deformation temperatures vorticity of flow and strain symmetry in the loch eriboll mylonites nw scotland implications for the kinematic and structural evolution of the northernmost moine thrust zone

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

extrusion and radial spreading beyond a closing channel abstract the development of experimental models of an extruding spreading zone beyond a closing channel is studied and a possible natural example described the strain pattern folding and radial displacements within the extruding spreading zone are analysed folds developed in the closing channel are either passively rotated or refolded when carried into the spreading zone a simple theoretical model of the experiments demonstrates that the stretching lineation is radial and parallel to the displacement direction towards the base and the front of the extruding spreading zone but perpendicular towards the top an intermediate domain with no stretching lineation separates the radial and concentric domains and corresponds to a transition through a uniaxial oblate ellipsoid k 0

kinematics of orocline and arc formation in thin skinned orogens the regional traces of folds faults and foliations found in many fold thrust belts are bent in plan view it proves valuable to distinguish between two types of bent orogens based on their kinematic evolution oroclines or rotational arcs are bent orogens in which segments of the orogen change strike during the evolution of the bend and nonrotational arcs are bent orogens in which segments of the orogen do not change strike during development of the bend the kinematic evolution of a bend that forms in a thin skinned orogen can be described in terms of three parameters the displacement path trajectories followed by points along the strike of the orogen the magnitude and distribution of tangential extension along the strike of the orogen and the change in position of the endpoints of the orogen with respect to a reference line only slight differences in the displacement path trajectory pattern determine whether an orogen evolves as a nonrotational arc or as an orocline thus the distinction between these two types of bends is not always of major tectonic significance compressional deformation along irregular continental margins or the impact of indentors on continental margins during collisional orogenies more likely leads to formation of nonrotational arcs as can be simulated with a sand wedge model interaction between a fold thrust wedge with obstacles in the foreland or with a wrench fault more likely leads to orocline formation orocline formation is also associated with noncoaxial reactivation of thrust faults tangential extension is a necessary consequence of certain displacement path trajectory patterns and can accompany development of either nonrotational arcs or oroclines two examples one from the umbrian apennines and one from the makran range demonstrate how movement on cross strike fault arrays can accommodate tangential extension in thin skinned bends

temperature and strain gradients through lesser himalayan rocks and across the main central thrust south central bhutan implications for transport parallel stretching and inverted metamorphism in order to understand mass and heat transfer processes that operated during himalayan orogenesis we collected temperature finite and incremental strain and kinematic vorticity data through a 5 km thickness of lesser and greater himalayan rocks in southern bhutan this transect crosses two major shear zones the main central thrust mct and shumar thrust st raman spectroscopy on carbonaceous material and garnet biotite thermometry are integrated with deformation temperatures from quartz petrofabrics these data define inverted field gradients that correspond in structural position with the mct and st which are separated by sections in which temperatures remain essentially constant temperatures increase from 400 500 c to 700 750 c between 675 m below and 200 m above the mct this defines a 269 44 c km inverted gradient interpreted to have formed via high magnitude 100 250 km shearing on a discrete mct zone delineated by the limits of inverted metamorphism temperatures increase from 300 400 c to 400 530 c across the st which is attributed to differences in maximum burial depth of hanging wall and footwall rocks strain and vorticity data indicate that lesser and greater himalayan rocks were deformed by layer normal flattening transport parallel lengthening and foliation normal shortening increase from 38 71 and 36 49 respectively between 2 3 1 0 km below the mct the mct acted as a stretching fault with translation on the order of 100 s of km accompanied by transport parallel stretching of footwall and hanging wall rocks on the order of 10 s of km this demonstrates that stretching accommodated between major shear zones can make a significant contribution to cumulative mass transfer

fold axis parallel extension in an arcuate fold and thrust belt the case of the helvetic nappes abstract the helvetic nappes of western switzerland are discussed as an example of an arcuate foreland fold and thrust belt in which active fold axis parallel stretching occurred fold axis parallel extension is recorded by 1 1 incremental strain data from pressure shadow fibres the significance of pressure shadow fibres for the determination of the deformation history of a region is discussed pressure shadows are used to quantify the amount of and to describe the distribution of fold axis parallel extension occurring in the helvetic nappes 2 2 the extension directions of conjugate systems of en echelon veins it is shown that an analysis of the geometry of conjugate vein systems can reveal a regional deformation pattern the relative age of the conjugate en echelon vein systems in the helvetic deformation history can be assessed the geometry of the conjugate sets relative to the local anisotropy plane is described and the significance of the preferred orientation of their extension directions is discussed 3 3 fold axis parallel sections a comparison of the regional distribution of the fold axis parallel strain with the shape of the helvetic nappes in fold axis parallel sections shows that the fold axis parallel strain cannot be related to the footwall topography of the nappes it is concluded that the fold axis parallel extension in the helvetic nappes was induced by a change of direction of overthrust shear this change occurred late in the deformation history and was superposed on the already formed nappes the changing direction of overthrust shear is the expression of an overall anticlockwise rotation going on in the overthrusting alpine nappe pile relative to the european plate a rotation which lead to the arcuate shape of the western alps

spatial variations of ductile strain in fold and thrust belts from model to nature abstract fold and thrust belts ftbs accommodate tectonic convergence through folding and faulting of crustal rocks during a collisional event between two continental plates although evidence of distributed deformation is common in ftbs that usually leads to continuous foliations and regionally occurring ductile structures of multiple orders it has rarely been given much attention assuming that zones of localized deformation like shear zones and brittle faults accommodate the entire amount of tectonic convergence this study presents 3d laboratory scale models using viscous thin sheet as crustal layers to understand the evolution of ductile strain in a tectonic wedge we varied the degree of mechanical coupling at the basal decollement i e weak versus strong to investigate this issue at constant convergence velocity in all experiments to avoid the influence of rate dependence on viscous rheology our results reveal that the strength of basal decollement controls the mode of wedge growth and hence the strain pattern particularly towards the hinterland the weak decollement models yield a zone of constriction towards the central part of hinterland explaining the occurrence of isolated patches of l tectonites and cross folds in ftbs while the strong decollement condition allows gravity driven flow to be active in the hinterland leading to orogen parallel recumbent folds in contrast both weak and strong decollement models produce deformation that characterises the commonness of pervasive hinterland dipping ductile fabrics towards the mountain front we correlate our findings to show that spatio temporal variations in basal coupling are responsible for varying occurrence of ductile structures in ftbs

simple shear deformation and quartz crystallographic fabrics a possible natural example from the torridon area of nw scotland ln this paper the microstructures and crystallographic fabrics of a 30 cm wide deformed quartz vein from the torridon area of nw scotland are described the vein which lies along a joint separating two blocks of proterozoic gneiss was deformed by crystal plastic processes when the rigid joint blocks moved in opposite directions parallel to the vein margins mylonitic foliation within this quartzose ductile shear zone locally displays the classic orientation variation expected for heterogeneous simple shear deformation in thin section the vein is a mylonite composed of dynamically recrystallized elongate quartz grains aligned oblique to the mylonitic foliation fabrics within the central part of the vein have been examined by optical and x ray texture goniometry methods together with orientation distribution function odf analysis the a axis fabric consists of a dominant single point maximum orientated within the xz plane at 9 to the lineation x this point maximum occupies a pole position to the corresponding single girdle c axis fabric for individual positions on the c axis fabric skeleton the orientations of potential crystallographic slip systems slip plane and direction relative to shear zone co ordinates have been calculated by odf analysis the calculation of relative resolved shear stresses schmidfactors for the potential slip systems assuming a simple shear kinematic framework reveals an elegantly simple relationship between individual grain c axis orientation and correspond ing orientations of potential slip systems these results are readily interpreted in terms of bulk simple shear deformation

