Ductile Shear Research Articles

Calcite e-twins as a tectonic indicator, paleo stress pattern and structural evolution of the Zagros hinterland, SE Iran

Through the examination of calcite twins, this research outlines the tectonic development and paleo stress patterns of the Paleozoic Routshon complex situated in the southeastern segment of the Sanandaj–Sirjan zone, a hinterland region of the Zagros orogeny in southeastern Iran. The study of orogenic phase indicates that the deformation event affecting the southern sector of the Sanandaj-Sirjan zone aligns with the Cimmerian orogenic phase of the Late Triassic period. A variety of structural features at both map and outcrop scales highlight the importance of slip partitioning in the structural evolution of this region, driven by inclined transpression. Observations suggest that the deformation related to contractional components includes steeply to moderately plunging folds, dip-slip domain deformation primarily involving thrusts, and ongoing deformation by strike-slip component motion, which results in thrust-related ductile shear zones. The analysis of calcite c-axis fabrics from mylonite samples obtained from these shear zones indicates a low-temperature monoclinic pattern of non-coaxial deformation. This deformation type underscores the impact of the strike-slip component in the development of progressive simple shear within thrust-related shear zones in this segment of the Sanandaj-Sirjan zone. Dynamic analysis of c-axis fabric data reveals a NE-SW orientation for the principal compressive axes (σ1) in this area. This direction, corroborated by additional data such as fault surface, GPS, and earthquake focal mechanism data, confirms that the orientation of the compressive axes (σ1) has remained consistent from the Late Triassic to the present.

Evidence of back-folding in the Beas Valley puts Himalayan tectonic models on trial

Field structural observations from the Himachal Himalaya, in the northwest of the mountain belt, challenge existing tectonic models and raise questions as to their validity. Microstructures and geochronological data reveal two discrete episodes of Barrovian metamorphism, the earliest during the Eocene-Oligocene transition, before an early period of recumbent folding. This metamorphic event occurred in association with a km-scale extensional ductile shear zone that is itself now recumbently folded on the km-scale, with an axial plane pressure solution cleavage. The Eocene-Oligocene gneiss complex is thus exposed in its core. The second episode of Barrovian metamorphism occurred in association with another regional-scale extensional shear zone during the Oligo-Miocene transition, thus synchronous to the South Tibetan Detachment System. This transects the recumbent fold stack. Microstructures show that the Main Central Thrust was initiated after the second phase of extension, and the associated second episode of Barrovian metamorphism had ceased operating. Further, the previously unrecognized km-scale Phojal Back-fold affects all of the above structures. Confusion caused by the misidentification of this structure led to the tectonic-wedge model, but this hypothesis can be invalidated by the structural evidence presented here. Our data support an alternative hypothesis that requires tectonic mode-switches in association with a succession of accretion events as India indents into Eurasia.

Oroclinal arcs of the Variscan Belt: a consequence of transpression during the consolidation of Pangaea

Arcuate traces of large structures characterize many mountain chains. The Variscan Belt is not an exception and traces one of the tightest oroclines on Earth, the Ibero-Armorican Arc, as well as more open oroclines in the Eastern Moroccan Meseta, Central Iberia, the French Massif Central and the Bohemian Massif. All the Variscan arcs are considered to be true or secondary oroclines and are late orogenic features, but their timing and mechanisms of deformation differ. Models explaining their origin have been proposed for some individual arcs and are generally controversial. This paper aims to interprete the ensemble of Variscan arcs, highlighting their age relative to previous orogenic features as well as to those associated with arc development. Several mechanisms operated to form the arcs, the most important being ductile transcurrent shearing and rigid–plastic indentation, with shortening parallel to the orogen having a secondary role. These mechanisms acted at different time intervals, their participation or relative importance differs for each arc and they gave rise to distinct associated structures. The development of the arcs is viewed as related to late Variscan dextral transpression provoked by the displacement of Laurussia to the east relative to Gondwana during the Serpukhovian to early Permian.

