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Quantifying climatic, lithological and tectonic drivers on topography
No full textMountain ranges and fluvial landforms are shaped by a competition of natural forcings: tectonic forcings provide the uplift that causes mountains to grow, and rivers erode them down by downcutting into the bedrock material. Climate forcings, such as precipitation, vegetation or temperature favour or hinder erosional processes, and the lithological composition controls how these forcings are recorded on the landscape. As geomorphologists, we are only able to work with current topography, through field observations or digital elevation models. Much of the existing work in geomorphology has focused on creating metrics that allow us to quantify the imprint of tectonic, climatic and lithological signals from landscapes. One of the first formulations relating erosion and uplift incorporates a direct relation with the channel steepness and the amount by which this steepening occurs. This thesis focuses on quantifying these factors to detect areas of river channels and hillslopes that show an active uplift, sudden changes in lithology or a change in erosion rates due to climatic differences, through deviations from their steady state behaviour.
I start by exploring how a climate signal is recorded on the landscape, and whether it is possible to detect such a signal through a form of the stream power law that includes discharge instead of drainage area. I set up a range of numerical simulations with a rainfall gradient component from which I calculate the optimal concavity index (θ), the χ profiles and the disorder of each river basin. In a landscape with uniform lithology, it is possible to identify simulation runs with a rainfall gradient using the disorder metric. However, when heterogenous lithology is introduced, this signal becomes clouded. I test this method in eight natural landscapes with rainfall gradients and find that, similarly to the simulations with heterogenous lithology, it is not possible to distinguish between drainage-area-driven and discharge-driven incision using the disorder method. Nonetheless, I can quantify the distortion introduced on the channel steepness index (ksn) when using different erosion rules.
The next research question explores the role of lithology in landscapes through its effects on drainage divide migration. Most drainage divides are located in the centre of mountain ranges. However, using a simple formulation of the stream power law, I find that a small difference in rock erodibility should theoretically lead to larger drainage divide asymmetries. I explore this discrepancy through a combination of analytical solutions and numerical simulations based on Hack's law and the stream power law. I quantify the difference in basin sizes expected due to a lithological step change offset from the centre of an initially symmetrical drainage divide. I expand on this with a set of numerical simulations, including a 3D lithology component where I study the role of rock erodibilities as well as the location and orientation of rock layers in the motion of the drainage divide. I find that the angle of the rock layers and the overall landscape erodibility largely control the position of the divide. I conclude this study with the comparison of the analytical and numerical solutions to the Sierra Alhamilla, an asymmetric mountain range with heterogenous lithology in south-eastern Spain.
The final part of this thesis explores the interactions between fluvial and hillslope processes. Hillslopes have been found to record tectonic signals propagating up channel tributaries in the form of knickpoints, with a lag in the response time. I choose a landscape with active tectonics, the South Fork Eel River in California, USA, to explore this transient hilltop signal. The analysis toolkit to perform an exploration of hillslopes is based on different laws from fluvial incision, where the steady-state condition is defined based on the physical parameters of the hillslopes and soils, such as the hilltop curvature, hillslope length, relief, slope angle, and soil transport and density parameters. I extract these parameters from a 3 m digital elevation model of the study site and explore the hypothesised hillslope response lag with respect to the transient fluvial signal, recorded through changes in channel steepness, ksn.
In summary, this thesis explores the signature of climatic, lithological and tectonic signals on landscapes. I discuss under what circumstances these signals can be detected, and propose new approaches to quantifying their effect on landscapes through a combination of analytical, numerical and topographic methods
Homing female figures in English literatures of the Indian subcontinent: a trans/national focus on homeland, body and return
Get PDFIn postcolonial and diaspora literatures of the Indian subcontinent, the idea of home is explored in terms of multiplicity and fluidity alongside social, cultural and political rootedness. The critical questions and arguments that drive this research initiative engage with late twentieth century literatures in English from the Indian subcontinent and the transnational imagination of home in relation to developing female character arcs. This research endeavour would examine women and their relationship with national, cultural and diasporic affiliations of home while navigating discussions on migration – dispersal and return – within and beyond the nation.
