School of Earth Sciences - Theses
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ItemPetrogenesis of the Melba Flats Ni-Cu-PGE Deposit in Western Tasmania: Insights from a Geochemical and Geochronological Investigation( 2016)Since its discovery in 1893, the Melba Flats Ni-Cu-PGE deposit has produced 10,000 tons of Ni and Cu at an average grade of 9.7% and 4.7% respectively. It is a magmatic sulphide deposit located 8 km north-east of the township of Zeehan, along the eastern margin of the Dundas Trough in Western Tasmania. The deposit is associated with a suite of bifurcating mafic intrusions hosting magmatic Ni-Cu-PGE sulphides intruded into a sequence of volcaniclastic lithic greywackes, which are correlated to the Crimson Creek Formation. U-Pb detrital zircon geochronology was utilized to show that the Melba Flats sediments have a maximum depositional age of c. 582 Ma. The Melba Flats mafic intrusions were formed by primitive magmas with 13 to 16 wt% MgO and a sub-alkaline tholeiitic affinity. 40*Ar/39Ar hornblende geochronology was employed to establish that the mafic intrusions were emplaced at c. 568 Ma, along an attenuated continental margin characterized by a transitional rift setting, analogous to the early Paleogene break-up margin of East Greenland. Melba Flats Ni-Cu-PGE sulphides are characterized by massive-to-semi-massive sulphides that possess high Ni, Cu and PGE tenors and mantle-like δ34S values and S/Se ratios and disseminated sulphides that have low Ni, Cu and PGE tenors, along with crustal δ34S values and S/Se ratios. Geochemical data indicates that the massive-to-semi-massive sulphides were formed at depth before being transported to their current sites, whilst the disseminated sulphides were formed during transport as the primitive magma interacted with the S-bearing crustal rocks.
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ItemBiogeochemical mercury cycling in sea ice and geothermal springs( 2016)Microorganisms strongly influence the environmental form and fate of mercury. Recent advancements in culture-independent molecular techniques, such as high-throughput sequencing, allow us to delve deeply into the functional and phylogenetic composition of environmental microbial communities. Metagenomic techniques can be used to link the distribution of microbes equipped with mercury cycling genes to environmental processes, such as the production of the toxin, methylmercury. Presented in this thesis are metagenomic-sequencing data (Illumina Hiseq 2 x 100-bp paired-end technology) from two mercury-enriched environments: Antarctic sea ice and brine, and acidic geothermal springs. Metagenomic analyses are paired with geochemical analyses to examine how various physicochemical parameters may impact microbial community structure and function as it relates to mercury cycling. Both environments exhibit microbial communities equipped with various adaptive mechanisms, including the mercury detoxifying mer-operon amongst other metal and stress resistance pathways. Furthermore, close analogues of mercury methylation genes (hgcAB) are detected alongside the mer detoxification pathway in both environments, indicating an active methylation-demethylation-reduction cycling of mercury by these microbial communities. In polar marine environments, the long-range atmospheric transport of inorganic mercury, and an active atmospheric cycling of mercury following polar sunrise, results in the deposition of mercury onto sea-ice covered waters of the Southern Ocean. Sea-ice covered waters and the fauna that inhabit them tend to have inflated concentrations of the bioaccumulative neurotoxin, methylmercury, compared to open ocean waters. However, the role of sea ice in biogeochemical mercury cycling is poorly understood, particularly the production of methylmercury. In this thesis, deep metagenomic sequencing is used to identify potential microbial transformations of mercury in first-year sea ice and brine sampled from East Antarctica. Measured concentrations of total and methylated mercury in the sea-ice environment ranged from 1.01 to 895 pM, and <0.1 to 0.80 pM, respectively. Heterotrophic bacteria and photoautotrophic eukaryotes dominated the metagenomic datasets; however, chemoautotrophic archaea and bacteria were also present at lower abundance within the sea-ice and brine communities. Mercuric reductase genes (merA) closely related to those of Proteobacteria were identified in brine and ice communities. Metagenomic screening for mercury methylation genes (hgcAB) showed similarity (>50%) to highly conserved sites of hgcA. The putative methylation gene belongs to Nitrospina, and is predicted to encode an HgcA-like pterin-binding enzyme in the family of cobalamin-dependent methyltransferase. This study shows that the toxin, methylmercury, may be microbially produced within sea ice, constituting a source of the toxin to the Southern Ocean ecosystem. This work suggests, for the first time, that microbial mercury methylation can occur in a low oxygen environment, and implicates a common marine nitrite-oxidizer in the production of methylmercury. This thesis presents new