preferred orientation of plagioclase neutron diffraction and u stage data abstract pole figures of anorthosite mylonite an 65 from san juan bautista california were determined with neutron diffraction using a 23 position sensitive detector this novel technique enables us to deconvolute the complex diffraction spectrum of this triclinic mineral into fifteen separate peaks hkl which are measured simultaneously all pole figures display strong preferred orientation with complicated triclinic distributions from which it appears that 001 poles are concentrated normal to the schistosity plane of the specimen the triclinic mineral is well suited to discuss some fundamental issues of texture representation if crystals are measured individually e g on the u stage positive and negative ends of directions can be unequivocally identified and the whole sphere rather than a hemisphere is necessary to represent the distribution of positive axes this is illustrated for a plagioclase rich amphibolite from orno huvud sweden

natural deformation fabrics of plagioclase implications for slip systems and seismic anisotropy abstract the optical and tem microstructures and fabrics of twelve naturally deformed plagioclase containing rocks of compositions from an0 an80 have been studied with a view to determining the deformation mechanisms the optical indicatrix fabrics have been recorded in all samples with respect to the foliation and lineation in general and form a girdle in the foliation plane with a high concentration of normal to the foliation in some cases the orientation of 010 and 001 cleavages or albite and pericline twins has allowed the construction of pole figures in all cases 010 has been shown to be the dominant slip plane with either 001 or 100 the dominant slip direction tem shows that the dominant slip system is 010 001 with secondary 010 100 the seismic p wave anisotropy has been calculated for the first time from the lpo data of plagioclase the anisotropy coefficient is high at around 15 the minimum p velocity is normal to the lineation in the foliation plane and the maximum is sub parallel to the foliation normal the combined information on deformation kinematics and seismic anisotropy is valuable for interpretations of deep crustal seismic sections

fabric transitions in the saxony granulite terrain abstract quartz fabric transitions reported from the saxony granulite terrain and from the nearby ore mountains are interesting because of the changes in the pattern types and the variation in the opening angles of the fabric skeletons a short summary is presented of salient features in research documented in the east german literature between 1961 and 1978 the changes in fabric skeleton with decreasing metamorphic grade are proposed as mainly consequent to the deactivation of prism c glide systems as temperature decreases fabrics indicative of the appearance of the c burgers vector might be diagnostic for deformation in regimes of comparatively high temperatures or low strainrates or for deformation under conditions such that there is a relatively high kinetically effective hydroxyl content in the quartz structure

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

transposed high pressure granulite fabrics cabo ortegal nw spain implications on the scales of deformation localization abstract high pressure granulite facies rocks of the bacariza formation cabo ortegal nw spain were syn metamorphically deformed at the contacts with the bounding units peridotite and eclogite massifs this enabled the formation of meter thick spectacular shear zones with reworked and transposed foliations and lineations the texturally stable mineral assemblage of the new fabrics records an intense ductile deformation of the mineral aggregate at temperatures of 700 800 c associated with amalgamation of eclogite high pressure granulitic rocks and ultramafic sheets in deep portions of a subduction channel the lattice preferred orientation of the main constituent minerals garnet augite amphibole plagioclase quartz and biotite discloses the active deformation mechanisms at the scale of the mineral grains and the relationships with the deformation at larger scales overprinting relationships of the metamorphic assemblages demonstrates that partitioning and deformation localization occurred at different scales under similar high grade conditions complete macroscopic transposition in the shear zones was complementary to meso and microscopic partitioning of deformation intensity and mechanisms between different lithological layers and mineral species

preferred orientation and anisotropy of seismic and magnetic properties in gabbronorites from the bushveld layered intrusion abstract the preferred orientation of plagioclase and pyroxene in gabbronorites from the bushveld layered igneous intrusion south africa was investigated using electron backscatter diffraction in two localities 200 km apart belfast and rustenburg strong preferred orientation was observed with the principal fabric features aligned in the subhorizontal foliation plane defined by 010 planes of plagioclase and 100 planes of pyroxene the pattern appears to result from the orientation of tabular crystals during the dynamic intrusion events that formed the layered structure primary magmatic oxides are also influenced by the preferred orientation of plagioclase generating an oblate magnetic susceptibility fabric parallel to the foliation plane plagioclase preferred orientation gives rise to other directionally dependent geophysical properties including a large seismic anisotropy with p waves traveling 4 8 faster in the vertical than in the horizontal direction

numerical modelling of lithosphere asthenosphere interaction in a subduction zone we developed a new 2 d numerical approach to study solid fluid coupling applied to subduction zones the lithosphere is characterised by an elastic or elastoplastic behaviour and the asthenosphere by a homogeneous isoviscous fluid the temperature effects are ignored and viscosity and density are constant in time the solid and the fluid problem are discretised by the finite elements method fem the same solid code used in hassani et al hassani r jongmans d chery j study of plate deformation and stress in subduction processes using two dimensional numerical models j geophys res 1997 102 17951 17965 has been used to compute the solution of the solid problem the stokes problem is solved by a direct solver with a stabilisation procedure we used a very simple staggered coupling method where the fluid domain is regularly re meshing we observed numerical instabilities when the time step is not sufficiently small especially when strong coupling between the solid and the fluid occurs we have tested different configurations where the lithosphere is elastic or elastoplastic and show how the slab geometry the topography and the stress regime in the plates are affected by the viscous resistance of the mantle we observed that the asthenosphere viscosity is a fundamental parameter in the subduction process for subduction with an extensional regime in the upper plate we observe a linear decrease of the extensional stress as a function of the asthenospheric viscosity

