strength and anisotropy of foliated rocks with varied mica contents abstract we have shortened 15 schists and gneisses with varying compositions 15 75 mica by volume at angles of 45 and 90 to foliation s to investigate the influence of micas on the strength and anisotropy of foliated rocks at the conditions tested t 25 c p c 200 mpa and e 10 5 s 1 compressive strengths vary by a factor 4 in the 45 and 90 orientations and individual rock types exhibit directional responses ranging from isotropic to strongly anisotropic trends of decreasing strength decreasing anisotropy and increasing ductility are observed with increasing mica content strains in all samples were localized within inclined shear zones accommodated by dislocation slip and grain scale microkinking in micas and microcracking in all other silicates stress strain response and grain scale deformation microstructures both indicate that mechanical behavior is strongly influenced by the concentration and spatial arrangement of micas shear zone formation is associated either with discrete brittle fracture transitional strain softening or steady strength ductile shear in both brittle and transitional samples extensive microcracking of strong quartzo feldspathic bridges occurs and stress drop magnitudes decrease with decreasing spacing and increasing overlap of adjacent critically oriented micas steady strength ductile shear zones develop in samples containing domains of interconnected micas and isotropic mechanical response is observed in those in which micas are contiguous in nearly all directions

the tertiary structural and thermal evolution of the central alps compressional and extensional structures in an orogenic belt the western alpine arc has been created during the cretaceous and the tertiary orogenies the interference patterns of the tertiary structures suggest their formation during continental collision of the european and the adriatic plates with an accompanying anticlockwise rotation of the adriatic indenter extensional structures are mainly related to ductile deformation by simple shear these structures developed at a deep tectonic level in granitic crustal rocks at depths in excess of 10 km in the early palaeogene period of the tertiary orogeny the main tertiary nappe emplacement resulted from a nw thrusting of the austroalpine penninic and helvetic nappes heating of the deep zone of the upper cretaceous and tertiary nappe stack by geothermal heat flow is responsible for the tertiary regional metamorphism reaching amphibolite facies conditions in the lepontine gneiss dome geothermal gradient 25 c km the tertiary thrusting occurred mainly during prograde metamorphic conditions with creation of a penetrative nw se oriented stretching lineation xi finite extension parallel to the direction of simple shear earliest cooling after the culmination of the tertiary metamorphism some 38 ma ago is recorded by the cooling curves of the monte rosa and mischabel nappes to the west and the suretta nappe to the east of the lepontine gneiss dome the onset of dextral transpression with a strong extension parallel to the mountain belt and the oldest s vergent backfolding took place some 35 to 30 ma ago during retrograde amphibolite facies conditions and before the intrusion of the oligocene dikes north of the periadriatic line the main updoming of the lepontine gneiss dome started some 32 30 ma ago with the intrusion of the bergell tonalites and granodiorites concomitant with s vergent backfolding and backthrusting and dextral strike slip movements along the tonale and canavese lines argand s insubric phase subsequently the center of main updoming migrated slowly to the west reaching the simplon region some 20 ma ago this was contemporaneous with the westward migration of the adriatic indenter between 20 ma and the present the western aar massif toce culmination was the center of strong uplift the youngest s vergent backfolds the glishorn anticline and the berisal syncline fold the 12 ma rb sr biotite isochron and are cut by the 11 ma old rhone simplon line the discrete rhone simplon line represents a late retrograde manifestation in the preexisting ductile simplon shear zone this fault zone is still active today the oligocene neogene dextral transpression and extension in the simplon area were concurrent with thrusting to the northwest of the helvetic nappes the prealpes 35 15 ma and with the jura thin skinned thrust 11 3 ma it was also contemporaneous with thrusting to the south of the bergamasc 35 5 ma and milan thrusts 16 5 ma the oligocene neogene dextral transpression in the central alps is of the same age as the thrusting to the west and southwest in the external french part and to the east in the internal italian part of the alpine arc of the western alps

fluid flow and volume loss during mylonitization an origin for phyllonite in an overthrust setting north carolina u s a abstract in the western blue ridge province of the southern appalachians north carolina granite gneisses are converted to phyllitic mylonites phyllonites along the base of the rector branch thrust sheet the mylonites show evidence for fluid infiltration and large volume losses with decreasing distance from the thrust and local shear zones feldspars undergo grain size reduction by cracking followed by chemical breakdown to muscovite sericite and zoisite the following reaction was important during mylonitization 3 k feldspar 2 h muscovite 2 k 6 sio 2 quartz displays evidence for dynamic recrystallization whereas feldspar deforms brittely which together with the secondary mineral assemblages suggest that deformation occurred at temperatures of between 300 c and 500 c in the presence of a fluid the mylonites are enriched by a factor of at least 2 5 in elements such as tio 2 p 2 o 5 zr y v la ce and depleted in sio 2 in comparison to their protolith gneisses the immobile element enrichments are attributed to enrichment in residual phases such as ilmenite zircon apatite and epidote in the mylonites and are interpreted as due to minimum volume losses of 60 mass balance calculations based on major element chemistry indicate the mylonites lost large amounts of sio 2 and smaller amounts of alkalies and were infiltrated by fluid rock weight ratios of between 10 2 and 10 3 phyllonitization as a process involves several deformation mechanisms including cataclasis and solution of feldspar as well as dislocation creep of quartz and micas reaction enhanced softening was apparently important in strain localization

constraining the relative strengths of high grade metamorphic rocks using foliation refraction angles an example from the northern new england appalachians foliation refraction angles are used to estimate effective viscosity contrasts between metapelitic and metapsammitic layers in amphibolite facies metaturbidites in the presidential range of the white mountains in eastern new hampshire an early formed foliation developed during km scale nappe folding consistently displays larger bedding foliation angles in metapelitic units than metapsammitic units suggesting that the metapelitic units had higher effective viscosities during this deformation we collected bedding foliation angle measurements from a combination of outcrops and locally derived boulders to test the hypothesis that foliation refraction angles could be used to estimate the effective viscosity ratio between different rock types at high metamorphic grades our results show that the metapelitic layers were between 2 and 3 times more viscous than the metaspsammitic layers which we attribute to the presence of large up to 15 cm long abundant up to 30 vol effectively rigid andalusite porphyroblasts in the metapelitic layers we present a methodology for using foliation refraction angles to determine the effective viscosity ratios of different rock types address the practical limitations we encountered and suggest this method is an easy way of estimating the strength contrast between different rock types finally we hypothesize that metamorphism resulting in porphyroblast growth can strengthen large crustal volumes during orogenesis

formation and maintenance of shear zones the processes that initiate and maintain prograde ductile shear zones are not well understood we have performed shear experiments 1 to 3 on a fine grained 100 150 m gneiss 59 quartz 28 plagioclase 13 aligned but not interconnected biotite to determine the evolution of deformation mechanisms that produce dramatic strain weakening and localization at conditions 1 5 gpa 800 c dot gamma 2 10 5 s where pure quartz aggregates deform homogeneously by dislocation creep initial yield occurs where stress concentrations at the tips of weak biotite grains produce semibrittle deformation in intervening quartz or plagioclase allowing local biotite interconnection by slip on 001 and initiating strain weakening after yield the interconnected biotites kink and thus strengthen but the highly strained parts of grains react to a fine grained mixed phase assemblage which deforms by grain size sensitive creep allowing further strain weakening and localization at 3 3 the strain and strain rate in the 100 m thick shear zone are 100 times that of the enclosing host rock similar to the localization observed in natural shear zones thus the processes that initiate strain localization are not necessarily the same as those that preserve the weak shear zone and once formed a shear zone may be permanently weakened

mechanisms of weak phase interconnection and the effects of phase strength contrast on fabric development abstract to determine how deformation mechanisms and fabrics evolve during the strain weakening and localization that accompany the transition from a load bearing framework to an interconnected weak phase shear experiments were conducted on a fine grained gneiss 58 quartz interconnected 28 plagioclase dispersed 13 biotite aligned but dispersed at 745 and 800 c 1500 mpa and two strain rates 2 10 5 and 2 10 6 s 1 in experiments with a high phase strength contrast hpsc 25 and 45 between dispersed biotite and framework quartz grains biotite grains become interconnected due to stress concentration at their tips that allow local semi brittle flow of intervening quartz and cataclasis of intervening plagioclase in the higher temperature hpsc samples the biotite partially dehydrated in high strain zones a single narrow shear zone formed because the psc between the quartz and biotite reaction product layers remained high in the lower temperature hpsc samples there was no reaction and a penetrative s c fabric formed the fabric is defined by the many multiply interconnected biotite strands that formed because the psc between the quartz and biotite decreased as the biotites kinked in slower strain rate experiments with a much lower psc 10 biotite interconnection occurs by shearing into quartz quartz boundaries where new weak strain free recrystallized grains form at low strain a weak s c fabric forms but it evolves to an s c fabric as the psc decreases with strain thus the magnitude of the strength contrast between a weak phase and its matrix influences the mechanism of weak phase interconnection the degree of strain localization and the fabric

fluid rock reaction weakening of fault zones the presence of weak phyllosilicates may explain the low shear strengths of fault zones if they define well developed fabrics the growth of phyllosilicates is favored in meteoric water dominated granitic fault systems where mineral aqueous fluid equilibria predict that modal phyllosilicate will increase via feldspar replacement reactions in deeper more alkaline rock dominated regimes the reactions reverse and feldspars tend to replace phyllosilicates in mg rich mafic rocks however phyllosilicates chlorite biotite can replace stronger framework and chain silicates in both shallower approximately 10 km meteoric h sub 2 o dominated and in deeper alkaline rock dominated regimes where these phyllosilicates precipitate in active fault zones they contribute directly to reaction softening because low temperature deformation of phyllosilicates is not governed by frictional processes alone but can occur by pressure independent dislocation glide the strength of phyllosilicate rich fault rocks can be low at all depths low strain rate creep during interseismic periods can align phyllosilicate grains in foliated gouge and phyllonites where preferred orientations are strong and contiguity of phyllosilicates is large strengths of rocks within fault zones may approach minimum strengths defined by single phyllosilicate crystals fault zones containing localized high concentrations of phyllosilicates with strong preferred orientations in well defined folia can exhibit aseismic slip especially where mafic mg rich rocksmore occur along the fault like parts of the san andreas fault 104 refs 6 figs 1 tab less