information on how the structure and functionality of sea-ice microbial communities impact mercury transformations in polar environments. Few studies have explored the microbial underpinnings of mercury transformations in geothermal settings. Yet, hot springs and fumaroles release significant quantities of aqueous and gaseous mercury into the environment. Presented here are results from metagenomic sequencing of geothermal microbial communities cycling mercury, focusing on the connections between putative metabolisms and mercury methylation, and mercury demethylation and reduction in contaminated sediments. Presented are data from two adjacent, acidic (pH<3), mesothermal (22-40.5 °C) hot springs of the Ngawha Geothermal Field (Northland, New Zealand), extremely enriched in total mercury (>1000 ng L-1), and varying MeHg concentrations (1-10 ng L-1). Acidophilic, thermophilic, sulfur-cycling, and iron-cycling bacteria and archaea dominate microbial communities of both springs. Although genes of the mer operon were detected in both springs (at high abundance), mercury methylation genes (hgcAB) were only detected in the cooler spring (ΔT~10°C), with an order of magnitude greater methylmercury. The hgcAB genes have no known closest relative, but most likely belong to an uncultured acidophilic, sulfate-reducing bacterium. This study shows that geothermal microbial communities are capable of net production of methylmercury, alongside active demethylation-reduction by mer-capable microbes, despite selective pressures from low pH and high mercury levels. However, temperature may be a major limiting factor on the presence and activity of mercury methylating microorganisms in the environment.
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ItemGravitational instabilities beneath the continents( 2016)The evolution of dense lower crust or lithosphere sinking from below continental crust is analysed using numerical and analytical models. The dripping and delamination mechanisms are typically used to describe modern instabilities, but their relative growth has not been quantified. Delamination is an exponentially growing instability which scales in the same way as for dripping, but grows more quickly by an order of magnitude. Its growth-rate is highly dependent on the viscosity and thickness of the lower crust which is captured in a scaling law. If specific conditions for its initiation are not completely met, a third mechanism called triggered dripping is activated. It is practically indistinguishable from delamination, but its slower growth is more similar to dripping. The characterisation of these mechanisms leads to a general model in which relatively stronger dense bodies must be recycled by delamination, requiring tectonic activation, whereas weaker bodies are likely to drip. The contrasting growth-rates of dripping and delamination can be used to understand why modern instabilities often appear to grow quickly and then stall. If an instability begins under conditions suitable for delamination, but migrates into a zone of higher lower crustal strength, it will revert to dripping and slow down by at least a factor of five. The recycling of dense restitic material from beneath Archean crust is explored by predicting how its growth would compare to the intra-crustal instabilities preserved as granite dome and greenstone keel structures. Using the wavelength and lack of overprinting deformation of the preserved granite domes as constraints, a specific crustal viscosity profile can be inferred. Restite recycling and dome and keel formation can plausibly occur simultaneously at the same wavelength, as described by the growth of a single instability. Restite could be trapped by a strong lithosphere and prevented from recycling, but once reaching twice the thickness of the granite, would rapidly sink through. The instability models are consistent with constant, rapid recycling while the felsic lower crust forms, before dome and keel formation is activated by weakening of the greenstone layer and occurs simultaneously with the recycling of any remaining restite. During the transition from stagnant lid to mobile lid (plate tectonics) convection, a large volume of lithospheric mantle is likely to have been recycled. It is proposed that this generated a uniquely high magnitude continental stress state which formed the cratonic keels. Through mantle convection scale modelling, it is demonstrated that depleted and therefore strong and buoyant lithosphere forming within the lid can be shaped into thick keels and survive the lid breaking event. The stress during their formation is four times higher than the convective stresses during mobile lid immediately afterwards. It would take a Rayleigh number decrease of almost two orders of magnitude from the initiation of tectonics to the present, to return the keels to this stress state and predict their destruction. The stress at subduction zones through time may have peaked at 75% of the lid breaking stress 1Ga ago. Any keel destruction on the modern Earth is then predicted to require significant keel weakening, which is consistent with observation.
