School of Earth Sciences - Theses
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The structure and evolution of the northern Australian margin: Insights from the Papuan Fold and Thrust Belt, Papua New Guinea
The Papuan Fold and Thrust Belt (PFTB) in Papua New Guinea (PNG), located on the leading edge of the northern Australian continental margin, has been subject to complex tectonism as a result of its location throughout much of the Cenozoic between the obliquely converging Australian and Pacific plates. The remoteness and inhospitable terrain characterising the PFTB make it one of the least well-known fold and thrust belts on Earth. The architecture of the northern Australian continental margin has been affected by both extensional and compressional tectonic forces, which first formed, and subsequently deformed, the Papuan Basin in the period from the early Mesozoic through to the present-day. Defining the geology, structure and evolution of the PFTB and Papuan Basin is central to our understanding of the geological and tectonic evolution of the northern Australian margin. In this thesis, a multidisciplinary approach is used to investigate the evolution of the PFTB, Papuan Basin and northern Australian continental margin. Field mapping and structural analysis within the remote Western Fold and Thrust Belt (WFTB) provide significantly improved constraints on the geology, structure and evolution of the fold belt. New geological constraints acquired over > 100 km of traverses suggest that the exposed Cenozoic Darai Limestone has very low shortening between ~ 12-22% yet structures in the Muller Range are elevated up to 7 km above regional. Structural work utilising regional-scale geological observations suggest that the inversion of pre-existing rift architecture on the northern Australian continental margin is the primary influence on the evolution of the area. The huge structural relief is produced by both tectonic inversion on deep-rooted normal faults and their linkage to the surface via triangle zones that form within the incompetent Mesozoic passive margin sedimentary sequence. Local- and regional-scale heterogeneities within the northern Australian continental margin, such as accommodation-zones and transfer structures are now expressed in the fold belt structure as discontinuities and cross-cutting structural features that are recognised throughout the PFTB. The 2018 Mw 7.5 PNG Highlands earthquake and aftershock sequence has provided an unprecedented opportunity to observe and analyse the crustal processes that have ultimately controlled the evolution of the PFTB. Seismological, GPS and remote sensing data offer constraint on the complex nature and spatiotemporal distribution of crustal deformation during the event, revealing that the PFTB experienced up to 1.2 m of uplift and ground deformation over 7,500 km2. Remarkable spatial and morphological similarities exist between the distribution of coseismic ground deformation associated with the event, and the less-inverted and uninverted extensional architecture that is well-constrained in the foreland across the Stable Platform. This suggests that the 2018 Highlands earthquake sequence was related to tectonic inversion along a previously unidentified extensional fault system beneath the PFTB, indicating the northern Australian passive margin has had a primary control on the evolution of structural styles observed throughout the PFTB. New low-temperature thermochronology data from extensive field surveys in the Muller Range were combined with legacy data in modern thermal history modelling tools to investigate the thermotectonic evolution of the WFTB and Papuan Basin. In particular, the Late Cretaceous to Oligocene history of the region is largely unknown due to the absence of a continuous stratigraphic record. Thermal history models based on these data suggest two major Cenozoic cooling episodes. The youngest, and best constrained, is clearly recorded in the stratigraphic record and relates to Neogene collision at the northern margin of the Australian continent. An older episode of comparable or greater magnitude occurred in the Eocene to Oligocene and may relate to the removal of 1,500-3,000 m of Late Cretaceous to Eocene stratigraphic section across the Muller Range prior to the widespread deposition of the shelfal Darai Limestone. It is suggested that extension along major faults beneath the Muller Range accommodated sedimentation from the Late Cretaceous to the Eocene, consistent with long-lived extensional structures observed in the foreland across the Stable Platform. The selective removal of this sequence across the Muller Range suggests it was uplifted in the Eocene to Oligocene, possibly in part facilitated by the inversion of extensional faults in the Muller Range area. This inversion is interpreted to have resulted from the Eocene to Oligocene collision of the expansive Sepik Terrane to the northwest of the PNG margin, an interpretation that has significant implications for the tectonic evolution of PNG and Southeast Asia. The studies presented in this thesis provide several key insights that significant advance our understanding of the geological, structural and tectonic evolution of the PFTB, Papuan Basin and northern Australian margin. An ongoing theme relates to the complex interplay between spatial variations in the architecture of the margin and spatial and temporal variations in the compressional stress field associated with an evolving tectonic setting between the Australian and Pacific plates.
