influence of grain fabric and lamination on the anisotropy of hydraulic conductivity in unconsolidated dune sands in local scale groundwater flow problems like for instance in contaminated groundwater studies it becomes increasingly important for proper prediction of contaminant migration to incorporate small scale geological and associated parameter heterogeneity in groundwater flow models of the micro scale geological heterogeneities the effect of grain fabric and lamination in unconsolidated sediments on the vertical anisotropy of effective hydraulic conductivity has received relatively little attention this is mainly due to the low conductivity contrast between laminae in combination with the friable nature of unconsolidated sediment which makes it sensitive to disturbances during experimental procedures as an alternative digital image analysis of sediment thin sections was applied to separate and quantify both causes of anisotropy the effective value of vertical anisotropy ratio of unconsolidated eolian dune sand as determined experimentally on distributed orientated core plugs ranges between 1 10 and 1 50 this wide range is partly caused by artificial noise introduced by distributed sampling in a heterogeneous medium and assuming thereby that the sampled material is homogeneous by applying digital image analysis of thin sections high resolution logs of hydraulic conductivity were generated simple upscaling of these values to effective conductivity parallel and perpendicular to sedimentary laminae provided quantitative estimates of the vertical anisotropy caused by lamination anisotropy values range between 1 0 and 1 075 the anisotropy ratio due to grain fabric ranges in value between 1 05 and 1 45 assuming that both anisotropy due to grain fabric and lamination are orientated identically if this is not the case the effective anisotropy is the weighted mean of anisotropies of the constituent layer types this study shows that for eolian sands the anisotropy owing to grain fabric dominates the effective value of anisotropy if conductivity contrast between laminae increases the contribution of anisotropy caused by grain fabric to the effective value of anisotropy decreases

numerical modelling of deep dipole dipole mhd sounding abstract distortion effects of surface geoelectrical inhomogeneities on deep mhd soundings must be taken into account in the interpretation of the measurements computer programs which calculate these effects for the steady state part of the mhd pulses using the thin sheet approximation and which present the results graphically are used to study the distribution of apparent resistivity for various kinds of models in which the mhd generator is located off the coast of the kola peninsula

seismic and geoelectric evidence for collisional and extensional events in the fennoscandian shield implications for precambrian crustal evolution extensive deep seismic and electromagnetic sounding programs have resulted in tens of deep refraction seismic profiles normal incidence reflection profiles several hundred magnetotelluric soundings and several magnetovariational array observations results are compiled into four maps 1 moho depth 2 thickness of the lower crustal high velocity layer vp 7 km s 3 depth to the upper interface of the high velocity layer and 4 distribution of upper and middle crustal electrical conductors the moho depth is approximately 40 km in the archaean domain and varies between 40 and 65 km in the svecofennian domain most of the thickness variation 0 24 km is concentrated in the lower crustal high velocity layer electrically the crust is formed of several rather homogeneous either resistive or conductive blocks that are bounded by upper and middle crustal conductive zones the cratonized archaean crust experienced an extensional phase during which a thin mafic lower crustal high velocity layer was produced in association with mafic dykes 2 2 2 1 ga and well conducting volcanogenic and metasedimentary belts 2 5 2 0 ga the moho of the archaean domain was consequently reformed in the early proterozoic major thickening of the crust 50 65 km occurred when a svecofennian island arc and the archaean craton collided the svecofennian orogeny 1 76 1 9 ga resulted in a collage of metasedimentary rocks squeezed between crustal blocks of the island arc the internal terrane boundaries are seen as good reflectors and or inclined or vertical conductors that extend at least to the middle crust the thick high velocity 7 km s lower crust is a combination of mafic lower crust of the island arc crustal blocks and mafic additions from either delamination of the lithosphere after the collision or from a period of late svecofennian subduction the collisional boundary is also preserved as a discontinuous conductive zone between the archaean and svecofennian domains the thickened crust was subsequently thinned to 42 km along e w striking zones during an extensional period characterized by the intrusion of anorogenic rapakivi granitoid batholiths coeval mafic dykes and gabbro anorthosite bodies 1 6 1 5 ga the crust of the central part of the shield has since remained virtually intact

vertical averages of rheology of the continental lithosphere relation to thin sheet parameters abstract we calculate vertically averaged rheologies for continental lithosphere subject to biaxial states of stress corresponding to extensional compressional and strike slip regimes we assume that brittle deformation occurs by byerlee s law and that ductile deformation occurs by steady state creep mechanisms under these conditions the vertically averaged rheology may be approximated by a single power law over the range of strain rates from 10 13 to 10 17 s 1 this is true not only when either brittle or ductile behavior dominates the strength of the lithospere but also when the contributions of brittle and ductile behavior are comparable the value of the power law exponent n and the strength of the lithosphere depend on the depth and stress difference at the brittle ductile transition the temperature at the moho and the geothermal gradient in the range of vertically integrated stress between 1012 and 1013 n m 1n reaches a minimum of between 5 and 10 under other circumstancesn increases rapidly as brittle behavior dominates over ductile behavior the strength of the lithosphere depends most strongly on the thermal state of the upper mantle but also is influenced by the stress supported in brittle portions of the lithosphere

the exponential flow law applied to necking and folding of a ductile layer summary the uniaxial exponential law has been applied to experimental results for steady state creep including that of wet and dry olivine pyroxenite and carbonates at a deviatoric stress greater than 100 mpa such stress levels likely occur in the upper mantle lithosphere and middle crust in a layered rock in layer parallel extension or shortening high deviatoric stress can occur in the stiffest layers for example a dolomite layer in fine grained marble under assumptions of isotropy and incompressibility the uniaxial law yields where j2 sijsij 2 is an invariant of the deviatoric stress tensor sij linearization about a homogeneous basic state of flow yields equations identical in form to those obtained for the familiar power law this establishes expressions for an effective viscosity and stress exponent where is an invariant of the basic state deformation rate these results allow application of existing analytical folding and necking solutions for a rock layer of power law fluid to a rock layer with an exponential flow law the effective stress exponent for the exponential flow law increases with decreasing temperature through the dependence of c on the latter and weakly with increasing deformation rate for dry olivine effective stress exponents are between 10 and 30 for temperatures between 400 and 600 c with little dependence on deformation rate finite element simulations employing full non linear forms of the flow laws show that large strain necking is nearly identical for power law and exponential flow laws the results suggest that the instability in necking and folding in ductile rock layers can be considerably stronger than inferred from results based on flow laws representing diffusion and dislocation creep the large values of the effective stress exponent ne 15 that may be attained for exponential flow laws can account for observed outcrop scale ductile necking

numerical modelling of spontaneous slab breakoff dynamics during continental collision slab detachment or breakoff is directly associated with phenomena like morphological orogenesis occurrenceofearthquakesandmagmatism atdepththedetachmentprocessisslowand characterized by viscous rheolgy whereas closer to the surface the process is relatively fast and plastic using a 2d mantle model 1500 km deep and 4000 km wide we investigated with finite difference and marker in cell numerical techniques the impact of slab age convergence rate and phase transitions on the viscous mode of slab detachment in contrast to previous studies exploring simplifiedbreakoffmodelsinwhichtheblockageresponsibleforinducingbreakoffiskinematically prescribed we constructed a fully dynamic coupled petrological thermomechanical model of viscousslabbreakoff inthismodel forcedsubductionofa700 km longoceanicplatewasfollowed bycollisionoftwocontinentalplatesandspontaneousslabblockingresultingfromthebuoyancyof thecontinentalcrustonceithadbeensubductedtoadepthof100 124 km typically fivephasesof model development can be distinguished a oceanic slab subduction and bending b continental collision initiation followed by the spontaneous slab blocking thermal relaxation and unbending in experiments with old oceanic plates in this phase slab roll back occurs c slab stretching and necking d slab breakoff and accelerated sinking and e post breakoff relaxation our experiments confirm a correlation between slab age and the time of spontaneous viscous breakoff as previously identified in simplified breakoff models the results also demonstrate a non linear dependence of the duration of the breakoff event on slab age a positive correlation being characteristic of young 50 ma slabs while for older slabs the correlation is negative the increasing duration of the breakoff with slab age in young slabs is attributed to the slab thermal thickness which increases both the slab thermal relaxation time and duration of the neckingprocess inolderslabsthistendencyiscounteractedbynegativeslabbuoyancy whichgen eratehigherstressesthatfacilitateslabneckingandbreakoff apredictionfromourbreakoffmodels isthattheolivine wadsleyitetransitionplaysanimportantroleinlocalizingviscousslabbreakoffat depths of 410 510 km due to the buoyancy effects of the transition

