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Earth's plate tectonic behaviour arises from lithospheric ductile weakening and shear-localization. The ubiquity of mylonites at lithospheric shear zones is evidence that localization is caused by mineral grain size reduction. Most lithospheric mylonites are polymineralic, suggesting that the interaction between mineral phases by Zener pinning promotes grain size reduction and weakening. Yet this interaction only occurs where mineral phases mix at the grain scale. Phase mixing and its effect on microstructure and strength have been shown in deformation experiments and natural field samples. Our theory for the interaction between phase mixing (treated as a stress driven diffusion) with two-phase grain damage has been compared to lab experiments. But using processes at the tiny grain-scale embedded within the small hand-sample and lab scales to model large-scale lithospheric processes, requires an upscaling scheme that captures the physics from micro- to macrostructures. For example, weakening from grain-damage in zones of mixing can lead to banded viscosity structure at the small scale that manifests as viscous anisotropy at the large scale. Here we provide a new framework for self-consistently upscaling from microscopic (grain) scales, to mesoscopic (petrological heterogeneity) scales to macroscopic (tectonic) scales. The first upscaling step models phase mixing and grain size evolution in a small 'mesoscopic' lab-scale volume or 'patch', which is equivalent to a point in the macroscopic space. Within this mesoscale patch, stress driven diffusive mixing is described by an analytical solution for mineral phase fraction, provided a minimalist Fourier representation of phase fraction, and a transformation to the patch frame of reference as well as to the principal stress directions at that point. The orientation and volume fraction of mixed-phase regions can then be extracted from the analytical solution for phase fraction. The grain size and viscosity in the mixed bands are determined by two-phase grain-damage theory; the unmixed zone properties follow from mono-phase grain damage theory. The mesoscale banded viscosity field leads to a macroscale anisotropic viscosity at that point in space. But, the evolution of properties at each macroscale point involves tracking only a few quantities (phase fraction, grain sizes) rather than modelling each patch of mesoscale space as its own 2-D or 3-D system. For the final upscaling, the anisotropic viscosity field is used in a macroscale lithosphere flow model. We show an example of this scheme for a lithospheric Rayleigh–Taylor drip driven by ridge-push compressive stress, which can cause anisotropic weakening via grain mixing and damage that may help initiate subduction and passive margin collapse. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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Metamorphic reactions are commonly driven to completion within shear zones thanks to fluid circulation, making the re‐equilibration of the mineral assemblage one of the dominant processes. Despite the important role of H2O in such processes, forward thermodynamic modelling calculations commonly assume either H2O‐saturated conditions or only fluid loss during prograde evolution to peak conditions. These assumptions influence the understanding of shear zones during the retrograde evolution. Here, we investigate the P–T–MH2O retrograde evolution of the Mt. Bracco Shear Zone (MBSZ), an Alpine ductile tectonic contact which marks the boundary between two HP units in the Dora‐Maira Massif (Western Alps, Italy). After the eclogite‐facies peak (at 500–520°C and 1.8–2.2 GPa), the subsequent mylonitic event is constrained at amphibolite‐facies conditions, continuing its evolution at decreasing pressure and temperature during rock exhumation, from ~590°C, 1.0 GPa down to ~520°C, 0.7 GPa. The P/T–MH2O forward modelling highlights different behaviour for the two analysed samples. After reaching a minimum H2O content at the transition from eclogite‐ to amphibolite‐facies conditions, a significant fluid gain is modelled for only one of the two analysed samples just before the mylonitic event. The MBSZ then evolves towards H2O‐undersaturated conditions. This work thus underlines the necessity of investigating the H2O evolution within shear zones, as the H2O content is susceptible to change through the P–T path, due to dehydration reactions or fluid infiltration events. Furthermore, lithological heterogeneities influence possible different fluid circulation regimes in shear zones, resulting in externally or internally derived fluid gain. [ABSTRACT FROM AUTHOR]

The Etam Shear Zone is a N to NNE trending strike-slip zone, and it displays a sinistral shearing mylonitic (ductile) zone and a brittle zone. Field observations and microstructural studies reveal that the ductile zone experienced strain partition. Primary minerals (quartz, K-feldspar, Plagioclase, and biotite) in wall rock (granodiorite) are gradually reduced towards the intensively deformed shear zone center. Quartz recrystallization also increases in the same direction. The structural evolution reveals the following: (1) a first phase of deformation D1 underlined by S1 foliation and mylonitic schistosity (Sm). This phase is oriented N10°E to N22°E and is associated with L1 lineation (NNE-SSW, average plunge of 10° NE), F1 fold (fold axis~ S60E), and asymmetric β1 boudins of domino's type; (2) a second phase D2 that develop F2 folding, C2 shearing planes (N60E to N75E and dipping strongly W to NW), and a late D2 brittle event. Microstructures such as recrystallization, sub-grain boundary, myrmekites, and undulose extinction characterize the sub magmatic and post magmatic deformation. Kinematic markers (rotated mineral, shearing planes, and fold vergence) reveal an earlier dextral shearing movement in biotite gneiss followed by sinistral shear sense recorded in mylonites. The mica-chlorite-epidote assemblage defines a greenschist facies in the Etam Shear Zone. Metasomatic reactions (Plagioclase + K-feldspar + water = Quartz + Albite + Epidote + Chlorite) during pervasive alteration of some minerals have probably released silica during deformation that was carried by hydrothermal fluids and precipitated in pull-apart fracture to form hydrothermal syn-kinematic quartz veins. [ABSTRACT FROM AUTHOR]

The occurrence of plate tectonics on Earth is rooted in the physics of lithospheric ductile weakening and shear-localization. The pervasiveness of mylonites at lithospheric shear zones is a key piece of evidence that localization correlates with reduction in mineral grain size. Most lithospheric mylonites are polymineralic and the interaction between mineral phases, such as olivine and pyroxene, especially through Zener pinning, impedes normal grain growth while possibly enhancing grain damage, both of which facilitate grain size reduction and weakening, as evident in lab experiments and field observations. The efficacy of pinning, however, relies on the mineral phases being mixed and dispersed at the grain scale, where well-mixed states lead to greater mylonitization. To model grain mixing between different phases at the continuum scale, we previously developed a theory treating grain-scale processes as diffusion between phases, but driven by imposed compressive stresses acting on the boundary between phases. Here we present a new model for shearing rock that combines our theory for diffusive grain mixing, 2-D non-Newtonian flow and two-phase grain damage. The model geometry is designed specifically for comparison to torsional shear-deformation experiments. Deformation is either forced by constant velocity or constant stress boundary conditions. As the layer is deformed, mixing zones between different mineralogical units undergo enhanced grain size reduction and weakening, especially at high strains. For constant velocity boundary experiments, stress drops towards an initial piezometric plateau by a strain of around 4; this is also typical of monophase experiments for which this initial plateau is the final steady state stress. However, polyphase experiments can undergo a second large stress drop at strains of 10–20, and which is associated with enhanced phase mixing and resultant grain size reduction and weakening. Model calculations for polyphase media with grain mixing and damage capture the experimental behaviour when damage to the interface between phases is moderately slower or less efficient than damage to the grain boundaries. Other factors such as distribution and bulk fraction of the secondary phase, as well as grain-mixing diffusivity also influence the timing of the second stress drop. For constant stress boundary conditions, the strain rate increases during weakening and localization. For a monophase medium, there is theoretically one increase in strain rate to a piezometric steady state. But for the polyphase model, the strain rate undergoes a second abrupt increase, the timing for which is again controlled by interface damage and grain mixing. The evolution of heterogeneity through mixing and deformation, and that of grain size distributions also compare well to experimental observations. In total, the comparison of theory to deformation experiments provides a framework for guiding future experiments, scaling microstructural physics to geodynamic applications and demonstrates the importance of grain mixing and damage for the formation of plate tectonic boundaries. [ABSTRACT FROM AUTHOR]

