Timescale of Emplacement and Rheomorphism of the Green Tuff Ignimbrite (Pantelleria, Italy)

We present a multidisciplinary study based on Differential Scanning Calorimetry (DSC), paleomagnetic analysis, and numerical modeling to gain information on the timescales of synand post-depositional ductile deformation of the strongly welded and rheomorphic Green Tuff ignimbrite (GT; Pantelleria, Italy). DSC measurements allow the determination of glass fictive temperatures (T-f; i.e., the parameter accounting for the cooling dependence of glass structure and properties). Using a T-f-based geospeedometry procedure, we infer the cooling rate (q(c)) experienced by the glassy phases in different lithofacies within the GT formation. Glass shards from the basal pumice fall deposit record a fast q(c) of similar to 10 degrees C/s. In contrast, the ignimbrite body returns slow q(c) values depending on the stratigraphic position and lithofacies (basal/upper vitrophyres, fiamme-rich and rheomorphic layers), ranging from similar to 10 (-2) to similar to 10(-6) degrees C/s. Moreover, paleomagnetic analyses of the natural remanent magnetization of ignimbrite matrix and embedded lithic clasts indicate an emplacement temperature higher than 550-600 degrees C. By integrating calorimetric and paleomagnetic datasets, we constrain a conductive cooling model, describing the ignimbrite's temperature-time-viscosity (T-t-eta) evolution from the eruptive temperature to below T-f. Outcomes suggest that the upper and basal vitrophyres deformed and quenched over hours, indicating that the entire GT underwent intense syn-depositional ductile deformation. Furthermore, the central body remained above T-f for a much longer timespan (>1 month), enabling post-emplacement rheomorphic flow. Lastly, we discuss the critical role of mechanisms such as shear heating and retrograde solubility of volatiles, in locally controlling the rheological behavior of the GT.Plain Language Summary Pyroclastic density currents (PDCs), one of the most dangerous occurrences caused by volcanic eruptions, are hot mixtures of ash, gas, and rocks that travel rapidly and propagate around the volcanoes. In some conditions, favored by the high temperature of the erupted material, the associated deposit (ignimbrites) may continue to deform viscously during and after deposition, experiencing welding (sintering and compaction of pyroclasts) and rheomorphism (formation of pervasive lava-like structures). Studying rheomorphic ignimbrites can improve our knowledge of the hazard posed by the emplacement of PDCs. Here we constrain the timescales at which this type of ductile deformation occurs, focusing on the pantelleritic Green Tuff ignimbrite emplaced during the 45 ky eruption at Pantelleria Island (Italy). Distinct experimental approaches (i.e., calorimetry and paleomagnetism) allow constraining a model to describe the cooling history of this deposit. We define different deformations regimes experienced by the ignimbrite, highlighting that intense viscous deformation occurred at timescales like those of deposition (hours), and rheomorphic flow can also continue for a longer time (months) after the PDC deposition. Finally, we discuss the possible causes that lead to the formation of the observed rheomorphic structures.