defect structures in naturally deformed clinoamphiboles a tem study abstract naturally deformed clinoamphiboles from the selbu tydal and forsback tarnaby areas of the scandinavian caledonides exhibit a well defined subgrain microstructure from a transmission electron microscopy study tem the subgrain boundaries are shown to consist of arrays of positive and negative screw dislocations with burgers vector b 001 locally expanded loops are present having long screw segments the subgrain boundaries are parallel to rational crystallographic planes of the type hk 0 the density of isolated dislocations within the subgrains is low in addition planar defect structures parallel to 010 and bounded by screw dislocations with b x 001 x are observed based on metamorphic criteria the p t values at the time of the amphibole growth have been estimated at 450 600 c and 4 6 kbar and these represent maximum conditions for the deformation the present results indicate that slip on hk 0 001 is an operative deformation mechanism in naturally deformed clinoamphiboles

island arc and mid ocean ridge volcanism modelled by diapirism from linear source regions abstract the gravitational instability of buoyant regions of partial melt in the upper mantle has been proposed as the underlying cause of regularly spaced volcanism at both island arcs and mid ocean ridges motivated by this suggestion we report a laboratory study of diapirism initiated by the release of buoyant fluid from a linear source into a homogeneous viscous environment fixed volume and fixed flux releases of buoyant fluid are studied a fixed volume release was obtained by rapidly towing a point source of fluid through a body of denser fluid the horizontal cylinder of fluid which was left behind by the source subsequently underwent a gravitational instability yielding plumes with a characteristic spacing expressions for the mean spacing of the plumes and for the growth rate of instability are derived by dimensional arguments and are compared with the experimental results both quantities are found to be independent of the viscosity of the buoyant fluid a surprising result which we have confirmed theoretically in contrast the plume spacing and growth rate produced by the gravitational instability of a layer of buoyant fluid each depend on the cube root of its viscosity in the light of these differing results we conclude that the geometry of the buoyant region of partial melt must be considered when developing gravitational instability models of island arc and mid ocean ridge volcanism we also conclude that if the expressions for a cylindrical region are appropriate then the observed spacing of volcanoes cannot be used to infer the viscosity and hence the melt fraction of the source region a fixed flux release of fluid was achieved by injecting buoyant fluid through a narrow slot in the base of a large tank of denser fluid the interface between the two fluids was found to be unstable and diapirs were formed however the rising diapirs were followed by a continuous uniform sheet of buoyant fluid we suggest therefore that thermal or rheological effects need to be included in fluid mechanical models if volcanism which is periodic in position and time is to be explained

resurfacing of deeply buried oceanic crust in naga hills ophiolite north east india petrofabric microstructure and seismic properties abstract we have studied the microstructure texture and seismic anisotropy of a foliated blueschist eclogite rock in the naga hills ophiolite nho complex north east india the rock is composed essentially of omphacite sodic amphibole glaucophane sodic calcic amphibole barroisite phengite and garnet and represents oceanic crust that experienced neo tethyan subduction and metamorphism before it resurfaced and accreted in the ophiolite omphacite and glaucophane show weak shape preferred orientation spo while phengite displays a strong spo the crystallographic preferred orientation cpo of omphacite is characterized by the 001 axes gridle within the foliation and the 010 poles concentrated sub perpendicular to the foliation for glaucophane the 001 axis aligns parallel to lineation and the 100 axis and 110 pole plunge perpendicular to foliation these cpos correspond to sl type fabrics related to plane strain deformation geometry based on our analysis we propose that the observed texture had developed from plastic deformation through dislocation creep employing the textural data we have presented the seismic anisotropies of the individual minerals blueschist eclogite domains and discussed their contributions in the bulk rock anisotropy the calculated bulk rock seismic anisotropies avp and avs of the rock are 12 8 and 8 1 respectively which correlate with the observed seismic anisotropy of the subducting oceanic crust

chemically induced grain boundary migration in calcite temperature dependence phenomenology and possible applications to geologic systems chemically induced grain boundary migration cigm is a solid state reaction mechanism which involves grain boundary migration initially straight grain boundaries in calcite bicrystals exhibit cigm when exposed to a srco3 or baco3 rich melt at temperatures of 680 840 c in a 1 bar co2 atmosphere under these conditions cigm is apparently a much more efficient way to form solid solutions than lattice diffusion the maximum observed rate of migration 1 10 9 m s occurred at about 760 c below that temperature migration was thermally activated but above it the process was thermally inhibited some portions of the boundary swept through a given crystal volume more than once in the ca sr co3 system the solute concentrations incorporated during a single pass of the boundary were far less than values expected at chemical equilibrium and departed further from equilibrium at higher temperatures cigm may be geologically important when the distances characteristic of grain boundary migration and diffusion are much larger than those characteristic of lattice diffusion the great similarity between results in ionic calcite and metals systems suggests cigm is a very general process which may be expected to occur in a large number of geologic systems

late holocene solifluction lobe activity in the mount rae area southern canadian rocky mountains solifluction lobes are tongue shaped accumulations of sediment resulting from localized periglacial mass wasting radiocarbon records from beneath two turf banked lobes in the mount rae area of the southern canadian rockies indicate that solifluction processes have been continuously active for at least the last 2000 years the long term rates of frontal movement at both sites average 0 49 cm year but vary in magnitude from 0 35 to 1 50 cm year both lobes terminate above soil pedons progressively overridden by their advance estimates of the apparent mean residence time of the contemporary soil ranges from 962 100 years in one case to 1600 100 years in the other this information was used to reconstruct a chronology of lobe activity collectively the radiocarbon records indicate that solifluction lobes in this area were advancing quite rapidly between 1900 1750 years bp but declined to a much slower but relatively constant pace up until the present

sediment movement rates and processes on cinder cones in the maritime subantarctic marion island sediment transport in the scoria areas of marion island is primarily the result of needle ice induced frost creep associated with diurnal soil frost cycles clasts move most rapidly in ne textured areas 532 mm a 1 sd 382 more slowly in stony areas 161 mm a 1 sd 179 and most slowly in blocky areas 26 mm a 1 sd 23 movement rates increase with increasing frost susceptibility of sediments slope angle and altitude the heave of dowels indicates that frost heave is active in all the scoria areas examined the depth of effective frost heave increases with increasing altitude with frost heave being restricted to the upper 100 mm of the soil in low altitude areas 200 m the heave of 150 mm dowels at the higher altitude sites provides evidence for segregation ice formation at depths greater than those associated with needle ice and diurnal soil frost cycles vertical movement pro les show a concave downslope pro le with sediment movement rates being most rapid at the soil surface and decreasing rapidly with depth this pro le shape is typical of areas dominated by diurnal freeze thaw cycles and needle ice the capture of sediments moving downslope in troughs and the sampling of material lifted by needle ice suggest that sediment transport by needle ice under present conditions is extremely effective observations suggest that although both ne material and clasts are transported downslope some preferential transport of clasts occurs experiment results and observations of soil frost processes suggest that frost creep associated with needle ice activity is the dominant slope process in the scoria areas of marion island other slope processes such as slopewash and debris ows appear to play a relatively minor and localized role in sediment transport it is suggested that needle ice activity is likely to be the dominant geomorphic agent in other areas of the subantarctic with similar climatic characteristics to marion island copyright 2004 john wiley sons ltd

dynamic recrystallization and chemical evolution of clinoamphibole from senja norway clinoamphibole from a mylonitic amphibolite exhibits microstructures characteristic of dynamic recrystallization including porphyroclasts in a finer grained matrix of needle shaped amphibole the matrix amphibole defines an ls fabric and porphyroclasts have core and mantle structures with a core containing undulose to patchy extinction and 100 deformation twinning surrounded by a mantle of recrystallized grains in addition intragranular grains also occur within the cores tem analyses of the porphyroclasts reveal that they contain a wide variety of lattice defects including high densities 5 108cm 2 of free dislocations and dislocation arrays dissociated dislocations stacking faults and 100 micro twins tem also shows that matrix grains and intragranular grains have relatively low defect densities and that the intragranular new grains occur at localities in the porphyroclasts characterized by high densities of dislocations these observations along with the chemical and orientation relationships between the recrystallized grains and porphyroclasts indicate that the new grains may have formed by heterogeneous nucleation and that further growth probably occurred by both strain assisted and chemically induced grain boundary migration or liquid film migration this recrystallization event is interpreted to be synkinematic based on the fact that no recrystallization textures are present in the matrix grains and that the matrix grains define an ls fabric however the low defect densities in the matrix grains and the lack of intracrystalline strain in other phases indicate that post kinematic recovery processes were active

