The Rössing Uranium Deposit: a product of late-kinematic localization of uraniferous granites in the Central Zone of the Damara Orogen, Namibia

The Rössing granite-hosted uranium deposit in the Central Zone of the Pan-African Damara Orogen, Namibia, is situated in the “SJ area” to the south of the Rössing Dome. The coincidence of a number of features in this area suggests that mineralization is closely linked to late-kinematic evolution of the Rössing Dome. These features include: (1) the rotation of the dome's long axis (trend of 017°), relative to the regional F₃ trend of 042°; (2) southward dome impingement, concomitant with dome rotation, producing a wedge-shaped zone of alkali-leucogranites, within which uranium mineralization is transgressive with respect to granites and their host lithologies; uranium mineralization and a high fluid flux are also confined to this arcuate zone to the south and south-east of the dome core and (3) fault modeling that indicates that the SJ area underwent late-D₃ to D₄ brittle–ductile deformation, producing a dense fault network that was exploited by leucogranites. Dome rotation and southward impingement occurred after a protracted period of transtensional tectonism in the Central Zone, from ca. 542 to 526 Ma, during which I- and S-type granites were initiated in a metamorphic core complex. Late-kinematic deformation involved a rejuvenation of the stresses that acted from ca. 600 to 550 Ma. This deformation overlapped with uranium-enriched granite intrusion in the Central Zone at 510 ± 3 Ma. Such late-kinematic, north–south transpression, which persisted into the post-kinematic cooling phase until at least 478 ± 4 Ma, was synchronous with left-lateral displacement along NNE-trending (“Welwitschia Trend”) shears in the vicinity of Rössing. Late-kinematic deformation, causing block rotation, overlying dome rotation and interaction of the more competent units of the Khan Formation with the Rössing Formation in the dome rim was pivotal in the localization of uranium-enriched granites within a highly fractured, high-strain zone that was also the site of prolonged/high fluid flux.     

Structural and remote sensing analysis of the Betsimisaraka Suture in northeastern Madagascar

The Betsimisaraka Suture (B.S.) of Madagascar is an important structural zone defining the collision between Eastern and Western Gondwana. It is represented by highly deformed high-grade metamorphic rocks with mineral assemblages typical of ophiolitic material, including chromitite and nickel bearing rocks consistent with a suture zone setting. Analysis of satellite imagery coupled with field investigations has helped to elucidate the structure and evolution of the B.S. Digital image processing of Landsat ETM+ data was integrated with Synthetic Aperture Radar (SAR) to reduce the effects of the dense vegetation cover. Enhanced false color composite (7-4-2 and 4-2-3), single Landsat bands and band ratio composites including 2/7-1/7-2/5, 5/3-5/1-7/3, 5/1-7/1-4/1 and 5/7-5/1-5/4 × 3/4 (in RGB) improve the lithologic contrast, reduce the effects of topography and enhance the structural lineaments. Ductile deformation deduced from structural features mapped on Landsat enhanced images indicates three generations of folding (F₁, F₂, and F₃) coupled with shearing: (1) F₁ folds with NE striking axial surfaces; (2) F₂ related with N–S striking axial surfaces, (3) and F₃ associated with ENE–WSW axial surfaces, indicating NNW and SSE contractional strain similar to the deformation in the southern B.S. Mapping these structures enables three types of shearing to be delineated: (1) NW–SE dextral shearing as seen in the Befandriana region; (2) NW–SE sinistral shearing defined by sigmoidal bodies in Mandritsara and Ankijanilava-Marotandrano regions, (3) and NE–SW striking dextral shears recorded in the Lake Alaotra region. Several faults, joints and fractures represent brittle deformation events. Lineaments analyzed within the B.S. are divisible into two groups of brittle structures: (1) N–S trending lineaments correlated with the Gondwanan collision events and (2) much younger NE and NW trending lineaments that are mainly found in the Antananarivo block. The latter may represent an active tectonic event in the central plateau that bounds the western part of the B.S.     

