Planar, non-planar and refolded sheath folds in the Phulad Shear Zone, Rajasthan, India

Progressive ductile shearing in the Phulad Shear Zone of Rajasthan, India has produced a complex history of folding, with development of planar, non-planar and refolded sheath folds. There are three generations of reclined folds, F₁, F₂ and F₃, with a striping lineation (L₁) parallel to the hinge lines of F₁. The planar sheath folds of F₁ have long subparallel hinge lines at the flanks joining up in hairpin curves at relatively small apices. L₁ swerves harmoniously with the curving of F₁ hinge line. There is a strong down-dip mineral lineation parallel to the striping lineation in most places, but intersecting it at apices of first generation sheath folds. Both the striping and the mineral lineation are deformed in U-patterns over the hinges of reclined F₂ and F₃. Folding of axial surfaces and hinge lines of earlier reclined folds by later folds was accompanied by very large stretching and led to the development of non-planar sheaths. The reclined folds of all the three generations were deformed by a group of subhorizontal folds. Each generation of fold initially grew with the hinge line at a very low angle with the Y-axis of bulk non-coaxial strain and was subsequently rotated towards the down-dip direction of maximum stretching. The patterns of deformed lineations indicate that the stretching along the X-direction was extremely large, much in excess of 6000 percent.     

Transpressional deformation,strain partitioning and fold superimposition in the southern Chinese Altai,Central Asian Orogenic Belt

Transpressional deformation has played an important role in the late Paleozoic evolution of the western Central Asian Orogenic Belt (CAOB), and understanding the structural evolution of such transpressional zones is crucial for tectonic reconstructions. Here we focus on the transpressional Irtysh Shear Zone with an aim at understanding amalgamation processes between the Chinese Altai and the West/East Junggar. We mapped macroscopic fold structures in the southern Chinese Altai and analyzed their relationships with the development of the adjacent Irtysh Shear Zone. Structural observations from these macroscopic folds show evidence for four generations of folding and associated fabrics. The earlier fabric (S₁), is locally recognized in low strain areas, and is commonly isoclinally folded by F₂ folds that have an axial plane orientation parallel to the dominant fabric (S₂). S₂ is associated with a shallowly plunging stretching lineation (L₂), and defines ∼NW-SE tight-close upright macroscopic folds (F₃) with the doubly plunging geometry. F₃ folds are superimposed by ∼NNW-SSE gentle F₄ folds. The F₃ and F₄ folds are kinematically compatible with sinistral transpressional deformation along the Irtysh Shear Zone and may represent strain partitioning during deformation. The sub-parallelism of F₃ fold axis with the Irtysh Shear Zone may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation (F₃) in fold zones. The strain partitioning may have become less efficient in the later stage of transpressional deformation, so that a fraction of transcurrent components was partitioned into F₄ folds.     

Deformation history of the Paleoproterozoic Liaohe assemblage in the eastern block of the North China Craton

The Paleoproterozoic Liaohe assemblage and associated Liaoji granitoids represent the youngest basement in the Eastern Block of the North China Craton. Various structural elements and metamorphic reaction relations indicate that the Liaohe assemblage has experienced three distinct deformational events (D₁ to D₃) and four episodes of metamorphism (M₁ to M₄). The earliest greenschist facies event (M₁) is recognized in undeformed or weakly deformed domains wrapped by the S₁ schistosity, suggesting that M₁ occurred before D₁. The D₁ deformation produced small, mostly meter-scale, isoclinal and recumbent folds (F₁), an associated penetrative axial planar schistosity (S₁), a mineral stretching lineation (L₁) and regional-scale ductile shear zones. Concurrent with D₁ was M₂ metamorphism, which occurred before D₂ and produced low- to medium-pressure amphibolite facies assemblages. Regionally divergent motion senses reflected by the asymmetric F₁ folds and other sense-of-shear indicators, together with the radial distribution of the L₁ lineation surrounding the Liaoji granitoids, imply that D₁ represents an extensional event. The D₂ deformation produced open to tight F₂ folds of varying scales, S₂ axial crenulation cleavages and ENE-NE-striking thrust faults, involving broadly NW–SE compression. Following D₂ was M₃ metamorphism that led to the formation of sillimanite and cordierite in low-pressure type rocks and kyanite in medium-pressure rocks. The last deformational event (D₃) formed NW-WNW-trending folds (F₃), axial planar kink bands, spaced cleavages (S₃), and strike–slip and thrust faults, which deflect the earlier D₁ and D₂ structures. D₃ occurred at a shallow crustal level and was associated with, or followed by, a greenschist facies retrograde metamorphic event (M₄).The Liaohe assemblage and associated Liaoji granitoids are considered to have formed in a Paleoproterozoic rift, the late spreading of which led to the occurrence of the early extensional deformation (D₁) and the M₁ and M₂ metamorphism, and the final closing of which was associated with the D₂ and D₃ phases of deformation and M₃ and M₄ metamorphism.     

