gold graphite association in granulite terrains implications for ore genesis abstract the gold occurrences of sri lanka are closely associated with the graphite deposits and are genetically related the quartz graphite margins of the vein graphite are shown to be metal accumulating the movement of co 2 rich fluids formation of graphite and the concentration of au are closely related whereas the co 2 rich fluids may well be the carbon carriers in the formation of graphite the original source of co 2 is still debated hydrothermal solutions when in contact with the graphite tend to activate the carbon thereby producing sites for the enrichment of gold the genetic link between the graphite veins and gold deposition has useful implications in the investigation of ore genesis in high grade metamorphic terrains and the exploration for gold

rifting of a passive margin and development of a lower crustal detachment zone evidence from marine magnetotellurics 1 rifting of passive margins has in some places led to the development of extensive submerged continental platforms crustal thinning by a factor of two or more is at least partially accommodated by high angle normal faults that extend through the brittle upper crust to the ductile parts of the lower crust however an approximately horizontal detachment zone has been postulated to explain the width of the shelf and differences between the amounts of upper crust and lower lithosphere extension here we present magnetotelluric mt and geomagnetic depth sounding gds measurements across the exmouth plateau a deep 1500 m and wide 500 km continental platform on the northwest shelf of western australia mt responses across all of the plateau in the bandwidth 102 104 s exhibit a pronounced apparent resistivity low of less than 1 m at 2 103 s two dimensional smooth inversion of all mt and gds data images a very low resistivity layer 0 1 m at a depth of 10 15 km dipping less than 1 landward from the continent ocean boundary below this layer resistivity increases to 103 m beneath the moho at 20 km depth the low resistivity mid crustal layer coincides with a laterally extensive seismic reflector at 10 15 km observed through much of the exmouth plateau we suggest that this layer represents a decoupling zone between ductile and brittle continental crust with a concentration of conducting mineralisation along shear zones the most probable conducting mineral is graphite this is known to precipitate along shear zones and would significantly facilitate extensional strain

seismic images of continental lithosphere seismic reflection profiles in usa and west germany over thirty years ago showed strong reflections from the lower crust but technical advances through the 1970s were needed for the method to image the continental lithosphere the british institutions reflection profiling syndicate birps found advantage in working offshore and has obtained over 12 000 km of profiles around britain during the past decade giving the densest regional coverage anywhere reflections are observed from strata in sedimentary basins from low angle faults that can be followed from surface to mid crust from a laterally variable reflective zone in the lower crust from the moho and from fault like features in the upper mantle there is evidence around britain that faults bounding sedimentary basins are older structures which have been reactivated it is argued that lower crustal reflectivity around britain is mainly due to anastomosing ductile shears which accommodate extension in the lower crust commensurate with extension in the upper crust by faulting and block rotation there is also evidence of reflections preserved from older orogenic features so that reflections interpreted as extensional ductile shears in the lower crust may also derive from older features that have been reactivated most reflections in the lower crust arise from rocks that have been highly strained evidence of lateral variability in the reflective character of the moho suggests that the moho has been actively involved in the structural evolution of the lithosphere upper mantle reflections are interpreted as shear zones which transfer crustal deformation through the lithosphere

electromagnetic induction studies in the eyre peninsula south australia magnetic field fluctuations have been recorded by an array of portable three component magnetometers at 60 sites across the eyre peninsula in south australia between december 1993 and march 1995 an additional 54 magnetometer data records collected prior to 1989 and described by milligan 1989 and milligan white chamalaun 1989 were included in the analysis a major conductive feature in the crust first noted by white milligan 1984 as the eyre peninsula anomaly epa is re examined to assess its continuity to the north of the original arrays and to investigate its relationship with major tectonic features magnetic field time series were converted to induction arrows in the frequency domain these induction arrows were initially inverted using the minimum structure 2 d occam approach to estimate the electrical conductance of the crust following this thin sheet forward modelling was used to examine the relationship between the conductance and the dominant tectonic features the principal results of the modelling are that a narrow conductive feature extends inland from the coast about 160 km before terminating and the conductance is in the range 3000 to 10 000 s which decreases inland a strong correlation exists between the electrical conductance of the eyre peninsula and bouguer gravity anomalies and in particular the epa is coincident with a significant bouguer gravity gradient there is also good agreement between the locations of the foci of earthquakes of magnitude greater than 4 0 and the epa we believe that the anomaly is associated with a geological fracture in the precambrian upper crust as a result of crustal extension prior to the rifting of australia from antarctica in the jurassic 160 ma

fluid evolution and graphite genesis in the deep continental crust metamorphism in deep continental crust is often accompanied by carbon rich h2o poor fluids1 5 and is characterized by the development of granulite facies mineral assemblages3 5 graphite a minor phase in many high grade metamorphic rocks6 17 has also been recognized in recent xenoliths of deep crustal material18 demonstrating its presence in modern deep continental crust although graphite is most common in biotite schists it has also been recorded in pyroxene gneisses two feldspar quartz pegmatites marbles and quartzo feldspathic gneisses11 15 19 the presence of supracrustal sequences in granulite facies regions6 19 suggests that the latter developed through progressive metamorphism and dehydration of shallow level crustal material much of this material must have experienced a metamorphic recrystallization in the amphibolite facies accompanied by the development of hornblende although graphite in the high grade rock may develop from oxidation of organic material this may not be the most common means for graphite genesis in much of west greenland14 and in southern norway12 the low grade hornblende bearing material does not contain significant quantities of organic compounds although graphite is found in higher grade metamorphic equivalents this strongly suggests that introducing co2 into deep crustal material leads to fluid evolution and graphite formation i outline here the evolution of fluid composition in the c o h system which is in equilibrium with common crustal mineral assemblages thermodynamic data for co2 h2o mixtures20 and mineral components21 are used to model the chemical changes in fluid composition which accompany introduction of co2 into various types of rock the results demonstrate that graphite genesis is a direct consequence of deep crustal metamorphism in the presence of a co2 rich fluid phase in addition the calculations demonstrate that the deep continental crust will not be a reservoir for large volumes of methane rich gas

grain boundary graphite in rocks and implications for high electrical conductivity in the lower crust the origin of zones of high electrical conductivity in the lower continental crust is a long standing mystery possible explanations include the presence of brines1 2 partial melt3 serpentine4 and graphite5 when discussing the occurrence of graphite in the crust many petrologists have considered phase relations as they would have existed at the peak of metamorphism or during igneous emplacement6 10 here we show that a fine film of graphite is present on the grain boundaries in three rocks from the laramie anorthosite complex in two of these rocks graphite was stable during igneous crystallization but in the other it was not we maintain that in all of the rocks the grain boundary graphite precipitated from a co2 rich fluid during cooling the chemical processes that produced the grain boundary graphite in these rocks are likely to operate in many lower crustal rocks we therefore contend that because the films we observe are capable of producing the high conductivity that is seen in the lower crust grain boundary graphite should be considered as a possible cause for some conductivity anomalies

electrical conductivity and carbon in metamorphic rocks of the yukon tanana terrane alaska electrical conductivity of a water saturated quartz mica garnet schist collected from a surface outcrop near the denali fault zone in the yukon tanana terrane of east central alaska increases slightly with pressure to about 200 mpa this behavior is unlike that exhibited by other yukon tanana samples or by most rocks from other locations detailed petrographic examination of the sample revealed the presence of a stringer of carbonaceous material generally less than 10 m thick enclosed in and intergrown with one of the muscovite layers and extending for about 2 cm along the foliation the stringer is probably responsible for the anomalous conductivity change with pressure making the sample the first for which anomalous electrical conductivity behavior can be attributed to carbon associated with a specific feature the carbonaceous stringer together with its host muscovite layer are deformed and broken around a rotated garnet porphyroclast the deformation was accommodated by plastic deformation of quartz and therefore occurred in the ductile regime under conditions at least equivalent to greenschist facies metamorphism we interpret the textural relations to indicate that the carbonaceous material was formed by fluid deposition in a fracture formed within the muscovite layer possibly during the main phase of metamorphism and deformation and that the mica and carbon stringer were then deformed by the noncoaxial deformation responsible for rotation of the garnet porphyroclasts together the facts that the deformation resulting in garnet rotation was ductile and that the garnet rotation disrupted the stringer demonstrate that the carbonaceous stringer was present at depth i e 10 km brittle deformation on the microscopic scale is observed to have broken the connectivity of the carbon stringer explaining in part why the rock does not exhibit anomalously high conductivity at 0 1 mpa 1 atm pressure the brittle deformation is interpreted to have been caused by unloading due to uplift the observations indicate that carbonaceous material may exert a primary control on crustal electrical conductivity because it may be present as interconnected arrays in grain boundaries or microfractures or in megascopic throughgoing fractures