PORPHYRY COPPER RECURRENCE IN THE ANDES OF CHILE AND ARGENTINA

Abstract Porphyry Cu deposits in the Chilean and Argentinian central Andes occur in a series of orogen-parallel magmatic arcs, which migrated episodically eastward since the Early Cretaceous. The three Cenozoic belts, corresponding to Paleocene-early Eocene, middle Eocene-early Oligocene, and Miocene-early Pliocene epochs, cut obliquely across a composite belt of subeconomic porphyry Cu mineralization formed at several times during the Late Carboniferous to Triassic interval. Based mainly on U-Pb zircon and Re-Os molybdenite ages, 10 Cenozoic porphyry Cu centers, including major deposits in all three of the Cenozoic belts, occupy the same sites as late Paleozoic-Triassic porphyry Cu mineralization where their respective magmatic arcs cross one another. The sites of recurrent porphyry Cu mineralization are believed to be underlain by long-lived dike conduits that were utilized at least twice—first in the late Paleozoic-Triassic and then again in the Cenozoic—to rapidly transmit hydrous magma from deep to shallow levels of the crust. Contenders for preferential dike localization include arc-oblique fault and subjacent ductile shear zones—transcrustal discontinuities—long hypothesized to be present in the region, particularly where they intersect magmatic arcs and associated porphyry Cu belts. Regardless of the controls on porphyry Cu recurrence, alteration zones hosted by late Paleozoic-Triassic volcanic and plutonic rocks in the central Andes of Chile and Argentina must be considered prime exploration targets for potentially large, high-grade Cenozoic porphyry Cu deposits.

Evolution of the rare-metal mineralization system associated with collision-related pegmatites in the western Altyn Tagh Orogen, Tugeman, NW China

On a regional scale, rare-metal pegmatite groups (pegmatites of common origin) display a zonal distribution with different chemical compositions. Whether the regional distribution of such pegmatites dike in a convergent zone is related to dehydration melting of different micas remains unclear. In recent years, the Altyn Tagh Orogen (ATO) in the northwest China has gained attention due to economically significant rare-metal mineralization. Our study focuses on the geochronology of columbite-group minerals (CGMs) and geochemistry of tourmaline from the Be-mineralized pegmatites in the Tugeman and Ayage Be deposits. CGM dating shows that these Be-mineralized pegmatites formed between 485 and 484 Ma. Subsequent hydrothermal activity, occurring around 453–450 Ma, is related to the development of a ductile shear zone. Evidence from tourmaline mineralogy and major elements reveals that the evolution of the Be-mineralized pegmatite is influenced by crystallization differentiation and subsequent fluid exsolution. Tourmalines from these pegmatites exhibit B isotope compositions ranging from –12.2 ‰ to –14.8 ‰, indicating a crustal metasediment source. Based on these findings, we propose a model for the rare-metal mineralization system in the Tugeman area: the sequential formation of Be to Li-mineralized pegmatites likely results from dehydration melting of muscovite and biotite during prolonged subduction and collision between the South Altyn Subduction–Collision Belt (SAB) and Central Altyn Block (CAB).

A clockwise P‐T‐t-D path of the northern Patagonia basement: implications for the Permian evolution of the Panthalassan margin of Gondwana

ABSTRACT The geologic evolution of the Gondwanide orogen recorded during the late Palaeozoic along the Panthalassan border of Gondwana is related to an active continental margin, though it is still debatable whether it was built by collision or accretion. To disentangle its orogenic processes and provide constraints on the orogen type, we characterize the physicochemical conditions of metamorphism and associated deformation of the Mina Gonzalito Metamorphic Complex from northern Patagonia (41°28’30” S-65°40’30” W). New field mapping and comprehensive petrochronologic analysis constrain the evolution of a thick-skinned fold and thrust belt in a retroarc setting, which is spatially and temporally related to the geometry and kinematics of the right-lateral reverse El Jagüelito ductile shear zone. Metamorphic evolution resulted in a clockwise P‐T‐t-D path reflecting three stages of continuously changing P-T conditions under high-pressure (8.0–9.9 kbar) amphibolite facies (540–680°C) during a single-phase progressive ductile deformation event. Monazite ages ranging from 303 ± 5 to 252 ± 6 Ma account for the single-phase regional tectono-metamorphic event spanning ~ 50 My. Our results, integrated with regional data, led to interpreting the late Palaeozoic Gondwanide orogen developed along the Panthalassan margin of Gondwana as accretionary in advancing-mode.

