Carboniferous normal faults in the Eastern Pyrenees: evidences and age constrains of syn-orogenic Variscan extension

In low grade rocks of the Eastern Pyrenees syn-orogenic Variscan extension is achieved by kilometric scale low-angle brittle normal faults. Evidence of these faults is generally depicted by subtractive contacts between Devonian upon Cambro-Ordovician rocks. Normal faults are cut by a Variscan granodiorite pluton and U-Pb available geochronologic data of the granodiorite, 305 Ma ± 3 [30], indicates that the age of extension can be attributed to Moscovian times. Extension postdates the main period of Variscan crustal thickening and occurs in N-S to NE-SW direction, roughly perpendicular to the trend of the main Variscan compressional structures. Such relationships point out that the onset of Variscan extension occurs after compression and prior to the granodiorite emplacement and to the deposition of post-orogenic volcanics.     

The structure of the Swansea Valley Disturbance between Clydach and Hay-on-Wye,South Wales

The Swansea Valley Disturbance is one of four NE-SW belts of faulting and folding which cross the northern limb of the South Wales Coalfield syncline at variance with the normal E-W Variscan structures. The Disturbance extends from Hay-on-Wye (Herefordshire) southwestwards to Clydach (near Swansea) and may extend northeastwards to Titterstone Clee Hill (near Ludlow) and southwestwards along the Tircanol Fault to Swansea Bay. The main structural elements of the Disturbance are: impersistent NE-SW folds; NE-SW normal faults; and NE-SW and NNW-SSE wrench faults. The NE-SW structures are confined to a narrow zone which seldom exceeds two kilometres in width. It is suggested that this narrow belt of faulting and folding has been controlled mainly by sub-Devonian basement structures, which involve faulting and/or folding. The effect of the Variscan compression was to reactivate the basement structure, which had the effect of resolving this compression along the disturbed zone to produce sinistral wrench movements. The structure of the Disturbance has been complicated by folding, produced by the Variscan force driving the Upper Palaeozoic rocks against the Lower Palaeozoic block. It is concluded that the main movements are of Variscan age and that vertical movements may have taken place in post-Carboniferous and post-Neogene times.     

Evidence for Devono-Carboniferous extensional tectonics in the central Pyrenees

Abstract

Detailed mapping of the prc-Variscan rocks exposed in the Southern Central Pyrenees reveals the occurrence of extensional tectonic contacts overprinted by main phase and pre-main phase folds. The interpretation of the former contacts as normal faults and the reinterpretation of other contacts described in the literature lead us to suggest an extensional event during Devonian and/or early Carboniferous times, prior to the contrac-tional Variscan paroxysm.     

Tectonic evolution of the deep crust: Variscan reactivation by extension and thrusting of Precambrian basement in the Bragança and Morais massifs (Tras-os-Montes,NE Portugal)

Abstract

The Braganca amd Morais Massifs (NE Portugal) comprise a pile of four nappes on lop of the Autochthon of the Central-Iberian Zone : a Parautochthon (PTC), a Lower Allochthon (LATC), an Ophiolitic Complex (OTC) and an Upper Allochton (UATC). This article focuses on the tectonic evolution of the prc-Variscan basement preserved in the Upper Allochthonous Thrust Complex. (1) In the Morais Massif, the UATC is mainly composed of orthogneisscs, micaschists and high-grade melamorphic rocks restricted to a small duplex between the orthogneisses and the ophiolitic complex. The orthogneisses are pervasively deformed by D6 (Variscan D1). characterized by NNW-SSE stretching lineation, C-S structures, and sense of shear to the SSE. The high-grade melamorphic rocks show at least three ductile deformation phases older than the gneisses deformation. The micaschists and the orthogneisses are cut by mafic sills and dykes transformed into amphibolites by the Variscan tec-tonometamorphic evolution. In a restricted domain where dykes arc less deformed, two deformation events can be recognized and arc considered to be pre-Variscan. The walls of the dykes show a N-S stretching and mineral lineation interpreted as resulting from D6 (Variscan D1). (2) In the Braganca Massif, the UATC comprises mafic to ultrainafic igneous and high-grade melamorphic rocks, and paragncisses with ky-eclogite lenses. Six ductile deformation phases are recognized. The D1 to D4 events may correspond to a complete pre-Variscan orogenic cycle, from subduction (D1) to collision (D2-DS) and thrusting of the high-grade metamorphic rocks to upper levels in the crust (D3-D4); D5 may result from the Lower Palaeozoic extensional event that marks the begining of the Variscan Wilson cycle; D6 is interpreted as the first Variscan orogenic event with southward movement. The UATC of the Cabo Ortegal anil Braganca Massifs comprise mainly upper mantle/lower erustal rocks. By contrast, the UATC of the Ordenes and Morais Massifs is mainly composed of middle to upper erustal rocks. Vic propose that this is the result of a regional ductile normal fault (extensional event) that was active prior to the Variscan orogeny, in Lower Palaeozoic times, and affected a Precambrian basement.     