deformation history of a subducted continental crust gran paradiso western alps continuing crustal shortening during exhumation eclogite facies continental basement in the western alps outcrops as tectonic windows below the oceanic units in the gran paradiso massif eclogite facies assemblages in mafic rocks display a n s stretching lineation da1 the main phase foliation formed under epidote amphibolite facies conditions da2 and defines a regional dome structure for the whole gran paradiso massif structural data including new detailed mapping microstructural analyses and metamorphic studies in the northern part of the gran paradiso cogne valley reveal the occurrence of major thrusts in this continental basement associated with the da2 deformation e w trending stretching lineations and fold axes are associated with this second and main deformation nappe stacking of the different units was achieved by top to the west shearing during exhumation of the eclogite facies basement the antiformal doming of the gran paradiso basement is thus produced by the nappe stacking during exhumation under epidote amphibolite facies

reaction enhanced ductility the role of solid solid univariant reactions in deformation of the crust and mantle abstract in collision belts and subduction zones penetrative deformation of thrust sheets may be catalyzed by metamorphic reactions if the reactions have a significant effect on the mechanical properties of rocks reaction enhanced ductility may result if a reaction produces a significant decrease in grain size thus allowing the rock to deform superplastically the product grain size of a reaction is a function of the ratio of nucleation and growth rates this ratio varies along any pressure temperature time ptt path and only attains a significant value at a certain distance from an equilibrium boundary a comparison of rates of change of p and t along typical segments of ptt paths for crustal rocks leads to the conclusion that a high nucleation rate a small grain size and resulting superplastic deformation are more likely to be achieved when reaction boundaries are crossed during underthrusting rather than during the thermal readjustments of a perturbed geotherm following crustal thickening or during uplift an example of reaction enhanced ductility occurs in eclogite facies metagranitoids in the sesia lanzo zone western alps where medium grained plagioclase reacted to fine grained jadeite quartz during early alpine underthrusting microstructural evidence from shear zones shows that the jadeite quartz aggregates were highly ductile and it is inferred that the small grain size allowed superplastic deformation further evidence suggests that this process operated during regional deformation of metagranitoids rubie 1982 the reaction of olivine to spinel or the beta phase by metastable overstepping of the two phase stability fields in a subducting plate ringwood 1975 may result in fine grained products and thus may be analogous to the albite jadeite quartz reaction the consequent decrease in effective viscosity is likely to have important consequences for plate dynamics at and below the depths of major phase transitions

u pb age trace element and hf isotope compositions of zircon in a quartz vein from eclogite in the western dabie mountains constraints on fluid flow during early exhumation of ultrahigh pressure rocks quartz veins in high pressure hp to ultrahigh pressure uhp rocks are the products of fluid rock interaction and thus provide insight into fluid processes in subduction zones in this paper we report an integrated study of mineral inclusion trace element u pb age and lu hf isotope compositions of hydrothermal zircon grains from a quartz vein within an uhp eclogite outcrop from the hong an area western dabie mountains these data are used to decipher the age conditions of formation and source of fluid for zircon formation during the exhumation of uhp rocks zircon grains from the vein have perfect euhedral shape and show sector zoning or weak zoning indicating that they precipitated from the aqueous fluid responsible for the vein formation raman spectroscopy analysis reveals that the zircon grains contain inclusions of garnet omphacite rutile quartz and h2o implying that they crystallized from aqueous fluid under hp eclogite facies conditions the zircon grains show low th u and lu hf ratios nearly flat hree patterns absent eu anomalies and low lree contents these characteristics are consistent with their precipitation in the presence of garnet and epidote and absence of feldspar and thus suggest that trace element concentrations in hydrothermal zircon are controlled by co precipitation of mineral assemblages crystallization temperatures of 670 to 712 c which were calculated using the ti content of zircon are consistent with their formation under eclogite facies conditions and may correspond to the temperature of the infiltrating fluid the weighted mean 206pb 238u age of 224 7 1 3 ma is taken as the best estimate for the age of quartz vein formation and records aqueous fluid flow during the early exhumation stage of uhp rocks the zircon grains in the quartz vein have hf compositions similar to those in the host eclogite which demonstrates isotopic equilibrium between fluid and rocks and that the fluid rock ratio was likely low

how large are departures from lithostatic pressure constraints from host inclusion elasticity minerals trapped as inclusions within other host minerals will develop non lithostatic pressures during both prograde and retrograde metamorphism because of the differences between the thermoelastic properties of the host and inclusion phases there is only a single possible path in p t space the entrapment isomeke along which no residual pressure would be developed in a host inclusion system non lithostatic pressures are developed in inclusions as a result of the external pressure and temperature deviating from the isomeke that passes through the entrapment conditions with modern equation of state and elasticity data for minerals now available it is possible to perform precise calculations of the isomekes for mineral pairs these show that isomeke lines are not straight lines in p t space at metamorphic conditions we show that silicate inclusions in silicate hosts tend to have flat isomekes with small values of because of the small range of thermal expansion coefficients of silicate minerals as a consequence the general behaviour under decompression is for soft silicate inclusions in stiffer hosts to develop excess pressures whereas a stiff silicate inclusion in a softer matrix will experience lower pressures than lithostatic pressure the opposite effects occur for compression after entrapment on the prograde path the excess pressures in inclusions including allowance for mutual elastic relaxation of the host and inclusion are most easily calculated by using the isomeke as a basis analysis of the simplest possible model of a host inclusion system indicates that deviations from lithostatic pressure in excess of 1 gpa can be readily produced in quartz inclusions within garnet in metamorphic rocks for softer host minerals such as feldspar the pressure deviations are smaller because of greater elastic relaxation of the host the maximum pressure deviation from lithostatic pressure in the host phase around the inclusion is one third of the pressure deviation in the inclusion routines for performing these calculations have been added to the eosfit7c software package

a model for garnet and plagioclase growth in pelitic schists implications for thermobarometry and p t path determinations numerical models of the progressive evolution of pelitic schists in the ncmnkfmash system with the assemblage garnet biotite chlorite staurolite plagioclase muscovite quartz h2o are presented with the goal of predicting compositional changes in garnet and plagioclase along different p t paths the numerical models support several conclusions that should prove useful for interpreting the p t paths of natural parageneses i garnet may grow along p t vectors ranging from heating with decompression to cooling with compression p t paths deduced from garnet zoning that are inconsistent with these growth vectors are self contradictory ii there is a systematic relation between garnet and plagioclase composition and growth such that for most p t paths garnet growth requires plagioclase consumption furthermore mass balance in a closed system requires that as plagioclase is consumed the remaining plagioclase becomes increasingly albitic inclusions of plagioclase in the core of garnet should be more anorthitic than those near the rim and zoned matrix plagioclase should have rims that are more albitic than the cores complex plagioclase textures may arise from the local variability of growth and precipitation kinetics iii a decrease of fe fe mg in a garnet zoning profile is a reliable indicator of increasing temperature for the assemblage modelled however there is no single reliable p monitor and inferences about p can only be made by considering plagioclase and garnet together iv consumption of garnet during the production of staurolite removes material from the outer shell of a garnet and may make recovery of peak metamorphic compositions and p t conditions impossible low peak temperatures typically recorded by staurolite bearing assemblages may reflect this phenomenon v diffusional homogenization of garnet affects the computed p t path and results in a clockwise rotation of the computed p t vector relative to the true p t path

a least squares approach to mass transport calculations using the isocon method a statistically rigorous approach based on the isocon method grant 1986 to evaluate mass transport is presented chemical analyses of multiple samples of unaltered parent and altered rock are used to calculate the average oxide or element concentration and its standard deviation for each rock population in a typical application the uncertainties in element concentration associated with the lack of homogeneity in each population outweigh the analytical uncertainty an efficient algorithm is presented to select the immobile elements by identifying the maximum number of elements that are within their uncertainties compatible with the same isocon the actual isocon is constructed by forcing a line through the origin and optimizing its slope using a weighted least squares procedure for the selected immobile elements thus individual uncertainties for each immobile element are taken into account as an illustration the procedure is applied to a published set of data on potassic and sericitic alterations of quartz monzonite of the bingham porphyry copper system the surprising result indicates that the apparent increase of potassic alteration phases is a consequence of acid leaching of calcium and sodium no mass transport of potassium is indicated silica water and sulfur were added during alteration the best fit isocon requires a small mass increase during each alteration process which however is within the uncertainty obtained for the slope of the isocon a software package is available from the authors in the form of a fortran 77 source code

electrodynamic parameters in the nighttime sector during auroral substorms the characteristics of the large scale electrodynamic parameters field aligned currents facs electric fields and electron precipitation which are associated with auroral substorm events in the nighttime sector have been obtained through a unique analysis which places the ionospheric measurements of these parameters into the context of a generic substorm determined from global auroral images a generic bulge type auroral emission region has been deduced from auroral images taken by the dynamics explorer 1 de 1 satellite during a number of isolated substorms and the form has been divided into six sectors based on the peculiar emission characteristics in each sector west of bulge surge horn surge middle surge eastern bulge and east of bulge by comparing the location of passes of the dynamics explorer 2 de 2 satellite to the simultaneously obtained auroral images each pass is placed onto the generic aurora the organization of de 2 data in this way has systematically clarified peculiar characteristics in the electrodynamic parameters an upward net current mainly appears in the surge with little net current in the surge horn and the west of bulge the downward net current is distributed over wide longitudinal regions from the eastern bulge to the east of bulge near the poleward boundary of the expanding auroral bulge a pair of oppositely directed fac sheets is observed with the downward fac on the poleward side this downward fac and most of the upward fac in the surge and the middle surge are associated with narrow intense antisunward convection corresponding to an equatorward directed spikelike electric field this pair of currents decreases in amplitude and latitudinal width toward dusk in the surge and the west of bulge and the region 1 and 2 facs become embedded in the sunward convection region the upward fac region associated with the spikelike field on the poleward edge of the bulge coincides well with intense electron precipitation and aurora appearing in this western and poleward portion of the bulge the convection reversal is sharp in the west of bulge and surge horn sectors and near the high latitude boundary of the upward region 1 fac in the surge the convection reversal is near the low latitude boundary of the upward region 1 with a near stagnation region often extending over a large interval of latitude in the eastern bulge and east of bulge sectors the region 1 and 2 facs are located in the sunward convection region while a spikelike electric field occasionally appears poleward of the aurora but usually not associated with a pair of fac sheets in the eastern bulge magnetic field data show complicated fac distributions which correspond to current segments and filamentary currents

