model for the evolution of the avalonian cadomian belt the inception and termination of the ca 670 550 ma avalonian cadomian orogenic cycle are interpreted in terms of the amalgamation and breakup of a late precambrian supercontinent synorogenic arc related volcano sedimentary and granitoid rocks are attributed to oblique southward directed subduction along the margin of the supercontinent after its amalgamation breakup of the supercontinent and inception of the iapetus cycle led to termination of subduction and its replacement by transform activity the resulting closure of late precambrian arc related basins and the development of early cambrian platform successions with acado baltic fauna accompanied the formation of iapetus but took place on the active margin of a preexisting ocean and do not record the evolution of its rifted european flank

microstructure and fabric development in ice lessons learned from in situ experiments and implications for understanding rock evolution abstract in this contribution we present a review of the evolution of microstructures and fabric in ice based on the review we show the potential use of ice as an analogue for rocks by considering selected examples that can be related to quartz rich rocks advances in our understanding of the plasticity of ice have come from experimental investigations that clearly show that plastic deformation of polycrystalline ice is initially produced by basal slip interaction of dislocations play an essential role for dynamic recrystallization processes involving grain nucleation and grain boundary migration during the steady state flow of ice to support this review we describe deformation in polycrystalline standard water ice and natural ice samples summarize other experiments involving bulk samples and use in situ plane strain deformation experiments to illustrate the link between microstructure and fabric evolution rheological response and dominant processes most terrestrial ice masses deform at low shear stresses by grain size insensitive creep with a stress exponent n 3 however from experimental observations it is shown that the distribution of plastic activity producing the microstructure and fabric is initially dominated by grain boundary migration during hardening primary creep followed by dynamic recrystallization during transient creep secondary creep involving new grain nucleation with further cycles of grain growth and nucleation resulting in near steady state creep tertiary creep the microstructural transitions and inferred mechanism changes are a function of local and bulk variations in strain energy i e dislocation densities with surface grain boundary energy being secondary except in the case of static annealing as there is a clear correspondence between the rheology of ice and the high temperature deformation dislocation creep regime of polycrystalline quartz we suggest that lessons learnt from ice deformation can be used to interpret polycrystalline quartz deformation different to quartz ice allows experimental investigations at close to natural strain rate and through in situ experiments offers the opportunity to study the dynamic link between microstructural development rheology and the identification of the dominant processes

carbonado revisited insights from neutron diffraction high resolution orientation mapping and numerical simulations abstract one of the most controversial diamond types is carbonado as its origin and geological history are still under debate here we investigate selected carbonado samples using neutron diffraction and high resolution orientation mapping in combination with numerical simulations neutron diffraction analyses show that fine grained carbonado samples exhibit a distinct lack of crystallographic preferred orientation quantitative crystallographic orientation analyses performed on transmission electron microscope tem sections reveal that the 2 10 m grains exhibit locally significant internal deformation such features are consistent with crystal plastic deformation of a grain aggregate that initially formed by rapid nucleation characterized by a high number of nucleation sites and no crystallographic preferred orientation crystal plastic deformation resulted in high stress heterogeneities close to grain boundaries even at low bulk strains inducing a high degree of lattice distortion without significant grain size reduction and the development of a crystallographic preferred orientation observed differences in the character of the grain boundary network and internal deformation structures can be explained by significant post deformation annealing occurring to variable degrees in the carbonado samples differences in intensity of crystal bending and subgrain boundary sharpness can be explained by dislocation annihilation and rearrangement respectively during annealing grain energy is reduced resulting in distinct changes to the grain boundary geometry grain scale numerical modelling shows that anisotropic grain growth where grain boundary energy is determined by the orientation of a boundary segment relative to the crystallographic orientation of adjacent grains results in straight boundary segments with abrupt changes in orientation even if the boundary is occurring between two triple junctions forming a zigzag pattern in addition in diamond anisotropic grain growth results in triple junctions that rarely show 120 angles our results support the interpretation that carbonados may have undergone at least 2 or 3 stages of development with rapid nucleation crystal plastic deformation to low strains and variable degrees of post deformation annealing such a history is commonly observed in earth s crustal or mantle rocks hence for carbonados it is not necessary to invoke an extraordinary and or extraterrestrial origin and history the combination of methods utilized here promises to help advance our understanding of diamond and diamond aggregates in the future

west african proximity of the avalon terrane in the latest precambrian considerable debate surrounds the late neoproterozoic paleogeographic position of the avalon terrane specifically whether it was adjacent to west africa or amazonia new paleomagnetic results from upper neoproterozoic rocks in the avalon terrane challenge the latter position samples collected from the ca 580 570 ma marystown group in the southern part of the burin peninsula of newfoundland canada yield high temperature magnetic components including dual polarity directions which are considered to be primary on the basis of positive fold and agglomerate tests the resultant tilt corrected inclination is 538 representing a paleolatitude of deposition of 34 81 88 78 for the marystown group given the likelihood that amazonia and laurentia were still juxtaposed around 580 570 ma the marystown group results reveal that the paleolatitude of avalon is significantly lower than would be expected if it was part of amazonia in fact avalon was separated from northern amazonia by at least 1100 km at ca 580 ma if west africa was juxtaposed to amazonia opposite laurentia by this time these results place avalon at the same paleolatitude as the northern margin of the west african craton

ti distribution in quartz across a heterogeneous shear zone within a granodiorite the effect of deformation mechanism and strain on ti resetting abstract the study of a heterogeneous ductile shear zone that developed at 500 c and 0 2 gpa during post magmatic cooling of a granodiorite has allowed the effect of strain and recrystallization on ti re equilibration of quartz to be assessed understanding this effect is critical for applying ti in quartz thermobarometry to mylonites differently strained quartz across the shear zone shows a heterogeneous distribution of ti concentrations ti measured by secondary ion mass spectrometry sims ranging between 2 and 45 ppm quartz cathodoluminescence cl is proven by spectral analysis to be correlated with ti allowing cl images to be calibrated as ti maps using sims measurements coarse grained weakly deformed domains consist of magmatic quartz extensively recrystallized by grain boundary migration gbm and mostly 65 75 area contain 20 38 ppm ti resetting to lower ti occurred locally i in haloes surrounding titanite and biotite inclusions ti as low as 6 ppm ii along grain boundaries and iii towards the interface of quartz domains with other mineral domains with increasing strain quartz underwent progressive grain size reduction and developed a bimodal microstructure with elongate grains 100 s m long surrounded by mantles of new grains 10 30 m in size recrystallized by subgrain rotation sgr dynamic recrystallization by sgr associated with prism slip became increasingly dominant over gbm as strain increased towards the shear zone core significant resetting of ti in quartz only occurred in high strain domains at shear strain probably 10 in the shear zone core where fine recrystallization amounts to 50 60 by area and coarser cores are strongly sub structured these domains are not compositionally homogeneous and still show a range of ti mostly between 2 and 10 ppm in all strain facies of the shear zone quartz filled pressure shadows associated with feldspar show an almost constant ti of 2 ppm the pristine ti content of the magmatic quartz mylonitized in the shear zone core is therefore significantly reset and converges asymptotically towards the equilibrium 2 ppm ti shown by new quartz precipitated in pressure shadows it is inferred that extensive recrystallization by sgr and repeated cycles of dislocation creep and rearrangement provided fluid access to quartz grain interiors promoting chemical buffering and leading to partial re equilibration to low ti these observations imply limitations on the use of the ti in quartz thermobarometry to constrain ambient conditions of ductile deformation

potential field imaging of palaeozoic orogenic structure in northern and central europe abstract the trans european suture zone tesz is the most fundamental lithospheric boundary in europe separating the ancient crust of the fennoscandian shield east european craton from the younger crust of central europe and extending deep into the mantle geophysical potential field images provide an overview of the entire palaeozoic orogenic system of northern and central europe for the first time the tesz is largely concealed by sedimentary basins of permian cenozoic age geological observations are largely restricted to local basement highs and deep boreholes and the coverage of deep seismic surveys is widely spaced despite experiments recently acquired within the europrobe programme by contrast the potential field data offer a relatively detailed coverage of standardised observations throughout the tesz while some features of the images may be sourced in the near surface particularly in the gravity image much of their content reflects the structure of the underlying palaeozoic basement at the scale presented the images highlight the most fundamental features of the crustal structure of the tesz these include the strong contrast between the highly magnetic crust of the east european craton and the less magnetic palaeozoic accreted terranes of central europe the lateral continuity of terranes and their internal structure particularly where arc magmatic complexes are involved and the location and geometry of the terrane boundaries oceanic sutures and strike slip zones that separate them

boreal summer continental monsoon rainfall and hydroclimate anomalies associated with the asian pacific oscillation with the twentieth century analysis data 1901 2002 for atmospheric circulation precipitation palmer drought severity index and sea surface temperature sst we show that the asian pacific oscillation apo during boreal summer is a major mode of the earth climate variation linking to global atmospheric circulation and hydroclimate anomalies especially the northern hemisphere nh summer land monsoon associated with a positive apo phase are the warm troposphere over the eurasian land and the relatively cool troposphere over the north pacific the north atlantic and the indian ocean such an amplified land ocean thermal contrast between the eurasian land and its adjacent oceans signifies a stronger than normal nh summer monsoon with the strengthened southerly or southwesterly monsoon prevailing over tropical africa south asia and east asia a positive apo implies an enhanced summer monsoon rainfall over all major nh land monsoon regions west africa south asia east asia and mexico thus apo is a sensible measure of the nh land monsoon rainfall intensity meanwhile reduced precipitation appears over the arid and semiarid regions of northern africa the middle east and west asia manifesting the monsoon desert coupling on the other hand surrounded by the cool troposphere over the north pacific and north atlantic the extratropical north america has weakened low level continental low and upper level ridge hence a deficient summer rainfall corresponding to a high apo index the african and south asian monsoon regions are wet and cool the east asian monsoon region is wet and hot and the extratropical north america is dry and hot wet and dry climates correspond to wet and dry soil conditions respectively the apo is also associated with significant variations of sst in the entire pacific and the extratropical north atlantic during boreal summer which resembles the interdecadal pacific oscillation in sst of note is that the pacific sst anomalies are not present throughout the year rather mainly occur in late spring peak at late summer and are nearly absent during boreal winter the season dependent apo sst relationship and the origin of the apo remain elusive