The relationship between Pan-African metamorphic and mylonitic evolutions is poorly documented in the Pan-African Fold Belt (PAFB) in Cameroon, probably due to lack of coexistence of regional metamorphic and mylonitic rocks in the same area as it is the case in the Fotouni-Kékem Shear Zone (FKSZ). The FKSZ belongs to the N30E–N50E branch of the Central Cameroon Shear Zone (CCSZ), a mylonitic corridor in the PAFB. Rocks cropping out in the FKSZ include amphibolites, charnockites, and kyanite-, sillimanite-, and garnet-bearing gneisses. They display structures, microstructures, and critical metamorphic indicators (garnet, sillimanite, kyanite, and pyroxene) that were used to reconstruct the P–T-t path of their tectonic and metamorphic evolution. Structural field-based and microstructural investigations evidenced three deformation phases: D1, D2, and D3, respectively, dated at 640–613 Ma, 613–590 Ma, and 590–542 Ma. D1 is a NE-SW trending flattening phase that induced a HP-HT metamorphism whose peak was attained in the granulite facies. D2 is a sinistral shear phase initiated in the N-S direction and progressively rotated toward the NNE-SSW (N25E) and locally toward the NE-SW as result of the convergence of the West African Craton on the West Cameroon. It is responsible for the emplacement of the NE-SW (N30E–N45E) Fondanti charnockitic intrusion and syn-kinematic Pan-African Fomopéa and Bandja neighboring plutons. D3 is a NE-SW to ENE-WSW (N45E–N70E) trending shear phase displaying a dextral movement; it locally curved by progressive rotation at WNW-ESE (N120E). Microstructures and mineral parageneses in the garnet-kyanite-sillimanite gneiss are consistent with a clockwise metamorphic P–T path followed by a retrograde one. The peak of the prograde phase was attained under granulite facies conditions (at about 11.5–13.5 kbar/850–900 °C) during D1 (640–613 Ma) deformation phase. The retrograde phase evolved under the amphibolite facies conditions (at about 6–7.5 kbar/700–800 °C) during D2 (613–590 Ma) and D3 (590–542 Ma) deformation phases. The regional metamorphism was interrupted during its retrograde phase by a mylonitic event, whose P–T conditions within the FKSZ were estimated on the basis of microstructures found in the mylonitized rocks. This mylonitic event displays a gradation from the border (3–5 kbar/350–500 °C) toward the core (6.5–10 kbar/700–900 °C). [ABSTRACT FROM AUTHOR]

The central portion of the Kalinjala Shear Zone, in the Port Neill region, South Australia, is recognised as a northeast-trending anomalously magnetic corridor coinciding with a sub-vertical transpressional dextral shear zone. This shear zone is a primary lithospheric structure, separating the nucleus of the Archean Gawler Craton from a suite of Paleoproterozoic granites (ca 1850 Ma) and earlier paragneiss. At Port Neill, there is an apparent 17 km dextral offset of a pre-existing Bouguer gravity anomaly by the Kalinjala Shear Zone. This is attributed to Kimban-aged (ca 1740–1690 Ma) deformation that is accompanied by peak granulite facies metamorphic conditions reaching pressures of ∼1.0 GPa and temperatures of ∼800 °C. This is consistent with the presence of leucosomes, within the sheared fabric, in the axial surface of mylonitic folds and in mafic gneisses. Strain has been localised within and adjacent to mafic dykes, predominantly hosted in the granitic rocks, with the highest strained areas having the greatest evidence of leucosome extraction. The local orientation of stretching lineations in this shear zone do not necessarily correlate with the transport direction but may reflect along- and across-strike variations in finite shear strain into alternating zones dominated by either constrictional or a superimposed flattening strain. Overprinting the dominant dextral movement are sinistral shears, with horizontal strike-slip movements, which post-date the peak metamorphic assemblages and are generally confined to sites containing late K-feldspar-rich pegmatites. The major movement recorded in the Kalinjala Shear Zone was both horizontal and vertical, occurring in response to regional exhumation of the Cornian-aged (ca 1850 Ma) sequences in the east, during the Kimban Orogeny, and was basically controlled by the nature of lithospheric plate convergence during accretion of a continental backarc onto the Gawler Craton. Dextral transpression with late-sinistral along-strike extension and east-up exhumation is associated with 17 km of horizontal offset along the central section of the Kalinjala Shear Zone. Deformation is dominated by three major Kimban-aged (ca 1730–1690 Ma) events, which overprint older ensialic backarc metasedimentary rocks, orthogneiss and granites of the Donington Suite. Granulite facies metamorphic conditions control the nature of deformation and strain distribution within the shear zone. The Kimban events record the collision of the Gawler Craton nucleus with the southern margin of the North Australian Craton. [ABSTRACT FROM AUTHOR]

Crystallographic preferred orientations of quartz aggregates in mylonitic orthogneisses from the highest strained central region of the Kalinjala Shear Zone, Port Neill South Australia, are products of a combination of constrictional and flattening strains. These rocks were deformed under granulite facies peak metamorphic conditions, reaching pressures of ∼1.0 GPa and temperatures of ∼800 ± 50 °C. Neutron diffraction textural and fabric analysis methods reveal a dominant single c-axis maximum parallel to the Y-strain axis. This is accompanied by a symmetric orthorhombic pattern, where the c-axis fabrics and corresponding pole figure densities of {a}, {m}, {r} and {z} planes are arranged around the XY foliation plane. These fabrics from coexisting adjacent layers suggest that the activation of prism- slip is characteristic for these high-temperature deformation conditions. A small asymmetry in the [c], {a} and {m} pole figures suggests a minor component of dextral shearing. The c-axis pattern is weakened where the foliation is dominated by secondary phases, such as concentrations of biotite mica. The opening angles obtained from cross-girdle c-axis patterns produce temperature estimates that are ∼100 °C lower than the peak metamorphic temperature of ∼800 ± 50 °C, but closer to temperatures related to a component of flattening strain. These c-axis patterns are consistent with other structural characteristics and can be related to the late-stage flattening strains during the Kimban Orogeny. Mylonitic orthogneisses are deformed under granulite facies conditions. Quartz fabrics were measured by bulk neutron diffraction and optical methods. Mylonite c-axis fabrics can be related to the high-temperature deformation and activation of the prism-< a > slip system. Crystal preferred orientations indicate a large component of flattening strain superimposed on the dextral shearing observed in the Kalinjala Shear Zone. [ABSTRACT FROM AUTHOR]

• First in situ SIMS oxygen isotope study in metamorphic core complex rocks. • Heterogeneous resetting of quartz and epidote δ18O at the grain scale. • Exchange with low-δ18O fluids at high-T during footwall mylonitization. • Inferred infiltration of meteoric fluids to midcrustal depths (>10 km). • No evidence of footwall refrigeration beneath the whipple mountains detachment. The extensional detachment systems of metamorphic core complexes (MCC) have the potential to facilitate significant fluid movement between Earth's surface and the warm, ductile middle crust. One long-standing and influential detachment fault-fluid interaction model called the "footwall refrigeration hypothesis" (Morrison and Anderson, 1998) postulates that cool meteoric fluids circulating downward along a detachment can facilitate large-scale heat loss at depth before rocks are exhumed by faulting. Stable isotopes of oxygen and hydrogen have long been recognized as potentially useful tracers of surface-to-depth fluid flow in core complexes; however, typical sampling methods and the competing effects between temperature and fluid composition have made oxygen isotope signatures of meteoric fluid infiltration difficult to detect and/or interpret in the rock record. Here, we use in situ δ18O measurements by secondary ion mass spectrometry (SIMS) to address the rock record of meteoric fluid interactions and temperature variation within the Whipple Mountains MCC footwall beneath the Whipple detachment fault (WDF). The micro-scale sampling of SIMS (10 µm diameter x 1–2 µm deep pits) allows investigation of isotope variability as a function of mineral geochemistry and microstructure and enables more accurate interpretation of the causes of isotope variability. In the Whipple footwall mylonites, quartz and epidote show grain-to-grain oxygen-isotope variability within samples and as a function of distance from the main detachment. Approaching the WDF, both quartz and epidote show increasing spread toward low δ18O values. The lowest SIMS-measured δ18O epidote values require exchange with a low δ18O, meteoric fluid at high temperature. However, SIMS data also reveal that meteoric fluid-rock interactions were spatially heterogeneous at the scale of individual grains, resulting in local preservation of metamorphic quartz-epidote oxygen isotope equilibrium unaffected by infiltrating meteoric fluid. Within preserved metamorphic domains at all investigated structural levels of the WDF footwall, quartz and epidote δ18O values are tightly clustered and yield similar fractionations (∆18O qz-ep). For samples from the longest footwall traverse, these ∆18O qz-ep values are 4.0 ± 0.4‰, consistent with δ18O equilibrium at 521+43/-37 °C. We conclude that there is no evidence of a large paleo-thermal gradient associated with the Whipple Detachment Fault (i.e., no evidence for footwall refrigeration). [ABSTRACT FROM AUTHOR]

