F2 Folds Research Articles

Kilometre-scale sheath folds in the western Betics (south of Spain)

The three-dimensional exposures of the Sierra Blanca marbles and neighbouring rocks in the western Betic cordillera of the south of Spain show tight to isoclinal F3 folds re-folded by a larger sheath-like structure. The relationships between both structures are as follows: (1) The mean axial surface for Sierra Blanca sheath fold can be defined as the great circle of best fit through the s-axes of sub-cylindrical F3 folding domains. (2) The asymmetric F3-folds indicate a single mainly eastward normal sense of shear. (3) The mean attitude of the axial-plane crenulation foliation, Sc, is also sub-parallel to the mean axial surface for Sierra Blanca sheath fold. The regional context of the Sierra Blanca sheath fold is finally discussed, and a model of a heterogeneous high-strain extensional zone proposed.

Consequences of Stabilizing the Natively Disordered F Helix for the Folding Pathway of Apomyoglobin

The F helix region of sperm whale apomyoglobin is disordered, undergoing conformational fluctuations between a folded helical conformation and one or more locally unfolded states. To examine the effects of F helix stabilization on the folding pathway of apomyoglobin, we have introduced mutations to augment intrinsic helical structure in the F helix of the kinetic folding intermediate and to increase its propensity to fold early in the pathway, using predictions based on plots of the average area buried upon folding (AABUF) derived from the primary sequence. Two mutant proteins were prepared: a double mutant, P88K/S92K (F2), and a quadruple mutant, P88K/A90L/S92K/A94L (F4). Whereas the AABUF for F2 predicts that the F helix will not fold early in the pathway, the F helix in F4 shows a significantly increased AABUF and is therefore predicted to fold early. Protection of amide protons by formation of hydrogen-bonded helical structure during the early folding events has been analyzed by pH-pulse labeling. Consistent with the AABUF prediction, many of the F helix residues for F4 are significantly protected in the kinetic intermediate but are not protected in the F2 mutant. F4 folds via a kinetically trapped burst-phase intermediate that contains stabilized secondary structure in the A, B, F, G, and H helix regions. Rapid folding of the F helix stabilizes the central core of the misfolded intermediate and inhibits translocation of the H helix back to its native position, thereby decreasing the overall folding rate.

Oligocene–Miocene thrusting in central Aegean: insights from the Cycladic island of Amorgos

In the central Aegean, the Cycladic island of Amorgos consists of two high‐pressure (HP) units, the marble‐rich Amorgos unit, which is correlated to the Mesozoic ‘cover’ sequence of the Menderes Massif, and the Cycladic Blueschist unit. New structural data show that the deformation history of the Amorgos HP‐rocks was principally governed by early Oligocene (or late Eocene)–early Miocene ductile to brittle thrusting (D1–D3) followed by middle–late Miocene oblique contractional movements (D4–D5). The D1 phase caused syn‐blueschist‐facies ductile thrusting of the Cycladic Blueschist unit over the Amorgos unit, with ambiguous kinematics. Progressive deformation under continuous NW–SE compression produced a sequence of imbricate NW‐directed thrusts (D2/3) characterized by a stratification of fault‐related rocks, with mylonitic zones (D2) giving way downwards to cataclastic zones (D3). Ductile D2 thrusting synchronous to greenschist‐facies retrogression, was accompanied by mega‐sheath folding during constrictional and general shear deformation. Brittle D3 thrusting was associated with NW‐verging F3 folds trending at a high‐angle to the transport direction. Orthogonal contraction gave way to transpression during which the compression orientation changed from NW–SE (D4) to NE–SW (D5). Back‐arc related NW–SE pure extension (D6) seems to have been established in post‐late Miocene times and related high‐angle normal faulting affected HP‐rocks only after they had already reached the uppermost crustal levels. Oligocene–early Miocene deformation history is interpreted to indicate syn‐compressional exhumation of HP‐rocks possibly in an extrusion wedge. In this case, Amorgos HP‐rocks should have occupied the base of the extrusion wedge. Copyright © 2011 John Wiley & Sons, Ltd.

