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Supplementary material 1: Isotopic data

Supplementary material 2: Grain size analysis

Fractured and karstified carbonate units are key exploration targets for the hydrocarbon industry as they represent important reservoirs. Furthermore, large water reserves and geothermal systems are hosted in carbonate aquifers. This paper documents the relationships between stratigraphy, structural patterns, silicification, and the spatial-morphological organization of a 3D multistorey cave system developed in a Neoproterozoic mixed carbonate-siliciclastic sequence. We found that the combination of lithology, silicification, fracture patterns (controlled by lithostratigraphic variability), and petrophysical properties control the formation of high or low permeability zones; their distribution was fundamental for the spatial organization of dissolution and the compartmentalization of the resulting conduit system in different speleogenetic storeys. We propose a deep-seated hydrothermal origin for the fluids involved in the main phases of karst formation. Warm and alkaline hydrothermal fluids caused silica dissolution, followed by chalcedony and quartz reprecipitation in pore space and fractures. Rising fluids concentrated along through-going vertical fracture zones in the lower storey, whereas sub-horizontal bedding-parallel fluid flow was focused on sedimentary packages containing highly silicified dolostones (SiO2>80 wt%) characterized by high permeability. The Calixto Cave is an enlightening example for the complex speleogenetic history affecting a mixed carbonate-siliciclastic succession where the combined effect of silicification and hydrothermal karst dissolution can potentially generate high-quality reservoirs.

The main issue addressed in this work is the process leading to fluid subsurface entrapment and pressure increase up to hydrofracturing and, possibly, to paroxysm in a hydrothermal setting, in order to envisage such processes and mitigate their effects in the volcanically active study area and elsewhere. A field and laboratory multidisciplinary approach is used in the fossil (late Pleistocene) portion of an active hydrothermal system (Colli Albani volcano, Rome, Italy). In this area, sulfate and sulfide mineralizations and strongly altered ignimbrites are exposed. The alteration acme occurs on top of a buried normal fault, where abundant degassing is still active, and fades away in 2–3 km. Based on pervasive versus discrete alteration styles, mineral assemblages, and further evidence, proximal and distal alteration domains are recognized. Both domains underwent steam-heated advanced argillic alteration with likely temperatures up to ~400 °C in the proximal domain and less than 150 °C in the distal domain. The process of hydrothermal alteration progressively and severely depleted many elements from the most permeable rock units, whereas the lowest-permeable unit (Tufo Lionato) underwent fracture and porosity healing accompanied by both mass and volume gain. In the proximal domain, the advanced argillic hydrothermal alteration eventually formed a substantial barrier to fluids. The hydrothermal fluids accumulated in and below this barrier, which was then suddenly hydrofractured when heat-driven hydraulic pressure overcame the effective stress, thus possibly leading to hydrothermal paroxysm. The decompression associated with hydrofracturing enhanced gas exsolution and mineral precipitation from the entrapped overpressured fluids. Mineral precipitation contributed, in turn, to fracture healing and to reinitiation of a new cycle of hydrothermal fluid entrapment. The key preconditions for the occurrence of the inferred processes are the contrasting compositions of K-alkaline host rocks and acidic alteration fluids, as also previously documented in other similar settings elsewhere.

In this paper we report a synthesis of the structural data collected during the WISE campaign on north Victoria Land, Antarctica. These data are integrated with the available apatite fission track data from the neighbour regions in order to place constraints to the kinematics and chronology of the brittle deformation along the western shoulder of the Wilkes Basin. Brittle faulting is consistent with the regional Cenozoic tectonic pattern of north Victoria Land and respond to the increasing phases of denudation that accompanied extension/transtension of the Rennick Graben shoulders since 40-50 Ma. The discovery of pseudotachylyte-bearing fault rocks at the Pacific termination of the Rennick Graben is also reported. The dating of the pseudotachylyte-bearing fault zone opens new perspectives for the reconstruction of the brittle tectonic evolution of the region. Results coming from these studies with the analyses of the new samples collected for the fission track dating may generate permissive kinematic and geodynamic models for the Cenozoic tectonic evolution of the entire northern Victoria Land region.