1995
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No abstract available
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A new expression with superior consistency in the calculation of magnitude from earthquake coda duration has been developed. The need to take into consideration the site, the seismograph characteristics and the source has been investigated. The expression is valid for distances up to 1000 km in southeast Australia except where the source is located on some other major geological foundation, for example the Australian shield or the Tasman Sea. The expression takes the form: MD = p1 + p2(log(SDD))P3 + p4Re-p5R . MD is duration magnitude, and the parameters in the above expression are determined by regression based on local magnitude ML. Duration, D, is in seconds, R is the hypocentral distance in kilometres, and pn are parameters. Parameter p1 zeroes the function to give values similar to ML and accounts for the definition of duration; p2 and p3 give the shape of the variation with duration; and p4 and p5 give the variation with distance. SD is the duration site correction factor , which varies from site to site and can be easily determined by comparing particular site durations with average durations. If the network data are electronically recorded, the value of SD for all sites can be continually monitored and updated. Because of the different methods of estimating D for the analogue and digital seismographs, a value of the parameter p1 for each seismograph type is necessary. analogue MD = - 0.46 + 0.45(log(SDD))2.4 + 0.0045Re-0.002R . digital MD = - 0.20 + 0.45(log(SDD))2.4 + 0.0045Re-0.002R . The above expressions present the relationship between MD, D (measured to double the background level) and R in southeast Australia for magnitudes between MD 0 and 5 and for distances from a few km to 1000 km.
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Thirty-seven spicule-dominated clasts from limestone breccia in the lower part of the Upper Ordovician Malongulli Formation of the Cliefden Caves area, central New South Wales, have yielded a collection of 2657 conodonts. These have been assigned to 48 species, dominated by Belodina confluens Sweet, Besselodus sp., Dapsilodus mutatus (Branson and Mehl)?, Drepanoistodus suberectus (Branson and Mehl), Oistodus cf. venustus Stauffer, Panderodus gracilis (Branson and Mehl), Paroistodus? sp. A Nowlan and McCracken, Scabbardella altipes subsp. B Orchard and Walliserodus amplissimus (Serpagli). Two new species are described, Pseudobelodina? anceps and Taoqupognathus tumidus. This allochthonous assemblage is a mixture of North American Midcontinent and North Atlantic-type pelagic elements, reflecting derivation from warm shallow and cooler deeper zones of the low-latitude, offshore Malongulli site . These spicule-dominated clasts probably formed initially as periplatform-ooze deposits at the outer margins of an island platform, then were incorporated in debris flows and transported basinward to become associated with the basal, in situ, graptolitic siltstone-shale Malongulli succession. The graptolite horizons are late Eastonian in age (Zone of Dicranograptus hians kirki). There is little evidence of reworking of the conodonts from older horizons. Twelve species have close North American Midcontinent affinities and may be correlated, using graphic methods, with the lower half of the Zone of Oulodus velicuspis, i.e. within the North American mid-upper Edenian Stage. This establishes upper limits for the age of the underlying pre-Malongulli carbonate succession, and confirms the Malongulli Formation as distinctive and much younger than the Darriwilian-early Gisbornian Malongulli-type succession to the east of Cliefden Caves.
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Pb-isotope data for samples of lead-bearing minerals collected in central Australia provide useful age information, based on recently developed Pb-isotope mixing models. The Strangways Metamorphic Complex and the Bonya Metamorphics in the Arunta Block are hosts to stratabound base-metal lodes with model Pb-ages clustered with that of vesicle-filling galena from the lower Hatches Creek Group in the southern Tennant Creek Block. It is suggested, therefore, that these units in the northern and central Arunta Block are time-depositional equivalents of the Hatches Creek Group, the age of which has been independently established by zircon geochronology as 1820-1810 Ma. This time interval was an episode of extension, rift-style deposition and generation of volcanogenic base-metal deposits in the northern and central Arunta Block. The model Pb-age of the Oonagalabi prospect, and hence its host, the Bungitina metamorphics, appears younger than that for the Strangways Metamorphic Complex, but field evidence requires a more complex interpretation, involving metasomatic addition of radiogenic lead. Other deposits in the northern Arunta Block have Pb-isotope characteristics that show they are younger than their host rocks, and are thus related to granite intrusion or later tectonism. Home of Bullion lead has a range of compositions that can be interpreted to indicate formation probably in the Late Proterozoic. The isotopic composition of lead in the gold lodes of the Tennant Creek district indicates model Pb-ages consistent with introduction shortly after deposition of their host rocks, during folding and granite emplacement.