This study focused on collecting structural data orientations of a crustal-scale shear zone (Palmi Shear Zone, PSZ, southern Calabria, Italy) by integrating various analytical and field-based techniques. The PSZ consists of deformed metamorphic rocks (migmatitic biotitic paragneiss, marbles, and skarns) showing multiple folding phases, and Hercynian tonalites and pegmatites (306-290 Ma), crosscut by Late Hercynian leucocratic dykes (ca. 290 Ma). Multi-sized clasts composed of different lithologies are preserved on clean outcrop surfaces, and are sheared into both σ - and δ -type objects that collectively suggest opposing senses of shear. The study incorporates structural analysis of folds, field and aerial surveys (UAV), digital mapping, and microcomputed tomography. Various kinematic indicators were observed in the PSZ, indicating a mix of factors influencing the shear strain patterns (e.g. fold interference patterns, different rock types with high viscosity contrast). The findings suggest a clear consistency between structural data inferred from 3D VOM (Virtual Outcrop Model) and those collected directly in the field, confirming the occurrence of both sinistral and dextral shear in the PSZ, providing important insights into the tectonic evolution of the Calabrian-Peloritani Terrane. [Display omitted] • This study provides new structural data of the Palmi Shear Zone (PSZ) in southern Italy, Calabria. • Analyzing VOM (virtual outcrop models) has allowed for the construction of a complete structural dataset of crystalline rocks. • We speculate that earlier sinistral shearing have been followed by dextral one. Opposite shear sense indicators suggest shear zone reactivation in reverse sense. [ABSTRACT FROM AUTHOR]

Transient creep during an earthquake cycle is very important to understand the rheology of fault and deformation mechanisms in the brittle–plastic transition zone. In this paper, we analyzed the microstructures of samples for mylonites, pseudotachylyte, and cataclasite under optical microscope, SEM, and EBSD, which were collected from the Red River fault in southwest of China, where we uncovered a series of ductile to brittle deformed rocks which recorded transient creep related to earthquakes. The results show that mylonites formed at the inter-seismic creep were overprinted by pseudotachylyte and cataclasite which were produced during co-seismic rupture, and cracks in cataclasite were healed during the post-seismic relaxation. Based on the analysis of the microstructures and deformation mechanism of fault rocks, we propose the oscillation deformation model to explain transient creep of the brittle–plastic transition zone during the seismic cycle in the Red River fault.

The main central thrust (MCT) is one of the major thrusts in Himalayas. In central Himalaya, MCT was defined as a contact between underlying Lesser Himalayan Sequence (LHS) and overlying higher Himalayan crystallines (HHC). However, in the Kashmir Himalayas, the main central thrust zone (MCTZ), shear zone associated with MCT, is overlain by Kashmir Tethyan Sequence suggesting that the MCTZ has been deformed through a mechanism different than the mechanism responsible for MCTZ evolution in other parts of the Himalayas. In the present study we used structural, microfabric and kinematic analyses to investigate the deformation kinematics of MCTZ. Microstructural investigation revealed that the quartz in orthogneiss mylonites of MCTZ was dynamically recrystallized by grain boundary migration (GBM) and sub-grain rotation recrystallisation (SGR) with top-to-SW sense of shear. The mean kinematic vorticity number (Wm) just above the thrust ranges from 0.72 to 0.84 (40%–52% pure shear component) decreasing upwards to 0.65–0.71 (35%–50% pure shear component). Deformation in the MCTZ is characterized by Rxz strain ratio varying from 2.7 to 8. The present study suggested that the MCTZ suffered 3%–40% vertical shortening and 3%–66% transport-parallel elongation. The results suggested that the HHC's were not completely exhumed to the topographic surfaces in the Kashmir Himalayas. Along the basal decollement, i. e., the main Himalayan thrust (MHT), the deformation continued until MCTZ reached the brittle-ductile transition where deformation mechanism changed to the brittle and the MCTZ rocks were transported to the surface through slip on brittle MCT. [ABSTRACT FROM AUTHOR]

Abstract The Guacuí and Batatal lineaments in the southern region of the state of Espírito Santo (southeastern Brazil) can be traced from satellite images for tens of kilometers. They are considered shear zones associated to the final tectonic stages of the Araçuaí-West Congo (A-WC) Orogen during the Ediacaran-Cambrian period. In order to understand their nature and tectonic implications for the development of the orogeny, we integrated geological and structural data based on fieldwork and detailed microstructural analysis. Data was collected along two profiles located at the central segments of both structures. The results demonstrated that the Guaçuí shear zone (GSZ) is a typical ductile transpressive shear zone with approximately 9 km width that shows numerous dextral kinematic indicators and records metamorphism of upper amphibolite facies. The GSZ shows different levels of intensity of dynamic deformation. The Batatal shear zone (BSZ) is a narrow brittle shear zone that shows essentially brittle microstructures (intercrystalline microfractures) revealing deformation at low-grade conditions (frictional regime). Based on their characteristics and the level of erosion of the rocks exposed in the region, we interpret that GSZ developed prior to BSZ.

The Guacuí and Batatal lineaments in the southern region of the state of Espírito Santo (southeastern Brazil) can be traced from satellite images for tens of kilometers. They are considered shear zones associated to the final tectonic stages of the Araçuaí-West Congo (A-WC) Orogen during the Ediacaran-Cambrian period. In order to understand their nature and tectonic implications for the development of the orogeny, we integrated geological and structural data based on fieldwork and detailed microstructural analysis. Data was collected along two profiles located at the central segments of both structures. The results demonstrated that the Guaçuí shear zone (GSZ) is a typical ductile transpressive shear zone with approximately 9 km width that shows numerous dextral kinematic indicators and records metamorphism of upper amphibolite facies. The GSZ shows different levels of intensity of dynamic deformation. The Batatal shear zone (BSZ) is a narrow brittle shear zone that shows essentially brittle microstructures (intercrystalline microfractures) revealing deformation at low-grade conditions (frictional regime). Based on their characteristics and the level of erosion of the rocks exposed in the region, we interpret that GSZ developed prior to BSZ. [ABSTRACT FROM AUTHOR]

Linking the deformation history of mylonitized continental rocks to the progress of devolatilization reactions that trigger reaction softening is critical for the understanding of crustal scale processes. We have analysed the field geometries and microstructures of deformed rocks within the southern Hercynian belt in Calabria, as well as modelled the pressure–temperature–deformation (P–T–d) trajectory of a main ductile shear zone that tectonically coupled the deeper crustal Mammola Paragneiss Unit with the upper crustal Stilo–Pazzano Phyllite Unit. P–T modelling of the mylonitic Mammola Paragneiss Unit was performed through calculation of phase equilibrium diagrams with the software thermocalc in the MnNCKFMASHTO model system. The prograde P–T–d trajectory is based on the zoning profiles of garnet porphyroblasts and their mineral inclusions, primarily barroisite and epidote. P–T modelling shows that peak metamorphic conditions of ~0.9 GPa and 585°C were reached during a Dn‐1 under‐thrusting event. The following exhumation during the Dn mylonitic event, and contact metamorphism during Dn+1 and Dn+2 folding events, have also been modelled because they are essential to restore the previous tectono‐metamorphic history. The exhumation trajectory was modelled down to 0.3 GPa with temperatures of 440–460°C, under fluid‐deficient conditions, as well as the final late Carboniferous contact metamorphism up to Tmax of 680–720°C. The prograde path shows clear evidence for thermal buffering during garnet growth at the expense of chlorite, with a heating‐dominated stage after chlorite breakdown. Subsequently, a rheological change associated with epidote breakdown (i.e. reaction softening) occurred, highlighted by a net steepening of the P/T trajectory towards the pressure peak. On the basis of the barroisite inclusions within garnet porphyroblasts as well as the 'hairpin' shape of the reconstructed P–T–d path (before contact metamorphism), we infer that the unusual low T/P gradient for the Hercynian crust exposed in the Mammola Paragneiss Unit records its involvement in the Palaeotethys–Gondwana subduction beneath Laurussia during Dn‐1 under‐thrusting. We present a new palaeotectonic interpretation along the southern Hercynian belt in Calabria during the Upper Mississippian–Lower Pennsylvanian, that is consistent with previous geochronology studies. [ABSTRACT FROM AUTHOR]

