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

microstructural deformation record of an orogen parallel extension in the vepor unit west carpathians abstract deformation microstructures of quartzo feldspathic rocks have been used to analyse the orogen parallel stretching of the vepor unit in the west carpathians during cretaceous convergence this process occurs contemporaneously with the burial of the vepor basement related to the overthrusting palaeozoic volcano sedimentary sequences of the gemer unit the microstructures develop along a metamorphic gradient indicating an increase of pressure and temperature towards the structural footwall the recrystallization of quartz occurs in the field of subgrain rotation recrystallization sgr while the plagioclase and k feldspar disintegrate into a neocrystallized muscovite albite matrix palaeopiezometry of the recrystallized quartz in conjunction with p t estimates yield strain rates of 9 10 12 to 6 10 14 s 1 depending on temperature 380 480 c and choice of piezometer and flow law calibration strain analysis of the recrystallized quartz aggregates and quartz texture analysis indicate an overprint of the variscan magmatic and high grade fabrics by a cretaceous plane strain deformation resulting in vertical shortening and horizontal orogen parallel east west stretching spatial distribution and conflicting sense of shear of the asymmetrical quartz lattice preferred orientation patterns indicate that the large scale pure shear deformation of the vepor unit is locally accommodated by simple shear zones observed metamorphic gradients across the vepor unit are interpreted to result from later large scale folding and heterogeneous exhumation of the orogen parallel deformation fabric

deformation enhanced fluid distribution in feldspar aggregates and implications for ductile shear zones experiments at 900 c and 1 4 gpa show that the distribution of aqueous fluid in fine grained feldspar aggregates changes from isolated pores under hydrostatic conditions to mostly wetted grain boundaries during deformation the isolated pore distribution is rapidly regained during annealing following deformation the deformation enhanced fluid distribution causes a switch from dislocation creep to diffusion creep accompanied by a significant decrease in strength it also increases the bulk transport rate through the aggregate by more than an order of magnitude a change in fluid distribution is not observed in fine grained quartz aggregates deformed at similar conditions if deformation enhanced fluid distribution occurs in naturally deformed feldspathic rocks it could help to explain the localization of strain and enhancement of bulk transport in ductile shear zones

crystallization of interstitial liquid and latent heat buffering in solidifying gabbros skaergaard intrusion greenland new compositional profiles across plagioclase grains from the layered series ls marginal border series mbs and upper border series ubs of the skaergaard intrusion are used to understand the mechanisms of cumulate rock solidification and the fate of the interstitial liquid the data show that plagioclase crystals display three types of compositional profile over the whole intrusion as follows 1 grains with normal zoning which dominate the mbs and ubs these are interpreted as having formed at the top of a crystal mush and then buried in the cumulate pile crystallization of the interstitial melt resulted in liquid differentiation and produced normally zoned rims on plagioclase cores 2 unzoned crystals which dominate the upper part of the ls also crystallized at the top of the mush and were then buried in mush with a low interstitial liquid fraction or one experiencing convective movements that kept the liquid to a constant composition 3 crystals with a mantle of decreasing an content followed by a rim of constant composition grains showing this complex zoning mostly occur in the lower parts of the ls depending on the stratigraphic position within the intrusion the composition of the rim can be an56 an51 or an40 in the main magma body these compositions an contents correspond to those of plagioclase primocrysts e g cores at the appearance of cumulus clinopyroxene an56 fe ti oxides an51 and apatite an40 compositional buffering of plagioclase rims is interpreted as being a consequence of enhanced release of latent heat of crystallization at the appearance of new interstitial phases in the crystal mush when a new phase saturates the latent heat contribution to the global enthalpy budget of the system becomes sufficiently high to keep the interstitial melt at its liquidus temperature for a period of time that could exceed thousands of years under these conditions equilibrium adcumulus growth together with diffusion and possibly advection of chemical components result in the formation of plagioclase rims of constant composition an content efficient thermal buffering of the mush liquid depends on the porosity i e fraction of liquid within the mush and the degree of compositional homogeneity of the mush in a heterogeneous and highly porous mush saturation of the new phase occurs in the coldest part of the mush and the enhanced latent heat release at the saturation of this phase is quickly dissipated to the whole volume of liquid including the warmest part that is not yet saturated in a new phase as a consequence no thermal buffering occurs and interstitial crystallization produces grains with normal zoning the distribution of the various types of plagioclase grains throughout the skaergaard intrusion can therefore be used to infer the spatial variability in the physical properties of the crystal mush such as the residual porosity both at an intrusion wide scale and at a millimetre to centimetre scale

engineering geology of residual soil derived from mudstone in zimbabwe abstract as increasing amounts of civil engineering work are carried out on mudstone derived residual soils it has become important to systematically assess their geological engineering properties thus the properties of mudstone derived residual soils were evaluated in this study via a series of comprehensive laboratory tests to assess physical mechanical mineralogical and microstructural variations results show that the physical properties of these soils are inadequate in terms of engineering applications these soils can be classified as highly plastic clays that are hard to compact natural mudstone derived residual soils undergo severe disintegration underwater while remolded examples are similar but tend to exhibit more stable responses natural mudstone derived residual soils behave in a structured way when subjected to shear and compression these soil types possess superior natural shear resistance but their strength decreases significantly following saturation as well as in wetting and drying cycles data show that the microstructures of mudstone derived residual soils are characterized by the presence of aggregations with fissures although iron bearing cementation between these aggregations is responsible for high soil strength bonds can be damaged or even destroyed when samples are saturated or subjected to wetting and drying cycles leading to a reduction in shear strength the results of this research provide clear parameters for related engineering applications and enhance our understanding of residual mudstone derived soils

dissolution precipitation creep of k feldspar in mid crustal granite mylonites abstract the deformation of k feldspar within lower amphibolite facies granite mylonites from the gran paradiso nappe north western alps italy is primarily accomplished by dissolution replacement and precipitation processes with little or no evidence for dislocation glide or creep new elongate grains of k feldspar precipitate in dilational domains within and around k feldspar porphyroclasts as a part of the process of myrmekite formation that progressively consumes the porphyroclasts the new grains grow in epitaxial continuity with the parent porphyroclasts which hinders the development of a bulk crystallographic preferred orientation cpo in the aggregates of new grains when large amounts of relict k feldspar are still present in mylonites the magmatic k feldspar is eventually transformed into 100 300 m thick nearly monomineralic fine grained 20 50 m in size aggregates of new grains showing an oblique shape fabric and a cpo the observed cpo is not consistent with the activity of any slip system in k feldspar instead it is interpreted to result from dissolution precipitation creep with a slow reaction rate parallel to 010 and 001 crystallographic axes and a fast reaction rate parallel the 100 axis consistent with such a dissolution precipitation mechanism boundaries of new k feldspar grains are highly corroded when oriented approximately parallel to the extensional instantaneous stretching axis whereas boundaries approximately orthogonal to the same stretching axis show well developed crystal facets the cpo developed is weak which suggests that the anisotropy in the dissolution growth rate of k feldspar is also weak

ephemeral gully erosion in southern navarra spain abstract ephemeral gully erosion is a common type of water erosion in the loamy soils formed on the miocene marls of southern navarra a study has been conducted to describe the different types of ephemeral gullies and to determine their origin evolution and importance as sediment sources three main types were identified according to their origin classic ephemeral gullies formed within the same field where runoff starts drainage ephemeral gullies collecting the runoff from another upstream area and discontinuity ephemeral gullies due to some abrupt slope change in the landscape during 2 consecutive years of normal precipitation ephemeral gully erosion has caused significant soil losses locally over 2 66 kg m 2 yr 1 which is above tolerable levels in each year one short and intense rainfall event has been responsible for almost all soil losses from ephemeral gullies the main cause of gully formation is the lack of any proper waterway for conveying water excess conventional tillage practices provoked gully occurrence while the maintenance of stubble or vegetation cover completely prevented their formation although farmers refill gullies every year they reappear at the same position if no preventive measures are taken early winter has been the most critical period for gully formation when the soil is wetter and the surface cover is scarce but a strong summer thunderstorm also caused severe gully erosion in unprotected soils two simple topographic indices based on the area and the slope of the watersheds are useful to describe total soil losses

channelized fluid flow through shear zones during fluid enhanced dynamic recrystallization northern apennines italy geochemical and petrographic studies of the triassic portoro limestone of the tuscan nappe in liguria italy indicate that fluid flow was channeled through interlayered bedding parallel shear zones during miocene shearing and low grade metamorphism carbon oxygen and strontium isotopic compositions and trace element concentrations in the portoro indicate that it was precipitated from normal marine waters in sheared and unsheared layers these isotopic compositions are indistinguishable yet sheared layers of microspar contain less than half the amount of strontium preserved in undeformed layers wavy grain boundaries and a dimensional preferred orientation of elongated grains indicate that calcite within sheared zones was dynamically recrystallized on the basis of these observations we suggest that during burial extraformational fluids were buffered into oxygen isotopic but not strontium concentration equilibrium with the portoro these syndeformational fluids were channeled through discrete 1 to 15 cm thick shear zones in which strontium was expelled from calcite and incorporated into grain boundary fluids during dynamic recrystallization

