the granite solidus (c. 3.2–3.4 kbar and 650–700 °C). Subsequently the terrane cooled into the andalusite stability field. Geochronological constraints suggest that: (a) peak‐M2 conditions were reached at c. 1594 Ma; (b) the garnet–orthopyroxene intergrowths in unmelted metapsammites probably developed between c. 1594 Ma and c. 1586 Ma; and (c) upper amphibolite facies rehydration occurred between c. 1586 Ma and 1568 Ma. The lack of petrological evidence for multiple dehydration and rehydration of the rocks suggests that the three episodes of mineralogical recrystallization can be linked to yield a single continuous retrograde P–T–t path of minor initial decompression (c. 1.5 kbar) from the M2 peak, followed by cooling (c. 100 °C) to the granite solidus over a period of c. 26 Ma. Late kyanite‐bearing shear zones that dissect the terrane are unrelated to this event and formed during the c. 300–400 Ma Alice Springs Orogeny. The shape of the P–T–t path and the duration of M2 metamorphism suggests that advective heating was not the major cause of high‐grade metamorphism, and that some other, longer lived heat source, such as the burial of anomalously radiogenic, pre‐tectonic granites, is required.

Crustal architecture of central Australia based on deep seismic reflection profiling

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.

Phanerozoic Denudation History of the Mount Isa Inlier, Northern Australia: Response of a Proterozoic Mobile Belt to Intraplate Tectonics

The lack of preserved Phanerozoic units within the Proterozoic Mount Isa Inlier of northern Australia renders it difficult to determine its Phanerozoic tectonic history. However, thermo-chronological methods provide a means for assessing this problem. Apatite fission-track data from the central and southern parts of the Inlier reveal periods of post-early Carboniferous accelerated cooling. Apatite fission-track ages vary from 235 to 390 Ma and corresponding mean track lengths range from 11.76 to 13.55 microns. These results record a protracted cooling history below about 110 ± 10° C. The earlier period of cooling revealed by the data occurred during middle Carboniferous time. The event resulted in >2 km of exhumation across the Inlier and probably was in response to intra-continental deformation associated with the Alice Springs Orogeny and tectonics in the adjacent Tasman Orogen. A high proportion of partly annealed fission tracks in the samples suggests that rocks now exposed across the Inlier resided at the top of the apatite partial annealing zone (approximately 60° C to 70° C) following the mid-Carboniferous cooling. Modeling of the fission-track age and length parameters suggests that approximately 30° C to 50° C of cooling occurred over the past 100 Ma. Assuming a geothermal gradient of 25° C/km, this corresponds to 1.2-2.0 km of exhumation. The post-Middle Cretaceous cooling possibly is related to extensional tectonics at the southern and eastern margins of the Australian plate during the Mesozoic and Tertiary periods and to the more recent collision at the northern margin of the plate. The spatial variation of apatite fission-track data within the Inlier indicates that the three major structural belts-the Western fold belt, Kalkadoon-Leichhardt belt, and the Eastern fold belt-exhibit similar thermal histories on a regional scale. It also indicates that the main N-S fault zones bounding the belts have not been reactivated in a vertical sense along their entire length since ∼350 Ma. However, adjacent smaller-scale fault-bounded blocks within the belts demonstrate variable cooling histories, suggesting that reactivation of favorably oriented minor faults within the Inlier, including segments of the major faults, probably occurred during this time interval. Variations in apatite fission-track data along the 1994 Australian Geological Survey Organization/Australian Geodynamics Co-operation Research Center (AGSO/AGCRC) Mount Isa seismic traverse indicate that up to 1 km of vertical displacement has occurred between two major intrabelt fault zones since middle Carboniferous time.

