phyllosilicate orientation demonstrates early timing of compactional stabilization in calcite cemented concretions in the barnett shale late mississippian fort worth basin texas u s a abstract calcite cemented zones in the prolific gas producing barnett shale ft worth basin texas preserve very early compactional fabrics that contrast markedly with fabrics in the enclosing host rocks the compactional state of these zones based upon alignments of phyllosilicates as determined by high resolution x ray goniometry hrxtg complemented by x ray powder diffraction xrpd back scattered electron imaging and elemental x ray mapping points to a prelithification near syndepositional timing of initial authigenic calcite emplacement thus these carbonate cemented zones are of particular interest for the information they provide on the character of grain assemblages and sediment types at the time of deposition and also on the early post depositional environmental chemistry of the barnett shale strata

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 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

microstructures deformation mechanisms and strain patterns in a vertical profile inner appalachian fold thrust belt alabama a core from the american anniston no 1 well drilled in the pell city thrust sheet calhoun county alabama provides an excellent scientific opportunity to study in a vertical profile the deformation mechanisms and strain patterns associated with large scale structures the core contains stratigraphic units from cambrian to mississippian in age and two major thrust faults a detailed structural analysis revealed two deformation episodes a pre orogenic deformation and alleghanian orogenic deformation in the pre orogenic deformation synsedimentary folds and growth faults are the characteristic early structures and normal faults formed in lithifield rocks later during alleghanian deformation earlier fractures are overprinted by later s1 penetrative structures in limestone shale and siltstone within thrust sheets s1 is solution cleavage dolomites in the thrust sheets were deformed by fracturing and no penetrative cleavages formed in major fault zones s1 mylonitic foliation formed in limestone shale and siltstone fault related dolomites were deformed cataclastically and no s surfaces formed strain magnitude increases towards major thrust faults in both the hanging walls and footwalls for the typical fault configuration in the core with limestone dolomite in the hanging wall and shale siltstone in the footwall strain is mainly caused by pressure solution and cataclasis in the hanging wall and by plastic deformation in the footwall

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

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

permeability of illite bearing shale 2 influence of fluid chemistry on flow and functionally connected pores 1 bedding parallel permeability of illite rich shale of the wilcox formation has been investigated using distilled water and 1 m solutions of nacl kcl and cacl2 as pore fluids despite low modal concentrations of swelling clays specimens expand upon fluid saturation and permeabilities depend on fluid composition permeabilities to flow of 1 m cacl2 are 3 5 times greater than values measured for the other pore fluids suggesting sensitivity to exchange of divalent cations for monovalent cations at clay mineral surfaces permeabilities of individual samples exhibit nonrecoverable changes with sequential changes in composition of incoming fluid permeabilities k at varying effective pressure pe fit a cubic law k k0 1 pe p1 m 3 where m and p1 are independent of fluid composition and k0 is greater for transport of 1 m cacl2 than that for transport of the other pore fluids assuming that fluid conduits have crack like dimensions the lack of sensitivity of m and p1 to fluid composition suggests that surface roughness and asperity stiffness of conduits are unaffected by cation exchange while changes in k0 reflect changes in the clay fluid interfaces of the connected pore space index terms 5114 physical properties of rocks permeability and porosity 5139 physical properties of rocks transport properties 1832 hydrology groundwater transport 3947 mineral physics surfaces and interfaces 1045 geochemistry low temperature geochemistry keywords permeability shale fluid chemistry citation kwon o b e herbert and a k kronenberg 2004 permeability of illite bearing shale 2 influence of fluid chemistry on flow and functionally connected pores j geophys res 109 b10206 doi 10 1029 2004jb003055