On the State of Reduced Complexity Climate Modelling
Reduced complexity climate models are the most computationally efficient end of the climate model hierarchy. They are widely used in climate science research, particularly at the science-policy interface. However, unlike the Coupled Model Intercomparison Project (CMIP) that focuses on more comprehensive global Earth System models, there has never been a systematic intercomparison project targeted specifically at reduced complexity climate models. In this PhD research, I introduce the Reduced-Complexity Model Intercomparison Project (RCMIP) and discuss the state of reduced complexity climate modelling. Beyond the intercomparison and evaluation efforts of RCMIP, I also consider reduced complexity model development and application. In the first part of the PhD, before introducing RCMIP, I present a CMIP-derived dataset. This dataset solves the many data-handling challenges which researchers face when they wish to extract regional-aggregate timeseries from raw CMIP data. Within the PhD, these regional-aggregate timeseries are a key resource for comparing reduced complexity climate models with the more comprehensive models participating in CMIP. The dataset, which currently comprises over 40 000 timeseries (approximately 8 million model years) for 83 variables from over 50 CMIP5 and CMIP6 models, 40 experiments and up to 100 ensemble members per model, has been made freely available. Whether for overall comparison purposes or for development and calibration of individual modules, the distilled Earth System Model data is a prerequisite for robust development of reduced complexity models. In the evaluation component of the thesis, I introduce RCMIP and perform two pieces of reduced complexity climate model evaluation. The first part focuses on the ability of reduced complexity climate models to emulate the response of more comprehensive models. I find that reduced complexity climate models can emulate the response of more comprehensive models to within a root-mean-square error of 0.2C over a range of experiments. However, there are clear differences between idealised and scenario-based experiments, with noticeably worse emulation ability in scenario-based experiments. The second evaluation part focuses on reduced complexity climate models' probabilistic distributions. Probabilistic distributions are used to make projections of not only our best-estimate of e.g. global-mean temperature but also the range of possible outcomes e.g. 5th or 95th percentiles. Across the reduced complexity climate models, there is notable variation in their agreement with other lines of evidence and their resulting projections. The variation emphasises the importance of evaluating reduced complexity climate models' probabilistic distributions they are using to identify any key biases before using them. Under the low-emissions SSP1-1.9 scenario median peak warming projections range from 1.3 to 1.7C (relative to 1850-1900, using an observationally-based historical warming estimate of 0.8C between 1850-1900 and 1995-2014). There is a clear need for further research into constraining model projections, particularly given the international community's goal of limiting warming to below 1.5C above pre-industrial in the long-term. In the application component of the thesis, I consider the question of the remaining carbon budget. In the IPCC's Special Report on Global Warming of 1.5C (SR1.5), the remaining carbon budget was calculated on the assumption that there is a strictly linear relationship between cumulative CO2 emissions and CO2-induced warming in its calculation of the remaining carbon budget. I examined the validity of this assumption using a reduced complexity climate model which explicitly quantifies feedbacks and non-linearities within the climate system. I find that considering non-linearities between cumulative CO2 emissions and CO2-induced warming increases remaining carbon budget estimates by approximately 10%. To provide continuity with existing approaches, I propose an update to the SR1.5 remaining carbon budget assessment framework which allows such non-linearities to be included. In the development component of the thesis, I present an updated methodology for deriving probabilistic distributions with reduced complexity climate models. Compared to previous work, I add extra variables to the historical constraining analysis and add an extra step, which I refer to as subsampling. With the subsampling, it is possible to derive probabilistic distributions which better reflect other, independent lines of evidence whilst also maintaining the covariance between model parameters e.g. equilibrium climate sensitivity and aerosol effective radiative forcing. The outputs from the development are used for the MAGICC reduced complexity model submission to RCMIP's probabilistic evaluation phase (RCMIP Phase 2). This is a key feature when reduced complexity models are used as `integrators of knowledge' i.e. as ways of transferring insights from one domain to another in a computationally efficient way. The reproducible pipeline used for the derivation provides a basis for future work focussed on each component within the probabilistic derivation methodology e.g. how the likelihood of certain parameter values are given the reduced complexity model output and observations, how the Monte Carlo Markov Chain is configured and how trade-offs between different lines of evidence can be considered in the subsampling step. I conclude with suggested next steps for reduced complexity climate modelling, based on my experiences performing each step in the development-evaluation-application cycle. There are many areas for future development. I think the key next step is in the evaluation and development of two key reduced complexity climate model parameterisations: the carbon cycle and aerosol effective radiative forcing. The thesis provides new insights into the behaviour and performance of reduced complexity climate models, particularly understanding of their limitations. Thanks to the methods and approaches developed in the thesis, I was also able to co-develop the assessment pipelines used within the forthcoming IPCC AR6 report - both for the creation of a probabilistic version of MAGICC7 that captures key characteristics of the climate system (such as equilibrium climate sensitivity and transient climate response) and for the categorisation of WG3 emissions scenarios in terms of the their global-mean temperature implications. The insights from the thesis allow users of reduced complexity climate models, most notably at the science-policy interface, to have greater confidence when interpreting reduced complexity climate model results.
