the development of the central european continental crust since the early archaean based on conventional and ion microprobe dating of up to 3 84 b y old detrital zircons abstract archaean components up to 3 84 b y old have been detected by ion microprobe dating of zircon from various sediments and metasediments of the european hercynides moldanubian of northeastern bavaria and the massif central the early archaean age of 3 84 b y was obtained within the euhedral core of a detrital grain from a moldanubian paragneiss the same detrital zircon grain reveals at least three metamorphic overprints at 2 59 b y b p 1 94 b y b p and 460 m y b p and at least two cycles of mechanical abrasion during surface recycling in the early proterozoic and after the pan african cycle it is thus the crystal with the most complex geological history ever detected further archaean ages for the crystallization of primary magmatic zircons probably in the course of continental crust formation in the first cycle provenances of the two studied areas were found at 3 15 2 9 2 76 and 2 65 b y b p and in a few cases between 2 6 and 2 5 b y b p early proterozoic crust forming events between about 1 75 and 2 15 b y b p play an important role especially for the sediments and metasediments of the montagne noire at the southern rim of the massif central middle and late proterozoic ages cluster around 1 0 b y b p and 600 m y b p for all analysed samples therefore the deposition of the metasedimentary precursors took place after the pan african cycle which is in line with the rb sr whole rock systematics of central european metasediments the ages of metamorphism in the different source areas could be inferred in some cases by lower intercept ages obtained on single detrital grains the following sequence coinciding with times of magmatic events has been so far established 2 59 1 94 and 1 0 b y b p and 600 m y b p thus ion probe dating is also capable of defining secondary effects in the respective provenances which are typical for orogenic cycles the ubiquitous presence of pan african detrital zircons in european metasediments is a strong indication that the detrital material was derived from gondwana however from the presence of metamorphic units including eclogitefacies rocks which probably formed at different continental margins and at various times between about 500 m y and 330 m y ago it is probable that various gondwana derived microcontinental plates contributed to the different basin fillings the gondwana derived detritus is also supported by the observation of two periods of relative quiescence or non activity around 2 3 b y ago and especially around 1 5 b y ago which is a typical feature observed in gondwana cahen et al 1984 but not in laurasia the other possible source of the detritus the lack of zircons crystallized between 3 84 and 3 15 b y ago might have different causes thus the continental crust of the european hercynides probably developed since the early archaean and grew via many crustal accretion events to its present state

dual role of microcracks toughening and degradation one of the methods of improving the mechanical properties of ceramics is to introduce a defective structure that acts as a restraint for the propagation of cracks in the present study a detailed investigation was carried out by introducing a defective structure in rock to determine if there is any improvement in properties similar to ceramics granite was chosen for the investigation and the microcracks were introduced by a heating and cooling cycle uniaxial compression tests have shown that granite thermally treated to 200 c shows the highest strength and the strength of granite treated to 400 c is comparable to that of unheated granite both ultrasonic images and acoustic emission monitoring show that for thermally treated samples the stress induced microcrack and macrocrack nucleation and their growth are retarded the variations in mechanical properties are explained based on the concept of toughening and degradation uniaxial compression tests on unheated and thermally treated granite samples have clearly established the dual role of microcracks which operate in the toughening and degradation mechanisms

on thickening ice the large ice sheets in greenland and antarctica play an important role in global climate much attention has focused on the west antarctic ice sheet in his perspective alley 1 highlights the report by joughin et al 2 who show that in the region of the west antarctic drainage into the ross sea on average the ice sheet is thickening slowly earlier studies indicated a net thinning in this region the results may indicate the stabilization or readvance of the ice sheet but the system is complex and existing data are insufficient to identify a trend 1 http www sciencemag org cgi content full 295 5554 451 2 http www sciencemag org cgi content short 295 5554 476

a 3 d study of the effects of thermomechanical loads on fracture slip in enhanced geothermal reservoirs heat extraction from enhanced geothermal systems egs can greatly affect the behavior of joints and other discontinuities in the reservoir fracture permeability can change in response to fluid injection extraction rock cooling variations of stress field and mineral dissolution precipitation the reduction in effective stress caused by pore pressure increase can cause the slippage of discontinuities thus inducing seismicity studies have shown that thermal stresses generated by cold water injection have a similar effect in order to assess the influence of thermal stresses on fracture opening and slippage a 3 d coupled heat extraction thermal stress elastic displacement discontinuity model is used in this study the effects of each mechanism on fracture slip is estimated with particular reference to the coso geothermal field the results indicate that under typical field conditions a substantial increase in fracture slip is observed when thermal stresses are taken into account the temporal evolution of the thermal stresses suggests that the rock mass deformation will not cease upon stoppage of water injection which can be a cause of delayed seismic activity

fabric analysis using the intercept method abstract the intercept method is a convenient yet powerful method for the numerical analysis of fabrics for geological applications it can be used when a set of objects e g pores grains or aggregates of a specific mineral lineaments etc with unique characteristics can be identified in an image the intercept method analyses boundaries of objects as a population of lines how intercepts are definined is therefore important in order to establish the significance of the intercept counts when the method is applied to digital images the subdivision of the information into square pixels must also be considered we apply a linear filter which minimizes the effects of grid anisotropy on the counting of intercepts analysis of the resulting rose of intercept counts by a fourier series considerably facilitates the interpretation of the data a rose of directions can readily be derived for the phase boundaries if a population of objects can be assumed to have started with an originally isotropic orientation and to have been deformed passively the fourier components of the rose of intercepts also give the sectional strain the fourier components also permit an objective test of the validity of describing a fabric as resulting from a homogeneous strain on an initially isotropic section two computer programs for the counting and analysis of intercepts on digital images are presented and applied in four examples which illustrate the usefulness of the intercept method for magmatic fabric measurement strain analysis and lineament analysis we also illustrate the importance of fourier series decomposition of rose of intercept counts for fabric interpretations intercept results are also shown to be compatible with those derived by other methods eigenvector surfor inertia tensor autocorrelation methods