Deformation Characteristics and Tectonic Evolution of the Eastern Qinling Orogenic Belt

The Qinling Mountains separating the northern from the southern China plate is a key region for the study of structural evolution of eastern Asia. It is composed of the Palaeozoic fold belt in its northern part and the Variscan and Indosinian fold belts in its southern part. The evolution of the former is marked by the closure of a northward subducting oceanic basin in the early stage, followed by southward obduction of ophiolites and intracontinental thrusting during the Variscan; whereas that of the latter is represented by intracontinental, shallow crustal deformation on the basis of a large-scale detachment structure(with a horizontal slip of at least of 100 km). Since the late Palaeozoic, however, both of the belts have been cut by a series of east-west sinistral strike-slip faults.     

Variscan granites of the Pyrenees revisited: their role as syntectonic markers of the orogen

Structural studies of Variscan plutons of the Pyrenees demonstrate their syn-orogenic character, contrasting with their post-orogenic Permian Rb-Sr datation. The Bassiès and Mont-Louis-Andorra plutons were emplaced before the main Variscan D2 phase and behaved as rigid markers during D2. Emplacement of the Néouvielle pluton was coeval with D2 as attested by its behaviour, first as a deformable body, then as a rigid body. The structures in plutons, combined with those of the country rocks, demonstrate that the whole Pyrenees acted as a large dextral ductile shear zone during the D2 main phase. This conclusion points out to the interest of the granitoids as kinematic markers.     

Basement-cover relations in the Appalachian fold and thrust belt

The external massifs along the Appalachian orogen include Precambrian basement rocks with attached cover. To the northwest (cratonward), in the Appalachian foreland fold and thrust belt, Palaeozoic sedimentary rocks, but no basement rocks, are exposed; that belt was the subject of the classic debate about thin-skinned (deformed cover rocks detached from undeformed basement) and thick-skinned (basement deformed with attached cover) structural styles. Presently available data indicate detached cover rocks and thin-skinned style in the fold and thrust belt: large-scale thrusting occurred late in the orogenic history. In the external basement massifs, late Precambrian graben-fill sedimentary and volcanic rocks indicate early basement faults; and within the craton, steep basement faults bound graben blocks of Cambrian age. Distribution of known basement faults suggests that basement rocks beneath the fold and thrust belt may also be faulted. Local episodic synsedimentary structural movement through much of the Palaeozoic is documented by stratigraphy in the fold and thrust belt. Axes of early synsedimentary structures are approximately coincident with axes of late folds and thrust fault ramps, but stratigraphic data show that magnitude of the early structures was much less than that of the late structures. These relations suggest the interpretation that early low-magnitude structures formed in cover rocks over basement faults and that the early structures, or the basement faults, significantly influenced the geometry of later detachment structures during large-scale horizontal translation.     

