The Central Iberian arc, an orocline centered in the Iberian Massif and some implications for the Variscan belt

An arcuate structure, comparable in size with the Ibero-Armorican arc, is delineated by Variscan folds and magnetic anomalies in the Central Iberian Zone of the Iberian Massif. Called the Central Iberian arc, its sense of curvature is opposite to that of the Ibero-Armorican arc, and its core is occupied by the Galicia-Trás-os-Montes Zone of NW Iberia, which includes the Rheic suture. Other zones of the Iberian Massif are bent by the arc, but the Ossa-Morena and South Portuguese zones are not involved. The arc formed during the Late Carboniferous, at final stages of thermal relaxation and collapse, and an origin related with right-lateral ductile transpression at the scale of the Variscan belt is proposed. The Central Iberian arc explains the width of the Central Iberian Zone, clarifies the position of the allochthonous terranes of NW Iberia, and opens new perspectives for correlations with the rest of the Variscan belt, in particular, with the Armorican Massif, whose central zone represents the continuation of the southwest branch of the arc detached by strike-slip tectonics.     

Stretching lineation and transport direction in the Ibero-Armorican arc during the siluro-devonian collision

Abstract

Large structures, lineations, foliations and sense of shear criteria are examined on the scale of the whole Ibero-Armorican Arc. Four sections (Galicia, Brittany-Vendée, Limousin and Eastern Massif Central) exemplify the major thrust sheets observed around the Arc. Stretching lineations are contemporaneous with the siluro-devonian metamorphism and are either transverse, oblique or parallel to the collision zone. A kinematic analysis shows that these lineations have resulted from a dominanüy transverse shear deformation which was followed by, or combined with, a longitudinal shear direction. On the scale of the entire Arc, this variation in the shear direction is interpreted as resulting from an early head on thrusting relative movement evolving to large scale movements parallel to the plate boundaries. Experiments with sand-silicone models support a model which generates the Arc by interaction between a transform sinistral direction, and a converning zone at a high angle to the transform direction.     

Genesis of the Ibero-Armorican arc

Abstract

The Ibero-Armorican arc is continuous between Iberia and Armorica; its curvature increased with time due to subduction followed by continental collision; indentation produced left lateral transpression in Iberia and right-lateral transpression in Armorica. It is argued that whereas the antithetic shear is predominant in Iberia, in Armorica a synthetic shear prevailed because the identer rotated anticlockwise between the opposed forelands of the Variscan Fold Belt. It is proposed that the major Rheic ocean, closed by subduction towards the inner part of the arc, solving the space problem of centripetal vergences.     

Structural pattern and ascent model in the Central Extremadura batholith,Hercynian belt,Spain

The Central Extremadura batholith constitutes an important alignment of plutonic rocks occupying part of the internal zones of the Iberian Hercynian belt. It comprises 13 plutons and numerous minor intrusions, and covers a wide compositional range from quartz-diorites to alkali-feldspar granites. Structural study of the individual plutons reveals that their internal structural patterns may be correlated. Correlation between the different structural patterns in the plutons and interpretation of the superimposed deformation in the metasedimentary host rocks indicate that the plutons were emplaced and deformed in an E-W, dextral, intracontinental shear zone developed during the second deformation phase (302–320 Ma). A qualitative ascent model has been established as follows: (1) development of extensional fractures at 45° to the E-W, dextral, shear zone; (2) intrusion of granitic magmas along these fractures and (3) rotation of the earlier intrusions, in a dextral sense, inducing antithetic shear zones (N-S sinistral) and a transverse shortening, perpendicular to the major axis of the earlier, deformed plutons as well as the forced emplacement of the later plutons.     

