F2 Folds Research Articles

Hydrothermal alteration events controlling magnetite-rich iron ore at the Matthew Ridge prospect, Jack Hills greenstone belt, Yilgarn Craton

Archean Banded Iron Formation (BIF) hosts high-grade (>55% Fe) iron ore at the Matthew Ridge prospect, in the Jack Hills greenstone belt, of the Narryer terrane, Yilgarn Craton. The ca 3000 Ma (SHRIMP U–Pb on zircons) Algoma-type BIF contains magnetite–hematite ore zones that are a product of successive overprinting hydrothermal alteration events. Rock types in the prospect area include Archean gneiss, metasedimentary rocks and BIF that are interlayered with dolerite. All rocks record peak amphibolite metamorphic mineral assemblages that are variably replaced by greenschist facies metamorphic minerals. Hypogene iron ore formed owing to: (1) the replacement of primary silica-rich bands in the quartz–magnetite BIF by Stage 1 hypogene magnesite and magnetite alteration; and (2) subsequent removal of Stage 1 magnesite to concentrate residual Stage 1 magnetite. These magnetite-rich ore bodies define <50 m-long by 20 m-wide lenses that trend NE and coincide with the hinge zones of Z-shaped, tight F1 folds, which are most likely parasitic folds to the regional NE-trending, steeply NE-plunging anticline in the Jack Hills greenstone belt. Magnetite-rich ore zones are enriched in Fe and depleted in SiO2, with only minor changes in other major oxides and trace elements, compared with least-altered BIF. Magnetite-rich ore zones are locally cut by shear zone-hosted, talc–magnetite hydrothermal alteration overprinted by talc–microplaty hematite alteration. The shear zones and both talc mineral assemblages formed during a second deformation event that was characterised by NNE–SSE shortening. The third deformation event resulted in at least two generations of NE- to NW-trending extensional veins and strike-slip faults that locally cut the talc-rich shear zones. Supergene goethite–hematite replaces magnetite-rich ore bodies within 80 m of the present surface. The strong structural control and distinct hydrothermal alteration assemblages of the Matthew Ridge prospect are the best exploration indices for high-grade magnetite-rich ore in the region.

Mega-fold interference patterns in the Beishan orogen (NW China) created by change in plate configuration during Permo-Triassic termination of the Altaids

Kilometer-size fold interference patterns in the Beishan Orogenic Collage (BOC) in the southernmost Altaids formed by fold superimposition in fossiliferous Permian sedimentary rocks. First-phase (F1), upright and almost north-trending folds, were refolded by E- to ENE-trending F2 folds, whose axial planes and axes are vertical or subvertical. From east to west there is a regional change in style of interference patterns from lobate–cuspate-, to crescent- to mushroom-shape. This variation is accompanied by a westward decrease in the F2 interlimb angle and related to a higher percentage of coarse-grained clastic rocks, suggesting a dependence of the F2 deformation on lithology. Axial planar slaty cleavages are well developed in F1 and poorly developed in F2 folds. The superposed folds mainly underwent flexural-slip and flexural flow folding to give rise to the lobate–cuspate pattern, and to the crescent pattern caused by flattening and flexural flow folding where the sediments were unconsolidated and enriched in fluids. The two folding events are interpreted to result from a major change in plate configuration that caused the inversion of an inter-arc basin during the final amalgamation of the BOC in the latest Permian to early to mid-Triassic. The two folding events bracketed between a maximum detrital zircon age of <273 Ma, and the youngest age of an intruded dyke at 219.0 ± 1.2 Ma suggest rapid plate reconfiguration related to final amalgamation of the Altaids orogen.

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.

Paleoproterozoic transpressional shear zone, eastern Black Hills, South Dakota: Implications for the late tectonic history of the southern Trans-Hudson Orogen