The influence of a sub-lithospheric decoupling layer on subduction zone surface motion, mantle circulation and slab dynamics
The lithosphere consists of a strong surface layer fragmented in a series of major and minor tectonic plates moving over a weaker and more buoyant layer of asthenospheric material. The boundary between the lithosphere and asthenosphere, the LAB, is a critical element of plate tectonics as it mechanically decouples the motion of strong tectonic plates from the ow in the underlying weaker mantle. Recent geophysical observations report an abrupt seismic velocity decrease and electrical conductivity increase beneath certain segments of the subducting Pacific plate, which may represent the channelised base of the subducting oceanic lithosphere. This inferred sub-lithospheric weakness is predicted to lubricate the base of subducting plates and enforce the decoupling at the LAB, with potential implications for our understanding of subduction geodynamics. However, little is known about the impact of this feature on subduction zone dynamics. This research work for the first-time compiles worldwide observations of a very thin and weak sub-lithospheric layer (SLL) embedded beneath the base of a descending oceanic lithosphere and computationally simulates SLL effects on subducting plate and mantle dynamics. In the last few decades, the rapid progress in technology has contributed to increasing complexity of geological observations and capacity of computational infrastructures. With direct geological observations limited in both time and space scales, computational modelling offers the extraordinary opportunity to test different hypotheses for Earth's evolution through geologic time and ongoing processes, whilst promoting the development of multidisciplinary approaches. This study, therefore, employs subduction numerical models and addresses the SLL impact on slab deformation style including subduction surface motions, induced mantle ow and patterns of seismic anisotropy. Our results show that the physical properties of the sub-lithospheric layer fundamentally control the partitioning of surface motions between trench migration and horizontal plate velocities, but also the partitioning of the induced mantle ow as it splits into its poloidal and toroidal components. A regional SLL mechanism firstly provides a novel explanation for the high convergence and low trench migrations rates observed globally at natural subduction zones. Then, an SLL also delivers an alternative answer for plate margins characterised by some degree of toroidal ow and for the apparent lack of a direct link between buoyantly driven convection (which drives only vertical and divergent motion) and the generation of toroidal energy (which corresponds to strike slip motion and plate spin). Additionally, a very thin and weak sub-lithospheric layer promotes the alignment of the maximum strain axes in the direction parallel to plate motion within the oceanic lithosphere and mantle directly beneath it, and in the direction parallel to the trench deeper in the sub-slab mantle. As a result, these findings contribute to the resolution of the slab anisotropy debate on whether an SLL can substantially influence three-dimensional ow around the subduction zone. These outcomes reveal that a very weak and thin sub-layer embedded in the Earth's uppermost asthenosphere impacts the complexity of current natural subduction systems, for which little predictive and holistic modelling exists. By acting as a slippery base for the motion of the subducting lithosphere, this layer sharpens the decoupling between the strong tectonic plates and the weaker mantle underneath. This has significant consequences on slab subduction style and patterns of seismic anisotropy as observed in several subduction regions, such as Izu-Bonin-Mariana, Central America and Kamchatka- Aleutian among others, where the presence of this layer is confirmed by geophysical studies. This thesis work ultimately aims to provide a novel conceptual framework to reinterpret the geological record of complex plate-margins, with broader implications for better understanding the evolution of the Earth's dynamic system.
Assimilation of the column-averaged CO2 concentrations from the Orbiting Carbon Observatory-2 (OCO-2) satellite data to improve our knowledge of Australian carbon flux estimates
Existing estimates of carbon fluxes for Australia primarily rely on process-based terrestrial ecosystem model simulations. Even though they are built to consider important ecosystem processes that control the exchange of CO2 between the land surface and the atmosphere, such as the connection between carbon uptake and water use by plants, their carbon flux exchange estimates are highly uncertain. Improving carbon flux estimates from global ecosystem models and its uncertainties is essential for advancing our understanding of the Earth system and carbon cycle-climate feedback. This dissertation contributes to solving this challenge through atmospheric data assimilation, also known as inverse modelling. The main structure of this thesis consists of three studies. The first study involved running a series of simulation experiments (OSSEs) to assess the potential of the Orbiting Carbon Observatory-2 (OCO-2) satellite retrievals to reduce the uncertainties in CO2 fluxes over Australia for 2015. In this study, we assumed that most of the uncertainties in the Australian carbon