Journal of Structural Geology最新文献

{"title":"Surface rupture signatures of historical earthquakes (16th and 19th centuries) from Kumaon-Garhwal, central Himalaya: Implications for seismic hazard assessment","authors":"Javed N. Malik ,&nbsp;Mitthu Dhali ,&nbsp;Mahendrasinh S. Gadhavi ,&nbsp;Prabhat Kumar ,&nbsp;Eshaan Srivastava ,&nbsp;Mohd Azhar Ansari ,&nbsp;Nayan Sharma ,&nbsp;Santiswarup Sahoo ,&nbsp;Pankaj Kumar","doi":"10.1016/j.jsg.2025.105567","DOIUrl":"10.1016/j.jsg.2025.105567","url":null,"abstract":"<div><div>This study presents integrated paleoseismic and geodetic studies from the Kaladungi Fault (KF) in the Kumaon-Garhwal Himalaya to constrain the rupture history of major historical earthquakes. A paleoseismic trench excavated at the base of a 15 m high fault scarp at Nandpur revealed geological evidence of four surface-rupturing paleoearthquakes. In total 24 Optically Stimulated Luminescence (OSL) and 24 radiocarbon (14C) ages allowed robust bracketing of these events.</div><div>Based on offsets of sedimentary units and ages we infer that <strong>Event-I</strong> took place between BCE 2945-1990 and BCE 1480-862, and <strong>Event-II</strong> was between BCE 1264-643 and BCE 793-287. These events could not be correlated with any paleo-earthquakes due to broad age brackets and nonavailability of any pre-historic records. <strong>Event-III</strong> occurred between CE 1242–1408 and CE 1415–1529, while <strong>Event-IV</strong> (MRE) was between CE 1692–1761 and CE 1689–1822. To constrain and correlate the rupture lengths of Event III and Event IV, we considered historical records and regional paleoseismic data from Nepal, Kumaon-Garhwal to the western Himalaya. The Event-III corresponds to the CE 1505 earthquake and Event-IV (MRE) represents the CE 1803 earthquake. Moreover, the long-term slip-rate for the KF is estimated to be ∼7.1 mm/yr, which suggests the ∼40 % of the total slip (∼17.7 mm/yr, estimated from geodetic) being partitioned on this fault.</div><div>Paleoseismic evidence for the CE 1803 event indicates surface rupture, as demonstrated by fault displacements and tightly bracketed ages, countering previous claims of a blind rupture. Supporting this interpretation, geodetic modeling and seismicity patterns constrain the downdip locking extent of the Main Himalayan Thrust to ∼100 km north of the Main Frontal Thrust. The reassessment of rupture dimensions for CE 1803 event aligns with a seismic moment magnitude Mw ∼8.1–8.2 rupture.</div><div>With updated strain budget of ∼3.9 m of elastic strain accumulated since 1803, the region now represents a mature seismic gap with potential for another great earthquake. This study underscores the importance of integrating trench-based paleoseismic chronologies with GPS-constrained crustal deformation models to enhance seismic hazard assessments along the Himalayan front.</div></div>","PeriodicalId":50035,"journal":{"name":"Journal of Structural Geology","volume":"202 ","pages":"Article 105567"},"PeriodicalIF":2.9,"publicationDate":"2026-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145528924","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":"How variations in intraplate stresses affect slip and dilation tendencies of faults: The onshore United Kingdom example","authors":"T.G. Blenkinsop","doi":"10.1016/j.jsg.2025.105571","DOIUrl":"10.1016/j.jsg.2025.105571","url":null,"abstract":"<div><div>Stresses in plate interiors vary in magnitude and orientations on various scales, implying that shear and normal stresses on faults may vary regionally. The effects of intraplate regional stress variations are analysed using a recent compilation of in-situ stress to calculate slip and dilation tendencies of United Kingdom onshore faults. The tendencies are normalised to maximum possible values. Stress in UK can be characterised by a strike slip stress field with variable orientations of maximum horizontal stress, S_H. Throughout southern UK, S_H is orientated NW, giving rise to very low slip and dilation tendencies on NE striking Caledonian faults. North and E to ESE striking faults have very high slip tendencies, and intermediate dilation tendencies. At a major boundary in stress orientation (the Highland Boundary Fault in Scotland), S_H changes abruptly to a northerly trend. Steeply dipping Caledonian faults in northern UK such as the Great Glen Fault have very high slip tendencies and intermediate dilation tendencies. Faults with low dips (e.g. the Moine Thrust) have intermediate slip and dilation tendencies. The dramatic change in slip tendencies on steeply dipping Caledonian faults from southern to northern UK illustrates some of the profound consequences of regional scale stress variations. These conclusions are robust to reasonable uncertainties, but the coarse results of this study indicate that more detailed knowledge of stress and fault geometry is necessary for applications such as pump-storage schemes, nuclear power plants, radioactive waste disposal, mining, and carbon sequestration, as well as for seismic hazard analysis.