cadomian terranes wrench faulting and thrusting in the central europe variscides geophysical and geological evidence sixty five per cent of the paleozoic basement of western and central europe is hidden by a sedimen tary cover and or sea this work aims to remove that blanket to detect new structures which could used to build a more comprehensive model of the variscan or ogeny it is based on the interpretation of various forms of data a published gravity maps corrected for the ef fects of the crust mantle boundary topography and light sedimentary basins b aeromagnetic maps c measurements of densities and d induced and rema nent magnetizations on rocks from paleozoic outcrops of the upper rhenish area from the northern b ohem ian massif to the eastern paris basin the saxothurin gian is characterized by a 500 km long belt of gravity highs the most important being the kraichgau high most of the corresponding heavy bodies are buried un der a post early visdan cover they are interpreted as relics of late proterozoic terranes overlain by an early to middle paleozoic sequence equivalent to the bo hemian t errane in the bohemian massif the most probable continuation of these dense bohemian ter ranes toward the west is the southern channel north ern brittany cadomian terrane the gravity lows are correlated with variscan granites and pre and early variscan metagranites gravity and magnetic maps demonstrate large scale displacement in devonian early carboniferous times along the parallel and equidistant nw se striking vistula elbe bavarian bray and south armorican dextral wrench faults in the vosges schwarzwald and central massif the faults continue with the east west striking lalaye lubine ba den baden and marche faults and with south vergent thrusts the bavarian faults shift the kraichgau terrane by 150 km relative to the bohemian terrane whereas the offset of the north ern brittany cadomian relative to the northern vosges kraichgau terranes is estimated at 400km along the bray fault sinistral wrench faults are the ne sw striking sillon houiller rheingraben rodl vitis and diendorf faults the southern vosges schwarzwald devonian dinanti an basin is interpreted as a pull apart basin at the south easterly extremity of the bray fault the bohemian and kraichgau body form allochthonous terranes which were thrust over the saxothuringian crust thrusting to the north west was accompanied by back thrusting and led to the forma tion of pop up structures contemporaneous dextral and sinistral wrench faulting resulted in transpressive strain during collision the zonal structure of the varis cides in the sense of kossmat 1927 is relevant only to the rhenohercynian foreland belt kossmat 1927 al ready spoke of a moldanubian region because it dis plays no real zonal structure the saxothuringian zone was formed by terrane accretion their apparent zonal structure is not a pre collisional feature but only the result of accretion and collision

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

models of magnetic anisotropy variations in sedimentary thrust sheets abstract magnetic anisotropy variation in sedimentary thrust sheets was modelled through the extension of strain models in thrust sheets coward and kim 1981 sanderson 1982 using the empirical relationship between the magnetic anisotropy and strain and respecting the pre deformational anisotropy of sedimentary rocks in thrust sheets deformed by simple shear and lateral lengthening the magnetic foliations remain near the bedding while in thrust sheets deformed by simple shear and lateral shortening the magnetic foliation may deviate strongly from the bedding even creating a girdle pattern in its poles

magnetic susceptibility of the rock matrix related to magnetic fabric studies non ferromagnetic minerals constitute what is called the rock matrix whose susceptibility k1 is directly accessible using only high magnetic fields measurements on minerals and a wide range of rock types show that k1 is mainly due to paramagnetism and hardly exceeds 10 3 si with an anisotropy degree p less than 1 35 different methods to estimate the role of the matrix component in low field susceptibility k and its anisotropy include petrological and chemical analysis evolution of p vs k low temperature studies and comparison of k with remanent magnetizations

rapid burial and exhumation during orogeny thickening and synconvergent exhumation of thermally weakened and thinned crust variscan orogen in western europe we present a model demonstrating that the presence of a thinned and thermally weakened crustal domain is a prerequisite for the formation of an orogenic root in mountain belts furthermore this mechanism permits burial to 50 to 70 km in time periods of less than 20 myr and exhumation from these depths in less than 10 myr our example from the variscan belt is based on field geology petrology of metamorphic rocks and detailed geochronology it describes an evolution from i sedimentation and magmatism during back arc extension ii compression leading to burial thickening of the lithosphere and granulite facies metamorphism followed by iii exhumation formation of dehydration melts migmatites and late intrusive granites the existing u pb ar ar data constrain this evolution to no more than 20 to 25 myr thermorheological modelling starts from a hot thinned crust forming a narrow 50 km wide orogenic root within 15 myr at a maximum of 50 km depth and subsequent exhumation by extrusion within 7 myr the model explains exhumation rates of 3 to 4 mm year by compressional extrusion and shows that extension cannot do the same job within the same time extrusion transported the internal thermal structure into the upper crust within a short time followed by rapid cooling whereas exhumation through extension would cause mantle upwelling and the formation of a long lived thermal anomaly the latter would contradict the geochronological and petrological database the model explains orogeny as a thermorheological process without needing to involve an external heat source from the mantle and presents a new explanation of the high temperature and low pressure conditions very typical of the variscan orogenic crust

microstructure and magnetic susceptibility applied to emplacement kinematics of granites the example of the foix pluton french pyrenees abstract the variscan foix granite 40 km 2 central pyrenees was subjected to detailed microstructural magnetic susceptibility and anisotropy studies magnetic susceptibility is shown to be of dominant paramagnetic origin its magnitudes clearly define a zoning which compares with the petrographic zoning and reveals that the pluton is the southern remnant of a normally zoned pluton truncated during alpine tectonics magnetic susceptibility anisotropy characterizes the planar and linear preferred orientations of the main iron bearing silicates namely biotite and accessory amphibole it is argued that these fabrics also reflect the flow pattern of the granite microstructural observations aid in distinguishing the magmatic and late magmatic fabric signatures main body from the post magmatic solid state signature leucogranite in order to account for the consistently subhorizontal and ne sw directed magmatic lineations in the northern sector of the foix granite an oblique intrusion is proposed it may be coeval with the early variscan regional sw trending shearing event steepening of the structures in at least part of the southern sector may be coeval with the late development of upright folds and vertical cleavage in the country rocks the utility of combined microstructural and magnetic susceptibility studies is demonstrated as an aid in the structural interpretation of granite intrusions which act as kinematic markers of their host terranes

tectonic evolution of the european variscan belt constrained by palaeomagnetic structural and anisotropy of magnetic susceptibility data from the northern vosges magmatic arc eastern france geochronology structural and anisotropy of magnetic susceptibility data from the northern vosges batholith which belongs to the northern vosges mid german crystalline rise arc show contrasting emplacement modes of southern granodiorites and northern granites the ene wsw trending fabrics of granodiorites 346 334 ma are parallel to the metamorphic cleavage affecting the host rocks developed during regional compression the nnw sse trending fabrics of younger granitoids c 330 ma reveal an extensional emplacement mode associated with a normal shear zone separating the two magmatic suites palaeomagnetism shows that the switch from a compressive to an extensional regime coincides with a regional counterclockwise rotation the 330 325 ma extension is further supported by palaeomagnetic and seismic data indicating southeastward tilt of the whole batholith finally the system is rotated clockwise without any structural overprint data from the early carboniferous northern vosges magmatic arc and remote bohemian massif suggest that this evolution is valid for the whole eastern branch of the european variscan belt for which the following model is proposed 1 late devonian early carboniferous east west shortening of the variscides above the rhenohercynian subduction zone 2 axial nnw sse shortening of the assembled variscan orogen associated with 3 internal sinistral rotation of inherited rhenohercynian transform faults and shortening of intervening blocks 4 east west extension and dextral external rotation of blocks between dextrally reactivated transforms supplementary materials analytical procedures for geochronology magnetism anisotropy of magnetic susceptibility and palaeomagnetism are available at www geolsoc org uk sup18635