flexural rigidity of fennoscandia inferred from the postglacial uplift the earth s response to glacial loading unloading offers exceptional promise for the study of the physical properties of the lithosphere in particular tilting of paleoshorelines is very sensitive to the lithosphere rigidity to determine the flexural rigidity the isostatic response to deglaciation in fennoscandia is modeled using an earth model with a layered mantle viscosity overlain by an elastic lithosphere the flexural rigidity and asthenosphere viscosity is allowed to vary to get a match between theoretical and observed present rate of uplift and tilting of paleoshorelines five different ice thickness models are used for a relatively thin ice 2500 m in central areas the resulting flexural rigidity is more or less uniform over fennoscandia with a value of 1023 n m elastic thickness te 20 km this is regarded as minimum for the flexural rigidity of central fennoscandia the pattern of the present rate of uplift and the tilts of the paleoshorelines of the area also sets an upper bound of the flexural rigidity 2 5 1025 n m te 110 km in more central areas of fennoscandia the flexural rigidity at the western coast of norway does not seem to exceed 1023 n m te 50 km the most likely glacier model gives a flexural rigidity of 1023 n m te 20 km at the norwegian coast increasing to above 1024 n m te 50 km in central parts of fennoscandia

albite deformation within a basal ophiolite shear zone abstract albite mylonites at the base of the white hills peridotite ophiolite fragment at st anthony newfoundland formed during obduction at lower crustal p t conditions optical microstructures preserve a history of intracrystalline distortion recovery and cyclic dynamic recrystallization associated with hot working the crystallographic fabric is consistent with shear of the polycrystalline aggregate dominated by glide on 010 planes as is also indicated by tem observations slip systems implied by trace analysis and invisibility experiments using tem are b 101 010 b 001 010 and b 1 2 112 201 dissociated dislocations have separations ranging from 8 3 15 nm these separations are notably smaller than in more calcic plagioclases and indicate a relatively higher ductility for albite in the case of cross slip controlled creep the observed dislocation densities relate to a high stress strain rate pulse imposed on the dominant high temperature creep textures

natural deformation and recrystallization of some intermediate plagioclase feldspars abstract intermediate an25 an48 plagioclase feldspars from the nordre stromfjord shear zone central west greenland have undergone extensive ductile deformation by intracrystalline slip and by twinning the plagioclases were deformed under low granulite facies conditions optical substructures include undulatory extinction deformation bands subgrains and deformational twins mainly on the albite law dynamic recrystallization occurred primarily by subgrain rotation with bulge nucleation migration of a high angle boundary as a secondary mechanism in some cases recrystallization is accompanied by a small decrease in the an content which is thought to indicate prograde metamorphic conditions the plagioclases exhibit a good crystallographic preferred orientation which is compatible with 010 001 being the principal slip system quartz has in the same rocks undergone more plastic deformation and more extensive recovery than plagioclase this is at least in part due to the fact that most dislocations in plagioclase are dissociated

zabargad st john s island an uplifted fragment of sub red sea lithosphere zabargad is a small island located in the red sea about 50 km w of its axis between 23o and 24on the island is not volcanic but probably represents an uplifted block of upper mantle and crustal rocks and yields information on the nature of the underlying lithosphere peridotites are the main rock type consisting of exceptionally fresh spinel lherzolites which equilibrated last at a depth 30 km in the mantle other ultramafic facies some representing rocks which mixed with a basaltic melt fraction and some which underwent metasomatic exchange at depth are also present the peridotite bodies are in tectonic contact with a metamorphic formation consisting of gneissic and amphibolitic rocks similar to those outcropping in the pan african metamorphic basement of eastern egypt peridotites and metamorphic rocks are both crossed by several generations of basaltic dykes the sedimentary zabargad formation of probable cretaceous or palaeocene age consists of alternations of silicified limestones sandstones black shales and phosphoritic lenses and is overlain by evaporites probably of miocene age and by reef limestone units one probably of early pleistocene age the other probably deposited during the last interglacial various formations are affected by localized metasomatic mineralizations recrystallized large gem quality olivine cancrinite scapolite garnierite the uplift of upper mantle ultramafic bodies with fragments of crust which created the island might have occurred in connection with the early stages of development of the rift which preceded the formation of the red sea an alternative explanation that the zabargad peridotites are remnants of a pan african palaeozoic ophiolitic complex is possible but less likely

c slip in quartz from subsolidus deformed granite abstract the slip behaviour of quartz from a granite and granitic veins during subsolidus high temperature conditions 700 800 c of deformation is studied by lattice preferred orientation lpo and microstructural techniques lpos with a point maximum of c axes parallel to the shear direction which is close to the stretching lineation are presented and interpreted with the aid of light and electron microscopy in terms of c as the dominant slip direction during plastic deformation microscopy of such samples reveals the presence of 0001 tilt boundaries and free 0001 burgers vector dislocations in edge orientations a change in lpo in a decreasing temperature environment from c axes parallel to the shear direction to c axes normal to the shear direction documents a fabric transition from the rare c slip to the more common a slip fabric it is concluded that conditions of high temperature and partial pressure of water are required to initiate dominant c slip during deformation at geological strain rates