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

a comparison of quartz c axis preferred orientations in experimentally deformed aplites and quartzites the effect of feldspar on the development of quartz c axis preferred orientations was investigated by experimentally deforming aplite 30 quartz and pure quartzite under equivalent conditions and comparing the quartz c axis patterns axial compression experiments were performed at temperatures of 800 and 900 c strain rates of 10 6 and 10 5s 1 and a confining pressure of 1500 mpa under these conditions both quartz and feldspar deform by dislocation creep with only minor syntectonic recrystallization the quartz c axis preferred orientations produced in the two rock types at equivalent deformation conditions and quartz grain strains were found to be essentially identical in pattern and strength these results indicate that the presence of feldspar has little or no effect on the operative slip systems in adjacent quartz grains in quartzo feldspathic rocks and that quartz c axis preferred orientation analyses can be performed on a wide range of crustal rock types

plagioclase microfabrics in a ductile shear zone from the jotun nappe norway abstract gabbroic and anorthositic rocks from the jotun nappe are transected by small ductile shear zones in which a high grade paragenesis and a new foliation are formed most plagioclase grains show evidence of plastic deformation and have recrystallized by subgrain rotation and bulge processes to fine grained mylonite during these processes a plagioclase grain shape fabric was destroyed and with increasing strain a pre existing plagioclase crystal fabric was successively modified enhanced and finally obliterated this could be evidence of superplastic flow inverse pole figure analysis of the typical plagioclase crystal fabric in the shear zone indicates that 021 a could be an operative slip system and planes close to 132 could be possible slip planes

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

compactional behavior of fine grained sediments examples from deep sea drilling project cores laboratory experiments statistical analysis of field data and theoretical models are combined in the attempt to derive standard compaction curves for various deep sea sediments which are in compaction equilibrium with the overburden a revised and generalized version of athy s law together with laboratory experiments allows the classification of sediments in terms of the amount of porewater structurally bound to the particles surfaces especially suction pressure experiments are useful in quantifying the forces that bind water on the particle surfaces the sediments analyzed almost range from systems of suspended grains to nearly perfect colloidal systems the methods and equations required to derive the relevant parameters are given for both downhole porosity density measurements and laboratory experiments

simple quantitative evaluation of porosity of argillaceous sediments at various depths of burial abstract relationship of porosity to depth of burial d in m geologic age a in millions of years and lithology ratio of thickness of shales total thickness of terrigenous deposits r has been developed by the writers which gives satisfactory results 0 exp 0 014 13 3 log a 83 25 log r 2 79 10 3 d where 0 initial porosity of the argillaceous sediment a nomogram has been prepared for an easy solution of the above equation

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

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

compaction driven evolution of porosity and permeability in natural mudstones an experimental study this paper describes a series of experiments designed to investigate the influence of lithology on the compactional loss of porosity and permeability in mudstones two intact samples of london clay with clay fractions of 40 and 67 were compacted to 33 mpa effective stress clay fraction permeability porosity pore size distribution and specific surface area were measured and their evolution was monitored throughout the compaction process electron microscopy was combined with mercury porosimetry to trace the collapse of the pore structure with increasing effective stress in both cases porosity loss occurred primarily by the collapse of large pores this process is more obvious in the coarser grained sample because throughout the compaction process it has a much broader range of pore radii and a much greater mean pore radius consistent with the pore size distributions the permeability of the coarser sample ranges from 10 10 m s 1 to 10 12 m s 1 while that of the finer grained sample ranges from 4 10 12 m s 1 to 5 10 14 m s 1 during progressive compaction from 2 to 33 mpa the compressibility of the finer grained sample is greater than that of the coarser grained sample 0 15 as opposed to 0 07 however in both cases the compressibility is much lower than that inferred for lithologically similar samples compacted over geological timescales the demonstration that both porosity and lithology clay fraction influence the permeability of mudstones should allow the development of more realistic porosity permeability relationships which take into account lithological variations exhibited by mudstones

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

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

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

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

seaweeds through time morphological and ecological analysis of proterozoic and early paleozoic benthic macroalgae abstract benthic marine macroalgae or seaweeds are key ecological players in oceans today and have been since the proterozoic to date however morphological and evolutionary patterns for precambrian macroalgae have been documented only in rather broad terms to refine our understanding in this critical interval we updated a dataset of proterozoic to early paleozoic noncalcified macroalgal fossils preserved as macroscopic carbonaceous compressions the data were analyzed using non parametric multidimensional scaling nmds and functional form group ffg approaches in order to characterize morphological and ecological evolutionary patterns of these macroalgae at the broadest scale the results show progressive increases in thallus size morphospace range and aspects of ecological complexity such as canopy heights surface area volume ratios and functional form groups through time these trends however are not always monotonous in particular significant increases in proterozoic macroalgal morphological diversity and maximum size appear to have occurred in the neoproterozoic tonian and ediacaran periods respectively analysis of ediacaran macroalgal assemblages shows that macroalgae broadly follow the ediacara like fossil assemblages with a significant drop in most studied metrics in the terminal ediacaran stage 550 539 ma the latter event occurs during the kotlinian crisis experienced by ediacara type macro organisms highlighting the possibility that this extinction event during the ediacaran cambrian transition affected both macroalgae and ediacara type macro organisms our analysis also confirms an ordovician increase in the morphological and ecological diversity of macroalgae