a reconsideration of the mass balance of a portion of the ross ice shelf antarctica des estimations raisonnables du degre de diminution de la vitesse juste a l amont de la zone de glace reposant sur le sol indiquent qu il n est plus possible de conclure a un epaississement a l aide de la bande d ecoulement originale de thomas et bentley 1978 c est pourquoi une nouvelle bande d ecoulement situee plus a l est que celle de thomas et bentley et non influencee par quelque zone de glace reposant sur le sol a ete choisie le bilan de masse de cette bande d ecoulement n est pas significativement different de zero une difference de vitesse entre l epaississement et l accretion a la base depassant environ 0 2 m a 1 n est pas vraisemblable

acoustic emission uncovers thermal damage evolution of rock abstract the thermal damage evolution of rock closely relates to many important scientific and engineering problems such as the mechanisms of earthquake and the effective extraction of deep geothermal energy however there is lack of rational thermal damage evolution model and the mechanisms of thermal damage are still unclear especially during cooling by use of a specially designed acoustic emission ae test system we explored the thermal damage evaluation in a rock and established a thermal damage evolution model which takes into account both heating and cooling processes the mechanisms of rock damage caused by heating and cooling were analyzed by using a lattice spring model and two different microstructural explanations were found i e heterogeneous microstructure and microdefect structure the damage evolution model can be used for more accurate modeling of these problems which involve rock thermal damage such as induced seismic activity during deep geothermal exploration

on the porphyroblastic eclogites of the śnieznik mountains in the polish sudetes abstract a rare textural variety of eclogite characterized by a two fold occurrence of omphacite as older microlites and as later porphyroblasts enclosing numerous microlites has been found at only two localities in the polish sudetes wojtowka and nowa wie the chemical compositions of 27 mineral grains in four samples of such eclogites have been determined by electron microprobe spot analysis all omphacite analyses of both porphyroblasts and microlites show almost identical compositions except for one microlite enclosed in an armoured relic of tremolite this microlite displays a lower na content and is considered to belong to an earlier stage of metamorphic evolution preceding the stage of eclogitization it is deduced that these porphyroblastic eclogites experienced three successive stages of metamorphic evolution 1 1 pre eclogite stage when the primary rocks developed the mineral composition of a low temperature metabasite greenschist facies 2 2 eclogite stage when the rock gained typical eclogite mineral compositions as a consequence of a strong increase in total pressure unaccompanied by an increase in ph2o eclogite facies 3 3 post eclogite stage when the garnet omphacite association ceased to be stable and was transformed into a hornblende plagioclase association as a result of the decrease of total pressure which became equal to ph2o amphibolite facies

variszische duktile tektonik im bohmischen massiv die verformungsanalyse und richtungsverteilung der streckungslineationen in den duktilen bereichen des bohmischen massiv zusammen genommen mit der lage der foliationsflachen deuten darauf hin das die blattverschiebungen wahrend der variszischen deformation die entscheidende rolle gespielt haben die kompatibilitat der deformation im gesamtbereich wurde durch uberschiebungen und transpression in den durch die blattverschiebungen begrenzten blocke erreicht das variszische tektogen wurde als blattverschiebungsorogen auf einer kontinentalen oberproterozoischen kruste angelegt die zonen mit intensiver krustenverkurzung und damit groster krustenmachtigkeit sind durch weitraumige anatexis und granitische intrusionen gekennzeichnet

force budget i theory and numerical methods a practical method is developed for calculating stresses and velocities at depth using field measurements of the geometry and surface velocity of glaciers to do this it is convenient to partition full stresses into lithostatic and resistive components the horizontal gradient in vertically integrated lithostatic stress is the driving stress and it describes the horizontal action of gravity the horizontal resistive stress gradients describe the reactions resistive stresses are simply related to deviatoric stresses and hence to strain rates through a constitutive relation a numerical scheme can be used to calculate stresses and velocities from surface velocities and slope and from ice thickness there is no mathematical requirement that the variations in these quantities be small

on the pulling power of ice streams gravity wants to pull an ice sheet to the center of the earth but cannot because the earth s crust is in the way so ice is pushed out sideways instead or is it the ice sheet sees nothing preventing i t from spreading out except air which is much less massive than ice therefore does not ice rush forward to fill this relative vacuum does not the relative vacuum suck ice into i t because nature abhors a vacuum if so the ice sheet is not only pulled downward by gravity i t is also pulled outward by the relative vacuum this pulling outward will be most rapid where the ice sheet encounters least resistance the least resistance exists along the bed of ice streams where ice bed coupling is reduced by a basal water layer especially if the ice stream becomes afloat and the floating p art is relatively unconfined around its perimeter and unpinned to the sea floor ice streams are t herefore fast currents of ice that develop near the margins of an ice sheet where t hese conditions exist because of these condit ions ice streams pull ice out of ice sheets and have pulling power equal to the longitudinal gravitational pulling force multiplied by the ice stream velocity t hese boundary conditions beneath and beyond ice streams can be quantified by a basal buoyancy factor that provides a life cycle classification of ice streams into inception growth mature declining and terminal stages during which ice streams disintegrate the ice sheet surface profiles of ice streams are diagnostic of the stage in a life cycle and hence of the vitality of the ice sheet