a flow law for dislocation creep of quartz aggregates determined with the molten salt cell we have used the molten salt cell to conduct an experimental study on the rheology of a natural quartzite containing 0 15 wt water co axial deformation experiments were conducted at constant piston displacement rates approximating constant strain rates at low strain the strengths of our natural quartzite measured in the molten salt cell are approximately half those measured at the same conditions in solid media because unlike solid confining media molten salt does not contribute to the strength of the sample it reduces the friction on the moving piston and it allows clear identification of the hit point we have limited the experimental conditions to those required for dislocation creep and have used only steady state flow stresses measured during climb accommodated dislocation creep to calculate the flow law parameters two flow laws were determined one for samples containing minor amounts of melt 1 2 and one for melt free samples in both cases the power law stress exponent n is 4 0 0 9 which is greater than that previously reported in flow laws for dislocation creep of quartz aggregates determined in solid media the activation energy q is 137 34 kj mol 1 for samples with melt and 223 56 kj mol 1 for those without within the range of previously determined values for quartz aggregates containing 0 1 wt water the pre exponential term a is 1 1 10 4 2 mpa ns 1 for samples without melt and 1 8 10 8 2 mpa ns 1 for those with melt the lower strengths measured in the molten salt cell indicate that previous piezometer relations for quartz experimentally determined in solid media are not correct extrapolation of the flow law for melt free aggregates to natural strain rates predicts higher strengths than most previous quartz flow laws however accurate extrapolation requires determining the dependence of flow stress on fh2o and or ah

retrograde evolution in the central vosges mountains northeastern france implications for the metamorphic history of high grade rocks during the variscan orogeny abstract metamorphism in the vosges massif is characterized by rocks formed under a wide range of pressures and temperatures high pressure rocks garnet peridotites and eclogites facies rocks are found together with low pressure rocks garnetcordierite stability field hitherto metamorphism has been interpreted within the framework of an identical p t evolution for all rock types corresponding to either two separate geotectonic events or a unique p t evolution from high pressure to low pressure conditions we studied gneisses from the vosges massif in order to constrain the p t t path they followed p t estimation indicates that gneisses equilibrated between 5 and 7 kbar during peak metamorphism time rocks later experienced cooling from temperatures above 650 c down to 550 c it is possible that the entire p t evolution of the gneisses took place within the lower part of the amphibolite facies which is inconsistent with the p t evolution of the high pressure rocks the noteworthy absence of muscovite staurolite assemblages is attributed to conditions of low h2o and low o2 activity the consequences for the geodynamics in the vosges massif are that at least two separate geotectonic events are required in order to explain the juxtaposition of high pressure rocks with low pressure amphibolite facies rocks

magnetic fabrics and strain in pencil structures of the knobs formation valley and ridge province us appalachians pencil structures which are found in weakly deformed mudrocks reflect the bedding cleavage intersection in weak to moderatelycleaved rocks their presence indicates conditions where bedding and cleavage fabrics are approximately equal in intensity we have determined the anisotropy of magnetic susceptibility ams in pencil structures from a sequence of mudstones of the ordovician knobs formation in the valley and ridge province of the us appalachians magnetic mineralogy was determined by x ray analysis and lowtemperature susceptibility measurements distribution of the magnetic ellipsoid axes is consistent with the incipient tectonic fabric of the pencil mudstones the maximum susceptibility axes parallel the pencils long axes while the minimum axes of susceptibility are normal to the primary sedimentary fabric independent strain quantification permits a correlation between magnetic fabric and tectonic strain an exponential relationship between the ams shape parameter t and tectonic shortening has been found for the interval of 10 25 shortening shortening 17 p exp t this relationship appears to be supported by tectonic strains up to 40 the t parameter t lnf 2 lnl lnl lnf where l lineation is kmax kint and f foliation is kint kmin describes the shape of the magnetic susceptibility ellipsoid which appears more sensitive to strain than past correlation attempts with the magnetic intensity parameter p or p 0 whereas this correlation between strain and ams is only valid within a restricted window of strain 10 40 shortening it establishes the magnitude and directions of tectonic strain in weakly deformed clay rich rocks where strain indicators are otherwise lacking or are poorly developed q 2003 published by elsevier science ltd

characteristics of internal contacts in the tuolumne batholith central sierra nevada california usa implications for episodic emplacement and physical processes in a continental arc magma chamber internal contacts along the eastern margin of the tuolumne batholith central sierra nevada california and structures preserved along these contacts are highly variable the contacts range from relatively sharp to gradational boundaries to sheeted zones to very complex boundaries formed by multiple processes fractional crystallization kilometer scale mixing within broad transition zones voluminous magmatic stoping along sharp contacts and downward return flow and or margin collapse of older magma units were important large scale processes along these contacts during chamber construction in contrast sheeting extensional cracking and diking represent only second order small scale complexities formation of the most complex zones along internal contacts resulted from the interaction of the sequential emplacement of different units with the irregular geometry of these contacts which often resulted from removal of earlier phases by stoping our results indicate that multiple processes are likely during emplacement of large magma bodies within one another and that it is unlikely that evidence for all internal processes during batholith construction are preserved we also argue that fairly large magma chambers existed in this batholith and thus that large accumulations of magma may exist in upper crustal chambers for significant periods of time

the granite pluton of zahedan se iran a petrological and magnetic fabric study of a syntectonic sill emplaced in a transtensional setting abstract the 32 ma nw se elongate 75 km long granite pluton of zahedan is one of the most conspicuous geological features of southeast iran it is composed of metaluminous to weakly peraluminous rocks corresponding to magnetite and ilmenite bearing i type granitoids ranging from diorites to granites all rocks including the late andesitic to dacitic dykes belong to the high k calk alkaline series field and structural relationships as well as evidence for mixing mingling can be used to unravel the magmatic history of the different rock types microstructural examinations and magnetic fabric measurements were performed on a collection of oriented specimens from the northern part of the pluton ubiquitous flat lying foliations argue in favour of a sill like body except for the diorite the biotite bearing granitic facies at the pluton s periphery was the first batch of magma which initiated the sill this sill probably behaved as a n s directed decollement zone marked by conspicuous magmatic cataclastic microstructures and n s treading lineations the intrusion of the hornblende bearing granodiorite with typical magmatic structures inflated the sill deformation occurred at the same time with e w to nw se stretching marked by lineations the main dioritic stock forming the floor of the sill is considered to be a remnant of the feeder zone an abundant dyke swarm marks the end of the syntectonic emplacement of the zahedan sill a three dimentional model of emplacement is proposed in the context of the latest episodes in the closure of the neo tethys during which the lut block ended its northward migration with respect to the afghan blocks

significance of ams analysis in evaluating superposed folds in quartzites quartzites tend to be compositionally homogeneous and because of this deformation related fabric elements foliations and lineations are poorly developed in them this makes structural analysis of deformed quartzites challenging the measurement of anisotropy of magnetic susceptibility ams is useful for recognizing structural imprints in rocks that lack mesoscopic fabrics and the present study is carried out with an aim to demonstrate the robustness of ams in analysing such deformation imprints in quartzites ams data of samples from folded quartzites located in an approximately 10 km 2 area around galudih eastern india are presented although on a regional scale superposed deformation and ductile shearing are known from the area the investigated quartzites do not preserve mesoscopic evidence of these large scale features and have developed folds that plunge gently towards the se with a vertical nw se striking axial plane the magnetic foliation recorded from ams analysis is parallel to the axial plane while the orientation of the magnetic lineation varies from se through vertical to nw this is similar to the large scale fold axis variations recorded in various regional domains mapped over an area of about 200 km 2 it is concluded that although the imprint of regional superposed deformation is not obvious on the mesoscopic scale in the quartzites around galudih this imprint can be detected from the magnetic fabric the present study thus highlights the usefulness of ams in analysing superposed folds in quartzites

