Structures in the banded iron-formation of the southeastern Bababudan Hills, Karnataka, India

The banded iron-formation in the southeastern Bababudan Hills display a macroscopic synformal bend gently plunging towards WNW. The bedding planes in smaller individual sectors show a cylindrical or conical pattern of folding. The dominant set of minor folds has WNW-ESE trending axial planes and the axes plunge towards WNW at gentle to moderate angles, though there is considerable variation in orientation of both axes and axial planes. A later set of sporadically observed folds has N-S trending axial planes. The macroscopic synformal bend within the study area forms the southeastern corner of a horseshoe shaped regional synformal fold closure which encompasses the entire Bababudan range. The minor folds are buckle folds modified to a varying extent by flattening. In some examples the quartzose layers appear to be more competent than the ferruginous layers; in others the reverse is true. The folds are frequently noncylindrical and the axes show curvature with branching and en echelon patterns. Such patterns are interpreted to be the result of complex linking of progressively growing folds whose initiation is controlled by the presence of original perturbations in the layers. Domes and basins have at places developed as a result of shortening along two perpendicular directions in a constrictional type of strain. Development of folds at different stages of progressive deformation has given rise to nonparallelism of fold axes and axial planes. The axes and axial planes of smaller folds developed on the limbs of a larger fold are often oriented oblique to those of the latter. Progressive deformation has caused rotation and bending of axial planes of earlier formed folds by those developed at later stages of the same deformational episode. Coaxial recumbent to nearly reclined fold locally encountered on the N-S limb of the macroscopic fold may belong to an earlier episode of deformation or to the early stage of the main deformation episode. The E-W to ESE-WNW strike of axial plane of the regional fold system in the Bababudan belt contrasts with the N-S to NNW-SSE strike of axial planes of the main fold system in the Chitradurga and other schist belts of Karnataka.     

Grenzen quantitativer Untersuchungen bei tektonischen Experimenten

Quantitative experimental and theoretical investigations, except basic research, should focus only to such parameters which can be measured in the field. This is shown by three examples.

1. A model of a discontinuum composed of particles, between which friction occurs during shear is presented. Even at simple shear external and internal stresses are in equilibrium; no rigid body rotation occurs. The theory is examinated experimentally.
2. In deformation experiments with clay and sand a development of anisotropy is observed, which can cause shear fractures. The orientation of the normal stress directions can be discovered from the shear sets, their angular distribution, and the rotation of the individual blocks.
3. Most folds can be traced back to the following three types, a) Similar folds, b) concentric folds, and c) box folds. b) and c) occur only near the surface and near shear planes, as shown by experiments and field observations. In all other cases the more energy-consuming similar folds are formed. Kink bands are presumably transitional between folds and shear fractures.

     

The geometry and kinematics of flow perturbation folds

Minor folds formed synchronous with ductile deformation in high strain zones can preserve a record of the scale and kinematics of heterogeneous flow. Using structures associated with WNW-directed Caledonian thrusting in N Scotland, we show that localised perturbations in flow resulted in the generation of predominantly cylindrical minor folds with hinges lying at low angles to the transport direction. These define a series of larger-scale fold culminations (reflecting ‘surging flow’) or depressions (reflecting ‘slackening flow’) that are bisected by transport-parallel culmination and depression surfaces. The fold patterns suggest a dominance of layer-normal differential shearing due to gradients in shear strain normal to transport. Culmination surfaces are marked by along-strike reversals in the polarity of structural facing and vergence of minor folds which, contrary to classic fold patterns, define reverse asymmetric relationships. Culmination surfaces separate folding in to clockwise (Z folds) and anticlockwise (S folds) domains relative to the transport lineation. The dip of fold axial planes systematically increases as their strike becomes sub-parallel to transport resulting in a 3D statistical fanning arrangement centred about the transport direction. Thus, mean S- and Z-fold axial planes intersect precisely parallel to the transport lineation and potentially provide a means of determining transport directions in cases where lineations are poorly preserved. Culminations display convergent fold patterns with fold hinges becoming sub-parallel to transport towards the culmination surface and underlying detachment, whilst axial planes define overall concave up listric geometries which are bisected by the culmination surface. Thus, around culminations and depressions there are ordered, scale-independent relationships between transport direction, shear sense, fold facing, vergence and hinge/axial plane orientations. The techniques described here can be applied and used predictively within any kinematically coherent system of ductile flow.     