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

a shear zone related greenstone belt hosted gold mineralization in the archean of west greenland a petrographic and combined pb pb and rb sr geochronological study abstract widespread gold showings have been found in an archean greenstone belt on storo in godthabsfjord west greenland the storo greenstone belt sgb comprises metasedimentary and metavolcanic rocks with minor magnetite rich bands gold mineralization is hosted by amphibolites mica schists and garnetite rocks gold occurs in various mineral associations but its presence in lollingite arsenopyrite pyrrhotite parageneses is prevalent to a lesser extent native gold occurs within seriticized plagioclase in sheeted quartz veins and as inclusions in garnet based on petrographic observations and comparison with similar ore mineral textures from other deposits we propose the following model for the deposition of the gold bearing lollingite arsenopyrite pyrrhotite assemblage on storo in the first stage arsenopyrite likely formed with gold in solid solution in the second stage gold bearing arsenopyrite was replaced by lollingite and pyrrhotite this took place during regional prograde metamorphism gold accumulated within lollingite as discrete grains during subsequent retrograde metamorphic conditions lollingite and pyrrhotite reacted and formed arsenopyrite and gold was subsequently exsolved and preferably deposited as discrete grains or films along arsenopyrite lollingite grain boundaries during a later event a garnet overgrowth partly affected the gold bearing lollingite arsenopyrite pyrrhotite assemblage the isotopic composition of pb released from these garnets by pb stepwise leaching experiments pbsl and the bulk pb isotopic compositions of arsenopyrite and pyrrhotite indicate a common source of pb in the fluid from which first the sulfides and subsequently garnet were formed bulk pb analyses and pbsl experiments of arsenopyrite and pbsl experiments of garnet yield ages of 2863 24 ma 2 mswd 0 2 for arsenopyrite and 2748 62 ma 2 mswd 220 for garnet the 2863 24 ma age is thus a minimum age for the mineralization rb sr isotopic analyses of cogenetic biotite from within strongly mineralized zones reveal middle proterozoic ca 1 65 ga mineral ages well in accordance with a regional reheating event known to have affected the area and are thus interpreted as cooling ages thereof this regional reheating event has apparently not affected the pb pb isotopic system of arsenopyrite implying that the blocking temperature of the u pb system in arsenopyrite is likely to be above 300 c the late archean ages for arsenopyrite and garnet are consistent with the regional amphibolite facies metamorphism related to major terrane assemblies in the study area occurring at that time the ore textures preserved today are interpreted as those formed under this regional metamorphic event

engineering geology of the tropical volcanic soils of la laguna tenerife abstract geological factors controlling the engineering properties of the volcanic soils of la laguna have been analysed emphasis has been placed on the influence of mineralogy and fabric on the strength and deformation properties and particularly on the results of the classification test the unusually high strength and low compressibility in relation to the high plasticity fine particle size and active mineral composition has been explained the soil fabric is considered to be the main factor leading to these unusual properties but geochemical environment and associated mineralogy also play an important part since la laguna soils show similar engineering geological characteristics to other widely distributed tropical volcanic soils a review has been made of the different hypotheses which attempt to explain the abnormal properties of these soils

influence of microscale weak zones on bulk strength shear zones have different rheological properties than the surrounding rocks indicating that the bulk strength of regions containing shear zone networks cannot be determined by considering the the host rock rheology alone we demonstrate the value of this concept at the microscale we first consider the phase arrangements in naturally deformed rocks and document that weak phases exhibit little interconnection within a microstructure rather three dimensional weak zones analogous to viscous shear zones can interconnect or bridge weak phases these zones typically form at high stress sites comprise multiple minerals and deform by mechanisms independent of those in the surrounding minerals the presence of weak zones strongly affects the bulk strength of the rock disproportionate to the mode of the weak zones for example the development of 1 mode of a weak zone at a high stress site can reduce the bulk strength of the rock nearly an order of magnitude calculation of the bulk strength of the rock by some averaging algorithm of the deformation mechanisms operating outside the weak zones will overestimate strength instead accurate calculations and predictions of bulk strength require accounting for the presence and geometry of weak zones for this reason we advocate use of the scale independent conceptual rheological model of interconnected weak zones or layers rather than that of interconnected weak phases more generally the way forward in improving quantification of the mechanical properties of the lithosphere requires recognizing and explicitly accounting for the spatial and temporal distribution of deformation mechanisms operating throughout a rock this article is protected by copyright all rights reserved

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

fluid infiltration and volume change during mid crustal mylonitization of proterozoic granite king island tasmania the development of 10 30 m wide mylonite zones at mid crustal depths in late proterozoic granitoids on king island tasmania was associated with pervasive infiltration of low 18o fluids 5 to 7 on the scale of the shear zones syndeformational fluid rock interaction produced substantial differences in mineral composition and bulk rock chemistry among several adjacent shear zones which are hosted by the same granite in a shear zone at cape wickham with a normal slip component changes in whole rock chemistry between granite and mylonites indicate a gain of ca and losses of k and na during deformation which was nearly isovolumetric notable losses of k rb and si occurred during partial retrograde alteration of mylonites near the western margin of this shear zone the alteration suggests a component of up temperature fluid flow in contrast 3 km to the south east in a strike slip shear zone at disappointment bay complete albitization of plagioclase was associated with na gain and ca loss deformation also involved losses of mg and fe up to 60 volume gain occurred during the formation of closely spaced mesoscopic to microscopic quartz veins during mylonitization the substantial silica gain in this as well as in two mylonite zones further south east is interpreted to have been associated with upward flow of aqueous fluids along these shear zones on the basis of a gradient reaction model minimum time integrated fluid fluxes of 106 m3 m2 are estimated for the disappointment bay west shear zone

engineering geology of basaltic residual soil in leiqiong southern china the physical mechanical chemical mineralogical and microstructural properties of basaltic residual soil obtained from leiqiong area in southern china were examined emphasis was placed on the influences of minerals oxides and the soil fabric on engineering geological characteristics particularly on strength and deformation properties distinctive properties such as high strength and low compressibility relative to high void ratios high liquid limits and low natural densities were explained the soil fabric with the cement bond formed by iron oxides was considered as the main factor that caused to peculiar mechanical behavior whereas mineralogy and climate processes also played important roles under the influence of repeated cycle of alternate wetting and drying the residual soil generated a large collapsible deformation upon soaking and its strength significantly decreased because of the derogation of cement bond the formation of microcracks and the weakening of microstructure failure zones

relationships between chemical weathering indices and physical and mechanical properties of decomposed granite abstract chemical weathering is one of the dominant mechanisms for weathering of granite in subtropical and tropical regions resulting in modifications of its chemical physical and mechanical properties thus the changes in chemical properties may provide insight about the changes in physical and mechanical properties of weathered granite in this paper three quantitative chemical weathering indices are correlated to the dry density of decomposed granite a linear relationship is found between each chemical weathering index and dry density for granitic saprolites mobiles index i mob gives the highest coefficient of correlation among the three chemical weathering indices further investigations were conducted to establish the relationships between i mob and other physical and mechanical properties of granitic saprolites the test results showed that fines content increases with increasing i mob but peak strength increases with decreasing i mob the peak states of granitic saprolites are consistent with rowe s stress dilatancy relationship dilatancy increases with decreasing i mob that contributes to a higher peak strength based on some test results presented in this study it seems that i mob could be a good indicator to quantify the effects of chemical weathering on physical and mechanical properties of granitic saprolites

characterisation of a volcanic residual soil and its implications for large landslide phenomena application to tenerife canary islands large landslides are common processes during the evolution of volcanoes and individual events can exceed several cubic kilometres in volume volcanic slope failures are a significant risk for the neighbouring population due to their huge volumes and great runout distances around the canary archipelago a total of seventeen deposits of large landslides have been found and on tenerife seven large landslides have affected the subaerial and submarine morphology during the last 6 ma however the causes of such mass movements are still poorly understood this work analyses the events around the canary islands and focuses on the ones that occurred on tenerife in order to obtain new insights into the mechanisms of large volcanic landslides the study is divided into a first part that includes site investigations examining the general features favouring large scale failures at volcanoes the second part describes the laboratory tests used to analyse a residual soil that may be the potential slip surface of the slides on tenerife the site investigation revealed that regional tectonics and the climate have a significant influence on the spatial distribution of the landslides moreover morphological and geological features such as deep fluvial canyons a high coastal cliff and persistent dike intrusion may favour the initiation of slope failure a typical residual soil sample from the lateral scarp of the la orotava amphitheatre on tenerife was studied by carrying out standard laboratory tests the microstructure was analysed using environmental scanning electron microscopy and a particular bonding was found this bonding was also detected by the geotechnical tests consolidation tests and direct shear tests revealed that the mechanical behaviour of the residual soil changes greatly if the bonding of the soil is broken the bonded structure generally fails when the effective normal stress surpasses the yield strength of the bonding in the case of large volcanic landslides with thicknesses up to several hundred meters the high overburden easily exceeds this yield strength and generates a broken bonding therefore volcanic residual soils such as the one analysed in this study are perfect candidates for the potential failure surfaces of large volcanic landslides referring to the la orotava events we assume that residual soil layers and morphological geological and climatic features reduced the slope stability to critical conditions whereas a strong earthquake associated with a caldera collapse episode may have finally triggered the landslide the results obtained indicate that the residual soils play an important role in affecting the stability of volcano slopes and their destabilising influence significantly favours large scale sliding we suggest that the results obtained from this study can be applied to other locations since volcanic residual soils are common in volcanic areas

dissolution and replacement creep a significant deformation mechanism in mid crustal rocks abstract zoning patterns and zoning truncations in metamorphic minerals in a granodioritic orthogneiss indicate that strain and s c fabrics in these rocks were produced by dissolution precipitation and replacement processes even at epidote amphibolite facies metamorphic conditions the metamorphic fabric is defined by alternating layers and folia dominated by quartz feldspars and biotite epidote zoning patterns in most metamorphic plagioclase orthoclase epidote and sphene are truncated at boundaries normal to the shortening direction suggesting dissolution interfaces of relict igneous orthoclase phenocrysts that face the shortening direction are embayed and replaced by biotite epidote and myrmekitic intergrowths of plagioclase and quartz metamorphic plagioclase grains are also replaced by epidote we interpret these microstructures to reflect strain enhanced dissolution the cores of many grains show asymmetric overgrowths with at least two generations of beards all oriented on the ends of grains that face the extension direction we interpret these textures to reflect precipitation of components dissolved by deformation enhanced dissolution while biotite and quartz probably deformed by dislocation creep the overall deformation was accommodated by dissolution perpendicular to the shortening direction and precipitation parallel to it these chemical processes must have been activated at lower stresses than the dislocation creep predicted from extrapolations of data from experiments in dry rocks thus wet crust is likely to be weaker than calculated from these experimental studies

