Thermomechanical behavior of large ash flow calderas

Abstract

Abstract. 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.

Stress Field Control during Large Caldera-Forming Eruptions

“…However, we would like to remark that here we focus on the syn-eruptive dynamics of magma ascent inside the shallow dyke conduit during LCFEs and not on magma chamber dynamics and mechanics able to trigger such eruptions that are likely related to magma chamber roof failure and have been explored by other authors (e.g., Burov and Guillou-Frottier, 1999;Folch and Martí, 2004;Gray and Monaghan, 2004;Gregg et al, 2012Gregg et al, , 2015Geyer and Martí, 2014). For the purposes of this study, aimed at describing the first-order features only, chamber and reservoir shapes are not crucial and for simplicity, as in Costa et al (2011), we assume them as elongated bodies with circular cross-sections.…”

The Neapolitan Yellow Tuff caldera offshore the Campi Flegrei: Stratal architecture and kinematic reconstruction during the last 15ky

“…Smith and Bailey, 1968;Lipman, 1984Lipman, , 1997Lipman, , 2000Walker, 1984;Cole et al, 2005) which allow identification of the caldera-forming products and the resulting caldera structure. A significant advance in the understanding of caldera formation has been made in the last decades by the application of mathematical, finite element and scaled analogue models (Druitt and Sparks, 1984;Burov and Guillou-Frottier, 1999;Acocella et al, 2000Acocella et al, , 2001Roche et al, 2000).…”

The possible contribution of circumferential fault intrusion to caldera resurgence

“…These conflicting factors may lead to the development of thick ring and arcuate dyke complexes often exposed at deeply eroded calderas. Burov and Guillou-Frottier (1999) find that ring faults are more conductive than the surrounding rocks, 'channelizing' part of the heat flow. Repeated intrusion into the base of the ring fracture zone, together with occasional eruptions due to high overpressure or melt mobilisation following more basic injections into or along the margin of the chamber, may contribute to the high heat gradient.…”

“…Circumferential faults are usually related to caldera formation and may be initiated in a number of ways (e.g. Anderson 1937;Marti et al 1994;Chadwick and Dieterich 1995;Branney 1995;Gudmundsson 1998;Burov and Guillou-Frottier 1999;Roche et al 2000;Acocella et al 2000;Guillou-Frottier et al 2000;Walter and Troll 2001;Troll et al 2002;Stix et al 2003). Traditionally, circumferential faults are referred to as ring faults when substantially complete and circular or ovoid in plan view.…”

“…The term bounding faults has been applied when a subsided block departs significantly from a simple circular or elliptical shape in plan view. The term peripheral faults is used for all those fractures away from the centre of a caldera which are circumferential (the border faults of Burov and Guillou-Frottier 1999) or radial and encompasses faults both inside and outside the collapse faults. In this paper, the term ring fault is used where the traditional term is appropriate, the term circumferential faults is used to describe any short arcuate fault or fault zone, and the term bounding faults is used for any significantly non-annular fracture system that surrounds a caldera block.…”