an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

interpretation of porphyroblast inclusion trails limitations imposed by growth kinetics and strain rates porphyroblast inclusion trails provide important information about the tectonometamorphic evolution of a metamorphic rock however there remains considerable controversy over whether porphyroblasts rotate during bulk non coaxial deformation with reference to an area of the scandinavian caledonides and utilizing existing data from theoretical and experimental modelling this study demonstrates that both straight and s shaped inclusion trails are consistent with an interpretation in terms of syndeformational porphyroblast growth in a regime approximating to newtonian simple shear at crustal strain rates of 10 14 s 1 and porphyroblast growth times of 0 1 1 0 ma it is shown that a maximum of 5d 9d angular rotation would occur during growth at faster strain rates of 10 12 s 1 e g those in a shear zone porphyroblast angular rotations of 90d are shown to occur in 0 1 0 25 ma i e times comparable with or faster than porphyroblastesis in view of this s shaped inclusion trails are to be expected in porphyroblasts growing in active shear zones or other situations of high shear strain whereas straight inclusion trails can be interpreted as static overgrowth of an existing fabric or as syndeformational porphyroblastesis at low strain rates

numerical modelling of hydrodynamics of permeable pile groins using swash this paper focuses on a specific form of groins permeable pile groins ppgs consisting of a single or double rows of wooden piles with only few experiments and simulations available to study the hydraulic functioning of ppgs the correlation between the effectiveness and the characteristics of the groin system has yet to be fully understood this paper presents the application of swash a non hydrostatic wave flow model to simulate flow fields affected by ppgs the swash model was calibrated to correctly reproduce longshore current fields in the laboratory then introducing ppgs in the model the simulation results are compared with available experimental measurements to investigate current ppg interaction the simulation results which generally agree well with the measurements show that ppgs hardly attenuate wave energy but considerably retard longshore currents within the groin fields rip currents are predicted to develop at both flank sides of each pile groin due to local positive water level gradients toward the pile groins

kinematic significance of mingling rolling structures in lava flows a case study from porri volcano salina southern tyrrhenian sea a basaltic andesite lava flow from porri volcano salina southern tyrrhenian sea is composed of two different magmas magma a 51 vol of crystals has a dacitic glass composition and magma b 18 vol of crystals a basaltic glass composition magma b is hosted in a and consists of sub spherical enclaves and boudin like banding and rolling structures rs four types of rs have been recognized type type complex types and transitional structures between sub spherical enclaves and rolling structures an analysis of the rs has been performed in order to reconstruct the flow kinematics and the mechanism of flow emplacement rolling structures have been selected in three sites located at different distances from the vent in all sites most rs show the same sense of shear kinematic analysis of rs allows the degree of flow non coaxiality to be determined the non coaxiality is expressed by the kinematic vorticity number wk a measure of the ratio sr between pure shear strain rate and simple shear strain rate the values of wk calculated from the measured shapes of microscopic rs increase with increasing distance from the vent from approximately 0 5 to 0 9 results of the structural analysis reveal that the rs formed during the early intermediate stage of flow emplacement they represent originally sub spherical enclaves deformed at low shear strain at higher strain rs deformed to give boudin like and stretched banding structures results of the kinematic analysis suggest that high viscosity lava flows are heterogeneous non ideal shear flows in which the degree of non coaxiality increases with the distance from the vent in the vent area deformation is intermediate between simple shear and pure shear farther from the vent deformation approaches ideal simple shear lateral extension processes occur only in the near vent zone where they develop in response to the lateral push of magma extruded from the vent lateral shortening processes develop in the distal zone and record the gravity driven movement of the lava the lava flow advanced by two main mechanisms lateral translation and rolling motion lateral translation equals rolling near the vent while rolling motion prevailed in the distal zones

structure mineralogy and pb isotopic composition of the as au ag deposit rotgulden eastern alps austria significance for formation of epigenetic ore deposits within metamorphic domes the abandoned as au ag mining district rotgulden is located within the eastern tauern window of the eastern alps and was reinvestigated in order to evaluate ore deposition during alpine late orogenic tectonic processes four major ore types have been recognized 1 quartz sulfide veins within variscan basement rocks 2 deformed metamorphic massive ores within fold hinge zones saddle reefs of permian to mesozoic cover sequences 3 ores in tension gashes of the cover sequence and 4 irregular replacement ore bodies in marbles of the cover sequence ore deposition was intimately related to late orogenic exhumation by stretching of footwall sequences within the tauern metamorphic core complex during late oligocene and neogene hydrothermal systems developed and metals from apparently distinct sources were deposited under decreasing temperature conditions lead is distinctly radiogenic and resembles the lead in au quartz veins of the mesozoic cover sequence of the hohe tauern

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

the dilemma of asymmetric porphyroclast systems and sense of shear abstract asymmetric porphyroclast systems have long been used as independent shear sense indicators based on their clast tail asymmetry and or stair stepping tail geometry over the last two decades advances in exploring the mechanical behavior of rigid clasts in a viscous flow has shed new light on understanding the formation of porphyroclast systems embedded in noncoaxial viscous flow and subsequently quantifying many aspects of flow kinematics using examples from naturally deformed rocks combined with results from recent advances our study indicates that 1 in general only porphyroclast systems composed of equant shaped clasts can be treated as independent shear sense indicators 2 special attention should be given to type porphyroclast systems composed of high angle intermediate aspect ratio clasts which may contain meaningful information on bulk flow kinematics 3 bookshelf structures cannot serve as independent shear sense indicators multi criteria and thorough three dimensional shape analyses are suggested for a proper interpretation of flow kinematics in ductile shear zones