multiple episodes of continental subduction during india asia convergence insight from seismic tomography and tectonic reconstruction abstract high wavespeed tomographic anomalies shallower than 1100 km beneath the india asia collision zone are interpreted as continental slabs subducted during collision combining anomaly positions with paleogeographic reconstructions of india we constrain the spatio temporal evolution of multiple episodes of continental subduction likely related to these anomalies this study highlights the different evolution at lithospheric scales of the western and eastern parts of the collision zone the evolution of the western part is characterized by two episodes of steep subduction of the northern margin of india the first episode involving an area with a lateral extent as large as 1500 km started at about 40 30 and ended by a slab break off process at 15 ma the second episode consists on subduction beneath the hindu kush mountains since 8 ma to the east of the collision zone no anomaly related to steep subduction along the northern edge of india is found we interpret two tomographic anomalies beneath the eastern border of the indian plate beneath burma and beneath the andaman sea as the result of two successive episodes of southeastward extrusion followed by subduction we suggest that both extruded portions were initially located along the northern margin of india and that they slid around the eastern syntaxis then southward along the eastern boundary of indian plate both portions subducted along the eastern border of india south of the eastern syntaxis we provide a rough estimate of the amount of indian lithosphere consumed during these subduction and extrusion episodes by comparing this amount with the total amount of indian lithosphere at the onset of collision we conclude that these processes accommodated most of the india asia convergence during collision

continental collision and slab break off a comparison of 3 d numerical models with observations abstract conditions and dynamics of subduction collision and subsequent 3 d slab break off and slab tear propagation are quantified for the first time using fully dynamic numerical models model results indicate that collision after the subduction of old strong subducting oceanic slab leads to slab break off at 20 25 myr after the onset of continental collision and subsequently a slab tear migrates more or less horizontally through the slab with a propagation speed of 100 150 mm yr in contrast young weak oceanic slabs show the first break off already 10 myr after continental collision and can experience tear migration rates up to 800 mm yr slab strength plays a more important role in the timing of slab break off and the speed of a propagating slab tear than negative slab buoyancy does slab break off is viable even for slabs that are supported by the viscosity jump and phase change between the upper and lower mantle the density of the oceanic slab and the subducting continental block is important for the amount of continental subduction and the depth of slab break off a 40 km thick continental crust can be buried to depths greater than 200 km although this maximum depth is significantly less for younger or very weak slabs or thicker continental crust slab break off typically starts at a depth of 300 km mostly independent of mantle rheology but like continental crustal burial can be shallower for young or buoyant plates our 3 d models illustrate how due to the difference in necking in 2 d and 3 d break off has an intrinsic small preference to start as a slab window within the slab s interior rather than as a slab tear at the slab edge however any significant asymmetry in the collision setting e g earlier collision at one end of the subduction zone would override this and leads to slab tearing starting near one edge of the slab these results put important new constraints on the dynamics of the collision and subsequent slab break off for modern collision belts for a proposed timing of the initial arabia eurasia collision at 35 ma break off of the 200 myr old neo tethys slab is unlikely to have occurred before 15 10 ma furthermore our results illustrate that shallow early break off of weak slabs provides a viable explanation for the absence of blueschists and ultra high pressure metamorphism in the precambrian geological record

source field problems in northern parts of the baltic shield abstract theoretical calculations using a stationary ionospheric electrojet model are presented for a resistivity model of interest in studies of the baltic shield the earth model includes the undistorted east european platform resistivity versus depth model and crustal and asthenospheric conducting layers the results show that close to the source region for periods 15 min the magnetic field depends only weakly on the earth s conductivity accordingly the resolution of apparent resistivity curves obtained by the horizontal spatial gradient hsg technique becomes poor at longer periods close to the source region a comparison with experimental hsg apparent resistivity from kiruna and kevo in northern scandinavia supports the existence of high conductivity in the lower crust the sharp decrease of apparent resistivity at periods 1 h may within the limits of experimental error be a result of the non uniform source rather than an indication of a good conductor in the upper mantle

slab breakoff a model for syncollisional magmatism and tectonics in the alps slab breakoff is the buoyancy driven detachment of subducted oceanic lithosphere from the light continental lithosphere that follows it during continental collision in a recent paper davies and von blanckenburg 1994 have assessed the physical conditions leading to breakoff by quantitative thermomechanical modeling and have predicted various consequences in the evolution of mountain belts breakoff will lead to heating of the overriding lithospheric mantle by upwelling asthenosphere melting of its enriched layers and thus to bimodal magmatism breakoff will also lead to thermal weakening of the subducted crustal lithosphere thereby allowing buoyant rise of released crustal slices from mantle depths in this paper we present a test of this model in the tertiary evolution of the european alps in the alps both basaltic and granitoid magmatism occur between 42 and 25 ma following the closure of oceanic basins by subduction and continental collision the granitoids are now well established to result from mixing of basalt with assimilated continental crust to identify the tectonically crucial origin of the partial mantle melts we have compiled all published geochemical and isotopic data of numerous mafic dykes occurring throughout the whole alpine arc their trace element and isotopic composition suggests that they have been formed by low degree melting of the mechanically stable lithospheric mantle we see no evidence for melting of asthenospheric mantle it was thus not decompressed to depths shallower than 50 km once initiated rapid lateral migration of slab breakoff will result in a linear trace of magmatism in locally thermal weakened crust this explains why all alpine magmatic rocks intruded almost synchronously along a strike slip fault the periadriatic lineament a compilation of ages from penninic high pressure rocks subducted to depths of up to 100 km shows that subduction took place at circa 55 40 ma followed by uplift at 40 35 ma from the short time interval between their uplift and the onset of magmatism we infer that both processes have been induced by the breakoff the slab breakoff model fulfills its predictions in the case of the alps and therefore supports the assumptions made in the theoretical model on a geological basis we believe that the characteristic association of magmatic activity with the return of high pressure rocks to the surface allows the identification of this process in the earth s mountain belts

dynamics and implications of slab detachment due to ridge trench collision 1 the approach of a buoyant spreading ridge to a subduction zone may lead to detachment of a subducted slab previous work has called upon the detachment process as an explanation for observed ridge abandonment and slab window related magmatism e g in baja california but such a scenario has not been tested using fully dynamic numerical models we use dynamic two dimensional models including a non newtonian rheology to study the approach of a trench parallel spreading ridge to a subduction zone we find that before the ridge approaches within 100 km of the trench detachment of the subducted slab occurs due to the combined effects of increased buoyancy and reduced strength of the lithosphere with ridge proximity in models exploring effects of subducted slab length distance of the ridge from the trench shear zone strength and lithospheric yield strength we find the following 1 a decrease in subduction velocity as the ridge approaches the trench 2 a shrinking surface plate that maintains a uniform plate like subduction velocity 3 ridge abandonment distances 100 275 km from the trench and 4 slab gap distances 195 285 km from the trench these results are consistent with observations in baja california where detachment of the cocos slab may explain abandonment of observed segments of the east pacific rise 50 200 km outboard of the trench and the presence of nonarc related magmatism located 100 250 km inboard of the trench ridge subduction does not occur within any of these two dimensional cases of ridge trench collision indicating that 3 d geometric effects may be responsible for ridge subduction

description of the polar profile transect display abstract the polar profile transect display attempts to integrate all the existing aeromagnetic gravimetric geological and geochemical investigations with the results of the deep seismic and electromagnetic studies carried out along the polar profile our description is based on the stage of joint interpretation reached at the second earth science study centre the nearly 440 km long ne sw trending polar profile traverses archaean and early proterozoic parts of the baltic shield the thickness of the crust varies between 40 and 50 km and consists of three distinct crustal layers a geological interpretation based on the various geophysical results is given for the uppermost 10 14 km thick layer in the central part of the profile the overthrust early proterozoic lapland granulite belt forms a wedge shaped body reaching the middle crust the upper crust is underlain by a transparent layer 16 18 km thick with a smooth increase in velocity and density a reflective lowermost 10 21 km thick layer has a high velocity gradient the crust mantle has a pronounced topography

