Fold Zone Research Articles

Fragmentation of the Adriatic Promontory: New Chronological Constraints From Neogene Shortening Rates Across the Southern Alps (NE Italy)

AbstractThe eastern Southern Alps are located at the northern tip of the Adria microplate, which imposes 2.0–2.5 mm/year of N‐S‐convergence relative to stable Eurasia. We map surface evidence of recent folding/faulting in this area from a 5‐m Digital Elevation Model (DEM). In the eastern part of the belt, observations reveal a 30‐km‐wide zone of active folding composed of at least five growing anticlines. The most recent ones warped the postglacial alluvial surface by <10 m. Combining these findings with published geological and geophysical data allows us to infer that active thrusting occurs along a single deeply rooted thrust, which accommodates the indentation of the Adriatic crust. Resolving the observed pattern of uplift on the inferred fault geometry indicates that NNE‐SSW shortening across the eastern Southern Alps has occurred at a rate of about 1.5 mm/year over the postglacial period. On the other hand, a balanced cross section for the eastern Southern Alps at the scale of the upper crust constrains a minimum of 43 km of finite shortening over the last 14 Ma, yielding a shortening rate of about 3 mm/year, which is 2 times higher than the postglacial shortening rate. This decrease in the shortening rate is associated to the Pleistocene activation of new thrusts that is compatible with a change in the direction of compression. The inferred local change in the kinematics of thrusting during the Pleistocene is consistent with a change from Nubia‐imposed to Adria‐imposed convergence indicating that the fragmentation of the Adriatic promontory could have occurred 1–2 Ma ago.

Carbon isotope variations in inorganic carbon materials: Implications for mud volcanic carbon cycling in the northern Tianshan fold zone, Xinjiang, China

Carbon isotopes of inorganic carbon-bearing materials (gaseous CO2, dissolved inorganic carbon (DIC) and solid carbonates) sampled from four mud volcanoes in the northern Tianshan fold zone were investigated to study the mud volcanic carbon cycling process. The analysed carbon isotopes showed 13C enriched characteristics, the δ13C values of CO2, DIC and solid carbonates were in the range of 10.99–32.11‰, 17.02–36.22‰, and −7.28–12.40‰, respectively. The 13C enrichment of CO2 could be attributed to carbon isotope fractionation during the partial reduction of CO2 by methanogenic bacteria. The positive δ13C values in DIC may be caused by the carbon isotope fractionation during CO2-water-rock interactions. The mixture of carbonate precipitates from DIC in the water and carbonate minerals from the host rock resulted in a relatively less positive δ13C value in carbonates of the mud samples. The reduction of CO2 by methanogenic bacteria could be the primary mechanism for the 13C enrichment of inorganic carbon during the carbon cycling process in the mud volcanic systems in the northern Tianshan fold zone.

Strain energy density distribution of a tight gas sandstone reservoir in a low-amplitude tectonic zone and its effect on gas well productivity: A 3D FEM study

The tight gas sandstone reservoirs in the Paleozoic of the Sulige gas field in China are highly heterogeneous, and fractures are key factors for stable reservoir production. Low-amplitude folds or nose-like structures are widely developed in the Upper Paleozoic strata in this area. To effectively predict gas well productivity, in this paper, a 3D FEM numerical simulation based on the deformation and energy variation of the rock mass was used to predict the "sweet spots" of gas well productivity in a tight gas sandstone reservoir using the He8 segment of the Middle Permian Xiashihezi Formation in the Central Sulige block as an example. The paleotectonic stress field of the study area during the maximum episode of compression in the Yanshanian movement was restored, and the two rupture parameters of the integrated rupture rate (IF) and strain energy density (U) were constructed. The strain energy density distribution has a high correlation with gas well productivity, indicating that it can better predict the rock rupture degree in low-amplitude tectonic zones. A complex relationship exists between the strain energy density distribution and low-amplitude folds. The high strain energy density zones are mainly distributed among the high positions and wing areas of the low-amplitude fold zone, but the top area of the low-amplitude fold does not necessarily have a high strain energy density. Portions of the high strain energy density zones are located in the gentle tectonic zone, located near but outside the low-amplitude fold zone. The strain energy in these gentle tectonic zones with a high strain energy density value is relatively high, and the rock mass is prone to rupture. This study is of great value in enriching the prediction of "sweet spots" in tight gas sandstone reservoirs in low-amplitude tectonic zones worldwide.

