Analogue models of collapse calderas and resurgent domes

Abstract

Collapse calderas and resurgent domes are a common association related to inflation–deflation processes in volcanic systems. The structure of calderas and domes depends upon the permitted, relative movements of crustal volumes at depth (the so-called “space problem”). 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 first-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.