strain path partitioning within thrust sheets microstructural and petrofabric evidence from the moine thrust zone at loch eriboll northwest scotland abstract quartz c axis fabrics and microstructures have been investigated within a suite of quartzites collected from the loch eriboll area of the moine thrust zone and are used to interpret the detailed processes involved in fabric evolution the intensity of quartz c axis fabrics is directly proportional to the calculated strain magnitude a correlation is also established between the pattern of c axis fabrics and the calculated strain symmetry two kinematic domains are recognized within one of the studied thrust sheets which outcrops immediately beneath the moine thrust within the upper and central levels of the thrust sheet coaxial deformation is indicated by conjugate mutually interfering shear bands globular low strain detrital quartz grains whose c axes are aligned sub parallel to the principal finite shortening direction z and quartz c axis fabrics which are symmetric both in terms of skeletal outline and intensity distribution with respect to mylonitic foliation and lineation non coaxial deformation is indicated within the more intensely deformed and recrystallized quartzites located near the base of the thrust sheet by single sets of shear bands and c axis fabrics which are asymmetric with respect to foliation and lineation tectonic models offering possible explanations for the presence of kinematic strain path domains within thrust sheets are considered

vorticity analysis in calcite tectonites an example from the attico cycladic massif attica greece abstract although calcite tectonites are widespread in nature their use to quantify flow vorticity is limited we use new micro structural petrofabric and vorticity data to analyse the kinematics of flow in outcrop scale calcite mylonite zones these zones are genetically related to a crustal scale ne directed ductile thrust basal thrust that emplaced the blueschist over the basal unit during the exhumation of the attico cycladic massif calcite microstructures reveal that the last stage of deformation occurred at temperatures 200 300 c achieved by mild heating which is possibly related with the reburial of the basal thrust s footwall vorticity analyses were based on the degree of asymmetry of calcite c axis fabrics as well as on the assumption that the orientation of the long axes of calcite neoblasts within an oblique foliation delineates the direction of instantaneous stretching axis both methodological approaches provide consistent estimates with a simple shear component between 55 and 82 w n 0 76 0 96 the use of the stress axis 1 orientation recorded by twin c axis pairs to quantify vorticity generally gives significantly lower simple shear component comparison of our vorticity estimates with previous estimates inferred from quartz fabrics and rigid porphyroclasts reveals that exhumation related deformation in the nappe pile was steady state

heterogeneous sub simple deformation in the gol e gohar shear zone zagros sw iran insights from microstructural and crystal fabric analyses petrofabrics and microstructural characteristics of the quartz rich mylonites were investigated in the nw se trending gol e gohar shear zone with length of ca 25 km within the zagros transpression zone sw iran the semi quantitative vorticity analysis using quartz c axis fabric rxz method and rigid porphyroclast geometry rigid grain net method reveals that mean kinematic vorticity number wm increases towards the baghat thrust fault displaying the occurrence of spatial and temporal variations of flow during progressive deformation the wm values show pure shear dominated flow progressively was changed to a simple shear dominated flow toward to the late stages of ductile deformation implying that the accelerating flow path was accompanied with progressive cooling and strain localization at the lower parts of the zone the measured vertical thinning dip parallel elongation suggests that thrust emplacement and metamorphic rock extrusion likely occurred by a combination of simple shear with a pure shear component of deformation i e general shear the estimated deformation temperatures based on the quartz c axis fabric patterns and their opening angles highlight greenschist to amphibolite facies conditions

determining vorticity axes from grain scale dispersion of crystallographic orientations kinematic shear sense indicators are critical tools for tectonic interpretation of shear zones the appropriate plane in which to interpret shear sense indicators is the vorticity normal surface which is often inferred using fabric foliation lineation orientation strain modeling however suggests that such fabrics can be unreliable for determining a kinematic framework we demonstrate the application of a new quantitative method crystallographic vorticity axis cva analysis that utilizes rotation statistics to calculate dispersion axes from crystallographic orientations at the grain scale we apply cva analysis to samples from three shear zones that exhibit distinct kinematics and deformation geometries in all cases the aggregate of calculated grain scale dispersion axes yields a preferred axis of crystallographic vorticity at the specimen scale that is coincident with the independently determined bulk vorticity axis this new method allows the position of vorticity axes to be evaluated independently of foliation and lineation information and without assumptions about the kinematics of deformation

temperature and strain rate dependent microfabric evolution in monomineralic mylonite evidence from in situ deformation of norcamphor norcamphor c7h10o was subjected to plane strain simple shear in a see through deformation rig at four different strain rate and temperature conditions two transient stages in the microfabric evolution to steady state are distinguished the grain scale mechanisms associated with the microstructural and textural evolution vary with the applied temperature strain rate and strain in high temperature low strain rate experiments computer integrated polarization microscopy reveals that the texture evolution is closely related to the crystallographic rotation paths and rotation rates of individual grains high c axis rotation rates at low to intermediate shear strains are related to the development of a symmetrical c axis cross girdle by the end of the first transient stage 1 5 to 2 during the second transient stage 1 5 to 6 the cross girdle yields to an oblique c axis single girdle as c axis rotation rates decrease and the relative activity of grain boundary migration recrystallization increases steady state 8 is characterized by a stable end orientation of the sample texture and the cyclic growth rotation and consumption of individual grains within the aggregate

testing thrust tectonic models at mountain fronts where has the displacement gone the alternative relationships that can exist between a mountain front and the adjacent foreland basin have been recognized for many years however seismic reflection data from such areas are commonly of poor quality and therefore structural models may contain large uncertainties in view of scientific and commercial interest in mountain belts we have reviewed the methods for discriminating between alternative interpretations using a case study from the montagna dei fiori in the central apennines italy in this area mesozoic and tertiary carbonate sediments are juxtaposed with a foredeep basin containing up to 7 km of messinian and plio pleistocene siliciclastic sediments a new structural model for this area demonstrates how the structures in this area form a kinematically closed system in which displacement is transferred from the thrust belt to blind structures beneath the present day foreland growth strata show that pliocene shortening was initially rapid 15 mm a 1 followed by slower rates during the final stages of deformation variations in structural elevation indicate a component of basement involvement during thrusting and this is further supported by magnetic modelling the results illustrate the interaction of thin and thick skinned structures in the central apennines and the methods for discriminating between alternative structural models

the relation between regionally consistent stretching lineations and plate motions abstract regional stretching lineations in the himalayas and southern tibet coincide with the known relative motions of the converging plates this relation is used to infer plate motions from the stretching lineations in the variscan and caledonian belts of europe and in the late proterozoic belts of northeast and east africa in each region high angle convergence of plates produced transverse stretching lineations and high angle collision was followed by or combined with large displacements first by ductile shear and later by brittle fractures parallel to the plate boundaries apparent stretching as a result of volume loss ramsay wood 1973 is common but recognisable as such reorientation by later deformations must be allowed for ramsay 1967 in ductile shear zones simple shearing leads to the progressive approach towards the shear plane of the xy plane of the strain ellipsoid and progressive approach of x the extension axis to the shear line ramsay graham 1970 thus large extensions approximate to the shear line large extensions tend to produce sheath folds whose acutely curved axes approach the stretching direction quinquis et al 1978 also the pre existence of a strong linear fabric predisposes the development of later folds with axes parallel to the earlier linear fabric for all of these reasons the extension axis x rather than the fold axis is of prime significance

