School of Earth Sciences - Theses
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ItemA low-temperature thermochronology investigation of the Turkana Depression: Implications for the development of the East African Rift System( 2018)The Turkana Depression is one of the most important segments of the East African Rift System (EARS) for studying the onset of intracontinental rifting and the influence of pre-existing lithospheric heterogeneities on focussing later magmatism and strain. Defining the topographic lowlands separating the Ethiopian and East African Domes, the Turkana Depression hosts the earliest manifestations of EARS-related volcanism (Eocene) and basin formation (Paleogene), as suggested by seismic data. Here in northern Kenya and southern Ethiopia, the EARS is expressed by a difuse region (~300 km) of deformation and highly attenuated crust (~ 20 km), in marked contrast to the narrow rift trends (~50 km) and thicker crust (~35-40 km) of the surrounding plateaux. The anomalous morphology and crustal architecture of the Turkana Depression may be attributed to earlier Cretaceous-early Paleogene Anza and South Sudan rifting. The similar age, geometry and gravity response of the ~NW-SE trending Anza and South Sudan rifts preserved to the east and west of Turkana has led to the hypothesis that these systems were once linked. However, due to the obscuring effect of subsequent volcanism and rifting and the scarcity of subsurface and geophysical data in the western Turkana Depression, the proposed connection is poorly constrained. This study presents a low-temperature thermochronology (apatite fission track, apatite (U-Th-Sm)/He, and zircon (U-Th)/He) survey of the Turkana Depression, constraining the nature and extent of pre-EARS tectonism and the subsequent late Paleogene onset and propagation of EARS faulting in the region. Thermal history modelling shows that the Turkana basement records significant Cretaceous denudational cooling, coeval with significant Anza-South Sudan syn-rift sedimentation, suggesting that this area may have initially acted as a basement high and axial sediment source between the rift systems at that time. In the Late Cretaceous-early Paleogene however, parts of Turkana began to subside, in places accommodating up to 500 m of infill. This signified an important period of crustal thinning that may have facilitated the subsequent Eocene commencement of plume-related volcanism, marking the initiation of the EARS. Nonetheless, the discontinuous nature and relatively shallow depths of these depocentres argue against the Anza and South Sudan rifts having achieved a hard linkage within Turkana. The Eocene extrusion of voluminous lavas in the northern Turkana Depression coincided with the abandonment of these early depocentres, possibly suggesting that dynamic topography associated with the arrival of a mantle plume was responsible for terminating early Paleogene subsidence in the region. Thermochronology data from the footwall of a major boundary fault in southern Turkana record a pronounced Eocene onset of cooling related to the development of the ~N-S Lokichar Basin. The Eocene nucleation of strain in the Turkana Depression, significantly predating faulting elsewhere in the EARS, highlights the importance of pre-existing rheological heterogeneities and mantle processes during the initiation of intracontinental rifting. By constraining the spatio-temporal commencement of ~E-W extension in East Africa, this study provides valuable insight into the causal mechanisms for EARS inception.
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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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ItemThermochronology of Tasmania and the South Tasman Rise: implications for the dynamic evolution of a complex rifted continental block( 2013)The crustal architecture of previously adjacent basement terranes in SE Australia, Tasmania and northern Victoria Land, Antarctica is a legacy of late Neoproterozoic-Paleozoic subduction along the east Gondwana margin, highlighting the Cambro-Ordovician Delamerian-Ross orogeny. Structures in this ancient crust were reactivated during late Mesozoic-Cenozoic Gondwana breakup. Tasmania and the offshore South Tasman Rise (STR) lay in a crucial location at the centre of these continental fragments and potentially contain clues regarding the nature of Gondwana dispersal. This study reports results of a systematic thermochronological study that has been carried out on STR dredge samples and Tasmanian dolerites to uncover the history of the ancient subduction system and the post-break up thermal history recorded in their crust. Oceanic investigations (ODP, IODP, magnetic and seismic surveys and swath mapping, etc) have been conducted across the STR and the western transform boundary over the past few decades. Important results have been previously reported and form the foundation towards understanding the tectonic significance of this region. In particular, the interpreted seismic profiles along with basement dredge materials provide essential structural, stratigraphic and petrological controls as in situ investigations are not possible. Nevertheless, only limited geochronological data are available and the regional thermal history is still sparse. In this study a systematic low-temperature thermochronological study has been carried out, applying 40Ar/39Ar, AFT, ZHe and AHe dating techniques to the STR dredge samples to disclose the tectonic evolution of study area. Results suggest that the western and eastern terranes of the STR (W-STR and E-STR, respectively) evolved differently prior to east-Gondwana breakup. While the 40Ar/39Ar data from the W-STR suggest late Cambrian-early Ordovician ages (~495-460 Ma), results from the E-STR suggests a much younger early Carboniferous (Mississippian) age range (325-357 Ma). The data allow temporal comparisons to be made between existing 40Ar/39Ar and K-Ar datasets obtained from S-SE Australia and NVL, and results obtained here from the thermochronologically less studied offshore STR region. The data support reconstructions, which indicate that W-STR shared provenance with Wilson Terrane prior to the continental breakup, and furthermore, refine its paleo-position. Based on the age pattern as well as petrological evidence, it is proposed that it was most likely situated west of the Lanterman Fault Zone (LFZ) and extended further west to the western flank of the pop-up structure bounded by Wilson and Exiles Thrust. E-STR,40Ar/39Ar ages are synchronous with a phase of major granite emplacement and mineralization, which occurred in western Tasmania and are correlated with deformation post Tabberabberan Orogeny in Tasmania and the Victoria. Results from this study provide more solid time-temperature constraints for late Neoproterozoic-Cambrian subduction-related processes and the more recent evolution of the transform Tasman Fracture Zone during separation between Antarctica and Australia. The thermal history models suggest a strong correlation between rapid cooling and tectonic activity in the STR block. Since mid-Cretaceous time these can be summarized as follows: (1) onset of the opening of the Tasman Sea at ~80 Ma, (2) amalgamation of the W- and E-STR blocks and a shift in the relative motion between Australia and Antarctica and (3) final clearance of continental breakup and onset of the opening of the Tasman Gateway. This study is also reports results from the first application of apatite (U-Th-Sm)/He (AHe) thermochronometry to the Tasmanian region. The data not only provide further spatial and temperature constraints, but also examine the quality of AHe ages obtained from mafic lithologies with lower U and Th content than felsic rocks and yielding less age dispersion. Mid Jurassic (~175-180 Ma) dolerite is widely distributed across onshore Tasmania making such a study possible. The dolerite forms part of the Ferrar Group continental flood basalt (CFB) emplaced prior to eastern Gondwana breakup. Its spatial/temporal significance and chemical composition make it an ideal rock-type to aid in documenting the regional post-continental breakup history and to test the influence of different parameters such as -radiation damage and U and Th zonation on AHe age dispersion from a low eU perspective. Low-temperature thermal modelling reveals two distinct cooling episodes. (1) Mid-Cretaceous cooling, which involved km-scale denudation (~3-4 km) in response to continental extension prior to the actual seafloor spreading in the Tasman Sea. (2) Late Cretaceous-early Tertiary cooling, restricted to the west margin of Tasmania in response to transform margin tectonism to the west. In addition, no correlation could be found between AHe ages and potential factors influencing age dispersion such as radiation damage, grain size, U-Th zonation. However, zircons derived from Tasmanian dolerites having high eU suggest an effective upper dosage limit (0.2-0.31018/g) for obtaining meaningful ZHe ages when studying Tasmanian dolerites or possibly other similar mafic lithologies.