Wilson, Penelope Irene Rose (2015) Direct linking of host rock deformation structures to the emplacement, morphology and accommodation of high-level igneous intrusions : the Henry Mountains, Utah. (PhD thesis), Kingston University, .
Abstract
Most studies of magmatic intrusions concentrate on geometry and internal architecture: only a few pay particular attention to emplacement-related deformation structures in the host rock that record how magma is accommodated within the crust. This research aims to develop a greater understanding of how igneous intrusive bodies are emplaced and accommodated within the shallow crust, using classic exposures found in the Henry Mountains, Utah. Two satellite intrusions to the Mt Hillers intrusive centre show highly contrasting geometries, host-rock deformation and accommodation structures and apparent emplacement mechanisms. Trachyte Mesa, the most distal satellite intrusion of Mt Hillers, has a relatively simple elongate (NW-SE) geometry, concordant with the Entrada Sandstone it intrudes. The intrusion is comprised of multiple, stacked intrusive sheets. Syn-emplacement deformation structures observed in the host rocks consist of a conjugate set of intrusion margin-parallel deformation bands and extensional brittle faults, the latter occurring at the tips of intrusive sheets. These structures, along with a post¬emplacement set of intrusion margin parallel and perpendicular tensile joints, indicate extensional strain normal to the intrusion margin, consistent with a two-stage growth mechanism for individual sheets as well as the overall intrusion. In comparison, Maiden Creek shows a more complex intrusion geometry, including: lobate morphologies; steps and broken bridges; inclined sill sheets; and concave-upwards 'Iaccolithic' morphologies. A new model is proposed for the emplacement, evolution and final geometry of the intrusion, with a central elongate NE-SW lobe resulting from a principal north-easterly propagating magma flow. Lateral growth of the Maiden Creek intrusion resulted from radial spreading of magma from this main north-easterly flow trend towards the east and north-west. It is proposed that the southern Maiden Creek intrusion is comprised of two westerly-derived (saucer-shaped?) sills. Overlying these deeper-rooted sills is the newly identified Maiden Creek Shear Zone (MCSZ). This structure is an antithetic accommodation structure to magma flow. Substantial amounts of strain observed through microstructural analysis of shear zone samples suggest that the MCSZ played a critical role in accommodating magma emplacement. This study suggests that much can be learnt about intrusion geometries and emplacement mechanisms through detailed structural and kinematic analyses of the host rocks and intrusion-host rock contact.
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