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

dynamic recrystallization processes in plagioclase porphyroclasts abstract recrystallization processes in plagioclase porpyroclasts were studied in samples from an anorthositic gabbro of the jotun complex south norwegian caledonides the deformation took place at a temperature of approximately 700 c and at pressures below 900 mpa two populations of plagioclase porphyroclasts can be distinguished based on microstructural observations the crystallographic orientation and grain shapes for the two populations are interpreted as soft and hard orientations of the dominant slip system 010 001 with respect to the kinematic reference system the orientations of porphyroclasts and recrystallized grains were measured with a u stage microscope analysis of misorientations between porphyroclasts and adjacent recrystallized grains yields different results for the two types of porphyroclasts in type 1 porphyroclasts soft orientation the misorientation is inferred to be caused by subgrain rotation due to the activation of the 010 001 slip system resulting in a host control orientation relationship whereas in type 2 porphyroclasts hard orientation no dominant misorientation relationship could be identified the slip systems of the dislocations inferred from transmission electron microscopy tem analysis correspond well with the results of the u stage misorientation analysis in type 2 porphyroclasts hard orientation micro fracturing was observed in the tem whereas microfractures are absent in type 1 porphyroclasts soft orientation micro fracturing is likely to be the mechanism for nucleation of new grains in type 2 porphyroclasts resulting in a no host control orientation relationship between porphyroclasts and recrystallized grains

the rate of water penetration in experimentally deformed quartzite implications for hydrolytic weakening abstract in order to determine the rate at which water penetrates the grains of a quartzite we performed several types of experiments including water added deformation and hydrostatic experiments on vacuum dried samples and hydrostatic annealing experiments on previously deformed quartzites samples of heavitree quartzite with an average grain size of 200 m and intragranular water concentration of 1750 420 h 10 6 si as measured by ftir were vacuum dried at 800 c 6 pa for 12 h after vacuum drying the grains showed thermal cracks with an average spacing of 50 m and an intragranular water concentration of 240 60 h 10 6 si one constraint on the rate of water penetration into quartz was obtained by deforming vacuum dried samples with 0 3 wt 20 000 h 10 6 si water added at 800 c 1500 mpa and a strain rate of 10 6 s conditions at which as is quartzite undergoes climb accommodated dislocation creep the time between reaching experimental conditions and yielding of the sample was between 2 and 9 h the samples had yield stresses of 100 250 mpa values well below the yield stress of 1500 mpa for a vacuum dried sample deformation of the grains was homogeneous on both optical and tem scales indicating that the grains had fully equilibrated with the water related defect that is responsible for water weakening ftir measurements are consistent with this interpretation one of the deformed samples has an intragranular water concentration of 820 160 h 10 6 si and vacuum dried samples hydrostatically annealed for 3 or 10 h at the same conditions as the deformed samples have an average intragranular water concentration of 730 180 h 10 6 si to obtain a second constraint on the rate of water penetration hydrothermal annealing experiments were performed on portions of a sample of heavitree quartzite which were previously deformed without water added at 700 c 1200 mpa and 10 5 s and have a high 10 16 m 2 and homogeneous dislocation density two samples were annealed at 700 c 1500 mpa for 24 h one with 0 2 wt water and one with no water added the grains in the water added sample have a rim approximately 15 m wide with dislocation recovery microstructures indicating equilibration with the water related defect the sample with no water added has a homogeneous dislocation density throughout the grains which is similar to that of the original deformed sample the 15 m wide recovered rim suggests a penetration rate approximately an order of magnitude faster than the rate of oxygen diffusion under hydrothermal conditions the observed rate of penetration of the water related species is consistent with theoretical considerations and calculations which predict a diffusion rate for h 2 o that is an order of magnitude faster than the rate of hydrothermal oxygen diffusion we infer from our experiments that the rate of water penetration into quartz is consistent with diffusion of h 2 o

geochronology and geochemistry of late cenozoic basalts from the leiqiong area southern china abstract the leiqiong area which includes the leizhou peninsula and the northern part of the hainan island is the largest province of exposed basalts in southern china ar ar and k ar dating indicates that incipient volcanism in the leiqiong area may have taken place in late oligocene time and gradually increased in tempo toward the miocene and pliocene epoch volcanic activities were most extensive during pleistocene and declined and ended in holocene based on radiometric age dating and geographic distribution pliocene and quaternary volcanism in hainan island can be grouped into two stages and six eruptive regions the early volcanism is dominated by flood type fissure eruption of quartz tholeiites and olivine tholeiites whereas the later phase is dominated by central type eruption of alkali olivine basalts and olivine tholeiites the systematic decrease of mgo feo and tio 2 with increasing sio 2 content for basalts from hainan island indicates that fractional crystallization of olivine clinopyroxene and ti bearing opaques may have occurred during magmatic evolution from coexisting fe ti oxide minerals it is estimated that the equilibrium temperatures range from 895 986 c and oxygen fugacities range from 10 13 4 to 10 10 7 atmospheres in the basaltic magmas the incompatible element ratios and the chondrite normalized ree patterns of basalts from the leiqiong area are generally similar to oib the nb u ratios less than 37 in most of the tholeiitic rocks and the negative nb anomaly observed in the spidergram of some basalts indicated that the influence of a paleo subduction zone derived component can not be excluded in considering the genesis of the basalts from the leiqiong area the tholeiites in the leiqiong area may have mixed with a more enriched lithospheric mantle component as well as undergone relatively larger percentages of partial melting than the alkali basalts

the preservation of seismic anisotropy in the earth s mantle during diffusion creep summary seismic anisotropy in the earth particularly in the mantle is commonly interpreted as the result of solid state deformation by dislocation creep that induces a lattice preferred orientation lpo diffusion creep operates where stress levels are lower and or grain sizes smaller it is often assumed that diffusion creep induces grain rotations that eventually destroy any existing lpo a new numerical test of this assumption shows that it is not necessarily the case diffusion creep will create some relative grain rotations but rotation rates decrease through time hence when microstructural change due to diffusion creep dominates that due to grain growth defined here as type p behaviour the converse being type o behaviour the model indicates that lpo will be weakened but preserved for a variety of strain paths including both pure and simple shear one measure of anisotropy is the proportional difference in shear wave velocities for different polarization vectors avs a model olivine microstructure with equant grains and initial maximum avs of 10 0 percent has this value reduced to 6 7 per cent when rotational steady state is attained other models with different initial maximum avs values exhibit final maximum avs values more than half the initial values if the grains are initially elongate by a factor of 2 maximum avs is reduced just slightly to 8 5 per cent thus when grain growth plays a subordinate role to the deformation diffusion creep weakens seismic anisotropy by a factor of less than 2 using maximum avs as a measure and olivine as an example consequently the link between seismic anisotropy and deformation mechanism in the mantle requires reappraisal regions with lpo may comprise material which once deformed by dislocation creep but is now deforming by diffusion creep in a rotational steady state

the preservation potential of microstructures during static grain growth grain growth simulations using the microstructure simulation system elle have been performed in materials with a pre existing grain shape foliation as might be expected the foliation is destroyed by the end of the experiment and grain areas have increased by a factor of seven the area of material swept by the migrating grain boundaries was monitored and it was found that at every stage virtually all of the grains which survived the grain growth process contain one and only one core of unswept material remarkably these remnant unswept cores preserve a useable record of the initial grain size and the orientation of the grain shape foliation this work suggests that even for samples where no equivalent protolith can be found it may be possible to see past a grain growth episode to estimate the original grain shape and grain size of the rock and perhaps even reconstruct the grain boundary kinematics in addition the identification of unswept cores has the potential to help unravel the evolution of grain boundary chemistry in rocks during metamorphism as an example of a natural system showing these microstructures we describe a peridotite from almklovdalen norway this peridotite was infiltrated by aqueous fluids at several stages during late caledonian exhumation and retrogressive metamorphism grain boundary migration associated with the last of these infiltration events swept off abundant intragranular fluid inclusions in the original chlorite peridotite at the grain scale microstructural mapping of the fluid inclusion rich areas shows that as with the numerical simulations many of the grains retain exactly one core of unswept material examples of other natural systems discussed include dislocation density distributions and trace element zoning

geochemical and stable isotope resetting in shear zones from taschalp constraints on fluid flow during exhumation in the western alps fluid flow at greenschist facies conditions during exhumation of the western alps occurred in several penecontemporaneous systems including shear zones at lithological contacts deformed contacts between serpentinite bodies and metabasalts albite veins within metabasalts and calcite quartz veins within calcareous schists fluid flow in shear zones that juxtapose metasediments and ophiolitic rocks within the piemonte unit reset o and h isotope ratios 18o values are buffered by the wall rocks however calculated fluid 2h values are similar within all the shear zones suggesting that they formed an interconnected network the similarity of 2h values of the sheared rocks and those of unsheared calcareous schists suggests that the fluids were derived from or had equilibrated with the schists that envelop the ophiolite rocks time integrated fluid fluxes at the sheared contacts estimated from changes in si in metabasalts were up to 105 m3 m 2 with the fluid flowing up temperature driven either by topography or seismic pumping individual shear zones were active for c 2 3 myr implying average fluid fluxes of up to 10 9 m3 m 2 s 1 rocks in shear zones within the ophiolite away from contacts with the metasediments show much less marked isotopic and geochemical changes implying that fluid volumes decreased into the ophiolite unit consistent with the source of fluids being the metasediments fluids were generated by dehydration reactions that were intersected during exhumation and while many rocks show the affects of fluid rock interaction large scale fluid flow between major units was not common

some garnet microstructures an illustration of the potential of orientation maps and misorientation analysis in microstructural studies the microstructures of two contrasting garnet grains are mapped using automated electron backscatter diffraction in both cases there is a very strong crystallographic preferred orientation with measurements clustered round a single dominant orientation each garnet grain is divided into domains with similar orientations limited by boundaries with misorientations of 2 or more in both samples most of misorientation angles measured across orientation domain boundaries are significantly lower than those measured between random pairs of orientation domains one sample is a deformed garnet that shows considerable distortion within the domains lines of orientation measurements within domains and across domain boundaries show small circle dispersions around rational crystallographic axes the domain boundaries are likely to be subgrain boundaries formed by dislocation creep and recovery the second sample is a porphyroblast in which the domains have no internal distortion and the orientation domain boundaries have random misorientation axes these boundaries probably formed by coalescence of originally separate garnets we suggest that misorientations across these boundaries were reduced by physical relative rotations driven by boundary energy the data illustrate the potential of orientation maps and misorientation analysis in microstructural studies of any crystalline material

variability of residual soil properties rainfall induced slope failures are commonly observed in residual soil due to weathering the residual soil properties vary with depths especially in tropical countries such as singapore therefore it is important to characterize the properties of residual soil with depth index properties soil water characteristic curve and saturated and unsaturated shear strength tests were carried out on residual soils from sedimentary jurong formation bukit timah granite and old alluvium in singapore the variations of residual soil properties in singapore were determined from the laboratory test results and evaluated as a function of soil inherent variability typical upper and lower bounds of soil properties for the residual soils in singapore were described using confidence interval approach and coefficient of variation cov in this paper the variations in residual soil properties can be incorporated in design based on risk or reliability approach the cov of index and engineering properties of residual soils in singapore indicate that residual soils from bukit timah granite and old alluvium are coarser than residual soil from sedimentary jurong formation the particle size distribution of residual soil from old alluvium is more uniform than that from bukit timah granite on the other hand the particle size distribution of residual soil from bukit timah granite is more uniform than that from sedimentary jurong formation the shear strengths of residual soils from bukit timah granite and old alluvium are higher than that from sedimentary jurong formation

