Analogue models of collapse calderas and resurgent domes

Collapse calderas and resurgent domes are a common association related to i nflation-deflation processes in volcanic systems. The structure of calderas and domes depends upon the permitted, relative movements of crustal volume s at depth (the so-called "space problem"). In order to study the structure s of collapse calderas and resurgent domes and to take the space problem in to account, several analogue models were made. Dry-quartz sand was used to simulate the rheology of the brittle crust, while newtonian silicone putty, located at the base of the sand-pack, simulated the ductile behaviour of t he magma. A piston moved the silicone putty downward or upward, inducing co llapse and doming within the sand. Three separate sets of experiments simul ated: (1) caldera collapse; (2) resurgence; and (3) superimposition of resu rgence on collapse and Vice versa. Collapse experiments are characterized b y the development of two concentric depressions; the first-formed depressio n is bordered by outward dipping reverse ring faults; the subsequent, outer concentric depression is bordered by inward dipping normal ring faults. Th e deformation pattern during resurgence is a function of the overburden thi ckness (T) and the dome diameter (D). For higher T/D ratios a dome forms, b ordered by inward dipping high angle reverse ring faults; outward dipping n ormal ring faults develop at late stages. For lower T/D ratios, the dome sh ows, at late stages, a crestal depression accompanied by radial fractures; subsequently, an apical extrusion of silicone occurs. The superimposition o f resurgence over collapse land vice versa) is characterized by the complet e reactivation, with opposite kinematics, of all the pre-existing ring faul ts during inversion. Both in caldera and resurgence, reverse ring faults fo rm in the early stages due to differential uplift; extensional structures s ubsequently form to accommodate gravitational collapse during the activity of the reverse faults. The experiments and the overall similarities with na ture suggest that the activity of both reverse and normal faults constitute s a possible solution to the space problem during major collapses or resurg ences. (C) 2000 Elsevier Science B.V. All rights reserved.