kotuikan formation assemblage a diverse organic walled microbiota in the mesoproterozoic anabar succession northern siberia abstract abundant and diverse microfossils from shales of the lower member of the 1500 ma old kotuikan formation in northern siberia document early mesoproterozoic life along the siberian platform similar to many meso and neoproterozoic microbiota the kotuikan assemblage is dominated by prokaryotic cyanobacteria both filamentous oscillatorialeans and nostocaleans which are represented primarily by cellular trichomes and by empty sheaths and coccoidal chroococcaleans including solitary and colonial specimens however unlike mesoproterozoic microbiota of shallow water settings the kotuikan compressed organic walled microfossil assemblage in the open shelf facies includes diverse microscopic eukaryotes large up to 1 mm diameter megasphaeromorph acritarchs branched filaments that are most likely of eukaryotic algae and coccoidal monostromatic colonies of chlorococcaleans as well as other morphologically complex microorganisms the kotuikan assemblage contains numerous taxa previously recorded from neoproterozoic successions e g elatera eosolena palaeastrum pterospermopsimorpha rugosoopsis and navifusa but these are long ranging genera and no neoproterozoic index fossils have been recorded the microbiota is considered to be mesoproterozoic in age because it contains a mixed assemblage of prokaryotic and eukaryotic microorganism remains including new taxa unknown in younger deposits the kotuikan assemblage documents the status of the marine biosphere at an early mesoproterozoic time preceding the primary radiation of eukaryotes and representing an evolutionary stage transitional between the predominantly prokaryote dominated paleoproterozoic and the eukaryote dominated neoproterozoic microbiota as reported in this study 35 taxa were identified in the kotuikan assemblage of which five forms are described informally that are assigned to 18 genera of microscopic prokaryotes and eukaryotes two new genera and five new species are proposed hirudiforma lancetica gen and sp nov h simmetrica gen and sp nov lineaforma elongata gen and sp nov elatera minor sp nov eosolena minuta sp nov

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

organic walled microfossils from the tonian tongjiazhuang formation of the tumen group in western shandong north china craton and their biostratigraphic significance abstract neoproterozoic successions in western shandong of north china craton contain fine grained siliciclastic rocks which offer a unique opportunity to explore the neoproterozoic biosphere in this study we have revealed a well preserved and diversified microfossil assemblage from the tongjiazhuang formation tumen group in western shandong north china craton which is characterized by abundant smooth walled sphaeromorphic acritarchs and cyanobacterium like filamentous forms and relatively low abundance of more complex acritarchs a total of 40 microfossil taxa belonging to 20 morphological genera have been identified most of which are described for the first time from the tongjiazhuang formation of the tumen group in western shandong including simia sp trachyhystrichosphaera aimika t botula germinosphaera bispinosa unnamed sp a jacutianema solubila and pololeptus rugosus the tongjiazhuang assemblage revealed in this study is much more diverse than previously reported and well comparable to the tonian liulaobei and gouhou assemblages in the xuhuai region particularly the occurrence of t aimika t botula g bispinosa j solubila and p rugosus indicates a late mesoproterozoic to tonian age for the tongjiazhuang formation in addition the occurrences of macroscopic carbonaceous compressions including chuaria tawuia assemblage from the tongjiazhuang formation and tawuia and sinosabellidites from the shiwangzhuang formation are consistent with geochronological data suggesting a tonian age for the tongjiazhuang fulaishan and shiwangzhuang formations of the middle and upper tumen group while the heishanguan and erqingshan formations of the lower tumen group are likely late mesoproterozoic to tonian in age thus available biostratigraphic data lithostratigraphic correlation and geochronological data suggest that the whole tumen group is late mesoproterozoic to tonian in age rather than cryogenian or ediacaran this study significantly improves our knowledge about the diversity of the tonian biosphere and implies a 200 300 million years unconformity the great unconformity between the late mesoprotoerozoic to tonian tumen group and the early cambrian liguan formation in western shandong

