Rheomorphic structures in a high-grade ignimbrite: The Nuraxi tuff, Sulcis volcanic district (SW Sardinia, Italy)

Deposits of the 15.8 Ma Nuraxi explosive eruption crop out in the Sulcis volcanic district, southwestern Sardinia. They consist of a decimeter-thick pumice fallout layer overlain, with no apparent temporal break, by several tens of meters thick, high-grade ignimbrite. The eruption was from a vent probably located ∼20-30 km north of S. Pietro island, and it tapped a compositionally uniform, rhyolite magma reservoir (SiO2 70-72 wt.%) bearing about 20 vol.% crystals. The ignimbrite consists of: (a) a Lower Ignimbrite (LI), <2-m-thick, fine-grained and glassy grading upward into a partly devitrified lapilli-tuff, and (b) an Upper Ignimbrite (UI) that is an intensely welded to lava-like lapilli-tuff. The deposit has an inferred maximum runout of about 80 km and overall dispersal/thickness characteristic of a low aspect-ratio ignimbrite. Over most of its outcrop the tuff rests on a flat, horizontal paleosurface and forms an ignimbrite plateau with uniform thickness and structural characteristics. However, in some areas it shows extensive folding and over-thickening as a result of rheomorphic flow into topographic depressions. Structural analysis of the ignimbrite reveals the presence of a pervasive foliation and lineation as well as macroscopic and microscopic flow structures. Occurrence of abundant, widely distributed stretched cavities in the tuff indicates that a substantial amount of gas was trapped in the tuff during aggradation. The observed pattern of structures of the ignimbrite plateau accords well with a conceptual model in which syn-depositional structures are formed during non-particulate, basal laminar flow due to agglutination of juvenile particles within the lower part of the pyroclastic density current. The flow was controlled by the substantial transfer of momentum from the high-speed, particulate upper part of the current to the top of the non-particulate part of the current and the onset of a complex relationship between welding, flow and sedimentation processes. It is proposed in particular that elongated gas pockets played a major role in guiding the laminar shear within the non-particulate part of the current in flat, horizontal areas, eventually promoting brittle failure of the deforming tuff mass along shear planes.