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1.
The western Anti-Atlas was formed by a Precambrian basement in the core of anticlines, surrounded by a Neoproterozoic and Palæozoic cover. The structural study of the Tata regional rocks shows a heterogeneous deformation, characterised especially by two types of folds in two orthogonal directions: north-south to north-northeast-south-southwest-trending and east-west-trending.The north-south structures are present in all of the Palæozoic cover and belong to the major Variscan compression of Late Carboniferous age by a comparison of the other domains of the western Anti-Atlas. Alternatively, east-west folding is assigned only to the lower part of the cover and consists of a ductile heterogeneous deformation, especially marked at the basement-cover interface. These folds are associated with a subhorizontal cleavage, indicating a southern vergence of the structures. A discussion of the age and the tectonic style of these structures is proposed, as well as their significance within the Variscan belt along the northern margin of the West African Craton.  相似文献   

2.
Stratigraphic and structural correlations between the Palaeozoic massifs of eastern Morocco and northern Algeria allow three tectonic domains to be distinguished: (1) The cratonic zone, i.e. the West African platform which remained outside the Variscan chain and its peripherical margin (Moroccan Anti-Atlas and Algerian Ougarta); (2) a WSW-ENE trending zone, over 1500 km from Marrakech to Kabylia and Calabria (in their assumed Palaeozoic location). — This zone was characterized during the Late Palaeozoic by a continuous instability indicated by the development of successive turbiditic basins and a major orogeny at the Devonian-Carboniferous boundary; and (3) central and western Morocco, which corresponds to the external zones of the European Hercynides.The Marrakech-Kabylia zone separates the Variscan domain from the stable and undeformed West African craton. During Early Palaeozoic times it began as an extensive or transtensive zone. It has been deformed by the Late Devonian orogeny and by Carboniferous and Permian reactivation. The zone represents the southern limit of the Hercynian chain and is distinguished by its transcurrent regime throughout the Late Palaeozoic. Correspondence to: A. Piqué  相似文献   

3.
《Geodinamica Acta》2013,26(5):309-329
The metamorphic basement of the Asinara island represents a key area of the Sardinia Variscan segment, because it displays an almost complete cross-section through the inner part of the Sardinia Variscan belt, where different tectono-metamorphic complexes have been juxtaposed along narrow belts of high-strain concentration. Detailed field mapping coupled with preliminary studies on the structural and metamorphic features of this small island, allow to draw a better picture of the structural frame issued from the Variscan collision in the inner zone of the belt. Three deformation phases related to crustal thickening in a compressive and transpressive, partitioned tectonic regime, followed by a later phase of extensional deformation have been recognised. In spite of a general HT/LP metamorphic overprint, linked to the post-collisional deformation phases, a relic Barrovian zoneography is still detectable. The Barrovian assemblages are preto syn-kinematic with respect to the D2 deformation phase, and pre-date the third, contractional tectonic event.

The HT/LP assemblage indicates a static growth of weakly deformed by the last deformation events. The complex geometry of the fabric associated to the D2 and D3 deformation events suggests an heterogenous deformation history with a monoclinic geometry characterized by switching of the stretching lineation orientation and a contrasting sense of displacement, probably controlled by a northward partitioned pure shear.  相似文献   

4.
The Adoudounian Basal Series within the western part of the Moroccan Anti-Atlas Mountains was deposited in a varying palæogeographical setting. The first deposits of volcaniclastic and carbonate sediments accumulated in small shallow basins under tectonic control. Then, sedimentation became siliciclastic and volcano-detrital with coastal and deltaic sedimentation in the western area and lagoon-lacustrine in the eastern area. Synsedimentary alkaline volcanism, associated with normal faulting, indicates a within-plate extensional tectonic regime related to rifting, which affected the northern margin of the West African Craton, during Late Neoproterozoic-Early Cambrian times.  相似文献   

