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Fracture patterns in cooling intrusions

Ellis-Evans, Jennifer 2019. Fracture patterns in cooling intrusions. PhD Thesis, Cardiff University.
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Understanding fracture patterns in intrusions has multiple industrial applications where knowledge of geotechnical stability or sub-surface fluid flow is required. This study advances the understanding of the influence of thermal stress on the evolution of early fractures, important as they are likely to influence early fluid flow and later fracturing events, in cooling plutonic bodies. This study involves numerical modelling, field work and analogue modelling studies. Finite element modelling in COMSOL® is used to predict thermal stresses that result from the cooling of hot intrusive bodies within a cooler crust. The models extend existing two-dimensional approaches into the third dimension. Models predict that stress magnitudes can cause fracturing and the minimum principal stress direction may change during cooling. The change in principal stress orientation is influenced by the geometry and depth of the modelled intrusive body and may result in multiple fracture orientations forming. Analogue modelling, using drying corn-starch patties, produces both radial and tangential fractures, which supports the idea that multiple fracture orientations may result from the cooling of intrusive bodies. Numerical models are applied to the Alta stock and Bingham Canyon copper porphyry deposit in Utah. Fracture orientations inferred from principal stresses and failure criterion are comparable to the field data. Application of the Griffith failure criterion to models of the Alta stock predicts that shear and hybrid failure conditions always precede tensile failure. High fluid pressure is the only parameter that may alter the sequence of failure, allowing tensile failure to occur independently of shear or hybrid failure. Hybrid and tensile failure-modes are inferred from the field observations. Application of the models at Bingham Canyon, a system in which high fluid pressures are likely, highlights that thermal stress may have controlled early vein orientations, providing an explanation for the spatial and temporal variation of early formed fractures.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Earth and Environmental Sciences
Date of First Compliant Deposit: 18 March 2020
Date of Acceptance: 18 March 2020
Last Modified: 11 Dec 2020 02:29

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