An Andean-type retro-arc foreland system beneath northwest South China revealed by SINOPROBE profiling

In the Mesozoic, South China was situated along the convergent margin between the Asian and Pacific plates, providing an excellent laboratory to understand the interactions between deformation, sedimentation and magmatism in a retroarc environment. The crustal architecture of northwest South China is displayed along the ∼600-km-long SINOPROBE deep seismic reflection profiles and reveals from east to west: (1) highly folded and truncated reflectors in the upper crust of the Yangtze Fold Zone, which correspond to thin- and thick-skinned thrust systems, and document large-scale intraplate structural imbrication and shortening; (2) a crustal-scale flat-ramp-flat structure, termed the Main Yangtze decollement, which forms a weak, viscous layer to accommodate strain decoupling and material transport in the thin- and thick-skinned systems; and (3) nearly flat-lying reflectors in the Sichuan Basin, which support interpretation of the basin as a weakly deformed depocentre. The Yangtze Fold Zone and the Sichuan Basin represent a retro-arc foreland basin system that is >800 km away from the continental-margin magmatic arc. We suggest that tectonic processes across the arc and retro-arc systems, including arc magma flare-up, basin sedimentation, retroarc thrust propagation, lithosphere underthrusting, root foundering, and extension-related magmatism were interrelated and governed mass transfer. Age data and geological relations link the tectonic processes to evolving geodynamics of the subducting Paleo-Pacific plate.

Late Paleozoic – Mesozoic tectonic evolution of the Eastern Taimyr-Severnaya Zemlya Fold and Thrust Belt and adjoining Yenisey-Khatanga Depression

Combined structural, thermochronological and geochronological studies were carried out to unravel the complex late Paleozoic – Mesozoic tectonic evolution of the eastern Taimyr – Severnaya Zemlya Fold and Thrust Belt, along with the adjoining Yenisey-Khatanga Depression and Olenek Fold Zone. New detailed field mapping, a fault and fold geometry and kinematic study, and paleostress reconstruction were undertaken for key areas within the Central and Southern Taimyr domains and the Olenek Fold Zone. Thirty seven samples were used for apatite fission track (AFT) analysis along with two samples for U-Pb zircon granite dating, and three samples for Ar-Ar metamorphic muscovite dating.The late Paleozoic (Early Carboniferous – Permian) tectonic event is recognized only in a relatively narrow zone including the Northern Taimyr Domain and part of the Central Taimyr Domain. It started in the Early Carboniferous (Visean) and is synchronous with the earliest stages of collision in the Uralian Orogen. Our results reveal a widespread distribution of latest Triassic – earliest Jurassic and Cretaceous deformations. The latest Triassic – earliest Jurassic tectonic event significantly overprinted the Northern and Central Taimyr domains and original unaltered late Paleozoic AFT ages are identified only locally. New completely reset AFT cooling ages group at 195-180 Ma, in reasonable agreement with structural data. Older Neoproterozoic and late Paleozoic Ar-Ar ages of metamorphic muscovite are retained, demonstrating that the dated rocks were not heated above 300 °C. An Early Cretaceous tectonic event is inferred from both the structural study and ca. 115–125 Ma AFT cooling ages, and is most widespread in the Southern Taimyr Domain, Olenek Fold Zone and adjoining parts of the Yenisey-Khatanga Depression, modifying older geological structures. A younger Late Cretaceous tectonic event at ca. 100-60 Ma is also inferred from AFT cooling ages, but its distribution is poorly constrained.Our study reveals that the distribution of late Mesozoic tectonic events has been significantly underestimated in previous structural studies of the region. The latest Triassic – earliest Jurassic and Cretaceous tectonic events are easily correlated with those in the Pai-Khoi – Novaya Zemlya and northern Verkhoyansk fold and thrust belts.

