School of Earth Sciences - Theses

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    Arc segmentation and landscape evolution in the Bhutan Himalaya
    Wood, Matthew Peter ( 2022)
    The principal structural elements of the Himalayan arc can be traced nearly continuously for 2500 km. Historically, along-strike variations in structure and denudation have not received the same attention as equivalent arc-normal trends. Yet research has demonstrated that arc segmentation can be controlled by lateral variations in the geometry of the Main Himalayan Thrust (MHT). The Bhutan Himalaya has a distinctive physiography and hosts nominal instrumental seismicity despite experiencing long-term strain accommodation comparable to the wider arc. This enigmatic section of the orogen presents an opportunity to test the case for local arc segmentation through applied tectonic geomorphology. By integrating low-temperature thermochronology – including apatite fission track (AFT), and apatite and zircon (U-Th-(Sm))/He thermochronometry of in situ bedrock, synorogenic sediments and modern detrital samples – cosmogenic radionuclide methods (10Be concentrations from detrital quartz samples that add to a nation-wide compilation of published data) and quantitative geomorphometry, this study documents the spatial and temporal variability of denudation to infer partitioning of deformation across crustal structures. Results show prominent along- and across-strike variation in denudation within Bhutan. Contiguous geomorphic zones are defined based on millennial-scale erosion rates and their morphometric proxies, including the physiographically distinct low-relief belt and southeast range front. Profile curvature statistics of central range front ridges are linked to earthquake-triggered landsliding and define the Naka Zone, which overlies a historically seismogenic MHT decollement flat. Synorogenic detrital thermochronometers provide information on source area bedrock cooling and the thermal evolution of the Indo-Gangetic paleo-basin. Linking the depositionally-adjusted age spectra of Siwaliks thermochronometers to an analogous modern detrital suite allows the estimation of sedimentary provenance. The dominant Lesser Himalaya source has been tectonically constructive since at least ~5 Ma, while the secondary Greater Himalaya source is ‘steady state’, as evidenced by a persistent ~4 Ma ZHe age peak. Range front and Siwaliks thermochronometers show that the structural succession is complicated near the eastern border. Southward decreasing detrital AFT exhumation rates in hinterland mountain catchments document progressive, semi-horizontal, post-cooling translation across the basal thrust flat. Decoupled intra-catchment millennial-scale erosion rates are a transient response to geologically recent rock uplift westward of the southeast range front. Exhumation rate modelling of individual in situ samples and elevation sampling transects show that geothermal gradients of at least 35 degrees C km-1 – and long-term erosion rates of ~1 km Myr-1 – have persisted across much of the study area. Multi-thermochronometric thermal history modelling results indicate that the locus of exhumation is offset by ~20 km towards the front southeast of Drangme Chu, predictive of a mid-crustal ramp beneath the Tawang River valley and Lumla window. An oblique ramp is invoked to reconcile differing orogenic sections. A synthesis of findings leads to the proposal of an obliquely oriented, second-order segment boundary within Eastern Bhutan, which may help constrain the seismic potential of adjacent arc segments.
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    Neotectonic Evolution of an Incipient Continental Plate Boundary Fault Intersection, Hope-Kelly Fault System, New Zealand
    Vermeer, Jessica LeeAnne ( 2022)
    The Hope-Kelly fault system forms the intersection between the plate boundary Hope and Alpine faults in the South Island of New Zealand. New fault mapping, paleoseismology, slip-rates, and low temperature thermochronology provide insights into the structure, kinematics and evolution of this fault intersection zone. Lidar, photogrammetry and field-based fault mapping reveals the transition from a dextral fault zone in the east to a splay-like zone of distributed oblique dextral-normal faults that abut the Alpine fault in the west. Structural interactions between the Hope-Kelly faults and the Alpine fault influence surface rupture geometries and kinematics, accommodate differential orogenic growth, and facilitate N-S extension that enables a slip rate change between the central and northern Alpine fault sections. Radiocarbon (14C), optically stimulated luminescence (OSL; quartz), and infrared stimulated luminescence (IRSL; feldspar) ages of fault-proximal sedimentary deposits are combined with geomorphic surface displacement measurements to derive fault slip-rates. Dextral slip-rates on the Hope Fault decrease westward from 5.6 (+2.0/-0.8) mm/yr to 1.7 (+1.0/-0.5) mm/yr. Dextral slip-rates on the Kelly Fault vary from 6.2 (+2.5/-1.2) mm/yr (east) to 2.0 (+2.5/-0.7) mm/yr (central) to 6.4 (+7.8/-1.4) mm/yr (west). Subsidiary faults have minimum slip-rates of 1.3 (+0.1/-0.4) mm/yr. Spatial variations in apparent slip rates are proposed to reflect complexities in slip localization and transfer across the complex deformation zone, slip on unrecognized, buried, and/or blind faults, and possible temporal transience in slip behaviours. Paleoseismic trenching and 14C dating of dead trees provides preliminary evidence for the most recent surface rupturing earthquake on the Taramakau section of the Hope Fault between ca. 1680 and 1840 AD, with a preferred age of ca. 1800-1840 AD. Coulomb fault stress transfer modelling of the 3D Hope-Kelly-Alpine fault intersection zone shows that slip on either the central Hope, Kelly, or central Alpine (source) faults increases Coulomb stress on the other (receiver) faults in the network, highlighting the potential for earthquake spatio-temporal clustering in this region. Zircon and apatite (U-Th)/He thermochronology is used to investigate the thermal-exhumational evolution of rocks in the Hope-Kelly-Alpine fault interaction zone. Late Miocene exhumation (3.4 - 0.8 km/Myr, assuming geothermal gradients of 33 - 40 degrees C/km) through crustal depths of approximately 5-6 km is interpreted to be controlled by proximity to the Alpine Fault, with rocks more proximal to the fault recording faster exhumation rates relative to more distal samples in the east. Establishment of the Hope-Kelly fault system in the Quaternary structurally juxtaposed rocks with discordant cooling histories. Rocks throughout the study region record increased cooling rates from circa 2 Ma. Possible causal mechanisms include increases in rock uplift and denudation rates associated with kinematic changes along the Australia-Pacific plate boundary, Quaternary glaciation, and/or increases in rock mass erodibility associated with Hope-Kelly fault system. This thesis provides new insights into a structurally complex plate boundary, with implications for analogous settings globally.
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    The structure and evolution of the northern Australian margin: Insights from the Papuan Fold and Thrust Belt, Papua New Guinea
    Mahoney, Luke George ( 2021)
    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.