white mica crystallinity finite strain and cleavage development across a large variscan structure nw spain x ray diffraction analyses were performed on white micas formed under strain and low grade metamorphic conditions from sandstones and siltstones along a transect through the narcea antiform a large structure that separates the external from the internal zones of the variscan belt of nw spain the results indicate a close spatial relationship between the onset of metamorphism and the cleavage front at the cleavage front strain is characterized by oblate ellipsoids that have x y z ratios from 1 to 1 5 the x y z ratio increases westward up to 3 0 as does the grade of metamorphism and is accompanied by an increase in the 9crystallinity9 index of the white mica from very low to low metamorphic grade this increase corresponds qualitatively to an increase in penetrativeness of the axial planar cleavage in the western part of the narcea antiform the major deformation event was the development of kilometre scale reverse shear zones because of polyphase deformation these rocks are not suitable for quantitative strain analysis but qualitatively finite strain can be observed to be prolate and invariably larger in this region than in the eastern sector the metamorphic grade is higher in the western part of the narcea antiform with local maxima occurring next to the base of the shear zones the b 0 cell parameter a semi quantitative geobarometer measured in the white mica is chaotic in the eastern sector showing a clear detrital inheritance in the western sector it is rational due to complete re equilibration during thermal resetting the inverse correlation between finite strain and white mica 9crystallinity9 is close showing that the anchizone epizone limit can be related to the increase of finite strain above values of r s 1 5 the close relation between white mica 9crystallinity9 and strain is suitable for tracking strain variations in extensive fine grained siliciclastic rocks where no appropiate strain markers are found

slip system domains 2 kinematic aspects of fabric development in polycrystalline aggregates abstract in the first paper of the present series cobbold and gapais 1986 we develop theoretical models of plane strain domainal deformation by slip along inextensible and indestructible fibres for such extreme mechanical conditions we show that deformation is largely controlled by kinematic factors we use our models to explain domainal patterns of natural slip systems including bedding and foliation planes faults ductile shear zones and lattice planes the present paper focusses on kinematic aspects of fabric development in polycrystalline aggregates a three dimensional kinematic model is developed it reveals that common quartz fabrics may develop domainally by reorientation and selection of inextensible slip directions a axes or slip planes with initially random orientation stable orientations of slip directions are close to both surfaces of no finite extension and orientations of large amount of shear we explain 1 1 common patterns of quartz a and c axis fabrics e g small circles and crossed girdles 2 2 characteristic differences between fabric patterns according to the shape of the finite strain ellipsoid 3 3 development of asymmetric fabrics during non coaxial deformation histories this last process involves selective destruction or replacement of certain orientation domains so that a preferred orientation of slip lines develops close to the bulk shear direction rotation recrystallization is shown to be an efficient natural selection mechanism the models also account for many microstructural features found in oriented polycrystalline rocks especially 1 1 alternating band like or kink like domains of twinned preferred orientations 2 2 individual grains or clusters of grains with retort like shapes and extensive recrystallisation at boundaries 3 3 small scale shear bands formed by strain partitioning and shear localization

relations between thrust belt evolution grain scale deformation and metamorphic processes cape smith belt northern canada abstract an important step towards understanding how continental thrust belts evolve is to study the relationship between the map scale structures and the associated grain scale deformation mechanisms and metamorphic processes this relationship is examined in the early proterozoic cape smith thrust belt in the context of the belt s p t t history and fluid rock interactions overprinting deformation metamorphism relationships allow the thrust belt s evolution to be described in terms of four events 1 1 piggyback thrusting above a basal decollement localized at the basement cover contact 2 2 greenschist to amphibolite grade metamorphism and development of a ductile basal shear zone 3 3 re imbrication out of sequence faulting of the internal zone at pre to syn thermal peak conditions 4 4 post thermal peak out of sequence thrusting and retrograde metamorphism out of sequence thrusting was accommodated by the development of ductile shear zones in general relatively broad ductile shear zones developed adjacent to thrusts during prograde metamorphism while relatively narrow shear zones developed adjacent to thrusts during retrograde metamorphism the width of the shear zones was controlled by factors such as the competition between softening and hardening processes prograde shear zones and the extent of fluid influx retrograde shear zones four processes were primarily responsible for accomplishing penetrative deformation and accommodating thrust faulting during the belt s history 1 1 partitioning of strain into pre existing weak zones or into relatively weak layers in a compositionally stratified sequence 2 2 growth and or rotation of metamorphic products into orientations of maximum resolved shear stress 3 3 transitions in dominant deformation mechanism from brittle or semi brittle to crystal plastic 4 4 retrograde metamorphism involving fluid influx and hydration reactions producing weaker products than reactants

microstructural and fabric studies from the rocks of the moine nappe eriboll nw scotland abstract microstructures and quartz c axis fabric diagrams from mylonites and psammitic moine schists collected in traverses across the lower levels of the moine nappe in the eriboll area are presented on approaching the moine thrust from the kyle of tongue the following microstructural sequence is encountered interlayered coarse grained biotite psammitic and schistose tectonites being in part mylonitic with two platy slide zones one containing biotite and the other only muscovite and chlorite and both showing quartz microstructures indicative of post tectonic relaxation these pass into more mylonitic rocks nearer the thrust zone which in turn passes into the main chlorite grade mylonite belt and finally adjacent to the moine thrust into reworked lower chlorite grade mylonites although there is some local variation the overall quartz c axis fabric is an incomplete asymmetric type i girdle the main variation is the development of type ii girdles in the reworked ultrafine grained mylonites the extent of the mylonitization is more extensive than previously reported studies of folds within the mylonite belt have revealed eye structures and small scale folds many are sheath folds they cannot be unequivocally correlated with large scale recumbent folds within the moine nappe the results presented indicate that mylonitization is not limited to a single phase and raises the possibility that there may be earlier caledonian or possibly precambrian structural elements present in the eriboll region moines prior to much of the mylonitization

microstructures and strain variation across the footwall of the main central thrust zone garhwal himalaya india abstract the microstructural variation with a progressive change in the strain pattern are described in the rocks occurring across the footwall of the main central thrust mct in an area of the garhwal himalaya in the western garhwal himalaya the mct has brought upper amphibolite facies metamorphic rocks southward over the greenschist facies rocks of the lesser himalaya the progressively increasing flattening strain towards the mct changes either to plane strain or in some cases to constrictional strain this change in strain is well recorded in the microstructures the zone dominated by flattening strain is expressed as bedding parallel mylonites the grain reduction in this zone has occurred by dynamic recrystallization and quartz porphyroclasts were flattened parallel to the mylonite zone the maximum finite strain ratio observed in this zone is 2 2 1 8 1 the zone where the flattening strain changes either to plane strain or constrictional strain record an increase in finite strain ratio up to 3 8 1 9 1 this zone represents deformation fabrics like s c microstructures simultaneously developed during mylonitization in an intense ductile shear zone the above zone is either near the mct or adjacent to crystalline klippen occupying the core of the synforms in the footwall of the mct the microstructural evolution and the finite strain suggest that the mct has evolved as the result of superposition of southward directed simple shear over the flattening strain the simple shear has played an active role in the rapid translation which followed the mylonitization at deeper levels

tectonic evolution of kishtwar window with respect to the main central thrust northwest himalaya india abstract the crystalline rocks of the himalaya high himalayan crystallines central crystallines etc are bounded to the north by the south tibetan detachment zanskar shear zone trans himadri fault system at the top and to the south by the main central thrust mct at the base to the south and tectonically below the mct lies the lesser himalaya comprising low grade meta sedimentary thrust sheets and the crystallines klippen north and tectonically above the zanskar shear zone lies the sedimentary succession of the tethys himalaya however there is a different geological set up to the northwest of beas valley which defies this normal set up i e the tethyan rocks either lie in contact with the lesser himalayan thrust sheets or they to a large extent surround the high himalayan crystallines hhc this unique set up is documented around areas comprising chamba kishtwar and zanskar regions in the northwest himalaya the lesser himalayan imbricated sequence extending progressively into deeper structural level beneath and crops out as kishtwar window within the hhc this tectonic contact along the window was described as the mct however the present study of the window indicates that its western side is bounded by a high angle ne dipping reverse fault kishtwar thrust the structural analyses reveal that this fault is younger than the mct and is interpreted here as breach thrust present observations suggest that the rocks of the window and the northeasterly lying hhc have been folded and exhumed together as an antiform along the kishtwar thrust the hhc occurring south of the kishtwar window have actually been thrust over the lesser himalayan thrust sheet this contact is observed and marked for the first time and is accordingly described as the main central thrust as per convention along the length of the himalayan belt northeast and in the hangingwall of the mct the repetition of the contact of the lesser himalayan rocks and the hhc is interpreted here as the deeper part of the mct brought to the surface by kishtwar thrust

microstructural evolution of the serido belt ne brazil the effect of two tectonic events on development of c axis preferred orientation in quartz the polyphase evolution of the serido belt ne brazil includes d crust formation at 2 3 2 1 ga d 2 thrust tectonics at 1 9 ga and crustal reworking by d 3 strike slip shear zones at 600 ma microstructural investigations within mylonites associated with d 2 and d 3 events were used to constrain the tectono thermal evolution of the belt d 2 shear zones commenced at deeper crustal levels and high amphibolite facies conditions 600 650 degreesc through grain boundary migration subgrain rotation and operation of quartz q prism slip continued shearing and exhumation of the terrain forced the re equilibration of high t fabrics and the switching of slip systems from c prism to positive and negative a rhombs during d 3 enhancement of ductility by dissipation of heat that came from syn d 3 granites developed wide belts of amphibolite facies mylonites continued shearing uplift and cooling of the region induced d 3 shear zones to act in ductile brittle regimes marked by fracturing and development of thinner belts of greenschist facies mylonites during this event switching from a prism to a basal slip indicates a thermal path from 600 to 350 degreesc therefore microstructures and quartz c axis fabrics in polydeformed rocks from the serido belt preserve the record of two major events which includes contrasting deformation mechanisms and thermal paths c 2003 elsevier ltd all rights reserved

