reply to bunte et al 2011 discussion of daniels and mccusker 2010 operator bias characterizing stream substrates using wolman pebble counts with a standard measurement template geomorphology 115 194 198

electrical resistivity image of the south atlantic continental margin derived from onshore and offshore magnetotelluric data we present a deep electrical resistivity image from the passive continental margin in namibia the approximately 700 km long magnetotelluric profile follows the walvis ridge offshore continues onshore across the kaoko mobile belt and reaches onto the congo craton two dimensional inversion reveals moderately resistive material offshore atypically low for oceanic lithosphere reaching depths of 15 20 km such moderate resistivities are consistent with seismic p wave velocity models which suggest up to 35 km thick crust the neoproterozoic rocks of the kaoko mobile belt are resistive but nnw striking major shear zones are imaged as subvertical conductive structures in the upper and middle crust since the geophysical imprint of the shear zones is intact opening of the south atlantic in the cretaceous did not alter the middle crust the transition into the cratonic region coincides with a deepening of the high resistive material to depths of more than 60 km

transport properties of olivine grain boundaries from electrical conductivity experiments grain boundary processes contribute significantly to electronic and ionic transports in materials within earth s interior we report a novel experimental study of grain boundary conductivity in highly strained olivine aggregates that demonstrates the importance of misorientation angle between adjacent grains on aggregate transport properties we performed electrical conductivity measurements of melt free polycrystalline olivine fo90 samples that had been previously deformed at 1200 c and 0 3 gpa to shear strains up to 7 3 the electrical conductivity and anisotropy were measured at 2 8 gpa over the temperature range 700 1400 c we observed that 1 the electrical conductivity of samples with a small grain size 3 6 m and strong crystallographic preferred orientation produced by dynamic recrystallization during large strain shear deformation is a factor of 10 or more larger than that measured on coarse grained samples 2 the sample deformed to the highest strain is the most conductive even though it does not have the smallest grain size and 3 conductivity is up to a factor of 4 larger in the direction of shear than normal to the shear plane based on these results combined with electrical conductivity data for coarse grained polycrystalline olivine and for single crystals we propose that the electrical conductivity of our fine grained samples is dominated by grain boundary paths in addition the electrical anisotropy results from preferential alignment of higher conductivity grain boundaries associated with the development of a strong crystallographic preferred orientation of the grains

grain growth in partially molten olivine aggregates grain growth rates in partially molten olivine aggregates were determined experimentally at a pressure of 1 gpa the aim of the experiments with fine grained starting material solution gelation derived fo90 olivine with a grain size of 1 m with 2 and 4 wt added basaltic glass was to determine growth rates during steady state grain growth a series of experiments was conducted at a fixed temperature of 1 350 c for durations from 2 to 700 h and a second series at fixed duration 168 h at temperatures from 1 250 to 1 450 c the resulting activation energy e 390 kj mol and growth exponent n 4 3 are comparable to melt free results nichols and mackwell 1991 the observed grain size distributions are broader than those predicted for normal grain growth or ostwald ripening and most similar to the log normal distribution the observed melt distribution is influenced by the continuous rearrangement of neighboring grains during growth the relatively large growth exponent observed in this study and by nichols and mackwell 1991 where secondary phases are present suggests that grain growth in the upper mantle is slower than predicted by karato 1989

integrated two dimensional lithospheric conductivity modelling in the pyrenees using field scale and laboratory measurements abstract recent magnetotelluric mt studies have shown that the lower crust in the pyrenees contains a high conductivity zone consistent with a subducting continental slab whose conductivity is 0 33 s m partial melting has been interpreted to be the most plausible explanation for this high conductivity here we report a two dimensional conductivity model of the lithosphere by integrating field scale and laboratory determinations of the conductivity of continental crustal and mantle rocks the laboratory data provide empirical formulas which allow us to determine the fluid saturated rock and melt conductivity when temperature pressure and lithology are known consequently we have also calculated the density lithostatic pressure and several alternative temperature profiles for use in the model from gravity seismic and thermal field data these can be used with a prescribed melt fraction to predict the electrical conductivity at depth which can be compared with the mt conductivity data alternatively the laboratory data can be combined with the mt conductivity data to predict the melt fraction at depth the primary outputs of the modelling are conductivity and melt fraction prediction profiles for six mixing models i waff s model hashin shtrikman hs upper bound ii hs lower bound iii parallel layers iv perpendicular layers v random melt areas and vi a modified archie s law that takes account of the presence of two conducting phases the modelling results indicate that a good match to the mt data can be obtained along the whole profile by the influence of pressure temperature and the fluid phase with the only exception being the subducted slab where a minimum of 4 7 melt fraction is necessary to explain the data