Patterns of deformed early lineations over later folds formed by buckling and flattening

The patterns of deformed early lineations (L₁) over later folds (F₂) can be classified into several morphological types depending on the nature of variation of L₁ F₂ over the folds. The field relations indicate that the folds under consideration are neither shear folds nor parallel folds modified by flattening. The lineation patterns are therefore interpreted in terms of an empirical model of simultaneous buckling and flattening in which it is assumed that (i) the central surface of the folded layer remains a sine curve in transverse profile, (ii) the ratio of rates of buckle shortening to homogeneous strain is proportional to sin 2a, with a as the dip angle and (iii) the progressive deformation is coaxial with the Z-axis of bulk strain parallel to the planar segments of the early folds. The model gives an insight into the relative importance of different physical factors which control the development of dissimilar lineation patterns. Not all lineation patterns are explicable by this simplified model. Thus complex patterns with variable L₁ F₂ along the fold axis may develop by a progressive rotation of the geometrically defined fold hinge through successive material lines. The theoretical results have been applied to interpret the lineation patterns in Central Rajasthan, India. It is concluded that L₁ was initially very close to the E-ESE trending subhorizontal Z-axis of bulk deformation during F₂-folding and that the X-axis was subhorizontal or gently plunging with a N-NNE trend.     

构造置换及其控矿规律——以吉林板石沟铁矿为例

 强烈的塑性变形使华北地台东北部太古宙鞍山群中的吉林板石沟铁矿发生强烈的构造置换;造成原始仅有二三层的铁矿褶皱重复,在X(包络线)、Y(枢纽线)方向均被拉断,形成透镜状的复式褶皱勾状体。现有的19个矿组均为这种复式褶皱的转折端,并多呈"Z"型不对称形式。根据以上控矿规律本文提出两个找矿方向,一是包络线方向,另一是枢纽线方向,对1、3矿组具体地做了勘探设计。目前本文的勘探设计已得到勘探验证,新增铁矿储量数千万吨。     

The structure of the Palaeozoic schists in the Southern Menderes Massif,western Turkey: A new approach to the origin of the Main menderes metamorphism and its relation to the Lycian Nappes

The Early Eocene to Early Oligocene tectonic history of the Menderes Massif involves a major regional Barrovian-type metamorphism (M₁, Main Menderes Metamorphism, MMM), present only in the Palaeozoic-Cenozoic metasediments (the so-called “cover” of the massif), which reached upper amphibolite faciès with local anatectic melting at structurally lower levels of the cover rocks and gradually decreased southwards to greenschist facies at structurally higher levels. It is not present in the augen gneisses (the so called “core” of the massif), which are interpreted as a peraluminous granite deformed within a Tertiary extensional shear zone, and lie structurally below the metasediments. A pronounced regional (S₁) foliation and approximately N-S trending mineral lineation (L₁) associated with first-order folding (F₁) were produced during D₁ deformation coeval with the MMM. The S₁ foliation was later refolded during D₂ by approximately WNW-ESE trending F₂ folds associated with S₂ crenulation cleavage. It is now commonly believed that the MMM is the product of latest Palaeogene collision across Neo-Tethys and the consequent internal imbrication of the Menderes Massif area within a broad zone along the base of the Lycian Nappes during the Early Eocene-Early Oligocene time interval. However, the meso- and micro-structures produced during D₁ deformation, the asymmetry and change in the intensity and geometry of the F₂ folds towards the Lycian thrust front all indicate an unambiguous non-coaxial deformation and a shear sense of upper levels moving north. This shear sense is incompatible with a long-standing assumption that the Lycian Nappes were transported southwards over the massif causing its metamorphism. It is suggested here that the MMM results from burial related to the initial collision across the Neo-Tethys and Tefenni nappe emplacement, whereas associated D₁ deformation and later D₂ deformation are probably related to the northward backthrusting of the Lycian nappes.     