observed velocity fluctuations on a major antarctic ice stream recent ice velocity measurements on the major antarctic ice stream b are compared with those collected 10 years ago the results show that the ice flow in the mouth of ice stream b has decelerated by about 20 percent the possible causes of this deceleration are discussed on the basis of present knowledge of current regional dynamics and the possible effects of this deceleration on future ice stream behavior are examined

changes in compressional and shear wave velocities and dynamic moduli during operation of a hot dry rock geothermal system compressional and shear wave velocities measured during cross well seismic surveys are found to decrease during operation of a hot dry rock geothermal system the relative p and s wave velocity decrease is very close to values predicted by o connell and budiansky 1974 who modeled velocities in rocks containing fluid saturated and dry cracks because our data fit their model quite well we attribute the velocity decrease to an increased density of fluid filled fractures in the reservoir fracture density parameters of o connell and budiansky range as high as 0 365 near the main heat transfer areas of the geothermal reservoir the dynamic shear and young s modulus are significantly lower near the principal heat transfer surfaces but the bulk modulus is largely unaffected these observations suggest that the velocity differences are caused by fully saturated fractures we demonstrate that the fractures inferred by this survey may have lengths as large as 70 cm these fractures may have been caused by shear failure microcracking associated with thermal drawdown or thermal stress cracking

west antarctic ice streams draining into the ross ice shelf configuration and mass balance extensive airborne radar soundings closely tied to accurately located ground stations made over a portion of the west antarctic inland ice sheet and the ross ice shelf have enabled us to map the boundaries of the ice streams and their flow bands on the ross ice shelf the surfaces of the active ice streams a and b are heavily crevassed but there are no visible crevasses on ice stream c however the existence of numerous crevasses at a depth of about 35 m implies that ice stream c ceased to be active about 250 years ago there is a complex zone at the head of ice stream b that suggests that ice stream b is currently widening and advancing toward the interior of the ice sheet the grounding lines between the inland ice and the ice shelf were mapped from the surface elevations using the requirement of hydrostatic balance and the character of the radar echoes the inland ice is more extensive than previously mapped and includes a large weakly grounded area downstream of ice streams a and b where the surface elevations are barely above hydrostatic equilibrium crary ice rise extends 70 km upstream of the previously mapped position to relate surface elevation data from satellite observations to sea level three geoidal models were tested we conclude that the gem 10c model is the best of the three for this section of antarctica a new map of surface elevation shows deep marginal troughs and longitudinal ridges along ice streams a and b but not on ice stream c whose surface contours show signs of stagnation ice stream c shows a strongly positive and ice stream b a significantly negative net balance rate ice streams a and d also show a significantly negative net flux whereas ice streams e and f do not the overall net balance of this part of the west antarctic inland ice is suggestively negative 23 15 km3 yr 1 from the measured flux into the ross ice shelf and previous measurements we calculate an average basal melt rate from beneath the ice shelf of 0 12 0 03 m yr 1

quantitative measure of the variation in fault rheology due to fluid rock interactions we analyze friction data from two published suites of laboratory tests on granite in order to explore and quantify the effects of temperature t and pore water pressure pp on the sliding behavior of faults rate stepping sliding tests were performed on laboratory faults in granite containing gouge granite powder both dry at 23 to 845 c lockner et al 1986 and wet pp 100 mpa at 23 to 600 c blanpied et al 1991 1995 imposed slip velocities v ranged from 0 01 to 5 5 m s and effective normal stresses were near 400 mpa for dried granite at all temperatures and wet granite below 300 c the coefficient of friction shows low sensitivity to v t and pp for wet granite above 350 drops rapidly with increasing t and shows a strong positive rate dependence and protracted strength transients following steps in v presumably reflecting the activity of a water aided deformation process by inverting strength data from velocity stepping tests we determined values for parameters in three formulations of a rate and state dependent constitutive law one or two state variables were used to represent slip history effects each velocity step yielded an independent set of values for the nominal friction level five constitutive parameters transient parameters a b1 and b2 and characteristic displacements dc1 and dc2 and the velocity dependence of steady state friction ss ln v a b1 b2 below 250 data from dry and most wet tests are adequately modeled by using the slip law ruina 1983 and one state variable a 0 003 to 0 018 b 0 001 to 0 018 dc 1 to 20 m dried tests above 250 can also be fitted with one state variable in contrast wet tests above 350 require higher direct rate dependence a 0 03 to 0 12 plus a second state variable with large negative amplitude b2 0 03 to 0 14 and large characteristic displacement dc2 300 to 4000 m thus the parameters a b1 and b2 for wet granite show a pronounced change in their temperature dependence in the range 270 to 350 c which may reflect a change in underlying deformation mechanism we quantify the trends in parameter values from 25 to 600 c by piecewise linear regressions which provide a straightforward means to incorporate the full constitutive response of granite into numerical models of fault slip the modeling results suggest that the succeptibility for unstable stick slip sliding is maximized between 90 and 360 c in agreement with laboratory observations and consistent with the depth range of earthquakes on mature faults in the continental crust

large scale ice flow over a viscous basal sediment theory and application to ice stream b antarctica recent seismic studies of ice stream b antarctica and field analysis of mid latitude glacial deposits suggest that deformable basal sediments e g water saturated till are important in determining ice sheet flow if the ratio of till viscosity to effective ice viscosity is small vertical shear associated with horizontal flow is confined to the deforming bed alone ice flow over a deformable bed is thus akin to that of floating ice shelves because ice shelves flow over inviscid seawater for some antarctic ice streams and possibly for portions of the late wisconsin ice sheets in north america and eurasia basal drag associated with deforming basal sediment does not induce significant vertical gradients of horizontal velocity instead basal drag affects the flow as if it were a horizontal body force balanced by longitudinal and transverse deviatoric stress gradients comparison of the observed flow of ice stream b to finite element simulations incorporating a viscous basal till suggest that a simple till theology is sufficient to explain the current velocity profile these simulations also highlight the importance of horizontal deviatoric stress in regions where driving stress and basal stress do not locally balance while bed deformation is critical to ice stream existence sensitivity tests suggest that ice shelf back pressure is still a crucial control affecting ice stream response to atmospheric and oceanic climate