superposed deformations in the eastern alps strain analysis and microfabrics abstract superposition mainly of two eoalpine cretaceous deformation events can be observed in vertical sections and horizontal traverses throughout a decollement zone in the eastern alps consisting of crystalline basement and autochthonous and allochthonous cover stacked in several nappes the first event is an episode with a non coaxial strain history caused by w directed thrusting the second a n directed shortening by folding and fold imbrication strain analysis based both on deformed pebbles and on the preferred orientation of phyllosilicate grains march theory microfabric analysis and observations of the relative timing of deformation and metamorphism together indicate that the superposition of structures caused by the two events amounts to one continuous progressive act of deformation we attribute gradual and consistent variations in intensity and axial ratio of the tectonic strain to a history influenced by a the position of samples in the pile b the relative importance of the strain increments caused by the two events and c rock ductility we interpret variations of the c axis fabrics of quartz in the same way and draw tentative kinematic conclusions for the eoalpine orogeny in the eastern alps

spatiotemporal variation in exhumation of the crystallines in the nw himalaya india constraints from fission track dating analysis abstract during himalayan orogeny coeval thrusting along the main central munsiari thrust mct mt and extension along the south tibetan detachment system stds are widely responsible for rapid exhumation of the higher himalayan crystalline hhc zone apatite and zircon fission track data along the kaliganga and darma valleys in the kumaon himalaya serve to document the shallow bedrock exhumation history of the hhc taking into account sample location within the hhc with respect to the mct mt the apatite fission track aft data sets along the darma 1 0 0 1 to 2 8 0 3 ma and kaliganga 1 4 0 2 to 2 4 0 3 ma which are sharing same structural setting and rock types and being separated by 40 km show very similar patterns of exhumation histories since plio quaternary in the kumaon himalaya data sets along darma and kaliganga are very similar to data set of adjacent traverse 50 km away along the goriganga valley studied by patel and carter 2009 whole data sets within the hhc in kumaon himalaya provide clear evidence for plio quaternary tectonic activity along the vaikrita thrust vt precipitation in this region exerts a strong influence on erosional surface processes fluvial erosional unloading along the himalaya is focused on the high mountainous region of the hhc where the orographic barrier forces out the maximum percentage of annual rainfall ft cooling ages reveal coincidence between rapid erosion and exhumation that is focused in a 25 30 km wide sector of the hhc rather than covering the entire orogen similarity of aft age pattern and exhumation rates along all three major traverses goriganga darma and kaliganga indicates that the region has been experiencing constant rate of crustal uplift and erosion since long time comparison of fission track ages from the kumaon himalaya with other segments of the nw himalaya shows spatiotemporal variation in exhumation it is described due to the development of local structures such as dome window in jammu and kashmir and himachal himalayas and quaternary active thrusting along the vt and mct mt in the garhwal kumaon himalaya

shear zones developed in an experimentally deformed quartz mylonite specimens of an experimentally deformed quartz mylonite show the development of shear zones these shear zones are of two types type a shear zones are discrete and within the specimen whereas type b shear zones occur where the specimen has been sheared past the corner of the piston type a shear zones are comprised of elongate recrystallized grains with long axes oriented 30 40 to the shear zone boundary type b shear zones consist of strongly flattened and recrystallized ribbon grains augen occur where the deformation history is coaxial around the piston corner where the deformation history is non coaxial only strongly flattened grains occur the c axis fabrics which develop in the shear zones comprise girdles or maxima approximately normal to the foliation defined by the grain elongation shape fabric an asymmetry with respect to the shape fabric is present in many of the fabrics this asymmetry results from either using a quartz mylonite with a strong preferred orientation as the starting material or as a result of the non coaxial deformation where it can be demonstrated that the asymmetry results from the non coaxial deformation the asymmetry is a leading edge with respect to the sense of shear the results of this study suggest that care must be taken in interpreting asymmetric quartz c axis fabrics in natural shear zones particularly if it is suspected that the protolith to the shear zone had an initially strong preferred orientation type a shear zones originate as brittle zones extending from the specimen corners which become recrystallized as pressure and temperature are increased these shear zones show initial non coaxial deformation dominantly simple shearing followed by later flattening parallel to the shortening direction type b shear zones form when the piston punches into the barreling specimen the strain history of these shear zones is characterized by initial flattening up to 30 followed by shearing past the piston corner

microscopic anatomy of a hot on cold shear zone insights from quartzites of the main central thrust in the alaknanda region garhwal himalaya

evaluation of a regional strain gradient in mylonitic quartzites from the footwall of the main central thrust zone garhwal himalaya india inferences from finite strain and ams analyses abstract the footwall of the main central thrust mct zone along the bhagirathi valley comprises a wide zone of mylonitic quartzite and deep level tectonites the systematic variation of finite strain parameters es k and v in the mylonites indicates heterogeneous deformation which is determined to vary between simple shear and non coaxial flattening type in such a strain regime the outer boundary of the quartz clasts are no longer preserved thus leading to an error in finite strain measurement in order to supplement the finite strain studies anisotropy of magnetic susceptibility ams analyses were carried out on the mylonitic quartzites a systematic variation in degree of anisotropy p with distance from the mct is documented and is interpreted to be tectonic in origin based on these results it is concluded that p can be used as a strain intensity gauge at least on an outcrop scale where a systematic variation in p values from one part of the outcrop to the other can be established however the quantitative relation between principal axes of finite strain ellipsoid and ams axes magnitude of principal susceptibility difference k1 and k3 and finite strain magnitude e1 ln 1 e1 and e3 ln 1 e3 were related by a logarithmic relationship with a correlation coefficient of 0 844

kinematic vorticity flow analysis and 40ar 39ar geochronology related to inclined extrusion of the hp lt metamorphic rocks along the zagros accretionary prism iran abstract the zagros accretionary prism in southwestern iran is exposed along the nw se trending of the zagros thrust system and inclined zagros transpression zone this accretionary prism consists of two units the upper sedimentary melange unit on top and the high pressure metamorphic melange unit at the bottom both units show characteristics of a tectonic wedge the upper unit consists of type i ii and iii melanges which display s c and c type shear band cleavages quartz ribbons and rectangular or fish head boudins the lower unit fabrics display and type porphyroclasts and quartz ribbon mylonites these fabrics formed from a combination of 60 5 simple shear and 39 5 pure shear both components were involved in a lateral exhumation of the high pressure low temperature metamorphic rocks in an inclined transpression wedge shaped geometry the estimated kinematic vorticity number wk was calculated from quartz c axis patterns rotation of porphyroclasts and orientation of finite strain with respect to shear zone boundaries using the mean estimated wm value of 0 84 the inclination angle for the thrust wedge on top of the ne subducting neo tethyan oceanic lithosphere is 18 the 40ar 39ar plateau ages of early generations of biotite from the lower metamorphic melange are 119 95 0 88 ma and 112 58 0 66 ma this late aptian age is related to early thrusting and formation of hp lt metamorphic rocks the dating of two amphibole samples from the amphibolite yields a weighted mean age of 91 23 0 89 ma this turonian cenomanian age suggests a later metamorphic event associated with subduction and obduction of the neyriz ophiolite and later lateral extrusion of hp lt metamorphic rocks along the inclined zagros accretionary prism

hercynian tectonics in the pyrenees a new view based on structural observations around the bassie s granite pluton abstract the structural study of the metasedimentary rocks surrounding the bassie s granite pluton allows definition of the succession and kinematics of the hercynian tectonic events in this part of the pyrenees and supports a model of syntectonic emplacement of the pluton the following sequence of events has been identified i an early phase of n s compression essentially resulting in southwards thrusting ii emplacement of the granite body along a ne sw sinistral strike slip shear zone which could be a late effect of the first phase iii a major dextral and transpressive phase responsible for themain fold structures trending on average n100 and following a sigmoidal pattern around the granite body this succession of events and their kinematics could be extended to the whole hercynian basement of the pyrenees

structural evolution of the cape smith thrust belt and the role of out of sequence faulting in the thickening of mountain belts out of sequence thrusting represents a mechanism capable of heterogeneous internal deformation in thrust belts and was largely responsible for internal thickening in the early proterozoic cape smith belt the south verging thrust belt is marked by a complex history of accretion internal deformation thickening induced metamorphism uplift and erosion an overall chronology for thrust belt evolution is built around the timing of thermal peak metamorphism prethermal peak deformation includes 1 initial piggyback stacking of basalt dominated thrust sheets above a basal decollement localized at the basement cover contact 2 greenschist to amphibolite grade metamorphism accompanied by development of a ductile shear zone at the base of the thrust belt and 3 out of sequence thrusting precipitating the metamorphic thermal peak both above and below these faults synthermal peak out of sequence thrusting was responsible for accretion of autochthonous crystalline basement slices and eventual abandonment of the external basal decollement postthermal peak deformation is characterized by the continued development of out of sequence thrusts which appear to young toward the hinterland north internal thickening by development of flattening cleavages or folding appears to be regionally insignificant when compared to that accomplished by the out of sequence faults this suggests that 1 an eroding and accreting thrust wedge may be able to maintain its critical shape by out of sequence thrusting and 2 this style of internal deformation is favored in belts dominated by relatively strong material e g basalt as in the cape smith belt

metamorphic reactions related to decompression and synkinematic intrusion of leucogranite high himalayan crystallines bhutan the high himalayan crystallines hhc of bhutan were penetratively deformed intruded by leucogranite and metamorphosed during the collision of the indian and asian plates metamorphic reaction textures in the hhc show that it experienced decompression while maintaining a laterally heterogeneous and locally inverted internal temperature range of c 600 750c this thermal structure was produced by thrusting hot migmatitic rocks over lower grade rocks within the hhc and by the advection of heat from the intrusion of leucogranite dykes and sills during decompression a variable velocity field within the hhc during exhumation and extrusion between india and tibet caused the inversion of top to the south sense of shear present throughout most of the hhc to top down to the north shear near its top

heterogeneous deformation and quartz crystallographic fabric transitions natural examples from the moine thrust zone at the stack of glencoul northern assynt quartz crystallographic fabric transitions in well exposed mylonites immediately beneath the moine thrust at the stack of glencoul nw scotland have been investigated by optical microscopy x ray texture goniometry and orientation distribution function analysis a progressive change is observed from asymmetrical kinked single girdle c axis fabrics at 0 5 cm beneath the moine thrust through asymmetrical type i cross girdle fabrics to symmetrical type i cross girdle fabrics at 30 cm beneath the thrust this c axis fabric transition is accompanied by a transition from asymmetrical single a axis maximum fabrics 0 5 cm beneath the thrust through asymmetrical two maxima fabrics to essentially symmetrical two maxima a axis fabrics odf analysis of these s l and l s tectonites indicates that c axis positions on the leading edge of the fabric skeleton are related by a common a direction oriented within the xz plane at a moderate angle to the lineation x in contrast c axis positions on the peripheral trailing edge are related by a positive r rhomb pole oriented close to z a directions lying within this common rhomb plane progressively change through 180 in orientation traced around the c axis fabric skeleton such contrasting single crystal rhomb a preferred orientations on the leading and trailing edges of the fabric skeleton are interpreted as indicating localized grain scale plane strain and flattening deformation respectively they result in tectonites with essentially symmetrical c and a axis fabrics which display strongly asymmetrical positive r and negative z rhomb pole figures the observed transition in quartz c and a axis fabrics is interpreted as indicating an increasing importance of non coaxial plane strain deformation as the moine thrust is approached even immediately 1 cm beneath the thrust however flow has still significantly departed from bulk simple shear and involved an important heterogeneous component of contemporaneous flattening deformation