development of microstructure during deformation of carnallite and bischofite in transmitted light during deformation of 5 5 0 1 mm samples of carnallite and bischofite in transmitted light the development of the microstructure was followed by time lapse photography in dry samples carnallite deforms by mechanical twinning intracrystalline slip and by cataclastic processes the addition of brine strongly enhances dynamic recrystallization by progressive misorientation of subgrains and by grain boundary migration bischofite shows similar behaviour in agreement with results from samples deformed in a triaxial apparatus important microstructural processes during dynamic recrystallization of wet bischofite are grain coalescence grain dissection and grain migration the oscillatory motion of grain boundaries and sudden large changes in grain boundary migration rate were important features

pre alpine evolution of a segment of the north gondwanan margin geochronological and geochemical evidence from the central serbo macedonian massif abstract the serbo macedonian massif smm represents a composite crystalline belt within the eastern european alpine orogen outcropping from the pannonian basin in the north to the aegean sea in the south the central parts of the massif i e southeastern serbia southwestern bulgaria eastern macedonia consist of the medium to high grade lower complex and the low grade vlasina unit new results of u pb la icp ms analyses coupled with geochemical analyses of hf isotopes on magmatic and detrital zircons and main and trace element concentrations in whole rock samples suggest that the central smm and the basement of the adjacent units i e eastern veles series and struma unit originated in the central parts of the northern margin of gondwana these data provided a basis for a revised tectonic model of the evolution of the smm from the late ediacaran to the early triassic the earliest magmatism in the lower complex vlasina unit and the basement of struma unit is related to the activity along the late cadomian magmatic arc 562 522 ma subsequent stage of early palaeozoic igneous activity is associated with the reactivation of subduction below the lower complex and the eastern veles series during the early ordovician 490 478 ma emplacement of mafic dykes in the lower complex due to aborted rifting in the middle ordovician 472 456 ma and felsic within plate magmatism in the early silurian 439 2 ma the third magmatic stage is represented by carboniferous late to post collisional granites 328 304 ma these granites intrude the gneisses of the lower complex in which the youngest deformed igneous rocks are of early silurian age thus constraining the high strain deformation and peak metamorphism to the variscan orogeny the permian triassic 255 253 ma stage of late to post collisional and within plate felsic magmatism is related to the opening of the mesozoic tethys

proterozoic and cambrian successions in upper silesia an avalonian terrane in southern poland all cambrian series and several cambrian biozones have been recognized using acritarch biochronology within the siliciclastic successions underlying upper silesia in southern poland the entire cambrian succession is around 580 m thick and contains rare l

hydrological signatures relating the asian summer monsoon and enso using the ncep ncar reanalysis for the 1950 2000 period differences in the atmospheric hydrological cycle between the extremes of enso i e la nina minus el nino are examined zonal vertically integrated moisture transport vimt across 1008e accounts for about half of the variability in net moisture convergence in the north indian ocean region between enso extremes when all enso events are considered changes in vimt across 1008e are associated with large changes in the strength of the pacific ocean trade wind regime during enso the bulk of the remaining vimt anomalies are from the arabian sea and appear to be associated with sea level pressure variations in the northern and western parts of the indian ocean basin this initial analysis therefore suggests that the interaction between the monsoon and enso may be more complex than the direct modulation of vimt by the pacific ocean trade winds alone the analysis is refined further by comparing the differences of the indian and pacific ocean hydrological cycles between enso extremes when they occur concurrently with anomalous monsoons enso anomalous monsoon years eam and when the monsoon is normal enso normal monsoon years enm for both eam and enm years similar differences exist in vimt across 1008e between enso extremes however major differences are noted in vimt anomalies from the west and south into the north indian ocean region thus the principal difference in moisture convergence in the north indian ocean between eam and enm years is associated primarily with vimt anomalies in the western indian ocean region and not those in the eastern indian or pacific oceans to test the hypothesis that pacific ocean sst anomalies occurring prior to the monsoon may be important in influencing the eventual nature of the monsoon the analysis is extended backward to the spring period while may sst differences in the nino 3 region between enso extremes are found to be similar for both eam and enm years vimt differences in both the indian ocean and the central and western pacific oceans are sig nificantly larger during eam years than enm years may sst differences in the central subtropical pacific ocean are also significantly larger during eam than enm years these results show that the anomalous sst gradient between the eastern equatorial and the central subtropical pacific ocean prior to the monsoon onset together with its associated vimts anomalies may be important factors in determining the degree of connection between monsoon and enso in addition the circulation in the indian ocean prior to and during the monsoon onset shares a strong association with the eventual intensity of the monsoon enso coupling

rheology of rocksalt abstract we review progress in experimental determinations of transient and steady state flow properties and processes of natural rocksalt aggregates focusing primarily on results from avery island louisiana domal salt the steady state flow field established from constant stress and constant strain rate tests at temperatures from 50 to 200 c strain rates from 10 5 to 10 9 s 1 and differential stresses from 20 7 to 2 5 mpa has been separated into two flow regimes each fit by a power law relation at the higher stresses and strain rates this relation is e 1 6 10 4 exp 68 1 rt 10 3 5 3 for which the pre exponential constant is expressed in mpa 5 3 s 1 and the apparent activation energy is in j mol 1 creep rates predicted by equation a do not differ appreciably from those predicted previously the relatively low stress low strain rate data are very well fit by e 8 1 10 5 exp 51 6 rt 10 3 2 4 which is comparable conditions predicts creep rates higher and equivalent viscosities lower than does equation a by two orders of magnitude the change in behavior from a to b is ascribed to a change in rate limiting mechanism from cross slip of screw dislocations to the climb of edge dislocations in this dry material subgrain formation dominates microstructural development during steady state flow of salt and from determinations of average subgrain diameters in crystals from 20 different rocksalt bodies flow stress levels between 0 6 and 1 4 mpa have been estimated these values do not bear directly on arguments concerning the nature of forces initiating salt pillow growth because apparently evidence relating to the early deformational history has been overprinted at that stage fluid assisted grain boundary diffusional processes might dominate dislocation creep leading to a linear stress strain rate mechanical response considering buoyancy forces alone behavior described by equation b rather than a reduces the relief necessary to initiate pillow growth but a 1000 fold amplification is required to produce stress differences near 1 mpa that forces other than buoyancy are important is indicated by the occurrence of these same paleostress levels in bedded salts and in shallow offshore concordant intrusions differential loading generally provides the most plausible initial driving force for the growth of diapiric salt structures

post deformational annealing at the subgrain scale temperature dependent behaviour revealed by in situ heating experiments on deformed single crystal halite the dynamics of substructures which encompass all structures present at the subgrain scale were investigated by static in situ annealing experiments deformed single crystal halite was annealed

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

palaeozoic terranes and their lithospheric boundaries within the trans european suture zone tesz a review the trans european suture zone tesz is a broad and complex zone of terrane accretion separating ancient lithosphere of the baltic shield and east european craton eec from the younger lithosphere of western and southern europe there is debate about the number of terranes involved and their origins the most significant terrane boundaries originally oceanic sutures are poorly exposed and are defined using faunal provinciality prominent geophysical lineaments may represent sub terrane boundaries within composite terranes the avalonia composite terrane was amalgamated to baltica in latest ician time the provenance of the malopolska and moravo silesian terranes whose neoproterozoic basement is suspect with respect to baltica but with which they nonetheless share faunal affinities is still debated the extent of the variscide rhenohercynian zone defines the southern limit of the early palaeozoic accreted terranes terranes within the bohemian massif exhibit hp metamorphism recording subduction driven collision prior to incorporation in the tesz collage itself associated with a hp event in romania the saxothuringian zone and rheic suture are directly juxtaposed with the eec and the crustal structure of the tesz has been much modified by post palaeozoic tectonism deep seismic reflection data where available show that the oceanic sutures are frequently associated with inclined reflectivity zones irz in the lithospheric mantle here inferred to mark relict subduction zones interpreting the age of subduction is complicated by offset of originally coplanar mantle and crustal segments of the irz resulting from early orogenic lithospheric delamination at the moho interface and or post orogenic reordering processes the mantle irz appear to indicate a n directed polarity for early palaeozoic subduction apparently incompatible with the geological evidence which indicates subduction of the tornquist sea beneath avalonia and a large amount of underthrusting of avalonia by baltica possible explanations for these apparently incompatible observations are evaluated