syngenetic thin graphite rich horizons in lower crustal rocks from the serre san bruno calabria italy and implications for the nature of high conducting deep crustal layers abstract in order to better understand the nature of deep crustal high electrical conductivity we studied the electrical properties of a tilted section of a former lower continental crust exposed in the calabrian arc of the alpine apennine mountain system geoelectric field measurements and impedance measurements on rock samples showed that these high grade metamorphic rocks are generally highly resistive as expected for crystalline electrolytically conducting rocks of low porosity this holds for graphite free metabasites as well as for metapelites which generally contain accessory up to 3 biogenic graphite in the form of isolated grains clearly as an exception a group of thin stratiform black horizons with thicknesses of 1 15 cm within the metapelitic series was detected by means of self potential sp measurements rock samples from these horizons exhibit high quasi metallic bulk conductivities of up to 50 s m 0 02 m in agreeement with up to 20 syngenetic graphite forming a network of interconnected streaks or crack fillings the high amount of carbon most probably originates from organic matter of c org rich black shales relative enrichment of the low mobility graphitic matter compared to the carbon content of the assumed protoliths may have been due to pressure solution and partial melting during prograde metamorphism without major contribution of a fluid phase resulting in isolated graphite flakes although enriched graphite in this form has little effect on electrical conductivity for the calabrian black horizons microscopic analyses make conceivable that in a further decisive step isolated graphite grains were mechanically smeared to continuous pathways during uplift by shearing producing hereby the observed graphitic network which is needed to generate high conductivity as c org rich black shales are common members of sedimentary sequences throughout the earth s history good conductors of this type may be expected in the continental crust at any depth depending on tectonic and metamorphic history with the exception of magmatic protoliths regarding the extremely high conductivity of the meta black shale samples containing syngenetic sheared graphite a total thickness of a few meters of such rocks is sufficient to explain magnetotelluric high conductivity anomalies in the deep crust

electrical conductivity of fluid bearing quartzite under lower crustal conditions abstract the electrical conductivity of fluid bearing quartzite was determined as function of temperature and fluid fraction at 1 gpa in order to assess the origin of the high conductivity anomalies observed in the middle to lower crustal levels dihedral angles of quartz fluid quartz determined from recovered samples were below 60 suggesting that fluid forms an interconnected network through the quartz aggregate the electrical conductivity of quartzite increases with increasing temperature which can be approximately expressed by arrhenius equation the apparent activation enthalpy decreases from 0 70 to 0 25 ev with increasing fluid fraction in volume from 0 00043 to 0 32 the electrical conductivity of the fluid bearing quartzite increased with fluid fraction proportionally to a power law 0 56 0 71 within the temperature range of 900 1000 k the electrical conductivity of the aqueous fluid bearing quartzite with the maximum fluid fraction 0 32 was found to be about three orders of magnitude higher than that of dry quartzite at 1000 k however its electrical conductivity was definitely lower than the geophysically observed values of high conductivity anomalies even if the quartzite contained large fluid fractions 0 32 the present results suggest that fluid bearing quartzite is unable to account for the high conductivity anomalies in terms of fluid fraction a significant amount of other ionic species such as na cl and al in aqueous fluid in addition to silica phases dissolved in fluid is required to increase conductivity

deep fluids and volatile recycling crust to mantle abstract there is now clear evidence that geofluids are involved in many stress release processes at many depths modern studies of crustal regions show that surface fluids may penetrate to depths of many kilometres at subduction zones volatiles may be recycled to mantle depths there is increasing evidence for the complexity of subduction structures with flaking off of major fragments which may devolatilize and revolatilize lower crust and upper mantle the same processes may be associated with massive magma underplating of continental crust the new techniques of modern surface chemistry open new possibilities for the study of fluid tracks thin films on mineral grains which may influence many properties like deep electrical conductivity

co 2 melts and granulite metamorphism the discovery that granulites contain co2 rich fluid inclusions while amphibolites in the same terrane contain h2o rich or mixed co2 h2o inclusions1 has led to a debate over the processes that produce granulite metamorphism one theory2 4 maintains that granulites form as the result of a massive influx of co2 from lower crustal or mantle sources such an influx could be responsible for the marked depletion many granulite terranes show in large ion lithophile elements2 5 others however maintain that granulite facies metamorphism is a necessary complement of crustal melt ing6 9 recently lamb and valley10 11 have argued that granulite metamorphism may be a product of any one of three processes co2 streaming partial melting or recrystallization of originally dry rocks here we suggest that these processes need not be independent and that they all may be related to the passage of melts through the crust

electrical conductivity of orthopyroxene and plagioclase in the lower crust the electrical conductivities of lower crustal orthopyroxene and plagioclase as well as their dependence on water content were measured at 6 12 kbar and 300 1 000 c on both natural and pre annealed samples prepared from fresh mafic xenolith granulites the complex impedance was determined in an end loaded piston cylinder apparatus by a solarton 1260 impedance gain phase analyzer in the frequency range of 0 1 106 hz the spectra usually show an arc over the whole frequency range at low temperature and an arc plus a tail in the high and low frequency range respectively at high temperature the arc is due to conduction in the sample interior while the tails are probably due to electrode effects different conduction mechanisms have been identified under dry and hydrous conditions for the dry orthopyroxene the activation enthalpy is 105 kj mol and the conduction is likely due to small polarons e g electrons hopping between fe2 and fe3 for the dry plagioclase the activation enthalpy is 161 kj mol and the conduction may be related to the mobility of na for the hydrous samples the activation enthalpy is 81 kj mol for orthopyroxene and 77 kj mol for plagioclase and the electrical conductivity is markedly enhanced probably due to proton conduction for each mineral the conductivity increases with increasing water content with an exponent of 1 and the activation enthalpies are nearly independent of water content combining these data with our previous work on the conductivity of lower crustal clinopyroxene the bulk conductivity of lower crustal granulites is modeled which is usually 10 4 s m in the range of 600 1 000 c we suggest that the high electrical conductivity in most regions of the lower crust especially where it consists mostly of granulites can be explained by the main constitutive minerals particularly if they contain some water contributions from other highly conducting materials such as hydrous fluids melts or graphite films are not strictly necessary to explain the observed conductivities

carbon deposition during brittle rock deformation changes in electrical properties of fault zones and potential geoelectric phenomena during earthquakes 1 to investigate potential mechanisms for geoelectric phenomena accompanying earthquakes we have deformed hollow cylinders of sioux quartzite to failure in the presence of carbonaceous pore fluids and investigated the resulting changes in electrical conductivity and carbon distribution samples were loaded at room temperature or 400 c by a hydrostatic pressure at their outer diameter increasing pressure at a constant rate to 290 mpa pore fluids consisted of pure co co2 and ch4 and a 1 1 mixture of co2 and ch4 each with pore pressures of 2 0 to 4 1 mpa failure occurred by the formation of mode ii shear fractures transecting the hollow cylinder walls radial resistivities of the cylinders fell to 2 9 to 3 1 m m for co tests and 15 2 to 16 5 m m for co2 ch4 tests compared with 23 m m for dry undeformed cylinders carbonaceous fluids had no discernable influence on rock strength on the basis of mapping using electron microprobe techniques carbon occurs preferentially as quasi continuous films on newly formed fracture surfaces but these films are absent from preexisting surfaces in those same experiments the observations support the hypothesis that electrical conductivity of rocks is enhanced by the deposition of carbon on fracture surfaces and imply that electrical properties may change in direct response to brittle deformation they also suggest that the carbon films formed nearly instantaneously as the cracks formed carbon film deposition may accompany the development of microfracture arrays prior to and during fault rupture and thus may be capable of explaining precursory and coseismic geoelectric phenomena

implications for the lithospheric geometry of the iapetus suture beneath ireland based on electrical resistivity models from deep probing magnetotellurics

lithospheric electrical structure in the central tibetan plateau and its tectonic significance abstract the himalaya tibetan plateau that is situated in the southwest china is an ideal natural field laboratory for continental dynamics in this study through the careful processing and analyzing of the magnetotellurics data the reliable 2 d electrical model was obtained to examine the relationship between the lithospheric electrical structure and dynamics in the central himalaya tibetan plateau six conductors mainly result from the partial melting and aqueous fluids and three resistors are related to different tectonic evolution in the model combined with other observations we summarize that the upwelling hot material partial melting aqueous fluids subduction erosion and crushed zone contribute to the error between the electrical location and the geographic location of the indus yarlung zangbo suture the subducted indian plate is shown by the different angles and different locations of its frontier in different areas the electrical structure beneath the lhasa terrane corresponds well with the geological features of an old locally reworked crust dominantly in the central lhasa terrane with two young juvenile crustal blocks surrounding the central terrane and it also shows the relationship among the dynamics magmatism and mineralization in the gangdise metallogenic belt meanwhile we indicate that the subducted polarity of the bangong nujiang tethyan ocean lithosphere was double sided and the smooth subduction of the longmu tso shuanghu tethyan ocean lithosphere may cut through the moho surface approximately 20 km above all these results can provide helpful constrains to understand the evolution mechanism of the himalaya tibetan plateau