relationships between dynamically recrystallized grain size and deformation conditions in experimentally deformed olivine rocks new microstructural data on experimentally deformed wet and dry natural olivine rocks anita bay and aheim dunite together with the other reliable experimental data indicate that the experimental stress recrystallized grain size relationship in olivine rocks is largely independent of water content and temperature and is only slightly dependent on the flow properties of the material the experimental data cover a stress range of 30 300 mpa water contents from 30 ppm to 300 ppm and temperatures in the range 1100 1650 c local melt contents of up to 10 volume cannot be demonstrated to have a significant effect on the stress grain size relationship

construction and destruction of the north china craton with implications for metallogeny magnetotelluric evidence from the hengshan wutai fuping region within trans north china orogen abstract the hengshan wutai fuping region within the trans north china orogen is an important corridor that preserves the records of precambrian evolutionary history and phanerozoic magmatic and metallogenic events in the north china craton here we present 2 d and 3 d inversion models of magnetotelluric data collected from the hengshan wutai fuping region and surrounding regions our data reveal nearly horizontal high conductivity layer at lower crustal and upper mantle depths of 35 45 km beneath the hengshan and wutai mountains providing direct evidence for crust mantle decoupling and magmatic underplating in the mesozoic yixingzhai gold deposits and boqiang cu mo au deposits two nearly upright conductors cut through the crust and connect the lower crustal high conductivity layer suggesting frozen channels of fluids and magmas a west dipping slab shaped conductor within the high resistivity lithosphere beneath the wutai complex appears to be the deep extension of the exposed longquanguan ductile shear zone these resistivity anomalies can provide new constraints on the subduction collision events during precambrian tectonic evolution of the trans north china orogen as well as on the phanerozoic tectonic magmatic and metallogenic events in the hengshan wutai fuping region the west dipping slab shaped conductor may indicate a westward oceanic subduction between the wutai and fuping complexes with subsequent collision associated with the formation of the trans north china orogen in the paleoproterozoic we interpret the crustal and upper mantle conductors to be part of an interconnected fluid melt transport channel which probably played an important role for the mesozoic tectonic magmatic and metallogenic events as well as the cenozoic basaltic volcanism in the hengshan wutai fuping region

faults shear zones in the earth s mantle abstract geodetic data support a short term continental deformation localized in faults bounding lithospheric blocks whether major faults observed at the surface affect the lithospheric mantle and if so how strain is distributed are major issues for understanding the mechanical behavior of lithospheric plates a variety of evidence from direct observations of deformed peridotites in orogenic massifs ophiolites and mantle xenoliths to seismic reflectors and seismic anisotropy beneath major fault zones consistently supports prolongation of major faults into the lithospheric mantle this review highlights that many aspects of the lithospheric mantle deformation remain however poorly understood coupling between deformation in frictional faults in the uppermost crust and localized shearing in the ductile crust and mantle is required to explain the post seismic deformation but mantle viscosities deduced from geodetic data and extrapolated from laboratory experiments are only reconciled if temperatures in the shallow lithospheric mantle are high 800 c at the moho seismic anisotropy especially shear wave splitting provides strong evidence for coherent deformation over domains several tens of km wide in the lithospheric mantle beneath major transcurrent faults yet it cannot detect narrow strain localization zones or shallowly dipping faults seismic profiling images shallow dipping seismic reflectors in the lithospheric mantle interpreted as the continuation of normal or less frequently inverse faults in the mantle however the nature of these reflectors is poorly understood there is a large discrepancy between the scale at which deformation processes are studied a few kilometers at most due to the limited size of continuous mantle exposures at the earth s surface and the scale inferred for strain localization in the mantle from geophysical observations tens of kilometers combining data on deformation microstructures and crystal preferred orientations in naturally deformed peridotites and seismologic observations allows nevertheless discussing strain localization processes that may play a role on the development of mantle shear zones

reflections from mantle fault zones around the british isles british institutions reflection profiling syndicate birps deep seismic reflection surveys show that the upper mantle around the british isles is reflective exhibiting discrete isolated reflecting zones within an otherwise seismically transparent mantle there are 20 known occurrences of reflectors within the upper mantle across the continental shelf of the british isles many delimited by intersecting or closely spaced profiles most of the mantle reflectors have similar reflection characteristics they dip 10 degree to 30 degree and are thin approximately 0 5 s approximately 2 km and discrete the geologic history of the continental shelf around the british isles an area 1000 x 1000 km is complex and the tectonic settings of these reflectors very each mantel reflector is associated with a major zone of deformation in the crust this association the dips of the reflectors and their high reflectivity all indicate a structural origin the birps mantle reflectors include probable early paleozoic thrusts and mesozoic normal faults in contrast to birps data deep reflection profiles in other areas of the world in a variety of geologic settings have only rarely imaged reflectors within the upper mantle