Age, geochemistry, and emplacement of the ~40-Ma Baneh granite–appinite complex in a transpressional tectonic regime, Zagros suture zone, northwest Iran

ABSTRACTThe Baneh plutonic complex is situated in the Zagros suture zone of northwest Iran between the Arabian and Eurasian plates. This complex is divided into granite and appinite groups. Zircon U–Pb dating shows that granites crystallized 41–38 million years ago but appinites experience more protracted magmatic evolution, from at 52 to 38 Ma. Whole-rock chemical compositions show significant major and trace element variations between the two lithologies. Granitic rocks are more evolved, with high contents of SiO2 (62.4–77.0 wt%), low contents of TiO2 (0.25 wt%), MgO (0.05–1.57 wt%), and Fe2O3 (0.40–4.06 wt%) and high contents of Na2O + K2O (≈10 wt%). In contrast, appinites have low contents of SiO2 (51.0–57.0 wt%) and K2O (<2.1 wt%) and high Fe2O3 (6.4–9.35 wt%), MgO (2.0–9.9 wt%), and Mg number (Mg# = 35–76). The concentration of rare earth elements in the appinites is higher than in granitic rocks, making it difficult to form granites solely by fractionation of appinite magma. (87Sr/86Sr)i and εNd(40 Ma) in both groups are similar, from 0.7045 to 0.7061 and −1.2 to +2.6, except for a primitive gabbroic dike with εNd(40 Ma) = +9.9. Appinites show mainly typical I-type characteristics, but granites have some S-type characteristics. The sigmoidal shape of the Baneh pluton and its emplacement into deformed Cretaceous shales and limestone showing kink bands, asymmetric and recumbent folds in a broad contact zone, with pervasive ductile to brittle structures in both host rocks and intrusion, indicate that magma emplacement was controlled by a transpressional tectonic regime, perhaps developed during early stages in the collision of Arabia and Eurasian plates.

Revisiting the Skin Lines on the Forehead and Glabellar Area.

This paper attempted to revisit the skin lines in forehead and glabellar area.Using PubMed, Scopus, and Google, papers describing skin tension lines of face were searched. Papers with illustrations or pictures of the facial lines were selected and reviewed. The studies of the skin lines in the forehead and glabellar area were analyzed.Langer used the term "cleavage of the cutis, anatomical lines." Over the forehead, he found a border zone of horizontal folds that were interrupted many times by the ascending folds coming from the flabella area. Cox used the terms "cleavage lines of the skin" and "lines of increased tension." In the glabellar area, cleavage lines were vertical and extended to the forehead. Rubin used the term "skin line." In the midline area over the nose, the skin wrinkled vertically. Kraissl used the term "normal wrinkle line." Above the nose, "curving vertical lines" were observed. Straith et al used the terms "normal tension line," and "Langer line." In the glabellar area, the horizontal lines from the upper eyelids became "curved vertically" until meeting the supra-brow horizontal line. Borges used the term "relaxed skin tension line." At the medial end of the eyebrow, his relaxed skin tension line ran obliquely upward and medially, to meet the contralateral line. Namikawa et al used the term "cleavage lines of the skin." It ran mediosuperiorly to inferolaterally, bordering the linea mediana anterior in glabellar area.Following the consensus of most authors, a curved vertical line following the glabellar frown is recommended for incisions for flaps or grafts.

Simulation of Paleotectonic Stress Fields and Distribution Prediction of Tectonic Fractures at the Hudi Coal Mine, Qinshui Basin

AbstractStudy on tectonic fractures based on the inversion of tectonic stress fields is an effective method. In this study, a geological model was set up based on geological data from the Hudi Coal Mine, Qinshui Basin, a mechanical model was established under the condition of rock mechanics and geostress, and the finite element method was used to simulate the paleotectonic stress field. Based on the Griffith and Mohr‐Coulomb criterion, the distribution of tectonic fractures in the Shanxi Formation during the Indosinian, Yanshanian, and Himalayan period can be predicted with the index of comprehensive rupture rate. The results show that the acting force of the Pacific Plate and the India Plate to the North China Plate formed the direction of principal stress is N‐S, NW ‐ SE, and NE ‐ SW, respectively, in different periods in the study area. Changes in the direction and strength of the acting force led to the regional gradients of tectonic stress magnitude, which resulted in an asymmetrical distribution state of the stress conditions in different periods. It is suggested that the low‐stress areas are mainly located in the fault zones and extend along the direction of the fault zones. Furthermore, the high‐stress areas are located in the junction of fold belts and the binding site of multiple folds. The development of tectonic fractures was affected by the distribution of stress intensity and the tectonic position of folds and faults, which resulted in some developed areas with level I and II. There are obvious differences in the development of tectonic fractures in the fold and fault zones and the anticline and syncline structure at the same fold zones. The tectonic fractures of the Shanxi Formation during the Himalayan period are more developed than those during the Indosinian and Yanshanian period due to the superposition of the late tectonic movement to the early tectonic movement and the differences in the magnitude and direction of stress intensity.