static recrystallization of vein quartz pebbles in a high pressure low temperature metamorphic conglomerate abstract microfabrics of quartz pebbles in hp lt c 1 gpa c 350 c metamorphic conglomerates are investigated the conglomerate was deformed by dissolution precipitation creep while the interior of the pebbles remained undeformed the different pebbles display a wide variety of quartz microstructures imported from the source rocks one type of pebble is derived from quartz veins it shows old grains with numerous fluid inclusions subgrains and undulatory extinction which are partly replaced by new grains devoid of inclusions and substructure free dislocation densities are on the order of 1012 m 2 in both grains we conclude that 1 the quartz vein underwent inhomogeneous crystal plastic deformation in the source rock 2 recrystallization took place by strain induced grain boundary migration starting from small crystalline volumes poor in defects 3 recrystallization was purely static and commenced during re burial of the conglomerate which 4 was simultaneously deformed by dissolution precipitation creep at low differential stress insufficient for crystal plastic deformation of quartz 5 fluid inclusions within old grains were eliminated and their fluid content was drained along the migrating high angle grain boundaries and 6 strain induced grain boundary migration ceased once the driving force became too low by static recovery concurrent to recrystallization within the deformed old grains

the western pacific subtropical high after the 1970s westward or eastward shift the interdecadal variation of the western pacific subtropical high wpsh in summer june july august during 1979 2009 relative to 1948 1978 was investigated based upon ncep ncar reanalysis data since the wpsh appeared to be much more stable and stronger in the lower troposphere compared to the middle troposphere and had great influences on the east asian summer climate with the lower level southerlies or southwesterlies at its western and southern edge the interdecadal changes of the wpsh at low levels were examined in this study over the area of interest 15 30 n 120 150 e the 850 hpa anticyclone circulation was weakened the sea level pressure decreased the relative vorticity at middle and lower levels 1 000 600 hpa increased and the maximum meridional wind at each latitude between 17 5 and 32 5 n moved eastward all the above decadal variations suggested that the wpsh recessed eastward during 1979 2009 relative to 1948 1978 which can be displayed in the 850 hpa eddy geopotential height field in association with the eastward recession of the wpsh the water vapor transport from the indian summer monsoon played a more important role in the summer precipitation over the middle and lower reaches of the yangtze river valley during 1979 2009 relative to 1948 1978

deformation of crystalline materials in thin section a method applicable to weak low melting point model materials permits observation through a petrographic microscope of large simple shearing deformation of samples as thin as ordinary thin sections examples of ductile brittle and intermediate behavior can be studied the method lends itself to teaching and may also prove valuable in research

decadal change of the spring snow depth over the tibetan plateau the associated circulation and influence on the east asian summer monsoon abstract the decadal change in the spring snow depth over the tibetan plateau and impact on the east asian summer monsoon are investigated using station observations of snow depth data and the ncep ncar reanalysis for 1962 93 during spring march april both the domain averaged snow depth index sdi and the first principal component of the empirical orthogonal function eof analysis exhibit a sharp increase in snow depth after the late 1970s which is accompanied by excessive precipitation and land surface cooling the correlation between sdi and precipitation shows a coherent remote teleconnection from the tibetan plateau northern india to western asia it is found that the increased snow depth over the plateau after the mid 1970s is concurrent with a deeper india burma trough an intensified subtropical westerly jet as well as enhanced ascending motion over the tibetan plateau additional factors for the excessive snowfall include more moisture supply associated with the intensification of the south

the margins of avalonia during cambrian and earliest ordovician times avalonia was an area forming an integral part of the huge gondwanan continent probably along the northern margin of amazonia until in early ordovician late arenig or llanvirn time it split off from gondwana leaving a widening rheic ocean to its south today its southern margin with gondwana extends northeast from east of cape cod massachusetts through nova scotia north of the meguma terrane and thence below sea level to the south of newfoundland on the eastern side of the present atlantic the southern margin may separate southwest portugal from the rest of the iberian peninsula it can be traced eastwards with more certainty from the south cornwall nappes to a line separating the northern phyllite belt on the southern margin of the rhenohercynian terrane and the mid german crystalline high there is no certain evidence of avalonian crust to the northeast of the elbe line the northern margin of avalonia extends westwards from south of denmark to the british isles where it merges with the iapetus ocean suture between scotland and england traced westwards it crosses ireland and reappears in northern newfoundland to the east of new world island where it may follow the trace of the dog bay line and the cape ray fault recent work suggests that the northern margin of avalonia may clip the northern tip of cape breton island in nova scotia and then enter the north american mainland at the bay of chaleur it may then be traced from north and west of the popelogan and bronson hill arcs to long island sound near newhaven connecticut the cambrian to devonian faunas reflect the history of avalonia initially they were purely gondwanan but as ordovician time proceeded more genera crossed firstly the tornquist ocean as it narrowed between avalonia and baltica to close in latest ordovician and early silurian times and secondly the iapetus ocean so that by the early silurian most of the benthic shelly faunas apart from the ostracods were the same round the adjacent margins of all three palaeocontinents

lattice preferred orientation in deformed then annealed material observations from experimental and natural polycrystalline aggregates in order to investigate the evolution of lattice preferred orientation lpo obtained during deformation we carried out post deformational annealing experiments for rock analog materials octachloropropane c3cl8 and norcamphor c7h10o polycrystalline aggregates deformed at low temperature high strain rate conditions show complete obliteration of grain boundary microstructures e g grain shape foliation during annealing but maintain the lpo by rapidly growing small grains which have inherited crystallographic orientations from the adjacent old grains on the other hand polycrystalline aggregates deformed at high temperature low strain rate conditions tend to maintain deformation microstructures during annealing due mainly to the presence of subgrain boundaries which pin the motion of grain boundaries the limited mobility of grain boundaries also allows preservation of the lpo obtained during deformation during grain growth at the stage when textural equilibrium is reached polycrystalline aggregates show no preferential removal of grains of certain crystallographic orientations therefore our experimental results suggest that lpo obtained during deformation is insensitive to post deformational annealing process a microstructural analysis of naturally deformed then annealed quartzite indicates that the similar processes may operate in natural rocks

experimental deformation of a single layer anhydrite in halite matrix under bulk constriction part 2 deformation mechanisms and the role of fluids abstract composite natural samples consisting of a single layer of anhydrite embedded in matrix of rock salt were experimentally deformed at temperature t 345 c and strain rate 10 7 s 1 the geometry and kinematics of folds and boudins which affect the stiff anhydrite layers were described in part 1 of the present study zulauf et al 2009 the microfabrics deformation mechanisms and the role of fluids are treated in the present paper anhydrite was deformed under brittle viscous conditions as is indicated by fracturing twinning and local recrystallization the latter based on strain induced grain boundary migration viscous deformation of halite was accommodated by slip on 110 110 which led to formation and rotation of subgrains and a striking 001 maximum parallel to the long axis x of the strain ellipsoid differential stress obtained from subgrain size of halite is largely consistent with stresses recorded by the load cells of the machine despite of the high deformation temperature and the low water content of the starting samples nacl brine and hydrocarbons expelled from fluid inclusions of both halite and anhydrite led to hydraulic fracturing and redistribution of matter the fluids migrated along newly formed microfractures in anhydrite and along halite anhydrite boundaries released hydrocarbons and nacl brine were redeposited in open space of neck domains in the form of black organic coatings condensate and fine grained halite respectively as these fluid controlled phenomena are common in salt domes of northern germany and released fluids may led to contamination of bed rock and biosphere the results of the present study should be important for workers who are dealing with radioactive waste deposits in rock salt

inclusion localised crystal plasticity dynamic porosity and fast diffusion pathway generation in zircon abstract a population of oscillatory zoned igneous zircon grains in a javanese andesite contains fluid and mineral inclusions up to 10 m across trapped during zircon growth orientation contrast imaging and orientation mapping by electron backscatter diffraction reveal that crystal plastic deformation overprints growth zoning and has localized around 1 10 m pores and inclusions cumulative crystallographic misorientation of up to 25 around pores and inclusions in zircon is predominantly accommodated by low angle 010 slip a distinctly different population of sub micron pores is present along subgrain boundaries and their triple junctions these are interpreted to have formed as a geometric consequence of dislocation interaction during crystal plasticity dislocation creep microstructures are spatially related to differences in cathodoluminescence spectra that indicate variations in the abundance of cl active rare earth elements the extent of the modification suggests deformation related fast pathway diffusion distances that are over five orders of magnitude greater than expected for volume diffusion this enhanced diffusion is interpreted to represent a combination of fast diffusion pathways associated with creep cavitation dislocations and along low angle boundaries these new data indicate that ductile deformation localised around inclusions can provide fast pathways for geochemical exchange these pathways may provide links to the zircon grain boundary thus negating the widely held assumption that inclusions in fracture free zircon are geochemically armoured once they are physically enclosed

deformation microstructures reveal a complex mantle history for polycrystalline diamond 1 detailed quantitative electron backscatter diffraction ebsd analysis of polycrystalline diamond rocks diamondites provides insights into their formation and significance the fine grain size of diamondites is usually attributed to rapid crystallization however ebsd reveals significant intragrain bending distinct low angle boundaries and straight to highly irregular high angle grain boundaries highly deformed grains may be in contact with others showing little or no deformation these features are typical for crystal plastic deformation in which differential stress generates dislocations according to the dominant slip systems in diamond dislocations accumulate to form subgrain boundaries with grain size reduction by rotation of subgrains nucleation and growth of new grains and migration of grain boundaries such features are seen in other polygranular materials such as deformed metals and quartzite deformed at high temperatures during this process interaction with fluids produced interstitial garnets with compositions different from those of the primary inclusions oscillatory cl zoning in diamonds developed through diffusion along subgrain boundaries rather than being a primary growth feature diamondites are thus not simply products of primary crystallization but may be strongly deformed recrystallized and modified integrated ebsd cl and in situ chemical and isotopic analysis provide the spatial control and new insights into mantle processes