Ductile shear damage micromechanisms studied by correlative multiscale nanotomography and SEM/EBSD for a recrystallized aluminum alloy 2198 T8

The damage mechanisms of ductile fracture under shear loading of an aluminum alloy 2198T8R were studied using flat thin-sheet samples. One sample was loaded until 85% of the failure displacement and then unloaded, and another one was loaded up to failure. To overcome the inherent shortcomings of nanotomography concerning the investigation of flat samples, synchrotron nano-laminography was applied to the pre-loaded sample and provided structural information down to the nanometer scale, allowing ductile damage nucleation and evolution to be studied. The damage features, including flat cracks and intermetallic particle-related damage, were visualized in 3D from the highly-deformed shear band region. Using nano-laminography, no nano-voids were found. The damaged shear ligament was also observed after polishing via destructive correlative scanning electron microscope (SEM) and electron back-scatter diffraction (EBSD) which suggests that the detrimental flat cracks were both intergranular and transgranular. The flat cracks were related to highly-deformed bands. No nano-voids could be found using SEM analysis. Fractography on the second broken sample revealed that the flat cracks contained hardly observable nanometer-sized dimples. The final coalescence region was covered by sub-micrometer-sized dimples, inside which dispersoid particles were present. The fact that no nano-void was found for the pre-deformed sample implies that the nucleation, growth and coalescence of these sub-micrometer-sized voids occur at late stages of the loading history.

Geochronological and sulfide geochemical evidence for gold mineralization related to post-collisional magmatism in the Wulonggou goldfield of the East Kunlun Orogen, northern Tibet

The relationships among magmatism, orogeny, and gold mineralization in lode gold deposits have been controversial for a long time. The East Kunlun Orogen in northern Tibet has experienced a complex Paleo-Tethyan tectono-magmatic evolution and hosts the largest Wulonggou lode gold deposits (>120 t). The Shuizhadonggou-Huanglonggou (SZDG-HLG) gold deposit is the largest in the Wulonggou goldfield and is an excellent candidate for solving the controversy through detailed field observations, magmatic and hydrothermal mineral in-situ U-Pb dating, and gold-bearing sulfide in-situ trace elements and sulfur-lead isotopes. Gold mineralization in the SZDG-HLG deposit includes sulfide disseminations and minor amounts of small quartz-sulfide veins in hydrothermal alteration assemblages along a ductile shear zone. Gold mineralization can be divided into four stages based on crosscutting relationships and associated alteration assemblages. U-Pb ages from magmatic zircon, hydrothermal rutile, and hydrothermal monazite support field relationships that gold mineralization was later than the ductile deformation but synchronous with the post-collisional porphyries. Gold mineralization evolved from high to low temperatures (560℃ to less than 300℃), accompanied by markedly decreased sulfur fugacities (–6.3 to –12.5) and oxygen fugacities (–20.7 to –40.5). The gradual increase of sulfur isotopes (δ34S) from stage 1 (–0.9 to 1.8 ‰ in Py1a) to stage 3 (up to 8 ‰) is ascribed to sulfide precipitation from a finite fluid reservoir, while the sharp decrease of δ34S values in stage 4 (down to –24 ‰) is caused by the fluid oxidation. The negative covariations of δ34S and Se/S values, positive correlations between Au and chalcophile elements (As, Sb, Te, and Bi) in most sulfides, and overlapping lead isotopes of sulfides and Late Triassic granodiorite-diorite confirm a post-collisional magmatic gold and sulfur source. This study demonstrates that lode gold deposits in Phanerozoic orogens having similar characteristics to their counterparts in Archean metamorphic terranes can be genetically related to post-collisional magmatism.

Using subtle variations in groundwater geochemistry to identify the proximity of individual geological structures: a case study from the Grimsel Test Site (Switzerland)

Understanding groundwater flow and the evolution of groundwater chemistry in networks of fractures in crystalline rock is of fundamental interest for geothermal projects, nuclear waste disposal, and groundwater resources. Groundwater chemistry at a given location is typically conceived of being of a specific ‘type’ (e.g. meteoric, juvenile, connate, marine), with associated chemical types controlled through water–rock interactions. Minor chemical variations between groundwater sample locations with the same chemical type are generally considered as ‘noise’ in the geochemical data. Here, we argue that this noise contains useful information on the mineral phases encountered by the groundwater as it travels through specific flow pathways. We analyse the spatial variability of groundwater chemistry around the Grimsel Test Site (GTS), Switzerland, where groundwater is hosted in two lithologies: the Central Aar Granite and the Grimsel Granodiorite, where flow occurs predominantly in a fracture network created by brittle reactivation of ductile shear zones. Groundwater chemistry is analysed using principal component and hierarchical cluster analyses, which identify two groundwater types based on their chemistry. The primary control on groundwater type is the host rock lithology (granite/granodiorite). While the spatial variability of groundwater chemistry within each of the two lithologies is small, statistical analysis of the data shows similar groundwater chemistry in borehole intervals that are crosscut by similar geological structures, implying a structural control on groundwater chemistry. Our research shows that subtle chemical variations in groundwater provide information on fracture network connectivity and the proximity of geological features, that specific volumes of groundwater has interacted with.