Pre‐kinematic greenschist facies metamorphism is often observed in granites and basement units of mountain belts, but rarely dated and accounted for in orogenic cycle reconstructions. Studying pre‐kinematic alteration is challenging because of its usual obliteration by subsequent syn‐kinematic metamorphism often occurring at conditions typical of the brittle–ductile transition. It is, however, to be expected that pre‐kinematic alteration has major implications for the rheology of the upper crust. In the 305 Ma‐old Variscan basement of the Bielsa massif (located in the Axial Zone of the Pyrenees), successive fluid–rock interaction events are recorded in granites below 350°C. Combined microstructural and petrographic analysis, low‐T thermobarometry and in situ U–Th/Pb dating of anatase, titanite and monazite show extensive pre‐orogenic (pre‐Alpine) and pre‐kinematic alteration related to feldspar sericitization and chloritization of biotite and amphibole at temperatures of 270–350°C at 230–300 Ma. This event is followed by a second fluid–rock interaction stage marked by new crystallization of phyllosilicates at 200–280°C and is associated with the formation of mylonitic shear zones and fractures parallel to the shear planes. U–Pb anatase and monazite ages as well as the microtextural relationships of accessory minerals suggest an age for this event at 40–70 Ma, consistent with independent regional geology constraints. The Variscan basement was therefore softened at late to post‐Variscan time, at least 150–200 Ma before the main Alpine shortening while Alpine‐age compression (c. 35–50 Ma) leads to the formation of a dense net of mylonites. The associated deformation, both distributed at the scale of the Bielsa massif and localized at decametric scale in mylonitic corridors, precedes the strain localization along the major thrusts of the Axial Zone. The Bielsa massif is a good example where inherited, pre‐orogenic upper crustal softening controls the deformation patterns in granitic basement units through low‐grade metamorphic reactions. [ABSTRACT FROM AUTHOR]

Weathering and lateritization processes and their implication to the fractionation of trace and rare earth elements (REEs) in soil materials developed upon mylonites in Kissi area of the Bétaré-Oya Gold District (Lom series, East Cameroon) were assessed through morphological (position on the slopes), mineralogical (X-ray diffraction) and geochemical (X-ray fluorescence and Inductively Coupled Plasma-Mass Spectrometry) investigations. Three (03) representative soil profiles were selected and described according to the topography (upslope, mid-slope and downslope). The resulted weathering products are rich in kaolinite (16–49%), gibbsite (3–17%), goethite (5–10%) and hematite (2–10%), with quartz (30–49%) and muscovite (4–31%) as inherited minerals. Geochemical proxies and related ternary diagrams signal an intense chemical weathering (CIA: 43.3–100%; PIA: 41.9–100%), but weak lateritization (IOL: 11.3–40.8%) of parent mylonite under warm and wet tropical climate, that induce the formation of kaolinitic laterites. These processes are characterized by a weak leaching of SiO 2 (79.7–52.2%), but strong one of labile elements (WIP: 48.7 to 0.8%; TRB: 228.9 to 15.1 mg kg−1). Al 2 O 3 (16.2–22.8%) and Fe 2 O 3 (4.2–14.0%) are the slightly enriched major oxides in the weathered materials. Some trace elements including Sc, V, Cr, Co, Ni, Cu, Ga, As, Li, Y, Mo, Cs, Hf, W, Pb, Th, Tl, U, Zr and REEs are reported as strongly enriched compared to mylonite, but weak mineralized according to their average contents in the earth's crust (Maximum Enrichment factor: 1.1–38.8). This gives rise of low mining potential of the current weathering products. Nevertheless, Ce (37.8–453.6 ppm), Ba (34.2–379.6 ppm), Cr (100.0–270.0 ppm), V (108.9–215.0 ppm), Zn (23.4–192.0 ppm), Ni (29.3–170.0 ppm) and Rb (2.5–140.0 ppm) are the most abundant in the soil profiles. Topographically, trace and REEs are preferentially enriched and mineralized at mid-slope and downslope of the interfluve. But, the overall behavior and enrichment of elements in Kissi are controlled by the dissolution of rock-forming minerals and their release in the residual soil system, their incorporation into the crystalline structure of newly formed secondary minerals and the lateral water runoff along the slope gradient. • An intense weathering combined to weak lateritization of mylonite weathering products is evidenced in the BOGD. • Leaching and reprecipitation mechanisms induce the preferential formation of secondary-rich Al and Fe minerals. • Low mineralization of trace and Rare Earth Elements is globally evidenced at mid-slope and downslope. • Elemental behavior is mostly controlled by lithology, landscape position, and water runoff. [ABSTRACT FROM AUTHOR]

A theory of the brittle‐ductile transition (BDT) is shown to be a direct consequence of a recently developed physics‐based constitutive law for rock friction (Aharonov & Scholz, 2018, https://doi.org/10.1002/2016JB013829), which assumes exponential creep on contacts. The theory was previously tested against experimental data for sliding at low ambient temperature and stress. Here, theoretical interpretation of experimental data at high temperature and stress shows that at some point the real area of contact reaches a maximum value beyond which it becomes fixed. The constitutive law shows that this marks the onset of the BDT, beyond which sliding changes from frictional to an exponential flow law for low‐temperature plasticity. Application to the Earth's crust shows that beyond this point, strength fall linearly with depth until it intersects the power law for bulk flow of the country rock, which marks the lower boundary of the BDT. Modeling, constrained by experimental data for granite, predicts that the BDT starts at a temperature of about 300°C, at a depth of 11–13 km in the continental crust, depending on fault slip rate and temperature gradient. The completion of the BDT is similarly calculated to occur around 475°, at 16–18 km, in agreement with laboratory and field observations. The BDT is thus found to be a region spanning about 175°C with a width of several kilometers. Within the exponential flow region, the structural outcome would be a relatively narrow mylonitized fault zone, which widens into a broader region of shear at the base of the BDT. Key Points: The friction constitutive law of Aharonov & Scholz (2018) is applied to high temperature and pressure: the experimental data are well fitThe experimentally‐constrained theory predicts transition from friction to low temperature plasticity at 300C for quartzo‐felspathic rocksThe transition from low to high temperature plastic flow will occur at 475C, forming a brittle ductile transition region several km thick [ABSTRACT FROM AUTHOR]

An accurate flow law for dislocation creep of quartz aggregates is critical for the understanding of continental rheology, numerical modeling of lithospheric processes, and the interpretation of microstructures of natural quartz‐bearing mylonites. Despite many decades of research, considerable discrepancies still exist among quartz flow laws determined from different experiments. We demonstrate that the key to reconcile these discrepancies is to consider the pressure dependence of the activation enthalpy. From existing high‐quality creep experiments on quartz aggregates, we critically identified test runs having microstructures and stress‐strain curves indicating steady state regimes 2 and 3 dislocation creep and used the data to obtain a set of flow law parameters most consistent among existing experiments. Because estimates of strain rate and stress from natural mylonites are bound with large uncertainties, they cannot be used to construct a flow law better than one determined from well‐controlled creep experiments. A large number of geological studies and modern GPS observations suggest that crustal scale ductile shear zones likely flow at a strain rate range between 10−13 and 10−11 s−1. In this strain rate range, our flow law predicts flow stresses broadly consistent with stress estimates from many natural mylonites based on well‐calibrated piezometers for dynamically recrystallized quartz grains. Key Points: Pressure dependence of activation enthalpy is key to reconcile discrepancies among flow laws for quartz derived from different experimentsA consistent flow law for climb‐accommodated dislocation creep for quartzite is obtained from critically selected high‐quality experimental dataThe new flow law is discussed in the context of stress estimates from natural mylonites based on various piezometric methods [ABSTRACT FROM AUTHOR]