imaging canary island hotspot material beneath the lithosphere of morocco and southern spain abstract the westernmost mediterranean has developed into its present day tectonic configuration as a result of complex interactions between late stage subduction of the neo tethys ocean continental collision of africa and eurasia and the canary island mantle plume this study utilizes s receiver functions srfs from over 360 broadband seismic stations to seismically image the lithosphere and uppermost mantle from southern spain through morocco and the canary islands the lithospheric thickness ranges from 65 km beneath the atlas mountains and the active volcanic islands to over 210 km beneath the cratonic lithosphere in southern morocco the common conversion point ccp volume of the srfs indicates that thinned lithosphere extends from beneath the canary islands offshore southwestern morocco to beneath the continental lithosphere of the atlas mountains and then thickens abruptly at the west african craton beneath thin lithosphere between the canary hot spot and southern spain including below the atlas mountains and the alboran sea there are distinct pockets of low velocity material as inferred from high amplitude positive sub lithospheric conversions in the srfs these regions of low seismic velocity at the base of the lithosphere extend beneath the areas of pliocene quaternary magmatism which has been linked to a canary hotspot source via geochemical signatures however we find that this volume of low velocity material is discontinuous along strike and occurs only in areas of recent volcanism and where asthenospheric mantle flow is identified with shear wave splitting analyses we propose that the low velocity structure beneath the lithosphere is material flowing sub horizontally northeastwards beneath morocco from the tilted canary island plume and the small localized volcanoes are the result of small scale upwellings from this material

structural evolution of intermediate crustal rocks in a strike slip and extensional setting variscan odenwald sw germany differential upward transport of metamorphic complexes and changing deformation mechanisms abstract the variscan basement exposed in the odenwald mountains southwest germany is composed of several allochthonous metamorphic complexes showing low pressure metamorphism with varying pt conditions and of syntectonic plutons i and s type during the late devonian early carboniferous times rapid uplift of these rocks caused rapid cooling before and during the thermal peak of metamorphism the metamorphic rocks experienced penetrative strain d 1 d 2 which was probably generated by crustal shortening and stacking the resulting micro structures infer distributed strain activation of c slip in quartz and postkinematic annealing during and after the thermal peak of metamorphism ne sw trending zones of normal oblique sinistral strike shearing d 3 and nne ssw trending zones of normal shearing d 4 developed these episodes of deformation are characterized by strong localization of strain strain softening was caused by reorientation of the lattice of quartz toward an orientation more favorable for easy glide along a and by a change in deformation mechanisms of feldspar from dislocation creep to grain size sensitive creep normal oblique strike shearing and normal shearing caused juxtaposition of mid crustal slivers of dissimilar pt and structural histories and provided mechanisms for their denudation detachment of rocks occurred along domains in which strain softening was operative further transtension and extension facilitated uplift and emplacement of syntectonic plutons which experienced also d 3 and d 4 strain in the plutonic rocks strain was broadly distributed in the initial hot stages and became increasingly localized as the deformation continued through cooling

plumes in a convecting mantle models and observations for individual hotspots the motion of hotspots and the deformation of their underlying plume conduits as calculated within models of global mantle flow are presented a new list of 44 possible hotspots with associated tracks has been compiled for all of them calculations have been performed under consideration of individual age and anomalous mass flux for three different models of plume buoyancy and mantle flow plume source depth has usually been assumed to be the top of d but an alternative source depth at the 670 km discontinuity has also been considered using models of relative plate motions and boundaries hotspot tracks on plates have been calculated and compared with age data ocean floor topography and distribution of volcanics on continents absolute plate motions have been redetermined under consideration of hotspot motion using a new least squares method for the hawaiian and yellowstone hotspots source locations and hotspot motion have been computed for a total of up to 23 different models the results show plume conduits being tilted with source regions at the d moving in the lowermost mantle flow generally toward large scale upwellings under southern africa and the south central pacific hotspot surface motion often represents the horizontal component of midmantle flow which is frequently opposite to plate motion toward ridges and away from subduction zones in particular almost all models tested predict southward motion of the hawaii and kerguelen hotspots and westward motion of the iceland hotspot for models including hotspot motion the agreement between calculated and observed hotspot tracks is frequently about as good as or better than for the fixed hotspot model but sometimes fixed hotspots give the best fit in some cases where the track ends at a subduction zone e g for the bowie hotspot results can give indications about the otherwise unknown age of the hotspot in other cases especially for the tahiti hotspot results suggest an origin shallower than d and in yet other cases particularly east africa the failure of the hotspot models used supports other evidence indicating the presence of comparatively broad upwellings rather than localized plumes

deformation mechanisms and phase distribution in mafic high temperature mylonites from the jotun nappe southern norway abstract shear zones and mylonites in anorthositic to gabbroic rocks from the jotun nappe in southern norway have been studied as an example of high temperature deformation in polyphase rocks the deformation has taken place at approximately 700 c and maximum confining pressures of 900 mpa the constituent phases amphibole pyroxene and plagioclase have recrystallized synkinematically reducing the grain size of the rock to about 20 to 70 m two different microstructures develop depending on the recrystallization mechanism dynamic recrystallization of plagioclase hornblende and pyroxenes produces monophase layers which extend from porphyroclasts heterogeneous nucleation of phases due to chemical disequilibrium produces phase mixtures of hornblende and plagioclase or clinopyroxene and plagioclase these phase mixtures extend from hornblende or clinopyroxene porphyroclasts the product of the deformation and simultaneous recrystallization is a well laminated microstructure of some monophase hornblende pyroxene and plagioclase layers which alternate with abundant layers of phase mixtures different deformation mechanisms operate in the different types of microstructures dislocation substructures dynamic recrystallization microstructures and a crystallographic preferred orientation measured by ji and mainprice 1988 on samples of the same shear zone indicate crystal plastic deformation in monophase plagioclase layers dynamic recrystallization in monophase hornblende layers indicates also crystal plastic deformation the phase distribution of plagioclase and hornblende grain shapes and lower dislocation densities in the phase mixtures suggest that the deformation mechanism is not crystal plasticity but rather a granular flow grain boundary sliding probably accommodated by diffusional mass transfer in the plagioclase hornblende mixtures the phases show an anticlustered non random distribution grain boundary sliding may contribute to phase mixing but cannot be the main reason for the anticlustering of phases and the complete phase mixing directly at the boundaries of the porphyroclasts the reason for the anticlustered distribution is not clear but could be explained by preferential heterogeneous nucleation of hornblende at plagioclase grain boundaries

instabilities of fluid conduits in a flowing earth are plates lubricated by the asthenosphere summary a laboratory and theoretical study of the stability of conduits of buoyant fluid in a viscous shear flow has been conducted the object of the study is to explain the formation of discrete islands in island chains such as the hawaiian emperor seamount chain and to investigate a new method by which the variation of shear with depth in the mantle may be determined the conduits were made by injecting oil into a more viscous oil of greater density initially a growing chamber of lower viscosity oil formed near the injector but when the chamber got sufficiently large it rose as a buoyant spheroid behind this trailed a vertical cylindrical conduit through which fluid could continue to rise to the surface as long as the source continued if the more viscous fluid was sheared laterally the conduit was gradually rotated to a more horizontal position the diameter of the conduit increased with time due to a decreasing component of gravitational force along the axis of the conduit when the conduit was tilted to more than 60 with the vertical it began to go unstable by developing bumps which ultimately initiated a new chamber which rose to a new spot in addition if the reynolds number of the conduit was greater than approximately ten an axisymmetric wavy instability appeared in the walls of the conduit and the conduit had to be tilted less before a new chamber was initiated if shear under the pacific plate has to tilt buoyant mantle plumes to as much as 60 to form the relatively regular island chains associated with hot spots most of the shear would be found in a zone with a vertical extent of less than 200 km

tracking denudation and sediment production and transport with cosmogenic 10be in arid high altitude himalayan half grabens zanskar northern india