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

density and visco elasticity of natrosol 250 hh solutions determining their suitability for experimental tectonics abstract analogue models often require that materials with specific physical properties be engineered to satisfy scaling conditions to achieve this goal we investigate the rheology of aqueous solutions of natrosol 250 hh a rheology modifier employed in various industries to thicken viscous solutions we report the rheological properties as functions of the concentration and temperature and discuss the advantages and limitations of these materials in view of their use in analogue modelling experiments the solutions are linear visco elastic for low stresses or strain rates becoming shear thinning for larger stresses for the typically slow analogue experiments of tectonics the solutions can be considered linear visco elastic with a maxwell relaxation time much smaller than the characteristic observation time this simplification is even more appropriate when the solutions are employed at temperatures higher than 20 c since the solutions then display a behaviour that is more viscous less elastic at the same shear rate while the newtonian viscosity reduces and the shear rate limit between newtonian and shear thinning behaviours increases the newtonian viscosity is shown to increase non linearly with concentration and decrease non linearly with temperature with concentrations between 0 and 3 and temperature between 20 and 40 c the viscosity varied between 10 1 and 4000 pa s while the density remained close to the density of water natrosol 250 hh thus offers the possibility to control the viscosity of a solution without significantly affecting the density thereby facilitating the design and setup of analogue experiments

deformation and recrystallization of plagioclase along a temperature gradient an example from the bergell tonalite abstract syn to post mylonitic tilting of the bergell tonalite allows investigation of the deformation and recrystallization of plagioclase grains along a temperature gradient from the lower to the upper amphibolite facies at the lowest temperatures recrystallization occurs by nucleation and growth of new grains having a different composition from the old grains in contrast at the highest temperatures recrystallization occurs by subgrain rotation associated with grain boundary migration the temperature increase is inferred to induce this transition in two ways first at higher temperatures the anorthite content of the new grains is higher and thus the comositional difference between old and new grains is lower hence the chemical driving potential is small compared with recrystallization at lower temperatures second higher temperatures facilitate climb of dislocations and thus subgrain formation as well as grain boundary mobility the temperature increase is also associated with a transition in the dominant deformation mechanism intracrystalline plasticity dominates deformation at lower temperatures whereas at higher temperatures deformation occurs primarily by diffusion accommodated grain boundary sliding as indicated by the weakening of the indicatrix preferred orientation and the formation of a mixed plagioclase biotite matrix with increasing deformation therefore the strength of the mid to lower crust may be overestimated by dislocation creep flow laws for plagioclase lower crust rheology for polyphase rocks is better approximated by constitutive relationships involving diffusion accommodated grain boundary sliding

reply to comment by w fjeldskaar what about the asthenosphere viscosity sea level change glacial rebound and mantle viscosity for northern europe viscosity profiles that best match the observational database include a low viscosity channel in the upper mantle fjeldskaar neither point to a well developed low viscosity channel in the 2000 in our paper we clearly noted that we examined only a upper mantle and the second model compares well with the part of the possible earth model space and that we restricted results from the sea level analysis only ourselves to a three mantle layer model comprising a lithothe second reason for not pursuing the more detailed sphere an upper mantle extending from the base of the solutions for the scandinavian region is that a major limitation lithosphere down to the 670 km seismic discontinuity and a in the modelling is the inadequate knowledge of the temporal lower mantle a broad range of parameters defining these three and spatial distribution of the ice cover over the region and layers was explored and no a priori values were attached to based on tests with different ice models we concluded that this any one layer this is important in that considerable trade off was more important than differences in predictions between between parameters can result we were careful to note that the threeand five layer mantle models unless some modification analysis yielded only first order results and that the resulting of the ice sheet was permitted in the inversion this would earth model parameters are effective values that while they result in a very considerable dependence of earth model results appear to give an adequate description of the rebound evidence on the adopted ice model for example compare figs 16 a the relative sea level change and shoreline migration may and b and fig 33 b in lsj we concluded that further not be a true reflection of the actual physical layering within the improvements in the knowledge of the spatial and temporal earth we also noted that further work is required to examine distribution of the ice are required before it becomes worthwhile whether a higher degree of mantle layering is appropriate to explore the earth structure in greater detail subsequent e g p 135 inversions in lsj therefore placed greater emphasis on the ice the reason we did not conduct a more exhaustive search of model than on the detailed earth structure although care was the model space was two fold the first was that our earlier taken at all times to ensure that an adequate separation of the experience with the british rebound modelling lambeck et al earthand ice model parameters occurred 1996 led to the conclusion that five layer mantle models three inversions of independent sea level data for ice sheet including the low viscosity asthenosphere option yielded only parameters and three layered mantle parameters led to a marginal improvement in the overall comparison of preconsistent conclusions about likely ice thicknesses during latedictions of sea level change with observations fig 1 illustrates glacial times these data sets are the lateand postglacial the two sets of effective earth model parameters that were geological evidence for sea level change across the region found to be optimal for the threeand five layer models the lsj the tide gauge data for the region lambeck et al five layer model is physically more intuitive but in the british 1998b and the timing and elevation data of various baltic lake isles case it does not lead to a significant improvement in the stages lambeck 1999 in all cases thick quasi parabolic ice ability to predict lateand postglacial sea levels and shoreline models in which ice thickness increases rapidly with distance locations compared to the three layer model in this particular from the ice margin yield predictions that are inconsistent case the observational database has only limited resolving with the sea level and shoreline observations for the southern power for isolating the higher resolution structure in the mantle and southeastern scandinavia and baltic regions see also and for all predictive purposes of sea level change and shoretushingham peltier 1991 and lambeck et al 1990 with line evolution three layer or five layer models are essentially the exception of some of the norwegian coastal zones the