</div></div>","PeriodicalId":50035,"journal":{"name":"Journal of Structural Geology","volume":"202 ","pages":"Article 105571"},"PeriodicalIF":2.9,"publicationDate":"2026-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145528926","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":"Richard Lisle in Oviedo (Spain): Contributions to the analysis of folding – a review","authors":"Fernando Bastida ,&nbsp;Jesús Aller ,&nbsp;Nilo C. Bobillo-Ares ,&nbsp;Francisco J. Fernández ,&nbsp;Omar Menéndez","doi":"10.1016/j.jsg.2025.105553","DOIUrl":"10.1016/j.jsg.2025.105553","url":null,"abstract":"<div><div>The work carried out by Richard Lisle, stemming from his stay in Oviedo and his integration into the fold analysis group at the local university, addressed a wide range of topics concerning fold geometry and kinematics. Regarding 2D fold geometry, Richard led the development of a method to relate the various classification schemes based on fold fitting functions, using a parameter defined as the normalized area bounded by the folded surface profile. Non-cylindrical folds were analysed based on the properties of developable surfaces and curvature analysis. In the field of kinematics, special attention was given to the neutral surface folding, which led to the distinction between two modes: “parallel tangential longitudinal strain” and “equiareal tangential longitudinal strain”. A method was also developed to estimate the total bulk shortening associated with the formation of flattened parallel folds. The 2D kinematics of various fold types was explored through numerical simulations, based on the displacement laws governing different folding mechanisms. This methodology was applied to the study of chevron folds, folded oblique surfaces, asymmetric folds, recumbent folds, and similar folds. The investigation led to the resolution of certain problems related to rock deformation and resulted in the description of a new deformation mode: “rotation shear”. The study of these topics also involved the development of computational tools to automate and facilitate the strain calculations. The formation mechanisms of several structures associated with folding were likewise examined, including crenulation cleavage, so-called “saw-tooth structures”, curved veins related to folds, and cleavage in pre-existing folds. Finally, we evaluate the impact of this body of work, its limitations, and future prospects for extending the research, particularly by using the results obtained as a foundation for dynamic modelling.</div></div>","PeriodicalId":50035,"journal":{"name":"Journal of Structural Geology","volume":"201 ","pages":"Article 105553"},"PeriodicalIF":2.9,"publicationDate":"2025-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145050129","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":"Out-of-sequence recent thrusting revealed by surface and subsurface data under the Po Plain, Italy","authors":"A. Tibaldi ,&nbsp;D. Barrera ,&nbsp;F.L. Bonali ,&nbsp;N. Corti ,&nbsp;G. Toscani","doi":"10.1016/j.jsg.2025.105556","DOIUrl":"10.1016/j.jsg.2025.105556","url":null,"abstract":"<div><div>Recognizing recent activity at buried out-of-sequence thrusts is a complex task but has important implications for seismic hazard and risk assessment. Here we propose an example of methodology that includes subtle geomorphological and hydrological observations at the surface correlated with upper crustal 3D reconstruction by seismic sections and stratigraphic logs. The densely inhabited Po Plain (Italy) hides the front of the Northern Apennines fold-and-thrust belt, buried under a thick succession of Plio-Pleistocene deposits. The Emilia Arc, located in the center of this front, is composed of a series of folds and south-dipping thrusts. Hindward of the thrust front, at the surface, high resolution Digital Elevation Models and GPS measurements allowed to discover topographic anomalies that denounce recent uplift and broad tilting. These have been accompanied by ample late Pleistocene-Holocene migration of the river network, and river over-excavation. In correspondence of these features, subsurface data show north-verging folds and reverse faults involving Pleistocene deposits. From this integration, a scenario where two out-of-sequence thrust zones contributed to modulate shortening during the late Pleistocene and, possibly, Holocene, emerges. Hinterland migration of deformation has been favored by the higher sedimentation load above the outer thrusts and their close confrontation with the thickened Southern Alps buried fronts.