Asymmetric Folds Research Articles

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.

InSAR derived co-seismic deformation triggered by the Mihoub (Tell Atlas of Algeria) 28 May 2016 (Mw = 5.4) earthquake combined to geomorphic features analysis to identify the causative active fault

In this work,we applied Sentinel 1A Synthetic Aperture Radar (InSAR) data from the European Space Agency, acquired before and after the event, to study the surface deformation induced by the Mihoub 28 May 2016 earthquake (Mw = 5.4) occurred in the Tell Atlas of Algeria.The deformed area is 10 km-long trending NE-SW and located about 6 km southwest of the locality of Mihoub, the most affected urban locality (I0= VIII, EMS scale). The displacement signal in the InSAR data showed up to 4 cm uplift and up to 12 cm subsidence zones indicating where the earthquake took place. The fault corresponds to the boundary between the uplift (hanging wall) and subsidence (footwall) zones.The short time between the two used images minimises uncertainties, and the used SentiNel Application Platform software (SNAP V7.0) ensures the topographic and orbital phases corrections. The geomorphic features analysis, including the shape of the uplifted area and the drainage diversion, indicate that the deformed area corresponds to an asymmetrical fold, a typical active structure in the Tell Atlas region. The slip model indicates one single patch with a max slip of ̴ 20 cm at a depth of 5 km on 60° northwest dipping and N°60 striking reverse fault. The results show that the Mihoub fault is most likely NW-dipping.

Cтруктурно-петрологическая эволюция Cнежнинского габбро-сиенитового массива (Южное Прибайкалье)

The article deals with the structural and petrological evolution of one of the many gabbroid massifs of the Southern Baikal region - Snezhninsky, located near the river of the same name, characterized by the most typical structural and metamorphic paragenesis: comfort with layering of containing rocks, located in a steep twisted wing of asymmetrical folds of latitudinal extension with horizontal hinges and axial planes inclined to the north. the array is complicated by a longitudinal oblique upsurge with a powerful zone of tectonites. The northwestern end of the massif is complicated by the upsurge of the northeastern extension. To a depth, the massif sinks to 4-5 km and there is cut off by another fault of the northwestern extension. The internal structure of the array is characterized by a rather monotonous development of small structural forms: the structures of the magma flow, emphasized by the elongation of flat xenoliths, the asymmetrical folds of the right pattern in the plan, the gradual intertransitions between all varieties of rocks composing the array, indicating a significant warming of all rocks during their formation and assimilation.

Late orogenic tectonics in the Ligurian Alps (Italy): constraints from syntectonic sedimentary deposits at the top of an exhumed plate interface

ABSTRACT This new map at 1:25,000 scale covers an area in the Western Alps (Italy), where the HP metamorphic basement is covered by a (predominantly clastic) late- and post-orogenic upper Eocene-lower Miocene cover. This provides unequivocal evidence that exhumation of the metamorphic units was accomplished by the lower Oligocene. The basement is made up of tectonic units with different metamorphic peaks that coupled during the exhumation path. Both basement and cover are affected by long-wavelength, asymmetric folds and thrust faults with top-to-E-NE vergence; this deformation phase has long been interpreted as linked to the rotation of the Corsica-Sardinia block and contemporary opening of the Liguro-Balearic basin and beginning of the Apennine orogeny. We particularly investigated some of these structures that crop out in the mapped area to give further constraints on their distribution, type and kinematics and to frame them in a larger picture of left-hand strike-slip tectonics.

Paleoproterozoic polyphase deformation in the Helanshan Complex: Structural and geochronological constraints on the tectonic evolution of the Khondalite Belt, North China Craton