tests of glacial rebound models for fennoscandinavia based on instrumented sea and lake level records summary evidence for changing sea levels in northwestern europe related to glacial rebound is found in both the geological record of the past millennia and in the instrumental records of the past two centuries the latter records are of two types records of sealevel change primarily from the baltic and the gulfs of finland and bothnia and records of the tilting of some of the larger lakes in both finland and sweden the sealevel records are particularly important because of their long duration and high quality their large number and good spatial distribution and the spatially coherent background noise the two instrumental data types are complementary and provide constraints on the upper mantle rheology and on the distribution of ice during the late glacial stage comparisons of the observed rates of change of the water levels with models for glacial rebound yield earth models with a lithospheric thickness of 80 100 km and an uppermantle viscosity of 4 5 1020 pa s evective parameters that are consistent with those obtained from the analysis of the geological evidence for the same region the mareograph results support ice sheet models in which the late weichselian ice thickness over the eastern and southern parts of fennoscandia is relatively thinner than that for the western region also consistent with the interpretation of the geological evidence for sea level change in addition the instrumental records provide constraints on the eustatic sea level change for about the past 100 years a satisfactory separation of the earth rheology parameters from this rate of change can be achieved by estimating the latter only from those records for which the predicted isostatic evects are small a check on these results is possible by using the lake level records to establish constraints on the earth model parameters and the sea level records to constrain also the eustatic change all approaches lead to an average eustatic sea level rise for the past century of about 1 1 0 2 mm yr 1

viscosity and thickness of the asthenosphere detected from the fennoscandian uplift the earth s response to the deglaciation in fennoscandia is modelled using a layered viscous model with an elastic lithosphere the modelled tilting of palaeoshorelines at particular locations peripheral to the former ice load and the pattern of the present rate of uplift consistent with the observations strongly suggest a low viscosity asthenosphere earth models without a low viscosity asthenosphere give significant mismatch to the present rate of uplift and are unable to mimic the observed tilting of palaeoshorelines models with a viscosity of the lower mantle of 1 0 2 0 1021 pa s and a viscosity of the upper mantle of 0 7 1 0 1021 pa s overlain by low viscosity asthenosphere with a thickness ranging from 25 to 100 km fit equally well with the observations it is suggested strongly that the asthenosphere has a thickness of less than 150 km and a viscosity of less than 7 0 1019 pa s

high temperature plastic deformation of quartz plagioclase multilayers by layer normal compression it is a common practice in rheological modeling to take the extrapolated compressive flow strengths of quartzite and plagioclase rock as the bulk flow strengths of the upper and lower continental crusts respectively such a practice implies that the bulk flow strength of polyphase rocks is identical to that of the pure weak phase to test this assumption we performed deformation experiments on synthetic layered and particulate quartz anorthite 50 50 composites and aggregates of the end members in a paterson gas medium apparatus at a confining pressure of 300 mpa a constant strain rate of 10 5 s 1 and temperatures from 1273 to 1473 k under these conditions anorthite deforms by recrystallization accommodated dislocation creep while quartz is semibrittle electron backscatter diffraction measurements show a strong lattice preferred orientation lpo of anorthite developed in deformed specimens and the lpo pattern is interpreted as a result of dislocation slip on the 010 100 system our experimental results show that the bulk flow strength of layered composites increases with decreasing thickness of the layers thinlayered composites are significantly stronger than pure anorthite aggregates and a homogeneous quartz anorthite mixture i e particulate composites with the same modal composition but weaker than pure quartz aggregates our experimental results together with the theoretical overview presented previously in the materials science literature demonstrate that 1 compressive flow strength of a polyphase rock cannot be represented by that of the weak phase and 2 thin layered rocks compressed normal to the layering are rheologically stronger than homogeneous isotropic mixtures under the same deformation conditions thus weak phase based rheology will lead to an underestimation of the bulk flow strength of the continental crust in which polyphase rocks dominate

fluid circulation related to deformation in the zabargad gneisses red sea rift abstract a fluid inclusion study in relation to textures has been performed on pan african granulite facies gneisses from zabargad island red sea rift egypt the gneiss samples were collected at a maximum distance of 60 m above the contact with mantle peridotites with which they are co structured by a high t plastic deformation during an isothermal decompression starting from conditions estimated at 35 km depth and 850 c base of the pan african crust a first metasomatism of the crustal section during decompression is represented by co2 fluid inclusions trapped at minimum conditions of 15 km depth and 800 c the co2 fluid inclusion planes are controlled in orientation by the high t plastic flow structures but have been subsequently deformed during medium t plastic deformation in quartz the second metasomatism is represented by solid rich aqueous fluid inclusions which were trapped at minimum p t conditions of 3 7 km depth and 450 c corresponding to a very high thermal gradient they trace the introduction either of seawater or of fluids resulting from leaching of the middle miocene evaporites within the gneisses during their residence at a shallow depth below the seafloor two models are proposed in the literature in order to reconstruct the geodynamic history of zabargad island one brueckner h k elhaddad m a hamelin b hemming s kroner a reisberg l seyler m 1995 a pan african origin and uplift for the gneisses and peridotites of zabargad island red sea a nd sr pb and os isotope study j geophys res 100 22283 22297 suggests that most of the decompression p t path is pan african in age and only the aqueous metasomatism is related to the rifting history of the red sea the second nicolas a boudier f montigny r 1987 structure of zabargad island and early rifting of the red sea j geophys res 92 461 474 boudier f nicolas a ji s kienast j r mevel c 1988 the gneiss of zabargad island deep crust of a rift tectonophysics 150 209 227 supposes the existence of an asthenospheric diapir intrusive through the deep pan african crust leading to a continuous deformation event during early rifting of the red sea although it is impossible to date trapping of fluid inclusions and to know with certainty the origin of the fluids our preference is for the second model but slightly modified the high t decompression path traced by co2 fluid inclusions relayed by solid rich aqueous inclusions traces a continuous process of crustal thinning such a continuous path would assign to a single event the thinning and uplift of the deep crust and uppermost lithosphere to shallow basin bottom during the red sea early rifting in such an interpretative model the northern and central peridotite bodies are representative of the uppermost continental lithosphere and only the southern body could belong to an ascending asthenospheric diapir centred on the southeast of zabargad island