Tracing home through female characters in late twentieth century Indian and transnational writers’ narratives, the chapters in my thesis explore national, global and cosmopolitan conflicts of border crossing. This relationship develops in various ways: Firstly, while the imagination of the homeland, primarily catered to the male gaze, marginalises women and sidelines them to the periphery of the nation’s body politic, it also exclusively allegorises and deifies them as custodians of belief systems. They are disparaged due to their economic and political backgrounds and culturally ostracised due to rigorous societal norms. However, their ability to give birth and continue with the humankind remains essential to the formation of a collective such as a nation. Women are borne by other women and raised to become emblematic and authentic portrayals of national customs and traditions, thus, occupying the public as well as the private realm. Secondly, women’s historical relation to the larger political space of the nation is often challenged by marginalised and subaltern narratives in which their bodies develop as prominent figures of home. Finally, homes have been associated with women; Their presence as the carriers and upholders of tradition and culture in diasporic journeys across nations contribute to the ongoing process of ‘being written for’ and ‘writing back’ to a point of origin, hope and nostalgia. These interventions have developed into a major counteractive discourse in postcolonial and transnational literary traditions. This extensive study of the oeuvre of late twentieth century literatures in English emerging from the Indian subcontinent critically analyses female characters in the literary engagements of the period highlighting narratives and counter-narratives by writers such as Salman Rushdie, Amitav Ghosh, Gayatri Chakravorty Spivak, Mahāśvetā Debī, Jhumpa Lahiri and Kiran Desai.
The conclusions that my research draws aid in the exploration of gender relations and the transnational and dialectic associations of home in the late twentieth century literary corpus of the Indian subcontinent. My findings examine the relationship between women and nation deploying discussions on the fixity of home, the corporeality of female bodies, ethnic and cultural nostalgia, and migrant and diasporic return. Elucidating various power structures – narrative, space, culture and identity, it sets up a dialogue between female characters, narratives and contemporary concerns of existence, journey and belonging that span the idea of the nation
Essays on political economics
No full textThis thesis consists of three chapters on political economics:
The first chapter focuses on behavioural differences between elected and selected public officials. Specifically, I investigate the response of Russian regional governors to the gubernatorial appointment reform of 2004, which changed governors' selection and retention mechanism from direct elections to presidential appointments. The reform allowed elected governors to stay in office and ask to be appointed for the next terms. This unique setting allows me to isolate the confounding effects of the selection systems channelled through a change in public officials' composition that plagues previous studies in this field. To evaluate governors' response to the reform, I construct a novel measure of governors' cooperation with the federal center derived from a large dataset of federal media articles using a range of natural language processing methods and supervised learning classifications. Applying RD in time methodology, I find that governors react to a change in their selection mechanism by increasing cooperation with the federal government and providing it with electoral support. In the context of Russian politics, this chapter documents the transition to a more centralized Russia with less political competition at the regional level.
The second chapter studies the effect of FDI on the quality of governance. Focusing on Vietnam, I investigate the effect that an increase in foreign economic engagement following the accession to WTO had on the quality of governance in the host provinces. Using the WTO accession as an exogenous shock to provincial foreign investment, I construct a shift-share instrumental variable and apply a two-stage least squares (2SLS) approach. I find some evidence that foreign investment raises corruption and expropriation risks, while simultaneously increasing the competence of provincial government officials.