tertiary extension of continental crust and uplift of psiloritis metamorphic core complex in the central part of the hellenic arc crete greece kinematic analysis of the deformation in central crete suggests that the structural evolution and exhumation of the high pressure low temperature hp lt rocks outcropping at the mount psiloritis metamorphic core complex are associated with a regional miocene north south extension and thinning of the continental crust this tectonic regime developed under bulk coaxial strain conditions with ductile deformation in the lower and brittle deformation in the upper crust and followed on the decompressional path a north south compression associated with a hp lt metamorphism in the lower crust this compressional event took place during oligocene early miocene and led to overthickening of the accretionary wedge in the hellenic arc an east west directed compression accompanied in the final stages the miocene north south extension of the continental crust

effects of groyne layout on the flow in groyne fields laboratory experiments this research is aimed at finding efficient alternative designs in the physical economical and ecological sense for the standard groynes as they are found in the large rivers of europe in order to test the effects of various groyne shapes on the flow in a groyne field experiments were performed in a physical model of a schematized river reach geometrically scaled 1 40 four different types of schematized groynes were tested all arranged in an array of five identical groyne fields i e standard reference groynes groynes with a head having a gentle slope and extending into the main channel permeable groynes consisting of pile rows and hybrid groynes consisting of a lowered impermeable groyne with a pile row on top flow velocities were measured using particle tracking velocimetry the design of the experiment was such that the cross sectional area blocked by the groyne was the same in all cases depending on the groyne head shape and the extent of submergence variations in the intensity of vortex shedding and recirculation in the groyne field were observed the experimental data are used to understand the physical processes like vortex formation and detachment near the groyne head it is demonstrated that the turbulence properties near and downstream of the groyne can be manipulated by changing the permeability and slope of the groyne head it is also observed that for submerged conditions the flow becomes complex and locally dominated by three dimensional effects which will make it difficult to predict by applying depth average numerical models or by three dimensional models with a coarse resolution in the vertical direction

rolling structures at large shear strain abstract at large shear strain shear criteria are often obliterated or become ambiguous from examination of both natural examples and experimental models we describe a new criterion called rolling structure widely represented in sheared rocks a typical rolling structure is composed of a rigid or competent object e g porphyroclast boudin fossil etc with two tails asymmetrically disposed around it in most cases tails are of the same material as that of the rotating object and result from strain softening and grain size reduction at the object periphery forming a mantle z and s asymmetries of rolling structures represent dextral and sinistral senses of shearing respectively tails must not be confused with pressure shadows which usually present an opposite asymmetry for a given shear sense besides the determination of the sense of shear the occurrence of these structures allows a minimum estimate of the strain intensity in strongly sheared rocks since rolling structure length is proportional to shear strain

progressive eclogitization under fluid present conditions of pre alpine mafic granulites in the austroalpine mt emilius klippe italian western alps metabasites from the austroalpine mt emilius klippe preserve different stages of the evolution from pre alpine granulite to alpine eclogite facies assemblages the pre alpine protolith includes layered garnet i clinopyroxene i plagioclase hornblende granulites and gabbros the eclogitic paragenesis comprises garnet ii clinopyroxene ii epidote ca amphibole chlorite eclogitic deformation is composite and involves two main episodes d1a d1b d1a produces a layer parallel foliation and incomplete transformation of granulites and is concluded by an episode of brittle deformation leading to the formation of eclogitic veins d1b gives rise to widespread s l tectonites and mylonites the eclogitic veins and surrounding haloes enriched in fluids during veining control the nucleation of d1b shear zones deformation is accomplished through ductile flow of reaction weakened plagioclase and hornblende sites and cataclasis of garnet i and clinopyroxene i both meso and microscale observations indicate that fluids play a primary role during eclogitization in both enhancing deformation and the kinetics of metamorphic reactions fluids influence plastic flow rates in matrix aggregates and promote fracturing of hard minerals the extent of metamorphic re equilibration is closely related to the density of the fluid pathways

swash an operational public domain code for simulating wave fields and rapidly varied flows in coastal waters abstract a computational procedure has been developed for simulating non hydrostatic free surface rotational flows in one and two horizontal dimensions its implementation in the publicly available swash an acronym of simulating waves till shore is intended to be used for predicting transformation of surface waves and rapidly varied shallow water flows in coastal waters this open source code http swash sourceforge net has been developed based on the work of stelling and zijlema 2003 stelling and duinmeijer 2003 zijlema and stelling 2005 zijlema and stelling 2008 the governing equations are the nonlinear shallow water equations including non hydrostatic pressure and provide a general basis for describing complex changes to rapidly varied flows typically found in coastal flooding resulting from e g dike breaks and tsunamis and wave transformation in both surf and swash zones due to nonlinear wave wave interactions interaction of waves with currents and wave breaking as well as runup at the shoreline the present paper provides a complete description of the numerical algorithms currently used in the code the code is benchmarked using some analytical problems moreover the numerical results are validated with various cases of laboratory data with the principal aim to convey the capabilities of the swash code in particular emphasis is put on an analysis of model performance and associated physical implications serial and parallel performance scalings are also presented

efficient computation of surf zone waves using the nonlinear shallow water equations with non hydrostatic pressure abstract a numerical method for non hydrostatic free surface irrotational flow governed by the nonlinear shallow water equations including the effects of vertical acceleration is presented at the aim of studying surf zone phenomena a vertical boundary fitted grid is used with the water depth divided into a number of layers a compact finite difference scheme is employed for accurate computation of frequency dispersion requiring a limited vertical resolution and hence capable of predicting the onset of wave breaking a novel wet dry algorithm is applied for a proper handling of moving shoreline mass and momentum are strictly conserved at discrete level while the method only dissipates energy in the case of wave breaking the numerical results are verified with a number of tests and show that the proposed model using two layers without ad hoc assumptions enables to resolve propagating nonlinear shoaling breaking waves and wave run up within the surf and swash zones in an efficient manner

