Intrafolial Folds Research Articles

Late Paleozoic to Mesozoic tectonic evolution of the Qinling Orogenic Belt: Structural and geochronological constraints from the Devonian Liuling Group, eastern Qinling area, China

The Late Paleozoic to Mesozoic tectonic evolution of the Qinling Orogenic Belt is not fully understood. To address this, structural analysis and rutile U-Pb dating were performed on the Middle–Late Devonian Liuling Group located along the northern margin of the South Qinling Belt (SQB). Detailed structural analysis reveals that the Liuling Group experienced four main deformation events (D1–D4). Initial D1 deformation is characterized by intrafolial folds (F1) and a regional penetrative foliation (S1). Subsequent D2 is marked by SW-vergent asymmetric, overturned folds and related crenulation lineation (L2). D3 and D4 are represented by, respectively, nearly E-W trending open folds and NNE-SSW trending centimeter-scale kink folds, both of which slightly modify D1 and D2 structures. Taking into account published chronological data, we propose that D1 and D2 occurred at 363–349 Ma and 332–318 Ma, respectively, and were associated with NE-SW oriented shortening during continental subduction of the SQB. Our new rutile U-Pb ages of 243–240 Ma provide a lower age limit for D3 deformation, which was related to N-S oriented shortening triggered by continental collision between the South China Block (SCB) and SQB along the Mianlue Suture Zone (MLSZ). Finally, D4 deformation was associated with the far-field effects of Late Jurassic subduction of the Paleo-Pacific Plate, which generated a WNW-ESE oriented shortening. Our findings suggest that continental subduction of the SQB continued until the earliest Late Carboniferous. Subsequently, the northern margin of the SQB experienced a Triassic metamorphic overprint triggered by collision between the SCB and SQB.

Configuration of Carbonatite Constrained in Preintrusion Transpositional Foliation in the Bayan Obo Giant Rare Earth Element Deposit, China

Abstract The Bayan Obo ore deposit is the largest rare earth element (REE) deposit in the world and has been assumed to be hosted in dolomite that was folded in a syncline. This has been challenged by results from drill holes and low-resistivity, controlled-source audio-frequency magnetotellurics (CSAMT) survey. In this paper, we present structural analysis of clastic sedimentary rocks from around the deposit, their relationship with carbonatite, and the orientations of the carbonatite bodies and dikes to constrain the possible configuration of carbonatite. Clastic sedimentary rocks underwent mylonitic deformation to slate, metasandstone, and metaconglomerate, displaying dramatic changes of thickness along strike. Slates locally preserve intrafolial folds and hook folds bounded by foliations; metasandstones have parallel layers of recrystallized and preferred-orientated quartz aggregations; metaconglomerates contain flattened pebbles with Flinn k values of 0.01 to 0.05 and 0. The above structures and foliations were crosscut and intruded by carbonatite and associated fenite, demonstrating preintrusion transposition of original bedding to steep foliation. Preintrusion foliation provides zones of weakness that were exploited by upwelling of carbonatite magma. Northeast-SW–striking left-stepping en echelon carbonatite dikes and E-W–striking carbonatite bodies indicate that the carbonatite was emplaced in a sinistral transtensional zone. The northern and southern segments of the carbonatite bodies are parallel to the steep foliation at shallow depths and merge together at depth, constraining a Y-shaped configuration, consistent with the low resistivity result of the CSAMT survey. The newly recognized Y-shaped morphology indicates that the carbonatite extends deeper than 1,775.4 m, more than twice the previously inferred maximum depth of the syncline model, and contributes to a significant >1.78 times increase in global potential RE2O3 resources than previously estimated in 2021.

Formation of the Hengshanli granitic gneiss dome in the Paleoproterozoic Jiao-Liao-Ji Belt, North China Craton