The Khondalite Belt is widely regarded as a Paleoproterozoic collisional orogen between the Yinshan and Ordos Blocks in the Western Block of the North China Craton. However, the deformation history of the Khondalite Belt is poorly understood, and many aspects of the tectonic processes involved in the collision remain enigmatic. In this paper, we carried out detailed field-based structural analyses integrated with geochronological studies in the Helanshan Complex, one of the multiply deformed high-grade gneiss terrains in the Khondalite Belt. Three major phases of deformation (D1-D3) were identified in the Helanshan Complex. D1 mainly produced S(SW)-verging recumbent to overturned intrafolial isoclinal folds F1, originally sub-horizontal penetrative transposition foliations S1 and SSW-plunging mineral lineations L1. D1 deformational fabrics occurred coevally with the granulite-facies peak metamorphism. D2 dominantly generated steeply SE(E)-plunging asymmetrical upright open folds F2 that successively refolded the previous D1 structures. D3 gave rise to a series of NE- to E-trending ductile shear zones, featured by top-to-the-(N)NW oblique thrusting with a component of sinistral strike-slip movement. LA-ICP-MS U-Pb zircon dating results of seven deformation-related samples reveal that D1 occurred at some time from 1954 Ma to 1911 Ma, most likely at 1954–1949 Ma. D2 successively happened in the period of 1911–1870 Ma, and D3 had broadly taken place at ∼ 1870 Ma. The D1-D3 deformation sequences recorded tectonic processes including the initial top-to-the-SSW thrusting and crustal thickening, subsequently followed by exhumation of the khondalites in the Paleoproterozoic. Combined with previous studies, these new structural and geochronological data indicate that the Khondalite Belt underwent a long-term (>100 Myr) orogenic history and developed polyphase deformation, high-grade metamorphism as well as magmatism in response to the NNE-SSW-directed oblique collision between the Yinshan and Ordos Blocks at ∼ 1.95 Ga.

Double-Eyelid Blepharoplasty: Dermis-Outer Orbicularis Fascia-Orbicularis-Levator Fixation Technique with Preorbicular Venous Network Preservation.

This article presents a new method using a dermis-outer orbicularis fascia-orbicularis-levator (DOOL) fixation technique for double-eyelid blepharoplasty. Our surgical technique preserves the preorbicular venous network (POVN) and uses mattress sutures to fix the dermis, outer fascia of the orbicularis oculi muscle, and orbicularis oculi muscle with pretarsal levator aponeurosis (DOOL). Between January 2016 and July 2018, 335 patients were treated with this POVN-preserving DOOL technique (321 women and 14 men; mean age, 29.6 y). The patients were followed up for 6-30 months. The complications were documented, and the overall outcomes of the upper eyelid folds were evaluated by both surgeons and patients as good, fair, or poor. Among 335 patients, 307 (91.6%) had good results, 17 (5.1%) had fair results, and 11 (3.3%) had poor results. Postoperative complications included partial (n=4) or complete (n=3) loss of the double-eyelid line and asymmetric folds (n=4). Hypertrophic/depressed scars did not occur. With less invasiveness and secure internal fixation, the DOOL fixation technique with POVN preservation can achieve a stable and natural double-eyelid appearance. This journal requires that authors assign a level of evidence to each article. For a full description of these Evidence-Based Medicine ratings, please refer to the Table of Contents or the online Instructions to Authors www.springer.com/00266 .

Structural and rheological features of the northeastern Sanyang shear zone in the eastern Jiangnan orogen: Indications for early Paleozoic orogeny in the South China Block

The N55°E−trending Sanyang shear zone is located in the eastern Jiangnan orogen, and the northeastern junction between the Cathaysia and Yangtze blocks is the key to understanding the evolution of the South China Block (SCB) for early Paleozoic crustal reworking. Most shear-sense indicators (e.g., asymmetric folds, porphyroclasts, S–C fabrics, and mica fish) indicate that the Sanyang shear zone is characterized mainly by oblique sinistral shear and locally by dextral shear. Mineral deformation behavior, quartz C-axis fabrics, and the Kruhl thermometer demonstrate that shear deformation developed under high greenschist-to low amphibolite facies conditions with temperatures ranging from 450 to 550 °C. Differential stresses deduced from recrystallized quartz grain sizes were approximately 22.4–46.5 MPa, and strain rates were on the order of 10−9 to 10−10 S−1. Three dimensional (3D) strain ellipsoid measurements and the Flinn diagram of the Sanyang shear zone indicate that all samples yield oblate and flattened strain ellipsoids, which imply a contractional regime. The kinematic vorticity (Wk) 0.716–0.999, estimated by using 3D strain ellipsoid-based methods, indicates a high simple shear contribution in the shear zone. All structural, strain, and vorticity analyses suggest that the Sanyang shear zone experienced general shear with transpression. The numerical estimates allow kinematic modeling of the Sanyang shear zone, which provides important information about the structural framework for the SCB. The development of the Sanyang shear zone was a response to the oblique convergence of the Yangtze and Cathaysia blocks during the early Paleozoic orogeny.