multiple magmatic fabrics in the sazava pluton bohemian massif czech republic a result of superposition of wrench dominated regional transpression on final emplacement abstract the sazava pluton 354 ma of the central bohemian plutonic complex was emplaced syntectonically into the upper crustal neo proterozoic and lower paleozoic host rock along the se margin of the tepla barrandian zone bohemian massif czech republic the host rock is characterized by transpressional fabric dominated by steep se nw dipping foliations and subhorizontal ne sw stretching lineations associated with flattening to a plane strain ellipsoid the strain intensity degree of oblateness and temperature conditions of deformation increase in the host rock towards the pluton margin in the pluton quartz microfabric indicates progressive cooling of the syntectonically emplaced magma the ams method revealed multiple magmatic fabrics and lateral variations in the shapes of the ams ellipsoid and the magnetic fabric intensity in the pluton the central part of the pluton yields prolate shapes of the ams ellipsoid and a low degree of anisotropy with preserved steep magnetic lineations whereas along the pluton margins oblate ams ellipsoids are associated with a high degree of anisotropy we interpret the multiple fabrics and the partially decoupled fabric pattern between the host rock and the pluton as being a result of superposition of regional tectonic strain during highly oblique transpression on emplacement related intrusive strain where outer solidified and more rigid edges of the pluton enabled preservation of older steep fabrics in its interior

some remarks on fabric overprints and constrictional ams fabrics in igneous rocks during the ascent emplacement and post emplacement deformation of igneous rocks two or more phases of deformation that overprint each other are often depicted these overprints when magnetic minerals are present are recorded in magnetic fabric in this contribution overprints are studied by means of numerical modeling following several basic scenarios common to igneous rocks biotite and amphibole that occur often together in igneous rocks are considered as carriers of the anisotropy of magnetic susceptibility modeling shows that 1 a constrictional fabric with a low degree of anisotropy as commonly recorded in magmatic rocks may result from a deformation overprint and not necessarily from an extensional transtensional regime and 2 that the constrictional ams fabrics originates from orthogonal superimposition of a deformation event on an ams fabric inherited from earlier magma emplacement history therefore the interpretation of a constrictional fabric must be performed with caution numerical modeling may provide a suitable help in strengthening the interpretation of real magnetic fabric data

the emplacement of the granitic las tazas complex northern chile the relationship between local and regional strain abstract the coastal batholith of northern chile grew under transtensional extensional conditions that prevailed along the andean margin during the mesozoic the batholith hosts the atacama fault zone a major arc parallel fault system which was characterised by sinistral transtensional shearing during the early cretaceous the las tazas complex is a composite granitoid intrusion that was emplaced syntectonically along the atacama fault zone at 130 ma syntectonic emplacement is indicated by a consistent kinematic history between the complex and its wall rocks together with synchronous crystallisation and shearing ages in contrast to regional patterns the las tazas complex was emplaced during a local change from vertical east side down to dextral transcurrent displacement along the fault zone during intrusion strain was partitioned between non coaxial simple shearing within country rock mylonites and a flattening strain across the crystallising complex this combination indicates that the pluton was emplaced under temporary transpressive conditions that were localised around the pluton probably induced by magma emplacement such a difference between local and regional strain suggests that emplacement related structures should only be related to regional strain states with great care

strain distribution and fabric development modeled in active and ancient transpressive zones 1 a model based on kinematics of transpression allows the measurable internal parameters simultaneous strain fabric and vertical elevation to be simulated in relation to the macroscopically determined external parameters of transpressive zones i e zone width velocity angle of convergence and zone depth we present a diagram of convergence angle against time for homogeneous transpression where isolines of strain intensity d strain symmetry k and vertical elevation of rock samples are superposed however the distribution of internal strain parameters is sensitive to three types of strain partitioning 1 discrete partitioning results in general decrease of finite strain accumulations and in increase of pure shear component 2 ductile partitioning splits the transpressional domain into a pure shear zone where strain accumulations decreases and in a wrench dominated zone where strain accumulations increases and 3 viscosity partitioning is marked by different strain rates in zones of different viscosity and therefore by different strain parameters

interpreting magmatic fabric patterns in plutons abstract most plutons have widespread magmatic fabrics the interpretation of which remains controversial we propose a method to constrain likely causes of fabric patterns the application of which indicates the following 1 preserved fabric patterns often form after chamber construction and only rarely provide information about ascent or emplacement 2 fabrics are poor recorders of total strain and are easily reset preserving only the last increment of strain during crystallization 3 in magmatic systems mechanically decoupled from host rocks patterns may result from strain during internally driven flow filter pressing or porous flow in relatively static chambers or by final increments of strain during emplacement 4 with greater emplacement depths fabric patterns increasingly reflect strain caused by regional deformation and 5 given that magmatic fabrics are easily reset and reflect only the last increment of strain of comparatively weak materials they may provide a relatively direct record of paleostress in orogenic belts

mechanisms of continental growth in extensional arcs an example from the andean plate boundary zone during the late triassic jurassic and early cretaceous an extensional magmatic arc was formed in the andean margin of northern chile plutons emplaced at ramps within a hinterland propagating extensional duplex were fed by dikes that transferred magma through the lower crust from a reservoir in the mantle phases of volcanism separated phases of plutonism and our model demonstrates that plutonism was favored only when upper plate extensional fault systems were active when they were not dikes cut across inactive faults and magma rose directly to the surface during volcanic phases of arc growth

ams record of brittle dilation viscous stretching and gravity driven magma ascent in area of magma rich crustal extension vosges mts ne france orogenic compression related fabrics 340 335 ma were reworked during regional extensional deformation 328 325 ma in a large anatectic crustal domain of the central vosges ne france the extension was first accommodated by brittle dilation affecting vertically anisotropic high grade rocks associated with emplacement of subvertical granitic sheets the ams fabric of granitoids is consistent with highly partitioned transtensional deformation marked by alternations of flat and steep foliations and development of orthogonal lineations this deformation passes to top to the southwest ductile shearing expressed in southerly migmatitic middle crust the ams fabric revealed moderately west dipping foliations bearing subhorizontal nnw sse trending lineations and predominantly plane strain to prolate shapes this fabric pattern is interpreted as a viscous response of stretched partially molten crust during continuous ductile extension vertical ascent of voluminous granites and stoping of the upper crust occurs further south this gravity ascent triggered by extension leads to development of south dipping ams foliations south plunging lineations and oblate fabrics in various crustal granites vertical shortening related to ascent of these 325 ma granitoids and persistent n s stretching is responsible for reworking and remelting of originally vertical compression related fabric in roof supracrustal granites 340 ma and development of highly prolate fabrics in these rocks this work shows that the finite shape of ams fabric ellipsoid is highly sensitive to both strain regime and superpositions of orthogonal deformation events

influence of melt induced mechanical anisotropy on the magnetic fabrics and rheology of deforming migmatites central vosges france in the central vosges france we describe and model the structural relationships existing between orthogneiss domains and their surrounding viscous migmatites these domains show a structural and anisotropy of magnetic susceptibility ams zonation associated with an increasing amount of melt from the core to their margins a pre extension oblate to plane strain fabric is preserved in the core of the orthogneiss domains where the granite veins were emplaced along a dilated mechanical anisotropy the internal margin of the orthogneiss bodies exhibits development of a prolate fabric and active buckling of an orthogneiss granite multilayer the external margin of the orthogneiss bodies is characterized by oblate fabrics resulting from a layer perpendicular shortening of a melt orthogneiss multilayer pinch and swell structures and extensional kink bands along which granite veins have been injected the surrounding metasedimentary migmatites show a similar structural history but the degree of fabric resetting is significantly higher this systematic fabric and structural succession is due to contrasting rheological evolution between orthogneiss and metasedimentary migmatites during progressive melting and continuously evolving degree of mechanical anisotropy in the stronger orthogneiss numerical modeling confirms the role of the relationship between initial mechanical anisotropy and superimposed deformation overprints on finite ams fabrics in migmatites