Structural evolution of the Irtysh Shear Zone (northwestern China) and implications for the amalgamation of arc systems in the Central Asian Orogenic Belt

The NW–SE Irtysh Shear Zone is a major tectonic boundary in the Central Asian Orogenic Belt (CAOB), which supposedly records the amalgamation history between the peri-Siberian orogenic system and the Kazakhstan/south Mongolia orogenic system. However, the tectonic evolution of the Irtysh Shear Zone is not fully understood. Here we present new structural and geochronological data, which together with other constraints on the timing of deformation suggests that the Irtysh Shear Zone was subjected to three phases of deformation in the late Paleozoic. D₁ is locally recognized as folded foliations in low strain areas and as an internal fabric within garnet porphyroblasts. D₂ is represented by a shallowly dipping fabric and related ∼ NW–SE stretching lineations oriented sub-parallel to the strike of the orogen. D₂ foliations are folded by ∼ NW–SE folds (F₃) that are bounded by a series of mylonite zones with evidence for sinistral/reverse kinematics. These fold and shear structures are kinematically compatible, and thus interpreted to result from a transpressional deformation phase (D₃). Two samples of mica schists yielded youngest detrital zircon peaks at ∼322 Ma, placing a maximum constraint on the timing of D₁–D₃ deformation. A ∼ NE–SW granitic dyke swarm (∼252 Ma) crosscuts D₃ fold structures and mylonitic fabrics in the central part of the shear zone, but is displaced by a mylonite zone that represents the southern boundary of the Irtysh Shear Zone. This observation indicates that the major phase of D₃ transpressional deformation took place prior to ∼252 Ma, although later phases of reactivation in the Mesozoic and Cenozoic are likely. The late Paleozoic deformation (D₁–D₃ at ∼322–252 Ma) overlaps in time with the collision between the Chinese Altai and the intra-oceanic arc system of the East Junggar. We therefore interpret that three episodes of late Paleozoic deformation represent orogenic thickening (D₁), collapse (D₂), and transpressional deformation (D₃) during the convergence between the Chinese Altai and the East Junggar. On a larger scale, late Paleozoic sinistral shearing (D₃), together with dextral shearing farther south, accommodated the eastward migration of internal segments of the western CAOB, possibly associated with the amalgamation of multiple arc systems and continental blocks during the late Paleozoic.     

Comparative behavior of Sr, Nd and Hf isotopic systems during fluid-related deformation at middle crust levels

We have carried out a comparative Rb-Sr, Sm-Nd and Lu-Hf isotopic study of a progressively deformed hercynian leucogranite from the French Massif Central, belonging to the La Marche ductile shear zone, in order to investigate the respective perturbation of these geochronometers with fluid induced deformation. The one-meter wide outcrop presents a strongly deformed and mylonitized zone at the center, and an asymmetric deformation pattern with a higher deformation gradient on the northern side of the zone. Ten samples have been carefully collected every 10 cm North and South away from the strongest deformed mylonitic zone. They have been analyzed for a complete major, trace element data set, oxygen isotopes, Rb-Sr, Sm-Nd and Lu-Hf isotopic systematics.We show that most of major and trace elements except SiO₂, alkaline elements (K₂O, Rb), and some metal transition elements (Cu), are progressively depleted with increasing deformation. This depletion includes REE + Y, but also HFS elements (Ti, Hf, Zr, Nb) which are commonly considered as immobile elements during upper level processes. Variations in elemental ratios with deformation, e.g. decrease in LREE/MREE- HREE, Nd/Hf, Th/Sr, increase in Rb/Sr, U/Th and constant Sr/Nd, lead to propose the following order of element mobility: U ? Th > Sr = Nd ? Hf + HREE. We conclude in agreement with previous tectonic and metallogenic studies that trace element patterns across the shear zone result from circulation of oxidizing F-rich hydrothermal fluids associated with deformation. A temperature of the fluid of 470-480 °C can be deduced from the δ¹⁸O equilibrium between quartz-muscovite pairs.Elemental fractionation induces perturbation of the Rb-Sr geochronometer. The well-defined ⁸⁷Rb/⁸⁶Sr-⁸⁷Sr/⁸⁶Sr correlation gives an apparent age of 294 ± 19 Ma, slightly younger than the 323 ± 4 Ma age of leucogranites in this area. This apparent age is interpreted as dating event of intense deformation and fluid circulation associated with mass transfer, and exhumation of the ductile crust shortly after the leucogranite emplacement. Sm-Nd and Lu-Hf isochron-type diagrams do not define any correlation, because of the low fractionated Sm/Nd and Lu/Hf ratios. Isotopic data demonstrate that only the Lu-Hf geochronometer system is not affected by fluid circulation and gives reliable T_DM age (1.29 ± 0.03 Ga) and ε_Hf signatures. By contrast, the Sm-Nd geochronometer system gives erroneous old T_DM ages of 2.84-4 Ga. There is no positive ε_Nd-ε_Hf correlation, because of decreasing ε_Nd values with deformation at constant ε_Hf values. However, ε_Nd-ε_Hf values remain in the broad ε_Nd-ε_Hf terrestrial array, which strongly indicates that fluid-induced fractionation can contribute to the width of the terrestrial array. The strong ε_Hf negative values of the leucogranite are similar to metasedimentary granulitic xenoliths from the French Massif Central and confirm the generation of the leucogranite by several episodes of reworking of the lower crust.     