α β phase transition in quartz monitored using acoustic emissions summary it is usually suggested that thermal cracking in a quartz bearing rock results from the anomalously high volumetric expansion coefficients of quartz e g simmons cooper 1978 it has also been recognized that thermal expansion mismatch and mineral anisotropy contribute to thermal cracking in materials that consist of a polycrystalline aggregate composed of several anisotropic minerals even in the absence of a temperature gradient friedrich wong 1986 experiments investigating thermal cracking in rocks commonly involve imaging and quantitative stereology of crack populations induced in rocks treated to various peak temperatures e g friedrich johnson 1978 homand etienne troalan 1984 atkinson mcdonald meredith 1984 meredith atkinson 1985 here we report on acoustic emission experiments that monitor the process of thermal cracking as it occurs during heating supported by measurements of crack surface area pore fluid permeability porosity and surface conductivity carried out on rock samples treated to various peak temperatures the acoustic emission measurements show a strong peak of microcracking at the phase transition temperature for quartz 573 c superimposed upon a background of microcracking due to thermal expansion there is also a clear peak of microcracking at higher temperatures 800 c that can be attributed to oxidation dehydroxylation reactions of hornblende and chlorite measurements of fluid permeability pore surface area porosity and electrical conductivity made on samples that have been heat treated to various maximum temperatures show increases associated with a major episode of cracking in the 500 600 c temperature range indicating that the new cracks form a well interconnected network this has been confirmed by sem and optical microscopy these results have implications for the electrical conductivity of the continental crust providing a mechanism enabling the high pore fluid connectivity needed to explain zones of high electrical conductivity at depth providing that cracks opened in this way remain open at the high pressures existing at depth it should be recognized however that th se measurements are limited in their direct application since they were obtained under initially dry conditions at laboratory pressures

fracture toughness and fracture roughness interrelationship in thermally treated westerly granite this paper presents an experimental work aimed at assessing the correlation between fracture toughness k ic and fracture roughness for a series of westerly granite specimens thermally treated up to 850 c mode i fracture toughness as a function of thermal treatment is determined using cracked chevron notched brazilian disc specimens the degree of roughness of the resultant fracture surfaces is analyzed with the aid of a high accuracy high precision stereo topometric measurement system roughness and toughness values display a negative correlation as a function of temperature fracture toughness decreases with increasing temperature due to the gradual opening of grain grain boundaries in response to thermal stresses mode i fractures preferentially follow these weakened grain grain boundaries which in addition to the thermal expansion of individual grains result in rougher failure profiles with increasing temperature at low temperature a distinct anisotropy in roughness was observed in all fracture surfaces with higher roughness value perpendicular to the direction of fracture propagation however higher treatment temperatures resulted in the homogenization of fracture roughness in all directions these results confirm the important link among petrofabric analysis fracture toughness and fracture roughness in response to thermal treatment

thermohaline circulation below the ross ice shelf a consequence of tidally induced vertical mixing and basal melting the warmest water below parts of the ross ice shelf resides in the lowest portion of the water column because of its high salinity vertical mixing caused by tidal stirring can thus induce ablation by lifting the warm but dense water into contact with the ice shelf a numerical tidal simulation indicates that vertically well mixed conditions predominate in the southeastern part of the sub ice shelf cavity where the water column thickness is small basal melting in this region is expected to be between 0 05 and 0 5 m yr and will drive a thermohaline circulation having the following characteristics high salinity shelf water at 1 8oc formed by winter sea ice production in the open ross sea flows along the seabed toward the tidal mixing fronts below the ice shelf and meltwater at 2 2oc produced in the wellmixed region flows out of the sub ice shelf cavity along the ice shelf bottom sensitivity of this ablation process to climatic change is expected to be small because high salinity shelf water is constrained to have the sea surface freezing temperature

quantitative three dimensional description of a rough surface and parameter evolution with shearing note roches reference lmr article 2002 005view record in web of science record created on 2006 11 09 modified on 2016 08 08

hercynian thrust related shear zones and deformation of the varied group on the contact of granulites southern moldanubian bohemian massif the three deformation phases inferred from the detailed structural analysis of the cesky krumlov varied group record the hercynian development of the southern moldanubian of the bohemian massif the deformation is related to the nw se thrusting of the large crustal units including granulites the formation of the ns and nw se trending shear zones is connected with the thrust movement the structural development begins with f1 isoclinal fold formation that could originate in unmetamorphosed sediments in the final stage they were strongly flattened and b1 boudinage developed in the rocks aplite dykes and migmatitization of paragneisses occurred at the same time the rocks were metamorphosed the d2 deformational phase was produced by the simple shear deformation of the unit and folds of various styles around rigid inclusions and the strike slip shear zone near the boundary of the granulite and the varied group were formed the f1 and f2 folds are parallel with the stretching and mineral lineation indicating a ns to nw se direction of the thrusting the youngest deformation is characterized by spectacular boudinage and by folding of the vertically oriented planes

