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ItemThe tectonic evolution of the Songpan Garzê Fold Belt, Southwest ChinaHarrowfield, Mathew James ( 2001)Structural observations suggest that all pervasive deformation of the Songpan Garzê Fold Belt occurred continuously and progressively during the Indosinian Orogeny (c.230-200 Ma), and prior to widespread post-tectonic magmatism c.195-180 Ma. Development of the arcuate structural grains and discrete high-grade metamorphic complexes that characterise the southeast Songpan Garzê Fold Belt can be explained entirely in terms of this progressive Indosinian deformation. Likewise, all greenschist-amphibolite facies Barrovian-style metamorphism within the southeast Songpan Garzê Fold Belt is correlated with Indosinian orogenesis. Thermodynamic modelling of micaceous, garnet- and aluminosilicate-bearing metamorphic assemblages, all of which define Indosinian-aged tectonic fabrics, suggests a continuous textural evolution that occurred under the influence of a simple clockwise P-T-t path. There is no structural or metamorphic evidence to substantiate the pervasive Yenshanian (Jurassic-Cretaceous) or Himalayan (Tertiary-Recent) orogenic overprints that previous studies of the terrane have inferred. The distribution of progressive structure and locally variable timing relationship between progressive deformation and peak metamorphism imply that Indosinian deformation occurred diachronously across the southeast Songpan Garzê Fold Belt. Such deformation was characterised by the westward migration of progressive structural ‘fronts’, towards the interior of the terrane, in response to lateral growth of the transpressive interface between the Songpan Garzê Fold Belt and adjacent Yangtze Craton. In contrast, consistent timing relationships between peak metamorphism, basement anatexis and the diapiric exhumation of high-grade metamorphic rocks suggest that the thermal maturation of the terrane was spatially and temporally uniform across the southeast Songpan Garzê Fold Belt. Post-Indosinian modification of the Songpan Garzê Fold Belt was limited to brittle strike-slip dissection, predominantly focussed into the discrete west-northwest-trending Xianshui He and Kunlun fault zones. Sinistral displacement of these two fault zones, from at least the Miocene onward, transported two large tracts of the terrane towards the southeast, effectively intact. This displacement was absorbed by reactivation of the Longmen Mountains Thrust Nappe Belt and transpressive exhumation of the Min Shan Uplift Zone. Modification of the Songpan Garzê Fold Belt did not accommodate any significant vertical distortion that might have contributed to isostatic rebound of the present-day 4500-metre Tibetan Plateau. Nevertheless, uplift of the greater Songpan Garzê Fold Belt occurred from at least the Neogene onward, accommodated by reactivation of the Songpan Garzê/Yangtze interface and present-day boundary of the northeast Tibetan Plateau. New 40Ar/39Ar and Rb/Sr thermochronology from southeast Songpan Garzê Fold Belt identifies two discrete cooling episodes: 1) late-Triassic-Jurassic cooling from peak-metamorphic conditions of >500° C to temperatures of around 350° C; and 2) rapid Tertiary-Recent cooling to near-surface temperatures. Both cooling episodes are interpreted to record erosional exhumation of the terrane. Whilst Mesozoic cooling is correlated with limited post-Indosinian rebound of the thin-skinned Songpan Garzê accretionary wedge, Tertiary-Recent cooling is thought to reflect an isostatic potential associated with Himalayan Orogenesis in central Asia. Surprisingly, this latter event does not seem to have been coupled to the rapid Neogene uplift of the Tibetan Plateau. Tertiary-Recent exhumation, uplift and structural modification of the Songpan Garzê Fold Belt are inferred to have been bolstered by a pervasively thickened viscous lower crust. This thickening is attributed to penetrative east-directed deformation of deeper lithosphere beneath the detached thin-skinned veneer of the Songpan Garzê Fold Belt. Such an interpretation favours penetrative deformation of the mid-lower crust as the dominant mechanism by which India/Eurasia intracontinental convergence was absorbed - not lateral escape or extrusion of eastern Asia - and highlights the role that the Yangtze Craton has played throughout Mesozoic and Tertiary orogenesis: acting as a rigid backstop to penetrative deformation of the more plastic Songpan Garzê Fold Belt.
ItemExtreme temperature events in AustraliaTrewin, Blair C. ( 2001)A high-quality set of historical daily temperature data has been developed for Australia. This data set includes 103 stations, most of which have data from the period between 1957 and 1996, and some for longer periods. A new technique, involving the matching of frequency distributions, is presented for the adjustment of temperature records for inhomogeneities at the daily timescale, and this technique is used in the development of the data set. A number of additional findings are presented on the impact of changing times of observation and accumulation of observations over periods longer than one day on the Australian temperature record. This data set was used for an extensive study of extreme temperature events in Australia. Widespread changes in the frequency of extreme temperature events in Australia were found over the 1957-1996 period. These changes were found both by an analysis or trends at individual stations and by analysis of spatial averages of indices of extreme temperature. In general, increases were found in the frequency of high maximum and high minimum temperatures, and decreases in the frequency of low maximum and low minimum temperatures. The changes were greatest for low minimum temperatures and least for high maximum temperatures, and were generally greatest in winter. The greatest decreases in the frequency of extreme low minima were found in Queensland. The trends were not universal, with trends opposite to those for Australia as a whole being found in some regions in some seasons. It was found, after examination of several possible models, that the frequency distribution of Australian daily maximum and minimum temperatures was best represented by a composite of two or three Gaussian distributions with different parameters. Using this model, it was found that the observed changes in temperature primarily resulted from changes in the means of the component distributions, indicating that the changes resulted principally from overall warming of the atmosphere rather than changes in circulation or air-mass incidence. The relationship between the frequency of extreme temperatures and the Southern Oscillation Index (SOl) was examined, with strong relationships being found in some seasons in many parts of Australia for most extreme variables, particularly high maximum temperatures. The weakest relationships were found for low minimum temperatures. Many of these relationships, except in winter, were as strong (or stronger) with the value of the SOl one season previously as they were with the SOl of the current season, indicating potential useful skill in the forecasting of seasonal frequencies of extreme temperatures in many cases.