5.
In the Ligurian Alps, the Barbassiria massif (a Variscan basement unit of the Briançonnais domain) is made up of orthogneisses derived from K‐rich rhyolite protoliths and minor rhyolite dykes. However, on account of subsequent Alpine deformation and a related blueschist facies metamorphic overprint that are pervasive within the Barbassiria Orthogneisses, little evidence of the earlier Variscan metamorphism is preserved. In this study, new U–Pb laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) dating of zircon from the Barbassiria Orthogneisses and dykes was undertaken to unravel the relationships between protolith magmatism and the Variscan metamorphic overprint. The results suggest a protolith age for the Barbassiria Orthogneisses of ~315–320 Ma (i.e., Early/Late Carboniferous), and constrain the age of a subsequent rhyolite dyke emplacement event to 260.2 ± 3.1 Ma (i.e., Late Permian). The Variscan high‐temperature (greenschist–amphibolite facies) metamorphic event that affected the Barbassiria Orthogneisses was likely associated with both tectonic burial and compression during the final stages of the Variscan collision during the Late Carboniferous period. Emplacement of late‐stage rhyolite dykes that cut the Barbassiria Orthogneisses is linked to a diffuse episode of Late Permian rhyolite volcanism that is commonly observed in the Ligurian Alps. The age of this dyke emplacement event followed a ~10–15 Ma Mid‐Permian gap in the volcano‐sedimentary cover sequence of the Ligurian Alps, and represents the post‐orogenic stage in this segment of the Variscides. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

6.
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7.
The Lufilian arc of Central Africa (also called Katangan belt or Copperbelt) is a zone of low to highgrade metasedimentary (and subsidiary igneous) rocks of Neoproterozoic age hosting highgrade CuCoU and PbZn mineralizations. The Lufilian arc is located between the Congo and Kalahari cratons and defines a structure which is convex to the north. Three major phases of deformation characterize the construction of the Lufilian arc. The first phase (D1) called the “Kolwezian phase” developed folds and thrust sheets with a northward transport direction. D1 deformation occurred in the Lufilian arc between ca. 800 and 710 Ma, with a peak in the range 790–750 Ma. It is here correlated with the main deformation in the Zambezi belt. Southward-verging folds with the same trends as the D1 structures were previously linked to a second tectonic event named Kundelunguian phase of the Lufilian orogeny. We show in this paper that they are backfolds developed during D1 along Katangan ramps and especially along the Kibaran foreland. The second phase (D2) of the Lufilian orogeny is the “Monwezi phase” including several large leftlateral strikeslip faults which have been activated successively. During this deformation phase, the eastern block of the belt rotated clockwise, giving the present day NWSE trend of D1 structures in this part of the Lufilian arc, and generating its convex geometry. The Mwembeshi dislocation, the major transcurrent shear zone separating the Zambezi and Lufilian arc, was mostly active during the D2 deformation phase. D2 deformation occurred between ca. 690 and 540 Ma. Such a long time interval is attributed to the migration of strikeslip faults developed sequentially from south to north, and probably to a slow convergence velocity during the collision between the Congo and Kalahari cratons. The third phase (D3) of the Lufilian orogeny is a late event called the “Chilatembo phase”, marked by structures transverse to the trends of the Lufilian arc. This deformation and the post-D2′ uppermost Kundelungu sequence (Ks3 Plateaux Group), are younger than 540 Ma and probably early Paleozoic.  相似文献   

8.
The Paleozoic massif of Tichka in the southern part of the Western High Atlas of Morocco constitutes a structural transition between the Meseta and the Anti-Atlas domains. It was affected by a complex network of fractures noticeable at different scales. Using Landsat ETM+ imagery permits detecting the main fracture directions. Various techniques of lineament’s extraction were applied, including the colored compositions, spectral band ratios, and directional filters applied to the principal component analysis. Lineament’s extraction is based on visual interpretation and completed by field observations. The resulted map allows recognizing at least four trending fracture system, with average N-S, NE-SW, E-W, and NW-SE orientations. The surrounding rocks of the granitic massif show a high fracture density. Tectonic indicators show that this massif is initially affected by NW-SE Variscan tectonic extension, followed by NW-SE Variscan compression. This regime is being maintained until the late Variscan period corresponding to the relaxation of the NW-SE major Variscan stress. A clockwise rotation of the latter stress, which became N-S to NNE-SSW, related to the late Variscan deformation, is responsible for reworking preexisting faults.  相似文献   