Quaternary tectonic evolution of the Pamir‐Tian Shan convergence zone, Northwest China

AbstractThe Pamir‐Tian Shan collision zone in the western Tarim Basin, northwest China, formed from rapid and ongoing convergence in response to the Indo‐Eurasian collision. The arid landscape preserves suites of fluvial terraces crossing structures active since the late Neogene that create fault and fold scarps recording Quaternary deformation. Using geologic and geomorphic mapping, differential GPS surveys of deformed terraces, and optically stimulated luminescence dating, we create a synthesis of the active structures that delineate the timing, rate, and migration of Quaternary deformation during ongoing convergence. New deformation rates on eight faults and folds, when combined with previous studies, highlight the spatial and temporal patterns of deformation within the Pamir‐Tian Shan convergence zone during the Quaternary. Terraces spanning ~130 to ~8 ka record deformation rates between ~0.1 and 5.6 mm/yr on individual structures. In the westernmost Tarim Basin, where the Pamir and Tian Shan are already juxtaposed, the fastest rates occur on actively deforming structures at the interface of the Pamir‐Tian Shan orogens. Farther east, as the separation between the Pamir‐Tian Shan orogens increases, the deformation has not been concentrated on a single structure, but rather has been concurrently distributed across a zone of faults and folds in the Kashi‐Atushi fold‐and‐thrust belt and along the NE Pamir margin, where shortening rates vary on individual structures during the Quaternary. Although numerous structures accommodate the shortening and the locus of deformation shifts during the Quaternary, the total shortening across the western Tarim Basin has remained steady and approximately matches the current geodetic rate of 6–9 mm/yr.

A contribution of gravity and seismic data in understanding the geometry of the Zouaraa - Ouchtata dune (NW Tunisia): Hydrogeological implications

As it is located in a very particular and complex domain within the Tellian fold and thrust belt zone in northwestern Tunisia, the Nefza area has always been challenging. Geological, hydrogeological and geophysical studies were conducted in the region. A multidisciplinary study was performed by combining geological and geophysical techniques.Gravity data processing revealed the continuity of the outcropping series of Argoub Er Romane and Jebel Hamra underneath the dune deposits building a high zone separating the dune of Zouaraa and Ouchtata into two asymmetric basins. It forms a threshold zone that controls the geometry of the dune reservoir in the area. The distribution of the gravity anomaly along the dune of Zouaraa proved the heterogeneity of this dune reservoir. Gravity data modeling for this area confirmed these results and showed a preferential tendency of subsidence to the northwest and thus the thickening of Zouaraa dune sequence as compared to that of Ouchtata.

Mesozoic structural evolution of the New Siberian Islands

Abstract The New Siberian Islands are affected by a number of Mesozoic tectonic events. The oldest event (D1a) is characterized by NW-directed thrusting within the South Anyui Suture Zone combined with north–south-trending sinistral strike-slip in the foreland during the Early Cretaceous. This compressional deformation was followed by dextral transpression along north–south-trending faults, which resulted in NE–SW shortening in the Kotelny Fold Zone (D1b). The dextral deformation can be related to a north–south-trending boundary fault zone west of the New Siberian Islands, which probably represented the Laptev Sea segment of the Amerasia Basin Transform Fault in pre-Aptian–Albian times. The presence of a transform fault west of the islands may be an explanation for the long and narrow sliver of continental lithosphere of the Lomonosov Ridge and the sudden termination of the South Anyui Suture Zone against the present Laptev Sea Rift System. The intrusion of magmatic rocks 114 myr ago was followed by NW–SE-trending sinistral strike-slip faults of unknown origin (D2). In the Late Cretaceous–Paleocene, east–west extension (D3) west of the New Siberian Islands initiated the development of the Laptev Sea Rift System, which continues until today and is largely related to the development of the Eurasian Basin.