the geology and structural evolution of a caledonian fold and ductile thrust zone kyle of tongue region sutherland northern scotland the middle to upper proterozoic metasedimentary rocks of the northern highland moine in sutherland are disposed in a series of major caledonian c 470 430 ma metamorphic thrust nappes produced by nw wnw directed shortening and reworking of parts of a pre existing precambrian c 1000 ma orogen lewisian inliers occur throughout sutherland and represent tectonically interleaved units of the original basement gneisses upon which the moine cover sequence was deposited in the western moine nappe of the kyle of tongue region the rocks are deformed by two main episodes d1 and d2 which are thought to represent the effects of precambrian and caledonian orogenesis respectively little basement cover interleaving or large scale folding occurred during the d1 event in the moine nappe but the associated metamorphism to ut leust garnet grade produced peak metamorphic mineral assemblages subsequent caledonian reworking d2 produced a foreland propagating system of wnw directed ductile thrusts and related major f2 sheath folds both of which resulted in much of the observed moine lewisian interleaving metamorphism associated with d2 decreases in grade westwards occurring at similar or slightly lower grades than the earlier d1 event this retrogressive overprint is most obvious at the w margin of the moine nappe the d2 folds and ductile thrusts are geometrically and kinematically similar to the caledonian structures recognized within the moine thrust zone to the west the very low 5 ramp angles suggesting that the thrusts must have very shallow dipping trajectories through the crust f2 fold formation is attributed to an early phase of wnw directed overthrusting compression which was then progressively superseded by a regional ductile extensional strain contemporaneous with continued wnw directed thrusting the occurrence of internal extension in the orogenic wedge during active thrust related foreshortening implies that the shear strength of the material close to the basal detatchment s was continuously reduced i e wedge motion became supercritical in this context strain softening processes within the ductile thrust mylonites were probably of major significance

strain and fabric analyses based on porphyroclast interaction abstract two dimensional computer models of the interaction of elliptical clasts in a variety of kinematic conditions are used to study how clast imbrication and shape fabric development relate to strain and strain history the models provide templates for a known clast density and clast ellipticity that correlate clast imbrication angle of collision and clast shape fabric to strain and kinematics of deformed rocks modeling also shows that the mode of clast interaction plays a significant role in the accumulation of imbricated clasts and the development of shape fabric upon collision between clasts the trains formed can remain fixed or rotate if they rotate imbricated clasts can become independent when they are no longer forced together by the flow based upon the mode of clast and train rotation using the march rotation of passive lines and the jeffery rotation of rigid particles as end members three models are defined in an attempt to simulate the conditions of clast interaction under varying conditions the march fixed train march rotating train and jeffery rotating train models simulate solid state intermediate and magmatic conditions respectively modeling shows that the two march models produce similar results but the jeffery model of train rotation always produces a lower proportion of imbricated clasts and a much weaker shape fabric application of the modeling to the study of a syntectonic granite in the sierra nevada suggests that deformation was dominated by simple shear some k feldspar imbrication and fabric development took place initially in the magmatic stage and culminated in the solid state to produce a large proportion of imbricated k feldspar porphyroclasts which define a strong shape fabric this type of analysis provides a rationale to determine syn vs post emplacement deformation in the granite

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

variations in the kinematics of deformation along the zagros inclined transpression zone iran implications for defining a curved inclined transpression zone abstract the combination of inclined collision and plate boundary shape can control the nature of deformation and the sense of shear along a transpression zone the present study investigated the effects of a boundary zone with curvilinear shape along a transpression zone on the kinematics of deformation the kinematics of the zagros transpression zone varies with the orientation of the zone boundary detailed structural and microstructural studies showed sinistral sense of shear on the southeastern part of the zagros inclined transpression zone fars arc but dextral sense of shear on the northwestern part of the zone it is inferred that the both senses of shear were developed coevally under a bulk general shear regional scale deformation along a curved inclined transpression miming the shape of the fras arc of the zagros and the reentrant of the bandar abbas syntaxis the zagros transpression zone formed by inclined continental collision between the afro arabian continent and iranian microcontinent

thermal effects and timing of thrusting in the moine thrust zone illite crystallinity studies in the foreland to the moine thrust belt show a widespread heating event which is interpreted as a thermal response to thrust sheet emplacement temperatures reached 275 c approximately 24 ma after the main thrusting events based on the relationships between syn thrusting intrusions and the thrust planes combined with radiometric dates from the moine sheet it is argued that movement on the moine thrust started between 440 and 430 ma ago contractional slip in the moine thrust belt ceased at about 425 ma ago and it is proposed that illite metamorphism attained temperatures of 275 c at about 408 ma ago

late brittle deformation in a caledonian ductile thrust wedge new evidence for gravitational collapse in the moine thrust sheet sutherland scotland abstract in the ductile regions of orogenic wedges late brittle structures formed at shallow crustal depths have received little attention however they are important as they of ten record the final stages in deformational history during uplift and erosion of the orogen in the moine rocks of sutherland late caledonian brittle folds and low angle faults post date ductile structures formed during wnw nw directed thrust movements earlier in the same orogenic event in the western moine nappe the late structures display geometries which indicate that they formed during regionally ese directed extensional movements down the dip of the pre existing regional foliation these late structures are correlated with similar wnw directed extensional low angle faults recognised in the moine thrust zone south of the assynt culmination both are thought to have formed in response to late caledonian gravity induced thinning of the moine thrust wedge and both pre date devonian extension associated with the formation of the west orkney basin along strike changes in the location and sense of extensional movements north and south of the assynt culmination are explained by changes in the pre existing structural geometry of the caledonian ductile thrust nappes within the moine hence earlier deformation patterns are thought to exert important controls over later events in an evolving orogenic wedge

forward modeling of non steady state deformations and the minimum strain path reply structural analyses of shear zones often rely on an assumption of steady state behavior i e that the ratio of pure shear strain rate s to simple shear strain rate s remains fixed throughout deformation however geological deformations are not necessarily steady state non steady state deformation paths can be theoretically modeled if certain deformation parameters such as strain or offset are specified we have analyzed a two dimensional case of specified offset and geometry termed the minimum strain path the minimum finite strain needed to produce a fixed offset across a shear zone is neither simple shear nor pure shear but a combination of the two the minimum strain path if this deformation accumulates with a steady state deformation the kinematic vorticity number w of the minimum strain path varies with the amount of finite offset although w approaches 0 7 at high offset values because of this relation between w and finite offset the minimum strain path is better modeled as a non steady state deformation in which case deformation history starts close to simple shear but rapidly changes to a more pure shear dominated deformation it is expected that the minimum strain path is applicable to geological deformation zones with relaxed boundary conditions such as basal parts of spreading nappes or extensional detachment systems 0 1997 elsevier science ltd

radial gravitational gliding on passive margins abstract gravitational gliding of uppermost sediments down a passive margin is possible if there is a basal layer of evaporite or other soft material to allow detachment in examples from the gulf of mexico and the brazilian margin gliding has produced three main structural domains an uppermost domain of downdip extension an intermediate domain of rigid gliding and a lowermost domain of downdip contraction domain boundaries are established by changes in slope in this paper we examine three kinds of gravitational gliding depending on the paths followed by material particles in ideal parallel gliding particle paths are parallel straight lines trending downslope this should occur where the margin is perfectly straight in ideal radial gliding particle paths are radii of a circle and the margin is shaped like a circular cone natural margins will not have ideal shapes but divergent gliding will tend to occur off coastal salients convergent gliding off coastal re entrants a simple kinematic model based on ductile behaviour illustrates some essential features of radial gliding changes in radius during divergent gliding produce strike parallel extension during convergent gliding they produce strike parallel contraction vertical strains also differ divergent gliding produces an uppermost domain of strong vertical thinning balanced by extensions in all horizontal directions similarly convergent gliding produces a lowermost domain of strong vertical thickening balanced by contractions in all horizontal directions these deformed states cannot be restored by simple techniques based on section balancing we have done three experiments using analogue materials sand to model the brittle behaviour of sediments silicone putty to model the ductile behaviour of basal layers of evaporite the experiments were properly scaled to account for gravitational forces experiment i reproduced convergent gliding above a basement with a conical upper surface strike parallel contraction was responsible for radial folds on the lower slopes the lowermost domain had a complex fold pattern indicating contraction in all horizontal directions experiment 2 reproduced divergent gliding above a basement with a pyramidal upper surface strike parallel extension produced radial rifts on the upper slopes the uppermost domain had a complex polygonal pattern of normal faults indicating extension in all horizontal directions experiment 3 also reproduced divergent gliding but due to non uniform sediment thickness we have examined the many seismic profiles both downdip and alongstrike available for the brazilian margin alongstrike profiles show structures which we attribute to strike parallel strains around the santos re entrant syn sedimentary growthfolds indicate strike parallel contraction increasing downslope this argues for a mechanism of convergent gliding in contrast around the campos salient normal growth faults and associated salt rollers indicate strike parallel extension decreasing downslope this argues for a mechanism of divergent gliding the pattern of oil fields in the campos area including the giant marlim field seems to be closely controlled by divergent gravitational gliding and associated processes radial gliding should occur on other passive margins as well especially where the coastline is sinuous structures around the western gulf of of mexico seem to indicate convergent gliding at the scale of the entire gulf structures at a smaller scale around the mississippi delta seem to indicate divergent gliding