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

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

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

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

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

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

a global transition to ferruginous conditions in the early neoproterozoic oceans eukaryotic life expanded during the proterozoic eon1 2 5 to 0 542 billion years ago against a background of fluctuating ocean chemistry2 3 4 after about 1 8 billion years ago the global ocean is thought to have been characterized by oxygenated surface waters with anoxic and sulphidic waters in middle depths along productive continental margins and anoxic and iron containing ferruginous deeper waters5 6 7 the spatial extent of sulphidic waters probably varied through time5 6 but this surface to deep redox structure is suggested to have persisted until the first neoproterozoic glaciation about 717 million years ago8 9 10 11 here we report an analysis of ocean redox conditions throughout the proterozoic using new and existing iron speciation and sulphur isotope data from multiple cores and outcrops we find a global transition from sulphidic to ferruginous mid depth waters in the earliest neoproterozoic coincident with the amalgamation of the supercontinent rodinia at low latitudes we suggest that ferruginous conditions were initiated by an increase in the oceanic influx of highly reactive iron relative to sulphate driven by a change in weathering regime and the uptake of sulphate by extensive continental evaporites on rodinia we propose that this transition essentially detoxified ocean margin settings allowing for expanded opportunities for eukaryote diversification following a prolonged evolutionary stasis before one billion years ago

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

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

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

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

petrology of scourian supracrustal rocks and orthogneisses from stoer nw scotland implications for the geological evolution of the lewisian complex

taxonomy and affinity of early mesoproterozoic megascopic helically coiled and related fossils from the rohtas formation the vindhyan supergroup india abstract a well preserved assemblage of compressed straight circular to sinuously coiled megascopic and helical carbonaceous fossils and other varied megascopic morphoforms are known from the early mesoproterozoic rohtas formation semri group within vindhyan supergroup exposed in katni district of central india these megascopic remains are preserved as impressions compressions partially mineralized remains and or epi relief some of the forms are typical filamentous empty sheaths and others are trichomes with cell like entities under various stages of degradation this study based on fresh collections and also of the topotype material of the helically coiled megascopic fossils straight forms and related fossilized remains occurring as epi relief from katni indicate that the two morphotaxa are distinct entities and possibly appear to be prokaryotes grypania spiralis and katnia singhii are most likely of cyanobacterial origin spirally coiled and circular fossils with epi relief and which probably represents a tissue grade organism are considered as spiroichnus beerii mathur 1983 linear sheet like carbonaceous solitary form has been placed in the morphotaxon proterotainia and described as p katniensis n sp certain rare circular carbonaceous forms are considered as chuaria sp a few circular disc like forms found in the assemblage are treated as dubiofossils

evidence of organic structures in ediacara type fossils and associated microbial mats ediacara type fossils represent a group of soft bodied organisms mainly known from imprints in proterozoic coarse grained siliciclastic sedimentary rocks circular compressions of beltanelliformis brunsae and remains related to ediacara type fossils such as cucullus fraudulentus and mucuplagum primitivum are reported here in an organic mode of preservation from the neoproterozoic doushantuo and liulaobei formations of china they can be interpreted as prokaryotic colonies a charnid fossil with circular attachment disc and stalk but torn off frond is documented in a three dimensional and partly organic mode of preservation from the neoproterozoic ust pinega formation white sea coast russia according to their morphology and structure the charniidae are not regarded by us as pennatulaceans modern myxobacteria illustrate that macroscopic size complexity and even compartmentalization can also be developed by prokaryotic colonies part of the ediacara type fossils may therefore represent prokaryotic colonies or symbiotic organisms involving prokaryotes finally direct evidence indicates that biofilms with associated prokaryotic sheaths preserved in both organic and pyritic fashion form the wrinkled surfaces elephant skin that were preferentially colonized by ediacara type fossils this finding supports previous interpretations based on comparative morphological and sedimentological approaches that ancient wrinkle structures were microbial mats

