Axial Plane Schistosity Research Articles

The Cabo Frio Thrust: A folded suture zone, Ribeira belt, SE Brazil

Kinematic evolution of thrust systems in which vergence changes along strike result in divergent interpretations regarding the subduction polarity. The Ribeira belt, located in southeastern Brazil, resulted from progressive amalgamation of several terranes against the southeastern margin of the São Francisco Craton (SFC) between ca. 620 and 580 Ma. This study brings new structural analysis of the Cabo Frio Thrust (CFT) located in the Ribeira belt, that displays strike variations and apparently double thrusting vergence, generating current debate on subduction vergence for the region. Four major stages of deformation (Dn-1, D1cf, D2cf and D3cf) have been identified in the southwestern segment of the CFT. The Dn-1 affected only the metasedimentary rocks of the Oriental Terrane previously the collision. The D1 phase is associated with the thrusting of orthogneisses of the Cabo Frio Terrane onto the metasedimentary rocks of the Oriental Terrane, with the development of a penetrative S1 foliation and L1 stretching lineation. The D2 deformation phase produced a recumbent synform in the early/syn-D2 stages that overturned the CFT. This resulted in F2 isoclinal folds and S2 axial plane schistosity with F2 axis plunges towards W-SW, parallel to the L1 stretching and L2 intersection lineations. During the late-D2 stage, the recumbent F2 fold was sheared and refolded, leading to horizontal rotation of linear and planar elements, changing the CFT strike from W-SW to N–S. A series of irregular upright folds were developed in the D3 deformation phase. The double vergence and strike variations of the CFT are a result of the major F2 fold geometry, influenced by the granitic batholith (Tinguí Complex) that acted as an indenter during continuous crustal shortening towards northwest.

Prograde polyphase regional metamorphism of pelitic rocks, NW of Jamshedpur, eastern India: constraints from textural relationship, pseudosection modelling and geothermobarometry

AbstractThe study area belongs to the Singhbhum metamorphic belt of Jharkhand, situated in the eastern part of India. The spatial distribution of the index minerals in the pelitic schists of the area shows Barrovian type of metamorphism. Three isograds, viz. garnet, staurolite and sillimanite, have been delineated and the textural study of the schists has revealed a time relation between crystallization and deformation. Series of folds with shifting values of plunges in the supracrustal rocks having axial-planar schistosity to the folds have been widely cited. Development of these folds could be attributed to the second phase of deformation. In total, two phases of deformation, D1 and D2, in association with two phases of metamorphism, M1 and M2, have been lined up in the study area. Chemographic plots of reactant and product assemblages corresponding to various metamorphic reactions suggest that the pattern of metamorphic zones mapped in space is in coherence with the temporal-sequential change during prograde metamorphism. The prograde P–T evolution of the study area has been obtained using conventional geothermobarometry, internally consistent winTWQ program and Perple_X software in the MnNCKFMASHTO model system. Our observations suggest that the progressive metamorphism in the area is not related to granitic intrusion or migmatization but that it was possibly the ascending plume that resulted in the M1 phase of metamorphism followed by D1 deformation. The second and prime metamorphic phase, M2, with its possible heat source generated by crustal overloading, was preceded by D1 and it lasted until late- to post-D2 deformation.

The Horto-Baratinha itabirite-hosted iron ore: A basal fragment of the Espinhaço basin in the eastern São Francisco Craton