The Sha’it–Nugrus shear zone separating Central and South Eastern Deserts,Egypt: A post-arc collision low-angle normal ductile shear zone

The northerly dipping Sha’it–Nugrus shear zone (SNSZ) is the boundary separating the Central Eastern Desert from the South Eastern Desert of Egypt. The hangingwall of this shear zone is composed of low-grade metavolcanics and ophiolitic nappes of the Central Eastern Desert, while the footwall consists of South Eastern Desert high-grade metapsammitic gneisses (Migif-Hafafit gneissic complex). The SNSZ is about 700 m thick and represents the shear foliated lower parts of the hangingwall and upper parts of the footwall. A significant part of the SNSZ has been truncated by a later normal fault along Wadi Sha’it, however the SNSZ is well-preserved along Wadi Nugrus. Features of the SNSZ include shear-related schistosity (termed Ss), mylonite zones, sheared syn-kinematic granitoid intrusions, diverse metasomatism and metamorphic effects (higher T overprinting of hangingwall lithologies and retrogression of footwall lithologies). Shear-sense indicators clearly show top-to-N or NW displacement sense. SNSZ structures overprint arc collision related nappe structures (～680 Ma) and are therefore post-arc collision. SNSZ syn-kinematic intrusives have been dated at ～600 Ma. The SNSZ is deformed (regionally and locally folded and thrust dissected) during later NE–SW compressive tectonism. The SNSZ had an originally approximately E–W strike, low-angle N-dip and a normal shear sense, making this an example of a low-angle normal ductile shear (LANF) or detachment fault. The steep NE dip of Ss foliations and low-pitching slip lineations along Wadi Nugrus are due to NW–SE folding of the SNSZ, and do not indicate a sinistral strike-slip shear zone. The normal shear sense activity is responsible for juxtaposing the low-grade Central Eastern Desert lithologies against South Eastern Desert gneisses. A displacement of 15–30 km is estimated on the SNSZ, which is comparable to LANF displacements in the Basin and Range province of the western USA. Frictional resistance along this shear was probably reduced by high magmatic fluid pressure and hydrothermal fluid pressure. The vastness and diversity of the hydrothermal activity along this shear zone is a characteristic of other LANFs in the Eastern Desert, e.g. at Gabal El-Sibai, and may be Gabal Meatiq. The SNSZ formed during the Neoproterozoic extensional tectonic phase of Eastern Desert that began ～600 Ma, and followed arc collision and NW-ward ejection of nappes.     

Contribution of structural analysis to understanding the geodynamic evolution of the Calabrian arc (Southern Italy)

A marked curvature of crustal structures characterizes the Calabrian arc in Southern Italy. The overall deformation of the arc seems mostly controlled by the Sangineto shear zone to the north and by the Mt. Kumeta-Alcantara shear zone to the south, which both separate different crustal sectors. Other important fault systems cut the Iblean foreland (Scicli-Ragusa fault zone) and many others dissect the crystalline units of Central Calabria. Neotectonic structural analyses have been carried out in order to recognize the character of the Plio-Pleistocene tectonic phases and their bearing on the present configuration of the arc.After the Middle Miocene extensional phase an Early-Middle Pliocene compressional phase is detectable in many parts of the arc. Right- and left-lateral displacements respectively characterize the Mt. Kumeta-Alcantara and Sangineto shear zones and right-lateral movements are also detectable within the Scicli-Ragusa fault system.Finally, the Pleistocene tensional regime seems to have been controlled mainly by uplift. The structural and neotectonic data allow us to propose a model of the recent evolution of the arc, which was bent mainly as a result of opposed wrench faulting along the Sangineto and Mt. Kumeta-Alcantara shear zones.     

Transpressional deformation in northwestern Sardinia (Italy): insights on the tectonic evolution of the Variscan BeltDéformation transpressive dans le Nord-Ouest de la Sardaigne (Italie) : considérations sur l'évolution tectonique de la chaı̂ne Varisque

Structural investigations in northwestern Sardinia highlight the occurrence of a regional D2 transpressional deformation related both to NNE–SSW direction of compression and to a NW–SE shear displacement. The deformation is continuous and heterogeneous, showing a northward strain increase, indicated by progressively tighter folds and occurrence of F2 sheath folds. D2 transpression is characterised by the presence of a crustal-scale shear deformation overprinting previous D1 structures, related to nappe stacking and top-to-the-south and southwest thrusting. The L2 prominent stretching lineation points to an orogen-parallel extension and to a change in the tectonic transport from D1 to D2. It is attributed to the position of Sardinia close to the northeastern part of the Cantabrian indenter during the progressive evolution of the Ibero-Armorican Arc. D1 phase developed during initial frontal collision, whereas D2 deformation characterised the progressive effect of horizontal displacement during the increasing curvature of the arc. To cite this article: R. Carosi, G. Oggiano, C. R. Geoscience 334 (2002) 287–294.     