</div></div>","PeriodicalId":50035,"journal":{"name":"Journal of Structural Geology","volume":"201 ","pages":"Article 105556"},"PeriodicalIF":2.9,"publicationDate":"2025-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145057155","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":"Going with the flow — Changes of vorticity control gold enrichment in Archean shear zones (Shebandowan Greenstone Belt, Superior Province, Canada)","authors":"Tobias Stephan ,&nbsp;Noah J. Phillips ,&nbsp;Hanna Tiitto ,&nbsp;Adrian Perez ,&nbsp;Michael Nwakanma ,&nbsp;Robert Creaser ,&nbsp;Pete Hollings","doi":"10.1016/j.jsg.2025.105542","DOIUrl":"10.1016/j.jsg.2025.105542","url":null,"abstract":"<div><div>Archean orogenic gold deposits are commonly hosted in brittle-ductile shear zones, where gold is locally remobilized and reprecipitated due to changes in physio-chemical conditions. However, in Archean rocks of the Superior Craton, these shear zones are often sparsely exposed and inferred primarily through spatial correlation, hence these shear zone architectures remain poorly understood. Here, we apply geochronology, kinematic analysis, and vorticity analysis techniques to study the Shebandowan Greenstone Belt in Northern Ontario, Canada. Geochronological results from zircon U-Pb CA-ID-TIMS and molybdenite Re-Os NTIMS dating yielded an age of 2717.35 ± 0.48 Ma for a proximal felsic intrusion and 2708 ± 12 Ma for auriferous and molybdenite-bearing vein mineralization, respectively. Field observations and regional age data indicate that both mineralization and intrusive activity predate deformation. Spatial interpolation of ductile foliation orientations reveals that gold mineralization is spatially associated with abrupt changes in shear zone orientation. Vorticity analysis — including rigid porphyroclast and crystallographic vorticity analysis — shows that these changes induce strain partitioning in releasing and restraining bends. These observed localized deflections in shear zone strike are attributed to the competency contrast between the old, feldspar-rich, isotropic, and syenitic plutonic bodies (rheologically stronger) and gold-bearing, intermediate metavolcanic and diorite units (rheologically weaker). Transitions from non-coaxial to coaxial flow dominated areas appear to have localized fluid pathways, allowing for fluid migration and gold precipitation during deformation. Quantifying deflections of shear zones adjacent to rigid geological bodies, combined with analyzing the deformation flow types, represents a powerful and cost-effective tool for gold exploration in granite-greenstone belts.</div></div>","PeriodicalId":50035,"journal":{"name":"Journal of Structural Geology","volume":"201 ","pages":"Article 105542"},"PeriodicalIF":2.9,"publicationDate":"2025-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145010724","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":"Microstructural evolution of micaceous mylonites","authors":"Katherine Billings,&nbsp;Philip Skemer","doi":"10.1016/j.jsg.2025.105555","DOIUrl":"10.1016/j.jsg.2025.105555","url":null,"abstract":"<div><div>In crustal shear zones, the grain size and geometric arrangement of mica is observed to evolve significantly with strain. The transition from tabular, interconnected mica layers to a well-mixed fine-grained ultramylonitic microstructure is inferred to reduce rock viscosity. This evolution is predicted to play an important role in the strength of the crust, particularly at plate boundaries. However, the mechanisms of mica layer disaggregation and phase mixing at high strains have not been widely explored. In this study, we conducted deformation experiments (up to shear strains of <span><math><mrow><mi>γ</mi><mo>&gt;</mo><mn>100</mn></mrow></math></span>) on synthetic composites of mica (fluorphlogopite) and either calcite or fluorite in the Large Volume Torsion Apparatuses (LVTs) at Washington University in St. Louis. Experiments were performed at T = 765 °C, P = 1.5 GPa, and strain rates of ∼1x10⁻⁴ s⁻¹. Deformation microstructures were analyzed using optical and electron microscopy and electron backscatter diffraction (EBSD). Microstructural evolution is sensitive to shear strain (γ) but largely insensitive to the composited material. Mica grain size is reduced through brittle mechanisms and mixing may be enhanced by phase boundary sliding. Interconnected layers of mica disaggregate and are fully mixed with the composited material at <span><math><mrow><mi>γ</mi><mo>&gt;</mo><mn>50</mn></mrow></math></span>. This sequence of deformation processes is also inferred from the complementary analysis of specimens from the Kuckaus Mylonite Zone in the Namaqua Metamorphic Complex in southwestern Namibia. This process of mica grain size reduction and phase mixing may contribute to the transition from mylonite to ultramylonite in high temperature crustal shear zones.