interpretation of gravity anomalies along the polar profile in the northern baltic shield abstract the polar profile in the northern baltic shield was analyzed by two and three dimensional quantitative gravity modelling interpretation of fourier amplitude spectra qualitative interpretation using bouguer anomalies and second vertical derivative and horizontal gradient maps and reference to petrophysical data densities of outcropping crustal units show several distinct modes ranging from 2600 to 2990 kg m3 in relation to the mean background density of 2690 kg m3 used in the gravity modelling the granitic crustal units 2600 kg m3 produce well defined gravity lows of up to 20 mgals and the granulites 2750 kg m3 and greenstones 2940 kg m3 cause the most pronounced gravity highs of 30 and 45 mgal the estimated thickness of the kittila greenstone belt is about 6 km in the cross section along the polar profile the base of the lapland granulite belt dips to the northeast and reaches a maximum thickness of about 16 km the thickness of the vainospaa granite body is approximately 6 km evidence of buried anomaly sources also exists our interpretation predicts units of increased density in the basement under the varanger peninsula sedimentary rocks the general pattern of a gravity high near the barents sea coast coupled with a gravity low seawards implies a thick layer of sedimentary rocks under the sea which may be compensated by crustal thinning according to the seismic refraction interpretation there is a depression in the moho under the southwestern edge of the lapland granulite belt explaining together with other features of the seismic model the regional background on which the gravity anomaly of the granulite belt is superposed according to the fourier analysis of gravity anomalies there are two dominant depth clusters of density discontinuities in the southwestern karelian province and middle parts lapland granulite belt of the polar profile the shallower discontinuity varies between 6 and 10 km and the deeper one rises from about 20 km in the southwest to about 10 km in the middle area in the northwestern part inari and sorvaranger terrains and varanger peninsula there are many separate depth values and clusters exist only at depths between 13 and 15 km

a back of the envelope approach to boudinage mechanics abstract smith has shown from linear hydrodynamic stability that pinch and swell boudinage is possible only in a non newtonian material by assuming uniaxial stress and strain and large viscosity contrast between the competent layer and the incompetent matrix smith s result can be derived in a very simple way the one dimensional force balance also predicts growth rates although not dominant wavelengths however we question the ability of even linear stability analysis to accurately predict boudin widths

viscous heating allows thrusting to overcome crustal scale buckling numerical investigation with application to the himalayan syntaxes article i nfo the eastern and western himalayan syntaxes are crustal scale antiforms whose metamorphic evolution is coeval in the india asia collisional history finite element modelling of lithospheric shortening and the resultingcrustal scale folds havepreviously been used todebate the structuralconditions for the development of such structures however numerical models could not account for the metamorphic evolution associated with folding in continuation of previous work we present two dimensional finite element modelling of lithospheric shortening inwhich thermal effects are implemented the new models are consistent with earlier interpretations that crustal folding is the plausible mechanism that shaped the two himalayan syntaxes they further lend support to the concept that lithospheric buckling is a basic response to large scale continental shorteningandan efficientmountainbuilding process introducingviscous shearheating shows however that the buckling mode shifts to a thrusting mode within few shortening depending on the initial thermal rheological structure of the deforming lithosphere the change to the thrusting mode prohibits further fold amplification and lateral fold propagation this mayexplainwhy in continental lithosphere crustal scale folds are isolated whereas regular periodic crustal fold trains are rare focusing deformation on through limbs thrust zones accompanies the establishment of inverted metamorphic gradients these results offer new working hypotheseson howlargethrusts like thehimalayamain centralthrustnucleate results further show that the thermal structure of the lithosphere strongly controls three fundamental deformation and metamorphic modes 1 a cold lithosphere mainly deforms by thermally activated thrusting and exhibits large areas with significant tectonic overpressures twice lithostatic 2 a warm lithosphere is essentially buckled and significant tectonic overpressure builds up in the upper crust and 3 a hot lithosphere tends to thicken homogenously and mainly records lithostatic pressures below the upper crust

the crustal structure along the polar profile from seismic refraction investigations abstract seismic refraction investigations along a 440 km long profije on the northern baltic shield have resolved the crustal structure in this area of archaean to early proterozoic lithosphere formation the profile called the polar profile extends approximately along a sw ne oriented line from the karelian province in northern finland across the lapland granulite belt and the kola peninsula province to the varanger peninsula in northeastern norway at six shotpoints large explosions 200 1680 kg and at three shotpoints small explosions 80 kg were detonated and recorded at an average station spacing of 2 km providing high quality record sections a two dimensional cross section of the crust was obtained by forward modelling using ray tracing techniques high velocity bodies are found in the upper crust related to the karasjok kittila greenstone belt and the lapland granulite belt they extend to a depth of 6 13 km in the karelian province in the southwest a low velocity zone was found between the depths of 8 and 14 km the middle crust shows a slight increase in the average velocities from the southwest to the northeast and a small velocity jump is found along a mid crustal boundary between 18 and 21 km the thickness of the middle crust varies between 16 and 18 km the lower crust and the crust mantle boundary moho show considerable lateral variation the top of the lower crust lies between 26 and 33 km while its thickness decreases from 21 km in the southwest to 10 14 km beneath the lapland granulite belt and the inari terrain reaching 20 km again in the extreme northeast the velocities also change laterally the thin lower crust is characterized by rather low velocities 6 8 6 9 km s whereas in the southwest and northeast the velocities 6 9 7 3 km s resemble more typical shield structures the moho is found at 47 km in the karelian province rises to 40 km beneath the lapland granulite belt and descends to 46 km in the northeastern part of the kola peninsula province the upper mantle velocities at the moho range from 8 1 km s in the region of the thin crust to 8 5 km s and more beneath the karelian province it is tempting to suggest that the anomalous lower crust underlying the lapland granulite belt and the inari terrain may represent the remnants of an early proterozoic back arc basin that was active prior to the 2 0 to 1 9 ga plate convergence event during which the lapland granulite belt was thrust onto the archaean basement of the karelian province another explanation is to assume that the velocity reduction in the anomalous lower crust was caused by a rather pronounced uplift of this region following the 1 9 ga collision event

heat flow and thermal structure of the lithosphere across the baltic shield and northern tornquist zone abstract terrestrial heat flow data from archean and proterozoic provinces of the baltic shield its southwestern margin and the tectonic transition to the younger tectonic units of central europe northern segment of the european geotraverse are compiled and examined for local and regional variations a 2 d numerical lithospheric temperature and heat flux model is presented covering 1800 km of the deep seismic fennolora profile across the baltic shield and 500 km of eugeno s northern eugemi seismic lines across the northern tornquist zone and the danish and north german basins thermal lithosphere defined as the outer layer with relatively low temperatures t c in which heat transfer is dominantly by conduction is found to vary markedly in thickness from 85 110 km beneath basins and tornquist zone to about 150 km in central shield areas and 200 250 km in areas of low heat flow along the northern shield profile these results are in very good agreement with seismological information from surface wave dispersion analysis and the refraction wide angle seismic interpretation of fennolora data the combined thermal and seismologic evidence indicates that the uppermost mantle region stratified in seismic velocity is restricted to the upper relatively cold thermal layer which is interpreted to constitute shield lithosphere local and to some extent regional variations in surface heat flow are found to be closely related to variations in near surface radiogenic heat production for the southern baltic shield a heat flow heat production relationship of q 0 32 8 7 6 h 0 is derived regional increase in heat flow from northern and central shield provinces to the shield margin and the tornquist zone is interpreted to be related partly to differences in upper crustal heat generation partly and in southernmost areas mainly to an increased heat flux from the upper mantle upper mantle heat flux is estimated at 20 25 mw m 2 along the northern and central shield transect increasing to 30 40 mw m 2 beneath the shield margin basins and tornquist zone lithospheric thickness variations in this region of northern europe are found to be closely associated with variations in heat flux from the uppermost mantle and perhaps to a large extent controlled by geodynamically determined lateral differences in upper mantle heat flux earthquake focal depth data and information on crustal strength show baltic shield seismicity to be limited generally to the upper 20 25 km brittle part of the crust at temperatures less than 300 400 c maximum source depth of magnetic anomalies at temperatures near 600 c is in the deepest part of the crust near the crust mantle boundary in the southern region about 25 km depth along the tornquist zone and about 35 40 km in southern shield areas and in the uppermost mantle 60 70 km depth in the northern shield areas some petrologic implications are discussed the present deep crust is supposed to be at lower temperature pressure conditions than at early stages of formation however also the present deepest parts of the shield crust 35 40 km are within the stability field of eclogites the general absence of significant gravity anomalies in areas of marked crustal thickness variations indicate high density eclogites to be present in significant amounts in the deep shield crust