shear criteria in granite and migmatite deformed in the magmatic and solid states microstructural criteria for the determination of the sense of shear in rocks homogeneously deformed in the partially melted state are similar to those which apply to solid state deformation sense of shear determination is either direct deduced from the sense of rotation of markers or indirect involving the obliquity between the shear and foliation planes or between the successive foliations imprinted at different stages of progressive deformation this study is a by product of the detailed structural and microstructural investigation of a high grade metamorphic rock pile variscan vosges massif france which underwent subhorizontal shearing during partial melting and further solidification depending on the rock chemistry on the position in the pile and the relative timing of progressive deformation layered migmatites and homogeneous granites were variously deformed in the partially melted and solid states the sense of shear obtained from these rock types using the criteria presented here consistently gives a top to sw direction

pluton emplacement within an extensional transfer zone during dextral strike slip faulting an example from the late archaean abitibi greenstone belt abstract the lake abitibi area within the late archaean abitibi greenstone belt exhibits an interlinked plutonic structural and metamorphic evolution that may characterize segmented strike slip faults at upper to mid crustal levels along the major southeastward propagating macamic d 2 dextral strike slip fault theological and preexisting d 1 structural heterogeneities induced the development of nnw trending dextral oblique splays which evolved into an extensional trailing fan and created an extensional nnw dipping stepover magma flowing upwards from deeper parts of the macamic fault spread towards the southeast at upper crustal levels along both the oblique slip and extensional d 2 splays and built several plutons in a pull apart domain between 2696 and 2690 ma different emplacement and material transfer mechanisms operated simultaneously in different parts of the system including fault dilation and wedging lateral expansion wall rock ductile flow and stoping transfer of movement between d 2 splays occurred under ductile conditions during syn emplacement amphibolite grade metamorphism 500 700 c during cooling d 2 faults appear to have been closely linked during the superior province wide late transpressional event

the influence of mechanical anisotropy on the behaviour of the lower crust abstract the aim of this paper is to determine what can be deduced about the rheology of the lower crust undergoing extension the state of stress existing there and how the finite strain pattern is developed from a consideration of the theoretical analyses of layer normal compression analogue models and outcrops of rocks from the lower crust it is concluded that the extended lower crust contains units with a significant competence contrast has a high mechanical anisotropy and experiences bulk rheological changes during deformation based on a consideration of hydraulic fracturing in anisotropic materials it is argued that depending on the magnitude differential stress fluids will move through and out of the lower crust by fluid fracturing along the planar horizontal stretching fabric and gently inclined normal kink bands extensional shears or by the formation of vertical fractures

chronology petrogenesis and heat sources for successive carboniferous magmatic events in the southern central variscan vosges mts ne france plutonic bodies of the central and southern vosges mts can be assigned to two major early carboniferous magmatic events a visean mg k event c 345 and 340 336 ma and a younger s type event 329 322 ma new petrological geochemical and sr nd isotopic data highlight the existence of two groups of mg k intrusions that might be related to the nature of their primary magma sources that is chur like and enriched mantle which interacted with juvenile and mature crustal material respectively the differences between these two groups are explained by a geodynamic scenario involving deep subduction and relamination of the saxothuringian continental crust under the moldanubian continent the relaminated radiogenic saxothuringian material is thought to have been responsible for dehydration melting of both subducted crust and underlying metasomatized mantle thereby generating the mg k magma subsequently emplaced at middle crustal depth during their ascent the mafic magmas interacted with crustally derived felsic melts significantly later c 10 15 myr a widespread mid crustal anatexis occurred generating voluminous granite intrusions from mixed crustal sources paragneisses and or immature felsic intermediate metaigneous rocks mixed with mg k plutons the principal heat source for such a major melting event is related to the presence of mg k plutons rich in heat producing elements which were responsible after the time lag specified for a temperature increase at mid crustal levels by in situ radiogenic heat production the current study underlines the importance of deep continental crust subduction and relamination for the magmatism and development of collisional orogens supplementary material analytical methods and data and supplementary figures are available at http www geolsoc org uk sup18795

palaeozoic evolution of the variscan vosges mountains a geological synthesis of the palaeozoic vosges mountains ne france is presented using existing observations and new data the geodynamic evolution involves 1 early palaeozoic sedimentation and magmatism 2 late devonian subduction triggering back arc spreading 3 early lower carboniferous continental subduction continent continent collision and polyphase deformation and metamorphism of the orogenic root and 4 late lower carboniferous orogenic collapse driven by thermal weakening of the middle crust the evolution is integrated within the framework of the european variscan belt the northern vosges comprise sediments of rhenohercynian affinity separated from tepla barrandian metasediments by a lower carboniferous magmatic arc the latter is correlated with the mid german crystalline rise and is ascribed to the south directed subduction of the rhenohercynian basin the saxothuringian moldanubian suture is thought to be obliterated by the magmatic arc while the lalaye lubine fault is interpreted as the tepla barrandian moldanubian boundary the central vosges are paralleled with the moldanubian domain of the bohemian massif where identical lithologies record the devonian carboniferous se directed subduction of the saxothuringian passive margin below the moldanubian upper plate the southern vosges represent the upper moldanubian crust and are linked to the southern black forest the presence of an oceanic domain to the south of the vosges black forest remains unclear

fabric evolution of rigid inclusions during mixed coaxial and simple shear flows abstract in a slowly moving viscous fluid a population of non interacting rigid inclusions evolves in an oscillatory fashion or creates a stable fabric depending on the flow geometry and the shape of the inclusions using jeffery s equations for the motion of a rigid ellipsoidal inclusion a mixed coaxial and simple shear flow was investigated both analytically and by numerical modeling exact results were obtained for spheroidal inclusions depending on the axial ratio of the inclusion and the amount of the coaxial component of flow individual inclusions can move cyclically or rotate to a stable orientation in one of two mutually orthogonal directions equations were found which allow us to distinguish between these types of motion and to determine the stable inclusion orientation the relative motions of individual inclusions determine the preferred orientation in a population of inclusions in a heterogeneous population with diverse axial ratios the fabric evolves as the superposition of subfabrics corresponding to subsets with different axial ratios the orientations of stable subfabrics depend on axial ratios and could thus be used as kinematic indicators in cases of a sufficiently large coaxial component of flow the fabric created by a heterogeneous population may comprise two stable mutually orthogonal subfabrics no exact results are available for general i e non spheroidal ellipsoidal inclusions and numerical modeling must be used if the coaxial component of flow is sufficiently strong the non spheroidal inclusions move much like the spheroidal ones orthogonal subfabrics may appear as transient features in the evolution of a homogeneous population of non spheroidal ellipsoidal inclusions

close spatial relationship between plutons and shear zones this paper explores the relationship between granitic plutons and transcurrent shear zones using examples from the borborema province northeastern brazil syntectonic plutons of this province generally have en cornue shapes and were emplaced preferentially within or at the margins of wide regional scale transcurrent shear zones particularly where shear zones intersect major lithological boundaries it is argued that the plutons shapes and their closeness to shear zones are indicative of a causal relationship between granite plutons and shear zones with the support of numerical models it is further argued that pluton emplacement in transcurrent terranes is controlled by low mean pressure magma traps at the shoulders of shear zones resulting from strain incompatibility caused by regional variations in rheological properties