fluxes were driven by the net primary productivity estimated by the CABLE land surface model (Australian land biosphere model). After performing OSSEs, we found that Australian carbon flux uncertainties can be reduced by up to 90 percent at a grid-point resolution over productive ecosystems. Given that the first study showed promising results about the potential of OCO-2 data to constrain fluxes around Australia, the second study focused on the quantification of CO2 sources and sinks over the continent. The main results of this study suggest that Australia acted as a carbon sink of -0.41 +- 0.08 PgC/y compared to the prior estimate 0.09 +- 0.2 PgC/y (excluding fossil fuel emissions). Analysis of the seasonal variation of the posterior CO2 fluxes aggregated by bioclimatic regions shows that the savannas in northern Australia and the sparsely vegetated ecosystem in central Australia were the primary drivers of stronger carbon uptake in 2015. Examination of the enhanced vegetation index (EVI) indicates that the primary reason for the stronger posterior carbon uptake (relative to the prior) registered over the savanna ecosystem was due to an increase of vegetation productivity (positive EVI anomalies) caused by an anomalous increase of rainfall in summer period. Additionally, we found that a slight increase of carbon over areas with sparse vegetation (the largest ecosystem by area in Australia), was also driven by a slight increase in land productivity and had a substantial impact on the Australian carbon budget for 2015. Underestimation of the gross primary productivity flux simulated by the CABLE model over the savanna and sparsely vegetated ecosystem was also a contribution of why OCO-2 lead to a stronger carbon estimate in 2015. The final study was built upon the second study and focused on understanding the Australian carbon flux variability from 2015-2019 and evaluating how Australian semi-arid ecosystems responded to changes in rainfall and temperature anomalies. This study suggests the 2015 carbon sink's size over Australia increased in 2016 due to increased vegetation productivity in this period. Australia's 2016 carbon uptake contributed almost all the long-term mean terrestrial sink estimated for 2015-2019 (-0.33 +- 0.09 PgC/y). The ecosystems that most contributed to this carbon sink were savanna and sparsely vegetated regions driven by a higher than expected greenness in vegetation (expressed by positive EVI anomalies) strongly influenced by positive rainfall anomalies and negative air temperature anomalies.
Stress interactions in intraplate earthquakes
Australia is a seismically active continent with an intraplate compressive stress field primarily driven by far-field plate boundary interactions. Major (magnitude >= 5) surface-rupturing earthquakes are primarily sourced from reverse faults. Major earthquakes perturb local-to-regional stress fields and influence the spatiotemporal properties of subsequent earthquakes. This thesis first contextualizes Australian earthquakes against global comparatives by investigating 260 finite-fault rupture models for 137 moment magnitude (Mw) 4.1 to 8.1 continental earthquakes worldwide. I find that: (i) Australian earthquakes are amongst the most kinematically and geometrically complex for their Mw, (ii) upper-bounds and variance of the number of faults that rupture co-seismically increase with increasing Mw, and (iii) multi-fault rupture populations show no dependency on strain rate or proximity to plate boundaries. The thesis then presents a suite of studies that model static and viscoelastic coulomb stress changes (dCFS) imparted by major Australian earthquakes on to receiver faults and in the surrounding crust. I find that static dCFS models provide an informative physical-statistical basis for characterising many seismic sequences in Australia, with some exceptions. Aftershocks occur predominantly within positive static stress lobes and close to the advancing viscoelastic positive stress lobes, especially over the first few decades after major earthquakes in these regions. Earthquake triggering appears to occur under stress perturbations as small as approx 0.001 to 0.01 bar, suggesting cratons contain regions of critically stressed lithosphere. The effects of varying source fault geometries and kinematics on dCFS fields and subsequent seismicity are used to show how progressive refinement of source fault models using emergent data can reduce epistemic uncertainties in the role of dCFS in earthquake triggering. In some instances, increased source model complexity does not significantly impact on dCFS results and possible relationships to aftershocks relative to simple source models. The thesis finally investigates the role of lithospheric-scale flexural bending due to eustatic sea level changes, and whether these impart dCFS perturbations on finite faults and in regions similarly to stresses imparted by preceding earthquakes. Preliminary age distributions of large paleo-earthquakes on these reverse faults are concurrent with dCFS peaks imparted by eustatic sea-level changes. dCFS of > 0.1 to > 1 bar on faults during the low-stand Marine Isotope (MIS) Stage 2 to Stage 4 interval (ca. approx 30 to approx. 70 ka) provides tentative evidence that earthquake clusters could be stimulated by sea-level lowstands. This thesis demonstrates the power and utility of dCFS modelling to improve understanding of intraplate earthquakes.