slab detachment beneath eastern anatolia a possible cause for the formation of the north anatolian fault in this paper we discuss the possibility that the north anatolian fault naf results from the deep deformation of the slab beneath the bitlis hellenic subduction zone we described the tectonic evolution of the anatolia aegean area in three main steps before during and after the formation of the naf we remark that the tectonic conditions that are assumed to have triggered the formation of the naf i e collision to the east and extension to the west was already achieved before the onset of that strike slip fault system we also highlight that the formation of the naf was accompanied by the uplift of the turkish iranian plateau and by a surge of volcanism in the eastern anatolia collisional area and probably by the acceleration of the aegean trench retreat we show tomographic images from global p wave model of piromallo and morelli c piromallo a morelli p wave tomography of the mantle under the alpine mediterranean area j geophys res 108 2003 doi 10 1029 2002jb001757 showing that the slab beneath the bitlis collisional belt is not continuous and that its possible rupture pursues to the west at least up to cyprus and possibly up to the eastern end of the hellenic trench all these observations suggest that the plate tectonic re organization occurred in the late miocene early pliocene in the region results from slab break off in the bitlis area and from its lateral propagation to the west this idea is tested in analogue laboratory experiments which confirm that the break of the slab under the collisional belt may trigger 1 the acceleration of slab retreat to the west due to the increase in slab pull force 2 the indentation of the continent in the collisional area and 3 produce the conditions that permit the lateral escape of material towards the west and the formation of the naf

a simple analytical solution for slab detachment abstract an analytical solution is presented for the nonlinear dynamics of high amplitude necking in a free layer of power law fluid extended in layer parallel direction due to buoyancy stress the solution is one dimensional 1 d and contains three dimensionless parameters the thinning factor i e ratio of current to initial layer thickness the power law stress exponent n and the ratio of time to the characteristic deformation time of a viscous layer under buoyancy stress t tc tc is the ratio of the layer s effective viscosity to the applied buoyancy stress the value of tc n specifies the time for detachment i e the time it takes until the layer thickness has thinned to zero the first order accuracy of the 1 d solution is confirmed with 2 d finite element simulations of buoyancy driven necking in a layer of power law fluid embedded in a linear or power law viscous medium the analytical solution is accurate within a factor about 2 if the effective viscosity ratio between the layer and the medium is larger than about 100 and if the medium is a power law fluid the analytical solution is applied to slab detachment using dislocation creep laws for dry and wet olivine results show that one of the most important parameters controlling the dynamics of slab detachment is the strength of the slab which strongly depends on temperature and rheological parameters the fundamental conclusions concerning slab detachment resulting from both the analytical solution and from earlier published thermo mechanical numerical simulations agree well indicating the usefulness of the highly simplified analytical solution for better understanding slab detachment slab detachment resulting from viscous necking is a combination of inhomogeneous thinning due to varying buoyancy stress within the slab and a necking instability due to the power law viscous rheology n 1 application of the analytical solution to the hindu kush slab provides no order of magnitude argument against slab detachment and therefore supports existing studies suggesting a currently ongoing slab detachment in the hindu kush slab

reflection seismic measurements across the granulite belt of the polar profile in the northern baltic shield northern finland abstract reflection seismic measurements were carried out in 1985 on the central part of the polar refraction seismic profile in northern finland the survey was planned and executed jointly by the universities of helsinki uppsala and hamburg using digital equipment with a total of 144 channels by repeating the shots and moving the geophone spreads every day we were able to observe a 42 50 km recording spread referred to reflection elements a line of 84 km total length through the source receiver midpoints was recorded we processed the results up to normal moveout corrected time sections a number of reflectors dipping 8 15 to the northeast were recorded in the lapland granulite belt range these represent a system of sheared granulites which were observed on the surface in the southern part of the profile from gravity modelling the bottom of the dipping layers coincides with the lower boundary of the granulites between the depths of 22 and 35 km the crust in this area seems to be transparent to seismic signals this leads to the assumption that the middle part of the crust is characterized by gentle velocity and density gradients the crust mantle boundary seems to be a layered moho with good reflectors lying at depths between 40 and 44 km

modeling the evolution of continental subduction processes in the pamir hindu kush region several geological and geophysical studies suggest the presence of two converging subduction zones in the western syntaxis of the india eurasia collision zone with steep northward subduction of indian lithosphere beneath the hindu kush and southward subduction of asian lithosphere under the pamir we investigate the geometry and the timing of these subduction processes seismic tomography images are used to constrain the geometry of the two slabs under the pamir and the hindu kush by measuring the tomographically inferred length of the indian slab under the hindu kush region and by comparison with paleomagnetic reconstructions we estimate that the process of indian slab break off most likely occurred at the early stage of the collision at ca 44 48 ma we infer that after slab break off western india continued its northward motion until its northern boundary reached the present hindu kush region where it began to subduct we estimate an age for the initiation of subduction in this region of about 8 ma we apply 2d thermo kinematic and rheological numerical modeling to compute the temperature distribution and brittle field along two vertical sections across the hindu kush and pamir regions for the hindu kush we introduce parameters reproducing fast and near vertical subduction whereas in the pamir region we use the results from published studies to reproduce slower subduction modeling results indicate that faster subduction under the hindu kush results in a deeper brittle region which is consistent with observed deeper seismicity than in the pamir region

thermo mechanical controls on the mode of continental collision in the se carpathians romania abstract the carpathians orogenic system with its along arc variations in topography developed in the aftermath of continental collision is associated with unusual foredeep basins large scale strain and seismicity concentration and high velocity mantle bodies the east carpathians continental collision was non cylindrical leading to large scale variations in thrust nappe kinematics orogenic uplift patterns and foredeep subsidence controlled by the mechanics and geometry of the lower plate thermo mechanical modelling demonstrates that in this low rate convergence regime the subducted lithosphere had enough time to interact with the mantle to advance towards a thermal resettlement this is favored by the low degree of metamorphism mechanical weakness of the lower plate and the lack of active surface processes at the contact with and in the upper plate in contrast low buoyant thick lower crust and active surface processes keep the continuity of the slab intact and promote the development of typical foredeep basins the model explains in a self consistent manner the unusual geometry of the vrancea seismogenic slab in the bend zone of the romanian carpathians the model is also consistent with the presence of two high velocity bodies inferred from seismic tomography studies and explains the depth zonation of seismicity in the vrancea area differences between the northern part of east carpathians and the southeastern bend of the carpathians arc are largely controlled by lateral variations in crustal structure topography emplacement and surface processes along the arc mechanical heterogeneity of the carpathians subduction leads to the development of two end member modes of collision allowing a study of these states and their transition lithospheric configuration and tectonic topography appear to be prime factors controlling variations in slab behavior in the se carpathians at the terminal phase of continental convergence slab delamination roll back and depocenter migration appear to play a more limited role at shallow and lithospheric levels

slab detachment in laterally varying subduction zones 3 d numerical modeling understanding the three dimensional 3 d dynamics of subduction collision systems is a longstanding challenge in geodynamics we investigate the impact of slab detachment in collision systems that are subjected to along trench variations high resolution thermomechanical numerical models encompassing experimentally derived flow laws and a pseudo free surface are employed to unravel lithospheric and topographic evolutions first we consider coeval subduction of adjacent continental and oceanic lithospheres sco this configuration yields to two stage slab detachment during collision topographic buildup and extrusion variable along trench convergence rates and associated trench deformation the second setting considers a convergent margin which is laterally limited by a transform boundary stb such collisional system is affected by a single slab detachment little trench deformation and moderately confined upper plate topography the effect of initial thermal slab age on sco and stb models are explored similarities with natural analogs along the arabia eurasia collision are discussed