The last chapter investigates the effect of foreign direct investment on institutional quality in the host country. Building a theoretical model, I find that conflicting interests between governments and foreign investors can lead to a nonlinear effect of FDI on institutional quality. In empirical exercise, to address a complex causal structure between institutional quality, FDI, and economic development, I employ a structural equation model that allows for bi-causal effects among these variables. The findings reveal a large, significant U-shape effect of FDI on institutional quality, both in aggregate and when considering its corruption and expropriation risks aspects independently. FDI is estimated to decrease institutional quality at low levels of investment but is expected to have a positive effect past a certain threshold. One possible driver of the observed nonlinear effect is the exponentially increasing costs that governments face when interfering with foreign firms
Pancho y las visitadoras: neocolonial challenges and transnational aspirations in Francisco Lombardi’s Peruvian-Spanish coproductions (1988-2006)
No full textPeruvian cinema has remained active over the last thirty years despite the country’s own
financial, legal and political challenges, and the overwhelming competition from Hollywood.
On the international festival circuit, Peru has been represented with emblematic exports like
those directed by Francisco Lombardi between 1988 and 2006. These titles have been
naturally recognised as part of an aspirational Peruvian “national” cinema. However, these
films challenge the former conception due to their partial funding by production companies
from Spain, Peru’s former colonial ruler and present-day capitalist trailblazer. Thus, aside
from depicting narratives mostly based in Lima or conveyed from a Lima-born protagonist’s
viewpoint, they mostly foreground Spanish actors alongside light-skinned criollo Peruvians
over those of the country’s paradoxically predominant mestizo or indigenous populations.
This thesis argues that these coproductions could be ideologically influenced by
longstanding tropes linked to Hispanidad, the neocolonial discourse that exalts Spain’s
linguistic and cultural legacy. At the same time, as transatlantic coproductions, any cultural
dissonance might be justified with the transnational discourse of Ibero-American cinema. To
unpack this perceived dichotomy in Peruvian-Spanish coproductions, the thesis addresses
historical backgrounds, economic inequalities, racial allegiances and cultural expressions that
infuse the production processes and the films themselves. Analysing Lombardi’s coproduced
oeuvre as a case study, the thesis elucidates the extent to which they have enabled the
creative agency of a financially dependent film industry; whether the involvement of Spanish
producers in the Peruvian film industry necessarily implies a neocolonial venture; and the
significance of Ibero-American coproductions within an already globalised cinema
Effects of geometry on the controlled failure of architected joints
No full textIn the study of fracture mechanics, fracture is always treated as a non-desirable phenomenon. Through the presented studies, we aim to embrace fracture and treat it as a feature. We consider architected materials, aka mechanical metamaterials, as the platform for applying the proposed failure by design concepts. The mechanical behaviour of architected materials, independent of the base material, is tailored by the careful tuning of their geometry and microstructure. In that way, architected materials can exhibit extraordinary mechanical properties i.e., negative Poisson’s ratio as well as superior stiffness-to-weight and toughness-to-weight ratios. Hence, architected materials offer limitless possibilities for exploration with the goal of pre-programming fracture behaviour and exploiting geometrically driven fracture processes.
The studies we present, are mainly focused around the concepts and applications of adhesive bonding and confinement, where load transfer needs to be optimal. Adhesive joints eliminate the use of traditional fasteners, hence decreasing the amount of points prone to structural failure. However, the extensive use of adhesives in large scale applications (primarily wind energy and aerospace) gives rise to various challenges including ease of disassembling during maintenance, sustainability, recyclability as well as their dependence on fossil fuels to name a few. Therefore, through the presented studies, we aim to incorporate architected materials in confined systems, having thick adhesive bondlines in mind, and to understand the underlying physics driving the fracture processes in such systems. We then attempt to quantify the effects of the small-scale structure on fracture initiation and propagation.
Although the studies presented can be seen as independent problems and case studies, the underlying motivation, objectives and overall narrative is shared. The common question being: "How geometric manipulations can control crack growth in confined systems?", directs our focus around the areas of fracture mechanics, composite materials and architected materials. We then introduce all above areas, highlighting the followed research aspects and laying out the necessary developments that took place. That is achieved by reviewing the literature of the individual modules while simultaneously developing the needed methodology for each module.