Determination of crustal structure in central Australia by inversion of traveltime residuals

Teleseismic traveltime anomalies for events recorded on portable seismograph arrays across the northern and southern margins of the Amadeus Basin in central Australia are inverted to obtain a picture of the velocity structure at a scale on the order of 5–10 km. A modified sirt algorithm is used to iteratively redistribute traveltime anomalies along incoming ray paths, subject to regularization constraints, to obtain a direct estimate of the lithospheric velocity field beneath the arrays. Model structure is assumed to be 2-D, based on the strong east-west strike of surface geology and gravity structure in the area. Spurious structures commonly generated in sirt inversions are suppressed using a filter based on the density of rays. A weighting towards near-surface structure is also applied to test the robustness of the inference of deep structures. The most prominent feature of the solutions for all three lines is a sharp interface between slow and fast regions, dipping away from the basin and beneath the adjacent basement blocks. The fastest region, on the upper side of the interface, corresponds to a belt of high-grade metamorphic rocks where they crop out at the surface. The interface between slow and fast regions extends to at least 50 km depth in all cases, dipping at about 50–60° on the northern lines and 60–80° on the southern line. On the northern lines the interface can be correlated with the Redbank Thrust identified in deep seismic reflection data, but the velocity interface seems to dip more steeply. Secondary features on the profiles include a possible change in dip of the interface on the southern line and a more complex structure on one of the northern lines, suggesting that one or more steeper faults cut through the hanging wall of the thrust there. Predicted gravity profiles derived from the velocity sections using empirical velocity—density relations resemble observed gravity surprisingly well in their style and approximate magnitude, providing support for the general features of the models. The results are consistent with a thick-skinned thrusting model for the deformation of the central segment of the basin margins and indicate that the style of deformation in the Late Proterozoic-Cambrian compressional event at the southern edge of the Amadeus Basin was similar to that in the later Alice Springs Orogeny in the north.

Neoproterozoic stratigraphy of the Centralian Superbasin, Australia

The basement of central Australia includes the Arunta Block and the 1200-1100 Ma Musgrave Block succeeded by 1075-1000 Ma bimodal volcanic rifts and dolerite dykes. A regional hiatus at 1000-800 Ma, interpreted as reflecting a central post-volcanic (underplated) upland, was followed at 800 Ma by a second swarm of dykes in the Musgrave Block, Gawler Craton and Stuart Shelf, associated with the crustal sagging that initiated the Centralian Superbasin. The Centralian Superbasin is defined here to encompass the Neoproterozoic fill of the Amadeus, Georgina, Ngalia, Officer and Savory Basins. The intra-cratonic superbasin was disrupted internally 540–600 Ma ago (“Petermann Ranges Orogeny”) by a central uplift and associated thrusts, and by mid-Carboniferous (320 Ma) tectonism (“Alice Springs Orogeny”) to form the structural basins listed above. The stratigraphy and sedimentology of the relatively well known areas can be used as a basis for predicting those of the poorly known regions. New results from acritarch biostratigraphy and isotope chemostratigraphy, in conjunction with conventional lithostratigraphy and sequence analysis, allow better stratigraphic resolution than previously possible. Field observations in the recently discovered Savory Basin in Western Australia provide a basis for correlations with the adjacent basins.

Paleozoic deformation and isotopic disturbance in the southeastern Arunta Block, central Australia

Proterozoic metamorphic rocks of the Arunta Block, central Australia, were thrusted southward over the northern margin of the Neoproterozoic to mid-Paleozoic Amadeus Basin during the 400-300 Ma Alice Springs Orogeny. The extent and nature of mid-Paleozoic ductile deformation in the southeastern Arunta Block is examined in light of available UPb, 40Ar 39 Ar , KAr, RbSr and SmNd isotopic age data. Most of the Proterozoic amphibolite and granulite facies rocks north of the zone of basement-cover interaction were exhumed and cooled from above ∼ 500 to below ∼ 125°C as a result of mid-Paleozoic nappe formation. This suggests that deformation early in the cooling history occurred at amphibolite facies conditions and progressed to greenschist facies conditions later in the cooling history. In general, shear zone rocks yield the youngest isotopic ages, indicating that deformation-induced recrystallization, grain size reduction, or localized thermal perturbations are responsible for the relatively young ages. We conclude that shear zones of mid-Paleozoic age were active under greenschist and amphibolite facies conditions, though further work is needed to differentiate whether amphibolite facies shear zones are Paleozoic or Proterozoic. Some Proterozoic shear zones were uplifted passively in the mid-Paleozoic, resulting in final closure and mixed ages in key minerals like hornblende.