Modelling Australia’s transition to a low carbon electricity system with optimised transmission networks and renewable energy exports
In order to meet the Paris Agreement and pursue efforts for a 1.5 degrees Celsius target, the electric power system worldwide must undergo a fundamental transformation as the main pillar for decarbonisation. The decarbonisation will almost certainly be driven largely by the massive adoption of variable renewables. The objective of this PhD project is to develop a co-optimisation model with the ability to perform comprehensive analysis to simultaneously explore the least-cost configurations of generation, storage and transmission over large geographic regions to facilitate the transition to a low carbon power system. Although applicable to any region, the capacity expansion model we developed focuses on Australia’s National Electricity Market. The capacity expansion calculated by the model satisfies prescribed demand projections and emission abatement targets from 2020 to 2050, subject to constraints including resource adequacy, system inertia and reliability, unit commitment, economic dispatch and transmission capacity. The model also takes into account the decommissioning of the existing fossil fuel generation fleets and explores a great range of renewable generation and storage options. The transmission model employs a Direct Current power flow approximation with 21 major load centres over the eastern states of Australia. We have used the model to examine various carbon abatement scenarios for Australia. We show that wind turbines and solar PV technologies combined with pumped hydro energy storage dominate the solutions, and that gas is unlikely to be a transition fuel towards a 100% renewable system by 2050. The 100% renewables systems simulated are not only technically feasible but also economically achievable. Australia’s vast wind and solar resources become increasingly valuable assets as the world decarbonises. Not only does this study consider meeting local requirements for electricity demand, but also the potential for export of green energy. Our research confirms the very real and great potential for Australia to produce cost-competitive hydrogen. In the pathways to carbon neutrality, we demonstrate there are substantial synergies between renewable hydrogen export and domestic energy transition in Australia. In particular, we show if the hydrogen industry co-locates within the National Electricity Market with shared renewable generation facilities (i.e. sector coupling), not only can the hydrogen industry produce cost-competitive hydrogen, but also that domestic electricity consumers could enjoy lower electricity costs, thereby benefiting the whole economy in the long run. Besides hydrogen export, we also conducted the first detailed least-cost optimisation studies of power system decarbonisation planning for both Australia and Indonesia that considers HVDC interconnections. We show direct electricity export to Indonesia could also benefit Australia’s domestic energy transition as any wind and solar energy not used by Indonesia could then be fed to the National Electricity Market. This could help both countries to achieve decarbonisation. Overall, our modelling suggests that a long-term system-wide perspective and orientation are critical for the design of energy policies that can secure broad energy system benefits. If planned well, Australia could become a major supplier of green hydrogen, direct renewable electricity and energy-intensive products as energy vectors in the global market. Particularly, this could be driven and strengthened by a robust commitment to “net-zero by 2050”.
Further development and application of the speleothem U-Pb geochronometer
Speleothems are valuable archives recording information on past climates and processes of landscape evolution. In fact, the high preservation potential of speleothems and their ubiquity across the globe are key attributes and speleothems are now extensively used in palaeoclimate modelling as complements to the more traditional marine and ice core climate records. The real ’engine’ of speleothems science, however, lies in the ability to reliably and robustly place these archives into chronological context. Historically this has been facilitated by the highly successful U-Th geochronometer, which can accurately date speleothems that formed within the last 650 thousand years. However, many speleothems – and by extension their proxy archives – are far older than this 650 ka limit and so require a significantly longer-lived radiometric methodology: the U-Pb geochronometer has recently filled this void. The U-Pb geochronometer is well known in the geological sciences, but its application to the analytically-challenging medium of speleothems has been relatively recent. Modern mass spectrometry methods now allow the accurate and precise analysis of the extremely low-levels of U and Pb within speleothems, thus providing access to such ’deep-time’ (i.e. multi-million years and further) palaeoclimatic and landscape evolution archives. Despite these significant advances, there remain fundamental impediments to the whole-scale production of speleothem U-Pb ages. This thesis investigates some of these shortfalls with the ultimate aim of advancing the speleothem U-Pb methodology and application such that future speleothem-based studies may more rapidly and more reliably place their reconstruction models into a more robust chronology. The first half of this thesis addresses two fundamental limitations of the speleothem U-Pb method: the large expenditure of time and effort required for producing calcite U-Pb ages, and the potential limitations in the correction for initial isotopic disequilibrium effects in the U-Pb decay chain. To address the former, I provide new software for the rapid production of publication-quality U-Pb isochron figures and ages. Additionally, a new chemical separation (i.e. chromatography) procedure for generating high-purity U and Pb fractions is developed. This new ’stacked resin’ protocol can produce mass spectrometry-ready U and Pb sample aliquots in a single working day – roughly half the time of the previous method – and so accelerates the acquisition of U-Pb ages. In terms of subsequent data deconvolution, the largest hurdle to obtaining accurate speleothem U-Pb ages arguably relates to the uncertainty added from having to estimate an initial isotopic disequilibrium value after the system has returned to equilibrium. As a potential remedy to this problem, this research program develops a framework in which to calculate appropriate disequilibria values for the production of disequilibrium-corrected U-Pb ages. The second part of this thesis is designed to highlight new applications of the calcite U-Pb chronometer made possible by the methodological advances documented in the first half. This research program investigates the utility of speleothems as proxies for regional uplift rates in karst terranes. This is accomplished by producing chronologies for 120 speleothems from the Buchan karst along southeast Australia’s passive margin. The results of this study indicate that SE Australia experienced a renewed period of uplift occurring at a maximum rate of 76 +/- 7 m / Ma beginning at least 3.5 million years ago. This speleothem-derived uplift rate is consistent with other independent regional uplift estimates thus adding credibility to the utility of speleothems as proxies for uplift. The relatively straight-forward method developed here is likely applicable in many future karst studies. Additionally, this research program also investigated the potential applicability of U-Pb geochronology to another form of carbonate archive – stromatolites. The methodological advances described in this thesis were applied to stromatolites from Lake Turkana, Kenya, and the palaeo-lake Bungunnia in southern Australia. The results of this experiment suggest that stromatolites contain too much inherited Pb that simply overwhelms the radiogenic ’age signal’ and so are unlikely to be successful candidates for U-Pb dating, at least for stromatolites formed during much of the Cenozoic period.