Major Late-Triassic strike-slip displacement in the Seven Devils terrane, Oregon and Idaho: A result of left-oblique plate convergence?

The shortening direction in rocks deformed in collision or subduction zones is not directly related to the plate-convergence vector; rather, it is perpendicular to the collision zone or subduction zone, even in cases where plate convergence is oblique. The component of convergence parallel to the subduction/collision zone is expressed by strike-slip displacement in the arc region behind the subduction zone. Such strike-slip shear zones have been recognized in the Seven Devils terrane of northeastern Oregon and adjacent Idaho. One of these (the Oxbow shear zone consisting of cataclasite, mylonite, and ultra-mylonite) trends northeasterly from Oxbow, Oregon to Cuprum, Idaho. The original rock types of the shear zone were plagiogranite, gabbro, diabase, bassalt, and keratophyre. The age of the mylonitization is constrained by ⁴⁰Ar/³⁹Ar dates as Late Triassic. Meso- and microscopic structures (textures and quartz c-axes fabrics) indicate that the shear zone was formed by left-lateral, strike-slip motion. A minimum left-lateral displacement of 65 km has been estimated, but the true displacement may have been much larger. The Oxbow shear zone is interpreted as an intra-arc strike-slip zone of the Seven Devils terrane, related to left-oblique plate convergence during the Triassic.     

Pluton accommodation at high strain rates in the upper continental crust. The example of the Central Extremadura batholith,Spain

Emplacement in the tensional bridge of a stepped dextral shear zone system is proposed for the Central Extremadura batholith (Spain). The country rocks show a pervasive anisotropy that conditioned the style of the structures developed as a consequence of the transference of displacement from the stepped shear zones to the releasing area. The kinematic evolution of the resulting megakink fold provided the volume increase necessary for the granite emplacement. Thermal and kinematic models suggest that the growth of individual plutons took place in periods of no more than several hundred to a few thousand years. Fast strain rates (10⁻¹⁰–10⁻¹¹ s⁻¹) must concentrate in local structures (e.g. initiation of kink folds) even in zones deforming as a whole under typical strain rates (10^−14±1 s⁻¹). Granite plutons might be used as strain-rate gauges for syn-plutonic structures.     

大别山花凉亭-弥陀剪切带与超高压变质岩片的斜向差异折返

超高压变质岩的折返过程是陆陆碰撞边界演化的关键问题。南倾的花凉亭-弥陀剪切带位于南大别低温-超高压变质 带和中大别中温-超高压变质带之间,矿物拉伸线理倾伏向为SE,逆冲和走滑分量大致相等。电子背散射衍射分析表明： 花凉亭-弥陀剪切带大多数样品的石英组构记录了上盘向NW的剪切变形,反映了中大别超高压变质岩向SE的快速折返, 而部分样品的石英具有上盘向SE的剪切指向,与早白垩世花岗岩穹隆发育导致的区域伸展有关。对前人的岩石学和年代学 成果进行总结,发现大别山进变质和超高压变质峰期/退变质的锆石U-Pb年龄从南往北逐渐变新,南大别和中大别在215~ 225 Ma同时经历了高压榴辉岩相退变质作用,在191~195 Ma经历了绿片岩相变质作用。超高压变质岩的白云母和黑云母的 40Ar/39Ar年龄靠近郯庐断裂时偏年轻,可能受到郯庐断裂活动的影响。南大别和中大别变质峰期温压的等值线与花凉亭-弥 陀剪切带的走向斜交,反映了超高压变质岩的斜向折返。因此,南大别低温-超高压变质带在~236 Ma最先开始折返,之后 中大别和北大别依次发生快速折返,具有不同折返速率和折返角度的构造岩片通过韧性剪切带调节相对运动。     

博格达山晚石炭纪造山活动的变形地质记录

主要由钙碱性火山岩、火山碎屑岩组成的博格达古岛弧是天山缝合造山带的重要组成部分 ,是一个发育较成熟的山链 ,其演化经历了晚古生代的韧性剪切收缩 ;中生代伸展调整及新生代再造山过程。晚古生代的造山活动在博格达山有很好的地质记录 ,并以显著的韧性剪切变形带的形成和发育同造山的褶皱构造为特点。剪切变形带内同构造的石英脉中的锆石U PbSHRIMP测年结果与山链中花岗岩、辉长岩年龄颇为一致 (311～ 316Ma) ,这个年龄反映在结束洋盆散聚、碰撞焊接的晚华力西期造山过程中 ,博格达古岛弧内存在一次虽不甚强烈 ,但又较为明显的构造岩浆事件 ,其成因可能与引起石炭纪大规模裂陆式喷发的深部断裂构造重新活动有关。     