A group of granitic gneiss domes that may provide constraints on the tectonic evolution of early Precambrian orogenic belts are well preserved in the LiaoJi segment (or LJOB-Liaoji orogenic belt) of the Jiao-Liao-Ji Orogenic Belt (JLJOB), eastern North China Craton. To deeply understand the genesis of the domes and how the crustal mass flowed in collisional orogenic belt, we carried out structural analysis of the Hengshanli dome in the western part of the LJOB. Detailed analyses of deformation structures, microstructures and fabrics, and geochronology of syntectonic granitic dykes demonstrate that three episodes of structural deformation contributed to the formation of the dome structure. During D1, a pure shear-dominated deformation, penetrative foliations (S1), symmetrical lenses or boudinages, intrafolial folds (F1) and mineral lineations (L1) developed in the interlayered rocks of viscosity contrasts. The D2, the WNW-ESE orientated sub-horizontal simple shear-dominated deformation, is characterized by asymmetrical A-type folds (F2), stretching lineations (L2) , mylonitic foliations (S2), localized ductile shear zones, axial planar foliations, rotated porphyroblasts, L>S tectonites and S-C fabrics that indicate high-strain simple shear deformation. A D3 brittle deformation is evidenced by E-W directed open folds (F3) by flexural folding mechanism. LA-ICP-MS U-Pb dating of zircons from the syntectonic granitic dykes reveals that the main deformation event occurred from ca. 1946 to 1844 Ma. The dating also reveals that the Liaoji granites at the core were emplaced at ca. 2147 Ma and experienced anatexis at ca. 1864 Ma and 1846 Ma. It is suggested that the mechanism of doming is a combination of middle and lower crustal sub-horizontal flow, diapirism and folding superposition. Convergent flow of remobilized magma at depth and sub-horizontal simple shearing of metasedimentary rocks triggered middle and lower crustal flow in association with diapirism during the doming. The granitic gneiss domes are the products of dominant lateral and subsidiary vertical crustal mass flow under E-W directed compressional regime in the LJOB. We suggest that the orogenic belt experienced peak deformation at ca. 1946-1844 Ma, accompanied with syntectonic metamorphism and magmatism at nearly the same time.

Pan African Nappe system: evidence of thrust structures from Okemesi, southwestern Nigeria

The Okemesi area, southwestern Nigeria, is part of the Ilesha schist belt and play host to the Okemesi fold belt and the Ifewara Shear Zone. Nappe structures had been proposed for some parts of Southwestern Nigeria without very clear meso or micro structural evidence. This study provides a lot of new structural evidences to substantiate the inferred Nappe structures within the Okemesi fold belt. Mapping of the Okemesi fold has been hampered by thick vegetation in the past. However, a major deformation zone was exposed by the recent landslide activities in the area, structures and lithologies exposed by the landslide were studied and structural features that are typical of transtensional and transpressional settings were observed. This paper presents the result of the systematic filed mapping which revealed the complete, fold transposition, elements of internal strain such as cleavages and mineral lineation, intrafolial folds, flexure slip, sheath, recumbent and conjugate kink folds, winged porphyroblast, faults (low angle reverse faults, dextral faults, normal faults), ramp and flat structures, etc. The characteristics and the geometry of the observed thrust structures make it an important piece of evidence, not only to infer the tectonic events in the area, but also to constrain the timing and nature of movement associated with thrusting in the area.

Mineral chemistry and deformation in a temperature gradient in the Sierras Pampeanas of Córdoba (Argentina): The Chicamtoltina Tonalite–Trondhjemite Orthogneiss

On the eastern margin of the Sierra Chica, in a gneissic host rocks metamorphosed under medium to high amphibolite facies (‘Sil-in’ isograd) during the Neoproterozoic–Cambrian M2-D2 event, a body of tonalitic-trondhjemitic composition was discordantly emplaced, cutting the S2 metamorphic foliation and the ‘Sil-in’ isograd. A strong deformation, with along the temperature gradient of the host rocks, transformed the igneous rocks into an orthogneiss during a M3-D3 event. Based on textural, microstructural and mineral chemistry changes along the mentioned metamorphic transition and age determination, the orthogneissification process is related to the deformational and thermal history in the Sierras Pampeanas of Córdoba. The M3-D3 event would have reworked the S2 foliation, generating an S3 foliation associated with folding and boudinage in the gneissic host rocks, forming a major intrafoliar fold in the tonalitic-trondhjemitic body, with its flanks on both sides of the ‘Sil-in’ isograd. The M3-D3 event is recorded in the textural and mineral chemistry variations, following a medium-high temperature gradient during deformation, from the southeast to the northwest of the body. In that direction, the ‘core and mantle’ texture in plagioclase increased the proportion of the recrystallized mantle, which was also enriched in anorthite content. Quartz textures evolved from polycrystalline ribbons, with internal deformation bands, to dismembered ones, with internal chessboard pattern. Recrystallization of the entire plagioclase-quartz aggregate ended in an interlobate to amoeboid texture, implying high-temperature conditions (~650-750 °C) along the mentioned temperature gradient during the M3-D3 event. Regarding micas, the primary biotite progressively increased in Mg and decreased in Fe towards the northwest, without appreciable textural changes. Concomitantly, the primary Ti-rich ferrimuscovite increases its content in Mg and becomes poor in Ti, at the same time that it developed symplectitic quartz-plagioclase intergrowths. During a final D4 deformational event, also under high ductility conditions, asymmetric intrafolial folds (drag folds, with geometrical S4 axial planes) were developed, associated with imbrication and folding of boudins within the S3 (// S2) foliation. The D2, D3, and D4 deformation events are part of a continuum (progressive deformation) in a combined simple shear and flattening regimen (transcompression = convergent oblique shear). The contrasting rheology between the stiffer Chicamtoltina igneous body and its gneissic host rocks explain the different behavior during high ductility deformation. Internally, this small igneous body, with plagioclase as stress-supporting mineral framework (plagioclase-quartz ratio 2:1), mirrored the rheological behavior of the middle to lower crust deformation. A concordia age of 535.81 ± 0.79 Ma (U–Pb LA-ICP-MS on zircon) obtained for the Chicamtoltina orthogneiss is interpreted as the crystallization age of the tonalitic-trondhjemitic protolith (igneous event Ig1). By correlation with the tonalitic orthogneisses from the Sierra Chica and Sierra Norte of Córdoba, this protholith is linked to the Pampean magmatic arc, with an age range of 541–515 Ma and a peak of magmatic activity at 530–535 Ma.