How the Severity and Mechanism of Recurrent Laryngeal Nerve Dysfunction during Monitored Thyroidectomy Impact on Postoperative Voice.

Simple SummaryRecurrent laryngeal nerve (RLN) dysfunction remains a major source of morbidity after thyroid surgery. Intraoperative neuromonitoring can qualify and quantify RLN function according to the laryngeal electromyography (EMG) response evoked by electrical stimulation of the RLN. To the best of our knowledge, this is the first report to discuss the severity and mechanism of RLN dysfunction and postoperative voice in patients who have received monitored thyroidectomy. For optimal voice and swallowing outcomes after thyroid surgery, thermal injury must be avoided, especially when using energy-based devices, and mechanical injury must be identified early to avoid a more severe dysfunction. Adherence to standard intraoperative neuromonitoring (IONM) procedures for thyroid surgery is suggested, including standard procedures for acquiring and interpreting intraoperative RLN signals, for identifying and classifying RLN injury mechanisms, for performing laryngeal examinations and comprehensive voice assessments (subjective and objective voice analysis) before and after surgery, and for performing standard follow-up procedures.Intraoperative neuromonitoring can qualify and quantify RLN function during thyroid surgery. This study investigated how the severity and mechanism of RLN dysfunction during monitored thyroid surgery affected postoperative voice. This retrospective study analyzed 1021 patients that received standardized monitored thyroidectomy. Patients had post-dissection RLN(R2) signal <50%, 50–90% and >90% decrease from pre-dissection RLN(R1) signal were classified into Group A-no/mild, B-moderate, and C-severe RLN dysfunction, respectively. Demographic characteristics, RLN injury mechanisms(mechanical/thermal) and voice analysis parameters were recorded. More patients in the group with higher severity of RLN dysfunction had malignant pathology results (A/B/C = 35%/48%/55%, p = 0.017), received neck dissection (A/B/C = 17%/31%/55%, p < 0.001), had thermal injury (p = 0.006), and had asymmetric vocal fold motion in long-term postoperative periods (A/B/C = 0%/8%/62%, p < 0.001). In postoperative periods, Group C patients had significantly worse voice outcomes in several voice parameters in comparison to Group A/B. Thermal injury was associated with larger voice impairments compared to mechanical injury. This report is the first to discuss the severity and mechanism of RLN dysfunction and postoperative voice in patients who received monitored thyroidectomy. To optimize voice and swallowing outcomes after thyroidectomy, avoiding thermal injury is mandatory, and mechanical injury must be identified early to avoid a more severe dysfunction.

Structural overprinting and style of deformation at sierra Beauvoir and sierra de Apen: A geometric and kinematic model for the evolution of the internal thrust-fold belt, Fuegian Andes, Argentina

A detailed study of the structural fabrics and overprinting relationships exposed at sierra Beauvoir and sierra de Apen, which include folds, foliations and thrust faults, allowed us to settle a deformation sequence of the internal thrust-fold belt of the Fuegian Andes. Our results, together with previous geochronology results, indicate that ductile structures developed during a Late Cretaceous simple shear deformation that formed asymmetric folds and related foliations under very-low grade metamorphism. This deformation affected rocks not younger than the Campanian, and is here related to the front of the orogenic wedge formed during arc-continent collision associated with the closure of a prior back-arc basin. Thrust systems overprint this ductile deformation, and even though they were considered in previous theoretical models, this is the first time their detailed geometry and time constraints are reported from field data. We identified an older, brittle-ductile thrust system formed during the Danian, with the slight penetrative deformation and very gentle folding of the Maastrichtian-Danian Policarpo Formation representing the frontal part of this thrust wedge. This thrust system formed at intermediate structural levels and was linked with basement-involved thrusting in the hinterland, which caused the shortening accommodated in the internal thrust-fold belt. The Eocene Apen-Malvinera thrust system, formed by brittle faults, was responsible for later exhumation of the Cretaceous rocks deformed by prior brittle-ductile structures. This deformation sequence implies that a significant supply of detritus to the foreland basin, originated from the erosion of the Cretaceous cover of the brittle-ductile structures of sierra Beauvoir, should be not older than the Eocene.