new geophysical constraints on the deep structure of the pyrenees a 2d modeling of geoid quasigeoid data across the pyrenees is consistent with electromagnetic images of the pyrenean lithospheric structure showing a subducted iberian lower crust using new magnetotelluric data acquired through the orogen a 3d electrical conductivity model of the pyrenean lithospheric structure is obtained the most outstanding feature is the presence of a high conductivity zone at lower crustal depths along the orogen in the contact zone between the iberian and european plates the integration of various geophysical parameters suggests that the simplest and most reasonable mechanism to explain the observed data is partial melting of the subducted iberian lower crust

fast diffusion along mobile grain boundaries in calcite experimental measurements of grain boundary diffusion are usually conducted on static boundaries despite the fact that grain boundaries deep in the earth are frequently mobile in order to explore the possible effect of boundary mobility on grain boundary diffusion rates we have measured the uptake of 44ca from a layer of 44ca enriched calcite powder during the static recrystallization of a single crystal of calcite at 900 c a region about 500 m wide adjacent to the powder layer is heterogeneously enriched in 44ca and complex zoning patterns including sharp steps in composition and continuous increases and decreases in 44ca content are developed in metamorphic rocks these would normally be interpreted in terms of changes in pressure or temperature rayleigh fractionation or episodic fluid infiltration these explanations cannot apply to our experiments and instead the zoning patterns are interpreted as being due to variations in grain boundary migration rate we have applied an analytical model which allows the product of grain boundary diffusion coefficient and grain boundary width dgb to be calculated from the grain boundary migration rate and the compositional gradient away from the powder layer the value of dgb in the mobile grain boundaries is at least five orders of magnitude greater than the published value for static boundaries under the same conditions in order to allow the scale of chemical equilibrium and hence textural evolution to be predicted under both experimental and geological conditions we present quantitative diffusion regime maps for static and mobile boundaries in calcite using both published values and our new values for grain boundary diffusion in mobile boundaries enhanced diffusion in mobile boundaries has wide implications for the high temperature rheology of earth materials for geochronology and for interpretations of the length and time scales of chemical mass transport moreover zones of anomalously high electrical conductivity in the crust and mantle could be regions undergoing recrystallization such as active shear zones rather than regions of anomalous mineralogy water or melt content as is generally suggested

styles of till deposition at the margin of the last glacial maximum north sea lobe of the british irish ice sheet an assessment based on geochemical properties of glacigenic deposits in eastern england the geochemical composition of last glacial maximum lgm tills and glacitectonites in east yorkshire and lincolnshire was analysed to assess vertical changes in their geochemical signature with respect to the current regional stratigraphic division and generic models of till deposition results fail to precisely differentiate the traditional basement skipsea and withernsea till types instead there is often more variability within these till types than between them the geochemical signatures are also laterally discontinuous and are often repeated vertically at a number of sites suggesting that the till sequences are composed of a number of lithologically distinct rafts which have been tectonically stacked or elevated to higher levels in the sediment pile at dimlington the production of a glacitectonically folded and stacked moraine is proposed as a mechanism to explain the remarkably thick sequence of withernsea till and the repetitions in geochemical composition we argue that the changes in till matrix geochemistry are related to changes in the dominance of ice lobes of different provenance which reworked pre existing quaternary sediments excavated local bedrock and progressively mixed these sediments with more far travelled material from further north the most significant implication of these findings is that the current lgm till stratigraphy of east yorkshire and lincolnshire is too simplified and therefore does not recognise the sedimentary and structural complexity produced by repeated onshore possibly surging flow by a dynamic north sea lobe at the eastern margin of the british irish ice sheet

magnetotelluric constraints on the tectonic setting of grenville aged orogenesis in central australia abstract magnetotelluric mt data were collected over a 480 km long profile in central australia extending from the mesoarchaean palaeoproterozoic gawler craton into the mesoproterozoic neoproterozoic musgrave province the two regions have contrasting geological histories and many reconstruction models of proterozoic australia propose that they were juxtaposed during the mesoproterozoic despite the significant differences in geological background between the gawler craton and the musgrave province the mt data did not image an electrical structure or any change in electrical character between them mt models suggest that southern central australia is electrically separated into two parts the first comprising the northernmost gawler craton and the musgrave province has a 5 km thick layer of moderate resistivity overlying highly resistive crust the second comprising the central northern gawler craton displays moderate resistivity to the base of the crust the lithospheric mantle underlying both electrical regions is uniformly highly resistive our results suggest that the musgrave province and gawler craton are more closely related than is currently recognized we propose that the gawler craton forms basement to the musgrave province and that the grenvillian musgrave orogeny was an intracratonic reworking event that overprinted the older basement

