Significance of contrast in the early stages of the structural history of the Delhi and the pre-Delhi rock groups in the Proterozoic of Rajasthan, western India

Structures of four generations are decipherable both in the pre-Delhi rocks of central Rajasthan, and in the Delhi rocks of Khetri in northeastern Rajasthan and around Todgarh in central Rajasthan. There is a remarkable identity in the later phases of the deformational history of the two groups, with gravity-induced structures followed by conjugate folds due to longitudinal shortening (N-S in northeastern Rajasthan and NE-SW in central Rajasthan). The earlier stages of the structural history of the two groups are, however, significantly different. The E-W-trending reclined folds of the first generation in the pre-Delhi rocks are absent in the Delhi rocks throughout Rajasthan. The NNE- to NE-trending folds of the second generation in the pre-Delhi groups are upright, whereas these structures in the Delhi rocks are of two phases—recumbent folds, followed by coaxial upright folds. The folds of the first and the second phases in the Delhi rocks plunge gently NE or SW where they are not affected by subsequent deformations. But the NE-trending folds in the pre-Delhi rocks show an extreme variation in axial plunge from horizontal to vertical, even where they are unaffected by later movements. Evidence has been adduced to suggest that these differences in the earlier phases of the structural evolution of the two groups are due to an angular unconformity between the Delhi and the pre-Delhi rocks.     

Deformational and crystallization history of the Precambrian rocks in north-central Aravalli Mountain,Rajasthan, India

Large-scale structures, textures and mineral assemblages in the Precambrian rocks of the Banded Gneissic Complex and the overlying Delhi Group in north-central Aravalli Mountain reveal a complex deformational-crystallization history. In the basement Gneissic Complex at least three deformational events, D₀, D₁ and D₂, and two separate episodes of metamorphism, M₁ and M₂, are recognized. The supracrustal Delhi Rocks display only two phases of deformation, D₁ and D₂, associated with a single protracted period of metamorphism, M₂.The first phase of deformation (D₁) of the Delhi orogeny (1650-900 m.y.) produced large isoclinal folds that are overturned towards the southeast and have gentle plunges in NE and SW directions. The second phase of deformation (D₂) gave rise to tight open folds on the limbs and axial-plane surfaces of the D₁ folds. These folds generally plunge towards the N and NNW at 30°–80°. In the Basement Complex one more deformation (D₀) of the Pre-Delhi orogeny (> 2000 m.y.) is recorded by the presence of reclined and recumbent folds with W to WNW trending fold axes. The D₀ folds were superimposed by D₁ and D₂ folds during the Delhi orogeny.The three deformational events have been correlated with the crystallization periods of minerals in the rocks and a setting in time is established for this part of the Aravalli range.     

Inversion of fold-hinge in superposed folding: An example from the Precambrian of central Rajasthan,India

Axial culminations and depressions of folds are common in regions of superposed deformations involving two sets of folds at high angles to each other. If the intensity of the later folding in these cases exceeds a particular limit, plunge reversal of the early folds gives way to “plunge inversion”. In such instances, segments of early folds rotate through end-on or reclined geometry while being refolded. And instead of plunge reversal at the hinge zones of later folds, the early folds plunge in the same direction in both limbs of the later folds. As a result, an antiform will pass along the axial trend to a synform. A particularly clear instance of plunge inversion has been noted from the “Sawar outlier” comprising a metasedimentary sequence within the older Banded Gneissic Complex in central Rajasthan. In Sawar, the southern segment of a south-southwest-trending synformal early fold has been inverted to attain an antiformal geometry because of superposition of a later fold at high angles to the early fold axes and axial planes. The deformation history of the large-scale folds has been traced and the stratigraphic implications of the plunge inversion discussed. From the movement pattern, it seems justifiable to correlate the metasedimentary sequence of the outlier with the Late Precambrian Delhi Group of parametamorphic rocks.     

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.     