Structural characteristics of the rocks of the Delhi Supergroup with special reference to interference patterns: An appraisal with some examples from Central Rajasthan

The rocks of the Delhi Supergroup, which occur around Barr-Sendra and Phulad-Deogarh regions in Central Rajasthan, show three phases of deformational episodes: (i) phase D₁—tight-to-long limbed isoclinal fold (F₁); phase D₂—open, asymmetric fold (F ₂) controlling the map pattern of the formational boundaries; and (iii) phase D₃—major warps (F₃). Interference between nearly coaxial F₁ and F₂ on northerly axes produced hook-shaped and crescent patterns whereas superimposition of easterly trending F₃ on F₂ produced dome-and-basin patterns. The thermal peak was achieved during the second phase of deformation when the rocks were constructively metamorphozed and granites (850−750 m.y.), late synkinematic with respect to second phase of deformation, were emplaced. The sequence of deformation and the structural pattern of the rocks of the Delhi Supergroup in Central Rajasthan strikingly resemble those in northeastern Rajasthan. Structurally the characteristics of the Delhi Supergroup as verified in the entire region from NE to Central Rajasthan are: (a) the same sequence of development of folds, F₁, F₂ and F₃, interspersed with nearly identical phases of recrystallization, (b) hook-shaped interference pattern due to near-coaxial refolding of F₁ by F₂, and (c) variation in axial plunge of F₂ resulting in culminations and depressions. Lastly, phases of the recrystallization history indicates little time gap between F₁ and F₂, and a considerable gap between F₂ and F₃.     

Thick‐skinned folding in the eastern Lachlan Fold Belt,Shoalhaven River Gorge,New South Wales

In the Shoalhaven River Gorge, in the eastern Lachlan Fold Belt, the Ordovician quartz‐turbidite succession (Adaminaby Group) is affected by one major phase of deformation with northerly trending, gently plunging, upright, close to tight folds (F₁) characterised by a range in half wavelengths up to 3 km. Several anticlinoria and synclinoria are developed and folds occur in at least four orders; these characteristics are consistent with buckling occurring at several scales and are controlled by the thickness of competent units in the multilayered succession. F₁ folding is thick‐skinned in style with the whole crust probably having been affected by deformation. D₁ occurred during the Silurian to Middle Devonian interval and was associated with crustal thickening and the shallowing of depositional environments over time. Locally, F₁ is overprinted by south‐southeast‐trending, steeply to moderately inclined F₂ that reorients F₁ to recumbent attitudes. D₂ is of Early to Middle Carboniferous age. Both deformations are related to convergence in an intra‐arc to backarc region and occurred inboard of a subduction zone, remnants of which occur in the New England Fold Belt.     

Metamorphic history of the Jutogh Series in the Simla klippe,lower Himalayas

The metamorphic rocks of the Jutogh Series around Simla, structurally overlying the less metamorphosed rock groups along a thrust contact, have been involved in three phases of deformation and two episodes of metamorphism. The first metamorphism is in the albite-epidote-amphibolite facies in a major part of the area, reaching the amphibolite facies locally in the central part. This metamorphism is late-to post-kinematic with reference to the F ₁ movement, the thermal peak having been reached in a post-F ₁ pre-F ₂ static phase. The second metamorphism, syn-to post-tectonic with respect to F ₂ but preceding F ₃, is generally in the greenschist facies, and only locally in the albite-epidote-amphibolite facies in the higher structural levels. Metamorphic overprinting has caused widespread retrogression and disequilibrium assemblages. As the large scale recumbent folding and thrusting of F ₁ and F ₂ phases belong to the Tertiary Himalayan orogeny, the metamorphism in the Jutogh Series could not have been Precambrian in age.     

On the deformation sequence in the New Harbour Group of Holy Island,Anglesey, North Wales

Four phases of deformation are recorded by minor structures in the New Harbour Group (NHG) of southern Holy Island. The regional schistosity in these rocks is a differentiated crenulation cleavage of D₂ age. An earlier preferred orientation (S₁) is commonly preserved as crenulations within the Q-domain microlithons of the S₂ schistosity and is demonstrably non-parallel to bedding. F₃ folds are widely developed in S₂ and, to a lesser extent, in bedding. S₃ crenulation cleavage is sporadically developed but can be intense locally. A major antiformal fold exists in the NHG near Rhoscolyn. This fold is of D₃ age since it clearly deforms S₂ schistosity and is consistent with the vergence of F₃ minor structures. All planar structures are deformed by folds of D₄ age. © 1997 John Wiley & Sons, Ltd.     