Utility of Clinical and Ultrasonographic Hip Screening in Neonates for Developmental Dysplasia of the Hip.

Developmental dysplasia of the hip (DDH) is a musculoskeletal condition occupying any point along a spectrum of anatomical abnormalities that alter the stability of the newborn hip. Presentation varies throughout infancy and the majority of cases, especially those that are mild in nature, tend to resolve without intervention. An analysis of outcomes was conducted on infants born over a two-year period at a single-center, community hospital in East Toronto. The unwritten norm at the institution has become to order hip ultrasonography for all infants born in the breech position through C-section. Given the healthcare expenditure associated with routine radiographic screening, a careful analysis was undertaken to ascertain whether this screening regimen was effective in preventing late-stage detection of advanced DDH and improving organization in patient management. There were a total of 4236 babies delivered over the two years. One-hundred sixty-four (164) babies were born breech and through C-section. Eight (8) babies had abnormal hip examinations, one of whom was ultimately diagnosed with DDH. Forty-six (46) babies showed abnormal hip ultrasound at six weeks. Seventeen (17) referrals were made to the orthopedic surgeon. This resulted in a total of seven cases of DDH being diagnosed over the two years. The sensitivity and specificity of clinical hip screening were 14.3% and 95.5%, respectively, while that for ultrasound screening was 100% and 75.2%.To improve the quality of care and detection of DDH, a risk factor analysis was conducted to retrospectively analyze which DDH cases would have been missed if a higher threshold to ordering hip ultrasonography had been used. Based on the test characteristics of clinical and ultrasonographic screening, held in conjunction with the risk factor analysis results, an altered screening regimen was proposed with the intention of being just as sensitive but more cost-effective. This regimen integrates clinical screening using Barlow and Ortalani maneuvers until the eight to 10-week period and examines for limited abduction from eight weeks onward. Adjuncts like the Galeazzi test and that for asymmetrical skin folds should also be included to increase the sensitivity of clinical screening. Ultrasonography is proposed for high-risk individuals, with the criteria for stratification as high-risk being extracted from the risk factor analysis. Ultrasound is also proposed to be done in a serial fashion prior to orthopedic surgery referral in cases where the age of the infant allows, which serves to better evaluate the risk for lasting DDH and understand the longitudinal trajectory of the patient. This serves the additional purpose of decreasing the psychosocial burden on families. This can be particularly significant for infants for whom the initial abnormalities are due to self-resolve with the maturation of the hip joint and the infant’s growth.

Spatial distribution of ultrahigh-temperature granulites of the Highland Complex of Sri Lanka: Lowermost continental crust above an ultrahot palaeo-Moho

The spatial distribution of isolated ultra-high-temperature (UHT) granulites in Sri Lanka corresponds to the exposure level of the deepest granulites of the Highland Complex (HC). Their spatial distribution, and that of ultramafic bodies in the HC, defines a relatively thin (several km) lowermost crustal layer that has been folded into a N-plunging asymmetric fold (F3 in our scheme), Z-shaped looking N. We postulate that the UHT granulites represent suitable lithologies within this lowermost crustal layer above an ultrahot palaeo-Moho. Prior to D3, this layer had already been imbricated and intensely folded with the HT granulites and with ultramafic units after peak metamorphism, leading to a thick package of HT granulites with isolated UHT assemblages. This concept has some important implications for granulite terrains in general: (i) UHT granulites elsewhere may also represent a thin zone above an ultrahot palaeo-Moho and overlain by HT granulite terrains, and their spatial relationships may have been complicated by ductile deformation during early cooling; (ii) many other HT granulite terrains may be underlain by a similar zone of UHT granulites, and, if so, new discoveries of UHT terrains may be expected.