evolution of pinch and swell structures in a power law layer analytical and finite element fem solutions for the necking of a single power law layer up to large finite amplitude are obtained continuous necking of the layer produces pinch and swell structures the layer is either a free plate or embedded in a homogeneous medium an analytical solution for finite amplitude necking based on the assumption that plane sections remain plane psrp agrees well with the fem result for a layer power law stress exponent n 5 and for a ratio of layer to medium effective viscosities m 100 fem simulations for embedded layers verify that psrp for m 20 the presented numerical experiments generate localized necking and pinch and swell structures similar to natural ones for n 5 and m 20 additional weakening mechanisms such as strain softening although likely to be operative in nature are not required to generate natural pinch and swell structures fem experiments with random perturbation of the layer interfaces show that even with strong necking instability the layer is thinned at the swell as well as at the necks affecting strain estimation from pinch and swell geometry 2008 elsevier ltd all rights reserved

magnetotellurics in southwestern sweden evidence for electrical anisotropy in the lower crust magnetotelluric data from measurements conducted on the southwestern part of the baltic shield are analyzed with respect to crustal and upper mantle structures the interpretations have been done using ordinary two dimensional and transverse anisotropic layered models in which distortions of the impedance tensors due to near surface electrical structures are included in the interpretation procedures the anisotropic model is able to explain the gross features of the data whereas the two dimensional model only displays features observed at long periods the two main results from the study with anisotropic models are the interpretation of a possible transverse anisotropic lower crust and a depth to the uppermost conductive layer within the mantle of more than 200 km the minimum depth to the conductive mantle layer is found from a most squares analysis

on induction effects at eiscat and image magnetometer stations summary when ground based magnetometer data are used for ionospheric magnetospheric studies it is important to know the external ionospheric magnetospheric and internal telluric contributions to the observed total variation to study them we calculate geomagnetic induction vectors and apply the siebert kertz separation method using data from the eiscat and image magnetometer stations in fennoscandia induction vectors in the period range 20 2560s show the expected and dominating ocean effect near the arctic ocean at masi a small scale inland anomaly is detected in addition to the ocean effect at hankasalmi a man made anomaly due to the nearby stare radar is present the source effect on induction vectors at high latitudes is briefly investigated by separating the magnetic variations of several bay events at image stations into external and internal parts we study the importance of the large scale induction in the recordings depending very much on the particular event the internal horizontal part is 10 30 per cent of the total field in the auroral electrojet region and its relative contribution increases in more distant regions the internal contribution of the vertical field at inland stations is 10 40 per cent having an opposite sign to the total field or 10 30 per cent having the same sign as the total field depending on the location with respect to the ionospheric currents and to the ocean results are also discussed using a thin sheet model

three dimensionality of slab detachment due to ridge trench collision laterally simultaneous boudinage versus tear propagation the detachment of subducted tectonic plates is a process that has been increasingly associated with collisional scenarios and the end of subduction in various locations worldwide in particular the propagation of slab detachment tearing of a subducting plate has been described in conceptual models as a cause for spatially and temporally progressing surface effects such as slab gap volcanism and uplift however there is little understanding of the causes and dynamics associated with three dimensional 3 d slab tearing especially in the case of ridge trench collision here we show using fully dynamic 3 d numerical models that the process of detachment due to ridge trench collision depends on the geometry of the ridge segments approaching the trench for a finite laterally symmetric slab the 3 d detachment process occurs nearly simultaneously along strike by way of boudinage type necking and opening of holes central to the slab for a case involving the approach of two offset ridge segments to the trench slab tearing occurs in the form of 1 a vertical propagating separation along the age offset boundary within the slab that was previously weakened by a transform weak zone and 2 horizontal propagating detachment controlled by lateral transfer of slab pull to adjacent surface plate segments however lateral decoupling between offset adjacent plate segments and the propagating nature of the vertical and horizontal tearing are dependent upon fracture zones remaining weak through the subduction zone whether detachment occurs simultaneously along strike or propagates laterally the process is controlled by plastic yielding of the slab interior when young lithosphere entering the trench can no longer support slab pull

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

the hellenic subduction zone a tomographic image and its geodynamic implications new tomographic images of the hellenic subduction zone demonstrate slab penetration in the aegean upper mantle to depths of at least 600 km beneath greece the lower part of the slab appears to be detached at a depth of about 200 km whereas it still seems to be unruptured beneath the southern aegean schematically we derive minimum time estimates for the duration of the hellenic subduction zone that range from 26 to 40 ma this is considerably longer than earlier estimates which vary between 5 and about 13 ma

aspects of the geoelectric models of the baltic shield abstract the geoelectric structure of the baltic shield in finland has been studied since 1980 four types of geoelectric model have been considered so far numerical models for the galvanic distortions of the upper 10 km horizontal distribution of crustal inductive anomalies vertical cross sections above relevant crustal anomalies and vertical conductivity profiles on a regional scale the models are based on extensive on going measurements being conducted with magnetovariational magnetotelluric and frequency sounding methods geoelectric studies in finland are a part of both the international elas electrical conductivity of the asthenosphere and the egt european geotraverse projects in finland the most relevant crustal conductivity anomalies south of 67 n are a conductive band surrounding the central finland granite area in the west and south and the oulu mikkeli and outokumpu anomalies n s e w and n s respectively the anomalies are located in the middle crust typically from 6 12 km downwards several regional conductivity boundaries and the existence of a conductive layer in the lower crust at least in parts of central finland have also been noted a vertical cross section across the oulu anomaly shows a wide crustal conductor which seems to dip towards the west the anomaly has exceptionally good conductivity 0 5 5 m and cannot be attributed to the known sandstone formation or the schist zone on the surface along the sveka dss deep seismic sounding profile some increases in conductivity of the upper crust correlate with the kainuu and the kuhmo schist zones the upper crustal parts of the raahe ladoga zone has also a higher conductivity than the surroundings some local conductivity anisotropies seem to coincide with deep faults located by dss the area east of kuhmo seems to be more or less free of crustal conductivity anomalies allowing electric soundings down to the upper mantle a deep conductivity model based on the measurements in this area differs from earlier models for northern scandinavia where the presence of a good conductor at asthenospheric depths has been reported variations in the electrical conductivity of the earth s crust and upper mantle thus yield information about the structural composition and the tectonic development of the earth the results now presented are only preliminary and their detailed geological tectonic importance is not yet fully understood later the models will be completed with the results from work done in both the eastern and the western parts of the shield

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

evolutionary model of the himalaya tibet system geopoem based on new modelling geological and geophysical data a two dimensional thermo mechanical laboratory modelling of continental subduction was performed the subducting continental lithosphere includes a strong brittle upper crust a weak ductile lower crust and a strong upper mantle the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is denser than the asthenosphere the lithospheric layers are composed of material whose strength is sensitive to and inversely proportional to temperature throughout the experiment the model surface was maintained under relatively low temperature and the model base at higher temperature the subduction rate satisfied the peclet criterion modelling confirms that the continental crust can be deeply subducted and shows that slab break off delamination and tectonic underplating are fundamental events with drastic consequences on the subsequent evolution of the convergent system combining these results with previous purely mechanical modelling we elaborate a new evolutionary model for the himalaya tibet convergent system the principal successive stages are 1 subduction of the indian continental lithosphere to 200 250 km depth following subduction of the tethys oceanic lithosphere 2 failure and rapid buoyancy driven uplift of the subducted continental crust from ca 100 km depth to some depth that varies along the mountain belt 20 30 km on average 3 break off of the indian subducted lithospheric mantle with the attached oceanic lithosphere 4 subduction underplating of the indian lithosphere under asia over a few to several hundred kilometers 5 delamination roll back and break off of the indian lithospheric mantle 6 failure of the indian crust in front of the mountain belt formation of the main central thrust and underthrusting of the next portion of indian lithosphere beneath tibet for a few hundred kilometers at the beginning of stage 6 the crustal slice corresponding to the crystalline himalayas undergoes erosion activated uplift and exhumation fl 2000 elsevier science b v all rights reserved