Mega-fold interference patterns in the Beishan orogen (NW China) created by change in plate configuration during Permo-Triassic termination of the Altaids

Kilometer-size fold interference patterns in the Beishan Orogenic Collage (BOC) in the southernmost Altaids formed by fold superimposition in fossiliferous Permian sedimentary rocks. First-phase (F₁), upright and almost north-trending folds, were refolded by E- to ENE-trending F₂ folds, whose axial planes and axes are vertical or subvertical. From east to west there is a regional change in style of interference patterns from lobate–cuspate-, to crescent- to mushroom-shape. This variation is accompanied by a westward decrease in the F₂ interlimb angle and related to a higher percentage of coarse-grained clastic rocks, suggesting a dependence of the F₂ deformation on lithology. Axial planar slaty cleavages are well developed in F₁ and poorly developed in F₂ folds. The superposed folds mainly underwent flexural-slip and flexural flow folding to give rise to the lobate–cuspate pattern, and to the crescent pattern caused by flattening and flexural flow folding where the sediments were unconsolidated and enriched in fluids. The two folding events are interpreted to result from a major change in plate configuration that caused the inversion of an inter-arc basin during the final amalgamation of the BOC in the latest Permian to early to mid-Triassic. The two folding events bracketed between a maximum detrital zircon age of <273 Ma, and the youngest age of an intruded dyke at 219.0 ± 1.2 Ma suggest rapid plate reconfiguration related to final amalgamation of the Altaids orogen.     

Progressive and polyphase deformation of the Schistes Lustrés in Cap Corse, Alpine Corsica

In Cap Corse, progressive deformation during Late Cretaceous obduction of the ophiolitic Schistes Lustrés (sensu lato) as a pile of imbricate, lens-shaped units during blueschist facies metamorphism was non-coaxial. Two zones are recognized: a lower series emplaced towards the west is overlain by a series emplaced towards the south-southwest in Cap Corse. Equivalent structures (differing only in orientation) occur in both zones. The change in thrust direction was responsible for local refolding and reorientation of previously formed structures, parallel to the new stretching direction immediately below the thrust contact between the two zones, and within localized shear zones in the underlying series.Both zones are refolded about E-overturned F₂ folds trending between 350 and 025°. Local minor E-directed thrusts occur associated with the F₂ folds. This second deformation of Middle Eocene age is considered to be related to the backthrusting of an overlying klippe containing gneisses of South Alpine origin, and is followed by a third Late Eocene phase of upright 060°-trending F₃ folds accompanied by greenschist facies metamorphism.     

Transpressional deformation,strain partitioning and fold superimposition in the southern Chinese Altai,Central Asian Orogenic Belt

Transpressional deformation has played an important role in the late Paleozoic evolution of the western Central Asian Orogenic Belt (CAOB), and understanding the structural evolution of such transpressional zones is crucial for tectonic reconstructions. Here we focus on the transpressional Irtysh Shear Zone with an aim at understanding amalgamation processes between the Chinese Altai and the West/East Junggar. We mapped macroscopic fold structures in the southern Chinese Altai and analyzed their relationships with the development of the adjacent Irtysh Shear Zone. Structural observations from these macroscopic folds show evidence for four generations of folding and associated fabrics. The earlier fabric (S₁), is locally recognized in low strain areas, and is commonly isoclinally folded by F₂ folds that have an axial plane orientation parallel to the dominant fabric (S₂). S₂ is associated with a shallowly plunging stretching lineation (L₂), and defines ∼NW-SE tight-close upright macroscopic folds (F₃) with the doubly plunging geometry. F₃ folds are superimposed by ∼NNW-SSE gentle F₄ folds. The F₃ and F₄ folds are kinematically compatible with sinistral transpressional deformation along the Irtysh Shear Zone and may represent strain partitioning during deformation. The sub-parallelism of F₃ fold axis with the Irtysh Shear Zone may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation (F₃) in fold zones. The strain partitioning may have become less efficient in the later stage of transpressional deformation, so that a fraction of transcurrent components was partitioned into F₄ folds.     