Relation between intrusive and deformational processes in oblique subductive margins. The case of the zoned Flamenco pluton in northern Chile

Oblique convergent margins, like the subduction of Phoenix beneath the South American plate during Jurassic and Early Cretaceous times, are characterized by strain partitioning and a positive feedback loop between strike-slip deformation and magma ascent along the magmatic arc.Located in the Coastal batholith of northern Chile, the Flamenco pluton is one of the youngest Andean intrusives emplaced in the western active margin of South America. Besides an older SW domain of granodioritic rocks (c.a. 213 Ma), the NW and E domains of the Flamenco pluton were emplaced between 194 and 186 Ma. They present a normally zoned structure constituted by external gabbroic to Qtz-dioritic magmatic facies and tonalites and granodiorites located in inner areas of the intrusive body. These domains are separated by a central strip of stretched coarse-grained Crd-schists that presents ductile asymmetrical folding and S–C structures that point to the NW-directed displacement of the E domain of the pluton. This syn-emplacement shear zone shows kinematic compatibility and continuity to the north and south with folded and mylonitic metasediments out of the contact aureole of the pluton. Together, these segments constitute a large, steeply dipping sigmoidal structure of average N–S direction; the called here Chañaral transcurrent shear zone.Contemporary to the emplacement of the Flamenco pluton, slight variations in the trend of the crustal-scale structure generated strike-slip and transpressive sectors along the Chañaral shear zone, which favored the access of intruding magmas to the final emplacement level. As a paradigmatic example, the curviplanar Flamenco shear zone, an internal, magmatic branch of the main structure that traverses the E domain of the pluton, is defined by the sinistral and reverse shearing under magmatic conditions of the previously mingled mafic and felsic batches. Consequently, the transpressive Flamenco shear zone is interpreted as an ascent conduit where gabbroic and granodioritic liquids interacted during the building of the intrusive body. In addition, these sheared rocks were affected by late textural coarsening processes that evidence the slow and cyclical cooling of the growing magma reservoir. In contrast with the steeply dipping contacts and structures found to the east, the NW domain of the pluton shows sharp and gently dipping contacts between almost horizontal magmatic layers. We suggest that the western block of the Chañaral shear zone was a relatively passive footwall dominated by horizontal flow trajectories and lower replenishment rates according to the inverse emplacement sequence, i.e., late external mafic batches intruded along the margins of the felsic core. The variable structural arrangement of the crustal rocks that hosted the Flamenco pluton was the result of the complex interaction between far-field and local, magmatic forces during the emplacement process, besides the interference with pre-Andean structures.The presence of the Chañaral shear zone favoring the emplacement of the Flamenco pluton demonstrates that the Late Jurassic to latest Early Cretaceous Atacama Fault System had earlier precursors and both the magmatic arc axis and the transcurrent shear zones migrated landward during Jurassic times.

The Mesozoic Amdo micro-block and East Asian superconvergent tectonic system

The assembly of East Asia involved multi-directional convergence of several micro-blocks during the Mesozoic. However, the geodynamic and tectonic elements associated with the southwestern convergent tectonic system of East Asia involving the Bangong-Nujiang subduction-collision system remain unclear. The Bangong-Nujiang Suture Zone in central Tibet shows a prominent east–west-trending differentiation divided by the Amdo micro-block. Here we evaluate the structural architecture of the Amdo micro-block, together with stratigraphic and paleomagnetic data, geochronological and geochemical data on the multi-stage magmatic and metamorphic rock suites with a view to trace the subduction and closure history of the Bangong-Nujiang Ocean as well as the collisions among micro-blocks. We also present new data from outcrop structural analysis which suggest that the micro-block has undergone five episodes of deformation (abbreviated as D1 to D5), which are represented by respectively NW-striking isoclinal tight folds, NE-striking tight folds, E-trending asymmetric folds, V-type conjugate strike-slip shear zones, and top-to-the-southwest thrust faults. In the Early Jurassic, initial active collision occurred between the Amdo micro-block and the Qiangtang Block followed by rifting and back-arc extension of the Bangong-Nujiang Ocean south of Amdo and passive collision between the Amdo micro-block and the Qiangtang Block in the late Early Jurassic. In the Middle-Late Jurassic, the Bangong-Nujiang Oceanic lithosphere underwent flat subduction beneath the Amdo micro-block resulting in slab tearing along the western border of the micro-block, large-scale intracontinental orogeny and thrusting. The slab tearing induced eastward mantle and crustal flows, and along with the N-S-directed compression, V-shaped conjugate shear zones formed in the interior of the Amdo micro-block. The intracontinental destruction and deep mantle lithospheric delamination in East Asia triggered the coeval convergent drops of deep materials in late Mesozoic, which drove further collision between the Lhasa Block and the East Asian continent in the Early Cretaceous.

