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Strain localization is crucial in developing crustal‐scale shear zones; however, a systematic analysis of deformation characteristics and their impact on seismic properties is still lacking. This study provides a comprehensive analysis of migmatites and granitic mylonites in the Ailaoshan‐Red River shear zone (ASRR‐SZ), incorporating detailed field observations, microstructure analysis, mineral crystallographic preferred orientations, rheological parameters, and seismic properties. Pre‐existing compositional and mechanical anisotropies significantly influence strain localization in the ASRR‐SZ. The southern part of the ASRR‐SZ is primarily characterized by crustal anatexis, suggesting these regions as potential initiation sites for strain localization. Strain characteristics in the ASRR‐SZ manifest in deformation temperatures (three ranges, ∼400–440°C, ∼470–500°C, and ∼730°C), differential stress (concentrated at 15.9–65.1 MPa), and strain rate (concentrated at 10−13–10−11 s−1). Notably, strain localization significantly alters the rock fabric and further affects the seismic properties of rocks. Significant differences in Vp values and orientations are noted between melanosomes (anisotropy of P‐waves: AVp = 6.8%–17.9%, Max. Vp along the X‐axis) and leucosomes (AVp = 3.4%–3.7%, Max. Vp along the Y‐axis). The seismic velocities and AVp in granitic mylonites exhibit a linear correlation with quartz content, and deformation conditions strongly influence their orientation. For a middle to lower crust thickness of ∼25 km, the delay times between fast and slow polarized shear waves are 0.3–0.66 s for granitic mylonites, 0.37–0.7 s for melanosomes, 0.22–0.31 s for leucosomes, 0.27–0.58 s for amphibolites, and 0.57–2.7 s for schists. The average delay time (dt) of these rocks along the ASRR‐SZ accounts for the observed delay time (dt = 0.58 s). Plain Language Summary The Ailaoshan‐Red River shear zone (ASRR‐SZ) exemplifies a zone of strain localization that plays a critical role in accommodating the tectonic extrusion of the Indochina block. This study examines the rheological parameters and seismic properties of migmatites and granitic mylonites within the ASRR‐SZ. Results reveal deformation temperatures in the shear zone and quantitatively characterize strain localization through rheological parameters. Crustal anatexis modifies rock fabric, leading to an increase in seismic wave velocity anisotropy in melanosomes and a decrease in leucosomes. The seismic properties of granitic mylonites are influenced by mineral composition and deformation temperatures. This study compares the shear wave splitting delay times of different rock types and emphasizes that migmatites and granitic mylonites can produce significant delay times. These insights provide a valuable reference for interpreting geophysical observations. Key Points Inherited compositional and pre‐existing structural heterogeneity along the Ailaoshan‐Red River shear zone (ASRR‐SZ) significantly influence strain localization Crustal anatexis and ductile shearing within ASRR‐SZ substantially impact rock fabric and seismic properties The observed seismic anisotropy reflects the average delay times of various rock types in the heterogeneous ASRR‐SZ

Titanite U–Pb geochronology is a promising tool to date high-temperature tectonic processes, but the extent to and mechanisms by which recrystallization resets titanite U–Pb dates are poorly understood. This study combines titanite U–Pb dates, trace elements, zoning, and microstructures to directly date deformation and fluid-driven recrystallization along the Coast shear zone (BC, Canada). Twenty titanite grains from a deformed calc-silicate gneiss yield U–Pb dates that range from ~ 75 to 50 Ma. Dates between ~ 75 and 60 Ma represent metamorphic crystallization or inherited detrital cores, whereas ~ 60 and 50 Ma dates reflect localized, grain-scale processes that variably recrystallized the titanite. All the analyzed titanite grains show evidence of fluid-mediated dissolution–reprecipitation, particularly at grain rims, but lack evidence of thermally mediated volume diffusion at a metamorphic temperature of > 700 °C. The younger U–Pb dates are predominantly found in bent portions of grains or fluid-recrystallized rims. These features likely formed during ductile slip and associated fluid flow along the Coast shear zone, although it is unclear whether the dates represent 10 Myr of continuous recrystallization or incomplete resetting of the titanite U–Pb system during a punctuated metamorphic event. Correlations between dates and trace-element concentrations vary, indicating that the effects of dissolution–reprecipitation decoupled U–Pb dates from trace-element concentrations in some grains. These results demonstrate that U–Pb dates from bent titanite lattices and titanite subgrains may directly date crystal-plastic deformation, suggesting that deformation microstructures enhance fluid-mediated recrystallization, and emphasize the complexity of fluid and deformation processes within and among individual grains.