kinematic vorticity and strain rate patterns associated with ductile extrusion in the chelmos shear zone external hellenides greece abstract kinematic vorticity strain rate finite strain and structural data have been used to describe the heterogeneous nature of ductile deformation in an extruding crustal scale shear zone the chelmos shear zone csz of the external hellenides the csz has a curved shape which is marked by a frontal flat and a steep zone the spatial variation in the kinematic vorticity number is strongly correlated with the geometry and the structural depth of the shear zone the pure shear component of deformation decreases from back to front and from top to bottom the ductile thinning normal to the shear zone s boundaries is higher in the steep zone and decreases westwards quartz grain size and strain rate in the csz is distributed into two approximately homogeneous domains the higher strain rate is recorded in the steep zone and the base of the frontal flat zone with lower rates in the upper parts of the frontal flat zone based on mesoscopic structural data and the spatial distribution of the shear strain rate the existence of a west dipping back flow zone in the boarder zone between these two domains is inferred the overall pattern of deformation in the csz implies that tectonic extrusion from the steep zone occurred at the final stages of ductile deformation and was associated with the formation of the backflow zone in the front of the shear zone ductile deformation of rocks probably started close to simple shear on a planar low angle shear zone progressively became penetrative and more general shear occurred as the csz steepened up this progressive increase in pure shearing may have occurred in a heterogeneous manner and was higher close to both the eastern margin of the steep zone and the top of the frontal flat zone solid state extrusion in the csz was also associated with a heterogeneous and progressive increase in strain rate

geochemical and mineralogical evidence for sahara and sahel dust additions to quaternary soils on lanzarote eastern canary islands spain africa is the most important source of dust in the world today and dust storms are frequent on the nearby canary islands previous workers have inferred that the sahara is the most important source of dust to canary islands soils with little contribution from the sahel region soils overlying a late quaternary basalt flow on lanzarote canary islands contain in addition to volcanic minerals quartz and mica exotic to the island s bedrock kaolinite in the soils also likely has an exotic origin trace element geochemistry shows that the soils are derived from varying proportions of locally derived basalt and african dust major element geochemistry clay mineralogy and interpretation of satellite imagery suggest that dust additions to the canary islands come not only from the sahara desert but also from the sahel region

clast based kinematic vorticity gauges the effects of slip at matrix clast interfaces clast based methods for estimating the mean kinematic vorticity number wm are compromised by strain localization at the clast margins localization increases with modal matrix mica content as determined with samples from the sandhill corner mylonite zone a crustal scale high strain strike slip shear zone in maine using these samples we estimate wm with the oblique quartz shape preferred orientation and rigid clast rotation methods the rigid clast rotation method yields much lower values for wm than the quartz method to investigate whether or not slip at the matrix clast interface can explain the discrepancy in calculated wm we conducted numerical modeling of rigid clasts enveloped by a low viscosity layer both embedded within a shearing viscous matrix within this dynamic framework we carried out numerical sensitivity analyses in which we varied the viscosity ratio between the lubricating layer and the surrounding matrix the thickness of the lubricating layer and the kinematic vorticity number of the bulk flow our data and numerical results succeed in explaining why wm estimates from clast based rotational methods are typically lower than estimates from other methods and this has implications for testing hypotheses related for example to vorticity partitioning in oblique convergent settings crustal scale extrusion or channel flow and exhumation of ultra high pressure rocks all of which rely on robust estimates of wm the relation between the shape preferred orientations of clasts and modal mica content lead to the hypothesis that mica is the cause of the lubrication at clast matrix interfaces if so then we surmise that mica fish should be self lubricating and would therefore form an end member shape preferred orientation regardless of matrix modal mica content the unique role of mica allows us to speculate about the bounds on viscosity contrast between the matrix and lubricated clast interfaces

a 3 d kinematic model of fabric development in polycrystalline aggregates comparisons with experimental and natural examples abstract a 3 d generalization of the approach previously developed by etchecopar for the simulation of fabrics in polycrystalline aggregates is presented the model is based on a geometric minimization of gaps overlaps and boundary displacements between deformed neighbouring cells each cell may deform using a small number

orogen parallel extension and oblique tectonics the relation between stretching lineations and relative plate motions many collisional orogens exhibit two distinct deformation phases the first and relatively early phase forms within an obliquely subducting continent and is characterized by regionally consistent stretching lineations subparallel to the orogen and to associated isoclinal and sheath folds second phase folds and stretching lineations develop during and after imbrication of the subducting plate into the overriding plate and they become progressively disharmonic with stretching lineations oriented perpendicular to the orogen structural morphology and stretching lineations indicate that a fundamental kinematic transition occurs as material is imbricated from footwall to hanging wall kinematic modeling based on modern oblique convergent boundaries and considerations of buoyancy forces suggest that only the relatively early structures reflect the relative plate motion but that the correspondence is not simple in particular the orientation of stretching lineations is a function of finite strain kinematics of deformation and angle of subduction and convergence and it will consistently underestimate the angle of oblique convergence second phase structures reflect isostatic emergence of the hanging wall and are independent of relative plate convergence examples are drawn from the south central canadian rockies the western alps the variscides of ireland and france and the himalaya

contribution of ductile flow to exhumation of low temperature high pressure metamorphic rocks san juan cascade nappes nw washington state the san juan cascade sjc nappes were subducted to a depth of 18 km metamorphosed under low temperature high pressure conditions and then exhumed all within 16 m y during exhumation penetrative deformation by solution mass transfer smt resulted in a widespread spaced cleavage strain directions were determined for 27 sandstone samples and absolute strain measurements for a subset of 19 samples z directions generally plunge moderately to the ne and x and y directions are scattered in the plane perpendicular to z smt deformation is constrictional at the local scale but the tensor average indicates plane strain uniaxial shortening at the regional scale with sx sy and sz equaling 1 01 0 91 and 0 58 respectively the average flattening plane xy dips 30 to the ne and the average x direction plunges 20 to the north the large shortening in z was compensated by a mass loss volume strain of 47 we present a simple one dimensional model that illustrates the relationship between finite strain and ductile exhumation for a steady state convergent wedge assuming depth dependent ductile flow and no reversal of principal strain rates with depth this model indicates that ductile thinning of the sjc nappes accomplished only 13 of the total exhumation despite a vertical shortening strain of 36 there is no evidence that normal faulting contributed significantly to exhumation we conclude that erosion operating at an average rate of 1 1 km m y 1 was the dominant exhumation process

stable positions of rigid objects in non coaxial flow a study in vorticity analysis abstract distortions of a homogeneous fabric which occur around rigid objects in rocks deformed by non coaxial ductile flow are useful to determine the sense of shear but also contain information on other aspects of the flow rigid objects in a non coaxial flow fall into two categories those which are permanently rotating and those which can reach stable positions at high finite strain the orientation of such immobilized objects with respect to eigenvectors of instantaneous flow is a function of the vorticity number of the flow the ratio of instantaneous stretches and the axial ratio of the object in naturally deformed rocks immobilized rigid objects may be recognized from the geometry of the surrounding fabric tails of recrystallized material around porphyroclasts in a mylonite rotate towards parallellism with the extensional eigenvector of the flow and the shape of tails reflects the rotational behaviour of the porphyroclast the axial ratio of immobilized porphyroclasts and their orientation with respect to the tails can theoretically be used to determine the vorticity number of the flow and deviations from isochoric plane strain in practice such an analysis is as yet difficult but a tentative example is given of the way in which the vorticity number can be calculated from a population of feldspar porphyroclasts in a quartzite mylonite from the french pyrenees