Here, we study the relationships of titanite-hosting microdomains, intragrain chemical variations, microstructure and fluids with the aim of deciphering the reliability of titanite U–Pb dating to constrain the age of deformation in mylonitic rocks. We investigate these relationships in a post-Variscan amphibolite-facies shear zone developed in the mid-low continental crust (Ivrea-Verbano Zone, Southern Alps, Italy). Quantitative orientation analyses along with textural imaging of titanite are combined with trace-element analyses and U–Pb age dating. Titanite is studied in mm- to cm-scale layered rocks showing compositional variation consisting of alternating 'amphibole-rich' (i.e., amphibolites) and 'clinopyroxene/plagioclase-rich' domains (i.e., calc-silicates). Titanite from amphibole-rich domains shows predominance of crystal–plastic deformation features, as abrupt or progressive core-to-rim structures characterized by increasing lattice distortion and local dislocation density, associated with the development of abundant subgrains and rare newly nucleated grains. We suggest that these microstructures form while interacting with small amounts of fluids circulating along the grain boundaries. Consequently, locally the chemistry of titanite is changing. In the clinopyroxene/plagioclase-rich domains, titanite is mostly undeformed and rarely shows bending localized in discontinuous narrow rims/tips. In these domains, fluid-mediated replacement reactions are either rare or absent, as also indicated by weak chemical variations across and among grains. These observations suggest different reactivities with respect to the same P-T-fluid conditions of the two compositional domains, which coexist within the same sample at the thin section scale. U–Pb data show correlations with chemical and microstructural domains that differ as function of the composition of the microdomain. This correlation is more apparent within amphibole-rich domains where microstructures characterized by high lattice distortion/dislocations and/or subgrains show significant variations of REE, Zr, Y, Nb, U with respect to the low deformed portion of grains. These titanite domains define an isotopic population providing the youngest (Jurassic) lower intercept age. A less clear correlation between titanite chemistry and microstructures is observed in clinopyroxene/plagioclase-rich domains. Here, the rare titanites showing lattice distortion and minor Sr depletion define a population providing a similar Jurassic lower intercept age. Therefore, our results demonstrate that microstructurally and chemical calibrated U–Pb dating of titanite provides realistic ages of shear zone activity, only in case of predominance of crystal-plastic processes and of local interaction of titanite with small amounts of fluids focused along grain boundaries. Finally, the different footprints recorded by titanite grains strongly depend on the composition of cm- or mm-scale interlayered domains in which titanite occurs. • Petrochronological and microstructural study on titanite from mylonitic rocks. • Titanite records heterogeneous features as function of microdomain composition. • Deformation microstructures exert significant control on petrochronological record. • Fluid availability governed the development of certain microstructural features. • Lithologies deforming in the presence of fluids are best for dating deformation. [ABSTRACT FROM AUTHOR]

The results of the study of mediosiliceous and mafic metasomatites located in ultramafic and mafic rocks Nyaschevo massif presents in the paper. The texthural and structural features of these rocks allow to attribute them to tectonites - protomylonites and mylonites. Mineralogical and geochemical characteristics of the rocks indicate to polycyclic process of metamorphism accompanied by metasomatic transformation. The metasomatites of medium composition have been characterized the primary of magmatic origins followed by metamorphic and regional deformation processes associated with metasomatic transformation (gedrite, cummingtonite ( T = 800-700°С, P = 2-5 kbar)).The metasomatites of mafic composition are the result of mylonitization and metasomatism under the influence of fluid induced the mobility of elements, such as Al, Ti, К, Ba and LREE. The input Mg from host serpentinites on the final stages of conversion is clear recorded in the forming of gedrite and height-magnesium enstatite. The metasomatites of mafic composition is showed the petrogeochemical connection with the rocks of syenite-carbonatite association.

Abstract Shear zones play a major role in the deformation of the crust at a variety of scales, as expressions of strain localization during orogeny and rifting, and also as reactivated structures. They influence the geometry and evolution of orogenic belts and rifts, crustal rheology, magma ascent and emplacement, and fluid flow. Consequently, assessing the timing of shear zone activity is crucial to reconstruct the tectonometamorphic evolution of the lithosphere. The interpretation of thermochronologic data from shear zones is, however, not straightforward. In the first place, closure temperatures depend on a number of factors (grain size, cooling rate, mineral composition and pressure, among others). On the other hand, deformation-related processes such as dynamic recrystallization, neocrystallization and fluid circulation seem to be crucial for isotopic systems and, thus, the obtained ages cannot be solely interpreted as a function of temperature in sheared rocks. For this reason, geochronologic data from shear zones might not only record cooling below closure temperature conditions but may also be affected by neo- or recrystallization, fluid-assisted deformation and inheritance of the protolith age(s). In order to robustly reconstruct P-T-ε-t paths of long-term crustal-scale shear zones, structural, microstructural and petrologic data from mylonites need to be integrated with ages from different thermochronometric systems. In addition, geochronologic data from associated intrusions and adjacent blocks can provide further irreplaceable constraints on the timing of deformation and its regional implications. One of the most challenging aspects that future lines of investigation should analyze is the quantitative evaluation of so far poorly explored aspects of isotopic diffusion, particularly the coupling with deformation processes, based on natural, theoretical and experimental data. Future works should also investigate the role of strain partitioning and localization processes in order to constrain the timing of deformation in different parts of a shear zone or in different branches of anastomosing shear zone networks. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Shear localization in the lithosphere is a characteristic feature of plate tectonic boundaries, and is evident in the presence of small grain mylonites. Localization and mylonitization in the ductile portion of the lithosphere can arise when its polymineralic material deforms by a grain-size sensitive rheology in combination with Zener pinning, which can impede, or possibly even reverse, grain growth and thus promotes a self-softening feedback mechanism. However, the efficacy of this mechanism is not ubiquitous and depends on lithospheric conditions such as temperature and stress. Therefore, we explore the conditions under which self-weakening takes place, and, in particular, the effect of temperature and deformation state (stress or strain-rate) on these conditions. In our model, the lithosphere-like polymineralic material is deformed in a two-dimensional simple shear driven by constant stress or strain rate. The mineral grains evolve to a stable size, which is obtained when the rate of coarsening by normal grain growth and the rate of grain size reduction by damage are in balance. Damage involves processes by which some of the deformational energy gets transferred into surface energy. This can happen by (i) dynamic recrystallization (grain damage) and (ii) stretching, deforming and stirring the material interface (interface damage). The influence of temperature enters through rheological laws (which govern the rate of work and damage), grain growth kinetics, and the damage partitioning fraction, which is the fraction of deformational work that goes into creating new surface energy. We demonstrate that a two-phase damage model, in which the partitioning fraction depends on both temperature and roughness of the interface between the phases, can successfully match the field data, including the reported correlation of grain size and temperature, the increasing dominance of dislocation creep at higher temperatures and a large range of grain sizes observed across the depth of a single shear zone. [ABSTRACT FROM AUTHOR]

In Southern Brazil there is a series of strike-slip shear zones that represent the late-collisional event of Western Gondwana in the Neoproterozoic-Eopaleozoic transition. The Curitiba Shear Zone (CSZ) is a part of this strike-slip regime separating two different domains: the supracrustal metasedimentary rocks and the basement composed of orthogneiss, migmatite and quartzite. The absence of detailed structural data for this discontinuity has motivated us to study this important part of the geology of southern of Brazil. The main purpose of this work is the characterization of meso and microstructures and determination of crystallographic fabric of quartz in orthogneisses and quartzites along the CSZ southern block. Structural data suggest that the CSZ consists of one ductile-brittle strike-slip deformation phase leading to the deformation of orthogneiss and quartzites of the Atuba Complex. The ductile regime was responsible for the formation of low-grade mylonite in greenschist facies. Crystallographic texture indicate a restricted recrystallization of quartz mainy via dislocation creep through activation ob basal slip system. The brittle regime led to the formation of cataclasites, faults and joints, with a mean orientation of N50E/75SE and N33E/80SE, respectively. Extensive dissolution-precipitation creep affected the rocks and infilled veins with carbonate and quartz. [ABSTRACT FROM AUTHOR]