Recent crustal deformation observations have revealed a right-lateral strain-concentration zone trending ENE–WSW in southwest Japan: the San-in shear zone (SSZ). Numerous large earthquakes—such as the 1943 Tottori earthquake (M JMA 7.3), the 2000 Western Tottori earthquake (M JMA 7.3), and the 2016 Central Tottori earthquake (M JMA 6.6)—have occurred in the SSZ. Therefore, understanding the mechanisms of the SSZ is essential in investigating the processes responsible for generating such large earthquakes. In the San-in region, the geothermal gradient is large along the volcanic front where Quaternary volcanoes exist. This study models the development of the SSZ using a finite element method with nonlinear viscoelasticity and Mohr–Coulomb plasticity, considering a heterogeneous thermal structure and strike-slip stress regime. Based on the observed geothermal gradient distribution, we assume that temperature increases linearly with depth. Our model generates a shear zone within the high geothermal gradient zone along the volcanic front. Geological investigations indicate that faults in the San-in region are immature, suggesting that the accumulated shear strain in the SSZ is small. In our model, when the cumulative shear strain is small, on the west side of the SSZ, small fault zones are generated in the oblique direction to the strike of the SSZ in the areas of the local high thermal gradient. As the simulated SSZ develops, fault zones become connected along it. Observations also show that fault zones that are oblique to the SSZ are dominant on the west side of the SSZ. Simulation results indicate that the SSZ is an immature shear zone that has developed along the volcanic front. Our model also reproduces the fault zone for the 1943 Tottori earthquake on the east side of the SSZ. We also investigate the role of localized thermal anomalies in fault formation and find that conjugate faults are formed around regions with a local high geothermal gradient. Many major earthquake faults in the San-in region are thought to be influenced by rheological structures caused by thermal anomalies. Graphical Abstract

The South Indian Granulite Terrane is traversed by several crustal scale shear zones, however the tectonic significance of the shear zones are poorly understood. The tectonic relevance of the Bavali Shear Zone (BSZ) ‒ in the WNW extremity of the Moyar Shear Zone ‒ at the interface between the Paleoarchean to Neoarchean Western Dharwar Craton (WDC) in the north and the late Neoarchean Nilgiri block in the south is poorly constrained. The most conspicuous feature in the WDC is a set of N-striking gently-plunging upright folds and N-striking dextral shear zones (deformation D 4 ). These D 4 structures are superposed on a shallowly-dipping D 3 recumbent folds and gently-dipping mylonite fabrics in a suite of anatectic gneisses, lower-grade supracrustal rocks and foliated granitoids. In regional scale, the D 3 fold axes curve into the WNW-striking BSZ (D 5 deformation), a steep-dipping transpressional shear zone with dextral kinematics. The BSZ is characterized by steeply-plunging stretching lineations sub-parallel to the hinges of reclined folds on the pre-shearing fabrics in the lithologies of the adjacent cratons. Syn-D 5 charnockite veins suggest the BSZ formed at T > 850 °C. Existing U–Pb (zircon) dates and monazite chemical dates (this study), indicate that the deformation-metamorphism-magmatism in the WDC and the Nilgiri block occurred between 3400 and 2500 Ma; by contrast the high-T D 5 oblique crustal shortening in the BSZ contemporaneous with multiple felsic emplacements was active between 830 and 720 Ma. The BSZ collision orogeny possibly preceded the eventual integration of the Greater India landmass with the Gondwanaland during the early-Palaeozoic. Graphical abstract

We investigate the role of the Africa‐Eurasia convergence in the recent tectonic evolution of the central Mediterranean. To this end we focused on two sectors of the Adriatic‐Hyblean foreland of the Apennine‐Maghrebian chain as they allow tectonic evidence for relative plate motions to be analyzed aside from the masking effect of other more local tectonic phenomena (e.g., subduction, chain building, etc.). We present a thorough review of data and interpretations on two major shear zones cutting these foreland sectors: the E‐W Molise‐Gondola in central Adriatic and the N‐S Vizzini‐Scicli in southern Sicily. The selected foreland areas exhibit remarkable similarities, including an unexpectedly high level of seismicity and the presence of the investigated shear zones since the Mesozoic. We analyze the tectonic framework, active tectonics, and seismicity of each of the foreland areas, highlighting the evolution of the tectonic understanding. In both areas, we find that current strains at midcrustal levels seem to respond to the same far‐field force oriented NNW‐SSE to NW‐SE, similar to the orientation of the Africa‐Eurasia convergence. We conclude that this convergence plays a primary role in the seismotectonics of the central Mediterranean and is partly accommodated by the reactivation of large Mesozoic shear zones.