significance of low symmetry fabrics in magmatic rocks abstract the significance of low symmetry fabrics monoclinic and triclinic in magmatic rocks is studied using theoretical models describing the motion of rigid markers embedded in a slowly deforming ductile matrix in agreement with curie s symmetry principle the development of these fabrics requires a non coaxial strain path simulations of monoclinic fabrics indicate that their characteristics depend on a the strain regime and the amount of strain and b the morphological composition of the population of rigid markers natural monoclinic fabrics contain precious information about the strain regime they result from type of the non coaxial strain path involved simple shear non coaxial flattening etc orientation of its axes and sense of shear the magnitude of the finite strain may eventually be estimated provided that the shape of the markers is precisely known

the development of contrasting structures during the cooling and crystallisation of a syn kinematic pluton structural elements of a syn kinematic pluton in the pilbara craton of western australia reflect evolving rheology during progressive crystallisation representing a transition from magmatic through high temperature sub solidus to low temperature deformation magmatic elements include 1 nne trending sub vertical kilometre scale folds outlined by xenolith trains that have a magmatic axial planar foliation defined by aligned k feldspar 2 cross cutting ene trending sub vertical shear zones intruded by numerous metre scale granite sheets and 3 a narrow concordant magmatic k feldspar foliation in the granite host rock adjacent to the shear zones microstructures such as fractured feldspars healed by quartz that is continuous with matrix grains further suggest that deformation occurred while the pluton was magmatic high temperature structural elements include checkerboard sub grain boundaries in quartz whereas deformation at lower temperatures is indicated by kinked feldspar recrystallised microcline and undulose quartz a nne trending sub vertical sub solidus foliation defined by elongate quartz aggregates overprints all the above mentioned features generation of these sequential structural elements correspond to the transition from newtonian fluid through bingham type to newtonian solid behaviour as the crystal fraction of the felsic magma increased the orientation of the structures is consistent with a regional ese wnw shortening direction throughout magma crystallisation this study indicates how evolving magma rheology and regional strain regimes control the micro to macro structural elements of a syn tectonic pluton

overprinting of magnetic fabrics in granites by small strains numerical modelling abstract the anisotropy of magnetic susceptibility ams is used to determine weakly defined mineral preferred orientation fabrics in granites however the effects of small strains superimposed on the low intensity ams fabrics typical of these rocks is not well understood numerical simulations are presented which investigate the effects of small superimposed strains on the shapes magnitudes and orientations of pre existing ams fabrics which are similar to those reported in the literature for undeformed granites a computer generated biotite petrofabric is subjected to progressive strain using march s strain response model for pure shear plane strain axially symmetric shortening and simple shear regimes results indicate that a pre existing ams fabric might be significantly modified or overprinted by small strains 10 15 strain in the pure shear and axially symmetric shortening regimes 0 5 1 0 for simple shear a new ams fabric may develop with an orientation similar to that expected for shearing of an initially isotropic petrofabric at low strains the evolution of the shape of the ams ellipsoid depends strongly on the orientation of the initial petrofabric with respect to the strain framework the ams ellipsoid may evolve towards either oblate or prolate shapes and the evolution in the ellipsoid shape may be reversed during progressive strain this modelling suggests that small strains affecting a pluton following its emplacement could mask ams fabrics developed earlier the results also suggest that in simple shear the orientation of the ams ellipsoid may serve as a shear sense indicator for small strains even if the earlier fabric is not isotropic

significance of a hiatus in down temperature fabric development within syn tectonic quartz diorite complexes channel islands uk detailed fabric development histories have been deduced for neoproterozoic foliated quartz diorite complexes of the channel islands uk the plutons were emplaced during regional d2 deformation magmatic fabrics formed at low crystal contents pre rheologically critical melt percent are only locally preserved due to overprinting by solid state deformation which occurred at moderate temperatures 400 550 c this is demonstrated by brittle microcracking of plagioclase and amphibole with only local ductile bending and dynamic recrystallization marginal recrystallization and myrmekite development on alkali feldspar ductile deformation of quartz to form types 2 3 ribbons quartz recrystallization by sub grain rotation associated with a slip but not c slip biotite recrystallization within micro shears and variable amphibole recrystallization the deformation histories display a distinct division between magmatic state and moderate temperature solid state deformation features with little evidence for intervening high temperature solid state fabrics this contrasts with criteria widely considered diagnostic of syn tectonic plutons which envisage development of a continuum of magmatic through high temperature solid state deformation fabrics however we consider the plutons syn tectonic and attribute the hiatus to contrasting strain and pluton cooling rates length and nature of deformation events and host rock ambient temperatures intermediate magmas emplaced at moderate to shallow levels in the crust may be more prone to develop such apparently punctuated deformation histories than sio 2 richer granitoids

fabric evidence for granodiorite emplacement with extensional shear zones in the variscan gredos massif spanish central system abstract three granitoid bodies in the central part of the gredos massif spanish central system batholith are tabular around 1 km in thickness and intruded into a migmatitic middle crust during the d3 deformation phase of the variscan orogeny petrologically they are composed of bt granodiorite and crd monzogranite and they show varying abundance of large cm scale feldspar megacrysts a detailed study of the shape preferred orientation spo magmatic fabric defined by these megacrysts together with a kinematic analysis of the structures due to interactions between them and the measurement of quartz c axis fabrics in migmatites and granitoids suggests that granitic magma and country rocks were mechanically coupled during deformation the emplacement took place along large scale extensional shear zones active during the first stages of the d3 phase the shape of the spo ellipsoids varies from constrictional at the centre of the granitic bodies to flattening or even oblate at their external contacts with the migmatitic host rocks the favoured interpretation of this spatial fabric variation is the overprinting of the emplacement fabrics by a constrictional tectonic regime associated with the growth of tabular magma chambers along extensional detachments followed by shear zone development commonly at the top of the granitic bodies the entire structure was later folded during the last stages of the d3 phase

internal fabric and strike slip emplacement of the pan african granite of solli hills northern nigeria the solli hills pluton 250 km2 in northern nigeria is an example of a pan african granite emplaced circa 580 ma ago in the jos adamawa block its micro structures and internal fabrics have been studied in order to help constrain the kinematics of the pan african orogeny in this block we used field observations and measurements petrographic examinations and measurements of the magnetic susceptibility and anisotropy of magnetic susceptibility of 124 regularly spaced stations most samples are ferromagnetic i e belong to the magnetite series granites and present predominantly magmatic microstructures particularly remarkable are the asymmetrical zoning of the petrographic types the susceptibility magnitudes and the planar linear magnetic fabrics the magnetic foliations and lineations are very consistent in the pluton around n 20 e 60 w and n 205 e 10 respectively field and microscopic observations reveal that deformation occurred everywhere in the pluton and immediate country rock envelope during a dextral strike slip synplutonic episode it is suggested that the solli hills pluton was emplaced in a late pan african dextral tear fault system the striking fabric asymmetry of the pluton strongly suggests a shear zone termination emplacement model