Intrusion of the Ediacaran multi-pulse Quatro Ilhas Granitoids under inclined transpression, Dom Feliciano Belt, Southern Brazil

The Quatro Ilhas Granitoids are syntectonic to a major NE-SW dextral strike-slip shear zone at the northeastern part of the Dom Feliciano Belt in Southern Brazil. They comprise high-K calc-alkaline, coarse-grained porphyritic granodiorite to monzogranite emplaced at 625 to 615 Ma. Their foliation is marked by the alignment of biotite and euhedral to stretched K-feldspar phenocrysts and dips gently to steeply to the SE. Mineral and stretching lineations plunge at a low angle to both NE and SW. Several fine-grained dioritic to granitic synplutonic sheets crosscut the porphyritic granites parallel or oblique to the foliation. This assemblage is affected by regional tectonics, resulting in centimetre-to metre-scale, NW-verging asymmetrical folds, whose axis is parallel to the observed lineations. Progressive verticalization of foliation due to folding leads to local steeply-dipping, dominantly dextral shear zones. The observed structures and coeval strike-slip and contractional structures point to oblique/inclined transpressive deformation of the monzogranite. The fine-grained granites are geochemically grouped into low- and high-K types. The first group is relatively depleted in K2O and enriched in Ba and Sr, whereas the second resembles A-type granitoids. The low-K intrusions are interpreted to be crustally-derived, and the high-K type results from differentiation of tholeiitic magmas with crustal contamination. The presence of both gently-dipping and steeply-dipping structures active during the emplacement of these granitoids is further evidence of the transpressive character of the regional tectonic event. It is interpreted as a marker of the transition between the main collision and overall oblique transpression (650–635 Ma) and strict strike-slip tectonics (≤611 Ma) during the post-collisional period of the Dom Feliciano Belt construction.

Geology of Bajalia Anticline of the Low Folded Zone of Iraq

Bajalia Anticline is located about 60km northeast of central Amarah city in Al-Teeb area near Iraq-Iran border. Field and laboratory works were conducted to study topography, geomorphology, stratigraphy, and structural geology of Bajalia Anticline. The Anticline has a longitudinal shape with about 29km in length and 5-7km in width. Injana, Mukdadiya, and Bai Hassan formations are the three formations that were recognized in the study area. The geometrical structural analysis depicts that the Anticline is non-cylindrical, asymmetrical, close, sub-horizontal, steeply inclined, and linear fold. Most of the fractures in the Anticline are joints. These joints were classified based on the tectonics axes, which are a, b, and c, into ac, bc, and hol. A major reverse fault is located at the margin of the southwestern limb parallel to the fold axis with about 25km length. This fault is responsible on the vergence of the Anticline and overturned part of the southwestern limb. The Anticline was formed as a result of the collision between the Arabian and Iranian plates during the Late Tertiary. The maximum stress axis, which is caused by collision, is perpendicular to the hinge line. The geometrical and genetic classification indicates that the Anticline was formed by the high folding intensity and with a role of evaporites layers.

Multi-stage, Upper Eocene-Oligocene anatexis in the Xolapa metamorphic belt (Puerto Escondido, Mexico): Dynamics of the Xolapa Complex as the decoupled lower crust of the Chortís Block upper crust during its tectonic migration