st john s island red sea a new geophysical model and its implications for the emplacement of ultramafic rocks in fracture zones and at continental margins abstract new gravity and magnetic data from the northern red sea reveal the extent of the large gravity anomaly 164 mgal and the presence of significant magnetic anomalies over st john s island spectral transformation and three dimensional potential field modelling delineate the surface configuration and vertical extent of the causative body and the enormous density contrast required 1 2 g cm 3 suggests that it is composed of unserpentinised peridotite density 3 4 g cm 3 to a depth of at least 8 km st john s island is uniquely located not only at a passive continental margin but also within a fracture zone at the transition from plate separation by seafloor spreading to extension by lithospheric attenuation this precludes several suggested mechanisms for the emplacement of ultramafic bodies in fracture zones thermal contraction serpentinite diapirism and changes in the poles of rotation do not seem possible mechanisms in this tectonic environment and the emplacement is most probably related to the spreading readjustment necessary to create a continent to continent fracture zone a post mesozoic age of emplacement associated with the onset of continental rifting and the rejuvenation of a pre existing continental fracture seems most probable

sea level change glacial rebound and mantle viscosity fornorthern europe northwestern europe remains a key region for testing models of glacial isostasy because of the good geological record of crustal response to the glacial unloading since the time of the last glacial maximum models for this rebound and associated sea level change require a detailed knowledge of the ice sheet geometry including the ice thickness through time existing ice sheet reconstructions are strongly model dependent and inversions of sea level data for the mantle response may be a function of the model assumptions thus inverse solutions for the sea level data are sought that include both ice and earth model parameters as unknowns sea level data from fennoscandia the north sea the british isles and the atlantic and english channel coasts have been evaluated and incorporated into the solutions the starting ice sheet for fennoscandia is based on a reconstruction of a model by denton hughes 1981 that is characterized by quasi parabolic cross sections and symmetry about the load centre both global northwestern europe as a whole and regional subsets of the data solutions have been made for earth model parameters and ice height scaling parameters the key results are as follows 1 the response of the upper mantle to the changing ice and water loads is spatially relatively homogenous across scandinavia the north sea and the british isles 2 this response can be adequately modelled by an effective elastic lithosphere of thickness 65 85 km and by an effective upper mantle viscosity from the base of the lithosphere to the 670 km depth seismic discontinuity of about 3 4 1020 pa s the effective lower mantle viscosity is at least an order of magnitude greater 3 the ice thickness over scandinavia at the time of maximum glaciation was only about 2000 m much less than the 3400 m assumed in the denton hughes model 4 the ice profiles are asymmetric about the centre of the ice sheet with those over the western part being consistent with quasi parabolic functions whereas the ice heights over the eastern and southern regions increase much more slowly with distance inwards from the ice margin

seismic reflectivity of a finely layered granulite facies ductile shear zone in the southern grenville province quebec abstract lithoprobe reflection profile 54 across the granulite facies morin shear zone msz in the grenville tectonic province quebec canada reveals a west dipping zone of strong reflections the origin of these reflections has been investigated by detailed field mapping laboratory measurements of p wave velocities and densities of representative rock samples from the main lithological units and forward synthetic modeling petrofabric and microstructural analyses were performed in order to interpret the measured seismic properties synthetic seismograms generated from the velocity and density data demonstrate that reflections originate from lithologic thin layers and thin layer clusters likely resulted from intensive tectonic transposition and metamorphic differentiation lattice preferred orientation lpo induced anisotropy of granulite facies mylonites except for those containing abundant sillimanite biotite or amphibole is very low and hence unlikely to be the primary cause for reflections confining pressure does not significantly affect the reflectivity of the shear zone because the reflection coefficients are not sensitive to pressure up to at least 600 mpa since composite reflections of thin layer clusters can cause high reflectivity the strong reflectivity from some deep crustal sections with low refraction velocities may be interpreted to be due to the presence of thin layer mafic clusters within dominantly felsic rocks

comparison of geoelectric and seismic reflection models of the zambezi valley basins northern zimbabwe the mana pools and lower zambezi karoo sedimentary basins lie within the zambezi mobile belt in northern zimbabwe the subsurface apparent resistivities measured at both locations are extremely low magnetotelluric mt data along a profile across part of the lower zambezi basin have been inverted using rapid relaxation inversion smith booker 1991 to find the minimum structure needed to fit the data and compare with an earlier forward model the resistivity models of both the mana pools and the lower zambezi basins are then compared with their structure revealed from seismic reflection data the resistivity structure of the mana pools basin is well modelled as a series of different resistivity layers whose boundaries are defined by the seismic data however the resistivity structure of the lower zambezi basin cannot be matched easily to the seismic structure additional structure with no seismic expression is required there is a conductive feature in the two basins in the upper karoo sandstone layer that extends below the seismic basement beneath the lower zambezi basin this indicates that the conductors may represent different types of features in the two basins consistent with their proposed different tectonic origins a resistive unit is present within the sediments in the lower zambezi basin that may represent intercalated basalt dykes giving an anisotropic mt response it has been suggested that there might be similar thin basalt layers within the sediments of the mana pools basin but these could not be resolved by mt methods the low resistivity of the basement particularly beneath the lower zambezi basin is remarkable and may result from a high degree of either chemical or tectonic alteration to the underlying rocks due to metamorphic processes and tectonic disruption during rift formation the presence of the lower zambezi basin conductor at depths greater than the seismic basement is consistent with observations to the west in the adjacent damara mobile belt where similar conductors have been seen in regions with no sedimentary cover