thermomechanical modelling of slab detachment abstract slab detachment or breakoff is appreciated as an important geological process as shown by recent tomographic imaging using a 2 d upper mantle model 660 km deep and 2000 km wide we have investigated with a 2 d finite difference and marker in cell numerical technique the multi resolutional character of the thermomechanical phenomena related to this complex geological process our experiments show that this process can be initiated in form of slab necking by a prolonged 8 30 my period of slab weakening due to thermal diffusion 60 c my due to strong positive thermal feedback from shear heating detached slab fragments sink rapidly with a tendency for coherent rotation the influence of temperature and pressure dependent thermal conductivity on the process of thermal weakening of the slab is quite significant this supports the idea that the breakoff process is triggered by thermal diffusion on a time scale linearly dependent on heat conductivity rapid topographic changes and increasing volcanic activities due to the melting of subducted oceanic crust are possible scenarios of this vigorously driven geodynamic process

geoelectric studies and conductivity structures of the eastern and northern parts of the baltic shield abstract large resistive regions ideally having a horizontal size of about 200 km are needed to study reliably the conductivity distribution of the lower crust of the baltic scandinavian shield by deep electromagnetic em methods the overburden of the shield consists of quaternary sediments and is thin with a low conductance this makes the shield favourable for em studies the numerous conductive zones e g schist belts in the uppermost crust however require careful two dimensional modelling in order to obtain reliable electrical cross sections of the earth s crust electromagnetic methods magnetometer array studies and magnetotelluric profiling have proven to be a useful tool in studies of the lithospheric geophysics of the shield the crust of the finnish and soviet baltic shield can be divided into geoelectrically different blocks in central finland the blocks are surrounded by narrow highly conductive band like formations they are located in the middle crust and their upper surface often occurs in the 6 10 km region the resisitivities are typically 5 10 m the greatest anomalous crustal regions have a conductance of more than 20 000 s the lower crust is very conductive in most parts of the shield two dimensional models of long magnetotelluric profiles indicate the great variety and complexity of the geoelectric structure of the crust

an introduction to the european geotraverse project first results and present plans abstract the european geotraverse egt is an international multidisciplinary project focused on a north south orientated lithospheric profile 4000 km long and of varying width extending from northernmost scandinavia to north africa this profile consists of three interlinking segments northern central and southern comprising a continuous succession of tectonic provinces ranging from the oldest precambrian areas of the baltic shield to the currently active area of the western mediterranean the broad aim of the egt project is to obtain a better three dimensional picture of the structure state and composition of the continental lithosphere to use as a basis for an understanding of its evolution and dynamics all of the 12 major projects that constitute the egt joint programme have now been initiated and several of these projects are nearing completion along the northern segment data from the fennoscandian long range project fennolora a 2000 km long seismic profile across the precambrian baltic shield show that except beneath southern sweden the shield is characterized by a high velocity 40 50 km thick crust including a 5 10 km thick crust mantle transition zone an alternating series of 4 6 high and low velocity zones is present in the subcrustal lithosphere the base of which increases in depth from ca 110 km to ca 230 km from south to north beneath the shield the top of the mantle transition zone lies at a depth of about 450 km the second major project along this segment the eugeno s european geotraverse northern segment southern part project is a multidisciplinary study of the fennoscandian border zone and was largely completed in 1984 with the realization of a large scale seismic experiment preliminary interpretation of the excellent data obtained indicate the presence of strong lateral variations in internal crustal structure beneath the danish basin field work for a third major project a multidisciplinary transect of the archaean and early proterozoic terrains in the northernmost part of the shield the polar profile was carried out in 1985 a series of deep seismic reflection lines has so far been realized in the area of the central segment in the context of german national programmes first interpretations of the seismic data from a 260 km long profile across the two main intra variscan hercynian lineaments have shown the presence of numerous horizons making up a highly reflective zone in the lowermost 10 km of the crustal section studied and distinct changes in reflectivity between the main variscan tectonic zones in 1986 the entire segment will be investigated in detail in an ambitious international programme of integrated geological and geophysical studies a series of seismic experiments termed egt s have been carried out across the southern segment in 1982 1983 and 1985 interpretation of data from the 1982 and 1983 experiments have led to several interesting results including 1 1 the suggestion that two crust mantle like interfaces exist beneath the po basin at depths of about 35 and 50 km and adjacent tectonic units these interfaces marking a deep contact zone between the adriatic and european plates 2 2 in the area between genoa and corsica the ligurian sea is underlain by a greatly thinned distinctly layered section of continental crust and 3 3 corsica and sardinia are underlain by bowl shaped typically variscan continental crusts the 1985 phase of seismic surveys focused on crustal structure beneath tunisia and the adjacent seas in addition two off traverse projects are being realized first a wide aperture network of autonomously recording seismic stations nars installed along the line gothenburg malaga between 1982 and 1984 is already yielding high quality data on the upper 600 700 km of the mantle second an investigation of lithospheric seismic anisotropy in the area of the iberian peninsula is being organized for 1987 1988 finally of great importance are the systematic compilation of existing data and where needed in critical regions collection of new geophysical and geological data presently being carried out for the entire area encompassed by the egt it is expected that these compilations will be completed by 1987 at about the same time that full results from the main large scale seismic experiments become available enabling the construction of an integrated lithospheric cross section along the egt requiring a final phase of intensive multidisciplinary collaboration

low temperature high stress deformation of olivine under water saturated conditions abstract low temperature high stress rheology of olivine was investigated under water saturated conditions by large strain shear deformation experiments using the griggs type apparatus samples at t 1273 1373 k and p 2 gpa under water saturated conditions deformed under a significantly lower applied stress than expected from the power law relation and the strain rate increases more rapidly with stress these observations suggest that an exponential flow law operates under the present experimental conditions in which the stress dependence of activation enthalpy is characterized by the peierls stress the peierls stress is determined between 1 6 and 2 9 gpa depending on the choice of the form of stress dependence of the activation enthalpy these values are significantly smaller than the peierls stress under dry conditions 9 1 gpa we suggest that water hydrogen incorporated in the olivine lattices decreases the peierls stress and hence the rate of deformation in the exponential creep is significantly enhanced by the presence of water the reduction of the peierls stress by water results in the highly anisotropic weakening effect of water that causes the fabric transitions in olivine the transition between the power law and the exponential creep occurs at relatively low stress 100 mpa at a geologically relevant strain rate of 10 15 s 1 when temperature is lower than 1273 k and under water rich conditions hence the exponential creep might be dominated in olivine rich rocks at low temperature and high stress under water rich conditions and such regions could be widespread in the subduction zone upper mantle where the cold lithospheric plates sink into the mantle

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

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

tethyan subducted slabs under india abstract tomographic imaging of the mantle under tibet india and the adjacent indian ocean reveals several zones of relatively high p wave velocities at various depths under the hindu kush region in northeastern afghanistan and southern tajikistan a regional northward dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the indian plate under northern pakistan this same slab shows a roll over structure with the deeper portion overturned and dipping southward as can also be seen in the distribution of earthquake hypocenters farther east southeast e g in the vicinity of nepal a well resolved anomaly below 450 km depth is connected to the slab under the hindu kush but seems to be separated from the lithosphere above 350 km these upper mantle anomalies are interpreted as the remnants of delaminated sub continental lithosphere that went down when greater india continued to converge northward with asia after 45 ma the deeper high velocity anomalies under the indian sub continent appear clearly separated from the shallower ones as well as from each other and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the indian plate they occur at depths between 1000 and 2300 km and occasionally descend down to the core mantle boundary the anomalies form three parallel wnw ese striking zones we interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the neo tethys ocean closed between india and tibet in the cretaceous and earliest tertiary the northern deep mantle zone under northern afghanistan the himalayas and the lhasa block in southern tibet may represent the last subducted remnant of the paleo tethys ocean which is thought to have closed before the hauterivian stage of the early cretaceous the middle zone continues southeastward as a rather straight high velocity zone towards sumatra where it becomes convex southward and parallel to the subduction zone under the sunda arc comparison of this straight middle zone near india with the shallower upper 600 1000 km northern zone which displays a cusp like shape near the yunnan sw china syntaxis of the eastern himalayas supports the notion that the shallow northern zone represents later subduction than the deeper middle zone the suggestion of a counterclockwise rotation 20 of the indian plate during tertiary indentation of asia is supported by these features the present day latitudes of 5 35 n of the deep slabs under india and adjacent areas correspond to the approximate paleolatitudes of the cretaceous subduction zones the slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey which implies that lateral movements in the deeper mantle were not large