Deformation history of the Hengshan–Wutai–Fuping Complexes: Implications for the evolution of the Trans-North China Orogen

The Trans-North China Orogen separates the North China Craton into two small continental blocks: the Eastern and Western Blocks. As one of the largest exposure in the central part of the orogen, the Hengshan–Wutai–Fuping Complexes consist of four lithotectonic units: the Wutai, Hengshan and Fuping Complexes and the Hutuo Group. The Hengshan Complex contains high pressure mafic granulites and retrograded eclogites. Structural analysis indicates that most of the rocks in these complexes underwent three distinct episodes of folding (D₁ to D₃) and two stages of ductile thrust shearing (STZ₁ between D₁ and D₂ and STZ₂ after D₃). The D₁ deformation formed penetrative axial planar foliations (S₁), mineral stretching lineations (L₁), and rarely-preserved small isoclinal folds (F₁) in the Hengshan and Fuping Complexes. In the Wutai Complex, however, large-scale F₁ recumbent folds with SW-vergence are displayed by sedimentary compositional layers. Penetrative transposition resulted in stacking of thrust sheets which are separated by ductile shear zones (STZ₁). The kinematic indicators of STZ₁ in the Hengshan and Wutai Complexes show top-to-the-S230°W thrusting likely related to northeastward, oblique pre-collisional subduction. D₁ resulted in crustal thickening with resultant prograde peak metamorphism. The Hutuo Group did not undergo the D₁ deformation, either because sedimentation was coeval with the D₁ deformation or because it was at a high structural level and was not influenced directly by the early deformation. The D₂ deformation produced NW-verging asymmetric and recumbent folds. The D₂ deformation is interpreted to have resulted from collision between the Eastern and Western Blocks of the North China Craton. In the Hutuo Group and the Fuping Complex, the development of ESE-verging asymmetric tight folds is associated with D₂. The structural pattern resulting from superimposition of D₁ and D₂ is a composite synform in the Hengshan–Wutai–Fuping Complexes. All four lithotectonic units were superposed during the later D₃ deformation. The D₃ deformation developed NW-trending open upright folds. Ongoing collision led to development of transpressional ductile shearing (STZ₂), forming the transpressional Zhujiafang dextral ductile shear zone between the northern Hengshan Complex and the southern Hengshan Complex, and generating the sinistral Longquanguan ductile shear zone between the Fuping Complex and the Wutai Complex, respectively. The STZ₁ and D₂ deformation were possibly responsible for fast syn-collisional exhumation of the high pressure mafic granulites and retrograded eclogites. The structural patterns and elucidation of the deformation history of the Hengshan–Wutai–Fuping Complexes places important constraints on the tectonic model suggesting that an oceanic lithosphere between the Eastern and Western Blocks underwent northeastward-directed oblique subduction beneath the western margin of the Eastern Block, and that the final closure of this ocean led to collision between the two blocks to form the coherent basement of the North China Craton.     

Structural events and metamorphic consequences in Almora Nappe,during Himalayan collision tectonics

Almora Nappe in Uttarakhand, India, is a Lesser Himalayan representative of the Himalayan Metamorphic Belt that was tectonically transported over the Main Central Thrust (MCT) from Higher Himalaya. The Basal Shear zone of Almora Nappe shows complicated structural pattern of polyphase deformation and metamorphism. The rocks exposed along the northern and southern margins of this nappe are highly mylonitized while the degree of mylonitization decreases towards the central part where the rocks eventually grade into unmylonitized metamorphics.Mylonitized rocks near the roof of the Basal Shear zone show dynamic metamorphism (M₂) reaching upto greenschist facies (～450 °C/4 kbar). In the central part of nappe the unmylonitized schists and gneisses are affected by regional metamorphism (M₁) reaching upper amphibolite facies (～4.0–7.9 kbar and ～500–709 °C). Four zones of regional metamorphism progressing from chlorite–biotite to sillimanite–K-feldspar zone demarcated by specific reaction isograds have been identified. These metamorphic zones show a repetition suggesting that the zones are involved in tight F₂ – folding which has affected the metamorphics. South of the Almora town, the regionally metamorphosed rocks have been intruded by Almora Granite (560 ± 20 Ma) resulting in contact metamorphism. The contact metamorphic signatures overprint the regional S₂ foliation. It is inferred that the dominant regional metamorphism in Almora Nappe is highly likely to be of pre-Himalayan (Precambrian!) age.     