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

instantaneous mapping of high latitude convection with coherent hf radars coherent hf radars at goose bay labrador and schefferville quebec are used to study plasma convection in the high latitude ionosphere maps of the two dimensional flow pattern are obtained by merging simultaneous sets of radial velocity data each with a time resolution of a few minutes from a time sequence of such maps we have separated the changes in flow due to magnetic local time mlt variations over the region of observation from those due to ut time variations we study in detail the convection in the early morning sector observed on october 15 1989 when the interplanetary magnetic field imf reversed from southward to northward this imf reversal was not associated with a clear response in the nightside convection but rather with several sudden changes some of which anticipated the bz reversal we suggest that these changes are associated with delayed and superposed ionospheric responses to previous imf perturbations or to local effects after the imf reversal from south to north our observations of westward and southwestward velocities in the 71 77 invariant latitude range are consistent with the earlier simulations for bz 0 and by 0 during the period of steady northward imf after the reversal the convection pattern was observed to reconfigure slowly a region of large westward velocities progressively moved poleward while convection in the low latitude part of the field of view faded away the time constant of this slow reconfiguration was about 1 hour and varied with mlt such that it was larger closer to midnight these data combined with particle data from successive passes of the dmsp satellites provide information on the contraction of the polar cap after the imf bz reversal and on the mlt dependency of the velocity at which this contraction occurs they show that the polar cap contracts more rapidly in the daytime than in the nightime and more rapidly in the postmidnight sector than in the premidnight sector

the strain dependent spatial evolution of garnet in a high pressure ductile shear zone from the western gneiss region norway a synchrotron x ray microtomography study reaction and deformation microfabrics provide key information to understand the thermodynamic and kinetic controls of tectono metamorphic processes however they are usually analysed in two dimensions omitting important information regarding the third spatial dimension we applied synchrotron based x ray microtomography to document the evolution of a pristine olivine gabbro into a deformed omphacite garnet eclogite in four dimensions where the 4th dimension is represented by the degree of strain in the investigated samples which cover a strain gradient into a shear zone from the western gneiss region norway we focused on the spatial transformation of garnet coronas into elongated garnet clusters with increasing strain the microtomographic data allowed quantification of garnet volume shape and spatial arrangement evolution with increasing strain the microtomographic observations were combined with light microscope and backscatter electron images as well as electron microprobe empa and electron backscatter diffraction ebsd analysis to correlate mineral composition and orientation data with the x ray absorption signal of the same mineral grains with increasing deformation the garnet volume almost triples in the low strain domain garnet grains form a well interconnected large garnet aggregate that develops throughout the entire sample we also observed that garnet coronas in the gabbros never completely encapsulate olivine grains in the most highly deformed eclogites the oblate shapes of garnet clusters reflect a deformational origin of the microfabrics we interpret the aligned garnet aggregates to direct synkinematic fluid flow and consequently influence the transport of dissolved chemical components ebsd analyses reveal that garnet shows a near random crystal preferred orientation that testifies no evidence for crystal plasticity there is however evidence for minor fracturing neo nucleation and overgrowth microprobe chemical analysis revealed that garnet compositions progressively equilibrate to eclogite facies becoming more almandine rich we interpret these observations as pointing to a mechanical disintegration of the garnet coronas during strain localization and their rearrangement into individual garnet clusters through a combination of garnet coalescence and overgrowth while the rock was deforming

analysis of substorm expansion and surge development we have used simultaneous magnetogram radar and viking imager data from evening local time to study several substorm expansion phase onsets and an expansion phase auroral surge that formed near the radar field of view at the onsets auroral brightenings developed within the region of the electric field gradient of the harang discontinuity and if not already present a westward electrojet began poleward of the eastward electrojet immediately following onsets the poleward boundary of the westward electrojet moved poleward along with the poleward boundary of the aurora however no consistent motion of the harang discontinuity was found in the ionosphere the auroral surge formed along the poleward boundary of the westward electrojet and of the aurora we argue that the distortion of the poleward boundary of the aurora associated with the surge was a manifestation of a distortion of the separatrix between open and closed magnetic field lines the distortion being caused by the field aligned currents within the head of the surge we suggest that rapid surge development may result from a tapping of the westward electrojet current at the surge head we also suggest that the separatrix distortion could lead to the escape of trapped plasma sheet particles along open field lines in the tail

the single slip hypothesis revisited crystal preferred orientations of sheared quartz aggregates with increasing strain in nature and numerical simulation this study discusses the fabric development in naturally sheared quartz aggregates in comparison to results from texture modeling according to the polycrystalline plasticity theory with particular emphasis on the formation of a single c axis maximum the investigated natural shear zone samples were deformed at about 650 50 c with increasing strain up to 14 and show dynamic recrystallization microstructures of grain boundary migration recrystallization neutron diffraction texture analysis results in c axis pole figures with a single maximum at the periphery of the pole figure this maximum does not align with the shear plane normal towards higher strain but rotates towards an inclined orientation in accordance with the sense of shear such a rotation is inconsistent with the single slip hypothesis and suggests that the formation of this c axis pattern is controlled by multi slip on several slip systems based on the polycrystalline plasticity theory this quartz fabric can develop if combined view the mathml source r view the mathml source z and view the mathml source prism slip dominates and must not be related to the commonly proposed view the mathml source basal slip the multi slip texture development is in agreement with the shear sense interpretation from the asymmetry between well defined quartz fabrics and the foliation for dominant view the mathml source basal slip in quartz and 2 numerical simulations predict a single peripheral maximum perpendicular to the shear plane and two a maxima with a 30 inward position parallel to the shear plane this simulation corresponds to naturally observed cpo patterns of quartz formed at different deformation conditions and it is in agreement with the single slip hypothesis hence our combined natural and numerical data suggest that the single slip hypothesis is a possible explanation for a single c 0001 maximum but not universally true

al2sio5 bearing segregations in the lepontine alps switzerland aluminum mobility in metapelites a genetic model is proposed for pegmatitic al2sio5 bearing segregations in regionally metamorphosed rocks of the lepontine alps components that form the segregations were derived from the host rocks by dissolution of minerals in alkali bearing aqueous fluids produced by metamorphic dehydration reactions the segregations formed syntectonically within extensional dilatant fractures infiltrations of fluids into the fractures was driven by fluid pressure gradients tectonic pumping precipitation of aluminum silicates from alkali aluminum complexes rather than alkali silicates was favored by reduced f h2o within the segregations the transformation of kyanite to andalusite within the segregations resulted from localized pressure reduction via dilatancy during the later stages of decompression of the pennine nappes subsequent to the main alpine metamorphism

implications of the fresnel zone texture for seismic amplitude interpretation abstractthe 3d reflection seismic response is associated with a zone the fresnel zone rather than with a single point used in the idealized 1d convolution model unlike a point of incidence the fresnel zone is complicated by its textural characters that are defined by the dip and azimuth of microreflectors in the zone the fresnel zone texture makes seismic amplitude interpretation more complicated than previously documented a conceptual model suggests that seismic amplitude variations with offset avo azimuth avaz and frequency spectral decomposition were physically related to textural roughness textural anisotropy and textural scale of the fresnel zone respectively textural roughness is defined by the dip deviation of microreflectors and contributes to the avo intercept and gradient textural anisotropy is defined by the degree of the preferred orientation of the microreflectors and directly affects the avaz signature textural scale is defined by the spacing of the microreflectors and c

enhanced mass transfer through short circuit diffusion growth of garnet reaction rims at eclogite facies conditions in the monte rosa area northern italy the assemblage garnet phengite quartz was formed in polymetamorphic metapelites from pre existing biotite and plagioclase during an eclogite facies alpine metamorphic overprint while phengite nucleated within plagioclase garnet formed 10 to 20 m wide continuous rims along the original biotite plagioclase interphase boundaries garnet formation involved diffusion of ca fe and mg bearing species across the growing rims the garnet rims show an overall asymmetric compositional zoning and distinct nanometer scale chemical patterns across grain boundaries within the garnet polycrystal the compositional patterns resulted from the interplay of diffusion along grain and interphase boundaries and volume diffusion individual garnet crystals are separated by low angle grain boundaries which are predominantly oriented perpendicular to the reaction rim giving rise to an overall palisade microstructure grain boundaries contain arrays of closely spaced channels about 2 nm wide which are filled with an amorphous phase these channels represent direct links between the garnet biotite and garnet plagioclase reaction fronts and provide pathways for fast diffusion from numerical simulation of the observed chemical patterns diffusion of ca and fe is inferred to have been 5 6 orders of magnitude faster within grain boundaries than through the interior of garnet grains although the amorphous grain boundary phase only takes up a small fraction 2 5 per mil of the total volume of the garnet polycrystal short circuit diffusion along grain boundaries contributed substantially to material transfer across the growing garnet rim despite the presence of fast pathways bulk diffusion was too slow to allow for chemical equilibration of the phases involved in garnet rim formation even on a micrometer scale relying on published garnet volume diffusion data d gb where gb grain boundary is estimated to be on the order of 10 19 20 m 2 s at the estimated reaction conditions of 650 c and 12 5 kbar where d gb ca is slower than d gb fe by a factor of ten

deformation mass transfer and mineral reactions in an eclogite facies shear zone in a polymetamorphic metapelite monte rosa nappe western alps this study analyses the mineralogical and chemical transformations associated with an alpine shear zone in polymetamorphic metapelites from the monte rosa nappe in the upper val loranco n italy in the shear zone the pre alpine assemblage plagioclase biotite kyanite is replaced by the assemblage garnet phengite paragonite at eclogite facies conditions of about 650 c at 12 5 kbar outside the shear zone only minute progress of the same metamorphic reaction was attained during the alpine metamorphic overprint and the pre alpine mineral assemblage is largely preserved textures of incomplete reaction such as garnet rims at former grain contacts between pre existing plagioclase and biotite are preserved in the country rocks of the shear zone reaction textures and phase relations indicate that the alpine metamorphic overprint occurred under largely anhydrous conditions in low strain domains in contrast the mineralogical changes and phase equilibrium diagrams indicate water saturation within the alpine shear zones shear zone formation occurred at approximately constant volume but was associated with substantial gains in silica and losses in aluminium and potassium changes in mineral modes associated with chemical alteration and progressive deformation indicate that plagioclase biotite and kyanite were not only consumed in the course of the garnet and phengite producing reactions but were also dissolved congruently during shear zone formation a large fraction of the silica liberated by plagioclase biotite and kyanite dissolution was immediately re precipitated to form quartz but the dissolved aluminium and potassium bearing species appear to have been stable in solution and were removed via the pore fluid the reaction causes the localization of deformation by producing fine grained white mica which forms a mechanically weak aggregate