the polar profile revisited combined p and s wave interpretation abstract the polar profile project studied the crustal structure in a 440 km long swath extending across the northern baltic shield in a southwest northeasterly direction the obtained seismic data comprise p waves and very clear s waves the results of forward modelling using the raytracing technique together with synthetic seismograms for the s waves necessitated slight modification of a previously published velocity model obtained from p waves only the profile crosses the lapland granulite belt with granulite facies rocks at the surface the lapland granulite belt and a part of the karasjok kittila greenstone belt are recognized as high velocity bodies in the upper crust both for p and s waves the v p v s ratio and poisson ratio respectively seems to be slightly lower than that in the surrounding archaean gneisses which also prevail in the middle crust down to a depth of about 30 km the lower crust 30 45 km is marked by a pronounced velocity contrast of the p waves in respect to the middle crust but only a small velocity contrast for the s waves is seen in the southwestern part of the profile unusually high velocities 8 6 8 7 km s are found in the upper mantle at depths of 46 to 50 km

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

the geoelectric model of the polar profile northern finland abstract electromagnetic soundings have been made in order to construct a geoelectrical conductivity model of the crust along the european geotraverse egt polar profile forty magnetotelluric mt soundings eighteen audiomagnetotelluric amt soundings and ten magnetohydrodynamic mhd soundings were made on the main polar profile polar i and ten more mt soundings on a parallel profile polar ii 40 km to the southeast of the main profile analysis of simultaneous recordings by the eiscat magnetometer chain and thin sheet modelling of the effect of the barents sea indicate that neither the source field effects nor the presence of the ocean are significant at periods below 200 s in the measurement area the magnetotelluric data have been modelled with two dimensional models representing the regional structure along the profiles in addition to the regional structure a thin inhomogeneous surface layer is included in the models in order to explain some local features of the measured response functions although details of the surface electrical structures are poorly resolved the gross features of the geoelectrical cross section are considered to be reliable the results divide the polar profile into three different blocks the better conducting karasjok kittila greenstone belt in the south has an average resistivity of less than 10 m the more resistant lapland granulite belt with a resistivity between 100 and 200 m is underlain by conductive the geoelectric cross section agrees in gross detail with the corresponding gravity refraction seismic and reflection seismic cross sections of the polar profile all methods indicated a similar shape for the southwestern part of the lapland granulite belt i e granulites have a gently ne dipping boundary against the underlying karelian province in the northeastern part of the granulite belt the geoelectric model and the gravimetric model show a rather steep s dipping boundary against the inari terrain northeast of the granulite belt

structure and composition of the precambrian crust along the polar profile in the northern baltic shield abstract along the 500 km long polar profile the continental crust of the northern baltic shield consists of several archaean and early proterozoic crustal segments in the south in the karelian province the upper crust consists of a 3 1 2 6 ga old archaean substratum with a 0 6 km thick low to medium grade volcanosedimentary cover here shallow basin facies rocks of the central lapland complex have been thrust northwards onto the karasjok kittila greenstone belt the repeatedly folded cover rocks form linear troughs between domes and culminations of granitoids the 47 48 km thick crust of the karelian province is of a shield type it is characterised by a low velocity layer interpreted as a horizon of decollement below the supracrustal and granitoid rocks in the lapland granulite belt early proterozoic high grade metapelites and metapsammites with minor metaigneous intercalations have been thrust to the southwest they are separated from the underlying karelian province by the tectonic mixture zone of the tanaelv belt in the interpretations two alternative models are given for the shape of the lapland granulite belt it is either a synformal nappe or a ne dipping body pinching out at depth and overthrust from the north by the archaean inari terrain both the lapland granulite belt and the inari terrain have a 42 44 km thick crust with a relatively thin lower crust marked by lower seismic velocities the early proterozoic polmak pasvik pechenga belt separates the archaean inari terrain from the archaean sorvaranger terrain in the north this belt is interpreted as a suture zone which continues to depth it may either dip steeply towards irregularities in the moho or form a sw dipping low angle plane below the lapland granulite belt the northernmost part of the baltic shield is concealed beneath a 0 7 km thick very low grade metamorphic late proterozoic cover this cover is divided into two parts by the trollfjord komagelv strike slip fault here the crust is again of a normal shield type with a thickness of 42 46 km

decomposition analysis of the bear magnetotelluric data implications for the upper mantle conductivity in the fennoscandian shield decomposition analysis of the bear magnetotelluric data implications for the upper mantle conductivity in the fennoscandian shield

a tomographic image of indian lithosphere break off beneath the pamir hindukush region summary the p and s seismic anomalies in the upper mantle beneath the pamir hindukush collision zone and surrounding areas are investigated using more than 4 10 5 traveltimes from 5775 events located in the study region recorded by the stations of the worldwide seismic network and reported in the isc catalogue all available epicentral distance ranges of seismic rays are considered all sources are relocated using an algorithm which includes the double difference method in addition source parameters are corrected in the main inversion step simultaneously with the velocity models the high reliability of the resulting p velocity images is confirmed by various tests the s model provides much smoother patterns which are however generally consistent with p anomalies our model shows a complex image of lithospheric plate under the study region under sw hindukush the northwards subducting indian plate is imaged throughout the upper mantle and transition zone further to the ne within a 300 km wide zone the upper part of the plate overturns and then the slab breaks off after it narrows at 250 km depth the tomographic model also provides evidence for the southward subduction of the lithosphere beneath the southern tien shan down to at least 250 km depth to the north of tarim we observe a northward dipping of the tarim plate down to 500 km depth at the crustal depth our model shows alternating belts of high and low velocities parallel to the main thrusting faults which indicate complex crustal structure in this region

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

crustal structure beneath southern norway imaged by magnetotellurics abstract we use data from two magnetotelluric profiles tosca10 and tosca 09 over the scandinavian mountains to study the crustal structure in southern norway the profiles cross the major tectonic structures of the caledonian orogen as well as the western margin of the precambrian baltica dimensionality and strike analyses indicate generally 3 d behavior of the data however the majority of the used data distinguishes a preferable strike direction which is supported by the geology of the region hence we employ 2 d inversion and choose to invert the determinant of the impedance tensor to mitigate 3 d effects in the data on our 2 d models magnetotelluric data from both profiles are inverted using a damped least squares solution based on a singular value decomposition we improved the solution by defining the inverse model covariance matrix through gradient or laplacian smoothing operators the two dimensional inversion models of the tosca 09 and tosca 10 field data from southern norway derived from the damped least squares scheme with the laplacian inverse model covariance matrix are presented resistive rocks extending to the surface image the autochthonous southwest scandinavian domain and the allochthonous western gneiss region near surface conductors which are located between the resistive caledonian nappes and precambrian basement delineate highly conductive shallow sea sediments so called alum shales they exhibit a decollement along which the caledonian nappes were overthrust a deeper upper to mid crustal conducting layer in the southwest scandinavian domain may depict the remnants of closed ocean basins formed during the accretions and collisions of various sveconorwegian terranes in tosca 10 the caledonian nappes the conducting alum shales and the deeper conductor are terminated in the west by the faltungsgraben shear complex which represents a crustal scale boundary between the western gneiss region in the west and the southwest scandinavian domain in the east

