The Maures-Tanneron Massif and the Corsica-Sardinia Block are two segments of the southern European Variscan belt that separated during the Late Oligocene-Miocene due to the opening of the Western Mediterranean basin. Correlation between the two regions, based mainly on petrologic similarities, is still debated. However, there are no detailed structural and petrochronological constraints on their potential relationships. In northern Sardinia there is well-documented evidence for a dextral transpressive shear zone that initiated after the first stage of frontal collision. In the Maures-Tanneron Massif, despite recognition of an important episode of transpressive deformation, it is still unclear which structures were active during this tectonic regime. We investigate in detail the kinematic of flow, finite strain and the timing of the deformation of the Cavalaire "Fault" (CF), a major ductile shear zone in the Maures-Tanneron Massif. In contrast to previous models, we argue that the CF is a transpressive shear zone characterized by a prevalent component of pure shear, while in-situ monazite geochronology reveals that the CF is initiated at ~ 323 Ma. The new data presented here, based on a multidisciplinary approach document, for the first time, the vorticity of the flow, finite strain and timing of this sector of the East Variscan Shear Zone, a regional-scale shear zone that characterized the Southern European Variscan belt during the late Carboniferous.

Detailed geological field mapping, integrated with meso- and microstructural investigations, kinematic of the flow and finite strain analyses, combined with geochronology, are fundamental tools to obtain information on the temperature–deformation–timing path of crystalline rocks and shear zone. The Posada-Asinara shear zone (PASZ) in northern Sardinia (Italy) is a steeply dipping km-thick transpressive shear zone. In the study area, located in the Baronie region (NE Sardinia), the presence of mylonites within the PASZ, affecting high- and medium-grade metamorphic rocks, provides an opportunity to quantify finite strain and kinematic vorticity. The main structures of the study area are controlled by a D2 deformation phase, linked to the PASZ activity, in which the strain is partitioned into folds and shear zone domains. Applying two independent vorticity methods, we detected an important variation in the percentage of pure shear and simple shear along the deformation gradient, that increases from south to north. We constrained, for the first time in this sector, the timing of the transpressive deformation by U–(Th)–Pb analysis on monazite. Results indicate that the shear zone has been active at ~325–300 Ma in a transpressive setting, in agreement with the ages of the other dextral transpressive shear zones in the southern Variscan belt.

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In recent decades, constraining the timing of shear activity has been one of the main topics of research about the tectono-metamorphic evolution of orogenic belts. We present a review of a combined structural and geochronological approach to two major ductile regional shear zones, in two collisional orogens: the first one affecting the Variscan basement in northern Sardinia (Italy) and the External Crystalline Massifs of the Alps (East Variscan Shear Zone; EVSZ), and the second one deforming the medium- to high-grade rocks of the metamorphic core of the Himalaya (High Himalayan Discontinuity). High-resolution, texturally and chemically controlled monazite geochronology applied in separated shear zones of the Variscan belt allowed recognizing a similar timing of activity ranging between c. 340–330 and 300 Ma. This approach led to a better understanding of the evolution of the EVSZ, supporting a model where several branches were active according to a growth by linkage model. Following a similar approach, in situ U-Th-Pb analysis of monazite constrained the timing of top-to-the-S/SW shearing of a regional-scale High Himalayan Discontinuity in the Himalayan belt to between c. 28 Ma and 17 Ma. Earlier exhumation of the hanging wall was triggered by shear zone activity, whereas at the same time, the footwall was still experiencing burial with increasing P-T conditions. The timing of shearing of this shear zone fits with an in-sequence shearing tectonic model for the exhumation of the Himalayan mid-crust.