Influence of differential basement mobility on contrasting structural styles in the cover rocks: An example from Early Precambrian rocks east of Udaipur,Rajasthan

Detailed structural and lithological mapping of the Aravalli rocks overlying the Mewar Gneiss in the area east of Udaipur, Rajasthan, suggests presence of blocks bounded by faults, showing a contrasting structural pattern. The contrast is reflected in the differential development and in the orientation of AF1, AF2 and AF4 folds in different blocks. In the central Umra block, the rocks constitute a virtually homoclinal sequence showing one dominant orientation of bedding and axial planar schistosity. Fold axes, lineations andβ orientations indicate presence of reclined folds of AF1 generation. AF2 folds are either absent or have developed only locally. The two other blocks which border the Umra block show development of large AF2 synforms and local minor antiforms having N-S or NNE-SSW trend. The folds interfere with AF4 folds producing irregular domes and basins in the western Kanpur-Kalarwas Block and minor plunge reversals in Bagdara-Dhamdhar Block. It is argued that the constituents of the different blocks which formed a collage of rift basins and horsts during sedimentation, responded differentially to deforming forces because of differential mobility of the underlying basement.     

Transpressional regime in southern Arabian Shield: Insights from Wadi Yiba Area, Saudi Arabia

Detailed field-structural mapping of Neoproterozoic basement rocks exposed in the Wadi Yiba area, southern Arabian Shield, Saudi Arabia illustrates an important episode of late Neoproterozoic transpression in the southern part of the Arabian-Nubian Shield (ANS). This area is dominated by five main basement lithologies: gneisses, metavolcanics, Ablah Group (meta-clastic and marble units) and syn- and post-tectonic granitoids. These rocks were affected by three phases of deformation (D₁–D₃). D₁ formed tight to isoclinal and intrafolial folds (F₁), penetrative foliation (S₁), and mineral lineation (L₁), which resulted from early E-W (to ENE-WSW) shortening. D₂ deformation overprinted D₁ structures and was dominated by transpression and top-to-the-W (?WSW) thrusting as shortening progressed. Stretching lineation trajectories, S-C foliations, asymmetric shear fabrics and related mylonitic foliation, and flat-ramp and duplex geometries further indicate the inferred transport direction. The N- to NNW-orientation of both “in-sequence piggy-back thrusts” and axial planes of minor and major F₂ thrust-related overturned folds also indicates the same D₂ compressional stress trajectories. The Wadi Yiba Shear Zone (WYSZ) formed during D₂ deformation. It is one of several N-S trending brittle-ductile Late Neoproterozoic shear zones in the southern part of the ANS. Shear sense indicators reveal that shearing during D₂ regional-scale transpression was dextral and is consistent with the mega-scale sigmoidal patterns recognized on Landsat images. The shearing led to the formation of the WYSZ and consequent F₂ shear zone-related folds, as well as other unmappable shear zones in the deformed rocks. Emplacement of the syn-tectonic granitoids is likely to have occurred during D₂ transpression and occupied space created during thrust propagation. D₁ and D₂ structures are locally overprinted by mesoscopic- to macroscopic-scale D₃ structures (F₃ folds, and L₃ crenulation lineations and kink bands). F₃ folds are frequently open and have steep to subvertical axial planes and axes that plunge ENE to ESE. This deformation may reflect progressive convergence between East and West Gondwana.     

Characteristics of pegmatoidal granite exposed near Bayalan,Ajmer district,Rajasthan

The study involves the characterization of pegmatoidal granite, southeast of Beawar, Ajmer district, Rajasthan. Earlier researchers had described this granite as part of the BGC, basement to the Bhim Group of the Delhi Super Group rocks. However, the present study indicates that it is younger than the rocks of Bhim Group of South Delhi Fold Belt, into which it is intrusive. The intrusion is structurally controlled and the outcrop pattern is phacolithic. The granite had intruded post-D₂ deformation of the Delhi orogeny along the axial planes of D₂ folds. The intrusion has also resulted in the formation of a contact aureole about the calc gneisses.     

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.     

Die tektonische Baugeschichte von Süd-Devonshire als Beispiel einer mehrphasigen variszischen Prägung