The Horto-Baratinha (HBD) iron ore deposit is located at the eastern border of São Francisco Craton, comprising BIF-hosted high-grade bodies (>60 wt.% Fe) associated with polydeformed quartz-mica-schists, amphibole-schist of Statherian maximum deposition age, enclosed by Statherian granitoids of the Borrachudos Suite and Neoarchean gneiss. All the sequence is crosscut by undeformed dikes and sills of pegmatitic bodies probably formed during Late Ediacaran-Cambrian. The metasedimentary sequence is stratigraphically correlatable with the Orosirian-Statherian Serra da Serpentina and Serra de São José Groups that comprise the basal units of the Espinhaço Supergroup and was intensively segmented into distinct tectonic blocks. The sedimentary/diagenetic bedding of the metamorphosed BIF (itabirite) is generally transposed by an axial planar schistosity. The lamellar hematite from itabirite is the oldest iron oxide generation, which was formed during the syn-deformational stage, parallel-oriented to the rock foliation. The (keno)magnetite grains from itabirite, iron ore and pegmatite bodies developed as idioblasts that grew over the foliation formed during late and post-deformational stages. Magnetite oxidizes subsequently to martite and granular hematite. Coarse lamellar hematite crystals randomly oriented in the border of the pegmatitic bodies also formed during the post-deformational stage due to hydrothermal reaction with itabirite. The country rocks have undergone at least three stages of deformation developed during the syn-collisional and late-collisional (Ediacaran to early-Cambrian) phases of the Brasiliano Orogeny: stage 1 with the development of a pervasive foliation (S1), parallel to axial plane to tight folds and transposition of all sedimentary structures; stage 2 with folding of S1; stage 3 with refolding of S1. Both fold systems interfere with each other making up a dome and basin refolding shape. During the late-collisional (Ediacaran to early-Cambrian) and post-collisional/gravitational collapse (Cambrian) the sequence was intruded by anatectic pegmatitic bodies, which are part of the Eastern Brazilian Pegmatite Province, one of the most significant pegmatitic regions worldwide. The fluid related with these intrusions could be related with the Si leaching, crystallization of magnetite and granular hematite, and consequent formation of high-grade iron bodies.

A NEW CONSIDERATION ABOUT THE ALMOPIA-PAIKON BOUNDARY BASED ON THE GEOLOGICAL MAPPING IN THE AREA OF NEROSTOMA-LAKKA (CENTRAL MACEDONIA, GREECE)

Geological mapping along the boundary of Almopia and Paikon zone in the 'Nerostoma' region, NNW of Lakka village defines mafic volcanogenic rocL·, meta-pelites and radiolarites, thick-bedded to massive micritic limestones and flysch meta-sediments that dip mainly towards SW. Middle-Late Cretaceous fossiliferous limestones overlie unconformably the flysch meta-sediments and are characterised only by a primary foliation SO that dips at shallow angles to the NNW-N. Secondary foliations SI (sub-parallel to SO) and S2 are observed in the meta-clastic rocks. A Dl event caused Fl and progressively F2 folds to which S2 is the axial plane schistosity dipping to SW. This event which has not affected the fossiliferous limestones is related to an oblique convergence or inclined transpression during the Late Jurassic-Early Cretaceous. A D2 event dated in Early Tertiary caused an intense NE-thrusting and S-C cataclastic fabric defining top-to-the NE sense-of-shear. At many parts, the unconformity contact between the fossiliferous limestones and the underlying flysch is obliterated by this thrusting event. As a result, the boundary between Almopia and Paikon zones as paleo-geographically defined in this area seems to be meaningless for the Jurassic-Cretaceous times.

Tectonic Setting of the Kadiri Schist Belt, Andhra Pradesh, India

AbstractPlate tectonic activity has played a critical role in the development of petrotectonic associations in the Kadiri schist belt. The calc alkaline association of basalt, andesite, dacite and rhyolite (BADR) is the signature volcanic rock suite of the convergent margin. The N‐S belt has gone below the unconformity plane of Cuddapah sediments. In the northern part geochemical and structural attributes of the Kadiri greenstone belt is studied along with microscopic observations of selected samples. Harker diagram plots of major elements generally indicate a liquid line of descent from a common source, such that BADR rocks are derived from a common parent magma of basaltic to andesitic composition. These calc‐alkaline volcanic rocks are formed at convergent margins where more silicic rocks represent more highly fractionated melt. All the litho‐units of this greenstone belt indicate crush and strain effects. The stretched pebbles in the deformed volcanic matrix with tectonite development along with associated greenschist facies metamorphism, alteration and hydration is remarkable. Flow foliation plane with N‐S strike and very low angle (5° to 10°) easterly dip and N‐S axial planar schistosity formed due to later phase isoclinal folding can be clearly identified in the field. Basic intrusives are quite common in the surrounding area. All the observations including the field setting and geochemistry clearly demonstrate ocean‐continent subduction as the tectonic environment of the study area.