In the third chapter, “Micropolar Elastic Foundations”, we formulate a complete theoretical framework for incorporating micropolar elasticity in an “augmented Winkler” elastic formulation. We focus on deriving closed-form solutions for the restoring linear and rotational stiffness based on stress potentials. We then present double cantilever beam solutions on the derived framework, and we quantify the effects of micropolar elasticity on the deflection distribution responses. Lastly, we lay the benefits and limitations of the method based on measured material parameters from existing literature.
In the fourth chapter, “Testing framework for Architected Joints”, we develop a testing framework for evaluating mode I fracture toughness that targets confined 3-dimensional architected materials. We focus on providing design guidelines for conducting such experiments. The testing setup is based on the classical double cantilever beam configuration, but the geometry of the pre-crack is designed specifically for avoiding any stress concentrations at corners, ensuring a cohesive crack propagation. The proposed configuration is benchmarked by numerical simulations incorporating micropolar elasticity and the phase field fracture model, and can successfully capture the crack paths.
In the fifth chapter, “Gyroids: Can size control crack growth?”, we performed an experimental study based on the framework developed in the third chapter. The obtained experimental results are then complemented by the analytical and numerical methods presented at the second chapter, highlighting a dependence of micropolar size effects on the observed crack paths. We provide an estimation for the critical strain energy release rate of the gyroid lattice, based on micromechanical assumptions. Furthermore, we define a crack shape index for characterising crack growth in confined lattices, that enables the quantification of crack shapes without the need for precise coordinates of the crack shape.
In the sixth chapter, “The Toughness of Auxetic Foams”, we delve into the low stiffness limit by testing open-cell poly-urethane foams. Next, thermomechanical treatment is performed to convert the foams and give them auxetic properties. The converted foams are then tested in uniaxial tension/compression, and their mechanical response is modelled with the Ogden hyperelastic model, complemented, within the fracture mechanics framework, by a traction-separation law. We then formulate a 1-dimensional semi-analytical Euler-Bernoulli beam model integrating the measured material properties to predict fracture behaviour under the double cantilever beam configuration. The model demonstrates a very good agreement with experimental results, quantifying the fracture toughness of the auxetic foam, revealing the influence of thermomechanical conversion.
In the last chapter, we summarise the research undertaken in this thesis, and we explore possible further directions that can be taken
Discovery of tool compounds for study of the ubiquitin-proteasome system
No full textUbiquitin (Ub) is conjugated to proteins through the Ubiquitin-proteasome system
(UPS), acting as a post-translational modification that tunes signalling pathways and
controls protein longevity. While the UPS is thought to be dysregulated in many types
of disease, there is a lack of existing small molecule tools with which it can be studied.
Developing new tool molecules would provide alternative approaches to examine the
mechanisms of the UPS and unlock opportunities to discover future drug candidates.
A structure-based approach was employed to identify a ligand that would act as a
molecular glue, binding between UBE2D1 and Ubiquitin, an interaction which
enhances substrate ubiquitination. The Sidhu group produced a ubiquitin variant,
UbV.D1.1, which bound tightly and selectively to UBE2D1, even over its close
homologues in the UBE2D family.† In particular, the mutation of a -Leu-Thr- sequence
in ubiquitin to a -Phe-Trp- sequence strongly contributed to the enhanced binding,
with the residues engaging additional key interactions. Therefore, a library of cyclic
dipeptides based on the chemistry of this -Phe-Trp- sequence of UbV.D1.1 were
synthesised as a starting point for the identification of molecular glues stabilising the
Ub/UBE2D1 interaction. The compounds were tested for their ability to perturb
ubiquitination reactions consisting of purified E1, E2, E3 and substrate proteins.
Visualisation using SDS-PAGE revealed that cyclo[ᴅ-Phe-ʟ-Trp] (CBS006) inhibited
ubiquitination at high μM concentrations as typically used in primary screening.
Further examination revealed that, rather than acting as a molecular glue to the
Ub/UBE2D1 interaction, CBS006 inhibited activation of Ub by the E1 enzyme.