Five movement-phases during the Variscan tectogenesis shaped the structural cast of the Devonian rocks in South Devon. The first movement-phase, i. e. the main tectonic phase, resulted in the mappable fold system trending E-W or ENE-WSW in the west accompanied by related minor folds, and the first cleavage (s ₁) parallel to the axial planes. The cleavage planes dip to the south as far as the line Slapton-Bigbury, while farther to the south they dip to the north and finally to the south again, thus forming a huge fanning. In the second movement-phase a second cleavage (s ₂) with E-W strike associated with minor folds, was superimposed on the older structures south of the line Berry Head-Cornworthy. The trend of these folds is, more or less E-W. Furthermore the second cleavage shows a fanning which does not coincide with that of the first cleavage. South-vergent minor folds of bedding and cleavage planes, associated with small-scale southward thrusts represent structures of a third movement-phase. The fourth movement-phase was the kinking of mainly the first cleavage in the southern part of South Devon. There are two groups: one shows flat-lying kink-bands affecting mainlys ₁; seldoms ₂, and having a southward thrusting sense of movement while the other consists of nearly vertical kink-bands trending N-S which displace to the south on their eastern sides. Subsequently with the commencement of the New Red deposition, repeated tectonic stretching took place, resulting in N-S and WSW--ENE trending faults. The succession of the different tectonic events led to occasionally very complicated superimpositions. The rocks in the middle and southern part of South Devon suffered a regional metamorphosis that increases slowly towards the south. Finally, the tectonic structures of South Devon are compared with those in South Cornwall where the same movements-phases caused a completely different structural style.     

Sedimentational,structural and migmatitic history of the Archaean Dharwar tectonic province,southern India

The earliest decipherable record of the Dharwar tectonic province is left in the 3.3 Ga old gneissic pebbles in some conglomerates of the Dharwar Group, in addition to the 3.3–3.4 Ga old gneisses in some areas. A sialic crust as the basement for Dharwar sedimentation is also indicated by the presence of quartz schists and quartzites throughout the Dharwar succession. Clean quartzites and orthoquartzite-carbonate association in the lower part of the Dharwar sequence point to relatively stable platform and shelf conditions. This is succeeded by sedimentation in a rapidly subsiding trough as indicated by the turbidite-volcanic rock association. Although conglomerates in some places point to an erosional surface at the contact between the gneisses and the Dharwar supracrustal rocks, extensive remobilization of the basement during the deformation of the cover rocks has largely blurred this interface. This has also resulted in accordant style and sequence of structures in the basement and cover rocks in a major part of the Dharwar tectonic province. Isoclinal folds with attendant axial planar schistosity, coaxial open folds, followed in turn by non-coaxial upright folds on axial planes striking nearly N-S, are decipherable both in the “basement” gneisses and the schistose cover rocks. The imprint of this sequence of superposed deformation is registered in some of the charnockitic terranes also, particularly in the Biligirirangan Hills, Shivasamudram and Arakalgud areas. The Closepet Granite, with alignment of feldspar megacrysts parallel to the axial planes of the latest folds in the adjacent schistose rocks, together with discrete veins of Closepet Granite affinity emplaced parallel to the axial planes of late folds in the Peninsular Gneiss enclaves, suggest that this granite is late-tectonic with reference to the last deformation in the Dharwar tectonic province. Enclaves of tonalite and migmatized amphibolite a few metres across, with a fabric athwart to and overprinted by the earliest structures traceable in the supracrustal rocks as well as in a major part of the Peninsular Gneiss, point to at least one deformation, an episode of migmatization and one metamorphic event preceding the first folding in the Dharwar sequence. This record of pre-Dharwar deformation and metamorphism is corroborated also by the pebbles of gneisses and schists in the conglomerates of the Dharwar Group. Volcanic rocks within the Dharwar succession as well as some of the components of the Peninsular Gneiss give ages of about 3.0 Ga. A still younger age of about 2.6 Ga is recorded in some volcanic rocks of the Dharwar sequence, a part of the Peninsular Gneiss, Closepet Granite and some charnockites. These, together with the 3.3 Ga old gneisses and 3.4 Ga old ages of zircons in some charnockites, furnish evidence for three major thermal events during the 700 million year history of the Archaean Dharwar tectonic province.     

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.     