sense of nappe emplacement from quartz c axis fabrics an example from the betic cordilleras spain preferred crystallographic orientation patterns fabrics in zones of ductile deformation beneath nappes are a potentially useful indicator of the sense of nappe translation quartz c axis fabrics from a shear zone beneath the pile of higher betic nappes in the betic cordilleras spain have distinctly asymmetric topologies revealed by constructing a fabric skeleton these skeletons closely resemble those produced by computer simulations of quartz fabric development basedon taylor bishop hill theory the fabric asymmetry indicates a northerly sense of transport for the nappes consistent with field evidence from the same area analyses of both c and a axis fabrics based on asymmetric intensity distributions within the fabric give equivocal results and may reflect the presence of earlier preferred orientation patterns predating nappe emplacement

prism c slip in the quartzites of the oakhurst mylonite belt california abstract quartz c axis fabrics from 21 amphibolite facies quartzites of the oakhurst mylonite belt california suggest that prism c prism a and basal a slip systems were activated during deformation qualitative two dimensional kinematic models consistent with meso and microscopic shear criteria are constructed based on a regime of combined pure and simple shear in this kinematic framework most of the observed c axis fabric patterns of the oakhurst mylonite belt can be explained if prism c slip was dominant during the early increments of deformation until a transition to dominant basal a slip occurred an apparent relationship between grain size and type of crystallographic fabric is interpreted as resulting from some quartz syntectonic vein quartz having recorded only the latest increments of deformation comparison with the metamorphic evolution of fault zones of similar tectonic origin in california suggests that prism c slip was enhanced by the circulation of hot fluids which hydrolytically weakened quartz and allowed prism c slip to occur at relatively low temperature

fabric asymmetry and shear sense in movement zones asymmetry between lattice preferred orientation and shape fabrics is commonly observed in microstructural studies of many deformed mineral aggregates e g ice carbonate rocks peridotites and quartzites theoretical predictions are limited by the nature of the models available and more can be gained by discussion of the mechanisms which give rise to such asymmetry in specific natural and experimental situations

magnetic properties of the high himalayan leucogranites structural implications the magnetic properties of the high himalayan leucogranites have been investigated on 527 specimens in three plutons everest makalu 6 sites and manaslu 40 sites in nepal and gangotri 43 sites in india susceptibility varies between 2 and 100 10 6 si with an anisotropy ratio up to 1 16 high field and low temperature magnetic measurements together with comparison with weight percent iron demonstrate that anisotropy of magnetic susceptibility is carried by paramagnetic biotite and tourmaline the latter produces an inverse fabric i e with the minimum axis parallel to mineral lineation the magnetic fabric demonstrates complex patterns of stretching lineations during magmatic emplacement and its usefulness in semi quantitatively estimating petrofabric intensity is demonstrated for the biotite bearing facies natural remanent magnetization was measurable at only two sites in everest makalu where there are well defined reverse directions carried by titanomagnetite and pyrrhotite comparison of these preliminary results with predicted directions for stable india suggests northward tilting of about 10 and a small clockwise rotation of this massif

strain and shape fabric variations associated with ductile shear zones abstract the foliated and compositionally banded granitic orthogneisses in the central core of the maggia nappe a lower pennine basement nappe of the central swiss alps are shown to be the sheared equivalent of late hercynian age granitic intrusions these ductile shear zones show mineral assemblages in amphibolite facies are alpine in age and form an anastomosing network enclosing remnant lozenge shaped pods of relatively undeformed rock the foliation developed within the shear zones concomitantly with a change in shape of quartz grain aggregates from initially equidimensional through tear drop shapes to ribbon like aggregates these shape changes occurred by intracrystalline glide together with intercrystalline slip on deformation induced planar surfaces

geometrical analysis of mesoscopic shear zones in the crystalline rocks of mct zone of garhwal higher himalaya abstract 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

late caledonian northeastward ophiolite thrusting in the shetland islands u k abstract kinematic analysis of the deformation in the shetland ophiolitic nappe unst and fetlar islands and in the underthrust caledonian basement indicates a northeastward sense of nappe emplacement this deformation is interpreted as silurian in age it occurred in greenschist facies metamorphic conditions and it post dated the initial thrusting of the ophiolite in the oceanic domain the similarities found between the emplacement histories of the shetland and scandinavian ophiolites can be accounted for by a three plate model for the caledonian orogen the northeasterly direction of nappe emplacement in shetland parallel to the axis of the caledonian belt is attributed to the proximity of a zone of large sinistral transcurrent motion

tectonic zonation of the central himalaya and the crustal evolution of collision and compressional belts abstract the central himalayan segment is divided from south to north into sub parallel structural facies zones the main boundary fault mbf plays the role of an important tectonic element dividing the siwalik molasse along the foothills of the himalaya against the para autochthonous and allochthonous tectonic units of the southern himalaya on the basis of recent researches i suggest that the himalayan region should be divided geologically into two major geologic zones i propose to call the dividing structural line between them the main axial zone maz of the himalaya which is situated in the crystalline complex of the higher himalaya this deep seated structure in the himalaya has played a crucial role in the history of geological development as well as incorporating the root zone of southward pushed thrust sheets the main central thrust mct tectonically separates the carbonate para autochthonous zone of southern himalaya from the metamorphosed crystalline vaikrita complex the vaikrita central crystalline complex in its turn is separated from the huge pile of the sedimentary tethyan complex by the tethyan thrust tt farther north the great himalaya suture ghs called the indus tsangpo suture divides the northern extremity of himalaya from the karakoram erogenic belt the ghs has provided excellent conditions for the study of mantle remains on the oceanic crust with the occurrence of ophiolitic melange which was the result of subduction of a continental type plate and obduction of oceanic material in recent years a large number of data has been recorded especially as a result of sino french and other scientific expeditions in the tibetan region reversed critical wide angle reflection profiles of the crust mantle boundary south of the great himalaya suture ghs or the indus tsango yarlung zangbo suture in tibet reveal a deep 70 km moho extending north of the higher himalaya whilst to the south the moho is 15 km higher thus in the southern region of tibet the crust gets thinner gradually towards the south and reduces to about 38 km in the gangetic plain of india the 300 km of terrain between the ghs and the gangetic plain represent the transitional collision and compressional belt the magnetolluric and explosive seismographic data indicate that the layered structures are developed within the crust transisting to the upper mantle the moho discontinuity exhibits step terraces the low velocity and resistance layer within the crust indicates that the crust is overlapping and superimposed thus the intense compression from south to north caused by the indian plate caused the crust to overlap shorten thicken and become isostatically adjusted to give rise to the tibetan plateau the successive occurrence of nappe shear zones led to progressive superimposing and thickening of the crust it also indicates intra continental subduction and destruction of the lithosphere after plate collision

microstructural and fabric heterogeneities in fault rocks associated with a fundamental fault the redbank deformed zone of central australia displays heterogeneous deformation on all scales microstructural and fabric observations indicate that the heterogeneities arise not only from inhomogeneous deformation during individual tectonic events but also as a result of fault reactivation superimposed fabrics from different deformation events can either cause the intensification of an earlier l s tectonite fabric or locally produce linear type fabrics fault reactivation is preferentially localized in those regions where the superimposed fabrics are parallel or sub parallel to the original fabric early foliations that are unfavourably oriented in relation to later superimposed fabrics may be left as relatively less deformed pods or lenses in the centre of the fault zone quartz c axis fabrics show systematic variations with the type of fault rock forming both symmetrical and asymmetrical girdles with respect to the foliation zones of high strain where grain refinement has been greatest display characteristic asymmetrical fabrics those areas in which grain refinement has been less intense and retrogression least noticeable generally exhibit symmetrical fabrics

metamorphism of the izu tanzawa collision zone abstract the difference in pressure condition of progressive metamorphism established by schreinemakers analysis of mineral assemblages in metabasalts makes it possible for the low grade metamorphism of the izu tanzawa fujigawadani collision zone to be divided into three types type i is characterized by prehnite epidote hematite suggesting the lowest pressure type whereas type ii is defined by prehnite epidote actinolite the intermediate pressure type and finally the distinctive assemblage of type iii is pumpellyite epidote actinolite the higher pressure type the pressure conditions estimated are about 1 kbar for type i 1 2 kbar for type ii and 2 3 kbar for type iii the metamorphic rocks of type iii occur in the southwestern part of the tanzawa mountains and the metamorphic rocks of type i occur in the central izu peninsula and the northeast tanzawa mountains therefore the upward displacement of the accretion mass due to collision deformation is most significant at the southwest tanzawa mountains this suggests that the accretion of the tanzawa and izu blocks is accompanied with large scale tilting of the mass

polyphase deformation of mylonites from the renco gold mine zimbabwe identified by crystallographic preferred orientation of quartz abstract gold mineralization at renco mine in the northern marginal zone of the limpopo belt zimbabwe is hosted by a set of intersecting steep and shallow dipping shear zones locally termed reefs both steep and shallow reefs are closely associated with quartz mylonites the crystallographic preferred orientation of quartz from samples of these quartz mylonites indicate that deformation in steep reefs occurred under amphibolite facies conditions pole figures and microstructures of samples from shallow reefs in contrast show evidence for an origin of the auriferous mylonites at amphibolite facies conditions that were subsequently overprinted by deformation under lower greenschist facies conditions since mineralization is hosted by both reef geometries gold mineralization is inferred to have occurred during deformation and associated fluid infiltration along mylonites at amphibolite facies grades coeval with late archaean thrusting of rocks of the northern marginal zone onto rocks of the zimbabwe craton the crystallographic preferred orientation patterns of quartz point to a normal sense of movement during reactivation of the shallow reefs probably related to the meso proterozoic tectonometamorphic event that has affected the central parts of the limpopo belt at ca 2 0 ga normal faulting along the boundary between the northern marginal zone and zimbabwe craton has to date not yet been documented and may provide some important constraints for the understanding of the kinematics of the meso proterozoic tectonics in the limpopo belt