the competition between rates of deformation and solidification in syn kinematic granitic intrusions resolving the pegmatite paradox abstract while fully crystallized granites rich in feldspar generally serve to strengthen the continental crust their precursor melts are assumed to be important agents of crustal weakening many syn tectonic granitic pegmatites are deformed within shear zones but ubiquitously preserve undeformed primary magmatic textures implying that they were largely molten during shearing yet the shapes of pegmatite bodies indicate that they deformed with a greater competence than their surroundings this co located pair of material behaviours is paradoxical we interpret field relationships in a typical pegmatite shear zone association torrisdale nw scotland and propose a mechanism by which syn tectonic granitic melts may in effect act as competent bodies while not yet fully crystallized competence was rapidly increased by preferential crystallization on intrusion margins that served to encapsulate residual melt inside stiff rinds further crystallization may have been pulsed as the concentrations of crystallization inhibitors fluxes increased in residual fluids postulating the existence of initial stiff rinds also consistent with modern estimates for rates of feldspar crystallization cms yr from undercooled hydrous silicic magma to form pegmatites these greatly outpace strain rate estimates for shear zones thus fully liquid granitic melts may only be present fleetingly and have little opportunity to weaken deforming crust before crystallization begins

rotating micro structures in antarctic cold basal ice implications for glacier flow and its interpretation structural analyses were conducted in the basal zone of an antarctic glacier the studied basal ice sequence was retrieved from a 20 m long subglacial tunnel dug at the margin of the glacier and is at the temperature of 17 c for the first time rotating clast systems embedded within debris rich ice were thin sectioned using specially designed cutting techniques the observed structures reflect the occurrence of pervasive shearing at the base of the glacier and can be used as shear sense indicators in addition some of these structures provide evidence for the presence of thin liquid films at the time of formation despite the marked freezing temperature of the ice it is showed here that cautious analysis of deformation structures present in debris bearing ice may bring insights not only into the flow dynamics of the embedding matrix but also into the behaviour of the interstitial fluid network at the base of cold glaciers and ice sheets

kinematics of cretaceous subduction and exhumation in the western rhodope chalkidiki block in the chalkidiki peninsula of northern greece a thrust complex made of a basement vertiskos unit a cover circum rhodope belt and arc back arc units chortiatis magmatic suite and eastern vardar ophiolites is exposed in the chalkidiki peninsula of northern greece the complex forms the western part of the rhodope metamorphic province and lies on the hanging wall of the kerdylion detachment the structure responsible for the exhumation of the southern rhodope core complex and the most prominent and visible ductile structure related to the tertiary aegean extension the chalkidiki thrust complex arguably preserves a complete record of cretaceous deformation and related fabrics in this contribution we describe the geometry of foliation stretching lineation and shear sense s on a regional scale the regional foliation strikes nw se and displays different patterns in the three studied units i dominantly dipping at low angle in the vertiskos unit ii affected by upright folding in the circum rhodope belt and iii systematically steeply dipping to the ne in the chortiatis magmatic suite stretching lineation trend dominantly sw ne in the three mentioned units on the basis of new mapping neglecting local perturbations and deformation related to tertiary extension we infer the regional kinematics of cretaceous syn metamorphic thrusting and subsequent exhumation of the metamorphic units thrusting took place toward the sw in present day coordination and the related fabrics are recorded throughout the metamorphic pile on the contrary exhumation related fabrics are related to shear toward the ne and are preferentially recorded in the uppermost part of the metamorphic pile suggesting that extension was more localised and of less finite intensity compared to thrusting

surface electrical resistivity tomography and hydrogeological characterization to constrain groundwater flow modeling in an agricultural field site near ferrara italy a portion of an unconfined alluvial aquifer located in the padana plain italy was characterized following an integrated hydro geophysical approach initially an electrical resistivity tomography ert survey was employed to localize the boundaries of a modest paleo channel body and to design the installation of a groundwater monitoring network multilevel slug tests were performed to estimate the aquifer s saturated hydraulic conductivities determined permeability values together with electrical resistivity data were correlated the correlation resulted in a site specific bi logarithmic linear relationship based on this relationship punctually determined hydraulic conductivities were spatially extended over the studied flow domain finally continuously measured piezometric heads were used to calibrate a 3d flow model sensitivity analysis was performed to confirm the reliability of the reconstructed permeability field as well as to assess the minimum number of direct measurements needed to safely characterize the selected aquifer portion the integration of the ert survey results with the classical hydrogeological tests can be conveniently applied to constrain the permeability field during flow model calibration although the applicability of the determined relationship is site specific the followed procedure is useful especially when there is a need to optimize the available resources and in case of small scale pilot studies

rotation of rigid elliptical cylinders in viscous simple shear flow analogue experiments we used experiments to investigate the behaviour of rigid elliptical cylinders embedded in a viscous fluid of finite thickness and for very small ratios s between shear zone width z and the shortest principal axis e2 of the ellipse that constitutes the base of the elliptical cylinder 3 5 s z e2 1 2 former theoretical and experimental work on the rotation of rigid inclusions immersed in a viscous matrix considered infinite shear zone thickness and or a very large ratio between shear zone thickness and inclusion size s 1 and concluded that inclusions rotate continuously and synthetically with the applied bulk simple shear flow except for inclusions with infinite axial ratio our results depart greatly from the analytical predictions experiments were carried out at constant shear zone thickness but variable s and degree of coupling at the inclusion matrix interface the results show that 1 confined inclusion rotation always deviates from theoretical predictions for infinite shear zone thickness even for synthetic rotation 2 the deviation in the angular velocity and or in the sense of rotation increases as s approaches 1 and antithetic rotation is possible from a position with ellipse longest axis parallel to the shear plane and 3 a slipping inclusion matrix interface greatly enhances deviation from theoretical predictions with antithetic rotations to as much as 20