Lateral subhorizontal middle to lower crustal flow in response to continental collision: Evidence from the Diancang Shan complex along the Ailao Shan-Red River belt, Southeastern Tibetan Plateau

How middle to lower crust in Southeastern Tibetan Plateau flowed in response to the Indian-Eurasian continental collision remains hotly debated in the last decades. Metamorphic complexes along crustal mobile zones, e.g., the Diancang Shan (DCS) complex along the Ailao Shan-Red River (ASRR) metamorphic belt may provide key knowledge on our understanding of the middle-lower crustal flow during the collision. In this contribution, integrated structural, microstructural, kinematic, and thermo-chronological studies of the DCS complex reveal that the complex constitutes a destructed A-type dome. High-grade metamorphic rocks of Proterozoic protoliths form the core (Unit I) that is covered by low-grade metamorphic rocks of Mesozoic age in the limbs (Unit II). The eastern limb of the DCS dome is cut by a normal shear zone (Unit III). Three phases (D1, D2 and D3) of Cenozoic deformation structures are recognized in the complex. The D1 and D2 structures constitute the dominant structural style of the Units I and II. The D1 structures consist of penetrative S1 foliations, mineral lineations (L1), symmetrical intrafolial folds (F1) and tectonic lenses of granitic and quartz veins in rocks from the Units I and II. Mylonitic foliations (S2//S1), NNW-SSE plunging subhorizontal stretching lineations (L2), widespread A-type folds (F2), asymmetric porphyroclasts and tectonic lenses form the dominant structural styles in the D2 deformation. D3 structures are new foliations S3 and hot striae L3 in the normal-slipping shear zone (Unit III). The S2//S1 foliations define the domal framework of the DCS complex, which is truncated by the D3 shear zone. The dome is NW-SE elongated and has a hinge line paralleling to the L2//L1 stretching lineations. Synthetic analysis of structural and microstructural styles, and kinematic vorticity data estimated by rotated rigid porphyroclast method (0.38–0.69) and oblique grain-shape/quartz c-axis-fabric method (0.72–0.98) implies that the D1 and D2 deformations formed a progressive process from early pure shear-dominated (D1) to simple shear-dominated (D2) shearing. Shear sense indicators from different parts (Units I and II) of the dome are compatible with top-to-the southeast shearing during D2 deformation. It is shown that the D1-D2 deformation was characterized by a successive process from penetrative flow (D1) and ductile shearing at middle to lower crustal levels to doming (D2) that was coeval with exhumation of the DCS complex since 27 Ma or earlier. We conclude that these processes, i.e., the penetrative flow, ductile shearing, doming and exhumation of the DCS complex, were attributed to lateral subhorizontal middle to lower crustal flow along the ASRR metamorphic belt in the Southeastern Tibetan Plateau during the Indian and Eurasian plate collision.

