Fabric development during shear deformation in the Main Central Thrust Zone, NW-Himalaya, India

Quartz microfabrics and associated microstructures have been studied on a crustal shear zone—the Main Central Thrust (MCT) of the Himalaya. Sampling has been done along six traverses across the MCT zone in the Kumaun and Garhwal sectors of the Indian Himalaya. The MCT is a moderately north-dipping shear zone formed as a result of the southward emplacement of a part of the deeply rooted crust (that now constitutes the Central Crystalline Zone of the Higher Himalaya) over the less metamorphosed sedimentary belt of the Lesser Himalaya. On the basis of quartz c- and a-axis fabric patterns, supported by the relevant microstructures within the MCT zone, two major kinematic domains have been distinguished. A noncoaxial deformation domain is indicated by the intensely deformed rocks in the vicinity of the MCT plane. This domain includes ductilely deformed and fine-grained mylonitic rocks which contain a strong stretching lineation and are composed of low-grade mineral assemblages (muscovite, chlorite and quartz). These rocks are characterized by highly asymmetric structures/microstructures and quartz c- and a-axis fabrics that indicate a top-to-the-south sense that is compatible with south-directed thrusting for the MCT zone. An apparently coaxial deformation domain, on the other hand, is indicated by the rocks occurring in a rather narrow belt fringing, and structurally above, the noncoaxial deformation domain. The rocks are highly feldspathic and coarse-grained gneisses and do not possess any common lineation trend and the effects of simple shear deformation are weak. The quartz c-axis fabrics are symmetrical with respect to foliation and lineation. Moreover, these rocks contain conjugate and mutually interfering shear bands, feldspar/quartz porphyroclasts with long axes parallel to the macrosopic foliation and the related structures/microstructures, suggesting deformation under an approximate coaxial strain path.On moving towards the MCT, the quartz c- and a-axis fabrics become progressively stronger. The c-axis fabric gradually changes from random to orthorhombic and then to monoclinic. In addition, the coaxial strain path gradually changes to the noncoaxial strain path. All this progressive evolution of quartz fabrics suggests more activation of the basal, rhomb and a slip systems at all structural levels across the MCT.     

Mylonitic microfabrics from the rocks of MCT zone in Alakhnanda valley,Garhwal Himalaya

MCT Zone of Alakhnanda valley is a major ductile shear zone in Garhwal Himalaya, which is characterised by different types of mylonite rocks. On the basis of grain size and the percentage of matrix in the rock, zones comprising protomylonite, augen mylonite, mylonite and ultramylonite have been identified. The study of microstructures, grain size and crystallographic preferred orientation of quartz c-axis fabric reveals that the rocks of the MCT zone were deformed by a combination of intracrystalline creep (power law creep) and grain boundary migration (sliding super plasticity).     

Inverted metamorphism and the Main Central Thrust: field relations and thermobarometric constraints from the Kishtwar Window, NW Indian Himalaya

Following the early Eocene collision of the Indian and Asian plates, intracontinental subduction occurred along the Main Central Thrust (MCT) zone in the High Himalaya. In the Kishtwar–Zanskar Himalaya, the MCT is a 2 km thick shear zone of high strain, distributed ductile deformation which emplaces the amphibolite facies High Himalayan Crystalline (HHC) unit south‐westwards over the lower greenschist facies Lesser Himalaya. An inverted metamorphic field gradient, mapped from the first appearance of garnet, staurolite and kyanite index minerals, is coincident with the high strain zone. Petrography and garnet zoning profiles indicate that rocks in the lower MCT zone preserve a prograde assemblage, whereas rocks in the HHC unit show retrograde equilibration. Thermobarometric results derived using THERMOCALC indicate a P–T increase of c. 180 °C and c. 400 MPa across the base of the MCT zone, which is a consequence of the syn‐ to postmetamorphic juxtaposition of M1 kyanite grade rocks of the HHC unit on a cooling path over biotite grade footwall rocks, which subsequently attain their peak (M2) during thrusting. Inclusion thermobarometry from the lower MCT zone reveals that M2 was accompanied by loading, and peak conditions of 537±38 °C and 860±120 MPa were attained. M1 kyanite assemblages in the HHC unit, which have not been overprinted by M2 fibrolitic sillimanite, were not significantly affected by M2, and conditions of equilibration are estimated as 742±53 °C and 960±180 MPa. There is no evidence for dissipative or downward conductive heating in the MCT zone. Instead, the primary control on the distribution of peak assemblages, represented by the index minerals, is postmetamorphic ductile thrusting in a downward propagating shear zone. Polymetamorphism and diachroneity of equilibration are also important controls on the thermal profile through the MCT zone and HHC unit.     

Strain in an Archean greenstone belt of Minnesota

We measured strain at more than 60 locations in metasedimentary and metavolcanic rocks of the Vermilion district, an E-W trending Archean greenstone belt in Minnesota. Strain ellipsoid orientations and shapes correlate strongly with N-S location in the belt, but magnitudes do not. Flattening strains occur near the present Vermilion fault (which bounds the greenstone belt to the north) with constrictional strains to the south. The observed strain patterns can be mathematically modeled by deformation paths which produce the flattening strains (with west plunging λ₁ axes) by dextral shear of the constrictional strains (with east plunging λ₁ axes). Using reasonable geologic constraints, the shear plane must dip to the north with a subhorizontal shear direction. Structures throughout the district also indicate dextral shear. A geometrical finite element program uses the measured strains to destrain the rocks and find the configuration which most closely satisfies strain compatibility equations. The linear E-W strain patterns and minor rotations about horizontal axes during the deformation preclude origin of the greenstone belt by infolding and shear off the flanks of a rising granitic diapir. By accounting for rotations which result in the (deformed) curvature of the original surface, a true estimate of 50% N-S shortening across the belt can be made. The data and deformation models favor the origin of the Vermilion district rocks at a convergent margin, most likely as a N-dipping subduction zone complex with shallow slab dip. The origin of the constrictional strains remains enigmatic.     

Dynamics of Saxothuringian subduction channel/wedge constrained by phase‐equilibria modelling and micro‐fabric analysis

Subduction and exhumation dynamics can be investigated through analysis of metamorphic and deformational evolution of associated high‐grade rocks. The Erzgebirge anticline, which forms at the boundary between the Saxothuringian and Teplá‐Barrandian domains of the Bohemian Massif, provides a useful study area for these processes owing to the occurrence of numerous meta‐basites preserving eclogite facies assemblages, and coesite and diamond bearing quartzofeldspathic lithologies indicating subduction to deep mantle depths. The prograde and retrograde evolution of meta‐basite from the Czech portion of the Erzgebirge anticline has been constrained through a combination of thermodynamic modelling and conventional thermobarometry. Garnet growth zoning indicates that the rocks underwent burial and heating to peak conditions of 2.6 GPa and at least 615 °C. Initial exhumation occurred with concurrent cooling and decompression resulting in the growth of amphibole and zoisite poikiloblasts overgrowing and including the eclogite facies assemblage. The development of clinopyroxene–plagioclase–amphibole symplectites after omphacite and Al‐rich rims on matrix amphibole indicate later heating at the base of the lower crust. Omphacite microstructures, in particular grain size analysis and lattice‐preferred orientations, indicate that the prograde evolution was characterized by a constrictional strain geometry transitioning into plane strain and oblate fabrics during exhumation. The initial constrictional strain pattern is interpreted as being controlled by competing slab pull and crustal buoyancy forces leading to necking of the subducting slab. The transition to plane strain and flattening geometries represents transfer of material from the subducting lithosphere into a subduction channel, break‐off of the dense slab and rebound of the buoyant crustal material.     

Strain and Crystallographic Fabric in Mesoscopic Ductile Shear Zones of Garhwal Himalaya

The MCT Zone of Bhagirathi valley of Garhwal Himalaya is characterized by numerous mesoscopic ductile shear zones. These shear zones are developed in response to nearly NNE-SSW maximum horizontal compression and provide an opportunity to study the variation in strain and crystallographic fabrics within the ductile shear zones.The grain shape and orientation of quartz under microscope reflect that strain is higher in the center and it progressively decreases towards the shear zone boundary. The preferred orientation of quartz c-axes across the shear zone suggests that the single girdle of the quartz c-axes are probably first developed at the shear zone boundary and become prominent in the center of shear zone with increase in the intensity of deformation. The strong crystallographic preferred orientation normal to foliation suggests that the internal deformation of the quartz might have taken place by dislocation creep mechanism exhibiting a non-coaxial deformation history.     

Strain distribution in a fold in the West Norwegian Caledonides

Strain has been measured from clasts within a deformed conglomerate layer at 17 localities around an asymmetric fold in the Rundemanen Formation in the Bergen Arc System, West Norwegian Caledonides. Strain is very high and a marked gradient in strain ellipsoid shape exists. To either side of the fold, strain within the conglomerate bed is of the extreme flattening type. In the fold, especially on the lower fold closure, the strain is constrictional. Mathematical models of perturbations of flow in glacial ice have produced folds of the same geometry as this fold, with a strikingly similar pattern of finite strain. The fold geometry and strain pattern, as well as other field observations, suggest that the fold developed passively, as the result of a perturbation of flow in a shear zone, where the strain was accommodated by simple shear accompanied by extension along Y.     

MAIN CENTRAL THRUST ZONE IN THE KATHMANDU AREA, CENTRAL NEPAL, AND ITS TECTONIC SIGNIFICANCE

MAIN CENTRAL THRUST ZONE IN THE KATHMANDU AREA, CENTRAL NEPAL, AND ITS TECTONIC SIGNIFICANCE1　AritaK ,LallmeyerRD ,TakasuA .TectonothermalevolutionoftheLesserHimalaya ,Nepal:constraintsfrom 4 0 Ar/3 9AragesfromtheKathmandunappe[J].TheIslandArc ,1997,6 :372～ 384. 2　RaiSM ,GuillotS ,LeFortP ,etal.Pressure temperatureevolutionintheKathmanduandGosainkundregions ,CentralNepal[J].JourAsianEarthSci ,1998,16 :2 83～ 2 98. 3　SchellingD ,KArita .…     

大山口剪切带大理岩糜棱岩的变形机制分析

大山口韧性剪切带是大别造山带"结晶轴带"内平行造山带延长方向的走滑型剪切带,其内发育的大理岩糜棱岩呈现了各种在绿片岩相变质条件下形成的显微构造,如碎斑构造、S-C组构、核幔构造等。糜棱岩化作用是以方解石颗粒粒度的减小为特征的,这一过程表现了大别造山带古老中地壳应变软化带和局部化带的岩石流变行为和物理状态。根据各种显微标志,并综合宏观特征,得出大山口剪切带总体为以右行剪切为主的走滑型剪切带,并被以正向滑动为主的近水平剪切带所改造。
作者在综合分析宏观及微观地质资料的基础上,初步建立了广水地区主要地质及变形事件序列,并指出:大别造山带三维空间上的变形及结构图象,主要是印支-燕山期陆-陆俯冲、碰撞造山运动的变形及造山期后的揭顶、塌陷等作用的综合结果。      

海南岛戈枕脆、韧性叠加剪切带基本特征及其成矿意义

戈枕脆、韧性叠加剪切带为一条呈北东向横贯海南岛土外山金矿区的窄长线性强应变带,具有显著的塑性流变、剪切应变及递进演化的特点。带内岩石表现出强烈的韧性变形和糜棱岩化及叠加的碎袭岩化,以及绿片岩相退化变质作用,形成一套由不同时代不同类型岩石的糜棱岩、碎袭岩所组成,呈似透镜状平行产出的剪切带。其中较显著的是产生一套类似混合岩外貌的长英质糜棱岩系列岩石。戈枕脆、韧性叠加剪切带的发现和确定对研究土外山金矿床的成因以及进一步找矿具有十分重要的意义。     

Intrafolial folds and associated structures in a progressive strain environment of Darjeeling-Sikkim Himalaya

The Dating rocks and Darjeeling gneisses, which constitute the Sikkim dome in eastern Himalaya, as well as the Gondwana and Buxa rocks of ‘Rangit Window’, disclose strikingly similar sequences of deformation and metamorphism. The structures in all the rocks belong to two generations. The structures of early generation are long-limbed, tight near-isoclinal folds which are often intrafolial and rootless. These intrafolial folds are associated with co-planar tight folds with variably oriented axes and sheath folds with arcuate hinges. Penetrative axial plane cleavage and mineral lineation are related structures; transposition of bedding is remarkable. This early phase of deformation (D ₁) is accompanied by constructive metamorphism. The structures of later generation are open, asymmetrical or polyclinal; a crenulation cleavage or discrete fracture may occur. The structures of early generation are distorted by folds of later generation and recrystallized minerals are cataclastically deformed. Recrystallization is meagre or absent during the later phase of deformation (D ₂). The present discussion is on structures of early generation and strain environment during theD ₁ phase of deformation. The concentration of intrafolial folds in the vicinity of ductile shear zones and decollement or detachment surface (often described as ‘thrust’) may be considered in this context. The rocks of Darjeeling-Sikkim Himalaya display minor structures other than intrafolial folds and variably oriented co-planar folds. The state of finite strain in the rocks, as observed from features like flattened grains and pebbles, ptygmatic folds and boudinaged folds indicate combination of flattening and constrictional type strain. The significance of the intrafolial folds in the same rocks is discussed to probe the environment of strain during progressive deformation (D ₁).     

An evaluation of the inverted metamorphic gradient at Langtang National Park, Central Nepal Himalaya

Abstract The crystalline core of the Himalayan orogen in the Langtang area of Nepal, located between the Annapurna-Manaslu region and the Everest region, contains middle to upper amphibolite grade pelitic gneisses and schists. These rocks are intimately associated with the Main Central Thrust (MCT), one of the major compressional structures in the northern Indian plate, which forms a 3.7-km-wide zone containing rocks of both footwall and hangingwall affinity. An inverted metamorphic gradient is noticeable from upper footwall through hangingwall rocks, where metamorphic conditions increase from garnet grade near the MCT zone to sillimanite + K-feldspar grade in the upper hangingwall. Petrographic data distinguish two metamorphic episodes that have affected the area: a high-pressure, moderate-temperature episode (M1) and a moderate-pressure, high-temperature episode (M2). Comparison with appropriate reaction boundaries suggests that conditions for M1 in the hangingwall were approximately 900–1200 MPa and 425–525°C. Thermobarometric results for 24 samples from the footwall, MCT zone and hangingwall reflect P-T conditions during the M2 phase of 400–1200 MPa and 490–660° C. The decrease in estimated palaeopressures from footwall to hangingwall approximate a lithostatic gradient of 27 MPa km^-1, with slight fluctuations in the MCT zone reflecting structural discontinuities. In contrast to the palaeopressures, palaeotemperatures are indistinguishable across the entire area sampled. Although field evidence suggests the presence of the inverted palaeothermal gradient well known in the Himalaya, quantitative thermobarometry indicates that temperatures of final equilibration were all within error of each other across 17 km of section. At Langtang, change in pressure is responsible for the presence of the sequence of index minerals through the section. I interpret these data to reflect diachronous attainment of equilibrium temperature conditions in a lithostatic palaeopressure profile after ductile faulting of the sequence.     

Transtensional folding

Strain modeling shows that folds can form in transtension, particularly in simple shear-dominated transtension. Folds that develop in transtension do not rotate toward the shear zone boundary, as they do in transpression; instead they rotate toward the divergence vector, a useful feature for determining past relative plate motions. Transtension folds can only accumulate a fixed amount of horizontal shortening and tightness that are prescribed by the angle of oblique divergence, regardless of finite strain. Hinge-parallel stretching of transtensional folds always exceeds hinge-perpendicular shortening, causing constrictional fabrics and hinge-parallel boudinage to develop.These theoretical results are applied to structures that developed during oblique continental rifting in the upper crust (seismic/brittle) and the ductile crust. Examples include (1) oblique opening of the Gulf of California, where folds and normal faults developed simultaneously in syn-divergence basins; (2) incipient continental break-up in the Eastern California-Walker Lane shear zone, where earthquake focal mechanisms reflect bulk constrictional strain; and (3) exhumation of the ultrahigh-pressure terrain in SW Norway in which transtensional folds and large magnitude stretching developed in the footwall of detachment shear zones, consistent with constrictional strain. More generally, folds may be misinterpreted as indicating convergence when they can form readily in oblique divergence.     

POLYPHASE METAMORPHISM AND INVERTED THERMAL GRADIENT IN THE LESSER HIMALAYA OF CENTRAL NEPAL: CONSTRAINTS FROM WHITE MICA COMPOSITIONS

POLYPHASE METAMORPHISM AND INVERTED THERMAL GRADIENT IN THE LESSER HIMALAYA OF CENTRAL NEPAL: CONSTRAINTS FROM WHITE MICA COMPOSITIONS     

High-Field Strength Element Geochemistry of Mafic Intrusive Rocks from the Bhagirathi and Yamuna Valleys,Garhwal Himalaya,India

Incompatible high-field strength element geochemistry divides mafic intrusive rocks of Garhwal Himalaya, exposed in the Bhagirathi and Yamuna valleys, into two distinct types, viz. older Garhwal Mafic Intrusive Rocks (GMIR1) and younger Garhwal Mafic Intrusive Rocks (GMIR2). GMIR1 is mainly associated with the Central Crystallines, whereas, GMIR2 belongs to the Garhwal Group. They outcrop close to the Main Central Thrust (MCT) Zone and extend in a NW-SE direction, following the strike direction. Both types are metamorphosed to amphibolite facies and classified as sub-alkaline, high-Fe tholeiitic basalts. They are enriched in large-ion lithophile and high-field strength elements relative to a primitive mantle source. GMIR2 has higher concentrations of incompatible high-field strength elements than the GMIR1. Further, on multi-element spidergrams, GMIR1 samples do not show any elemental anomaly but GMIR2 samples show distinct negative Nb and Sr anomalies. GMIR1 multi-element and rare-earth element trends are similar to N-MORB, whereas, GMIR2 follows trends observed in the within plate basalts (continental flood basalts type). Discrimination diagrams, based on incompatible trace elements, corroborate the N-MORB nature of GMIR1 and CFB (WPB) nature of GMIR2. Geochemical modeling indicates that these mafic intrusive rocks were derived by close-system fractional crystallization of depleted (for GMIR1) to variably enriched parental basalts (for GMIR2). Probably these two contrasting mafic rock types have been juxtaposed during the Himalayan orogeny. Proterozoic mafic rocks of similar geochemical characteristics have been reported from different parts of the northern Indian lithosphere. Observed composition suggests its genetic association with the sub-continental lithosphere rather than effect of crustal contamination.     

Geometrical analysis of mesoscopic shear zones in the crystalline rocks of MCT zone of Garhwal Higher Himalaya

The crystalline rocks of the MCT Zone of Garhwal Higher Himalaya exhibit well-preserved mesoscopic shear zones. Majority of these shear zones are of ductile and brittle ductile type with both sinistral and dextral sense of movement. Detailed analysis of mesoscopic shear zones reveals that sinistral shear zones exhibit a strike variation from NNE to ENE and dextral shear zones exhibit variation from NNW to WNW directions thus forming a conjugate pair. The bisectors of statistically preferred orientations of the two sets of the shears indicate that they generated due to NNE–SSW horizontal compression. These dextral and sinistral shear zones exhibit strike–slip geometry developed during progressive ductile shearing.     

辽西兴城—台里韧性剪切带北段变形特征

辽西兴城—台里地区发育系列花岗质岩石,强烈构造变形特征均显示其具有韧性剪切带的特点。对剪切带北段进行详细宏微观构造解析,结合岩石变形强度差异性分析、有限应变测量、石英C轴EBSD测试以及古差异应力值估算等研究,结果表明剪切带内花岗质片麻岩和眼球状花岗质片麻岩具有NEE向左行剪切变形特征,变形岩石为S-L构造岩,应变类型属于平面应变,古差异应力值介于30~40 MPa之间。长石-石英矿物温度计以及石英C轴EBSD组构指示剪切带以中低温变形为主,温度在400℃~500℃,属绿片岩相变质,具中-低温韧性剪切带特征。韧性剪切带内普遍存在变形分解现象,弱变形带内岩石残斑含量较高,眼球状构造和S-C组构较为发育;强变形带岩石残斑含量较低,剪切面理较为发育,糜棱面理发育较弱或者不发育。     

A structural transect in the Lower Dolpo: Insights on the tectonic evolution of Western Nepal

We present the results of a structural transect in Lower Dolpo, cross-cutting the upper part of the Lesser Himalaya (LH), the Higher Himalayan Crystallines (HHC) and the lower part of the Tibetan Sedimentary Sequence (TSS). The MCT zone affects the upper part of the LH as well as the lower part of the HHC and shows a later brittle reactivation. Mean vorticity in the MCT points to non-coaxial deformation. These data, together with available kinematic data along the belt, on the South Tibetan Detachment System (STDS) and in the core of the HHC, point to increasing simple shear toward the tectonic boundaries. A top-to-the-SW high-temperature shear zone (Toijem Shear Zone) is recognized in the middle part of the HHC at the boundary between Units 1 and 2. It developed during the earlier stages of exhumation of the HHC, enhancing the decompression of the hanging wall and the emplacement of leucogranite dykes and sills. Its development could be explained by a change in the velocity profile during the extrusion of the HHC, triggered by first order changes in rock types of the tectonic unit. The STDS is marked by a wide zone of high strain and by a metamorphic jump from amphibolite facies in the carbonate rocks of the upper part of the HHC to greenschist facies marbles in the lower part of the TSS. The development of a pervasive foliation towards the bottom of the TSS indicates increasing strain, related to top down-to-the-NE tectonic transport. A Low P metamorphic event, marked by the growth of post-D1 biotite porphyroblasts at the base of the TSS, is related to the conductive heating from the underlying HHC.     

Geometry and kinematics of the Mosha Fault, south central Alborz Range, Iran: An example of basement involved thrusting

Field investigation of the western part of the Mosha Fault in several structural sections in the south central Alborz Range showed that the fault has a high angle of dip to the north, and emplaces Precambrian to Cenozoic rocks over the Eocene Karaj Formation. Study of the kinematics of the Mosha Fault in this area, based on S–C fabric and microstructures, demonstrates that it is a deep-seated semi-ductile thrust. Strain analysis on rock samples from different sections across the Mosha Fault shows a flattening pattern of deformation in which the long axis of the strain ellipsoid is aligned in the fault shear sense. The Mosha Fault is associated with a large hanging-wall anticline, cored by Precambrian rocks, and series of footwall synclines, formed of late Tertiary rocks. This geometry, together with several low angle short-cut thrusts in the fault footwall, implies that the Mosha Fault is an inverted normal fault which has been reactivated since the late Tertiary. In the study area, the reverse fault mechanism was associated with the rapid uplift and igneous activity in the central Alborz Range during the late Tertiary, unlike in the eastern portion of the fault, where the fault kinematics was replaced by a strike-slip mechanism in the Late Miocene.