Deformation style in the Granulite-charnockite province of South India: An example from the Kusulmalai area in Salem district,Tamil Nadu,India

A multiple-deformation sequence is established for different types of gneisses, mafic-paleosomes and banded magnetite quartzites (BMQ) exposed within the area. In gneisses, the basin-shaped map pattern represents the type-i interference structure formed due to the overprinting of F₃ folds with ENE striking axial planes on F₂ folds with axial planes striking NNW. The BMQ band occurring as an enclave within the gneissic country, represents a large scale F₁ fold with relatively smaller scale F₂ folds developed on its limbs. Mafic-paleosomes within the streaky-charnockitic-gneisses exhibit structures formed due to the interference of more than two phases of folding (F₁,F_la,F₂,F₃). It is shown that the deformation plan in these rocks is consistent with the generalized deformation scheme for Granite-greenstone belts. The difference in the map pattern of Granite-greenstone belts and Granulite-charnockite terrains is ascribed to the variance in Theological properties, layerthickness ratios and local displacement directions during different phases of folding. These differences apart, both the Granite-greenstone and Granulite-charnockite provinces in South India are deformed by an early isoclinal folding which is successively overprinted by folding on NNW and ENE striking axial planes.     

Tectonic Evolution of the Southern Part of Aravalli Mountain Belt and its Environs: Possible Causes and Time Constraints

Structural studies on Proterozoic rocks belonging to the Lunavada Group, Southern Aravalli Mountain Belt (SAMB), India, have shown that they underwent three episodes of deformation which have led to the formation of various regional scale interference patterns. Whilst the northern parts of the SAMB underwent brittle-ductile deformation, the southern portion underwent more ductile deformation. On the basis of structural as well as metamorphic studies it has been established earlier that the region was subjected to uplift orogenesis during its evolutionary history. In the present paper an attempt is made to visualize the possible causes that led to deformation of the SAMB, the structural geology of which has been established by the authors, and to constraint the timing of these events on the basis of already available geochronological data. A “working-hypothesis” is proposed according to which it is suggested that deformation of the SAMB is a result of the accretion of the three protocontinents viz. Aravalli, Dharwar and Singhbhum during the Mesoproterozoic. It is envisaged that the accretion of Aravalli and Singhbhum Protocontinents occurred between 1600 and 1400 Ma along the NE-SW trending Son Suture and this event led to development of NE-SW trending structures in the SAMB. Suturing of Aravalli and Dharwar Protocontinents between 1400 and 935 Ma along the E-W Narmada Suture was responsible for the E-W to NW-SE trending D₃ structures of the SAMB. It is postulated that the Satpura orogeny which resulted in deformation of rocks in Satpura mountain range lying to the south of Narmada Suture was coeval with the accretion of Aravalli and Dharwar Protocontinents.     

Structural pattern in the Precambrian rocks of Sonua-Lotapahar region,North Singhbhum,eastern India

In the western part of the North Singhbhum fold belt near Lotapahar and Sonua the remobilized basement block of Chakradharpur Gneiss is overlain by a metasedimentary assemblage consisting of quartz arenite, conglomerate, slate-phyllite, greywacke with volcanogenic material, volcaniclastic rocks and chert. The rock assemblage suggests an association of volcanism, turbidite deposition and debris flow in the basin. The grade of metamorphism is very low, the common metamorphic minerals being muscovite, chlorite, biotite and stilpnomelane. Three phases of deformation have affected the rocks. The principal D1 structure is a penetrative planar fabric, parallel to or at low angle to bedding. No D1 major fold is observed and the regional importance of this deformation is uncertain. The D2 deformation has given rise to a number of northerly plunging major folds on E-W axial planes. These have nearly reclined geometry and theL ₂lineation is mostly downdip on theS ₂surface, though some variation in pitch is observed. The morphology of D2 planar fabric varies from slaty cleavage/schistosity to crenulation cleavage and solution cleavage. D3 deformation is weak and has given rise to puckers and broad warps on schistosity and bedding. The D2 major folds south of Lotapahar are second order folds in the core of the Ongarbira syncline whose easterly closure is exposed east of the mapped area. Photogeological study suggests that the easterly and westerly closing folds together form a large synclinal sheath fold. There is a continuity of structures from north to south and no mylonite belt is present, though there is attenuation and disruption along the fold limbs. Therefore, the Singhbhum shear zone cannot be extended westwards in the present area. There is no evidence that in this area a discontinuity surface separates two orogenic belts of Archaean and Proterozoic age.     