The structure, rheology, and rupture mechanics of seismogenic faulting in continental lithosphere
This thesis explores relationships between the structure, rheology, and rupture properties of seismogenic faulting in continental lithosphere from four different perspectives. First, I derive a scaling relationship that links the spacing between two nearly parallel strike-slip faults to the frictional strength, fault width, and lower crust viscosity for strike-slip shear zones. Based on the scaling law, I estimate a possible range of lower crust viscosity in the San Andreas Fault system (California), the Marlborough Fault Zone (New Zealand) and central Tibet (China). Second, using numerical modelling methods, I explore how lower crust rheology contrast may affect the long-term evolution of a major plate boundary fault. The case study is based on the San Andreas Fault, which is found to vary dipping angles (~50-90 degree) along strike. The moderately dipping strike-slip fault is not consistent with Anderson faulting theory. This inconsistency may be reconciled if there are lateral variations in the lower crustal rheology across the fault plane that decrease fault dip with time. Third, in addition to strike-slip faults in active tectonic settings, my study also includes reverse faults in stable cratonic regions, especially for the cratonic areas of western and southern Australia. I apply statistical methods to investigate the co-seismic slip distribution of 11 surface-rupturing earthquakes in Australian stable regions and provide a link between the shape and characteristics of co-seismic slip distributions and the geophysical properties of the host crust. Fourth, using a comprehensive geophysical survey with the co-located 13GA-EG1 and 12GA-AF3 seismic reflection profiles and magnetotelluric profiles and regional gravity and magnetic maps in the Nullarbor Plain (Australia), I find that faults initiated back to the Proterozoic could still be reactivated in a Cenozoic convergent setting, especially for those major faults cutting to deep crust. Those deep-penetrating faults at terrane boundaries could be a potential channel for fluids to pass through, and thus further weakened by the fluids, which is revealed by high-conductivity anomaly in magnetotelluric profiles. The last research chapter of this thesis addresses a technical issue in the particle-in-cell finite element method, which is widely used in geodynamic numerical modelling. As mixing materials with contrasting viscosity within one element results in stress fluctuations, I assess different smoothing methods to reduce the spurious stress in mixed-material elements.
A taxonomic revision of the tertiary echinoid genus monostychia
For over 100 years the genus Monostychia (Echinoidea: Clypeasteroida) and its type species M. australis Laube, 1869 have been a taxonomic home for a wide range of genera and species with the commonality of a rounded to pentagonal, discoidal test and a submarginal periproct. The specimens comprising this group are all extinct and from the Tertiary strata of southern Australia. While there have been a few minor species identified beyond M. australis, notably M. etheridgei Woods, 1877 and P. loveni (Duncan, 1877), it has been clear to many researchers that the variability remaining in M. australis was representative of numerous other taxa awaiting discovery. Recent taxonomic works on the Clypeasteroida suggested that the number of interambulacral plates on the oral surface of the test of some species was a useful diagnostic character. Of interest were the plates that first come into contact with the periproct. However, there appeared little evidence in the literature that it had been established that the number of such plates remained constant with test length and age, or that the variability in each taxon, of those plate numbers, has been determined. Without understanding those two issues the utility of plate numbers was questionable. This study set out to resolve some of those issues for Monostychia and its relatives. It was found that the number of interambulacral and ambulacral plates on the oral surface was fixed and did not change with increasing test length and therefore there was potential utility for plate numbers as a taxonomic tool. However, there was substantial variability in the numbers. As a result, the use of plate numbers in the paired interambulacra, paired ambulacra, and ambulacrum III on the oral surface appears to have limited utility at genus level. At the species level, however, such numbers can be quite useful diagnostically, particularly when paired interambulacral, paired ambulacral and ambulacrum III plate numbers are used in combination. The plates that first come into contact with the periproct was shown to have little value taxonomically at the genus or species level within the monostychioids, largely because most species had the same plate number dominating, but also because of the variability. At subfamily level the taxonomic value of this feature is yet to be established. A