Major Structural Features and the Tectonic Evolution of the Bossangoa-Bossembele Basement, Northwestern Central African Republic

Planar structures (foliations and shear zones) and linear structures (lineations, fold axes) around the Bossangoa and Bossembele area (northwestern part of the Central African Republic) in the North Equatorial fold belt have been investigated and analyzed with the aim of unraveling the major structural elements and the tectonic evolution of the basement. The foliations show two major trends, an older flat-lying granulitic layering of uncertain age reworked by later, Neoproterozoic NE-SW ductile, sinistral shearing and mylonitization. The related lineation is conspicuous and corresponds to a mineral and stretching lineation. It trends N0-30°E with a moderate plunge (0-15) towards the north. F1 folds occur only as decimeter-sized isoclinals folds; their axial traces are roughly parallel to the lineation. The brittle phase characterized by NW-SE-trending open and partially filled fractures is younger than the mylonitization. Quartzo- felsdpathic anatectic veins or planes are commonly boudinaged, and the shape of boudins rather suggests non-coaxial deformation. The mylonitic foliation with associated stretching lineation and the ductile shear zone are recognized for the first time and support a tectonic evolution in which ductile deformation was overprinted by brittle failure accompanied by the emplacement of some intrusives.

The structural, metamorphic and temporal evolution of the country rocks surrounding Venetia Mine, Limpopo Belt, South Africa: Evidence for a single palaeoproterozoic tectono-metamorphic event with implications for a tectonic model

This paper presents the results of a detailed and fully integrated pressure–temperature–time-deformation (P–T–t-D) study of the country rocks around Venetia Mine in the Central Zone of the Limpopo Belt, South Africa. Detailed structural mapping around Venetia Mine delineates four deformation events (D1–D4). Relict S1 comprises quartzofeldspathic bands in biotite gneiss and amphibolite. S2, defined by biotite in biotite schist, gneissic banding in biotite gneiss, long axes of quartzofeldspathic augen, and the long axes of amphibolite lenses/boudins, is axial planar to F2. F3 occurs predominantly at the contacts between biotite gneiss and biotite schist, forming open to closed, upright to inclined, E-W- to ENE-trending, shallowly plunging folds. D4, which was constrictional-prolate in nature, refolded S1, S2, F2 and F3. F4 folds and an L4 mineral lineation, defined predominantly by sillimanite in metapelitic schist, plunge moderately NE to NNE and overprint all previous fabrics. Poles to refolded S2 foliations show a characteristic great circle distribution. In turn, the pole to this great circle coincides with the orientation of L4. Such patterns are also found in the Avoca, Bellevue and Ha-Tshansi sheath folds in the Central Zone, albeit that the plunge of the lineations at Venetia are predominantly NE-wards, rather than SW-wards. The moderately NE- to NNE-plunging F4 sheath folds and L4 lineations accord with the NE-SW trend of sheath folds and associated mineral elongation lineations observed elsewhere in the Central Zone. However, PbSL dating of syntectonic garnets constrain the minimum and maximum age of D2 and D4 structures respectively to c. 2037–2040 +/− 22Ma. The metamorphism developed in metapelitic lithologies is characterized by peak amphibolite facies conditions of ∼6.5kbar and 680°C. Pseudosection modelling of growth zoning in garnet provides evidence of a prograde pressure and temperature increase from ∼5kbar at 600°C to ∼6–6.5kbar at 650–680°C, which is interpreted to be a consequence of tectonic thickening. Collectively, the integrated P–T–t-D data unequivocally demonstrates that the country rocks around Venetia experienced a structural–metamorphic event in the palaeoproterozoic. Coupled with published data we suggest a simple two-fold tectonic model, which involves the Central Zone as separate terrane docking with the Kaapvaal during the Neoarchean and later, during the Palaeoproterozoic this Central Zone-Kaapvaal amalgam collides with the Zimbabwe Craton.