post deformational annealing of calcite rocks abstract the evolution of microstructure and crystallographic preferred orientation cpo during post deformational annealing was studied on three calcite rock types differing in purity and grain size carrara marble 98 calcite mean grain size of 115 m solnhofen limestone 96 5 m and synthetic calcite aggregates 99 7 m samples were first deformed in torsion at 727 c at a shear strain rate of 3 10 4 s 1 to a shear strain of 5 and subsequently heat treated at 727 c for various durations between 0 and 24 h microstructures and cpos were analysed by optical microscopy image analysis and electron backscatter diffraction ebsd all rock types deformed in the dislocation creep field at the same applied conditions but their microstructures and cpos after deformation and after annealing differed depending on starting grain size and material composition in carrara marble and in the synthetic calcite aggregate a strong cpo developed during deformation accompanied by dynamic recrystallisation with significant changes in grain size during annealing widespread grain growth and subtle changes of cpo occurred and equilibrated foam microstructures were approached after long annealing times the cpo is the only feature in annealed samples indicating an earlier deformation phase although it is not always identical to the cpo formed during deformation in the more impure solnhofen limestone secondary phases on grain boundaries suppressed grain boundary mobility and prevented both the formation of a recrystallisation cpo during deformation and grain size modification during deformation and annealing

lattice rotation in polycrystalline aggregates and single crystals with one slip system a numerical and experimental approach abstract in this study lattice rotations in polycrystals and single crystals with one slip system have been analysed for pure shear and simple shear by re visiting previously published data and by conducting new numerical models that are compared with results from experiments on polycrystalline ice the numerical models are based on the finite difference method and on the assumption that dislocation glide on one slip system is the dominant crystalline deformation mechanism and is controlled by the critical resolved shear stress law such a deformation scheme corresponds to the operation of glide on 0001 in polycrystalline ice used in the physical deformation experiments the results show that lattice rotation is primarily controlled by the bulk deformation kinematics in both the polycrystalline aggregates and the single crystals in the single crystals the lattice rotation is entirely consistent with the vorticity of the bulk deformation kinematics whereas in the polycrystalline aggregates extensive grain interactions significantly modify the local lattice rotations and may even lead to the lattice planes of individual grains rotating in an opposite sense to that of the bulk deformation these results can reasonably explain the development of crystallographic preferred orientations widely reported in the literature

evolution of late neoproterozoic to early paleozoic tectonic elements in central and southeast european alpine mountain belts review and synthesis abstract late neoproterozoic to early paleozoic cadomian tectonic elements are widespread in the southeastern alpine mediterranean mountain belts with discontinuous exposure extending from the alps to the menderes massif in turkey the sequences include voluminous plutonic island and continental arc successions some ophiolites of variable late neoproterozoic and cambrian to early ordovician ages high to medium grade metamorphic sequences and subordinate metasediments geological and geochronological data suggest that these cadomian elements partly experienced tectonothermal activity including metamorphism and magmatism between ca 650 and 600 ma followed by further i and s type plutonism in an andean type continental margin setting ca between 570 and 520 ma subsequently a major rift zone developed which resulted in continental stretching and subsequent formation of a back arc basin which is dated as late cambrian rifting is followed by oceanic spreading which commenced ca at the cambrian ordovician boundary possibly in a back arc setting this tectonic scenario suggests that alpine mediterranean cadomian tectonic elements were accreted to gondwana during early cadomian events ca 600 650 ma subsequently they were likely part of a long lasting outer subduction zone of gondwana at the margin of a panthalassa type ocean during late neoproterozoic iii and cambrian due to back arc spreading continental pieces started to split off from gondwana ca at the cambrian ordovician boundary

upper ordovician palaeogeography of the bohemian massif implications for armorica summary the palaeogeographic position of armorica in the upper ordovician is still rather unclear due mainly to the lack of reliable palaeomagnetic data to help resolve this a palaeomagnetic study of caradocian and ashgillian sediments and volcanics of the barrandian basin in the central bohemian massif czech republic has been carried out a total of 29 sites were sampled including four from a diabase sill intruded into hirnantian age sediments thus providing a baked contact test three components of magnetization labelled a b and c have been resolved in addition to a low temperature component which corresponds to the present day direction of the local geomagnetic field d i 360 67 component a southerly and shallow before bedding correction is only sporadically present and corresponds to a remagnetization event of permo carboniferous age which is well known throughout hercynian europe component b present as stable endpoints in the sill and as a secondary component in the host rock sandstones yields an overall mean direction of 360 39 95 9 3 k 52 3 in situ and 355 21 95 9 3 k 52 3 after bedding correction component c identified as stable endpoints in sedimentary and volcanic rocks passes both a fold test and a baked contact test and is interpreted as being the primary direction of magnetization the overall mean direction for c is 183 59 95 9 5 k 30 3 nine sites after bedding correction which corresponds to a palaeopole position of 80 s 360 e this translates into palaeolatitudes of 40 s for the area studied in latest ordovician times but indicates large amounts of rotation up to 170 of the bohemian massif before final consolidation of hercynian europe although such large amounts of rotation are difficult to accommodate they are consistent with palaeomagnetic results obtained from lower ordovician and upper silurian rocks elsewhere in the barrandian basin if the bohemian massif is interpreted as being an integral part of the armorican microplate then these results indicate that prior to caradoc times armorica rifted away from the northern margin of gondwana and that by ashgillian times it was located at intermediate palaeolatitudes

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

the development of shear zones in polycrystalline camphor urai j l and humphreys f j 1981 the development of shear zones in polycrystalline camphor in g s lister h j behr k weber and h j zwart editors the effect of deformation on rocks tectonophysics 78 677 685 thin polycrystalline specimens of rhombohedral camphor have been deformed in pure shear in the temperature range 283 343k the deformation processes dynamic recrystallization and the development of microstructure were followed by transmission polarized light microscopy when the stress distribution in the specimens was favourable shear zones were initiated the development of microstructure changed drastically above 310320k the shape of the dynamically recrystallized grains which was equiaxed below this temperature became strongly elongated this change probably due to the development of a marked anisotropy in grain boundary mobilities and the activation of new slip systems made the development of shear zones a more frequently occurring phenomenon above 310 320k

the effect of pre tectonic reaction and annealing extent on behaviour during subsequent deformation insights from paired shear zones in the lower crust of fiordland new zealand granulite facies pargasite orthogneiss is partially to completely reacted to garnet granulite either side of narrow 20 mm felsic dykes in fiordland new zealand forming 10 80 mm wide garnet reaction zones the metamorphic reaction changed the abundance of minerals and their shape and grain size distribution the extent of reaction and annealing temperature related coarsening and nucleation is greatest close to the dykes whereas further away the reaction is incomplete as a consequence grain size and the abundance of garnet decreases away from the felsic dykes over a few centimetres the aspect ratios of clusters of s1 pyroxene and pargasite in the orthogneiss which are variably reacted to post s1 garnet decrease from high in the host to near equidimensional close to the dyke post reaction deformation localized in the fine grained partially reacted areas this produced a pattern of paired shear zones located at the outer parts of the garnet reaction zone our study shows that grain size sensitive deformation occurs where the grain size is sufficiently reduced by metamorphic reaction the weakening of the rock due to the change in grain size distribution outweighs the addition of nominally stronger garnet to the assemblage

pacific east asian teleconnection how does enso affect east asian climate observational evidence is presented to show a teleconnection between the central pacific and east asia during the extreme phases of enso cycles this pacific east asian teleconnection is confined to the lower troposphere the key system that bridges the warm cold events in the eastern pacific and the weak strong east asian winter monsoons is an anomalous lower tropospheric anticyclone cyclone located in the western north pacific the western north pacific wind anomalies develop rapidly in late fall of the year when a strong warm or cold event matures the anomalies persist until the following spring or early summer causing anomalously wet dry conditions along the east asian polar front stretching from southern china northeastward to the east of japan kuroshio extension using atmospheric general circulation and intermediate models the authors show that the anomalous philippine sea anticyclone results from a rossby wave response to suppressed convective heating which is induced by both the in situ ocean surface cooling and the subsidence forced remotely by the central pacific warming the development of the anticyclone is nearly concurrent with the enhancement of the local sea surface cooling both the anticyclone and the cooling region propagate slowly eastward the development and persistence of the teleconnection is primarily attributed to a positive thermodynamic feedback between the anticyclone and the sea surface cooling in the presence of mean northeasterly trades the rapid establishment of the philippine sea wind and sst anomalies implies the occurrence of extratropical tropical interactions through cold surge induced exchanges of surface buoyancy flux the central pacific warming plays an essential role in the development of the western pacific cooling and the wind anomalies by setting up a favorable environment for the anticyclone sst interaction and midlatitude tropical interaction in the western north pacific

fabrics produced mimetically during static metamorphism in retrogressed eclogites from the zermatt saas zone western italian alps abstract lattice preferred orientations lpos are commonly interpreted to form by dislocation creep consequently they are used to infer deformation at the metamorphic grade at which the minerals were stable especially if those minerals show a shape fabric here we show that lpos can occur through mimicry of a pre existing lpo so they formed statically not during deformation omphacite and glaucophane lpos occur in eclogite facies rocks from the zermatt saas unit of the northwest italian alps barroisite grew during greenschist facies retrogression and has an lpo controlled significantly by the eclogite facies omphacite and glaucophane lpos rather than directly by deformation using spatially resolved lattice orientation data from the three key minerals collected using electron backscatter diffraction we deploy a new technique of interphase misorientation distribution analysis to prove this barroisite lpo develops by mimicry of omphacite via a particular lattice orientation relationship and by direct topotactic and epitactic replacement of glaucophane lpo in turn influenced anisotropic grain growth resulting in a barroisite grain shape fabric thus regional retrogression during exhumation of the zermatt saas high pressure rocks was in large part static rather than dynamic as previously interpreted in general the possibility of mimetic fabrics forming during metamorphic reactions must be borne in mind when interpreting direct structural observations and seismic anisotropy data in terms of deformation in both crust and mantle