Structural and hydrothermal evolution of the shear-zone-related iron-oxide enrichment in metavolcano-sedimentary rocks of the intracontinental araçuaí orogen: The case of the espírito santo iron deposit

The Rio São João metavolcano-sedimentary sequence is located in the eastern border of the northern Espinhaço fold thrust belt, in the intracontinental Araçuaí-West Congo Orogen. Mafic schist, quartz-, amphibole- and carbonate itabirites, as well as a carbonate–silicate rock, occur in the study area. These contain mineral assemblages compatible with the amphibolite-facies metamorphic conditions. Magnetite- and hematite-rich itabirites form a series of iron deposits in the region near Igaporã-Caetité, Bahia state, Brazil. The Espírito Santo iron deposit has compact hematite layers truncating quartz itabirite with magnetite varying from 10 to 55 vol% of the rock volume; siliciclastic and mafic rocks also host iron-rich domains, variably enriched in magnetite and hematite. The deposit is hosted in compressional duplexes with structural top towards the SW. These structures have an Sn-1//Sn foliation represented by compositional banding and parallel schistosity that is observed at all scales. Other common features are S/C/C, boudins, pinch-and-swell structures, as well as sheath, syn-tectonic folds, and a mineral stretching lineation (Lxn). Hydrothermal alteration was contemporaneous to the development of duplex and the Sn-1//Sn foliation. A distal alteration zone is dominated by a potassic (muscovite and-or biotite) ± chlorite assemblage and the intermediate zone by carbonate development. The proximal alteration zone constitutes the iron oxide (magnetite and hematite) domains; sulfide minerals as well as quartz veins (silicification) in association with other phases also formed. Hydrothermal magnetite is associated with C’ structures, which grows either including silicates and skeletal carbonates, or forming corrosion borders in these minerals. Lamellar hematite forms polycrystalline aggregates, characterizing the S/C structure in the Sn-1//Sn foliation, as well as the axial planar foliation (Sn) in intrafolial folds and the mineral stretching line Lxn. Data obtained by the LA-ICPMS technique show that, in general, in quartz-rich and amphibole itabirites hydrothermal magnetite is richer in REE than an early-stage magnetite. Hydrothermal magnetite REE pattern is close to that of the country rock of the mineralization. The formation of hematite- and magnetite-rich domains is related to the percolation of hydrothermal fluids that leached all components of the original silicates, quartz and the carbonates of all host rocks, and precipitated new hydrothermal phases taking advantage of the Ediacaran shear structures.

A missing link of the Proto-Tethys Ocean between the Qinling and Qilian orogens, China: Insights from geochronology and structural geology

The Qinling-Qilian connection zone (QQCZ) is a key area to reveal the relationship and to make a link of the North Qinling and the North Qilian orogens, China. Here we present U–Pb dating data of detrital zircons from four sedimentary/metasedimentary rocks in the QQCZ and the southwestern North China Block (NCB) and detailed regional structural data. Three episodes of fold deformation (D1, D2 and D3) are distinguished in the QQCZ, with the former two occurred during the early Paleozoic. The D1 deformation is mainly characterized by regionally penetrative schistosity and some residual rootless intrafolial folds due to the intensive superpositions by the subsequent D2 and D3 deformations. The D2 deformation characterized by tight folds, associated axial plane foliations and crenulation lineations indicates a stress field characterized by NNE–SSW-directed compression, which may be induced by the collision between the NCB and the southern blocks. The D3 deformation which might occur during the Mesozoic is marked by upright open folds and kink bands. The similarity of the detrital zircon age spectra of the Huluhe Group in the North Qilian Orogen and the Erlangping Group in the North Qinling Orogen suggests that the two groups have similar provenance, which may indicate that the North Qilian Orogen corresponded to the North Qinling Orogen in a regional tectonic framework. In addition, the remarkable age peak at ~435 ​Ma of the detrital zircon age spectrum of the Duanjiaxia Formation in the southwestern NCB indicates that this formation obtained the provenance of the North Qilian and North Qinling orogens, which may be generated by the collage of the southwestern NCB and the QQCZ during the Late Ordovician–Early Silurian. Based on structural, detrital zircon and metamorphic data, we suggest that the North Qilian and North Qinling orogens underwent similar evolution during the early Paleozoic due to the closure of the North Qilian and the Kuanping oceans which located at the northern boundary of the Proto-Tethys Ocean.

Characterization of the Rapanui mass‐transport deposit and the basal shear zone: Mount Messenger Formation, Taranaki Basin, New Zealand