</div></div>","PeriodicalId":50035,"journal":{"name":"Journal of Structural Geology","volume":"201 ","pages":"Article 105555"},"PeriodicalIF":2.9,"publicationDate":"2025-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145106204","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":"Influence of tectonic regime superposition on damage zone structures in a basin-boundary fault, Jatobá basin, NE Brazil","authors":"D.L. Vasconcelos ,&nbsp;F. Balsamo ,&nbsp;L.C. Palhano ,&nbsp;F.C.C. Nogueira ,&nbsp;F.H.R. Bezerra ,&nbsp;F.O. Marques ,&nbsp;R.G. Oliveira ,&nbsp;W.E. Medeiros ,&nbsp;R.A. Fuck ,&nbsp;A.C. Tavares ,&nbsp;R.R.S. Dantas","doi":"10.1016/j.jsg.2025.105552","DOIUrl":"10.1016/j.jsg.2025.105552","url":null,"abstract":"<div><div>We integrated magnetic, gravity, and structural data to investigate the Ibimirim Fault and the evolution of its damage zones within the Jatobá Basin, NE Brazil. Our findings indicate that the magnetic anomalies observed within the Pernambuco and Cruzeiro do Nordeste shear zones are consistent with the strike of the Ibimirim Fault in the study area. Integrated with structural analysis of the Precambrian fabric and rift fault, we suggest that the basement fabric influences the position and geometry of the Ibimirim Fault. Gravity inversion revealed that the Jatobá Basin contains three depocenters, with the main one situated in the central part of the Ibimirim Fault. A fault throw analysis indicates that the Ibimirim Fault comprises three main fault segments, with this study focusing on the area where the central and eastern segments connect. In this overstep area, extensional and compressional fault sets occur in the fault damage zone. The compressional structures crosscut and displace the extensional older structures. The superposition of these structures enhances structural fracturing connectivity without increasing the damage zone width. Paleostress characterization shows extensional structures with sub-vertical σ1 and sub-horizontal σ3 (05/061). In contrast, compressional structures have sub-vertical σ3 and sub-horizontal σ1 (03/349), indicating a non-colinear basin inversion likely driven by far-field stresses from the Andes and the Mid-Atlantic Ridge. This research highlights the intricate interplay between tectonic regime superposition and the evolution of extensional and compressional structures within the same fault damage zone, with significant implications for understanding the formation of sub seismic structures in these regions.</div></div>","PeriodicalId":50035,"journal":{"name":"Journal of Structural Geology","volume":"201 ","pages":"Article 105552"},"PeriodicalIF":2.9,"publicationDate":"2025-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145106203","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":"Strain localization and rheological weakening along mid-crustal anisotropies as recorded in the Itapetim and Tendó shear zones (northeastern Brazil)","authors":"Raylline Silva ,&nbsp;Rhander Taufner ,&nbsp;Gustavo Viegas ,&nbsp;Claudia Trepmann ,&nbsp;Lauro Montefalco ,&nbsp;Carlos J. Archanjo ,&nbsp;Tiago Lyra ,&nbsp;Thais Silva","doi":"10.1016/j.jsg.2025.105558","DOIUrl":"10.1016/j.jsg.2025.105558","url":null,"abstract":"<div><div>The Central Borborema Province (CBP) in NE Brazil hosts well-preserved mid-crustal shear zones that formed during the Pan-African orogeny in a strike-slip tectonic setting. These structures are key to investigate the strength-controlling processes in high-temperature continental shear zones, which are often obliterated from the geological record during late exhumation. We have analyzed mylonites and ultramylonites from the Itapetim and Tendó shear zones (ISZ and TSZ), two major NE-trending shear zones in the CBP, using a combination of field mapping, microfabric analysis and mineral chemistry. Both the ISZ and TSZ are primarily composed of granitic mylonites with discrete, thin ultramylonitic layers. In mylonites, K-feldspar porphyroclasts form a load-bearing framework that deforms through microfracturing, dislocation-glide-controlled deformation and grain boundary migration. Plastic strain is preferentially accommodated in discontinuous monomineralic quartz ribbons alternating with plagioclase-rich layers. The ultramylonitic layers are spatially and genetically associated with quartz veins and aplitic dykes, in which fluid-rock interactions induce strain softening. This process leads to the development of myrmekite-derived K-feldspar neoblasts and biotite-rich layers, further enhancing rheological weakening. The data gathered in this study suggest that strain is preferentially localized along lithological boundaries, resulting in the development of sinistral shear zones that conform to the overall tectonic setting of the central block of the Borborema Province. Our findings demonstrate strain compatibility between these NE-trending sinistral shear zones and the major E-W dextral lineaments that bound them, offering valuable insights into the mechanisms of rheological weakening in the post-collisional stage of continental orogens.</div></div>","PeriodicalId":50035,"journal":{"name":"Journal of Structural Geology","volume":"201 ","pages":"Article 105558"},"PeriodicalIF":2.9,"publicationDate":"2025-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145106205","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":"Interaction between diagenesis and faulting in sandstone reservoirs: Inferences from the Groningen gas field","authors":"Job P.B. Arts ,&nbsp;Dallyn Rodrigues ,&nbsp;Fadi H. Nader ,&nbsp;Martyn R. Drury ,&nbsp;Liviu C. Matenco ,&nbsp;Ernst Willingshofer ,&nbsp;André R. Niemeijer","doi":"10.1016/j.jsg.2025.105554","DOIUrl":"10.1016/j.jsg.2025.105554","url":null,"abstract":"<div><div>We investigated the progressive diagenetic evolution of fault zones in the Rotliegend reservoir of the Groningen gas field (NE Netherlands) through microstructural and petrological analysis of cataclastic bands and the only retrieved fault gouge from the reservoir. Our observations show that authigenic quartz, ferroan dolomite, siderite and anhydrite are spatially associated with the presence of faults in the reservoir.