Very low-temperature metamorphism in Ordovician metasedimentary rocks above and below the Sardic unconformity, SW Sardinia, Italy

In the Iglesiente region, the Cabitza and Monte Argentu Formations are separated by an angular unconformity known as the Sardic unconformity. This is related to an early Ordovician mild compressional phase, known as the “Sardic phase”. The pelitic samples from the structurally lower Cabitza Formation consist of alternating reddish phyllosilicate-rich and whitish phyllosilicate-poor, sialic layers, whose S0 bedding plane is parallel to a pre-Variscan S1P schistosity overprinted by the Variscan S1V schistosity. Pelitic samples from the Monte Argentu Formation are characterized by a Variscan S1V axial plane schistosity. Samples from the two formations consist of quartz and phyllosilicates. The latter are potassic white mica, chlorite, paragonite, locally kaolinite, and pyrophyllite. The illite crystallinity values determined for the Cabitza samples are 0.25–0.31, with an average of 0.29; meanwhile, the Monte Argentu samples produce values of 0.33–0.38, with an average of 0.35. The chlorite crystallinity and b0 of potassic white mica values show greater heterogeneity in the Cabitza than the Monte Argentu samples. The b0 values and P–T pseudosections allow us to confirm that there is no significant difference in the P–T metamorphism conditions between the Cabitza and Monte Argentu samples. The Iglesiente region, which is considered to be the rift zone behind the Middle Ordovician Sarcidano–Barbagia volcanic arc, underwent the “Sardic phase”, giving rise to E–W folds. These were first overprinted by weak E–W, and then by stronger N–S-oriented Variscan deformation events.

Survival Conditions of Folding in Different Depth During Orogenesis‐Deformation in Texas Creek and Chester Dome, USA

AbstractA succession of 5 FIA trends (foliation intersection or inflection axes in porphyroblasts) preserved in high temperature‐low pressure regime PreCambrian rocks in the Texas Creek, Arkansas River region reflected by the fold axial plane traces and schistosity data in this region. Similar fold axial plane trace data measured in Palaeozoic rocks in Chester Dome, Vermont, which is high temperature to medium pressure regime, only preserve the effects of the youngest FIAs of the all 5 FIA sets that obtained in this region. The other three FIA sets have no equivalent fold axial planes. This difference from shallow to deeper orogenic regimes reflects decreasing competency at greater pressure with collapse and unfolding of earlier formed folds. The greater overlying load of rocks has tended to flatten all but the very largest early‐formed structures, preserving only those folds that were more recently developed.

Structural evolution of the Agnew–Wiluna greenstone belt, Eastern Yilgarn Craton and implications for komatiite-hosted Ni sulfide exploration