p wave velocity across a noncoaxial ductile shear zone and its associated strain gradient consequences for upper crustal reflectivity in order to simulate a normal incidence reflection profile across a noncoaxial ductile shear zone we determined p wave velocities of samples cut parallel and normal to mylonite foliation along a closely spaced profile 27 cm long through a transition zone and its associated strain gradient the ductile shear zone developed within an aplitic leucogranite was sampled from a kilometer wide ductile transcurrent fault in the northern french massif central strain analysis indicates that the sample experienced heterogeneous and progressive simple shear deformation shear strain systematically increases from zero in the undeformed protolith to 30 in the mylonite the transition zone thickness t is about 30 cm and the mylonite thickness m is about 10 cm the amount of quartz and mica increases relative to feldspar toward the mylonite indicating that a mineralogical composition change accompanied mylonitization mica and quartz developed a strong crystallographic preferred orientation cpo in the least strained domain seismic anisotropy is low and mean vp is 6 km s at 600 mpa anisotropy increases up to 10 and vp decreases up to 5 35 km s for propagation normal to the mylonite foliation through the transition zone this systematic velocity change correlates with the increasing through the transition zone and can be directly related to the cpo of mica and the increase in volume percent mica within the mylonite zone these results indicate that velocity and anisotropy gradients may in some cases be associated with ductile shear zones and that mylonite boundaries may not represent first order discontinuities the reflectivity of a ductile shear zone depends on the thickness of the transition zone relative to the seismic wavelength and on the t m ratio synthetic seismograms show that for a given seismic wavelength the reflectivity decreases when the transition zone thickness increases and when the ratio t m increases we show that layers with second order boundaries velocity gradients in transition zones are only seismically detectable within a narrow thickness range extrapolation to thicker shear zones is based on the assumption that the strain gradient thickness relative to shear zone thickness is to a first approximation scale independent in granitic domains ductile shear zones with similar geometrical and petrophysical features to the example studied here will be detected on deep seismic profiles only if their width is between 20 and 400 m development of ductile shear zones with strain gradients of the appropriate thickness to enhance reflectivity is favored under low temperature conditions in the granitic upper crust indeed low temperature strain gradient may explain the high seismic reflectivity of the upper crust in the scandinavian caledonides whereas high temperature strain gradient may explain in part the relative transparency of the european variscan upper crust

evolution of nappes in the eastern margin of the bohemian massif a kinematic interpretation the entire pile of nappes in the eastern margin of the bohemian massif is characterized by two stages of variscan nappe emplacement each exhibiting a different kinematic and metamorphic evolution

structural and thermochronological studies of the almora klippe kumaun nw india implications for crustal thickening and exhumation of the nw himalaya

ductile thrusting in the himalayas shear sense criteria and stretching lineations the most useful criteria used for the determination of the sense of shear can be found in the huge himalayan ductile shear zone known as m c t they allow the determination of the sense of shear in relation with an unequivocal southward directed intracontinental thrust the various criteria analyzed at different scales of observation show the permanence of the shear flow regime throughout the metamorphic evolution however most of the observed shear criteria are due to late nappe emplacement postdating the climax of the metamorphism the most striking feature is the stretching lineation which is roughly normal to the thrust trace all along the belt it parallels everywhere the shear direction and thus indicates the transport direction of the nappes the observed radial pattern of stretch trajectories implies that the local motion does not correspond to the average plate convergence

shear zone origin of quartzite mylonite and mylonitic pegmatite in the coyote mountains metamorphic core complex arizona abstract mylonites derived largely from granite pegmatite and sedimentary quartzite occupy a 500 m thick gently n dipping zone along the northern flank of the coyote mountains west of tucson in southeastern arizona the quartzite mylonites are exceptionally well developed and occur as discrete layers and lenses 2 5 m thick within yet thicker boudinaged sill like lenses of mylonitic pegmatite mylonitization took place in the tertiary within a normal slip ductile shear zone the shear zones formed in response to regional extension of continental crust extension is along a north south line and n directed sense of shear is revealed by mica fish oblique foliations in dynamically recrystallized quartz aggregates and asymmetric quartz c axis fabrics the microstructures and c axis fabrics taken together disclose that ductile and brittle deformation was achieved by intense penetrative non coaxial laminar flow dominated by progressive simple shear

ductile extrusion of the higher himalayan crystalline in bhutan evidence from quartz microfabrics quartz textures measured from deformed quartz tectonites within the lesser himalaya and higher himalaya crystalline of bhutan show similar patterns orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense the asymmetry of c axis and a axis patterns consistently indicates top to the south shearing the obliquity of the texture and the inferred finite strain plane strain to moderately constrictional suggest the strain regime had a combination of rotational and irrotational strain path in most of the samples from the bhutan himalaya the inferred deformation mechanisms suggest moderate to high temperature conditions of deformation that produced the observed crystallographic preferred orientation much higher temperature of deformation is indicated in the quartz veins from a leucogranite the observed ductile deformation is pervasively developed in the rocks throughout the investigated area the intensity of deformation increases only slightly in the vicinity of the main central thrust simultaneous southward shearing within a large part of the higher himalaya crystalline near and above the main central thrust and normal faulting across the south tibetan detachment is explained by the tectonically induced extrusion of a ductily deforming wedge the process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes channel flow accounts for some observed phenomena in the himalayan orogen such as inverted metamorphic sequences near the main central thrust not related to an inversion of isotherms and the syntectonic emplacement of leucogranites into the extruding wedge locally leading to an inversion of isotherms due to heat advection

fabric development in experimentally sheared quartzites abstract a fine grained quartzite was deformed under a combination of simple shear and axial compression to varying amounts of shear strain 1 3 to 2 9 the experiments were conducted at 800 c 1500 mpa and an axial strain rate of 10 6 s in the dislocation creep regime shear zone boundaries were oriented at 45 or 30 to the compressior axis the progressive development of deformation microstructures including planar fabrics and c axis preferred orientations was studied as a function of increasing shear strain elongate original grains define a foliation s which rotates from 45 to 0 to the shear zone boundary with increasing strain producing a sigmoidal foliation in a single shear zone a second foliation invariant in orientation is defined by elongate recrystallized grains which lie at about 30 40 to the shear zone boundaries no c surfaces parallel to the shear zone boundaries were observed however extensional crenulation cleavages c defined by parallel zones of recrystallized grains as well as impurity and melt segregations were observed even at low strain lying at approximately 30 to the shear zone boundaries and these could easily be mistaken for c surfaces c axis preferred orientations of original grains define symmetric type i crossed circle girdles at low strain 2 with the greater concentration of c axes in the direction of shear these features are very similar to those observed in natural ductile shear zones

microfabrics of calcite ultramylonites as records of coaxial and non coaxial deformation kinematics examples from the rocher de l yret shear zone western alps microfabrics were analysed in calcite mylonites from the rim of the pelvoux massif western alps france wnw directed emplacement of the internal penninic units onto the dauphinois domain produced intense deformation of an eocene age nummulitic limestone under lower anchizone metamorphic conditions slightly below 300 c two types of microfabrics developed primarily by dislocation creep accompanied by dynamic recrystallisation in the absence of twinning coaxial kinematics are inferred for samples exhibiting grain shape fabrics and textures with orthorhombic symmetry their texture crystallographic preferred orientation cpo is characterised by two c axis maxima symmetrically oriented at 15 from the normal to the macroscopic foliation non coaxial deformation is evident in samples with monoclinic shape fabrics and textures characterised by a single oblique c axis maximum tilted with the sense of shear by about 15 from the analysis of suitably oriented slip systems for the main texture components under given kinematics it is inferred that the orthorhombic textures which developed in coaxial kinematics favour activity of b10 11n and b02 21n slip along the f and r planes respectively with minor contributions of basalban slip in contrast the monoclinic textures which developed during simple shear are most suited for duplex ban slip along the basal plane the transition between the dominating slip systems for the orthorhombic and monoclinic microfabrics is interpreted to be due to the effects of dynamic recrystallisation upon texture development since oblique c axis maxima documented in the literature are most often rotated not with but against the shear sense calcite textures alone should not be used as unequivocal shear sense indicators but need to be complemented by microstructural criteria such as shape preferred orientations grain size estimates and amount of twinning

reflection characteristics of a mylonite zone based on compressional wave velocities of rock samples summary a combined field and laboratory study of the seismic reflection response and seismic properties of the mylonite zone capping the kettle dome metamorphic core complex provides important constraints on reflectivity of mylonites because of its structural and petrologic characteristics this mylonite zone can be regarded as a model for mid to deep crustal mylonite zones cdp reflection profiling revealed that the mylonite zone is reflective laboratory measurements of compressional wave velocities v at high confining pressures in samples collected directly up dip from the cdp reflection line revealed a wide variation of v in common mid crustal lithologies mafic and carbonate rock mean v values range from 6 67 to 7 31 km s at 600 mpa and quartzite and quartzo feldspathic values range from 6 16 to 6 52 km s seismic anisotropy at low pressures can be related to open microcracks and preferred mineral orientation whereas the latter effect controls anisotropy at high pressures the laboratory data provide the basis for a seismic model which indicates that the kettle dome mylonite zone can generate reflections from zones where high and low velocity lithologies are interlayered these reflections correlate with those observed on the cdp data thus demonstrating that the reflectivity of the mylonite zone is a direct result of its lithologic diversity and layered nature in this case it is not necessary to explain the reflections by the effects of anisotropy retrogression high pore pressure or constructive interference

midcrustal discontinuities and the assembly of the himalayan midcrust detailed quartz lattice preferred orientation lpo data define two structural discontinuities in the exhumed high grade metamorphic core of the himalaya exposed in the upper tama kosi region of east central nepal the structures are marked by abrupt breaks in a general trend of up structural section increasing quartz lpo defined deformation temperatures deformation associated with the upper structural discontinuity which occurs within sillimanite grade rocks is postpeak metamorphism in both the hanging wall and the footwall new geochronologic data constrain the timing of metamorphism in the hanging wall of the upper discontinuity to between 24 and 16 ma indistinguishable from previously published ages for the footwall movement across this structure represents early miocene strain localization and thickening in the himalayan midcrust movement across the lower discontinuity which occurs between staurolite and kyanite grade rocks appears to be synmetamorphic with material in its footwall at approximately 10 ma but postpeak metamorphism for material in its hanging wall this movement is interpreted to reflect the underplating and incorporation of material into the metamorphic core the recognition of two thrust sense discontinuities in the exhumed himalayan core in the tama kosi region is consistent with other similar structures recognized along the himalaya the widespread nature of these structures reinforces that they are important to our understanding of the evolution of the kinematics of large hot orogens