Early Precambrian stratigraphy of central and southern Rajasthan,India

Structural, stratigraphic and petrologic studies between Amet and Sembal in the Udaipur district of southcentral Rajasthan indicate that all the rocks belonging to the Banded Gneissic Complex, the Aravalli Group and the Raialo Formation have been involved in isoclinal folding on a westerly trend, co-axial refolding, and upright folding on a north to north-northeast trend. There is neither an unconformity nor an overlap between the Aravallis and the Raialos. The conglomerates supposed to mark the erosional unconformity above the Banded Gneissic Complex near Rajnagar is a tectonic mélange of folded and torn quartz veins in mica schist within the Aravalli Group. The Aravalli—Raialo metasediments have been migmatized synkinematically with the first folding to give rise to the Banded Gneissic Complex; the gneissic complex does not have any separate stratigraphic entity. By contrast, there is an undoubted erosional unconformity between the type Aravalli rocks and the underlying Sarara granite to the south. These relations, coupled with the continuity of the Aravalli rocks of Udaipur northward to the metasedimentary rocks of the Sembal—Amet area along the strike, and a comparable structural history, point to granitic rocks of at least two generations in the Early Precambrian of central and southern Rajasthan. Preliminary radiometric dating of rocks of known stratigraphic—structural relationship seems to confirm the presence of granitic rocks of two ages in the Early Precambrian, and of a considerable interval between the deposition of the Aravalli—Raialo rocks and the Delhi rocks. The Udaipur granite, post-dating the first deformation but preceding the upright folding on the northerly trend, provides evidence for granitic activity of a third phase before the deposition of rocks of the Delhi Group.     

Structural geometry of the early Precambrian terrane south of Coimbatore in the “Palghat Gap”, southern India

The ENE-plunging macroscopic folds, traced by calc gneiss interbanded with marble and sillimanite schist within the Peninsular Gneiss around Suganapuram in the ‘Palghat gap’ in southern India, represent structures of the second generation (D₂). They have folded the axial planes of a set of D₁ isoclinal folds on stratification coaxially, so that the mesoscopic D₁ folds range from reclined in the hinge zones, through inclined to upright in the limb zones of the D₂ folds. Orthogonal relation between stratification and axial planar cleavage, and ‘M’ shaped folds on layering locate the hinge zones of the D₁ folds, whereas folds on axial planar cleavage with ‘M’ shaped folds are the sites of the D₂ fold hinges. Extreme variation in the shapes of the isoclinal D₁ folds from class 1B through class 1C to nearly class 2 of Ramsay is a consequence of buckling followed by flattening on layers of widely varying viscosity contrast. The large ENE-trending structures in this supracrustal belt within the Peninsular Gneiss in the ‘Palghat gap’ could not have evolved by reorientation of NS-trending structures of the Dharwar tectonic province to the north by movement along the Moyar-Bhavani shear zone which marks the boundary between the two provinces. This is because the Moyar and Bhavani faults are steep dipping reverse faults with dominant dip-slip component. Deceased     

The detection and significance of early deformation in the southern Variscan Pyrenees,Spain; implications for regional Paleozoic structural evolution

Detailed structural analysis of part of the Variscan southcentral Pyrenees revealed the occurrence of several deformation generations, of which the most important one, called the mainphase folding and striking WNW-ESE, seems to be the oldest. Directional analysis of structural elements related to mainphase folding (sedimentary bedding, mainphase cleavage, small-scale foldaxes and intersection lineations) shows, however, that sedimentary bedding must have been non-planar before mainphase deformation took place. This observation suggests that premainphase folding occurred as well, and indeed the areal distribution of intersection lineations in the studied area demonstrates the existence of two early Variscan fold systems. They are characterized by very open NNW-SSE and WSW-ENE folds and have subvertical axial planes and subhorizontal foldaxes. In strong contrast to mainphase folds, penetrative axial plane foliations did not develop during deformation. Pre-mainphase folds in varying orientations have been reported from many other areas in the central Variscan Pyrenees, but a reinterpretation of existing maps and other data shows that also in these cases two pre-mainphase deformation generations must be present, rather than just one as suggested in most previous work. Again, the interference pattern of the two fold systems as well as field evidence indicates that axial planes are steep and strike approximately N-S and E-W, but locally strong reorientation due to Alpine deformation (mainly thrusting) has taken place. The significance of pre-mainphase folding in the Variscan Pyrenees is discussed in the light of an overall dynamic/ kinematic model involving alternating convergent and divergent right-lateral oblique-slip movements along the north-eastern boundary of the Iberian (micro-)plate. The occurrence of pre-mainphase folds is related to

1. the transition from divergent to convergent obliqueslip movement (NNW-SSE folds), and
2. initial oblique convergence of the Iberian and European plates (WSW-ENE folds) prior to mainphase collision.