Syncline contribution to mountain peak building: examples from Western Zagros, Kurdistan Region, Northeastern Iraq

The current study focuses on roles of the synclines in building mountains of the Zagros Fold-Thrust Belt from Northeastern Iraq, Kurdistan Region. The studied area is located in the Imbricated Zone, which is a continuation of Simply Folded Zone of Iran. In some parts of this zone, the synclines contribute to building nearly half of the mountains and increase the relief contrast of the landscapes. The role of syncline in building mountains is opposite to its apparent intrinsic shape which is located in low land and normally forms synclinal valleys. In studied area, this phenomenon is observable in the areas that are complexly deformed, deeply exhumed, and predominately composed of the carbonate rocks. In these areas, exhumation and stream erosion have removed high thickness of overburden rocks and the tectonic stresses have crenulated carbonate rocks in the synclines. Due to these processes, these synclines are observable on the peak of the mountains now. The contribution of synclines in mountain building is related to three properties: the first is folding of synclines in confined condition (relative to anticlines) by which they have transformed to highly compact and stiff rocks. The second is accumulation of hydrothermal or meteoric water in the synclines due to its downward bending which aids precipitation of minerals and heals the possible tectonically and lithostatically induced fractures. The third is ductile deformation of synclines in confined and deeper level as compared to brittle deformation of anticlines in relatively shallower level and unconfined condition during deformation. Consequently, these factors result in an inversion of topography by which synclines are remained as mountains (high topography) above surface during exhumation while erosion removes anticlines. The result of this study is applicable for geological, geomorphological, and structural mapping in highly deformed and deeply exhumed terrains. The possible prediction of geomorphological position of synclines and anticlines on the mountain peaks and valley bottoms respectively is the applicable result in the above terrains.

Geología del cuadrante Tapantí (1:50 000), Costa Rica

Se presenta un mapa geológico, escala 1:50 000 del cuadrante Tapantí, el cual se encuentra en la región central de Costa Rica y forma parte del Cinturón Deformado del Centro de Costa Rica (CDCCR), producto de la interacción de la cordillera de Coco y la región W de la microplaca Panamá. Las rocas más antiguas cartografiadas son miocenas y están representadas por las formaciones sedimentarias Pacacua, Peña Negra, San Miguel y Coris, que forman el borde oriental de la cuenca Candelaria. Se distinguieron 3 eventos magmáticos; el arco volcánico Mioceno, representando por la Formación La Cruz y clastos volcánicos de la Formación Pacacua; un nuevo periodo de actividad ígnea entre 6 Ma y 4,2 Ma, representado por las Formaciones Grifo Alto, Doán y el Intrusivo de Tapantí y, finalmente, se desarrolla el arco volcánico actual, representado por las formaciones Paraíso y Cervantes, con un rango de edad entre los 0,6 Ma y 0,02 Ma. Desde el punto de vista geoquímico, se observa un fuerte cambio entre los 10 y 6 Ma, lo cual se expresa por medio del enriquecimiento de K y Si, y los cambios en la concentración de U, Sr, Ba, La. Dicho cambio corresponde temporalmente con el arribo de la cordillera de Coco a la Fosa Mesoamericana y es reflejado en la región por medio del arco magmático del Plioceno y el arco volcánico actual. El modelo estructural sugiere la existencia de 2 episodios de deformación, el primero ocurrió durante el periodo comprendido entre el Mioceno Superior y el Plioceno y produjo la Zona de Plegamiento Orosi-Patarrá mientras que el segundo corresponde con la instauración del CDCCR, desde el Plioceno hasta la actualidad, conformado por fallas sinestrales con orientación ENE y dextrales con orientación NW.