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.     

Contrasting pressure–temperature–deformation history across a vestigial craton–mobile belt boundary: the western margin of the Eastern Ghats Belt at Deobhog, India

At Deobhog, migmatitic gneisses and granulites of the Eastern Ghats Belt are juxtaposed against a cratonic ensemble of banded augen gneiss, amphibolite and calcsilicate gneiss, intruded by late hornblende granite and dolerite. In the migmatitic gneiss unit, early isoclinal folds (syn‐D_1M and D_2M) are reoriented along N–S‐trending and E‐dipping shear planes (S_3M), with (S_1M–S_3M) intersection lineations having steep to moderate plunges. The near‐peak P–T condition was syn‐D_3M (≥900 °C, 9.5 kbar), as inferred from syn‐D_3M Grt+Opx‐bearing leucosomes in mafic granulites, and from thermobarometry on Grt (corona)–Opx/Cpx–Pl–Qtz assemblages. The P–T values are consistent with the occurrence of Opx–Spr–Crd assemblages in spatially associated high‐Mg–Al pelites. A subsequent period of cooling followed by isothermal decompression (800–850 °C, c. 7 kbar) is documented by the formation of coronal garnet and its decomposition to Opx+Pl symplectites in mafic granulites. Hydrous fluid infiltration accompanying the retrograde changes is manifested in biotite replacing Opx in some lithologies. The cratonic banded gneiss–granite unit also documents two phases of isoclinal folding (D_1B & D_2B), with the L_2B lineation girdle different from the lineation spread in the migmatitic gneiss unit. Calcsilicate gneiss (Hbl–Pl–Cpx–Scap–Cal) and amphibolite (Hbl–Pl±Grt±Cpx) within banded gneisses record syn‐D_2B peak metamorphic conditions (c. 700 °C, 6.5 kbar), followed by cooling (to c. 500 °C) manifested in the stabilization of coronal clinozoisite–epidote. The D_3B shear deformation post‐dates granite and dolerite intrusions and is characterized by top‐to‐the‐west movement along N–S‐trending, E‐dipping shear planes. Deformation mechanisms of quartz and feldspar in granites and banded gneisses and amphibole–plagioclase thermometry within shear bands in dolerites document an inverted syn‐D_3B thermal gradient with temperature increasing from 350 to 550 °C in the west to ≥700 °C near the contact with the migmatitic gneiss unit. The thermal gradient is reflected in the stabilization of chlorite after hornblende in S_3B shears to the west, and post‐D_2B neosome segregation along D_3B folds and shears to the east. The contrasting lithologies, early structures and peak metamorphic conditions in the two units indicate unconnected pre‐D₃P–T –deformation histories. The shared D₃ deformation in the two units, the syn‐D₃ inverted thermal gradient preserved in the footwall cratonic rocks and the complementary cooling and hydration of the hanging wall granulites across the contact are attributed to westward thrusting of ‘hot’ Eastern Ghats granulites on ‘cool’ cratonic crust. It is suggested that the Eastern Ghats migmatitic gneiss unit is not a reworked part of the craton, but a para‐autochthonous/allochthonous unit emplaced on and amalgamated to the craton.     