networking of shear zones at the brittle to viscous transition cap de creus ne spain a crustal scale shear zone network at the fossil brittle to viscous transition exposed at cap de creus ne spain evolved by coeval fracturing and viscous mylonitic overprinting of an existing foliation initial fracturing led to mylonitic shearing as rock softened in ductilely deformed zones surrounding the fractures mylonitic shear zones widened by lateral branching of fractures from these shear zones and by synthetic rotation of the existing foliation between the fractures and shear zones shear zones lengthened by a combination of fracturing and mylonitic shearing in front of the shear zone tips shear zones interconnected along and across their shearing planes separating rhomb shaped lozenges of less deformed rock lozenges were subsequently incorporated into the mylonitic shear zones by widening in the manner described above in this way deformation became homogeneous on the scale of initial fracturing metre to decametre scale in contrast the shear zone network represents localisation of strain on a decametre length scale the strength of the continental crust at the time of coeval fracturing and viscous shearing is inferred to have decreased with time and strain as fracturing evolved to mylonitic shearing and as the shear zones coalesced to form a through going network subparallel to the shearing plane crustal strength must therefore be considered as strain and scale dependent

2635 ma amphibolite facies gold mineralisation near a terrane boundary suture on storo nuuk region southern west greenland abstract gold on storo in the nuuk region of southern west greenland occurs in a slice of strongly deformed amphibolite facies neoarchaean quartzo feldspathic metasedimentary rocks and amphibolites in tectonic contact with the eoarchaean faeringehavn terrane and the meso to neoarchaean akia terrane the gold is associated with either lollingite feas arsenopyrite or pyrrhotite which belong with the amphibolite facies silicate mineral assemblages zircons were u pb dated by shrimp to constrain the timing of mineralisation the 2700 2830 ma oscillatory zoned high th u volcano sedimentary zircon as whole grains and cores are interpreted to give the maximum depositional age of the rocks hosting the gold a granite sheet barren of gold that cuts gold mineralised rocks gives an age of ca 2550 ma which is the absolute minimum age of mineralisation metamorphic zircons associated with auriferous arsenopyrite particularly close to secondary lollingite pyrrhotite and in the coexisting amphibolite facies silicate minerals were dated in situ in polished thin sections these have lower average th u and yielded dates of ca 2635 ma which is interpreted as the age of the gold bearing mineral assemblages and hence probably the mineralisation supracrustal packages containing abundant quartzo feldspathic sedimentary rocks with depositional ages of ca 2800 ma and metamorphosed at 2650 2600 ma also occur as folded tectonic intercalations along the western edge of the faeringehavn terrane south of storo moreover in the 3080 2960 ma kapisilik terrane to the north and east of storo high grade metamorphic events also occurred between 2650 and 2600 ma however the eoarchaean faeringehavn terrane to the south and east of storo shows widespread zircon growth and recrystallisation during 2720 2700 ma regional metamorphism but not at 2650 2600 ma the storo gold prospect is thus located near an important tectonic terrane boundary cryptic suture between domains with different neoarchaean metamorphic history storo mineralisation at ca 2635 ma probably occurred during deformation of this boundary shortly after it was created and at storo maybe was focussed in an area of low strain in a major antiform the general tectonic frameworks for ca 2635 ma gold mineralisation on storo and in the yilgarn craton are similar these gold provinces are compared and contrasted and the likely reason why so much less gold occurs in the nuuk region is discussed

behavior of unsaturated weathered residual granite soil with initial water contents abstract there is a significant difference between the swccs response for drying and wetting curves in unsaturated state in other words the initial water content condition can influence the behavior of unsaturated soils to find this out a series of triaxial compression tests was performed on samples of compacted granite soils in a modified triaxial cell that can separately control pore air pressure u a and pore water pressure u w unsaturated granite soil samples were prepared with different initial water contents dry wet and optimum moisture by compaction in a mould these samples were tested at different suction and confining stresses the volume change of an unsaturated soil during shearing is more sensitive to the confining pressure compared to the initial water content and the matric suction the volume expands during shearing and the volumetric strain is much larger at the smaller confining pressure and at the higher matric suction the variation of the internal frictional angle with the initial water content and the matric suction is negligible but the effective cohesion increases with matric suction

assessment of soil losses by ephemeral gully erosion using high altitude stereo aerial photographs the objective of this study is to explore in a critical way the potential of high altitude stereo aerial photographs for the assessment of ephemeral gully erosion rates on 28 may 1995 an intensive rainfall event 30 mm h 1 during 30 min return period 3 years occurred in central belgium ephemeral gullies formed within an area of 218 ha study area 1 were mapped and measured both in the field and by high altitude aerial photos taken at the same time comparison of these two methods shows that if only one of the two surveying techniques had been used only 75 per cent of the total ephemeral gully length would have been detected so that the combination of aerial and field data leads in fact to the best possible determination of total gully length within the selected area a correction factor c is proposed so that the results of an ephemeral gully erosion survey based on high altitude stereo aerial photos can be adjusted for the undetected gullies next a sequential series of high altitude stereo aerial photographs taken in six different years was analysed in order to determine ephemeral gully erosion rates in three selected study areas study areas 2 3 and 4 selection criteria were chosen so that these three areas were similar to study area 1 and representative for the cultivated areas in central belgium where intense soil erosion regularly occurs ephemeral gullies were mapped and their total length was measured from the aerial photos using a mean gully cross section of 0 2635 m2 determined in study area 1 the average eroded volume is 1 89 m3 ha 1 in six months for study area 1 0 86 m3 ha 1 in six months for area 2 1 44 m3 ha 1 in six months for area 3 and 2 37 m3 ha 1 in six months for area 4 according to the correction factor c these mean ephemeral gully erosion volumes have to be increased by 44 per cent the ephemeral gully erosion rates based on high altitude stereo aerial photos correspond well with the results of other surveys carried out in the belgian loess belt copyright 1999 john wiley sons ltd

deformation of granitoids at low metamorphic grade i reactions and grain size reduction abstract deformation and chemical reaction in granitoids at greenschist facies conditions were investigated in samples from three shear zones two from the alps corvatsch granodiorite and aiguilles rouges granite switzerland and one from eastern australia wyangala granite when examined by light and electron microscopy all show similar features of deformation that progressed from coarse parent rock through to fine grained mylonite quartz deformed plastically while both k feldspar and plagioclase fractured and recrystallized in conjunction with chemical change which had as its end point an assemblage albite quartz white mica caal silicates k feldspar and calcic plagioclase depending on fluid chemistry from the parent rock were unstable at low metamorphic grades in the presence of aqueous fluid since ca bearing plagioclase was not stable in these environments myrmekite did not replace k feldspar in any of the rocks examined recrystallization of feldspar which should perhaps be termed neocrystallization was initiated predominantly at clast margins or along microfractures that are marked by fluid inclusions and twin offsets in old feldspar grains particularly in plagioclases dislocations exist in wall like structures that are commonly associated with voids suggesting origins in microfractures these dislocations had severely limited mobility and subgrain rotation does not appear to have contributed to recrystallization we conclude that recrystallization mainly occurred via a classical nucleation mechanism with minor contributions from twin boundary migration we argue that the 450 550 c lower limit for recrystallization of feldspar referenced throughout the literature is not applicable to the rock systems under investigation and should be discarded as a universal limit microfracturing microboudinage and reaction are seen as the prerequisites for mylonite formation from coarse grained granitic rocks at low metamorphic grades see part ii of this paper neocrystallization fluid access and nucleation appear to have accompanied or immediately followed the first deformation increments and were primarily responsible for continued grain size reduction transgranular fracture was comparatively important only at the onset of deformation since the formation of shear zones in granitoids commonly takes place under conditions of low metamorphic grade in the presence of aqueous fluids in the earth s crust breakdown reactions and nucleation recrystallization in feldspars may be important influences upon localization of deformation in granitoids

characteristics of residual soils in singapore as formed by weathering residual soils are weathering products of rocks that are commonly found under unsaturated conditions the properties of residual soils are a function of the degree of weathering a series of index properties engineering properties mercury porosimetry tests and scanning electron microscope sem examinations were performed on residual soils from two major geological formations in singapore the results indicate that the variation in the index and engineering properties as well as microstructural characteristics of the residual soils with depth can be related to the degree of weathering an increase in the degree of weathering results in an increase in pore volume and produces a larger range of pore size distribution the variation in the pore volume and the pore size distribution through a profile of weathered rock can be used as an indicative measure of the variation in the degree of weathering with depth

intragranular dynamic recrystallization in naturally deformed calcite marble diffusion accommodated grain boundary sliding as a result of subgrain rotation recrystallization abstract intragranular microshear zones within a greenschist facies calcite marble were studied to try to constrain better the processes of dynamic recrystallization as well as the deformation processes that occur within newly recrystallized grains intragranular recrystallized grains within large twinned calcite porphyroclasts can be related to the host from which they have recrystallized and are the focus of an electron backscatter diffraction study lattice distortions low angle boundaries and some high angle boundaries 15 in the microshears within a porphyroclast have the same misorientation axes suggesting that deformation occurred by climb accommodated dislocation creep involving subgrain rotation recrystallization changes in the ratio of host and twin domain as the deformation zone is entered show that twin boundary migration also occurred recrystallized grains have similar sizes 10 60 m to subgrains suggesting that they formed by subgrain rotation however within the intragranular microshear zones the misorientations between recrystallized grains and porphyroclasts are considerably larger than 15 and misorientation axes are randomly oriented moreover recrystallized grain orientations average around the porphyroclast orientation we suggest that the recrystallized grains once formed are able to deform partly by diffusion accommodated grain boundary sliding which is consistent with predictions made from lab flow laws