role of fluid in strain softening within the main central thrust in sikkim the origin of quartz rich mylonites abstract the presence of fluids promotes strain softening and profoundly affects the evolution of shear zones the main central thrust mct is a major shear zone that accommodated at least 90 km of shortening and played a significant role in cenozoic evolution of the himalaya surprisingly no information exists on the role of fluid in evolution of the mct this study integrates mineralogical and geochemical analyses with mass balance calculations to explore the role of fluids in evolution of the mct shear zone from four exposures which span 40 km distance along the transport direction in sikkim eastern india our analyses reveal significant fluid induced strain softening in the investigated exposures attested by retrogression of biotite to chlorite and feldspar to muscovite and quartz the retrogressed feldspar grains are surrounded by recrystallized quartz and the muscovite and chlorite produced due to retrogression define the mylonite foliations which indicate fluid activity was syn tectonic we propose a three stage conceptual model to analyze the role of fluid in the evolution of the mct in sikkim first the shear zone nucleated along pre existing structurally weak and compositionally distinct zones following which the proto shear zone developed in closed system conditions and deformation localized by dynamic recrystallization and dislocation creep resulting in formation of 200 1000 m thick protomylonites then fluids infiltrated the shear zone and induced retrogression and element mobility in the hinterland most exposures significant fluid induced retrogression transformed the calc silicate and paragneiss protoliths to mylonites resembling micaceous quartzites in the frontal most and structurally intermediate exposures the muscovite and chlorite grains underwent further dissolution leaving behind a quartz rich residue thus transforming tonalite orthogneiss and paragneiss protoliths to mylonites resembling monomineralic quartzites the deformation front narrowed through time and became progressively localized within the 120 410 m thick strain softened mylonites at the shear zone core whereas deformation ceased or continued slowly in the 100 880 m thick protomylonites at the margins the study improves our understanding of the strain softening mechanisms operative within the mct and the origin of quartz rich mylonites

elastic geobarometry for anisotropic inclusions in cubic hosts abstract mineral inclusions entrapped in other minerals may record the local stresses at the moment of their entrapment in the deep earth when rocks are exhumed to the surface of the earth residual stresses and strains may still be preserved in the inclusion if measured and interpreted correctly through elastic geobarometry they give us invaluable information on the pressures p and temperatures t of metamorphism current estimates of p and t of entrapment rely on simplified models that assumes that the inclusion is spherical and embedded in an infinite host and that their elastic properties are isotropic we report a new method for elastic geobarometry for anisotropic inclusions in quasi isotropic hosts the change of strain in the inclusion is modelled with the axial equations of state of the host and the inclusion their elastic interaction is accounted for by introducing a 4th rank tensor the relaxation tensor that can be evaluated numerically for any symmetry of the host and the inclusion and for any geometry of the system this approach can be used to predict the residual strain stress state developed in an inclusion after exhumation from known entrapment conditions or to estimate the entrapment conditions from the residual strain measured in real inclusions in general anisotropic strain and stress states are developed in non cubic mineral inclusions such as quartz and zircon with deviatoric stresses typically limited to few kbars for garnet hosts the effect of the mutual crystallographic orientation between the host and the inclusion on the residual strain and stress is negligible when the inclusion is spherical and isolated assuming external hydrostatic conditions our results suggest that the isotropic and the new anisotropic models give estimations of entrapment conditions within 2

mapping using the tsyganenko long magnetospheric model and its relationship to viking auroral images the tsyganenko long magnetospheric model 1987 has been used in conjunction with ultra violet images taken by the viking spacecraft to investigate the relationship of the auroral distribution to different magnetospheric regions the model describes the large scale structure of the magnetosphere reasonably well for dipole tilt angles near zero but it appears to break down at higher tilt angles even so a wide variety of auroral configurations can be accurately described by the model it appears that the open closed field line boundary is a poor indicator of auroral arc systems with the possible exception of high latitude polar arcs the auroral distribution typically called the oval maps to a region in the equatorial plane quite close to the earth and can be approximately located by mapping the model current density maximum from the equatorial plane into the ionosphere although the model may break down along the flanks of the magnetotail the large scale auroral distribution generally reflects variations in the near earth region and can be modeled quite effectively

scales of disequilibrium and rates of equilibration during metamorphism mounting evidence suggests that partial disequilibrium meaning disequilibrium for some elements but not for others may be a common but rarely detected phenomenon during metamorphic mineral growth even in ordinary prograde reactions that progress to completion detailed examination of compositional variations in garnet crystals over a range of metamorphic grade suggests distinctly different scales of equilibration for common elements with strong temperature dependence under lower greenschist facies conditions fe and mg may equilibrate at hand sample scale whereas mn and ca may not equilibrate even at millimeter scale although mn may achieve hand sample scale equilibration under upper greenschist facies conditions ca and many trivalent cations e g rees may not do so until temperatures exceed those of the middle amphibolite facies even in the lower granulite facies some elements e g y yb show indications of disequilibrium at sub centimeter scales during garnet growth analysis and numerical modeling of undisputed disequilibrium textures demonstrate that the most common impediment to equilibration during metamorphism is the sluggishness of intergranular diffusion the same mechanism known to govern porphyroblast crystallization in many metamorphic environments despite this importance to petrology few quantitative determinations exist of inter granular diffusion rates under metamorphic conditions using numerical simulations of coupled intergranular and intracrystalline diffusion processes in coronal textures around partially resorbed garnet crystals from the llano uplift u s a a very precise and relatively accurate estimate is obtained for the rate of intergranular diffusion of al in fluid undersaturated systems this result and an earlier estimate for fluid saturated systems provide bracketing values for al diffusivity during metamorphism

metamorphic veining and mass transfer in a chemically closed system a case study in alpine metabauxites western vanoise in western vanoise french alps karstic pockets of triassic to jurassic metabauxites embedded in carbonate rocks and containing several generations of metamorphic veins were studied during blueschist facies metamorphism a cumulative amount of 13 vol of water is inferred to have been produced locally by successive dehydration reactions and part of this fluid remained in the bauxitic lenses during most of the metamorphic cycle field and geochemical evidence show that these rocks have been isolated from large scale fluid flow closed system behaviour it is proposed that the internally derived fluid has promoted the opening of fluid filled open spaces as attested by the euhedral habits of vein minerals and served as medium for mass transfer from rock to vein indeed the vein infill is obviously the result of chemical interactions at the millimetre to centimetre scale between the rock minerals and the locally produced aqueous fluid two vein types can be distinguished based on mineralogical and textural features i some veins are filled with newly formed products of either prograde chloritoid or retrograde chlorite metamorphic reactions in this case fluid filled open spaces seem to offer energetically favourable nucleation growth sites ii the second vein type is infilled with cookeite or pyrophyllite that were present in the host rock prior to the vein formation in this closed chemical system the components for the vein infill minerals have been transferred from rock to vein through the fluid in a dissolution transport precipitation process possibly stress assisted these different vein generations all contain al rich mineral infills suggesting that al was a mobile element cm scale during metamorphism in these hp rocks fluid flow may have been restricted and if so mass transfer occurred by diffusion in an almost stagnant fluid metamorphic veins can be seen as witnesses of fluid and mass redistribution that partly accommodate the rock deformation lateral segregation

dating polygenetic metamorphic assemblages along a transect across the western alps multichronometric analyses were performed on samples from a transect in the french italian western alps crossing nappes derived from the brianconnais terrane and the piemonte liguria ocean in an endeavour to date both high pressure hp metamorphism and retrogression history twelve samples of white mica were analysed by 39ar 40ar stepwise heating complemented by two samples from the monte rosa nappe 100 km to the ne and also attributed to the brianconnais terrane one sm nd and three lu hf garnet ages from eclogites were also obtained white mica ages decrease from c 300ma in the westernmost samples zone houille re reaching c 3008c during alpine metamorphism to 548ma in the internal units to the east which reached c 5008c during the alpine orogeny the spatial pattern of eocene k ar ages demonstrates that si rich hp white mica records the age of crystallization at 47 48ma and retains ar at temperatures of around 5008c paleocene early eocene lu hf and sm nd ages recording prograde garnet growth before the hp peak confirm eclogitization in eocene times petrological and microstructural features reveal important mineralogical differences along the transect all samples contain mixtures of detrital syn d1 and syn d2 mica and retrogression phases d3 in greatly varying proportions according to local variations in the evolution of pressure temperature fluid activity deformation p t a d conditions samples from the zone houille re mostly contain detrital mica the abundance of white mica with si46 45 atoms per formula unit increases eastward across the whole traverse phengitic mica grown during hp metamorphism defines the d1 foliation syn d2 mica is more si poor and associated with nappe stacking exhumation and hydrous retrogression under greenschist facies conditions syn d1 phengite is very often corroded overgrown by or intergrown with syn d2 muscovite most importantly syn d2 recrystallization is not limited to s2 schistosity domains micrometrescale chemical fingerprinting reveals muscovite pseudomorphs after phengite crystals which could be mistaken for syn d1 mica based on microstructural arguments alone the cl k ratio in white mica is a useful discriminator as d2 retrogression was associated with a less saline fluid than eclogitization as petrology exerts the main control on the isotope record constraining the petrological and microstructural framework is necessary to correctly interpret the geochronological data described in both the present study and the literature our approach which ties geochronology to detailed geochemical petrological and microstructural investigations identifies 47 48ma as the age of hp formation of syn d1 mica along the studied transect and in the monte rosa area cretaceous apparent mica ages which were proposed to date eclogitization by earlier studies based on conventional thermochronology are due to ar inheritance in incompletely recrystallized detrital mica grains the inferred age of the probably locally diachronous greenschist facies low si syn d2 mica ranges from 39 to 43ma coexistence of d1 and d2 ages and the constancy of non reset d1 ages along the entire transect provides strong evidence that the d1 white mica ages closely approximate formation ages volume diffusion of ar in white mica activation energy e 250 kj mol 1 pressure adjusted diffusion coefficient d 050 03 cm2 s 1 has a subordinate effect on mineral ages compared with both prograde and retrograde recrystallization in most samples