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ItemMomentum transport by organised deep convection( 2016)Deep convection is an important process that influences the vertical redistribution of heat, moisture, and momentum. Convective momentum transport (CMT) is composed of multiscale dynamical processes, including convective-scale CMT associated with updrafts and downdrafts, and mesoscale CMT associated with the quasi-steady circulation from organised systems. This multiscale nature of CMT is an active area of research especially regarding the influence of momentum transport on the mesoscale organisation of convective systems and vice-versa; these mesoscale processes and their associated CMT are the focus of this study. Firstly, the CMT budgets of simulated idealised mesoscale convective systems are examined, and their sensitivity to horizontal resolution, domain size, and the boundary conditions, investigated. This is followed by an examination of a case study during the Tropical Warm Pool - International Cloud Experiment (TWP-ICE). The effect of horizontal resolution, time evolution, and cloud regime on the CMT were evaluated to determine the momentum profiles on both scales. These results are then compared to observational data derived from Doppler radar. The idealised simulations reveal that for relatively large domains, horizontal gradient terms are still important, including the mesoscale pressure gradients; such terms are neglected in all CMT parameterisations. Like most convection parameterisations, current CMT parameterisations only represent convective-scale processes through relatively simple plume models. Thus, they do not properly represent the transports associated with organised systems, even though the tilted circulation associated with organised convection is a fundamental aspect of CMT. The horizontal pressure gradient and the sign of the momentum flux change sign as the system develops. Small domain calculations, which have become common for radiative-convective equilibrium experiments, are shown to suppress organisation through artificially large compensating subsidence and hence provide unrealistic representations of the processes involved. Finally, examination of the cross-updraft/downdraft pressure gradients demonstrates significant errors in their representation in current schemes. The convective and mesoscale momentum transport profiles from the real cases reveal how these scales change at various stages of their evolution. Grid spacing $>$ 4 km does not capture the convective scale CMT and the coarsest resolution ($\sim$ 30 km) produces the wrong sign of CMT. Mesoscale organisation is highly influential when evaluating the CMT, as these tilted circulations are responsible for much of the momentum transport. These findings indicate that rather than just parameterising the convective or small-scale dynamics, these schemes must take into account the larger, mesoscale processes. The observational data reveals differences in the dynamical structures to those identified using the model data. Analysing momentum transport profiles identifies a downshear-tilted system, suggesting that the model is not representing the full diversity of systems that actually occur. Another simulation using a different microphysics scheme and analysis of the model's thermodynamics shows that both microphysics schemes create overly strong cold pools which preferentially generate upshear-tilted systems. The usefulness of this radar dataset has also allowed insights into the variety of systems and may be used to reproduce these systems in models.
