Thermo-mechanical behavior of large ash-flow calderas
Résumé
The fundamental thermal and mechanical processes that occur within the "ash flow caldera-magma chamber" systems remain largely enigmatic. To date, the only models of caldera collapse are simple, mostly elastic or viscoelastic mechanical models that can predict some of the conditions preceding the collapse. They cannot, however, predict the collapse itself because they are incapable either of reproducing the formation of faults or of accounting for the brittle-ductile transitions and fault-related thermal anomalies. We have constructed analytical and numerical therrnomechanical models that account for both elastic-plastic-ductile rheology and physical properties of the caldera rocks. The overpressured magma is evacuated through a central vent and transforms into ash flow units deposited within the forming caldera. Brittle deformation, faulting, and subsequent collapse of the structure are reproduced. The results show that in the absence of a regional stress field the collapse on both sides will occur only for aspect ratios (i.e., caldera diameter to the depth of the magma chamber) exceeding 5 and that internal embedded faults may also appear when the aspect ratio exceeds 10. Thermal conductivity contrasts in ash flow calderas give rise to strong heat refraction that localizes deep seated thermal anomalies to the outer sides of the faults. In the presence of regional extension the border faulting can be attenuated or disappear, and the faults tend to localize around the central part of the chamber roof. Coupled therrnomechanical modeling suggests that the outer sides of the border faults have high trapping capabilities for hydrothermal fluids. The geometry of the brittle-ductile transition largely controls that of the fractured zones within and around the chamber roof, thus justifying a new "mechanical definition" of magma chamber geometry.
Domaines
Sciences de la Terre
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