new and improved determinations of velocity of ice streams b and c west antarctica measurements of velocity have been made on and next to ice streams b and c west antarctica the results are more precise than previous work and constitute a 93 increase in the number of values these velocities are used to describe the confluence of flow into the ice streams and the development of fast ice stream flow the onset of fast streaming flow occurs in many separate tributaries that coalesce down glacier into the major ice streams for those inter stream ridges that have been studied the flow is consistent with steady state along ice stream b gradients in longitudinal stress offer little resistance to the ice flow the transition from basal drag control to ice shelf flow is achieved through reduced drag at the glacier base and increased resistance associated with lateral drag velocities in the trunk of ice stream c are nearly zero but those at the up glacial head are similar to those at the head of ice stream b

an ice shelf model test based on the ross ice shelf a standard numerical experiment featuring the ross ice shelf antarctica is presented as a test package for the development and intercomparison of ice shelfmodels the emphasis of this package is solution of stress equilibrium equations for an ice shelf velocity consistent with present observations as ademonstration we compare five independently developed ice shelf models based on finite difference and finite element methods our results suggest thatthere is little difference between finite element and finite difference methods in capturing the basic large scale flow features of the ice shelf we additionallyshow that the fit between model and observed velocity depends strongly on the ice shelf temperature field for which there is presently little observationalcontrol the main differences between model results are due to the equations being solved the boundary conditions at the ice front and the discretisationmethod finite element vs finite difference

ice shelf response to ice stream discharge fluctuations i unconfined ice tongues abstract unsteady thickness and flow observed on the ross ice shelf are thought to result primarily from fluctuations of ice streams and outlet glaciers that feed the ice shelf at its inland boundaries ice stream discharge fluctuations constitute an independent means of forcing unsteady ice shelf behavior and their effect must be distinguished from those of oceanic and atmospheric climate to understand ice shelf change ice stream generated fluctuations of a one dimensional ice shelf are found to propagate through the ice shelf environment along the two characteristics of the hyperbolic governing equations one characteristic permits instantaneous transmission of grounding line velocity changes to all points downstream the other characteristic represents slow transmission of grounding line thickness changes along lagrangian particle paths

upper paleozoic oceanic crust in the polish sudetes ndsr isotope and trace element evidence the two main mafic ultramafic complexes outcropping in the polish sudetes around the sowie gory high grade massif mt sleza and nowa ruda have been studied for trace elements and nd sr isotopes upper paleozoic sm nd whole rock isochron ages are obtained for both massifs 353 21 ma 2 with i 8 8 0 1 mt sleza and 351 16 ma with i 8 5 0 1 nowa ruda the high initial nd point to a source characterized by a strong time integrated lree depletion and along with incompatible trace element patterns they substantiate the oceanic derivation of these massifs although a subduction related marginal basin setting cannot be totally precluded trace element and isotopic data rather suggest a normal mid ocean ridge origin for the sudetic ophiolites while these results are in marked contrast with previously inferred precambrian or early paleozoic ages remnants of oceanic crust as young as the early carboniferous are consistent with the local sedimentary record of pelagic facies from the frasnian to the tournaisian and they may provide a suitable explanation for the contrasted evolution displayed by different domains of the sudetes the sudetic ophiolites might tentatively be correlated with the upper paleozoic prasinites of the saxon lineament if a late hercynian ca 150 km dextral offset is assumed along the elbe fracture in that case they might represent the latest stage remnants of an oceanic suture zone between the saxo thuringian and moldanubian zones together with several other examples throughout europe these ophiolites provide compelling evidence for the involvement of oceanic crust and plate tectonic processes in the variscan belt

rockfall triggering by cyclic thermal stressing of exfoliation fractures exfoliation of rock deteriorates cliffs through the formation and subsequent opening of fractures which in turn can lead to potentially hazardous rockfalls although a number of mechanisms are known to trigger rockfalls many rockfalls occur during periods when likely triggers such as precipitation seismic activity and freezing conditions are absent it has been suggested that these enigmatic rockfalls may occur due to solar heating of rock surfaces which can cause outward expansion here we use data from 3 5 years of field monitoring of an exfoliating granite cliff in yosemite national park in california usa to assess the magnitude and temporal pattern of thermally induced rock deformation from a thermodynamic analysis we find that daily seasonal and annual temperature variations are sufficient to drive cyclic and cumulative opening of fractures application of fracture theory suggests that these changes can lead to further fracture propagation and the consequent detachment of rock our data indicate that the warmest times of the day and year are particularly conducive to triggering rockfalls and that cyclic thermal forcing may enhance the efficacy of other more typical rockfall triggers some rockfalls occur without obvious triggers such as seismicity or freeze thaw conditions temperature and deformation patterns on a granite cliff suggest that cyclical thermal forcing can progressively open fractures and trigger rockfalls

influence of thermal treatment on mode i fracture toughness of certain indian rocks abstract an experiment was carried out to measure the static mode i fracture toughness of manoharpur sandstone bellary dolerite and dholpur sandstone after thermal treatment at different temperatures that ranged from ambient conditions to 600 c the three point bending method was applied using cracks straight through semi circular bending cstscb specimens that were fabricated according to international society for rock mechanics isrm standards three different rocks were used to measure the degree of influence of thermal treatment on fracture toughness in different rocks in addition petrographic and x ray diffraction xrd analyses were carried out to identify the compositions of these rocks finally scanning electron microscope sem analysis was performed in order to measure the micro cracks that were induced within these rocks as a result of thermal treatment the experimental results demonstrated that up to a temperature range of 100 c the fracture toughness of manoharpur sandstone bellary dolerite and dholpur sandstone increased by 40 25 and 65 respectively when compared to the ambient condition and thereafter decreased with a gradual increase of temperature at 600 c when compared to the ambient condition the fracture toughness of these rock types decreased by 59 36 and 30 for manoharpur sandstone bellary dolerite and dholpur sandstone respectively