The newly discovered Attu carbonatite of West Greenland: A Mesoproterozoic dyke intrusion enriched in primary Sr-Ba-REE minerals

A newly discovered ca. 1.5 Ga old carbonatite dyke in central West Greenland is characterized by very high contents of rare earth elements (REE), Sr and Ba (up to 12.4 wt% total REE, 14.6 wt% SrO and 12.4 wt% BaO). The dyke occurrence is named ‘Attu carbonatite’ after the nearby village Attu. The carbonatite is primarily composed of carbonate minerals such as Sr-rich calcite (CaCO3) and dolomite (CaMg(CO3)2), huntite (Mg3Ca(CO3)4), strontianite (SrCO3), alstonite (CaBa(CO3)2), burbankite ((Na,Ca)3(Sr,Ba, Ce)3(CO3)5) and daqingshanite ((Sr,Ca,Ba)3(Ce, La)(PO4)(CO3)3−x(OH, F)x). In addition to the wide range of carbonate minerals, the carbonatite contains coarse-grained monazite(Ce), apatite and magnetite. Barite occurs as discrete crystals together with the rock-forming carbonates. Texturally, the carbonatite rock displays abundant intimately intergrown fine- to medium-grained Ca, Sr, Ba and REE carbonate minerals, which exhibit prominent exsolution textures within calcite, burbankite, strontianite and dolomite hosts, as well as in the apatite crystals. Several different exsolution textures are observed: 1) alstonite in calcite; 2) daqingshanite in calcite; 3) daqingshanite in burbankite; 4) MgBa carbonate in strontianite; 5) MgBa carbonate in calcite and strontianite in dolomite; 6) strontianite in apatite; and 7) monazite(Ce) in apatite. The carbonatite dyke is foliated and exsolution textures are observed internally in the foliation-defining minerals indicating that exsolution occurred after the main deformation event.The carbonatite magma intruded into Archaean basement gneisses that had been affected by the Nagssugtoqidian tectono-metamorphic event at approximately 1850 Ma. Magmatic monazite(Ce) crystals from the carbonatite yield a UPb age of 1565 ± 53 Ma, which is the current best estimate of dyke emplacement. Monazite(Ce) that exsolved from apatite and calcite yields a UPb age of 1492 ± 33 Ma, which is within the analytical uncertainty of the primary magmatic monazite UPb age. However, the UPb age determinations suggest that mineral exsolution occurred a few million years after carbonatite magma emplacement, in response to further cooling of the deep-seated dyke (tectonically-induced uplift?). Dyke emplacement may have occurred within an active ductile shear zone, which helps to explain the foliation of the carbonatite rock, predating cooling-related mineral exsolution. Country rock fenitization by fluids that emanated from the carbonatite dyke intrusion is recorded by the increasing abundance of mafic silicates such as Ba-rich phlogopite at the contact zone.

Geochronology of the Paleoproterozoic Helanshan ductile shear zones: Insights into temporal framework of polyphase deformation in the Khondalite Belt, North China Craton