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

development of lattice preferred orientation in clinoamphiboles deformed under low pressure metamorphic conditions a sem ebsd study of metabasites from the aracena metamorphic belt sw spain the mechanical properties of the lower continental crust are intimately related to the rheology of calcic clinoamphibole naturally deformed clinoamphibole typically shows brittle behaviour under a wide range of deformation conditions whereas crystal plasticity is rarely observed in this study the microfabric of clinoamphiboles from metabasites deformed under low pressure medium to high temperature metamorphic conditions is presented amphiboles from all samples have developed lpo that can be attributed to different deformation mechanisms depending on deformation temperature fluid content structure and phase strength contrasts rigid body rotation of amphibole prisms within a weaker plagioclase matrix corresponds to metabasites deformed under medium deformation temperatures that show a weak layer structure and a high phase strength contrast between plagioclase and clinoamphibole lower temperatures and the presence of fluids mean that one of the studied rocks was affected also by dissolution precipitation creep and cataclastic flow in the final sample dislocation creep was accommodated by recovery and subgrain rotation dynamic recrystallization in monominerallic hornblende layers at higher temperatures which could be representative of the rheology of amphiboles in a lower continental crust at high temperature conditions

semibrittle and cataclastic deformation of hornblende quartz rocks in a ductile shear zone abstract hornblende quartz rock of the coniston shear zone i csz i at the grenville front canada was deformed dominantly by semibrittle flow of hornblende quartz aggregate and locally by cataclastic flow of hornblende aggregates fracturing and slip along fractures produced mesoscopic domino style deformation void formation associated with domino style deformation was prevented by leveling the corners of the slid segments of individual mesodominoes producing narrow shear zones the shear zones eventually coalesced to form the dominant mylonitic c foliation on the microscopic scale semibrittle flow was achieved by dynamic recrystallization of quartz concomitant with microcracking and microfaulting of hornblende into microdominoes and rotation shear and separation of the microdominoes plastic deformation of quartz accommodated the rotation and shear of the microdominoes thereby obviating the space problem local cataclastic flow occurred in the hornblende domains and produced r 1 riedel shear surfaces the observed processes are consistent with greenschist grade retrograde metamorphism accompanying post orogenic uplift along this zone

petrofabric p wave anisotropy and seismic reflectivity of high grade tectonites abstract ji s salisbury m h and hanmer s 1993 petrofabric p wave anisotropy and seismic reflectivity of high grade tectonites tectonophysics 222 195 226 in order to understand seismic anisotropy and its influence on the reflectivity of lower crustal fault zones we have undertaken experimental measurements of p wave velocities up to 600 mpa for 20 granulite and upper amphibolite facies mylonites from the snowbird tectonic zone canada the rocks whose composition ranges from ultramafic to felsic have experienced extremely large ductile strain it is found that the amphibolitic mylonites exhibit significant vp anisotropy 6 13 at 600 mpa whereas the pyroxene bearing granulite facies mylonites and the quartz or feldspathic tonalitic granitic and diatexitic mylonites are quasi isotropic in order to constrain interpretation of the measured vp properties microstructural analyses have been systematically performed in the samples studied it is shown that the interaction between feldspar and quartz or pyroxene and the absence of amphibole cause anisotropy in the granulite facies mylonites to be low the amphibolitic mylonites are strongly anisotropic because of a large volume fraction of hornblende with strong lpo 001 lineation and 100 foliation the anisotropy pattern in the amphibolitic mylonites is consistently orthorhombic vp x vp y vp z where x is parallel to stretching lineation y is normal to the lineation in the foliation plane and z is normal to the foliation plane but the anisotropy pattern in the mica rich mylonites is transversely isotropic vp x vp y vp z if sufficiently large volumes of the crust display such patterns this difference may be an important indicator for kinematic analysis in the middle to lower crust using modern seismic techniques the study demonstrates that while seismic reflectivity is strongly lithology controlled fabric induced anisotropy can both enhance and decrease seismic reflectivity hence the discontinuity of seismic reflections along ductile shear zones in the lower crust is not necessarily indicative of the discontinuity of the shear zones but may indicate changes in composition or metamorphic grade

the effect of dislocation density on the aqueous solubility of quartz and some geologic implications a theoretical approach the internal energy associated with dislocations is calculated in quartz as a function of dislocation density these calculations show that at dislocation densities of 109 1010 1011 and 1012 cm dislocation cm3 the energy stored per mole of quartz is 2 2 19 7 170 and 1426 j respectively the density of free dislocations in natural quartz rarely exceeds 109 line cm cm3 and the resulting increase in free energy is probably too small to affect any geochemical processes significantly this applies to the overwhelming volume of rocks in the earth s crust the density of dislocations in tangles may exceed 1011 line cm 2 which is significant but the volume of quartz containing tangles is a very small fraction of most quartz grains nevertheless where fluid rock ratios are low the aqueous activity of sio2 may be significantly increased by the dissolution of quartz containing dislocation tangles such dissolution could be geologically significant in active greenschist facies fault zones and mylonites where repeated cold working would produce a sustained source and large population of dislocation tangles in such a setting the locally high internal energies of quartz containing dislocation tangles would lead to strain solution the resulting high activity of aqueous silica would enhance the stability of feldspars and would establish chemical potential gradients of sio2 which would drive the diffusive mass transfer of sio2 out of the fault zone

deformation mechanisms in second phase affected microstructures and their energy balance based on the relationship zener parameter z second phase size second phase volume fraction vs calcite grain size dg second phase controlled aggregates and microstructures that are weakly affected by second phases are discriminated the latter are characterized by large but constant grain sizes high calcite grain boundary fractions and crystallographic preferred orientations cpo while calcite grain size and calcite grain boundary fraction decrease continuously and cpo weakens with decreasing z in second phase controlled microstructures these observations suggest that second phase controlled microstructures predominantly deform via granular flow because pinning of calcite grain boundaries reduces the efficiency of dynamic recrystallization favoring mass transfer processes and grain boundary sliding in contrast the balance of grain size reduction and growth by dynamic recrystallization maintains a steady state grain size in microstructures that are only weakly affected by second phases promoting a predominance of dislocation creep with increasing temperature the relationship between z and dg persists but the calcite grain size increases continuously based on microstructures the energy of each modifying process is calculated and its relative contribution is compared with energies of the competing processes surface energy dragging energy dynamic recrystallization energy the steady state microstructures result from a temperature dependent energy minimization procedure of the system

frontal advance of turf banked solifluction lobes kluane range yukon territory canada humus horizons associated with thirteen solifluction lobes on a single slope were radiocarbon dated at fixed distances behind the risers as well as ahead of the lobes the dated horizons together with stratigraphic observations of planar discontinuities inside the lobes and shallow burial of vegetation ahead of the lobes all suggest rapid advance of lobe fronts we infer that lobe advance at this site results from slow accumulation of soliflucted material behind a relatively rigid riser progressive steepening of the riser and build up of stress and finally rupture of the front and its collapse onto the slope rebuilding of the lobe usually then takes place the time for a full cycle of development is estimated to be in the order of a few hundred years long term frontal movement appears inversely related to lobe size and is mainly controlled by soil moisture and sediment characteristics these findings call into question the usefulness of solifluction lobe advance rates for paleoclimate reconstructions at least at this site

processes controlling the shrinkage of porphyroclasts in gabbroic shear zones abstract this paper presents microstructural and geochemical data on amphibole and clinopyroxene porphyroclasts collected in amphibolite facies ultramylonites of the ivrea verbano zone italy the well rounded porphyroclasts show a concentration of defects including 100 twins stacking faults free dislocations subgrains and microcracks within their rims that indicate stress concentrations localised deformation within the margins of porphyroclasts increases reaction kinetics and supports amphibole clinopyroxene replacement reactions and heterogeneous plagioclase nucleation in addition to microcracking and or subgrain rotation these metamorphic reactions lead to a marginal grain size reduction and the shrinkage of porphyroclasts in order to correlate the defect concentration at the rim of porphyroclasts with stresses a finite element inclusion matrix model is used it depends on the degree of bonding between inclusion and matrix whether strong stress concentrations within the rim of the inclusion occur for well coupled two phase systems shear stresses within the rim of the inclusion can reach a magnitude many times the magnitude of the matrix stresses marginal stress concentrations do not evolve if inclusion and matrix are decoupled this may indicate that mechanically decoupled porphyroclasts can survive to very large strains