previously unreported structural feature was identified in many of the specimens. This was a thin circumferential wall of stereom present on the right-hand side of the test, lying half way between the marginal and central buttressing. It was a form of intestinal buttressing referred to hereafter as the intermurum. Its presence enabled the establishment of a new subfamily, Monostychinae in the family Arachnoididae. Four genera have been placed in the Monostychinae; Monostychia Laube, 1869, Quinquestychia gen. nov., Rotundastychia gen. nov. and Deltoidstychia gen. nov. A key to these genera is provided. Several species have been established and others redescribed in this study. In Monostychia there are seven species; M. australis Laube, 1869, M. macnamarai Sadler et al. 2017, M. alanrixi Sadler et al. 2017, M. merrimanensis Sadler et al. 2019, M. etheridgei Woods, 1877, M. glenelgensis Sadler et al. 2019 and M. robheathi sp. nov. A new genus, Quinquestychia gen.nov., has also been established. It contains four species: Q. mannumensis sp. nov., Q. gigas sp. nov., Q. berylmorrisae sp. nov. and Q. robertirwini (Sadler et al. 2017). The last of these species was published as a Monostychia earlier in this study but reassigned later on the basis of further data. A second new genus, Rotundastychia gen. nov., has also been established. It contains three species: R. pledgei sp. nov., R. aquilaensis sp. nov. and R. eyriei sp. nov. A third new genus, Deltoidstychia gen. nov., has also been established. It currently contains a single species, D. erioaster sp. nov. In addition to the above, two other new genera were established but they do not belong in the subfamily Monostychinae. Instead they are tentatively placed in the subfamily Ammotrophinae. The first of these is Obscurostychia gen. nov. with two new species: O. spirographica sp. nov. and O. curtus sp. nov. Keys to all the genera discussed above that contain more than one species have been provided.
Radiocarbon dating of Kimberly rock art
Throughout the world, ancient rock art records some of the earliest attempts at complex human communication. However, constraining the age of older rock art has remained a largely intractable scientific problem thereby limiting our ability to integrate rock art into the rest of the archaeological record. Researchers have studied the globally significant Aboriginal rock art in the Kimberley region of Western Australia for more than 40 years and have comprehensively documented a sequence of rock art stylistic periods. It has long been thought that the oldest styles in this sequence date back to the Pleistocene but only two such dates, relating to identifiable motifs, have been published and both are problematic. The surviving pigment in paintings of all but the most recent Kimberley rock art style contains no material that can be radiometrically dated. There are, however, mineral accretions and mud wasp nests in the same Kimberley rock shelters that house rock art and, occasionally, these under or overlie paintings. This study explores the development of radiocarbon dating techniques to reliably date remnant mud wasp nests found to be in contact with rock art. Recently constructed mud wasp nests were collected and analysed to understand the source and age of carbon-bearing material they contain. Unburned plant material and charcoal were found in similar volumes, but charcoal is the carbon-bearing constituent most likely to provide a reliable radiocarbon age for old nests. Old wasp nests were analysed using a wide range of techniques to determine how taphonomic processes alter their physical and chemical composition. These results guided experimentation with pretreatment methods designed to remove sources of carbon contamination while preserving as much of the carbon in the original charcoal as possible. A total of 120 old mud wasp nests were prepared for radiocarbon dating of which 75 contained sufficient carbon for measurement. The distribution of the 75 ages indicated nests were built quasi-continuously over, at least, the last 20,000 years. The motifs in contact with the 75 nests were classified into one of the six main Kimberley rock art stylistic periods by two subject matter experts. Just 3 nests overlay motifs from each of the Cupules and Wanjina periods suggesting only that some motifs in these styles are older than 7,200 years and 500 years, respectively. The 4 dates available for each of the Static Polychrome and Painted Hand periods permit a very tentative hypothesis for their chronology while the 16 dates relating to Irregular Infill Animal Period (IIAP) motifs and the 20 dates for Gwion motifs provide a more secure estimate. The concise hypothesis proposed for the chronology of the Kimberley rock art styles is that the IIAP style was in use from at least 17,000 to 13,000 years ago. It was followed by the Gwion period from 13,000 – 12, 000 years ago and then the Static Polychrome period 11,000 to 9,000 years ago. The Painted Hand period followed at around 8,500 to 9,000 years ago.