electromagnetic geophysics and global tectonics lateral variations of electrical conductivity in the lithosphere and below it can be informative concerning both contemporary and ancient global tectonics the methods of investigation include classical four electrode resistivity depth probing which is of special value in continental shields magnetotellurics long used on land and recently in the pacific to show that in electrical terms the lithosphere is thicker near an oceanic ridge than had been expected and magnetometer arrays which are useful primarily in mapping highly conductive structures examples of the use of all three methods will be reviewed with emphasis on the last in conjunction with other geophysical geological and geochemical data magnetometer arrays have been useful in mapping partial melt under the western united states in discovering an extension of the southern end of the african rift system to the atlantic coast in detecting a major fracture zone in central north america which may be a proterozoic interplate collision feature and in discovering a conductive belt under southernmost africa which may result from accumulation of serpentinized marine crustal rocks at the top of a subduction under proterozoic gondwanaland inherent difficulties of quantitative modeling limit the utility of magnetometer arrays largely to discovery and mapping of two and three dimensional conductive structures this remains valuable however particularly when other physical properties can be used to limit the range of possible interpretations of the conductive structures recent array studies now in progress in western canada have defined two new conductivity anomalies one of these may represent a local partial melt concentration under the rocky mountains trench the other a precambrian rift valley in the lower crust previously suggested by others on the basis of seismological gravity and static magnetic data both are being investigated by more concentrated arrays

stretching instabilities and lithospheric boudinage a competent layer with a nonlinear rheology can under extension exhibit pinch and swell instabilities such instabilities can explain small scale regular deformations of rock recently they have also been invoked in relation to the distribution of basins and ranges fletcher and hallet 1983 and the undulation of the bouguer map froidevaux 1986 in the western part of north america for the case of a simple stratified structure different authors have expressed mathematical solutions describing these instabilities in this paper we elucidate the general physical meaning of these nonhomogeneous deformations in the simplest case where the competent layer is fully embedded in a softer material a fundamental mode develops with a preferred wavelength equal to 4 times the thickness of a competent layer the novelty is the occurrence of overtones with alternate symmetrical and antisymmetrical deflections of the interfaces between layers these modes have a large amplitude when the rheology tends to perfect plasticity the presence of a free surface like the earth s surface for the upper crust is shown to double the preferred wavelength of the fundamental mode and to forbid one overtone out of two this new effect is caused by gravity which inhibits the growth of surface topography another shift is of opposite sign to that caused by a free surface numerical solutions for multilayer structures can elucidate the nature of couplings between various layers the case of a three layer lithosphere having a soft lower crust and overlying a mantle asthenosphere has been analyzed in some detail it shows that in the range of wavelengths larger than 30 km up to five preferred modes may be superimposed this should encourage further analysis of the western u s topography and gravity pattern which is known to contain at least two wavelengths about 40 km for the individual basins and ranges and about 200 km for the bouguer troughs in nevada and their associated broad topographies

structure of the scandes lithosphere from surface to depth abstract in this review we give an overview of geophysical data and models available for the scandinavian mountains and adjacent areas as they are of relevance to the debate about the existence and cause of neogene uplift emphasis is given to potential field and petrophysical data of which the earth science communities of norway and sweden have a long tradition of acquisition and interpretation this is reflected in the wealth and dense coverage of data available the topography of the scandes mountain chain correlates to a large degree with a bouguer gravity low which suggests isostatic compensation but comparison with magnetic and petrophysical data shows that the gravity low is partly influenced by the trans scandinavian igneous belt signals due to the effects of surface geology can be further eliminated by considering the near surface density distribution as provided by petrophysical sampling to illuminate the entire lithospheric structure active and passive seismic and magnetotelluric data can be used recent and ongoing experiments are aimed at improving the existing models of the lithosphere integration of these different geophysical data sets allows the structure of the lithosphere of the scandes and its surroundings to be addressed seismic models show no pronounced crustal root below the scandes which could provide isostatic compensation but instead an increase of crustal thickness towards the central fennoscandian shield from the integrated model isostatic compensation is inferred to be largely controlled by the density distribution of the crust and that a high density lower crust to the east of the scandes is necessary to achieve isostatic equilibrium on a regional scale we demonstrate the typical crustal structure with profiles crossing from the scandes into the shield and a combined onshore offshore basement map

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

lithospheric necking a dynamic model for rift morphology abstract rifting is examined in terms of the growth of a necking instability in a lithosphere consisting of a strong plastic or viscous surface layer of uniform strength overlying a weaker viscous substrate in which strength is either uniform or decreases exponentially with depth as the lithosphere extends deformation localizes about a small imposed initial perturbation in the strong layer thickness for a narrow perturbation the resulting surface topography consists of a central depression and uplifted flanks the layer thins beneath the central depression the width of the rift zone is related to the dominant wavelength of the necking instability which in turn is controlled by the layer thickness and the mechanical properties of the lithosphere for an initial thickness perturbation with a width less than the dominant wavelength deformation concentrates into a zone comparable to the dominant wavelength if the initial perturbation is wider than the dominant wavelength then the width of the zone of deformation is controlled by the width of the initial perturbation deformation concentrates in the region of enhanced thinning and develops periodically at the dominant wavelength a surface layer with limiting plastic stress exponent n behavior produces a rift like structure with a width typical of continental rifts for a strong layer thickness consistent with various estimates of the maximum depth of brittle deformation in the continental lithosphere the width of the rift is essentially independent of the layer substrate strength ratio for a power law viscous surface layer n 3 the dominant wavelength varies with layer substrate strength ratio to the one third power and is always larger than for a plastic surface layer of the same thickness the great widths of rift zones on venus may be explained by unstable extension of a strong viscous surface layer

electromagnetic studies in the fennoscandian shield electrical conductivity of precambrian crust abstract electromagnetic em investigations of the 1980s in the fennoscandian baltic shield produced an unique and unified em data set studies include regional investigations by the magnetovariational mv method with large lateral sampling distance investigations of anomalous conductivity structures by magnetotelluric mt soundings and other em and electrical methods audio mt soundings d c dipole dipole and vlf resistivity profilings with shorter sampling distance and studies of the near surface conductivity by airborne em surveys the variety of methods provide an ability to map efficiently crustal conductivity structures from a regional scale of hundreds of kilometres down to local details of some metres in the anomalous structures the precambrian of the fennoscandian shield is characterized by roughly nw se directed elongated belts of conductors which separate more resistive crustal blocks the latter serve as transparent windows through which to probe deep electrical structure and belts of conductors as tectonic markers of ancient orogenic zones including 1 the kittila vetrenny poyas conductor 2 the lapland granulite belt and inari pechenga imandra varzuga conductors 3 the archaean proterozoic boundary conductor and 4 the southern finland conductor the conductive belts orogenic conductors indicate places where crustal masses collided and were finally sealed together enhanced conductivity in the orogenic conductors is caused primarily by an electronic conducting mechanism in graphite and sulphide bearing metasedimentary rocks estimations of the lower crustal conductivity indicate a laterally heterogeneous lower crust in the fennoscandian shield archaean lower crust seems to be in general more resistive than the early proterozoic lower crust of the karelian and svecofennian domains the lower crust in the southwestern part of the svecofennian domain and in the sveconorwegian domain seems to be more resistive than in the central part of the svecofennian domain

rheologic controls on the dynamics of slab detachment abstract the detachment of subducted lithosphere slabs has been proposed in a number of locations as the cause for observed volcanism and regional tectonic evolution for example along strike progression of uplift in the adriatic sea and west to east progression of volcanism in the trans mexican volcanic belt may be caused by propagating detachment of subducted slabs an understanding of the dynamics of slab detachment is necessary to relate the timing of detachment to surface observations we demonstrate using two dimensional 2d numerical models that the timing occurrence and surface effects resulting from the detachment of subducted lithosphere are strongly dependent upon the stiffness i e stress supported viscously of the lithosphere and rheology of the upper mantle in particular in models with a newtonian upper mantle where plastic yielding is the only weakening process within the slab detachment does not occur models accounting for non newtonian upper mantle deformation demonstrate two modes of detachment 1 slow and deep detachment of strong slabs 500 mpa or greater maximum yield strength controlled by heating of the slab and 2 fast and shallow detachment of weaker slabs 300 mpa controlled by yielding within the slab interior for slabs with a maximum yield strength of 500 mpa the time to detachment increases with slab age for ages ranging from 40 to 120 my indicating that detachment time is controlled by the integrated stiffness of the slab which limits the sinking rate however the detachment time for weaker slabs 300 mpa is independent of age because weaker slabs sink rapidly and the time to detachment is limited by the rate of flow of the surrounding mantle