祁漫塔格韧性剪切带中绢云母40Ar-39Ar定年及地质意义

祁漫塔格韧性剪切带是祁漫塔格蛇绿混杂岩带与北昆仑岩浆弧的区域主构造边界,对剪切带内花岗质糜棱岩中绢云母40Ar-39 Ar法年龄测定,获得了(271.1±2.2) Ma的坪年龄,相应的36Ar/40 Ar-39 Ar/40 Ar反等时线年龄为(270.5±3.7)Ma (MSWD=2.7),39Ar/36 Ar-40 Ar/35 Ar正等时线年龄为(270.9±4.7) Ma (MSWD=0.22);坪年龄(271.1±2.2) Ma接近于绢云母矿物的形成年龄,也代表了祁漫塔格韧性剪切带的形成年龄.通过韧性剪切变形带内运动学特征的研究,表明剪切带具有右旋斜冲的性质.祁漫塔格韧性剪切带的形成与古特提斯洋的俯冲作用有关,是古特提斯洋向北俯冲造山过程的远程效应.     

Palaeozoic collisional tectonics and magmatism of the Chinese Tien Shan, central Asia

The Chinese Tien Shan range is a Palaeozoic orogenic belt which contains two collision zones. The older, southern collision accreted a north-facing passive continental margin on the north side of the Tarim Block to an active continental margin on the south side of an elongate continental tract, the Central Tien Shan. Collision occurred along the Qinbulak-Qawabulak Fault (Southern Tien Shan suture). The time of the collision is poorly constrained, but was probably in in the Late Devonian-Early Carboniferous. We propose this age because of a major disconformity at this time along the north side of the Tarim Block, and because the Youshugou ophiolite is imbricated with Middle Devonian sediments. A younger, probably Late Carboniferous-Early Permian collision along the North Tien Shan Fault (Northern Tien Shan suture) accreted the northern side of the Central Tien Shan to an island arc which lay to its north, the North Tien Shan arc. This collision is bracketed by the Middle Carboniferous termination of arc magmatism and the appearance of Late Carboniferous or Early Permian elastics in a foreland basin developed over the extinct arc. Thrust sheets generated by the collision are proposed as the tectonic load responsible for the subsidence of this basin. Post-collisional, but Palaeozoic, dextral shear occurred along the northern suture zone, this was accompanied by the intrusion of basic and acidic magmas in the Central Tien Shan. Late Palaeozoic basic igneous rocks from all three lithospheric blocks represented in the Tien Shan possess chemical characteristics associated with generation in supra-subduction zone environments, even though many post-date one or both collisions. Rocks from each block also possess distinctive trace element chemistries, which supports the three-fold structural division of the orogenic belt. It is unclear whether the chemical differences represent different source characteristics, or are due to different episodes of magmatism being juxtaposed by later dextral strike-slip fault motions. Because the southern collision zone in the Tien Shan is the older of the two, the Tarim Block sensu stricto collided not with the Eurasian landmass, but with a continental block which was itself separated from Eurasia by at least one ocean. The destruction of this ocean in Late Carboniferous-Early Permian times represented the final elimination of all oceanic basins from this part of central Asia.     