micro and meso scale deformation structures in experimental fault zones abstract kinematic structures of brittle brittle ductile and ductile shear zones were developed in experimental simple shear deformation transparent films of paraffin wax with different initial fabrics were progressively deformed in a manually driven stage mounted shear apparatus producing heterogeneous simple shear 0 36 2 64 at variable strain rates of from 3 3 10 5 to 4 08 10 5 s 1 en echelon fracture arrays compressive and tensile bridges spaced cleavage stylolite like cleavage and sigmoidal cleavage trails were developed in brittle and brittle ductile deformation structures produced in ductile experiments were c s and c s fabrics and type asymmetric tails shear lenses and active folds of pre existing and induced anisotropy planes the experiments offer insight into the microstructural processes involved and demonstrate development of a progression of shear structures in a single non steady deformation

the rotation of garnet porphyroblasts around a single fold lukmanier pass central alps abstract the orientation of the straight internal foliation s i within large 5 mm garnet porphyroblasts has been measured relative to the orientation of the external foliation s e around a single antiform of 0 5 m wavelength which folds the dominant regional foliation the internal foliation is not constant in orientation but varies consistently both with position around the fold and with the porphyroblast ellipticity the dip of s i hinge dip taken as zero is consistently less than the dip of s e it increases with increasing dip of s e and with increasing ellipticity of the porphyroblasts s i effectively defines a fold with an opening angle greater than that in the external foliation the opening angle of this fold in s i decreases with increasing porphyroblast ellipticity the observed variation in the orientation of s i can be explained qualitatively by a flattened flexural flow model for fold development as could be expected for folding of a pre existing strongly anisotropic foliation the measurements clearly demonstrate that rotation of porphyroblasts relative to geographical co ordinates did occur during the development of this fold and that a model based on the classical theories of rotation of stiff inclusions in a weaker viscous matrix is most appropriate

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

variscan intracontinental deformation the coimbra cordoba shear zone sw iberian peninsula the coimbra cordoba shear zone is a major lineament roughly parallel to the nw se variscan structures of the southern iberian peninsula inside the shear zone evidence for a non coaxial strain path exists from structural microstructural and lattice preferred orientation data on quartz plane strain is suggested by the study of microstructures in the xy and yz sections in all formations occurring in the shear zone with increasing strain a mylonitic series develops the deformation contemporaneous with epizonal to mesozonal metamorphism is responsible for 1 1 the regional subvertical foliation oblique to the shear zone 2 2 the well defined subhorizontal lineation striking nw se 3 3 the discrete planes of shear roughly parallel to the shear zone the data are interpreted as resulting from a left lateral slip motion with a displacement of at least 72 km in the light of plate tectonics it is inferred that the coimbra cordoba lineament could have been a suture zone which evolved into an intracontinental left lateral shear zone during the variscan orogeny of the ibero armorican arc

natural rotated inclusions in non ideal shear abstract kinematic analysis of rotated winged inclusions is often couched in terms of ideal simple shear with reference to a specific model based upon porphyroclasts of circular section however many natural winged inclusions are not porphyroclasts commonly their geometries as well as the orientation distribution of the inclusions themselves reflect the non ideal nature of the shearing deformation during which they formed stair stepped sigma shaped geometries may reflect the immaturity of a structure developed in non ideal shear or a mature structure developed in ideal simple shear stair stepped delta shaped geometries indicate that deformation approximated to ideal simple shear in plane delta shaped geometries and skewed inclusion orientation distributions are indicators of non ideal shear

the use of asymmetric pressure shadows in mylonites to determine the sense of shear abstract asymmetric pressure shadows aps on both sides of a rigid porphyroclast are commonly observed in mylonites along the median tectonic line mtl in japan it is one of the most noticeable asymmetric microstructures showing that the porphyroclasts have rotated during non coaxial laminar flow in a ductile shear zone the shadow domains are filled with recrystallized quartz and k feldspar excepting aps various asymmetric microstructures in the mylonites indicate a sinistral sense of displacement throughout the ductile shear zone along the mtl based on the shape analysis of aps in xz section parallel to the mylonitic lineation and normal to the mylonitic foliation the following results were obtained 1 the relative position of the aps with respect to a porphyroclast is not a reliable criterion for deducing the sense of shear and 2 the drag angle of the shadow boundaries with respect to the mylonitic foliation in each quartered domain is diagnostic of the sense of shear when the shearing is sinistral in upper right and lower left hand side of a porphyroclast is larger than in upper left and lower right hand side and vice versa for dextral shearing these results demonstrate that the drag patterns of aps around porphyroclasts in mylonites are highly reliable indicators for the determination of the sense of shear

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

2 d shape preferred orientations of rigid particles in transtensional viscous flow ghosh and ramberg tectonophysics 34 1976 1 70 studied the two dimensional 2 d rotational behaviour of individual rigid particles embedded in a viscous medium subject to simple shear and transpression sr 0 at low medium shear strains we now extend this theoretical study to transtension sr 0 by deriving a new analytical solution to non interacting populations of rigid particles and to high shear strains we used different initial orientations of particles in the same graph to simulate populations of particles in an originally isotropic rock our results show that 1 shape preferred orientations spos developed in viscous simple shear flow are transient and cyclical except for particles with aspect ratio or material lines which tend to the shear direction 2 stable spos can develop in transpression they dip towards the shear sense but when defining an s c structure with the shear foliation this composite fabric is always antithetic and thus indicates the wrong sense of shear 3 stable spos can also develop in transtension they dip opposite to the shear sense but when defining an s c structure with the shear foliation this composite fabric is always synthetic and thus indicates the correct sense of shear 4 spos produced in transpression or transtension can be used as vorticity gauges only when they represent stable orientations which must be demonstrated a priori

