Depositional and welding processes in low aspect ratio ignimbrites: examples from the Sulcis Volcanic District(Sardinia, Italy)

The rheomorphic, high-grade, welded ignimbrites are a special type of pyroclastic density current (PDC) deposits usually associated with high intensity volcanic explosive activity (VEI >4). They are characterized by a high variability of physical features and sedimentological structures that may testify different emplacement mechanisms from a PDC and a different response to topography during and after the end of the depositional processes. When the temperatures of the deposits are higher than the glass transition temperature (Tg) and the deposits stay hot for long periods, they can undergo important local readjustments. The purpose of this PhD thesis is to investigate the processes which control the deposition and deformation of rhyolitic to comenditic welded ignimbrites, how and when the different processes occurred and which factors influenced the formation of the main structures. The Sulcis Volcanic District (SW of Sardinia - Italy) is a good field laboratory where sixteen, well exposed, well conserved and not affected by recent tectonic processes, welded ignimbrites crop out. All these features permit to study and analyze the main processes, despite their Miocene age. The research focalized its attention on three ignimbritic units: Punta Senoglio Unit, Ventrischio Unit and Monte Ulmus Unit. In particular, attention was pointed on the Monte Ulmus Ignimbrite (MUI), where all these processes are particularly well recorded along the numerous available outcrops . A systematic and detailed study of the deposits was carried out. The description at different scales of the main features of the deposits and the analysis of the products with different methodologies, permitted to recognize the main eruptive phases and the main processes driving the formation of the ignimbrite, together with their relative timing. The deposits of all the three studied ignimbrites show evidences of syn- and post-emplacement welding and rheomorphism. The analysis of the micro-structural features on thin sections clear evidences an upward variations in strain rate, suggesting deposition varying from gradual to step-wise aggradation. The MUI well preserves primary structures typical of high grade ignimbrites (foliations, lineations, welding and facies variations), structures typical of rheomorphic ignimbrites (rotated solid fragments, different scale folds, ramps, etc) and structures related with cooling and degassing processes (blisters, degassing pipes, devitrification facies). The MUI is a high-grade, rhyolitic to trachytic, locally rheomorphic, low-aspect-ratio welded ignimbrite. It crops out mainly in Sant’Antioco and San Pietro islands and in the Sulcis, on an area of 300 km2 with a minimum estimated volume of 2.5 km3. The most likely position of the source area is in the northern sector, few kilometers N of the San Pietro Island. Four main pyroclastic flow units (in the sense of Smith 1960) that cooled together, named A0, A1, A2 and B and preceded by a basal fall-out deposit (F), have been distinguished. In addition, seven main lithofacies were recognized inside the MUI, reflecting the different flow regimes of PDC (from turbulent to laminar) and the different depositional processes (from gradual to step-wise aggradation). All the described lithofacies are suggestive of welding and deformation processes are syn- and post emplacement. MUI physical features are strongly related to the paleotopographic conditions (topographic highs, topographic lows or flat conditions). Locally, the deposits record quick syn-emplacement en-masse readjustment, also on gently dipping slopes (~10°), probably due to generally low viscosity of the products or to the large amount of gas entrapped inside the deposits. The low values of Tg estimated for the MUI products ( ~415 °C) possibly resulted in protracted, nearly plastic conditions of the deposits, enhancing the remobilization of the deposits also during the final phases of emplacement. The first, turbulent, steady flows were characterized by high depositional rates and the related deposits were emplaced by gradual aggradation. The flows became progressively laminar with the increase of the run-out, and different traction carpets were emplaced en-masse by step-wise aggradation. The second half of eruption was characterized by an unsteady mass flow, possibly related with a caldera-forming phases. The deposits grew up by quick gradual aggradation from laminar flows. Welding processes of the upper portion of MUI are mainly post-emplacement and they were driven principally by lithostatic load.