The Agnew–Wiluna greenstone belt in the Yilgarn Craton of Western Australia is a narrow package of complexly deformed Archean supracrustal rocks that hosts two of the world's largest komatiite-hosted nickel sulfide deposits, the Mt Keith and Perseverance deposits. These deposits and several others in the belt are centred on thick lenses of adcumulate-textured komatiite interpreted to represent areas of channelised magma flow. The large nickel sulfide deposits are located in parts of the belt associated with ca 2720 to 2700 Ma felsic volcanism (e.g. the Leinster and Mt Keith nickel camps). In these areas, felsic to intermediate volcanic rocks are intercalated with syn-volcanic massive sulfides of inferred exhalative origin. While these primary magmatic features are clearly first-order controls on the distribution of Ni sulfide deposits in the belt, several regional-scale deformation events have significantly complicated the interpretation of primary stratigraphic relationships. The earliest recorded deformation events (D1,2,3) resulted in an east–west trending greenstone belt with recumbent isoclinal folds and ductile shear zones. Subsequent west-southwest–east-southeast shortening during the D4 event at ca 2666 Ma involved the refolding of the tectono-stratigraphy to produce belt-scale, north- to north-northwest-trending upright folds, a pervasive axial planar schistosity in all rocks, and the present-day steeply dipping, overturned supracrustal sequences, and emplacement of granitoids in major antiformal fold hinges. Polyphase folding of supracrustal rocks produced Type 2 fold interference patterns with multiple facing reversals at various scales across the belt. West-southwest–east-southeast extension during the D5 event at ca 2665 Ma triggered the development of terrestrial basins (i.e. Scotty Creek and Jones Creek) in areas flanking major antiforms, resulting in the deposition of the Jones Creek Conglomerate. Subsequent west-southwest–east-southeast shortening during the D6 event resulted in the folding of the Jones Creek Conglomerate and formation of gold-bearing veins in the Agnew gold camp. Belt-wide relaxation in east–west shortening during the D7 event caused open, recumbent F7 folding of the steeply dipping stratigraphy. Broadly east–west shortening during the D8 to D10 events resulted in the tightening of existing fold hinges, the dismemberment and displacement of panels of supracrustal rocks by sinistral (e.g. Perseverance shear zone) and then dextral (Waroonga) shear zones. The Agnew–Wiluna belt displays (para)autochthonous associations within the belt, with district-scale heterogeneities caused by primary volcano-sedimentary facies changes combined with polyphase deformation. Importantly, nickel sulfide-bearing sequences identified in nickel camps can potentially be traced to different parts of the belt by unravelling the effects of polyphase deformation.

Structural geology of the Mount Cook Range and Main Divide, Hooker Valley region, New Zealand

The structure of the Mt Cook Range is dominated by large, north-facing, steeply plunging, late folds. The Main Divide, on the western side of Hooker Valley, is formed of west-dipping beds, broken by west-northwest-dipping faults. Four episodes of folding occur in the Main Divide:F1 are possibly soft-sediment folds; F2 are isoclinal and display an axial plane schistosity; F3 are southwest-trending upright folds similar to those further west in the Alpine schists; and F4 folds are related to faulting. The rocks in the Main Divide are warped about steep axes; these structures may correlate with the late folds in the Mt Cook Range, but their relationship to the other folds in the Main Divide is unknown. The relationship between the large steeply plunging folds in the Mt Cook Range and similar structures elsewhere is discussed; we consider that the structures in the Mt Cook Range formed with initially steep plunges.

Análise Estrutural do Grupo Rio Pardo - Sudeste do Estado da Bahia

O Grupo Rio Pardo esta localizado no sudeste do Estado da Bahia e geologicamente, marca a transição entre o Cráton do São Francisco e a Faixa Araçuaí. Trata-se de uma sequência metassedimentar de baixo grau metamórfico, de idade meso a neoproterozoica, depositadas sobre um embasamento de idade paleoproterozoica a arqueana. O Grupo Rio Pardo é constituído das seguintes unidades litoestratigráficas, da base para o topo: Formação Panelinha formada por metapsefitos e metagrauvacas; Subgrupo Itaimbé, compreendendo as Formações Camacã (metapelitos e lentes carbonáticas), Água Preta (metarenitos e metassiltitos), Serra do Paraíso (metacarbonatos e quartzitos) e Santa Maria Eterna (metacarbonatos e quartzitos). No topo desta sequência, discordantemente sobre as anteriores, depositou-se a Formação Salobro, constituída de metapelitos e metapsefitos. O Grupo Rio Pardo esta dividido em duas unidades litoestruturais, separadas pela falha inversa Rio Pardo-Água Preta com transporte tectônico para nordeste. A Unidade Litoestrutural 1, que abrange o setor norte, possui características autóctones com estruturas associadas à primeira fase de deformação. A Unidade Litoestrutural 2, localizada a sudoeste da falha inversa, caracteriza um bloco subautóctone com dobramentos fechados relacionados à segunda fase de deformação. Um terceiro evento deformacional esta registrado na borda oeste da bacia. A vergência tectônica do Grupo Rio Pardo para NE se explica pela mudança do regime deformacional e da orientação dos eixos principais da deformação no decorrer do evento orogênico colisional brasiliano no setor norte da faixa Araçuaí, onde o transporte tectônico para norte serviu de motor para a deformação no Grupo Rio Pardo.