9.
The amalgamation of Pangea during the Carboniferous produced a winding mountain belt: the Variscan orogen of West Europe. In the Iberian Peninsula, this tortuous geometry is dominated by two major structures: the Cantabrian Orocline, to the north, and the Central Iberian curve (CIC) to the south. Here, we perform a detailed structural analysis of an area within the core of the CIC. This core was intensively deformed resulting in a corrugated superimposed folding pattern. We have identified three different phases of deformation that can be linked to regional Variscan deformation phases. The main collisional event produced upright to moderately inclined cylindrical folds with an associated axial planar cleavage. These folds were subsequently folded during extensional collapse, in which a second fold system with subhorizontal axes and an intense subhorizontal cleavage formed. Finally, during the formation of the Cantabrian Orocline, a third folding event refolded the two previous fold systems. This later phase formed upright open folds with fold axis trending 100° to 130°, a crenulation cleavage and brittle–ductile transcurrent conjugated shearing. Our results show that the first and last deformation phases are close to coaxial, which does not allow the CIC to be formed as a product of vertical axis rotations, i.e. an orocline. The origin of the curvature in Central Iberia, if a single process, had to be coeval or previous to the first deformation phase.  相似文献   

10.
New structural data obtained on the Birimian terranes of the Kolia-Boboti sedimentary Basin, the eastern part of the Dialé-Daléma Supergroup in the Kédougou-Kéniéba inlier show two major phases of Eburnean compressional deformation: (1) a D1 phase of thrusting tectonics affected the Lower Birimian B1 tourmalinized sediments. This first tectonic phase is characterized by isoclinals overturned to recumbent folds P1 with N040° 20°NE trending axis, associated with axial plane schistosity S0S1 which is mainly transposed in the bedding; (2) a D2 phase of compressional (D2a) and transpressional (D2b) tectonics is responsible for the crossfolds P2a-P2b exhibiting curved axes. These P2 folds are associated with the major schistosity S2, north-south to SW-NE trending, mainly dipping to the south-east. The S2 schistosity is mostly displayed in the large shear zones corridors where it steeply dips locally toward the north-west. A north-west vergence thrusting phase (D2c) of flats and ramps, associated with reverse folds, represents the last Eburnean event. This geometrical feature is characteristic of a “positive flower structure”. These different Eburnean compressional phases are separated by extensional deformation which is characterized by sedimentary deposits and volcanic flows.  相似文献   

11.
The Anglona and SW Gallura regions represent key places to investigate the tectonic evolution of medium‐ and high‐grade metamorphic rocks cropping out in northern Sardinia (Italy). From south to north we distinguish two different metamorphic complexes recording similar deformation histories but different metamorphic evolution: the Medium Grade Metamorphic Complex (MGMC) and the High Grade Metamorphic Complex (HGMC). After the initial collisional stage (D1 deformation phase), both complexes were affected by three contractional deformational phases (D2, D3 and D4) followed by later extensional tectonics. The D2 deformation phase was the most significant event producing an important deformation partitioning that produced localized shearing and folding domains at the boundary between the two metamorphic complexes. We highlight the presence of two previously undocumented systems of shear belts with different kinematics but analogous orientation in the axial zone of Sardinia. They became active at the boundary between the MGMC and HGMC from the beginning of D2. They formed a transpressive regime responsible for the exhumation of the medium‐ and high‐grade metamorphic rocks, and overall represent a change from orthogonal to orogen‐parallel tectonic transport. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

12.
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.  相似文献   

13.
Apúlia is a small Portuguese sector in NW of Central-Iberian Zone, that have been deformed in a non-coaxial sinistral transpressive regime during the first and main Variscan tectonic event (D1). This deformation give rise to a major NW–SE anticline, where the S1 N–S cleavage transect the inverted short NE limb; two and three-dimensional strains analysis have been done in the low metamorphic grade Ordovician quartzites of this limb using Fry and Rf/ϕ methods. The data show that most deformation was due to intergranular deformation mechanisms. The intragranular deformation leading to the distortion of strain markers and to cleavage was very incipient and a latter event in the D1 phase. The apparent plane strain ellipsoids (if no volume change is assumed) related to the intragranular mechanisms contrast with the more prolate strain ellipsoids related to the bulk deformation of Apúlia Quartzites. This constrictional bulk strain fabrics are characteristic of the sinistral transpressive regimes dominant in the northern sectors of the Central-Iberian Zone.  相似文献   