Metamorphic events in the eastern Arunta Inlier, Part 2. NdSrAr isotopic constraints

In the Harts Ranges, the Central Province of the eastern Arunta Inlier is characterised by upper amphibolite facies ortho- and paragneisses. Previously published UPb dating of zircons, as well as new SmNd and RbSr data indicate that the intrusive ages of both mafic and felsic orthogneisses are in the range 1780-1740 Ma (Strangways Orogeny). These rocks had initial ϵ Nd values of 0 to − 3 and depleted mantle model ages of 2.5-2.1 Ga and they intruded supracrustal rocks which are now preserved as paragneisses with initial ϵ Nd of about − 15. SmNd and RbSr whole-rock isotope data indicate that the hornblende-rich amphibolitic gneisses and mylonites in the Harts Range Detachment Zone (Ding and James, 1989) were equilibrated at ∼ 1473 Ma (± 133) when the rocks experienced hydration, isotopic homogenisation and thrust faulting. This is the Anmatjira Event described elsewhere in the Arunta Inlier and is a predominantly structural influence with thrust faults and hydrous fluid flux. Accordingly the record of this event by radiogenic geochronometers has been diffuse, a problem also exacerbated by the probable influence on these rocks, of least one more subsequent lower-temperature amphibolite facies metamorphic event. In spite of the indications of ∼ 1470 Ma isotopic homogenisation of amphibolites in the Harts Ranges, RbSr isotopic analyses of all minerals and 40Ar 39Ar from hornblendes yield ages in the range 450-300 Ma. Using the SmNd system, a few garnet-hornblende pairs give 1400-1250 Ma ages, but several of these pairs also yield early Palaeozoic ages. These data suggest that the Central Province of the eastern Arunta Inlier also experienced an early Palaeozoic thermal event, having cooled from temperatures > 550°C during and immediately before the Carboniferous Alice Springs Orogeny. In the Southern Province, south of the Illogwa Shear Zone, micas record Mesoproterozoic ages (1500-1250 Ma) for the RbSr, KAr and ArAr systems, equivalent to the whole-rock ages obtained from the Central Province, to the north of the Illogwa Shear Zone. Clearly the exposed southern basement cooled to <350°C, presumably as a result of uplift and exhumation during the Anmatjira Event, when basement now exposed in the Central Province remained at depth, undergoing one or more additional amphibolite facies events. The spectacular contrast in cooling histories across the Illogwa Shear Zone emphasises that this is an important crustal scale tectonic structure.

BURIAL HISTORY OF THE EASTERN OFFICER BASIN, SOUTH AUSTRALIA

The intracratonic Officer Basin of central Australia was formed during the Neoproterozoic, approximately 820 m.y. ago. The eastern third of the Officer Basin is in South Australia and contains nine unconformity-bounded sequence sets (super-sequences), from Neoproterozoic to Tertiary in age. Burial history is interpreted from a series of diagrams generated from well data in structurally diverse settings. These enable comparison between the stable shelf and co-existing deep troughs. During the Neoproterozoic, subsidence in the north (Munyarai Trough) was much higher than in either the south (Giles area) or northeast (Manya Trough). This subsidence was related to tectonic as well as sediment loading. During the Cambrian, subsidence was much higher in the northeast and was probably due to tectonic and sediment loading (carbonates over siliciclastics). During the Early Ordovician, subsidence in the north created more accommodation space for the last marine transgression from the northeast. The high subsidence rate of Late Devonian rocks in the Munyarai Trough was probably related to rapid deposition of fine-grained siliciclastic sediments prior to the Alice Springs Orogeny. Rates of subsidence were very low during the Early Permian and Late Jurassic to Early Cretaceous, probably due to sediment loading rather than tectonic sinking. Potential Neoproterozoic source rocks were buried enough to reach initial maturity at the time of the terminal Proterozoic Petermann Ranges Orogeny. Early Cambrian potential source rocks in the Manya Trough were initially mature prior to the Delamerian Orogeny (Middle Cambrian) and fully mature on the Murnaroo Platform at the culmination of the Alice Springs Orogeny (Devonian).