However, when CBS006 was resynthesized via a different synthetic route, the new
sample did not reproduce the initially observed inhibition. This disparity is under
further investigation in the Hulme group.
Next, fluorine-labelled Ub-mimicking peptides (UMPs) based on those studied by
Zhao and co-workers‡ were synthesised. Fluorine NMR experiments including a
CPMG pulse sequence were performed on the peptides to study their sensitivity to
the presence of the E1 enzyme. Preliminary results highlighted one of these peptides
as being particularly sensitive towards binding to the E1, having a C2 value of 56%.
This peptide has potential for future use as a spy molecule in screening small
molecule libraries to discover E1 binders in a high-throughput manner.
Finally, a novel cleavable linker for peptide synthesis, termed COLAZ, was designed
and synthesised. The COLAZ linker bestows peptides with a fluorescent label that it
stores in a photostable state throughout peptide synthesis. The suitability of the
COLAZ linker for peptide synthesis was demonstrated by tracking the synthesis of a
model dipeptide using LCMS, TLC and IR. Finally, the COLAZ linker was used to
synthesise novel side-chain derivatised UMPs
Boiling point: how the UK and US governments crafted immigration environments of extreme unwelcoming in the Brexit and Trump eras and their impact on immigrant inclusion efforts
Get PDFBoth the Leave campaigns of the Brexit referendum in the UK and presidential campaign of Donald Trump in the US trafficked the idea that immigrants and unbridled immigration had severely diminished their country’s domestic and international autonomy, power, and prosperity (Hinojosa Ojeda & Telles, 2021; Newth, 2023). Each promised that, if successful, they would drastically curb immigration, restructure their respective immigration systems, and root out, deport, and deter ‘undesirable’ immigrants in order to create a (more) unwelcoming immigration environment (Gonzalez & Carranza, 2020; Zapettini, 2019). This was framed by the Brexit and Trump campaigns as necessary to restoring and preserving their countries.
Following the Brexit vote and Trump’s inauguration, the governing Conservative Party under the premierships of Theresa May and Boris Johnson in the UK and the Trump Administration in the US acted to fulfil these campaign promises of promulgating a (more) unwelcoming immigration environment. These harsh tactics have been well documented (e.g. Bolter et al, 2022; Griffiths & Yeo, 2021; Taylor, 2018; Waslin, 2020), but did they achieve their intended effect of generating a (more) unwelcoming environment for immigrants and immigration in the UK and the US during the Brexit and Trump eras and, if so, what were the repercussions of this for immigrant inclusion efforts?
Scant literature exists on how immigrant inclusion practitioners, those working on the front lines in a variety of everyday contexts to welcome and include immigrants, perceived the national government’s actions and their implications. This study employs original quantitative and qualitative survey data collected from immigrant inclusion practitioners operating in the UK and the US during the Brexit and Trump eras along with content analysis to respond to these questions.
It establishes that the participating immigrant inclusion practitioners almost unanimously perceived their national governments’ immigration approaches to have been extremely unwelcoming during the Brexit and Trump eras. Yet, a striking difference emerges from the data that alludes to distinctive paths taken by their national governments to accomplish this. Distilled through the metaphor of bringing a pot to boil on a stove top, this research charts the distinct paths of the UK Government’s ‘slow boil’ graduated intensification and the US Government’s ‘fast boil’ rapid transformation that ultimately allowed each government to bring their country’s immigration environment to a boiling point of extreme unwelcoming in these time periods. The thesis constructs profiles, based on participant comments and substantiated by the literature, of key components of the Brexit and Trump Governments’ immigration approaches, facilitating a comparative examination and identifying seven common features of these approaches. This research underscores the pivotal role that a national government plays in crafting immigration environments of un/welcoming.
Analysis of quantitative data shows that the Brexit and Trump Governments’ immigration approaches had a decisive impact on immigrant inclusion efforts overall and in three critical areas: messaging, funding, and daily operations.