electrical properties of fine grained olivine evidence for grain boundary transport 1 to investigate the mechanisms responsible for electrical conduction and deduce the relation between grain size and conductivity the electrical conductivity of synthetic polycrystalline forsterite with grain sizes between 1 1 0 4 and 4 7 2 4 m was measured at temperatures up to 1470 c and at 0 1 mpa pressure the complex impedance plots display one clear arc indicating a single dominant conduction mechanism bulk conductivity is inversely proportional to the grain size of the different samples this relation suggests that the electrical conductivity of the samples is controlled by grain boundary diffusion of the charge carriers the apparent activation energy q for diffusion of the charge carriers between 1180 and 1470 c lies between 315 39 and 323 15 kj mol this resembles previous data on grain boundary diffusion of mg in forsterite a geometrical model of less conducting cubic grains and more conducting grain boundaries agrees well with the experimental data this model is applied to predict the conductivity contrast between fine grained shear zones and less deformed regions in the lithosphere upper mantle shear zones are predicted to have 1 5 2 orders of magnitude higher conductivity than less deformed regions in the lithosphere this means grain boundary transport probably has an important role in the existence of anomalously high conductivity zones in the upper mantle and that fine grained shear zones might be detected using magnetotelluric methods

magnetotelluric constraints on subduction polarity reversing reconstruction models for proterozoic australia two dimensional lithospheric scale magnetotelluric imaging in the central australian proterozoic has constrained the large scale architecture of terrane assembly during paleoproterozoic accretion and collision the comparatively conductive north australian craton consisting of rocks between 2500 and 1730 ma in age has been imaged to extend for 150 km under the 1690 1620 ma warumpi province which forms part of a large comparatively juvenile terrane in central southern australia collision between the north australia craton and warumpi province occurred ca 1640 ma the boundary between these domains is modeled to be subvertical at crustal scale but dips south at 45 in the mantle to depths of 150 km we interpret this geometry to reflect lithospheric scale underthrusting of the north australian craton beneath the warumpi province and suggest that it provides a first order constraint on subduction polarity during collision ca 1640 ma in contrast most contemporary models for the evolution of paleoproterozoic australia propose that the north australian craton was located on the overriding plate of a long lived ca 1800 1550 ma north directed subduction system the polarity of these models is not consistent with the lithospheric scale geophysical architecture

evidence of electrical anisotropic structures in the lower crust and the upper mantle beneath the rhenish shield abstract we present the results of a long period electromagnetic investigation of the crust and upper mantle beneath the rhenish shield in western germany the magnetotelluric phase data reveal a frequency dependent regional strike that varies only smoothly across the array at short periods t 1000 s there is a consistent striking in we direction which provides strong evidence of an anisotropic structure in the upper mantle too geomagnetic data were also used whereas we reference the magnetic field components of all sites to an arbitrarily chosen field site that provides a direct view of the anomalous current flow we show that in case of a non one dimensional electrical substructure of the reference site all magnetic transfer functions of the other sites can be affected by lateral conductivity contrasts beneath the reference site a simple method to remove such effects is introduced applying this method the magnetic data show a distinct anomaly in the northwestern part of the area finally we present a 3d model of the conductivity structure beneath the rhenish shield which contains a superposition of two anisotropic structures in the lower crust 2000 s and the upper mantle 20 000 s respectively and a local anomaly in the upper crust 4000 s

the electric moho since mohorovicic1 discovered a dramatic increase in compressional seismic velocity at a depth of 54 km beneath the kulpa valley in croatia the moho has become arguably the most important seismological horizon in earth owing to its role in defining the crust mantle boundary it is now known to be a ubiquitous feature of the earth being found beneath both the continents and the oceans and is commonly assumed to separate lower crustal mafic rocks from upper mantle ultramafic rocks electromagnetic experiments conducted to date however have failed to detect a corresponding change in electrical conductivity at the base of the crust although one might be expected on the basis of laboratory measurements2 here we report electromagnetic data from the slave craton northern canada which show a step change in conductivity at moho depths such resolution is possible because the slave craton is highly anomalous exhibiting a total crustal conductance of less than 1 siemens more than an order of magnitude smaller than other archaean cratons we also found that the conductivity of the uppermost continental mantle directly beneath the moho is two orders of magnitude more conducting than laboratory studies on olivine would suggest inferring that there must be a connected conducting phase

archaean cratonic roots mantle shear zones and deep electrical anisotropy the extent to which the mantle participated in the growth and stabilization of ancient cratons is central to our understanding of the evolution of the continents1 the detection of seismic anisotropy beneath precambrian north america for example has been interpreted as showing that strain induced orientation of mantle minerals in subcontinental lithospheric mantle can preserve a record of ancient episodes of deformation2 here we present mag netotelluric measurements from the superior province of the canadian shield which reveal pronounced electrical anisotropy in the upper 100 km of the underlying mantle we argue that this anisotropy is best explained by conducting graphite films oriented within fractures or on grain boundaries and associated with metasomatism of the mantle roots of major archaean shear zones which transect the entire superior province the uppermost mantle beneath the canadian shield has therefore remained fixed to the crust and isolated from significant tectonic reworking since the late archaean