monitoring periglacial processes towards construction of a global network this paper outlines the strategy for constructing a global monitoring network for periglacial processes the monitoring system should be designed with appropriate choices of parameters and techniques which depend on the purpose of monitoring e g modelling individual processes or assessing the sediment budget of a catchment acquisition of comparable data from globally distributed sites requires standardized techniques and instruments in addition expansion of the monitoring network to poorly accessible periglacial sites benefits from compact cold resistant and maintenance free instruments priority is given to automatic acquisition of year round data that promote understanding of interactions between ground movements and environmental factors examples of previous process monitoring frost wedging solifluction and permafrost creep suggest the significances of 1 high resolution of data for short term small scale processes 2 combination of movement and its major variables and 3 long lasting monitoring that distinguishes long term trend from interannual fluctuation as the first stage of the monitoring network a model experimental site is under construction in svalbard where a variety of periglacial processes coexist in a small area to test and compare various techniques to be standardized

transient discontinuities revisited pseudotachylyte plastic instability and the influence of low pore fluid pressure on deformation processes in the mid crust abstract occurrences of pseudotachylyte cyclically introduced as melts into the outer hebrides thrust are demonstrably synkinematic with crystal plastic mylonites classical interpretations of these rock types as respectively products of pressure dependent frictional melting and thermally activated intracrystalline deformation create a paradox in that these processes are to a large extent mutually exclusive detailed microstructural and microcompositional analyses of host mylonites and primary and deformed pseudotachylyte were carried out by light and electron microscopy the ambient shear zone environment in which pseudotachylyte formed was determined to comprise temperatures in the order of 500 c and stresses of 140 210 mpa on which were imposed much higher transient stresses in response to heterogeneous non uniform flow the latter conditions are sufficient for the development of plastic instabilities in feldspar rich crust if pore fluid pressures are sufficiently low the latter is consistent with the absence of hydration during exhumation observed in the rocks under study low pore fluid pressure during thrust exhumation of deep crust enables activation of high strength ductile processes dominated by dislocation glide which may be a prerequisite to instability whereas other studies have demonstrated the possible occurrence of such melt generating instabilities it is believed that this study provides the first example in which the calculated potential for instability formation is consistent with the deformation microstructures and estimated pressure temperature conditions grain size reduction to produce ultramylonites dominated by grainsize sensitive flow is achieved by both deformation induced dynamic recrystallization and crystallization of instability generated melts

diffusion induced grain boundary migration in calcite grain boundaries in synthetic calcite bicrystals and slabs of yule marble exposed to srco3 at temperatures between 650 and 800 c undergo diffusion induced grain boundary migration digm as the boundary migrates through the crystal it produces a ca sr co3 solid solution digm is known to occur in metals at temperatures as low as one third the melting temperature where lattice diffusion rates are insignificant compared to grain boundary diffusion rates the similarity between microstructures in calcite and metals systems is striking in both systems grain boundaries migrate to increase their total area and reduce their radius of curvature migration occurs in the absence of significant storud internal strain energy and boundary migration is accompanied by changes in the chemical composition of the solid although a widely accepted theory for digm is lacking it is clear that the process involves the formation of a compositionally distinct phase which grows by material transport along an advancing grain or interphase boundary grain boundary migration during digm must be driven at least in part by the reduction of chemical free energy associated with the chemical composition change that takes place behind the advancing boundary although the reaction mechanisms by which multicomponent multiphase systems in the earth reach equilibrium are likely to be numerous and diverse it is possible that some mechanisms are analogous to the digm as observed in laboratory experiments on calcite bicrystals

computational approaches to studying non linear dynamics of the crust and mantle we outline a mathematical formulation for mantle convection which can deal with the viscoelastic plastic rheology of the cool parts of the lithosphere this formulation is then analyzed to expose the numerical challenges inherent in the equations and a suitable solution strategy is outlined with this strategy in place we discuss a parallel implementation explaining how we maintain computational efficiency in a tiered and modular software environment we show two examples from geodynamics which demonstrate the capability of the numerical method and its implementation c 2007 elsevier b v all rights reserved

deformation fabrics of natural blueschists and implications for seismic anisotropy in subducting oceanic crust abstract investigations of microstructures are crucial if we are to understand the seismic anisotropy of subducting oceanic crust and here we report on our systematic fabric analyses of glaucophane lawsonite and epidote in naturally deformed blueschists from the diablo range and franciscan complex in california and the hida mountains in japan glaucophanes in the analyzed samples consist of very fine grains that are well aligned along the foliation and have high aspect ratios and strong crystal preferred orientations cpos characterized by a 1 0 0 0 0 1 pattern these characteristics together with a bimodal distribution of grain sizes from some samples possibly indicate the occurrence of dynamic recrystallization for glaucophane although lawsonite and epidote display high aspect ratios and a strong cpo of 0 0 1 0 1 0 the occurrence of straight grain boundaries and euhedral crystals indicates that rigid body rotation was the dominant deformation mechanism the p wave av p and s wave av s seismic anisotropies of glaucophane av p 20 4 av s 11 5 and epidote av p 9 0 av s 8 0 are typical of the crust consequently the fastest propagation of p waves is parallel to the 0 0 1 maxima and the polarization of s waves parallel to the foliation can form a trench parallel seismic anisotropy owing to the slowest v s polarization being normal to the subducting slab the seismic anisotropy of lawsonite av p 9 6 av s 19 9 is characterized by the fast propagation of p waves subnormal to the lawsonite 0 0 1 maxima and polarization of s waves perpendicular to the foliation and lineation which can generate a trench normal anisotropy the av s of lawsonite blueschist 5 6 9 2 is weak compared with that of epidote blueschist 8 4 11 1 calculations of the thickness of the anisotropic layer indicate that glaucophane and lawsonite contribute to the trench parallel and trench normal seismic anisotropy beneath ne japan but not to that beneath the ryukyu arc our results demonstrate therefore that lawsonite has a strong influence on seismic velocities in the oceanic crust and that lawsonite might be the cause of complex anisotropic patterns in subduction zones

deformation mechanisms and reaction of hornblende examples from the bergell tonalite central alps abstract hornblendes in the bergell tonalite begin to crystallize relatively early during the crystallization history of the pluton and become resorbed during the late stages of crystallization during the crystallization of the magma deformation commences by magmatic flow in this stage the hornblendes behave as rigid particles in a viscous matrix melt the rotation and alignment of hornblende as elongate rigid particles have produced a strong preferred orientation of both particle long axes and crystallographic directions with progressive solidification of the melt there is a gradual transition from magmatic flow to solid state deformation in the bergell tonalite the crystallographic and shape preferred orientation that have developed during solid state deformation are identical to those of magmatic flow this identical fabric development during solid state and magmatic flow deformation can be explained by the same deformation mechanism that has prevailed in hornblends during both deformation periods i e hornblendes have always acted as rigid particles in a viscous matrix tem observations show no evidence for intracrystalline plasticity in hornblende grains the modal abundance of hornblende decreases progressively with increasing solid state deformation to yield more biotite epidote and quartz small hornblende matrix grains form by cataclastic processes the observed changes in grain size shape and abundance of hornblende occur mainly by fracturing dissolution and reaction so that the solid state deformation of the tonalite is a complex process to which fracturing dissolution of hornblende and metamorphic reactions all contribute the relationship between deformation and reaction represents an example of incongruous pressure solution or diffusive mass transfer involving reaction it is inferred that generally hornblende does not appear to deform significantly by intracrystalline plasticity at temperatures below 650 750 c in the presence of an aqueous fluid crystal plasticity could become dominant at higher temperatures and or lower aqueous fluid activities

on the dynamics of long lived plume conduits in the convecting mantle abstract long lived plumes responsible for mantle hotspots and associated chains of volcanism will be deflected from the vertical as they penetrate through a mantle which is overturning on much larger length scales in response to surface cooling recent experiments have shown that plume inclination leads to entrainment of surrounding material as a result of coupling between conduction of heat and laminar stirring we develop an analysis for entrainment and its effects on the structure and dynamics of continuing steady conduits in a mantle undergoing horizontal shear it is found that the stirring of surrounding mantle with plume source material can result in major modifications of the diameter temperature and mass flux in plumes this model is a departure from the axisymmetric vertical pipe models of plumes and predicts that the temperature in a defected plume will decrease with height thus reducing the plume temperature in the upper mantle while allowing a sizeable temperature difference at the source the cooling of plume conduits after they are deflected by plate motion can lead to a lower maximum temperature and lower mgo contents of basalts along some linear hotspot tracks compared with picrites from their associated flood basalt provinces the effects of entrainment depend primarily on the buoyancy or heat flux carried by the plume with greater cooling for smaller fluxes strong continuing plumes such as hawaii easter island macdonald tahiti and reunion are predicted to produce picrites having mgo contents comparable to the picrites found in continental flood basalt provinces weak plumes such as crozet bouvet and juan de fuca will give only cooler melts through entrainment long lived plumes can sample the whole depth of the mantle from the source to the surface and stirring within the plume leads to compositional zonation which may contribute to compositional heterogeneity in hotspot melts we also predict that plumes originating at the core mantle boundary and having buoyancy fluxes less than about3 103 n s 1 will not give surface hotspots in good agreement with the lower limit of hotspot buoyancy fluxes inferred from seafloor topography