AbstractSubmarine mass‐transport deposits are important in many ancient and modern basins. Mass‐transport deposits can play a significant role in exploration as reservoir, seal or source units. Although seismic data has advanced the knowledge about these deposits, more outcrop studies are needed to better understand gravity mass flows and predict the properties of their resultant deposits. It is proposed that sufficiently well‐exposed outcrops of mass‐transport deposits can be divided into three strain‐dominant morphodomains: headwall, translational and toe. The outcrops of the Rapanui mass‐transport deposit, part of the Lower Mount Messenger Formation in the Taranaki Basin, New Zealand, are exposed along a ca 4 km transect in coastal cliffs that enable the identification of the three morphodomains. The aim of this study is to characterize the stratigraphic and sedimentological nature of the Miocene‐age Rapanui mass‐transport deposit outcrops and the evolution of its basal shear zone. The basal shear zone of a mass‐transport deposit is defined as the stratal zone formed in the interface between the overriding mass flow and the underlying in situ deposits or sea floor. Accordingly, the deformation structures in the Rapanui mass‐transport deposit and the basal shear zone were documented in an established spatial framework. Traditional methodologies were used to characterize the sedimentology of the Rapanui mass‐transport deposit. Data collected from intrafolial folds, rafted blocks and samples from the Rapanui mass‐transport deposit were used to investigate strain and matrix texture evolution, estimate palaeoflow direction, and calculate yield strength and overpressure at time of deposition. Additionally, a one‐dimensional numerical model was used to test sedimentation‐driven overpressure as probable trigger. This work demonstrates that the basal shear zone, as well as the matrix texture of a mass‐transport deposit, can vary spatially as sediments from underlying deposits are entrained during shear‐derived mixing. This phenomenon can impact the seal potential of mass‐transport deposits and their interaction with fluids in the subsurface.

Geochemistry and origin of the Rhyacian tholeiitic metabasalts and meta-andesites from the Vila Nova Greenstone Belt, Guyana Shield, Amapá, Brazil

The origin and significance of the metavolcanic rocks of the Vila Nova Greenstone Belt (VNGB) is an important issue in order to understand the evolution of the Guyana Shield in Paleoproterozoic times. This work presents new LA-MC-ICPMS U-Pb zircon data and geochemical analyses carried out on metavolcanic rocks of the VNGB. This information was supplemented by petrography, stratigraphic and structural data acquired through geologic mapping and boreholes descriptions. In the Vila Nova region, the VNGB units rest on the Archean basement composed of orthogneisses of the Tumucumaque Complex. The lower portion of the VNGB consists of metabasalts and andesitic metabasalts, covered by an upper metasedimentary domain with subordinate metavolcanic and exhalative chemical rocks. The metavolcanic rocks include metabasalts, meta-andesites, amphibolites and amphibole schists, deformed and elongated according to the regional NW-SE structure. U-Pb zircon data yielded an age of 2154 ± 6 Ma for a meta-andesite of the lower portion of VNGB. One orogenic metamorphism followed by three deformational events were recognized. The D1 and D2 events formed the schistosity (S1), preserved as intrafolial folds (F2) and the crenulation cleavage (S2) associated with the development of the oblique shear zones. Two distinct assemblages: 1) garnet, hornblende and biotite, and 2) hornblende, plagioclase and diopside define two distinct metamorphic peaks: M1 and M2 with temperatures of 450 and 650 °C, and lithostatic pressure between 4 and 6 kbar. The metabasites comprise geochemical compositions of Fe- and Mg-tholeiites with komatiitic affinity. The first group is enriched in LREE and HFSE. The second group shows MORB-like REE patterns in association with negative Nb, Ti and P anomalies. The observed features indicate that the metavolcanic rocks of the VNGB represent the evolution of back-arc basins and island arcs associated with the Trans-Amazonian Orogeny. These rocks characterize the Rhyacian evolution of the Maroni-Itacaiunas Province in the eastern portion of the Guyana Shield.

Early Paleozoic tectonic reactivation of the Shaoxing-Jiangshan fault zone: Structural and geochronological constraints from the Chencai domain, South China

The Shaoxing-Jiangshan fault zone (SJFZ), as a fundamental Neoproterozoic block boundary that separates the Yangtze Block from the Cathaysia Block, is the key to understanding the evolution of South China from Neoproterozoic block amalgamation to early Paleozoic crustal reworking. New structural observations coupled with geochronological ages from the Chencai domain indicate that intense ductile deformation and metamorphism along the SJFZ occurred at ∼460-420 Ma, in response to the early Paleozoic orogeny in South China. To the east of the SJFZ, the deformation involves widespread generations of NE-striking foliation, intrafolial folds, and local development of sinistral-oblique shear zones. The shearing deformation occurred under amphibolite facies conditions at temperatures of >550 °C (locally even >650 °C). To the west of the SJFZ, the deformation corresponds to sinistral-oblique shearing along NE-striking, steep-dipping zones under greenschist facies conditions at temperatures of 400–500 °C. These deformation styles, as typical mid-crustal expressions of continental reworking, reflect tectonic reactivation of the pre-existing, deeply rooted Neoproterozoic block boundary in the early Paleozoic. We infer that the tectonic reactivation, possibly induced by oblique underthrusting of north Cathaysia, facilitated ductile shearing and burial metamorphic reactions, giving rise to the high-strain zones and high-grade metamorphic rocks. With respect to pre-existing mechanical weakness, our work highlights the role of tectonic reactivation of early structures in localizing later deformation before it propagates into yet undeformed domains.