Tangled up in folds: tectonic significance of superimposed folding at the core of the Central Iberian curve (West Iberia)

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.     

From thickening to extension in the Variscan belt — kinematic evidence from Sardinia (Italy)

The Variscan nappe stack of SE Sardinia originated as a result of several stages of nappe imbrication during the Lower Carboniferous phases of the Variscan orogeny. The crustal shortening caused regional SSW-and W-directed thrusting, greenschist facies metamorphism and open-to-isoclinal polyphase folding. The final stage of shortening produced large-scale antiforms and synforms.
Post-collisional deformation resulted in inversion of earlier thrusts as normal faults, development of low-angle normal faults, and refolding of earlier foliation and thrust planes by asymmetric folds with subhorizontal axial planes. Facing directions of these latest folds are directed horizontally outward from the hinge zones of main antiforms, suggesting that they cannot be regarded as parasitic folds of the latest thickening phase, but instead are the consequence of vertical shortening during gravitational collapse of dome-like km-scale antiforms, leading to denudation of antiformal culminations.     

Structural pattern and strain history of a superposed fold system in the Precambrian of Central Rajasthan,India. II. Strain history

Analysis of shapes of folds, together with other structures such as axial plane foliation boudinage, mullions and cross joints, show that the F₁ folds in the ‘main Raialo syncline’ were formed by buckling, and were subsequently modified by flattening normal to the axial planes and lengthening along the axis. The apparent buckle shortening of the F₁ folds generally ranges between 70 and 80%. The folds were formed by simple shear (giving place to pure shear at certain stages) in an almost north-south direction on subhorizontal beds. Progressive deformation in the later stage of F₁ folding resulted in gentle upright folding of F₁ axial planes on F₁′ axes slightly oblique to F₁. The F₂ folds, whose average shortening ranges from 20 to 30%, were also formed by buckling caused by horizontal compression in a nearly northwest-southeast direction. This folding was preceded and followed in some instances by homogeneous strain, as deduced fro mthe shapes of the F₂ folds and the nature of variation of the F₁ lineations. The F₃ conjugate structures developed when the maximum compressive strain was vertical and the intermediate compressive strain northwest-southeast, almost normal to the subvertical F₂ axial planes. The increase in the amplitude of the F₂ folds in the last phase of F₂ folding in certain zones resulted in an excess of vertical load, which dissipated with the formation of the F₃ structures. In the last stage of movement (F₄) the maximum compressive strain became horizontal along the strike of F₂ axial planes, whereas the minimum compressive strain was normal to them. The F₄ structures, therefore, point to a longitudinal shortening with reference to large scale F₂ folding.     

Charriage crustal,anatexie et decrochements ductiles dans les Maures orientales (Var,France) au cours de l'orogenese varisque

The kinematics of the deformational events recorded in the catazonal gneisses within the eastern part of the Maures massif (Variscan basement of Provence, southeastern France) has been established. These events can be correlated with both the metamorphic and the magmatic evolution, and the orogenic history of the eastern Maures then consists of the following stages:

1. -HP-metamorphism preserved only in relict eclogites and quartzites rich in calc-silicates,
2. -Catazonal metamorphism involving anatectic melting, broadly contemporaneous with large-scale horizontal transport towards the NNE. During this event, the lithologic units were disrupted and intensively mylonitized.
3. -Intrusion of a first generation of anatectic granitoids.
4. -A second tectonic event under epi-to mesozonal metamorphism conditions is responsible for the northward displacement of the eastern Maures relative to the western part along a sinistral strike-slip fault 4 km wide (Ramatuelle — Plan de la Tour fault) in which the early anatectic granitoids have been mylonitized. Outside the fault zone, this event is marked by upright to W-vergent open folds trending N-S, i.e. parallel to the transport direction.
5. -A moderate cataclastic reactivation of the Ramatuelle — Plan de la Tour fault with a dextral sense of shear, locally accompanied in the eastern part of the area by minor lowtemperature thrusting towards the south.
6. -Intrusion of a second generation of anatectic granites about 320 Ma ago.

The metamorphic, magmatic and tectonic evolution of the eastern Maures suggests a continuous orogenic history in an area of rapid crustal thickening by large-scale thrusting within the continental crust. This evolution may be related to the development of continental subduction during the continent/continent collision responsible for the Variscan orogeny in southern Europe.     

Variscan tectonics in the Iberian Pyrite Belt, South Portuguese Zone

This paper aims to discuss the structural evolution of the Iberian Pyrite Belt during the Variscan Orogeny. It provides new structural data, maps and cross sections from the eastern part of the Iberian Pyrite Belt. Regional geology of the South Portuguese Zone and lithostratigraphy of the Iberian Pyrite Belt are first briefly summarised. Three roughly homoaxial deformation phases are distinguished, and are mainly characterised by south-verging multi-order folds, axial planar cleavages and thrusts. Three structural units are distinguished: the La Puebla de Guzmán and Valverde del Camino antiforms are rooted units related to the propagation of southward-directed thrust systems that may branch onto the lower décollement level of the South Portuguese Zone; El Cerro de Andévalo is a structurally higher unit, mainly composed of allochthonous D1 thrust nappes. No evidence of sinistral transpression has been found in the transected cleavage and the strike of S3 with respect to S2. Better evidence of transpression is the moderately to steeply westerly plunging folds that show S-type asymmetry in down-plunge view. Variscan deformation in the Iberian Pyrite Belt is defined as the combination of a dominant southwards shear and a sinistral E-shear caused by oblique continental collision between the South Portuguese plate and the Iberian Massif.     

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.     

Distinguishing syn-cleavage folds from pre-cleavage folds to which cleavage is virtually axial planar: examples from the Cambrian core of the Lower Palaeozoic Anglo-Brabant Deformation Belt (Belgium)

Within the Cambrian Jodoigne Formation in the easternmost part of the Anglo-Brabant Deformation Belt, sub-horizontal to gently plunging folds occur within the limbs of steeply plunging folds. The latter folds are cogenetic with cleavage and are attributed to the Brabantian deformation event. In contrast, although cleavage is also (1) virtually axial planar to the sub-horizontal to gently plunging higher-order folds, shows (2) a well-developed divergent fanning across these folds, (3) an opposing sense of cleavage refraction on opposite fold limbs, and (4) only very small cleavage transection angles, an analysis of the cleavage/bedding intersection lineation suggests that these higher-order folds have a pre-cleavage origin. On the basis of a comparison of structural and sedimentological features these higher-order folds are interpreted as slump folds. The seemingly ‘normal’ cleavage/fold relationship across the slump folds within the limbs of the large steeply plunging folds is due to the very small angle between cleavage and bedding.As such, a ‘normal’ cleavage/fold relationship is no guarantee for a syn-cleavage fold origin. It is not unlikely that also within undeformed, recumbent slump folds, a well-developed compaction fabric, formed parallel to the axial surface of the slump folds, may show fanning and contrasting senses of cleavage refraction on opposite fold limbs.     