ItemGeochemistry of sedimentary opal, Hebel, Southern QueenslandGallacher, Andrew David ( 2001)Previous studies on Australian opal were geologically uncontrolled and analysed specimens were commonly from an unknown source. This research is the first integrated geologically-controlled study of sedimentary opal in Australia with the aim of refining models for opal genesis. The work was undertaken in the recently-discovered Hebel field of southern Queensland. Mapping, underground sampling and RAB drilling specifically for this project were undertaken. After petrography, weathered host rocks and opal were analysed by XRF, XRD, INAA, LA-ICP-MS mass spectrometry. Geologically-constrained precious and non-precious opal samples were analysed by using mass spectrometry, EMPA, IR, NMR, SEM and bioluminescence. Opal is hosted by porous, low density kaolinite-bearing Early Cretaceous argillaceous sediments overlain by Tertiary rudites and arenites. The sequence is highly weathered and mapping, mineralogical, geochemical and isotope studies show that weathering becomes less intense with depth. Deep weathering took place in the Late Cretaceous and silicification associated with tropical weathering took place in the Early Tertiary. Later Tertiary cooler more arid weathering features overprint the earlier weathering. Geochemical and isotopic studies show that during the long period of weathering, there was constant re-equilibration in an open dynamic evolving system as it still is today. Opal occurrences are stratigraphically controlled by smectitic-bearing clay horizons. There are possible local structures such as faults, joints and bedding planes which allowed the plastic deformation of opaline gel by dehydration into the enveloping smectitic sediments. Opal underwent post-consolidation brittle deformation. Continued long term weathering produced bleaching of and trace element stripping from of the outer rims of opal. This study confirmed structural differences between nonprecious and precious opal and showed that opal operates as a molecular sponge. Hence it was not possible to obtain a RblSr age of formation of opal but it was possible to show, on the basis of 0 and H isotopes, that there were several periods of isotopic overprinting, probably related to different episodes of weathering. NMR, IR and EMP A studies showed that the molecular structure of opal is governed by impurities, water occurs as OH- and possible short chain aliphatic organic compounds occur in the structure. Short chain aliphatic compounds are of biological origin. SEM studies show bacterial microfossil silica pseudomorphs. No DNA was detected in opal. Opal formed from long-term intense tropical weathering of porous permeable clastic sediments. Descending silica-laden fluids were dehydrated by expanding-lattice clays and formed as gel in structural sites in favourable stratigraphic horizons. Ubiquitous microorganisms probably played a role in opal deposition, possibly providing the locus for structural distortion during colloidal precipitation thereby promoting the formation of nonprecious opal. It is suggested that precious opal has formed from colloidal silica precipitation in the absence of bacteria thereby explaining the rarity of precious opal.
ItemAcid mine drainage at Rosebery Pb-Zn mine, TasmaniaHale, Cindy Therese ( 2001)Rosebery underground base metal (Pb, Zn, Cu, Fe, Ag, Au) sulphide mine in Tasmania, Australia has been in production for over 100 years. It is a massive sulphide deposit hosted in a fractured rock aquifer. It produces an average of 60L/sec of acidic waste water contaminated with Pb, Zn, Cu, Fe, Mn, Mg and sulphate. The study objectives were to firstly determine the water sources and flow pathways within the mine. Secondly, by chemical analysis and extensive pH and electrical conductivity testing, to determine the deterioration of water quality over time, within acid generating areas. Time was measured from the closing down of a production area (or the end of work) to sampling time. The oldest area measured ceased production thirty years ago. Testing of the objectives involved chemical sampling, stable isotope analysis, water flow measurements, pH, temperature and electrical conductivity measurements, and extensive mapping of underground levels. The results confirmed the sources of water recharge, which included rainfall, a fault - bound creek, and high pressure sealed water bearing faults intersected by underground workings and drilling. Water flow pathways within the mine include extensive secondary permeability as a result of workings, fallen abandoned zones acting as water conduits; an extensive fault in the south linking the creek to underground workings, and fractures and surface workings channelling rainfall. Results also indicated a strong seasonal pattern to groundwater recharge at the southern end of the mine, due to an extensive, and previously undocumented fault system. The seasonal variation in rainfall correlated with discharge underground and to the total mine dewatering rates. Water quality was assessed as a function of pH and electrical conductivity. There was a strong correlation found between water quality deterioration and time. In sulphide zones, AMD generation occurred within two years. This relationship is an easily applied tool for the prediction of water quality deterioration in any other producing base metal mine.