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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ItemThermochronological insights into the morphotectonic evolution of Zimbabwe, southern Africa( 2017)The Zimbabwe Craton and surrounding mobile belts that make up Zimbabwe form the north-eastern part of the Southern African Plateau, which is of great scientific interest due to its anomalous elevation. The Phanerozoic history of Zimbabwe is largely unresolved and is difficult to unravel using conventional field methods due to the fragmentary nature of the preserved geological record and lack of structural controls in the dominantly granitic lithologies. Low-temperature thermochronology systems provide an invaluable toolkit for understanding upper crustal processes and in turn deciphering cryptic morphotectonic histories. Despite their value, thermochronology studies within Zimbabwe are considerably lacking, especially within the cratonic interior. In this work, a multiple low-temperature thermochronology approach— including the first apatite and zircon (U-Th)/He data and a more spatially extensive apatite fission track dataset—is employed together with inverse thermal history modelling to unravel the Phanerozoic histories of the different tectonic provinces of Zimbabwe. The data reveal that structural reactivation, largely caused by stress transmission and associated with uplift and denudation of different crustal blocks, has played a major role in the morphotectonic evolution of Zimbabwe, albeit spatially and temporally variable. The new dataset allows for a more clearly defined spatial and temporal structural reactivation pattern and suggests that the cratonic interior experienced reactivation in the Paleozoic but has since remained tectonically stable. Cratonic Zimbabwe preserves a Pan-African signature associated with Gondwana amalgamation, whereas the eastern cratonic margin and neighbouring mobile belts are dominated by Jurassic Gondwana breakup signals. The spatial extent and trend of the Pan-African rejuvenation signature suggest that the anomalous topography of Zimbabwe may have an ancient component. The regional dataset suggests unroofing of a previously more extensive sedimentary cover over the craton that began in the Paleogene. The zircon (U-Th)/He dataset in this work provides significant methodological insight. The unexpectedly recurrent ‘inversion’ of low-temperature thermochronology ages suggests that moderately-extremely radiation-damaged zircons can, in certain geological settings, act as ultra-low-temperature thermochronometers and provide insight into the more recent morphotectonic history. However, at present, the current zircon α-radiation damage accumulation and annealing model (ZRDAAM) does not adequately capture the He diffusion behaviour of the majority of the dated zircons. The exact source of this issue in the zircon (U-Th)/He system is uncertain, but could be associated with a ZRDAAM calibration issue, an unaccounted source of error and/or a currently unrecognised factor affecting He diffusion and retentivity within zircon.
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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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ItemThermochronological and structural insights into the Mesozoic-Cenozoic tectonic evolution of the eastern Tibetan Plateau( 2013)This thesis explores the evolution of the eastern Tibetan Plateau (TP) in select areas, i.e. Yidun Arc, eastern Songpan-Ganze terrane (SGT), Longmen Shan (LMS) and Sichuan Basin (SB) using a combination of structural and thermochronological techniques. Results highlight the role of: (i) flat subduction of the Meso-Tethys in triggering crustal refrigeration and exhumation in the Yidun Arc; (ii) crustal strength discontinuities in transferring deformation in central Asia; (iii) inherited crustal architecture in forming the eastern TP margin; (iv) crustal extrusion in forming the post-collisional stress-regime and high elevations in the eastern TP; and (v) Late Cenozoic onset of the Asian monsoon in enhancing river incision in the southeastern TP. Thermochronology data from the Yidun Arc indicate a distinct phase of Late Jurassic-Early Cretaceous crustal refrigeration and exhumation, which is interpreted as resulting from flat subduction of the Meso-Tethys and subsequent Lhasa-Qiangtang collision along the Bangong suture. Such an interpretation is consistent with lithospheric features imaged by seismic data. Thermochronology data from the eastern SGT (including a deep >7 km borehole) point to a ubiquitous phase of Late Jurassic-Early Cretaceous cooling. This cooling is best explained by regional post-orogenic denudation, as a far-field response to Lhasa-Qiangtang collision to the south and extensive sinistral shearing along major faults bounding the SGT. Projecting these aforementioned events onto an Early Cretaceous paleogeographic terrane reconstruction, results in a new tectonic model, where shearing along the faults transferred strain related to the Lhasa-Qiangtang collision into central Asia. Structural and geochronological results from the southern LMS suggest that Early Cretaceous-Early Paleogene deformation of the LMS included a phase of crustal extension along its hinterland, and a phase of crustal shortening along its front, forming ~2-3 km thick foredeep deposits in the southwestern SB. This deformation assemblage is interpreted as the combined effect of failure of the LMS crustal wedge and clockwise rotation of the SGT. These results indicate that the pre-Cenozoic LMS was underlain by a thickened crust, which was further thickened by Late Cenozoic crustal shortening along a series of listric reverse-faults merging into a detachment seated at a depth of ~20-30 km. Thermochronology data from deep boreholes across a W-dipping reverse fault in the eastern SB point to a distinctive cooling episode in the hanging wall commencing at ~28 ± 3 Ma. This age constrains the timing of a Cenozoic shortening component along this structural belt corroborating that a phase of W-E shortening occurred in the eastern TP and SB. This finding is consistent with evidence supporting extrusion of the eastern TP. Enhanced river incision in the Yidun Arc (part of the southeastern TP) was initiated in the Early Miocene (~15-22 Ma). This timing is older than previous proposals for Late Miocene plateau formation and river incision elsewhere in the same region. It is concluded that the Early Miocene inception of river incision heralds the onset of surface uplift by continental subduction and extrusion, whereas Late Miocene incision was triggered by commencement of the Asian monsoon.