Subsidence of ash-flow calderas: relation to caldera size and magma-chamber geometry |
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| Authors: | Peter W Lipman |
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| Institution: | (1) U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, USA e-mail: plipman@mojave.wr.usgs.gov, US |
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| 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
plate-subsidence 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.
Received: 13 February 1997 / Accepted: 9 August 1997 |
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| Keywords: | Volcanoes Caldera geometry Caldera subsidence Ash-flow tuff |
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