Sediment creep on slopes in pelagic limestones: Upper Jurassic of Northern Calcareous Alps, Austria

Slump structures in Upper Jurassic pelagic limestones of the Northern Calcareous Alps were studied using methods of ductile structural geology. The early deformation observed cannot be explained by conventional models for slumping, commonly describing slump complexes with an extending upper and contracting lower part. Instead, the structures suggest distributed bedding-parallel surficial stretching, and folding localized by roll-over normal faulting. The initially isoclinal and recumbent folds are pervasively overprinted by stretching, causing reorientation of fold axes into the downslope direction. Transport of folds separates antiformal and synformal hinges, and the resulting isolated hinges resemble intrafolial folds in metamorphic terranes. Only folds formed late in the process of lithification allow insight in the early stages of fold evolution. We suggest that this type of early deformation represents sediment creep that may be characteristic for slump structures in pelagic carbonates.

Shear Sense Analysis of the Higher Himalayan Crystalline Belt and Tectonics of the South Tibetan Detachment System, Alaknanda-Dhauli Ganga Valleys, Uttarakhand Himalaya

In the central parts of Uttarakhand Himalaya, more than 20 km thick homoclinally NE-dipping Higher Himalayan Crystalline (HHC) Belt is thrust over the Lesser Himalaya Sedimentary Belt along the Main Central Thrust (MCT), and is almost continuously exposed between Helang and Malari along the Alaknanda and Dhauli Ganga valleys. The upper contact of this belt with the Martoli Formation of the Tethyan Himalayan Sequence is demarcated by the South Tibetan Detachment System (STDS). The belt is ubiquitously marked by small-scale asymmetrical structures like S-C and S-C' foliation, porphyroclasts and porphyroblasts, mineral fishes, intrafolial folds, duplex structures, ductile-brittle shear zones, and asymmetric shear boudins. Sense of ductile to brittle- ductile shearing has been determined from these structures across the whole belt, the MCT and the STDS, and reveals two phases of shear deformation: (a) an older top-to-SW upwards phase throughout the HHC, having an overall thrust geometry (DS1), and (b) a younger superposed top-to-NE downwards phase with normal fault sense from the middle to upper parts (DS2). These shear senses provide invaluable constraints on various tectonic models currently in use for the evolution of the Himalayan metamorphics.

Winged inclusions: Pinch-and-swell objects during high-strain simple shear

In this study, we compare natural examples of isolated pinch-and-swell objects, which have been deformed in simple shear, with results of high-strain numerical models. Such structures, which have geometrical similarities with δ-clast systems and rolling structures, have been called winged inclusions. We suggest a new mechanical explanation for the evolution of winged inclusions, which form when pinch-and-swell shaped objects consisting of a core and pre-existing wings rotate out of the shear plane. The viscosity ratio, the stress exponent, and the shape of the winged inclusion have a significant influence on the rotation rate. The rotational behaviour of winged inclusions differs significantly from the rotational behaviour of simple elliptical objects with comparable aspect ratios. During the early stages of formation, winged inclusions may resemble mirror images of sigmoids and misinterpretations may lead to a wrong determination of the shear sense. During progressive shear to large strains, the structures may be approximately described as consisting of a pulsating rotating core and thinning wings that rotate with different rates. During rotation of the structure, the core of the winged inclusions records an overall decrease of the aspect ratio influencing the rotation rate of the inclusion. The wings are subject to ptygmatic folding, when they rotate through the field of instantaneous shortening and may unfold again in the field of instantaneous stretching. During ongoing shearing, the wings on both sides of the core rotate with the core, may change their positions and finally unfold after rotation about 180° resulting again in a pinch-and-swell shaped object. Therefore, winged inclusions record ambiguous information about the finite strain. Rotating winged inclusions create a significant flow perturbation in the matrix resulting in nucleation of folds, refolding and propagation of shear zones. Rootless intrafolial folds with opposing closures have strong geometrical similarities with winged inclusions and we therefore speculate that the formation of winged inclusions might be an efficient mechanism to produce rootless intrafolial folds.