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

the deformation of plagioclase investigated using electron backscatter diffraction crystallographic preferred orientation data abstract extensive plagioclase grain orientation datasets have been collected using the rapid and accurate electron backscatter diffraction sem technique the crystallographic preferred orientation cpo of a porphyroclastic rich plagioclase layer and an equigranular plagioclase layer in an amphibolite facies shear zone from harris scotland were investigated in order to study their cpos and deformation mechanisms the layers have random cpos together with other microstructures and misorientation distributions these suggest dynamic recrystallisation followed by dominant grain boundary diffusion creep this is surprising given the coarse 100 250 m recrystallised grain size of the layers plagioclase within tectonites of these grain sizes in the amphibolite facies are generally inferred to deform by dislocation creep from microstructures and observed strong cpos deformation mechanism maps show that this style of deformation is possible at very slow strain rates

dislocation creep of fine grained recrystallized plagioclase under low temperature conditions abstract lattice preferred orientation lpo and dislocation substructure of fine grained plagioclase were investigated in ultramylonite from a shear zone along the hatagawa fault zone in northeast japan in the shear zone the orthoclase mole fraction of the recrystallized alkali feldspar is not less than 0 94 this composition suggests that deformation occurred below 500 c selected area diffraction by transmission electron microscopy tem for an electron transparent foil and an x ray texture goniometer xtg were used to determine the lpo of plagioclase burgers vectors of dislocations were determined by invisibility criteria the 100 010 and 001 pole figures reveal strong preferred orientations the xtg results are consistent with the lpo determined by tem the tem observations confirmed the presence of at least six burgers vectors these burgers vectors or the net slip directions of multiple systems are almost parallel to the lineation thus the present study indicates ductility by dislocation creep for fine grained plagioclase in ultramylonite deformed at relatively low temperature the lpo pattern and the burgers vectors determined in this study are different from those of deformation at much higher temperature thus the present study also suggests a switching of the slip systems for plagioclase depending on the temperature

plagioclase preferred orientation in layered mylonites evaluation of flow laws for the lower crust thesis s m joint program in marine geology and geophysics massachusetts institute of technology dept of earth atmospheric and planetary sciences and the woods hole oceanographic institution 2008

upper mantle beneath a young oceanic rift peridotites from the island of zabargad red sea information on the nature of the upper mantle in a young rift zone with lithosphere transitional from continental to oceanic was obtained from the island of zabargad probably an uplifted fragment of red sea lithosphere the island exposes exceptionally fresh mantle derived peridotite spinel iherzolite bodies the mineralogy and major element rare earth element and mineral chemistry of the spinel iherzolites suggest that they equilibrated last at depth 30 km in the mantle and that they have oceanic rather than continental affinity it is unlikely that the zabargad ultramafics are part of a late precambrian early paleozoic ophiolite as found elsewhere in circum red sea regions rather they were uplifted probably in connection with the development of the red sea rift the zabargad peridotites are in tectonic contact with continental metamorphic basement rocks it appears therefore that regions of an embryonic rift such as the red sea can be covered with continental type crust but be underlain by an upper mantle having already oceanic affinities

on the measurement of plagioclase lattice preferred orientations abstract there are two major difficulties in measuring plagioclase lattice preferred orientations lpos by u stage the first which is caused by dependence of the lpo determination on anorthite content cannot be overcome using the u stage method at present the second difficulty which is caused by the dependence on orientation of thin sections can be overcome by measuring all twinned grains in any two orthogonal sections compared with previous methods published in the literature the present method using two rather than three orthogonal sections is more statistically meaningful and time saving in practice this method was used for measuring lpos of plagioclase an 50 from a typical anorthosite mylonite deformed under the granulite facies conditions in the morin ductile shear zone within the canadian grenville tectonic province

the gneiss of zabargad island deep crust of a rift abstract the gneisses of the zabargad island are in contact with lherzolites that were emplaced by mantle diapirism during the early rifting of the red sea the gneisses and the peridotites at their contact develop similar structures kinematics and common granulite metamorphism away from the contact the shear strain and associated recrystallizations decrease rapidly thus the zabargad gneisses are metamorphosed and deformed by the mantle diapiric intrusion this is confirmed by preliminary isotopic dates the gneisses have a tripartite polygenic composition of felsic granulites metagabbros and amphibolites the metagabbros and amphibolites are metamorphosed only by the mantle diapir intrusion which also induces a partial anatexis of the gneisses the felsic granulites record a proceeding high p granulite facies metamorphism they are assumed to represent screens of ancient continental crust

microstructures in quartz and feldspars of the bomdila gneiss from western arunachal himalaya northeast india implications for the geotectonic evolution of the bomdila mylonitic zone abstract microstructures of the bomdila gneiss bg that intruded the metapelites of the lesser himalayan crystallines lhc in the western arunachal himalaya were studied to understand the deformation mechanisms and temperatures during deformation the lhc is delimited at the base by the bomdila thrust bt and the main central thrust mct at the top the microstructural features of quartz and feldspar in the bg indicate that the rocks underwent an early stage of deep level ductile deformation superimposed by shallow level brittle deformation during a later stage of deformation grain boundary migration and subgrain rotation recrystallization are the main deformation mechanisms for recrystallization of quartz and feldspars pressure solution and solution transfer were also important at the base of the bomdila mylonitic zone both the feldspars plagioclase and k feldspar and quartz show dynamic recrystallization with growth of myrmekite at the upper level while the development of quartz subgrains and fractured in feldspars are dominate at the lower level the deformation microstructures of bg suggest that the bomdila mylonitic zone shows an inverted thermal profile towards the mct i e increase in deformation temperature from 400 c at the lower zone to 500 600 c at the upper zone of the gneiss the microstructural variations and decreasing temperatures of deformation from west to east indicate that the lhc initially buried below higher himalayan crystallines hhc along the mct have been exhumed along the bt shear criteria indicates top to the se displacement in agreement with the kinematics of the lhc along the bt producing of an inverted thermal profile in the study area may be related to imbrication and post metamorphic shearing