hangingwall strain a function of duplex shape and footwall topography abstract the concept of piggy back thrust tectonics implies that foreland is progressively accreted onto a developing thrust sheet during duplex formation lateral shape changes in duplexes in the hangingwall of a thrust and corrugations in the footwall will fold higher thrust sheets to give culminations and depressions balancing of parts of high level thrust sheets with lower sheets and foreland requires a sequence of extensional and compressional strains orientated normal to the thrust transport direction culmination walls will be sites of strike parallel extension subsequent adjacent culminations will compress early culmination walls which will result in a sequence of irrotational strains examples of this geometry are given from the moine thrust zone of northwest scotland the model allows a re examination of strains and hangingwall evolution in some thrust sheets in the helvetic and external zones of the alps

the concept of steady state foliation abstract a steady state foliation is thought of as a foliation with constant orientation and intensity in a rock undergoing steady state flow the problem discussed is whether such straininsensitive foliations exist in rocks and how they might be distinguished from the more familiar strain sensitive foliations steady state foliations would be useful indicators of stress or of strain rate yet misleading if used as indicators of finite strain one requirement for maintenance of steady state foliations is operation of processes that locally weaken grain shape preferred orientation or reduce grain aspect ratios or destroy lithological layering during flow these are the inverses of the processes normally discussed in connection with foliation development many such processes are known though the details of how and when they operate efficiently enough to stabilize a foliation are unknown there is however a tendency for some of them to operate more efficiently at high strains high temperatures and in flow regimes with high vorticity natural and experimental high temperature shear zones are accordingly one environment in which steadystate foliations may be encountered an experiment is described in which a thin specimen of para dichlorobenzene was given a large shearing deformation at 90 of its absolute melting point a steady state foliation was not produced but the technique is a promising one for further microstructural studies where in situ observation of material undergoing large deformations is desirable

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

quantitative kinematic flow analysis from the main central thrust zone nw himalaya india implications for a decelerating strain path and the extrusion of orogenic wedges quantitative kinematic indicators from the main central thrust zone mctz in the nw himalaya have been used to characterize the type of flow during deformation different generations of tension gashes have been rotated by variable angles with respect to the mylonitic foliation forming associated fringe folds these record the late stage brittle ductile flow and reveal that a strong pure shear component of deformation occurred throughout the mctz to characterize earlier deformation increments fabrics from highly deformed quartz ribbons were analyzed well developed shape and lattice preferred orientation patterns show a systematic change of the glide systems and suggest inverted palaeotemperatures within the mctz investigations of the c axes patterns reveal a strong asymmetry at the top of the mctz whereas the samples from the base of the mctz show almost perfectly symmetrical type i crossed girdles deformation within the mctz probably started close to simple shear flow at higher temperatures which progressively became a more general shear during cooling and ended in a pure shear dominated flow during the final stages of brittle ductile deformation i e a decelerating strain path using the higher himalaya crystalline as an example a kinematic model for the extrusion of crustal wedges above major thrust zones is suggested

the evolution of high temperature mylonitic microfabrics evidence from simple shearing of a quartz analogue norcamphor plane strain simple shearing of norcamphor c7h10o in a see through deformation rig to a shear strain of 10 5 at a homologous temperature of th 0 81 yields a microfabric similar to that of quartz in amphibolite facies mylonite synkinematic analysis of the norcamphor microfabric reveals that the development of a steady state texture is linked to changes in the relative activities of several grain scale mechanisms three stages of textural and microstructural evolution are distinguished 1 rotation and shearing of the intracrystalline glide planes are accommodated by localized deformation along three sets of anastomozing microshears a symmetrical c axis girdle reflects localized pure shear extension along the main microshear set sa oblique to the bulk shear zone boundary abbreviated as szb 2 progressive rotation of the microshears into parallelism with the szb increases the component of simple shear on the sa microshears grain boundary migration recrystallization favours the survival of grains with slip systems oriented for easy glide this is associated with a textural transition towards two stable c axis point maxima whose skeletal outline is oblique with respect to the sa microshears and the szb and 3 at high shear strains 8 the microstructure texture and mechanism assemblage are strain invariant but strain continues to partition into rotating sets of microshears steady state is therefore a dynamic heterogeneous condition involving the cyclic nucleation growth and consumption of grains

underplating related finite strain patterns in the gran paradiso massif western alps italy heterogeneous ductile strain superimposed on a nappe stack a finite strain study in the central and northern gran paradiso massif of the italian western alps has been carried out to elucidate whether ductile strain shows a relationship to nappe contacts and to shed light on the nature of the subhorizontal foliation typical of the gneiss nappes in the alps the r f and fry methods were used on feldspar porphyroclasts from 98 samples of the gran paradiso unit upper tectonic unit of the gran paradiso massif and 12 samples from the underlying erfaulet unit lower unit of the gran paradiso massif microstructures and thermobarometric data show that feldspar ductility at temperatures higher than c 450 c occurred only during high pressure metamorphism when the rocks were underplated beneath the overriding adriatic plate therefore the finite strain data can be related to high pressure metamorphism in the alpine subduction zone the augen gneiss was heterogeneously deformed and axial ratios of the strain ellipse in xz sections range from 2 1 to 69 8 the long axes of the finite strain ellipsoids trend west wnw and the short axes are subvertical associated with a subhorizontal foliation the strain magnitudes do not increase towards the nappe contacts the data indicate flattening strain type in the gran paradiso unit and constrictional strain type in the erfaulet unit and prove deviations from simple shear we conclude that nappe stacking occurred early during subduction probably by brittle imbrication and that ductile strain was superimposed on and modified the nappe structure during high pressure underplating in the alpine subduction zone the accumulation of ductile strain during underplating was not by simple shear and involved a component of vertical shortening which caused the subhorizontal foliation in the gran paradiso massif

kinematics and vorticity of flow associated with post collisional oblique transpression in the variscan inner zone of northern sardinia italy abstract quartz c axis fabrics vorticity of flow and deformation temperatures were analyzed in samples collected in the central sector of the variscan inner zone in sardinia italy in order to constrain the kinematics of exhumation of medium and high grade metamorphic rocks during d2 post collisional deformation we investigated two shear belts with opposing shear senses these shear belts are developed parallel to the trend of the inner zone which marks the boundary between two metamorphic complexes our data suggest that the sinistral shear belt initiated during the early stage of d2 deformation in a simple shear dominated regime whereas the dextral shear belt initiated later in a pure shear dominated regime we document a general non coaxial flow regime with contemporaneous pure and simple shear components geometrical relationships reveal the operation of oblique extrusion during a compressive regime where the coaxial component acted sub horizontal and orthogonal to the trend of the axial zone changes in the kinematics vorticity of flow and strain geometry that occurred during d2 deformation may be related to post collisional orogen parallel displacement caught up in the variscan orogeny