The Khondalite Belt has been regarded as a Paleoproterozoic collisional orogen in the northwestern North China Craton, but the geochronological framework of polyphase deformation in the Khondalite Belt still remains unclear, particularly the timing of orogen-parallel ductile shearing. In this study, we conducted field-based structural analysis and LA-ICP-MS zircon U-Pb geochronology on the Helanshan ductile shear zones (HDSZ) in the Khondalite Belt. The results show that three pre-kinematic leucocratic dykes have been reworked by the shear zones and yielded magmatic zircon ages of 1934 ± 13 Ma, 1924 ± 5 Ma and 1920 ± 24 Ma. Two syn-kinematic leucocratic dykes appeared along the mylonitic foliations and were dated at 1887 ± 21 Ma and 1871 ± 28 Ma. A post-kinematic leucocratic dyke cut the mylonitic foliations and gave a crystallization age of 1816 ± 28 Ma. These emplacement ages of shear zone-related dykes constrain the development of the HDSZ at some time between ∼ 1920 Ma and ∼ 1816 Ma. Meanwhile, zircon overgrowth rims from three intrusions that suffered the shear deformation exhibited metamorphic ages of 1904 ± 11 Ma, 1840 ± 13 Ma and 1826 ± 31 Ma. Similar metamorphic ages of 1901 ± 7 Ma, 1888 ± 8 Ma and 1849 ± 29 Ma were also obtained from high-temperature felsic, pelitic and calcitic mylonites. These results further unravel that the HDSZ most likely developed at ca. 1904–1826 Ma. Additionally, we have summarized available deformation-related geochronological data in the Khondalite Belt, and proposed that this belt underwent three major phases of deformation (D1-D3) in the Orosirian. D1 mainly produced overturned to recumbent isoclinal folds F1, penetrative transposition foliations S1 and mineral lineations L1 at ca. 1.97–1.93 Ga. Subsequently, D2 dominantly resulted in tight to open upright folds F2 at ca. 1.93–1.90 Ga. Later, D3 generated a series of NE- to E-trending orogen-parallel ductile shear zones at ca. 1.90–1.82 Ga. Together with multiple high-grade metamorphism and magmatism, the D1-D3 deformation recorded a complex protracted orogeny (>100 Myr) in response to the collision between the Yinshan and Ordos blocks.

Tectonic evolution of the South Tazin Lake shear zone; a major Arrowsmith-age (ca. 2.36 Ga) structure in the SW Rae craton, Laurentia

The Rae Province records the effects of multiple, staggered orogenies along both margins, leaving a complex record of tectonism within major crustal-scale shear zones. While the recently recognized 2.5–2.3 Ga Arrowsmith orogeny is now known to have affected a large tract of the western Rae, few related structures have been described. In this paper, we present the first detailed investigation of such a structure, the ‘South Tazin Lake shear zone’, localized along the Nolan-Zemlak lithotectonic domain boundary in the SW Rae. Multi-scale structural analysis undertaken herein documents the lithostructural zonation and strain progression across this structure and evaluates its context in the regional tectonic evolution. Strain intensity increases dramatically from north to south, with onset of thick, continuous intervals of protomylonite to ultramylonite toward and into a 3–5 km wide corridor that straddles the Nolan-Zemlak domain boundary and constitutes the core of the shear zone. New U-Pb zircon igneous crystallization ages are consistent with previous results for units across the Nolan-Zemlak domain boundary while 40Ar-39Ar thermochronology of hornblende and biotite provides temporal constraints on deformation and a more detailed thermal evolution than previous studies. An 40Ar-39Ar date of 2361 ± 12 Ma for hornblende overgrowing the sheared and recrystallized matrix in blastomylonitic rocks places a minimum age on D2 mylonitization. This, together with similar ages in the core of the shear zone from foliation-parallel hornblende, definitively links ductile shearing and the related S2 fabric along the South Tazin Lake shear zone with Arrowsmith orogenesis. Cooling rates suggest an initial accelerated phase (∼2360–2350 Ma) of post-Arrowsmith exhumation, but the absence of extensional shear-sense indicators implies control by structures outside of the immediate area; or masking by younger (Taltson-age) structures. This was likely followed by a much slower erosion-related (≤2.35 Ga) phase. All-told, the South Tazin Lake shear zone represents a significant Arrowsmith-age tectonic front that was reactivated only along its southern margin under ductile–brittle conditions during the ca. 1.93 Ga Taltson orogeny. These dual fronts were later transposed to the northeast in concert with deformation along the Black Bay fault during the ca. 1.90 Ga Snowbird orogeny.