microstructure and mechanical properties of halite coarse muscovite synthetic aggregates deformed in torsion abstract we investigated the behavior of a synthetic two phase aggregate composed of 80 halite 20 coarse muscovite under torsion deformation at 100 200 and 300 c a confining pressure of 250 mpa and a constant strain rate of 3e 4 s 1 at all temperatures the two phase aggregate deformed homogeneously at the sample scale the strength of the aggregate and muscovite deformation depended on temperature strain rate and initial orientation of muscovite greatest dimension relative to the shear plane with muscovite initially parallel to the shear plane halite flowed plastically and long muscovite grains were not strong enough to behave as a rigid inclusion rotating in a ductile matrix instead high aspect ratio grains deformed by folding which was visibly accomplished by slip along mica 001 cleavage and by great flattening of halite in the core of the folds when initially normal to the shear plane high aspect ratio muscovite grains passively rotated towards the shear plane in both cases small and low aspect ratio muscovite grains behaved mostly as mica fish with mica grains tilted opposite to shear sense similarly to natural mylonites porphyroclast systems and rolling structures were also common in the microstructure a strong foliation made of halite ribbons and aligned muscovite flakes rapidly developed but did not make the composite aggregate weaker when compared with single phase halite comparison with synthetic aggregates of single phase halite shows that the two phase aggregate was much stronger than single phase halite in all cases comparison with a synthetic aggregate with calcite porphyroclasts shows that the strength of the aggregate with muscovite at 100 c was lower below a strain of ca 2 6 and higher beyond this value and that the aggregate with calcite was stronger at 200 c strain rate stepping tests 1e 5 s 1 2e 3 s 1 indicate that the two phase aggregate behaved as power law viscous with stress exponents of ca 12 and 10 at 100 and 200 c respectively the mechanical data obtained in this study represent the actual rheological behavior of the aggregates because we used soft polymer jackets

control of shear zone location and thickness by initial grain size variations in upper mantle peridotites abstract the turon de tecouere peridotite north pyrenean fault zone france contains protomylonites grading to ultramylonites grain size reduction to 0 005 0 025 mm took place by reaction during deformation within the protomylonite domain the size of the porphyroclasts suggests an initial grain size in this domain of 2 10 mm in the ultramylonite domain porphyroclasts are polycrystalline with an internal grain size of 0 1 0 2 mm indicating that the ultramylonite domain had a finer initial grain size than the protomylonite domain lattice preferred orientations and misorientations within a polycrystalline porphyroclast in the ultramylonite domain suggest an early episode of dislocation creep accommodated dynamic recrystallization restricted predominantly to the present day ultramylonite domain later grain size reduction by reaction resulting in very fine grained ultramylonites was concentrated in the same domain for reaction involved deformation initial grain size may be crucially important as it controls the surface area available for nucleation of new grains as a result of the finer initial grain size within the present day ultramylonites a critical percentage of fine grained matrix developed in the present day ultramylonite domain earlier than in the present day protomylonite domain resulting in localization of further deformation in the ultramylonites this study suggests that zones of locally smaller grain size can act as heterogeneities that control the location and width of shear zones formed by reaction softening

from xy tracking to buckling axial plane cleavage fanning and folding during progressive deformation folding of axial plane cleavage can occur during progressive deformation without a change in the overall background flow two field examples of upright lachlan fold belt se australia and recumbent naukluft nappe complex central namibia folds are presented in which strongly refracted pressure solution cleavage in competent layers on the fold limbs is buckled as a result of ongoing fold amplification finite element modelling confirms that cleavage refraction on limbs can be sufficient for cleavage planes to be subsequently shortened and therefore folded cleavage refraction is unequally developed on opposite limbs of asymmetric folds formed by oblique shortening of a layer in coaxial flow or by folding in a more general shear environment the differences in finite strain on opposite limbs can be quite marked even when the fold shapes themselves are not obviously asymmetric for folding in simple shear flow as specifically modelled here refraction is only strong on the fold limb that rotates against the imposed sense of shear in known shear environments this provides a potential kinematic indicator in folded units at relatively low strain e g in simple shear of around one where other higher strain indicators typical of mylonites are not yet sufficiently developed or are equivocal

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

the curved translation path of the parpaillon nappe french alps the parpaillon nappe is one of the two helminthoid flysch nappes emplaced on the external dauphinois zone of the western alps a structural analysis of the nappe is presented two superposed deformations d1 and d2 are described that are mainly characterized by large scale recumbent folds whose axes are quasi orthogonal ne sw for d1 and nw se for d2 their vergence is northwestward for d1 and southwestward for d2 during the d2 deformation the nappe was separated into two units one of these being thrusted over the other an analysis of incremental strain using quartz and calcite fibre growth indicates that d2 follows d1 without discontinuity therefore the superposition of d1 and d2 structures is interpreted as a progressive deformation instead of two distinct phases of deformation the emplacement of the nappe is discussed under two aspects the relations between displacement and strain and the role of gravity it is concluded that the translation has been twofold first towards the nw and then towards the sw and that the displacement result essentially from gravity forces kinematic implications for the alpine collision are suggested

winged inclusions pinch and swell objects during high strain simple shear abstract in this study we compare natural examples of isolated pinch and swell objects which have been deformed in simple shear with results of high strain numerical models such structures which have geometrical similarities with clast systems and rolling structures have been called winged inclusions we suggest a new mechanical explanation for the evolution of winged inclusions which form when pinch and swell shaped objects consisting of a core and pre existing wings rotate out of the shear plane the viscosity ratio the stress exponent and the shape of the winged inclusion have a significant influence on the rotation rate the rotational behaviour of winged inclusions differs significantly from the rotational behaviour of simple elliptical objects with comparable aspect ratios during the early stages of formation winged inclusions may resemble mirror images of sigmoids and misinterpretations may lead to a wrong determination of the shear sense during progressive shear to large strains the structures may be approximately described as consisting of a pulsating rotating core and thinning wings that rotate with different rates during rotation of the structure the core of the winged inclusions records an overall decrease of the aspect ratio influencing the rotation rate of the inclusion the wings are subject to ptygmatic folding when they rotate through the field of instantaneous shortening and may unfold again in the field of instantaneous stretching during ongoing shearing the wings on both sides of the core rotate with the core may change their positions and finally unfold after rotation about 180 resulting again in a pinch and swell shaped object therefore winged inclusions record ambiguous information about the finite strain rotating winged inclusions create a significant flow perturbation in the matrix resulting in nucleation of folds refolding and propagation of shear zones rootless intrafolial folds with opposing closures have strong geometrical similarities with winged inclusions and we therefore speculate that the formation of winged inclusions might be an efficient mechanism to produce rootless intrafolial folds