matrix mosaics brittle deformation and elongate porphyroclasts granulite facies microstructures in the striding athabasca mylonite zone western canada abstract the late archean striding athabasca mylonite zone western canadian shield is a 400 km long linked system of granulite facies mylonite belts developed in the continental crust the granulite facies mylonites 800 1000 c at 0 8 1 1 gpa developed in subsets of the anhydrous mineral assemblage clinopyroxene orthopyroxene garnet plagioclase quartz hornblende comparison of the microstructures of the striding athabasca mylonites with published descriptions of natural and experimental deformation of geological and analogue materials at elevated homologous temperatures provides some insight into the role played by different processes in the development of the striding athabasca mylonites in addition to dislocation creep and dynamic recrystallisation extensive mass transfer occurred contemporaneously with brittle fracture and cataclasis during granulite facies metamorphism the microstructure and extreme phase dispersal in anhydrous polymineralic matrix mosaics is indicative of efficient grain scale and aggregate scale diffusion grain boundary mobility and mass transfer at relatively slow strain rates despite their annealed appearance the granoblastic matrix mosaics developed synkinematically deformation induced dilation may enhance metamorphic reactions where products are more voluminous than reactants in the porphyroclast population highly elongate 20 1 monocrystalline orthopyroxenes appear to be fragments of dismembered kinked parent grains rather than stretched porphyroclasts granulite facies mylonites should not be treated as direct analogues of greenschist facies mylonites in particular it is essential to evaluate the potential positive feedback between structural and metamorphic processes in highly strained high temperature shear zone rocks

euxinia linked to the cambrian drumian carbon isotope excursion dice in australia geochemical and chemostratigraphic evidence abstract the cambrian drumian carbon isotope excursion dice event is a negative carbon isotope 13 c excursion it is associated with the first appearance datum fad of the agnostoid arthropod ptychagnostus atavus whose first appearance in the global boundary stratotype section and point gssp defines the base of the cambrian series 3 drumian stage although this isotopic event has been clearly identified in the great basin usa it has not been consistently reported in other areas of the world here we report a study integrating isotopic and geochemical data from middle cambrian series 3 sections from the northeastern georgina basin australia to investigate the occurrence of the dice event in australia and the paleoenvironmental conditions associated with this event the similarity between the isotopic excursions observed in the georgina basin 2 to 3 2 13 c excursion and that observed in the great basin 2 to 3 6 13 c excursion highlighted the presence of the dice in australia and emphasized the potential use of this event for refining global isotopic correlations the multi proxy correlation used in this study also showed a pulse of euxinia in association with the dice suggesting strongly euxinic conditions near the base of the drumian stage

development of shape and lattice preferred orientations application to the seismic anisotropy of the lower crust abstract we review the physical basis of the development of fabrics in plastic and viscous flow and illustrate the typical fabrics formed by these processes in the main rock forming silicates of the lower crust feldspar quartz pyroxene and amphibole the orientation process in plastic deformation where a single slip is dominant is recalled and the role of the constraint of neighbouring grains is emphasized the fabric development of anisometric crystals in viscous flow is discussed as a function of the main controlling parameters shear strain aspect ratio and interference between crystals the same sense of fabric asymmetry is introduced by plastic and viscous flow between the flow plane and the shape preferred orientation and hence coherent kinematic analysis can be undertaken in both modes of flow in order to assess the role of such fabrics in the seismic laminations of the lower continental crust we have calculated the seismic p wave properties of typical fabrics for hypothetical monomineralic and polymineralic rocks the calculations show that the strongest anisotropies develop in monomineralic rock with values between 5 and 16 compared with 5 and 8 for typical rock compositions the strongest anisotropies for layered monomineralic rocks generated by fabrics is only 6 compared to the 14 suggested by model studies of the observed seismic laminations we suggest that other effects such as compositional layering and or constructive interference of seismic waves are responsible for augmenting the apparent anisotropy

feldspar fabrics in a greenschist facies albite rich mylonite from electron backscatter diffraction abstract the current database of plagioclase cpos in naturally deformed rocks is small and is not supported by cpos from experimentally deformed rocks or by numerical modelling so that the current understanding of plagioclase cpos is limited electron backscatter diffraction ebsd has the potential of measuring the full crystallographic orientation of small domains of any crystalline material including plagioclase we generate and test a file of the relative electron diffraction intensities for lattice planes in the albite structure this enables rapid and accurate individual orientation measurements of albite using ebsd ebsd is used to collect 360 data points from 7 areas across a 2 cm by 1 mm pure albite band in a greenschist facies albite mylonite from a gabbro protolith the cpos are unlike any published the a b and c axes of albite are not symmetrically distributed with respect to the kinematic axes of deformation and do not possess an internal monoclinic or orthorhombic symmetry there is some microstructural evidence for the operation of dislocation creep although this interpretation conflicts with the generally held view that crystal plastic deformation does not occur in plagioclase below about 500 c an alternative mechanism is one of granular flow accommodated by solution transfer however this mechanism would be expected to give a random cpo for either mechanism we need to explain the triclinic nature of the cpo it is possible that the cpo represents the modification by deformation of a pre existing cpo probably defined by a small number of original plagioclase grains