Analysis of qualitative data reveals several conditions produced by these approaches (1) a surge in demand for inclusion work, 2) significant reductions in public funding and state capacity for immigrant inclusion work, and 3) more extreme reactions from politicians and sections of the public towards immigrant inclusion) that significantly impacted the field’s capacity and practitioners’ health, well-being, and safety. These impacts were largely detrimental to the field though opportunities did emerge for some UK and US participants’ inclusion efforts to grow. However, this increased capacity was highly contextual, often influenced by the geographic scope and type(s) of inclusion work undertaken and localised responses to it. Subnational governmental responses appear to either amplify or muffle the national government’s approach and impact based on their supportive or resistant positions to it.
Findings from this study suggest that both the UK and US Governments during the Brexit and Trump eras extended their immigration authority at subnational levels through direct (e.g. executive action, resource reallocation, funding cuts, etc.) and indirect (e.g. extending ‘everyday bordering’ (Yuval-Davis et al, 2018) and ‘deputisation’ practices (Walsh, 2014)) means. This adds to assertions made by Emilsson (2015) and Coker (2018) of a ‘national turn’ in immigrant inclusion (traditionally handled and increasingly led predominantly by subnational governments) that compromises the ability of subnational governments to set and implement localised policy.
In addition to demonstrating the substantial impact of the national government’s approach on crafting immigration environments and impacting inclusion efforts, this study contributes to the mapping of the immigrant inclusion field in the UK and the US, which is heavily under-researched. It also develops and employs a comprehensive typology of immigrant inclusion activity that may be used in future research
Bayesian inference for model-based clustering and RNA velocity in single-cell data
No full textSingle-cell RNA sequencing (scRNA-seq) enables the measurement of gene expression activity at the single-cell level, offering insights beyond bulk RNA sequencing that only provides average expression levels and masks cell-to-cell heterogeneity. Clustering single cells based on transcriptome similarity is crucial for identifying putative cell subtypes. However, most existing clustering methods focus solely on gene expressions, while incorporating additional cellular dynamic information may further enhance the identification of subpopulations. For instance, RNA velocity, defined as the first derivative of the spliced mRNA counts with respect to latent time, is essential for elucidating the directed trajectories of cells in scRNA-seq data. Latent time can also be used as a covariate, describing a cell’s position in a biological process. Moreover, integrating scRNA-seq datasets collected across different time points, tissues, or experimental conditions remains an ongoing challenge.
To address these challenges, we develop a covariate-dependent hierarchical Dirichlet process (C-HDP). This method allows for borrowing information across multiple experiments and can accommodate covariates in an interpretable manner through bounded kernel functions and normalized weights. We design an efficient Gibbs sampling scheme for posterior inference, employing data augmentation tricks. For applications, we focus on two types of data, 1) high-dimensional scRNA-seq data with two experimental conditions, aimed to study the influence of the transcription factor, Pax6, in the development of embryonic cells and 2) calcium imaging data measuring neuronal activity at fixed time frames from the hippocampus of the brain. For the Pax6 data, we use latent time as the covariate and identify covariate-dependent homogeneous clusters. The relationship between the cluster and latent time is found to vary across experiments. For calcium imaging data, we uncover recurring clusters aligned with the spatial location and navigation of the animal.
While C-HDP successfully integrates covariate information into clustering, it assumes that latent time is known. To more accurately capture cell dynamics, we develop a Bayesian hierarchical model that estimates latent time and RNA velocity. Existing methods often lack proper uncertainty quantification and many adopt unrealistic assumptions or rely on black-box models that are difficult to interpret. We propose ConsensusVelo, combining Markov chain Monte Carlo (MCMC) and consensus approaches, to infer RNA velocity with appropriate uncertainty. Applied to scRNA-seq data from mouse embryonic stem cells, ConsensusVelo correctly models unspliced-spliced mRNA relationships, outperforming the popular methods in the field, scVelo and VeloVI. In particular, our method provides a smoother estimate for gene-shared latent time, whereas large noise is detected in scVelo.
