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Sardinia (Italy) represents one of the most comprehensive cross-sections of the Variscan orogen. The metamorphic and structural complexity characterizing its axial zone still presents many unresolved issues in the current state of knowledge. The data presented from the structural study of the entire axial zone of this area have allowed the authors to propose a subdivision into two new structural complexes. In particular, a younger complex is identified as the New Gneiss Complex, containing remnants of an older and higher-grade metamorphic complex defined as the Old Gneiss Complex. The structural and geometric relationships between the two complexes suggest the redefinition of the axial zone of Sardinia as part of the intracontinental East Variscan Shear Zone/medium-temperature Regional Mylonitic Complex. Comparable relationships are also highlighted in many other areas of the Variscan chain (e.g., Morocco, Corsica, Maures Massif, and Argentera Massif). Extending this new structural interpretation to other axial zones of the South European Variscan orogen could provide new hints for reconstructing the collision boundaries between Gondwana and Laurussia in the late Carboniferous to the early Permian periods.

In the Western Alps, a steeply dipping km-scale shear zone (the Ferriere-Mollières shear zone) cross-cuts Variscan migmatites in the Argentera-Mercantour External Crystalline Massif. Structural analysis joined with kinematic vorticity and finite strain analyses allowed to recognize a high-temperature deformation associated with dextral transpression characterized by a variation in the percentage of pure shear and simple shear along a deformation gradient. U–Th–Pb dating of syn-kinematic monazites was performed on mylonites. The oldest ~ 340 Ma ages were obtained in protomylonites, whereas ages of ~ 320 Ma were found in mylonites from the core of the shear zone. These ages indicate that the Ferriere-Mollières shear zone is a still preserved Variscan shear zone. Ages of ~ 320 Ma obtained in this work are in agreement with ages of the dextral transpressional shear zones occurring in the Maures-Tanneron Massif and Corsica-Sardinia. However, transpression in the Argentera-Mercantour Massif started earlier than in other sectors of the southern Variscan Belt. This is possibly caused by the curvature of the belt triggering the progressive migration of shear deformation. Our data allow a correlation between the Argentera-Mercantour Massif and other segments of the Southern European Variscan Belt, in particular with Maures-Tanneron Massif and Corsica-Sardinia, and contribute to fill a gap in the age of activity and in the kinematics of the flow of the system of dextral shear zones of the southern portion of the EVSZ.

This work presents an integrated structural, kinematic, and petrochronological study of the Monte Grighini dome within the Variscan hinterland–foreland transition zone of Sardinia (Italy). The area is characterised by dextral transpressive deformation partitioned into low- and high-strain zones (Monte Grighini shear zone, MGSZ). Geothermobarometry of one sample of sillimanite-bearing mylonitic metapelite indicates that the Monte Grighini shear zone developed under high-temperature (~ 625 °C) and low-pressure (~ 0.4–0.6 GPa) conditions. In situ U–(Th)–Pb monazite geochronology reveals that the deformation in the shear zone initiated at ca. 315 Ma. Although previous studies have interpreted the Monte Grighini shear zone to have formed in a transtensional regime, our structural and kinematic results integrated with constraints on the relative timing of deformation indicate that it shows similarities with other dextral ductile transpressive shear zones in the Southern European Variscan belt (i.e., the East Variscan Shear Zone, EVSZ). However, dextral transpression in the Monte Grighini shear zone started later than in other portions of the EVSZ within the framework of the Southern European Variscan Belt due to the progressive migration and rejuvenation of deformation from the core to the external sectors of the belt. Graphical abstract