Structural styles and tectonic evolution of the Kolia-Boboti sedimentary Basin, Kédougou-Kéniéba inlier, eastern Senegal

New structural data obtained on the Birimian terranes of the Kolia-Boboti sedimentary Basin, the eastern part of the Dialé-Daléma Supergroup in the Kédougou-Kéniéba inlier show two major phases of Eburnean compressional deformation: (1) a D1 phase of thrusting tectonics affected the Lower Birimian B1 tourmalinized sediments. This first tectonic phase is characterized by isoclinals overturned to recumbent folds P1 with N040° 20°NE trending axis, associated with axial plane schistosity S 0S 1 which is mainly transposed in the bedding; (2) a D2 phase of compressional (D2a) and transpressional (D2b) tectonics is responsible for the crossfolds P2a-P2b exhibiting curved axes. These P2 folds are associated with the major schistosity S 2, north-south to SW-NE trending, mainly dipping to the south-east. The S 2 schistosity is mostly displayed in the large shear zones corridors where it steeply dips locally toward the north-west. A north-west vergence thrusting phase (D2c) of flats and ramps, associated with reverse folds, represents the last Eburnean event. This geometrical feature is characteristic of a “positive flower structure”. These different Eburnean compressional phases are separated by extensional deformation which is characterized by sedimentary deposits and volcanic flows.

A Variscan continental collision of the West Sudetes and the Brunovistulian terrane: a contribution from structural and metamorphic record of the Stronie Formation, the Orlica-Śnieżnik Dome, SW Poland

Structural, petrological and geochronological data from marbles and mica schists combined with those from highly disputable gneisses and eclogites of the Orlica-Śnieznik Dome (OSD), gave new insights into Variscan evolution of eastern borderland of the West Sudetes. It is shown that the Variscan tectonometamorphic evolution of the OSD began with E–W oriented subhorizontal shortening (D1 stage) related to the collision of the West Sudetes terranes and the Brunovistulian terrane. The shortening led to generally upright folding, which resulted in formation of the steep N–S trending metamorphic planar fabric S1, thickening of the lithosphere and burial of the Stronie Formation under greenschist facies conditions. As a consequence of subsequent, near-coaxial gravity-controlled vertical shortening (D2), the S1 foliation was deformed in tight recumbent folds F2. The flattening strain was associated with the progression to amphibolite-facies conditions (from ca. 510 upto ca. 620°C) and uplift from depths corresponding to 9–10 kbar to depths corresponding to 7–8 kbar. On microscale, the flattening strain is documented by rotation of the mineral fabric overgrown by syn-D2 prograde garnet porphyroblasts. The Sm–Nd Grt–WR isochron age, correlated with the D2 event yields 346.5 ± 4.4 Ma. Further deformation and progressive metamorphism led to development of the subhorizontally disposed S2 axial plane schistosity, which terminated at the metamorphic temperature peak. During retrogression but still under ductile conditions of deformation, the S2 planes were reactivated during successive top-to-the-N shear movement of the OSD (D3 stage). Due to the mutual interaction of the OSD with the adjacent terranes the shear deformations were localized within marginal parts of these units. Finally, as a result of the NE–SW and the NW–SE oriented shortenings (D4 stage and D5 stage, respectively), both the structural surfaces and metamorphic isograds were regionally folded with W(NW)-ward plunges. A synthesis of new and existing data shows an overall similarity in sequence of Variscan deformations (D1–D2–D3) in the West Sudetes borderland to that observed in the eastern flank of the Moldanubian domain.