The Iwaraja shear zone is an easterly splay to the Ifewara shear zone in the eastern part of Ilesha schist belt, southwestern Nigeria. Results of field and microstructural investigations reveal that the Iwaraja shear zone was developed during the second deformation phase that affected the region under retrograde metamorphic conditions. Deformation within the shear zone involved the mylonitisation of the granitic gneiss generating rocks ranging from weakly deformed granitic gneiss to ultramylonite under amphibolite to greenschist facies, respectively. The shear zone is characterized by NNE-SSW trending, sub-vertical mylonitic foliation (ca. 022°/79°) with shallowly dipping stretching lineations (ca. 021°/10°). Minor folds in the shear zone include early and late folds of pegmatite dykes in the granitic gneiss mylonite. The early folds are typically open to close folds but the late folds show typical geometry of synthetic folds characterized by thinned short limbs. Kinematic indicators in the granitic gneiss mylonite include sigma-type (σ-type) porphyroclasts and displaced fractured feldspar porphyroclasts indicate dextral shear sense. These features show that dextral, transcurrent displacement was important in the final amalgamation of different crustal blocks during the closing stages of the Pan-African Orogeny. [ABSTRACT FROM AUTHOR]

The mineralogical and geochemical investigation of mylonitic weathering profiles in Fodjomekwet-Fotouni was done to better trace the occurrence of minerals and chemical elements in this area. Four representative soil profiles were identified in two geomorphological units (upland and lowland) differentiating three weathering products (organo-mineral, mineral and weathered materials). Weathering of these mylonites led to some minerals association such as vermiculite, kaolinite, goethite, smectite, halloysite, phlogopite and gibbsite. The minerals in a decreasing order of abundance are: quartz (24.2%–54.8%); kaolinite (8.4%–36.0%); phlogopite (5.5%–21.9%); goethite (7.8%–16.1%); vermiculite (6.7%–15.7%); smectite (10.2%–11.9%); gibbsite (9.0%–11.8%) and halloysite (5.6%–11.5%) respectively. Patterns of chemical elements allow highlighting three behaviors (enriched elements, depleted elements and elements with complex behavior), depending on the landscape position of the profiles. In the upland weathering products, K, Cr and REEs are enriched; Ca, Mg, Na, Mn, Rb, S and Sr are depleted while Si, Al, Fe, Ti, Ba, Co, Cu, Ga, Mo, Nb, Ni, Pb, Sc, V, Y, Zn and Zr portray a complex behavior. Contrarily, the lowland weathering profiles enriched elements are Fe, Ti, Co, Cr, Cu, V, Zr, Pr, Sm, Tb, Dy, Er and Yb; while depleted elements are Ca, Mg, K, Na, Mn, Ba, Ga, S, Sr, Y, Zn, La, Ce and Nd; and Si, Al, Mo, Nb, Ni, Pb, Rb, Sc evidenced complex behaviors. In all the studied weathering products, the REEs fractionation was also noticeable with a landscape-position dependency, showing light REEs (LREEs) enrichment in the upland areas and heavy REEs (HREEs) in lowland areas. SiO 2 , Al 2 O 3 and Fe 2 O 3 are positively correlated with most of the traces and REEs (Co, Cu, Nb, Ni, Mo, Pb, Sc, V, Zn, Zr, La, Ce, Sm, Tb, Dy, Er, Yb), pointing to the fact that they may be incorporated into newly formed clay minerals and oxides. Ba, Cr, Ga, Rb, S, Sr, Y, Pr and Nd behave like alkalis and alkaline earths, and are thus highly mobile during weathering. [ABSTRACT FROM AUTHOR]

The generation of plate tectonics on Earth relies on complex mechanisms for shear localization, as well as for the retention and reactivation of weak zones in the cold ductile lithosphere. Pervasive mylonitization, wherein zones of high deformation coincide with extensive mineral grain size reduction, is an important clue to this process. In that regard, the grain-damage model of lithospheric weakening provides a physical framework for both mylonitization and plate generation, and accounts for the competition between grain size reduction by deformation and damage, and healing by grain growth. Zener pinning at the evolving interface between mineral components, such as olivine and pyroxene, plays a key role in helping drive grains to small mylonitic sizes during deformation, and then retards their growth once deformation ceases. The combined effects of damage and pinning, however, rely on the efficiency of inter-grain mixing between phases (e.g., olivine and pyroxene) and grain dispersal, which likely depends on grain size itself. Here we present a new model for inter-grain mixing and damage and the onset of rapid mixing. The model considers the competition between the formation of new grains behind a receding interphase triple junction (e.g., olivine growing into a boundary between two pyroxene grains) and their severance or spalling during progressive deformation and damage. The newly formed grains of one phase are then transported along the opposing phase's grain-boundaries and the two phases become dispersed at the grain-scale in a growing mixed layer. The small intermixed grains also affect the grain evolution of the surrounding host grains by Zener pinning, and hence influence the rheology and growth of the mixed layer. As the grains in the mixed layer shrink, subsequently spalled new grains are also smaller, causing a feedback that leads to more rapid mixing and shear localization in the mixed layer. The early stages of mixing can be compared to laboratory experiments, but the transition to a steady-state localized mixed layer, as a proxy for a mylonitic zone, can take a few million years at mid lithospheric conditions. Moreover, a transition in mixing efficiency occurs as grain size reduces, which can induce hysteretic behavior, wherein strong, slowly deforming regions can co-exist with weak rapidly deforming zones, analogous to plate tectonic states with large strong plates and narrow weak boundaries. [ABSTRACT FROM AUTHOR]

The estimation of 3D grain size distributions (GSDs) in mylonites is key to understanding the rheological properties of crystalline aggregates and to constraining dynamic recrystallization models. This paper investigates whether a common stereological method, the Saltykov method, is appropriate for the study of GSDs in mylonites. In addition, we present a new stereological method, named the two-step method, which estimates a lognormal probability density function describing the 3D GSD. Both methods are tested for reproducibility and accuracy using natural and synthetic data sets. The main conclusion is that both methods are accurate and simple enough to be systematically used in recrystallized aggregates with near-equant grains. The Saltykov method is particularly suitable for estimating the volume percentage of particular grain-size fractions with an absolute uncertainty of ±5 in the estimates. The two-step method is suitable for quantifying the shape of the actual 3D GSD in recrystallized rocks using a single value, the multiplicative standard deviation (MSD) parameter, and providing a precision in the estimate typically better than 5%. The novel method provides a MSD value in recrystallized quartz that differs from previous estimates based on apparent 2D GSDs, highlighting the inconvenience of using apparent GSDs for such tasks. [ABSTRACT FROM AUTHOR]

This petrographic study of rock samples from the area of a large-scale shear zone, the Sottung-Jurmo shear zone, in SW Finland, illuminates the thermal development of and strain distribution within the rocks during the last development stages of the shear zone. The results indicate relatively high temperatures during the deformation phases that created the gneisses and the mylonites, strain partitioning through time and continued transpression from S-SW until at least c. 1.79 Ga. This has implications to the latest tectonic models since the results together with previous studies suggest a compartmentalisation of regional stresses between the area SW of the shear zone and central Fennoscandia. The results also suggest that the uplift rate increased during the late stage of the transpression.

La Faja de Deformación La Horqueta (FDLH) es la sexta faja de cizalla reconocida en la sierra de Velasco, Sierras Pampeanas, NO de Argentina. Esta faja separa dos tipos de metagranitoides de características petrográficas y geoquímicas contrastadas: metagranodioritas, hacia el SO, consisten en granodioritas con biotita-allanita-titanita débilmente peraluminosas que se asemejan a los granitoides tipo I del sur de la sierra de Velasco y de sierras aledañas pertenecientes al arco magmático famatiniano, mientras que los metagranitos, hacia el NE, consisten en monzogranitos con biotita-muscovita fuertemente peraluminosos idénticos al Ortogneis Antinaco del norte de la sierra de Velasco, el cual pertenece a un cinturón de granitoides tipo S interno, respecto del arco famatiniano. La FDLH, desarrollada allí a partir de estos dos protolitos, tinen un espesor de 1 a 2 km y está compuesta por protomiolonitas y milonitas que desarrollan una fuerte foliación milonítica de rumbo NNO y fuerte inclinación al E. El análisis cinemático indicaría un régimen deformacional transpresivo principal con componente inverso-sinestral, acompañado por una componente de transcurrencia destral N-S subordinada, que habría puesto en contacto ambos ambiente geotectónicos.