the development of δ clasts in non linear viscous materials a numerical approach abstract winged porphyroclasts are often used as kinematic or strain indicators in shear zones in particular it has been suggested that the geometry of clasts in mylonites can give information about the rheological behaviour of the matrix around the clast passchier et al 1993 we tested this hypothesis with a numerical finite element simulation of the flow field of a viscous matrix around a relatively rigid clast power law viscous flow properties of the matrix with stress exponents of 1 linear viscous 3 and 5 were tested as well as three different sets of approximately simple shear boundary conditions which are relevant to experimental and or natural situations the simulations show that the flow field is primarily determined by the boundary conditions the stress exponent is of secondary importance in general the different boundary conditions produce two types of flow fields about the rigid clast an eye shaped flow field and a bow tie shaped flow field the former is produced by boundary conditions that approximate simple shear applied at an infinite distance from the rigid clast and the latter is produced by boundary conditions which are equivalent to a shear box or ring shear apparatus although both the shear box and ring shear boundary conditions produce bow tie flow patterns the wing geometries which result from the flow patterns for these two cases can be significantly different the geometry of the wings that develop on a clast is a direct function of the flow field around the clast for linear and non linear rheologies only the ring shear boundary conditions produce wings that show stair stepping i e wings that are parallel but not in the same plane in natural shear zones clasts both with and without stair stepping are observed in comparing geologic observations with experimental or numerical modelling it is essential to understand which boundary conditions are appropriate

kinematics of compressional and extensional ductile shearing deformation in a metamorphic core complex of the northeastern basin and range abstract analysis of shear criteria enables the kinematics of two main ductile shearing events d 1 and d 2 to be established in the raft river grouse creek and albion metamorphic core complex the first event d 1 is a nne thrusting and corresponds to mesozoic shortening a well developed non coaxial ductile deformation d 2 of cenozoic age is marked by the occurrence of opposing eastward in raft river and westward shear criteria in albion grouse creek these characterize an arch structure where the shear strain increases outwards in the axial zone of the complex d 2 seems coaxial cenozoic extension is considered to be related to gravitational instability induced by mesozoic overthickening of the crust involving uplift erosion and abnormal heating brittle extension occurs in the upper part of the uplifted domain it is transformed laterally above undeformed basement towards stretched domains of the middle and lower crust through the ductile shear zones localized at the precambrian paleozoic interface this extension of the middle and lower crust occurred in the vicinity of the root zones of the former mesozoic thrusts which may thus have been reactivated as ductile normal faults during cenozoic extension

experimental observations on the effect of interface slip on rotation and stabilisation of rigid particles in simple shear and a comparison with natural mylonites for axial ratios r not vert similar3 porphyroclasts from three mylonites all show a very strong spo with the long axis of the best fit ellipse at an antithetic angle of 5 10 to the shear direction this is more consistent with a stable end orientation than with the transient fabrics predicted by theory for elliptical rigid particles in simple shear the cause of this divergence is investigated in a series of high simple shear strain 15 analogue experiments performed in a ring shear machine couette flow using a linear viscous matrix pdms the rotational behaviour of elongate r not vert similar5 rigid particles with elliptical and rhomboidal shapes comparable with the natural examples is modelled for both coherent and slipping particle matrix interfaces interface slip causes a dramatic reduction in the rotation rate of the elliptical particle compared with theory when the long axis is close to the shear direction but not stabilisation interface slip does result in stabilisation of the rhomboidal particle with the long diagonal oriented at a small antithetic angle to the shear direction for monoclinic particles mirror image shapes referred to here as types 1 and 2 show different rotational behaviour for the type 1 particle with a shape comparable to porphyroclast systems and mica fish the long side rotates asymptotically into parallelism with the shear direction natural examples of type 1 particles such as hornblende and olivine porphyroclasts measured from the finero mylonites southern alps show a very strong preferred orientation for r not vert similar3 with the long side parallel or at a small 5 antithetic angle to the mylonitic foliation for type 2 particles the short side stabilises close to the shear direction or at a small synthetic angle as also observed for sillimanite porphyroclasts from the mont mary mylonites western alps in this natural case stabilisation of the short sides is against an extensional crenulation cleavage rather than the mylonitic foliation the analogue experiments establish that interface slip is one mechanism for stabilisation of elongate rhomboidal particles in natural examples decoupling from the matrix may be affected by extensional crenulation cleavage or c planes in s c fabrics

shear zones between rock units with no relative movement abstract shear zones are normally viewed as relatively narrow deformation zones that accommodate relative displacement between two blocks that have moved past each other in opposite directions this study reports localized zones of shear between adjacent blocks that have not moved past each other such deformation zones which we call wakes form due to the movement of exotic blocks within a viscous medium denser blocks sinking within a salt structure the paths between separated boudins melt in partially molten surroundings melt movement during migmatisation or solid blocks sinking through a partially molten magma body stoping from the fluid dynamics perspective these shear zones can be regarded as low reynolds number deformation zones within the wake of a body moving through a viscous medium while compact moving bodies aspect ratio 1 1 1 generate axial symmetric cone like shear zones or wakes elongated bodies vertical plates or horizontal rod like bodies produce tabular shear zones or wakes unlike conventional shear zones across which shear indicators usually display consistent symmetries shear indicators on either side of the shear zone or wake reported here show reverse kinematics thus profiles exhibit shear zones with opposed senses of movement across their center lines or planes we have used field observations and results from analytical and numerical models to suggest that examples of wakes are the transit paths that develop where denser blocks sink within salt structures bodies of melt rise through migmatites between boudins separated by progressive extension and perhaps where slabs of subducted oceanic lithosphere delaminate from the continental crust and sink into the asthenosphere we also argue that such shear zones may be more common than they have been given credit for and may be responsible for some reverse kinematics reported in shear zones

