Resurgent Calderas: analogue models and field data.

Abstract

Caidera collapse and resurgence are common phenomena in several volcanic areas of the world. In order to better understand these phenomena, in the last decades, analogue models have been carried out by many authors using variable methodologies. Almost all the experiments did not take into consideration the effects of the regional stress field on caldera resurgence, a role which has been well analyzed recently by Marotta (2001) and Acocella et alii (2004). Caldera resurgence in areas ìndeformed (Acoceila et alii 2000) or previously deformed (Marotta 2001 Acocelia el alii 2004) by enensional tectonism has been investigated through analogue modelling. In these experiments, according to scaiing laws, dry-quartz sand and Newtonian silicone putty bave been used to simulate the brittie behaviour of the Fiarth’s crust and the ductue behaviour of the magma. respecrively. The resuits of the experiments, performed lo simulate only resurgence, show mainly a dome-like intumescence. Such a behaviour has been described for many large calderas in Japan, United States and New Guinea. In these calderas the deformation generates a radial symmetry and a dome-like intumescence (resurgent dome) of the central portion of the caldera floor (Smith and Bailey 1968). The results of the experiments, performed to simulate caldera resurgence in a simple-graben structure, suggest that resurgence always occurs through formation of a discrete number of differently displaced blocks. The deformation generates a compressional stress regime within one portion of the resurgent block and an extensional stress regìme along the opposite portion, facilitating magma intrusion. This behaviour has been recogrnzed at the medium-size resurgent calderas of Ischia, Campi Flegrei and Pantelleria. At each of these structures, caldera collapse and resurgence depend also upon the characteristics of both structural setting and regional stress field, and occur through the formation of a discrete number of differentially displaced blocks (Orsi et alii 1991, 1996, 1999).