Structural anatomy of the Kushtagi schist belt, Dharwar craton, south India—An example of Archaean transpression

The NW–SE trending Kushtagi schist belt in the eastern Dharwar craton records the tectonic consequences of a transpression regime. It was formed due to Late Archaean oblique convergence in the Dharwar craton. This relatively unknown schist belt is characterized by two major episodes of deformation. The dominant D 1 deformation forming tight to isoclinal non-cylindrical folds and a regional axial planar schistosity is a consequence of transpression. The transpression-related sinistral shear regime is followed by development of a conjugate set of F 2 folds and S 2 cleavages that are superimposed on the D 1 structures. The maximum shortening direction associated with the F 2 folds is NW–SE, which suggests longitudinal compression subsequent to transpression. Riedel shear fractures were developed in the sinistral shear regime and dolerite dykes were emplaced along the conjugate Riedel shear fractures. Intrusive granite bodies within and outside the schist belt were emplaced in the late phase of D 1 transpression.

SINCLINAL OURO FINO REVISITADO, QUADRILÁTERO FERRÍFERO, MINAS GERAIS: UMA HIPÓTESE SOBRE A SUA ORIGEM E EVOLUÇÃO

The Ouro Fino Syncline is a regional structure with a controversial origin. Two structural models represent the synthesis to its tectonic evolution. On the one hand, according to Ladeira & Viveiros (1984), the Ouro Fino syncline is the result of the first folding event which affected the Minas Supergroup. These folds present E-W trending axes and associated penetrative cleavage. The tectonic transport vector is northward. On the other hand, Fonseca (1990), interprets Ouro Fino syncline's origin as a result of compressive forces directed to westward. The axis has N-S trending and westward vergence according to Dorr (1969). This second point of view is the most accepted model. However the present investigation of the Ouro Fino syncline allowed lo characterize four families of structures and to present a third hypothesis for its origin and evolution. The first family is characterized by tight and reclined folds with moderate ESE plunging fold axis. The other structures of this phase are the axial plane schistosity and the mineralization parallel to intersection lineation. The mineral lineation is characterized as a "b" type fabric. The iron mineralization is closely related to the strain parameters of the first deformation phase. The fold closure occurs in the southern region of the Ouro Fino Syncline defined by structural relationships between bedding and schistosity and by subsidiary "S" and "Z" folds in the west and east limbs respectively. These relations allow to redefine the Ouro Fino structure as a reclined antiformic synclinorial fold with southward vergence and axis plunging to ESE. In this case the paleoproterozoic Minas Supergroup units occupy the fold nucleus being enveloped by the archean Rio das Velhas Supergroup metapelitic sequence. The second deformation phase is characterized by tight to open folds with sub-horizontal N-S trending axes genetically related to the Córrego das Flechas fault. The notable fold for this phase is the footwall Córrego das Flechas synform with westward vergence. The other families are represented by crenulations and E-Wand N-S axial trending gentle folds.

Sinclinal Ouro Fino revisitado, Quadrilátero Ferrífero, Minas Gerais : uma hipótese sobre a sua origem e evolução.