The geodynamic evolution of the Southern European Variscides: constraints from the U/Pb geochronology and geochemistry of the lower Palaeozoic magmatic-sedimentary sequences of Sardinia (Italy)

The high-grade metamorphic complex of northern Sardinia consists of a strongly deformed sequence of migmatitic ortho- and paragneisses interlayered with minor amphibolites preserving relic eclogite parageneses. The protolith ages and geochemical characteristics of selected gneiss samples were determined, providing new constraints for reconstructing the Palaeozoic geodynamic evolution of this sector of the Variscan chain. The orthogneisses are metaluminous to peraluminous calcalkaline granitoids with crustal Sr and Nd isotopic signatures. One orthogneiss from the high-grade zone and one metavolcanite from the volcanic belt in southern Sardinia were dated by LAM-ICPMS (and SHRIMP) zircon geochronology. The inferred emplacement ages of the two samples are 469 ± 3.7 and 464 ± 1 Ma, respectively. The analysed paragneisses are mainly metawackes with subordinate metapelites and rare metamarls. Three paragneiss samples were dated: zircon ages scatter between 3 Ga and about 320 Ma, with a first main cluster from 480 to 450 Ma, and a second one from about 650 to 550. Variscan zircon ages are rare and mostly limited to thin rims and overgrowths on older grains. These data indicate that the high-grade complex principally consists of middle Ordovician orthogneisses associated with a thick metasedimentary sequence characterised by a maximum age of deposition between 480 and 450 Ma. The association of nearly coeval felsic-mafic magmatic rocks with immature siliciclastic sedimentary sequences points to a back-arc setting in the north Gondwana margin during the Early Palaeozoic. The Variscan metamorphic evolution recorded by the high-grade gneisses (Ky-bearing felsic gneisses and mafic eclogites) testifies to the transformation of the Late Ordovician–Devonian passive continental margin into an active margin in the Devonian–Early Carboniferous.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

New data on the stratigraphy,structure and petrology of the Precambrian/Cambrian inlier at Martley,Worcestershire

A disused Victorian gravel pit [SO 7450 5956] 1 km west of Martley, Worcestershire formerly exposed an inlier of Neoproterozoic meta-igneous rocks and early Palaeozoic quartz arenite. The pit is back-filled, but trenching at the site between 2010 and 2014 re-exposed the rocks of the inlier and the surrounding Silurian and Carboniferous cover rocks. The site lies on the East Malvern Fault (EMF) and the work has proved the relationships between the meta-igneous rocks, quartz arenite and cover rocks, and revealed a complex of thrust faults in the footwall of the EMF. The thrusts are interpreted as footwall shortcuts and provide evidence of the Variscan inversion and compressive events resolved along this fault line (the Malvern Lineament) which has a prolonged and complex history of activation and reactivation. The structures at Martley provide a model in microcosm for other Variscan compressional structures along the Malvern Lineament.     

Thermal evolution of the orogenic lower crust during exhumation within a thickened Moldanubian root of the Variscan belt of Central Europe

At the eastern margin of the Bohemian Massif (Variscan belt of Central Europe), large bodies of felsic granulite preserve mineral assemblages and structures developed during the early stages of exhumation of the orogenic lower continental crust within the Moldanubian orogenic root. The development of an early steep fabric is associated with east–west-oriented compression and vertical extrusion of the high-grade rocks into higher crustal levels. The high-pressure mineral assemblage Grt-Ky-Kfs-Pl-Qtz-Liq corresponds to metamorphic pressures of ∼18 kbar at ∼850 °C, which are minimum estimates, whereas crystallization of biotite occurred at 13 kbar and ∼790 °C during decompression with slight cooling. The late stages of the granulite exhumation were associated with lateral spreading of associated high-grade rocks over a middle crustal unit at ∼4 kbar and ∼700 °C, as estimated from accompanying cordierite-bearing gneisses. The internal structure of a contemporaneously intruded syenite is coherent with late structures developed in felsic granulites and surrounding gneisses, and the magma only locally explored the early subvertical fabric of the felsic granulite during emplacement. Consequently, the emplacement age of the syenite provides an independent constraint on the timing of the final stages of exhumation and allows calculation of exhumation and cooling rates, which for this part of the Variscan orogenic root are 2.9–3.5 mm yr⁻¹ and 7–9.4 °C Myr⁻¹, respectively. The final part of the temperature evolution shows very rapid cooling, which is interpreted as the result of juxtaposition of hot high-grade rocks with a cold upper-crustal lid.     