proterozoic ophiolites and mafic ultramafic complexes marginal to the istanbul block an exotic terrane of avalonian affinity in nw turkey abstract among the proterozoic inliers in the istanbul block the lowest structural levels are exposed in the sunnice massif north of bolu amphibolite facies mafic and subordinate ultramafic rocks of the cele meta ophiolite underlie the greenschist facies ediacaran calc alkaline yellice metavolcanics which are intruded by the 565 576 ma dirgine granitoids hornblende gneisses of the cele meta ophiolite comprise island arc meta tholeiites and transitional to calc alkaline metabasalts which together with minor serpentinite are disposed in a broadly antiformal structure the meta ophiolitic rocks are therefore the oldest ophiolites in nw turkey and are themselves thrust on to a putative pre existing continental margin now represented by the metasedimentary migmatites of the demirci gneisses which may thus be the oldest rocks of the complex the istanbul block is an exotic terrane unlike other western turkish terranes it lacks variscan metamorphism its different provenance indicated by its geological record faunal affinities and inherited mid proterozoic isotopic dates suggests a former link with avalonian basement in england nw europe and the maritime provinces of canada hence together with other terranes now situated further east than the avalonian terranes of nw europe the istanbul block may represent the easternmost extremities of avalonia which were detached during its end ordovician collision with the bruno silesian promontory on the sw margin of baltica subsequent migration of the istanbul block to its present location occurred by eastward displacement by sinistral transpression along the southern margin of baltica to a point east of the dobrogea and south of the scythian platform followed by collision with the sakarya block in the mesozoic and late cretaceous southward displacement with the opening of the black sea basin

water enhanced dynamic recrystallization and solution transfer in experimentally deformed carnallite cylindrical samples of polycrystalline carnallite kmgcl3 6h2o were deformed in a triaxial apparatus at 60 c at confining pressures between 0 1 and 31 mpa and at strain rates between 10 4 and 10 8 s 1 in a number of cases small amounts of saturated carnallite brine were added samples without added brine deform by intracrystalline slip mechanical twinning cracking and by frictional sliding on crack surfaces stress strain curves of these samples are strongly dependent on confining pressure addition of brine has a dramatic effect on both microstructural development and mechanical properties grain boundary migration is strongly enhanced at lower strain rates additional intracrystalline effects start to appear together with the onset of solution transfer rapid compaction in samples deformed with added brine causes high fluid pressures to develop at higher strain rates addition of brine results in a decrease of the flow stress by a factor of two this weakening will increase even further at strain rates below about 10 9 s 1 when solution transfer becomes rate controlling it is argued that deformation of carnallite in nature is adequately described by the flow law found for samples deformed with added brine

the neoproterozoic pan african basement from the alpine lower danubian nappe system south carpathians romania abstract the south carpathians which were thrust to the moesian platform in the alpine orogeny late cretaceous to tertiary include the danubian nappe system the danubian pre alpine basement comprises two variscan nappes each one made up of partially retrogressed amphibolite facies rocks intruded by granitoids and capped by an ordovician devonian volcano sedimentary cover no lithological correlation can be established between the pre ordovician basements of these two units the metamorphic basement from the first variscan nappe the dr gsan group is composed of banded amphibolites with some augen and aluminous gneisses intruded by granodioritic to tonalitic plutons the banded structure of the amphibolites together with their geochemistry suggests a volcano sedimentary sequence zircon u pb data on an intercalated augen gneiss have given an age of 777 3 ma for the emplacement of the protolith of this gneiss nd model ages t dm for the amphibolites range from 717 ma to 817 ma at 777 ma nd values cluster is at 8 3 to 9 8 and sr initial ratios range between 0 7007 and 0 7023 indicating an oceanic origin without continental crust contamination major and trace elements from the dr gsan amphibolites consistently display an island arc signature with three differentiation trends evolving from an early tholeiitic trend to a more differentiated low k calc alkaline one the dr gsan terrane is similar to the early pan african juvenile terranes of the sahara the basement of the second variscan nappe the lainici p ius group is made up of metasedimentary rocks mainly quartzites marbles and graphitic mica gneisses cut by early leucogranitic dykes medium k calc alkaline and alkali calcic mainly granitic plutons and late medium k porphyry diorite dykes this magmatism can be bracketed between 588 ma and 567 ma u pb zircon ages the best preserved pluton tismana 567 ma old displays a composite alkali calcic very high k calc alkaline magmatic sequence ranging from gabbro diorite to monzogranite including an ultramafic pod ages and geochemical signatures resemble the saharan late pan african granitoids the existence of late precambrian partly juvenile terrains is thus confirmed within the basement of south carpathians which renders then a segment of the european alpine belt that can be successfully compared to the pan african trans saharan belt

syndeformational grain growth microstructures and kinetics abstract grain growth can be an important high temperature process in fine grained polycrystalline materials we studied its effect on texture during deformation using a two dimensional computer model and a see through experiment with octachloropropane syndeformational grain growth in geological materials can slow down the development of a grain shape preferred orientation or even obliterate it important processes are preferentially oriented deformation induced neighbour switches the presence of a grain shape preferred orientation slows down the increase in grain size due to grain growth

seven types of subgrain boundaries in octachloropropane abstract subgrain boundaries in thin sheets of octachloropropane deformed at 0 7 0 8 t m on the stage of a microscope are seen to appear in the material in seven different ways type i boundaries show the classical evolution by polygonization of bent crystals type ii are essentially kink boundaries which migrate sideways during deformation to reach their present positions in the crystals type iii develop at the sites of former grain boundaries by reduction of misorientation of adjacent grains type iv and v originate by impingement of migrating grain boundaries or subgrain boundaries respectively type vi propagate in their own planes behind migrating grain boundaries to which they are attached type vii develop statically from optically strain free grains by a process probably otherwise similar to the type i process two thirds of the boundaries are types i or ii in view of the variety of subgrain boundary histories in ocp interpretation of similar features in minerals ought to be undertaken cautiously criteria are needed for telling the different types of subgrain boundaries apart in situations where only a final view of the structure is available as in optical and electron micrographs of rocks

caledonian and proterozoic terrane accretion in the southwest baltic sea abstract a marine seismic reflection survey was carried out in 1996 by dekorp potsdam and bgr hannover in the sw baltic sea several tectonic lineaments were crossed nearly perpendicularly for example the caledonian deformation front which is assumed to mark the northern boundary of the terrane avalonia which accreted to baltica ca 400 ma here a bivergent collision is clearly observed confirming early ideas from the babel survey the ne dipping reflections in the uppermost mantle are interpreted as signs of the subducted tornquist ocean a similar tectonic style of compression and indentation is observed in the proterozoic crust northeast of bornholm where in addition a remarkable crustal thickening and a strong increase of reflection power is observed the three dekorp lines in this area provide a certain three dimensional control and allows extension of similar observations from the babel line a southward this proterozoic terrane accretion seems to be connected to major tectonic lineaments in southern sweden either to the gothian thrust or the sveconorwegian front

riphean rifting and major palaeoproterozoic crustal boundaries in the basement of the east european craton geology and geophysics abstract the east european craton which occupies the northeastern half of the european continent is characterized by the presence of giant even transcratonic systems of riphean meso to neoproterozoic rifts aulacogens the largest of these riphean rift systems tend to follow the course of palaeoproterozoic sutures and junction zones that subdivide the craton into three crustal segments with autonomous development histories these segments named fennoscandia volgo uralia and sarmatia are marked by distinct t magsat anomalies genetically the intersegment junction zones represent different types of collisional and accretional interaction between the component crustal segments of the craton in accordance with this the riphean rift systems vary in width the presence or absence of moho uplifts and other properties like for instance heat flow values however all of them follow belts of weakly magnetized crust studded with local magnetic maxima due to the presence of mafic rocks associated with the rifting the rift system intersegment junction zone relationships in consequence similarly represent a range of variation and cannot be referred to one single model

subsidence analysis of eastern avalonian sequences implications for iapetus closure basin subsidence analysis employing the backstripping method indicates that fundamentally different basin generating mechanisms controlled silurian basin development in northern england and in wales accelerating subsidence with time suggests that the dominant control on basin formation in northern england was flexure of the eastern avalonian lithosphere mid wales however displays dominantly decelerating subsidence suggesting an extensional control the errors involved with the backstripping method particularly concerning lower palaeozoic application have been addressed in conjunction with sedimentological and tectonic data the subsidence patterns for the windermere supergroup indicate deposition in a flexurally loaded foreland basin which migrated southwards with time similar data for the powys supergroup of mid wales imply an extensional control on silurian deposition possibly in a strike slip setting however later poidoli and early devonian welsh sedimentation may represent the onset of foreland basin deposition the new data are consistent with a diachronous closure of the iapetus ocean related to rotational collision of eastern avalonia with laurentia during the early silurian early silurian oblique collision produced transtension in wales subsequent rotation of eastern avalonia produced more orthogonal collision thus continental flexure was felt initially in northern england in late wenlock times followed by migration of the resultant foreland basin to the southeast into wales in early devonian times the versatility of the backstripping method is shown by this application to lower palaeozoic sequences