kinematics of oceanic thrusting and subduction from basal sections of ophiolites the structural analysis of high stress deformation at the base of the peridotite section in many ophiolites in suture zones enables the kinematics of oceanic convergence responsible for the suture to be defined this deformation results from the thrusting over oceanic crust of a young lithosphere involved in a subduction process flow plane flow line sense of shear related to the thrusting are geometrically deduced from the study of mineral preferred orientations in the basal peridotite and in the quartzites belonging to the amphibolite sole

asymmetric quartz c axis fabrics and flow vorticity a study using rotated garnets abstract the development of a type i crossed girdle c axis fabric with an asymmetric density distribution and an internally asymmetrical skeletal outline is examined in a specimen of garnet bearing mylonitic quartzite from the betic movement zone in southern spain the garnets are pre mylonitic skeletal porphyroblasts that rotated during mylonitic deformation but preserved a strain free equilibrium microstructure and a random c axis fabric it follows that the asymmetry of the density distribution must result from the deformation history itself the sense of asymmetry of the c axis population and that of the skeletal outline are consistent with the sense of vorticity inferred from extensional crenulations and asymmetric microboudinage and also with the sense of rotation of the garnets analysis of the rotation angles with aspect ratio of the garnet porphyroblasts demonstrates that a pure shear component was involved in an essentially non coaxial flow this may explain the internal asymmetry of the fabric skeleton and confirms the idea that a spectrum exists of type i fabrics intermediate between the theoretically predicted fabrics for pure and simple shear a tentative reconstruction of the kinematic framework suggests that the central segment of the fabric skeleton is linked to the extensional apophysis of the flow

a new estimate of the conditions for transition from basal a to prism c slip in naturally deformed quartz abstract quartz c axis fabrics of naturally deformed quartz in metacherts from the ryoke metamorphic belt in the yanai district southwestern japan suggest that the prism c prism a and basal a slip systems were activated during the plastic deformation under high temperature metamorphic conditions with increasing temperature quartz c axis fabric patterns change from girdle type through x maximum with a faint crossed girdle to x maximum the dominant orientation of subgrain boundaries in the quartz also changes from prism to basal with increasing temperature closely correlated with the change in quartz c axis fabric the fabric transition is also accompanied by an abrupt increase of recrystallized quartz grain size moreover observations with transmission electron microscopy tem clearly identify the dominant activation of c slip through the presence of basal subgrain boundaries and free dislocations with the 0001 burgers vector in the quartz from the metachert samples exhibiting x maximum c axis fabric based on these observations it is concluded that the mechanism switch from basal a to prism c slip systems occurred with increasing temperature in the metacherts the transition temperature is petrologically estimated to be ca 550 600 c at the natural strain rate

role of inversion tectonics in structural development of the himalaya abstract analysis of surface and subsurface structures variation of shortening amounts obtained by restoration of deformed cross sections and occurrence of younger hangingwall rocks over the older footwall rocks across the vaikrita thrust in the higher himalaya suggests reactivation of early normal faults as thrusts based on this an inversion tectonics model is proposed for structural development of the himalaya the model explains the geometrical shape of the himalaya as primary arcuation and helps in resolving superimposed deformation in the region

sense of shear in high temperature movement zones from the fabric asymmetry of plagioclase feldspars abstract a new method for determining the sense of shear in plagioclase bearing tectonites from the 010 orientation of plagioclase feldspar is presented the method is based on the asymmetry of the 010 plane with respect to the structural frame foliation and lineation and the dominant activity of the 010 slip plane in the high temperature plasticity of plagioclase feldspar using examples from the zabargad gneisses red sea the method is applied to plagioclases of an25 an45 and compared with other methods of shear sense determination quartz c axis fabrics and microstructural criteria

deformational and strain patterns of an intracontinental collision ductile shear zone an example from the higher garhwal himalaya the higher himalayan metamorphic belt of garhwal appears to have evolved in a major 15 20 km wide ne dipping ductile shear zone of the overthrust type due to intracontinental crustal shortening during cenozoic continental collision within the indian plate out of the four major distinct deformation phases the most widespread d2 deformation phase is marked by a prominent s2 foliation axial planar to the reclined and recumbent f2 folds a coaxial ne plunging l2 stretching lineation and syntectonic growth of index metamorphic minerals these deformational structures are developed during the sw directed d2 ductile shear deformation irrespective of the orientation of the s2 foliation and thrust zones regional strain patterns in quartzite of the lesser himalayan garhwal group beneath the main central thrust mct and the central crystalline zone using rf data on quartz clasts and aggregates reveal that the maximum values of finite strain es are attained within narrow mylonitic shear zones within the broad ductile shear zone during its progressive translation towards higher structural levels in late d2 and d3 deformation phases the shortening was accommodated by development of thrust sheets with the mct and jutogh thrust surfaces coinciding with zones of mylonite and maximum strain at higher levels ductile shear zones became compressional brittle ductile zones which resulted in the development of sw verging f3a and f3b folds during the southward migration of the thrust sheets in the lesser himalaya

kinematic analysis of rock flow and deformation temperature of the sirjan thrust sheet zagros orogen iran microstructural finite strain and vorticity analyses of quartz rich mylonites were used in order to investigate kinematics of rock flow and deformation temperature in the sirjan thrust sheet exposed in a structural window within the sanandaj sirjan high pressure low temperature hp lt metamorphic belt that forms part of the hinterland of the zagros orogenic belt of iran a dominant top to the sw sense of shear in the study area is indicated by several shear sense indicators such as asymmetric boudins rotated porphyroclasts mica fish and s c fabrics quantitative analyses reveal approximately plane strain deformation conditions with r xz values ranging from 2 5 to 4 3 and increasing towards the sirjan thrust opening angles of quartz c axis fabrics and recrystallization regimes suggest deformation temperatures vary from 430 to 625 50 c in the hanging wall rocks oblique grain shape and quartz c axis fabrics were used to estimate the degree of non coaxiality during deformation the obtained vorticity profile indicates a down section increase in kinematic vorticity number w m from 0 6 to 0 89 this range of vorticity numbers confirms contributions of both simple 41 68 and pure shear 32 59 deformation components the structural characteristics of the study area ultimately were controlled by oblique motion of the afro arabian plate relative to the iranian plate

isotopic and structural constraints on the location of the main central thrust in the annapurna range central nepal himalaya five isotope enhanced geologic transects in the southern annapurna range of central nepal elucidate structural geometries near the main central thrust whole rock nd isotopes and u pb ages of detrital zircons unambiguously distinguish greater himalayan hanging wall and lesser himalayan footwall metasedimentary rocks nd 0 values for lower lesser himalayan rocks typically range from 20 to 26 whereas greater himalayan rocks usually have nd 0 values of 19 to 12 lower lesser himalayan rocks yield detrital zircons with an age peak at ca 1880 ma and no detrital zircons younger than ca 1550 ma in contrast greater himalayan rocks yield detrital zircons with a prominent broad peak of ages at ca 1050 ma and no detrital zircons younger than ca 600 ma the protolith boundary between greater and lesser himalayan rocks is up to 1 km farther south than usually mapped on the basis of lithology field and microstructural observations indicate the presence of a top to the south ductile shear zone superimposed on this boundary confirming this shear zone as the main central thrust no evidence exists for large scale structural mixing of greater and lesser himalayan rocks along the main central thrust in the annapurna range

secondary lineation in a high t quartzite galicia spain an explanation for an abnormal fabric using refractory inclusions in quartz grains as a marker for the x finite direction xf of the strain undergone by the quartzites it is shown in our case that the shape preferred orientation of the quartz grains xq is perpendicular to xf within the foliation plane and is a secondary lineation according to that reinterpreted frame the lattice preferred orientation patterns become normal resulting from the dominant activity of the m a slip systems in a non coaxial shear regime and not from c slip development of xq is attributed to grain growth during a post tectonic thermal event with a faster rate of boundary migration parallel to c and without substantial change in the lattice fabric tem data give support to the latter interpretation but do not demonstrate the absence of c slip this example suggests that a close examination of the significance of the grain shape lineations is necessary in case of high temperature deformations operation of c as a dominant slip direction identified in experimental deformation of high water content synthetic quartz crystals is still to be established in natural quartz tectonites

late mesozoic strike slip movement on the border ranges fault system in the eastern chugach mountains southern alaska preliminary studies of mylonitic rocks from the haley creek metamorphic assemblage in south central alaska suggest that the border ranges fault system was a strike slip ductile shear zone during part of its late mesozoic history evidence for this strike slip event is limited to the hanging wall of the border ranges fault system where deformed crystalline rocks were apparently subjected to a retrograde mylonitic deformation prior to subduction related underthrusting of the latest cretaceous valdez group but subsequent to a late jurassic plutonic event the strike slip interpretation contrasts with previous interpretations which related mylonitic fabric development to multiple deformational events during underthrusting of the chugach terrane a conflict resulting from the interpretation of lineation as an intersection lineation parallel to the y axis of strain versus an extension lineation evidence for horizontal stretching on a vertical flattening plane includes 1 a parallelism between all lineations including intersection lineations and unequivocal extension lineations developed within systems of strike slip ductile shear zones 2 extensional fragmentation of feldspars parallel to lineation 3 local rodding and development of l tectonites 4 boudin axes at a high angle to lineation 5 crystallographic preferred orientations of quartz and 6 syn kinematic crack seal veins with fibers parallel to lineation the local parallelism between intersection lineations and extension lineation is interpreted as a consequence of sheath folding in a high strain zone and we suggest that it is this complication that is largely responsible for confusion on the kinematic history despite evidence for a strain pattern consistent with a strike slip shear zone evidence for noncoaxial strain and a clear sense of shear is elusive nonetheless coaxial deformation seems unlikely given the structural position of the mylonitic rocks along an ancient plate boundary regional correlation of rock types is suggestive of a dextral offset of at least 50 km although this offset could be due to younger faulting thus further work will be needed to clearly distinguish a dextral versus sinistral sense of shear important absolute age problems still remain and thus the tectonic significance of the strike slip event is uncertain however the general late mesozoic age places the event within an important time period in the history of the northern cordillera thus the strike slip event may have 1 played a role in a transform history preceding early cretaceous thrusting along the border ranges fault system 2 been a complication from collision of wrangellia with north america and 3 been important in the northward transport of outer cordilleran terranes that is implied by paleomagnetism