lynchets in eastern belgium a geomorphic feature resulting from non mechanised crop farming abstract this paper investigates soil stratigraphy of lynchets in sloping lands in eastern belgium and rates of their development by comparison to tillage translocation by non mechanised agriculture in belgium detailed topographical surveys and augerings were carried out at three sites that were under cropland till 1900 1950 rates and diffusion constants of current non mechanised tillage erosion were monitored in ethiopia using tracers and by precise surveys of tillage steps of known ages the lynchets in belgium consist of colluvium that was removed at the upper border of previously farmed plots repeated tillage with animal drawn implements results in soil translocation fluxes to the lower plot borders in the order of 11 to 91 kg m 1 year 1 for slope gradients between 0 03 and 0 35 m m 1 the studied lynchets in eastern belgium would have required cropping agriculture for a total of 217 585 years on steep and 1037 1867 years on gentle slopes the establishment of lynchets hence took place without any deliberate action of digging but due to repeated tillage even with simple implements and water erosion calculated time spans of tillage indicate that some parts of the study area had already been under cultivation during the gallo roman period others since the middle ages

dislocation generation slip systems and dynamic recrystallization in experimentally deformed plagioclase single crystals abstract three samples of gem quality plagioclase crystals of an60 were experimentally deformed at 900 c 1 gpa confining pressure and strain rates of 7 5 8 7 10 7 s 1 the starting material is effectively dislocation free so that all observed defects were introduced during the experiments two samples were shortened normal to one of the principal slip planes 010 corresponding to a hard orientation and one sample was deformed with a schmid factor of 0 45 for the principal slip system 001 010 corresponding to a soft orientation several slip systems were activated in the soft sample dislocations of the 001 010 and 110 001 system are about equally abundant whereas 110 111 and 101 in 131 to 242 are less common in the soft sample plastic deformation is pervasive and deformation bands are abundant in the hard samples the plastic deformation is concentrated in rims along the sample boundaries deformation bands and shear fractures are common twinning occurs in close association with fracturing and the processes are clearly interrelated glissile dislocations of all observed slip systems are associated with fractures and deformation bands indicating that deformation bands and fractures are important sites of dislocation generation grain boundaries of tiny defect free grains in healed fracture zones have migrated subsequent to fracturing these grains represent former fragments of the fracture process and may act as nuclei for new grains during dynamic recrystallization nucleation via small fragments can explain a non host controlled orientation of recrystallized grains in plagioclase and possibly in other silicate materials which have been plastically deformed near the semi brittle to plastic transition

reaction enhanced permeability during retrogressive metamorphism abstract retrogressive metamorphism is in most cases associated with influx of external fluids into igneous or metamorphic rocks with low to negligible initial permeabilities thus fluid migration is associated with some process that creates the necessary channels for migration one such mechanism is associated with the volume changes resulting from volatilization reactions petrographic studies of a partly serpentinized troctolite from the duluth complex in minnesota reveal how the increase in solid volume associated with hydration of olivine phenocrystals generates an extensive network of microfractures in the surrounding plagioclase dominated matrix this microfracture network allows hydrous fluids to penetrate further into the rock a 2 d spring network model illustrates how expanding polygons or non spherical grains in an elastically deforming matrix cause dilatant microfracturing and porosity generation even if the overall stresses are compressive the model emphasises how fluid infiltration may be promoted by stress heterogeneities set up by metamorphic reactions

detailed recording of gully morphology in 3d through image based modelling abstract the ability to understand gully erosion development is closely related to our ability to quantify the morphology of gullies at present various technologies are at hand to collect data at increasing levels of detail however many of the developed technologies are time consuming difficult to apply or expensive as an alternative image based modelling offers a cost efficient flexible and rapid method to quantify gully morphology from photographs taken in the field in this study the use of image based modelling was tested and fine tuned to quantify the morphology of four gully heads in contrasting biophysical environments prone to gully erosion two bank gullies in central belgium and two permanent gullies in northern ethiopia ground photographs n 88 235 were taken with a reflex canon eos 450d camera having a 20 mm wide angle lens with a fixed focal length the data collection occurred during days of 30 100 cloud cover and after removing excessive vegetation in the gullies processing of the photographs occurred in photoscan 1 0 2 software using the semi automated structure from motion multi view stereo sfm mvs workflow and allowed to produce 3d digital elevation models with accuracies that range from millimetres to centimetres in addition for the same surface 2 5d models were created in arcgis gully morphological properties were derived and included cross sections total volume and volume of undercut walls and soil pipe inlets for the volume calculation optocat software was used cross sections were also quantified by tape meter measurements when compared to 3d models cross sections quantified from tape meter measurements and from 2 5d models underestimate the cross sectional area by

the influence of grain boundary fluids on the microstructure of quartz feldspar mylonites quartz rich domains in pre alpine water deficient amphibolite facies 510 580 c 250 450 mpa pegmatite mylonites from mont mary mm western alps preserve a fine dynamically recrystallized grain size without significant annealing despite the high synkinematic temperatures and subsequent static greenschist facies alpine overprint the microstructure is dramatically different from more typical water rich amphibolite facies mylonites such as from the simplon fault zone in the central alps where the recrystallized grain size is on the millimetre scale the difference reflects the dominant strain induced recrystallization mechanism 1 progressive subgrain rotation and grain boundary bulging for the dry mm examples and 2 fast grain boundary migration for the wet simplon examples the grain boundary microstructure imaged with sem is also very different with most grain boundaries in the dry mm samples lacking porosity whereas grain boundaries in the wet samples are decorated by a multitude of pores quartz grain boundaries from both wet and dry samples are locally coated by thin 100 s of nanometres possibly amorphous silica films despite the differences in microstructure the crystallographic preferred orientations cpos of quartz rich domains from both areas are very similar water deficient conditions hinder grain boundary mobility and thereby modify the dominant recrystallization mechanism s but apparently have little influence on the intracrystalline slip systems as reflected in the cpos strong c axis y maxima in mm mylonites both k feldspar and plagioclase an33 38 dynamically recrystallize consistent with the inferred metamorphic conditions under water deficient conditions mid to lower crustal rocks can deform heterogeneously under transitional ductile brittle conditions at high differential stress for mm ca 300 500 mpa as estimated for dry mohr coulomb failure and preserve this high stress microstructure because of the low mobility of dry grain boundaries

effects of check dams reforestation and land use changes on river channel morphology case study of the rogativa catchment murcia spain this work has been financially supported by the project respuesta hidrologica y geomorfologica de los sistemas fluvio torrenciales afectados por proyectos de restauracion hidrologico forestal en cuencas semiaridas del sureste peninsular of the fundacion instituto euromediterraneo de hidrotecnia ce the first author was financially supported by an i3p postdoc contract csic ce and a ramon y cajal contract from the education and science ministry spain gonzalo g barbera also was supported by i3p postdoc contract

geomechanical and geochemical changes during early stages of weathering of karamu basalt new zealand abstract variations in geochemical and geomechanical characteristics were measured through a basalt weathering sequence exposed at an abandoned quarry at karamu near hamilton new zealand geochemical measurements show that at the early stages of weathering fresh to slightly weathered reductions in the concentrations cao and mgo are rapid although they are not accompanied by major mineralogical change during later stages of weathering secondary clay minerals form following well described mineralogical changes fresh basalt is strong uniaxial compressive strength 262 mn m 2 point load index 5 59 with well developed closely spaced columnar jointing slightly weathered material is distinguished in the field only by slight discolouration along joint planes but laboratory tests show a dramatic loss of strength in slightly weathered rock point load index 0 41 cbr 43 a causal relationship between the loss of cations and loss of strength in the early stages of weathering is suggested this stage of weathering is diffusion controlled whereby cations are lost from the constituent primary minerals and replaced by h this process disrupts the lattice structure and causes a marked loss of strength implications are that slightly weathered rock which appears competent in the field is likely to have much lower strength and durability than assumed from field evidence

experimental evidence for water weakening of quartzite by microcracking plus solution precipitation creep natural quartzites have been experimentally deformed at a temperature of 1073 k a confining pressure of 1 2 0 1gpa strain rates of 10 5 10 6 and 10 7 s 1 and in the presence of 0 4 wt of added water microstructures after deformation indicate that at strain rates of 10 5 10 6 s 1 crystal plastic deformation is predominant this is evident from abundant deformation lamellae in the original grains and the fact that these grains show a weak crystallographic preferred orientation new grains are absent or of relatively minor volumetric importance in contrast samples deformed at 10 7 s 1 show numerous arrays of fine new polygonal to euhedral quartz grains developed in axially aligned transgranular and grain boundary microcracks these new grain aggregates are characterized by abundant microscale voids fine intergranular channels and fluid inclusion trails syntaxial overgrowth structures on crack walls are also widespread the aggregates of new grains are thus interpreted to have formed by precipitation from solution in addition old grain boundaries oriented perpendicular to the shortening direction show evidence for dissolution the old grains show almost no deformation lamellae subgrains or c axis preferred orientation it therefore appears that at strain rates of 10 7 s 1 microcracking and solution precipitation creep are the dominant deformation mechanisms

plastic flow of omphacite in eclogites at temperatures below 500 c implications for interplate coupling in subduction zones in subduction zones oceanic crust of the downgoing plate presumably forms a continuous interlayer between the upper and the lower plate during subduction the basaltic material is transformed to eclogite thus the flow strength of eclogite must pose an upper bound to shear stresses across the plate boundary for the deeper levels of subduction zones up to now experimental flow laws for wet diopside have been applied to predict the strength of eclogites however based on experimental calibration the material would be essentially undeformable by dislocation creep at 650 c even at 700 c a strain rate of 10 14 s 1 would still imply a differential stress of ca 150 mpa omphacite microstructures in eclogites from the piemonte zone western alps reveal shape preferred orientation subgrains and sutured high angle grain boundaries due to migration recrystallization and a pronounced crystallographic preferred orientation these textures indicate that the omphacite was deformed by dislocation creep in contrast garnet forms rigid inclusions within the weaker pyroxene matrix fe mg exchange thermometry for garnet omphacite pairs indicates temperatures of 465 50 c valle di locana and 475 50 c vallone di saint marcel for a pressure of 1 5 gpa these results are not consistent with the predictions based on experimental flow laws for diopside our data suggest that the flow strength of sodic pyroxene is significantly lower than that of diopside this finding is consistent with the homologous temperature concept with the melting temperature of jadeite at 3 gpa being approximately 350 c lower than that of diopside in addition dynamic migration recrystallization of omphacite in the investigated samples can be linked to small chemical changes this indicates that mobile grain boundaries formed an effective pathway for the exchange of ions and that this exchange was fast compared to the rate of boundary migration thus the combined reduction of stored strain energy and chemical free energy provided the driving force for recrystallization the potential effects of these chemical changes on flow strength remain to be explored