the evolution of high temperature mylonitic microfabrics evidence from simple shearing of a quartz analogue norcamphor plane strain simple shearing of norcamphor c7h10o in a see through deformation rig to a shear strain of 10 5 at a homologous temperature of th 0 81 yields a microfabric similar to that of quartz in amphibolite facies mylonite synkinematic analysis of the norcamphor microfabric reveals that the development of a steady state texture is linked to changes in the relative activities of several grain scale mechanisms three stages of textural and microstructural evolution are distinguished 1 rotation and shearing of the intracrystalline glide planes are accommodated by localized deformation along three sets of anastomozing microshears a symmetrical c axis girdle reflects localized pure shear extension along the main microshear set sa oblique to the bulk shear zone boundary abbreviated as szb 2 progressive rotation of the microshears into parallelism with the szb increases the component of simple shear on the sa microshears grain boundary migration recrystallization favours the survival of grains with slip systems oriented for easy glide this is associated with a textural transition towards two stable c axis point maxima whose skeletal outline is oblique with respect to the sa microshears and the szb and 3 at high shear strains 8 the microstructure texture and mechanism assemblage are strain invariant but strain continues to partition into rotating sets of microshears steady state is therefore a dynamic heterogeneous condition involving the cyclic nucleation growth and consumption of grains

quantification of mineral behavior in four dimensions grain boundary and substructure dynamics in salt here we present the first four dimensional time and three dimensional space resolved experiment on a strongly deformed geological material results show that even complicated microstructures with large continuous and discontinuous changes in crystallographic orientation can be resolved quantitatively the details that can be resolved are unprecedented and therefore the presented technique promises to become influential in a wide range of geoscientific investigations grain and subgrain scale processes are fundamental to mineral deformation and associated earth dynamics and time resolved observation of these processes is vital for establishing an in depth understanding of the latter however until recently in situ experiments were restricted to observations of two dimensional surfaces we compared experimental results from two dynamic in situ annealing experiments on a single halite crystal a 2d experiment conducted inside the scanning electron microscope and a 3d x ray diffraction experiment this allowed us to evaluate the possible effects of the free surface on grain and subgrain processes the extent to which surface effects cause experimental artifacts in 2d studies has long been questioned our study shows that although the nature of recovery processes are the same the area swept by subgrain boundaries is up to 5 times larger in the volume than observed on the surface we suggest this discrepancy is due to enhanced drag force on subgrain boundaries by thermal surface grooving our results show that while it is problematic to derive absolute mobilities from 2d experiments derived relative mobilities between boundaries with different misorientation angles can be used

the east carpathian crystalline mesozoic zone romania paleozoic amalgamation of gondwana and east european craton derived terranes abstract the alpine orogen of the eastern carpathians contains pre alpine terranes framed by younger rocks the geological data on these terranes are reviewed and interpreted in order to elucidate their geodynamic evolution during the paleozoic the rebra tulghes terrane shows a stratigraphic sequence similar to avalonia neoproterozoic platformal metasediments rebra group overlain by late neoproterozoic volcano sedimentary rocks pietrosu bistritei porphyroids and lower paleozoic cover tulghes group the bretila terrane made up of orthogneisses pelitic gneisses granites amphibolites and micaschists shows affinities with the basement of the east european craton the modern juxtaposition of the bretila terrane and the rebra tulghes terrane may be explained in terms of collision between the latter and the east european craton during the ordovician times as an effect of collision the bretila terrane was thrust over the rebra tulghes terrane and induced the occurrence of thrust faults within it a lower paleozoic emplacement of the east carpathian pre alpine thrust nappes is thus inferred

effect of confining pressure on dilatation recrystallization and flow of rock salt at 150 c microstructural evidence for fluid assisted dynamic recrystallization fadrx is widespread in naturally deformed rock salt however the principal experimental evidence for fadrx in salt has been obtained from stress relaxation experiments and it is unclear whether the process occurs during steady state dislocation creep and what its effect might be here we report deformation experiments performed on natural rock salt at a constant strain rate of 3 5 10 7 s 1 150 c and confining pressures pc of 3 30 mpa samples deformed at pc 3 mpa showed continuous work hardening and minor dilatation microstructurally they exhibited intergranular cracking plus slip band and subgrain structures indicative of dislocation glide creep electron backscatter diffraction analysis revealed a high frequency of boundaries with low angle misorientations in the range 5 10 in contrast samples deformed at pc 6 5 mpa showed work hardening followed by steady state flow at strains 6 7 these samples compacted slightly and crystal plastic deformation was accompanied by extensive fadrx with a predominance of high angle boundaries 30 50 over low angle boundaries we infer that fadrx is suppressed by dilatation at low pressures as a result of grain boundary disruption at pressures high enough to prevent dilatation however fadrx acts as a recovery mechanism counteracting work hardening the results offer a possible explanation for rheological variability seen in previous experiments conducted at pressures up to 30 mpa a simple rate model for diffusion and interface controlled fadrx indicates that fadrx should become increasingly important toward natural halokinetic conditions although the effect on flow stresses is likely to be small

microstructural evolution during initial stages of static recovery and recrystallization new insights from in situ heating experiments combined with electron backscatter diffraction analysis we present in situ observations performed using electron backscatter diffraction of static recovery and recrystallization processes occurring in a plastically deformed geological material namely rocksalt static heating experiments carried out in a scanning electron microscope at temperatures of up to 450 c allowed direct detailed observations of grain boundary migration between substructured grains in deformed polycrystalline samples contrary to expectations crystallographic orientation maps reveal that behind slowly migrating grain boundaries new subgrain boundaries form while pre existing subgrain boundaries are sometimes inherited moreover the crystallographic character especially the preferred orientation of misorientation axes of the new substructures reflects the character of the previous deformation history these results imply that substructural features such as subgrain misorientation distributions may be relatively robust indicators of deformation mechanisms and conditions even in tectonites that have undergone late static recrystallization in addition our observations suggest that the process of static grain boundary migration does not necessarily reset earlier deformation microstructures as commonly assumed the kinematic observations of boundary migrations are inconsistent with simple models in which migration is achieved by single atoms jumping across the boundary

new 40ar 39ar laser single grain ages of muscovites from mylonitic schists in the rodna mountains eastern carpathians romania correlations with microstructure the rodna mountains afford the most internal structural window into the crystalline units of the eastern carpathians in romania the rodna mountains consist of variscan metamorphic nappes that were restacked in the alpine phase of carpathian development forming the subbucovinian and infrabucovinian nappes in order to evaluate age of deformation ten samples were taken from the zone of greenschist facies mylonitic schist that marks the alpine tectonic boundary between the subbucovinian and infrabucovinian nappes and 40ar 39ar laser single grain ages determined for schistosity forming muscovite microstructural assessment of quartz and muscovite distinguished two deformation events single grain ages from the microstructurally most strongly reworked samples four samples give a tight clustering of ages at ca 95 ma the least reworked schists have a broader clustering of ages spanning ca 200 280 ma with a late permian peak and some samples showing outlier ages in the range 200 100 ma the relative development of the outliers which correlates with evidence for increased microstructural reworking is interpreted to mark progressive isotopic resetting the ca 95 ma ages for the most reworked schists are estimates for the age of the alpine nappe stacking the ca 200 280 ma ages are similar to those of magmatism metamorphism and sedimentation thought to mark post variscan pre alpine rifting and ocean basin formation in parts of the alps and may be the thermal imprint of a related event in the eastern carpathians

the petrological significance of misorientations between grains misorientation analysis quantifies microstructural features in tectonites metamorphic and igneous rocks and allows hypotheses on their formation to be tested the misorientation between two lattices can be expressed by a rotation axis and rotation angle for lattices with symmetry it is conventional to take the minimum angle that enables one lattice to be rotated into the other for a group of lattice measurements two types of misorientation distribution can be calculated selecting random pairs of grains gives the random pair misorientation distribution selecting neighbouring pairs gives the neighbour pair misorientation distribution the forms of both distributions are visualised using histograms or cumulative frequency diagrams they are strongly influenced by any overall crystallographic preferred orientation and by intrinsic crystal symmetry in many rocks the random pair misorientation distribution and neighbour pair misorientation distribution are statistically significantly different quantified using the kolmogorov smirnov test differences between the random pair misorientation distribution and neighbour pair misorientation distribution imply that adjacent grains have physically interacted or are inherited from a precursor microstructure interactions include 1 reduction in surface energy by lattice alignment we show this may have occurred in garnet clusters in schist and olivine in a cumulate it is well known in metals and may be a common geological process 2 nucleation where those nuclei have influenced the orientation of adjacent nuclei 3 mechanical rotations of facetted grains in compacting crystal mushes so that faces become parallel 4 growth twinning inheritance includes 1 subgrain rotation recrystallisation in tectonites deforming by crystal plastic processes 2 mechanical and transformation related twinning 3 domainal microstructures e g where grains have formed from a few large original grains may give rise to spurious correlations when the orientation data cover more than one domain with this proviso misorientation analysis can be used to investigate many important microstructural processes

dynamic recrystallization and fabric development during the simple shear deformation of ice abstract in situ observations of polycrystalline ice deformed in simple shear between 10 and 1 c are presented this study illustrates the processes responsible for the deformation the development of a preferred crystallographic orientation and the formation of a preferred dimensional orientation intracrystalline glide on the basal plane accompanying grain rotations and dynamic recrystallization helps to accommodate the large intragranular strains these are the most important mechanisms for crystallographic reorientation and produce a stable fabric that favours glide on the basal plane localized kinks developed in grains unfavourably oriented for easy glide are unstable and are overprinted by dynamic recrystallization dynamic recrystallization is a strain softening process with nucleation occurring in the form of equiaxed grains that grow subparallel to pre existing grain anisotropies and become elongate during deformation plots of grain axial ratio against orientation r indicate a weak shape fabric which does not correspond to the theoretical foliation and elongation for the appropriate increment of shear strain we argue that estimates of the strain magnitude made from orientation of elongate grains are unreliable in high temperature shear zones these results are applicable to both geological and glacial shear environments