deformation of leucogranites of the crystalline main central sheet in southern tibet china abstract towards the top of the main central sheet in southern tibet china leucogranites intrude metamorphic rocks related to rocks of the main central thrust in the himalayas our study shows that the leucogranites suffered a northward shear deformation the significance of this event is discussed in terms of gravity driven decollement of the 10 000 m thick tethyan sedimentary pile over its crystalline basement

fluid inclusions tectonic indicators abstract during the first half of the 20th century fluid inclusions have been studied in ore geology in order to determine the chemistry and physical parameters density of the mineralising fluids apart from a few pioneering works in the middle of the century fluid inclusions were not used as structural markers before the late 1980s during the same time experiments as well as observations of natural samples have shown that the volume shape and density of fluid inclusions may change when submitted to pressure temperature conditions different from their trapping isochores in view of these experiments and based on natural examples from the higher himalaya metamorphic rocks it is evident how fluid inclusions can be used as thermobarometric structural and tectonic markers points of interest for further studies on fluid inclusions in nature and experiments are also stressed

tonalite sill emplacement at an oblique plate boundary northeastern margin of the bohemian massif abstract a tonalitic sill has been examined at the variscan transpressive boundary of the lugian and silesian plates at the ne margin of the bohemian massif a structural petrological and geochronological study reveals that it was emplaced syn tectonically with major ductile shearing in lower crustal rocks magmatic and pre rheological critical melt percentage rcmp fabrics are concordant with the hanging wall structures but discordant with those of the footwall the ams study shows the predominance of flattening strain at the margins and plane strain fabrics in the core numerical modelling of ams fabrics is in good agreement with the hypothesis of magma flow and deformation in oblique transpression a tectonic model was developed explaining emplacement and syn tectonic deformation of progressively cooled tonalitic intrusion

how useful are millipede and other similar porphyroblast microstructures for determining synmetamorphic deformation histories oppositely concave microfolds ocms in and adjacent to porphyroblasts can be classified into five nongenetic types type 1 ocms are found in sections through porphyroblasts with spiral shaped inclusion trails cut parallel to the spiral axes and commonly show closed foliation loops type 2 ocms commonly referred to as millipede microstructure are highly symmetrical the foliation folded into ocms being approximately perpendicular to the overprinting foliation type 3 ocms are similar to type 2 but are asymmetrical the foliation folded into ocms being variably oblique to the overprinting foliation type 4 ocms are highly asymmctrical only one foliation is present and this foliation is parallel to the local shear plane type 5 ocms result from porphyroblast growth over a microfold interference pattern types 1 and 2 are commonly interpreted as indicating highly noncoaxial and highly coaxial bulk deformation paths respectively during porphyroblast growth however theoretically they can form by any deformation path intermediate between bulk coaxial shortening and bulk simple shearing given particular initial foliation orientation and timing of porphyroblast growth type 3 ocms can also form during these intermcdiatc deformation paths and are commonly found in the same rocks as type 2 ocms type 4 ocms may indicate highly noncoaxial deformation during porphyroblast growth but may be difficult to distinguish from type 3 ocms thus types 1 3 and possibly 4 reflect the finite strain state giving no information about the rotational component of the deformation s responsible for their formation furthermore there is a lack of unequivocal independent evidence for the degree of noncoaxiality of deformation s during the growth of porphyroblasts containing ocms type 2 ocms that occur independently of porphyroblasts or other rigid objects might indicate highly coaxial bulk shortening but there is a lack of supporting physical or computer modelling it is possible that microstructures in the matrix around ocms formed during highly noncoaxial and highly coaxial deformation histories might have specific characteristics that allow them to be distinguished from one another however determining degrees of noncoaxiality from rock fabrics is a major long standing problem in structural geology

quartz fabric transition in a cooling syntectonic granite hermitage massif france abstract the hermitage granite situated in the northwest massif central france is a syntectonic hercynian leucogranite emplaced along an active transcurrent shear zone during emplacement and cooling the progressive deformation is marked by the development of a primary homogeneous foliation gradually affected by ductile shear bands c s mylonites increase in strain heterogeneity during cooling corresponds to a change of dominant deformation mechanism of the quartz phase from grain growth and migration recrystallization to intracrystalline a slip and rotation recrystallization migration recrystallization is characterized by preferred orientations of c axes close to the principal extension direction we discuss relevant deformation mechanisms and rheological implications for syntectonic plutons in particular we argue that the transitions between homogeneous and heterogeneous accumulation of strain cannot generally be correlated with transition between magmatic and solid state flow

defining the himalayan main central thrust in nepal an inverted metamorphic field gradient associated with a crustal scale south vergent thrust fault the main central thrust has been recognized along the himalaya for over 100 years a major problem in himalayan structural geology is that recent workers have mapped the main central thrust within the greater himalayan sequence high grade metamorphic sequence along several different structural levels some workers map the main central thrust as coinciding with a lithological contact others as coincident with the kyanite isograd up to 1 3 km structurally up section into the tertiary metamorphic sequence without supporting structural data some workers recognize a main central thrust zone of high ductile strain up to 2 3 km thick bounded by an upper thrust mct 2 vaikrita thrust and a lower thrust mct 1 munsiari thrust some workers define an upper lesser himalaya thrust sheet that shows similar p t conditions to the greater himalayan sequence others define the main central thrust either on isotopic nd sr differences differences in detrital zircon ages or as being coincident with a zone of young

the contact between the higher himalaya crystallines and the tibetan sedimentary series miocene large scale dextral shearing space and time evolution of the synmetamorphic structures across the metamorphic pile have been studied in several areas along the himalayan belt east and central nepal garhwal zanskar from one area to the other the evolution is very similar 1 at the base of the pile in the main central thrust mct shear zone the stretching lineation penetrative and regularly oriented n0 e to n30 e indicates the mct transport direction very constant all along the belt from the eohimalayan main metamorphic development up to the late metamorphic movements 2 at the top of the pile at the contact between the crystalline unit and its sedimentary cover gravity driven structures are confirmed north vergent folds ductile normal faulting however there are numerous local indications of late miocene syn to late emplacement of the leucogranitic plutons dextral shearing 3 in between across the medium part of the pile the stretching lineation shows a conspicuous progressive regional clockwise rotation clearly indicated by the strain trajectories mapped in nepal and garhwal points 1 and 2 show that the crystalline sedimentary boundary despite its apparent structural and metamorphic continuity is not only a normal fault but also an important dextral shear zone which has acted since the upper miocene as the main southern limit of the eastward extruding tibetan block

thermal structure and exhumation history of the lesser himalaya in central nepal the lesser himalaya lh consists of metasedimentary rocks that have been scrapped off from the underthrusting indian crust and accreted to the mountain range over the last 20 myr it now forms a significant fraction of the himalayan collisional orogen we document the kinematics and thermal metamorphism associated with the deformation and exhumation of the lh combining thermometric and thermochronological methods with structural geology peak metamorphic temperatures estimated from raman spectroscopy of carbonaceous material decrease gradually from 520 550 c below the main central thrust zone down to less than 330 c these temperatures describe structurally a 20 50 c km inverted apparent gradient the ar muscovite ages from lh samples and from the overlying crystalline thrust sheets all indicate the same regular trend i e an increase from about 3 4 ma near the front of the high range to about 20 ma near the leading edge of the thrust sheets about 80 km to the south this suggests that the lh has been exhumed jointly with the overlying nappes as a result of overthrusting by about 5 mm yr for a convergence rate of about 20 mm yr this implies underthrusting of the indian basement below the himalaya by about 15 mm yr the structure metamorphic grade and exhumation history of the lh supports the view that since the mid miocene the himalayan orogen has essentially grown by underplating rather than by frontal accretion this process has resulted from duplexing at a depth close to the brittle ductile transition zone by southward migration of a midcrustal ramp along the main himalayan thrust fault and is estimated to have resulted in a net flux of up to 150 m 2 yr of lh rocks into the himalayan orogenic wedge the steep inverse thermal gradient across the lh is interpreted to have resulted from a combination of underplating and post metamorphic shearing of the underplated units

diachronous deformation along the base of the himalayan metamorphic core west central nepal geologic mapping combined with microstructural and geochronologic analyses from the lower himalayan metamorphic core in west central nepal record along strike similarity in flow style despite variability in the timing of metamorphism and deformation the main central thrust zone at the base of the himalayan metamorphic core varies in thickness tectonostratigraphy and metamorphic gradient along the 250 km of strike length studied in situ u th pb geochronology of monazite sampled from an along strike transect at the top of the high strain zone records eocene oligocene prograde metamorphism followed by miocene retrograde metamorphism the timing of prograde and retrograde metamorphism and the muscovite 40 ar 39 ar dates gradually decrease along strike from northwest to southeast this age trend is punctuated by an abrupt 3 8 m y decrease in the age of prograde and retrograde metamorphism and muscovite 40 ar 39 ar dates near the marsyandi river in central nepal quartz crystallographic preferred orientation fabrics from a parallel transect along the base of the high strain zone document similar flow style at 440 c in central nepal muscovite 40 ar 39 ar ages interpreted to approximate the age of deformation at this structural level decrease from ca 7 to 4 ma along strike from northwest to southeast diachronous deformation and metamorphism along strike in west central nepal demonstrate the necessity of incorporating more than a single transect into tectonic models along strike tectonometamorphic variability in west central nepal spatially corresponds to faults in the indian basement bounding the subsurface faizabad ridge highlighting the possible influence of inherited basement faults on the geometry of the basal himalayan detachment the main himalayan thrust as well as the tectonic evolution of the structurally overlying himalayan metamorphic core this study highlights the potential influence of inherited structures on the overlying orogenic wedge and the probability of along strike diachroneity of deformation in the himalaya