Diapirism and the Fold Zone controversy of the Ordovician glaciomarine Pakhuis Formation, South Africa

Abstract The Fold Zone controversy centers around firstly, the relationship between the direction of ice-flow and the geometry of the fold structures, and secondly the relative importance of subglacial or proglacial buckling as a folding process compared to the role of load casts or ball-and-pillow structures. The more recent recognition of diapirs associated with the Fold Zone resulted in further field investigations. Understanding the advance-retreat sequence constrains the interpretation of the glaciotectonic processes. The Sneeukop Member, the unconformity and Steenbras Member are interpreted as a continuum ice-expansion sequence without a major retreat interlude. The palaeo-ice-flow data are reinterpreted to accommodate the regional variation of ice-flow patterns. It is concluded that the Pakhuis ice sheet expansion overriding the Peninsula basin took place by means of tributary troughs converging on the main Peninsula trough thus explaining the relationship between the direction of ice-flow and fold trend variations across the basin. The folds also vary regionally in style between upright and overturned. Diapirs were recognized both as blind mushroom structures below the unconformity and piercing upwards. Chaotic breccias and lit-par-lit intermixing of tillite and deformed, attenuated banded sandstone are interpreted as blowout material presumably formed by a piercement diapir venting at the surface. Diapirism in the Pakhuis glacial context is understood as the consequence of an ice sheet expansion over a water-logged Peninsula sand aquifer. The over-pressurized sand aquifer gave rise to diapirs and associated structures. The main features of the Fold Zone are compared to modern examples as described in the literature. Push moraine and crevasse-filling examples are considered as the best-fit models to explain the genesis of the Fold Zone.

Structural evolution of Iwaraja shear zone, southwestern Nigeria

The Iwaraja shear zone is an easterly splay to the Ifewara shear zone in the eastern part of Ilesha schist belt, southwestern Nigeria. Results of field and microstructural investigations reveal that the Iwaraja shear zone was developed during the second deformation phase that affected the region under retrograde metamorphic conditions. Deformation within the shear zone involved the mylonitisation of the granitic gneiss generating rocks ranging from weakly deformed granitic gneiss to ultramylonite under amphibolite to greenschist facies, respectively. The shear zone is characterized by NNE-SSW trending, sub-vertical mylonitic foliation (ca. 022°/79°) with shallowly dipping stretching lineations (ca. 021°/10°). Minor folds in the shear zone include early and late folds of pegmatite dykes in the granitic gneiss mylonite. The early folds are typically open to close folds but the late folds show typical geometry of synthetic folds characterized by thinned short limbs. Kinematic indicators in the granitic gneiss mylonite include sigma-type (σ-type) porphyroclasts and displaced fractured feldspar porphyroclasts indicate dextral shear sense. These features show that dextral, transcurrent displacement was important in the final amalgamation of different crustal blocks during the closing stages of the Pan-African Orogeny.

Глубинный геологический разрез и тектоника Северного Урала

Geological cross section of the Northern Urals to a depth of 7 km is presented. The author offers an interpretation of the tectonic structures in accordance with plate tectonics theory. West Urals Folding Zone is interpreted as a Thrust Zone of the Russian Plate formed during Kungurian Age of the Lower Permian. Polyudovo-Kolchimskiy anticlinorium is termed as an allochthon klippe of the Russian Plate. Central Urals Arc is identified as an accretionary prism of the Russian Plate. The Tagil-Magnitogorsk Depression is referred to as an obduction ophiolite zone. Two major tectonic faults were identified. The Western Urals Thrust combines the series of the platform autochthone thrusts and coincides to the north with the Timan-Urals Fault. The General Urals Fault is the obduction plane.

Petrophysical Evaluation of the Sarvak Formation Based on Well Logs in Dezful Embayment, Zagros Fold Zone, South West of Iran

Petrophysical Evaluation of the Sarvak Formation Based on Well Logs in Dezful Embayment, Zagros Fold Zone, South West of Iran

Petrophysical Evaluation of the Sarvak Formation Based on Well Logs in Dezful Embayment, Zagros Fold Zone, South West of Iran

Dezful embayment in the south west of Iran is part of Zagros fold zone which despite the small area includes almost all of Iran oil production. Sarvak Formation with middle cretaceous (Albin-Turunin) is the second biggest reservoir of Zagros basin, after the Asmari formation. In this study, with well logging data of two wells, by Geolog software, the petrophysical parameters of Sarvak Formation in the Dezful embayment was studied using the contingent Probabilistic method. Petrophysical parameters such as determination of porosity, water saturation, shale volume and lithology were calculated using this data and common cross plots. Based on this evaluation and utilizing cross plots result, the lithology of Sarvak formation in this zone was determined to be Limestone with traces of dolomite and shale. According to the calculated petrophysical properties of the reservoir, and for more accurate assessment, Sarvak Formation was divided to 6 sections. Findings of this research show that Sarvak Formation includes good reservoir parameters only in half its sections (sections 4, 2 and 6).