disequilibrium for ca during growth of pelitic garnet compositional zoning in hundreds of almandine rich garnets in amphibolite by facies micaceous quartzites from the picuris range north central new mexico usa indicates that although mn mg and fe achieve chemical equilibrium at hand sample scale during garnet growth ca does not instead ca concentrations at the surface of growing garnets appear to depend strongly on kinetic factors that govern the local chemical environment yielding disequilibrium for ca at scales larger than the region immediately surrounding an individual porphyroblast detailed zoning profiles were obtained for 371 garnet crystals in a small volume of a single sample of garnetiferous quartzite and core analyses were made of 97 additional crystals each analysis was made on a section that passed precisely through the morphological centre of the crystal located by means of 3 d imagery from computed x ray tomography the data reveal strong correlations between crystal size and concentrations of mn mg and fe but not ca in garnet cores a relationship between crystal size and isolation rigorous cross correlations among concentrations of mn mg and fe but not ca and systematic variations in ca concentrations as a function of crystal size and core composition that are anomalous in comparison to the behaviour of the other divalent cations we interpret these observations as the result of thermally accelerated diffusion controlled garnet growth in circumstances that promoted rapid intergranular diffusion and thus rock wide equilibration of mn mg and fe but that prevented equilibration at similar scale for ca because of its more sluggish intergranular diffusion the anomalous behaviour of ca is made evident in these garnets by the presence of sharp spikes in ca concentration which are demonstrably not a consequence of any simultaneous rock wide event such as a change in pressure temperature or some other intensive parameter instead ca concentrations probably reflect the local extent of reaction in the immediate vicinity of each porphyroblast to the degree that such kinetic factors introduce departures from chemical equilibrium for ca thermobarometric estimates that involve grossular contents of pelitic garnet will be in error

elasticity of pyrope and majorite pyrope solid solutions to high temperatures abstract majorite garnet solid solutions are major mineral phases in the earth s upper mantle and transition zone here we present the first brillouin scattering measurements of the elasticity of majorite mj mg 4 si 4 o 12 pyrope py mg 3 al 2 si 3 o 12 solid solutions mj 50 py 50 and mj 80 py 20 and single crystal elasticity of pure synthetic pyrope at temperatures up to 800 c the temperature derivatives of the adiabatic bulk k s and shear moduli for all compositions along the mj py join are the same within the experimental uncertainties k s t 14 0 14 5 20 mpa k t 8 3 9 2 10 mpa k the temperature dependence of the acoustic velocities for mj py solid solutions is about half that of other major transition zone minerals this implies that temperature variations in the transition zone inferred from lateral velocity heterogeneity can be significantly underestimated if the properties of majoritic garnet are not taken into account

experimental deformation of muscovite abstract the strength and mechanical anisotropy of muscovite have been investigated by shortening single crystals at 45 0 and 90 to the basal plane 001 at temperatures t from 20 to 400 c confining pressures pc from 10 to 400 mpa and strain rates e from 2 5 10 7 to 2 3 10 4 s 1 samples deformed as 45 to 001 exhibit smooth undulatory extinction in thin section associated with dislocation glide with few well developed low angle kink boundaries basal shear strengths depend on confining pressure at pc e a exp a exp q rt results from temperature and strain rate stepping experiments performed at pc 200 mpa yield an activation energy q of 47 19 kj mol 1 and an exponential constant a of 0 5 0 2 mpa 1 samples loaded parallel to the basal plane shorten by the development of sharply defined kink bands strengths associated with kink band formation are higher than those associated with glide in samples compressed at 45 to 001 and exhibit significant pressure dependence 0 8 up to confining pressures of 200 mpa kink bands are generated at the ends of samples where contact is made with the pistons samples loaded at 90 to 001 are much stronger than samples of all other orientations and deformation is accomplished by fracture fracture strengths exhibit a strong dependence on confining pressure 0 9 similar in magnitude to other silicates that are brittle at the same conditions by comparison with biotite muscovite is weaker in shear on 001 at the experimental conditions tested with an activation energy for glide q that is significantly smaller than that for glide in biotite

metasomatic coronas around hornblendite xenoliths in granulitefacies marble ivrea zone n italy i constraints on componentmobility centimeter to decimeter thick reaction bands occur at hornblendite marble interfaces in val fiorina in the granulite facies metamorphic ivrea zone from hornblendite to marble the reaction bands show a consistent succession of sharply bounded mineral layers comprising a monomineralic clinopyroxene layer a garnet clinopyroxene layer and a scapolite clinopyroxene layer reaction band formation occurred as a response to gradients in the chemical potentials of calcium and magnesium as defined by the hornblendite assemblage and the marble matrix the metasomatic corona primarily replaced the hornblendite and only minor amounts of marble were consumed the reaction band behaved as an open system with net transfer of calcium from the marble into the reaction band and a net transfer of iron and magnesium in the opposite direction mass balance considerations allow us to constrain a range of feasible mass balance scenarios for which major element fluxes across the boundaries of the reaction band may be quantified modeling of layer growth as a steady diffusion process yields ratios of the phenomenological diffusion coefficients for si al mg and ca of l sisi over l caca 2 5 kern 1pt rm l alal over l caca 1 the relative diffusivities are primarily constrained by the sequence of mineral layers of the reaction band and by the relative thickness of the layers the results of steady state diffusion modeling are relatively insensitive with respect to variations in the major element boundary fluxes

neogene lateral extension during convergence in the central alps evidence from interrelated faulting and backfolding around the simplonpass switzerland abstract the simplon fault zone occurs at the western end of the lepontine metamorphic dome in the central alps and represents a major low angle 25 dip normal fault associated with wsw ene directed lateral extension movement began in the early miocene and continued until at least 3 ma accumulating a total vertical throw of around 10 km corresponding to 24 km displacement parallel to the fault this displacement is partitioned between a broad zone of ductile mylonites in the footwall and a more discrete cataclastic detachment fault the simplon line in the central region simplon village to zwischbergen some 3 km of the total vertical displacement is accommodated by the discrete cataclastic zone due to the higher exposed structural level the importance of the cataclastic component increases towards the northwest in the northern steep belt around the simplonpass retrograde greenschist facies mylonite formation and younger cataclastic faulting associated with lateral extension occurred contemporaneously with sse vergent large scale backfolding representing nnw sse shortening and crustal thickening the combined effect was an overall shortening across the orogen accommodated by an extension parallel to the orogen allowing lateral escape of continental material normal faulting and the resultant lateral escape was not just superficial it extended to depths of at least 15 km and probably to depths exceeding 25 km the overall geometry and kinematics of the simplon fault zone and in particular the interplay between folding and faulting is very similar to that described from the metamorphic core complexes and detachment faults of the cordillera of the southwestern usa within the central alps extension was associated with rapid exhumation of the lepontine metamorphic dome as evidenced by very rapid cooling rates determined from mineral ages in the footwall similar to the cenozoic tectonics of the himalayas and tibet orogen parallel extension in the mountain belt may have been promoted by an overall reduction in the horizontal compressive stress in this direction more limited extension of the same age and orientation is known from a large area of the european foreland greater extension in the mountain belt compared with the foreland was accommodated by major strike slip transfer fault systems and was linked to coeval overthrusting of the lowlands towards the southwest in the external southwestern alps of haute provence france coeval folding and orogen parallel extension such as at the simplonpass were developed in a dextral transpressive regime due to oblique wnw ese convergence across the alps during the neogene

sequence and style of major post nappe structures simplon pennine alps abstract in the upper pennine nappe complex of the simplon pennine alps switzerland and italy at least three phases of major post nappe folding in places associated with thrusting can be distinguished these are superimposed on an earlier formed partly chaotic complex of tectonic units including the bernhard and monte rosa continental flakes and the zermatt saas and antrona ophiolite complexes the earliest post nappe folds were essentially isoclinal throughout the whole region and were accompanied by a strong schistosity which is the main foliation in most areas later two successive phases of back folding led to the present overall structure both phases typically show rapid variations in style from open folds lacking axial planar schistosity to very tight structures with complete foliation transposition this has been demonstrated by systematically mapping the major axial traces over the whole region successively removing the major structures in reverse order shows that the ophiolite complexes were originally part of a continuous unit marking an important suture between the bernhard and monte rosa nappes

the role of deformation reaction interactions to localize strain in polymineralic rocks insights from experimentally deformed plagioclase pyroxene assemblages abstract in order to study the mutual effect of deformation and mineral reactions we have conducted shear experiments on fine grained plagioclase pyroxene assemblages in a griggs type solid medium deformation apparatus experiments were performed at a constant shear strain rate of 10 5 s 1 a confining pressure of 1 gpa and temperatures of 800 850 and 900 c while the peak stress of plagioclase orthopyroxene assemblages reaches values between those of the end member phases the strength of polymineralic materials strongly decreases after peak stress and reaches flow stresses that stabilize far below those of the weaker phase plagioclase this weakening correlates with the coeval development of high strain shear zones where new phases are preferentially produced including new pyroxene plagioclase and amphibole the reaction products mostly occur as intimately mixed phases within fine grained and interconnected shear bands together with different compositions with respect to the starting material this indicates that deformation significantly enhances the kinetics of mineral reactions which in turn strongly weaken the deforming sample here attributed to a switch to grain size sensitive diffusion creep through phase nucleation and grain size reduction such an interplay between deformation and mineral reactions may have strong implications for the initiation development and durability of shear zones in the lower crust

reaction induced fracturing in a hot pressed calcite periclase aggregate abstract the chemo mechanical feedbacks associated with hydration of periclase immersed in a calcite matrix were investigated experimentally dense calcite periclase aggregates with