effect of cooling on the water content and mechanical behavior of metamorphosed rocks calculations of the decline in f h2o in equilibrium with typical metamorphic assemblages during cooling indicate that there will be a large drop in pore water pressure within 100 of the metamorphic maximum the loss of pore fluid and the generation of large effective pressures across grain boundaries will inhibit rock deformation by those mechanisms that depend on the presence of an aqueous fluid phase in many metamorphic belts extensive deformation occurs below the metamorphic maximum during heating that is while fluid is being generated whereas rocks do not deform in the same ductile manner on cooling even while temperature is high this may reflect changing mechanical properties of the rocks rather than merely changes in regional stresses mobile belts could be made rigid in this way in about 20 m y

thermobarometry of garnet clinopyroxene hornblende granulites from the kapuskasing structural zone amphibole mixing properties and standard state properties for the end members tremolite tschermakite pargasite ferrotremolite ferrotschermakite and ferropargasite have been refined through analysis of relevant phase equilibrium experiments coexisting cummingtonite hornblende pairs and natural mineral parageneses when merged with the mineral properties given by r g berman these amphibole properties allow the calculation of numerous equilibria suitable for thermobarometry in a wide range of bulk compositions at amphibolite to granulite metamorphic grades the method was applied to texturally equilibrated garnet pyroxene hornblende gneisses and compared both to results based on anhydrous assemblages and to those for garnet biotite gneisses from the kapuskasing structural zone thermometer resetting is apparent only in garnet biotite gneisses whereas touching garnet pyroxene pairs in mafic gneiss show little evidence of retrograde fe mg exchange computed metamorphic temperatures garnet clinopyrox

electromagnetic images of crustal structures in southern and central canadian cordillera data from more than 400 magnetotelluric soundings made since the early 1980 s in the canadian cordillera over a 300 000 km2 area between 49 and 53 5 n are used to image qualitatively regional three dimensional crustal variation in electrical conductivity by means of phase maps phase frequency sections and maps of resistivity at depth two hundred of the soundings were acquired as part of lithoprobe southern canadian cordillera transect activities and their locations were coordinated with the seismic reflection and refraction experiments the lower crust has a generally pervasive low resistivity 1 100 m throughout the cordillera west of the foreland belt within this canadian cordilleran regional conductor the magnetotelluric data reveal both two dimensional structures with highest conductivities along the coast belt and omineca belt and three dimensional variation along geological strike this conductor mapped over a volume in excess of 106 km3 is most probably caused by fluids saline wat

electrical structure of the crust below the deccan flood basalts india inferred from magnetotelluric soundings summary the indian shield has experienced major stable continental region scr earthquakes in the recent past which occurred in the deccan volcanic province until now the deeper crust below the deccan traps where thick basaltic layers cover large areas of the indian shield remains poorly understood a magnetotelluric mt study covering a broad period range was conducted along a n s running profile of 330 km length to achieve more insights into the nature of the crustal electrical structure below the flood basalts after dimensionality analysis and decomposition 2 d inversions for te and tm modes were carried out the basalt cover characterized by at first glance surprisingly low electrical resistivities is found to have an average thickness of 400 m with the exception of central parts of the profile where it reaches up to 700 m the crust is in general highly resistive but several subvertical zones of enhanced conductivity were delineated in the middle to lower crust which are tentatively explained as images of hidden partly reactivated faults fractures of the precambrian basement sensitivity analysis indicates a high resolution of these features an alternative approach incorporating macro anisotropy is additionally considered and corresponding 2 d anisotropic forward modelling confirms the existence of successions of conductive dykes in the deep crust

magnetotelluric studies in the central india tectonic zone implications for intraplate stress regimes and generation of shallow earthquakes abstract the central india tectonic zone citz dissects the indian peninsula into the northern and southern crustal blocks the citz has been a seismically active region since the precambrian whereas the relatively deep crustal earthquakes near the narmada faults in the eastern part of the citz have been well investigated the mechanisms for the shallow earthquakes in the western part remain unknown here we present results from a new magnetotelluric study to derive the crustal structure and to understand its implications our data show a thick and highly resistive 500 ohm m crust in the south of tapti river as against a less resistive one in the north these results in conjunction with heat flow values indicate that the crust below the southern part has stable continental cratonic signatures on the northern side of the tapti river we infer the ascent of basaltic magmas from the mantle into the shallow crust and crystallization into layered intrusions these mafic ultramafic bodies could be a potential cause for the shallow earthquakes in the western part of the citz the mafic ultramafic bodies below the crust of citz would locally modify the intraplate stresses which in turn would facilitate the occurrence of earthquakes due to reactivation of pre existing faults thus the large accumulation of strain energy in the deep crust beneath the region can be attributed to the presence of high stress bodies emplaced at depth during the deccan volcanic activity

electrical resistivities of crustal fluids results of calculations on the electrical resistivities of crustal fluids as a function of temperature pressure and fluid composition indicate that crustal fluids can range in resistivity from approximately 1 m the dominant contribution to decreasing the resistivities of crustal fluids comes from increased concentrations of chloride and bicarbonate salts and increased temperature and density of the fluids calculations indicate that the sharpest decrease in fluid resistivity occurs at shallow depths 10 km and temperatures 300 c can not be readily explained as a product of variations in resistivities of crustal fluids these results indicate that a transition in mineralogy or pore shape geometry is more likely responsible for the observed decrease in electrical resistivity of the middle crust than a major change in fluid resistivity

electrical imaging of narmada son lineament zone central india from magnetotellurics the narmada son lineament nsl zone is the second most important tectonic feature after himalayas in the indian geology magnetotelluric mt studies were carried out in the nsl zone along a 130 km long nne ssw trending profile the area of investigation extends from edlabad 20 46 16 7 5 59 05 in the south to khandwa 21 53 51 7 6 18 05 i n the north the data shows in general the validity of a two dimensional 2d approach besides providing details on the shallow crustal section the 2d modeling results resolved four high conductive zones extending from the middle to deep crust spatially coinciding with the major structural features in the area namely the gavligarh tapti barwani sukta and narmada south faults the model for the shallow section has brought out a moderately resistive layer 30 150 m representing the exposed deccan trap layer overlying a conductive layer 10 30 m inferred to be the subtrappean gondwana sediments the latter resting on a high resistive basement upper crust the deccan trap thickness varies from around a few hundred meters to as much as 1 5 km along the traverse a subtrappean sedimentary basin like feature is delineated in the northern half of the traverse where a sudden thickening of subtrappean sediments amounting to as much as 2 km is noticed the high resistive upper crust is relatively thick towards the southern end and tends to become thinner towards the middle and northern part of the traverse the lower crustal segment is conductive over a major part of the profile considering the generally enhanced heat flow values in the nsl region coupled with characteristic gravity highs and enhanced seismic velocities coinciding with the mid to lower crustal conductors delineated from mt presence of zones of high density mafic bodies intrusives with fluids presumably associated with magmatic underplating of the crust in the zone of major tectonic faults in nsl region are inferred 2004 elsevier b v all rights reserved

evidence from borehole samples for the role of accessory minerals in lower crustal conductivity the high electrical conductivity of the lower continental crust as inferred from geophysical measurements1 has defied a simple explanation both pore saturating brines2 and interconnected carbon films3 5 have been proposed as conducting pathways but their relative importance remains uncertain and may vary from place to place6 so far attempts to address this question in the laboratory have had to rely on samples of metamorphic rock collected from surface exposures7 10 although these rocks almost certainly originated in the lower crust chemical alteration during their transport to and residence at the surface is likely to have affected their conducting properties here we report conductivity data for pristine crustal rocks recovered from depths of up to 4 6 km in the ktb boreholes in southern germany our results show that the electrical conductivity of accessory phases such as fe ti oxides and sulphides can enhance or even surpass the high conductivity produced by saline fluids

magnetotellurics study to identify subsurface resistivity structure in the eastern part of kachchh little rann area of gujarat india a magnetotelluric mt survey has been conducted in the eastern part of the kachchh region of gujarat state crossing little rann starting from adesar village in the north to enjar village in the south where limited geophysical data is available the profile has been acquired in the n s direction having a total length of 52 km with the interstation spacing of 3 4 km from two dimensional 2d inversion of the mt data four conductive zones are identified the analysis reveals that the depth of the basement varies from 1 2 km in the south to 1 8 km in the north the first conductive zone is found at a distance of 3 km in the south of adesar and is identified at the location of the kanmer fault kf the eastern extension of south wagad fault swf and might be the kf the second conductive zone is found at a distance of 42 km in the south of adesar and 13 km north of enjar village and is located at the contact zone of kachchh and saurashtra peninsula at the location of the north kathiawar fault nkf it is inferred as the nkf the third conductive zone at a distance of 27 km in the south of adesar is found between the first and second conductor this is interpreted as a step fault formed during the rifting process of kachchh the lower crust below 20 km is found conductive 120 m and might indicate the presence of fluid filled mafic ultramafic rocks in the lower crust