The Tan-Lu fault zone (TLFZ) along the East China continental margin (ECCM) experienced a change from sinistral to normal faulting in the late Mesozoic. Thirty-four laser ablation (LA)-ICPMS zircon U-Pb dates for plutons and volcanic rocks along the TLFZ indicate that extension-related magmatism started as early as 136 Ma. The development of pre-eruption rift basins along the TLFZ during the earliest Early Cretaceous further constrains the onset time of the Tan-Lu normal faulting to the beginning of Early Cretaceous (ca. 145 Ma). Association of extensive rifts, metamorphic core complexes, and magmatism along the margin with the Tan-Lu normal faulting suggests an Early Cretaceous extensional regime for the ECCM that also started at the beginning of the Early Cretaceous, about 145 Ma. An undeformed granite dike that intrudes the sinistral ductile shear zone yields an LA-ICPMS zircon U-Pb age of 122 Ma. Seven 40Ar/39Ar plateau ages of mica samples from mylonites in the Tan-Lu sinistral ductile shear zone range from 129.5±0.8 to 101.8±0.6 Ma, and these are considered to represent cooling ages related to later normal faulting. A white mica 40Ar/39Ar plateau age of 149.8±0.9 Ma is interpreted as the cooling age of sinistral faulting. It is suggested that the sinistral faulting took place before 150 Ma (Late Jurassic), rather than in the Early Cretaceous, as previously proposed. The Tan-Lu sinistral faulting developed under a transpressive regime along the ECCM during the Late Jurassic. It is inferred that the switch from Late Jurassic transpression to Early Cretaceous extension is due to a shift from oblique, shallow subduction of the Izanagi Plate to orthogonal, steep subduction of the Pacific Plate.

Despite extensive research on fault rocks, and on their commercial importance, there is no non-genetic classification of fault breccias that can easily be applied in the field. The present criterion for recognizing fault breccia as having no ‘primary cohesion’ is often difficult to assess. Instead we propose that fault breccia should be defined, as with sedimentary breccia, primarily by grain size: with at least 30% of its volume comprising clasts at least 2 mm in diameter. To subdivide fault breccias, we advocate the use of textural terms borrowed from the cave-collapse literature – crackle, mosaic and chaotic breccia – with bounds at 75% and 60% clast content. A secondary breccia discriminant, more difficult to apply in the field, is the ratio of cement to matrix between the clasts. Clast-size issues concerning fault gouge, cataclasite and mylonite are also discussed.

Brittle reactivation of plastic shear zones is frequently observed in geologically old terranes. To better understand such deformation zones, we have studied the >700 Ma long structural history of the Himdalen–Ørje Deformation Zone (HØDZ) in SE Norway by K–Ar and 40Ar–39Ar geochronology, and structural characterization. Several generations of mylonites make up the ductile part of HØDZ, the Ørje Shear Zone. A 40Ar–39Ar white mica plateau age of 908.6 ± 7.0 Ma constrains the timing of extensional reactivation of the Ørje mylonite. The mylonite is extensively reworked during brittle deformation events by the Himdalen Fault. 40Ar–39Ar plateau ages of 375.0 ± 22.7 Ma and 351.7 ± 4.4 Ma from pseudotachylite veins and K–Ar ages of authigenic illite in fault gouge at c. 380 Ma are interpreted to date initial brittle deformation, possibly associated with the Variscan orogeny. Major brittle deformation during the Early–Mid Permian Oslo Rift is documented by a 40Ar–39Ar pseudotachylite plateau age of 294.6 ± 5.2 Ma and a K–Ar fault gouge age of c. 270 Ma. The last datable faulting event is constrained by the finest size fraction in three separate gouges at c. 200 Ma. The study demonstrates that multiple geologically significant K–Ar ages can be constrained from fault gouges within the same fault core by combining careful field sampling, structural characterization, detailed mineralogy and illite crystallinity analysis. We suggest that initial localization of brittle strain along plastic shear zones is controlled by mechanical anisotropy of parallel-oriented, throughgoing phyllosilicate-rich foliation planes within the mylonitic fabric.

Outcrop-scale structures and magnetic fabric anisotropy of the Bomdila Gneiss (BG) that intruded the Lesser Himalayan Crystallines (LHC) of the Arunachal Lesser Himalaya are studied to understand the BG deformation history and tectonic evolution. Detailed analysis of structures reveals that the LHC have undergone three phases of deformation, D1, D2 and D3. The S2 foliation developed during the second phase of deformation (D2) is the most penetrative planar fabric in the studied rock, which shows a general ENE–WSW strike with moderate NW dip. Mesoscopic evidence of a later phase of deformation (D3) in the BG is lacking. Evidence of D3 deformation in the form of F3 folds is only observed in the adjacent metasedimentary rocks of the LHC. The magnetic foliations recorded from anisotropy of magnetic susceptibility (AMS) analysis of the BG are mostly striking NW–SE with a moderate dip towards the NE or SW, and magnetic lineation is mostly sub-horizontal and dominantly plunging towards the SE. Our study shows that the magnetic fabric of the BG does not correspond to any visible outcrop-scale mesoscale foliation. However, the magnetic foliation of the BG is parallel to the axial plane of the F3 folds of the adjacent metasedimentary rocks of the LHC. Integration of AMS and outcrop-scale structural analysis helps us envisage the superposed deformation history of the BG. Our study emphasizes the importance of AMS to detect late-stage or feeble deformation events that leave no visible outcrop-scale imprint and are difficult to discern through conventional geological means.