weakening and strain localization produced by syn deformational reaction of plagioclase there are many observations in naturally deformed rocks on the effects of mineral reactions on deformation but few experimental data in order to study the effects of chemical disequilibrium on deformation we have investigated the hydration reaction plagioclase h2o more albitic plagioclase zoisite kyanite quartz we utilized fine grained 2 6 m plagioclase aggregates of two compositions an54 and an60 both dried and with 0 1 0 4 wt h2o present in shear deformation experiments at two sets of conditions 900 c 1 0 gpa in the plagioclase stability field and 750 c 1 5 gpa in the zoisite stability field dry samples and those deformed in the plagioclase stability field underwent homogeneous shearing by dislocation creep but samples with 0 1 to 0 4 wt water deformed in the zoisite stability field showed extreme strain localization into very narrow 1 3 m shear bands after low shear strain in these samples the microstructures of reaction products in the matrix differ from those in the shear bands in the matrix large up to 400 m zoisite crystals grew in the direction of finite extension and relict plagioclase grains are surrounded by rims of recrystallized grains that are more albitic in the shear bands the reaction products albitic plagioclase zoisite white mica and traces of kyanite form polyphase aggregates of very fine grained 0 1 m dislocation free grains most of the sample strain after 2 has occurred within the shear bands within which the dominant deformation mechanism is inferred to be diffusion accommodated grain boundary sliding dagbs the switch from dislocation creep in dry samples deformed without reaction to dagbs in reacted samples is associated with a decrease in flow stress from 800 to 200 mpa these experiments demonstrate that heterogeneous nucleation driven in part by chemical disequilibrium can produce an extremely fine grained polyphase assemblage leading to a switch in deformation mechanism and significant weakening thus localization of deformation in polyphase rocks may occur on any pressure p temperature t path where the equilibrium composition of the constituent minerals changes

fluid channelling during ductile shearing transformation of granodiorite into aluminous schist in the tauern window eastern alps ductile shearing in the core of the tauern window austria transformed metagranodiorite into si undersaturated garnet chlorite staurolite schist at a depth of c 35 40 km during the alpine orogeny four distinct zones have been recognized extending from the wallrock into the centre of the shear zone zone i unaltered metagranodiorite with subordinate amphibolite zone ii biotite white mica garnet schist zone iii biotite phengite schist zone iv quartz absent garnet chlorite staurolite schist with garnets up to 10 cm across whole rock analyses show a dramatic decrease in sio2 from 65 wt in zone i to 35 wt in zone iv ca na and sr also decrease across the shear zone whereas al ti fe mg p cr ni zn and rb all increase towards zone iv mass balance calculations indicate that shearing was accompanied by up to 60 volume loss near the centre of the shear zone comparison of the tauern window samples with other shear zones in granitic hosts indicates that silica loss accompanied by gains in mg fe and ti is typical for volume loss shear zones but is distinctly different from the element behaviour exhibited in shear zones that are thought to represent approximately isovolumetric behaviour in the samples studied here volume loss appears to have resulted from channellized fluid flow during shearing producing time integrated fluid fluxes of 108 cm3 cm 2 in zone iv this large volume of fluid may have originated in part from dehydration of flysch carried beneath the metagranodiorites during eocene movement on the north penninic subduction zone development of an inverted thermal gradient during subduction would have allowed the fluid to scavenge large amounts of silica from the shear zone during ascent and heating

shear zones and rock history abstract shear zones that have widened with time can display margin to interior microstructural profiles unlike the real microstructural history of the interior rocks shear zones that have not widened with time can provide more reliable views of microstructural history and therefore of deformation processes even the latter zones can mislead however where there has been an annealing overprint during or following the shearing interval these sources of uncertainty need to be acknowledged more regularly and if possible eliminated by workers interpreting deformation processes from shear zone strain gradients

beginning of melting in the granite system qz or ab an h2o the beginning of melting in the system qz or ab an h2 o was experimentally reversed in the pressure range p text h text 2 text o 2 15 kbar using starting materials made up of mixtures of quartz and synthetic feldspars with increasing pressure the melting temperature decreases from 690 c at 2 kbar to 630 c at 17 kbar in the an free alkalifeldspar granite system qz or ab h2o in the granite system qz or ab an h2o the increase of the solidus temperature with increasing an content is only very small in comparison to the alkalifeldspar granite system the solidus temperature increases by 3 c 7 c if albite is replaced by plagioclase an 20 an 40 the difference between the solidus temperatures of the alkalifeldspar granite system and of quartz anorthite sanidine assemblages system qz or an h2o is approximately 50 c

joint two dimensional observations of ground magnetic and ionospheric electric fields associated with auroral zone currents 32 auroral zone currents during the passage of a westward travelling surge

thermodynamic properties of white micas on the basis of high pressure experiments in the systems k2omgoal2o3sio2h2o and k2ofeoal3o3sio2h3o abstract experiments in the piston cylinder apparatus were undertaken to synthesize the assemblage of phengite garnet kyanite and quartz or coesite in the system k2o mgo al2o3 sio2 h2o kmash compositional data of phengite were obtained for 13 experimental runs in the range of 30 to 45 kbar and 850 c to 1100 c the same was achieved for 23 runs in the system k2o feo al2o3 sio2 h2o kfash between 600 c and 900 c and 15 to 55 kbar on the basis of the latter runs and 6 additional synthesis experiments the dependence of the lattice dimensions on the composition of 2m1 phengites in kfash was determined to derive thermodynamic data we evaluated the above experiments and those reported by massonne and schreyer 1987 1989 on kmash phengites coexisting with other assemblages considering simple activity model like ams ms we obtained for the mg al celadonite and fe2 al celadonite theoretical endmembers hf0 5832415 3040 and 5492287 6267 j mol s0 288 527 4 008 and 303 148 6 827 j k mol v0 13 870 0 027 and 13 962 0 067 j bar mol the nonideal behavior of the muscovite ms mg al celadomite series was modelled using an asymmetric margules formalism with the following parameters w12h w21h 0 w12s 58 598 8 804 w21s 15 920 3 393 w12v 0 735 0 118 w21v 0 187 0 026 with units as above for the muscovite fe2 al celadonite series we used a symmetric model with wh 0 ws 24 083 3 088 j k mol and wv 0 3394 0 1120 j bar mol the above data were applied to construct a petrogenetic grid the example chosen is related to the kfash system with potassic white mica quartz and h2o in excess it covers the t range from 150 c to 550 c at a p up to 28 kbar the new thermodynamic data are also discussed in regard to their use in geothermobarometry

phase relations and chemical composition of phengite and paragonite in pelitic schists during decompression a case study from the monte rosa nappe and camughera moncucco unit western alps the metamorphic evolution of metapelites from the eastern part of the monte rosa nappe and the camughera moncucco unit both situated in the upper penninic units sw of the simplon line were investigated using microstructural relationships and equilibrium phase diagrams the units under consideration experienced prealpine amphibolite facies conditions and underwent a complex metamorphic evolution during the alpine orogeny peak pressures during an early alpine high pressure stage of 12 5 16 kbar were similar in the monte rosa nappe and camughera moncucco unit a pronounced thermal gradient is indicated during decompression leading to an amphibolite facies overprint as the decompression paths went through the chlorite biotite and plagioclase stability fields in most of the monte rosa nappe through the staurolite field in the easternmost monte rosa nappe and in the camughera moncucco unit and through the sillimanite field in the easternmost camughera moncucco unit in high al metapelites the initial formation of staurolite is related to continuous paragonite breakdown and associated formation of biotite in the course of this reaction phengite becomes successively sodium enriched in low al metapelites in contrast the initial staurolite formation occurs via the continuous breakdown of sodium rich phengite in both low and high al metapelites the largest volume of staurolite is formed during the continuous breakdown of sodium rich phengite below p t conditions of about 9 5 kbar at 600 650 c during this reaction phengite becomes successively potassium enriched as sodium from phengite is used to form the albite component in plagioclase for normal pelitic chemistries phengite becomes na enriched during decompression through the breakdown of paragonite along a nearisothermal decompression path the na content in phengite reaches its maximum when paragonite is entirely consumed during further decompression the paragonite component in phengite decreases again because na is preferentially incorporated into the albite component of plagioclase

mineral reactions and element migration during metamorphism of calcareous sediments from the vassalboro formation south central maine k na and co 2 were progressively lost during the metamorphic event no mass transfer of fe mg ti si or ca sodium was probably lost through a mineral fluid hydrolysis reaction that destroyed the albite component of plagioclase and produced quartz anorthite and a soluble na bearing fluid species potassium was probably lost independently through a mineral fluid hydrolysis reaction that destroyed biotite and formed calcic amphibole sphene anorthite and a soluble k bearing fluid species h 2 o was also progressively lost observed amount of mass transport of k and na requires 1 10 rock volumes of fluid interacting with the carbonate rocks regional metamorphism considered as possible large scale acid metasomatism modified journal abstract

spatial correlation of deformation and mineral reaction in experimentally deformed plagioclase olivine aggregates abstract shear deformation of hot pressed plagioclase olivine aggregates was studied in the presence and absence of mineral reaction experiments were performed at 900 c 1500 mpa and a constant shear strain rate of 5 10 5 s 1 in a solid medium apparatus whether the mineral reaction between plagioclase and olivine takes place or not is controlled by choosing the appropriate plagioclase composition labradorite an 60 does not react anorthite an 92 does labradorite olivine aggregates deformed without reaction are very strong and show strain hardening throughout the experiment syndeformational reaction between olivine and anorthite causes a pronounced strain weakening the reaction produces fine grained opx cpx spinel aggregates which accommodate a large fraction of the finite strain deformation and reaction are localised within a 0 5 mm wide sample three representative samples were analysed for their fabric anisotropy r and shape preferred orientation fabric angle with the shear plane using the autocorrelation function acf fabric anisotropy can be calibrated to quantify strain variations across the sheared samples in the deformed and reacted anorthite olivine aggregate there is a strong correlation between reaction progress and strain regions of large shear strain correspond to regions of maximum reaction progress within the sample the derived strain rate variations range up to almost one order of magnitude