remote impacts of the 18 6 year period modulation of localized tidal mixing in the north pacific 1 the impact of the 18 6 year period modulation of the localized strong tidal mixing on the annual mean sea surface temperature sst in the open north pacific ocean is analyzed in a numerical experiment using an ocean general circulation model oscillatory sst anomalies induced in this model move eastward along the boundary between the subarctic and the subtropical gyres from the western boundary to the central pacific in about 10 years with a speed slower than that of the surface currents anomalous currents in the upper 300 m play an important role in inducing sst anomalies these anomalous currents are coupled to subsurface density anomalies and could be related to eastward moving low mode second or higher baroclinic long rossby waves these results suggest that the 18 6 year modulation of localized strong tidal mixing can influence large scale sst and or heat flux and thus climate as a result of remote impacts associated with changes in the mid depth density structure induced locally where mixing is strong that move horizontally into the open ocean

geoelectric structure of the great slave lake shear zone in northwest alberta implications for structure and tectonic history the study of ancient plate boundaries can provide insights into the past and present day tectonic processes here we describe a magnetotelluric mt study of the precambrian basement of the hay river fault hrf in northwest alberta which is the southwest segment of the great slave lake shear zone new broadband mt data were collected to give a clearer image of the crustal structure the western canada sedimentary basin was imaged as a low resistivity layer above the resistive crystalline basement four basement conductors were defined and correlate with the terrane boundaries delineated with aeromagnetic data these are 1 a major conductor in the kiskatinaw domain 2 a conductor on the boundary of the ksituan and chinchaga domains 3 a conductor on the boundary of the chinchaga and buffalo head domains and 4 a conductor near the hay river fault both 1 and 2 correspond to areas of high seismic reflectivity the low resistivity can be explained by interconnected grain boundary graphite or s

magnetotelluric soundings structure and fluids in the southern canadian cordillera the cordillera of western canada lies in a region of oceanic and island arc lithosphere accreted to north america during subductions of the last 200 ma magnetometer arrays have shown the crust of the region to be highly conductive magnetotelluric mt soundings across the intermontane and omineca tectonic belts between 50 n and 54 n reveal structure in terms of electrical resistivity pseudosections of phase and apparent resistivity and preliminary resistivity depth sections are shown for three transects the resistivity range is from less than one ohm metre to several thousands of ohm metres in old continental shields crustal resistivities cover a similar four decade range transposed up two decades i e 102 106 m we show that the observed resistivities can be produced by water with nacl and or co2 in solution at the high temperatures of the cordilleran crust in fractured rock of effective porosity 4 5 the resistivity variations may represent varying fracture densities by following structu

fluid deposited graphitic inclusions in quartz comparison between ktb german continental deep drilling core samples and artificially reequilibrated natural inclusions we used raman microsampling spectroscopy rms to determine the degree of crystallinity of minute 2 15 m graphite inclusions in quartz in two sets of samples experimentally reequilibrated fluid inclusions in a natural quartz grain and biotite bearing paragneisses from the ktb deep drillhole in se germany our sequential reequilibration experiments at 725 c on initially pure co2 inclusions in a quartz wafer and the j krautheim 1993experiments at 900 1100 c on organic compounds heated in gold or platinum capsules suggest that at a given temperature 1 fluid deposited graphite will have a lower crystallinity than metamorphosed organic matter and 2 that the crystallinity of fluid deposited graphite is affected by the composition of the fluid from which it was deposited we determined that the precipitation of more crystalline graphite is favored by lower f h2 higher f o2 and that the crystallinity of graphite is established by the conditions including gas fugacities that pertain as the fluid first reaches graphite saturation graphite inclusions within quartz grains in the ktb rocks show a wide range in crystallinity index reflecting three episodes of carbon entrapment under different metamorphic conditions isolated graphite inclusions have the spectral properties of totally ordered completely crystalline graphite such crystallinity suggests that the graphite was incorporated from the surrounding metasedimentary rocks which underwent metamorphism at upper amphibolite facies conditions much of the fluid deposited graphite in fluid inclusions however shows some spectral disorder the properties of that graphite resemble those of experimental precipitates at temperatures in excess of 700 c and at elevated pressures suggesting that the inclusions represent precipitates from c o h fluids trapped under conditions near those of peak metamorphism at the ktb site in contrast graphite that is intimately associated with chlorite and other presumably low temperature silicates in inclusions is highly disordered and spectrally resembles kerogens this graphite probably was deposited during later greenschist facies retrograde metamorphism at about 400 500 c the degree of crystallinity of fluid deposited graphite is shown to be a much more complex function of temperature than is the crystallinity of metamorphic graphite to some extent experiments can provide temperature calibration of the crystallinity index however the difference in time scales between experimental runs and geologic processes makes it difficult to infer specific temperatures for naturally precipitated graphite

magnetotelluric array data analysis from north west fennoscandia abstract new magnetotelluric mt data in north west fennoscandia were acquired within the framework of the project magnetotellurics in the scandes masca the project focuses on the investigation of the crustal and upper mantle lithospheric structure in the transition zone from stable precambrian cratonic interior to passive continental margin beneath the caledonian orogen and the scandinavian mountains in western fennoscandia an array of 59 synchronous long period and 220 broad band mt sites was occupied in the summers of 2011 to 2013 we estimated mt transfer functions in the period range from 0 003 to 10 5 s the q function multi site multi frequency analysis and the phase tensor were used to estimate strike and dimensionality of mt data dimensionality and strike analyses indicate generally 2 d behaviour of the data with 3 d effects at some sites and period bands in this paper we present 2 d inversion of the data 3 d inversion models are shown in the parallel paper we choose to invert the determinant of the impedance tensor to mitigate 3 d effects in the data on our 2 d models seven crustal scale and four lithospheric scale 2 d models are presented the resistive regions are images of the archaean and proterozoic basement in the east and thin caledonian nappes in the west the middle and lower crust of the svecofennian province is conductive the southern end of the kittila greenstone belt is seen in the models as a strong upper to middle crustal conductor in the caledonides the highly conductive alum shales are observed along the caledonian thrust front the thickest lithosphere is in the palaeoproterozioc svecofennian domain not in the archaean the thickness of the lithosphere is around 200 km in the north and 300 km in the south west

comment on electrical conductivity of albite quartz water and albite water nacl systems and its implication to the high conductivity anomalies in the continental crust by guo x yoshino t shimojuku a in their otherwise excellent study of the electrical conductivity of mineral systems including albite and aqueous fluids in the continental crust guo et al 2015 note that when archie s law archie 1942 is fitted to the data for bulk conductivity as a function of fluid fraction the exponent values are negative for all measurements made between 500 k and 900 k they note that the negative values indicate that the bulk conductivity decreases with increasing fluid fraction and that this is opposite to the accepted behaviour e g glover 2015 guo et al 2015 attempt to explain the behaviour by noting that the melting point of the ab qtz h2o system varies with increasing water content however there is an underlying problem with their use of the simple archie s law archie s law is given by the equation

why is the electrical resistivity around the ktb hole so low the low resistivity anomaly close to the ktb borehole coincides with both a self potential and a static magnetic anomaly if this coincidence is not accidental it may yield information about the conditions required for the existence of low resistivity anomalies in the deeper crust a possible answer to the question in the title of this paper is that the oxygen fugacity in the crust around the ktb hole must be low enough to stabilize graphite on a grain boundary scale this could partially explain the extremely low resistivities of the graphitized cataclastic zones the formation of magnetic pyrrhotite from 250 to 4000 m deep and the large self potential anomaly the latter requires a large vertical gradient of the oxygen fugacity and a continuous graphitic conductor through this gradient zone

seismic reflectors conductivity water and stress in the continental crust the uppermost 10 15 km of the earth s continental crust differs in several geophysical properties from the lower crust the upper crust is electrically resistive seismically transparent contains nearly all intracontinental earthquake hypocentres and responds to stress elastically with brittle fracture the lower crust is electrically conductive contains many seismic reflectors is aseismic and shows ductile response to stress i show here that the characteristics of both regions can be explained if the entire crust contains saline water in separated cavities in the compressively stressed rocks of the upper crust but in the lower crust forming an interconnected film on the crystal surfaces the model may apply principally to technically active parts of the continental crust beneath shields the lower crust may be dry