Lastly, we investigate the behavior of mixture models in high dimensions. From simulation study, we suggest mitigating the degeneracy of mixture models through calibration of prior parameters and model parametrization. Additionally, a scalable nonparametric sampling algorithm, posterior bootstrap (PB), is investigated to overcome the computational limitations of MCMC. We apply PB to infer the single-cell clustering model for one experimental condition in the Pax6 data. MCMC samples are found to show high correlation and a single chain is easy to get trapped in local modes, whereas PB’s parallel sampling scheme provides independent posterior samples and can appropriately identify multiple modes for mixture models
The last stronghold: US-Thai relations during the Vietnam war
Get PDFThis thesis examines the gradual decline of American power on mainland Southeast Asia
during the years of American effort to contain the spread of communism in Southeast Asia by
participating in the Vietnam War from the perspective of the US-Thai relations. Throughout the
Vietnam War, the relationship between the United States and Thailand was strengthened by their
mutual anti-communist sentiments that led to Thailand becoming a facilitator of American
military forces in Vietnam, chiefly through provision of airbases for the United States on Thai
territory. It was these bases that enabled the fullest extent of American military power to be
brought to bear all over Indochina, namely Vietnam, Laos, and Cambodia. By the mid-1960s,
Thailand arguably became the primary hub for American operations in Southeast Asia. Thailand, in
return, received an array of political, military, and economic aid packages that strengthened its
economy, but also its successive military governments. What began as a bilateral politicalmilitary
cooperation between the two nations, however, led to domestic changes in Thailand that
resulted in a democratic transition towards a civilian government whose foreign policy initiative
was to move away from its established alignment with the United States.
The dissertation follows a chronological structure that began with American interest in
working with the Thai military government in the late 1950s and ended with the complete
withdrawal of American personnel that had been based in Thailand in 1976. The first chapter
explores the root of US-Thai alignment and the Nixon Doctrine. Followed by a chapter on the
impact American bases had on Thailand, and the political transition in 1973. Chapter three
explores the changes that resulted from the political transition, and the resultant attempts by both
the United States and Thailand to adjust to each other under the changing circumstance. The last
chapter offers a narrative and analysis on how political turmoil in Thailand resulted in American
departure. The dissertation concludes that ultimately the loss of American stronghold in
Thailand, as a result of Thai politics, led to American containment arc against communism in
Southeast Asia being shifted further southwards towards the maritime nations of the region. The
methodology used in this study is rooted in archival research drawn from material in both
Thailand and the United States.
By studying the course of the US-Thai relations, the eventual decline of American power in
Southeast Asia could be seen through the scope of American engagement with a regional power
other than Vietnam. Thus, further contribute to the landscape of literature on American Cold
War in Southeast Asia
Geochemically enriched magmas at Etinde volcano, Cameroon Volcanic Line: new insights into enigmatic intraplate provinces
No full textIntraplate magmatic provinces occur globally, across the oceans and the continents.
The processes which drive magmatism at different intraplate locations are debated,
as their intraplate nature means their origins are not explicable by plate tectonic
theory. Conventionally, voluminous intraplate magmatic provinces are linked to a
mantle plume model, where melting occurs as anomalously hot material is brought
up from the deep Earth via convection. This model predicts observable
characteristics, such as time-progressive activity as a plate moves over a fixed mantle
‘hotspot’, elevated mantle potential temperatures, and geochemical evidence for a
primordial (deep) mantle source. Though the plume model explains magmatic activity
well in some locations, such as at Hawaii and Réunion, numerous enigmatic intraplate
settings across the world lack evidence of association with plumes. Many of the
proposed alternative models are overly specific to a given setting and so lack
testability.