Evolution of Eastern Ghats granulite belt of India in a compressional tectonic regime and juxtaposition against Iron Ore Craton of Singhbhum by oblique collision — transpression

Three major episodes of folding are evident in the Eastern Ghats terrain. The first and second generation folds are the reclined type; coaxial refolding has produced hook-shaped folds, except in massif-type charnockites in which non-coaxial refolding has produced arrow head folds. The third generation folds are upright with a stretching lineation parallel to subhorizontal fold axes. The sequence of fold stylesreclinedF ₁and coaxialF ₂, clearly points to an early compressional regime and attendant progressive simple shear. Significant subhorizontal extension duringF ₃folding is indicated by stretching lineation parallel to subhorizontal fold axes. In the massif-type charnockites low plunges ofF ₂folds indicate a flattening type of deformation partitioning in the weakly foliated rocks (magmatic ?). The juxtaposition of EGMB against the Iron Ore Craton of Singhbhum by oblique collision is indicative of a transpressional regime.     

Cataclastic lineations

This paper describes a stretching lineation developed on the S-foliations of cataclasites and fault gouges from the Akaishi Tectonic Line (ATL), central Japan. The lineation is called a ‘cataclastic lineation’, and appears to result from homogeneous laminae flow of cryptocrystalline matrix clay minerals and the preferred orientation of fractured porphyroclasts. Slickenlines overprint these textures forming cataclastic lineations. These two kinds of linear structures are developed at large angles to one another.The orthogonal projection of the perpendicular of the intersection of S-foliations, C-surfaces and R₁ shears (B-axis) onto the C-surface, which defines the shear direction, is parallel to the orthogonal projection of cataclastic lineation on the C-surface. In addition, planes cut parallel to the cataclastic lineation and perpendicular to the C-surface show distinctive asymmetric structures indicating left-lateral shear, which coincides with the main fault movement of the ATL as defined by displacement of tectonic units, S-C-surfaces and R₁ shears. In contrast, planes parallel to the slickenlines and perpendicular to the C-surface show no asymmetric fabrics. Thus, cataclastic lineations are considered to indicate the shear direction of the ATL during formation of the cataclasites, whereas the slickenlines indicate shear directions of later, superficial movements along the ATL which have no relationship to the main shear direction.     

Transpressive Structures in the Ghadir Shear Belt,Eastern Desert,Egypt: Evidence for Partitioning of Oblique Convergence in the Arabian‐Nubian Shield during Gondwana Agglutination

Transpressional deformation has played an important role in the late Neoproterozoic evolution of the ArabianNubian Shield including the Central Eastern Desert of Egypt. The Ghadir Shear Belt is a 35 km-long, NW-oriented brittleductile shear zone that underwent overall sinistral transpression during the Late Neoproterozoic. Within this shear belt, strain is highly partitioned into shortening, oblique, extensional and strike-slip structures at multiple scales. Moreover, strain partitioning is heterogeneous along-strike giving rise to three distinct structural domains. In the East Ghadir and Ambaut shear belts, the strain is pure-shear dominated whereas the narrow sectors parallel to the shear walls in the West Ghadir Shear Zone are simple-shear dominated. These domains are comparable to splay-dominated and thrust-dominated strike-slip shear zones. The kinematic transition along the Ghadir shear belt is consistent with separate strike-slip and thrustsense shear zones. The earlier fabric(S1), is locally recognized in low strain areas and SW-ward thrusts. S2 is associated with a shallowly plunging stretching lineation(L2), and defines ～NW-SE major upright macroscopic folds in the East Ghadir shear belt. F2 folds are superimposed by ～NNW–SSE tight-minor and major F3 folds that are kinematically compatible with sinistral transpressional deformation along the West Ghadir Shear Zone and may represent strain partitioning during deformation. F2 and F3 folds are superimposed by ENE–WSW gentle F4 folds in the Ambaut shear belt. The sub-parallelism of F3 and F4 fold axes with the shear zones may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation in fold zones. Dextral ENEstriking shear zones were subsequently active at ca. 595 Ma, coeval with sinistral shearing along NW-to NNW-striking shear zones. The occurrence of upright folds and folds with vertical axes suggests that transpression plays a significant role in the tectonic evolution of the Ghadir shear belt. Oblique convergence may have been provoked by the buckling of the Hafafit gneiss-cored domes and relative rotations between its segments. Upright folds, fold with vertical axes and sinistral strike-slip shear zones developed in response to strain partitioning. The West Ghadir Shear Zone contains thrusts and strikeslip shear zones that resulted from lateral escape tectonics associated with lateral imbrication and transpression in response to oblique squeezing of the Arabian-Nubian Shield during agglutination of East and West Gondwana.     