quartz veins deformed by diffusion creep accommodated grain boundary sliding during a transient high strain rate event in the southern alps new zealand abstract the crystallographic preferred orientations cpos and microstructures of deformed quartz veins were measured for four samples in the hanging wall of the alpine fault in the southern alps new zealand their deformation and exhumation has occurred since 4 ma the quartz veins have been ductilely sheared to finite shear strains of 5 15 in late cenozoic shear zones at 450 50 c 310 90 mpa and strain rates between 2 10 11 and 2 10 9 s 1 the sheared veins have a polygonal microstructure with few subgrains and an average grain size of 100 m the cpo of the veins is random to very weak within the shear zones we suggest that dislocation creep accommodated initial shear deformation at high stresses and strain rates the deformation must have created a strong cpo and concomitant dynamic recrystallization reduced the grain size significantly dissipation of stresses during initial deformation lead to a stress and strain rate drop required for a switch to diffusion creep accommodated grain boundary sliding gbs continued shearing accommodated by gbs destroyed the cpo post deformational grain growth gave rise to a final polygonal microstructure with a similar grain size in veins and in the wall rocks analysis of existing experimental data suggest that this sequence of events is possible in the time available rates of all processes may have been enhanced by the presence of a water rich fluid within the shear zones these observations of naturally deformed rocks provide a model for the processes that may occur during short lived deformation at transiently high stresses at mid crustal depths or deeper

arc generated blocks with crustal sections in the north atlantic craton of west greenland crustal growth in the archean with modern analogues abstract the ca 700 km long archean craton of west greenland consists of six meso neoarchean ca 3000 2720 ma shear zone bounded crustal blocks that display similar cross sections from south to north ivittuut kvanefjord bjornesund sermilik fiskefjord maniitsoq each block has a southerly upper and a northerly lower zone thus each faces upwards to the south upper zones have prograde amphibolite facies mineralogy and have never been in the granulite facies whereas lower zones reached granulite facies and were partly retrogressed to amphibolite facies upper and lower zones consist predominantly of tonalite trondhjemite granodiorite ttg orthogneisses geochemistry suggests generation by slab melting in subduction settings of island arcs and active continental margins the gneisses contain km thick metavolcanic amphibolite layers typically bordered by km thick layers containing anorthosite and leucogabbro most upper zones contain upper greenschist to amphibolite facies metavolcanic belts including volcaniclastic andesitic rocks the two most prominent metavolcanic belts in the fiskefjord block at qussuk andesitic volcaniclastic rocks garde a a 2007 a mid archaean island arc complex in the eastern akia terrane godthabsfjord southern west greenland journal of the geological society london 164 565 579 and ivisaartoq mafic ultramafic rocks and anorthosite leucogabbro from upper and lower parts of a supra subduction zone system polat a frei r appel p w u dilek y fryer b ordonez calderon j c yang z 2008 the origin and compositions of mesoarchean oceanic crust evidence from the 3075 ma ivisaartoq greenstone belt sw greenland lithos 100 293 321 have island arc geochemical signatures the 2 km thick fiskenaesset complex bjornesund block comprises chromite layered anorthosites leucogabbros and gabbros and local pillow bearing roof pendants from overlying metavolcanic amphibolite the style of deformation changes downwards within crustal blocks upper zones are characterised by linear metavolcanic belts deformed by mostly one major phase of isoclinal folding and lower zones by kilometre scale double triple fold interference patterns everywhere ttg protoliths have intruded anorthositic and volcanic rocks typically along ductile shear zones often so extensively that only anorthositic or amphibolitic lenses are preserved the meso neoarchean crust was thickened by a combination of thrusting isoclinal folding and continued ttg injection dissimilarities in the proportions of anorthositic and metavolcanic rocks in the six blocks suggest that they evolved in several different microcontinents but by similar processes these crustal blocks provide an exceptional example of how continents evolved in the meso neoarchean comparable archean examples in kapuskasing and pikwitonei canada and modern analogues in fiordland new zealand kohistan himalayas southern california batholith peruvian andes and hidaka japan demonstrate that processes of continental growth from island arc to continental arc magmatism and by implication to continental collision were broadly similar throughout most of earth history

micromechanisms of shear zone propagation at the brittle viscous transition our investigation of progressively strained rock samples from the margins of greenschist facies shear zones utilizes a space for time approach to reveal how the mylonitic overprint of metapsammitic and pelitic host rocks at the cap de creus involved brittle fracturing we present a set of microscale observations indicating that microfractures formed immediately prior to or coevally with a fine grained mylonite microfracturing dominated early stages of strain localization on the scale of the shear zones on the microscale centimeter long fractures facilitated strain softening by allowing enhanced fluid access thereby accelerating the dynamic recrystallization of quartz and a metamorphic reaction of biotite as these two processes produce a polyphase matrix of small dislocation poor grains that eventually form an interconnected rheologically weak phase fractures become inactive this represents a strain dependent brittle viscous transition we outline this transition in a conceptual model for the rheological evolution of mid crustal shear zones

re os and u pb constraints on gold mineralisation events in the meso to neoarchaean storo greenstone belt storo southern west greenland the storo greenstone belt in godthabsfjord southern west greenland consists of juxtaposed rock units of different age and origin and hosts gold mineralisation that is associated with arsenopyrite along a contact between lithological units and along the axial plane of a large fold core the age and origin of the gold is debatable but in this paper we present new arsenopyrite re os and zircon u pb data to constrain the age of the storo gold deposit a 2 71 0 05 ga arsenopyrite isochron and 2 707 0 008 ga highly radiogenic arsenopyrite from a mineralisation along a rock contact together with a 2 64 0 02 ga arsenopyrite isochron along the axial plane of the fold core indicate a two stage mineralisation process while the 2 707 0 008 ga highly radiogenic arsenopyrite provides firm support for an early mineralisation event a mixing origin cannot yet be excluded for the 2 71 0 05 ga isochron the 2 64 0 02 ga isochron is in perfect agreement with recent u pb zircon data nutman et al 2007 precambrian research 159 19 32 and these data are best explained by orogenic mineralisation during amphibolite facies metamorphism along structural weak planes the initial os 187 os 188 value of 0 56 0 16 for the 2 64 0 02 ga isochron indicates a crustal source for the metals whereas the initial os 187 os 188 0 1 0 6 for the 2 71 0 05 ga isochron remains unconstrained re os data are best explained by relatively short crustal residence times of less than 0 1 ga wherein the os and associated metals were extracted from the mantle at a time younger than 2 8 ga and in which the 2 64 0 02 ga stage formed by mobilisation of an earlier mineralisation detrital zircon constraints imply volcanism and sediment deposition for parts of the belt at 2 84 ga and that these units were tectonically juxtaposed to similar to 3 05 ga rocks within the belt the units that were deposited 2 84 ga record metamorphic zircon u pb ages of similar to 2 63 ga but evidence for 2 72 ga metamorphism which is ubiquitous in adjacent terranes is lacking c 2012 elsevier b v all rights reserved

symplectite formation in the presence of a reactive fluid insights from hydrothermal experiments this study describes the microstructural and chemical development of symplectites obtained in fluid mediated mineral replacement experiments during the experiments polymineralic feldspar rich samples were exposed to aqueous na sio2 solution at 600 c and 2 kbar confining pressures for durations of 12 hr to 20 days the resulting reaction rims display high mineralogical and structural complexity and contain two varieties of symplectites represented by nanometre scale intergrowths of gehlenite zeolite and grossular zeolite grains the experimental fluid was enriched in 18o isotope in order to trace oxygen redistribution during the reaction the elevated 18o concentration in the reaction products and the heterogeneity in its distribution suggest that symplectite formation was controlled by dissolution precipitation mechanisms rather than volume diffusion processes microstructural and chemical observations suggest that symplectite formation occurred in multiple stages in response to spatially heterogeneous and temporarily evolving fluid composition at the reaction interfaces hence our results shed light on the fundamental processes involved in symplectite formation improving our ability to interpret symplectite microstructures this article is protected by copyright all rights reserved

creep of garnet in eclogite mechanisms and implications abstract the rheological properties of subducting rocks represent a fundamental parameter in the dynamics of subduction zones making robust predictions about these properties and the general strength evolution of subducting plates is hampered by a relatively restricted understanding of the mechanisms by which rocks deform at high and ultrahigh pressure this uncertainty is relates to the discrepancy between experimental and field based observations of eclogite deformation and in particular the role of garnet to further investigate this important aspect we performed a textural and micro structural investigation applying optical microscopy element mapping and electron backscatter diffraction on deformed garnet polycrystals from an eclogite mylonite the results were compared to those from a study of undeformed polycrystals that formed from a supercooled frictional melt at hp conditions the mylonites polycrystals are flattened parallel to the main high pressure foliation in the rock and individual grains were shortened by an average 15 although dislocation creep is commonly presumed to dominate garnet straining in eclogites no record of this mechanism was found instead the garnet grains have dissolution surfaces indicative of deformation by intergranular pressure solution the observations provide compelling evidence for the role of fluids and syn tectonic porosity in the weakening of garnet a supposedly rigid eclogite component such weakening represents a crucial step in the fundamental feedback loop between fluid ingress metamorphism and near instantaneous competence loss in rocks undergoing deep subduction