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

substorm convection patterns observed by the super dual auroral radar network a study of convection patterns during substorms observed by the super dual auroral radar network superdarn radar network is presented specific attention is given to the growth phase and the time immediately following expansion onset it is found that the growth phase is characterized by the enhancement of the velocity shear near midnight and its extension to low latitudes and to local times across the midnight meridian the velocity shear was observed to diminish at expansion onset in addition the ionospheric flow velocity magnitude was observed to be enhanced for some period of time prior to expansion onset and to decrease at expansion onset possible explanations for the establishment of the midnight velocity shear during growth phase and its subsequent disappearance at expansion onset are presented also arguments are presented regarding the decrease of ionospheric flow velocity at expansion onset

toward an observational synthesis of substorm models precipitation regions and high latitude convection reversals observed in the nightside auroral oval by dmsp satellites and hf radars a combination of simultaneous measurements from high low and ground altitude instruments has been used to infer the rapid evolution of the coupled nightside magnetosphere ionosphere during substorms reversals from an eastward zonal convection to westward zonal convection become apparent inside the volume of the substorm bulge a few minutes after the intensification of the westward electrojet these reversals persist for periods of 10 20 min and appear to have a one to one correspondence with the occurrence of dipolarizations the changes in convection are accompanied by changes in precipitation flux depletion regions fdr are measured by the dmsp satellites inside the surge near the equatorward portion of the westward electrojet intensification the poleward boundary of every fdr is collocated with a convection reversal and an arc intensification that marks a poleward transition into a region initially dominated by intense discrete electron precipitation and velocity dispersed ion structures vdis convection in the fdr constitutes an eastward electrojet channel that may produce a transient recovery signature as observed by ground magnetometers the observations of fdr s and the fast westward flows that accompany them are consistent with the scenario of a rarefaction and or a neutral line in the near earth tail that produces fast earthward flows after the breakup the arc intensification at the poleward boundary of the depletion region and the collocated transient convection reversal favor an enhancement of the magnetosphere ionosphere coupling and thus the continuation of the substorm expansion in a multiple cell convection system arguments are presented to explain how a series of pseudobreakups modify the near earth magnetic field into an increasingly dipolar geometry until a breakup is possible

the determination of time stationary two dimensional convection patterns with single station radars at the present time most ground based radar estimations of ionospheric convection use observations from single station facilities this approach requires certain assumptions as to the spatial and or temporal uniformity of the convection in this paper the authors present a critical examination of the accuracy of these vector velocity determinations using realistic modeled flow patterns that are time stationary but not spatially uniform they find that under certain circumstances the actual and inferred flow fields show considerable discrepancy sometimes not even agreeing in the sense of flow direction specifically they show that the natural curvature present in ionospheric convection on varying spatial scales can introduce significant error in the velocity estimate particularly when the radius of curvature of the flow structure is less than or equal to the radar range to the scattering volume the presence of flow curvature cannot be detected by radars which determine velocities from measurements in two viewing directions and it might not be detected by radars using azimuth scanning techniques thus they argue that every effort should be made to measure the ionospheric convection by bidirectional or multidirectional observations of a common ionospheric volume and that a synthesis of coherent and incoherent radar observations from different sites ismore preferable to multidirectional single station observations using either radar alone these conclusions are applicable to any doppler measurement technique and are equally valid for high latitude wind patterns using fabry perot interferometer techniques less

identification of the scales of differential element mobility in a ductile fault zone two fundamentally different scales of element mobility have been identified in the upper amphibolite facies hunts brook fault zone of south central connecticut usa field relations mineral chemistry and 80 bulk rock analyses provide overwhelming evidence that the blastomylonitic schists and gneisses of the fault zone were derived from the enclosing granodioritic rope ferry orthogneiss two sample mass balance calculations between the rope ferry gneiss and fault rocks strongly suggest that the rope ferry gneiss was segregated into biotite rich schists and quartz plagioclase rich gneisses through the centimetre scale transfer of si al ca na ba and zr from schists to gneisses three and four sample mass balance calculations provide convincing evidence that al ca na and ba were lost and si and k gained by the entire fault zone both scales of metasomatism have been identified at two localities separated by 20 km implying that the metasomatic processes were pervasive throughout the fault zone from the mass balance calculations an apparent disparity in k mobility arises k appears to be gained by the entire fault zone but is immobile during the centimetre scale segregation this is because mass balance calculations can only detect differential element mobility differential element mobility requires that the mechanism of metasomatism produces chemical potential gradients therefore the identification of differentially mobile elements provides insight into the mechanisms of metasomatism the immobility of k during segregation of schists and gneisses implies that the segregation was not achieved through the extraction of partial melt however the element mobility is reconcilable with deformation driven metamorphic differentiation the gains of si and k and losses of ca na al and ba for the entire fault zone are also inconsistent with the intrusion or extraction of partial melt but may reflect the infiltration of a metasomatizing aqueous fluid

rheological weakening of high grade mylonites during low temperature retrogression the exhumed continental ailao shan red river fault zone se asia abstract we present a detailed case study of an exhumed continental strike slip fault zone the ailao shan red river asrr strike slip fault zone to investigate how deformation promotes strain localization and how the weak second phases and fluids trigger rheological weakening during retrogression near the ductile to brittle transition during exhumation along the asrr strike slip fault zone in the diancang shan dcs metamorphic massif high temperature ductile deformation d 1 pervasively occurred during shearing and exhumation since late oligocene the high temperature microstructures and textures are in part or entirely altered by subsequent low temperature shearing d 2 since late miocene which is under the conditions of frictional viscous transition of k feldspar ca 450 c during further exhumation to the upper crustal levels the formation of d 2 microstructures and shear bands overprinted high temperature intracrystalline plasticity phases d 1 in mylonitic rocks depending on the main rock forming minerals the results also demonstrate that the brittle ductile transition involves a combination of different deformation mechanisms and possible rheological paths in quartz rich rocks quartz was deformed in the dislocation creep regime and records transition of microfabrics and slip systems during decreasing temperature which lasted until retrogression related to exhumation as a result grain size reduction associated with fluids circulating within the asrr strike slip fault zone at brittle ductile transition leads to reaction and texture weakening rheological weakening is the consequence of the syntectonic deformation fluid flow reaction softening reaction creep and textural softening the hydrous fluids resulted in hydration of silicates decompression occurred during shearing and as a result of tectonic exhumation all these results demonstrate that the exhumation through the ductile to ductile brittle transition involves a combination of different deformation mechanisms rheological transition features and feedbacks between deformation decreasing temperature and fluids in the asrr fault zone

deep fluid circulation in alpine shear zones pyrenees france field and oxygen isotope studies a combined field stable isotope and whole rock chemical study was made on late cretaceous to tertiary metasomatic shear zones cutting hercynian gneisses in the aston massif pyrenees france mylonitisation occurred during the early stages of alpine compression under retrograde conditions at 400 450 c and about 10 km depth whole rock 18o values of 11 to 12 in the gneisses was lowered to 5 to 9 in the shear zones with the quartz muscovite 18o 16o fractionations of about 2 to 4 essentially unchanged these 18o 16o systematics together with d muscovite 40 to 50 indicate that large volumes of formation waters or d rich meteoric waters passed through the shear zones during deformation the same fluids also redistributed major elements as shown by the correlation of 18o shift with muscovitisation and albitisation reactions in granitic wall rocks however even though 18o was universally lowered within the shear zones the 18o 16o ratios were not homogenised nor do they correlate in detail with the presence or absence of muscovitisation suggesting that fluid flow was probably fracture controlled and episodic field mapping shows that along the length of a particular shear zone muscovitisation of granite gneiss dies out 150m above the contact with underlying sillimanite gneiss thus muscovitisation and albitisation of granite gneiss in shear zones and their wall rocks probably occurred during re equilibration of acidic chloride rich aqueous fluids that had previously moved upward within the shear zones through underlying sillimanite gneiss extremely high material balance fluid rock ratios 103 are required to explain the extent of muscovitisation along this shear zone implying integrated fluid mass fluxes of about 108 kg m2 this is probably close to the maximum value for other shear zones in the network similar volumes of a more chemically evolved fluid must have passed through the unmuscovitised mylonites showing that the absence of alteration cannot necessarily be used to infer low values of fluid flux for reasonable pressure gradients and time scales of fluid movement effective permeabilities of 10 15 to 10 17 m2 are required such values can be accounted for by short lived widely spaced cracks produced during seismic activity a model is presented in which formation waters were seismically pumped down an underlying shallow southward dipping decollement and then upward through the steeply dipping shear zone network

kinematics of ductile shearing from outcrop to crustal scale in the monte rosa nappe western alps significant ductile deformation has produced widespread mylonitic structures in the basement orthogneisses and the mesozoic cover quartzites and marbles of the monte rosa nappe in the inner part of the western alps we summarize here the results of a detailed microstructural study at several scales of observation in the northern part of the nappe and discuss the kinematic significance and compatibility of such results most of the observed deformations can be accounted for by progressive wnw vergent shearing during and after the late eocene within a ductile shear zone of crustal scale the displacement of the hanging wall may have been in excess of several tens of kilometers however some ese vergent shear criteria cannot be reconciled with this general picture and suggest that local backthrusting occurred later near the toe of the monte rosa nappe

mid crustal metasomatic reaction veins ina spinel peridotite in central dronning maud land east antarctica rare metresized lenses of spinel peridotite are enclosed in high grade metamorphic rocks the rocks experienced a medium p granulite facies metamorphism at 575ma and a low p amphibolite facies overprint at 530ma the latter is probably related to extensive granitoid magmatism between 530 and 500ma which produced large volumes about half of the outcrops today of granitic to syenitic rocks as well as abundant k feldspar quartz pegmatites one of the spinel peridotite lenses in the schirmacher oasis of central dronning maud land is crosscut by several small up to 10 cm wide veins with a characteristic zoned sequence of mineral assemblages which was formed by reaction of a hydrous sio2 saturated fluid or pegmatitic melt with the peridotite the zoned sequence consists of the following mineral assemblages from the centre of the vein towards the outer margin zone 0 plagioclase quartz zone 1 green biotite intergrown with zircon clinoamphibole zone 2 cummingtonite dark brown biotite intergrown with rutile clinoamphibole zone 3 cummingtonite light brown biotite spinel zone 4 olivine orthopyroxene spinel clinopyroxene unaltered peridotite this sequence was investigated with respect to its conditions of formation modal mineralogy mineral chemistry fluid inclusions and oxygen and hydrogen isotope compositions of selected minerals based on the stability of cummingtonite and on equilibrium calculations in the mgo sio2 h2o system and on quartz biotite oxygen isotope thermometry the reaction vein formed at 650 c which is in accord with typical pegmatite crystallization temperatures the pegmatite of zone 0 is interpreted to have formed in an open fissure whereas on textural grounds zone 3 replaces former peridotite on the basis of mass balance constraints the boundary between zones 1 and 2 is interpreted to approximately represent the former boundary between peridotite and the open fissure before reaction oxygen isotope systematics show that the infiltrating fluid had an isotopic composition of 9 101 2 smow all minerals of the reaction vein with the exception of the inherited spinel and olivine in the adjacent peridotite are in equilibrium with such a fluid spinel in the peridotite is depleted in o compared with coexisting olivine which suggests isotopic disequilibrium spinel in zone 3 has a distinctly different isotopic composition compared with that in the peridotite apparently approaching but not reaching equilibrium the combination of mineral chemistry and mass balance constraints of the modal mineralogy constrains the volume change during metasomatism and the direction of elemental diffusion it is indicated that mg cr and ni always diffused towards the vein whereas si al k na h2o and possibly fe diffused into the peridotite