东北非努比亚地盾金矿成矿地质特征与找矿标志

东北非金矿的矿床类型、成矿条件与努比亚地盾演化密切相关。努比亚地盾金矿类型,按成因可分为两大类:一类是与造山带有关的韧性剪切带金矿,即造山型金矿;另一类是弧-弧拼合过程中形成的火山成因块状硫化物金矿,即VMS型铜-金矿。受韧性剪切带控制的造山型金矿形成于闭合-碰撞造山阶段,与弧-弧碰撞缝合带展布方向一致的北东向韧性剪切带为造山型金矿的主要控矿构造,该类型矿床规模以中小型为主。沿韧性剪切带成带分布的火山成因块状硫化物(VMS型)金矿床形成于洋壳俯冲-岛弧的形成阶段,其形成环境为岛弧裂谷或弧后裂谷环境,矿床规模以大型为主,成群分布于5个矿集区内。本文总结了该地区造山带型和VMS型金矿的主要特征,并依此提出在该地区寻找此两种类型金矿的找矿标志。     

Relations between the emplacement and fabric-forming conditions of the Kapitan-Dimitrievo pluton and the Maritsa shear zone (Central Bulgaria): magnetic and visible fabrics analysis

The Kapitan-Dimitrievo pluton was emplaced within the 15 km wide Maritsa shear zone during the Late Cretaceous. It has well-known U–Pb zircon age (78.54 ± 0.13 Ma) and appears as a late-syntectonic intrusion that marked the last ductile deformation in the Maritsa shear zone. Magnetite is believed to be the main carrier of the magnetic fabric in this pluton, and crystallized mainly late, after the main rock-forming minerals. Two fabrics are recorded, a visible syn-magmatic fabric (due to magma flow) and magnetic late-magmatic fabric (related to regional stresses). Although different, both are mainly related to the shearing along this shear zone. These results constrain in age the dextral strike-slip controlled emplacement and evolution of the Late Cretaceous plutons from Central Bulgaria.     

Late Devonian–early Permian accretionary orogenesis along the North Tianshan in the southern Central Asian Orogenic Belt

The Palaeozoic orogenic process in the North Tianshan of the southern Central Asian Orogenic Belt is controversial. Systematic field study indicates that the ophiolitic fragments of the North Tianshan are mainly thrust slices and blocks of a late Palaeozoic accretionary complex, which was intruded by granitoids. U-Pb zircon dating of plagiogranites from the North Tianshan ophiolite yielded a mean age of 343.1 ± 2.7 Ma. These are typical oceanic plagiogranites but with a supra-subduction zone (SSZ) signature. Ophiolitic basalts display N-MORB, E-MORB, and OIB compositions. One gabbro with an age of 301.9 ± 2.2 Ma shows E-MORB geochemistry mixed with N-MORB and OIB. Some andesites show clear island arc characters indicated by enrichment of LILEs relative to HFSEs. Mean ages of 344.9 ± 4.2 and 298.7 ± 2.4 Ma were obtained for a granite porphyry and a mylonitic granite, respectively. The two granitoids display an island arc geochemical signature evidenced by enrichment of LILEs and depletion of HFSEs. Combined with an eastward migration of Late Devonian to Carboniferous arc magmatism and related Cu-Au-Mo deposits, we propose that trench retreat and slab roll-back took place during subduction of the Junggar Ocean spreading ridge beneath the North Tianshan arc, and that the accretion may have lasted into early Permian time, an important late stage of the long-lived accretionary orogenesis in the southern Central Asian Orogenic Belt.     

Oblique collision and evolution of large-scale transcurrent shear zones in the Kaoko belt,NW Namibia

Early structures in the central part of the Kaoko orogenic belt of NW Namibia suggest that the initial stage of collision was governed by underthrusting of the medium-grade Central Kaoko zone below the high-grade Western Kaoko zone, resulting in the development of an inverted metamorphic gradient. In the Western zone, early structures were overprinted by a second phase of deformation, which is associated with localization of the transcurrent Puros shear zone along the contact between the Western and Central zones. During this second phase, extensive partial melting and intrusion of ∼550 Ma granitic bodies occurred in the high-grade Western zone. In the Central zone, the second phase of deformation led to complete overprinting of the early foliation in the zone adjacent to the Puros shear zone, and to the development of kilometre-scale folds in the more distal parts. Strain partitioning into transcurrent deformation along the Puros shear zone and NE–SW oriented shortening in the Central zone is consistent with a sinistral transpressional regime during the second phase of deformation. Transcurrent deformation continued during cooling of the entire belt, giving rise to the localized low-temperature Village Mylonite Zone that separates a segment of elevated Mesoproterozoic basement from the rest of the Western zone in which only Pan-African ages have so far been observed. The data suggest that the boundary between the Western and Central Kaoko zones represents a modified thrust zone controlling the tectonic evolution of the Pan-African Kaoko belt.     