a geophysical perspective on mantle water content and melting inverting electromagnetic sounding data using laboratory based electrical conductivity profiles abstract this paper applies electromagnetic sounding methods for earth s mantle to constrain its thermal state chemical composition and water content we consider long period inductive response functions in the form of c responses from four stations distributed across the earth europe north america asia and australia covering a period range from 3 9 to 95 2 days and sensitivity to 1200 km depth we invert c responses directly for thermo chemical state using a self consistent thermodynamic method that computes phase equilibria as functions of pressure temperature and composition in the na 2 o cao feo mgo al 2 o 3 sio 2 model system computed mineral modes are combined with recent laboratory based electrical conductivity models from independent experimental research groups yoshino 2010 and karato 2011 to compute bulk conductivity structure beneath each of the four stations from which c responses are estimated to reliably allocate water between the various mineral phases we include laboratory measured water partition coefficients for major upper mantle and transition zone minerals this scheme is interfaced with a sampling based algorithm to solve the resulting non linear inverse problem this approach has two advantages 1 it anchors temperatures composition electrical conductivities and discontinuities that are in laboratory based forward models and 2 at the same time it permits the use of geophysical inverse methods to optimize conductivity profiles to match geophysical data the results show lateral variations in upper mantle temperatures beneath the four stations that appear to persist throughout the upper mantle and parts of the transition zone calculated mantle temperatures at 410 and 660 km depth lie in the range 1250 1650 c and 1500 1750 c respectively and generally agree with the experimentally determined temperatures at which the measured phase reactions olivine spinel and spinel ferropericlase perovskite occur the retrieved conductivity structures beneath the various stations tend to follow trends observed for temperature with the strongest lateral variations in the uppermost mantle for depths 300 km conductivities appear to depend less on the particular conductivity database conductivities at 410 km and at 660 km depth are found to agree overall with purely geophysically derived global and semi global one dimensional conductivity models both electrical conductivity databases point to 2 o in the upper mantle for transition zone minerals results from the laboratory database of yoshino 2010 suggest that a much higher water content up to 2 wt h 2 o is required than in the other database karato 2011 which favors a relatively dry transition zone 2 o incorporating laboratory measurements of hydrous silicate melting relations and available conductivity data allows us to consider the possibility of hydration melting and a high conductivity melt layer above the 410 km discontinuity the latter appears to be 1 regionally localized and 2 principally a feature from the yoshino 2010 database further there is evidence of lateral heterogeneity the mantle beneath southwestern north america and central china appears wetter than that beneath central europe or australia

electrical conductivity of olivine a dunite and the mantle laboratory studies of the electrical conductivity of rocks and minerals are vital to the interpretation of electromagnetic soundings of the earth s mantle to date the most reliable data have been collected from single crystals we have extended these studies with electrical conductivity measurements on a dunite from north carolina in the temperature range of 600 1200 c and under controlled oxygen fugacity observations of conductivity as a function of oxygen fugacity and temperature demonstrate that conduction in the dunite is indistinguishable from conduction in single olivine crystals thus the common practice of exaggerating the single crystal conductivities to account for conduction by grain boundary phases in the mantle is unnecessary because the dunite conductivity is consistent with that published for single crystals under similar conditions we have made a combined analysis of these data conductivity as a function of temperature between 600 and 1450 c displays three conduction mechanisms whose activation energies may be recovered by nonlinear least squares fitting yielding activation energies of 0 21 2 56 10 19 j 0 13 1 60 ev below 720 c 2 56 0 02 10 19 j 1 60 0 01 ev between 720 c and 1500 c and 11 46 0 90 10 19 j 7 16 0 56 ev above 1500 c the behavior of conductivity as a function of oxygen fugacity is well explained by a model in which an o2 independent population of charge carriers is supplemented at high oxygen fugacities with a population that is proportional to o20 3 this parametrization produces a clear correlation of the o2 dependent term with iron content which is otherwise obscured by variations in conductivity among olivines

lithosphere rheology a note of caution abstract it has become common practice to use laboratory determined low temperature frictional sliding data together with power law equations for steady state creep of rocks at high temperatures to construct inferred profiles of rock strength and mode of failure with depth in the lithosphere in some cases this may involve unwarranted extrapolation of rock mechanics behaviour beyond the region of its validity the form of transition from frictional behaviour to intracrystalline plastic flow in the earth is much more complex than is suggested by the above model within the regime of plastic flow substantial changes in flow strength and mode of failure whether flow becomes localized into shear zones may accompany microstructural changes developed over large strains when deformation is accompanied by metamorphic changes existing flow laws for rocks are likely to be wholly inapplicable until a clearer understanding has emerged of the full range of expression of the rheological behaviour of rocks existing models of lithosphere rheology should be treated with caution