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

the success of recent land management efforts to reduce soil erosion in northern france abstract soil erosion is an important problem in open field agricultural landscapes with almost no permanent vegetation in small headwater catchments and with few physical obstacles to reduce runoff velocities runoff concentration along linear landscape elements plot boundaries or thalwegs frequently causes ephemeral gullies to form the latter reflecting the poor hydrogeomorphic condition of the land and soilscape to address this problem and to remediate negative on and off site effects land management efforts have multiplied over the past decades in many regions this includes amongst other measures the implementation of vegetation barriers called fascines in the loess dominated aa river basin of northern france where cropland accounts for 67 of the cover we investigated the effect of fascines on ephemeral gully erosion dynamics together with rainfall characteristics and cropland management this was accomplished through a spatially explicit study of 269 sites prone to ephemeral gullying using a diachronic analysis of historical aerial photographs between 1947 and 2012 ephemeral gully densities at the scale of the aa river basin 643 km2 varied between 0 39 and 1 31 m ha 1 long term average of 0 68 m ha 1 with local maxima up to 9 35 m ha 1 densities are however much higher when only considering the most erosion vulnerable municipalities long term average of 2 23 4 30 m ha 1 those values should be used when comparing results from this study to other reports of ephemeral gully erosion fascines were introduced in 2001 and were present in 30 of the gully erosion sites by 2012 although the presence of fascines has an effect on gully length reduction spatial and temporal variations in gully length were mainly driven by cumulative precipitation measurement of sediment deposition at 29 fascines in 2016 showed that only 47 of the fascines functioned as sediment sinks they stored on average 1 7 mg of sediment per winter half year corresponding to 0 009 mg ha 1 the results suggest that fascines positively impact the landscape s resilience and reduce ephemeral gully erosion rates the use of vegetation barriers such as fascines are increasingly implemented for erosion control in western europe but pose problems for the management of open field landscapes

textural evolution of plagioclase feldspar across a shear zone implications for deformation mechanism and rock strength caledonian amphibolite facies shear zones developed in granulitefacies anorthosites and anorthositic gabbros of the bergen arcs western norway allow a detailed study of the relationships between fluid infiltration mineral reactions the evolution of microstructure and deformation mechanisms a sequence of rocks from the relatively pristine granulites into a shear zone has been studied by optical microscopy electron microprobe microanalysis empa scanning electron microscopy sem electron backscatter diffraction ebsd and transmission electron microscopy tem focusing on the progressive development of microstructure in the plagioclase feldspars leading up to their deformation in the shear zone at the outcrop scale fluid infiltration into the granulites is marked by a distinct colour change in the plagioclase from lilac brown to white this is associated with the breakdown of the intermediate composition plagioclase an50 in the granulite to a complex intergrowth of na rich and ca rich domains ebsd analysis shows that this intergrowth retains the crystallographic orientation of the parent feldspar but that the ca rich domains contain low angle grain boundaries within the shear zone this complex intergrowth coarsens by grain boundary migration annihilating these grain boundaries but retaining the na rich and ca rich zoning pattern analysis of nearestneighbour misorientations of feldspar grains in the shear zone demonstrates that local crystallographic preferred orientation cpo is inherited from the parent granulite grain orientations random pair misorientation angle distributions show that there is no cpo in the shear zone as a whole nor is there significant shape preferred orientation spo in single grains these observations are interpreted in terms of fluid induced weakening and deformation by dissolution precipitation pressure solution creep

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

deformation of granitic rocks experimental studies and natural examples crustal deformation is inherently complex because the crust is inhomogeneous and anisotropic on a wide variety of scales advances in our understanding have come from a combination of field observations laboratory experiments and theoretical models what necessarily links all these investigations is a focus on process crystal plasticity dislocation creep is one of several grain scale deformation processes or mechanisms each process is dominant over a certain range of conditions in the crust has a particular form of the flow law and produces a characteristic set of microstructures characterizing the deformation mechanisms for different minerals over a range of conditions allows one to link the mechanical behavior or flow law associated with that process to the microstructures it produces the information on microstructures can be used to help interpret the thermomechanical history of naturally deformed rocks and flow laws can be used to create more realistic models predicting crustal behavior in different tectonic settings iterations among field theoretical and experimental approaches are critical for making progress in our understanding the deformation process of crystal plasticity is the main focus of this volume crystal plastic deformation of the crust is in many ways more complex than that of the mantle because most crustal rocks are polyphase aggregates and the strengths of common crustal silicates such as quartz feldspar and mica are very different at a given set of conditions yield strength envelopes for the crust as shown schematically in figure 1 are commonly plotted using the dislocation creep flow law for quartz which inherently assumes that quartz is the controlling phase and undergoes steady state flow but is the flow law for dislocation creep of quartz a good approximation for the deformation of a granite let alone a crustal plum pudding of regions and layers of different lithologies the crust

ephemeral gully erosion in northwestern spain this study aimed to describe types of ephemeral gullies and to determine their origin evolution and importance as sediment sources in a coruna province northwest spain ephemeral gullies and or rills have been measured in a representative sample of medium textured soils most prone to crusting developed over basic schist this sample consisted of 11 small sites ranging from 0 63 to 7 34 ha a case study of concentrated rill gully erosion in a 0 47 ha catchment with coarse textured soils developed over granite was also reported the mean slope of the sites studied ranged from 6 1 to 16 8 main periods when soil surface was poorly covered were late spring maize seedbeds and late autumn early winter grassland and winter cereal seedbeds case studies where fields were left bare in winter were also investigated soil incision and channel formation were observed even with relatively low rainfall intensities when the soil surface was sealed but also after a single short intense rainfall event on recently tilled surfaces concentrated erosion took place mainly on seedbeds and newly tilled soils in late spring and by autumn or early winter but gullies also appeared in other seasons when the soil surface was left bare in most of the cases studied ephemeral gully erosion caused significant soil losses ranging between 2 and 5 m3 ha for a single season to locally over 25 m3 ha gully development was significantly affected by agricultural operations such as lineal elements often acting as initial axes of concentrated erosion main gullies tended to reappear at the same position

temperature dependent grain boundary migration in deformed then annealed material observations from experimentally deformed synthetic rocksalt abstract grain boundary migration between strained substructured grains and newly appearing strain free grains has been observed during static in situ annealing of pre deformed rocksalt in the sem with increasing temperature t the migration velocity increases and the character of grain boundary migration changes as temperature increases there is an increase in the length of individual migrating boundary segments that move at similar rates in addition the frequency of migrating boundaries that form traces of a 100 boundary plane of at least one of the crystals involved increases and moving grain boundaries between new and old grains change from highly irregular to smooth straight boundaries at the same time there is a decrease in the influence of the substructure of pre existing strained grains on the grain boundary movement resultant microstructures reflect these changes at 325 350 c the deformed then annealed microstructure is characterized by very irregular grain boundaries a high abundance of 5 50 m scale remnants of old substructured grains within new grains giving a poikilitic microstructure at 350 400 c grain boundaries often exhibit elongate embayments into the strained grains and most remnants of old strained grains are located at former grain boundaries at 400 c grain boundaries between new and old strained grains are straight to smoothly curved the grain boundary velocity observations are explained by the effect of temperature on mobility coupled with local driving force variations additionally at low annealing temperature impurity solute drag and driving force variations are influential while at high temperature the anisotropy in grain boundary energy with crystallographic orientation becomes more important transferring the knowledge from our experiments to geological samples enables us to recognize and interpret similar microstructures in rocks thereby making it is possible to relate microstructural characteristics to the pre annealing and post deformational annealing history

effects of drainage ditches and stone bunds on topographical thresholds for gully head development in north ethiopia abstract gully erosion is an extreme process of land degradation operating in different regions of the world a common way to quantify the susceptibility of land to gully incision is the use of topographical thresholds for different land use types however the impact of various management practices in cropland on these thresholds has not been studied to date although land management may significantly affect runoff production erosion processes and rates here the impact of different land management practices on gully head development in cropland is studied based on a standardized procedure for topographical threshold analysis s ka b where s represents the slope gradient of the soil surface a the drainage area at the gully head b an exponent and k a coefficient reflecting the resistance of the land to gully head development a case study area was chosen around wanzaye north ethiopia where three different cropland management practices were studied in 75 catchments i the catchment wide use of stone bunds on the contour ii the use of slightly sloping drainage ditches feses and iii the combined use of stone bunds and feses the lowest k values 0 078 0 090 are found for catchments treated with feses the highest k values 0 198 0 205 are observed for stone bund catchments and medium k values 0 092 0 099 are found for mixed catchments this finding implies that catchments with the exclusive use of drainage ditches are the most vulnerable to gully head development compared with mixed catchments and stone bund catchments however on site sheet and rill erosion rates are reduced by feses as they lower the gradient of the overland flow lines three trends in cropland management around wanzaye and the wider region are observed i feses are exclusively made on rather steep slopes where small drainage areas lead to the rapid development of gully heads ii stone bunds are constructed on both steeper and gentle sloping cropland and iii larger and gently sloping catchments seem to be most suitable for the combined use of drainage ditches and stone bunds

compositional zoning of calcite in a high pressure metamorphic calc schist clues to heterogeneous grain scale fluid distribution during exhumation calcite in former aragonite dolomite bearing calc schists from the ultrahigh pressure metamorphic uhpm oceanic complex at lago di cignana valtournanche italy preserved different kinds of zoning patterns at calcite grain and phase boundaries these patterns are interpreted in terms of lattice diffusion and interfacial mass transport linked with a heterogeneous distribution of fluid and its response to a changing state of stress the succession of events that occurred during exhumation is as follows as the rocks entered the calcite stability field at t 530 550 c p ca 1 2 gpa aragonite occurring in the matrix and as inclusions in poikilitic garnet was completely transformed to calcite combined evidence from microstructures and digital element distribution maps mn mg fe and ca k radiation intensity patterns indicates that transformation rates have been much higher than rates of compositional equilibration of calcite involving resorption of dolomite and grain boundary transport of mg fe and ca this rendered the phase transformation an isochemical process during subsequent cooling to t ca 490 c where lattice diffusion effectively closed grains of matrix calcite have developed diffusion zoned rims a few hundred micrometres thick with mg and fe increasing and ca decreasing towards the phase boundary composition profiles across concentrically zoned large grains in geometrically simple surroundings can be successfully modelled with an error function describing diffusion into a semi infinite medium from a source of constant composition the diffusion rims in matrix calcite are continuous with quartz phengite paragonite and dolomite in the matrix this points to an effective mass transport on phase boundaries over a distance of several hundred micrometres if matrix dolomite has supplied the mg and fe needed for incorporation in calcite in contrast diffusion rims are lacking at calcite calcite and most calcite garnet boundaries implying that only very minor mass transport has occurred on these interfaces over the same t t interval from available grain boundary diffusion data and experimentally determined fluid solid grain boundary structures inferred large differences in transport rates can be best explained by the discontinuous distribution of aqueous fluid along grain phase boundaries observed patterns of diffusion zoning indicate that fluid was distributed not only along grain edge channels but spread out along most calcite white mica and calcite quartz two grain junctions on the other hand the inferred non wetting of calcite grain boundaries in carbonate rich domains is compatible with fluid calcite calcite dihedral angles 60 determined by holness and graham 1995 for a wide range of fluid compositions under the p t conditions of interest whereas differential stress has been very low at the stage of diffusion zoning t 490 c it increased as the rocks were cooling below 440 c at 0 3 0 5 gpa dislocation creep and the concomitant increase of strain energy in matrix calcite induced migration recrystallisation of high angle grain boundaries for that stage the compositional microstructure of recrystallised calcite grain boundary domains indicates significant mass transport along calcite two grain junctions which at the established low temperatures is likely to have been accomplished by ionic diffusion within a hydrous grain boundary fluid film dynamic wetting of migrating grain boundaries