the influence of country rock structural architecture during pluton emplacement the loch loyal syenites scotland the late caledonian loch loyal syenites comprise three related but geographically separate composite intrusions the ben loyal ben stumanadh and cnoc nan cuilean bodies they were emplaced c 426 ma into moine and lewisian rocks of the moine and naver thrust sheets previously deformed by caledonian nw directed folding and thrusting at amphibolite facies metamorphic conditions the intrusions lie mainly within a zone of large scale cross folds in which the regional foliation trends nw se at high angles to regional orogenic strike nne ssw the cross folds are underlain by a broad ese dipping zone of high strain known as the ben blandy shear zone the nw basal contact of the ben loyal body dips generally se and strikes sub parallel to the foliation in the underlying ben blandy shear zone the higher parts of this syenite together with those of the ben stumanadh and cnoc nan cuilean intrusions form a series of variably coalesced dykes trending nw se sub parallel to the foliation in the pre existing zone of cross folds there is little evidence for significant country rock deformation during emplacement of the syenites minor components of top to the se extension occurred along the nw boundary of the ben loyal body whilst minor dextral shearing is associated with the emplacement of the nw se vertical sheet like units that are thought to form the se part of this intrusion and the other loch loyal syenites generally contact parallel magma tic state deformation fabrics are dominant in most of the intrusions and are thought to reflect limited magma pressure driven internal deformation of the igneous bodies during and immediately after emplacement the geometry of these fabrics and the loch loyal syenites as a whole has been directly controlled by the pre existing structural architecture of the surrounding country rocks this appears to be a previously overlooked feature of many plutons emplaced at mid to upper crustal depths below the region influenced by the free surface the siting and emplacement of the loch loyal syenites may have been initially facilitated by late caledonian gravity driven extension of the moine thrust sheets following the initial phase of intrusion it is also possible that the presence of magma actually promoted further nappe collapse in the adjacent country rocks there is no compelling reason to link the upper crustal extension observed during emplacement to the genesis of the alkaline magmas in the underlying mantle the siting and ascent of the syenites at depth may be linked to the presence of steeply dipping structures in the lewisian autochthon buried beneath the moine nappe

interpretation of deformation fabrics of infrastructure zone rocks in the context of channel flow and other tectonic models infrastructure zones are essentially horizontal to shallowly dipping crustal scale zones of non coaxial flow with two possible interpretations a a crustal scale shear zone transporting upper crust over lower crust and or mantle transport flow or b a zone of channel flow in which there is a flux of weak crust between relatively strong upper and lower crust and or mantle away from the centre of the orogen transport flow has a constant shear sense across the zone whereas in channel flow the sense of shear reverses across the zone channel flow may be driven by extrusion extrusive channel flow due to the two channel walls approaching one another or by a pressure gradient along the channd normal channel flow with no convergence of the walls necessary arguments based on strain compatibility and mechanics suggest that extrusive channel flow is unlikely kinematic vorticity numbers have been used to show that infrastructure and other shear zones have undergone flattening strains but we show that the numbers are incompatible with extrusion we also show that in addition to the problems inherent in determining kinematic vorticity numbers from fabric the numbers cannot be related to bulk flow in mechanically heterogeneous zones because of flow partitioning drag folds are a better indication albeit qualitative of whether a zone is thinning or not they also give a conservative estimate of the minimum accumulated shear strains and may inhibit strain localization like snowball garnets they indicate shear strains that are so large that the pure shear thinning component for a steady shear zone has to be small

evidence for non plane strain flattening along the moine thrust loch srath nan aisinnin north west scotland we report quartz c axis patterns grain shape fabrics and microstructures for 11 mylonitic quartzites and quartz phyllosilicate schists from a transect across the moine thrust at loch srath nan aisinnin north west scotland in the footwall samples collected more than 42 m normal distance from the thrust surface quartz c axis textures indicate a general flattening strain i e 0 k 1 samples within 19 m normal distance of the thrust are completely recrystallized and exhibit asymmetric c axis patterns recrystallized hanging wall fault rocks exhibit random c axis patterns on the scale of a standard thin section relict footwall grains provide the closest approximation of finite strain they have octahedral shear strains s between 1 10 and 1 47 and exhibit general flattening k values 0 0524 0 659 the long axis of the mean relict grain shape trends parallel to the regional transport direction and plunges gently to the ese in contrast recrystallized footwall grains have a mean grain shape with the longest axis oriented nearly perpendicular to the transport direction furthermore these samples have grain shape k value ranges from 0 157 to 0 295 recrystallized hanging wall grain shapes exhibit the lowest octahedral shear strains s 0 532 0 733 and largest mean k values 0 351 0 961 of this sample set the long axes of the mean recrystallized hanging wall grain shapes are parallel to transport similar to that of relict footwall grains unrecrystallized quartz overgrowths about opaque mineral grains suggest concurrent elongation in all directions within the mylonitic foliation and support the inference of general flattening deformation the mylonitic foliation and penetrative lineation are consistent with a wnw shearing direction however both were folded during later deformation increments recrystallized grains in footwall quartzites suggest a 305 320 azimuth for the shearing direction the best fit axis of the poles to the foliation is 18 145 close to the mean trend plunge of mesoscopic fold hinges at this site 18 151 both suggest a nnw azimuth for the shearing direction thus structural microstructural and fabric indicators suggest a change in the azimuth of the shearing direction from 286 to 331

vorticity analysis in a metachert from the sanbagawa belt sw japan abstract common tectonic features of deformed metachert include both sets of extended and shortened veins and quartz c axis preferred orientation patterns in such rocks information is also commonly available on the orientation and magnitude of the finite strain these features can be used to derive two independent estimates of the degree of non coaxiality during deformation in an example from the sanbagawa belt sw japan the two methods give consistent results and show that deformation was intermediate between simple and pure shear such types of progressive deformation distributed over a broad region could cause substantial subvertical thinning and be a significant factor in the exhumation of high p t rocks in the region

variation in vorticity of flow during exhumation of lower crustal rocks neoproterozoic ambaji granulite nw india abstract the exhumation of the neoproterozoic ambaji granulite in the aravalli delhi mobile belt nw india took place along nnw sse trending d2 shear zones the shear zones evolved from a high temperature 700 c thrust slip shearing event in the lower middle crust to a low temperature 450 c retrograde sinistral top to nw shearing event at the brittle ductile transition bdt the vorticity of flow wm along the shear zones is estimated with the rigid grain net and strain ratio orientation techniques the wm estimates of 0 32 0 40 and 0 60 coincide with the high temperature event and suggests pure shear dominated deformation the low temperature phase coincides with wm estimates of 0 64 0 87 and 1 0 implying two flow regimes the shear zone was first affected by general non coaxial deformation and gradually became dominated by simple shearing we interpreted that the high temperature event happened in a compressive tectonic regime which led to horizontal shortening and vertical displacement of the granulite to the bdt the low temperature event occurred in a transpressive tectonic setting that caused the lateral displacement of the granulite body at bdt depth the wm values indicate a non steady strain during exhumation of granulite this tectonic evolution is comparable with that of the himalayas

the strain and textural history of thin skinned tectonic zones examples from the assynt region of the moine thrust zone nw scotland abstract the moine thrust zone at assynt is a classical example of a foreland propagating thin skinned thrust zone ideal for an examination of thrust related folds deformation textures and strains and complex incremental strain history divergent transport directions oblique trending folds duplex zones and extensional strains normal to the main transport direction may all be explained in terms of thrust propagation leading to the development of oblique to lateral ramps the majority of thrusts cut up section from basement to cover in the transport direction but there is also localised extensional flow and thinning of the thrust sheets in northern assynt the thrust zone involves a wide vertical zone of sinistral shear within which forethrusts backthrusts and associated folds and cleavages are oblique to the general transport direction it is suggested that north of this shear the thrusts moved further probably under a thicker cover while to the south movement was more intermittent probably under a thinner cover this variation is probably due to a change in thrust geometry in the moines east of the moine thrust zone causing a variation in gravitational potential along the length of the moine thrust