Fluid migration and vein formation during deformation and greenschist facies metamorphism at Ormiston Gorge, central Australia

ABSTRACTDuring the Alice Springs Orogeny, deformation at Ormiston Gorge, central Australia, occurred under lower‐ to middle‐greenschist facies conditions. Dolomites of the Bitter Springs Formation and quartzites. metagreywackes, and metapelites of the Heavitree Quartzite contain abundant early‐, syn‐, and post‐tectonic veins. However, though vein densities locally approach 15%, the distribution of veins and the oxygen isotope geochemistry of wallrocks and veins suggest that fluid movement was on a local scale. The Heavitree Quartzite contains quartz veins that, even along the main thrust plane, have similar δ18O values (13.5–16.9%o) to those of their wallrocks (13.6–16.9%o), with Δ18O(vein‐wallrock) values of ‐0.6 to 0.4%o. In contrast, the Bitter Springs Formation contains predominantly dolomite veins that have δ18O values of 23.4 to 27.7%o. These differences are observed even at the boundary between the Heavitree and Bitter Springs rocks, implying that significant fluid exchange between these rocks has not occurred, or that fluid flow was channelled through areas outside those sampled for this study. By contrast with the Heavitree Quartzite, δ18O values of wallrocks in individual samples of the Bitter Springs Formation are significantly higher (23.3–29.1%o) than those of the veins, with δ18O(vein‐wallrock) values up to ‐4%o (average of ‐2.1%o). These systematic differences in δ18O values most likely result from oxygen isotope fractionation caused by fluid immiscibility or disequilibrium dissolution. Smaller differences in δ13C values between some dolomite veins and wallrocks [δ13C(vein‐wallrock) up to ‐1.9%o, average of ‐0.5%o] are also explained by these processes. This study indicates that large volumes of veins may be produced by repeated fracturing and fluid migration within particular rock units, without involving large volumes of externally derived fluids.

Neocratonic magmatic‐sedimentary basins of post‐Variscan Europe and post‐Kanimblan eastern Australia generated by right‐lateral transtension of Permo‐Carboniferous Pangaea

AbstractThe stress from the initial coalescence of Pangaea during the mid‐Carboniferous (320 Ma) collision of Gondwanaland and Laurussia in the Variscan (Sudetic) magmatic fold belt was transmitted through Pangaea to generate the nappes and thrusts that dismembered the intracratonic Centralian Superbasin during the Alice Springs Orogeny and the megakinks that terminally deformed the Lachlan fold belt along the subducted Andean‐type margin of eastern Australia. Definitive collision was followed by a lacuna on the Pangaean platform that reflects uplift. The first release of heat at ≅ 300 Ma from the self‐induced Pangaean heat anomaly weakened the hypersensitive neocratonic crust of the inactive but still hot European and eastern Australian magmatic fold belts to become stress guides for right‐lateral transtension during anticlockwise rotation of Pangaea. Deep transtensional fractures provided a way into and through the neocratonic crust for magma ranging from S‐type granite to rhyodacitic ignimbrite to basalt, with alkaline undersaturated rocks in some rifts. Lagging behind the magma, the platform subsided at ≅290 Ma in basins by differential weakening of the crust during the release of Pangaean heat. The Gondwana facies accumulated in the Gondwanaland province and the Stephanian‐Rotliegend succession in Europe. The basins of Europe and eastern Australia continued to grow by transtension followed by mid‐Permian (270–265 Ma) thermal sagging and rifting. Their histories then diverged. Europe maintained its post‐orogenic course except in the Alpine region, where rifting and sagging continued to accommodate the western Tethys. The eastern fringe of Australia entered a new, Innamincka, orogenic cycle that developed in embryo at 265 Ma to a fully developed magmatic arc and yoked foreland basin at 258 Ma.

