Analogue models of collapse calderas and resurgent domes V. Acocella * , F. Cifelli, R. Funiciello Dipartimento di Scienze Geologiche, Universita degli Studi Roma TRE, Largo San Leonardo Murialdo 1, 00146 Rome, Italy Received 9 December 1999; revised 10 April 2000; accepted 10 April 2000 Abstract Collapse calderas and resurgent domes are a common association related to in¯ation±de¯ation processes in volcanic systems. The structure of calderas and domes depends upon the permitted, relative movements of crustal volumes at depth (the so-called ªspace problemº). In order to study the structures of collapse calderas and resurgent domes and to take the space problem into 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 the magma. A piston moved the silicone putty downward or upward, inducing collapse and doming within the sand. Three separate sets of experiments simulated: (1) caldera collapse; (2) resurgence; and (3) superimposition of resurgence on collapse and vice versa. Collapse experiments are characterized by the development of two concentric depressions; the ®rst-formed depression is bordered by outward dipping reverse ring faults; the subsequent, outer concentric depression is bordered by inward dipping normal ring faults. The deformation pattern during resurgence is a function of the overburden thickness (T ) and the dome diameter (D). For higher T/D ratios a dome forms, bordered by inward dipping high angle reverse ring faults; outward dipping normal ring faults develop at late stages. For lower T/D ratios, the dome shows, at late stages, a crestal depression accompanied by radial fractures; subsequently, an apical extrusion of silicone occurs. The superimposition of resurgence over collapse (and vice versa) is characterized by the complete reactivation, with opposite kinematics, of all the pre-existing ring faults during inversion. Both in caldera and resurgence, reverse ring faults form in the early stages due to differential uplift; extensional structures subsequently form to accommodate gravitational collapse during the activity of the reverse faults. The experiments and the overall similarities with nature suggest that the activity of both reverse and normal faults constitutes a possible solution to the space problem during major collapses or resurgences. q 2000 Elsevier Science B.V. All rights reserved. Keywords: caldera; resurgence; analogue models; ring faults 1. Introduction Collapse calderas and resurgent domes are common structural features in volcanic systems (Fig. 1). The development of calderas and domes has been classi- cally interpreted in the light of an evolution character- ized by: (1) regional tumescence and generation of ring fractures; (2) major eruption and caldera collapse; (3) intra-caldera resurgent doming (Smith and Bailey, 1968; Henry and Price, 1984; Newhall and Dzurisin, 1988). Calderas and domes are commonly related to in¯ation±de¯ation processes inside magma chambers (Smith and Bailey, 1968; Lipman, 1984; Newhall and Dzurisin, 1988; Lipman, 1997); however, the precise modalities under which these structures form are still debated. Piston-like collapse calderas, related to the removal of magma from the chamber (Williams, 1941; Simkin Journal of Volcanology and Geothermal Research 104 (2000) 81±96 0377-0273/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S0377-0273(00)00201-8 www.elsevier.nl/locate/volgeores * Corresponding author. Tel.: 139-6-548-88082; fax: 139-6- 548-88201. E-mail address: acocella@uniroma3.it (V. Acocella).