Evaluating magnetic lineations (AMS) in deformed rocks

Magnetic lineation in rocks is given by a cluster of the principal axes of maximum susceptibility (K_max) of the Anisotropy of Magnetic Susceptibility (AMS) tensor. In deformed rocks, magnetic lineations are generally considered to be the result of either bedding and cleavage intersection or they parallel the tectonic extension direction in high strain zones. Our AMS determinations, based on a variety of samples that were taken from mudstones, slates and schists from the Pyrenees and Appalachians, show that strain is not the only factor controlling the development of magnetic lineation. We find that the development and extent to which the magnetic lineation parallels the tectonic extension direction depends on both the original AMS tensor, which in turn depends on the lithology, and the deformation intensity. Rocks having a weak pre-deformational fabric will develop magnetic lineations that more readily will track the tectonic extension.     

Structural studies in the Pre-Vindhyan rocks of Rajasthan: A summary of work of the last three decades

Multiple deformation in all the Precambrian metamorphic-migmatitic rocks has been reported from Rajasthan during the last three decades. But, whereas the Aravalli Group and the Banded Gneissic Complex show similarity in the style and sequence of structures in all their details, the rocks of the Delhi Group trace a partly independent trend. Isoclinal folds of the first generation (AF₁) in the rocks of the Aravalli Group had gentle westerly plunge prior to later deformations. These folds show reclined, inclined, and upright attitude as a result of coaxial upright folding (AF_la). Superposition of upright folds (AF₂) of varying tightness, with axial plane striking N to NNE, has resulted in interference patterns of diverse types in the scale of maps, and deformation of earlier planar and linear structures in the scale of hand specimens. The structures of the third generation (AF₃) are either open recumbent folds or reclined conjugate folds with axial planes dipping gently towards NE or SW. Structures of the last phase are upright conjugate folds (AF₄) with axial planes striking NNE-SSW and E-W. The Banded Gneissic Complex (BGC) underlies the Aravalli Group with a conglomerate horizon at the contact, especially in southern Rajasthan. But, for a major part of central and southern Rajasthan, migmatites representing BGC show a structural style and sequence identical with those in the Aravalli Group. Migmatization, broadly synkinematic with the AF₁ folding, suggests extensive remobilization of the basement. Very rare relict fabric athwart to and overprinted by structures of AF, generation provide tangible evidence for a basement. Although the structures of later phases in the rocks of the Delhi Group (DF₃ and DF₄) match with the late-phase structures in the Aravalli Group (AF₃ and AF₄), there is a contrast in the structural history of the early stages in the rocks of the two groups. The folds of the first generation in the Delhi Group (DF₁) were recumbent to reclined with gentle plunge towards N to NNE or S to SSW. These were followed by coaxial upright folds of varying tightness (DF₂). Absence of westerly trending AF₁ folds in the Delhi Group, and extreme variation in plunge of the AF₂ folds in contrast with the fairly constant plunge of the DF₂ folds, provide evidence for an angular unconformity between the Aravalli and the Delhi Groups. Depending on the importance of flattening attendant with and following buckling during AF₂ deformation, the lineations of AF₁ generation show different patterns. Where the AF₁ lineations are distributed in circular cones around AF₂ axes because of flexural-slip folding in layered rocks with high viscosity contrast, loci of early lineations indicate that the initial orientation of the AF₁ axes were subhorizontal, trending towards N280°. The orientation of the axial planes of the earlier folds has controlled the development of the later folds. In sectors where the AF, axial planes had N-S strike and gentle dips, or E-W strike with gentle to steep dips, nearly E-W horizontal compression during AF₂ deformation resulted in well-developed AF₂ folds. By contrast, where the AF, axial planes were striking nearly N-S with steep dips, E-W horizontal compression resulted in tightening (flattening) of the already isoclinal AF₁ folds, and probably boudinage structures in some instances, without the development of any AF₂ folds. A similar situation obtains when DF₄ deformation is superposed on earlier structures. Where the dominant S-planes were subhorizontal, N-S compression during DF₄ deformation resulted in either chevron folds with E-W striking axial plane or conjugate folds with axial plane striking NE and NW. In zones with S-planes striking E-W and dipping steeply, the N-S compression resulted in flattening of the earlier folds without development of DF₄ folds.     