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Altered basalt, dolerite and gabbro have been dredged from a previously unsampled portion of the Macquarie Ridge (47o-48° S), using the commercial fishing vessel Amaltal Explorer. These rocks are petrographically and geochemically similar to mafic-ultramafic volcanic and plutonic suites of MORB-like petrological affinity, collected along the ridge between 49° and 58° S by previous investigators. The northernmost part of the Macquarie Ridge (Puysegur Bank) is thus geologically related to the rest of the ridge, even though it is bathymetrically part of the New Zealand continental shelf. Sedimentary rocks dredged from 47o-48° S were derived from both oceanic Macquarie Ridge and continental New Zealand sources. There is no evidence of subduction-related magmatism along any part of the Macquarie Ridge.
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In a reconstructed supercontinent assembly at ~0.9 Ga, the Grenville orogen extends from Scandinavia through North America and Antarctica to Australia. Part of it, the 2000 km long Grenville Province, exposed in the southeastern Canadian Shield, is large enough to allow a comprehensive view of its tectonic character. It has an orogen-parallel zonation: older, reworked crust, representing Archaean and Palaeoproterozoic orogens exposed in adjacent parts of the shield, is restricted to its northwest side; supracrustal and plutonic rocks of Grenvillian age (~1.3-0.95 Ga) are limited to the southeastern half. The latter lie on or within late Palaeoproterozoic and earlier Mesoproterozoic crust, which is the deformed, temporal equivalent of terranes that form a substantial part of the buried North American craton south of the shield. A pre-Grenvillian period of quiescence at ~1.5 Ga may have followed an earlier continental assembly. Grenvillian calc-alkaline igneous rocks, limited in volume and distribution, represent arc accretion that terminated with ocean closure by ~1.2 Ga. New crust was added after continent-continent collision and attendant crustal thickening by emplacement of large gabbro-anorthosite massifs of mantle origin, associated with, and in part responsible for, granitoid magma derived from the lower crust. This magmatism, beginning at ~1.18 Ga, was accompanied or followed by high-grade metamorphism, except in parts of the Grenvillian supracrustal terranes, and by low-angle, thrust-sense, ductile deformation directed toward the north and northwest. Ductile followed by brittle extensional deformation between 1.05 and 1.0 Ga, along with terminal thrust-uplift along the northwest margin of the orogen, represent the closing stages of tectonic activity, leading to unroofing and cooling by ~0.9 Ga. There is little evidence in the Grenville Province for supercontinent break-up until ~600 Ma.
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Layered mafic-ultramafic intrusions of the Giles Complex, western Musgrave Block are confined to a southern granulite facies block thrust northward over an amphibolite facies block and southward over sediments of the Officer Basin. The Tomkinson Ranges to Jameson Range region, at the westernmost end of the southern granulite facies terrane, consists of 17 medium to large-scale faulted segments or intact layered mafic-ultramafic sills and lopoliths emplaced into felsic to intermediate or mafic granulite facies orthogneiss. Protoliths of these gneisses, giving ages of -1.55 and 1.3 Ga, were metamorphosed at -1.2 Ga. Emplacement of the mafic-ultramafic bodies into multiply deformed felsic granulites (D1 and D2-pure shear; T>750°C, P=5±1 kb), previously believed to have occurred at about 1200-1188 Ma, is now thought more likely to be of 1.08--1.06 Ga--coeval with the Tollu Group volcanics. The western Musgrave Block displays crustal zonation of near-contemporaneous units, from deep crustal ultramafic-dominated intrusions in the north (south of the Woodroffe Thrust), to gabbro-pyroxenite intrusions in the Tomkinson Ranges, to troctolite intrusions in the southwest, to upper crustal volcanics of the Tollu Group-i.e. a southward rise in crustal level. The layered intrusions include: (1) large olivine-clinopyroxene-plagioclase troctolite to troctolite-anorthosite bodies, with Fe-rich olivine and plagioclase as liquidus phases, crystallised from highly evolved silica-undersaturated liquids and representing high-pressure orthopyroxene fractionation prior