Paleoproterozoic granulites from the Xinghe graphite mine, North China Craton: Geology, zircon U–Pb geochronology and implications for the timing of deformation, mineralization and metamorphism

The Xinghe graphite deposit is located in the north-central part of the North China Craton (NCC) where Archean gray gneiss, metasedimentary rocks (khondalites) and mafic (high-pressure) granulites constitute the dominant rock types. In this study, we report results of detailed geological and structural mapping around the Xinghe graphite mine. We identify three stages of deformation (D1–D3) and the graphite mineralization is correlated with D1 or between D1 and D2. The first stage of deformation represented by rootless intrafolial folds whose axial surface cuts across the regional penetrative gneissosity (S2) of D2, was constrained at ca. 1.95–1.90Ga by U–Pb age of zircon grains from a mafic high-pressure granulite. The second stage of deformation is represented by tight folds with formation of regional penetrative foliation and strong ductile shearing, when garnet broke down to plagioclase+pyroxene. This stage is dated as ca 1.85Ga, from U–Pb analysis of zircons in leucosome from a migmatitic gneiss. The third stage of deformation is represented by large scale folding, which probably formed between 1.85 and 1.80Ga, as constrained from the ca. 1.80Ga emplacement age of an undeformed pegmatite that cuts across the host metasediments. Prior to the high-pressure granulite metamorphism, voluminous ca. 2.15–2.0Ga granites were emplaced within the khondalites. The graphitization postdates the emplacement of these granites as deduced from the presence of flaky graphites in the metagranites. The protolith of the mafic high-pressure granulite dike that intruded the khondalite at ca. 2.0Ga was also metamorphosed at 1.95–1.90Ga, and is part of the exploited graphite ore body. We therefore constrain the timing of graphite mineralization to be between ca. 1.95 and 1.85Ga. Our geological and geochronological data indicate that all the rock units in the Xinghe graphite mine were subjected to high-pressure granulite facies metamorphism at ca. 1.95–1.90Ga.

Tectonic and deformation history of the Gyeonggi Massif in and around the Hongcheon area, and its implications in the tectonic evolution of the North China Craton

The Precambrian rocks of the Gyeonggi Massif (GM) in and around the Hongcheon area, South Korea, consist of the Yongduri Gneiss Complex (YGC), Euiam Group (EG) and Euiam Gneiss Complex (EGC). The YGC and EG are composed mainly of partially migmatised metasedimentary rocks and the EGC is Paleoproterozoic intrusive rock that intruded the EG. At least three major folding (F1, F2 & F3) accompanied with the two-stages of ductile shearing and three-stages of metamorphic events (M1, M2 & M3) occurred in the study area. The F1 folds are extremely drawn out, isoclinal, intrafolial folds and have Class 2 to Class 1C geometry. The F1 folds and regional S1 foliation in the YGC and EG are the results of E-W compression during the D1 deformation. Ductile shearing in the southern part of the EG is marked by the Palbongsan Shear Zone that indicates top-to-the SW sheared movement during syn to post-F1 folding. The F2 folds are open to tight, SW plunging and inclined folds, and have Class 1A to 1C geometry. The F2 folding and subsequent NNE thrusting along multiple ductile shear zone parallel to S2 planar fabrics, represent the D2 deformation due to N-S progressive shortening. The D3 deformation was coaxial with the D1 deformation, leading to the development of the F3 kink bands in the mylonite zones. The SHRIMP zircon ages form the EG yield M1 metamorphism age of 1917–1925Ma. However ca. 1867–1883Ma, M2 metamorphism previously reported from the YGC is absent or weakly preserved in the EG representing that the M2 metamorphism was not strong enough to form new zircon in the EG. The igneous zircons from the augen gneiss in the EGC yield intrusion age of ca. 1867–1881Ma and the geochemistries of the EGC gneisses show post-collision tectonic origin. The D1 deformation observed in the YGC and EG but not in the EGC. The D1 deformation occurred during M1 metamorphism, and then the M2 metamorphism and 1867–1881Ma igneous activities could have occurred together during post-collision tectonic stage. The M1 metamorphism and ca. 1867–1881Ma post collision magmatism in the study area are well matched with the collision related metamorphism at ca. 1.90–1.93Ga and post collision igneous activities at ca. 1.80–1.86Ga along the Jiao-Liao-Ji belt in the North China Craton suggesting that M1 metamorphism could have caused by the collision along the Jiao-Liao-Ji belt. The M3 metamorphism occurred at ca. 230–260Ma and its peak metamorphic conditions were 720–730°C/13–14.5kbar. The D2 deformation and the M3 intermediate-P/T metamorphism occurred during the Permo-Triassic collision event between the North China Craton and South China Craton. The study area was not a collision boundary but located in the peripheral areas of the collision belt during the Paleoproterozoic and the Permo-Triassic time. The D3 deformation occurred at a time gap between the Permo-Triassic collision and the subduction related Jurassic intrusion in the Korean Peninsula.