Structural Control on Banded Iron Formation (BIF) and Gold Mineralization at Abu Marawat Area, Central Eastern Desert, Egypt

Abu Marawat area is considered as a promising site for exploration of gold mineralization, where many ancient gold mines, hydrothermal alteration zones, and intervening quartz veins are present. This study is a field-based structural analysis that aims at revealing the relationship between mineral occurrences and local structural setting. The area is a part of a back-arc volcanosedimentary sequence associated with banded iron formation (BIF) that has undergone extensive ductile and brittle deformation history. This multiple deformation is manifested by four phases. D1 and D2 are the product of compressional stresses and are expressed by F1, F2 and F3 folds in low-grade regionally metamorphosed rocks. D1 was a progressive deformational phase started with F1 folds, which in a later stage were overprinted by F2 folds. It resulted from NW-SE-oriented pure shear and is associated with imbricate thrust stacks, which control the locations of listwanite bearing gold. NE-SW-oriented compressive stress during D2 is displayed by F3-slip folds at the early stage followed by N-S- to NW-trending dip-slip normal faults and related shear zones. Mineralized quartz veins (MQV) were developed post-D2 and pre-D3. Folding and refolding in addition to thrust movement play a significant role in shortening and thickening of the iron formation bands located at the summit of Gebel Abu Marawat. D3 and D4 are expressed by brittle deformation. D3 is displayed by conjugate shear planes represented by sinistral-NW-oriented and dextral-NE–oriented strike-slip faults that led to the dislocation and redistribution of gold mineraliztion associated with both MQV and listwanite. Barren quartz veins trending E-W were also developed along gash fractures formed during this phase of deformation.

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 Sm (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.

Folding in orogens: a case study in the northern Iberian Variscan Belt

AbstractFolding during the Variscan deformation in NW Iberia has been analysed from the foreland to the hinterland of the orogen using several geometrical techniques complemented by the numerical simulation of kinematical folding mechanisms with the aid of a computer program (‘FoldModeler’). In the foreland, folds are related to thrusts; in the inner zones, folds can be attributed to three deformation phases.D1andD2involved deformation with horizontal foreland‐directed displacements.D1gave rise to closed or tight folds (F1) and cleavage (S1); development of large recumbentF1folds in the hinterland required a simple shear regime and a final coaxial strain component with sub‐vertical maximum shortening. Strain incompatibilities at deeper levels, together with high temperatures, favoured the concentration of ductile deformation in shear zones and development of mylonites duringD2. Local flow instabilities generatedF2folds that were passively amplified by a combination of simple shear, coaxial strain and area change.D3involved a change to a regime with dominant coaxial deformation and a sub‐horizontal maximum shortening; it gave rise to upright or steeply inclined folds (F3). Development ofD3structures was heterogeneous and depended on the previous dip of the bedding andS1, the presence or emplacement of granitoids, the stacking of thrust sheets or the previous development of large faults bringing into contact rocks with different competence.D3structures are mainly concentrated in metapelitic areas and appear distributed in bands on several scales. On a small scale, tectonic banding appears associated with small folds as a result of pressure‐solution processes. In areas with sub‐verticalS1, later sub‐horizontal kink bands were formed by a vertical compression. Copyright © 2010 John Wiley & Sons, Ltd.

Transposition of structures in the Neoproterozoic Kaoko Belt (NW Namibia) and their absolute timing

Three structural profiles across the Coastal Terrane, the Boundary Igneous Complex and the Orogen Core have been studied in the Kaoko Belt of northwestern Namibia. The oldest known Si fabric is inherited from an older tectono-metamorphic event. It occurs in the Coastal Terrane only and the extent of its reworking increases from south to north. The S1 foliation reactivates or folds Si fabric in the Coastal Terrane and appears as an early planar fabric in granitoids of the Boundary Igneous Complex and migmatites of the Orogen Core domain. Superimposed subvertical S2 fabric corresponds to axial plane cleavage of upright close to isoclinal folds and the extent of its development also increases from south to north. Active migration of partial melt during S2 development in the Orogen Core dates the onset of this deformation at ~550 Ma. Distribution of F2 fold axes and L2 stretching lineations suggests pure shear–dominated deformation associated with development of N–S trending S2 cleavage preserved in the central profile, followed by sinistral simple shear–dominated deformation on newly developed NW–SE trending pervasive cleavage in the northern part of the area. Such spatial variation in the deformation record is attributed to the irregular shape of the Congo Craton indenter that is reflected by heterogeneous development of the S2 cleavage front in the Coastal Terrane and the Boundary Igneous Complex. Common orientation of L1 and L2 stretching lineations and solid-state reworking on both S1 and S2 planes suggest single event of sinistral transpression since 550 Ma with strain partitioning into domains of oblique thrusting (reactivated S1) and transcurrent sinistral shearing (S2 and S3). Such succession of deformation structures suggests that major subvertical shear zones in the Kaoko Belt do not correspond to early crustal discontinuities, but rather reflect late strain localization during cooling.

Granitos peraluminosos intrusivos no Complexo Metamórfico Brusque: registro do magmatismo relacionado a colisão neoproterozóica no Terreno Tijucas, Itapema (SC)

Brusque Metamorphic Complex is one of the main units of the Tijucas terrane in Santa Catarina state. This complex, located in the Itapema region is constituted by metasedimentary association with subordinate presence of mafic and ultramafic metavolcanic rocks and peraluminous granites. The granites have tabular form from centimetric to metric thickness intruding metapelites, mafic and calc-silicate schists. These units are tectonically interposed and disposed to a low angle foliation from northeast-southwest direction. The granites are concordant with the S2 foliation and lie in according to axial surfaces of isoclinal F2 folds. The granite have a monzogranite composition with a K-feldspar, plagioclase and quartz, subordinates quantities of muscovite and tourmaline, with biotite, garnet, monazite, zircon, apatite and opaque minerals as accessories. The main structure is a magmatic banding of centimetric thickness alternating regular and continual levels from medium to coarse equigranular texture as well as pegmatitic texture levels. The banding is originated by successive fulfilling of fractures by granite injections. Geochemical studies characterize the granites as peraluminous magmatism related to high- K calc-alkaline series. The majors and traces elements patterns present values for the ratios LaN/YbN (1.34 to 4.66), K2O/Na2O (0.86 to 1.72), CaO/Na2O (0.11 to 0.44) and low values of Rb, Sr, Zr and RRE, are similar to peraluminous granites of other orogenic belts of Phanerozoic age. These type of granites are usually associated to rock generated by partial fusion of crustal sequences of pelitic composition with formation of a granulitic residue. Structural relations between the position of the granitic bodies and metamorphic evolution of the Brusque Metamorphic Complex associated to geochemical and geotectonic parameters are related to magmatism generator by anatexis connected to collisional event of the end of Brasiliano Cycle.

Neoproterozoic structural evolution of SE Sinai, Egypt: I. Re-investigation of the structures and deformation kinematics of the Um Zariq and Malhaq Formations, northern Wadi Kid area

The Wadi Kid area of the SE Sinai comprises Neoproterozoic arc metavolcanics and metasediments bordered to the N, W and S by post-kinematic granitoid plutons. The structure and tectonics of the Kid area remain controversial despite more than two decades of investigations. In this study the structural history and characteristics of the northern part of the Kid area (Um Zariq and Malhaq Formations) are described. The earliest tectonic event, D1, produced upright folds with probably originally WNW–ESE trending hinges. The F1 folding weakens to the north. The main deformation, D2, produced regionally penetrative shallow-dipping S2 foliations and semi-recumbent tight to isoclinal F2 folds. S2 cleavage is a product of pure flattening strain that involved pressure solution deformation mechanism, rather than a shear foliation that involved mylonitization as previously reported. Macroscopic F2 folds are dissected by top-to-SSE low-angle thrusts and mylonite zones that emplaced the high T Um Zariq Formation schists over the low-grade metavolcanics in the southern part of the Kid area. The SSE direction of thrusting is parallel to stretching lineations on the S2 cleavage planes. Later folds include D3 WNW–ESE to E–W trending upright gentle folds and D4 upright gentle N–S to NE–SW trending folds. In contrast to the extensional histories recently proposed this study finds the deformation events in the northern Kid area to comprise a compressional folding and thrusting history.

Structure and evolution of the Lewisian Gairloch shear zone: variable movement directions in a strike-slip regime

Synopsis The Gairloch shear zone (GSZ) has long been regarded as a dextral strike-slip shear zone with a northwesterly plunging shear direction. However, the original strain analysis applies only to the northeastern third of the zone and the structure of the remaining two-thirds is considerably more complex. The 6.5 km wide GSZ was formed during late Laxfordian (D3) deformation at c . 1.7 Ga. It was a product of the interaction of D3 folding and dextral strike-slip shear under retrogressive greenschist-facies metamorphism on rocks previously intensely deformed by early Laxfordian (D2) deformation in amphibolite-facies. The GSZ is divided into four units across strike: the Northeastern shear zone with NW-plunging D3 linear fabric; the Flowerdale belt with SE-plunging D2 linear fabric and low D3 strain; the Ard belt with steeply plunging F3 folds; and the Southwestern shear zone, with SE-plunging D3 lineation. Thus the GSZ consists of two high-strain D3 shear zones on either side of a central low-strain belt. Although consistently dextral strike-slip throughout, the D3 shear regime varies from NE-up in the Northeastern shear zone through sub-horizontal strike-slip in the Ard belt to SW-up in the Southwestern shear zone. This variation may have been caused by local uplift or depression induced by variations in geometry of the walls of the strike-slip system. A revised very approximate minimum estimate of the horizontal component of dextral shear is c . 9.5 km with a net NE-up vertical component of c . 5.5 km. Similarities in structure between the GSZ and the South Harris belt strengthen the case for these two zones to have been part of the same structure.

Pampean deformation in the Sierra Norte de Córdoba, Argentina: Implications for the collisional history at the western pre-Andean Gondwana margin

[1] In the Sierra Norte de Cordoba, Argentina, at the inferred western pre-Andean Gondwana margin, structural analyses show that Upper Proterozoic–Lower Cambrian strips of metamorphic clastic rocks underwent two stages of a compressive deformational history. The D1 deformation led to the formation of an S1 cleavage/foliation and affected an older granite. It is related to F1 folding around ∼N-S to NE-SW trending axes that partly record a steep plunge. Reconstructed fold structures display maximum wavelengths and amplitudes on a scale of hundreds of meters to kilometers. A steep to overturned dip of thick packages of metaconglomerates and metasandstones is related to this folding event. Also, because of variably oriented L1 lineations and dextral shearing, structures of the D1 deformation are assigned to different structural domains interpreted as the result of strain partitioning during dextral transpression. Strips of post-D1 granite and clastic mylonites with a dextral sense of shear are attributed to the second-stage deformational event. The late stage dextral shearing and mylonitization locally affected intrusions of granites and dacitic porphyries. In the country rocks it took place along preexisting S1 planes or locally developed S2/C2 planes related to F2 fold structures with steeply plunging axes. The regional metamorphism accompanying the deformation was in the range of the subgreenschist facies–greenschist facies transition and greenschist facies. The emplacement of synkinematic intrusions presumably followed active dextral shear zones. It cannot be excluded that during compressive deformation, older (normal) fault lines were reactivated and caused strain partitioning and later dextral shearing. After cessation of compressive deformation, granites and granodiorites intruded the deformed clastic succession and led to contact metamorphism of variable extent. Younger high-level intrusions record chilled margins also at angular xenoliths. Isotopic dates from granitoids, dacites, and metaclastic rocks in the Sierra Norte area and surroundings suggest that compressive deformations and metamorphism took place during the Early Cambrian Pampean orogeny. The deformational and metamorphic history can be attributed to either ridge subduction/collision or collision of the Pampean terrane with the western pre-Andean Gondwana margin. The two-stage kinematics suggests that (oblique?) collision was followed by a change to dextral displacements in the upper plate during the final state of and after collisional tectonics. The Sierra Norte area represented the upper (eastern) plate as supposed part of the Rio de la Plata craton and its cover, overriding the lower plate that consisted of an accretionary complex in the west.

The tectono-metamorphic record of the Tuscan Nappe from the Colline Metallifere region (Northern Apennines, Italy)

This paper deals with a structural and petrographic study carried out on key outcrops belonging to the Tuscan Nappe exposed in the Colline Metallifere region (Montieri-Poggio Prugnoli). Here, the Tuscan Nappe was affected by polyphase deformation characterised by superposed folding events (F1, F2 and F3) and related tectonic foliations (S1 and S2) in agreement with the structural setting described for the Tuscan Nappe exposed both in other places of southern Tuscany, and in the Alpi Apuane region. The development of F1 folds was accompanied by metamorphism which produced a sin-kinematic mineralogical assemblage developed on the S1 tectonic foliation. This paragenesis, consisting of calcite + quartz ± muscovite ± Fe- and Ti-oxides, is indicative for a temperature of about 200-300°C according to the T values estimated for the F1 folding event documented for the Tuscan Nappe of the Apuan Alps, suggesting a very low grade metamorphism which affected the Tuscan Nappe during its stacking in the Northern Apennines collisional belt

Towards a volcanic–structural balance: relative importance of volcanism, folding, and remobilisation of nickel sulphides at the Perseverance Ni–Cu–(PGE) deposit, Western Australia

Perseverance is a world-class, komatiite-hosted nickel sulphide deposit situated in the well-endowed Leinster nickel camp of the Agnew–Wiluna greenstone belt, Western Australia. The mine stratigraphy at Perseverance trends north-northwest (NNW), dips steeply to the west, and is overturned. Stratigraphic footwall units lie along the western margin of the Perseverance Ultramafic Complex (PUC). The PUC comprises a basal nickel sulphide-bearing orthocumulate- to mesocumulate-textured komatiite that is overlain by a thicker, nickel sulphide-poor, dunite lens. Hanging wall rocks include rhyodacite that is texturally and compositionally similar to footwall volcanic rocks. These rocks separate the PUC from a second sequence of nickeliferous, E-facing, spinifex-textured komatiite units (i.e. the East Perseverance komatiite). Past workers argue for a conformable stratigraphic contact between the PUC and the East Perseverance komatiite and conclude that the PUC is extrusive. This study, however, clearly demonstrates that these komatiite sequences are discordant, implying that the PUC may have intruded rhyodacite country rock as a sill with subsequent structural juxtaposition against the East Perseverance komatiite. Early N–S shortening associated with the regional DI deformation event (corresponding to the local DP1 to DP3 events at Perseverance) resulted in the heterogeneous partitioning of strain along the margins of the competent dunite. A mylonite developed in the more ductile footwall rocks along the footwall margin of the PUC, while isoclinal F3 folds, such as the Hanging wall limb and Felsic Nose folds, formed in low-mean stress domains along the fringes of the elongated dunite lens. Strata-bound massive and disseminated nickel sulphides were passively fold thickened in hinge areas of isoclinal folds, whereas basal massive sulphides lubricated fold limbs and promoted thrust movement along shallowly dipping lithological contacts. Massive sulphides were physically remobilised up to 20 m from their primary footwall position into deposit-scale fold hinges to form the 1A and Felsic Nose orebodies. First-order controls on the geometry of the Perseverance deposit include the thermomechanical erosion of footwall rocks and the channelling of the mineralised komatiitic magma. Second- or third-order controls are several postvolcanic deformation events, which resulted in the progressive folding and shearing of the footwall contact, as well as the passive fold thickening of massive and disseminated sulphide orebodies. Massive sulphides were physically remobilised into multiple generations of fold hinges and shear zones. Important implications for near-mine exploration in the Leinster camp include identifying nickeliferous komatiite units, defining their three-dimensional geometry, and targeting fold hinge areas. Fold plunge directions and stretching lineations are indicators of potential plunge directions of massive sulphide orebodies.

F4 二次元展開膜収納機構のための折り畳み解析(宇宙構造(1))

F4 二次元展開膜収納機構のための折り畳み解析(宇宙構造(1))

Geochemistry of magmatic rocks and time constraints on deformational phases and shear zone slip in the Méiganga area, central Cameroon

The Méiganga area is situated in the Adamawa–Yadé domain (AYD) of the Pan-African fold belt in Cameroon. The AYD is characterized by abundant plutonic rocks that intruded Palaeoproterozoic gneisses. It is cut by the transcurrent Central Cameroonian Shear Zone (CCSZ). The studied area is made up of metadiorite (MD), pyroxene-bearing granite (PGr) and biotite-muscovite granite (BMGr), hosted in a metasedimentary sequence of amphibole-biotite gneisses. The granitoids are metaluminous to slightly peraluminous, and mainly of I-type (ASI ≤ 1.1), representing a high-K calc-alkaline to shoshonitic suite. They were derived from crustal protoliths (BMGr), from rocks of mantle origin (MD, PGr), and/or from the remelting of metabasalt or metatonalite (MD, PGr). Four successive deformational phases (D1, D2, D3, and D4) are present in the Méiganga area. The S1 foliation is formed by tectonic transposition of the S0 primitive surface represented by contacts between the gneiss and intercalated amphibolites. The synmigmatitic D2 deformational phase is characterized by S2 banded schistosity, S2/C2 sinistral shear planes, and F2 folds with axes parallel to a L2 stretching lineation. Imprints of the D2 and subsequent deformational phases are similar in the metadiorite and host rocks, implying syn-D2 emplacement and crystallization of the metadiorite; therefore 614–619 Ma 207Pb/206Pb zircon evaporation ages obtained for this rock type date the syn-D2 magmagenesis. Similarly, the D3 phase of the PGr is 601 ± 1 Ma, dated by the 207Pb/206Pb evaporation method. D4 is a late-stage brittle deformational phase. Sinistral movement of the CCSZ is associated with D2, whereas its latest activity, characterized by dextral slip, cannot be older than emplacement of the 558 ± 2 Ma BMGr (207Pb/206Pb zircon evaporation age).

Alpine tectono‐metamorphic history of the continental units from Vardar zone: the Kopaonik Metamorphic Complex (Dinaric‐Hellenic belt, Serbia)

AbstractThe easternmost zone of the Dinaric‐Hellenic belt is represented by the Vardar Zone, in which the Kopaonik Metamorphic Complex (KMC) is regarded as the lowermost unit. This complex is topped by the unmetamorphosed Brzece unit and is intruded by the Oligocene Kopaonik Intrusive complex. The KMC is characterized by a stratigraphy that includes metapelites and meta‐carbonates of Late Triassic age, associated with metabasites. It is characterized by a complex deformation history that comprises four phases: D1 to D4. The D1 phase structures occur only as relict structures, whereas the D2 phase structures are represented by isoclinal F2 folds, associated with a well‐developed S2 foliation. The estimated P‐T conditions for the D1 and D2 metamorphism are consistent with the upper greenschist facies. The D3 phase is characterized by west‐verging thrusts associated with upright folds. In contrast, the D4 phase is characterized by open folds (F4) associated with low‐angle normal faults. The D1 and D2 deformation phases developed during the shortening related to continental collision, whereas the subsequent D3 and D4 phases can be related to the progressive exhumation of the KMC. The D4 phase probably developed during extensional tectonics during and after emplacement of the Kopaonik Intrusive Complex. The data show that the continental units belonging to the Vardar zone had a long‐lived deformation history that was more complex that previously thought. Copyright © 2009 John Wiley & Sons, Ltd.

Tectonic history of the Monti dell’Uccellina range, Southern Tuscany, Italy

Geological mapping, combined with macroscopic and microscopic structural analyses have been used to unravel the geometry and tectonic evolution of the Monti dell'Uccellina group (Southern Tuscany). A polyphase tectonic history has been recognized, characterized by three main tectonic events associated with the development of shear zones, folds and foliations. During the first tectonic phase, East verging F1 folds with axial plane foliation developed under very low-grade metamorphic conditions, as highlighted by illite crystallinity data and by the calcite-dolomite geothermometer.Several piled tectonic units, belonging to the Tuscan and Subligurian domains, constitute the backbone of the Monti dell'Uccellina range.These are, from bottom to top: 1) the Torre Cannelle Unit, made up by the Verrucano Group rocks; 2) the Talamone Unit, made up by the Calcare Cavernoso Formation; 3) the Monti dell'Uccellina Unit, represented by a passive margin sequence spanning from the Triassic Verrucano group at the base to the Tertiary Scaglia Fm. at the top; 4) the Vacchereccia Unit, represented only by Triassic Verrucano Group; 5) the Collelungo Unit, made up by the Calcare Nummulitico Fm. followed by the Macigno Fm; 6) the Canetolo Unit (Sub-Ligurian Unit Auctt.) represented by the «Argille & Calcari» Fm.Stacking of the nappe pile occurred during the first tectonic phase (D1) in two stages: during the first stage the main tectonic units were emplaced. The second tectonic stage is characterized by later thrusts that cut through the older ones, all of them with a top-to-the- East sense of movement, that led to the emplacement of three tectonic complexes. Each complex comprises three superposed tectonic units that are from top to bottom the Collelungo Unit, the Vacchereccia Unit and the Monti dell'Uccellina Unit.During the second tectonic phase (D2), the nappe pile was deformed by a kilometre-scale N-S trending upright antiform gently plunging towards the North.The last tectonic phase is characterized by the development of sub-horizontal folds and of low-angle detachment faults that were produced during the post-collisional extensional events.A detailed survey, that was achieved through compilation of a 1:10.000 scale map, has clarified the structural and stratigraphic position of the «Pseudoverrucano» Auctt. Formation, whose attribution has been debated for a long time. According to our data it can be referred to two different tectonic units: the Vacchereccia Unit, represented only by this formation, and the Monti dell'Uccellina Unit where it constitutes the bottom of a post-Triassic continental margin sequence.

Structural history and granite emplacement in the Rottenstone Domain during closure of the Trans-Hudson Orogen, Davin Lake, northern Saskatchewan

The Rottenstone Domain at Davin Lake northern Saskatchewan, exhibits structural and granite-emplacement evidence for crustal thickening, and possible Himalayan-style extrusion, overprinted by transpressional strain increasing toward the contact with the Wathaman Batholith. Three discrete Rottenstone subdomains parallel the regional strike of the Trans-Hudson Orogen: (i) the southeast Rottenstone subdomain (SERSD) with gently northwest-dipping migmatitic straight gneiss (S1) and white granitoid rocks with pinch-and-swell structures parallel to the straight gneissosity; (ii) the central Rottenstone subdomain (CRSD), which partly preserves the same NW-dipping fabric (S1) but is overprinted at its margins by tight upright F2 folds and includes a stockwork of pink monzogranitic aplites and pegmatites; and (iii) the northwest Rottenstone subdomain (NWRSD) in which the F2 folds are generally tighter and penetrative and its network of narrow white granitoid rocks is deformed and transposed by the F2 folds; but in the northwestern part, a wide, syn-D2 complex of schlieric white tonalitic and diatexite sheets strikes parallel the orogen. The SERSD D1 straight zone may be a remnant of Himalayan-type extrusion zone although it could be the lowest member of a stack of ductile thrust sheets. The CRSD stockwork may represent fluid-assisted magma injection into extensional fractures above the postulated extrusion zone. The increasing transpressional strain northwestward expressed primarily by the F2 folds in CRSD and NWRSD defines the Davin Lake shear zone, into which the NWRSD granitoid dyke complex represents syntectonic magma injection. Both the postulated extrusion and transpression are related to oblique convergence of the Archean Sask craton with the Archean Rae–Hearne craton.