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Structural Development of Calderas 301
collapsed area at surface coincides with the most emptied area within the reservoir
(Kennedy et al., 2004).
4.2. Towards a consistent experimental model
The consistency among the experiments simulating underpressure, under different
boundary conditions, permits to propose a comprehensive model describing the
evolution of experimental collapses. This can be adequately summarised through
four main stages, representing discrete moments within an evolutionary continuum
controlled by the amount of subsidence (Figure 10; Acocella, 2007).
Stage 1 is characterised by a broad depression, with inward tilted margins, at the
surface. This usually occurs with a subsidence of very few mm (in a caldera a few
cm wide), corresponding to several tens to few hundreds of m in nature. At the
same time, the upward propagation of the reverse ring fault starts at depth; as long as
the fault remains buried, the diffuse strain forms the inward tilt at surface. At this
stage, an experimental downsag caldera is formed (Figure 10a).
At Stage 2, the reverse ring fault reaches surface, replacing the downsag. This
usually occurs with a subsidence of several mm (in a caldera a few cm wide),
corresponding to several hundreds of m in nature. The structural boundary of the
caldera is completely defined, exhibiting a clear rim or scarp, above the reverse ring
fault. While the reverse fault is always outward dipping, the caldera rim above, due
to rapid decay of the overhanging reverse fault scarp, is subvertical or inward
dipping and may be located in an outer position. Additional reverse ring faults may
be present within the rim, accordingly with the above-mentioned mode of outward
incremental growth (Kennedy et al., 2004). At this stage, the basic structure of the
experimental caldera resembles a piston-type (with lower aspect ratios of the caldera
roof) or funnel-type (with higher aspect ratios) (Figure 10b; Roche et al., 2000).
Stage 3 results from the further increase in subsidence, usually in the order of
B1 cm (in a caldera a few cm wide), corresponding to B1 km in nature. This stage
develops an inward tilt on the outer periphery of the reverse ring fault. This is the
surface accommodation of the incipient, upward-propagating outer normal ring
fault. At this stage, a peripheral downsag forms (Figure 10c).
Figure 10 Schematic representation of the four stages of evolution of caldera collapse,
obtained in all the experiments, as a function of the amount of subsidence.