Paleoproterozoic rocks in the Black Hills of southwestern South Dakota record deformation associated with accretion of Proterozoic terranes from the south and suturing of the Wyoming and Superior cratons (ca. 1780–1715 Ma). Folding associated with suturing of these Precambrian terranes is well documented. However, penetrative fabrics related to shearing are characterized in only a few localities. The research presented herein identifies and characterizes a ≥4-km-wide, pervasive, thick-skinned shear zone, which deforms the regional-scale folding in the Black Hills portion of the Wyoming craton. Shearing is bracketed between ca. 1740 Ma (post regional-F2 folding) and ca. 1715 Ma (intrusion of the Harney Peak Granite). This high-strain zone is characterized by narrow anastomosing shear zones (m-scale) in the Paleoproterozoic rocks in the east-central Black Hills near Rockerville, South Dakota. The shear zones converge to form a km-wide zone of deformation in Paleoproterozoic and Archean rocks to the north near Nemo, South Dakota. Kinematic indicators such as composite foliations, microfolding, and asymmetric mantled porphyroclasts from within the shear zone support left-lateral, east-side-up transpression. Furthermore, strain associated with the shearing is coupled to vertically plunging, isoclinal F3 folds, commonly identified only as hinge areas preserved between strongly sheared limbs, within and adjacent to the shear zone in the study area. Correlation of this deformational event from the Paleoproterozoic rocks in the east-central Black Hills to older Paleoproterozoic and Archean rocks to the north has implications for these structures to have formed during a basement-involved deformational event. Evidence that stresses associated with this ∼1740–1715 Ma event were transferred inboard to the Wyoming Province is present to the southwest in the Hartville Uplift and Laramie Mountains of southeastern Wyoming. In both locations, structures with similar timing and kinematics overprint Cheyenne-Belt deformation. The regional extent and thick-skinned nature of these structures are interpreted to signify the change to continent-continent collisional tectonics during the final suturing of the Wyoming and Superior Provinces.

The Dalradian rocks of the central Grampian Highlands of Scotland

The central Grampian Highlands, as defined here, are bounded to the north-west by the Great Glen Fault, to the south-west by Loch Etive and the Pass of Brander Fault and to the south-east by the main outcrop of the Loch Tay Limestone Formation. The more arbitrary northern boundary runs north-west along the A9 road and westwards to Fort William. The detailed stratigraphy of the Dalradian Supergroup ranges from the uppermost Grampian Group through to the top of the Argyll Group, most notably seen in the two classic areas of Loch Leven–Appin and Schiehallion–Loch Tay; Southern Highland Group strata are preserved only in a small structural inlier south of Glen Lyon.Major F1 and F2 folds are complicated by co-axial northeast-trending F3 and F4 folding, as well as by locally important north- or NW-trending folds. In the Loch Leven area, nappe-like F1 folds verge to the north-west, whereas to the south-east the major recumbent F1/F2 Tay Nappe verges to the south-east. The trace of the upright Loch Awe Syncline lies between the opposing nappes, but in this region a large mass of late-Caledonian granitic rocks obscures their mutual relationship. Three tectonic ‘slides’ are identified that are certainly zones of high strain but which in part could be obscuring stratigraphical variations.The regional metamorphism ranges from greenschist facies on the western seaboard of Argyll to amphibolite facies in most of the remainder of the region. The study of garnets, together with kyanite and staurolite in the Schiehallion area, has enabled a detailed history of the metamorphism and structure to be unravelled.Stratabound mineralization occurs in the Easdale Subgroup, where there is also evidence of changes of sedimentary environment associated with volcanicity and lithospheric stretching. The region is dissected by a series of NE-trending, dominantly left-lateral, faults, subparallel to the Great Glen Fault, whose movement history is illustrated here by that of the Tyndrum Fault.

Mineralogical Study of Gabbro-Anorthosite from Dumka, Chhotanagpur Gneissic Complex, Eastern Indian Shield

Abstract The Chhotanagpur Gneissic Complex (CGC), bearing imprints of widespread high grade metamorphic and magmatic history since Palaeoproterozoic, represents an integral crustal segment of Eastern Indian Shield. The gabbro-anorthosite intrusives constitute a part of mafic-ultramafic magmatism in the CGC. The study area around Dumka (24°16' to 24°20'N: 87°13' to 87°22'E) predominantly comprises of granite gneiss and charnockitic country rocks within which gabbro-anorthosite intrusions occur as lenses. Field relations and structural studies reveal that the country rocks of Dumka have suffered three phases of deformation represented by F1, F2 and F3 folds. The gabbro-anorthosite intrusives maintain a sharp contact with the host rocks, deformed and metamorphosed. Relict igneous layering or primary igneous foliation (Sig) is recorded where metamorphic overprint is minimal. Mineral phases of gabbro-anorthosite rocks suggest that clinopyroxene compositions from gabrro correspond to diopside and clinoferrosilite, while those from anorthosite are clinoferrosilite. Amphiboles from the gabbro-anorthosite rocks are calcic, and range from ferroan pargasite in gabbro to ferroan pargasitic hornblende in anorthosite. Plagioclase from gabbro and anorthosite belong to bytownite and andesine respectively. Chemical composition of garnet in gabbro is almandine. Thermobarometric estimates for Dumka gabbro-anorthosites correspond to 511°C to 915°C and 5.0-7.5 kb pressure, comparable to that estimated for Bengal Anorthosite (593-795°C, 4.1-7.3 kb). Fractionation trend of plagioclase substantiates a single parental magma in the evolution of Dumka gabbro-anorthosite intrusives.