Sub-meter scale lithological heterogeneity and its influence on the CO2 trapping capacity in CO2 storage reservoirs
Lithological heterogeneity at mm- to cm-scale exists in the form of sedimentary structures such as cross bedding and planar bedding in heterogeneous siliciclastic reservoirs. At this scale, sedimentary structures typically consist of two lithologies with significant differences in their porosity, permeability, capillary entry pressure and mineral composition. The lithology with high porosity, high permeability, low capillary entry pressure and a high abundance of quartz and feldspar characterizes the reservoir rock and is conducive to fluid flow. On the other hand, the lithology with low porosity, low permeability, high capillary entry pressure and a high abundance of clay and carbonate minerals characterizes the intraformational baffle and acts as flow barrier. The presence of both lithologies at mm- to cm-scale govern local fluid flow and geochemical reactions. This is especially important in CO2 storage reservoirs where CO2 flow and mineralization might be significantly affected. Hence, it is necessary to account for lithological heterogeneity at sub-meter scale in regional scale dynamic simulations. However, intraformational baffles are typically neglected in static reservoir models primarily because small scale intraformational baffles are not commonly resolved by wireline logs. Therefore, their lithological properties are not accounted for in reservoir models. This study addresses the characterization and incorporation of intraformational baffles in reservoir models so that the reservoir scale estimation of carbon mineral trapping capacity can be improved. Firstly, rocks from the CO2CRC’s Otway Research Facility were characterised using a range of sample and data analysis methods. The variability in porosity, permeability, capillary entry pressure, grain size and mineral composition was used to classify five homogeneous rock type classes: coarse sandstone, fine sandstone, siltstone, mudstone and carbonate-cemented sandstone. A new workflow was developed where homogeneous rock types and composite rock types consisting of two lithologies characteristic of intraformational baffles were represented. Rock properties for composite rock types were derived so that mm-scale heterogeneity can be upscaled for integration in coarser discretised reservoir models. Lithotype logs of composite rock types were derived and used to build a high resolution 2D static reservoir model of the site. Multiphase fluid flow and reactive transport simulations were run on 120 and 8 realizations, respectively, to develop a detailed understanding of CO2 flow and geochemical reactions at mm-scale. It was found that the capillary entry pressure characteristic of the reservoir rock significantly governed the range of turning point saturations while the amount of clay and carbonate content of the baffle governed the amount of carbon mineralized. Reactive transport simulations were also run on two 2D reservoir models to quantify the error associated with the estimation of carbon mineral trapping at reservoir scales if intraformational baffles are not accounted for in dynamic simulations. The results show that the neglection of intraformational baffles in reservoir rocks might lead to an underestimation of mineral trapping capacity by nearly 2.5 times.
The relationship between climate and mechanisms of tropical cyclone formation
A major uncertainty in future projections of tropical cyclone (TC) frequency is due to inadequate understanding of the atmospheric mechanisms leading to a reduction in the TC formation in warmer climates. Although the recently proposed “marsupial pouch theory,” of TC formation indicates that a semi-enclosed recirculating region known as a “pouch” within large-scale disturbances provides necessary conditions for TC formation, it is important to link the frequency of the pouch environments to the evolving climate conditions. Therefore, this thesis examines the changes in the marsupial pouch TC formation environments and their relationship to the large-scale environmental conditions in the current climate and idealized warmer climates. Here we use ERA-interim reanalysis data and high-resolution Australian Community Climate and Earth-System Simulator (ACCESS) climate model simulations of the current climate and idealized warmer climates (aqua-planet simulations) with two fundamentally different TC tracking schemes. The first scheme is the Okubo-Weiss Zeta Parameter (OWZP), a phenomenon-based tracking scheme that detects TC-favorable locations within marsupial pouches using resolution-independent thresholds. The other scheme is the Commonwealth Scientific and Industrial Research Organization (CSIRO), a traditional TC tracking scheme that uses resolution-dependent thresholds. Environmental and structural composite analysis of developing and non-developing tropical depressions (TDs) 48 hours before TD formation, which are detected using the OWZP scheme in the reanalysis, showed that developing circulations have a strong protective layer around the core from the lower to mid troposphere that protects it from external disruptive influences. The relative importance of the environmental variables influencing tropical storm (TS) formation varies across ocean basins due to differences in the large-scale disturbances and surrounding environmental conditions. Statistical TS prediction schemes are also developed using the environmental conditions of developing and non-developing TDs. This analysis notes that random forests and support vector machine algorithms have higher accuracy than decision trees (DTs). Additionally, the hybrid approach of DTs and Markov decision process is proposed which indicates that developing TDs have a higher likelihood of remaining in more favorable environmental conditions than non-developing TDs. In current climate simulation using the climate model, the TCs detected within marsupial pouches are a subset of the traditionally detected TCs. Also, the OWZP scheme performs better in representing the TC frequency statistics and has stronger relationships with the surrounding environmental conditions in the current climate than the CSIRO scheme. In idealized aquaplanet model simulations, we observe both reduced marsupial pouch environments and traditionally detected TCs with increasing sea surface temperatures (SSTs). The increased saturation deficit and increased stability explain the reduction in the frequency of formations with increased SST. The present study also notes a decrease in the frequency of low-intensity storms and an increase in the frequency of intense storms with increasing SSTs. In a drier and more stable atmosphere, the initial vortices need to be stronger for TC formation to occur, with higher low-deformation vorticity and higher upward mass flux. This research indicates that the marsupial pouch theory may be a fundamental paradigm of TC formation due to its better relationships with the large-scale environmental conditions in different climates.