Subsidence of ash-flow calderas: relation to caldera size and magma-chamber geometry

Abstract

Diverse subsidence geometries and collapse processes for ash-flow calderas are inferred to reflect varying sizes, roof geometries, and depths of the source magma chambers, in combination with prior volcanic and regional tectonic influences. Based largely on a review of features at eroded pre-Quaternary calderas, a continuum of geometries and subsidence styles is inferred to exist, in both island-arc and continental settings, between small funnel calderas and larger plate (piston) subsidences bounded by arcuate faults. Within most ring-fault calderas, the subsided block is variably disrupted, due to differential movement during ash-flow eruptions and postcollapse magmatism, but highly chaotic piecemeal subsidence appears to be uncommon for large-diameter calderas. Small-scale downsag structures and accompanying extensional fractures develop along margins of most calderas during early stages of subsidence, but downsag is dominant only at calderas that have not subsided deeply. Calderas that are loci for multicyclic ash-flow eruption and subsidence cycles have the most complex internal structures. Large calderas have flared inner topographic walls due to landsliding of unstable slopes, and the resulting slide debris can constitute large proportions of caldera fill. Because the slide debris is concentrated near caldera walls, models from geophysical data can suggest a funnel geometry, even for large plate-subsidence calderas bounded by ring faults. Simple geometric models indicate that many large calderas have subsided 3-5 km, greater than the depth of most naturally exposed sections of intracaldera deposits. Many ring-fault platesubsidence calderas and intrusive ring complexes have been recognized in the western U.S., Japan, and elsewhere, but no well-documented examples of exposed eroded calderas have large-scale funnel geometry or chaotically disrupted caldera floors. Reported ignimbrite "shields" in the central Andes, where large-volume ash-flows are inferred to have erupted without caldera collapse, seem alternatively interpretable as more conventional calderas that were filled to overflow by younger lavas and tuffs. Some exposed subcaldera intrusions provide insights concerning subsidence processes, but such intrusions may continue to evolve in volume, roof geometry, depth, and composition after formation of associated calderas.

Nuevos antecedentes sobre la estructura interna de la caldera La Pacana mediante un estudio gravimétrico (Andes centrales, Chile).

“…4). The possible collapse geometries (Lipman, 1997;Cole et al, 2005) arising from the modeled ignimbrite infill are described in the discussion section. We speculate that the intracaldera ignimbrite infill in the southern part of the caldera is made up of several ignimbrites besides Atana (see discussion section), but due to the lack of density contrast we can not determine the boundary between the different volcanic deposits in the gravity model.…”

“…The ignimbrite infill is modeled with respect to the described topographic margin, limited by normal faults according to analog models (Acocella, 2007;Hardy, 2008;Martí et al, 2008) and with their sense of slip inferred from these models (Fig. 4), therefore more complex structures such as the caldera collapse collar (Lipman, 1997) are not taken into account Although andersonian normal faults are expected to dip ∼60º, the modeled dips are shallower and steeper along the interpreted structure. Finally, the forward model shows that the roof of shallow intrusive bodies is located at ∼1.5-2 km below the surface and the ignimbrite sheets.…”

“…These assumptions do not hold for the APVC calderas (Salisbury et al, 2011). For La Pacana caldera, the previous work has presented different erupted volumes, based on the assumption of different geometries for the caldera inner structure (Lipman, 1997). Gardeweg and Ramírez (1987) estimated the erupted volume to be ∼900 km 3 , while Lindsay et al (2001a) estimated ∼2,500 km 3 .…”

“…The large size of these eruptions (volumes larger than 450 km 3 and a Volcanic Explosivity Index (VEI) greater than 8) has attracted a growing global interest to understand the evolution and eruptive dynamics of the biggest calderas (Wilson, 2008), by studying their emitted products and the formation of their structure and geometry (Sparks et al, 2005;Acocella, 2007;Martí et al, 2008;Hardy, 2008). Due to the lack of observations of eruptions of this size, the caldera dynamics and the climatic impact of these supereruptions, including the effects over the human population, are poorly understood (e.g., Cole et al, 2005;Lipman, 1997;Self, 2006;Jones et al, 2007;Self and Blake, 2008). The largest ignimbrite f lareups in the world (e.g., Taupo volcanic zone, New Zealand; San Juan mountains, SW United States; Altiplano Puna Volcanic Complex APVC, southern central Andes of Bolivia, Chile and Argentina) are the places where the largest calderas in the world are located (Lipman, 2007;Rowland et al, 2010;De Silva, 1989a;De Silva et al, 2006;De Silva and Gosnold, 2007) and then constitute ideal study cases to investigate the dynamics of these supervolcanoes.…”

Introduction: from the British Tertiary into the future – modern perspectives on the British Palaeogene and North Atlantic Igneous provinces

“…Although it was long thought that the evolution of collapse caldera volcanoes begins with a largemagnitude uplift ('tumescence') of country rocks upon initial intrusion of a large pre-caldera magma body, evidence for such pre-collapse uplift has been elusive or absent in recent surveys of a range of volcanoes. This has sown doubt as to whether pre-caldera tumescence Introduction is important enough to be recorded in significant geological structures (Lipman, 1997). However, the Northern Marginal Zone of the Rum Central Complex has been shown to contain exceptional evidence for intrusioninduced uplift prior to caldera collapse (Troll, Emeleus & Donaldson, 2000).…”