Progressive and polyphase deformation of the Schistes Lustrés in Cap Corse, Alpine Corsica

In Cap Corse, progressive deformation during Late Cretaceous obduction of the ophiolitic Schistes Lustrés (sensu lato) as a pile of imbricate, lens-shaped units during blueschist facies metamorphism was non-coaxial. Two zones are recognized: a lower series emplaced towards the west is overlain by a series emplaced towards the south-southwest in Cap Corse. Equivalent structures (differing only in orientation) occur in both zones. The change in thrust direction was responsible for local refolding and reorientation of previously formed structures, parallel to the new stretching direction immediately below the thrust contact between the two zones, and within localized shear zones in the underlying series.Both zones are refolded about E-overturned F₂ folds trending between 350 and 025°. Local minor E-directed thrusts occur associated with the F₂ folds. This second deformation of Middle Eocene age is considered to be related to the backthrusting of an overlying klippe containing gneisses of South Alpine origin, and is followed by a third Late Eocene phase of upright 060°-trending F₃ folds accompanied by greenschist facies metamorphism.     

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.     

Geology,structural and exhumation history of the Higher Himalayan Crystallines in Kumaon Himalaya,India

The crystallines in the Kumaon Himalaya, India are studied along Goriganga, Darma and Kaliganga valleys and found to be composed of two high-grade metamorphic gneiss sheets i.e. the Higher Himalayan Crystalline (HHC) and Lesser Himalayan Crystalline (LHC) zones. These were tectonically extruded as a consequence of the southward directed propagation of crustal deformation in the Indian plate margin. The HHC and its cover rocks i.e. the Tethyan Sedimentary Zone (TSZ) are exposed through tectonic zones within the hinterland of Kumaon Himalaya. The HHC records history of at least one episode of pre-Himalayan deformation (D₁), three episodes of Himalayan deformation (D₂, D₃, D₄). The rocks of the HHC in Kumaon Himalaya are thoroughly transposed by D₂ deformation into NW-SE trending S_m (S₁+S₂). The extent of transposition and a well-developed NE-plunging L₂ lineation indicate intense strain during D₂ throughout the studied portion of the HHC. Ductile flow continued, resulting in rotation of F₁ and F₂ folds due NE-direction and NW-SE plunging F₃ folds within the HHC. The over thickened crystalline was finally, superimposed by late-to-post collisional brittle-ductile deformation (D₄) and exposed the rocks to rapid erosion.     

Structural and kinematic analysis of the Early Paleozoic Ondor Sum-Hongqi mélange belt,eastern part of the Altaids (CAOB) in Inner Mongolia,China

We present a structural and kinematic study of an Early Paleozoic subduction mélange and a magmatic arc that form the main elements of the Southern Orogen Belt of Inner Mongolia, which lies in the eastearn part of the Altaids or Central Asia Orogenic Belt. The structural analysis of the mélange conducted in the Hongqi and Ondor Sum areas (western Inner Mongolia) shows two phrases of ductile deformation. The D₁ event is responsible for the pervasive S₁ foliation, NW–SE striking L₁ stretching lineation and F₁ intrafolial folds. These microstructures are coeval with a greenschist facies metamorphism. During D₂, NW-verging F₂ folds associated with a S₂ axial planar cleavage deformed S₁ and L₁. The D₁ kinematic criteria indicate a top-to-the-NW sense of shear. D₁ and D₂ developed before the unconformable deposition of the Early Devonian shallow water sandstone. A lithosphere scale geodynamic model involving an Early Paleozoic southeast-directed subduction beneath the North China Craton and late Silurian collision of the North China Craton with an hypothetical microcontinent is proposed to account for the microstructural evolution.     