microbiotas from the late proterozoic chuar group northern arizona and uinta mountain group utah and their chronostratigraphic implications abstract samples of shales from the dominantly clastic upper proterozoic chuar and uinta mountain groups in northern arizona and utah yielded relatively abundant assemblages of acritarchs these are taxonomically varied and are interpreted as consisting of openwater cosmopolitan eukaryotic plankton the assemblages are compared with late proterozoic late riphean and early vendian c 800 700 ma sequences in the southern urals of the ussr the russian platform northern and southern scandinavia svalbard and east and north west greenland in all instances both the assemblages and discrete form taxa predate the varangerian vendian glacial event of the north atlantic region and this is taken to indicate a comparable age for glaciogenic rock units in western north america one new acritarch species vandalosphaeridium walcottii is erected and the partly megascopic species chuaria circularis is emended

the nature and age of mesoproterozoic strike slip faulting based on re os geochronology of syntectonic copper mineralization assynt terrane nw scotland in ancient basement regions such as the lewisian complex nw scotland the ages of brittle deformation events are commonly poorly constrained due to a lack of datable fills an array of nw se sinistral and antithetic e w dextral faults related to a regionally recognized episode of brittle shearing cut neoarchaean gneisses and c 2 25 ga quartz pyrite veins close to the trace of the unexposed regional scale nw se fault copper iron mineralisation occurs at an intersection between an antithetic dextral fault and an older c 2 25 ga quartz vein optical microscopy sem and xrd analyses reveal an array of intergrown co genetic copper iron sulphides hematite and barite complex mm thick zoned alteration rims rich in epidote occur at contacts between the sulphides and gneisses rhenium osmium copper iron sulphide geochronology yields an age of c 1 55 ga for the hydrothermal mineralization event associated with faulting fault movements demonstrably overlap with mineralisation based on the asymmetric fibrous growth forms of these minerals within local dextral shears which acted as local channelways for mineralizing fluids during and after faulting we tentatively propose that this regionally recognised strike slip faulting previously termed the late laxfordian should be referred to as the assyntian in order to distinguish it from kinematically distinct laxfordian events

assessment of β the compression coefficient of mudstones and its relationship with detailed lithology abstract the mechanical compaction of soils and fine grained sediments is commonly described in soil mechanics by a relationship between void ratio and effective stress the relationship is defined by two parameters derived from the slope of the line relating void ratio and log effective stress and e the void ratio at a chosen effective stress e g 100 kpa and e are strongly correlated and are related to the atterburg liquid limit which is itself related in a general way to the grain size and mineralogy of the sediment in general finer grained sediments have higher initial void ratios and compact faster than coarser sediments porosity depth trends for mudstones in sedimentary basins are therefore strongly influenced by lithology steps or changes in the slope of porosity depth trends can be caused by lithological change rather than changes in the pressure regime a method is presented to evaluate for mechanically compacted mudstones using porosities determined from the density log fdc and based on the relationship between effective stress and void ratio developed in soil mechanics the method allows one to obtain rapidly approximate values for materials for which it is essentially impossible to obtain compaction parameters experimentally calculated values for mudstones in wells 8 3 2 and 30 2 1 on the norwegian margin vary between 0 2 and 0 65 the values correlate positively with the clay particle content of the mudstones which we have determined for 50 mudstone samples from the same wells the strong link between mudstone lithology and suggests that a downhole log may be a useful way to visualize downhole variations in mudstone lithology the log also shows how lithology affects the rate of porosity loss with depth or effective stress for example porosity in mudstones within both wells 8 3 2 and 30 2 1 is essentially constant over 700 m sections of normally pressured mudstones the log and grain size data indicate that this reflects changes in the lithological character and of the muds and not as commonly inferred a change in the pressure regime