on the rise of strongly tilted mantle plume tails abstract the rise of an initially horizontal buoyant cylinder of fluid through a denser fluid at low reynolds number is used to look at the ascent of strongly tilted mantle plumes through the mantle such ascents are characterized by 1 the growth of instabilities and 2 the development of a thermal wake downstream three dimensional numerical experiments were carried out to examine these features an hybrid particle in cell finite element method was used to look at the rise of non diffusing cylinders and a standard finite element method was used to look at the diffusing case first the experiments show that the timescale of the fastest growing instability vary with the rayleigh number and the viscosity ratio in particular the growth rate decreases as the rayleigh number decreases in agreement with our analysis of the laboratory experiments of kerr et al 2008 second the experiments show that the length of the thermal wake increases with the rayleigh number but the change in viscosity has almost no influence on the wake length applied to strongly tilted mantle plumes we conclude that such plumes cannot be unstable given the plume timescales we also discuss the application of this conclusion to weakly tilted plumes besides this study allows to predict that mantle plumes are unlikely to have developed a significant thermal wake by the time they reach the surface finally the resolution that is required to allow for the growth of mantle plume tails by combined diffusion and thermal entrainment is shown to represent a challenge for the large scale mantle convection simulations

coupled caal nasi diffusion in plagioclase feldspar experiments and applications to cooling rate speedometry abstract the rate of caal nasi interdiffusion in plagioclase feldspar was determined under 1 atm anhydrous conditions over the temperature range 1400 to 1000 c in calcic plagioclase an80 81 by homogenizing coherent exsolution lamellae the dependence of the average interdiffusion coefficient on temperature is given by the expression d 10 99 cm 2 sec exp 123 4 kcal mol rt t in k this value is for diffusion perpendicular to the 03 1 interface of the lamellae caal nasi interdiffusion is 4 to 5 orders of magnitude slower than oxygen diffusion in the temperature range 1400 to 1200 c and possibly 10 orders of magnitude slower at subsolidus temperatures the large differences in diffusion rates explain the apparent contradiction posed by the plagioclases of large layered intrusions e g the skaergaard which retain delicate ca na compositional zoning profiles on the micron scale but have undergone complete oxygen isotopic exchange with heated meteoric groundwater from the surrounding wall rocks caal nasi diffusion is slow the closure temperature is high within the solidus liquidus interval and ca na zoning is preserved oxygen diffusion is faster the closure temperature is lower 350 400 c and the feldspars exchange oxygen with the low temperature hydrothermal fluids the complex micron scale oscillatory zones in plagioclase can also be used as cooling rate speedometers for volcanic and plutonic plagioclase cooling histories typical of large mafic intrusions e g the stillwater are slow begin at high initial temperatures 1200 c and result in homogenization of oscillatory zones on the scale of 10 microns the oscillatory zones found in the plagioclase of granodioritic plutons are preserved because cooling is initiated at a lower temperature 1000 c limiting diffusion to submicron length scales despite the slow cooling rate of the intrusion

spider texture and amphibole preferred orientations abstract foliation in blueschist facies chert from california is defined by layers of oriented alkali amphibole which consistently curve towards and converge on pyrite and possibly pyrrhotite crystals these foliation nodes not previously described are called here spider texture the texture is interpreted in terms of perturbations of the stress field in a matrix undergoing strain about rigid pyrite or pyrrhotite crystals and it has important implications for understanding the mechanisms of amphibole preferred orientation development geometrical relationships between spider texture pressure shadows and quartz preferred orientations suggest that amphiboles grew with a strong preferred orientation along planes of maximum shearing stress the mechanism of foliation and preferred orientation development probably involved competitive anisotropic growth of amphibole prisms within the small gaps that open at steps on shear planes followed by additional micro porphyroblastic growth the first stage of the mechanism is similar to slickenfibre development

benchmark of three dimensional numerical models of subduction against a laboratory experiment abstract in this paper we validate three dimensional numerical models of subduction against a laboratory experiment the design of our numerical models follows that of the laboratory and employs a single subducting plate fixed at its trailing edge without an overriding plate rheology and buoyancy are not thermally dependent but importantly the model has a free surface boundary all the models include a sticky air layer which is an approximation to a free surface boundary condition as opposed to a free slip condition that unrealistically binds the plate to the upper surface we use a parallel python particle in cell finite element scheme and present 8 different models that differ in their viscosity at the interfaces of the subducting plate we consider harmonic and arithmetic averaging of the viscosity together with a plate composite rheology associated with arithmetic averaging which similarly to the harmonic averaging results in two lubricated sub layers at the plate lower and upper interfaces despite showing a similar subduction induced poloidal and toroidal flow in the mantle the agreement of the models with the experiment is overall ordinary the rates at which the slab tip sinks and the plate hinge retreats are approaching the laboratory counterparts to within 7 18 at best when using the composite plate models errors in the characterization of the laboratory parameters can account for 5 of such differences as the composite plate sub layers are made stiffer the plate hinge also begins to depart from the steady retreat observed in the laboratory experiment all numerical models exhibit a secondary rayleigh taylor instability along the plate edge that is not observed in the laboratory experiment the development of this instability is further delayed when the numerical spatial resolution is increased or when the composite plate sub layers are made stiffer this benchmark exercise shows that 1 the glucose surface tension in the laboratory not taken into account in the numerical modelling is sufficient to prevent the flank instability during the retreat and 2 numerical models are usually free from it due to their use of stiffer plate sub layers and or their display of a limited amount of retreat compared to this study

photogrammetric reconstruction of glacier mass balance using a kinematic ice flow model a 20 year time series on grubengletscher swiss alps the kinematic boundary condition at the glacier surface can be used to provide glacier mass balance at individual points if changes in surface elevation horizontal and vertical surface velocities and surface slope are known vertical ice velocity can in turn be estimated from basal slope basal ice velocity and surface strain this relation is applied to reconstruct a 20 year mass balance curve of grubengletscher swiss alps largely using repeated aerial photogrammetry but only a minimum of field work for individual years the mass balance distribution on the glacier tongue was modelled with an accuracy of about 0 9 m a 1 ice mechanical assumptions and errors in glacier bed geometry markedly affect discrete mass balance patterns but are eliminated to a major extent when calculating year to year mass balance changes the resulting curve for the grubengletscher tongue 1973 1992 shows reasonable consistency with meteorological data and other glaciologically derived mass balance series large changes in measured ice speed on the glacier tongue 50 significantly governed the long term variability of ice thickness over the observational period

conduit diameter and buoyant rising speed of mantle plumes implications for the motion of hot spots and shape of plume conduits 1 mantle plumes are expected to be affected by large scale flow in the earth s mantle related to plate motions subducted slabs and possibly large scale upwellings motion of plume conduits will depend on both large scale flow and buoyant rising speed of the conduit through the mantle here we present a model of depth dependent plume conduit temperature viscosity radius and buoyant rising speed and use it to compute plume and hot spot motion results support a temperature anomaly of about 500 k at the plume base in this case sublithospheric temperature anomaly is about 150 k for the iceland plume transition zone anomaly is between 150 and 200 k for iceland about 250 k for samoa and about 300 k for hawaii thermal plume radii are about 100 km in the upper mantle increasing to about 200 km in the lower mantle beneath the lithosphere viscosity in the vicinity of plumes is substantially reduced compared to the underlying mantle and fast moving plates deflect plumes by 200 km or less corresponding to a plume conduit buoyant rise time of about 3 myr between 400 and 100 km depth where most of the shear flow due to plate motions occurs during 100 myr plume conduits rise buoyantly from about 1500 2000 km depth in many cases computed hot spot motion agrees with previous computations south southeastward motion of the hawaiian hot spot during the past 100 myr louisville hot spot motion in a similar direction but at slower speed during the past 50 myr and westward motion of the easter hot spot relative to hawaii and louisville however the previously computed substantial southward motion of the kerguelen hot spot is not confirmed if plume sources in the lowermost mantle are assumed to move with large scale flow they are predicted to be displaced toward the two large scale upwellings beneath africa and the pacific relative to hot spot surface locations for fixed sources predicted tilts of the lower part of conduits tend to be opposite to that