Metamorphism and kinematics of the early deformation in the Variscan suture of SW Iberia

The early (Devonian) collisional stage in SW Iberia has been investigated through the analysis of deformation in the Cubito‐Moura schists, the main lithology of an Allochthonous Complex putatively rooted in the suture between the Ossa‐Morena and South Portuguese zones. The first deformation in these schists (D₁) is recorded as a S₁‐L₁ mylonitic fabric well preserved in early quartz veins. Subsequent D₂ deformation caused the main folds and the main (S₂) foliation. After restoration, the stretching lineation (L₁) trends at a small angle with the Ossa‐Morena/South Portuguese suture. This trend, together with the top‐to‐the‐east kinematics determined from quartz microfabric is indicative of an oblique left‐lateral collisional scenario in SW Iberia. Chlorite–white K‐mica–quartz ± chloritoid multi‐equilibrium calculations yield P–T conditions in the range 0.9–1.2 GPa and 300–400 °C, during the first collisional stage. P–T conditions during D₂ were 0.3–0.8 GPa and 400–450 °C, thus indicating an important stage of exhumation of the Allochthonous Complex during these two collisional events, after subduction of the Ossa‐Morena Zone margin under the South Portuguese Zone continental crust. In the general context of the Variscan orogen, dominated by dextral collision, the left‐lateral convergence in SW Iberia can be explained in terms of the Avalonian salient represented by the South Portuguese Zone, which would impinge between Iberia and Morocco.     

Lithologic and structural mapping of the Abiete–Toko gold district in southern Cameroon,using Landsat 7 ETM+/SRTM

The geology of the Abiete–Toko gold district in South Cameroon is investigated using a combination of Landsat 7 ETM+/SRTM image processing techniques, conventional geologic field mapping and geostatistical analysis. The satellite images were treated using Principal Component Analysis and Sobel filters to separate the background noise from lithotectonic structures which were matched with field data. The results show that this area has been affected by a polyphase deformation represented by S₁ foliation, S_c1 schistosity, L₁ lineation, S₂ foliation, F₂ folds, and F₃ shear zones and faults. A detailed analysis of all the structures led to the identification of two major networks of dextral and sinistral shear zones oriented WNW–ESE and NE–SW, respectively. These results may serve in mining prospection, especially in the search for tectonically controlled primary mineralization and so may significantly guide the exploration of primary gold mineralization in the Abiete–Toko area subjected to years of artisanal gold mining.     

Non-axial planar S1 cleavage in the Hawick Rocks of the Galloway area,Southern Uplands,Scotland

S₁ cleavage in the Hawick Rocks of the Galloway area is non-axial planar, cutting obliquely across the F₁ folds in a predominantly clockwise sense. Individual S₁ cleavage planes within cleavage-fans in F₁ folds strike clockwise, locally anti-clockwise, of axial surfaces, and the mean plane to the S₁ cleavage-fans dips predominantly more steeply than the axial surface. F₁ folds investigated at scattered localities in Silurian and Ordovician rocks north of the Hawick Rocks are also transected by the S₁ cleavage, indicating that non-axial planar S₁ cleavage is widespread in the Southern Uplands. The S₁ cleavage is a composite fabric. Objects deformed within sandstones and tuffs indicate oblate strain. F₁ fold plunge varies from NE to SW and fold hinges locally are markedly curvilinear. Steeply plunging and locally downward-facing F₁ folds are present along the southeast margin of the Hawick Rocks. The non-axial planar S₁ cleavage relationships persist in the steeply plunging F₁ folds. Synchronous development of the non-axial planar S₁ cleavage and the variably plunging F₁ folds is proposed.     

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.     

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.