antarctic ice rises and rumples their properties and significance for ice sheet dynamics and evolution locally grounded features in ice shelves called ice rises and rumples play a key role buttressing discharge from the antarctic ice sheet and regulating its contribution to sea level ice rises typically rise several hundreds of meters above the surrounding ice shelf shelf flow is diverted around them on the other hand shelf ice flows across ice rumples which typically rise only a few tens of meters above the ice shelf ice rises contain rich histories of deglaciation and climate that extend back over timescales ranging from a few millennia to beyond the last glacial maximum numerical model results have shown that the buttressing effects of ice rises and rumples are significant but details of processes and how they evolve remain poorly understood fundamental information about the conditions and processes that cause transitions between floating ice shelves ice rises and ice rumples is needed in order to assess their impact on ice sheet behavior targeted high resolution observational data are needed to evaluate and improve prognostic numerical models and parameterizations of the effects of small scale pinning points on grounding zone dynamics

fracture toughness and subcritical crack growth during high temperature tensile deformation of westerly granite and black gabbro abstract the double torsion testing method has been used to determine catastrophic and subcritical crack propagation parameters for pre cracked specimens of westerly granite and black gabbro under a number of environmental conditions the critical stress intensity factor for catastrophic crack propagation fracture toughness of granite and gabbro has been measured at temperatures from 20 to 400 c in a vacuum at 20 c the fracture toughness of westerly granite was 1 79 0 02 mpa m 1 2 and for two blocks of black gabbro it was 3 03 0 08 mpa m 1 2 and 2 71 0 15 mpa m 1 2 respectively these values are very close to those reported by other investigators for tests conducted in air of ambient humidity at room temperature for both rocks fracture toughness at first increased slightly and then decreased steadily on raising the temperature above ambient conditions this behaviour is explained in terms of the density and distribution of thermally induced microcracks as determined by quantitative optical microscopy subcritical crack growth behaviour has been studied at temperatures up to 300 c and under water vapour at pressures of 0 6 to 15 kpa both the load relaxation and incremental constant displacement rate forms of the double torsion testing method were utilised to generate stress intensity factor crack velocity diagrams crack growth was measured over the velocity range 5 10 3 to 10 7 m s 1 increasing both temperature and water vapour pressure resulted in substantially higher crack growth rates the overall effect of raising the temperature over the range studied here 20 300 c was to increase the crack growth rate in granite and gabbro by 5 and 7 orders of magnitude respectively at constant stress intensity factor and vapour pressure of water for both rocks the slopes of stress intensity factor crack velocity curves were sensitive to changes in both temperature and water vapour pressure at low values of the latter parameter slopes fell substantially on raising the water vapour pressure but were relatively insensitive to changes in temperature at these higher pressures no subcritical crack growth limit was encountered estimates of the uncertainty in our experimental data are given from the results of multiple load relaxation experiments on westerly granite specimens we estimate the uncertainty in position of stress intensity factor crack velocity curves along the stress intensity axis to be c 10 of the fracture toughness and the uncertainty in slope of such curves to be c 12 problems associated with the extrapolation of our experimental data to regions of higher effective confining pressure in the earth s crust are discussed

observed velocity fluctuations on a major antarctic ice stream most of the antarctic ice sheet is drained by systems comprising an area of slow sheet flow which converges into a much faster valley glacier or ice stream on reaching the coast this ice then either enters an ice shelf confined by embayments and ice rises or calves off into the ocean the higher speed of ice streams enables them to respond to changes in their environment faster than the slower ice sheet in the west antarctic where the ice sheet moves across bedrock well below sea level the ice streams have the potential to rapidly increase or decrease the ice discharge into the sea1 the timescale for these changes has been placed at a few hundred years if this is true then changes should be detectable by careful measurements on a decadal timescale by comparing recent ice velocity measurements with those collected ten years ago we establish that the ice in the mouth of ice stream b has decelerated by 20 we discuss the possible causes of this deceleration on the basis of our knowledge of the current regional dynamics and the possible ramifications of this deceleration on future ice stream behaviour

behavior of flexural gravity waves on ice shelves application to the ross ice shelf ocean waves continuously impact floating ice shelves and affect their stress regime low frequency long period 75 400 s ocean waves are able to reach ice shelf cavities from distant sources and excite flexural gravity waves that represent coupled motion in the water of the cavity and the ice covering above analytic treatment of simplified geometric configuration and three dimensional numerical simulations of these flexural gravity waves applied to the ross ice shelf show that propagation and ice shelf flexural stresses are strongly controlled by the geometry of the system bathymetry of the ice shelf cavity and ice shelf cavity thickness the derived dispersion relationships group and phase velocities of these waves can be used to infer poorly constrained characteristics of ice shelves from field observations the results of numerical simulations show that the flexural gravity waves propagate as beams the orientation of these beams is determined by the direction of the open ocean waves incident on the ice shelf front the higher frequency ocean waves cause larger flexural stresses while lower frequency waves can propagate farther away from the ice shelf front and cause flexural stresses in the vicinity of the grounding line

force mass and energy budgets of the crary ice rise complex antarctica the stress mass and energy dissipation budgets of crary ice rise are analyzed using field data collected during the 1983 1985 austral summers and in previous field programs in addition the net back pressure and ice discharge rate along the grounding lines of ice streams are calculated to assess the effect of the ice rise on the surrounding flow comparison of the ice rise budgets with the analysis of grounding line data confirms the influence of the ice rise on ice sheet stability and suggests that crary ice rise may have formed recently in response to an acceleration of one of the ice streams it is concluded that feedback between ice stream acceleration and ice rise formation may control the future evolution of the above ice stream and promote long term grounding line stability in the face of strong natural fluctuations