variation in amphibole and plagioclase composition with deformation investigation of the mineral chemistry of a shear zone in an amphibolite facies metagabbro has shown that the composition of the amphibole and plagioclase varies with deformation chemical analyses of the rocks indicate that the shear zone approximates an isochemical system the amphibole varies progressively from an initial magnesio hornblende to a ferroan pargasitic hornblende in the shear zone alkalies ti and fe2 fe2 mg increasing while the plagioclase becomes more anorthitic varying from labradorite to bytownite these changes in composition of amphibole and plagioclase from metabasic rocks are normally associated with increasing temperature in prograde regional metamorphic terrains such chemical variations observed in localized shear zones would suggest that shearing stress in regional metamorphism may play an important role in producing the documented discrepancies in amphibole chemistry between different metamorphic belts

complete texture analysis of a deformed amphibolite comparison between neutron diffraction and u stage data abstract this paper describes a new technique of analysing complex neutron diffraction patterns allowing the quantitative texture analysis of each rock forming mineral up to trielinic lattice symmetry in polymineralic rocks the method is demonstrated using hornblende and plagioclase fabrics in alzenau amphibolite from the spessart mountains mid german crystalline rise hornblende and plagioclase exhibit a pronounced shape preferred orientation with respect to the macroscopic fabric elements for control purposes crystallographic preferred orientations were also determined independently by individual grain measurements using a u stage from the neutron texture analyses we obtain only superposed pole figures hornblende and hornblende hornblende and plagioclase and plagioclase with plagioclase of low indexed lattice planes therefore the critical crystallographic directions were recalculated by the orientation distribution function odf which is given by texture components the optical and neutron derived pole figures are in a good agreement all pole figures display strong preferred orientation although the plagioclase texture is less pronounced the hornblende texture consists of a 001 maximum parallel to the stretching lineation with 010 showing a tendency to develop a girdle around the lineation 100 axes are distributed with a clearly split maximum representing the monoclinic angle between 100 and 001 the plagioclase texture is a rare one with 001 subparallel within the foliation and 100 oriented subparallel to the lineation 010 of plagioclase is concentrated normal to lineation within the foliation the technique outlined here is potentially the most useful one for texture analysis of polyphase rocks independent of crystal symmetry

miocene exhumation of northeast pamir deformation and geo thermochronological evidence from western muztaghata shear zone and kuke ductile shear zone abstract the west muztaghata shear zone wmsz and kuke ductile shear zone ks are located at the western and eastern margins respectively of the muztaghata gneiss dome md in the northeast pamir we report new geological data including quartz fabric analyses zircon u pb ages and mica 40 ar 39 ar ages to constrain the geometry kinematics and evolution time of the wmsz and ks the presence of garnet and kyanite in the mylonites from the wmsz indicate that the shear zone roots into the lower crust the deformation characteristics of mylonites of the wmsz show top to w or sw shear sense and formed at mid high temperatures zircons from high grade mylonites and synkinematic leucogranites record a consistent u pb ages of 20 ma interpreted to reflect high grade metamorphism and partial melting in the northeast pamir in the early miocene prior to ductile shearing along the wmsz the adjacent undeformed 12 ma kuzigan pluton indicates deformation along the wmsz ceased by then 40 ar 39 ar dating results for mica show that the wmsz underwent continued exhumation to the middle upper crust until 12 8 ma to the east the ks has e dipping foliations with a top to e normal shear sense and preserves low to medium deformation temperature fabrics previous data and new biotite dating from the ks yield 40 ar 39 ar ages of 12 8 ma indicating broadly synchronous activity along the ks in the middle miocene integrated with previous studies our results suggest that continuous india asia convergence led to thickening of the pamir crust which propagated from south to north initiating in the south pamir in the late eocene and advanced through the north pamir around 20 ma this was immediately followed by rapid exhumation during east west stretching and dome formation broadly coeval but slightly outlasting exhumation of the other central pamir gneiss domes

interaction of chemical and physical processes during deformation at fluid present conditions a case study from an anorthosite leucogabbro deformed at amphibolite facies conditions we present microstructural and chemical analyses of chemically zoned and recrystallized plagioclase grains in variably strained samples of a naturally deformed anorthosite leucogabbro southern west greenland the recorded microstructures formed in the presence of fluids at mid crustal conditions 620 640 c 7 4 8 6 kbar recrystallized plagioclase grains average grain size 342 m with a random crystallographic orientation are volumetrically dominant in high strain areas they are characterized by asymmetric chemical zoning an80 cores and an64 rims that are directly associated with areas exhibiting high amphibole content and phase mixing analyses of zoning indicate anisotropic behaviour of bytownite plagioclase with a preferred replacement in the left langle 0 10 right rangle direction and along the 001 plane in areas of high finite strain recrystallization of plagioclase dominantly occurred by bulging recrystallization and is intimately linked to the chemical zoning the lack of cpo as well as the developed asymmetric zoning can be explained by the activity of grain boundary sliding accommodated by dissolution and precipitation creep dpc in low strain domains grain size is on average larger and the rim distribution is not related to the inferred stress axes indicating chemically induced grain replacement instead of stress related dpc we suggest that during deformation in high strain areas pre existing phase mixture and stress induced dpc caused grain rotations that allowed a deformation enhanced heterogeneous fluid influx this resulted in local plagioclase replacement through interface coupled dissolution and precipitation and chemically induced grain boundary migration accompanied by bulging recrystallization along with neocrystallization of other phases this study illustrates a strong interaction and feedback between physical and chemical processes where the amount of stress and fluids dictates the dominant active process the interaction is a cause of deformation and external fluid infiltration with a result of strain localization and chemical re equilibration at amphibolite facies conditions

contrasting microstructures and deformation mechanisms in metagabbro mylonites contemporaneously deformed under different temperatures c 650 c and c 750 c

non newtonian stagnant lid convection and magmatic resur facing on venus abstract mantle convection on venus is likely to occur in the regime known as stagnant lid convection we investigate this regime for internally heated convection with temperature and pressure dependent power law viscosity dislocation creep scaling relationships obtained for large aspect ratio convection are different from steady state square box calculations but agree well with scaling theory and boundary layer stability analysis results for arrhenius viscosity and pressure dependent viscosity show that the efficiency of heat transport is sensitive to the viscosity function at the bottom of the lid new scaling relationships are applied to parameterized convection calculations of the thermal history of venus assuming that plate tectonics could not occur during evolution the onset of convection beneath the lid is delayed even for initial potential temperatures near the solidus during the conductive regime melting is suppressed due to the development of a thick cold lid at the surface after convection begins the lid becomes thinner and the planet undergoes a period of widespread melting and volcanism the timing of the beginning and ending of melting depends on various factors such as the initial conditions and mantle rheology the episode of melting predicted by the models can be reconciled with the cessation of global resurfacing on venus 300 800 myr ago the models yield present day lithospheric thicknesses around 200 km which is similar to previously suggested estimates

development of shape and lattice preferred orientations of amphibole grains during initial cataclastic deformation and subsequent deformation by dissolution precipitation creep in amphibolites from the ryoke metamorphic belt sw japan amphibolites from the ryoke metamorphic belt sw japan were deformed initially by cataclasis and subsequently by dissolution precipitation creep initial cataclastic deformation produced a rather weak shape preferred orientation spo of brown amphibole grains with small aspect ratios as well as a poorly developed amphibole lattice preferred orientation lpo with na a 100 axes scattered subnormal to the foliation and ng or c 001 axes scattered around the lineation during later deformation by dissolution precipitation creep preferential dissolution at grain boundaries subparallel to the foliation and simultaneous compaction normal to the foliation have likely produced a distinct spo of elongate brown amphibole grains subparallel to the foliation as well as their lpo such that their ng or c axes are scattered around the lineation while na a and nb b 010 are spread along a girdle normal to the lineation also during this deformation green amphibole precipitated as isolated grains or in pressure shadow regions around brown amphibole grains nucleation and anisotropic growth of isolated green amphibole grains according to the orientations of the principal stress directions produced an lpo of these grains such that their na a are oriented normal to foliation nb b within the foliation normal to the lineation and ng or c axes are parallel to the lineation in addition there is an associated spo growth of green amphibole in pressure shadow regions around brown amphibole grains occurs either syntaxially or anisotropically according to the orientations of the principal stress directions q 2003 elsevier ltd all rights reserved