an assemblage of macroscopic and diversified carbonaceous compression fossils from the tonian shiwangzhuang formation in western shandong north china abstract macroalgae play a crucial ecological and biogeochemical role in modern marine ecosystems although eukaryotic algae have a deep history within the precambrian most pre cryogenian alga fossils are microscopic and macroalga fossils are poorly documented during this period here we report a well preserved assemblage of macroscopic and diversified carbonaceous compression fossils from the tonian shiwangzhuang formation in western shandong north china this assemblage consists of seventeen distinct morphotaxa including eleven macroalgae showing remarkable taxonomic diversity that is more diverse than many other tonian macrofossil assemblages all over the world three taxa from the shiwangzhuang assemblage show evidence of simple multicellularity with two of them consisting of giant cells and comparable to giant celled green algae and seven taxa with diagnostic transverse annulations are likely coenocytic algae the shiwangzhuang assemblage indicates that some eukaryotic clades had achieved macroscopic growth through simple multicellularity or coenocytism paving the way either ecologically or phylogenetically for the eventual appearance of complex multicellularity in addition there are some biostratigraphically significant genera including the chuaria tawuia and sinosabellidites protoarenicola pararenicola assemblages indicating a tonian age for the shiwangzhuang formation this study also suggests that the tonian diversity of multicellular or coenocytic macroalgae is probably underestimated pending for a systematic and thorough investigation of tonian macroscopic carbonaceous compressions macroalgae may have an initial ecological expansion at least locally if not globally in the tonian period

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

macroscopic worm like body fossils from the upper precambrian 900 700 ma huainan district anhui china and their stratigraphic and evolutionary significance abstract abundant and varied macroscopic worm like body fossils from the late precambrian liulaobei formation huainan group and the jiuliqiao formation feishui group in the huainan district northern anhui province china are further investigated through this progress report after the initial discovery by zheng wenwu and a recent study by wang guixiang sinosabellidites zheng in the liulaobei formation is a worm like organism of questionable metazoan origin it has merely a superficial resemblance with the tubes of late vendian early cambrian true sabelliditida but has much in common with the accompanying macroscopic algal remains of tawuia hofmann both pararenicola wang and protoarenicola wang are representatives of primitive worm like animals in the jiuliqiao formation their macroscopic size elongate cylindrical body anterior aperture and apparatus prominent and elastic annulations are characteristic for their metazoan origin although they lack sufficient evidence for their definite relationship with any known kind of living worms the liulaobei formation and the jiuliqiao formation are dated at about 850 ma and 740 ma respectively regional geology and stratigraphic correlation and the occurrence of the distinctive chuaria tawuia assemblage suggest a time range between less than 900 ma and over 700 ma for the whole shallow water marine sedimentary sequence of the huainan and feishui groups this latter group is disconformably covered by the drop stone facies diamictites and tillites of the late precambrian luoquan glaciation apart from the enigmatic sinosabellidites both pararenicola and protoarenicola are claimed to be the oldest multicellular animals so far discovered in the world and the first reliable evidence for the pre ediacarian evolutionary history of manifest metazoan life

the generation of quartz normative melts and corundum bearing restites by crustal anatexis petrogenetic modelling based on an example from the lewisian of north west scotland abstract rare layers of an aluminous muscovite rich rock from the lewisian complex at stoer north west scotland display evidence which suggests that the rock has undergone local partial melting to form quartz bearing veins and a corundum bearing restite the assemblages observed in these rocks match those predicted by modelling in the system kalo2 naalo2 al2o3 sio2 h2o knash where certain bulk compositions melt peritectically to give corundum bearing restites and quartz normative melts study of the model system shows that the observed parageneses could have formed from a range of bulk compositions with a variety of possible values of ah2o which could have been internally or externally buffered the knash petrogenetic grid together with another in the system cao na2o feo al2o3 sio2 h2o cnfash allows the p t path of the rocks to be delineated and an estimate to be made of the conditions at the peak of metamorphism as 11 kbar and 900 925 c this estimate is in agreement with p t estimates using thermobarometric methods on adjacent lithologies the activity of h2o in the system throughout metamorphism is calculated to have been 0 3

microfossils from the lower mesoproterozoic kaltasy formation east european platform abstract basinal shales of the lower mesoproterozoic kaltasy formation sampled from three boreholes drilled into the southeastern east european platform russia contain abundant and moderately well preserved microfossils 34 distinct entities have been identified most assigned to simple sphaeromorphic or small filamentous taxa found widely and characterized by long stratigraphic ranges ornamented microfossils found in coastal successions of other lower mesoproterozoic basins are absent but large filamentous microfossils interpreted as possible benthic photosynthetic eukaryotes are recorded drawing comparisons to relatively deep water shales in siberia in overall aspect the kaltasy microfossils are consistent with other broadly coeval assemblages but they highlight the importance of environment as well as age in determining the distributions of remains that record the early diversification of marine eukaryotes rectia magna is described as a new species