Three successive strain regimes spanning up to ~16 million years were identified from the field, structural and geochronological analyses in Upper Eocene-Oligocene migmatites of the Xolapa Complex around the Puerto Escondido longitude (~97°W), in southern Mexico. A west-facing asymmetric folding affecting diatexites at ca. 38–31 Mad defines Regime A. Regimes B and C affected younger metatexites. Regime B recorded an outward extension at ca. 31–25 Ma, whereas Regime C occurred at ca. 25–22 Ma as NE-directed thrusting. By coupling the Upper Cretaceous-Oligocene tectonic framework of southern Mexico with the dynamics and timing of the identified strain regimes, we interpret that Regimes A to C recorded distinct stages of the separation and eastward migration of the Chortís Block, viewed as a continuous geodynamic episode. According to our proposal, the upper and lower crust of Chortís adjacent to nuclear Mexico were vertically decoupled before its separation by the presence of pre-Eocene crustal anisotropies. Regime A occurred when the Chortís Block rode over the Xolapa Complex during its initial eastward escape. Since this episode, the Chortís Block (upper crust) was captured by the Caribbean plate, whereas the Xolapa Complex (lower crust) remained as the southernmost North America plate. Chortís eastward migration and the consequent unloading of its decoupled lower crust triggered anatexis, isostatic adjustments, and diachronous uplift of the Xolapa Complex (Regime B). The progressive substitution of Chortís by its thinner lower crust in the tectonic configuration next to nuclear Mexico gradually enhanced the local convergence rate between the Farallon/Cocos plate and just-severed North America. The Farallon/Cocos plate was accreted to North America (Regime C) after a critical lengthening of the uprising lower crust along the southern Mexican margin. The local increase of the convergence rate promoted the landward consumption of the Xolapa Complex by subduction erosion since 22 Ma.

Kinematic evolution of fold-and-thrust belts in the Yubei-Tangbei area: Implications for tectonic events in the southern Tarim Basin

The Yubei-Tangbei area in the southern Tarim Basin is one of the best-preserved Early Paleozoic northeast-southwest trending fold-and-thrust belts within this basin. This area is crucial for the exploration of primary hydrocarbon reservoirs in northwestern China. In this study, we constructed the structural geometric morphology of the Yubei-Tangbei area using geophysical logs, drilling, and recent two- and three-dimensional (2-D and 3-D) seismic data. The Early Paleozoic fault-propagation folds, the Tangnan triangle zone, fault-detachment folds, and trishear fault-propagation folds developed with the detachment of the Middle Cambrian gypsum–salt layer. According to a detailed chronostratigraphic framework, the growth strata in the Upper Ordovician–Lower Silurian layer formed by onlapping the back limb of the asymmetric fault-propagation folds, which therefore defines the timing of deformations. The changes in kink band hinges and amplitudes in the Permian–Carboniferous and Cenozoic folding strata suggest that the evolution of the fold-and-thrust belts followed a sequential evolution process rather than a simultaneous one. Above the pre-existing Precambrian basement structure, the Yubei-Tangbei fold-and-thrust belts can be divided into four tectonic evolution stages: Late Cambrian, Late Ordovician to Early Carboniferous, Carboniferous to Permian, and Cenozoic. The northwestern-verging Cherchen Fault is part of the piedmont fold-and-thrust system of the southern Tarim foreland basin. We interpreted its strata as a breakthrough trishear fault-propagation fold that developed in three phases: Mid–Late Ordovician, Silurian to Middle Devonian, and Triassic to present. These tectonic events are responses of the Altyn-Tagh and Kunlun collisional orogenic belts and the Indian-Eurasian collision. The inherited deformation and structural modification in the southern Tarim Basin may be an indicator of the growth and evolution of peripheral orogens.

Structural framework of the transpressive Mahanadi Shear Zone and reevaluation of the regional geology in the northern sector of the Eastern Ghats Mobile Belt, Eastern India