Polyphase late Palaeozoic deformation in the southeastern foreland and northwestern Piedmont of the Alabama Appalachians

Late Palaeozoic deformation in the southern Appalachians is believed to be related to the collisional events that formed Pangaea. The Appalachian foreland fold and thrust belt in Alabama is a region of thin-skinned deformed Palaeozoic sedimentary rocks ranging in age from Early Cambrian to Late Carboniferous, bounded to the northwest by relatively undeformed rocks of the Appalachian Plateau and to the southeast by crystalline thrust sheets containing metasedimentary and metaigneous rocks ranging in age from late Precambrian to Early Devonian. A late Palaeozoic kinematic sequence derived for a part of this region indicates complex spatial and temporal relationships between folding, thrusting, and tectonic level of décollement. Earliest recognized (Carboniferous(?) or younger) compressional deformation in the foreland, observable within the southernmost thrust sheets in the foreland, is a set of large-scale, tight to isoclinal upright folds which preceded thrafing, and may represent the initial wave of compression in the foreland. Stage 2 involved emplacement of low-angle far-traveled thrust sheets which cut Lower Carboniferous rocks and cut progressively to lower tectonic levels to the southwest, terminating with arrival onto the foreland rocks of a low-grade crystalline nappe. Stage 3 involved redeformation of the stage 2 nappe pile by large-scale upright folds oriented approximately parallel to the former thrusts and believed to be related to ramping or imbrication from a deeper décollement in the foreland rocks below. Stage 4 involved renewed low-angle thrusting within the Piedmont rocks, emplacement of a high-grade metamorphic thrust sheet, and decapitation of stage 3 folds. Stage 5 is represented by large-scale cross-folding at a high angle to previous thrust boundaries and fold phases, and may be related to ramping or imbrication on deep décollements within the now mostly buried Ouachita orogen thrust belt to the southwest. Superposed upon these folds are stage 6 high-angle thrust faults with Appalachian trends representing the youngest (Late Carboniferous or younger, structures in the kinematic sequence.     

Transpressive duplex and flower structure: Dent Fault System, NW England

Revised mapping along the Dent Fault (northwest England) has improved the resolution of folds and faults formed during Variscan (late Carboniferous) sinistral transpression. A NNE-trending east-down monocline, comprising the Fell End Syncline and Taythes Anticline, was forced in Carboniferous cover above a reactivated precursor to the Dent Fault within the Lower Palaeozoic basement. The Taythes Anticline is periclinal due to interference with earlier Acadian folds. The steep limb of the monocline was eventually cut by the west-dipping Dent Fault. The hangingwall of the Dent Fault was dissected by sub-vertical or east dipping faults, together forming a positive flower structure in cross-section and a contractional duplex in plan view. The footwall to the Dent Fault preserves evidence of mostly dip-slip displacements, whereas strike-slip was preferentially partitioned into the hangingwall faults. This pattern of displacement partitioning may be typical of transpressive structures in general. The faults of the Taythes duplex formed in a restraining overlap zone between the Dent Fault and the Rawthey Fault to the west. The orientations of the duplex faults were a response to kinematic boundary conditions rather than to the regional stress field directly. Kinematic constraints provided by the Dent and neighbouring Variscan faults yield a NNW–SSE regional shortening direction in this part of the Variscan foreland.     

