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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.
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