Structural analysis, petrochronology and metamorphic petrology enable identification and bracketing of the timing of a newly mapped high-temperature ductile shear zone (Jagat Shear Zone (JSZ)) in the Himalayan metamorphic core in Central-Western Nepal. In situ U-Th-Pb monazite petrochronology constrains the timing of top-to-the-S/SW shearing between 28-27 Ma and 17 Ma. Burial and prograde metamorphisms in footwall rocks were linked to thrust-sense movement along the JSZ, while the hanging wall rocks were retrogressed and exhumed. The identification and age of the JSZ (as part of a regional system of shear zones: the High Himalayan Discontinuity (HHD)) coupled with the localization and timing of activity of the Main Central Thrust (MCT) (i) fills a gap in tracing the HHD along orogenic strike, (ii) supports the identification of the position and timing of the long-debated MCT and (iii) helps to place the boundaries of the Himalayan metamorphic core and its internal architecture. Thus, our study is a significant step towards a precise identification of the burial, assembly and exhumation mechanisms of the Himalayan metamorphic core.

The Etam granite-gneissic complex is located in western part of the Tombel graben in the Central African Fold Belt in Cameroon. It includes plutonic rocks (coarse-grained granite, fine-grained granite and biotite granite) partially mylonitised and a metamorphic basement constituting gneiss and migmatites. Granites are syn- to post-tectonic and S-type, ferroan and strongly peraluminous. The high Rb/Sr (0.92–1.46) and low Sr/Ba (0.18–0.26) ratios coupled with an important negative Eu (Eu/Eu* = 0.37–0.59) anomaly characterize the lower degrees of crustal melting upon their formation accompanied with the dehydration of hydrous minerals as biotite. A CaO/Na2O ratio greater than 0.3 indicates that granites from Etam were derived from clay-poor, plagioclase-rich psammitic source material. Etam granites are peraluminous and display high potassic and alkali content. These characteristics are in line with the continental collision setting. The evidence of heterogeneous deformation in these rocks suggest syn to late collisional setting. The estimated crystallization temperature is resolved to be between 750 and 875 °C for Etam granites. Mylonites exhibit granitic composition with high SiO2 and Al2O3 contents similar to granites. Coarse-grained granite and mylonites show only little changes in magmatic minerals content. However, the grain size varies, decreasing from the coarse-grained granite to mylonite. The gneiss and mylonite plot in the igneous field in discrimination diagram. These characters suggest the igneous protoliths that are probably the surrounding coarse-grained granite for mylonite and an earlier intrusion for gneiss.

A blueschist-facies mylonite crops out between two high-pressure tectono-metamorphic oceanic units of the Ligurian Western Alps (NW Italy). This mylonitic metabasite is made up of alternating layers with different grain size and proportions of blueschist-facies minerals. The mylonitic foliation formed at metamorphic conditions of T = 220–310 °C and P = 6.5–10 kbar. The mylonite shows various superposed structures: (i) intrafoliar and similar folds; (ii) chocolate-tablet foliation boudinage; (iii) veins; (iv) breccia. The occurrence of comparable mineral assemblages along the foliation, in boudin necks, in veins and in breccia cement suggests that the transition from ductile deformation (folds) to brittle deformation (veining and breccia), passing through a brittle–ductile regime (foliation boudinage), occurred gradually, without a substantial change in mineral assemblage and therefore in the overall P–T metamorphic conditions (blueschist-facies). A strong fluid–rock interaction was associated with all the deformative events affecting the rock: the mylonite shows an enrichment in incompatible elements (i.e. As and Sb), suggesting an input of fluids, released by adjacent high-pressure metasedimentary rocks, during ductile deformation. The following fracturing was probably enhanced by brittle instabilities arising from strain and pore-fluid pressure partitioning between adjacent domains, without further external fluid input. Fluids were therefore fixed inside the rock during mylonitization and later released into a dense fracture mesh that allowed them to migrate through the mylonitic horizon close to the plate interface. We finally propose that the fracture mesh might represent the field evidence of past episodic tremors or ‘slow earthquakes’ triggered by high pore-fluid pressure.