Palaeozoic sequences and evolution of the Calabrian-Peloritan Arc (Southern Italy)

Three Palaeozoic sequences belonging to three different basins crop out in the Calabrian–Peloritan Arc. Their age covers the time span from middle (?) Cambrian to early Carboniferous. The sequences comprise terrigenous, volcanic and carbonate rocks, and show low-grade metamorphism. The basement is of pre-middle Cambrian age, crops out in the Calabrian–Peloritan Arc, and was metamorphosed prior to the opening to the Palaeozoic basins. The Palaeozoic basins existed no later than the middle Carboniferous, with inferred maximum crustal extension in the Cambro-Ordovician. By Devonian time, the tectonic regime was compressional overall with middle Devonian island-arc type volcanic activity that continued until the closure of the basins. Approximately 330 Myr, the Palaeozoic sequences experienced low-pressure greenschist facies metamorphism and continuing subduction controlled the Variscan tectonogenesis with 280 Myr island-arc type intrusive magmas. Subduction ceased and late-stage 280–270 Myr granitic magmas were emplaced during continental collision. The crustal sector carrying the Palaeozoic basins is interpreted as comprising fragments of an active continental margin, one of the several microplates, located at the southern margin of the Euro-Asia continent which faced a large (at least 1500 km in width) ocean basin.     

Two-sided Variscan thrust tectonics in the Vosges Mountains,northeastern France

Abstract

Variscan convergence produced two-sided (bivergent) crustal-scale thrusting in the Vosges Mountains. In the northern Vosges the central polymetamorphic crystallines were thrust to the NW over Cambrian to Silurian low-grade and very low-grade metamorphic clastics. Synorogenic upper Devonian - lower Carboniferous turbidites and volcanics were folded into NW-vergent structures which display SE-dipping slaty cleavage. The entire sequence shows increasing metamorphism and deformation from NW to SE. Late right-lateral strike-slip faulting along the Lalaye-Lubine fault zone outlasted thrusting. In the southern Vosges a lower Carboniferous turbiditic basin that was fringed on the south by a volcanic arc was tectonically shortened by south-directed tectonic imbrication of slivers of varied rocks including ultramafics, gneissic basement, and synorogenic elastics. The increasing degree of deformation and metamorphism towards the north suggests a thrust contact with the polymetamorphic gneisses of the central Vosges. The final stages of Variscan convergence were accompanied by voluminous granitic plutonism and by faulting along NNE-SSW and E-W-trending strike-slip faults. The tectonic evolution reflects progressive Variscan closure of a previously extended basinal crust in a high-temperature regime.     

Tertiary compressional deformation of the Iberian plate

The Tertiary deformation of the Iberian plate is here interpreted as the result of changes in the coupling between the Iberian–African plates. During the early stages of the Africa/Iberia subduction (Palaeocene), deformation was confined at the Betic plate boundary. From the Eocene, during the collision in the southern plate margin, compressional deformation delocalized and distributed throughout the Iberian plate. First, in the Pyrenees, where the main stage of thrusting occurred during the Late Eocene – Early Oligocene. Then (mainly Oligocene – Late Miocene), in the inner part of the Iberian plate, forming basement uplifts in the Iberian Chain and the Central System, in correspondence of pre-existing (Mesozoic and Variscan) structures. Finally, during the decay of compression inside the Iberian plate, extension took place the Mediterranean margin and the Alboran Sea.     

The eastern French Pyrenees: from mountain belt to foreland basin

The Pyrenees is a young mountain belt formed as part of the larger Alpine collision zone. This excursion explores the development of the Pyrenean Mountain Belt in southern France, from its early extensional phase in the mid‐Cretaceous and subsequent collisional phase, through its uplift and erosion in the Late Cretaceous and again in the Eocene, which led to the development of the Aquitaine‐Languedoc foreland basin. One of the complexities of the Pyrenean Belt is that thrusting, uplift and erosion during the Pyrenean orogeny exposed older Variscan basement rocks in the central core of the mountains, rocks which were metamorphosed during an earlier event in the late Carboniferous. Thus, this orogenic belt also tells the story of an earlier collision between Laurussia in the north and Gondwana in the south at c. 300 Ma, prior to the onset of the Pyrenean events at c. 100 Ma. Here we seek to unravel these two separate orogenic stories.     