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ItemGabbro magmatism in the Lachlan Orogen, southeastern Australia: implications for mafic–felsic associations and granitoid petrogenesis( 2016)The nature and role of mafic endmembers in granitoid petrogenesis is poorly constrained in the Lachlan Orogen of southeastern Australia. Most previous studies have focussed on intermediate–felsic magmas and attempted to model the nature of their primitive precursors. Ordovician–Devonian magmatic rocks of gabbroic composition are exposed as rare volumetrically minor intrusions throughout the Lachlan Orogen, and are temporally and spatially associated with more voluminous granitoids. A detailed study of tholeiitic and alkali gabbros exposed in the Kuark Zone of eastern Victoria provides new insights into gabbro petrogenesis and the source regions of such magmas which in turn have implications for the generation of granitoids, particularly I- and A-type magmas, throughout the Lachlan Orogen. The Arte Igneous Complex and Scrubby Flat Gabbro are poorly exposed mafic intrusions spatially associated with A-, I- and S-type magmas. Although major- and trace-element variations do not always show clear evidence for a geochemical link between these units, variations in Sr, Nd and Hf isotope compositions indicate a shared source for at least some of the magmas. Alkali gabbros in particular preserve considerable Sr-Nd isotopic heterogeneity ranging greater than 10 epsilon Nd units within a small geographic area interpreted to represent the root of the Arte intrusion. It is proposed herein that magmatic differentiation occurred via fractional crystallisation and cumulate processes; however, it is argued that much of the Sr-Nd isotopic variation was inherited from a heterogeneous source region. A model involving a small degree (<20%) of partial melting of greenstone basement can explain the variation within the alkali gabbro of the Arte Igneous Complex. Subsequent higher degrees of partial melting (>30%) can explain the more voluminous, less heterogeneous tholeiitic gabbros. Spatially associated A- I- and S-type granitoids and the gabbros is more cryptic, some geochemical and Sr-Nd isotope links are apparent. Importantly, the I-type intrusions are interpreted to have been derived magmas fromed by partial melting of more intermediate compositions within greenstone basement (c.f. ultramafic to mafic for the gabbros), thus they share a similar heterogeneous source region with the gabbro rocks. In contrast, the S-types intrusions have a more complex link to the gabbros and are interpreted to have been derived via partial melting and assimilation of Ordovician turbidites by tonalite magmas of the Arte Igneous Complex. Comparison of the magmatic rocks of the Kuark Zone with other twelve of the 20 known Lachlan Orogen gabbros reveals similar isotopic heterogeneity. This requires that the source heterogeneity is present on a local- and regional-scale. Cambrian greenstone basement exposed in rare fault-bounded belts throughout the Lachlan Orogen have the same isotopic heterogeneity. Moreover, the same heterogeneity is observed in I-type granitoids of the Lachlan Orogen. The implication is that I-type magmas may also be generated by partial melting of greenstone basement rocks, thus both gabbroic and I-type magmas image their source region. There is a correlation between the age of gabbros and Sr-Nd isotope values, with younger gabbros characterised by on average more isotopically juvenile compositions. The ca. 380 Ma Bingie Bingie Suite that approachs compositions of depleted mantle. A number of the gabbros exhibit arc-like trace-element characteristics, however, given that the greenstones were generated in a Cambrian arc environment, these signatures may be inherited from their source (greenstones) rather than the gabbro magmas themselves being generated in a subduction zone setting. The chemical characteristics of most of the gabbros are consistent with the partial melting of greenstone basement in a back arc basin setting under extension. Influx of new mantle-derived magma is only likely to have occurred to produce the youngest mafic rocks (e.g., Mount Buller Igneous Complex). The results of this study provide new insights into the source regions for mafic intrusions of gabbro/diorite composition in the Lachlan Orogen. In light of this new information, these insights present an opportunity to re-examine the petrogenetic models for I- and A-type granitoids in particular.