The Cadjebut Deposit as an example of mississippi valley-type mineralization on the Lennard Shelf, Western Australia; single episode or multiple events?

The Cadjebut Zn-Pb deposit is one of several Mississippi Valley-type resources hosted in Devonian carbonates of the Lennard shelf, Western Australia. Several dolomitization events and at least three pulses of mineralization are documented in the Cadjebut area, indicating multiple episodes of fluid expulsion during basin evolution. The data contrast with previous models which propose a single dolomitization and mineralization event for the Lennard shelf.Two major mineralization events are recorded in the Cadjebut area. The earlier more weakly developed mineralization event is characterized by finely crystalline sphalerite and marcasite laths disseminated in dolomite cements that occlude primary porosity in evaporitic sequences. The later, mineralizing event forming the Cadjebut orebody is represented in terms of diagenesis either in cloudy calcites that replace evaporitic minerals or within blocky calcite cements in large secondary vugs and cavities. Timing relationships relative to diagenetic events in the area indicate that the first mineralization event occurred early in the diagenetic history prior to significant burial and is interpreted to be Middle Devonian (late Givetian-early Frasnian) in age. The later event occurred subsequent to significant chemical compaction and maximum burial and is dated as mid-Carboniferous (late Tournaisian-Westphalian). Anomalous values of Zn and Pb in early Carboniferous sediments of the Lennard shelf, and the apparent absence of mineralization in late Carboniferous to Permian sediments, support a shelfwide mid-Carboniferous event. Stratigraphic constraints suggest an absolute age for Cadjebut of 325 + or - 25 Ma. This second mineralization event coincides with a period of reefal platform emergence, second-order cycle regression, and minor uplift of the Lennard shelf, all temporally coinciding with the waning stages of the Alice Springs orogeny centered in central Australia 700 km to the southeast of Cadjebut.The two main pulses of Mississippi Valley-type mineralization on the Lennard shelf have distinctive patterns of Pb isotope compositions. The early burial small resource mineralization has heterogeneous Pb isotope compositions whereas later economic and subeconomic mineralization has more homogeneous signatures. The Pb isotope data, and the overall intensity of mineralization, suggest that large volumes of fluid were expelled during the later period of platform emergence, sea-level regression, distal orogenic events, and mineralization.

Palaeomagnetism of the Brewer Conglomerate in central Australia, and fast movement of Gondwanaland during the Late Devonian

SUMMARY Palaeomagnetic analysis of 11 diamond-drill cores from the Late Devonian Brewer Conglomerate in the northeastern part of the Amadeus Basin, central Australia, yields three magnetic components: a drilling-induced remanence (C1) acquired during the industrial drilling process, a mid- to Late Carboniferous syndeformational overprint (C2) acquired during the Alice Springs Orogeny, and a primary remanence (C3). C3 passes a fold test and gives a latest Devonian pole (BC) at 47.1dS, 041.0dE with A95= 6.4d. The revised APWP for Gondwanaland requires West Gondwanaland to drift across the South Pole during the Late Devonian with a speed of ˜20cm yr-1, while East Gondwanaland remained at low-latitude positions. It is suggested that fast plate movement occurred far more often during the Palaeozoic than has previously been thought.

The Undoolya sequence: Late Proterozoic salt influenced deposition, Amadeus Basin, central Australia

Parts of the Late Proterozoic to Cambrian sequence along the northeastern margin of the Amadeus Basin were deposited under the influence of salt movement within the underlying Bitter Springs Formation. Later folding during the Devonian Alice Springs Orogeny and subsequent erosion has exposed salt‐influenced structures to provide a rare opportunity to observe the effects of diapiric growth on local facies and structure. Such effects are commonly only seen in seismic section. Salt withdrawal led to normal faulting and syn‐sedimentary thickening of adjacent units. The Undoolya Sequence, a previously undescribed 710 m section, was deposited within a salt‐withdrawal basin adjacent to a proposed diapiric structure. Periods of salt mobilization are recorded by syn‐depositional thickening and localized unconformities within units flanking the diapiric structure. This structure is representative of the influence salt movement had on deposition in the northeastern Amadeus Basin during the Late Proterozoic.