14.
U–Pb geochronological data, obtained on single zircon grains using the ion microprobe Cameca IMS 1270 (CRPG-CNRS, Nancy), indicate a Neoproterozoic age (625 ± 5 Ma) for the two intrusions of Wirgane granodiorite. This age is in variance with the previously published conclusions where the intrusions were supposed Variscan. Consequently, the Wirgane intrusions are interpreted as remnants of the Neoproterozoic basement of the Variscan belt. They are similar to the Anti-Atlas calc-alkaline intrusions of the collisional to post-collisional stages of the Pan-African orogeny. At that time, the Wirgane region was an area of continental growth to the north of the major South Atlasic Fault.  相似文献   

15.
After the Variscan Cycle, the global tectonic framework underwent three major adjustments. The first occurred in the Late Triassic-initial Jurassic, the second in the Late Jurassic-early Early Cretaceous and the third in the Late Cretaceous-Eogene. On that basis, the post-Variscan tectonic history is divided into three tectonic cycles——the Indosinian, Yanshanian and Himalayan Cycles. The post-Neocom(k_1~1) andpre-Aptian(k_1~2) tectonic movement marks the end of the Yanshanian orogeny in eastern China and the initiation of the subduction of the Tethyan ocean in western China and represents the boundary between the Yanshanian and Himalayan Tectonic Cycles.  相似文献   

16.
Rift‐related regional metamorphism of passive margins is usually difficult to observe on the surface, mainly due to its strong metamorphic overprint during the subsequent orogenic processes that cause its exposure. However, recognition of such a pre‐orogenic evolution is achievable by careful characterization of the polyphase tectono‐metamorphic record of the orogenic upper plate. A multidisciplinary approach, involving metamorphic petrology, P–T modelling, structural geology and in situ U‐Pb monazite geochronology using laser‐ablation split‐stream inductively coupled plasma mass spectrometry, was applied to unravel the polyphase tectono‐metamorphic record of metapelites at the western margin of the Teplá‐Barrandian domain in the Bohemian Massif. The study resulted in discovery of three tectono‐metamorphic events. The oldest event M1 is LP–HT regional metamorphism with a geothermal gradient between 30 and 50 °C km?1, peak temperatures up to 650 °C and of Cambro‐Ordovician age (c. 485 Ma). The M1 event was followed by M2‐D2, which is characterized by a Barrovian sequence of minerals from biotite to kyanite and a geothermal gradient of 20–25 °C km?1. D2‐M2 is associated with a vertical fabric S2 and was dated as Devonian (c. 375 Ma). Finally, the vertical fabric S2 was overprinted by a D3‐M3 event that formed sillimanite to chlorite bearing gently inclined fabric S3 also of Devonian age. The high geothermal gradient of the M1 event can be explained as the result of an extensional, rift‐related tectonic setting. In addition, restoration of the deep architecture and polarity of the extended domain before the Devonian history – together with the supracrustal sedimentary and magmatic record – lead us to propose a model for formation of an Ordovician passive continental margin. The subsequent Devonian evolution is interpreted as horizontal shortening of the passive margin at the beginning of Variscan convergence, followed by detachment‐accommodated exhumation of lower‐crustal rocks. Both Devonian shortening and detachment occurred in the upper plate of a Devonian subduction zone. The tectonic evolution presented in this article modifies previous models of the tectonic history of the western margin of the Teplá‐Barrandian domain, and also put constraints on the evolution of the southern margin of the Rheic ocean from the passive margin formation to the early phases of Variscan orogeny.  相似文献   