lithospheric structures and precambrian terrane boundaries in northeastern botswana revealed through magnetotelluric profiling as part of the southern african magnetotelluric experiment 1 within the framework of the southern african magnetotelluric experiment a focused study was undertaken to gain improved knowledge of the lithospheric geometries and structures of the westerly extension of the zimbabwe craton zim into botswana with the overarching aim of increasing our understanding of southern african tectonics the area of interest is located in northeastern botswana where kalahari sands cover most of the geological terranes and very little is known about lithospheric structures and thicknesses some of the regional scale terrane boundary locations defined based on potential field data are not sufficiently accurate for local scale studies investigation of the nnw sse orientated 600 km long zim line profile crossing the zimbabwe craton magondi mobile belt and ghanzi chobe belt showed that the zimbabwe craton is characterized by thick 220 km resistive lithosphere consistent with geochemical and geothermal estimates from kimberlite samples of the nearby orapa and letlhakane pipes 175 km west of the profile the lithospheric mantle of the ghanzi chobe belt is resistive but its lithosphere is only about 180 km thick at crustal depths a northward dipping boundary between the ghanzi chobe and the magondi belts is identified and two middle to lower crustal conductors are discovered in the magondi belt the crustal terrane boundary between the magondi and ghanzi chobe belts is found to be located further to the north and the southwestern boundary of the zimbabwe craton might be further to the west than previously inferred from the regional potential field data

surface diffusivity of cleaved nacl crystals as a function of humidity impedance spectroscopy measurements and implications for crack healing in rock salt 1 rock salt offers an attractive host rock for geological storage applications because of its naturally low permeability and the ability of excavation induced cracks to heal by fluid assisted diffusive mass transfer however while diffusive transport rates in bulk nacl solution are rapid and well characterized such data are not directly applicable to storage conditions where crack walls are coated with thin adsorbed water films to reliably predict healing times in geological storage applications data on mass transport rates in adsorbed films are needed we determined the surface diffusivity in such films for conditions with absolute humidities ah ranging from 1 to 18 g m3 relative humidities rh of 4 78 by measuring the surface impedance of single nacl crystals we use the impedance results to calculate the effective surface diffusivity s d cusing the nernst einstein equation thes values obtained lie in the range 1 10 27 m3 s 1 at very dry conditions to 1 10 19 m3 s 1 for the deliquescence point at 296 k which is in reasonable agreement with existing values for grain boundary diffusion under wet conditions estimates for the diffusivity d made assuming a film thickness of 50 90 nm and no major effects of thickness on the solubility c lie in the range of 1 10 14 to 8 10 12 m2 s 1 for the highest humidities studied 14 18 g m3 ah 60 78 rh for geological storage systems in rock salt we predict s values between 1 10 22 8 10 18 m3 s 1 these imply crack healing rates 6 to 7 orders of magnitude lower than expected for brine filled cracks

magnetotelluric investigations of the lithosphere beneath the central rae craton mainland nunavut canada new magnetotelluric soundings at 64 locations throughout the central rae craton on mainland nunavut constrain 2 d resistivity models of the crust and lithospheric mantle beneath three regional transects responses determined from colocated broadband and long period magnetotelluric recording instruments enabled resistivity imaging to depths of 300 km strike analysis and distortion decomposition on all data reveal a regional trend of 45 53 but locally the geoelectric strike angle varies laterally and with depth the 2 d models reveal a resistive upper crust to depths of 15 35 km that is underlain by a conductive layer that appears to be discontinuous at or near major mapped geological boundaries surface projections of the conductive layer coincide with areas of high grade archean metasedimentary rocks tectonic burial of these rocks and thickening of the crust occurred during the paleoproterozoic arrowsmith 2 3 ga and trans hudson orogenies 1 85 ga overall the uppermost mantle of the rae craton shows resistivity values that range from 3000 m in the northeast beneath baffin island and the melville peninsula to 10 000 m beneath the central rae craton to 50 000 m in the south near the hearne domain near vertical zones of reduced resistivity are identified within the uppermost mantle lithosphere that may be related to areas affected by mantle melt or metasomatism associated with emplacement of hudsonian granites a regional decrease in resistivities to values of 500 m at depths of 180 220 km increasing to 300 km near the southern margin of the rae craton is interpreted as the lithosphere asthenosphere boundary