A palaeomagnetic study of hydrothermal activity and uranium mineralization at Mt Painter, South Australia

Palaeomagnetic measurements indicate that the uraniferous Radium Ridge Breccias near Mt Painter in the northern Flinders Ranges of South Australia have been magnetized twice, both times in the Permo‐Carboniferous. The palaeomagnetic south pole for one remanence was estimated as 165.1°E, 65.7°S (A95 = 11.5°). The direction for the other remanence was not well defined; however samples collected from diamictite bodies within the breccia gave 133.9°E, 33.1°S (A95 = 6.9°) as the probable pole for the overprint. The latter is similar to overprint poles published from central Australia, which are generally attributed to the Alice Springs Orogeny. Magnetization directions interpreted as Permo‐Carboniferous were obtained also from the Mt Gee Sinter (a quartz‐hematite‐rich chemical sedimentary rock which overlies the Radium Ridge Breccias) and from U‐mineralized hematitic ironstone bodies within the breccia. The magnetizations of both units were probably coeval with the younger magnetization of the Radium Ridge Breccias. A positive fold test demonstrates that the remanence of the sinter is primary, indicating a major hydrothermal event in the Permo‐Carboniferous. It is not clear by how much this event post‐dates the deposition of the underlying Radium Ridge Breccias and hematitic ironstone. The latter may be Ordovician or older if earlier monazite U‐Pb data are correct, and may have been formed by granite‐related hydrothermal fluids. In that case, the older remanence in the breccia is also an overprint. Epithermal sinter formation and chemical (rather than purely thermal) resetting of the remanence in the underlying breccia were probably due to deep circulation of oxidized fluids during Permo‐Carboniferous tectonic activity. Chemical reaction of these fluids with pre‐existing magnetite‐bearing ironstone may have been responsible for uranium mineralization during the Permo‐Carboniferous. This interpretation is consistent with published textural, isotopic and fluid inclusion data, but an older age for the uranium concentration, as a primary part of the ironstone formation, cannot be excluded. Alternatively, if the monazite data are discarded, the entire hydrothermal process including iron, uranium and silica deposition could have occurred in the Permo‐Carboniferous.

The Palaeozoic history of an unusual intracratonic thrust belt in central Australia based on 40 Ar- 39 Ar, K-Ar and fission track dating

Intraplate thrusting, involving reactivation of a mid-Proterozoic province boundary within the basement metamorphic rocks of the Arunta block, occurs at the northern margin of the late Proterozoic to mid-Palaeozoic Amadeus Basin in central Australia. Earlier geological and geophysical studies show that the thrust system, referred to as the Alice Springs orogen, is 'thick-skinned' in the central part of the thrust belt and was dominated by the crust-cutting Redbank thrust zone. Several episodes of thrusting, defined stratigraphically, began about 400 Ma ago. Analysis and modelling of 40 Ar- 39 Ar age spectra on K-feldspar from the basement rocks, combined with regional geological constraints, point to cooling rates of 3–10 °C/Ma and moderate overall exhumation rates of 100–400 m/Ma during the Alice Springs orogeny, without appreciably increased heat flow. However, final uplift along the thrust front at 300–320 Ma was rapid, as the time of final closure of small domains in K-feldspar approximates apatite fission track ages. By using 40 Ar- 39 Ar age data as a constraint, together with available geological and geophysical data, crustal shortening across the thrust belt is thought to be approximately 20 to 30%. Overall convergence rates of 0.6–3 km/Ma are slow compared with rates of ≥ 5 km/Ma for continent-continent collision zones.