characterisation and prediction of thickness and material properties of periglacial cover beds tharandter wald germany abstract soils of central european subdued mountain ranges are structured by various layers of relic periglacial slope deposits pleistocene cover beds these have great impact on subsurface substrate properties therefore gaining extensive spatial information on material composition and thickness of slope deposits is essential here we present a gis supported approach to predict layer properties which goes beyond existing deterministic procedures in a section of the tharandter wald saxony germany we deduced subsurface structures from soil pits and auger cores substrates were analysed to clarify stratigraphical setting and relation to parent rock types we assume cryoturbation solifluction slope wash and aeolian sediment admixture to be important processes of periglacial cover bed formation these processes are mainly controlled by geological and geomorphological factors combining assumptions and field data we determine properties of periglacial cover beds based on relief parameters an integrated data set is generated which provides information for a geostatistical interpolation of measured layer thicknesses the composition of solifluidally dislocated substrate is approximated by a modified flow model modelling returned thicknesses of loess bearing layers i e upper layer and intermediate layer prediction of the upper layer involved an inverse distance weighted approach and yielded an inaccuracy of

effect of thermal diffusion on the stability of strongly tilted mantle plume tails 1 the effect of thermal diffusion on the stability of strongly tilted mantle plume tails is explored by investigating experimentally and numerically the gravitational instability of a rising horizontal cylindrical region of buoyant viscous fluid at large viscosity ratios we find that the instability is unaffected by diffusion when the rayleigh number ra is greater than about 300 when ra is less than 300 diffusion significantly increases the time for instability as the rising fluid region needs to grow substantially by entrainment before it becomes unstable when ra is less than about 140 and the rise height available h is less than about 40 times the cylinder radius the rising region of fluid is unable to grow sufficiently and instability is prevented when our results are applied to the earth we predict that thermal diffusion will stabilize plume tails in both the upper and lower mantle we also predict that some of the buoyancy flux in mantle plumes is lost during ascent to form downstream thermal wakes in any larger scale mantle flow

compositional changes of minerals associated with dynamic recrystallizatin the rate of compositional and isotopic exchange between minerals may be enhanced significantly if the rock is deformed simultaneously the enhanced exchange rate may result from a reduction in grain size shorter distance for volume diffusion dissolution and growth of grains by diffusion creep pressure solution or the movement of high angle grain boundaries through strained grains during recrystallization in the dislocation creep regime the migration of high angle grain boundaries provides high diffusivity paths for the rapid exchange of components during recrystallization the operation of the latter process has been demonstrated by deforming aggregates consisting of two plagioclases an1 and an79 at 900 c 1 gpa confining pressure and a strain rate of 2x10 6s 1 the polygonal recrystallized grains were analyzed using an analytical transmission electron microscope and have a variable but often intermediate composition at the conditions of these experiments the volume interdiffusion rate of nasi caal is too slow to produce any observable chemical change and microstructural chemical relations indicate that the contribution from diffusion creep was insignificant except for initially fine grained 2 10 m aggregates these results indicate that strain induced recrystallization can be an effective mechanism for enhancing the kinetics of metamorphic reactions and for resetting the isotope systematics of minerals such as feldspars pyroxenes and amphiboles

different slip systems controlling crystallographic preferred orientation and intracrystalline deformation of amphibole in mylonites from the neyriz mantle diapir iran abstract a deformed layered gabbro and a mylonitic gabbro sample from the marginal shear zone of the neyriz mantle diapir in iran were analyzed using electron backscatter diffraction ebsd both samples have the common amphibole crystallographic preferred orientation cpo in which 100 lies perpendicular to foliation and parallel to lineation amphibole grains in the layered gabbro sample have little internal deformation whereas in the mylonitic gabbro sample the amphibole grains are strongly distorted and contain low angle grain boundaries there is a subtle change in cpo as a function of grain size in the mylonitic gabbro coarse grains porphyroclasts have a 100 cpo oriented with the main foliation reference frame whilst fine grains have a 100 cpo oriented with the c shear bands detailed analysis of porphyroclast distortions and subgrain boundary trace analysis suggests that hard slip systems most particularly 110 control intracrystalline deformation schmid factor analysis suggest that these slip systems are not involved in foliation formation but are linked kinematically to c shear bands it is unlikely that the slip systems that control intracrystalline deformation are important in cpo formation we interpret that subgrain rotation recrystallization lead to grain size reduction and the elongate recrystallized grains were rotated towards the c shear bands by grain boundary sliding this rigid body rotation possibly in combination with easy slip on 100 are considered the main cause of cpo formation amphibole zonation patterns in the layered gabbro sample suggest that oriented growth of amphibole may have contributed to cpo

solifluction rates processes and landforms a global review abstract field data on the rates of solifluction and associated parameters are compiled from the literature in an attempt to evaluate factors controlling the spatial variability in solifluction processes and landforms with special attention on the climate solifluction relationship the analyzed data originate from 46 sites over a wide range of periglacial environments from antarctic nunataks to tropical high mountains solifluction broadly defined as slow mass wasting resulting from freeze thaw action in fine textured soils involves several components needle ice creep and diurnal frost creep originating from diurnal freeze thaw action annual frost creep gelifluction and plug like flow originating from annual freeze thaw action and retrograde movement caused by soil cohesion the depth and thickness of ice lenses and freeze thaw frequency are the major controls on the spatial variation in solifluction processes near the warm margin of the solifluction affected environment diurnal freeze thaw action induces shallow but relatively rapid movement of a superficial layer 5 10 cm thick on average often creating the thin stone banked lobes typically seen on tropical high mountains in addition to diurnal movement annual frost creep and gelifluction may occur on slopes with soil climates of seasonal frost to warm permafrost dislocating a soil layer shallower than 60 cm at a rate of centimeters per year and eventually producing medium size solifluction lobes in high arctic cold permafrost regions two sided freezing can induce plug like flow of a soil mass 60 cm or thicker the correlation between process and landform suggests that the riser height of lobes is indicative of the maximum depth of movement and prevailing freeze thaw type climate change may result in new different ground freezing conditions thereby influencing the surface velocity and maximum depth of soil movement soil moisture and topography also control solifluction high moisture availability in the seasonal freezing period enhances diurnal freeze thaw action and subsequent seasonal frost heaving the latter contributes to raising the moisture content of the thawed layer and promotes gelifluction during the thawing period the slope angle defines the upper limit of the surface velocity of solifluction a diagram correlating the potential frost creep with the actual surface velocity permits an inter site comparison of the relative magnitude of solifluction components physically based modelling of periglacial slope evolution requires synthetic and more detailed field monitoring and laboratory simulations of solifluction processes

rheology of petrolatum paraffin oil mixtures applications to analogue modelling of geological processes abstract paraffins have been widely used in analogue modelling of geological processes petrolatum and paraffin oil are commonly used to lubricate model boundaries and to simulate weak layers in this paper we present rheological tests of petrolatum paraffin oil and several homogeneous mixtures of the two the results show that petrolatum and all petrolatum paraffin oil mixtures are strain strain rate and temperature dependent under typical experimental strain rates 10 3 10 1 s 1 for the same conditions pure paraffin oil is a slightly temperature dependent linear newtonian fluid all mixtures have yield stress and flow stress strain softening values that decrease with decreasing shear rate and with increasing relative amounts of paraffin oil the degree of strain rate dependence shear thinning also decreases with increasing paraffin oil content because these materials have rheologies that can be characterized and controlled they are suitable for use in a large number of analogue model settings either as a lubricant or to simulate weak layers when used as a lubricant mixtures with higher paraffin oil content should perform better than pure petrolatum

rise and deflection of mantle plume tails 1 experimental observations by kerr and meriaux 2004 of the deflection of thermal plume tails by an imposed background shear are reanalyzed using a corrected velocity profile the deflected plumes are found to be accurately described by a constant vertical rise velocity equation 14 that depends primarily on the plume buoyancy flux and the ambient viscosity our results can be used to further improve the predictions of numerical models of the shearing of plume tails in the earth s mantle