The Ouro Fino Syncline is a regional structure with a controversial origin. Two structural models represent the synthesis to its tectonic evolution. On the one hand, according to Ladeira & Viveiros (1984), the Ouro Fino syncline is the result of the first folding event which affected the Minas Supergroup. These folds present E-W trending axes and associated penetrative cleavage. The tectonic transport vector is northward. On the other hand, Fonseca (1990), interprets Ouro Fino syncline’s origin as a result of compressive forces directed to westward. The axis has N-S trending and westward vergence according to Dorr (1969). This second point of view is the most accepted model. However the present investigation of the Ouro Fino syncline allowed to characterize four families of structures and to present a third hypothesis for its origin and evolution. The first family is characterized by tight and reclined folds with moderate ESE plunging fold axis. The other structures of this phase are the axial plane schistosity and the mineral lineation parallel to intersection lineation. The mineral lineation is characterized as a “b” type fabric. The iron mineralization is closely related to the strain parameters of the first deformation phase. The fold closure occurs in the southern region of the Ouro Fino Syncline defined by structural relationships between bedding and schistosity and by subsidiary “S” and “Z” folds in the west and east limbs respectively. These relations allow to redefine the Ouro Fino structure as a reclined antiformic synclinorial fold with southward vergence and axis plunging to ESE. In this case the paleoproterozoic Minas Supergroup units occupy the fold nucleus being enveloped by the archean Rio das Velhas Supergroup metapelitic sequence. The second deformation phase is characterized by tight to open folds with sub-horizontal N-S trending axes genetically related to the Corrego das Flechas fault. The notable fold for this phase is the footwall Corrego das Flechas synform with westward vergence. The other families are represented by crenulations and E-W and N-S axial trending gentle 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 (D 1 to D 3) and four episodes of metamorphism (M 1 to M 4). The earliest greenschist facies event (M 1) is recognized in undeformed or weakly deformed domains wrapped by the S 1 schistosity, suggesting that M 1 occurred before D 1. The D 1 deformation produced small, mostly meter-scale, isoclinal and recumbent folds (F 1), an associated penetrative axial planar schistosity (S 1), a mineral stretching lineation (L 1) and regional-scale ductile shear zones. Concurrent with D 1 was M 2 metamorphism, which occurred before D 2 and produced low- to medium-pressure amphibolite facies assemblages. Regionally divergent motion senses reflected by the asymmetric F 1 folds and other sense-of-shear indicators, together with the radial distribution of the L 1 lineation surrounding the Liaoji granitoids, imply that D 1 represents an extensional event. The D 2 deformation produced open to tight F 2 folds of varying scales, S 2 axial crenulation cleavages and ENE-NE-striking thrust faults, involving broadly NW–SE compression. Following D 2 was M 3 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 (D 3) formed NW-WNW-trending folds (F 3), axial planar kink bands, spaced cleavages (S 3), and strike–slip and thrust faults, which deflect the earlier D 1 and D 2 structures. D 3 occurred at a shallow crustal level and was associated with, or followed by, a greenschist facies retrograde metamorphic event (M 4). 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 (D 1) and the M 1 and M 2 metamorphism, and the final closing of which was associated with the D 2 and D 3 phases of deformation and M 3 and M 4 metamorphism.

Partial Melting of Metapelitic Rocks Beneath the Bushveld Complex, South Africa

Metapelitic rocks in the aureole beneath the Bushveld Complex preserve evidence for both high- and low-aH2O anatexis. The aureole is characterized by an inverted thermal structure in which suprasolidus rocks potentially interacted with an H2O-rich volatile phase derived from underlying, dehydrating rocks. At lower grade (T < 700°C) the rocks contain fibrolite mats and seams that record local redistribution of volatiles. Incongruent reactions consuming biotite produced small quantities (<1 mol %) of liquid and peritectic cordierite that remained trapped within the mesosome. Larger volumes of melt (3–4%), preserved as coarse-grained discordant leucosomes, were produced by congruent melting following a structurally focused influx of H2O. Subhorizontal volatile-phase flow was concentrated within thin (∼10 mm) metapsammite horizons that are preserved as stromatic quartz–sillimanite veins. Upward migration occurred along steep fibrolite seams that are subparallel to a variably inclined foliation. Discordant leucosomes are concentrated within antiformal fold closures of quartz–sillimanite veins and along the axial planar schistosity. Closer to the contact (T > 725°C), volatile-phase-absent, biotite-consuming melting and melt extraction produced coarse-grained garnet–cordierite granofels. At the contact, leucodiatexites devoid of peritectic phases suggest effective segregation of melt from an underlying source. Migmatitic metapelites and their lower-grade stratigraphic equivalents have similar bulk-rock oxygen isotope values, consistent with very limited volatile-phase infiltration and precluding the Bushveld Complex magmas as the source of the volatiles.