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

experimental mechanical compaction of clay mineral aggregates changes in physical properties of mudstones during burial dry and brine saturated clay aggregates ranging from pure smectite to pure kaolinite were compacted in the laboratory experiments were conducted by increasing vertical effective stress up to 50 mpa to study the changes in physical properties of mudstones during burial the results suggest that the physical properties porosity density acoustic velocity etc of mudstones vary greatly with increasing effective stress clay mineralogy and fluid content kaolinite aggregates are much more compressible than those composed of smectite brine saturated clay mixtures are much more compressible than dry clay mixtures brine may soften and lubricate the clay matrix compared to dry clays resulting in higher compressibility in the brine saturated state the lesser degree of compaction of dry smectite compared to dry kaolinite aggregates can be explained by the differences in grain size as kaolinitic clays have much larger grains than smectitic clays the extremely fine grained nature of smectite implies that the imposed vertical stress is distributed over a very large number of grain contacts so that the force per contact area is very low in smectite compared to that in coarse grained kaolinite at 20 mpa effective stress corresponding to about 2 km burial depth with hydrostatic pore pressure brine saturated pure kaolinite compacted to 20 porosity while pure smectite retained 41 porosity brinesaturated clay mixtures show higher vp and lower vs than dry clay mixtures a pronounced difference in vs was observed among the brinesaturated clay mixtures the lowest vs as a function of vertical effective stress was found for pure smectite for a given porosity value smectitic clays have higher velocities and elastic moduli than the kaolinitic clays the vp vs ratio is significantly higher in smectite than in kaolinite aggregates treating mudstones as a uniform class of rocks introduces significant errors in estimates of physical properties which are strongly affected by the clay mineralogy grain size the amount of other minerals present and fluid content our results have implications for well log interpretation and mudstone and shale property determinations from seismic data at shallow burial depth o 2k m 801c above depths where significant chemical compaction takes place r 2007 elsevier ltd all rights reserved

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

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

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

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

linking the sulu uhp belt to the korean peninsula evidence from eclogite precambrian basement and paleozoic sedimentary basins abstract considerable debate on whether and how the sulu orogenic belt extends eastward to the korean peninsula has remained over the past decade new results reported here include the following 1 an eclogite and retrograded eclogite bearing complex hongseong complex is discovered in south korea in which the eclogite occurs as lenses in circa 810 820 ma granitic gneiss shrimp zircon dating of the eclogite yields 230 ma for the metamorphic age and 880 ma for the protolith age 2 the basement of the rangnim gyeonggi and yeongnam massifs have affinities to the basement of the north china block ncb however the gyeonggi massif encloses a minor amount of large or small slabs of the hongseong complex that are similar to the rocks of the sulu belt 3 two main paleozoic basins within the rangnim and gyeonggi massifs have a similar paleozoic tectono stratigraphy to the ncb 4 the imjingang and ogcheon belts do not exhibit any metamorphic characteristics of collisional orogenic belts based on these facts we propose a crustal detachment and thrust model and suggest that the collision belt between the yangtze block yb and ncb sino korea craton is preserved along the western margin of the korean peninsula the lower part of the uhp metamorphosed lithosphere of the yb was subducted under the korean peninsula and not uplifted to the surface the lower crust of the yb the hongseong complex was detached from the subducted lithosphere and thrust over the korean peninsula and inserted into the basement rocks of the gyeonggi massif the upper crust of the yb possibly was detached from the lower crust and overthrusted along the honam and chugaryong shear zones the imjingang and ogcheon belts possibly represent the detached upper crust of yb and their present occurrences are controlled by a mesozoic strike slip shear structure all these detached lower and upper crustal slabs were strongly deformed during the late jurassic and early cretaceous tectonic event leading to their present geological distribution and characteristics