Transtensional origin of multi-order cross-folds in a high-grade gneiss complex, southwestern Grenville Province: formation during post-peak gravitational collapse

The well-mapped western part of the Ottawa River Gneiss Complex (ORGC; new name), a large metamorphic core complex, hosts a system of gently plunging cross-folds of outcrop to regional scale situated in the ductile detachment zone between the lower grade cover and high-grade core of the complex. The cross-folds are buckle structures that deform the attenuated gneissic layering, plunge parallel to the regional elongation lineation, range from upright to recumbent, and exhibit distinctive hinge-parallel elongation lineations, all features of extension-dominated ductile transtension. Our L–S fabric data are consistent both with kinematic modelling that predicts progressive constrictional strain in the hinge zones of transtensional folds, and with dynamic modelling that predicts rotation of flattened fold limbs into moderately dipping attitudes. On the basis of petrologic data, we show that cross-folding postdated the peak Ottawan metamorphism and took place during retrogression and exhumation of the thrust-sheet stack. The cross-folds form an inclined system that is principally developed in retrograde melt-weakened amphibolite-facies rocks, with the transtensional origin implying that exhumation and retrogression of the high-grade core of the complex took place in an oblique extensional setting. A transtensional origin for the cross-folds removes the need to appeal to orogen-parallel regional shortening, an implausible requirement of previous interpretations, and is compatible with data indicating that much of the visible fabric and structure of amphibolite-facies domains of the ORGC developed during post-peak exhumation, retrogression, and gravitational collapse of the thrust-sheet stack.

Metamorphism of volcanogenic massive sulphide deposits in the Urals. Ore geology

The Urals VMS province comprises a broad spectrum of variably metamorphosed deposits, from unmetamorphosed to those without any primary ore textures, which are the results of high-grade metamorphic processes. Contact metamorphism near large granite and granodiorite plutons caused the most significant changes of ores, with coarse-grained to pegmatoidal ores with magnetite closest to its contact with the intrusion, followed by pyrrhotite-enriched copper ores, and more distal zinc (±Pb±Ag) mineralisation. Koktau, Tarnyer and Vesenneye deposits are metamorphosed to the hornblende-hornfels and pyroxene-hornfels facies (t=400–800°C, P=1–6kbar). Metamorphism of Tash-Yar, Dzhusinskoe and Krasnogvardeiskoe deposits corresponds to the greenschist and albite-epidote-hornfels facies (t=250–450°C, P=1–4kbar).The regional metamorphism of VMS ores varies from prehnite-pumpellyite facies (t=150–300°C, P=0.5–4kbar) in the South Urals to the epidote-amphibolite and amphibolite facies (t=400–600°C (up to 700°C), P=1–6kbar) in the Karabash area in the Middle Urals. In the Magnitogorsk zone, the metamorphism of host rocks and VMS bodies increases to the north, reaching its peak near the Ufa promontory of the East European platform. With increased metamorphism, the morphology of orebodies evolves from gently dipping thick lenses (Alexandrinskoe and Uzelga fields), to subvertical and folded (Uchaly and Novo-Uchaly deposits) and pseudomonoclinal steeply-dipping vein-like bodies (Karabash district).The massive sulphide transformation in PTX-gradient fields led to partial redistribution of ore material. An enrichment in Cu, Zn, Ag and Au, ±Pb occur in the uppermost parts of large steeply-dipping massive sulphide lenses in wide tectonic zones (e.g., Gai deposit) or as gold-sulphide disseminated bodies near large metamorphosed VMS lenses, distal to a granite pluton (Tarnyer deposit). Partial melting probably occurred in some highly metamorphosed deposits (Tarnyer, Koktau and Mauk). Redeposition of base metals sulphides (chalcopyrite, tennantite, sphalerite, ±bornite, galena), as well as the presence of “visible” gold and tellurides, took place during retrograde metamorphism, which produced a transfer of ore matter towards the low stress areas, such as the outer parts of shear zones, the uppermost parts of steeply-dipping ore lenses, pressure shadows, hinge zones of small folds, and small extension fractures (i.e., Alpine-type veins) in deformed ore body or its immediate surroundings.