the recognition of multiple magmatic events and pre existing deformation zones in metamorphic rocks as illustrated by cl signatures and numerical modelling examples from the ballachulish contact aureole scotland the combination of cathodoluminescence cl analysis temperature and temperature time calculations and microstructural numerical modelling offers the possibility to derive the time resolved evolution of a metamorphic rock this combination of techniques is applied to a natural laboratory namely the ballachulish contact aureole scotland analysis of the appin quartzite reveals that the aureole was produced by two distinct magmatic events and infiltrated by associated fluids developing microstructures allow us to divide the aureole into three distinct regions region a 0 400 m 663 c t max 714 c exhibits a three stage grain boundary migration gbm evolution associated with heating fluid i and fluid ii gbm in region b 400 700 m 630 c t max 700 m of contact t max 630 c is characterised by healed intragranular cracks the combination of cl signature analysis and numerical modelling enables us to recognise whether grain size increase occurred mainly by surface energy driven grain growth gg or strain induced grain boundary migration sigbm gg and sigbm result in either straight bands strongly associated with present day boundaries or highly curved irregular bands that often fill entire grains respectively at a temperature of 620 c evidence for gbm is observed in the initially dry largely undeformed quartzite samples at this temperature evidence for gg is sparse whereas at 663 c cl signatures typical for gg are commonplace the grain boundary network approached energy equilibrium in samples that were at least 5 ka above 620 c

geodynamic evolution of the european variscan fold belt palaeomagnetic and geological constraints the variscan fold belt of europe resulted from the collision of africa baltica laurentia and the intervening microplates in early paleozoic times over the past few years many geological palaeobiogeographic and palaeomagnetic studies have led to significant improvements in our understanding of this orogenic belt whereas it is now fairly well established that avalonia drifted from the northern margin of gondwana in early ordovician times and collided with baltica in the late ordovician early silurian the nature of the gondwana derived armorican microplate is more enigmatic geological and new palaeomagnetic data suggest armorica comprises an assemblage of terranes or microblocks palaeobiogeographic data indicate that these terranes had similar drift histories and the rheic ocean separating avalonia from the armorican terrane assemblage closed in late silurian early devonian times an early to mid devonian phase of extensional tectonics along this suture zone resulted in formation of the relatively narrow rhenohercynian basin which closed progressively between the late devonian and early carboniferous in this contribution we review the constraints provided by palaeomagnetic data compare these with geological and palaeobiogeographic evidence and present a sequence of palaeogeographic reconstructions for these circum atlantic plates and microplates from ordovician through to devonian times

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

avalonian proximity of the ordovician miramichi terrane northern new brunswick northern appalachians paleomagnetic evidence for rifting and back arc basin formation at the southern margin of iapetus a paleomagnetic investigation of the middle ordovician tetagouche group in northern new brunswick was undertaken to determine the paleogeo aphi position of the miramichi terrane stepwise thermal demagnetization of pillow basahs reveals a high temperature characteristic gnet tion with a mean direction of b w z 69 k 22 aas 13 tih corrected n 7 sites 73 samples a positive fold test and the presence of antipodal normal and reversed polarity directions indicate that this ancient direction is ordovician in age with a paleopole position of 52 n 352 e the corresponding pateolatitude of 535 places these vokanic rocks near the southern margin of the iapetus ocean at paleolatitudes similar to those revealed by avalon for the middle to late ordovician the mafic and felsic volcanic rocks and marine sedimentary rocks of the tetagouche and fournier groups have been interpreted to be remnants of a rifted continental margin and a middle ordovician back arc basin our results show that the process of rifting and back arc basin formation occurred at the avalonian margin of iapetus which implies that ordovician subduction was not restricted to the laurentian margin but also marks the southern margin of japetus

late vendian early palaeozoic tectonic evolution of the baltic basin regional tectonic implications from subsidence analysis abstract subsidence analysis was performed on 43 boreholes penetrating the upper vendian lower palaeozoic sedimentary succession of the baltic basin the results were related to lithofacial and structural data to elucidate subsidence mechanisms and the regional tectonic setting of basin development tectonic subsidence patterns are consistent throughout the basin for the time period studied an extensional tectonic subsidence event possibly of two phases is indicated from the late vendian to the beginning of the middle cambrian this event is seen in the southwestern part of the baltic basin peri tornquist zone until the earliest cambrian after which it is also observed in the sw ne trending baltic depression part of the basin basin development during this time is interpreted as recording the latest stages of break up of the precambrian super continent rodinia and ultimately the formation of the tornquist sea the late middle cambrian to middle ordovician tectonic subsidence pattern of the baltic basin is characteristic of post rift thermal subsidence of the newly formed passive continental margin of baltica developed along its southwestern edge a gradual increase in subsidence rate is observed from the middle late ordovician and throughout the silurian particularly for ludlow and pridoli times creating subsidence curves with convex shapes typical of foreland basin development the rate of late silurian tectonic subsidence increases significantly towards the southwest margin of the baltic basin adjacent to the present location of the north german polish caledonides the baltic basin therefore appears to have developed primarily as a flexural foreland basin during silurian oblique collision of baltica and eastern avalonia a foreland setting is supported by the influx of distal turbidites into the basin from southwest sources in the late silurian compressional deformation structures of early devonian lochkovian age are seen in seismic sections in the central part of the baltic basin lithuania these together with a change in subsidence pattern mark the end of the caledonian stage of basin development of the baltic basin

microstructure and micromotion in recrystallization flow of octachloropropane a first look advance of the genetic theory of metamorphic fabric depends in part on improved understanding of dynamic recrystallization processes and the effects of these processes upon various fabric elements this paper describes aspects of the microstructural evolution and associated small scale motion in a dynamically recrystallizing sheet of octachloropropane the material was deformed in progressive pure shear at 2 10 3 sec 1 with interruptions for photography and u stage measurement at 70 of its absolute melting temperature the dominant recrystallization process is migration of grain boundaries with consequent major adjustment of the shapes and sizes of grains but no marked change in the average grain size and no creation of new grains typical grains in the fully deformed material 30 bulk shortening have a composite structure with primitive high strain regions inherited directly from the undeformed state rimmed by successively younger regions of lower strain accreted behind outwardmoving boundaries an array of second phase marker particles permits distinction between grain boundary motion with and through the material and reveals an unexpected ability of grains as a whole to migrate short distances up a bulk strain gradient the markers also show that the only important deformation process is intragranular deformation and that it is proceeding at somewhat different rates in different grains the central problem posed by the observations is to explain the direction and speed of each migrating boundary two hypotheses were tested neither with outstanding success one of these hypotheses is that grain boundary migration is part of a strain accomodation process that operates between grains stretching or shortening at different rates parallel to their mutual boundary

what happens to deformed rocks after deformation a refined model for recovery based on numerical simulations deformation in large parts of the middle crust results in strained rocks consisting of grains with variable dislocation densities and microstructures which are characterized by gradual distortion

deformation and ultrafine dynamic recrystallization of quartz in pseudotachylyte bearing brittle faults a matter of a few seconds abstract tectonic pseudotachylytes i e quenched friction induced silicate melts record coseismic slip along faults and are mainly reported from the brittle crust in association with cataclasites in this study we document the occurrence of recrystallization of quartz to ultrafine grained grain size 1 2 m aggregates along microshear zones 50 150 m thick in the host rock adjacent to pseudotachylytes from two different faults within quartzite schneeberg normal fault zone eastern alps and tonalite adamello fault southern alps in the brittle crust the transition from the host quartz to microshear zone interior includes i formation of high dislocation densities ii fine 0 3 0 5 m polygonization to subgrains defined by disordered to well ordered dislocation walls iii development of a mosaic aggregate of dislocation free new grains the crystallographic preferred orientation cpo of quartz towards the microshear zone shows a progressive misorientation from the host grain by subgrain rotation recrystallization to a nearly random cpo possibly related to grain boundary sliding these ultrafine aggregates appear to be typically associated with pseudotachylytes in nature we refer the crystal plastic deformation of quartz accompanied by dramatic grain size refinement to the coseismic stages of fault slip due to high differential stress and temperature transients induced by frictional heating microshear zones localized on precursory fractures developed during the stages of earthquake rupture propagation and the very initial stages of fault slip thermal models indicate that the process of recrystallization including recovery processes occurred in a time lapse of a few tens of seconds

dynamic recrystallization of wet synthetic polycrystalline halite dependence of grain size distribution on flow stress temperature and strain abstract it is often observed that dynamic recrystallization results in a recrystallized grain size distribution with a mean grain size that is inversely related to the flow stress however it is still open to discussion if theoretical models that underpin recrystallized grain size stress relations offer a satisfactorily microphysical basis the temperature dependence of recrystallized grain size predicted by most of these models is rarely observed possibly because it is usually not systematically investigated in this study samples of wet halite containing 10 ppm water by weight were deformed in axial compression at 50 mpa confining pressure the evolution of the recrystallized grain size distribution with strain was investigated using experiments achieving natural strains of 0 07 0 12 and 0 25 at a strain rate of 5 10 7 s 1 and a temperature of 125 c the stress and temperature dependence of recrystallized grain size was systematically investigated using experiments achieving fixed strains of 0 29 0 46 and one to a strain of 0 68 at constant strain rates of 5 10 7 1 10 4 s 1 and temperatures of 75 240 c yielding stresses of 7 22 mpa the microstructures and full grain size distributions of all samples were analyzed the results showed that deformation occurred by a combination of dislocation creep and solution precipitation creep dynamic recrystallization occurred in all samples and was dominated by fluid assisted grain boundary migration during deformation grain boundary migration results in a competition between grain growth due to the removal of grains with high internal strain energy and grain size reduction due to grain dissection i e moving boundaries that crosscut or consume parts of neighbouring grains at steady state grain growth and grain size reduction processes balance yielding constant flow stress and recrystallized grain size that is inversely related to stress and temperature evaluation of the recrystallized grain size data against the different models for the development of mean steady state recrystallized grain size revealed that the data are best described by a model based on the hypothesis that recrystallized grain size organizes itself in the boundary between the grain size sensitive solution precipitation and grain size insensitive dislocation creep fields application of a piezometer calibrated using the recrystallized grain size data to natural halite rock revealed that paleostresses can vary significantly with temperature up to a factor of 2 5 for t 50 200 c and that the existing temperature independent recrystallized grain size stress piezometer may significantly underestimate flow stresses in natural halite rock