신보광산 주변지역에서 변성광물의 성장과 변형작용 사이의 상대적인 시간관계: 우라늄 광화대의 상대적인 광화시기

옥천변성대의 남서부에 위치하는 전라북도 진안군 신보광산과 그 동부지역에서 고품위 우라늄 지화학 이상대가 보고된 바가 있다. 본 논문은 선캠브리아기 변성퇴적암류(규암, 변성이질암, 변성사질암)와 이를 관입하는 시대미상의 페그마타이트와 백악기 반암류 등이 분포하는 신보광산 동부지역에서 이들 구성암류가 경험한 변형작용과 변성광물의 성장 사이의 상대적인 시간관계에 대한 미구조의 연구결과를 보고하고, 기존 연구결과를 종합하여 광화작용의 상대적인 시기를 고찰하였다. D1 변형작용은 홍주석 반상변정 내에서 주로 신장된 석영, 흑운모, 불투명광물 등으로 정의되는 직선 형태의 내부엽리 Si 를 형성시켰다. Si 엽리를 미습곡시키고 S2 파랑엽리를 형성시키는 D2 변형작용은 전기 파랑습곡작용과 후기 파랑습곡작용으로 구분된다. 전자는 홍주석의 맨틀부에서 곡선 형태의 Si 를 형성시켰다. 후자는 홍주석을 포획하며 그 외부에서 S1-2 복합엽리를 형성시켰다. 홍주석 반상변정은 직선 Si 엽리에 해당하는 S1 엽리가 형성된 이후의 비변형작용 조건하에서 출현하여 후기 파랑습곡작용 이전까지 성장하였다. 시대미상의 페그마타이트는 D2 변형 이후에 관입하여 S1-2 복합엽리를 무질서하게 덮는 섬유상 규선석을 성장시켰다. D3 변형작용은 S1-2 복합 엽리와 D2 파랑습곡 그리고 섬유상 규선석을 미습곡시키는 F3 습곡을 형성시켰다. 이는 섬유상 규선석의 성장과 관련된 페그마타이트의 관입은 D2 변형과 D3 변형 사이의 비변형조건하에서 발생하였음을 지시한다. 흑운모의 녹니석화작용과 홍주석 반상변정 내부에서 S1-2 복합엽리와 F3 습곡축면에 평행한 두 방향의 벽개 라멜라에 의해 인지되는 후퇴변성작용은 각각 D2 후기변형과 D3 변형과 함께 발생하였다. 따라서 우라늄의 초생적 근원암을 페그마타이트로 간주하는 기존 연구결과를 고려해 볼 때, 시대미상의 페그마타이트는 지각이 상승하는 후퇴변성작용 동안에 해당하는 D2 변형과 D3 변형 사이의 비변형조건하에서 관입하여 신보광산 주변지역에 우라늄 광화대를 형성시킨 것으로 고찰된다. The geochemical high-grade uranium anormal zone has been reported in the Shinbo mine and its eastern areas, Jinan-gun, Jeollabuk-do located in the southwestern part of Ogcheon metamorphic zone, Korea. In this paper is reported the time-relationship between deformation and growth of metamorphic minerals in the eastern area of Shinbo mine, which consists of the Precambrian metasedimentary rocks (quartzite, metapelite, metapsammite) and the age-unknown pegmatite and Cretaceous porphyry which intrude them, and is considered the relative mineralization time on the basis of the previous research's result. The D1 deformation formed the straight-type Si internal foliation which is defined mainly as the arrangement of elongate quartz, biotite, opaque mineral in andalusite porphyroblast. The D2 deformation, which is defined by the microfolding of Si foliation, formed S2 crenulation cleavage. It can be divided into two sub-phases, early crenulation and late crenulation. The former occurs as the curvetype Si foliation in the mantle part of andalusite. The latter occurs as S1-2 composite foliation which warps around the andalusite. The andalusite porphyroblast began to grow under non-deformation condition after the formation of S1 foliation which corresponds to the straight-type Si foliation. It continued to grow before the late crenulation phase. The age-unknown pegmatite intruded after the D2 deformation and grew the fibrous sillimanite which random masks the S1-2 composite foliation. The D3 deformation formed F3 fold which folded the S1-2 composite foliation, D2 crenulation, fibrous sillimanite. It means that the intrusion of pegmatite related to the growth of the fibrous sillimanite took place during the inter-tectonic phase of D2 and D3 deformations. The retrograde metamorphism is recognized by the chloritization of biotite and two-way cleavage lamellae which is parallel to the S1-2 composite foliation and the F3 fold axial surface in the andalusite porphyroblast. It occurred during the D2 late crenulation phase and D3 deformation. In considering of the previous research's result inferring the most likely candidate for the uranium source rock as pegamatite, it indicates that the age-unknown pegmatite intruded during the inter-tectonic phase of D2 and D3 deformations, i.e. during the retrograde metamorphism related to the uplifting of crust, and formed the uranium ore zone around the Shinbo mine.

Genesis of superimposed hypogene and supergene Fe orebodies in BIF at the Madoonga deposit, Yilgarn Craton, Western Australia

The Madoonga iron ore body hosted by banded iron formation (BIF) in the Weld Range greenstone belt of Western Australia is a blend of four genetically and compositionally distinct types of high-grade (>55 wt% Fe) iron ore that includes: (1) hypogene magnetite–talc veins, (2) hypogene specular hematite–quartz veins, (3) supergene goethite–hematite, and (4) supergene-modified, goethite–hematite-rich detrital ores. The spatial coincidence of these different ore types is a major factor controlling the overall size of the Madoonga ore body, but results in a compositionally heterogeneous ore deposit. Hypogene magnetite–talc veins that are up to 3 m thick and 50 m long formed within mylonite and shear zones located along the limbs of isoclinal, recumbent F1 folds. Relative to least-altered BIF, the magnetite–talc veins are enriched in Fe2O3(total), P2O5, MgO, Sc, Ga, Al2O3, Cl, and Zr; and depleted in SiO2 and MnO2. Mafic igneous countryrocks located within 10 m of the northern contact of the mineralised BIF display the replacement of primary igneous amphibole and plagioclase, and metamorphic chlorite by hypogene ferroan chlorite, talc, and magnetite. Later-forming, hypogene specular hematite–quartz veins and their associated alteration halos partly replace magnetite–talc veins in BIF and formed during, to shortly after, the F2-folding and tilting of the Weld Range tectono-stratigraphy. Supergene goethite–hematite ore zones that are up to 150 m wide, 400 m long, and extend to depths of 300 m replace least-altered BIF and existing hypogene alteration zones. The supergene ore zones formed as a result of the circulation of surface oxidised fluids through late NNW- to NNE-trending, subvertical brittle faults. Flat-lying, supergene goethite–hematite-altered, detrital sediments are concentrated in a paleo-topographic depression along the southern side of the main ENE-trending ridge at Madoonga. Iron ore deposits of the Weld Range greenstone belt record remarkably similar deformation histories, overprinting hypogene alteration events, and high-grade Fe ore types to other Fe ore deposits in the wider Yilgarn Craton (e.g. Koolyanobbing and Windarling deposits) despite these Fe camps being presently located more than 400 km apart and in different tectono-stratigraphic domains. Rather than the existence of a synchronous, Yilgarn-wide, Fe mineralisation event affecting BIF throughout the Yilgarn, it is more likely that these geographically isolated Fe ore districts experienced similar tectonic histories, whereby hypogene fluids were sourced from commonly available fluid reservoirs (e.g. metamorphic, magmatic, or both) and channelled along evolving structures during progressive deformation, resulting in several generations of Fe ore.

Paleosol occurrences along the Archean–Proterozoic contact in the Aravalli craton, NW India

The Archean–Proterozoic contact (APC) comprises a large scale folded surface around Udaipur in NW India. The APC is exposed on both eastern and western flanks of a synclinorium formed during thick skinned thrusting of the Archean basement and the Paleoproterozoic cover units. Paleosol occurrences on both the limbs of this structure imply an originally continuous paleoweathering horizon and not individual pockets of paleosol as seen today. Preservation of paleosol sections along the Archean–Proterozoic interface in the Aravalli craton is strongly controlled by the structural grain. Complete paleosol sections are recognized in the eastern flank of the Udaipur synclinorium in low strain regions with respect to the regional F1 and F2 folding. In this sector the Tulsi Namla paleosol section represents an ancient lateritic weathering profile with an iron depleted pallid lower zone (lithomarge) and an iron enriched upper zone (mottled zone to ferricrete). Original kaolinite in the saprolite and pallid zone has been transformed into kyanite during metamorphism and subsequently to white mica during retrograde overprint, while Fe rich clay and goethite were probably the primary minerals for the chloritoid and hematite which now constitute the upper zone. At the western flank of the synclinorium the contact is heavily sheared and Archean basement and Proterozoic units show interfingering on a small scale. Here, the paleosol sections are incomplete.The weathering interval can be inferred by the stabilization of the craton at ∼2.5Ga and deposition of rift related sequences of the Aravalli basin (Delwara formation) at ∼2.15Ga. The Udaipur paleosols, especially in the eastern flank, represent the most complete Archean–Paleoproterozoic paleosol sections in India.

Kilometer‐scale refolded folds caused by strike‐slip reversal and intraplate shortening in the Beishan region, China

Spectacular refolded folds clearly visible from space deform Permian turbidite successions in a 180 km2 fault‐bounded compartment within the Beishan region of northwestern China. The folds formed after final terrane amalgamation of the Central Asian Orogenic Belt and record an important sequence of intraplate deformation events in central Asia during the Late Permian‐Jurassic. The fold interference patterns are dominantly Type 2 refolds with kilometer‐scale wavelengths. They are bounded to the north and south by regional E‐W faults with multiple displacement histories as revealed by crosscutting ductile and brittle fabrics and kinematic indicators indicating opposing senses of motion. The proximity of the refolded folds to the main bounding faults strongly suggests that the fold interference structures were caused by strike‐slip fault reactivation and shear related bending of F1 folds combined with components of N‐S compression across the compartment. Faulting within the folded compartment is limited to an array of right‐lateral strike‐slip faults which appear to have helped accommodate room problems during the refolding event. The driving force for the deformation was likely to be distant—including regional block collision to the south, large‐scale rotations between the Siberian and North China Cratons and plate boundary convergence along the eastern Chinese margin. Our study demonstrates that continental interior regions characterized by terrane collages are susceptible to reactivation by distant plate margin effects and that polyphase reactivation of major strike‐slip and oblique‐slip faults can generate refolding within upper crustal basinal compartments. In addition, the fault‐bounded basin containing the refolded folds in the Beishan region is surrounded by older more complexly deformed crystalline basement. Therefore, the deformed basin sequence represents a unique compartmental archive of the Permian‐Triassic history of intraplate deformation north of Tibet.

Superposition of Four Generations of Folds and Development of Horse Creek Dome, Black Hills, South Dakota

The study area is a part of the Precambrian basement of the Black Hills that was uplifted during Laramide Orogeny. Early Proterozoic meta-conglomerate and greywacke units occupy the study area and form a crescent fold which is unusual structural pattern, mapped in the Precambrian core of the Black Hills. This study aims to determine how the crescent shape was formed via fieldwork investigation and structural data analysis. Four Precambrian (F1, F2, F3 and F4) folding events affected the study area prior to the uplift. Minor folds indicated that F1 and F4 exhibited isoclinal folding, F2 reflected similar folding and F3 displayed kink band and chevron folding. F2 is associated with axial plane cleavage (S2), trends towards the west, and is associated with regional metamorphism. F3 trends NNW and is associated with spaced cleavage (S3). F4 trends WSW and is associated with spaced crenulation cleavage (S4). The area was structurally divided into five domains. Poles to bedding (S0) and axial plane cleavage (S2) equal area stereographic projections show steeply plunging maxima that have monoclinic symmetry, resulting from superposition of two oblique F2 and F3 folding events. Domain I contains the Horse Creek dome that resulted from superposition of F3 on F2. The meta-conglomerate unit formed a crescent fold, due to superposition of the first two generations of folds. The crescent fold follows the Horse Creek dome, trends and opens westerly, and it is parallel to F4 folds.

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.

Polystage deformation of the Gaoligong metamorphic zone: Structures, 40Ar/39Ar mica ages, and tectonic implications

The Gaoligong metamorphic zone is located southeast of the Eastern Himalayan Syntaxis in western Yunnan, China. The zone is characterized by four stages of deformation (D1–D4). D1 structures record early compressive deformation during the Indosinian orogeny, which formed tight to isoclinal F1 folds of bedding with a penetrative S1 foliation developed parallel to fold axial planes. Mid-crustal horizontal shearing during D2 resulted in overprinting of D1 structures. D1 and D2 structures are associated with granulite facies metamorphism. D3 doming resulted in late crustal thickening and the development of a regional NW–SE trending F3 antiform. Synchronous with or slightly subsequent to D3 deformation, the zone experienced D4 ductile strike-slip shearing, resulting in its exhumation to shallow crustal levels and retrograde metamorphism. Granitic D4 mylonites predominantly yield 40Ar/39Ar mica ages of 15–16 Ma, indicating that D4 dextral strike-slip shearing occurred in the Miocene. Weakly deformed leucogranite and protomylonite yield 40Ar/39Ar ages of 10–11 Ma, suggesting that ductile strike-slip shearing continued to the Late Miocene. The new 40Ar/39Ar data indicate that escape-related deformation along the Gaoligong strike-slip shear zone occurred in the Miocene. In association with recent geophysical studies, and on the basis of the structural, crystal preferred orientation (CPO), and geochronological data presented in this paper, we suggest that the Gaoligong metamorphic zone formed in response to intracontinental transpression in the southeast of Tibet, characterized as intense deformation and metamorphism at middle–upper crustal levels.

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.

Petrotectonic framework of granulites from northern part of Chilka Lake area, Eastern Ghats Belt, India: Compressional vis-à-vis transpressional tectonics

Granulite-facies rocks occurring north-east of the Chilka Lake anothosite (Balugan Massif) show a complex metamorphic and deformation history. The M1–D1 stage is identified only through microscopic study by the presence of S1 internal foliation shown by the M1 assemblage sillimanite–quartz–plagioclase–biotite within garnet porphyroblasts of the aluminous granulites and this fabric is obliterated in outcrop to map-scale by subsequent deformations. S2 fabric was developed at peak metamorphic condition (M2–D2) and is shown by gneissic banding present in all lithological units. S3 fabric was developed due to D3 deformation and it is tectonically transposed parallel to S2 regionally except at the hinge zone of the F3 folds. The transposed S2/S3 fabric is the regional characteristic structure of the area. The D4 event produced open upright F4 folds, but was weak enough to develop any penetrative foliation in the rocks except few spaced cleavages that developed in the quartzite/garnet–sillimanite gneiss. Petrological data suggest that the M4–D4 stage actually witnessed reactivation of the lower crust by late distinct tectonothermal event. Presence of transposed S2/S3 fabric within the anorthosite arguably suggests that the pluton was emplaced before or during the M3–D3 event. Field-based large-scale structural analyses and microfabric analyses of the granulites reveal that this terrain has been evolved through superposed folding events with two broadly perpendicular compression directions without any conclusive evidence for transpressional tectonics as argued by earlier workers. Tectonothermal history of these granulites spanning in Neoproterozoic time period is dominated by compressional tectonics with associated metamorphism at deep crust.

Structural control of El Sela granites and associated uranium deposits, Southern Eastern Desert, Egypt

The El Sela area is a part of the basement complex of the Eastern Desert of Egypt and the Pan-African Shield. The area comprises outcrops of dismembered ophiolites thrust over arc volcano-sedimentary sequence and intruded by different syn- to post-tectonic granitoids. Structural analysis of the area enabled the separation and definition of four structural episodes: (E1) folding–thrusting episode associated with the cratonization of the arc/inter-arc rock association and the intrusion of the syntectonic (Older) granites. (E2) Upright folding episode associated with the compression and shortening to the ENE–WSW direction which is different from the NNW–SSE shortening direction during E1; at the end of E2, late tectonic granites were intruded. (E3) Post-tectonic granitic intrusion episode: two mica granite and granitic dikes were intruded during this episode. (E4) Fracturing, faulting, and post-granitic dike extrusion episodes caused different faults that took place after cratonization until the present. There are three generations of folds during ductile deformation (E1 and E2). The F2 folds are nearly coaxial (along ENE–WSW trend) with the F1 folds. The F3 folding is displayed by folds generally trending NNW–SSE. Therefore, the ENE–WSW and NNW–SSE trends can considered as preexisting discontinuities and mechanical anisotropy of the crust in the following structure episodes. Brittle deformation (E3 and E4) reveals the importance of those trends which control the multi-injections and many alteration features in the study area. During reactivation, a simple shear parallel to the inherited ductile fabrics was responsible for the development of mineralized structures along the ENE–WSW and NNW–SSE trends. So they can be considered as paleochannel trends for deep-seated structures and can act as a good trap for uranium and/or other mineral resources. Most of the uranium anomalies are delineated along ENE–WSW and NNW–SSE shear zones where quartz-bearing veins bounded the lamprophyre dike and microgranites and dissected them in relation to the successive fracturation and brecciation corresponding to the repeated rejuvenation of the structures. Therefore, the structural controls of the uranium mineralizations in the El Sela area appear to be related to the interaction between inherited ductile fabrics and overprinting brittle structures.

Infracambrian superimposed tectonics in the Late Proterozoic units of Mount Ablah area, southern Asir Terrane, Arabian Shield, Saudi Arabia

Mount Ablah is a mining prospect, hosted by a dioritic igneous body that is bound to the east by greenschist grade metamorphic rocks and to the west by Ablah group volcano–sedimentary rocks. Rock units of Mount Ablah area were remapped through field investigations, petrological studies, and analysis of enhanced TM Landsat data. Ablah group rocks were divided into lower tuffaceous and upper epiclastic units. The epiclastic unit was divided into three subunits. During remapping, a sliver of serpentinite was discovered, which occurs between the dioritic igneous body and Ablah group rocks. The greenschist grade metamorphic rocks were intruded by Late Proterozoic quartz diorite prior to deposition of Ablah group rocks. The epiclastic unit is an infracambrian molassic unit that filled a graben, known as the Ablah graben. The Mount Ablah area was intruded by post-tectonic granitic rocks and affected by two superimposed F1-F2 folding events, associated with thrust and dextral faults, respectively. The first folding event involved N–S folding and thrusting. Simultaneously, stress partitioning at E–W accommodations zones produced E–W minor folds (F2) and associated E–W dextral faults. The F1–F2 folding events are contemporaneous with the Pan African deformation event, also known as the East African Orogeny (EAO). The EAO is infracambrian in age and culminated in development of the Najd sinistral fault system. The E–W dextral faults were probably reactivated during Cenozoic Red Sea rifting. The Ablah graben's infracambrian sedimentary rocks, such as siltstone, sandstone, and limestone that are mainly bound within the Ablah graben were not deformed prior to F1–F2 folding. Thus, the upper epiclastic unit of the Ablah group rocks is rheologically different from the surrounding greenschist rocks, responded to the late E–W compression in a more ductile manner than the surrounding greenschist rocks. Therefore, the Ablah graben was inverted, refolded, and crosscut by E–W dextral faults during the infracambrian EAO event, prior to development of Najd sinistral fault system, which are evident in Asir Terrane and crosscut Ablah graben.

The nature of the Ailao Shan–Red River (ASRR) shear zone: Constraints from structural, microstructural and fabric analyses of metamorphic rocks from the Diancang Shan, Ailao Shan and Day Nui Con Voi massifs

The structural geology, timing of shearing, and tectonic implications of the ASRR shear zone, one of the most striking lineaments in Southeast Asia, have been the topics of extensive studies over the past few decades. The Xuelong Shan (XLS), Diancang Shan (DCS), Ailao Shan (ALS) and Day Nui Con Voi (DNCV) metamorphic massifs along the shear zone have preserved important information on its structural and tectonic evolution. Our field structural analysis, detailed microstructural and fabric analysis, as well as the quartz, sillimanite and garnet fabric studies of the sheared rocks from the massifs demonstrate the dominant roles of three deformation episodes during Cenozoic tectonic evolution in the shear zone. Among the contrasting structural and microstructural associations in the shear zone, D2 structures, which were formed at the brittle to ductile transition during large-scale left-lateral shearing in the second deformation episode, predominate over the structural styles of the other two deformation episodes. Discrete micro-shear zones with intensive grain size reduction compose the characteristic structural style of D2 deformation. In addition, several types of folds (early shearing folds, F21, and late-shearing folds, F22) were formed in the sheared rocks, including discrete to distributed mylonitic foliation, stretching lineation and shear fabrics (e.g., mica fish, domino structures, as well as sigma and delta fabrics). A sequence of microstructures from syn-kinematic magmatic flow, high-temperature solid-state deformation, to brittle–ductile shearing is well-preserved in the syn-kinematic leucocratic intrusions. Deformation structures from the first episode (D1) are characterized by F1 folds and distributed foliations (S1) in rocks due to pure shearing at high temperatures. They are preserved in weakly sheared (D2) rocks along the eastern margin of the ALS belt or in certain low-strain tectonic enclaves within the shear zone. Furthermore, semi-brittle deformation structures, such as hot striae and discrete retrogression zones, are attributed to normal-slip shearing in the third deformation episode (D3), which was probably locally active, along the eastern flank of the DCS range, for example.There are four quartz c-axis fabric patterns in the mylonitic rocks, including type A point maxima, type B Y point maxima with crossed girdles superimposition, type C quadrant maxima, as well as type D point and quadrant maxima combination. They are consistent with microscopic observations of microstructures of high-temperature pure shearing, low-temperature simple shearing and their superimposition. Integrated microstructural analysis and fabric thermometer studies provide information on both high temperature (up to 750°C) and dominant low-temperature (300–600°C) deformations of quartz grains in different rock types. Sillimanite and garnet fabrics, especially the latter, were primarily formed at the peak metamorphism during high-temperature pure shearing.The above structural, microstructural and fabric associations were generated in the tectonic framework of the Indian-Eurasian collision. The low-temperature microstructures and fabrics are attributed to left-lateral shearing along the ASRR shear zone from 27 to 21Ma during the southeastward extrusion of the Indochina block, which postdated high-temperature deformation at the peak metamorphism during the collision.

The structure of the Glenelg-Attadale Lewisianoid Inlier and its relationship to the Moine Thrust Zone

Synopsis The Glenelg-Attadale Lewisianoid Inlier lies adjacent to the Moine Thrust Zone. The inlier is surrounded and divided into Eastern and Western units by Moinian metasediments. The Eastern Unit was emplaced against the Western Unit in the Grenvillian Orogeny. Moinian sediments were deposited unconformably on both units in the Neoproterozoic. During the Late Neoproterozoic Knoydartian and Lower Palaeozoic Caledonian orogenies both the inlier and the overlying Moinian rocks were deformed and metamorphosed in the amphibolite facies. Deformation affected particularly the western and eastern margins of the inlier and a strip of infolded Moinian rocks between Eastern and Western units, with the formation of blastomylonites and isoclinal folding (F1). Above a major décollement in the Moine strip and along the western margin of the inlier, the rocks were folded by easterly plunging reclined (F2) folds. This phase of folding affected mainly the Lewisianoid rocks of the Eastern Unit and the adjacent Moine sediments of the Mountain Belt. Deformation was associated with the formation of a large-scale mushroom interference structure and the extrusion of the inlier northwestwards, towards the foreland, marked by a dominant ESE stretching lineation. In Avernish, reclined F2 folds are cut by randomly-oriented hornblende porphyroblasts, which are altered to biotite and smeared out in localized mylonitic shear zones similar to the mylonites in the Moine Thrust Zone. If these mylonitic shear zones were formed contemporaneously with the mylonites in the Thrust Zone, folding and thrusting events superimposed on the mylonites, must have occurred later than events in the Mountain Belt.

A Neoproterozoic continental rift succession: The volcano-sedimentary Koivib Mountains deposits of Namibia

Neoproterozoic volcanic and sedimentary rocks compose the 750m thick Koivib Mountains succession, which forms part of the Rosh Pinah Formation of southwest Namibia. Detailed sedimentary facies analysis and physical volcanology enabled determination of four lithofacies and component twelve facies that record subaqueous deposition in a tectonically controlled shallow sea. The mafic volcanic lithofacies is composed of mafic: (1) flow, (2) volcaniclastic, and (3) intrusive facies characteristic of seamount building by effusive volcanic processes. The felsic volcanic lithofacies containing felsic: (4) flow and (5) volcaniclastic facies displays features characteristic of proximal to distal and marginal regions of subaqueous dome–flow complexes. The siliciclastic lithofacies composed of (6) shale, (7) thin- to medium-bedded sandstone, (8) thick-bedded sandstone, and (9) sedimentary breccia facies is consistent with a submarine fan environment in which subaqueous density current and turbidity flow processes were predominant. The silicilastic–carbonate lithofacies, which contains (10) massive bedded carbonate, (11) laminated to cross-bedded calcarenite, and (12) graded bedded calcarenite, formed mainly through erosion of primary carbonate and transport of material from a platform to a slope setting. The overall stratigraphy, combined with geochemical results indicating bimodal volcanism in a within-plate tectonic setting, signifies that the Koivib Mountains succession developed as part of a continental rift sequence associated with Neoproterozoic break-up of the Rodinia supercontinent. The structural geology of the Koivib Mountains revealed two generations of folds, including (i) NW trending tight, isoclinal folds (F1) and (ii) NNW trending open folds (F2). The second deformation event refolded F1 folds, producing periclinal structures. The deformation features are consistent with SE transpression associated with the continental collision that occurred during closure of the Adamastor Ocean and formation of Gondwana.