petrofabric of forsterite marbles and related rocks from a low pressure metamorphic terrain almaden de la plata massif ossa morena zone sw spain and its kinematic interpretation abstract a fabric study of forsterite marble and calc silicate rock paragenetic minerals is presented for samples collected in the almaden de la plata massif of sw spain that experienced variscan low pressure metamorphism related to the tectonic evolution of the suture between the ossa morena and south portuguese zones of the iberian massif two tectonically stacked metamorphic units contain marble metacarbonate and variably impure dolomitic rocks that record contrasting thermal conditions of the upper greenschist structuraly upper unit and the upper amphibolite granulite facies structuraly lower unit different reactional paths lead to formation of forsterite marbles in both units dolomite and calcite petrofabrics disclose their contrasting rheological behaviour during syn metamorphic ductile deformation under temperatures between 500 and 700 c dolomite formed harder porphyroclast systems and boudinaged bands whereas weaker calcite underwent dynamic recrystallization in both cases deformation was facilitated by activation of various intracrystalline slip systems complemented by other deformation mechanisms the fabrics of other paragenetic metamorphic minerals also denote crystal plasticity and exhibit close geometrical relationships with respect to the macroscopic foliation and mineral stretching lineation including a systematic asymmetry suggestive of non coaxial deformation components foliations and lineations are interpreted as solid state flow planes and directions bearing a kinematic significance in particular regional sinistral shear at mid deep crustal realms coeval with low pressure metamorphism explains the imbrication of metamorphic units and their exhumation concomitant with the overall wrench tectonic framework ruling the tectonic evolution of the ossa morena south portuguese suture between the late devonian 360 ma and the latest carboniferous 300 ma

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

crystallographic preferred orientation in albite samples deformed experimentally by dislocation and solution precipitation creep abstract the crystallographic preferred orientations of a series of experimentally deformed fine grained albite aggregates were measured by synchrotron source x ray diffraction most samples were deformed and extensively recrystallized by low temperature recrystallization accommodated dislocation creep in axial compression as well as simple shear these samples developed weak but distinct crystallographic preferred orientations consistent with intracrystalline slip on 001 the sheared samples have a marked asymmetry of the maxima with respect to the shear zone boundaries one sample was axially compressed by solution precipitation creep it developed a somewhat different but equally strong preferred orientation perhaps reflecting crystallographic anisotropy in rates of dissolution and growth

what about the asthenosphere viscosity comment on sea level change glacial rebound and mantle viscosity for northern europe by k lambeck c smither and p johnston lambeck et al 1998a hereafter lsj present a thorough in my own studies fjeldskaar 1994 1997 i have shown that the best fitting models have a low viscosity asthenosphere study of the sea level change connected to the deglaciation of fennoscandia they establish a comprehensive observational based on fjeldskaar 1994 the thickness and viscosity of the asthenosphere are not precisely determined but are within database on crustal rebound for the whole of northern europe this database is used in a high resolution modelling of the the range shown in fig 1 it is clearly demonstrated that the asthenosphere has a thickness of less than 150 km a thicker rebound based on the observed deglaciation pattern a key result is an estimate of the effective elastic thickness of the asthenosphere shows significant discrepancies from the observed uplift data lithosphere and the viscosity structure of the mantle lsj give a brief overview of previous results on the mantle models similar to the one favoured by lsj were tested in fjeldskaar 1994 model no 14 it was pointed out that models rheology models established by a similar approach lsj s comment to models with a low viscosity asthenosphere and with a lower mantle viscosity of 2 0 1021 pa s or higher without a low viscosity asthenosphere are not viable options the various other previously published viscosity models is that the differences are attributable to a number of factors these reason is that the short wavelength components decay too slowly giving significant deviations in central parts of the uplift dome factors include the fact that the other studies used only a subset of the data and that some of them assumed the ice sheet to be this is clearly demonstrated in the theoretical present uplift pattern calculated with the rheology parameters of lsj given perfectly known a third factor is that none of the studies has examined the entire range of possible earth model parameters in lambeck et al 1998b the predicted present day uplift centre lies further to the west than the observed centre this would this immediately gives the reader the impression that lsj do not have similar shortcomings and that the final resulting not be the case if a low viscosity asthenosphere was used fjeldskaar 1994 fig 6 furthermore the rheology model of earth rheology model is better constrained than the previous studies however this is not the case lsj have ignored models lsj cannot produce the observed shoreline tilts fjeldskaar 1994 this is also demonstrated in lsj figs 28 and 29 with a low viscosity asthenosphere on this basis it is rather peculiar that they 1 criticize others for not having used the in conclusion lsj have not been able to come up with a model for the earth rheology that gives optimum fit to entire range of possible earth models and 2 give the reader the impression that models with a low viscosity asthenosphere the observed rebound because models with a low viscosity asthenosphere have not been considered the lsj rheology is are unrealistic

scales of deformation partitioning during exhumation in a continental subduction channel a petrofabric study of eclogites and gneisses from nw spain malpica tui allochthonous complex

recrystallization and fabric development in plagioclase the microstructures and fabrics of naturally deformed intermediate composition plagioclase feldspars from the amphibolite granulitic gneisses from the lower crust have been studied special interest in the effects of dynamic recrystallization and deformation fabrics has been taken the plagioclase feldspars an40 50 have undergone extensive ductile deformation by intracrystalline slip and twinning they show strong crystallographic preferred orientation which was produced by dominant 010 001 slip dynamic recrystallization is indicated by fine grained new grains neoblasts interspersed with deformed relicts porphyroclasts the neoblasts have formed preferentially along grain boundaries kink band boundaries kbbs intracrystalline shear zones and around solid inclusions such as garnet and pyroxene optical microstructural evidence suggests that dynamic recrystallization occurred primarily by grain boundary migration with subsidiary subgrain rotation which appears to have occurred only at low st

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