Crustal thickening and attenuation as revealed by regional fold interference patterns: Ciudad Rodrigo basement area (Salamanca,Spain)

The structure of the Ciudad Rodrigo area (Iberian Massif, Central Iberian Zone) has been revisited in order to integrate new geological data with recent models of the evolution of the Iberian Massif. Detailed mapping of fold structures along with a compilation of field data have been used to constrain the geometry and relative timing of ductile deformation events in this section of the hinterland of the Variscan belt. The structural evolution shows, in the first place, the development of a regional train of overturned folds with associated axial planar foliation (D₁). Towards the lower structural levels, the deflection of the fold limbs and a subhorizontal crenulation cleavage depict the upper structural boundary of a superimposed low angle shear zone (D₂), which extends at least to the deepest parts of the basement exposed in the study area. The amplification and rotation of D₁ folds about a horizontal axis also occurred within this shear zone. The flat-lying character of the D₂ structures accounts for the attenuation of the previously thickened crust, which developed following gravity gradients during thermal re-equilibration. Subsequent deformation led to the formation of two orthogonal sets of upright folds (D₃), representing a new shift between crustal thinning and crustal thickening in the region.     

Basement deformation: tertiary folding and fracturing of the Variscan Bielsa granite (Axial zone,central Pyrenees)

The Bielsa thrust sheet is a south-verging unit of the Axial zone in the central Pyrenees. The Bielsa thrust sheet consists predominantly of a Variscan granite unconformably overlain by a thin cover of Triassic and Cretaceous deposits. During the Eocene–Oligocene, Pyrenean compression, displacement of the Bielsa thrust sheet generated a large-scale south-verging monocline. Low temperature deformation of the Bielsa thrust sheet resulted in the development of: (1) E–W trending, asymmetric folds in the Triassic cover with amplitudes up to 1.5 km; these folds of the cover are related with normal and reverse faults in the granite and with rigid-body block rotations. (2) Pervasive fracturing within the Bielsa granite is also attributed to Pyrenean deformation and is consistent with a NNE to ENE shortening direction; two main, conjugate fault systems are associated with this direction of shortening, as is a subvertical strike-slip system with shallow-plunging slickenside lineations and a moderately dipping fault system with reverse movement; and (3) in addition, we recognise strike-slip and reverse shear bands, associated with sericitisation and brittle deformation of quartz and feldspar in the granite, that enclose Triassic rocks. Basement deformation within the Bielsa thrust sheet can be related to movement of faults developed to accommodate internal deformation of the hanging wall. Several models are proposed to account for this deformation during the southward displacement of the thrust.     

Superposed folding in the Honakere arm of the Chitradurga-Karighatta schist belt in the Dharwar tectonic province,southern India,and its bearing on the Sargur-Dharwar relation