mechanical analysis of pinning points in the ross ice shelf antarctica ice shelves regulate the rate of ice sheet discharge along much of the antarctic coastline pinning points sites of localised grounding within floating ice can in turn regulate the flow and thickness of an ice shelf while the net resistive effect of ice shelves has been quantified in a systematic way few extant pinning points have been examined in detail here complete force budgets are calculated and examined for ice rises and rumples in the ross ice shelf west antarctica the diverse features have different effects on ice shelf mechanics that do not depend simply on their size but may we conclude depend on the properties of seafloor materials

water pressure coupling of sliding and bed deformation iii application to ice stream b antarctica geophysical studies and glaciological analyses suggest strongly that ice stream b west antarctica moves primarily by pervasive deformation of a meters thick subglacial till analysis of the longitudinal profile of the ice stream up stream of the ice plain suggests that basal sliding is slow everywhere that effective pressure decreases slowly down stream and that the strain rate of pervasive shear is proportional to the basal shear stress and inversely proportional to the square or cube of the effective pressure discrete shearing may occur beneath the pervasively deforming zone these and other hypotheses which build on the analyses of the first two papers in this series can be tested in the field

state of balance of the cryosphere the current state of balance of the terrestrial ice sheets and glaciers is poorly known what little data are available suggest that worldwide mountain glaciers have receded since about the mid nineteenth century with occasional interruptions of the retreat the interior part of the greenland ice sheet appears to be thickening or in near equilibrium but this ice sheet may be thinning in the coastal areas estimates of the mass balance of the antarctic ice sheet suggest that it is positive although the error limits allow for a slightly negative balance there is an urgent need to greatly improve the current estimates and to monitor the ice sheets continuously for changes in volume and extent a program based on satellite observa tion techniques in cooperation with ground based surveys repeated over long time periods many years or decades appears to be most opportune to achieve this almost 94 of all water present on the earth and in its atmosphere is stored in the world s oceans snow and ice in the cryosphere account for only about 1 5 of the total the bulk of the ice can be found in antarctica the volume of the ice sheet there is about 10 times that of the green land ice sheet and about 160 times as large as the com bined volume of mountain glaciers and small ice caps although the water volume of the cryosphere is much smaller than that of the world s oceans small changes in the volume of grounded ice can have dire consequences for global sea level this becomes more obvious when considering the volume of grounded ice in terms of sea level equivalent as given in table 1 it should be stressed that most of the numbers given in this table contain large uncertainties because considerable amounts of the ice in greenland and antarctica are below the present day sea level a complete melting of these ice sheets would lead to a smaller sea level rise than that corresponding to the total volume of ice stored there furthermore melting of the large floating ice shelves surrounding west antarctica has no direct effect on sea level and hence these are not included in table 1 changes in ice volume occur when the mass balance is nonzero a positive mass balance means that the glacier is growing expanding glacier retreat is caused by a negative mass balance thus a crucial issue for future sea level changes is the current mass balance of the terrestrial ice masses

coupled thermal hydrological mechanical analyses of the yucca mountain drift scale test comparison of field measurements to predictions of four different numerical models abstract the yucca mountain drift scale test dst is a multiyear large scale underground heating test designed to study coupled thermal hydrological mechanical chemical behavior in unsaturated fractured and welded tuff as part of the international cooperative code comparison project development of coupled models and their validation against experiments four research teams used four different numerical models to simulate and predict coupled thermal hydrological mechanical thm processes at the dst the simulated processes included heat transfer liquid and vapor water movements rock mass stress and displacement and stress induced changes in fracture permeability model predictions were evaluated by comparison to measurements of temperature water saturation displacement and air permeability the generally good agreement between simulated and measured thm data shows that adopted continuum model approaches are adequate for simulating relevant coupled thm processes at the dst moreover thermal mechanically induced rock mass deformations were reasonably well predicted using elastic models although some individual displacements appeared to be better captured using an elasto plastic model it is concluded that fracture closure opening caused by change in normal stress across fractures is the dominant mechanism for thermal stress induced changes in intrinsic fracture permeability at the dst whereas fracture shear dilation appears to be less significant this indicates that such changes in intrinsic permeability at the dst which are within one order of magnitude are likely to be mostly reversible

the effect of high temperature and pressure on rock friction coefficient a review the coefficient of friction is a parameter of the mechanical state which has an important role in the shear failure of rock in the fields of earth sciences in order to analyze the influence of temperature and pore pressure on the coefficients of friction between solid and gouge data from studies carried out in both china and international contexts are applied analyzed and discussed the survey results show that the coefficient of friction mainly depends on the pore pressure pressure and temperature under the supercritical water state the framework of silicate minerals might be substantially disintegrated which suggests a possible change in the frictional resistance from stick slip to stable sliding the temperature range of 300 450 c could constitute as the critical threshold for shear failure to collected rocks i e gabbro gouge pyroxene gouge gabbro solid biotite gouge structured gouge granitic mylonite mylonite plagioclase gouge granite gouge pre fractured granite westerly granite

eclogites in the french palaeozoic orogen geodynamic significance abstract eclogites are widespread in the devonian internal zone of the french palaeozoic orogen they occur in different tectonic settings either in high pressure low temperature or in high pressure high temperature environments they are tectonically incorporated into several major allochthonous units as demonstrated either by their blastomylonitic texture or by their being in evident disequilibrium with their country rocks the protoliths of the presently well identified eclogite bearing series present different typical associations depending on their present location the igneous series include within plate basalts normal and transitional morbs and even calc alkaline rocks they were erupted mostly during the palaeozoic although they sometimes contain inherited precambrian zircons further some of them may be remnants of old precambrian oceanic crust most of the eclogitic parageneses were generated during the earliest silurian tectonometamorphic climax either in low temperature or in high temperature conditions later they followed various retrograde courses in still geographically distinct low temperature or high temperature associations the basic rocks in the leptynolitic amphibolitic complex and particularly the eclogites thus provide sufficiently clear evidence to support a new geodynamic scenario implying one or several subordinate narrow tensional zones in a thinned but well developed continental crust rather than in a true oceanic domain