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

microstructural evolution during thermal annealing of ice i h we studied the evolution of the microstructure of ice ih during static recrystallization by stepwise annealing experiments we alternated thermal annealing and electron backscatter diffraction ebsd analyses on polycrystalline columnar ice pre deformed in uniaxial compression at temperature of 7 c to macroscopic strains of 3 0 5 2 annealing experiments were carried out at 5 c and 2 c up to a maximum of 3 25 days typically in 5 6 steps ebsd crystal orientation maps obtained after each annealing step permit the description of microstructural changes decrease in average intragranular misorientation at the sample scale and modification of the misorientation across subgrain boundaries provide evidence for recovery from the earliest stages of annealing this initial evolution is similar for all studied samples irrespective of their initial strain or annealing temperature after an incubation period 1 5 h recovery is accompanied by recrystallization nucleation and grain boundary migration grain growth proceeds at the expense of domains with high intragranular misorientations consuming first the most misorientated parts of primary grains grain growth kinetics fits the parabolic growth law with grain growth exponents in the range of 2 4 4 0 deformation induced tilt boundaries and kink bands may slow down grain boundary migration they are stable features during early stages of static recrystallization only erased by normal growth which starts after 24 h of annealing

the high temperature static recovery and recrystallization behaviour of cold worked carrara marble the static recovery and recrystallization behaviour of carrara marble has been investigated as a function of pre strain 8 27 and annealing temperature 500 700 c quantitative estimates of the rate of recovery were obtained by comparing the stress strain curves generated at a temperature of 426 c a confining pressure of 170 mpa and a strain rate of 3 10 4s 1 before and after a period of isostatic annealing curves of fractional recovery r as a function of log time t show a characteristic sigmoidal shape in which the rate of recovery dr d logt is initially small increases rapidly and finally becomes small again the acceleration of recovery occurs at shorter times both with increasing pre strain and with increasing annealing temperature microstructural examination of the annealed material shows that the acceleration of recovery corresponds to the onset of recrystallization and that prior to the onset of recrystallization recovery is controlled by the development of a subgrain structure whereas after recrystallization has gone to completion it is controlled by the intensification and growth of this substructure the results have been interpreted within the perspective of hart s state variable description of inelastic deformation this analysis shows that the decay of the mechanical state of the marble during annealing can be treated as a two stage static recovery process substructure formation substructure coarsening with the recrystallization marking the transition between these two stages

the palaeozoic evolution in the alps from gondwana to pangea more than 50 of the alps expose fragments of palaeozoic basement which were assembled during the alpine orogeny although the tectonic and metamorphic history of the basement units can be compared to that of the variscan crust in the alpine foreland most of the basement pieces of the alps do not represent the direct southern continuation of variscan structural elements evident in the massif central the vosges black forest or the bohemian massif the basement units of the alps all originated at the gondwana margin they were derived from a precambrian volcanic arc suture fringing the northern margin of gondwana from which they were rifted during the cambrian ordovician and silurian a short lived ordovician orogenic event interrupted the general rifting tendency at the gondwana active margin after the ordovician the different blocks drifted from the gondwana margin to their pangea position colliding either parallel to armorica with laurussia or with originally peri gondwanan blocks assembled presently in armorica from the devonian onwards many basement subunits underwent the complex evolution of apparently oblique collision and nappe stacking docking started in the external massifs the penninic and lower and middle austroalpine units in approximately devonian early carboniferous times followed by the upper austroalpine and the south alpine domains in the visean and the namurian times respectively wrenching is probably the best mechanism to explain all syn and post collisional phenomena since the visean followed by post orogenic collapse and extension it explains the occurrence of strike slip faults at different crustal levels the formation of sedimentary troughs as well as the extrusion and intrusion of crustal and mantle derived magmas and allows for contemporaneous rapid uplift of lower crustal levels and their erosion from the stephanian onwards all regions were deeply eroded by large river systems

in situ annealing experiments of octachloropropane as a rock analogue kinetics and energetics of grain growth abstract two dimensional in situ observation of the change of size of individual grains of octachloropropane ocp during post deformation annealing revealed that grain boundary migration occurred to reduce the grain boundary energy within the polycrystalline aggregate studied some grains show a cyclic change in grain size others have a stable grain size while most grains were consumed and disappeared in order that the mean grain size could increase during grain coarsening some grains were dissected then coalesced amalgamated with others and their centers migrated across the aggregate the average grain size d can be expressed by d k 0 exp q rt t n where k0 is a constant q is the activation energy r is the gas constant t is the temperature t is the annealing time and n is a constant we obtained n 0 1 0 2 and q 6 2 kcal mol the driving force p is the surface energy stored at grain boundaries the driving force can be expressed as p tm where m is the constant and hence the grain growth rate d can be expressed as d p n 1 m the value of n 1 m ranges from 4 to 10

geology and paleobiology of islands in the ordovician iapetus ocean review and implications a history of islands in the iapetus ocean and their destruction is recorded by widespread occurrences of lower to upper ordovician volcanic and epiclastic rocks and some limestone through the axial parts of the appalachian caledonide orogen in north america and northwestern europe assemblages of arenigian and llanvirnian benthic shelly fossils chiefly brachiopods from these rocks constitute the celtic biogeographic province that is distinguished by its endemic genera and by some that are older than in other biogeographic provinces some that are younger than elsewhere and others that have not hitherto been found together evidence from these fossils and from the geology of the rocks pertaining to them suggests that north american terranes that included islands of arenigian llanvirnian age lay at middle to high latitudes both in mid ocean and within the armorican continental margin whereas coeval rocks in west central ireland and norway that have toquima table head shelly fossils were originally part of the laurentian continental margin eradication of the celtic assemblage and the reduction of ordovician faunal provinciality in the appalachian caledonide region coincide with llanvirnian obduction subduction of hundreds of kilometres of iapetus ocean crust late llanvirnian and younger ordovician rocks in the remnant basin including some of peri insular origin have brachiopods of scoto appalachian affinities no discrete islands of ashgillian age have been recognized although fossiliferous volcaniclastic and polymictic conglomerates indicate intermontane basins in some places shelly fossils in these rocks and those in the siltstones and limestones from the gaspe peninsula are like those of ashgillian age elsewhere

an in situ optical investigation of the deformation behaviour of sodium nitrate an analogue for calcite abstract in situ observations have been made under the optical microscope of the deformation of thin polycrystalline specimens of sodium nitrate calcite structure at temperatures between 20 c 0 5 tm and 300 c 0 98 tm at low temperatures or high strain rates deformation twinning is observed between 20 c and 180 c inhomogeneous deformation together with dynamic recrystallisation by subgrain rotation results in a core and mantle microstructure at temperatures above 180 c rapid dynamic recrystallisation by grain boundary migration also occurs producing a coarse grained microstructure the results are interpreted in terms of impurity controlled grain boundary motion as proposed for halite and a strong similarity between the deformation behaviour of sodium nitrate and marble is noted

subgrain boundaries and related microstructural features in edml antarctica deep ice core subgrain boundaries revealed as shallow sublimation grooves on ice sample surfaces are a direct and easily observable feature of intracrystalline deformation and recrystallization statistical data obtained from the epica dronning maud land edml deep ice core drilled in east antarctica cannot detect a depth region of increased subgrain boundary formation grain boundary morphologies show a strong influence of internal strain energy on the microstructure at all depths the data do not support the classical view of a change of dominating recrystallization regimes with depth three major types of subgrain boundaries reflecting high mechanical anisotropy are specified in combination with crystal orientation analysis

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

rifting during separation of eastern avalonia from gondwana evidence from subsidence analysis subsidence curves for cambrian ordovician sequences from the anglo welsh segment of the paleocontinent of avalonia reveal two periods of regionally enhanced basement subsidence early cambrian 545 518 ma and late cambrian to early tremadocian 505 490 ma the earlier event may record transtension following the avalonian cadomian orogeny the second event may be a transtensional precursor to the late tremadocian volcanic arc on eastern avalonia however paleomagnetic faunal volcanic and sedimentary evidence suggests that the main separation of eastern avalonia from gondwana occurred after middle arenigian time rifting during separation is probably recorded by localized middle arenigian to llanvirnian 480 462 ma subsidence along the welsh basin margin but rifting must have occurred mainly on the now obscured southern margin of the avalonian continent pronounced caradocian 462 449 ma subsidence is associated with back arc rifting after separation from gondwana