Symmetric structures in ductile shear zones range widely in shapes and geneses. Matrix rheology, its flow pattern, its competency contrast with the clast, degree of slip of the clast, shear intensity and its variation across shear zone and deformation temperature, and degree of confinement of clast in shear zones affects (independently) the degree of symmetry of objects. Kinematic vorticity number is one of the parameters that govern tail geometry across clasts. For example, symmetric and nearly straight tails develop if the clast–matrix system underwent dominantly a pure shear/compression. Prolonged deformation and concomitant recrystallization can significantly change the degree of symmetry of clasts. Angular relation between two shear zones or between a shear zone and anisotropy determines fundamentally the degree of symmetry of lozenges. Symmetry of boudinaged clasts too depends on competency contrast between the matrix and clast in some cases, and on the degrees of slip of inter-boudin surfaces and pure shear. Parasitic folds and post-tectonic veins are usually symmetric.

The Lepontine dome (European Central Alps) is constituted by basement nappes under Alpine regional Barrovian metamorphism, yet the duration and pattern of its post‐peak amphibolite‐facies cooling remain uncertain. We examine garnet‐rim compositional re‐adjustment and employ inverse multicomponent diffusion modeling to estimate apparent cooling rates. We selected six garnet paragneisses at different tectonic levels within the Lepontine nappe pile. Garnet crystal rims are syn‐kinematic with respect to the amphibolite‐facies metamorphic foliation and display coupled Mn increase and Mg decrease, indicative of retrograde reactions. Using geothermobarometry, we estimate re‐equilibration post‐peak temperatures between 577 and 661°C at pressures between 0.5 and 1.3 GPa. Our inferred cooling rates reveal a spatial trend: the shear zone below the main nappe (Maggia‐Adula) records rapid cooling with rates between 100 and 400°C/Myr, the footwall cools slower with a rate of 2°C/Myr, and the migmatitic belt in the southern Lepontine dome shows intermediate cooling rates between 20 and 50°C/Myr. We perform one‐dimensional (1D) thermal and two‐dimensional (2D) thermo‐kinematic models. The observed cooling rate distribution, characterized by high rates along the shear zone and significantly lower rates in the footwall, matches the predictions of 2D models where a hotter nappe is emplaced onto a colder one. Both 1D and 2D models suggest that high cooling rates above 600°C and above 0.5 GPa observed along the main shear zone are likely not caused solely by heat advection and diffusion associated with nappe stacking and exhumation. Local heat sources, such as the percolation of hot fluids or shear heating, may have contributed to high cooling rates.

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.

The Nyakong-Manyi Shear Zone (NMSZ) is a NE-SW elongated corridor found to the northwest of the Foumban-Bankim Shear Zone (FBSZ) along the Central Cameroon Shear Zone. Controversial chronology models has been proposed for the kinematic evolution of the sinistral and dextral shear phases in the Tikar Plain, thus in the FBSZ; early dextral and late sinistral shear phases for some authors and early sinistral and late dextral shear for others. Moreover, the NMSZ kinematic evolution implication on the mylonitization P-T-t path in the area seem to be problematic and the present paper aim is to clear enough those problems; since this shear zone is the main mylonitic corridor that registered the left and right lateral movement in this area. The NMSZ comprises amphibolites, protomylonites, strict sensus mylonites (garnet-kyanite-sillimanite mylonite and garnet-pyroxene mylonite), ultramylonites kyanite-sillimanite and garnet-kyanite-sillimanite gneiss. Field structures testify that the investigated area recorded three deformation phases: (i) the D 1 deformation phase which is marked by NW-SE to N-S trending S 1 metamorphic foliation with low to moderate dips (15°–45°) that was transposed during the D 2 phase, is responsible for a regional metamorphism whose mineral paragenesis is garnet-kyanite-sillimanite; (ii) the early sinistral NNE-SSW to NE-SW shear phase D 2 marked by S 2 metamorphic and mylonitic foliations; responsible for, L 2 stretching mineral lineation, F 2 fold axes and B 2 boudins structures; (iii) the late dextral NE-SW shear phase D 3 , characterized by F 3 folds, B 3 boudins and ductile dextral C 3 shear planes. Mineral paragenesis garnet + kyanite + sillimanite and microstructures within gneiss testify that this rock underwent high grade regional metamorphism whose peak conditions are estimated at 11.5–13.5 kbar/850–900 °C. After the peak of metamorphism gneiss was overprinted by high grade pressure mylonitization during the early sinistral and late dextral shear deformations. Microstructural data here indicate a high-grade mylonitization whose P-T conditions are estimated at least at around 10 kbar/750 °C attained during the D 2 . Shear markers, indicates that the studied area underwent an intense mylonitization at deep crustal deformation level, probably at the ductile-brittle boundary structural level during a major dextral shear deformation.

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.