Rheomorphic diapirs in densely welded ignimbrites: The Serra di Paringianu ignimbrite of Sardinia, Italy

Abstract

Rheomorphic structures in welded ignimbrites are commonly associated with deposition from hot pyroclastic flows on inclined topography or by tractional shear on aggrading agglutinate by the pyroclastic density current. We describe a type of rheomorphic deformation involving the formation of diapirs in ignimbrite following the re-equilibration of internal gravitational instabilities within the deposit. We show that diapirs can develop in horizontal welded ignimbrites that have an inverted density stratification. The interpretation of these structures in terms of their dynamics and kinematics can help in defining the timing of the deformation history of ignimbrites.The rhyolitic Serra di Paringianu Ignimbrite is the uppermost, largest ignimbrite of Cenozoic volcanism in SW Sardinia. A detailed study at La Punta, north of San Pietro island (Sardinia; Italy), schematically characterized by a twin sequence of densely welded and partially welded ignimbrite flow units of the Serra di Paringianu Ignimbrite, revealed the presence of mushroom and pillow-shaped diapiric structures, constituted by material of an intermediate, partially welded, flow unit intruding into an upper, densely welded, unit. The diapirs are generally connected with their source region and show both vertical and lateral variations in the textural and physical features. The uppermost densely welded unit is deformed within a few meters of the diapiric structures, with the foliation pattern of the surrounding ignimbrite enveloping the cup region of diapirs.We used the existing numerical models of cooling–compaction of welded ignimbrites to constrain the time scale for the development of specific rheomorphic structures. We present a model in which the diapirism involves the buoyant rise of partially welded lithofacies into the densely welded cover in a time scale of a few months. The diapir model takes into account the rheology of the host and intruding layers and explains the textural and physical modifications both experienced by the partially welded lithofacies during rise and induced in the host layers. The model shows that the time scale of the diapiric rise well agrees with the time scale of the cooling process of the upper portion of the ignimbrite.