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ItemAn attribution study of southeast Australian wildfire risk( 2016)Extreme weather and climate-related events often have a serious impact on our economy, environment and society. This is particularly true in Australia where recurring heat waves, floods, droughts and wildfires have resulted in the loss of life, property and livelihoods. The 2009 'Black Saturday' wildfires in southeast Australia provides a tragic example, having resulted in the death of 173 people and the destruction of over 2000 homes. While there are a number of recorded attribution studies for Australian temperature and precipitation-related events, no such study exists for fire weather. This thesis presents a new climate modelling system for regional climate simulation and the attribution of weather and climate extremes over Australia and New Zealand. The system, known as weather@home Australia-New Zealand, uses public volunteers' home computers to run a moderate-resolution global atmospheric model with a nested regional model over the Australasian region. By harnessing the aggregated computing power of home computers, weather@home is able to generate an unprecedented number of simulations of possible weather under various climate scenarios. This combination of large ensemble size with high spatial resolution allows a range of extreme events to be examined over Australia and New Zealand with well-constrained estimates of sampling uncertainty. The model is seen to be capable of resolving many climate features that are important for the Australian and New Zealand regions, including the influence of El Nino-Southern Oscillation (ENSO) on driving natural climate variability. Using the new weather@home modelling framework, this thesis presents the first known attribution study of southeast Australian fire weather. By applying the McArthur Forest Fire Danger Index to large ensembles of regional climate model simulations generated for factual and counterfactual climate scenarios, this thesis reveals that anthropogenic climate change increased the likelihood of elevated wildfire risk over southeast Australia during the 2008-2009 fire season. Furthermore, the influence of anthropogenic climate change on wildfire risk is found to be greater in spring than summer. Through a series of further modelling experiments, this thesis also demonstrates a novel approach for separating the influence of ENSO and anthropogenic climate change within the context of an attribution study. Across southeast Australia, the increase in wildfire risk due to a change in ENSO phase (from La Nina to El Nino conditions) was identified to be much greater than the increase attributed to anthropogenic climate change. This was largely due to the strong increase in drought factor, and decrease in relative humidity, from La Nina to El Nino conditions.
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ItemGlobal-scale waves and their links with tropical convection( 2016)Analyses have found a substantial amount of tropical convection variability is attributable to the interaction of convection with waves that exist and propagate throughout the tropics. The waves that interact most strongly with convection in the tropics are those that have large surface convergence signatures and vertical structures which correspond closely to those of deep convection. However, it has been shown that tropical convection also significantly interacts with the 5-day Rossby-Haurwitz wave, a global-scale normal mode of the atmosphere with very small associated convergence and a barotropic vertical structure. The properties and processes of this interaction are currently not well understood, and this thesis seeks to improve the understanding of this interaction. Annual and interannual variability in the strength of the interaction between the 5-day wave and convection is examined through use of wavenumber-frequency spectra of coherence-squared between zonal wind data from ERA Interim and outgoing longwave radiation (OLR) from NOAA satellite datasets. This analysis finds that the association between the 5-day wave and tropical convection is strongest during equinoctial seasons and weakest during austral summer, but occurs in all seasons. No difference in the coherence-squared between 5-day wave zonal winds and OLR variations is found in opposing El Niño-Southern Oscillation (ENSO) and Quasi-Biennial oscillation (QBO) phases. The spatial distribution of convection associated with the 5-day wave is investigated through lag-regression composites of OLR and TRMM 3B42 precipitation against a filtered zonal wind timeseries, with significant local signals in tropical convection found over west equatorial Africa, the western Amazon basin, the eastern Pacific Ocean Inter-tropical Convergence Zone (ITCZ) near Panama, the north-eastern In- dian Ocean near Sumatra, and the ITCZ over the western Indian and Pacific Oceans. Enhanced convection occurs in phase with easterly wind anomalies in the western Amazon, in phase with westerly wind anomalies in the eastern Pacific, and approximately a quarter-cycle in advance of westerly wind anomalies elsewhere. These results are applied to analyse simulations of the 5-day wave and its interaction with tropical convection in 30 CMIP5 models. All models investigated simulate the horizontal wind anomalies associated with the wave well, and also consistently display significant coherences between 5-day wave zonal winds and precipitation. However, they perform less well at simulating the spatial distribution and magnitude of precipitation anomalies. A strong trend exists between model resolution and wave-associated precipitation variance in the western Amazon, but does not exist between model resolution and wave-associated precipitation elsewhere. Enhanced precipitation associated with the 5-day wave in the western Amazon basin, along the western coast of Sumatra and off the coast of the Philippines was found to be strongly associated with enhanced low-level moisture flux convergence on the windward edge of topographic barriers and resulting topographic uplift. Convective anomalies associated with the 5-day wave over the Gulf of Guinea and the eastern Pacific ITCZ are closely associated with anomalies in the total column water vapour, with the 5-day wave modulating convection in these regions by affecting moisture transports. In both situations, north-south oriented terrain features interact with the zonal wind anomalies associated with the 5-day wave to modulate convection.