to intrusion. These bodies are commonly magnetite-rich, representing high oxygen fugacities, and include little or no ultramafic component, e.g. Jameson, Blackstone, Cavenagh, and Bell Rock intrusions; (2) large orthopyroxene-clinopyroxene-plagioclase gabbronorite to norite intrusions, including a significant (up to about 30%) ultramafic component, e.g. Michael Hills, Mount Davies, Kalka; (3) small to medium-sized layered pyroxenite-peridotite-gabbro intrusions, e.g. Murray Range, Claude Hills, The Wart, Gosse Pile, Ewarara, and (4) stratiform anorthosites forming lenses and recrystallised tongues interlayered with felsic granulites, mainly around Teizi bore. Ultramafic increments crystallised from little-fractionated primitive basaltic magmas saturated with olivinespinel form late magmatic pulses injected into above-solidus resident gabbroic bodies. Intrusion was followed by isobaric cooling (Wingellina Hills: P=6±1 kb; Blackstone: ~4 kb). Near-coeval relations between the Giles Complex and the Tollu Group volcanics imply rapid uplift and erosion of deep crustal zones followed by volcanic activity. Feeders for the volcanics are represented by type-A dykes correlated with the ~1.05-1.07 Ga Kulgera swarm of the eastern Musgrave Block, and by extensive granite veining and related granulite facies recrystallisation of large sectors of the Giles Complex (D3-simple shear; early stage T=650-700°C, P=11 kb; late stage P=4.5±1.1 kb). Northward thrusting of the granulite facies block over amphibolite facies gneisses along the western extension of the Woodroffe Thrust -550 Ma was associated with elevated pressures along the fault zone (P=14.0±1.1 kb; T=750°C), contemporaneous with the Petermann Ranges deformation.
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The Mount Aloysius massif comprises complexly folded high-pressure Mesoproterozoic granulites, typical of those making up much of the western Musgrave Block. The granulites host the well-studied mafic-ultramafic layered intrusions of the Giles Complex, but up to now have not been mapped in detail. Previous lithological mapping and airphoto interpretation produced conflicting synformal and antiformal interpretations of the structure of the massif. New mapping reported here resolves this conflict, and shows that the massif preserves four episodes of folding. The first formed small, initially recumbent (now reclined), F1 folds, which are earlier than any previously known in the massif. Two major episodes formed a large west-inclined isoclinal, gently doubly plunging, F2 antiform, which was subsequently folded early in D3 to a steeply doubly plunging antiform and then bent late in D3 to an arcuate shape. With each episode, the intensity of deformation decreased, from isoclinal F1 folds with a strong axial-plane granoblastic-textured foliation to open F4* folds with spaced axial-plane cleavage and local greenschist retrogression, reflecting decreasing ductility as the rocks cooled. Formation of the F2 antiform was helped by the presence of a stiff buttress of relatively massive granulite in the east of the massif, against which well-layered granulite to the west was squashed and flattened. Subsequently, the same near-massive granulite provided a stiff auge-like core, around which the well-layered granulite was wrapped during D3.
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No abstract available
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The Woodroffe Thrust has until now been known in outcrop only in South Australia and the Northern Territory, where it forms part of a system of south-dipping thrust faults that penetrate the continental crust forming the Proterozoic Musgrave Block. Detailed mapping by AGSO in 1991 located new exposures of granite mylonite-previously mapped as sheared porphyry-in Western Australia, at a position predicted from geophysical data some 20 years earlier, and 120 km farther west than the most westerly known previous exposure. The Western Australian exposures of the thrust separate subeclogite facies metamorphic rocks and deformed granite in the south from amphibolite facies deformed granite in the north. The thrust dips gently south, and is marked by isolated exposures of mylonite derived from the granite north of the thrust. Lineations in the mylonite and in the northern and southern terranes are subhorizontal to gently plunging and east-northeast-trending, oblique to the strike of the thrust.