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Thèse Année : 2008

Kinematic, deformation and lava emplacement

Cinématique, déformation et mise en place des laves

Résumé

Kinematics and deformation in lava flows are addressed using three different approaches: descriptive volcanology, petrophysics and magnetism. Pāhoehoe and 'a'ā are two terms used in volcanology, first used to differentiate lava surfaces, but then to describe two different emplacement styles. Pāhoehoe lavas emplace themselves as a lobe stacks that may coalesce. In contrast, 'a'ā lavas emplace en masse and form upper clinker by rupture crust of their. This crust can reach the flow base by roll-over caterpillar motion. As they have different modes of emplacement, they contain in their interiors several diagnostic features that make it possible to discriminate the type in old lava flows where surfaces are missing or covered. 'A'ā flows present clinker and highly deformed vesicles, whereas pāheohoe flows display slightly deformed vesicles, lobe stacks and particular zonations in vesicles and phenocrysts. These discriminatory characteristics allowed the attribution of lava types to several old lava flows. This has provided constraints on emplacement histories, particularly in the French Massif Central where some pahoehoe flows have been clearly identified. Breccia associated with lava flows has been studied in detail, both in the field and in thin section. Detailed textural analysis, especially with scanning electron microscope (SEM) has revealed petrophysical information on breccia formation. These breccias include clinker (previously considered to be exclusive to 'a'ā lavas), fault cataclasites and peperites. The upper clinker are typical of 'a'ā flows, but lower clinker-like breccia can also develop in pāhoehoe flows according phreatomagmatism, or basal / lateral shearing. Peperites have phreatomagmatic features with more explosive textural patterns than clinkers. These breccia studies have revealed that phreatomagmatism is quite common during lava flow emplacement: the probability that a lava flow on its downslope movement encounters a watersaturated basement is quite high. This indicates that phreatomagmatic hazards associated with lava flows could be locally very significant. A methodology for the correct use of Anisotropy of Magnetic Susceptibility (AMS) is proposed. This requires the determination of the AMS carriers and the AMS fabric time acquisition compared to the microlite fabric. This methodology has been applied to two lava flows to check the feasibility. The results show that AMS can be used confidently, with the methodology proposed. The results obtained are interpreted in terms of different emplacement scenarios. Interpretations include the establishment of a vertical qualitative velocity profile and the detailed reconstruction of the last emplacement stage of two lava flows. A further improvement in understanding lava flow emplacement, with attendant hazard and aquifer knowledge improvement will be possible by continuing the investigation on the three topics on a broader range of lava flows, different compositions and environments.
La cinématique et la déformation des laves ont été traitées selon trois approches : volcanologie descriptive, pétrophysique et magnétisme. Les laves sont classées, selon leurs modes de mise en place, en aa ou pahoehoe. Elles contiennent dans leurs intérieurs différents critères qui permettent de classer les laves anciennes comme celles du Massif Central, afin de contraindre leurs mises en place. L'analyse texturale (MEB) des brèches associées aux laves a révélé des informations pétrophysiques sur leurs formations. Les clinkers sommitaux sont typiques des coulées aa alors que les clinkers basaux peuvent se développer dans des coulées pahoehoe par phréatomagmatisme, comme les pépérites, ou par cisaillement basal. Une méthodologie AMS est proposée et appliquée à deux coulées de lave. Les résultats obtenus sont interprétés en terme de scénarios de mise en place incluant l'établissement d'un profil qualitatif de vitesse et la reconstruction des derniers mouvements de la lave.
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Dates et versions

tel-00730519 , version 1 (10-09-2012)

Identifiants

  • HAL Id : tel-00730519 , version 1

Citer

Sébastien Loock. Cinématique, déformation et mise en place des laves. Sciences de la Terre. Université Blaise Pascal - Clermont-Ferrand II, 2008. Français. ⟨NNT : 2008CLF21849⟩. ⟨tel-00730519⟩
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