chronological constraints on the pre orogenic history burial and exhumation of deep seated rocks along the eastern margin of the variscan orogen bohemian massif czech republic key lithological units of the high grade eastern margin of the bohemian massif were dated using the u pb and pb pb methods on zircons in order to establish a chronological framework for the geodynamic evolution of the variscan orogenic root the protolith ages for metagranitoids orthogneisses and granulites of thickened lower and middle crust reveal the existence of magmatic activity that occurred over a 100 million year time interval from cambro ordovician to early devonian times probably related to discontinuous intracontinental rifting of neoproterozoic crust our geochronological data suggest that the eastern part of the orogenic root represents thermally softened and rifted neoproterozoic crust preserved farther to the east as the brunia microcontinent zircon ages for felsic granulites high grade gneisses of the lower crust and of a syn convergence granodioritic intrusion in the upper crust indicate that thickening and exhumation of the crust occurred during a narrow time interval between 370 and 340 ma exhumation of the lower crust to mid crustal levels was a localized process that occurred at 340 ma and was associated with crustal scale folding in the internal part of the root as well as orogenic channel flow along the eastern collisional margin both types of exhumation mechanisms were driven by deep level wedging indentation of the easterly brunia continent followed by deposition of heavy minerals and pebbles derived from high pressure rocks in the adjacent foreland basin final orogenic development was characterized by ne sw dextral transpressive shearing parallel to the brunia margin as well as dextral trans tension associated with activity along the elbe lithospheric fault these processes affected the marginal parts of the orogenic root and were accompanied by 330 to 325 ma old syntectonic granitoid intrusions along reactivated lithotectonic boundaries rotation of the assembled orogenic belt accompanied by lithospheric faulting driven by westerly subduction roll back may be the most plausible model to explain late deformation of the orogenic root

north south extension in the tibetan crust triggered by granite emplacement we combine zircon sensitive high resolution ion microprobe u pb spot dating and mica 4 0 ar 3 9 ar plateau ages with field geological and geochemical constraints from the malashan area of southern tibet to show that the deformed granite core of the north himalayan metamorphic domes in this area is not indian basement but was intruded and deformed during the himalayan orogeny microstructural observations reveal that a transition from top to the south thrust related to top to the north extension related deformation occurred during granite intrusion and related metamorphism this suggests that intrusion triggered the onset of extensional tectonics in the tibetan middle to upper crust expected positive feedback mechanisms between decompression melting leading to more intrusion and more extensional deformation suggest that this mechanism may have been important on a regional scale

tectonic evolution of the rehamna metamorphic dome morocco in the context of the alleghanian variscan orogeny structural and 40ar 39ar geochronological investigations of the rehamna massif meseta moroccan variscan belt provide new constraints on the tectonic evolution of the alleghanian variscan orogen during the upper paleozoic three main tectonic events have been recognized 1 southward thrusting of an ordovician sequence over the proterozoic basement its cambrian sedimentary cover and the overlying devono carboniferous basin this event caused subhorizontal shearing and prograde barrovian metamorphism of the buried rocks 2 continuous shortening resulting in the development of a synconvergent extrusion of metamorphosed units to form a dome elongated e w this was responsible for synconvergent detachment of the ordovician upper crustal sequence the timing of these two episodes is constrained to 310 295 ma by cooling and metamorphic amphibole and mica ages 3 a nw wnw convergence in a direction orthogonal to the previous one and characterized by the accretion of the rehamna dome to the continental basement in the east based on 40ar 39ar cooling ages from a syntectonic granitoid and its host rocks and metamorphic 40ar 39ar ages from greenschist facies mylonite the timing of this event falls between 295 and 280 ma the end of the variscan orogeny in the moroccan meseta is constrained by the 40ar 39ar cooling age of a posttectonic pluton dated at 275 ma the tectonic events highlighted in morocco coincide with the late variscan alleghanian tectonic evolution of southern europe and north america and can be correlated with the global reorganization of plates that accompanied suturing of pangaea at around 295 ma

the paleoproterozoic djebel drissa ring complex eglab shield algeria post collisional intrusions in a transtentional tectonic setting abstract the eburnean a type granitic djebel drissa ring complex is part of the 2 08 ga post collisional aftout magmatic suites that intruded the 2 22 2 09 ga old paleoproterozoic granitoids in the eglab shield in west african craton it is composed of unfoliated granitoids arranged in near concentric zones the zoning is interpreted as resulting from multiple intrusions of continuous magmatic pulses leading to in situ growth of the complex new field observations anisotropy of magnetic susceptibility ams gravimetry and aeromagnetic data suggest that the regional deformation controlled the emplacement of this complex the sub circular form of the granitic body and the ams pattern are consistent with a transtensional tectonic setting along a nw se major shear zone and with an emplacement also related to an associated wnw ese extension fault this well dated regional deformation corresponds to the last main structural event of the paleoproterozoic evolution of the eglab shield

the deformation matrix for simultaneous simple shearing pure shearing and volume change and its application to transpression transtension tectonics simultaneous simple shearing and pure shearing with or without additional volume change can be combined into a single upper triangular deformation matrix the off diagonal term is named the effective shear strain and is a simple function of the pure shearing and simple shearing components a three dimensional deformation matrix for the simultaneous combination of coaxial deformation with or without additional volume change and up to three simple shearing systems with mutually orthogonal shear planes is also presented by using this matrix one can easily extract the various properties of incremental as well as finite strain and the progressive as well as finite rotation of passive markers during deformation the case of transpression transtension is revised using the unified deformation matrix the orientation of the major axis of the strain ellipsoid 1 is always horizontal if the deformation is transtensional switches from horizontal to vertical during transpressional wrenching 1 wk 0 81 for constant vorticity deformations and is always vertical for highly transpressional deformations wk 0 81 for transpression material lines initially rotate towards the horizontal shearing direction but generally turn to rotate towards the vertical axis after a certain strain for transtension all material lines rotate towards a direction in the horizontal plane which is oblique to the shearing direction

the role of dynamic recrystallization in the development of lattice preferred orientations in experimentally deformed quartz aggregates abstract experiments on coaxially deformed quartz aggregates show that when dynamic recrystallization involves grain size reduction the lattice preferred orientation lpo of the recrystallized grains depends on the recrystallization mechanism at low temperatures and fast strain rates strain induced grain boundary migration recrystallization favors growth of grains oriented poorly for slip on the basal and prism planes resulting in an lpo of c axes parallel to the 1 direction at higher temperatures or slower strain rates progressive subgrain rotation produces recrystallized grains which inherit the small circle girdle lpo of their deformed host grains and which continue to re orient by slip at even higher temperatures and slower strain rates progressive subgrain rotation is accompanied by rapid grain boundary migration which does not favor grains of any particular orientation thus the lpo is again similar to that of non recrystallized grains in contrast when dynamic recrystallization involves grain growth driven by surface energy reduction a strong lpo of c axes parallel to 1 develops during grain growth but then evolves with increasing strain toward a small circle girdle the correlation of lpos with recrystallization mechanisms has important implications for the assumption of homogeneous strain in theoretical models of lpos and for interpreting natural lpos in recrystallized rocks

luminescence dating of early mounds in northeast louisiana abstract direct dating of the construction of ancient earthworks in eastern north america is needed to resolve the ambiguity surrounding more indirect methods such as radiocarbon sediment dating by luminescence can potentially fill this need luminescence dating of buried surface sediments from earthworks at two sites in northeastern louisiana show that the mound and or ridge sediments were not well exposed to light at the time of their construction or occupation nevertheless the structure of the luminescence data allows an evaluation of the age of construction at both sites and demonstrates that earthwork building occurred much earlier than was previously believed

extensional pluton emplacement models do they work for large plutonic complexes shallow crustal to mid crustal magma emplacement models commonly rely on local or regional extension to solve the room problem we argue that existing extensional models explain the emplacement of sheeted dike complexes or internally layered plutons but are not as easily reconciled with the emplacement of elliptically shaped plutons or batholiths for the following reasons 1 rock strengths and crustal stresses prevent large voids from existing at depth and therefore require wall rock extension to be balanced by magma influx 2 rates of extension are controlled by fault slip rates of millimetres to centimetres per year and thus require numerous small injections of magma and 3 because of their sheetlike shapes and small thicknesses these small magma injections will cool rapidly and form sheeted plutons or dike swarms extension may however facilitate emplacement of elliptically shaped plutons in concert with other mechanisms if certain kinematic constraints are satisfied