Geometry and evolution of superposed folding in the Zawar lead-zinc mineralised belt,Rajasthan

The lead-zinc bearing Proterozoic rocks of Zawar, Rajasthan, show classic development of small-scale structures resulting from superposed folding and ductile shearing. The most penetrative deformation structure noted in the rocks is a schistosity (S ₁) axial planar to a phase of isoclinal folding (F ₁). The lineations which parallel the hinges ofF ₁ folds are deformed by a set of folds (F ₂) having vertical or very steep axial planes. At many places a crenulation cleavage (S ₂) has developed subparallel to the axial planes ofF ₂ folds, particularly in the psammopelitic rocks. The plunge and trend ofF ₂ folds vary widely over the area. Deformation ofF ₂ folds into hook-shaped geometry and development of another set of axial planar crenulation cleavage are the main imprints of the third generation folds (F ₃) in the region. In addition to these, there are at least two other sets of cleavage planes with corresponding folds in small scales. More common among these is a set of recumbent and reclined folds (F ₄), developed on steeply dipping early-formed planes. Kink bands and associated sharp-hinged folds represent the other set (F ₅). Two major refolded folds are recognizable in the map pattern of the Zawar mineralised belt. The larger of the two, the Main Zawar Fold (MZF), shows a broad hook-shaped geometry. The other large-scale structure is the Zawarmala fold, lying south-west of the MZF. Both the major structures show truncation of lithological units along their respective east ‘limbs’, and extreme variation in the width of formations. The MZF is primarily the result of superimposition ofF ₃ onF ₂.F ₁ folds are relatively smaller in scale and are recognizable in the quartzite unit which responded to deformation mainly by buckle shortening. Large-scale pinching-and-swelling that appears in the outcrop pattern seems to be a pre-F₂ feature. The structural evolutionary model worked out to explain the chronology of the deformational features and the large-scale out-crop pattern envisages extreme east-west shortening following formation ofF ₁ structures, resulting in the formation of tight and isoclinal antiforms (F ₂) with pinched-in synforms in between. These latter zones evolved into a number of ductile shear zones (DSZs). The east-west refolding of the large-scaleF ₂ isoclinal antiforms seems to be the consequence of a continuous deformation and resultant migration of folds along the DSZs. The main shear zone which wraps the Zawar folds followed a curved path. Because of the penetrative nature of theF ₂ movement, the early lineations which were at high angles to the later ones (as is evident in the west of Zawarmala), became subparallel to the trend ofF ₂ folding over a large part of the area. Further, the virtually coaxial nature ofF ₂ andF ₃ folds and the refolding ofF ₃ folds by a new set of N-S folds is an indication of continuous progressive deformation.     

Structural assemblies and age of deformation in the western sector of the Anyui-Chukotka Fold System,Northeastern Asia

Indications of intense deformation in the Anyui-Chukotka Fold System and the South Anyui Suture Zone have been noted for a long time [3, 5, 19, 36]. The character and age of the deformation, however, remain a matter of debate. Using structural paragenetic and deformational kinematic analyses, we establish three deformation stages in the Anyui-Chukotka Fold System. The structural assembly comprising open folds and NW-trending axial-plane cleavage was formed during the stage of regional compression (D₁) related to the collision of the Chukotka-Arctic Alaska microcontinent with Eurasia. The assembly of the second stage in the Alyarmaut Rise is distinguished by isoclinal folds F₂, gently dipping metamorphic schistosity, and pervasive cleavage in combination with folded quartz veins and lenses. Planar structural elements of the second stage are disturbed by low-amplitude normal and reverse faults and kink folds of stage D₃. The U-Pb (SHRIMP-RG) and ^40/39Ar methods were used for determination of the isotopic age of the deformations. The Aptian-Albian zircon age (117–108 Ma) has been established for six postcollision granitic plutons of the Anyui-Chukotka Fold System and the South Anyui Suture. Syncollision deformation completed 125–117 Ma ago. The extensional tectonic stage D₂ accompanied by emplacement of the Lyupveem pluton occurred 120–105 Ma ago. The ^40/39Ar age of the biotite from the metamorphic rocks marks the age of syndeformation metamorphism (109–103 Ma). The lower limit of brittle failure and deformation D₃ is estimated at 105 Ma.     

Retrograde evolution of eclogites: evidences from microstructures and 40Ar/39Ar white mica dates,Münchberg Massif,northern Bavaria

Phengites from eclogites and pegmatites (3T, 2M₁, coarse-grained and recrystallized) of the Münchberg Massif (Weissenstein and Oberkotzau) have been dated by the ⁴⁰Ar/³⁹Ar method. 3T-micas from the eclogites yielded plateau and isochron ages of 365±7 Ma. 2M₁-micas show disturbed degassing spectra. Micas from pegmatites show a slight excess Ar component, with an isochron age of 353 to 351±3 Ma. An age component of approximately 300 Ma was also detected. In combination with age values from the literature, the cooling history of the Münchberg Massif from eclogite-facies conditions (390 Ma) to cooling below 350°C (350 Ma) is documented. The age component of 300 Ma is attributed to a low-grade stage of mineral growth accompanied by a transitional ductile-brittle deformation. The petrological effects include formation of pumpellyite-prehnite-facies minerals, frequently precipitated in microcraks and cleavage planes of earlier formed minerals. This stage has to be seen in conjunction with the intrusions of the Fichtelgebirge granite.     