modelling of mantled porphyroclasts using non newtonian rock analogue materials abstract the development of type porphyroclasts was modelled experimentally in a circular transparent shear rig using crystalline rock analogue materials rectangular camphor objects embedded in a matrix of octachloropropane were deformed in simple shear flow the camphor deformed into objects with progressive deformation up to a shear strain of 100 the development of these objects and of gradients of flow parameters in the matrix was followed from step to step during the deformation the objects in this experiment show stair stepping of wings that did not occur in earlier experiments with newtonian fluids analysis of the flow pattern indicates that this is due to an unusual flow perturbation geometry around the relatively rigid camphor objects which rotate more slowly than equivalent objects in newtonian flow the non newtonian rheology of the experimental materials may be responsible for this deviant behaviour this implies that non newtonian fluids are better analogues than newtonian fluids to study the development of structures in rocks with non newtonian rheology since the geometry of the resulting structures can be different

reduction of sand demand for shore protection beach nourishment is an environmentally preferred method of shore protection but the annual sand requirement may lead to substantial maintenance costs the shoreline processes involving the surf zone beach and dune are reviewed with the aim of reducing the annual sand requirement of eroding shorelines it is shown that surf zones with equilibrium profiles on which the wave energy conversion is evenly distributed across the surf zone from experience for given conditions indicate least loss of sand on steep eroding shorelines it may be difficult to establish an equilibrium profile for such cases the use of perched surf zones is recommended which are supported at the seaward limit by an underwater sill for reduction of littoral transport the use of pervious pile groynes is recommended these are arguably more efficient than impervious groynes the sand loss from a usually dry beach by raised water levels is shown to be a function of the beach slope and is least when the storm waves at raised water levels do not cut an erosion escarpment the loss of sand from a dune by infrequent severe storm tides can be prevented with the aid of a built in membrane these sand losses are usually large and constitute an uneconomic use of this sand resource the proposed concepts and measures are linked to existing knowledge augmented by data from the large wave flume lwf in germany and field data from the north and baltic sea coasts

the partitioning of deformation in flowing rock masses abstract most geological structures owe their development to heterogeneous and or non steady flow flow partitioning refers to the division of the instantaneous velocity field into components related to translation strain and rotation flow partitioning during the development of a geological structure usually changes with time and different histories of flow are involved at different points in the developing structure heterogeneity leads to the development of flow domains with different characteristics notably the degree of non coaxiality of deformation and different partitionings of the rotational component of flow into shear induced vorticity and spin during non steady flow there is often continual repartitioning of vorticity between the spin and shear induced components spin is an important aspect of flow leading to the development of structure this paper discusses the partitioning of deformation in a flowing rock mass coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries non coaxial flow is favoured by suitably oriented material anisotropy and or the presence of well defined boundary constraints involving parallel relative displacements a developing anisotropy e g a crystallographic fabric in a shear zone can induce a change from coaxial to non coaxial flow other effects are related to the scale of approximation for example bulk shear induced vorticity can be locally converted to spin flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response the factors that affect flow partitioning e g spin versus non coaxial deformation are therefore of interest

analysis of transpressional deformation from geometrical evolution of mesoscopic structures from phulad shear zone rajasthan india abstract transpressional deformation in ductile shear zones may be analyzed by simultaneous wall parallel simple shear with strain rate xy along with three dimensional coaxial deformation with strain rates x y and z several types of transpression may be distinguished on the basis of relative values of strain rate ratios x a y b and z c it is possible to identify the nature of transpressional deformation from a detailed analysis of shear zone structures in the phulad shear zone of rajasthan india evidence of general flattening a thrusting sense of shear occurrence of transport parallel stretching lineation frequent occurrence of sheath folds with apical direction parallel to the stretching lineation and occurrence of u shaped lineation patterns indicate that the deformation was transpressional with b a ratio ranging between 1 and 2 the bulk deformation was not dominated by simple shear but involved both simple and pure shear with extrusion along the transport direction much greater than along the vorticity vector rotation of long tectonic clasts caused reorientation of monoclinic rolling structures so that strongly monoclinic structures with opposite senses of asymmetry in different domains appear on subhorizontal outcrops parallel to the vorticity vector although the component of simple shear is zero on these surfaces

controls on the geometry of tails around rigid circular inclusions insights from analogue modelling in simple shear abstract we used analogue modelling to investigate the factors controlling tail geometry in porphyroclast systems results show that 1 inclusions can develop in both slipping and non slipping modes but inclusions only form in the latter 2 inclusions develop when the mantle production rate is constant and the mantle is transected by the separatrix inclusions form when the mantle is initially outside the separatrix and later comes inside this line 3 in the slipping mode the wedge shaped tail of inclusions always has a straight external surface parallel to the shear plane whereas in the non slipping mode the external surface is curved inwards external embayments 4 together with earlier theoretical results and inclusions always show stair stepping of tails when embedded in a viscous matrix under homogeneous simple shear deformation 5 maximum stair stepping occurs in the slipping mode and is at least equal to the inclusion diameter if our models bear significant similarity to nature then i the straight or curved character of inclusions could mean that they had respectively a slipping or non slipping interface with the surrounding recrystallized matrix and ii inclusions may result from shear deformation under retrogressive metamorphic conditions in thrust systems