relationships between syn tectonic granite fabrics and regional pttd paths an example from the gander avalon boundary of ne newfoundland abstract syn tectonic granitic plutons and their host rocks not only provide a potential wealth of information about the timing and nature of regional kinematics but also record information about thermal conditions successive syn tectonic plutons emplaced into the gander zone of ne newfoundland preserve fabric overprinting formed during cooling to regional ambient thermal conditions ambient conditions have been approximated for each pluton by careful analysis of microstructures and consideration of potential cooling histories and have illustrated regional exhumation during a tectonic reversal from silurian devonian sinistral transpression to devonian dextral transpression because the time period of emplacement is relatively short by comparison to orogenic events typically by more than an order of magnitude individual plutons may record a snap shot of the regional history by combining information from a number of plutons emplaced sequentially during a period of regional orogenesis a picture may be built up defining time temperature deformation ttd paths such information may compliment regional metamorphic pt studies in helping to establish orogenic pttd paths

modified jeffery model influence of particle concentration on mineral fabric in moderately concentrated suspensions 1 a modified jeffery model involving particle interactions allows the simulation of preferred orientation of crystals and the anisotropy of magnetic susceptibility in magmatic flow with up to 40 of rigid particle fraction numerical simulations are compared to analogue and natural fabrics a study of the role of particle concentration shows that the increasing percentage of crystal fraction in dilute suspensions leads to progressive attenuation of the fabric intensity and cyclicity at high strains in simple shear flow the lineation converges to a constant value close to the shear plane with increasing particle concentration the fabric diverges from the shear direction reflecting the effect of increased crystal tiling the model can be applied to quantify mineral preferred orientations of crystal mushes in magmatic fabric studies

hot contacts of garnet peridotites in middle upper crustal levels new constraints on the nature of the late variscan high t low p event in the moldanubian central vosges ne france p t paths based on parageneses in the immediate vicinity of former high temperature contact zones between mantle peridotites and granulitic country rocks of the central vosges ne france were derived by applying several conventional thermometers and thermobarometric calculations with an internally consistent dataset the results indicate that former garnet peridotites and garnet spinel peridotites were welded together with crustal rocks at depths corresponding to 1 1 2 gpa the temperature of the crustal rocks was about 650 700 c at this stage whereas values of 1100 c garnet peridotites and 800 900 c garnet spinel peridotites were calculated for the ultramafic rocks after emplacement of the mantle rocks exhumation of the lower crust took place to a depth corresponding to 0 2 0 3 gpa the temperatures of the incorporated peridotite slices were still high 900 1000 c at this stage this is indicated by the presence of high t low p parageneses c 800 c 0 2 0 3 gpa in a small 1 10 m contact aureole around a former garnet peridotite crustal rocks distant to the peridotites equilibrated in the same pressure range at lower temperature 650 700 c high cooling rates 102 103 c ma 1 were calculated for a garnet biotite rock inclusion in the peridotites and for the crustal rocks at the contact by applying garnet biotite diffusion modelling minimum rates of 0 75 7 5 cm a 1 are required for vertical ascent of rock units 30 km vertical distance derived from the crust mantle boundary resulting in a late variscan 340 ma high t low p event

model of successive granite sheet emplacement in transtensional setting integrated microstructural and anisotropy of magnetic susceptibility study 1 this study presents a model of successive emplacement of three granite plutons in transtensional deformation regime controlled by the preextensional synconvergent history of the lower and middle crust in the central vosges mountains france the complex compressional orogenic structure recorded a vertical exhumation related fabric in midcrustal levels which was overprinted by the regional extension three successively emplaced granite sheets exploited the inherited vertical anisotropy obliquely oriented with respect to the applied tensile stress this progressive opening of original steep fabric leads to the sequential generation of free spaces toward the south and emplacement and deformation of granite sheets repeated granite intrusions are responsible for reheating of southern margins of the already partially solidified granites which became reactivated under lower ambient thermal conditions related to overall 6 ma cooling of rapidly exhumed crust the results of anisotropy of magnetic susceptibility ams fabric modeling suggest a highly partitioned oblique extension divided into pure shear dominated deformation close to the central and northern margins of intrusions and wrench dominated shear along the southern margins the ams modeling suggests the importance of overprinting of the intrusive fabrics by the transtensional deformation

prism and basal plane parallel subgrain boundaries in quartz a microstructural geothermobarometer empirical data on quartz subgrain patterns from various metamorphic rocks show that at least up to 10 kbar in the stability field of low quartz prismatic subgrain boundaries are dominant whereas basal subgrain boundaries are not developed in the high quartz stability field both prismatic and basal subgrain boundaries occur and form typical rectangular chessboard patterns the likely reason behind the different occurrence of these subgrain patterns is that in high quartz prismatic glide becomes as easy as or probably even easier than basal glide the two types of subgrain patterns can be clearly distinguished by optical microscopy consequently the occurrence of chessboard subgrain patterns in quartz represents a practicable geothermobarometer the possibilities of its application are far reaching and include the specification of deformation conditions at high grade metamorphism the recognition of syntectonic intrusions and the distinction between pluton emplacement at lower and at higher crustal levels

a review of criteria for the identification of magmatic and tectonic foliations in granitoids abstract foliations in granitoids can form by magmatic flow submagmatic flow high temperature solid state deformation and moderate to low temperature solid state deformation a review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent diapiric emplacement and expansion emplacement during regional deformation or regional deformation post dating emplacement however a magmatic origin is favoured for foliations defined by the alignment of igneous commonly euhedral minerals particularly where the foliation is parallel to internal or external pluton contacts foliations formed during expansion or ballooning of diapirs may be strictly magmatic in origin although some studies suggest that solid state deformation also may occur if so we would hope to find evidence of deformation of crystal melt systems and that the solid state deformation occurred at high temperatures the inference of syntectonic foliations is most convincing where magmatic and high temperature solid state foliations are subparallel these foliations are continuous with regionally developed foliations in the wall rocks synkinematic porphyroblasts are present in the wallrocks and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage a strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals no alignment of igneous minerals occurs the foliation is locally at high angles to pluton wallrock contacts and the foliation is continuous with a regionally developed cleavage

submagmatic microfractures in granites microstructures in hercynian granite plutons of the pyrenees indicate fracturing of plagioclase crystals in the presence of melt this is represented mainly by quartz and or feldspar that fills fractures electron microprobe analyses demonstrate that in compositional agreement with the rims of the zoned plagioclase the feldspar infillings are more sodic with decreasing melt fraction fracturing resulted from concentration of stress at contacts between grains while the granite was submagmatic i e when the melt was below the critical fraction for magmatic flow material compositionally far from eutectic is petrologically the best candictate to develop submagmatic fractures because during cooling the volume fraction of melt remains low for a large temperature range a strong tendency for microfractures to be oblique to flow foliation may constitute a new criterion for shear sense in magmas