The structure of the Palaeozoic schists in the Southern Menderes Massif,western Turkey: A new approach to the origin of the Main menderes metamorphism and its relation to the Lycian Nappes

The Early Eocene to Early Oligocene tectonic history of the Menderes Massif involves a major regional Barrovian-type metamorphism (M₁, Main Menderes Metamorphism, MMM), present only in the Palaeozoic-Cenozoic metasediments (the so-called “cover” of the massif), which reached upper amphibolite faciès with local anatectic melting at structurally lower levels of the cover rocks and gradually decreased southwards to greenschist facies at structurally higher levels. It is not present in the augen gneisses (the so called “core” of the massif), which are interpreted as a peraluminous granite deformed within a Tertiary extensional shear zone, and lie structurally below the metasediments. A pronounced regional (S₁) foliation and approximately N-S trending mineral lineation (L₁) associated with first-order folding (F₁) were produced during D₁ deformation coeval with the MMM. The S₁ foliation was later refolded during D₂ by approximately WNW-ESE trending F₂ folds associated with S₂ crenulation cleavage. It is now commonly believed that the MMM is the product of latest Palaeogene collision across Neo-Tethys and the consequent internal imbrication of the Menderes Massif area within a broad zone along the base of the Lycian Nappes during the Early Eocene-Early Oligocene time interval. However, the meso- and micro-structures produced during D₁ deformation, the asymmetry and change in the intensity and geometry of the F₂ folds towards the Lycian thrust front all indicate an unambiguous non-coaxial deformation and a shear sense of upper levels moving north. This shear sense is incompatible with a long-standing assumption that the Lycian Nappes were transported southwards over the massif causing its metamorphism. It is suggested here that the MMM results from burial related to the initial collision across the Neo-Tethys and Tefenni nappe emplacement, whereas associated D₁ deformation and later D₂ deformation are probably related to the northward backthrusting of the Lycian nappes.     

Stratigraphy,structure and geochronology of ore leads in the Matsitama Schist belt of Northern Botswana

The Matsitama schist belt in northeastern Botswana comprises an area of metasediments, notably quartzites, limestones, shales and amphibolites that are bounded by granites and gneisses. The belt lies southwest of the Rhodesian cration and north of the Limpopo mobile belt.Stratigraphic, structural and lead isotopic evidence indicates that the Matsitama metasediments are equivalent to the Shashi metasediments in the Limpopo belt. There is strong evidence that the Matsitama and Shashi metasediments stratigraphically underlie volcanic rocks of the Tati belt which have been correlated with Archaean schist belts of about 2700 Ma of Rhodesia. Therefore, the Matsitama and Shashi rocks are at least as old as the schist belts of the Rhodesian craton and may represent a shallow-water facies that occurs only in the Limpopo area.There is no structural evidence that the Matsitama and Shashi metasediments were deposited unconformably on basement rocks, although the presence of gneiss, amphibolite and ironstone pebbles in a Matsitama conglomerate, as well as the presence of orthoquartzites, shows the existence of a basement source region. However, the surrounding granites intrude the Matsitama and Shashi metasediments and all underwent several deformation phases.The structural history of the Matsitama rocks can be described in terms of five phases of deformation. The main cleavage-producing deformation phase, F₂, folded the rocks into a major synform and intensely deformed them. Before this, however, the rocks had been folded and thrust so that part of the succession shows downward-facing F₂ structures and there are possibly repetitions of the stratigraphy due to imbrication. Structures of the F₃ and F₄ phases fold the main cleavage but locally are sufficiently intense to modify the shape of the finite strain ellipsoid. There is a major ductile shear zone of F₄ age, south of which F₄ folds are tight, while to the north, F₄ deformation is negligible. All of these structures can be correlated with deformation phases in the Tati schist belt to the east and in the northern part of the Limpopo mobile belt.Lead isotope evidence suggests that mineralization in the Matsitama metasediments occurred at least 2200 Ma ago, and that leads from Dihudi/Thakadu and Messina, in the centre of the Limpopo belt, underwent a two-stage history of events at 2600–2700 Ma and 2000–2100 Ma ago, agreeing with other geochronological evidence. The leads from Matsitama and Messina are isotopically distinct from leads from the Rhodesian schist belts, which show evidence of transfer to the crust some 3500 Ma ago. The absence of this 3500 Ma-old lead from the Matsitama and Messina environments may indicate different crustal conditions and possibly the absence of the Rhodesian-type early basement.     