Mahanadi Shear Zone (MSZ) is located in the northern segment of the Eastern Ghats Mobile Belt (EGMB), represented by extensive mylonitization. WNW-trending mylonitic foliation has a moderate-to-steep dip towards NNE. Kinematic indicators such as asymmetric folds, shear bands, quarter structures, suggest dextral ductile shear sense of the dominant strike–slip component and reverse sense of the less prominent dip slip component. Predominance of foliation over lineation indicates flattening strain. Reverse movement and flattening strain confirm shortening. The MSZ is a transpressional zone with simple shear along and pure shear (shortening) perpendicular to the mylonitic fabric. Wide variation in plunge of stretching lineation, presence of back-rotated clasts and 0 < Wk < 1 on two sections oblique to the mylonitic foliation, suggest triclinic symmetry of deformation. This study demonstrates that the width of the MSZ—located in the northern part of the Phulbani Domain—extends beyond 38 km. It covers the entire ‘Tikarpara Domain’ in the north of Phulbani Domain and put the very existence of Tikarpara Domain in question. Lithounits of Phulbani Domain continue to the north across the southern boundary of the MSZ, through Tikarpara Domain and Angul Domain up to the Northern Boundary Shear Zone (NBSZ). It is proposed that the entire area comprising the Phulbani Domain, the MSZ, and the region further north up to the NBSZ, form part of a single domain—Phulbani Domain. MSZ is an outcome of intra-terrane transpression. Thus, this study also questions the credibility of the domainal classification of the EGMB.

Petrographic, geochemical and structural characteristics of gold-bearing metasedimentary rocks from the Atacora structural unit, Northwestern Bénin Republic

The metasedimentary rocks of the Atacora structural unit around Natitingou in Benin are composed of quartzites, quartz muscovite schist and mica schist. Being auriferous, the petrographic, geochemical and structural features of these rocks were investigated to ascertain the source of gold and its relation to structural deformations. Quartz, muscovite and opaque minerals constitute the dominant minerals in the quartz muscovite schist. The quartzites and mica schists are predominantly composed of quartz, plagioclase, microcline, muscovite and chlorite in varying proportions, with biotite, garnet and epidote in mica schist. The rocks have quartzose sedimentary composition and plot within the passive margin setting on major element discrimination diagrams. The Pan-African ductile and brittle deformation phases produced predominantly folds, strike-slip faults, dextral shear plane and fractures within the area. Gold-bearing veins in the host rocks occur within and near the N-S to NE-SW trending microfolds and the major brittle E-W and NE-SW faults. Gold mineralization is hosted within the NE-trending shear zones and NE-SW trending asymmetric folds which are attributed to transpressional D2 phase of the Pan-African orogeny. Pathfinder elements for gold mineralization in the Atacora are W–Cu–As–Mo ± Sn. This may suggest interaction between host rocks and mineralizing fluids, probably from deformation and metamorphic processes during the late stages of the Pan-African Orogenic event. However, fluid inclusion and stable isotopic studies are required to constrain the origin of ore fluids.

Structure and chronology across the Achankovil terrain boundary shear zone system (South India), and its Madagascar connection in the Gondwanaland

In South India, the WNW-striking ~ 130-km-long Achankovil Shear Zone (AKSZ) separating the Trivandrum Block and the Madurai Block is speculated to continue within southern Madagascar in the East Gondwanaland. Two aspects continue to be debated, whether the AKSZ is a terrain boundary shear zone, and if so, when the accretion occurred. We present, for the first time, a detailed field-based analysis of contrasting mesoscale structures and deformation kinematics in the granulite facies rocks across the AKSZ. The mineralogical segregation layering (S2T) in the Trivandrum Block is sub-horizontal to gently inclined and describes F3T regional-scale open asymmetric WNW-trending sub-horizontal folds. By contrast, the S2M granulite facies layering in the Madurai Block describes regional-scale steeply plunging F3M folds. Both blocks share the NNW/NNE-trending sub-parallel sinistral and dextral S3 shears of the AKSZ. Deformation microstructures and garnet-bearing melt-hosted syn-S3 extensional shears attest to the persistence of high temperature during D2–D3 deformations. Monazite chemical dates and existing Pb–Pb zircon dates are overwhelmingly Pan-African in and across the AKSZ, but the Paleoproterozoic (2.1–1.8 Ga) dates in the Trivandrum Block, equivalent to those reported in Anosyan and Androyan domains of southern Madagascar, are lacking in the Madurai Block. By contrast, the Mid Neoproterozoic (800–750 Ma) Pb–Pb zircon dates in the Madurai Block, equivalent to the Antananarivo domain (Central Madagascar), are lacking in the Trivandrum Block. The structural–chronological contrasts between the two blocks suggest the AKSZ to be a Pan-African terrain boundary shear zone system that is continuous with the Ranotsara shear zone in Madagascar. [248]