unusual transition in quartzite dislocation creep regimes and crystal slip systems in the aureole of the eureka valley joshua flat beer creek pluton california a case for anhydrous conditions created by decarbonation reactions abstract microstructures and quartz c axis fabrics were analyzed in five quartzite samples collected across the eastern aureole of the eureka valley joshua flat beer creek composite pluton temperatures of deformation are estimated to be 740 50 c based on a modified c axis opening angle thermometer of kruhl j metamorph geol 16 1998 142 in quartzite layers located closest 140 m to the pluton wall rock contact flattened detrital grains are plastically deformed and partially recrystallized the dominant recrystallization process is subgrain rotation dislocation creep regime 2 of hirth and tullis j struct geol 14 1992 145 although grain boundary migration dislocation creep regime 3 is also evident complete recrystallization occurs in quartzite layers located at a distance of 240 m from the contact and coincides with recrystallization taking place dominantly through grain boundary migration regime 3 within the quartzites strain is calculated to be lowest in the layers closest to the pluton margin based on the aspect ratios of flattened detrital grains the c axis fabrics indicate that a slip operated within the quartzites closest to the pluton wall rock contact and that with distance from the contact the operative slip systems gradually switch to prism c slip the spatial inversion in microstructures and slip systems apparent high temperature deformation and recrystallization further from the pluton contact and apparent low temperature deformation and recrystallization closer to the pluton contact coincides with a change in minor phase mineral content of quartzite samples and also in composition of the surrounding rock units marble and calc silicate assemblages dominate close to the pluton wall rock contact whereas mixed quartzite and pelite assemblages are dominant further from the contact we suggest that a thick marble unit located between the pluton and the quartzite layers acted as a barrier to fluids emanating from the pluton decarbonation reactions in marble layers interbedded with the inner aureole quartzites and calc silicate assemblages in the inner aureole quartzites may have produced high x co 2 water absent fluids during deformation the presence of high x co 2 fluid is inferred from the prograde assemblage of quartz calcite and not wollastonite diopside k feldspar in the inner aureole quartzites we suggest that it was these dry conditions that suppressed prism c slip and regime 3 recrystallization in the inner aureole and resulted in a slip and regime 2 recrystallization which would normally be associated with lower deformation temperatures in contrast the prograde assemblage in the pelite dominated outer part of the aureole is biotite k feldspar these wet pelitic assemblages indicate fluids dominated by water in the outer part of the aureole and promoted prism c slip and regime 3 recrystallization because other variables could also have caused the spatial inversion of c axis fabrics and recrystallization mechanisms we briefly review those variables known to cause a transition in slip systems and dislocation creep regimes in quartz our conclusions are based on a small number of samples and therefore the unusual development of crystal fabrics and microstructures in the aureole to the ejb pluton suggests that further study is needed on the effect of fluid composition on crystal slip system activity and recrystallization mechanisms in naturally deformed rocks

detection of a weak late stage deformation event in granitic gneiss through anisotropy of magnetic susceptibility implications for tectonic evolution of the bomdila gneiss in the arunachal lesser himalaya northeast india outcrop scale structures and magnetic fabric anisotropy of the bomdila gneiss bg that intruded the lesser himalayan crystallines lhc of the arunachal lesser himalaya are studied to understand the bg deformation history and tectonic evolution detailed analysis of structures reveals that the lhc have undergone three phases of deformation d 1 d 2 and d 3 the s 2 foliation developed during the second phase of deformation d 2 is the most penetrative planar fabric in the studied rock which shows a general ene wsw strike with moderate nw dip mesoscopic evidence of a later phase of deformation d 3 in the bg is lacking evidence of d 3 deformation in the form of f 3 folds is only observed in the adjacent metasedimentary rocks of the lhc the magnetic foliations recorded from anisotropy of magnetic susceptibility ams analysis of the bg are mostly striking nw se with a moderate dip towards the ne or sw and magnetic lineation is mostly sub horizontal and dominantly plunging towards the se our study shows that the magnetic fabric of the bg does not correspond to any visible outcrop scale mesoscale foliation however the magnetic foliation of the bg is parallel to the axial plane of the f 3 folds of the adjacent metasedimentary rocks of the lhc integration of ams and outcrop scale structural analysis helps us envisage the superposed deformation history of the bg our study emphasizes the importance of ams to detect late stage or feeble deformation events that leave no visible outcrop scale imprint and are difficult to discern through conventional geological means

granite fabrics and regional scale strain partitioning in the serido belt borborema province ne brazil 1 fabrics of the brasiliano age granitoid plutons of the serido belt northeastern brazil were organized following the partitioned regional strain field which shaped larger areas of the basement and its metapelitic cover nine plutons located at the central and western portions of the belt have been investigated by means of anisotropy of magnetic susceptibility these plutons emplaced between 580 and 575 ma are mainly potassic calk alcalic magnetite bearing monzogranites the presence of primary coarse grained magnetite as the main magnetic mineral is indicated by optical and electronic microscopy magnetic susceptibilities hysteresis ratios and thermomagnetic curves the plutons show a strong internal coherence of magnetic fabric with foliation poles distributed around a zone axis parallel to the gently plunging mean lineation the magnetic fabric was used to trace the strain partitioning along the belt in the central transpressive shear belt the magnetic lineations are parallel to north trending regional stretching on the other hand in the western basement block a voluminous granitic magmatism was emplaced as a consequence of a transtensional extensional deformation lineations in these granites trend in a ne direction which is parallel to the transport inferred from kinematic indicators along the shear zones a crustal scale east trending dextral shear zone connects both the transtensional and transpressional domains

precise upb chronometry of 345 340 ma old magmatism related to syn convergence extension in the southern vosges central variscan belt abstract the southern vosges host a large volcano sedimentary basin which contains clastic sediments of famennian late devonian ca 365 ma to late visean ca 340 335 ma age the basin is bordered by an exhumed high grade basement complex and intruded by several granitoid plutons a chronology for basin evolution and magmatism was established dating volcanic rocks of the basin as well as cross cutting granitoid rocks by the u pb technique applied to zircon and titanite the basin evolution started with olistolithic deposits and overlying marine shales that have a famennian fauna ca 365 ma and contain detrital zircons with an age of 386 ma the lowermost rhyolite flows interlayered with the basin sediments and tholeiitic basalts yielded an age of 345 2 ma following a tectonic event at the bottom of the upper visean the rates of sedimentation and volcanism were strongly enhanced leading to basin emersion and were concluded by the extrusion of the uppermost ignimbrite molkenrain rhyolite emplaced at 340 2 ma the whole volcano sedimentary association was subsequently intruded by melts of the ballons and cre tes granites as well as by small monzodioritic and dioritic satellite intrusions along the border of the ballons granite within 3 2 myr 342 1 to 339 5 2 5 ma the zircons of most of the volcanic and plutonic rocks investigated contain inherited lead of 0 6 and 2 1 ga age the differences in initial nd isotopic composition of the ballons and cre tes granites can be explained by differential uptake of crustal material during magma genesis the precise age data prove that magmatism and basin formation in the southern vosges were extremely short lived and coeval to the exhumation of adjacent high grade gneiss terrains revealing an episode of extension at the end of the lower carboniferous between 345 and 340 ma visean the tectonic regime in the internal part of the variscan orogen is that of extension and strike slip while convergent tectonics were still ongoing in the external part of the variscan orogen

noncoaxial k feldspar and ams subfabrics in the land s end granite cornwall evidence of magmatic fabric decoupling during late deformation and matrix crystallization 1 a comparative study of the anisotropy of magnetic susceptibility ams and k feldspar phenocryst fabrics in the land s end granite demonstrates that the ams fabric predominantly reflects late magmatic deformation tensor analysis of the k feldspar fabric shows a complex pattern characterized by meter scale variations in orientation symmetry and intensity mainly related to heterogeneous flow of the phenocryst rich magma during emplacement in contrast the ams fabric is predominantly homogenous subhorizontal and oblate and is stable at the pluton scale quantitative microstructural analysis suggests that the ams fabric is controlled by deformation of the partially crystallized matrix resulting from the combined effects of late internal adjustment within the pluton and regional deformation a general model of fabric development associated with a vertical pure shear overprint on a variable vertical fabric is evaluated by numerical modeling the study demonstrates how the memory of different fabric elements may be dependent upon their grain size crystallization sequence and recorded previous strain