empirical high latitude electric field models electric field measurements from the dynamics explorer 2 satellite have been analyzed to extend the empirical models previously developed from dawn dusk ogo 6 measurements j p heppner 1977 the analysis embraces large quantities of data from polar crossings entering and exiting the high latitudes in all magnetic local time zones paralleling the previous analysis the modeling is based on the distinctly different polar cap and dayside convective patterns that occur as a function of the sign of the y component of the interplanetary magnetic field the objective which is to represent the typical distributions of convective electric fields with a minimum number of characteristic patterns is met by deriving one pattern model bc for the northern hemisphere with a y interplanetary magnetic field imf and southern hemisphere with a y imf and two patterns models a and de for the northern hemisphere with a y imf and southern hemisphere with a y imf the most significant large scale revisions of the ogo 6 models are 1 on the dayside where the latitudinal overlap of morning and evening convection cells reverses with the sign of the imf y component 2 on the nightside where a westward flow region poleward from the harang discontinuity appears under model bc conditions and 3 magnetic local time shifts in the positions of the convection cell foci the modeling above was followed by a detailed examination of cases where the imf z component was clearly positive northward neglecting the seasonally dependent cases where irregularities obscure pattern recognition the observations range from reasonable agreement with the new bc and de models to cases where different characteristics appeared primarily at dayside high latitudes the analysis of these differences for strong bz conditions using passes through different mlt zones led to finding that the deviations from both the bc and de models could be modeled by extending the evening cell foci toward and beyond noon with a deformation that roughly resembles a rotational twist of the extended evening cell the morning cell accommodation to the model bc deformation produces a narrow tongue of eastward convection near 75 invariant latitude on the dayside the model de deformation compresses the morning cell toward the night hours such that the sunward convection in the morning cell is diverted to polar latitudes in the 0400 to 0700 mlt sector in both cases the deformations of the two cell patterns lead to sunward convection in dayside polar regions while maintaining the integrity of the nightside convection pattern thus the nightside dilemmas that plague three and four cell models designed to explain sunward convection in polar regions under bz conditions do not appear

petrotectonics of lawsonite eclogite exhumation insights from the sivrihisar massif turkey 1 the sivrihisar massif of the tavsanli zone of turkey is 1 of less than 10 known lawsonite eclogite localities worldwide rocks of the sivrihisar massif consist of eclogite and blueschist in contact with metasedimentary host rocks and record decreasing maximum pressure conditions across three wnw ese striking belts from 16 24 kbar in the northern halilbagi belt to 14 16 kbar in the karacaoren belt and 8 10 kbar in the kertek belt where present sodic amphibole phengite chlorite and quartz define a pervasive s0 s1 foliation garnet omphacite and lawsonite define stretching lineations and kinematic indicators d1 and d2 structures are similar in the halilbagi and karacaoren belts but differ to those in the kertek belt d3 structures are uniform across the massif including fibrous calcite that occurs parallel to f3 fold axes shear sense indicators from field observations and asymmetric type i cross girdle of quartz c axes obtained from electron backscatter diffraction ebsd show top to the south thrusting throughout much of the massif d1 and d2 structures are interpreted to have formed during exhumation by extrusion along a 5 c km gradient the halilbagi and karacaoren belts were juxtaposed possibly as deep as 45 km within the subduction channel and exhumed by the arrival of the anatolide microcontinent at approximately 70 ma homogeneity of f3 axes and calcite fibers across the massif suggests that assembly occurred at blueschist conditions before exhumation through the aragonite calcite transition 350 c above 8 kbar

sm nd isotopic evidence on the age of eclogitization in the zermatt saas ophiolite rocks within the zermatt saas ophiolite of the western alps have undergone eclogite facies metamorphism during subduction prior to the alpine collision the metamorphic history of these rocks is well defined with eclogitic assemblages being followed by the limited growth of blueschist assemblages of glaucophane and paragonite subsequent greenschist alteration occurs adjacent to faults veins and metasediments away from such sources of water retrogression is very limited sm nd isotopic analyses of an essentially unretrogressed eclogitic metabasalt suggest that eclogite facies metamorphism occurred at 52 18 ma the large uncertainty is due to the presence of very small amounts of nd rich epidote present as inclusions within garnet as the closure temperature of garnet to sm nd is thought to be 600dc resetting due to post high pressure diffusion is thought to be insignificant given the fine grained protolith to the sample analysed and its extensive deformation under eclogite facies conditions incomplete homogenization of pre metamorphic isotopic variations is also considered unlikely to be responsible for the young age a tertiary age of eclogitization means that models of early alpine evolution based on the cessation of high pressure metamorphism in the cretaceous need to be revised

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

deformation of granitoids at low metamorphic grade ii granular flow in albite rich mylonites abstract the high strain deformation at low metamorphic grades of three investigated granitoids is dominated by the granular flow of albite rich polyphase aggregates these aggregates formed by retrograde breakdown reactions from intermediate plagioclase and k feldspar the predominant deformation mechanism changes in granitoids as the grain size is reduced coarse grained low strain examples are deformed by a combination of intracrystalline plasticity quartz and fracturing feldspar in the mylonites intracrystalline plasticity of quartz plays only a minor role and the dominant deformation mechanism is a non cataclastic granular flow of polyphase aggregates consisting largely of albite and quartz deformation appears to be stable probably because grain growth in albite quartz mixtures is inhibited these fine grained aggregates are mechanically weaker than pure quartz aggregates thus the change in deformation mechanism mainly due to feldspar breakdown reactions appears to be important for the localization of high shear strain deformation at low metamorphic grades in granitoids and other lithologies modally dominated by feldspar the rheological behaviour of albite dominated mineral aggregates may have two consequences for middle to upper crustal deformation of modally feldspar dominated lithologies 1 feldspar in the presence of aqueous fluids can be an important mineral controlling the rheology at low metamorphic grades that is below amphibolite facies p t conditions 2 the occurrence of granular flow of fine grained polyphase aggregates in low grade granitoids is probably common and calls for great care in the modelling of middle to upper crustal rheology based on flow laws for intracrystalline plasticity of single minerals such as quartz

subduction interface processes recorded by eclogite facies shear zones monviso w alps abstract the monviso ophiolite lago superiore unit constitutes a well preserved almost continuous upper fragment of oceanic lithosphere subducted at c 80 km depth thereby providing a unique opportunity to study mechanical coupling processes and meter scale fluid rock interactions occurring at such depths in present day subduction zones it is made of i a variably thick 50 500 m section of eclogitized basaltic crust associated with minor calcschist lenses overlying a 100 400 m thick metagabbroic body and of ii a c 1 km thick serpentinite sole we herein focus on the three major eclogite facies shear zones found at the top of the unit at the boundary between basalts and gabbros and between gabbros and serpentinites respectively strain localization occurred at lithological interfaces irrespective of material strength while ductile deformation dominates along the shear zones local brittle behavior is demonstrated by the existence of numerous eclogite breccias of fe ti metagabbros and widespread garnet fractures possibly linked with intermediate depth eclogite facies micro seismicity these m to hm sized fragments of fe ti metagabbros were later sheared and disseminated within serpentinite schists along the gabbro serpentinite boundary lower shear zone lsz pervasive and focused fluid flow is attested in the lsz by significant alteration of bulk rock compositions weakening of the rocks and widespread crystallization of hydrous parageneses by contrast the intermediate shear zone isz shows evidence for more restricted short range fluid flow the activity of both the isz and lsz ceased during early lawsonite eclogite facies exhumation when deformation localized deeper within the serpentinite sole allowing for the detachment and preservation of this large ophiolitic fragment

structural evolution of the contact between two penninic nappes zermatt saas zone and combin zone western alps and implications for the exhumation mechanism and palaeogeography the boundary zone between two penninic nappes the eclogite facies to ultrahigh pressure zermatt saas zone in the footwall and the blueschist facies combin zone in the hanging wall has been interpreted previously as a major normal fault reflecting synorogenic crustal extension quartz textures of mylonites from this fault were measured using neutron diffraction together with structural field observations the data allow a refined reconstruction of the kinematic evolution of the pennine nappes the main results are 1 the contact is not a normal fault but a major thrust towards northwest which was only later overprinted by southeast directed normal faulting 2 exhumation of the footwall rocks did not occur during crustal extension but during crustal shortening 3 the sesia dent blanche nappe system originated from a continental fragment cervinia in the alpine tethys ocean and the combin zone ophiolites from the ocean basin southeast of cervinia 4 out of sequence thrusting played a major role in the tectonic evolution of the penninic nappes

the effects of quartz recrystallization and reaction on weak phase interconnection strain localization and evolution of microstructure abstract we conducted axial compression and general shear experiments at t 900 c and p 1 5 gpa on samples of banded iron formation bif and synthetic aggregates of quartz hematite and magnetite to investigate how dynamic recrystallization of quartz promotes strain localization and the role of weak second phases oxides on the rheology and microstructural evolution of the aggregates experiments showed strain localization into oxide rich layers and that the oxide content and oxide distribution are key factors for the strength of the aggregate only 2 10 wt hematite leads to pronounced weakening and increasing hematite content above 10 has only a minor additional effect where oxide grains are dispersed the initial strength contrast with quartz induces stress concentrations at their tips promoting high stress recrystallization accommodated dislocation creep of quartz fine recrystallized quartz reacts with oxide forming trails of fine reaction product ferrosilite fayalite leading to the interconnection percolation of a weaker matrix the strength contrast between the quartz framework and these fine grained trails promotes strain localization into micro shear zones inducing drastic strain weakening thus dynamic recrystallization of quartz promotes syn deformational reactions leading to a microstructurally controlled evolution of phase strength contrast it results in a rheologic transition from load bearing framework to a matrix controlled rheology with transition from s c to s c fabric with increasing strain