     

East-west mafic dyke in Aravallis – a post-depositional event deciphered by aeromagnetic studies

Aravalli fold belt has witnessed major tectonism resulting in intense deformation and associated mafic magmatism. Recently acquired high resolution aeromagnetic data over central Aravalli fold belt brought out a conspicuous E-W trending magnetic anomaly extending for more than 35 km in length cutting across the whole succession of Aravalli Supergroup. This anomaly on ground is manifested as undeformed basic dyke intruding into metasediments of Aravalli Supergroup. The E-W trend and undeformed nature of these dykes suggest that they represent post Aravalli mafic magmatism which are emplaced sympathetic to the axial plane of F₃ folding.     

Geology,geochemistry and geochronology of the Archaean Peninsular Gneiss around Gorur,Hassan District,Karnataka, India

The Peninsular Gneiss around Gorur in the Dharwar craton, reported to be one of the oldest gneisses, shows nealy E-W striking gneissosity parallel to the axial planes of a set of isoclinal folds (DhF₁). These have been over printed by near-coaxial open folding (DhF₁₂) and non-coaxial upright folding on almost N-S trend (DhF₂). This structural sequence is remarkably similar to that in the Holenarasipur schist belt bordering the gneisses as well as in the surpracrustal enclaves within the gneisses, suggesting that the Peninsular Gneiss has evolved by migmatization synkinematically with DhF₁ deformation. The Gorur gneisses are high silica, low alumina trondhjemites enriched in REE (up to 100 times chondrite), with less fractionated REE patterns (Ce_N/Yb_N < 7) and consistently negative Eu anomalies (Eu/Eu* = 0.5 to 0.7). A whole rock Rb-Sr isochron of eight trondhjemitic gneisses sampled from two adjacent quarries yields an age of 3204 ± 30 Ma with Sr _i of 0.7011 ± 6 (2σ). These are marginally different from the results of Beckinsale and coworkers (3315 ± 54 Ma, Sr _i = 0.7006 ± 3) based on a much wider sampling. Our results indicate that the precursors of Gorur gneisses had a short crustal residence history of less than a 100 Ma.     

The thermotectonic evolution of a Proterozoic,low pressure,granulite dome,West Uusimaa,SW Finland

Three fold generations have been recognized in Svecofennian rocks (±1,800 Ma) from West Uusimaa, SW Finland. The first one (F1) might be related to thrusting and imbrication tectonics at plate collision contacts. The main generation (F2) is due to a N-S horizontal crustal shortening, which created at first E-W trending upright folds in the whole region and later tightened these F2 folds in the western part of the belt, whereas conjugate shear zones and tectonic lenses of competent rock bodies developed in the eastern part. Simultaneously the metamorphic conditions rose from amphibolite- to granulite-facies in this eastern part, which is known as the West Uusimaa Complex. The amphibolite- to granulite-facies transition zone along the western boundary of the granulite-facies complex is studied in detail. A number of prograde mineral reactions are telescoped in this transition zone: the breakdown of biotite and amphibole to ortho- ±clino-pyroxene in metaigneous rocks, the appearance of garnet in cordierite-bearing metapelites and the appearance of scapolite in calcareous rocks. Distinct mineralogical changes also occur in this zone which cross cuts all major structures and rock units and are only affected by late-F3 folding (open, disharmonic folds with approximately N-S trending axial planes) and young shear zones, associated with pseudotachylite generation. The absence of any evidence of block faulting and tilting of the crust that could be associated with the granulite complex suggests that the whole region represents one crustal level. A fluid-inclusion study indicates similar pressures for the amphibolite facies and the granulite facies domains. Application of various independent geothermobarometric methods suggest a low pressure (3–5 K bar) and a temperature increase from 550–650° C to 700–825° C, associated with a decreasing water activity (0.12O<0.4) and a general increasing CO₂ activity. Fluid inclusions strongly suggest an isobaric amphibolite/granulite transition. There-fore the granulite-facies complex is designated a thermal dome. Whole rock chemical data show that granulite-facies metamorphism is isochemical. Constraints for the Svecokarelian crustal evolution are discussed.