channel flow and the himalayan tibetan orogen a critical review the movement of a low viscosity crustal layer in response to topographic loading provides a potential mechanism for 1 eastward flow of the asian lower crust causing the peripheral growth of the tibetan plateau and 2 southward flow of the indian middle crust to be extruded along the himalayan topographic front thermomechanical models for channel flow link such extrusion to focused orographic precipitation at the surface isotopic constraints on the timing of fault movement anatexis and thermobarometric evolution of the exhumed garnet to sillimanite grade metasedimentary rocks support mid crustal channel flow during the early to mid miocene exhumed metamorphic assemblages suggest that the dominant mechanism of the viscosity reduction that is a requirement for channel flow was melt weakening along the upper surface defined by the south tibetan detachment system and strain softening along the base bounded by the main central thrust neotectonic extrusion bounded by brittle quaternary faults south of the main central thrust is positively correlated with the spatial distribution of precipitation across a north south transect suggesting climate tectonic linkage over a million year time scale a proposed orogen wide eastward increase in extrusion rate over 20 ma reflects current precipitation patterns but climate tectonic linkage over this time scale remains equivocal

models of strain variation in nappes and thrust sheets a review abstract strain within nappes and thrust sheets can be modelled using simple boundary conditions an original rectangular prism of material resting on a thrust plane is considered to be subject to various pure and simple shear components operating parallel to the thrust deformation gradient tensors are determined for the models and solutions found for the principal strains a two dimensional model considers heterogeneous simple shear combined with a stretch in the direction of shear simple shear zones arise as a special case where 1 recumbent fold nappes characteristic of gravitational collapse can be modelled with 1 and thrust sheets with layer parallel shortening and folding with differential transport of nappes gives rise to additional wrench type shears solutions for the principal strains in this type of model are found and used to interpret side wall ramps and steep zones parallel to nappe movements by including a stretch in the transport direction these differential transport models produce prolate strains 1 and oblate strains finally a brief consideration of bending strains produced in a sheet passing over a ramp indicates reversals in the shear direction during the strain history these predict possible crenulation of early thrust fabrics at ramps

evolution of c axis pole figures and grain size during dynamic recrystallization results from experimentally sheared quartzite 1 quartzite samples experimentally sheared at conditions where subgrain rotation and grain boundary migration recrystallization are active show a striking change in the c axis crystallographic preferred orientation cpo with increasing shear strain up to 8 and degree of recrystallization up to 100 we used optical methods to determine the c axis cpo for the bulk samples and for porphyroclasts and recrystallized grains separately and to track the strengths of the cpos in five different orientation domains with increasing the c axis pole figure evolves from a broad peripheral maximum indicative of basal a slip to an inclined single girdle with two maxima indicative of rhomb a slip and finally an elongate single maximum at the girdle center indicative of prism a slip throughout this sequence the fabric skeleton rotates with the sense of shear the bulk preferred c axis orientation results from growth and shrinkage of differently oriented crystallographic domains grain boundary density analysis indicates that the size of prism a recrystallized grains is 1 5 times that of average and 2 times that of rhomb grains indicating that prism a slip is significantly easier at our experimental conditions this inference is supported by autocorrelation function analysis of the various orientation domains and how they evolve with strain similar progressive changes in c axis cpo are observed in quartzites naturally sheared at conditions where grain boundary migration recrystallization operates these results indicate that achievement of microstructural and mechanical steady state requires high strain and complete recrystallization

moine thrust zone mylonites at the stack of glencoul i microstructures strain and influence of recrystallization on quartz crystal fabric development since the early descriptions published by callaway in 1884 the gently dipping mylonites exposed along the moine thrust at the stack of glencoul have drawn generations of geologists to the northern part of the assynt district these mylonites derived from cambrian quartzites footwall and moine pelites and psammites hanging wall have figured prominently in a early research into the influence of crystal plastic deformation and recrystallization on microstructural and crystal fabric evolution b debates on the kinematic interpretation of macro and micro structures and crystal fabrics and c debates on the tectonic significance of such kinematic data in this paper first we briefly review the historical aspects of this research and then using both previously published and unpublished data document the finite strain and quartz fabric development at this classic mylonite locality a tectonic interpretation of these data together with quantitative estimates of flow vorticities associated with mylonite formation at the stack of glencoul are presented in a companion paper by law 2010

strain models within spreading nappes abstract internal strain within nappes and thrust sheets is especially important in the understanding of emplacement mechanisms in this paper strain patterns within spreading nappes are investigated theoretically and experimentally three types of spreading nappe are distinguished using various boundary conditions spreading gliding spreading and extruding spreading nappes each type is placed in its proper natural environment strain components and strain gradients are summarized from the use of scale models performed with analogue materials mathematical computations from experimental modelling allow the calculation of 1 1 the schistosity trajectories in vertical flow planes 2 2 the relationship between stretch and transport direction and 3 3 the spatial distribution of different types of strain ellipsoid these provide some tests which make it possible to demonstrate in the field whether nappes are or are not gravity induced finally some field examples from the central alps and the pyrenees are briefly discussed they provide good support to the strain models presented in the present paper

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

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

the nappe rule why does it work the nappe rule stresses that imbrication emplaces nappes with higher metamorphic grade above those with lower grade this means that the overriding nappes must be exhuming during nappe stacking otherwise metamorphic grade would increase structurally downwards the high grade metamorphic internides of many orogens are characterized by a pervasive nappe emplacement related subhorizontal foliation indicative of vertical ductile shortening thinning by quantifying finite strain in the gran paradiso massif italian alps we show that vertical ductile thinning associated with a nappe emplacement related subhorizontal foliation caused sufficient exhumation of the overriding nappes and can thus explain the occurrence of higher grade rocks above lower grade ones

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

strain and kinematic analysis in general shear zones we present a unifying theory for the development and distribution of strain markers and kinematic indicators in zones of general shear and thus provide a framework in which data previously considered contradictory may be understood rigid and deformable porphyroclast systems including and complex grains are potential indicators of both strain and flow the shapes and distribution of such porphyroclast systems may be used to distinguish among different tectonic regimes general shear is divided into two fields sub simple shear in which the rotational component of the strain is less than that for simple shear and super simple shear in which the rotational component is greater than for simple shear sub simple shear may involve narrowing or broadening of shear zones super simple shear regimes are possible in local regions such as the vicinities of deformable porphyroclasts but must be enclosed by regimes of sub simple shear the polar mohr constructions for finite deformation and flow are useful to analyze general shear in theory the hyperbolic net is employed for practical plotting of real data and derivation of the kinematic vorticity number wn this number represents the relative contributions of pure and simple shear in steady flow in nature deformation is thought to build up and decay by processes that may invalidate the assumption of constant flow regime we therefore introduce the concept of accelerating deformation and analyze the implications of non steady flow for the shearing histories of deformed objects

structural expressions and tectonic implications of general noncoaxial flow in the midcrust of a collisional orogen the northern scandinavian caledonides mapping and strain analysis in the northern scandinavian caledonides indicate that one way in which contemporaneous upper crustal extension and deeper thrusting within collisional orogens may be linked in a kinematically compatible manner is through the ductile flow of rocks in the midcrust of these mountain belts in the ofoten efjorden region of north norway amphibolite facies metamorphism and strong penetrative strain occurred at deep levels of the caledonian nappe stack as it was thrust eastward onto the western margin of baltica kinematic analysis of deformation in the lower portion of the caledonian nappe stack indicates that the rock mass underwent heterogeneous noncoaxial flow with components of foreland directed simple shear and subvertically shortened pure shear general noncoaxial flow simultaneous simple shear pure shear can accomplish foreland directed tectonic transport subvertical thinning and transport parallel elongation at deep levels of a nappe stack this pattern of flow is significant technically for two reasons first it suggests spreading of the caledonian allochthon during thrust emplacement and second it can form a kinematic link between normal faulting at shallower structural levels and thrusting at the base of the nappes by analogy with the caledonides general noncoaxial flow of midcrustal rocks may represent an important process in other collisional and contractional orogens particularly in those that have undergone upper crustal extension while thrusting continued in the foreland