measurement of solifluction rates using multi temporal aerial photography a new method is presented to assess decadal solifluction lobe movement and volumetric changes at the catchmentscale the method is based on photogrammetrical analysis of orthorectified repeat aeria

orientation related deformation mechanisms of naturally deformed amphibole in amphibolite mylonites from the diancang shan sw yunnan china abstract sheared amphibolite rocks from diancang shan high grade metamorphic complex along the ailao shan red river shear zone southwestern yunnan china show typical mylonitic microstructures the mylonites are characterized by porphyroclastic microstructures and the ultramylonites are highly lineated with alternating amphibole and quartzofeldspathic domains microstructural analysis and p t estimation suggest that the amphibole grains in the mylonitic rocks are deformed and dynamically recrystallized at amphibolite facies in the mylonitic amphibolites there are two types of amphibole porphyroclasts i e type i hard and type ii soft porphyroclasts they have their 001 crystallographic orientations subnormal and sub parallel to the stretching lineation of the rocks respectively the two types of porphyroclasts show distinct deformation microstructures and sub microstructures formed by various deformation mechanisms which contribute in different ways to the generation of the fine grained matrix shape preferred orientation analysis misorientation analysis of the two types of porphyroclasts and new fine grains around them further prove the generation of the fine grains in matrix from the type ii porphyroclasts the type i hard porphyroclasts are deformed mainly by mechanical rotation work hardening and intragranular microfracturing in contrast the deformation of the type ii soft porphyroclasts is mainly attributed to crystalline plasticity i e twinning dislocation creep and dynamic recrystallization during the deformation of the type ii porphyroclasts the 100 001 slip system plays a dominant role during deformation and grain size reduction of amphibole twinning along the active 100 slip system in combination with dislocation creep gliding and climbing governs the nucleation of subgrains and formation of dynamically recrystallized fine grains a process here named twinning nucleation recrystallization

syndeformational recrystallization dynamic or compositionally induced dynamic recrystallization in the strict sense of the term is the reconstitution of crystalline material without a change in chemical composition driven by strain energy in the form of dislocations driving potentials additional to internal strain energy may contribute to the recrystallization of naturally deformed minerals which form solid solutions such as feldspar amphiboles and pyroxenes if they change their composition during recrystallization to estimate the relative importance of these driving potentials the chemical composition of porphyroclasts and recrystallized grains of plagioclase clinopyroxene and hornblende have been investigated in samples from a high grade shear zone of the ivrea zone italy the plagioclases show two different recrystallization microstructures bulging recrystallization at grain boundaries and discrete zones of recrystallized grains across porphyroclasts probably involving fracturing deformation took place under amphibolite facies conditions on a retrograde p t path porphyroclast and recrystallized compositions from bulging recrystallization microstructures differ only in their or content and yield a g between mean host grain and mean recrystallized grain composition at fixed p t conditions of approximately 5 joules 10 4 m3 extreme compositional variations yield approximately 60 j 10 4 m3 the increase of free energy due to dislocations calculated for common glide systems in plagioclase are on the order of 100 joules 10 4 m3 for high values of dislocation densities of 1014 m 2 thus the effect of chemically induced driving energies on grain boundary velocity appears small for mean compositions but may be as great as that of deformational energies for larger chemical differences in the other type of microstructure porphyroclasts and recrystallized grains in discrete zones differ in their anorthite content the maximum g induced by the compositional disequilibrium is on the order of 100 j 10 4 m3 this maximum value is of the same magnitude as the g derived from high dislocation densities of 1014 m 2 the resulting combined g is approximately twice as high as for deformational g alone and heterogeneous nucleation may become a feasible recrystallization mechanism which is evident from the microstructures the recrystallization mechanism depends on the nature of the driving potential grain boundary migration gbm and heterogeneous nucleation can release gibbs free energy induced by compositional disequilibrium whereas this is not likely for subgrain rotation therefore only gbm and heterogeneous nucleation may link metamorphism and deformation so that syndeformational recrystallization may represent a transitional process ranging from dynamic recrystallization to metamorphic reaction

quantitative texture analysis of glaucophanite deformed under eclogite facies conditions sesia lanzo zone western alps comparison between x ray and neutron diffraction analysis

fluid interaction and element mobility in the development of ultramylonites abstract shear zones are often characterized by the presence of mylonitic fabrics the textural development of such fabrics is enhanced by the presence of a fluid phase in a single specimen of rock from the brevard fault zone in north carolina we can demonstrate the development of ultramylonite domain by focused fluid flow the ultramylonite interface retains a sharp textural and chemical discontinuity this suggests that solute transport was dominantly parallel to the tectonic layering major chemical changes between the ultramylonite zone and the protolith include losses of si 2 na 2 0 and k 2 0 and gain of cao feo and hjo in the high strain domain introduction during the tectono thermal evolution of mountain belts zones of high strain are often recorded by the development of mylonites although mylonites and subcategories within mylonites sibson 1977 can be generated by numerous well documented physical processes their chemi cal and isotopic evolution is much more complex mechanical processes often associated with the development of high strain zones include micro fracturing dilatancy grain boundary sliding and dislocation glide these properties are strongly dependent on the modal mineralogy bulk compo sition heterogeneity of the rock and the temperature and pressure con ditions during deformation such variations in the mechanics of deforma tion have led to the more common classification of shear zones as either ductile or brittle sibson 1977 ramsay 1980 however most shear zones are characterized by the availability of fluids that cause a drastic change in the style of deformation by altering the behavior and assem blage of minerals hydration reactions and the rates of chemical and mechanical processes during progressive deformation the presence of a fluid phase during deformation enhances solution transport microfractur ing and recrystallization suc h chemically active zones generate mineral assemblages that record the metamorphic history of the mylonites al though it is not unusual to find minerals that persist through such an event these relict minerals if recognized can identify equilibrium vs nonequilibrium assemblages observed in shear zones clearly the pres ence of relict minerals would have a substantial impact on the trace element and isotopic signature of the rock to more carefully document the behavior of elements in a strain zone that has abundant fluid we have chosen to study a transect across the brevard fault zone near rosman north carolina within this tran sect one particular outcrop provided a single large specimen that over a distance of 25 cm showed the entire strain gradient fabric from proto mylonite to ultramylonite see sibson 1977 for mylonite series ob served by us as discontinuous patches in the longer 15 km transect we report petrographic textural and chemical data from this block and pro vide a physical model to explain these data

deformation related recrystallization processes abstract recrystallization is a common microstructural transformation that occurs during deformation metamorphism and diagenesis of rocks studies on minerals and rock analogues have demonstrated that a wide range of recrystallization mechanisms can occur the range of mechanisms is related to the various ways in which two basic processes grain boundary migration and new grain boundary formation combine to transform the microstructure two recent papers drury et al 1985 urai et al 1986 have proposed different schemes for the description of recrystallization mechanisms the purpose of this paper is to provide a unified framework for the description of mechanisms recrystallization mechanisms are divided into three main types rotation mechanisms which principally involve the formation of new grain boundaries migration mechanisms which principally involve grain boundary migration and general mechanisms which involve both basic processes a further distinction is made on the basis of the continuity of the microstructural transformation with respect to time each of the three main types of mechanism can be divided into a number of sub types depending on whether the processes of grain boundary migration new grain boundary formation and new grain formation occur in a discontinuous or continuous manner with respect to time as the terms continuous and discontinuous have been used in the metallurgical literature to signify the spatial continuity of the microstructural transformation the terms discontinuai and continual are used to refer to the temporal continuity of the transformation it is recommended that the following aspects should be specified if possible in a general description of recrystallization mechanisms 1 1 how do the basic processes combine to transform the microstructure 2 2 if new grain development occurs what is the development mechanism and does new grain formation occur in a continual or discontinuai manner 3 3 if grain boundary migration is involved in the transformation what is the migration mechanism i e fast solute escape migration slow solute loaded migration fluid assisted migration etc and is migration a continual or discontinuai process the application of the unified scheme is illustrated by reviewing studies that have provided detailed information on the recrystallization mechanisms involved the complicating effects of solid solution impurities dispersed second phase particles and grain boundary fluid films are also considered and it is demonstrated that variations in content of these types of impurity can significantly effect the types of recrystallization that occur in a given material