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

exhumed lower crust in nw ireland and a model for crustal conductivity granulite facies gneisses within the ox mountains inlier nw ireland are interpreted as an exhumed segment of sub dalradian lower crust formed at 605 37 ma and later wedged upwards by crustal imbrication along the n dipping highland boundary fault clew bay terrane boundary a lower crustal provenance for the gneisses is suggested by evidence for eclogite decompression and for the later replacement of sillimanite by kyanite in response to slow cooling at depth granulite facies rocks are believed to be present throughout much of the lower continental crust granulites are electrically resistant in contrast to the lower crust which is usually electrically conducting this paradox may be resolved by inferring that the granulite facies rocks form dry kilometre scale lenses mega augen wrapped around by anastomosing retrograded brine soaked electrically conducting shear zones retrogressive hydration is believed a to seal the margins of the dry mega augen with expanded reaction products and b to concentrate the residual brine amphibolite facies rocks associated with the lower crustal shear zones above about 350 c would equilibrate with strong unsaturated brine and become electrically conducting in the upper crust similar rocks would concentrate the brine to the point of saturation whereupon halite would precipitate all water be consumed and the rock would become an insulator

graphite and quartz petrofabrics examples from the ediacaran black quartzites of the ossa morena zone sw iberia abstract we study with the electron back scattered diffraction ebsd technique the fabric of metamorphosed and ductilely deformed phtanites or graphitic cherts a common lithotype in ediacaran supracrustals of the west european cadomian orogen currently they are black quartzites with a planolinear tectonite structure microscopically they present a sub mm scale alternation of coarse and fine grained dynamically recrystallized quartz bands we attribute intracrystalline plasticity partitioning to variations in graphite inclusion concentration constraining quartz grain boundary mobility during dynamic recrystallization under non coaxial strain regimes and moderate to elevated temperatures 400 650 c lattice preferred orientations of quartz c and axes are geometrically related to the external reference frame provided by foliations and lineations we also identify the involvement of 0001 r m and m c quartz intracrystalline slip systems deformation modes operated simultaneously and under identical temperatures in interleaved parallel domains mm to cm thick in adjacent coarse and fine grained bands medium to high t plasticity is congruent with the syntectonic temperature gradients associated with the amphibolite facies metamorphism of the country rocks we also present the first specific study published so far on natural deformation graphite lattice preferred orientation graphite inclusions as well as those of mica exhibit mineral shape fabrics that suggest operation of 0001 slip however ebsd measurements also record fabrics suggestive of m slip in spite of a rather small volumetric proportion graphite spatial organization at increased shear strains facilitated ductile deformation if a graphite network is established in the rock it can potentially increase rock electrical conductivity thus accounting for mid and lower crust anomalous electric conductivity

partial melt or aqueous fluid in the mid crust of southern tibet constraints from indepth magnetotelluric data summary the indepth project has applied modern geophysical techniques to the study of the crustal structure and tectonic evolution of the tibetan plateau in the lhasa block seismic reflection surveys in 1994 detected a number of bright spots at 15 20 km depths that indicate zones of crustal fluids aqueous fluids or partial melt coincident magnetotelluric mt data collected in 1995 detected a major zone of high electrical conductivity at the same depth as the brightspots using constrained inversion the mt data require a minimum crustal conductance of 6000 s this abnormally high electrical conductance can be best explained by a layered model with fluids partial melt aqueous fluids or a combination of partial melt and aqueous fluids the non uniqueness of the mt method means that a wide range of melt fraction thickness combinations for the above models could all explain the 6000 s conductance to distinguish between these three models other geophysical and geological data are required reflection seismic data suggest that a high fluid content 15 per cent is present at the top of the layer the amplitude versus offset data suggest that the top of this layer may be aqueous fluids rather than partial melt passive seismic data imaged a 20 km thick layer of lower fluid content that is probably partial melt petrological studies suggest that concentrations of aqueous fluids above 0 1 per cent at mid crustal depth cannot be sustained taken together these data show that the high conductivity in southern tibet is most probably the result of a relatively thin layer of aqueous fluids 100 200 m overlying a thicker zone of partial melt 10 km

causes of high crustal conductivity beneath the iapetus suture zone in great britain summary two magnetic variation anomalies in southern scotland and northern england have been linked to the position of the iapetus suture previous magnetotelluric measurements along a 140 km profile that crosses both anomalies have been supplemented by new high quality broad band observations groom bailey decomposition of the impedance and hypothetical event analysis of the magnetic variation data agree in defining different strike directions for the two halves of the mt profile n50 e for the northern part over the southern uplands and n90 e for the southern part over the northumberland trough te and tm mode apparent resistivity and phase data at representative frequencies have been inverted for the two sub profiles separately using the smooth inverse of smith booker 1990 the uppermost layer has a low resistivity is variable in thickness and correlates well with the known position and thickness of carboniferous sedimentary rocks the second layer has a very high resistivity thousands of m and reaches the surface where the lower palaeozoic metamorphic rocks of the southern uplands crop out a relatively rapid transition to low resistivities occurs at depths of between 8 and 16 km the conducting layer appears to be quasi continuous but where the profile crosses the anomalies identified by magnetic variation measurements the conductance increases and the upper surface is shallower the spatial coverage of the magnetic variation data has enabled us to extrapolate conductive features away from the line of section and project the electrical image onto the nec vertical incidence seismic reflection profile in the north sea there is excellent agreement between a number of features in the acoustical and electrical images the shallowing of the low resistivity layer to form a narrow wedge like feature corresponds to the offshore position of the stublick fault while to the north of the fault the top of the layer coincides with a south dipping reflector thought to be a thrust however the zones of high conductance and high lower crustal reflectivity do not in general correlate the good conductor beneath he northumberland trough spans two zones which were differentiated on ihe nec profile in terms of their reflectivity the shape of the conductor s upper surface in the vicinity of the stublick fault agrees well with the model of chadwick holliday 1991 who proposed that the iapetus suture was a whole crustal shear with a gently dipping central ramp the coincidence between the present day low resistivity layer and a surface of weakness that was active 300 400 myr ago is much more readily explained in terms of mineralogy the presence of graphite than the presence of fluids there is also strong support for interpreting the northern conductor in the same way its upper surface is relatively flat and occupies the position predicted as the horizontal detachment surface over which wedges of the southern uplands rocks were thrust its northeastward extension is sampled by a group of carboniferous vents from which xenolith suites of crust and upper mantle origin have been obtained the middle to upper crust is believed to be represented by quartzo feldspathic gneisses that contain abundant graphite

origin of fe ti oxide ores in mafic intrusions evidence from the panzhihua intrusion sw china economicconcentrationsof fe ti oxidesoccuras massive conform able lenses or layers in the lower part of the panzhihua intrusion emeishan large igneous province sw china mineral chemistry textures and quilfequilibria indicate that oxides in rocks of theintrusion were subjected to extensive subsolidus re equilibration andexsolution the primary oxide reconstructed from compositions oftitanomagnetite in the oresand associated intergrowths isan alumi nous titanomagnetite usp

magnetotelluric evidence of lithospheric mantle thinning beneath the southern sierra nevada a wideband 0 01 1000 s magnetotelluric survey across the southern sierra nevada has identified zones of enhanced conductivity in the lower crust and upper mantle that underlie the resistive batholithic rocks at depths greater than 10 20 km the eastern zone underlies the highest topography of the range and extends eastward this eastern conductive zone extends to depths in excess of 100 km based on model sensitivities and lies well below the moho depth of 32 38 km from seismological studies therefore the enhanced conductivity in the mantle cannot be attributed to conventional explanations for conductive continental crust and is instead likely due to partial melt such an interpretation is consistent with gravitational seismological and geologic evidence estimates of the partial melt fraction range from 2 to 5 at depths of 40 70 km which are consistent with fractions of melt inclusions observed in xenoliths from the eastern sierra nevada this partial melt accounts for approximately one third of the density decrease required in the mantle for support of the present high elevations the other two thirds of the density decrease could be due to thermal expansion of the upper mantle or due to mineralogical changes the western conductive zone also straddles the moho and extends into the mantle beneath western foothills of the sierra nevada and the great valley sediments to the west however xenoliths from this region indicate high velocity crustal rocks to depths in excess of 60 km and we therefore attribute the enhanced conductivity to graphitic metasediments and or dehydration of metaserpentinite emplaced by downward return flow of the host rocks during intrusion of the sierran plutons

electrical conductivity temperatures and fluids in the lower crust we have compiled both laboratory and worldwide field data on electrical conductivity to help understand the physical implications of deep crustal electrical profiles regional heat flow was used to assign temperatures to each layer in regional electrical conductivity models we avoided those data where purely conductive heat flow suggested temperatures more than about 1000 sup 0 c substantially higher than solidus temperatures and outside the range of validity of heat flow models the resulting plots of log conductivity sigma versus 1 t demonstrate that even low conductivity layers lcl have conductivities several orders of magnitude higher than dry laboratory samples and that the data can be represented by straight line fits in addition tectonically active regions show systematically higher conductivities than do shield areas because volatiles are usually lost in laboratory measurements and their absence is a principal difference between laboratory and field conditions these materials probably account for the relatively higher conductivities of rocks in situ in the crust free water in amounts of 0 01 0 1 in fracture porosity could explain crustal conductivities other possibilities are graphite hydrated minerals in rare instances and sulfur in combination with other volatiles as most of the temperatures are less than 700 sup 0 c partial melting seems likely onlymore in regions of highest heat flow where the conductive temperature profiles are inappropriate another result is that at a given temperature crustal high conductivity layes hcl are more conductive by another order of magnitude and show more scatter than do lcl s less