Diachronous deformation and a strain gradient beneath the Selkirk allochthon,northern Monashee complex,southeastern Canadian Cordillera

Structural relationships of granitoid rocks dated by the U–Pb method indicate that deformation was diachronous and a strain gradient exists in a 6-km-thick section beneath the Selkirk allochthon, in the northern Monashee complex, one of the deepest structural exposures in the southern Canadian Cordillera. At high structural levels, immediately beneath a crustal-scale thrust zone that transported the allochthon eastward, a metasedimentary-dominated cover sequence was strongly affected by kilometre-scale east-verging isoclinal folds (F₁) and outcrop-scale folds (F₂) that are associated with the dominant foliation and lineation. The F₂ folding occurred, at least in part, after 58 Ma and ceased by 55 Ma. In deeper levels of the cover sequence and the underlying orthogneiss-dominated basement, F₂ folding occurred, at least in part, after 52 Ma and ceased by 49 Ma. Proterozoic dykes in the basement were locally weakly affected by D₂. These new findings require that: (i) D₂ compression youngs structurally downward, synchronous with the thermal peak of metamorphism; (ii) D₂ in deeper levels is synchronous with extension above the complex that was partly responsible for its exhumation; and (iii) a D₂ strain gradient lies between strongly deformed cover rocks and weakly D₂-deformed basement rocks. We propose a model in which rocks that were tectonised at different places and times within the orogen were juxtaposed, likely during east-verging kilometre-scale F₁ folding and shearing along the isocline limbs (a similar model was previously proposed to explain a pattern of downward younging thermal peak ages and an inverted metamorphic sequence in higher rocks). The rapid downward decrease in deformation intensity suggests that the lower limit of significant Cordilleran strain lies in the exposed basement. Cessation of deformation at this level is attributed to the fact that the basement attained elevated temperatures and began straining when the Cordilleran tectonic regime changed from compressional to extensional.     

The role of sedimentary and tectonic brines in the Damara Orogen, Namibia

Along the southern margin of the Upper Proterozoic Damara Orogen, Namibia, an accumulation of extraordinarily large megacrystalline quartz-dolomite bodies occur. They were emplaced in tectonically controlled positions during an early deformational phase of the Damara Orogeny, where hot nappes were thrust over a fluvial-lacustrine and evaporitic metaplaya sequence (Kamtsas and Duruchaus Formations) which was deposited on the faulted EW-trending continental margin of the Kalahari craton. Individual occurrences of the quartz-dolomite bodies often cover several hundred square meters. Characteristic for the quartz-dolomite bodies is zoning with an outer shell of giant milky quartz crystals, some more than 15 m long along the c-axis, tightly intergrown or twinned (Brazilian twins) with a perfect cleavage parallel to the positive rhombohedral faces {1011}; there is an inner shell of crystalline dolomite and a central pipe of dolomitic breccia. Based on fluid inclusions studies the formation fluids of quartz-dolomite bodies can be related to the mobilization of interstitial fluids and to dehydration and leaching of evaporitic hydrate minerals of the metaplaya sequence. The fluids are characterized by extremely high salinities of up to 68 wt % total salt content. Minimum temperatures of formation, as determined in fluid inclusion studies, ranged from 150 to 250°C. At a later stage CO₂ derived from decarbonatization reactions was dissolved in the fluids. Changes in pressure and temperature led to effervescence and the formation of a quartz stockwork in the surrounding country rock. During the main phase of the progressive Damara Orogeny, the southward advancing accretionary nappe pile of the Khomas trough drove ahead large amounts of tectonometamorphic fluids, characterized by intermediate salinity and high CO₂ contents. When these fluids met with the previously established hypersaline fluid system, large amounts of CO₂ were released due to the mixing of the two fluids; if there is no mixing, each fluid then maintains its salinity and there is no CO₂ degassing. The CO₂ from this mixing is now present as secondary, high-density inclusions not only in the quartz-dolomite bodies but also in the surrounding country rock. Pressure estimations indicate at least 100–600 MPa as a minimum pressure of formation for these inclusions. The remaining aqueous fluid phase has produced local alterations and Cu, Pb, and Au mineralization.