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

structural characteristics and re os dating of quartz pyrite veins in the lewisian gneiss complex nw scotland evidence of an early paleoproterozoic hydrothermal regime during terrane amalgamation in the archaean basement rocks of the assynt and gruinard terranes of the mainland lewisian complex in nw scotland a regional suite of quartz pyrite veins cross cut regional palaeoproterozoic badcallian ca 2700 ma inverian ca 2480 ma fabrics and associated scourie dykes the quartz veins are overprinted by amphibolite greenschist facies laxfordian deformation fabrics ca 1760 ma and later brittle faults the hydrothermal mineral veins comprise a multimodal system of tensile hybrid hydraulic fractures which are inferred to have formed during a regional phase of nw se extension the almost orthogonal orientation of the quartz veins ne sw to the scourie dykes nw se are incompatible and must result from distinct paleostress regimes suggesting they are related to different tectonic events this hypothesis is supported by rhenium osmium dating of pyrite that yields an age of 2249 77 ma placing the vein hosted mineralisation event after the oldest published dates for the scourie dykes 2420 ma but before the youngest ages 1990 ma sulphur isotope analysis suggests that the sulphur associated with the pyrite is isotopically indistinguishable from primitive mantle the presence of the ca 2250 ma quartz pyrite veins in both the assynt and gruinard terranes confirms that these crustal units were amalgamated during or prior to inverian deformation the absence of the veins in the rhiconich terrane is consistent with the suggestion that it was not finally amalgamated to the assynt terrane until the laxfordian

neoproterozoic microfossils from the margin of the east european platform and the search for a biostratigraphic model of lower ediacaran rocks abstract a ca 600 m thick siliciclastic succession in northern russia contains abundant and diverse microfossils that document early to middle ediacaran deposition along the northeastern margin of the east european platform the vychegda formation is poorly exposed but is well documented by a core drilled in the timan trough region kel tminskaya 1 borehole vychegda siliciclastics lie unconformably above tonian to lower cryogenian strata and below equivalents of the late ediacaran redkino succession that is widely distributed across the platform the basal 10 m of the formation preserve acritarchs and fragments of problematic macrofossils known elsewhere only from pre sturtian successions in contrast the upper nearly 400 m of the succession contains abundant and diverse large acanthomorphic acritarchs attributable to the ediacaran complex acanthomorph palynoflora ecap this distinctive set of taxa is known elsewhere only from lower but not lowermost ediacaran rocks in between lies an additional assemblage of relatively simple filaments and stratigraphically long ranging sphaeromorphic acritarchs interpreted as early ediacaran in age bearing in mind that knowledge of late cryogenian post strurtian pre marinoan microfossils is sparse the vychegda record is consistent with data from australia and china which suggest that diverse ecap microfossil assemblages appeared well into the ediacaran period accumulating paleontological observations underscore both the promise and challenges for the biostratigraphic characterization of the early ediacaran period

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

neoproterozoic 900 ma sariwon sills in north korea geochronology geochemistry and implications for the evolution of the south eastern margin of the north china craton article the sariwon sills are distributed in the pyongnam basin at the center of the korean peninsula eastern part of the north china craton these sills are up to 150 m in thickness and up to more than 10 km in length baddeleyite grains separated from a 50 m thick sill give a sims 206 pb 207 pb age of 899 7 ma mswd 0 34 n 14 which is interpreted to be the crystallization age of this sill zircon grains from the same sill gives a lower intercept u pb age of 400 ma which is likely a close estimation of the greenschist facies metamorphism of this sill the sariwon sills are dolerites and have tholeiitic compositions they show enrichment of light rare earth element concentrations la ybn 1 4 2 8 and are slightly depleted in high field strength elements e g nb zr and ti in comparison to neighboring elements on the primitive mantle normalized spidergram the whole rock endt t 900 ma values are around 2 whereas in situ ehft t 900 ma values from zircon grains vary from 25 to 8 they are similar to the coeval sills in other parts of the north china craton e g the chulan sills xu huai basin shandong peninsula and the dalian sills lv da basin liaodong peninsula these sills possibly originated from a depleted mantle source e g asthenosphere rather than from the ancient lithospheric mantle of the north china craton and have experienced significant assimilation of lithospheric materials the strata and sills in the xu huai lv da and pyongnam basins are comparable moreover the three basins are geographically correlatable based on neoproterozoic geographical reconstruction we therefore propose that there is a xu huai lv da pyongnam rift system along the south eastern edge of the north china craton during neoproterozoic 900 ma with the closure of the rift at 400 ma as a result of a continent margin process it is possible that this southeastern margin of the ncc did not face the inland in the configuration of the supercontinent rodinia