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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.