Abstract. The dynamic evolution of fault zones at the seismogenic brittle–ductile transition zone (BDTZ) expresses the delicate interplay between numerous physical and chemical processes. Deformation and fluid flow at the BDTZ are closely related and mutually dependent during repeating and transient cycles of frictional and viscous deformation. Despite numerous studies documenting in detail seismogenic faults exhumed from the BDTZ, uncertainties remain as to the exact role of fluids in facilitating broadly coeval brittle and ductile deformation at that structural level. We combine structural analysis, fluid inclusion, and mineral chemistry data from synkinematic and authigenic minerals to reconstruct the temporal variations in fluid pressure (Pf), temperature (T), and bulk composition (X) of the fluids that mediated deformation and steered strain localization along BFZ300, a strike–slip fault originally active at the BDTZ. BFZ300 deforms the Paleoproterozoic migmatitic basement of southwestern Finland and hosts in its core two laterally continuous quartz veins formed by two texturally distinct types of quartz – Qtz I and Qtz II, with Qtz I older than Qtz II. Veins within the damage zone are formed exclusively by Qtz I. Mesostructural and microstructural analysis combined with fluid compositional data indicate recurrent cycles of mutually overprinting brittle and ductile deformation triggered by oscillations of fluid pressure peaking at 210 MPa. Fluid inclusion microthermometry and mineral pair geothermometry indicate that the two documented quartz types precipitated from different fluid batches, with bulk salinities in the 1 wt % NaCleq–5 wt % NaCleq range for Qtz I and in the 6 wt % NaCleq–11 wt % NaCleq range for Qtz II. The temperature of the fluids involved with initial strain localization and later fault reactivation evolved through time from > 350 ∘C during Qtz I precipitation to < 300 ∘C at the time of Qtz II crystallization. The peak fluid pressure estimates constrain pore pressure oscillations between 80 and 210 MPa during the recorded faulting episodes. Our results suggest variability of the physico-chemical conditions of the fluids steering deformation (Pf, T, X), reflecting the ingress and effects of multiple batches of fluid in the fault zone. Initial fluid-mediated embrittlement generated a diffuse network of joints and/or hybrid–shear fractures in the damage zone; subsequent strain localization led to more localized deformation within the fault core. Localization was guided by cyclically increasing fluid pressure and transient embrittlement of a system that was otherwise under overall ductile conditions. Our analysis suggests that fluid overpressure at the BDTZ can play a key role in the initial embrittlement of the deforming rock and steer subsequent strain localization.

Abstract. The microstructural record of fault rocks active at the brittle–ductile transition zone (BDTZ) may retain information on the rheological parameters driving the switch in deformation mode and on the role of stress and fluid pressure in controlling different fault slip behaviours. In this study we analysed the deformation microstructures of the strike-slip fault zone BFZ045 in Olkiluoto (SW Finland), located in the site of a deep geological repository for nuclear waste. We combined microstructural analysis, electron backscatter diffraction (EBSD), and mineral chemistry data to reconstruct the variations in pressure, temperature, fluid pressure, and differential stress that mediated deformation and strain localization along BFZ045 across the BDTZ. BFZ045 exhibits a mixed ductile–brittle deformation, with a narrow (<20 cm thick) brittle fault core with cataclasites and pseudotachylytes that overprint a wider (60–100 cm thick) quartz-rich mylonite. Mylonitic deformation took place at 400–500 ∘C and 3–4 kbar, typical of the greenschist facies metamorphism at the base of the seismogenic crust. We used the recrystallized grain size piezometry for quartz to document a progressive increase in differential stress, from ca. 50 to ca. 120 MPa, towards the shear zone centre during mylonitization and strain localization. Syn-kinematic quartz veins formed along the mylonitic foliation due to transiently high pore fluid pressure (up to lithostatic value). The overprint of the veins by dynamic recrystallization and mylonitic creep is further evidence of the occurrence of brittle events under overall ductile conditions. We propose a conceptual model in which the ductile–brittle deformation cycle was controlled by transient oscillations in fluid pressure and progressively higher differential stress, possibly occurring in a narrowing shear zone deforming towards the peak strength of the crust at the BDTZ.

The Etruscan site of Pyrgi extended over 10 ha along the Tyrrhenian coastline. Fieldwork has so far brought to light the sanctuary and a ceremonial complex at the southern edge of the settlement, whereas additional evidence is provided by a section cut by sea erosion across the site. The archaeological layer is about 2 m thick and includes dry masonry buildings arranged into lots; the most superficial features are disturbed by deep agricultural activities and the top layer includes scattered building material (limestone and tuff blocks). At this site, the efficacy of geophysical prospections are generally affected by the nearby presence of the beach's magnetic sand and the railway line. The use of dense ground penetrating radar (GPR) data proved to be the most suitable method in the mapping of buried archeological structures and made it possible to map out the topographic layout of the area which is currently under excavation. The dense three‐dimensional georadar (GPR) data highlight the presence of structures implanted at different topographic levels and provide preliminary information about the main building interventions and planimetric modifications inside the urban lots. The work was extremely useful for the future planning of the archaeological excavation as it makes it possible to reduce the costs and environmental impact of the archaeological activity. The georadar anomalies were confirmed by an archaeological test‐trench conducted in the surveyed area.

Si esaminano le diverpologie degli altari scavati o individuati nei due santuari di Pyrgi, sottolineando le peculiarità dell'area di culto C nel santuario monumentale e le diverse funzioni e tipologie dei numerosi altari portati in luce nel santuario meridionale. E' presentato il probabile altare rinvenuto a Nord del santuario monumentale nell'ultima campagna (2012). La sua presenza amplia notevolmente verso nord l'area di pertinenza del Santuario monumentale

Tourmaline nodules occurring in the Capo Bianco (Elba Island, Italy) aplitic rocks are here investigated by X‑ray microtomography 3D imaging. This non-invasive technique provides 3D images of the tourmaline nodules, revealing an irregular morphology consisting of branches that extend radially from the cores. The nodules present scale-invariant features that can be described by a box-counting fractal dimension. The value of the fractal dimension is proportional to the size of the nodules and tends asymptotically to a value of 2.5, in agreement with the results obtained from the simulation of virtual nodules, by means of a diffusion-limited aggregation model based on a Monte Carlo Metropolis algorithm, in which the growth probability at the tips of the nodule is an inverse function of the diffusion coefficient. The results support the hypothesis that tourmaline formed by a disequilibrium magmatic process, in which diffusion represents the rate-limiting step, inducing the formation of nodules with irregular shapes. This study shows the potential of X‑ray microtomography, in combination with numerical modeling, as a probe for accessing the 3D microstructural information of complex mineral morphologies with a non-invasive approach. The combination of numerical and experimental, non-invasive, 3D techniques represents a fundamental step forward in bridging the gap between the observation of microstructures and the interpretation of the associated processes