A restored section of the “southern Variscan realm” across the Corsica–Sardinia microcontinent

A complete section of the southern realm of the Variscan orogenic belt can be restored in the Corsica–Sardinia segment. Northern Corsica exposes a nonmetamorphosed Palaeozoic succession lying on Panafrican mica schist related to a microcontinent (most likely Armorica or from a microcontinent from the Hun superterrane) that had drifted away directly from Gondwana. These formations are thrust over the Variscan Internal Zone composed mainly of anatectic high-grade Palaeozoic formations that crop out from central Corsica to northern Sardinia; the metamorphic peak of the eclogite remnants has been dated at c. 420 Ma. The Variscan Internal Zone interpreted here as a collision zone, and also the Eovariscan suture, was intruded in Corsica by Mg–K granite from 345 to 335 Ma. The thrust of this Internal Zone onto the stack of parautochthonous nappes in central Sardinia is cross-cut by the Posada Asinara dextral shear zone. To the south, parautochthonous nappes overthrust the North-Gondwana margin which displays a possible Panafrican basement topped by an Iglesiente–Sulcis nonmetamorphic/anchimetamorphic Palaeozoic succession.     

Metamorpher Übergang Saxothuringikum-Moldanubikum östlich tirschenreuth/opf., nachgewiesen durch phasenpetrologische Analyse

Based upon the knowledge of the field relations the mineral assemblages of the various metamorphic rocks of the area are plotted in AKF-diagrams with the aim at determining possible mutual reaction relationships. For the eastern part a continuous metamorphic transition follows, starting with mica schists of the Lower Palaeozoic Saxothuringicum as characterized by the assemblage biotite-muscovite- and alusite through a series of biotite-sillimanite-gneisses with garnet of K feldspar into the high-grade gneisses and migmatites of the Moldanubicum as typified by cordierite-sillimanite-K feldspar parageneses. Hence the age of this metamorphism common to all units is probably early Variscan (= Hercynian). In the western part the Moldanubian unit, in particular, was affected by wide-spread later diaphthoresis creating mineral assemblages like chlorite-biotite-muscovite and chlorite-phengitic white mica, that represent a lower facies than that of the neighbouring mica schists. This diapthoresis is also of Hercynian age, because it is followed by the intrusion of igneous rocks probably during Sudetic (= Middle Carboniferous) times.     

米仓山、南大巴山前缘构造特征及其形成机制

在对四川盆地东北部盆山结合部地表地质和石油地球物理资料综合分析的基础上,阐述了米仓山前缘构造、南大巴山前缘构造的几何学、运动学特征;发现了二者的共性和不同,二者均以双重构造为主,通过古生代构造层的叠置,而迅速抬升出露地表,米仓山前缘以被动顶板双重构造为主,即典型的"三角带"构造,南大巴山前缘以主动顶板双重构造为其显著特征;初步分析了原因,区域滑脱层,特别是嘉陵江组-雷口坡组膏盐岩滑脱层及古生界泥页岩滑脱层,构成了顶板和底板逆冲断层,其间的台地相碳酸盐岩构成了断夹块,受米仓山早期基底隆升和侧向挤压,形成了被动顶板双重构造,南大巴山递进挤压变形,形成了主动顶板双重构造。     

沙垒田凸起前古近系基岩分布及源—汇过程

沙垒田凸起及围区凹陷是一个完整的源-汇系统.基于钻井岩心、三维地震及锆石测年等资料,刻画了沙垒田凸起前古近系基岩组成及分布并探讨了古近纪早期源-汇体系配置关系.研究表明,沙垒田凸起自南向北依次发育太古界-元古界混合花岗岩、奥陶-寒武系碳酸盐岩与碎屑岩、中生界火山碎屑岩;其中,凸起东段以元古代混合花岗岩为主,西段以奥陶-寒武系碳酸盐岩为主.结合古地貌可知,砂岩富集区与源-汇耦合系统相对应,即基岩组成、汇水区、物源通道及边界样式共同控制沉积砂体组成样式与规模.对比分析凸起东西段、南北侧各控制要素与沉积砂体间关联性,可知凸起东段南侧混合花岗岩基底、较大的汇水面积与物源通道及斜坡带对应高效源-汇系统.