Structural analyses in the northern part of the North Patagonia Massif, in the foliated Caita Có granite and in La Seña and Pangaré mylonites, indicate that the pluton was intruded as a sheet-like body into an opening pull-apart structure during the Gondwana Orogeny. Geochronological studies in the massif indicate a first, lower to middle Permian stage of regional deformation, related to movements during indentation tectonics, with emplacement of foliated granites in the western and central areas of the North Patagonian Massif. Between the upper Permian and lower Triassic, evidence indicates emplacement of undeformed granitic bodies in the central part of the North Patagonian Massif. A second pulse of deformation between the middle and upper Triassic is related to the emplacement of the Caita Có granite, the development of mylonitic belts, and the opening of the Los Menucos Basin. During this pulse of deformation, compression direction was from the eastern quadrant. [ABSTRACT FROM AUTHOR]

The Karakoram Shear Zone (KSZ) is a northwest-southeast trending dextral ductile shear zone that has mylonitized the Tangste and Darbuk granitoids of the southern margin of the Asian plate. Kinematic vorticity (W) has been estimated in 6 mylonitized Tangste granite samples, using Porphyroclast Hyperbolic Distribution (PHD) and Shear Band (SB) Analyses methods on well-developed quartz and feldspar porphyroclasts, and synthetic and antithetic shear bands respectively to visualize the overall deformation of the KSZ. The PHD and SB analyses yield W values ranging from W=0.29 to 0.43 and 0.45 to 0.93, respectively, thus indicating distinct pure and simple shear dominant regimes during different stages of the evolution of the KSZ. Strain has essentially been pure shear when southern edge of the Asian plate was initially juxtaposed against the Indian plate around 70 Ma, and changed to simple shear, possibly during the reactivation of this shear zone during 21-13 Ma to produce the shear bands. [ABSTRACT FROM AUTHOR]

U-Pb and Hf zircon (sensitive high-resolution ion microprobe -SHRIMP- and laser ablation-inductively coupled plasma-mass spectrometry -LA-ICP-MS-), Ar/Ar hornblende and muscovite, and Rb-Sr whole rock-muscovite isochron data from the mylonites of the Sarandí del Yí Shear Zone, Uruguay, were obtained in order to assess the tectonothermal evolution of this crustal-scale structure. Integration of these results with available kinematic, structural, and microstructural data of the shear zone as well as with geochronological data from the adjacent blocks allowed to constrain the onset of deformation along the shear zone at 630-625 Ma during the collision of the Nico Pérez Terrane and the Río de la Plata Craton. The shear zone underwent dextral shearing up to 596 Ma under upper to middle amphibolite facies conditions, which was succeeded by sinistral shearing under lower amphibolite to upper greenschist facies conditions until at least 584 Ma. After emplacement of the Cerro Caperuza granite at 570 Ma, the shear zone underwent only cataclastic deformation between the late Ediacaran and the Cambrian. The Sarandí del Yí Shear Zone is thus related to the syncollisional to postcollisional evolution of the amalgamation of the Río de la Plata Craton and the Nico Pérez Terrane. Furthermore, the obtained data reveal that strain partitioning and localization with time, magmatism emplacement, and fluid circulation are key processes affecting the isotopic systems in mylonitic belts, revealing the complexity in assessing the age of deformation of long-lived shear zones. [ABSTRACT FROM AUTHOR]

We analyse the microstructure and quartz LPOs of 36 layers of varying composition from a several-meter thick sequence of amphibolite-facies metacherts and related mica-garnet-plagioclase-quartz bearing schists from the central Southern Alps in the mylonite zone related to the Alpine Fault. Quartz contents vary from ∼10 to 100% and all of the LPO fabric skeletons are similar, featuring an asymmetric single girdle of [c]-axes inclined ∼30° away from the ZY plane. LPO strength is typically low at quartz contents <70% (M Index of ∼0.05) whereas it may be very high for nearly pure quartz rocks (M Index of up to 4.0). We attribute this change to a sparseness of interphase boundaries in the more quartzose rocks, a reduction in grain-boundary pinning, and a corresponding efficiency of grain boundary migration during dynamic recrystallization. The transition corresponds to a Zener parameter of approximately 700. In layers poor in quartz and rich in mica, the quartz grain size was kept small, and phase-boundary density, high. This may have promoted grain-size sensitive creep and dislocation glide in mica. Dislocation creep in the interspersed quartz grains was correspondingly reduced, and weaker quartz LPOs were produced. In highly quartzose layers, quartz grain boundaries experienced little drag or pinning from impurity phases and were able to migrate quickly into higher strain-energy grains. Preferential consumption of poorly oriented grains strengthened quartz LPOs, geometrically softened the dislocation creep process in these quartzose layers, and contributed to grain coarsening. The lack of evidence for instabilities in the thinly layered (<1 mm, quartz-rich vs. mica-rich) mylonite implies that a combination of deformation mechanisms, grain-size sensitive flow and dislocation creep, in the layers were able to accommodate a nearly homogeneous deformation between the different composition layers. [ABSTRACT FROM AUTHOR]

The wide distribution of mylonites and phyllonites in the Pohorje and Kobansko areas is the consequence of the extensive northward thrusting caused by the Cretaceous collision of the European and African plates. In the accretionary wedge meta-ultrabasites withparts of the oceanic crust were incorporated between the nappes and tectonically transported to higher levels. Ascending of the cizlakite and granodiorite melts resulted into the core structure of the Pohorje. The highest were emplaced the most metamorphosed rocksof the eclogite facies. Rocks of the Pohorje Formation continue on the other side of the Ribnica-Selnica stair-like structure in the Kobansko area. The influence of the thrusting is noticed also in the Pohorje granodiorite, which is slightly altered in the zeolite faciesconditions. Part of it (Pekrska gorca) was detached and probably in the final stage of thrusting displaced for at least 11 km toward NNE. The eastern prolongation of the Pohorje massif subsided toward the Panonian basin. The pivotal Labot fault separated Pohorje and Kobansko areas from Karavanke and Strojna in Neogene. Along it the NW part of the Pohorje block was downthrown.

Paleoproterozoic Lingtse orthogneiss shows three dominant structural associations in the Sikkim Himalayan fold thrust belt (FTB): (a) these rocks locally define the Pelling thrust (PT) zone that lies in the footwall of the Main Central thrust (MCT), (b) they occur as linear, almost N–S trending imbricates within the PT sheet, and (c) they form a N–S trending ∼13.5 km long, ∼2 km wide discontinuous linear mylonitic klippen within the Daling Formation of the lower Lesser Himalayan Sequence (LHS) that lie east of the N–S trending Teesta valley. The klippe records N–S trending, strong L-tectonites that are spatially associated with LS and SL tectonites; the stretching lineations are oriented along the transport direction. These rocks are exposed along three mylonite zones. From north to south these are the Phodong mylonite zone, the Tumin Eingee mylonite zone and the Rumtek mylonite zone. All the three mylonite zones are overall folded along gently plunging, transport-parallel, N–S trending synform with the strongest L fabric developed along the hinge zone, and its dominance decreases progressively toward the eastern and western structural contacts of the klippen. The highest constrictional strain is recorded in Phodong and Tumin-Eingee mylonite zones and decreases south toward Rumtek mylonite zone. The long axes ( X ) of the quartz strain ellipsoids show ∼N–S trend and gentle plunge that agrees with the stretching lineations observed in the field. Additionally, the XY plane of the generated ellipsoids overall agree with the observed variation of the mylonitic foliation planes in the field from all the three locations. Microstructural analysis from the mylonite klippe suggests that quartz and feldspar have undergone ∼35% and ∼76% grain size reduction with respect to the protolith. Dislocation creep and microfracturing are the dominant deformation mechanisms in quartz and feldspar, respectively. The grain-size reduction is more prominent near the eastern and the western boundaries as they mark structural contacts between the L tectonites and the Daling phyllites. Strain is partitioned between quartz and feldspar in the mylonite zones with quartz recording constrictional strain while feldspar records flattening strain. We explain this variation due to their different deformation mechanisms. Shear-sense analysis of feldspar porphyroclasts suggest dominantly transport-parallel flow along with a component of transport-perpendicular flow in the mylonite zones. Existing models on mechanism of L tectonite formation and transport-parallel folds do not completely explain our observations. We propose the possible existence of a lateral ramp beneath the PT sheet to explain the generation of L tectonites and transport-parallel fold in the study area. [ABSTRACT FROM AUTHOR]