resistivity structure of the central indian tectonic zone citz from multiple magnetotelluric mt profiles and tectonic implications the central indian tectonic zone citz is a major tectonic feature extending across the indian subcontinent it was formed in the paleoproterozoic when the bastar craton and the bundelkhand craton were sutured together this region is recognized in the geological record as a persistent zone of weakness with many tectonothermal events occurring over geologic time the weakness of this region may have caused the late cretaceous early tertiary deccan volcanism to have been localized in the citz the zone is still tectonically active as evidenced by sustained levels of seismic activity this paper presents the first systematic investigation of the resistivity structure of the citz using multiple magnetotelluric mt transects two dimensional 2d resistivity models were generated for five north south profiles that cross the citz and encompass an area of 60 000 km2 the models were based on the joint inversion of transverse electric te transverse magnetic tm and tipper hz data all the profiles showed a low resistive 10 80 m middle to lower crust beneath the citz with a crustal conductance of 300 800 s the presence of an interconnected fluid phase and or hydrous metallic minerals appears to be the most likely explanation for the elevated conductivity that is observed beneath the citz the presence of fluids is significant because it may indicate the cause of persistent weakness at crustal depths a northward dip of both the crustal conductive layer and coincident seismic reflections favor a northward polarity of the subduction process associated with the formation of the citz

magma underplating of continental crust experiments conducted over the past decade have shown that common mantle melting products have densities at moho pressures that exceed the density of average continental crust in a situation of perfect gravitational equilibrium they would not rise to or near the surface the exact behaviour reflects the coupling of crust to the mantle strong at early periods of magmatic cycle but weak at more evolved stages where crustal melting and degassing commences spreading of underplate magmas in rift zones above subduction zones or even hot spots may perturb the state of the crust over very large areas where dense magmas occur beneath normal 30 km continental crust they may crystallize to form gabbro peridotiteanorthosite complexes dense crustal components may founder with melting assimilation and degassing producing very high temperature fluids rich in h2o co2 co and s halogen species depending on compositions the process acts as density filter cleaning the crust and lowering its mean density this process must have been particularly efficient during the archean when komatiite magmas were common opportunities for magma contamination and mixing processes must abound in situations of large scale underplating when crust is overthickened underplate magmas may crystallize to products with eclogitic affinity with the possibility of their foundering in normal mantle

the plagioclase magma density paradox re examined and the crystallization of proterozoic anorthosites intermediate composition plagioclase an40 60 is typically less dense than the relatively evolved basaltic magmas from which it crystallizes and the crystallization of plagioclase produces a dens residual liquid thus plagioclase should have a tendency to float in these magmatic systems there is however little direct evidence for plagioclase flotation cumulates either in layered intrusions or in proterozoic anorthosite complexes the layered series of the poe mountain anorthosite southeast wyoming contains numerous anorthosite leucogabbro blocks that constrain density relations during differentiation all blocks are more mafic than their hosting anorthositic cumulates their plagioclase compositions are more calcic and each block is in strong sr isotopic disequilibrium with its host cumulate associated structures disrupted and deformed layering indicate that 1 a floor was present during crystallization and that plagioclase was accumulating and or crystallizing on the floor 2 compositional layering and plagioclase lamination formed directly at the magma crystal pile interface and 3 the upper portions of the crystal pile contained significant amounts of interstitial melt liquid densities are calculated for proposed high al olivine gabbroic parental magmas and fe enriched ferrodioritic and monzodioritic residual magmas of the anorthosites taking into account pressure oxygen fugacity p2o5 estimated volatile contents and variable temperatures of crystallization for all reasonable conditions calculated block densities are greater than those of the associated melt the liquid densities however are greater than those for an40 60 plagioclase which cannot have settled to the floor plagioclase must either have been carried to the floor in relatively dense packets of cooled liquid plus crystals or have crystallized in situ a sloping floor possibly produced by diapiric ascent of relatively light plagioclase rich cumulates is required to allow for draining and removal of the dense interstitial liquid produced in the crystal pile and may be a characteristic feature during the crystallization of many proterozoic anorthosites and layered intrusions

free carbon electrical conductivity in the earth s mantle laboratory measurements indicate that solid carbon either amorphous or as graphite enhances the electrical conductivity of rocks carbon in quantities of only a few ppm providing it forms an interconnected phase could give rise to electrical conductivities of the order seen in the high conductivity layers in the earth s mantle this possibility is consistent with carbon petrology in mantle rocks and puts the formation and migration of abiogenic carbon compounds into a broader context

the occurrence of graphite bearing fault rocks in the atotsugawa fault system japan origins and implications for fault creep abstract graphite in fault zones has received little attention even though it is a well known solid lubricant that could affect frictional properties of faults dramatically this paper reports the presence of abundant graphite in fault zones of the atotsugawa fault system central japan mesoscopic and microscopic observations of fault rocks revealed two processes of carbon enrichment in fault zones one is a pressure solution process or diffusive mass transfer in general which removes water soluble minerals such as quartz and carbonates from rocks resulting in the enrichment of insoluble minerals including carbon the other process is precipitation of graphite from a high temperature carbon rich fluid forming graphite filling fractures within cataclasitic fault zones the two processes have led to the concentration up to 12 wt of graphite in the atotsugawa fault zones compared to 0 to 3 wt of carbonaceous materials in the host rocks this concentration is high enough for graphite to affect frictional properties at wide range of slip rates the presence of graphite may provide an explanation for the low resistivity the patterns of microearthquakes and fault creep along the western part of the atotsugawa fault system graphite should receive more attention as a weakening and stabilizing agent of faults

electrical conductivity structure of north west fennoscandia from three dimensional inversion of magnetotelluric data abstract new magnetotelluric mt data in north west fennoscandia were acquired within the framework of the project magnetotellurics in the scandes masca the project focuses on the investigation of the crustal and upper mantle lithospheric structure in the transition zone from stable precambrian cratonic interior to passive continental margin beneath the caledonian orogen and the scandinavian mountains in western fennoscandia an array of 59 simultaneous long period and 220 broad band mt sites were occupied in the summers of 2011 to 2013 the 3 d inversion of the masca data was obtained using the modem 3 d code the full impedance and tipper data were used for the inversion the rocks of archaean and proterozoic basement towards east and the caledonian nappes towards west are modelled as resistive structures in the central and southern parts the whole crust is resistive and reflects the trans scandinavian igneous belt granitoids the middle to lower crust of the svecofennian province is conductive an uppermost crustal conductor is revealed in the skelleftea ore district the south end of the kittila greenstone belt is seen in the models as a strong upper to middle crustal conductor in the caledonides the highly conductive alum shales are observed along the caledonian thrust front a map of the crustal conductance for the north west fennoscandian shield is presented

electrical conductivity distribution of the lithosphere in the central fennoscandian shield abstract geoelectric information on the crustal structures of the central fennoscandian shield consists of magnetovariational mv and magnetotelluric mt data mv data are obtained from magnetometer arrays devised to map lateral variations of subsurface resistivity mt data are obtained from soundings yielding knowledge on vertical resistivity distribution in the form of cross sections the electromagnetic data base for fennoscandia includes data from about 350 magnetometer sites and about 600 magnetotelluric soundings the results show that the crustal resistivity of the archaean and palaeoproterozoic crust varies greatly both laterally and vertically the model resistivities range from several tens of thousands of m to below 1 m which are extreme values amongst those normally observed in crustal rocks several large elongated and inclined crustal conductors which mark fossil tectonic boundaries have been discovered in the shield these include 1 the lake ladoga outokumpu kainuu schist belt and oulu conductors that form a discontinuous conductive boundary between the domains of archaean and palaeoproterozoic crust 2 the southern finland conductor caused partly by nearly vertical several kilometres deep graphite bearing metasedimentary layers that were tilted and deeply buried in a svecofennian collision 3 the kokkola bothnian skelleftea storavan and oulu conductors representing a collisional belt in a northwards accreted palaeoproterozoic terrain and 4 the lapland granulite belt conductors imaging a sequence of sheared thrust sheets that developed as the granulites were upthrusted southwestwards onto the archaean craton zones of enhanced electrical conductivity delimit more resistive and homogeneous crustal blocks including the archaean kuhmo block in eastern finland and the svecofennian central finland granitoid complex cfgc highly resistive upper crust 30 000 m in the cfgc coincides with the granitoid intrusions groundwater which fills the fractures and pores of the granitoids is non conductive 100 m and is thus not able to enhance the bulk conductivity therefore the resistivity of the upper crust approaches that of dry rocks at depths of 10 to 14 km a decrease in resistivity by one order of magnitude coincides with a seismic low velocity vp zone that appears related to a porous fractured zone filled with saline water a lower crustal conductor beneath the cfgc at depths of 30 to 40 km has a thickness of 10 to 15 km and a resistivity of 20 to 100 m it is most probably caused by graphite that can have been precipitated from co2 rich fluids during the peak metamorphism of the svecofennian orogeny and the subsequent cooling period free fluids trapped in the lower crust appear less plausible magnetotelluric soundings in the resistive cfgc in the central fennoscandian shield have not provided indications of a well developed electrical asthenosphere within the uppermost 300 km