The Cameroon Volcanic Line (CVL) is an intraplate magmatic province in West Africa
which spans oceanic and continental lithosphere and has a strikingly linear
morphology. It lacks age-progression and primordial geochemical signatures and,
despite moderately high magma production rates sustained over ca, 65 m.y.,
evidence for high mantle potential temperature is lacking. Furthermore, a nearidentical
magmatic province is located in NE Brazil on the precisely conjugate South
Atlantic margin. Magmatic activity in the two provinces started at about the same time
(40-50 m.y. after continental separation) and continues to the present day in the CVL
and until very recently in NE Brazil. Most alternative models for the CVL fail to
reconcile all observed features, ignore the NE Brazil province, and lack applicability
to other intraplate magmatic provinces. More recently, however, the CVL and NE
Brazil have been used as the case study for a new model for intraplate magmatism
which links these provinces to a shallow, enriched asthenospheric source. This model
has applicability to other enigmatic intraplate magmatic settings worldwide.
In order to evaluate this new model, this PhD study aims to better understand
geochemical enrichment at the CVL. The small volcano Etinde stands out as being
composed of the most geochemically enriched igneous rocks of the CVL, with
incompatible trace element concentrations up to 1000 times greater than in primitive
mantle, and high concentrations of volatile elements such as sulfur and chlorine. Its
lavas are highly silica-undersaturated, spanning compositions from olivine
melanephelinite to leucite-schorlomite nephelinite. Magmas of this composition
cannot be generated by any realistic degree of melting of normal mantle, and must
result from partial melting of a geochemically enriched source. As such, in aiming to
understand geochemical enrichment at the CVL as a whole, Etinde is the focus of the
present study and serves as a window through which the nature of enrichment in the
CVL source region can be observed.
Several geochemical investigations are reported in this thesis, each aimed at building
upon the limited existing understanding of geochemical enrichment in the CVL source
region. For a broad suite of Etinde samples, groupings are identified based on their
petrography and geochemistry. Mafic samples fall into distinct groups; the feldsparfree
mafic nephelinites with rare, resorbed olivine and feldspar-bearing basanites with
abundant euhedral olivine. Intermediate nephelinites form three groups, each with a
distinct modal abundance of haüyne. Lastly, there is a group of felsic nephelinites that
contain schorlomite and phases such as nosean, leucite and melilite. Enigmatic trace
element behaviour in the suite is resolved by the measurement of new perovskitemelt
partition coefficients (KDs), specifically tailored to the Etinde system. These KDs
span a wide range from ~ 0.02 for aluminium to ~ 116 for thorium, with KDs > 10
recorded for many of the light rare earth elements (LREEs). Whole rock geochemistry
is used to better constrain enrichment in the melt source region. Targeted, in-situ
chemical analyses of olivine and titanaugite further build on these geochemical
insights and textural evidence in olivine-poor nephelinite demonstrates the clear role
of carbonate enrichment in magma genesis. The measurement of eight robust new
40Ar/39Ar ages shows that Etinde magmas form two distinct age clusters, nephelinite
magmas are older at ~ 0.55 Ma and likely represent the initial, small-degree melts of
an enriched source. This source was then diluted in the formation of the much larger
neighbouring Mount Cameroon volcano, where basanite magmas erupted from both
Mount Cameroon and a reactivated Etinde plumbing system after a gap of ~ 0.4 Ma.
Finally, an ambitious final study presents the first sulfur-isotope values for the CVL,
aimed at constraining the origin of enrichment. The δ³⁴S range at Etinde, after
accounting for magmatic processes, is demonstrated to be + 3.7‰ to + 6.3‰,
significantly higher than the range for typical asthenosphere. This indicates that sulfur,
which here serves as a proxy for wider enrichment, is of a recycled origin, delivered
to the mantle from the Earth’s surface in the past. The work presented in this thesis
facilitates critical evaluation of the origin of magmatism at this enigmatic intraplate
province, bringing us closer to the ultimate goal of a full understanding of intraplate
magmatic systems and their complex origins
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