magma driven hydraulic fracturing and infiltration of fluids into the damaged host rock an example from dronning maud land antarctica abstract excellent outcrops in dronning maud land antarctica provide unique insight into the mode and extent of fluid infiltration into metamorphic and plutonic rocks in the middle crust the fluids are liberated from pegmatitic veins and give rise to alteration halos in the alteration halos the conspicuous change in colour is correlated with 1 hydration mineral reactions and 2 high density of microcracks in quartz and feldspar exceeding that observed in the unaltered host rock by an order of magnitude the field relations indicate that the veins originated as melt driven hydraulic fractures sealed by pegmatite and aplite crystallising from volatile rich melts with the alteration halo being the wake of the process zone formed at the tip of the propagating fractures it is proposed that 1 the size of the alteration zone and the width of the vein are correlated resulting in higher values of both these quantities for cracks propagating at higher velocities and consequently higher crack propagation toughnesses 2 the damage zone is characterised by a transient state of high permeability which was short lived due to rapid healing and sealing of microcracks 3 the infiltration and retrogression of the high grade rocks can be considered as a quasi instantaneous process on geologic time scales with a duration of hours to weeks

changes in the west antarctic ice sheet the portion of the west antarctic ice sheet that flows into the ross sea is thinning in some places and thickening in others these changes are not caused by any current climatic change but by the combination of a delayed response to the end of the last global glacial cycle and an internal instability the near future impact of the ice sheet on global sea level is largely due to processes internal to the movement of the ice sheet and not so much to the threat of a possible greenhouse warming thus the near term future of the ice sheet is already determined however too little of the ice sheet has been surveyed to predict its overall future behavior

formulation of ice shelf dynamic boundary conditions in terms of a coulomb rheology coastal boundaries where fast flowing ice shelves shear past stagnant grounded ice are typically riven with surface crevasses seawater filled basal crevasses and tidal strand cracks here we formulate a boundary condition describing stress transmission through these fractured boundaries in terms of the coulomb law as a result of this formulation agreement between finite element simulations of the ross ice shelf flow and field observations is improved over agreement obtained with formulations which do not account for ice failure our results additionally suggest that shear stress transmitted through ice shelf boundaries is lower than previously thought

3d modelling of the effect of thermal elastic stress on grain boundary opening in quartz grain aggregates abstract during the exhumation of rocks and the associated temperature and pressure decrease the anisotropic thermoelastic properties of minerals lead to internal stresses on the grain scale which in turn cause fracturing and opening of grain and phase boundaries to gain deeper insight into the onset of grain boundary cracking and the subsequent evolution of grain boundary fracture networks after crack initiation 3d grain scale numerical modelling has been performed in detail the fracturing and opening of quartz grain boundaries during exhumation of quartzite with millimeter scale grains and randomly oriented crystallographic axes accompanied by cooling from 300 to 25 c and decompression from 300 to 22 mpa have been modelled with contact mechanics and the finite element method model grains have an anisotropic linear elastic rheology and an anisotropic thermal expansion tensor grain boundaries are modelled as contact surfaces with a non linear strain softening rheology in tension and a mohr coulomb rheology in shear grain boundary fractures nucleate at grain vertices and triple lines in agreement with observations in nature and experiment comparison with grain boundary opening data from natural quartzite indicates that the tensile yield strength y of quartz grain boundaries has an upper bound of 75 mpa with best fit results for y 25 mpa moreover the best fit model implies that quartz grain boundary opening initiates after a temperature and pressure decrease of around 80 c and 80 mpa

tectonometamorphic evolution of the bohemian massif evidence from high pressure metamorphic rocks the variscan orogenic belt of which the bohemian massif is a part is typically recognized for its characteristic low pressure high temperature metamorphism and a large volume of granites however there are also bodies of high pressure rocks eclogites garnet peridotites and high pressure granulites which are small in size but widely distributed throughtout the massif initially the high pressure rocks were considered to be relicts of a much older orogenic event but the increasing data derived from isotopic and geochronological investigations show that many of these rocks have palaeozoic protoliths metamorphic ages from the high pressure rocks define no single event instead a number of discrete clusters of ages are found between about 430 ma and the time of the dominant low pressure event at around 320 330 ma

flow law for ice in polar ice sheets theories of glacier flow are based commonly on the assumption that ice is not a newtonian fluid but has a non linear stress dependent viscosity1 5 here we re examine the spreading of antarctic ice shelves and suggest that the data cannot define a unique flow law tilt measurements in four boreholes in both the arctic and antarctic seem to show that a linear flow law e a1 where a1 10 15 s 1 pa 1 at 28 c where e is the effective strain rate and the effective shear stress may be just as appropriate for describing the flow of polar ice sheets

strain and tectonic styles related to variscan transpression and transtension in the moravo silesian culmian basin bohemian massif czechoslovakia abstract progressive variscan development of the n s to nne ssw trending dextral wrench zone in the moravo silesian culmian basin on the eastern border of the bohemian massif resulted in the westphalian asturian transpression followed by transtension of the area the kinematics of the structures are indicated by three phase folding n s to nne ssw and e w stretching of rocks and eastward oriented thrust tectonics the estimated displacement based on strain analysis of conglomerates and folds can reach several tens of kilometers and important stretching up to 160 is due to the perpendicular extension transtension in the most ductile part of the zone