</div><div>The uneven distribution of syntaxial quartz overgrowths in the host rock suggests that silica was readily available in the reservoir, but that quartz precipitation was inhibited by coatings and slow-growing euhedral faces. Quartz cementation was preferentially concentrated in cataclastic bands, where fresh fracture surfaces provided favourable nucleation sites.</div><div>Anhydrite and siderite cements are concentrated in dilatant fractures in cataclastic bands or in the host rock adjacent to it. Based on their association with dilatant fractures and the required sources for sulphates and reducing fluids, respectively, we interpret that they were introduced by extrinsic, fault associated fluids, during a late stage of the diagenetic history.</div><div>The preferential quartz cementation in cataclastic zones suggests that many Groningen faults experienced cementation-related strengthening, while this process was inhibited in the clay-rich fault cores as observed in the BIR-01 well, implying that fault reactivation is most likely to occur within the weak fault core.</div></div>","PeriodicalId":50035,"journal":{"name":"Journal of Structural Geology","volume":"201 ","pages":"Article 105554"},"PeriodicalIF":2.9,"publicationDate":"2025-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145060301","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":"Shear zone folds: Structural analysis and interpretation","authors":"G.I. Alsop,&nbsp;D.J. Condon","doi":"10.1016/j.jsg.2025.105544","DOIUrl":"10.1016/j.jsg.2025.105544","url":null,"abstract":"<div><div>Shear zones and associated folds form an important category of structures that are widely developed in the internal portions of orogenic belts. The analysis of such folds is crucial as they can be used to help determine kinematics, bulk strain and deformation histories of crustal-scale shear zones as well as influencing fluid flow and potential mineralisation. Our case study focuses on a Caledonian shear zone in NW Ireland that is marked by NW-directed crustal thickening (D1) that is followed by a later phase (D2) of SE-directed gravity-driven extensional collapse. We provide a detailed analysis of mesoscopic folds and fabrics formed during this greenschist-facies extension in order to answer some general questions including how to distinguish folds with hinges that initiate parallel to transport versus those folds that have rotated towards shear, the control that larger folds may have upon the geometry and orientation of smaller folds, and the influence that lithology plays in the evolution of shear zone folds. Our study comprises a mixed fold and fabric data set derived from a complex combination of i) <em>flow perturbation folds</em> with hinges that have initiated sub-parallel to transport, ii) <em>sheath folds</em> with hinges that have rotated towards transport during intense progressive shear, and iii) <em>spiral folds</em> with hinges that initiate normal to transport but fail to rotate as they roll and tighten. These mesoscopic folds form across a range of scales with smaller folds forming at a variety of angles, and in some cases transecting larger fold hinges indicating a non-Pumpellyan or incongruous relationship. Competent psammites host more open folds with hinges at higher angles to transport when compared to weaker pelites that contain tighter folds at variable angles to shear. Lithology and scale are therefore critical factors in controlling shear zone folds, with natural data sets commonly dominated by smaller-scale (&lt;10 cm) folds that are more prevalent and easier to measure at outcrop. A consequence of this bias in recording data is that the amount of fold rotation (and hence bulk strain) may be over-estimated. A further complication is that fold pairs preserve more open upper hinges compared to tighter lower hinges that are closer to underlying detachments, meaning that the exact position of the hinge within fold pairs should be routinely recorded. The flattening of buckle fold hinges within competent psammites reflects the influence of gravity-driven deformation with an absolute sense of top-down-to the SE shear, which we interpret as being focussed into the shear zone during orogenic collapse.</div></div>","PeriodicalId":50035,"journal":{"name":"Journal of Structural Geology","volume":"201 ","pages":"Article 105544"},"PeriodicalIF":2.9,"publicationDate":"2025-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145050119","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}