flooding of property by runoff from agricultural land in northwestern europe abstract in the last twenty years there has been an increase in the incidence of flooding of property by runoff from agricultural land in many areas of northwestern europe these events take the form of inundations by soil laden water associated with erision and the formation of ephemeral or talweg gullies developed in normally dry valley bottoms costs of such events may be considerable e g almost us 2m at rottingdean southern england in 1987 these costs are largely borne by individual house occupants insurance companies and local councils the distribution of flooding is widespread but areas of high risk can be identified the hilly area of central belgium parts of northern france the south downs in southern england and south limburg the netherlands all these areas have silty more or less loessial soils two types of flooding may be distinguished winter flooding associated with wet soils and the cultivation of winter cereals and summer flooding due to thunderstorm activity and runoff particularly from sugar beet maize and potato crops the distribution of these types of erosion varies in relation to the interaction between physical characteristics soils and topography climatic conditions and land use across the region the reason for the recent increase in flooding events appears to be changes in land use in the area of arable cropping and the continued intensification of farming such as the use of chemical fertilizers the decline in aggregate stability the increase in the size of fields and compaction by farm vehicles in some regions the risk of flooding has also increased because of expansion of urban areas in valley bottom locations communities have responded to the flooding hazard with emergency or protective measures usually involving engineered structures rather than land use change the policy response to the increased risk of flooding has been very limited especially at the national and provincial level the exception being plans developed with farming organisations in south limburg and the pays de caux in southern england initiatives have been few and largely consist of protective measures undertaken by urban municipalities

can metamorphic reactions proceed faster than bulk strain available constraints on metamorphic reaction rates derived from the study of natural systems are similar to or slightly lower than the bulk strain rates measured in the same rocks here we explore whether this apparent relationship is merely coincidence or due to a more fundamental mechanistic link between reaction and strain grain boundary migration accommodated dislocation creep gbmdc or grain boundary diffusion creep gbdc i e pressure solution both of which involve dissolution precipitation as we define it will occur simultaneously with mineral reactions involving dissolution precipitation in the presence of a non zero deviatoric stress the exact relationships between reaction and strain are different depending on whether gbmdc or gbdc is controlling strain but the mechanistic link exists in both cases we present theoretical arguments which show that bulk strain by gbmdc or gbdc which may additionally be accommodated by processes not involving dissolution precipitation such as dislocation glide and climb or grain boundary sliding should in most cases be somewhat faster than the bulk reaction rates as observed with few exceptions for natural metamorphic systems undergoing plastic deformation strain rates provide an upper limit for bulk reaction rates occurring simultaneously in the same rocks the data suggest that mineral reaction rates may typically be within one order of magnitude of the strain rate

rill and gully development during an extreme winter runoff event in norway erosion on agricultural fields in norway has earlier been reported to be mainly sheet and minor rill erosion exceptional rainfall events during january and february 230 of 30 years monthly mean in 1990 led to extreme runoff and widespread erosion also on areas normally not considered to have a high erosion risk this study presents the results of a field survey in three counties where rills and ephemeral gullies were measured after this event shifting weather conditions with freezing and thawing had given frozen subsoil topsoil conditions varied from ice and snow covered surface to thawed surface with frozen subsoil in one of the areas with a high clay content only the upper topsoil layer was unfrozen during this event extended sheet erosion and smaller rills occurred in the southernmost locations more of the topsoil profile was unfrozen combined with a high silt sand content more severe rilling and ephemeral gullies developed gullies developed down to the depth of the drainpipes in all locations this erosion equals soil losses of more than 100 tons ha 1 or 8 9 mm soil lost from the entire field area different mechanisms for gully development were registered gullies developed as enlarged rills with headward migration and sidewall sloughing knickpoints in the slope could also lead to gully development concentrated water flow entering the field from farmyards roads neighbouring fields and woods could be the starting point for uncontrolled rilling and ephemeral gullying the combination of frozen subsoil saturated soil with low strength and intense rainfall led to gully development also on areas with gentle slopes especially sandy soils management practices like crop cover tillage and lack of surface water control highly influenced the development of gullies

effectiveness of vegetation barriers for marly sediment trapping vegetation barriers can be effective in trapping eroded sediment but little knowledge exists on the characteristics of vegetation barriers efficient in trapping all sediments from an eroded zone upslope the objective of this study is to quantify the effectiveness of vegetation barriers for marly sediment trapping relationships between eroded zones and vegetation barriers composed of low vegetation that is to say herbaceous and under shrub layers located downslope and sufficient to stop all the sediments eroded above have been studied in the brusquet experimental marly catchment in the french southern alps 40 plots with surface areas less than 500 m were studied the eroded zones and the vegetation barriers were characterised by their surface areas between which an average linear relationship exists results also showed that for a marly plot with a given surface area a downslope vegetation barrier covering only 20 of this plot can be sufficient to trap all the sediments eroded above it

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

analysis of dynamic recrystallization and nucleation in a quartzite mylonite electron backscatter diffraction ebsd has been used to analyse a natural quartz mylonite from the stack of glencoul nw scotland this technique has been used to measure the misorientation between original protolith parent grains and recrystallized daughter grains the angle of misorientation is important because it has implications for the controlling recrystallization mechanism the sample exhibits approximately 65 a recrystallized microstructure the average neighbor daughter grain size is 12 m and the average subgrain size of the parent grain is 16 m the parent grains do not show a systematic increase in misorientation from the centre of the grain to the edges these data are inconsistent with the subgrain rotation recrystallization model we propose that recrystallization was facilitated by bulging during strain induced grain boundary migration however the majority of misorientations between the parents and their neighbouring daughter grains are in the range 10 30 requiring that another process has operated to rotate grains to higher misorientations with respect to their parents this process is likely to be grain boundary sliding

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

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

the replacement of plagioclase feldspars by albite observations from hydrothermal experiments oligoclase and labradorite crystals have been experimentally replaced by albite in an aqueous sodium silicate solution at 600 c and 2 kbars the replacement is pseudomorphic and is characterised by a sharp chemical interface which progresses through the feldspar while preserving the crystallographic orientation reaction rims of albite up to 50 m thick can be readily achieved within 14 days re equilibration of plagioclase in an 18o enriched sodium and silica bearing solution results in oxygen isotope redistribution within the feldspar framework structure the observed characteristics of the reaction products are similar to naturally albitised plagioclase and are indicative of an interface coupled dissolution reprecipitation mechanism chemical analyses demonstrate that the albitisation is accompanied by the mobilisation of major minor and trace elements also including elements such as al and ti which are commonly regarded as immobile during hydrothermal alteration the results contribute to developing our understanding of the close association between large scale albitisation and secondary ore mineralisation which is common in nature

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

a proposal of structural models for colluvial and lateritic soil profile from southwestern brazil on the basis of their collapsible behavior abstract this paper aims to propose structural models for understanding the collapsibility of a colluvial and lateritic soil by conducting oedometric tests with controlled suction executed in laboratory the undisturbed soil samples submitted to experiments were collected by opening a trial pit in experimental field for soil mechanics and foundations situated at the state university of campinas sao paulo brazil the soil samples were characterized by means of standard geotechnical tests chemical and mineralogical tests and mercury intrusion porosimetry oedometric tests with controlled suction showed the influence of matric suction on the compressibility of the soil samples it was also observed that the higher the values of matric suction the higher the laboratory collapse potential cp the mineralogical chemical tests indicated the presence of oxides and hydroxides of iron in the samples compounds usually associated with cementation the presence of mineral such as kaolinite gibbsite hematite and goethite was also observed the impregnated thin layer plates of the soil samples as well as the grain size distribution curves and mercury intrusion porosimetry indicated the presence of a structure formed by microaggregates structural models for soil collapse were proposed basing on the results obtained there were behavioral differences in each soil sample collected at different depths the behavior of the sample collected at a 1 5 m depth was basically influenced by matric suction as for the samples collected at 4 5 m and 6 5 m besides being both influenced by matric suction they were also influenced by the presence of cementing agents samples collected at 4 5 m and by the presence of more angular grains of quartz samples collected at 6 5 m

single crystal plagioclase feldspar dissolution rates measured by vertical scanning interferometry here we introduce a technique for simultaneous measurement of surface normal retreat rates of specific cleavage faces by vertical scanning interferometry and the bulk dissolution rate of a mineral powder a hydrothermal reactor is used to contain both a well characterized powder and oriented single crystals with a masked reference surface at elevated temperatures we show examples using both anorthite and albite reacted at temperatures between 150 and 200 c in the case of albite dissolution rates of fine grained powders are substantially enhanced compared to those prevailing on large single crystal cleavage surfaces rates developed on the 010 albite cleavage surface are also substantially faster than those on the 001 face where etch pit development was relatively modest and surface normal retreat was not detectable within the time frame of the experiment the reasons for this difference are not immediately clear but may be related to anisotropy in the distribution of al o si vs si o si bonds in the albite structure 010 twinning expressed on the 001 surface and possible disruption of kink propagation across the twin plane

synkinematic microscopy of transparent polycrystals abstract deformation of weak polycrystalline materials in thin section on the stage of a petrographic microscope offers insight into the origin of optical microstructures similar to microstructures in naturally deformed rocks sample materials used to date include several soft minerals ice carnallite bischofite nitratite and various organic plastic materials they can be simple sheared or pure sheared at temperatures to 300 c at a variety of strain rates using relatively simple and inexpensive apparatus the complete microstructural evolution can be recorded photographically along with the complete kinematic history research applications have focussed on dynamic recrystallization and progress has been made towards understanding how the average grain size is maintained constant in steady state recrystallization flow records have been obtained of the development of various crystal plastic microstructures and of lattice reorientation trajectories during flow teaching applications have begun to be developed and several videotapes are available future technical developments are expected to permit stress measurement or control during experiments and synkinematic recording of lattice reorientation numerous research areas invite attention these include investigation of the microstructural developments that precede faulting the behaviour of deforming crystal melt systems and the behaviour of dynamically reactive multi phase systems