When do folds unfold during progressive shear?

Folded layers in rocks can be stretched again, potentially unfolding the folds back to straight layers. Little is known, however, about how to recognize partly or even entirely unfolded layers. When folded layers can unfold, what determines their mechanical behavior, and how can we recognize them in the field? In order to address these questions, we present a series of numerical simulations of the stretching of previously folded single layers and multilayers in simple shear. Layers do not completely unfold when they undergo softening before or during the stretching process, or when adjacent competent layers prevent them from unfolding. Intrafolial folds and cusp-like folds adjacent to straight layers as well as variations in fold amplitudes and limb lengths of irregular folds are indicative of stretching of a fold train.

The evolution of the footwall to the Ronda subcontinental mantle peridotites: insights from the Nieves Unit (western Betic Cordillera)

Strongly heterogeneous deformation and extreme metamorphic gradients characterize the dominantly carbonate Nieves Unit in the footwall to the Ronda mantle extrusion wedge in the western Betic Cordillera. A well-developed foliation and mineral lineation, together with isoclinal intrafolial folds, occur in silicate-bearing, calcite or dolomite marbles within a c . 1.5 km thick metamorphic aureole underlying the peridotites. For the inferred maximum pressure of 300 MPa, petrological investigations allow us to define temperature ranges for the main zones of the metamorphic aureole: >510 °C (probably c. 700 °C) for the forsterite zone; 510–430 °C for the diopside zone; 430–360 °C for the tremolite zone; 360–330 °C for the phlogopite zone. Field structural analysis integrated with petrological, microstructural and electron backscatter diffraction textural data document large finite strains consistent with general shear within the metamorphic aureole, associated with NW-directed thrusting of the peridotites. On the other hand, post-kinematic silicate growth suggests that heat diffusion from the high-temperature peridotites continued after the final emplacement of the Ronda mantle extrusion wedge, leading to final zoning of the metamorphic aureole and to local partial annealing of calcite marble textures, particularly in the highest-temperature zone of the thermally softened footwall carbonates. Following substantial cooling, renewed crustal shortening affected the whole Nieves Unit, resulting in widespread development of NE–SW-trending meso-scale folds.

Geometry and kinematics of the late Proterozoic Angavo Shear Zone, Central Madagascar: Implications for Gondwana Assembly

This paper documents the 20 to 60km wide N–S trending Angavo Shear Zone (ASZ) in central Madagascar and its tectonic implications by examining its structural styles, kinematics and geometry. Our study indicates that the ASZ is characterized by at least two ductile Late Proterozoic deformation events (D1 and D2) followed by a brittle neotectonic deformation (D3). The early D1 event produced a regionally extensive S1 foliation, stretching/flattening mineral lineation L1 and symmetrical structural fabrics such as recumbent and isoclinal intra-folial folds (F1), implying a flattening deformation. D1 deformational fabrics are locally overprinted by D2 structures. D2 is characterized by a penetrative S2 foliation, shallow south plunging L2 lineation, asymmetric and sheath folds (F2) consistent with a right lateral sense of movement exhibited by delta- and sigma-type porphyroclast systems and asymmetric boudinage fabrics. D2 represents a non-coaxial flow regime formed in a dextral west over east shear zone during a partitioned transpression in response to east–west-directed compression during the assembly of Gondwana. A close resemblance with the Achankovil shear zone in India is noticed; however the continuation of the ASZ in Africa is uncertain.

Structural and kinematic analysis of the Early Paleozoic Ondor Sum-Hongqi mélange belt, eastern part of the Altaids (CAOB) in Inner Mongolia, China

We present a structural and kinematic study of an Early Paleozoic subduction mélange and a magmatic arc that form the main elements of the Southern Orogen Belt of Inner Mongolia, which lies in the eastearn part of the Altaids or Central Asia Orogenic Belt. The structural analysis of the mélange conducted in the Hongqi and Ondor Sum areas (western Inner Mongolia) shows two phrases of ductile deformation. The D1 event is responsible for the pervasive S1 foliation, NW–SE striking L1 stretching lineation and F1 intrafolial folds. These microstructures are coeval with a greenschist facies metamorphism. During D2, NW-verging F2 folds associated with a S2 axial planar cleavage deformed S1 and L1. The D1 kinematic criteria indicate a top-to-the-NW sense of shear. D1 and D2 developed before the unconformable deposition of the Early Devonian shallow water sandstone. A lithosphere scale geodynamic model involving an Early Paleozoic southeast-directed subduction beneath the North China Craton and late Silurian collision of the North China Craton with an hypothetical microcontinent is proposed to account for the microstructural evolution.