Time-space relationships of deformative structures in welded ignimbrites : the diapiric structures of the “Serra di Paringianu” Ignimbrite (SW Sardinia, Italy)

The rheology of ignimbrites during welding and compaction has been matter of interest in recent years for the ability of welded ignimbrites to develop a wide variety of rheomorphic structures. The existence of numerical models able to reproduce the cooling-compaction history of welded ignimbrites allows to constrain the time scale for the development of specific rheomorphic structures and to test existing viscosity models. The Serra di Paringianu rhyolitic ignimbrite represents the youngest major ignimbrite of the Cenozoic volcanism of SW Sardinia. It widely crops out on San Pietro and Sant’Antioco islands, while scattered outcrops are present in the Sulcis mainland. A detailed study of La Punta section, north of San Pietro Island, schematically characterized by a twin sequence of densely welded and partially welded ignimbrite units, reveals the presence of mushroom-shaped diapiric structures constituted by the partially welded intermediate unit intruding the upper densely welded cover.