how the crust meets the mantle lithoprobe perspectives on the mohorovicic discontinuity and crust mantle transition application of regional geophysical and geological methods throughout two decades of canada s lithoprobe project provides new opportunities to analyze the mohorovicic discontinuity moho and crust mantle transition the transect format employed during lithoprobe in which 10 specified regions of canada were targeted for approximately a decade each between 1984 and 2003 permitted teams of scientists to focus on geological geophysical and tectonic issues for each transect as a primary objective was to enhance knowledge of the structure of the crust and lithosphere an obvious target in each transect was the nature and origin of the moho and crust mantle transition accordingly the combined results provide new perspectives on the moho and the relationship of the moho to the crust mantle transition perhaps the most important result is that the continental geophysical moho is a deceptively simple feature it has a variety of signatures at different scales that preclude a single universally applicable interpretation in methods that provide large scale information such as regional seismic studies it is a relatively abrupt refraction velocity contrast that often displays a dramatic downward decrease in seismic reflectivity however its origin in a geological or tectonic sense is perhaps best determined by careful analyses of structural details near the geophysical moho which are complex and varied in some areas within canada it appears that the geophysical moho may be old and perhaps remains from the time the crust formed in other areas it appears to be a relatively young feature that was superimposed onto older crustal fabrics

pressure effect on electrical conductivity of mantle olivine abstract we report new measurements of electrical conductivity of olivine that were made in a multi anvil press using solid buffers mo moo2 to stabilize oxygen partial pressure the temperature range was 1000 c 1400 c and the pressure range was 4 10 gpa when pressure effects are interpreted as activation volumes in the arrhenius equation they yield values of order 0 6 0 6 cm3 mole values of the pre exponential factor 0 are of the order of hundreds of s m and the zero pressure activation energy is about 1 5 ev in agreement with laboratory measurements at lower pressure conditions in addition we analyzed older literature data which produced a similar result but with larger variation the common practice of assuming that the pressure effect on electrical conductivity of olivine in the earth s upper mantle depends weakly on pressure has rested on uncertain grounds these new observations suggest that for upper mantle conditions depths of 80 200 km neglecting pressure effects on olivine conductivity is justified a weak pressure effect also supports the small polaron fe3 model as the dominant conduction mechanism although the mainly positive activation volumes argue for a component of lattice deformation when applied to the depth range 200 400 km in the earth the laboratory data yield conductivities of order 10 2 s m slightly lower than the range of geophysical conductivity profiles but well within this range after allowing for probable effects of oxygen activity and temperature uncertainty

grain growth in the system forsterite diopside water abstract grain growth experiments in dunite clinopyroxenite and wehrlites having various forsterite diopside ratios under water saturated conditions were performed to investigate the effect of water on grain growth kinetics in the earth s upper mantle the experiments were conducted at 1200 c and 1 2 gpa for 1 5 763 h using a piston cylinder apparatus both normal grain growth ngg and abnormal grain growth agg proceeded in the rocks the addition of water to the rocks resulted in the inhibition of ngg in dunite and clinopyroxenite whereas it resulted in the acceleration of ngg of both forsterite and diopside in wehrlites agg of the first phase in the rocks tended to be promoted by increasing the amount of water added the values of the growth exponent n for forsterite and diopside in wehrlites were in the range 1 8 4 8 and 1 8 4 5 respectively the n values for the first phase in wehrlites tended to show low values 1 8 2 5 whereas higher n values 3 have been reported under dry conditions this suggests that faster ngg of the first phase with n 2 can proceed in peridotites under water saturated conditions whereas much slower ngg of the first phase with n 3 proceeds under dry conditions this could have the consequences that the effective viscosity of the water saturated peridotites can be higher than that under dry conditions in the earth s upper mantle when the creep mechanism is grain size sensitive creep