episodic gravitational collapse and migration of the mountain chain during orogenic roll on in the himalayas an w e belt of maximum bulk horizontal shortening the orogen core moved north relative to the overlying crust to form the himalayan syntaxes due to roll on of this portion of the indian plate this displacement occurred below a lengthy succession of gently dipping decollements that formed episodically at a depth of 30 km along the orogen core due to numerous periods of gravitational collapse and spreading of the overlying ductile crust successively developed basal decollements were deformed when continued bulk horizontal shortening of the orogen core below reasserted dominance over the effects of gravitational collapse above causing refolding about steeply dipping axial planes this resulted in northwards migration of the orogen core above depths of 30 km causing rocks metamorphosing at depths of 22 km on the north side of the orogen core to be moved to its south side with no change in depth as roll on progressed garnet porphyroblasts record this lengthy history of lateral migration across the orogen within their inclusion trails the 6 4 kbar average pressures accompanying it were obtained from the mn fe and ca contents of successive garnet cores garnet grew at depths of 22 km until movement towards the surface initiated on successively developed decollements that accommodated the volume constraints of gravitational collapse and spreading on both sides of the orogen the speed of extrusional displacement increased the further the rocks migrated from the orogen core developing mylonitic schists around the porphyroblasts this truncated inclusion trails against all matrix foliations as the porphyroblasts were carried towards the surface indeed these rocks were multiply deformed during at least four distinct periods of deformation after mylonitization began and prior to exposure above the main central thrust mct three or more sub vertical and sub horizontal foliations were formed during each of the five changes in fia trend foliation inflection intersection axes in porphyroblasts preserved in these rocks the inclusion trail asymmetries and p t of garnet core growth accompanying each fia reveal that the first four changes in fia trend which define periods of tectonism about one direction of horizontal bulk shortening relative plate motion occurred on the north side of the orogen core the fifth occurred on the south side of the orogen core and the switch in shear sense on gently dipping foliation planes that resulted from this shift to the south eventually led to the development of the mct when magnetic anomaly 22 that formed in the southern indian ocean ridge is taken into account these five changes in fia trend correlate markedly with changes in the motion of india relative to a constant eurasia from 50 to c 25 ma they reveal that eurasia moved nnw during fias 1 3 and 4 and sse during fia 5 when the shear sense on gently dipping foliations switched to top to the s they suggest collision of india with eurasia took place at 50 ma immediately prior to the development of fia 1

deformation temperatures vorticity of flow and strain in the moine thrust zone and moine nappe reassessing the tectonic evolution of the scandian foreland hinterland transition zone abstract examination of deformation temperature 3 d strain and flow vorticity wm in mylonites from the assynt loch more region of the moine thrust zone mtz allows quantitative kinematic and thermal characterization of shearing at the base of the scandian 435 425 ma orogenic wedge quartz microstructures and c axis fabric opening angles from mylonites in the immediate hangingwall and footwall to the moine thrust suggest that deformation temperatures are highest in the eastern part of the assynt region including mylonites close to alkaline intrusive complexes and decrease along strike both to the north stack of glencoul loch more and to the south knockan quartz c and a axis fabrics together with limited 3 d strain data indicate that deformation in both the footwall and hangingwall mylonites dominantly involved plane strain to general flattening although domains of more constrictional flow are identified adjacent to thrust transport parallel lineaments in the overlying moine nappe rigid grain analyses indicate a remarkably constant flow vorticity for tens of kilometers along orogenic strike 40 60 pure shear in both the hangingwall and footwall mylonites integration with previously reported strain and vorticity estimates from the assynt region indicates a 50 75 sub vertical shortening perpendicular to gently dipping foliation that must have been accommodated by either volume loss or extrusion of material toward the synorogenic topographic surface extrusion implies a causal link between upper and lower crustal processes with significant implications for the kinematic and geometric evolution of the scandian wedge

analysis of deformation paths in shear zones the geometry of ductile shear zones can be used to solve problems of regional tectonics if the deformation path of material in the zones is sufficiently understood flow in many shear zones may have deviated from simple shear and consequently data on the final deformation state such as finite strain and volume change are insufficient for reconstruction of the deformation path even if flow parameters were constant during progressive deformation additional information on the flow vorticity number is also needed and can be obtained from fabric elements such as sets of folded boudinaged veins rotated porphyroblasts and blocked rigid objects mohr circle constructions are presented as a tool to calculate deformation parameters from fabric data to represent the deformation path graphically and to reconstruct flow parameters from the shape of this path if the vorticity number or the volume change rate changed during progressive deformation the deformation path can be partly reconstructed using sets of fabric elements which register mean and final values of these parameters

channel flow ductile extrusion and exhumation in continental collision zones this collection of 27 review and research papers provides an overview of the geodynamic concepts of channel flow and ductile extrusion in continental collision zones the focal point for this volume is the proposal that the middle or lower crust acts as a ductile partially molten channel flowing out from beneath areas of over thickened crust such as the tibetan plateau towards the topographic surface at plateau margins this controversial proposal explains many features related to the geodynamic evolution of the plateau and for example extrusion and exhumation of the crystalline core of the himalayan mountain chain to the south in this volume thermal mechanical models for channel flow extrusion and exhumation are presented and geological and geophysical evidence both for and against the applicability of such models to the himalayan tibetan plateau system as well as older continental collision zones such as the hellenides the appalachians and the canadian cordillera are discussed

crustal structure restoration and evolution of the greater himalaya in nepal south tibet implications for channel flow and ductile extrusion of the middle crust

albitized microcline grains of post depositional and probable detrital origins in brottum formation sandstones upper proterozoic sparagmite region of southern norway untwinned albite ab 99 4 an 0 4 or 0 2 grains forming during diagenesis and very low grade metamorphism are common in sandstones of the brottum formation upper proterozoic in the sparagmite region of southern norway these untwinned albite grains are usually clouded by vacuoles and tiny inclusions which are mostly made of carbonaceous material although some albite grains also contain some combination of carbonates quartz phengite chlorite pyrite halite and anatase the presence of microcline grains replaced partly by albite indicates that the albite grains are pseudomorphs after microcline chessboard albite which occurs in the brottum formation is suggested to be detrital in origin

structures and fabrics in a crustal scale shear zone betic cordillera se spain abstract a broad zone of dominantly ductile high strain deformation lies beneath the aguilon nappe in the sierra alhamilla southern spain it forms part of a crustal scale movement zone traceable through much of the betic cordillera which separates the higher betic nappes from the underlying nevado filabride complex the zone is characterized in outcrop by a distinctive platy foliation and a strong nne trending stretching lineation microstructural characteristics include quartz ribbons mica fish augen of feldspar and other minerals in a matrix of dynamically recrystallized quartz and extensional crenulation cleavages narrow bands of ultramylonite and cataclasite occur within and on the margins of the movement zone deformation occurred under lower greenschist facies conditions and was accompanied by retrogression of earlier higher grade mineral assemblages structures in the movement zone developed in a temporal sequence beginning with isoclinal folding and transposition of older foliations this was followed by the formation of extensional crenulation cleavages and the progressive localization of strain into the ultramylonite bands mylonitic foliation in these bands is deformed by syn mylonite folds restricted to the bands all these structures were then deformed by s to se vergent small scale folds restricted to the movement zone as a whole cataclasis associated with alteration is localized along the ultramylonite bands and indicates a transition to late stage brittle deformation the lower boundary of the movement zone is gradational strain decreases recrystallized grainsize and the degree of recrystallization of quartz increases and pressure solution becomes the dominant deformation mechanism in mica schist asymmetric quartz fabrics in the movement zone indicate a nne sense of shear but variations in the degree of asymmetry suggest that flow was partitioned with the ultramylonite bands taking up much of the shear strain and the intervening rocks deforming more slowly and with a lower degree of non coaxiality diffuse fabrics in the fine grained ultramylonite bands may indicate a switch to a grainsize sensitive deformation mechanism and an overall downward increase in the opening angle of crossed girdle fabrics may reflect increased water activity at depth