D2 asymmetric folds and their vergence meaning in the Montagnola Senese metamorphic rocks (inner northern Apennines, central Italy)

This study describes asymmetric folds whose vergence is derived from refolding of a pre-existing inclined foliation. The study area is in the Montagnola Senese area where Mesozoic rocks in the greenschist facies crop out. These rocks were affected by deformation during the collisional stage (late Oligocene–early Miocene) of the Northern Apennines (D1 event), and during the post-collisional extensional tectonics (D2 event) that affected the inner zone of the Northern Apennines since the early–middle Miocene. During the D1 event, SW dipping axial plane schistosity and non-cylindrical folds indicative of a highly heterogeneous strain developed. During the second event westward verging folds developed. These folds are characterised by thick steep limbs. The S2 foliation, which is well developed in narrow and localised zones, is mainly a pressure solution cleavage with NE plunging stylolitic teeth. Micro- and meso-structural observations are used to discuss the relation between cleavage distribution and strain during the D2 event. The evolution of D2 folds was mainly controlled by a dissolution process and by a component of partitioned shear-strain. In the relatively ‘high’ strain domains, deformation took place by the combined effects of volume loss and shear displacement. In low strain domains, deformation took place by a veining and flattening process. In this way, the pre-existing foliation can be deformed in folds with vergence opposite to the sense of shear.

Ductile deformations of opposite vergence in the eastern part of the Guerrero Terrane (SW Mexico)

The Teloloapan volcanic arc in SW Mexico represents the easternmost unit of the Guerrero Terrane. It is overthrust by the Arcelia volcanic unit and is thrust over the Guerrero–Morelos carbonate platform. These major structures result from two closely related tectonic events: first, an eastward verging, ductile deformation (D1) characterized by an axial-plane schistosity (S1) supporting an E–W trending mineral stretching lineation (L1) and associated with synschistose isoclinal, curvilinear folds (F1). Numerous kinematic indicators such as asymmetrical pressure-shadows, porphyroclast systems, and micro-shear bands (S–C structures) indicate a top-to-the-east shear along L1. This first deformation was followed by another ductile event (D2) that produced a crenulation cleavage (S2) associated with westward overturned folds (F2), hence showing that the vergence of D2 is opposite to that of D1. Regionally, both D1 and D2 deformations have been identified east and west of the Teloloapan unit, in the Arcelia volcanic rocks as well as in the Mexcala flysch of Late Cretaceous age overlying the Guerrero–Morelos platform. This implies that all three units were deformed and thrust simultaneously, during the Late Cretaceous or Paleocene, prior to the deposition of the overlying, undeformed Eocene red beds of the Balsas group.

Dextral transpression in Late Cretaceous continental collision, Sanandaj–Sirjan Zone, western Iran

The Sanandaj–Sirjan Zone of western Iran is a metamorphic belt (greenschist–amphibolite) that was uplifted during Late Cretaceous continental collision between the Afro-Arabian continent and the Iranian microcontinent. In the June area, 300 km southwest of Tehran, the Late Palaeozoic–Mesozoic succession was affected by two major episodes of deformation. The first deformation formed tight folds and axial plane schistosity. These are strongly overprinted by second deformation structures that formed during Late Cretaceous continental collision under dextral transpression. The convergence has a low obliquity and has significant deformation partitioning into two domains. (1) A widespread schist and marble domain with intensely folded and foliated rocks that are cut by thrusts and have an overall south-southwest vergence. (2) A domain with wide zones of mylonitic granite, amphibolite and less common calcite mylonite that are affected by a foliation with the same orientation as in rocks of the schist and marble domain. Rocks of this domain also contain an intense sub-horizontal stretching lineation and abundant shear-sense criteria indicating dextral shear. This contrasts with many zones of transpression where strike-slip shearing is taken up along discrete faults. A syn- D 2 pluton (the Galeh–Doz pluton) has a major S-shaped bend within it, imparted during the dextral transpression.