The El Gigante Metamorphic Complex: A missing block with a Grenville-age basement in the Western Sierras Pampeanas, Argentina

The southwestern paleo-margin of Gondwana is interpreted as an accretionary margin that was active from the late Neoproterozoic to the late Paleozoic. The basement of this paleo-margin is widely exposed in the central-western part of Argentina (Sierras Pampeanas area), where a protracted evolution from the Mesoproterozoic to the late Paleozoic is recorded. Part of this evolution is preserved in the El Gigante Metamorphic Complex (Sierra de El Gigante; Western Sierras Pampeanas), a small rotated block within the Valle Fértil Lineament fault zone that separates the Western and Eastern Sierras Pampeanas. The complex is composed of medium-grade meta-siliciclastic and meta-carbonate rocks and medium- to high-grade meta-igneous rocks, affected by tight to isoclinal folds and a pervasive east–west foliation resulting from the Famatinian orogeny (broadly late Cambrian to early Devonian). Later events include localized ductile shear zones. The isotopic and geochronological data from El Gigante Metamorphic Complex reveal at least three distinct lithological assemblages: (1) metamorphosed felsic igneous rocks of ca. 1.11 Ga, i.e., late Mesoproterozoic, (2) a Neoproterozoic metasedimentary succession composed of quartzites and mica-schists, (3) possibly mid- to late-Cambrian marble and graphite-schist. The Grenvillian assemblage (1) was part of a large reworked Paleoproterozoic continental block called MARA (acronym of Maz-Arequipa-Río Apa) and was the basement over which the Neoproterozoic and Cambrian sedimentary successions were deposited. Based on U-Pb zircon ages, lithological similarities and Sr-isotope data, the three lithological assemblages of the El Gigante Metamorphic Complex can be correlated with similar ones in the nearby geologically better-known Sierra de Pie de Palo. The Neoproterozoic and the Cambrian metasedimentary successions are respectively equivalent to the Difunta Correa Metasedimentary Sequence and the Nikizanga-Caucete Groups, which are recognized throughout the Sierras Pampeanas, east and west of the Valle Fértil Lineament.

Failure analysis of additively manufactured AuxHex and hierarchical honeycomb-core sandwich panels subjected to projectile impact

Honeycomb sandwich panels (HSPs) are extensively employed across various industries owing to their outstanding strength and capacity for energy absorption. Despite their widespread use, the potential of additively manufactured (AM) honeycombs in ballistic protection remains relatively unexplored. This study aims to comprehensively assess the energy absorption performance of two distinct HSP configurations, namely AuxHex and Hierarchical, through high-velocity impact tests spanning velocities ranging from 220 to 270 m/s. The high-velocity impact experiments are conducted using a single-stage pneumatic gun, complemented by simulations executed via Abaqus/Explicit. Our findings reveal that the hierarchical HSP demonstrated a notable 9.6 % enhancement in energy absorption compared to panels featuring an AuxHex honeycomb. Interestingly, the hierarchical panels also necessitated 4.8 % more energy for perforation. In terms of failure mechanisms, both sheets of the HSPs experienced ductile shearing, with the formation of characteristic petal-like structures. Concurrently, the honeycomb core underwent fragmentation on the side facing the projectile, while plugs were ejected from the back sheet. Moreover, our analysis indicates that the residual deflection of the back sheet surpassed that of the striking sheet by 15–20 %. This disparity can be attributed to prolonged contact duration and variations in interaction forces between the sheets. Overall, this investigation provides valuable insights into the energy absorption capabilities of AM honeycomb-based HSPs under high-velocity impact conditions, shedding light on their potential for enhanced ballistic protection in various applications.

Shear strengthening of reinforced concrete beams completely wrapped by large rupture strain (LRS) FRP

An experimental work on the shear strengthening of reinforced concrete (RC) beams completely wrapped by large rupture strain (LRS) FRP is presented in this paper. A total of 14 three-point bending tests were carried out on simply supported RC beams, with a focus on the impacts of the shear span-to-depth ratio (1.53, 2.25, and 3.01) and FRP reinforcement ratio (0.37 %, 0.75 %, and 1.12 %) on the shear behavior of RC beams strengthened with LRS FRP. For the beams with a median length, the ductility coefficients increased by 81 %, 154 %, and 335 % with the increase in FRP reinforcement ratio. For the long beams, the ductility coefficients increased by 116 %, 286 %, and 470 % as the FRP reinforcement ratio improved. The ductility markedly improved with the increase in shear span-to-depth ratio. PET FRP’s fracture was not found at the ultimate state, and the improvement in mid-span deflection after the peak state was mainly contributed by the shear deformation. The strengthened specimens exhibited a ductile shear failure mode. For the short RC beams, the shear capacity was slightly enhanced, and the ductility was not improved after being strengthened. The strengthened short RC beams still showed a brittle diagonal compression failure mode. The experimental shear contribution of PETT FRP was compared with the prediction of the existing design guidelines. Finally, an effective strain model of PET FRP, including the shear span-to-depth ratio effect, was proposed and verified with the experimental results.