bounds on strain in large tertiary shear zones of se asia from boudinage restoration we have used surface balanced restoration of stretched boudinaged layers to estimate minimum amounts of finite strain in the mylonitic gneisses of the oligo miocene red river ailao shan shear zone yunnan china and of the wang chao shear zone thailand the layer parallel extension values thus obtained range between 250 and 870 we discuss how to use such extension values to place bounds on amounts of finite shear strain in these large crustal shear zones assuming simple shear these values imply minimum total and late shear strains of respectively 33 6 and 7 3 at several sites along the red river ailao shan shear zone for the wang chao shear zone a minimum shear strain of 7 4 is deduced assuming homogeneous shear would imply that minimum strike slip displacements along these two left lateral shear zones which have been interpreted to result from the india asia collision have been of the order of 330 60 km red river ailao shan and 35 20 km wang chao

subglacial deformation and water pressure cycles as a key for understanding ice stream dynamics evidence from the late ordovician succession of the djado basin niger subglacial deformation is crucial to reconstructing glacier dynamics sediments associated with the late ordovician ice sheet in the djado basin niger exhibit detailed structures of the subglacial shear zone three main types of subglacial shear zones ssz are discriminated the lowermost ssz developed on sandstones displays riedel macrostructures and cataclastic microstructures these resulted from brittle deformation associated with strong glacier bed coupling and low pore water pressure where they developed on a clay rich bed the overlying ssz display s c to s c fabrics sheath folds and dewatering structures these features indicate high ductile shear strain and water overpressure on fine grained sand beds the ssz exhibit homogenized sand units with sand stringers interpreted as fluidized sliding beds the succession of subglacial deformation processes depends on fluid pressure behavior in relation to subglacial sediment permeability fluid overpressure allows subglacial sediment shear strength and ice bed coupling to be lowered leading to ice streaming

kinematics of peridotite emplacement during north atlantic continental rifting galicia northwestern spain abstract the western part of the galicia passive continental margin western edge of spain is bordered by serpentinized peridotites over a n s distance of about 125 km these peridotites were emplaced at the end of continental rifting and or at the very beginning of oceanic accretion they have been observed in situ and sampled in five sites with the nautile submersible during the galinaute cruise 1986 some crustal continental rocks were also collected at two sites on the tilted blocks adjacent to the peridotite ridge most of the escarpments covered by the nautile on the peridotite ridge have yielded only ultramafic rocks except at dive 10 where the peridotites are overlain by a sheared chlonte bearing schist and at dive 14 where the ultramafic rocks are covered by basalts all the peridotites are plagioclase bearing harzburgites and iherzolites locally cross cut by rare plagioclase rich veins and dioritic intrusives some pyroxenites and gabbros are associated with the peridotites at dive 14 the plagioclase bearing peridotites are similar in composition to those drilled during odp leg 103 primary structures have not been found except in the northern part of the studied zone where a few samples exhibit magmatic textures elsewhere the peridotites show evidence of a strong mylonitization event which has overprinted a former primary high temperature fabric at these localities boudinage structures and ultramylonitic bands referred to as shear bands occur and the rocks have a disrupted mylonitic texture a few dioritic dykes have also suffered this mylonitization shearing occurred in a rotational regime at high and decreasing temperature 1000 850 c and under a high deviatoric stress 102 mpa i e under lithospheric conditions the structural pattern of the peridotite ridge deduced from measurements on oriented samples is complex the trend of the mylonitic foliation appears nearly constant from south to north n125 n105 but its dip increases 20 45 northward from south to north the stretching lineation pitch varies from subvertical to subhorizontal stretching has occurred along a shear zone acting as a normal fault in a northeasterly direction in the southern part of the studied area to the north it occurred in a northwesterly direction with a strike slip component stretching in the chlorite bearing schist and in the gneissose rocks of the continental basement seems to be unrelated to the peridotite deformation this study confirms the uplift of the galicia peridotites during the continental rifting of the north atlantic ocean in cretaceous times during their ascent the peridotites underwent some melting and high temperature low stress ductile deformation followed by intense mylonitization at decreasing temperature and increasing stress that has overprinted most of the primary textures the geometry of the mylonitic structures is related both to the ascent of the peridotite dome and to the extensive stress strain field prevailing in the continental lithosphere during the rifting episode it is consistent with that expected in the upper part of a dome locally offset by transverse faults in its northern part due to the vicinity of a triple junction point the kinematics of emplacement agree well with an e w opening of the north atlantic ocean

shear criteria and structural symmetry abstract during the last decade it has been shown that most relevant shear criteria within ductile rocks are asymmetric structures e g pressure shadows shear bands c s structures fabrics tension gashes folds veins the correspondence between coaxial or non coaxial deformation and symmetric or asymmetric particle velocity fields respectively accounts for the use of structural symmetry as an indicator of strain history the application of this symmetry concept to various field examples emphasizes that i the degree of symmetry of a given structural pattern reflects the bulk strain regime irrespective of the size and the mechanical behaviour of the considered system and ii the strain regime can also be inferred from the order of appearance and dominance of structures which contribute to the total deformation pattern even where the progressive deformation results in a complex pattern which cannot be directly interpreted

vertical structure of the undertow outside the surf zone the vertical structure of the undertow in the shoaling region outside the surf zone is quite different from that inside the surf zone inside the surf zone the undertow typically has a large seaward directed velocity near the bottom and either a shoreward directed or a small seaward directed velocity at trough level measurements show that outside the surf zone the undertow has a small seaward directed velocity near the bed and a large seaward directed velocity at trough level in this paper we develop theoretical expressions for the undertow outside the surf zone and show that the steady streaming from the bottom boundary layer which was earlier found to have a negligible effect in the surf zone svendsen et al 1987 does have a significant influence on the vertical structure of the undertow in that region