shear zones in the upper mantle evidence from alpine and ophiolite type peridotite massifs there is abundant field and microstructural evidence for localization of deformation in alpineand ophiolite type mantle massifs on the basis of field relationships and microstructures we recognize two types of tectonite shear zones mediumto coarseand fine grained as well as two types of mylonitic shear zones anhydrous and hydrous peridotite mylonites in tectonite shear zones softening processes responsible for localization are probably melt related weakening in the medium to coarse tectonites and a change in limiting slip system in the fine grained tectonites in peridotite mylonites the most likely cause for softening and localization is a change in dominant deformation mechanism from dislocation to grain size sensitive creep microstructural and petrological study of mylonite rocks reveals that reactions either continuous net transfer reactions anhydrous and hydrous or melt rock reactions play a key role in the formation of fine grained material that promotes grain size sensitive creep these reactions occur over a broad range of pressure temperature conditions encompassing a large part of the lithospheric upper mantle we conclude that mantle shear zones are widespread and that they reduce the bulk strength of the lithosphere significantly attempts to understand the dynamics of tectonic processes depend critically on quantitative knowledge concerning the mechanical properties of the lithosphere and the relative strength of the crust and upper mantle e g brun 2002 for many years it has been accepted that the upper part of the mantle lithosphere is strong compared to the crust e g kirby 1983 kuznir park 1986 the notion of a strong uppermost mantle is based on extrapolation of laboratory flow laws kohlstedt et al 1995 and the depth distribution of seismicity in the lithosphere chen rc molnar 1983 the inferred high strength of the shallow mantle implicitly refers to homogeneous flow however at large strains deformation of the mantle lithosphere may become localized within weak shear zones fig 1 so the application of low strain flow laws may seriously overestimate the strength of the mantle rutter brodie 1988 handy 1989 vissers et 01 1995 in a recent study on the seismicity and elastic thickness of continental lithosphere magi et al 2000 have suggested that the crust is in fact the strongest layer thus it is emerging that the strength of lithospheric upper mantle may vary considerably over the last decade we have studied a number of peridotite massifs both of orogenic alpine type and ophiolitic origin in order to establish under which conditions and to what extent deformation in mantle rocks is localized in this contribution we present an overview of shear localization features in these peridotite massifs from our data as well as published data from other workers it emerges that under lithospheric conditions localization of deformation in mantle rocks is a common phenomenon fig 1 table i in addition shear localization seems to occur over a wide range of pressure temperature conditions producing a number of distinct shear zone types the range of shear zone types is comparable to types of deformation structures found in crustal rocks e g carreras et al 1980 microstructural work has highlighted the importance of mineral reactions as a softening mechanism in the development of mantle mylonite shear zones handy 1989 drury et al from alsop g i holdsworth r e mccaffrey k j w hand m eds 2004 flow processes in faults and shear zones geological society london special publications 224 11 24 0305 8719 15 00 o the geological society of london 2004 12 a h dijkstra eta l

lithospheric rheological heterogeneity across an intraplate rift basin linfen basin north china constrained from magnetotelluric data implications for seismicity and rift evolution abstract we take the linfen basin which is the most active segment of the cenozoic intraplate shanxi rift as an example showing how to use magnetotelluric data to constrain lithospheric rheological heterogeneities of intraplate tectonic zones electrical resistivity models combined with previous rheological numerical simulation show a good correlation between resistivity and rheological strength indicating the mechanisms of enhanced conductivity could also be reasons of reduced viscosity the crust beneath the linfen basin shows overall stratified features in both electrical resistivity and rheology the uppermost crustal conductive layer is dominated by friction sliding type brittle fracturing the high resistivity mid crust is inferred to be high viscosity metamorphic basement being intersected by deep fault the plastic lower crust show significantly high conductivity feature seismicity appears to be controlled by crustal rheological heterogeneity micro earthquakes mainly distribute at the brittle ductile transition zones as indicated by high to low resistivity interfaces or the high pore pressure fault zones while the epicenters of two giant destructive historical earthquakes occur within the high resistivity and therefore high strength blocks near the inferred rheological interfaces the lithosphere scale lateral rheological heterogeneity along the profile can also be illustrated the crust and upper mantle beneath the ordos block luliang mountains and taihang mountains are of high rheological strength as indicated by large scale high resistivity zones while a significant high conductivity lithosphere scale weak zone exists beneath the eastern margin of the linfen basin according to previous geodynamic modeling works we suggest that this kind of lateral rheological heterogeneity may play an essential role for providing driving force for the formation and evolution of the shanxi rift regional lithospheric deformation and earthquake activities under the far field effects of the india eurasian collision

velocity conductivity relationships for mantle mineral assemblages in archean cratonic lithosphere based on a review of laboratory data and hashin shtrikman extremal bounds article i nfo can mineral physics and mixing theories explain field observations of seismic velocity and electrical conductivity and is there an advantage to combining seismological and electromagnetic techniques these two questions are at the heart of this paper using phenomenologically derived state equations for individual minerals coupled with multi phase hashin shtrikman extremal bound theory we derive the likely shear and compressional velocities and electrical conductivity at three depths 100 km 150 km and 200 km beneath the central part of the slave craton and beneath the kimberley region of the kaapvaal craton based on known petrologically observed mineral abundances and magnesium numbers combined with estimates of temperatures and pressures we demonstrate that there are measurable differences between the physical properties of the two lithospheres for the upper depths primarily due to the different ambient temperature but that differences in velocity are negligibly small at 200 km we also show that there is an advantage to combining seismic and electromagnetic data given that conductivity is exponentially dependent on temperature whereas the shear and bulk moduli have only a linear dependence in cratonic lithospheric rocks focussing on a known discontinuity between harzburgite dominated and lherzolitic mantle in the slave craton at a