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Progressive growth of the Cerro Bayo cryptodome, Chachahuén volcano, Argentina – implications for viscous magma emplacement
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
Y-TEC – CONICET. Av. Del Petroleo s/n - (Entre 129 y 143), Berisso (CP 1923), Buenos Aires, Argentina .
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.ORCID iD: 0000-0003-0717-4014
Physics of Geological Processes, The NJORD Centre, Department of Geosciences, University of Oslo, Box 1047, Blindern, 0316 Oslo, Norway.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Cryptodome and dome collapse is associated with volcanic hazards, such as, explosive eruptions, pyroclastic flows, and volcanic edifice collapse. Study of the growth and evolution of volcanic domes provides vital information on the link between dome growth and the development of weakness zones that may cause collapse. The Cerro Bayo cryptodome is superbly exposed in the eroded Miocene Chachahuén volcano in the Neuquén basin, Argentina. Cerro Bayo is a >0.3 km3 trachyandesitic cryptodome that intruded within the uppermost kilometer of the Chachahuén volcano. Here we investigate the emplacement of the Cerro Bayo cryptodome using structural mapping, photogrammetry, 3D structural modelling and measurement of magma flow indicators, brittle deformation features and magnetic fabrics with anisotropy of magnetic susceptibility (AMS). Magma flow fabrics near the margin are concentric and indicate contact-parallel flow and internal inflation of the body. Magmatic and magnetic fabrics and fracture patters in the interior of the cryptodome are more complex and outline several structural domains. These domains are separated by magmatic shear zones that accommodated intrusion growth. The shear zones locally overprint the earlier formed concentric fabric. The nature of the structural domains shows that emplacement of Cerro Bayo occurred in three stages that resemble the endogenous to exogenous growth of volcanic domes. The formation of magmatic shear zones during cryptodome formation may have a profound effect on cryptodome stability by creating weakness zones that increase the risk of collapse.

Keywords [en]
Cryptodome, Intrusion emplacement, AMS, magmatic breccia, magma flow, volcanic hazards, laccolith, dome collapse, viscous magma
National Category
Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
URN: urn:nbn:se:uu:diva-363407OAI: oai:DiVA.org:uu-363407DiVA, id: diva2:1256757
Available from: 2018-10-18 Created: 2018-10-18 Last updated: 2018-10-19
In thesis
1. The dynamic emplacement of felsic magma in the upper crust
Open this publication in new window or tab >>The dynamic emplacement of felsic magma in the upper crust
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Felsic magma intrudes earth’s upper crust through a variety of mechanisms. Magma intrusion growth and shape have mainly been explained in terms of host rock properties and intrusion depth, while considering the magma as an overpressurised fluid. However, volcanologists view a magma as a rheologically evolving fluid, which affects the magma flow in volcanic conduits. This thesis seeks to explore intrusion dynamics during magma emplacement by taking both the magma and the host rock into account. The first part of the thesis investigates the emplacement of the Sandfell laccolith/cryptodome, the Cerro Bayo cryptodome and the Mourne granite pluton. Both cryptodomes grew initially by inflation, which resulted in contact-parallel magma flow. Later during the emplacement, the rim of the intrusions viscously stalled as indicated by brecciation and fracturing in the intrusion rims, which then forced them to grow vertically. Our observations suggest that rheological changes in the magma during intrusion growth may control the shape of the cryptodomes/laccoliths. Previously proposed emplacement mechanisms of the Mourne Mountains granite pluton were tested by investigating host-rock deformation and the surrounding contact-metamorphic aureole. The aureole displays contact-metamorphic segregations that were later deformed by brecciation and shearing. The consistent regional fracture patterns in the pluton roof indicate that it was not widely domed, while the north-eastern wall of the pluton was deflected parallel to the strike of the contact. These observations suggest that multiple mechanisms emplaced the pluton, involving both floor subsidence and deflection of the roof and wall.

The last part of the thesis studies the magma plumbing system to the Holuhraun 2014-15 eruption with mineral and whole-rock geochemistry and thermobarometry. The Holuhraun eruption was accompanied by subsidence in the Bárðarbunga caldera but occurred in the Askja volcanic system. Our results show that the Holuhraun eruption was fed from a vertically extensive magma plumbing system in the Bárðarbunga volcanic system.

The works of this thesis highlight that felsic magma emplacement in the upper crust involves multiple and dynamic mechanisms that control the growth and shape of the intrusion and that the interplay between magma and host-rock properties needs to be considered.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 69
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1736
Keywords
laccolith, cryptodome, magma emplacement, magma flow, magma rheology, granite, rhyolite, pluton, AMS, XCT, thermobarometry
National Category
Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-363445 (URN)978-91-513-0483-0 (ISBN)
Public defence
2018-12-07, Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 09:00 (English)
Opponent
Supervisors
Available from: 2018-11-13 Created: 2018-10-19 Last updated: 2018-11-30Bibliographically approved

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