Structural Evolution, Exhumation Rates, and Rheology of the European Crust During Alpine Collision: Constraints From the Rotondo Granite—Gotthard Nappe

AbstractThe rheology of crystalline units controls the large‐scale deformation geometry and dynamics of collisional orogens. Defining a time‐constrained rheological evolution of such units may help unravel the details of collisional dynamics. Here, we integrate field analysis, pseudosection calculations and in situ garnet U–Pb and mica Rb–Sr geochronology to define the structural and rheological evolution of the Rotondo granite (Gotthard nappe, Central Alps). We identify a sequence of four (D1–D4) deformation stages. Pre‐collisional D1 brittle faults developed before Alpine peak metamorphism, which occurred at 34–20 Ma (U–Pb garnet ages) at 590 ± 25°C and 0.9 ± 0.1 GPa. The reactivation of D1 structures controlled the rheological evolution, from D2 reverse mylonitic shearing at amphibolite facies (520 ± 40°C and 0.8 ± 0.1 GPa) at 18–20 Ma (white mica Rb–Sr ages), to strike‐slip, brittle‐ductile shearing at greenschist‐facies D3 (395 ± 25°C and 0.4 ± 0.1 GPa) at 14–15 Ma (white mica and biotite Rb–Sr ages), and then to D4 strike‐slip faulting at shallow conditions. Although highly misoriented for the Alpine collisional stress orientation, D1 brittle structures controlled the localization of D2 ductile mylonites accommodating fast (∼3 mm/yr) exhumation rates due to their weak shear strength (<10 MPa). This structural and rheological evolution is common across External Crystalline Massifs (e.g., Aar, Mont Blanc), suggesting that the European upper crust was extremely weak during Alpine collision, its strength controlled by weak ductile shear zones localized on pre‐collisional deformation structures, that in turn controlled localized exhumation at the scale of the orogen.

Timing and Tectonic Implications of the Development of the Orosirian Qianlishan Ductile Shear Zones in the Khondalite Belt, North China Craton

Orogen-parallel ductile shear zones are conspicuous structures in the Khondalite Belt, but the timing of shearing remains poorly understood. Here, we present field-based structural and zircon U-Pb geochronological studies on the newly discovered Qianlishan ductile shear zones (QDSZ) in the Khondalite Belt. Our results show that the nearly E-W-trending QDSZ are characterized by steeply S(SW)-dipping mylonitic foliations and mainly display a top-to-N(NE) sense of shearing. Two pre-kinematic intrusions yielded zircon crystallization ages of 2055 ± 17 Ma and 1947 ± 9 Ma, providing the maximum age limit for the QDSZ. Additionally, zircon overgrowth rims from three high-temperature mylonites gave metamorphic ages of 1902 ± 8 Ma, 1902 ± 26 Ma and 1884 ± 12 Ma, interpreted to record the timing of development of the QDSZ. Integrated with previous studies, we propose that the Qianlishan Complex suffered three phases of Orosirian deformation (D1–D3), of which the D3 deformation led to the development of the QDSZ. Deformation events D1, D2 and D3 are considered to have occurred at ca. 1.97–1.93 Ga, 1.93–1.90 Ga and 1.90–1.82 Ga, respectively. These events document that the Khondalite Belt underwent a protracted (>100 Myr) orogenic history in response to the collision between the Yinshan and Ordos blocks.

Brittle initiation of dissolution–precipitation creep in plagioclase-rich rocks: insights from the Bergen arcs, Norway