structure of the central scandinavian caledonides and the underlying precambrian basement new constraints from magnetotellurics summary we investigate the structure of the accretionary wedge of the caledonian orogen the underlying autochthonous parautochthonous carbonaceous alum shales and the precambrian basement we have conducted 60 broad band magnetotelluric soundings along a 180 km long profile in j amtland sweden across the eastern section of the central scandinavian caledonides dimensionality analysis and regional strike estimates indicate that the conductivity structure can be approximated by a 2 d model having a n40 e strike direction consistent with the dominant geological strike the determinant average of the impedance tensor together with the tipper transfer function from the best 34 sites were inverted by the rebocc 2 d inversion code an electrically highly conducting layer beneath the caledonides images alum shales the autochthonous cambrian carbon bearing black shales on top of the precambrian basement based on the comparison of electrical conductivity and seismic reflectivity models we suggest that the caledonian accretionary wedge thickens in a step wise manner from ca 1 to 5 6 km towards the west in the east the wedge is composed of the lower allochthon in the west the wedge reaches the thickness of 15 km and is composed of the lower allochthon at the bottom the middle upper allochthons at the top and resistive allochthonous basement slices the upper crust of the autochthonous precambrian basement is homogeneous and resistive from surface

stress strain changes and triggered seismicity at the geysers california the principal results of this study of remotely triggered seismicity in the geysers geothermal field are the demonstration that triggering initiation of earthquake failure depends on a critical strain threshold and that the threshold level increases with decreasing frequency or equivalently depends on strain rate this threshold function derives from 1 analyses of dynamic strains associated with surface waves of the triggering earthquakes 2 statistically measured aftershock zone dimensions and 3 analytic functional representations of strains associated with power production and tides the threshold is also consistent with triggering by static strain changes and implies that both static and dynamic strains may cause aftershocks the observation that triggered seismicity probably occurs in addition to background activity also provides an important constraint on the triggering process assuming the physical processes underlying earthquake nucleation to be the same gomberg this issue discusses seismicity triggered by the mw 7 3 landers earthquake its constraints on the variability of triggering thresholds with site and the implications of time delays between triggering and triggered earthquakes our results enable us to reject the hypothesis that dynamic strains simply nudge prestressed faults over a coulomb failure threshold sooner than they would have otherwise we interpret the rate dependent triggering threshold as evidence of several competing processes with different time constants the faster one s facilitating failure and the other s inhibiting it such competition is a common feature of theories of slip instability all these results not surprisingly imply that to understand earthquake triggering one must consider not only simple failure criteria requiring exceedence of some constant threshold but also the requirements for generating instabilities

effect of water on the electrical conductivity of lower crustal clinopyroxene 1 the electrical conductivity of lower crustal clinopyroxene was measured at 6 12 kbar 250 1000 c and ni nio buffer conditions the dependence of electrical conductivity on water content was studied using both natural and preannealed samples separated from a fresh xenolith granulite with water contents from 0 to 375 ppm an end loaded piston cylinder apparatus and a solarton 1260 impedance gain phase analyzer were used in the study over a frequency range of 0 01 106 hz to obtain the complex impedance spectra the results show that the influence of pressure is very weak relative to temperature and water content and that two distinct mechanisms with different activation enthalpies dominate electrical conduction under dry and wet conditions for the dry sample the activation enthalpy is 102 kj mol and the main charge carriers are small polarons i e hopping of electron holes between ferrous and ferric irons for wet samples the electrical conductivity is significantly enhanced with an activation enthalpy of 70 kj mol and the charge carriers are likely to be protons under hydrous conditions the activation enthalpies are nearly independent of water content and the conductivity is a function of water content with an exponent of 1 as a major constituent of granulites with 60 modal volume in some regions clinopyroxene containing minor amounts of water may contribute significantly to the high electrical conductivity in the lower crust especially under stable continental regions

partial melting below the magmatic arc in the central andes deduced from geoelectromagnetic field experiments and laboratory data abstract magnetotelluric and geomagnetic deep soudings in northern chile revealed a pronounced high conductivity zone hcz below the western cordillera which constitutes the present magmatic arc with active volcanism of the south american continental margin conductivities in the range of 1 s m are observed the anomalously high conductivities in a broad depth range from approximately 20 km to at least 60 km are interpreted in terms of partial melting other geophysical observations such as a zone of low seismic velocities lvz at similar depths high heat flow values 100 mw m2 and a pronounced negative anomaly in the residual gravity field are also considered impedance spectroscopic laboratory experiments up to and in the temperature range of partial melting were performed under controlled oxygen fugacities at sub solidus temperatures electrical behavior is described by defect electrons with an activation energy of 1 34 ev and a conductivity of 2 5 ms m at 900 c model calculations using a modified brick layer model mbl were compared with experimental observations a good agreement between calculations and experiments is achieved with an electrical resistivity of the melt phase of 7 s m at 1250 c assuming an activation energy of 1 ev the same mbl model is used to calculate melt proportions beneath the western cordillera between 14 and 27 vol of interconnected melt are necessary to explain the observed hcz the stability of the melt rich crust is explained by a dynamic melting crystallisation behavior during crustal anatexis and by magma filled dikes

the electric moho underneath eastern indian craton 1 moho boundary discovered about a century back is well established on the basis of seismological studies electromagnetic studies using very low frequencies however have not been able to establish this boundary due to the presence of highly conducting continental lower crust magnetotelluric lithospheric study over the eastern indian craton 3 3 gyr resolves the lower crust and upper mantle boundary due to the absence of highly conducting continental lower crust underneath the craton from the present study it can be speculated that a dynamics other than the plate tectonics possibly existed up to the mid archaean

dehydration reaction and isotope front transport induced by co2infiltration at nuliyam south india the field relations from a quarry at nuliyam south india illustrate dehydration of an amphibolite facies gneiss to granulite facies charnockite by co2 influx over a scale of 30 m both the calc silicate source of the fluids and the full extent of their penetration into the gneiss are preserved in a continuous section fluid flow is by a hydraulic fracture mechanism but is thought to be pervasive the sharp reaction front predicted by the continuum mechanical theory for advective fluid transport is not observed the front spreading is on too large a scale for either diffusive or dispersive control and is due to local kinetic disequilibrium between the fluid and rock although the divariant nature of the reaction may also have a limited effect the time integrated fluid flux varies from the instantaneous porosity at the fluid front to 20 vol adjacent to the calc silicate carbon isotope budgets suggest that decarbonation of the calc silicate by a rayleigh fractionation process provides a sufficient source for the co2 influxing into the gneiss graphite abundances vary from 0 01 to 0 1 by weight it is principally derived by precipitation from the fluid and may be modelled from phase equilibria carbon isotope fronts coincide with the reaction front on the scale of sampling although isotopic disequilibrium between graphite and inclusion co2 also implies local fluid rock disequilibrium

crustal conductors along the sveka profile in the fennoscandian baltic shield finland summary electrical properties of the crust in the central fennoscandian baltic shield are examined along the 435 km magnetotelluric profile sveka a 2 d geoelectric model shows large variations in the crustal resistivity with the model resistivities ranging from 0 1 to 35 000 m the main geoelectrical features of the crust and upper mantle are 1 a resistive archaean lower crust conductance 30 s 2 a conductive palaeoproterozoic lower crust beneath the central finland granitoid complex 170 900s free h2o rich fluids trapped into the lower crust seem less plausible causes for enhanced conductivity as long retention times are required and no possible sources seem to be available to produce fluids into the dry lower crust most likely causes for permanent enhancement of lower crustal conductivity include the conductive fill of ductile shear zones conductive graphite bearing lamellae conduction in mafic rocks perhaps via conduction in magnetite or conductive solid remnants of an ancient shear zone between two crustal blocks thin graphitic grain boundary films precipitated from co2 rich fluids also seem less plausible 3 a very resistive 10 000 35 000 m upper crust of the palaeoproterozoic central finland granitoid complex and the archaean domain 4 narrow and elongated conductors 0 1 100 m beneath the palaeoproterozoic kainuu ksb and tampere tsb schist belts are caused by graphite bearing metasedimentary rocks buried deep into the crust in the collisions of crustal terranes during the formation of the palaeoproterozoic svecofennian crust the 500 km long tsb conductor represents an internal svecofennian terrane boundary whereas the ksb conductor is a part of a discontinuous archaean proterozoic boundary conductor 5 resistive upper mantle there are no indications of an upper mantle conducting layer within the uppermost 300 km in the central part of the fennoscandian shield