strain patterns within the late variscan granitic dome of velay french massif central the velay granites are intruded as a late variscan dome in the french massif central strain patterns within the velay granites reflect the geometry and kinematics of late carboniferous deformation at the site of final emplacement the internal deformation developed during granitic magmatism is not homogeneously distributed from inner to outer zones increasing deformation is marked by changes from high temperature foliations to s c orthogneisses strain concentration at the margins reflects a strength contrast due to cooling and subsequent magma hardening according to the internal strain field two zones can be distinguished in the velay dome i a central and southern zone where concentric patterns indicate a southward expansion of granites and ii a northern zone where the lateral expansion is limited and combined with top to the north extensional shearing along the mont pilat detachment zone and high temperature wrenching these asymmetric strain patterns relate to the expansion and deformation of granites emplaced below a detachment fault during the uplift of a thickened area post thickening combination of extensional and wrench tectonics is inferred

seismic monitoring and analysis of excessive gas emissions in heterogeneous coal seams abstract uncontrolled and excessive gas emissions pose a serious threat to safety in underground coal mining in a recently completed research project a suite of monitoring techniques were employed to assess the dynamic response of the coal seam being mined to longwall face advance at coal mine velenje in slovenia together with continuous monitoring of gas emissions two seismic tomography measurement campaigns and a microseismic monitoring programme were implemented at one longwall top coal caving panel over 2000 microseismic events were recorded during a period of four months over the same period there also was a recorded episode of relatively high gas emission in the same longwall district in this paper a detailed analysis of the processed microseismic data collected during the same monitoring period is presented specifically the analysis includes the spatial distribution of the microseismic events with respect to the longwall face advance the magnitude of the energy released per week and its temporal evolution examination of the spatial distribution of the recorded microseismic events has shown that most of the microseismic activity occurred ahead of the advancing face furthermore the analysis of the gas emission and microseismic monitoring data has suggested that there is a direct correlation between microseismicity and gas emission rate and that gas emission rate tends to reach a peak when seismic energy increases dramatically it is believed that localised stress concentration over a relatively strong xylite rich zone and its eventual failure which was also identified by the seismic tomography measurements may have triggered the heightened microseismic activity and the excessive gas emission episode experienced at the longwall panel monitored

linked fluid and tectonic evolution in the high himalaya mountains nepal fluid inclusions were studied in a quartz lens from the structurally highest unit of the himalaya mountains in nepal from a textural geometrical chemical and isotopic point of view six types of fluid inclusions were distinguished one of these types consists of annular inclusions this shape is attributed to a confining pressure increase in a non isotropic stress field two successive stress fields were deduced from the orientation of the inclusion planes relative to the schistosity the bulk composition of the fluid was dominated by co2 84 mol and h2o the composition remained constant during the whole history of the sample indicating that it was buffered by the carbonaceous host rock and or that one single fluid was reworked in situ by decrepitation stable isotope of fluids and minerals indicate 1 that fluids were buffered by surrounding rocks for o and c and 2 that at least two types of water metamorphic and meteoric were involved finally a p t t e path is proposed for the sample taking into account the southward thrusting along the main central thrust the northward tectonic denudation of the himalaya mountains inducing tectonic burying below the annapurna range and lastly rapid uplift

the origin of himalayan anatexis and inverted metamorphism models and constraints abstract the key to comprehending the tectonic evolution of the himalaya is to understand the relationships between large scale faulting anatexis and inverted metamorphism the great number and variety of mechanisms that have been proposed to explain some or all of these features reflects the fact that fundamental constraints on such models have been slow in coming recent developments most notably in geophysical imaging and geochronology have been key to coalescing the results of varied himalayan investigations into constraints with which to test proposed evolutionary models these models fall into four general types 1 the inverted metamorphic sequences within the footwall of the himalayan thrust and adjacent hanging wall anatexis are spatially and temporally related by thrusting 2 thrusting results from anatexis 3 anatexis results from normal faulting and 4 apparent inverted metamorphism in the footwall of the himalayan thrust is produced by underplating of right way up metamorphic sequences we review a number of models and find that many are inconsistent with available constraints most notably the recognition that the exposed crustal melts and inverted metamorphic sequences not temporally related the generalization that appears to best explain the observed distribution of crustal melts and inverted metamorphic sequences is that due to specific petrological and tectonic controls episodic magmatism and out of sequence thrusting developed during continuous convergence juxtaposing allochthonous igneous and metamorphic rocks this coincidental juxtaposition has proven to be something of a red herring unduly influencing attention toward finding a causal relationship between anatexis and inverted metamorphism

strain deformation temperatures and vorticity of flow at the top of the greater himalayan slab everest massif tibet this paper presents quantitative data on strain deformation temperatures and vorticity of flow at the top of the greater himalayan slab the data were collected from the tibetan side of the everest massif where two low angle normal faults bound the upper surface of the greater himalayan slab the earlier and structurally lower lhotse detachment and the later and structurally higher qomolangma detachment greenschist to sillimanite grade quartz rich metasedimentary rocks exposed in the rongbuk to north col region of the everest massif are characterized by cross girdle quartz c axis fabrics indicating approximate plane strain conditions fabric opening angles progressively increase with depth beneath the overlying lhotse detachment and indicate progressively rising deformation temperatures of 525 625 50 c at depths of 300 600 m beneath the detachment deformation temperatures of c 450 c are indicated by fabric opening angles in epidote amphibolite facies mylonites located closer to the overlying detachment a top down to the north normal shear sense is indicated by the asymmetry of microstructures and c axis fabrics but the degree of asymmetry is low at distances greater than 400 m beneath the detachment and sillimanite grains are drawn into adjacent conjugate shear bands but still appear pristine indicating that deformation occurred at close to peak metamorphic temperatures these quenched fabrics and microstructures indicate rapid exhumation in agreement with previous isotopic dating studies mean kinematic vorticity numbers w m were independently calculated by three analytical methods calculated w m values range between 0 67 and 0 98 and indicate that although a simple shear component is generally dominant particularly in greenschist facies mylonites located between the lhotse and overlying qomolangma detachments there is also a major component of pure shear in samples located at 400 600 m beneath the lhotse detachment pure and simple shear make equal contributions at w k 0 71 our integrated strain and vorticity data indicate a shortening of 10 30 perpendicular to the upper surface of the greater himalayan slab and confirm that the upper surface of the slab is a stretching fault with estimated down dip stretches of 10 40 assuming plane strain deformation measured parallel to the flow plane transport direction

folding and shear deformation in quartzites inferred from crystallographic preferred orientation and shape fabrics abstract folding and simple shear deformation frequently occur together on all scales the kinematic information from crystallographic preferred orientation cpo and microstructural information from shape fabrics are used to investigate the relationships of simple shear and folding in small scale folds three fold samples were analysed sample 1 with the fold axis perpendicular sample 2 oblique and sample 3 essentially parallel to the shear direction all folds have formed by buckling in a simple shear deformation regime the principal kinematic directions for each sample lie in a single plane normal to the fold axial plane and at variable angles to the fold axis the cpos allow a distinction to be identified between coaxial and simple shear deformation components in different parts of each fold the shear senses determined by cpos and shape fabrics indicate that the shear deformation and the buckling of the layers occurred approximately simultaneously in all samples cpo analysis of the flexural slip components of the folding suggests that the fold axes have not rotated substantially towards the extension direction since their initiation the variable orientations of the fold axes are explained by variable original orientations of the anisotropy with respect to the shear direction

retrograde evolution of quartz segregations from the dos picos shear zone in the nevado filabride complex betic chains spain evidence from fluid inclusions and quartz c axis fabrics synkinematic quartz veins are ubiquitous in the shear zone separating the veleta unit from the calar alto unit in the internal part of the betic cordilleras they have been studied with respect to quartz c axis fabrics microstructures and fluid inclusions veins were probably generated during syn metamorphic stacking of the units at p 500 600 mpa and t 400 500 c quartz displays two groups of microstructures in the shear zone 1 older coarse grained mosaics cgm resulting from exaggerated grain growth and 2 younger fine grained mosaics fgm developed at the expense of the former the fine grained mosaics show polygonal granoblastic and elongate mosaic microstructures in general with ribbon microstructures often found near the boundary of the units fluids contained in secondary inclusions vary from high salinity brines to different types of co2 brine mixtures and low density co2 fluids differences in composition and p t trapping conditions are indicated for the different types of inclusions some fluid inclusions are older than the fgm whereas others are younger thus constraining the p t conditions at which the two microstructural events took place fluid inclusion evidence suggests conditions of pfluid 170 mpa and t 370 430 c for the cgm and pfluid 20 80 mpa and t 340 c for the fgm the quartz c axis fabrics dealt with here correspond to the second recrystallization event as little evidence of older fabrics is preserved in the shear zone c axis patterns vary across the shear zone from slightly asymmetrical type i crossed girdles in the hanging wall and footwall to more asymmetrical crossed girdles at the boundary of the units this indicates a correlative increase in the magnitude of the heterogeneous shear strain in the same direction most of the deformation is concentrated at the top of the veleta unit the sense of movement is top to the west in agreement with other kinematic markers the quartz c axis fabrics resulted from dynamic recrystallization during simple shear the retrograde p t path inferred from fluid inclusion analysis along with other geological and geochronological evidence indicates that this deformation is coeval with a reduction in the crustal overburden geochronological and stratigraphic data show that the proposed dos picos extensional detachment separating the calar alto and veleta units took place during the early miocene synchronous with the intense thinning of the nevado filabride complex and of the whole continental crust underlying the alboran basin

the development of discrete shear zones in a granite stress strain and changes in deformation mechanisms abstract the development of millimetre scale continuous shear zones in the sidi ali bou nab algeria granite and their progressive transformation into discrete shear zones are presented increases in strain stress and strain rate magnitudes as well as changes of deformation mechanisms are suggested by an evolution of the microstructure across these narrow shear zones temperatures are assumed to be constant from boundaries to core the most striking aspects of this evolution are 1 1 a decrease in the angle between the foliation and the shear zone 2 2 a decrease in the recrystallized grain size of quartz together with the development of a crystallographic orientation characterized by a strong concentration of c axes around the y axis of the finite strain ellipsoid 3 3 the development of very fine grained feldspathic aggregates or layers probably through dynamic recrystallization of original large crystals and 4 4 the replacement of original biotite by muscovite and a new generation of biotite in the continuous shear zones the deformation is controlled by the behaviour of quartz this mineral undergoes extensive dynamic recrystallization which probably favours a higher strain rate and a change in the active slip system from a dominant basal one in the outermost part of the structures to a dominant prismatic one in its core paleopiezometry and strain computations show a shear strain up to 10 and an inferred steady state differential stress to 100 130 mpa respectively the high stress level is suspect because the estimates are derived from recrystallized grain size which may be sensitive to strain and variations of chemical environment discrete shear zones develop in relation with dynamic recrystallization of feldspars in the core of continuous shear zones dynamic recrystallization of feldspars produces very fine grained aggregates grain size of 6 7 m which coalesce and form a continuous layer at the central part of the shear zones this fabric may result from 1 1 a change in the driving flow mechanism from dislocation creep of quartz to dislocation creep or and grain boundary sliding of feldspar neoblasts and 2 2 a large strain softening that would favour strain concentration in the fine grained matrix and the development of sharp shear zone boundaries