The initiation of ductile shear zones commonly occurs spatially associated with fluid-rock reactions along brittle precursors. In many cases the relative timing of fracturing, fluid infiltration, reaction, and recrystallisation is unclear, making it difficult to disentangle mechanisms of shear zone initiation from subsequent deformation and recrystallisation. Here we present the study of the transition from a dry plagioclase-diopside-garnet-scapolite host granulite-facies lithology to (1) a low strain amphibolite-facies rock, and (2) a transition from low strain to high strain amphibolite-facies lithologies. Hydration of the granulite-facies precursor at amphibolite-facies conditions produces an assemblage comprised dominantly of plagioclase-amphibole-zoisite-clinozoisite-kyanite-scapolite-quartz. Detailed study of plagioclase chemistry and microstructures across these two transitions using Electron Backscatter Diffraction (EBSD) and Wavelength Dispersive Spectrometry (WDS) allows us to assess the degree of coupling between deformation and fluid-rock reaction across the outcrop. Plagioclase behaves dominantly in a brittle manner at the hydration interface and so the initial weakening of the rock is attributed to grain size reduction caused by fracture damage and fluid infiltration at amphibolite-facies conditions. Extensive fracturing-induced grain size reduction locally increases permeability and allows for continuing plagioclase and secondary mineral growth during shear. Based on plagioclase microstructures, such as, an inherited but dispersed crystallographic preferred orientation (CPO), truncation of chemical zoning, and the dominance of fine (5–150 µm), slightly elongate, polygonal grains we conclude that deformation is dominantly facilitated by dissolution–precipitation creep assisted by grain boundary sliding in the shear zone.

Kinematics of the Clarke River Shear Zone (northeastern Australia) and implications for the tectonic evolution of the Tasmanides

The Clarke River Shear Zone marks the boundary between the Thomson and Mossman orogens in the northeastern Tasmanides. To better understand the kinematic history of this east-northeast-trending shear zone, and its role during the tectonic history of eastern Gondwana, we investigated rock deformation along a segment of the Clarke River Shear Zone. Based on field observations, interpretation of aeromagnetic data and microstructural analysis, we show that the Clarke River Shear Zone experienced at least three phases of reactivation. Gently plunging stretching lineation and kinematic indicators, such as steeply plunging asymmetric folds and asymmetric porphyroclasts, correspond to an early phase of sinistral strike-slip ductile shearing. Subsequent general dextral shear deformation is suggested based on the presence of S-tectonites, symmetric porphyroclasts and kinematic indicators with dominant dextral and subordinate sinistral shear senses. This phase, which occurred under amphibolite-facies metamorphic conditions, was contemporaneous with the emplacement of the Craigie Tonalite, for which we obtained a new zircon U–Pb crystallisation age of 410.3 ± 1 Ma. A later reactivation phase along the Clarke River Shear Zone, under brittle conditions, is indicated by the presence of cataclasite zones and chlorite–epidote alteration. Based on our findings, and regional information on the tectono-magmatic evolution of northeastern Queensland, we suggest that the Clarke River Shear Zone was possibly a sinistral transform fault during the Silurian. During the Devonian, it is possible that deformation along the Clarke River Shear Zone was related to the transition between the non-retreating subduction zone in the north, and the highly mobile (retreating) plate boundary farther south. However, the dextral kinematic component associated with this deformation is not fully explained. Brittle deformation, likely associated with dextral transpression during the Carboniferous, marked the last stage of activity along the Clarke River Shear Zone.

Thrust-related metamorphism in Carboniferous slates of southern Patagonia (South America): the fate of forearc successions

The Devonian to early Carboniferous western margin of Patagonia (South America) includes a NW–SE-trending magmatic arc associated with a NE-dipping palaeo-subduction zone. Along the Andean region of southern Patagonia, the Eastern Andean Metamorphic Complex developed in a forearc position and consists of a succession of very low- to low-grade metaturbidite–metabasic rocks emplaced from the Devonian to Carboniferous. There are significant uncertainties surrounding this metamorphic complex, mainly related to the tectonosedimentary setting of the basin and the subsequent conditions of deformation and metamorphism, which hinder our understanding of the orogenic architecture. To reveal the links between tectonics and metamorphism, we conducted a structural analysis and sampled metapelites to measure the illite crystallinity along a regional structural cross-section in the Eastern Andean Metamorphic Complex. Our analysis reveals broadly lower to upper anchizonal metamorphism roughly synchronous with deformation along northwards-verging thrusts. These findings support the development of a forearc hyperextended basin that was subsequently closed during the Gondwanide Orogeny (late Paleozoic), a model that reconciles previous proposals suggesting passive margin v. back-arc basin models. This closure led to the emplacement of supra-subduction zone ophiolites and turbidites over the continent through the landwards migration of brittle–ductile reverse shear zones. Supplementary material: Measurements of the Kübler index are available at https://doi.org/10.6084/m9.figshare.c.7160849 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists