Journal of Structural Geology最新文献

{"title":"Integration of deformed objects into a composite object analogous to the strain ellipse: a novel graphic method of Rf/ϕ strain analysis","authors":"Yehua Shan","doi":"10.1016/j.jsg.2025.105470","DOIUrl":"10.1016/j.jsg.2025.105470","url":null,"abstract":"<div><div>The composite object of post-strain elliptical objects is defined as having a radial length that is the square root of the normalized sum of the squared radial lengths of the objects. It proves analogous to the strain ellipse and makes <em>R</em>_<em>f</em><em>/ϕ</em> strain analysis simple. The finite strain is estimated from the overall shape of the composite object. The discrepancy between the composite object and the estimated strain ellipse is used to appraise the assumption of the uniformly distributed major axes of pre-strain objects. Two strain methods, direct calculation and numerical estimation, are developed to determine the strain from the composite object. Synthetic and natural datasets are used to validate the methods.</div></div>","PeriodicalId":50035,"journal":{"name":"Journal of Structural Geology","volume":"199 ","pages":"Article 105470"},"PeriodicalIF":2.6,"publicationDate":"2025-05-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144255196","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Switching from contractional to extensional tectonics along the Alpine Front: structural and metamorphic evidence from the Balagne area (northern Corsica, France)","authors":"Maria Di Rosa ,&nbsp;Edoardo Sanità ,&nbsp;Alessandro Malasoma ,&nbsp;Luca Pandolfi ,&nbsp;Michele Marroni","doi":"10.1016/j.jsg.2025.105471","DOIUrl":"10.1016/j.jsg.2025.105471","url":null,"abstract":"<div><div>In the Corsica Island (France), the tectonic boundary along which the units of Alpine Corsica overlap the Hercynian Corsica represents a first-order structure which has not been investigated in depth yet. In this paper, the reconstruction of the Alpine Corsica Front in the Balagne area through the characterization of the deformation history and the associated peak metamorphism for the units of Alpine Corsica and for the Hercynian Corsica is provided. The collected data indicate that a gap in the peak metamorphism between the less metamorphic units located at the top of the unit pile, and those located at lower structural level which registered low-blueschist facies conditions exists. Also, a detailed structural analysis of the shear zones along the Alpine Corsica Front is supplied. The studied N-S trending, E-dipping shear zones documented at the western rim of the Balagne area are characterized by deformation history indicating a top-to-E sense of shear that overprint a former top-to-W kinematics. These findings indicate that the Alpine Corsica Front was affected by extensional tectonics after the shortening, which developed during the top-to-W thrusting of the Alpine Corsica onto the Hercynian Corsica. Framing our results in the regional tectonic setting, we suggest that the extensional tectonics documented in the Balagne area developed during the Oligocene-Early Miocene, i.e., during the transition from the latest stages of orogenic exhumation within the Alpine belt to the post-orogenic rifting stage in the Liguro-Provençal and Tyrrhenian Basins.</div></div>","PeriodicalId":50035,"journal":{"name":"Journal of Structural Geology","volume":"199 ","pages":"Article 105471"},"PeriodicalIF":2.6,"publicationDate":"2025-05-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144195700","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Deformation style and fluid flow behaviour in a faulted siliciclastic-carbonate sequence","authors":"Cyrill Labry ,&nbsp;Antonio Funedda ,&nbsp;Anita Torabi ,&nbsp;Claudio Arras ,&nbsp;Stefania Da Pelo","doi":"10.1016/j.jsg.2025.105468","DOIUrl":"10.1016/j.jsg.2025.105468","url":null,"abstract":"<div><div>Understanding fluid flow behaviour of faults is important for resource exploitation and management. Most studies analysing fluid flow behaviour in fault zones focus on either high-porosity or low-porosity sequences. In this study, we analyse various mechanical layers within alternating siliciclastic-carbonate sequences in fault zones in the Oligo-Miocene Logudoro basin, located in northern Sardinia (Italy). We combined a variety of methods, including field measurements of structures, in-situ permeability and Schmidt Hammer rebound measurements converted to uniaxial compressive strength. Furthermore, we performed thin section analysis, to characterize grain size, porosity, and mineral compositions. We used the gathered data to model the fluid flow conditions within deformation patterns present in one fault-zone. Deformation styles are related to mechanical properties of the rocks at the time of faulting, juxtaposition and differences in intergranular space and porosity in sandstones. The permeability is influenced by the presence of deformation structures such as fractures (high permeability), compactional shear bands (low permeability) and fault rocks within fault core (high and/or low permeability). The fluid flow model highlights the significant influence of deformation bands on the flow field and hydraulic gradient, demonstrating the importance of including these structures in the analysis.</div></div>","PeriodicalId":50035,"journal":{"name":"Journal of Structural Geology","volume":"199 ","pages":"Article 105468"},"PeriodicalIF":2.6,"publicationDate":"2025-05-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144212001","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Fault zone deformation and fracture intensity in chalk-dominated carbonates","authors":"David A. Ferrill ,&nbsp;Adam J. Cawood ,&nbsp;Kevin J. Smart ,&nbsp;Daniel J. Lehrmann ,&nbsp;Mark A. Evans ,&nbsp;Lisa D. Stockli ,&nbsp;Daniel F. Stockli","doi":"10.1016/j.jsg.2025.105469","DOIUrl":"10.1016/j.jsg.2025.105469","url":null,"abstract":"<div><div>Cretaceous Anacacho Limestone within the Balcones fault system (central Texas) is investigated to understand fracturing of chalk-dominated carbonates in a normal faulting deformation regime. Development of opening-mode fractures is highly sensitive to mineralogy and associated mechanical behavior. Low mechanical rebound beds that have &gt;7 % clay and &lt;90 % carbonate generally lack well-developed opening-mode fracture sets. High-rebound beds with &lt;7 % clay and &gt;90 % carbonate contain opening-mode fracture networks. Away from mapped faults (with ∼10 m or greater throw), deformation is represented by opening-mode fractures in two orthogonal sets with intensities of &lt;1 fracture/meter. The NE-SW-striking dominant set parallels regional normal-fault strike. Deformation adjacent to a ∼10-m-throw normal fault includes small-displacement (antithetic and synthetic) normal faults and opening-mode fractures. Near-fault fracture intensity reaches &gt;5 × background levels, and decays to background intensity ∼30 m from the fault. Fluid inclusion analyses of calcite from dilational crack-seal zones along normal faults and opening-mode fractures within the fault damage zone reveal: (i) liquid hydrocarbon inclusions with ∼28–36 API gravity oil; (ii) homogenization temperatures from two-phase inclusions used to estimate burial depths of 0.9–1.5 km (oil inclusions) up to 2.4–2.9 km (aqueous inclusions); and (iii) aqueous inclusion ice-melting temperatures indicating basinal brine rather than near-surface meteoric water during vein cementation. U-Pb ages from these veins indicate Paleocene-Eocene faulting and fracturing (57.9 ± 3.2 to 38.9 ± 4.1 Ma), and limited Miocene (16.6 ± 2.2 Ma) activity. This work documents the essential role of small-displacement faults and opening-mode fractures in the migration of hydrocarbons through low-permeability chalk dominated carbonates.</div></div>","PeriodicalId":50035,"journal":{"name":"Journal of Structural Geology","volume":"199 ","pages":"Article 105469"},"PeriodicalIF":2.6,"publicationDate":"2025-05-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144212019","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"A long history of ductile and brittle deformation of Eastern Pyrenees fault zones: coupling 40Ar/39Ar geochronology, chlorite geothermometry and structural observation","authors":"Gaétan Milesi ,&nbsp;Philippe Münch ,&nbsp;Delphine Charpentier ,&nbsp;Arthur Iemmolo ,&nbsp;Aoutmane Bouaoudi ,&nbsp;Andreï Lecomte ,&nbsp;Virginie Moutarlier ,&nbsp;Renaud Gley ,&nbsp;Michael Bonno ,&nbsp;Patrick Monié","doi":"10.1016/j.jsg.2025.105465","DOIUrl":"10.1016/j.jsg.2025.105465","url":null,"abstract":"<div><div>In orogenic settings, fault networks accommodate crustal deformation during the evolution of mountain ranges. Fault zones exhibit multiphase ductile and brittle activity, complicating their interpretation. Dating deformation in orogenic basement remains a challenge. In this study, we conducted structural and microstructural observations, coupled with ⁴⁰Ar/³⁹Ar dating on encapsulated mineral fractions and chlorite thermometry, on major fault zones in the Eastern Pyrenees. To overcome the presence of K-feldspar within fault gouge, a major issue for fault gouge argon dating, we present a method to estimate the contribution of two mixed K-rich phases from ⁴⁰Ar/³⁹Ar step heating. This method provides limit or maximum age for the deformation recorded by fault gouges in which illite polytypes are mixed with K-feldspar. Our results reveal a ductile-to-brittle transition between 40 and 35 Ma (Priabonian-Bartonian), characterized by 2<em>M</em>1 muscovite and evidence of strike-slip movement along the Py NE-SW fault and NW-SE secondary faults. The Py fault gouge contains muscovite formed at temperatures exceeding 200–250 °C. In the Têt NE-SW fault gouge, the coexistence of 2<em>M</em>1 and 1<em>M</em> illite polytypes suggests late-stage crystallization of 1<em>M</em> illite at 22.1 ± 1.4 Ma, at temperatures between 100 and 150 °C, as determined by chlorite thermometry. These findings confirm significant normal faulting activity on the Têt fault during the Oligo-Miocene, consistent with published low-temperature thermochronology data indicating early exhumation of the Canigou massif relative to the Carança massif, facilitated by normal displacement along the Py fault, and a later exhumation of both massifs in relation to the Têt normal fault activity.</div></div>","PeriodicalId":50035,"journal":{"name":"Journal of Structural Geology","volume":"199 ","pages":"Article 105465"},"PeriodicalIF":2.6,"publicationDate":"2025-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144212020","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Depth-dependent in-situ stress state of the Longmen Shan fault Belt: Evidence from Wenchuan earthquake fault scientific drilling project Hole-2 image logs and core","authors":"Chunrui Li ,&nbsp;Haibing Li ,&nbsp;Huan Wang ,&nbsp;Jialiang Si ,&nbsp;Lei Zhang","doi":"10.1016/j.jsg.2025.105467","DOIUrl":"10.1016/j.jsg.2025.105467","url":null,"abstract":"<div><div>The Longmen Shan Fault Belt, as a critical tectonic boundary along the eastern margin of the Tibetan Plateau, exhibits stress field characteristics that are important for understanding the kinematic features of the fault zone and its seismic activity. Recent studies have demonstrated significant variations in the shallow stress field of the fault zone, particularly when seismic ruptures propagate to shallow depths where the kinematic characteristics undergo marked changes. To investigate the stress states at different depths within the fault zone, this study systematically analyzed the in-situ stress field of the Yingxiu-Beichuan Fault through borehole wall image logs and core samples from the Wenchuan Earthquake Fault Scientific Drilling Project Hole 2 (WFSD-2). Combined with previous research findings, six depth intervals (424–569 m, 958–1163 m, 1271–1357 m, 1523–1623 m, 1870–2081 m, 2112–2178 m) were identified, with mean maximum horizontal principal stress orientations of 318°, 301°, 291°, 306°, 328°, and 190°, respectively, showing ∼20° inter-segment variations. Regional fault geometry analysis revealed distinct kinematic behaviors: Interval 1 exhibited pure thrust motion; Intervals 2–4 showed thrust-dominated motion with right-lateral strike-slip components; and Intervals 5–6 showed thrust motion with left-lateral strike-slip components. Integrated with existing studies, the 750–950 m and 1300–1500 m depth segments correspond to pure thrust and right-lateral strike-slip thrust regimes, respectively, matching the coseismic slip patterns of the Yingxiu-Beichuan Fault and Shenxigou Fault during the Wenchuan earthquake. These depth-dependent differential stress states, manifested as coseismic response features along the fault zone, result from the vertically heterogeneous rupture processes during the Wenchuan earthquake. This postsismic stress configuration reflects the kinematic adjustments of different fault segments, which fundamentally explains the structural complexity of the Longmen Shan Fault Belt and the multi-stage rupture characteristics observed in the 2008 Wenchuan earthquake.</div></div>","PeriodicalId":50035,"journal":{"name":"Journal of Structural Geology","volume":"199 ","pages":"Article 105467"},"PeriodicalIF":2.6,"publicationDate":"2025-05-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144178528","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Crystal preferred orientation of lawsonite and glaucophane in lawsonite blueschist deformed in general shear","authors":"Seungsoon Choi ,&nbsp;Haemyeong Jung,&nbsp;Sejin Jung","doi":"10.1016/j.jsg.2025.105464","DOIUrl":"10.1016/j.jsg.2025.105464","url":null,"abstract":"<div><div>The crystal preferred orientations (CPOs) of lawsonite and glaucophane in lawsonite blueschist reflect deformation processes in the cold subducting oceanic crust and are crucial for understanding subduction zone dynamics. However, the dominant CPOs of lawsonite and glaucophane and their formation mechanisms in cold subduction zones remain uncertain. We conducted deformation experiments on lawsonite blueschist in general shear at high pressures (1.0–2.5 GPa) and low temperatures (230–400 °C), with varying shear strains (γ = 1.1–4.0). Two types of lawsonite CPOs were found with increasing shear strain: Type-A, where the [010] axis is subparallel to the shear direction, and Type-B, where the [100] axis is subparallel to the shear direction, with the [001] axes subnormal to the shear plane for both types. Additionally, a single type of glaucophane CPO was observed regardless of the magnitude of shear strain, characterized by the [001] axes aligned subparallel to the shear direction and the [100] axes aligned subnormal to the shear plane. This pattern is consistent with those most commonly reported in previous studies. Cataclastic flow was observed in the lawsonite blueschist samples, as indicated by intragranular and shear fractures. In addition, the observation of weak intracrystalline deformation of lawsonite and glaucophane with a minor dislocation suggests that rigid-body rotation with a minor dislocation activity may be a mechanism underlying the CPO formation in these minerals. Our findings highlight that the magnitude of shear strain significantly influences the development of lawsonite CPOs in cold subduction zones.</div></div>","PeriodicalId":50035,"journal":{"name":"Journal of Structural Geology","volume":"199 ","pages":"Article 105464"},"PeriodicalIF":2.6,"publicationDate":"2025-05-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144189570","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Thickness of an extensional plate-boundary shear zone in the mantle: Implications for tectonic controls on strain localization and transient strain rates","authors":"Vasileios Chatzaras ,&nbsp;Julie Newman ,&nbsp;Basil Tikoff","doi":"10.1016/j.jsg.2025.105466","DOIUrl":"10.1016/j.jsg.2025.105466","url":null,"abstract":"<div><div>This study shows that constant displacement rate conditions – imposed by plate tectonics – is the best approach to consider the evolution of extensional plate-boundary shear zones, as stresses vary during deformation. The Turon de Técouère massif of the French Pyrenees preserves a Cretaceous, magma-poor hyperextended plate margin within the lithospheric mantle. The massif exposes an extensional shear zone hosted in lherzolite. The present-day structure of the shear zone, frozen at 750 °C, consists of a ∼40 m thick ultramylonite, bordered by a ∼200 m thick mylonite and a protomylonite &gt;100 m thick. The ultramylonite overprinted a ∼40 m-thick mylonite that was active between 1000 and 850 °C, while the shear zone became thicker with time. Using displacement rates, determined from tectonic analyses, and strain rate estimates, determined from microstructural analyses, the calculated thickness (8–20 m) of the shear zone is less than the observed thickness (40–200 m) at different temperature conditions experienced by these rocks. Therefore, the shear zone is thicker than necessary to accommodate the plate motion. This result indicates that the Turon de Técouère shear zone likely represents the entire divergent plate boundary at conditions of the lithospheric mantle. We introduce the concept of a “jump around” shear zone – in which a smaller zone of active shearing moves around within a thicker zone of deformation – to explain the discrepancy between the thinner shear zones predicted by strain rates and the thicker observed shear zone.</div></div>","PeriodicalId":50035,"journal":{"name":"Journal of Structural Geology","volume":"199 ","pages":"Article 105466"},"PeriodicalIF":2.6,"publicationDate":"2025-05-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144243318","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Quantitative characterization of fracture networks in the Permian organic-rich shales of the Whitehill formation in the Karoo Basin, South Africa and implication for hydrocarbon exploration, CO2 sequestration, and other fractured geofluid systems","authors":"Kenneth Chukwuma,&nbsp;Nicholas B. Harris,&nbsp;Elena Konstantinovskaya","doi":"10.1016/j.jsg.2025.105463","DOIUrl":"10.1016/j.jsg.2025.105463","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Mudstone formations (shales) are increasingly recognized as targets of hydrocarbon exploration and production, sites for CO&lt;sub&gt;2&lt;/sub&gt; storage, and repositories for nuclear and other waste. Due to their low matrix permeability, considerable amount of fluid flow in shales may occur within fractures, and consequently significant effort is placed on the characterization of fracture networks. We report results of a multidisciplinary field and borehole study of a fracture network in Permian organic-rich shales of the Whitehill Formation in the Karoo Basin, South Africa, developed in proximity to Lower Jurassic dolerite intrusive sills. The goal was to systematically document the different fracture types, differentiate the underlying fracturing mechanisms, and quantify their contribution to fracture networks in intrusion-related organic-rich shales. Our observations comprise high-resolution digital outcrop models constructed from ground- and drone-based photogrammetry, and petrographic and geochemical data from fracture cements and fluid inclusions contained in them.&lt;/div&gt;&lt;div&gt;Our results indicate that the fracture network in the studied shale developed from a suite of processes related to intrusion of the sills. These resulted in the formation of five distinctive fracture types, with varying orientation and fracture-filling material: (i) vertical solid bitumen veins; (ii) sub-vertical to horizontal solid bitumen-calcite veins; (iii) closely-spaced subvertical calcite veins; (iv) horizontal bitumen and calcite veins; and (v) sub-vertical joints. Based on the spatial distribution of these fracture types, three fracture domains are identified: upper, central, and lower domains, where the upper and lower domains are near the intrusions and the central domain at a greater distance.&lt;/div&gt;&lt;div&gt;The upper and lower domains are dominated by solid bitumen and solid bitumen-calcite veins that exhibit a strong vertical orientation with a N-S strike. In contrast, the central domain comprises nearly horizontal solid bitumen-calcite veins. The vertical tensile fractures in the upper and lower domains likely developed in response to increased pore fluid pressure associated with the mobilization hydrocarbon and other hydrothermal fluids caused by intrusive heating. The horizontal tensile fractures in the central domain may be related to the thermal maturation and poroelastic deformation of the organic-rich shales. The development of the N-S tensile fractures is consistent with the normal fault regime associated with the E-W Early Jurassic extensive tectonics that led to the breakup of Gondwana.&lt;/div&gt;&lt;div&gt;The relative contributions of the vertical and horizontal tensile fracture types in the Karoo shale to spatial variation of fracture orientation, intensity, and connectivity were evaluated using digital fracture network quantification. All observed fracture types contributed to hydrocarbon and fluid migration and storage. Our observations demonstrate that ou","PeriodicalId":50035,"journal":{"name":"Journal of Structural Geology","volume":"198 ","pages":"Article 105463"},"PeriodicalIF":2.6,"publicationDate":"2025-05-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144138839","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Unsteady transpression: How progressive variations in kinematic vorticity influence finite strain in shear zone evolution","authors":"Jorge Alonso-Henar ,&nbsp;Carlos Fernández ,&nbsp;Manuel Díaz-Azpiroz ,&nbsp;Elena Druguet","doi":"10.1016/j.jsg.2025.105462","DOIUrl":"10.1016/j.jsg.2025.105462","url":null,"abstract":"<div><div>Natural shear zones usually follow unsteady deformation histories. Concerning transpression zones, it is expected that the vorticity of the flow will vary not only in space but also in time. This work presents a study of unsteady flows that considers two ideal extreme cases: one, here named Type i, in which the vorticity increases with time, from virtual pure to simple shearing (<em>W</em>_k = 0.01 to 0.999), and another, Type d, in which vorticity decreases with time, from virtual simple to pure shearing (<em>W</em>_k = 0.999 to 0.01). To tackle the issue of vorticity changes during progressive strain, we modeled the evolution of a vertical transpression zone with left lateral movement and vertical extrusion, considering four basic types of shear zone growth. The results show that, in many cases, the resulting fabrics are difficult to distinguish from those of a steady flow. There are situations, however, in which the pattern of foliations, the orientation of stretching lineations, or an abnormally high magnitude of local non-coaxiality may be criteria to identify the unsteady character of the flow. Under Type i flows, the spin undergone by the instantaneous stretching axes, antithetic to the rotation imposed by the vorticity vector, should have consequences in an exceptionally high non-coaxiality of the fabric, and can induce an antithetic rotation of the foliation. In some cases, the resulting foliation pattern can be sigmoidal, whose tips indicate the direction of relative displacement of the adjacent blocks. Other pieces of evidence detected in the fabric patterns that may indicate unsteadiness in the flow are: a) zones with low deformation intensity that nevertheless show lineations parallel to the extrusion direction; b) local zones of very high non-coaxiality (reflected in rock fabrics - high asymmetric porphyroclasts or CPOs), higher than that imposed by the total vorticity; and c) the coexistence of mutually orthogonal stretching lineations without evidence of steady deformation trajectories to justify it. Our model also suggests that, when temporal strain partitioning is identified in a shear zone, the main evidence of unsteadiness of the flow would be found in bands where the non-coaxial component is eventually concentrated. For the rest of the shear zone, the variations in strain trajectories are so small compared to those of steady flows that the assumption that the flow is steady may be considered valid. We have tested the model into two natural shear zones: one where vorticity increase has been already noticed (Balaram Shear Zone in the northwestern part of the Indian Peninsula) and another one where unsteady transpression is presented as a simpler explanation than the provided steady analytic models (a high strain zone in Cap de Creus, Eastern Pyrenean Axial Zone).</div></div>","PeriodicalId":50035,"journal":{"name":"Journal of Structural Geology","volume":"198 ","pages":"Article 105462"},"PeriodicalIF":2.6,"publicationDate":"2025-05-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144147624","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}