the himalayan main central thrust pile and its quartz rich tectonites in central nepal as a consequence of the india asia collision a deeply rooted crustal slice has been moved southwards over the northern boundary of the indian plate at the present level of observation the main central thrust pile a rock pile of several kilometers thickness is affected by shear deformation which imprinted a flat foliation and a constant n s trending lineation these tectonites were deformed under a wide range of metamorphic conditions grading from lower greenschists up to the sillimanite field the m c t shear zone is studied from the view point of the quartz rich rocks over 100 samples helped to define a microstructural zonation roughly parallel to the thermal one 35 samples particularly rich in quartz have been used for the preferred orientation study of their c axes and a few of them for their a axes strong lattice orientations when undisturbed by mainly secondary recrystallization attest to large plastic strains in quartz the c axes are mostly distributed along unequally populated crossed girdles which may evolve into an inclined single girdle the a axes concentrate perpendicular to the more populated girdle concerning quartz the following is suggested 1 1 the deformation has a dominant shear component responsible for the dominant stretching lineation it could be simple shear in case of inclined single girdles 2 2 one set of a axes line up parallel to the shear direction slip on various planes in the a direction may be an important deformation mechanism 3 3 asymmetry in the lattice preferred orientations could be used to predict the sense of shear for the himalayan thrusting in more than 80 of cases

major late triassic strike slip displacement in the seven devils terrane oregon and idaho a result of left oblique plate convergence abstract the shortening direction in rocks deformed in collision or subduction zones is not directly related to the plate convergence vector rather it is perpendicular to the collision zone or subduction zone even in cases where plate convergence is oblique the component of convergence parallel to the subduction collision zone is expressed by strike slip displacement in the arc region behind the subduction zone such strike slip shear zones have been recognized in the seven devils terrane of northeastern oregon and adjacent idaho one of these the oxbow shear zone consisting of cataclasite mylonite and ultra mylonite trends northeasterly from oxbow oregon to cuprum idaho the original rock types of the shear zone were plagiogranite gabbro diabase bassalt and keratophyre the age of the mylonitization is constrained by 40ar 39ar dates as late triassic meso and microscopic structures textures and quartz c axes fabrics indicate that the shear zone was formed by left lateral strike slip motion a minimum left lateral displacement of 65 km has been estimated but the true displacement may have been much larger the oxbow shear zone is interpreted as an intra arc strike slip zone of the seven devils terrane related to left oblique plate convergence during the triassic

plio plistocene in sequence thrust propagation along the main central thrust zone kumaon garhwal himalaya india new thermochronological data abstract the kumaon and garhwal himalayas nw india have similar geologic tectonic topographic and precipitation pattern however low temperature thermochronological age pattern varies significantly along the strike as well as across the strike of major faults here we interpret the new set of thermochronological age data across the strike of major faults namely vaikrita thrust vt munsiari thrust mt berinag thrust bt and the baijnath nappe along pindari valley in the kumaon garhwal himalaya in the present work 18 new apatite fission track aft ages of samples collected along a north south transect have been reported ages in the hanging wall of vt which range from 2 1 0 2 to 4 2 0 7 ma have been found to be becoming younger linearly with distance from north to the vt this trend crosses the vt with significant jump in ages in the south of the vt ages are lying between 0 3 0 1 and 3 9 0 8 ma and show linear variation with distance from the vt to bt no significant jump in ages across the mt is observed and the linear trend of younging southward is continuing till the bt the break and displacement in age pattern with youngest ages close to thrusts possibly indicate progressive late thrust movement it reflects sequential uplift and cooling towards the south from the vt to bt which is consistent with an in sequence style of thrust propagation we compare our new data to the published thermochronological data from its adjacent traverses along the goriganga and dhauliganga valleys 25 km away from the present traverse towards east and west respectively in the kumaon garhwal himalaya in order to understand potential along strike variations in the kinematics our observations indicate a dynamic coupling between tectonic and surface processes in the garhwal kumaon himalaya and support that the geometry of crustal scale faults and their associated kinematics control exhumation pathways of rocks

thermal evolution of the lesser himalaya central nepal insights from k white micas compositional variation abstract the lesser himalayan low to medium grade metamorphic rocks in central nepal are rich in k white micas occurring as porphyroclasts and in matrix defining s 1 and s 2 porphyroclasts are usually zoned with celadonite poor cores and celadonite rich rims the cores are the relics of igneous or high grade metamorphic muscovites and the rims were re equilibrated or overgrown under lower t metamorphic conditions the matrix k white micas defining s 1 pre dating the main central thrust activity are generally celadonite rich they show heterogeneous compositional zoning with celadonite rich cores and celadonite poor rims they were recrystallized at lower t condition prior to the main central thrust activity most probably prior to the india asia collision pre himalayan metamorphism the matrix k white micas along s 2 synchronous to the main central thrust activity neohimalayan metamorphism are relatively celadonite poor and were recrystallized under relatively higher t condition k white micas defining s 1 also were partially re equilibrated during the neohimalayan metamorphism the average compositions of recrystallized k white micas defining both s 1 and s 2 become gradually poor in fe mg and si contents and rich in al and ti contents from south to north showing an increase of metamorphic grade from structurally lower to higher parts in the lesser himalaya this shows that the metamorphism is inverted throughout the inner lesser himalaya the tectono metamorphic significance of the published k ar and 40 ar 39 ar k white micas ages from the lesser himalaya need re evaluation in the context of observed intrasample compositional variation and zoning and possible higher closure temperature 500 c for k ar system

quartz microfabric of the laxfordian canisp shear zone nw scotland the canisp shear zone transects layered lewisian gneisses near lochinver nw scotland it is a vertical ductile shear zone with a dextral shear sense formed during laxfordian amphibolite facies metamorphism transposing the layering to new foliation and linear structures minerals in the layered gneisses show little or no shape fabric while a strong shape fabric defines the foliation for quartz this shape fabric is accompanied by development of a preferred crystal orientation with fabric patterns reflecting the geometry of the shear deformation the quartz fabric shows a pole free area around the lineation with the c axes concentrated in an asymmetric cross girdle or a point maximum perpendicular to the shear plane and a monoclinic symmetry consistent with the shear sense

enhanced hydrothermal activity on an ultraslow spreading supersegment with a seismically detected melting anomaly abstract seafloor hydrothermal venting fields occur on all ocean spreading ridges osrs regardless of spreading rates however the distribution of seafloor hydrothermal activity such as frequency and spacing on ultraslow spreading osrs are poorly known chinese dayang cruises from 2015 to 2016 conducted detailed water column surveys for seafloor hydrothermal activity using a towed system with an array of turbidity sensors and a near bottom camera along the ultraslow spreading southwest indian ridge here we report the discovery of multiple hydrothermal plumes overlying segments 28 29 and 30 between the indomed and gallieni fracture zones from these data and earlier explorations in segments 25 27 we identify nine active venting sites the spatial density fs sites 100 km of active sites along the 394 km of ridge axis in our study area is thus 2 8 nearly 3 higher than predicted by the global trend of fs for ultraslow osrs in the interridge database previous studies concluded that an enhanced magma supply to the central indomed gallieni supersegment 11 8 ma is now limited to segment 27 our results indicate that although hydrothermal activity may be most concentrated in segment 27 the discoveries of active venting in segments 25 30 implies the presence of additional magma bodies across a broad extent of the indomed gallieni supersegment

anatomy of the mondonedo nappe basal shear zone nw spain this paper analyses a major shear zone from the iberian hercynian belt which forms the basal thrust of the mondonedo nappe the shear zone developed by ductile deformation under amphibolite facies metamorphic conditions and later by brittle ductile deformation in greenschists facies folds in the shear zone are asymmetric very tight 1c or similar class and frequently developing sheath geometries the sheath folds originated by non coaxial flow superimposed on earlier irregularities the fabric of quartzitic rocks in the shear zone changes from bottom to top from ultramylonites through blastomylonitic rocks to non mylonitic tectonites c axis fabrics vary across the shear zone but show a dominant monoclinic symmetry the blastomylonitic rocks include the fabrics representing the highest temperatures the main foliation of the schists results from flattening of an earlier foliation recording occasional microfolds the use of different kinematic criteria has allowed an analysis of their validity as well as an assessment of movement direction towards the foreland of the orogen

textures and c axis orientations of deformed quartz crystals from porphyric dikes of the alpine root zone western alps deformation textures and c axis preferred orientations of quartz phenocrysts from porphyric dikes of the alpine root zone have been formed under conditions of greenschist to low amphibolite facies it is shown that the deformation textures as shear planes and kink bands boudins grain boundary sutures subgrains coarse and fine recrystallized grains deformation lamellae and fractures are developed in a chronological sequence during first increasing and later decreasing temperatures

simulation of fabric development in recrystallizing aggregates ii example model runs abstract this study investigates the role of the coupling of dynamic recrystallization and lattice rotations in fabric development a new two dimensional computer simulation of polycrystalline deformation is used which combines homogeneous straining internal lattice rotations and dynamic recrystallization processes five example runs of the simulation are described here which compare the progressive development of fabrics resulting from different recrystallization regimes and different straining geometries it is found that these fabrics can evolve significantly with progressive strain from one strong fabric pattern to another the effect of recrystallization is not only to create point maxima concentrations of c axes but also to modify and create girdle distributions correlations are made between the fabrics generated by this model and previously reported quartz and ice fabrics

regional shear variation in relation to diapirism and folding abstract regional structural trends in the levezou area southern massif central france delineate an antiformal structure deformed granitoids display changes along strike in s c plane relationships and therefore in sense of shear these variations are interpreted to be strain accommodation features that developed during the diapiric emplacement and folding of the granitoid sheets and so cannot be used as kinematic indicators on a regional scale

fabric development during shear deformation in the main central thrust zone nw himalaya india abstract 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