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ItemThe thermotectonic evolution of the southwest Yilgarn craton, Western Australia( 2016)The Yilgarn craton, lies in the southern part of Western Australia, became cratonized at around 2600 Ma. Its post-cratonisation history is somewhat fragmentary due to the paucity or absence of a stratigraphic record. However, the exposed Archean crystalline rocks can provide important constraints on the ‘missing’ thermotectonic history if appropriate thermochronological methods are used. Previously reported Rb-Sr biotite cooling ages from the southwestern Yilgarn craton suggest that it was subjected to late ‘Pan-African’ tectonism (~400-600 Ma) and E-W compression, resulting in tectonic loading (thrusting) of sediments onto basement rocks along its western margin. However, this proposed tectonic model is based largely on bulk Rb-Sr biotite analyses with minimal petrological and geochemical control. In order to provide new insights into the thermotectonic evolution of the southwestern Australian crystalline terranes, including the southwestern Yilgarn craton, the Albany-Fraser Orogen and the Leeuwin Complex, this study applied multiple thermochronometers that are sensitive to a broad temperature range (~500-40 ºС). 40Ar/39Ar results of muscovite, biotite and hornblende grains were obtained from well-documented sample sites broadly comparable to those sampled previously for Rb-Sr biotite analysis in the Yilgarn craton and surrounding terranes (e.g. Albany-Fraser Orogen and Leeuwin Complex). Along a north traverse (Perth traverse) extending from W to E across ~80 km in the Yilgarn craton, muscovites record consistent to slightly decreasing ages of ~2450-2220 Ma. However considerably younger muscovite ages of ~600-610 Ma were obtained from a southern traverse (Harvey traverse) across the craton extending from W to E for a distance of ~150 km. Coexisting biotite results from the Perth and Harvey traverses reveal significant age variations, with ages decreasing systematically from east to west. Based on 40Ar/39Ar biotite ages and their chemical composition, three age domains are identified: an easterly biotite domain in the craton interior with ages of ~2500 Ma; a transitional domain with average ages of ~1000-1100 Ma; and a western biotite domain with ages of ~530-860 Ma. It is noted that relatively consistent biotite ages of ~600-630 Ma occur only along the Darling Fault. The transitional zone identified in the Perth Traverse is not revealed along the Harvey traverse, probably due to less comprehensive sampling coverage. Petrographic and chemical studies indicate that the biotite from each domain are distinctly different in composition and origin, i.e. magmatic to the east versus hydrothermal to the west. The more scattered biotite ages in the transitional and western zones are therefore unlikely to represent cooling ages, but rather indicate probable fluid-induced partial or complete biotite recrystallization at ~600-630 Ma. In the adjacent Albany-Fraser Orogen, 40Ar/39Ar ages of ~1100 Ma in the east of orogen decrease to ~650 Ma towards the west. In the Leeuwin Complex however, both biotite and hornblende yield similar 40Ar/39Ar ages of ~500 Ma and these are only marginally younger than coexisting zircon U/Pb ages. Complementary zircon and apatite (U-Th)/He data (ZHe and AHe respectively) were obtained from similar areas in the Yilgarn craton where 40Ar/39Ar dating had been carried out. Zircons yield a wide range of He ages (~400-30 Ma), and only grains with low [eU] (effective uranium contents; [eU] = [U] + 0.235 × [Th]; a proxy for radiation damage) yield relatively similar ages of ~280-350 Ma. For grains with [eU] values of 900-2000 ppm, ZHe ages are negatively correlated with age and range from ~30-200 Ma due to the effect of radiation damage. This age dispersion is not observed in the Albany-Fraser Orogen and the Leeuwin Complex, where ZHe ages cluster around a narrower age range of ~280-380 Ma. AHe ages (~250-330 Ma) yield a broadly similar age range to the ZHe results. 40Ar/39Ar results from the cratonic interior suggest that most of the craton experienced slow cooling soon after initial cratonisation at ~2600 Ma. However, the western margin of the craton seems to have been affected by later tectonic events resulting in young 40Ar/39Ar ages (<1300 Ma). 40Ar/39Ar results from the transitional domain in the Yilgarn craton could be interpreted as partially reset ages due to hydrothermal alteration related to the Pan-African tectonism, as recorded in the Leeuwin Complex. Alternatively these ages may relate to the Pinjarra and/or Albany-Fraser Orogenic events. However, given the E-W strike the Albany-Fraser Orogen is unlikely to have caused thermal/hydrothermal effects along the western margin of the craton in a N-S direction. However, the timing of Pinjarra Orogen that lies to the west of the Yilgarn craton is temporally coincident with 40Ar/39Ar ages from the transitional zone. Therefore, the Mesoproterozoic Pinjarra Orogeny may have affected the western margin of the craton and reset the biotite 40Ar/39Ar ages in the transitional zone. The westernmost biotite recrystallisation ages of ~600–630 Ma support palaeomagnetic indications of oblique collision between Greater India and the Australian continent during Gondwana amalgamation in Late Neoproterozoic time. In view of the aforementioned 40Ar/39Ar data in the western margin of both Yilgarn craton and the Albany-Fraser Orogen, young biotite Rb/Sr ages are interpreted to result mainly from later hydrothermal fluid alteration instead of thermal diffusion process as previously suggested. During Late Palaeozoic, the Yilgarn craton experienced an episode of accelerated cooling (>4 ºС/Myr) indicated by thermal modelling results of (U-Th)/He data and previously unpublished AFT data. This cooling possibly resulted from the removal of several kilometres of sedimentary cover on the craton. Evidence for the sedimentary cover is also inferred from the Collie and Perth basins. The former is a fault-bounded Phanerozoic basin enclosed in the Yilgarn craton, and is assumed to represent an outlier of sediments that once extended over the craton. The Perth Basin, located along the western margin of the Yilgarn craton accumulated a thick sedimentary pile (up to 15 km) during the initial rifting of Gondwana in the Early Permian. However, U/Pb detrital zircons ages in both Collie and Perth basins show few Archaean ages, indicating that the Yilgarn craton was not a major source area despite its close proximity. Therefore, previous and current thermochronological results suggested that the Yilgarn craton may have been covered by early-mid Palaeozoic sedimentary rocks. The inferred sedimentation is also indicated by dynamic topography history, which showed that the craton underwent a history of protracted subsidence since the mid-Palaeozoic, thus providing accommodation space for the accumulation of a sedimentary succession over the craton. These sediments were removed later and caused accelerated cooling during the Late Paleozoic as revealed by thermochronology data in the Late Paleozoic. This accelerated cooling/denudation event may relate to one or all of possibilities listed: 1) mantle flow and the resultant dynamic topography; 2) Permo-Carboniferous glaciation and the isostatic effects of deglaciation; 3) a far-field response to continental collision between Gondwana and Laurussia, followed by intra-Gondwana rifting along the western margin of Western Australia. Therefore, the Late Palaeozoic accelerated cooling in the craton has experienced a more dynamic history than previously envisaged.
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ItemThe thermal expression of the newer volcanics province, Southeast Australia( 2016)Southeast Australia is an accretionary terrane that has experienced a complex tectonic and thermal history. Understanding its thermal structure can reveal insights to mechanical stability and rheology. While oceanic lithosphere generally behaves as a cooling thermal boundary layer, continental lithosphere contains many heterogeneities that complicate the development of accurate thermal models. We use surface heat flow data to explore the variations and controls on thermal regimes in Southeast Australia. From the increased density of surface heat flow data points, we identified three new heat flow provinces which are compositionally and thermally distinct. The Newer Volcanics heat flow province is the most recent (< 8 Ma) event to perturb the crustal geotherm, and overprints the Lachlan and Delamerian provinces. The thermal contribution of the Newer Volcanics is masked by the steady-state signal which, in itself, contains considerable uncertainty. To quantify the steady-state component, we minimised discrepancies between numerical solutions to the steady-state heat equation and surface heat flow observations using gradient-based inversion. Surface heat flow constrains the integrated heat sources and lower flux boundary condition, but our inversions highlight a need for additional geophysical constraints on subsurface temperature. Variations in seismic shear wave velocity have a strong sensitivity to temperature, but require the separation of anharmonic and anelastic components. Pseudosections were computed to estimate the anharmonic component, which we compared with a shear wave tomographic model from the WOMBAT seismic array. We found anelastic attenuation was significant in the upper crust due to the effects of apertures in the rock fabric at low confining pressures. Our coupled heat flow and shear wave velocity inversions reveal higher rates of heat production within Precambrian rocks than previous inversions constrained only by surface heat flow observations. In particular, the Selwyn Block generates approximately 1 μW m −3 . Moho temperatures within the Selwyn Block were above 800 ◦ C in over 95% of simulations, which has important implications on its mechanical ability to resist accretionary strain. By aggregating the posterior of multiple 2D inversions across Southeast Australia, we generated a 3D temperature model of the steady-state thermal regime. The amount of heat contributed by transient heat sources associated with the Newer Volcanics is estimated between 10–40 mW m −2 . However, the absence of any long wavelength heat flow trends coupled with observations of anomalous seismic and electric resistivity in the lower crust to upper mantle suggests that a large heat source is yet to diffuse to the surface. Our research contributes to the understanding on the thermo-tectonic evolution of the Tasman Orocline, and the surface expression of intraplate volcanism.
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ItemCharacterising the zonally asymmetric features of the Southern Hemisphere extratropical circulation and their influence on regional climate variability( 2016)The major zonally asymmetric features of the Southern Hemisphere (SH) extratropical circulation are the zonal wavenumber one (ZW1), zonal wavenumber three (ZW3) and the Pacific-South American (PSA) pattern. These tropospheric waveforms play a critical role in the meridional transport of heat and moisture and in the development of blocked flow, causing the regional surface climate to vary strongly depending on the strength, frequency and phase of their activity. The PSA pattern is widely regarded as the primary mechanism by which the El Nino-Southern Oscillation (ENSO) influences the high southern latitudes, and in recent years it has been suggested as a mechanism by which longer-term tropical sea surface temperature trends have influenced the Antarctic climate. This thesis presents novel approaches to identifying both the zonal waves and PSA pattern in reanalysis and model output. In comparison to existing wave identification methods, the approaches more fully exploit the information available from Fourier analysis. For the zonal wave analysis, this was achieved by adapting the wave envelope construct recently used in the identification of synoptic-scale Rossby wave packets. In order to apply similar methods to the non-zonal PSA pattern, a grid rotation method traditionally used in ocean modelling was used to orient the equator along the approximate great circle path of the pattern. These new wave identification methods were applied to ERA-Interim reanalysis data in order to analyse the climatological characteristics of the waveforms and their influence on regional climate variability. The results reveal that both the zonal waves and PSA pattern are important drivers of temperature, precipitation and sea ice variability in the mid-to-high southern latitudes. While ZW1 and ZW3 are both prominent features of the climatological circulation, the defining feature of highly meridional hemispheric states is an enhancement of the ZW3 component. Identified seasonal trends towards the negative phase of the PSA pattern were largely inconsistent with recent high latitude temperature and sea ice trends. Only a weak relationship was identified between the PSA pattern and ENSO, suggesting that the pattern might be better conceptualised as preferred regional atmospheric response to various external (and internal) forcings. The analysis of large datasets such as ERA-Interim typically requires extensive use of various software tools and packages, to the point where coding/programming is a major component of the research methodology. Despite this strong reliance on computation, traditional academic publishing formats and conventions do not allow for the documentation of computer software and code, which means it is impossible to replicate and verify much of today's academic literature. In an attempt to provide a practical solution to this so-called reproducibility crisis, the zonal wave and PSA pattern results have been presented in a reproducible manner. The procedure used to document the computational aspects of the research was developed to be consistent with recommended best practices in scientific computing and seeks to minimise the time burden on authors. It should provide a starting point for weather and climate scientists looking to publish reproducible research, and it is proposed that relevant academic journals could adopt the procedure as a formal minimum standard.