metamorphism of reduced granulites in low co 2 vapour free environment pervasive flooding of co2 has been proposed as the cause of granulite facies metamorphism that is capable of producing many distinctive characteristics of the deep continental crust reduced water activity orthopyroxene bearing assemblages depletion of large ion lithophile lil elements and dehydration1 6 the calculations presented here of c o h fluid composition for conditions of granulite facies metamorphism show that oxygen fugacity f o2 estimates from many terranes are sufficiently low that the addition of co2 rich fluid causes graphite to precipitate for values of pressure p and temperature t common to granulites and with f o2 slightly below the quartz fayalite magnetite buffer qfm the addition of co2 sufficient to grow 10 vol orthopyroxene requires the precipitation of 1 5 vol graphite as 0 1 vol graphite is readily recognizable but is not reported in most low f o2 granulites these rocks have not been flooded by co2 and low f h2o is probably due to extraction of a magma or recrystallization of an already dry rock7 8 granulite terranes may thus result from a combination of these three processes and the dominance of any one cannot now be demonstrated on a regional basis

wetting properties of anorthite aggregates implications for fluid connectivity in continental lower crust 1 the connectivity of c h o fluids in anorthite aggregate was assessed by measurements of the dihedral angle at conditions of 800 1000 c and 0 8 1 2 gpa for duration of 1 21 days the longer duration experiments show reduction of the effect of crystal anisotropy on pore geometry especially fluid filled small pores 5 m in anorthite aggregate quickly approach to textural equilibrium controlled by minimization of surface energies in 3 days anorthite aggregates with co2 fluid show large dihedral angle over 120 in the anorthite aggregates with aqueous fluid the dihedral angles are over 60 at pressure of 1 0 gpa and decrease across critical angle of 60 with increasing pressure over the range of experimental conditions isopleths of dihedral angle show nearly zero to negative dp dt slope with increasing temperature the observed reduction in dihedral angle with increasing pressure and temperature may be attributed to increase in plagioclase solubility in aqueous fluid natural anorthite with small amounts of albite component shows smaller dihedral angle than pure anorthite at higher temperature conditions the anorthite content of the run products deviates progressively from that of starting material this suggests that preferential solubility of albite component relative to anorthite affects the equilibrium fluid distributions of plagioclase by aqueous fluid experimental results point to the likelihood that plagioclase rich lower continental crust will not allow intergranular fluid flow of c h o fluids even in the presence of high geothermal gradient

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

secondary graphitization in mantle derived rocks the assemblage magnesite graphite ch4 has been identified by laser raman microprobe spectroscopy in fluid inclusions in olivine grains in the kao kimberlite and the duluth complex troctolite in both cases the assemblage essentially eggler s emog oxygen fugacity buffer is believed to arise from secondary reactions rather than from primary igneous processes the known stability field in p t f o2 space of the emog buffer coupled with some other petrologic constraints imposed by the host rocks indicates in both cases that the carbon bearing assemblage was precipitated at low pressures and temperatures 2 kbar 550 c at oxygen fugacities within about 1 log f o2 unit below the fayalite magnetite quartz fmq buffer the laser raman microprobe provides compositional and structural data on the coexisting phases including carbon that further constrain the stability field of the natural assemblages

deep magnetotelluric signatures of the early neoproterozoic cariris velhos tectonic event within the transversal sub province of the borborema province ne brazil abstract the borborema structural province in northeast brazil consists of a complex set of crustal blocks of different ages origin and evolution with final amalgamation during the late neoproterozoic brasiliano orogeny in the central part of the province its transversal sub province is characterized by elongate ne sw terranes largely affected by the brasiliano metamorphic overprinting and by a suite of early neoproterozoic metavolcanic and metaplutonic rocks reflecting a distinct tectonic event cariris velhos magnetotelluric mt data were collected at 64 stations along four profiles across this central region of the province to assess its deep electrical resistivity structure dimensionality analysis shows that the subsurface is electrically dominated by 3d structure and 3d inversions of the full impedance tensor and vertical magnetic transfer function tipper were carried out data fit is relatively good with an acceptable distribution of the misfit between the sites and the dominant features in the final model are north dipping crustal conductors which can be tentatively correlated between the different mt profiles and cross contrasting terranes at the surface of the province considering the geometry and distribution of the conductors it is proposed that they might reflect remnants of a north directed subduction zone with enhanced conductivity associated with graphitized biogenic material in metasedimentary rocks originally deposited in a restricted oceanic environment and subsequently metamorphosed and introduced into the deep crust by the subduction of the oceanic lithosphere constrained by isolated outcrops of early neoproterozoic rocks with oceanic affinity the proposed subduction event may have taken place during the cariris velhos orogeny with the crustal remobilization by the brasiliano orogeny being restricted to the brittle upper crust this interpretation gives support for an accretionary model to the neoproterozoic geodynamic evolution of the borborema province

lithospheric and upper mantle structures results of electromagnetic soundings in europe abstract information on electrical conductivity of the subsurface at a crustal scale is inferred mainly from magnetotelluric mt and magnetometer data the latter include both magnetometer array studies mv and geomagnetic depth soundings gds locally controlled source data e g data from d c and very low frequency vlf resistivity surveys frequency soundings and airborne electromagnetic mappings provide information on near surface structures e g on exposures of deeper conductors the variety of electromagnetic methods provides an efficient zooming ability for structures from a regional scale of hundreds of kilometres to small local details of some metres thus both the mapping of large scale crustal conductivity structures and a detail study of the anomalous structures are possible the electrical conductivity of the crust and upper mantle is described within the plate tectonic framework examples of structures which become electrically conductive at different phases of the process are shown using the results of electromagnetic soundings in various tectonic environments in europe it is shown how the crustal conductors can be interpreted as ancient and modern tectonic markers these include structures from the fennoscandian shield precambrian extensional basins and collisional zones the carpathians scotland and ireland suture zones and the pannonian basin extensional basin and thrust zones as well as conductors detected along the european geotraverse e g the terrain boundaries and the decollement of the variscan thrusting an example which considers the lower crustal conductivity from the fennoscandian shield shows rather large lateral variations in conductance of the lower crust varying from 1 s to several hundreds of siemens estimates of the depth of the asthenospheric conducting layer within the european territory also vary considerably shallowest depths are detected beneath extensional regions the pannonian basin and the rhinegraben 40 80 km whereas depths of 200 km or over or the absence of a conducting layer are detected beneath the alps and old precambrian cratons among the key areas where electromagnetic investigations should play an essential role are the deep structure of the east european platform boundaries of archaean and proterozoic terrains the structure of the teisseyre tornquist zone the nature of the present day continental lower crust and the characteristics of the electrical asthenosphere beneath europe

field petrophysical and carbon isotope studies on the lapland granulite belt implications for deep continental crust the palaeoproterozoic lapland granulite belt is a seismically reflective and electrically conductive sequence of deep crustal 6 9 kbar rocks in the northern fennoscandian shield it is composed of garnet sillimanite gneisses khondalites and pyroxene granulites enderbites which in certain thrust sheets form about 500 m thick interlayers the structure was formed by the intrusion of intermediate to basic magmas into turbiditic sedimentary rocks under granulite facies metamorphism accompanied by shearing of the deep crust about 1 93 1 90 gyr ago gal granulites were upthrust 1 90 1 87 ga and the belt was divided by crustal scale duplexing into four structural units whose layered structure was preserved the thrust structures are recognized by the repetition of lithological ensembles and by discordant structural patterns well distinguishable in airborne magnetic and electromagnetic data thrusting gave rise to clockwise pressure temperature evolution of the belt however some basic rocks possibly record an isobaric cooling path the low bulk resistivity of the belt 200 1000 m is caused by interconnected graphite and subordinate sulphides in shear zones on the basis of carbon isotope ratios this graphite is derived mostly from sedimentary organic carbon the seismic reflectivity of the belt may be caused by velocity and density differences between pyroxene granulites and khondalites as well as by shear zones

direct current resistivity and lotem model for the deep structure of the northern edge of the kaapvaal craton south africa abstract the transition from the high grade metamorphic southern marginal zone smz of the limpopo belt to the low grade kaapvaal craton was studied by means of geoelectrical techniques the d c data showed conclusively that the high grade rocks were thrust over the low grade rocks this contradicts earlier geological ideas but agrees with recent seismic reflection data the area where high grade rocks occur above low grade strata was mapped out and the southern boundary of the smz was established the conductor that occurs at lower crustal depths in the kaapvaal craton was traced into the smz using the lotem method the conductor shallows from 30 to 22 km over 12 km on the surface but does not reflect the difference in metamorphic depth of 30 km seen at the surface this indicates that the conductor is a transient feature of the crust the conductor is not associated with a seismically reflective crust the cause of the conductor remains enigmatic