The supracrustal enclave within the Peninsular Gneiss in the Honakere arm of the Chitradurga-Karighatta belt comprises tremolite-chlorite schists within which occur two bands of quartzite coalescing east of Jakkanahalli(12°39′N; 76°41′E), with an amphibolite band in the core. Very tight to isoclinal mesoscopic folds on compositional bands cut across in the hinge zones by an axial planar schistosity, and the nearly orthogonal relation between compositional bands and this schistosity at the termination of the tremolite-chlorite schist band near Javanahalli, points to the presence of a hinge of a large-scale, isoclinal early fold (F₁). That the map pattern, with an NNE-plunging upright antiform and a complementary synform of macroscopic scale, traces folds 'er generation (F ₂),is proved by the varying attitude of both compositional bands (S₀) and axial pranar schistosity (S ₁), which are effectively parallel in a major part of the area. A crenulation cleavage (S ₂) has developed parallel to the axial planes of theF ₂ folds at places. TheF ₂ folds range usually from open to rarely isoclinal style, with theF ₁ andF ₂ axes nearly parallel. Evidence of type 3 fold interference is also provided by the map pattern of a quartzite band in the Borikoppalu area to the north, coupled with younging directions from current bedding andS ₀ -S ₁ inter-relation. Although statistically theF ₁ andF ₂ linear structures have the same orientation, detailed studies of outcrops and hand specimens indicate that the two may make as high an angle as 90°. Usually, in these instances, theF ₁ lineations are unreliable around theF ₂ axes, implying that theF ₂ folding was by flexural slip. In zones with very tight to almost isoclinalF ₂ folding, however, buckling attendant with flattening has caused a spread of theF ₁ lineations almost in a plane. Initial divergence in orientation of theF ₁ lineations due to extreme flattening duringF ₁ folding has also resulted in a variation in the angle between theF ₁ andF ₂lineations in some instances. Upright later folding (F₃) with nearly E-W strike of axial planes has led to warps on schistosity, plunge reversals of theF ₁ andF ₂ axes, and increase in the angle between theF ₁ andF ₂ lineations at some places. Large-scale mapping in the Borikoppalu sector, where the supposed Sargur rocks with ENE ‘trend’ abut against the N-‘trending’ rocks of the Dharwar Supergroup, shows a continuity of rock formations and structures across the hinge of a large-scaleF ₂ fold. This observation renders the notion, that there is an angular unconformity here between the rocks of the Sargur Group and the Dharwar Supergroup, untenable.     

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.     

Late Precambrian series and structures in the Navalpino Variscan Anticline (Central Iberian Peninsula)

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 (D_v1) gave rise to a upright WNW - ESE trending folds on all scales, whereas the second (D_v2) gave rise to a brittle—ductile sinistral strike-slip shear zone tending subparallel to the axial trace of the D_v1 folds.     

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.     

Structural pattern and strain history of a superposed fold system in the Precambrian of Central Rajasthan,India. I. Structural pattern in the ‘main Raialo syncline’, Central Rajasthan

Marble, calc-silicate rock, quartzite and mica schist of Precambrian age in the ‘main Raialo syncline’ in the Udaipur district of central Rajasthan, India, have been affected by folding of four main generations (F₁–F₄), the first two of which are seen in the scale of map to microsection. The very tight to isoclinal F₁ folds with long limbs and thickened hinges are generally reclined or inclined, and plunge gently castward or westward where least reoriented. The axial planes of the F₁ folds have been involved in upright warps on east-west axes (F₁′), nearly coaxial with the F₁ folds, in some sectors. These folds have been overprinted by upright F₂ folding of varying tightness with the axial planes striking north to northeast, resulting in interference patterns of different types in all scales. A penetrative axial plane foliation related to F₁ folding and a crenulation cleavage parallel to the F₂ axial pianes are seen in the micaceous rocks. Two sets of conjugate folds and kink bands of smail scale have been superimposed on the F₁–F₂ folds in thinly foliated rocks. The first of these sets (F₃) has its conjugate axial planes dipping gently northeast and southwest, whereas the paired axial planes of the later set (F₄) are vertical with north-northwest and east-west strikes.     

Singhbhum Shear Zone: Structural transition and a kinematic model

The northern fold belt away from the Singhbhum Shear Zone displays a set of folds on bedding. The folds are sub-horizontal with E-W to ESE striking steep axial surfaces. In contrast, the folds in the Singhbhum Shear Zone developed on a mylonitic foliation and have a reclined geometry with northerly trending axes. There is a transitional zone between the two, where the bedding and the cleavage have become parallel by isoclinal folding and two sets of reclined folds have developed by deforming the bedding—parallel cleavage. Southward from the northern fold belt the intensity of deformation increases, the folds become tightened and overturned towards the south while the fold hinges are rotated from the sub-horizontal position to a down-dip attitude. Recognition of the transitional zone and the identification of the overlapping character of deformation in the shear zone and the northern belt enable the formulation of a bulk kinematic model for the area as a whole.