Opposed shear senses inferred from neotectonic mesofracture systems in the North Anatolian fault zone

The 1200-km long North Anatolian fault zone is a right-lateral, intracontinental transform boundary which was initiated in the Late Neogene. Sediments of Pliocene to Holocene age in basins between Cerkes and Erbaa, within the convex-northwards arc of the fault zone, are deformed by syn-sedimentary and post-depositional mesoscopic faults and joints. The mesofractures, which strike obliquely to the fault zone, include reverse faults, normal faults, normal shear joints, conjugate vertical joints and strike-slip faults. Each type of structure occurs in two geometrical groups, one comprises four systems of fractures, the other is made up of five systems. The directions of secondary compression and/or extension inferred from the first group of mesofractures, which are restricted to sediments of Pliocene to Early Pleistocene age, are interpreted as being related to left-lateral shear along the North Anatolian fault zone. The directions of compression and/or extension inferred from the second group of mesofractures, which cut sediments of Pliocene to late Holocene age, were generated during right-lateral shear.The presence of the second group of mesofractures is understandable because they are related to the shear sense which operates at the present-day, but those interpreted as being related to left-lateral shear are more puzzling: their development implies one or more reversals of the dominant sense of displacement. Several tentative models to explain such reversals are proposed, including regional and local influences, the latter related to mechanical constraints and/or the effects of other fault systems.     

K---Ar and Rb---Sr geochronology og igneous rocks from the Sierra de Paiman, northwestern Argentina

Epizonal igneous and metamorphic rocks in northwestern Argentina are exposed in the Sierra de Paiman. The metamorphic rocks are quartzites, phyllites, and slates with soft-body impressions and fossil traces that suggest a late Precambrian-Cambrian age. The igneous rocks were intruded during two major magmatic events according to K---Ar and Rb---Sr data. The older event is represented by different kinds of granitoids and gabbroids, intruded 437–459 Ma. These rocks were emplaced syntectonically in a shear zone that remained active after emplacement, causing extensive mylonitization on the east side of the range. The granitoids show Sr isotopic disturbances possibly related to magma mixing events. Petrologic, geochemical, and isotopic data for these rocks suggest a volcanic-arc setting, probably related to the back arc of the eastward-dipping continental arc of the Famatina Belt. During the younger event (ca. 379 Ma) stocks and dikes of leucogranites were emplaced post-tectonically with respect to the last episode of mylonitization. The leucogranites have syn-collisional signatures and may thus represent the culmination of the volcanic arc of the Famatina Belt.     

Cryogenian ophiolite tectonics and metallogeny of the Central Eastern Desert of Egypt

The Central Eastern Desert (CED) is characterized by the widespread distribution of Neoproterozoic intra-oceanic island arc ophiolitic assemblages. The ophiolitic units have both back-arc and forearc geochemical signatures. The forearc ophiolitic units lie to the west of the back-arc related ones, indicating formation of an intra-oceanic island arc system above an east-dipping subducted slab (present coordinates). Following final accretion of the Neoproterozoic island arc into the western Saharan Metacraton, cordilleran margin magmatism started above a new W-dipping subduction zone due to a plate polarity reversal. We identify two belts in the CED representing ancient arc–forearc and arc–back-arc assemblages. The western arc–forearc belt is delineated by major serpentinite bodies running ～NNW–SSE, marking a suture zone. Ophiolitic units in the back-arc belt to the east show an increase in the subduction geochemical signature from north to south, culminating in the occurrence of bimodal volcanic rocks farther south. This progression in subduction magmatism resulted from diachronous opening of a back-arc basin from north to south, with a bimodal volcanic arc evolving farther to the south. The intra-oceanic island arc units in the CED include coeval Algoma-type banded iron formations (BIFs) and volcanogenic massive sulphide (VMS) deposits. Formation of the BIFs was related to opening of an ocean basin to the north, whereas development of the VMS was related to rifting of the island arc in the south. Gold occurs as vein-type mineral deposits, concentrated along the NNW–SSE arc–forearc belt. The formation of these vein-type gold ore bodies was controlled by the circulation of hydrothermal fluids through serpentinites that resulted in Au mobilization, as constrained by the close spatial association of auriferous quartz veins with serpentinites along the western arc–forearc belt.