Page 318 - Caldera Volcanism Analysis, Modelling and Response

Structural Development of Calderas 293

inward dipping normal faults form. Inward tilted wedges at surface are bordered by
the reverse and normal faults. Both the normal and reverse faults nucleate at the
periphery of the silicone layer, along the point of maximum curvature of the sand-
silicone interface. Subsidence usually occurs asymmetrically. This set of experiments
may explain the development of coherent piston-like calderas delimited by reverse
faults, whereas the gravity-driven normal faults are interpreted to border an outer
zone, source of large landslides. For high aspect ratios (Z1, type B), multiple sets of
reverse faults propagate upwards concentrically, forming a series of nested cones
(Figure 4c). Normal faults, if present, are restricted to the periphery of the upper
cone, where the reverse faults reach surface. Accordingly with the authors, this set
of experiments may explain the development of funnel-like calderas with minor
explosive activity.
Acocella et al. (2000) use Newtonian silicone putty (same type as Roche et al.,
2000) sinking below a sand-pack through a descending piston (Figure 5a). The
5
length ratio is L B10 and the experiments last a few hours. Various shapes of the
sand-silicone interface (flat, symmetric dome, asymmetric dome) are considered.
All the experiments have roof aspect ratios falling in the A type of Roche et al.
(2000). The evolution of collapse is consistent with that observed by Roche et al.
(2000). Two sets of concentric ring faults form, the former being reverse and the
latter normal. Before the propagation at surface of each fault, the surface undergoes
a diffuse deformation with an inward tilt at surface. When faulting reaches
the surface, this turns into localised deformation. While the reverse faults are the
straightforward consequence of differential uplift (Sanford, 1959; Mandl, 1988), the
normal faults form as a gravitational response to the instability given by the slip
along the reverse faults. The activity of both the reverse and normal faults may solve
the room problem during large collapses (Figure 5b). The same structures also form
during the reactivation of the pre-collapse doming structures. Significant
asymmetric depressions (trapdoor calderas) are not common, unlike Roche et al.
(2000) and form only imposing asymmetric domes along the sand-silicone
interface. These silicone domes influence the position of nucleation of the faults,
which show differential slip and generate asymmetric collapses (Acocella et al.,
2001). The development of two concentric depressions may explain pairs of nested
calderas in nature (Acocella et al., 2001).

Figure 5 Experiments from Acocella et al., 2000. (a) Adopted apparatus; (b) evolution of an
experimental caldera (modi¢ed after Acocella et al., 2000).