The geodynamic framework of the South China Craton in the Early Paleozoic and Early Mesozoic has been modeled as developing through either oceanic convergence or intracontinental settings. On the basis of an integrated structural, geochemical, zircon U-Pb and Hf isotopic, and mica 40Ar/39Ar geochronologic study we establish that an intracontinental setting is currently the best fit for the available data. Our results suggest that widespread tectonomagmatic activity involving granite emplacement and mylonitic deformation occurred during two distinct stages: ~435-415 Ma and ~230-210 Ma. The coeval nature of emplacement of the plutons and their ductile deformation is corroborated by the subparallel orientation of the mylonitic foliation along the pluton margins, gneissose foliation in the middle part of pluton, the magmatic foliation within the plutons, and the schistosity in the surrounding metamorphosed country rocks. The 435-415 Ma granitoids exhibit peraluminous, high-K characteristics, and zircons show negative εHf( t) values (average −6.2, n = 66), and Paleoproterozoic two-stage model ages of circa 2.21-1.64 Ga (average 1.84 Ga). The data suggest that the Early Paleozoic plutons were derived from the partial melting of the Paleoproterozoic basement of the Cathaysia Block. The 230-210 Ma granites are potassic and have zircons with εHf( t) values of −2.8-−8.7 (average −5.4, n = 62), corresponding to TDM2 ages ranging from 2.0 to 1.44 Ga (average 1.64 Ga), suggesting that the Early Mesozoic partial melts in Cathaysia were also derived from basement. The geochemical distinction between the two phases of granites traces continental crustal evolution with time, with the Early Mesozoic crust enriched in potassium, silicon, and aluminum, but deficient in calcium, relative to the Paleozoic crust. Kinematical investigations provide evidence for an early-stage ductile deformation with a doubly vergent thrusting pattern dated at 433 ± 1 to 428 ± 1 Ma (40Ar/39Ar furnace step-heating pseudoplateau ages obtained on muscovite and biotite from mylonite and deformed granite) and a late-stage strike-slip movement with sinistral sense of ductile shearing at 232 ± 1 to 234 ± 1 Ma (40Ar/39Ar furnace step-heating pseudoplateau ages) along an E-W direction. The geological, geochemical, and isotopic signatures likely reflect far-field effects in response to continental assembly events at these times. [ABSTRACT FROM AUTHOR]

The Palaeozoic rocks of the Dingle Peninsula provide a record of the evolution of the Caledonides, Acadides and Variscides. The succession ranges from Early Ordovician deep-water sediments, through Silurian shallow marine to non-marine sediments and volcanic rocks to an Old Red Sandstone (ORS) succession topped by Carboniferous marine shales. Comparison of structural styles in the unconformity-bounded groups, together with a detailed analysis of fault zones, allows the tectonic history to be deduced. The older rocks record Caledonian processes on the margin of Avalonia during Early Ordovician time and convergence then soft collision with Laurentia during Silurian time. The Dingle Basin was developed during the late Silurian – Early Devonian transtension in the Iapetus suture zone and was inverted in the latest Emsian Acadian orogenic episode. Post-Dingle Group ORS groups in the north of the peninsula were deposited in a syn-rift footwall block to the main Munster Basin. The Acadian transpressional and Munster Basin extensional structures were reactivated or overprinted in the Variscan deformation such that Acadian folds are transected by Variscan cleavage in both plan and vertical views. After Iapetus closure, changes in the tectonic regime are believed to be a result of adjustments in the geometry of subduction of the Rheic Ocean. [ABSTRACT FROM PUBLISHER]

The structural-geological field survey (1:10,000 scale) of the south-western portions of the Aspromonte Massif (southern Calabria, Italy) clearly reveal a nappe-pile stack made of three main tectono-metamorphic units piled-up by means of tectonic contacts with associated cataclastic or mylonitic rocks developed at various crustal depth. These three units are from top to bottom: the Stilo Unit, the Aspromonte-Peloritani Unit (APU) and the Madonna di Polsi Unit. As part of an ongoing project, we have constructed a structural and metamorphic GIS database based on 285 sampling stations, which allows for metamorphic and structural maps to be obtained in the form of the metamorphic zoning of the SU.. This unit, made up of low greenschist- to low amphibolite-facies Palaeozoic metamorphic rocks, lies through a brittle tectonic contact over the APU, which is made up of amphibolite-facies metamorphic rocks intruded by late Hercynian peraluminous both locally overprinted by Alpine type metamorphism. The petrographic-derived metamorphic zonation chart here presented has been created after the detailed petrographic analysis of about 80 thin sections and the recognition of mineral assemblages of a thermal metamorphic aureole (biotite, muscovite and andalusite blastesis) produced by Late Hercynian magmatic bodies intruded into metapelites (Punta d'Atò granitoids, outcropping in the northern part of the mapped area). [ABSTRACT FROM PUBLISHER]

This study represents the first detailed tectono-metamorphic map of the metamorphic complexes cropping out in the inner portion of the Variscan belt in north-central Sardinia Island (Italy). The Main Map is at a 1:30.000 scale and covers an area ofc.148 km2. It is based on 1: 10.000 scale classic field mapping and represents an overview of the lithological and structural complexities documented in the metasedimentary and migmatite domes cropping out along the central transect of the Posada-Asinara Line (PAL). The PAL is a crustal scale discontinuity that divides migmatites from the metasedimentary sequences affected by greenschist- to amphibolite-facies metamorphism. The map shows the orientations of the superimposed foliations, fold axes and mineral lineations on the basis of geometric crosscutting relationships and, for the first time, the location of ductile-brittle and brittle shear zones developed during the long-lived activity of the Posada-Asinara Line. [ABSTRACT FROM PUBLISHER]

The Fodjomekwet-Fotouni shear zone (FFSZ) belongs to the western part of the Central Cameroon Shear Zone (CCSZ). From the border to the core, the FFSZ is made of: (i) protomylonites, (ii) garnet-sillimanite mylonites and (iii) garnet-sillimanite ultramylonites. Structural data indicate (1) a N40°E mylonitic foliation trend (strongly dipping toward NE or SW) with strike varying between N34°E–78°NW at Fotouni and N52°E–61°SE at Fodjomekwet; (2) a stretching lineation (plunging toward NE or SW) with best line varying between 224/4 at Fotouni and 232/10 at Fodjomekwet; (3) sinistral (N–S) and dextral (NE–SW) kinematic markers, respectively correlated to the D 2 and D 3 of the CCSZ deformation phases. The presence of relic sinistral shear markers wrapped by dominant dextral shear markers rendered difficult the understanding of the FFSZ evolution. Anisotropy of magnetic susceptibility in the FFSZ shows that magnetic fabrics are characterized by dominant NE–SW and locally N–S to NNE–SSW trending foliation and lineation. These directions show that mylonitisation was initiated in the N–S direction during the sinistral syn-D 2 shearing, evolved and reoriented toward NNE–SSW and NE–SW between 613 and 590 Ma. The sinistral shearing event was thereafter followed by a NE–SW trend dextral syn-D 3 reactivation of the N50°E fault between 580 and 552 Ma. The sinistral phase seems to have favoured the evolution of fractures in pull-apart structures. The pull-apart opening of fracture enables the upwelling of magma and the emplacement of the Fomopéa and the Bandja plutons (FBP). This synkinematic emplacement is witnessed by the elongated shape and the NE–SW trending direction of these massifs. Kinematic emplacement of FBP controlled by active shear zone is comparable to syntectonic plutonism in eastern Nigeria and NE Brazil. [ABSTRACT FROM AUTHOR]

Archaeological excavations and restoration of a palatial Roman housing complex in Ephesos, Turkey yielded 40 wall-decorating plates of folded mylonitic marble ( Cipollino verde ), derived from the internal Hellenides near Karystos, Greece. Cipollino verde was commonly used for decoration purposes in Roman buildings. The plates were serial-sectioned from a single quarried block of 1,25 m 3 and provided a research opportunity for detailed reconstruction of the 3D geometry of meterscale folds in mylonitized marble. A GOCAD model is used to visualize the internal fold structures of the marble, comprising curtain folds and multilayered sheath folds. The sheath folds are unusual in that they have their intermediate axis normal to the parent layering. This agrees with regional tectonic studies, which suggest that Cipollino verde structures formed by local constrictional non-coaxial flow. Sheath fold cross-section geometry, exposed on the surface of a plate or outcrop, is found to be independent of the intersection angle of the fold structure with the studied plane. Consequently, a single surface cannot be used as an indicator of the three-dimensional geometry of transected sheath folds. [ABSTRACT FROM AUTHOR]