17.
The Magba Shear Zone is made up of granites, migmatites, orthogneiss, metagabbro, mafic dyke and mylonites with coarse grained texture, porphyroblastic, granoblastic, cataclastic and mylonitic texture respectively. Structural features and kinematic indicators testify the syntectonic emplacement of Magba granitoids and also provide detailed information on the relative timing of deformation as follows: (1) D1 of tangential movement immediately followed by (2) the D2 phase which is heterogeneous simple shear in dextral transpressive context with a NW-SE direction (3) D3 tectonic phase is marked by sinistral transpressive tectonic and superposed folding with a NE-SW kinematic direction. Combined ductile NE-SW shear movements and NWSE compressional movements defined a transpressional tectonic regime during the D3 deformation (4) A brittle stage D4 is controlled by transcurrent tectonics and responsible for the emplacement of faults, and joints. The Magba granites would have intruded along sub-vertical mid-crustal feeder channels and were emplaced as a sheet or sheets along the shear zone during the early stage of the C3 shearing, followed by gabbro and mafic dyke at the late stage. Strike-slip dilatancy pumping under transpressive tectonic is suggested as a possible mechanism for the emplacement of the Magba granites.  相似文献   

18.
The Navalpino Anticline is a major Variscan structure in the Central Iberian Zone of Spain. Three lithological groups are defined in the pre-Ordovician rocks of this anticline. The Rifean or Lower Vendian Extremeño Dome Group is unconformably overlain by the Upper Vendian Ibor-Navalpino Group. This latter group presents two different facies separated by a NW-SE trending synsedimentary fault. The Lower Cambrian Valdelacasa Group unconformably overlies both the Extremeno Dome and the Ibor-Navalpino Groups.Three pre-Variscan episodes of deformation have been defined in the area of the Navalpino Anticline. A major asymmetrical fold with a subvertical east-west-striking limb is the result of the first deformation event of pre-Late Vendian age. The second deformation event is of Cadomian (Late Precambrian) age and is composed of two stages; (i) an early extensional stage including NW - SE trending extensional fault and basin development in the north-eastern block; and (ii) a second compressive stage giving rise to north-south trending upright folds. This second compressive stage possibly inverted the basin. A final pre-Variscan deformation event took place between the Early Cambrian and the Early Ordovician resulting in a 5–10° tilting to the north-east.There are two main phases of Variscan deformation in the area. The first deformation event (Dv1) gave rise to a upright WNW - ESE trending folds on all scales, whereas the second (Dv2) gave rise to a brittle—ductile sinistral strike-slip shear zone tending subparallel to the axial trace of the Dv1 folds.  相似文献   

19.
《Journal of Structural Geology》2001,23(6-7):1015-1030
The Malpica–Lamego Line (MLL) is a deformation zone in the Variscan belt of NW Iberia (NW Spain and N Portugal) that runs parallel to the chain for at least 275 km, bearing I-type granodiorite plutons along most of its length. The MLL affects previous structures by which high pressure and ophiolitic rocks were exhumed and emplaced on the Iberian plate during earlier deformation phases. Correlation and reconstruction of the stratigraphy of these sheets or tectonic units at both sides of the shear zone allows a preliminary estimate of the accumulated vertical and horizontal offsets after the tectonic activity of the fault. The value of the separations, of crustal-scale proportions, reaches a maximum 15 km of vertical offset that decreases gradually to the south. The structural record found in the rocks indicates a strike-slip regime that, in general, does not fit the geometry of the offsets. We suggest that the MLL went through two different stages during the same orogenic cycle: a first dip-slip episode, a reverse faulting event, overprinted by a later strike-slip reactivation.  相似文献   

20.
The Southern Variscan Front in the Tinerhir area involves Palaeozoic allochthonous units (Ouaklim and Tilouine units) thrust onto the northern edge of the West African Craton during late Carboniferous time. Illite crystallinity data highlight an anchizonal grade for the Ouaklim Unit, and a diagenesis-anchizone transition for the Tilouine Unit during deformation phase D1. The tectonic stack is crosscut by major dextral reverse faults bounding E–W trending domains of dominant shortening deformation (central domain) and strike-slip deformation (northern and southern domains), later segmented by a network of post-Variscan faults. This complex deformation pattern is the result of kinematic partitioning of dextral transpression along the Southern Variscan Front, coeval with the Neovariscan (300–290 Ma) oblique convergence observed at the scale of the whole Moroccan Variscides. Partitioning of dextral transpression described in the Tinerhir area is consistent with dextral wrench faulting along the Tizi n’ Test Fault, and with Appalachian-style south-directed thrusting in the Tinerhir and Bechar-Bou Arfa areas.  相似文献   

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