modelling the lower crust by simulation of the in situ conditions an example from the saxonian erzgebirge abstract seismic data supply an image of the earth s interior with more detail than other geophysical methods however interpretation of the data is a complicated process laboratory investigations of rock properties under simulated in situ conditions can make a valuable contribution for the material interpretation of the seismic profile dekorp 3 mve 90 along the saxonian erzgebirge in germany rock samples from this area were investigated under high pressure high temperature conditions the aim of the investigations was to find relations between longitudinal and shear wave velocities v p and v s and the mineral composition of the rocks under extreme conditions of pressure and temperature the experiments with different pressure chambers were made using two types of pressure apparatus one allows numerous experiments at relatively low pressure for measurements at extreme conditions up to 2 gpa and 1000 c with partial melting a special arrangement inside a gas pressure chamber was used the experimental data are used for the construction of velocity vs depth curves together with seismic velocity models and the geological interpretation of reflections these data allow the construction of a crustal model of the erzgebirge it can be shown that there are indications of partial melting zones in the lower crust the results provide an example of the importance of this method for the material interpretation of seismic data in general

velocity structure of the kapuskasing uplift northern ontario from seismic refraction studies a crustal scale seismic refraction experiment was conducted over the kapuskasing structural zone northern ontario in 1984 the zone cuts obliquely across the east west structural grain of the superior province in the canadian shield and has been proposed as a cross section of archean crust exposed by thrust faulting along the ivanhoe lake cataclastic zone during early proterozoic time five seismic refraction lines of 360 450 km were shot over the area there were 18 profile shots and two fan shots with a recorder spacing of 2 5 km we have modeled the travel times and amplitudes of the data from the profile lines and analyzed reflections from the crust mantle boundary on the fan shots we have imaged a low velocity zone under the abitibi greenstone belt ranging from 4 5 to 9 12 km depth that is underlain by a highly reflective zone there is a considerable deepening of the moho from 40 43 km to 50 53 km under and to the west of the southern end of the kapuskasing structural zone a high velocity anomaly of 6 6 6 7 km s has been imaged in the upper crust down to 20 km depth beneath the kapuskasing structure with a suggested dip of 15 2 to the west this corresponds well to the proposed location of a granulite zone thrust up from the middle or lower crust in the early proterozoic

the ordovician chondrite from brunflo central sweden ii secondary minerals abstract brunflo is an altered h4 h5 stony meteorite that fell in calcareous mud on the continental shelf of the iapetus ocean about 460 470 ma b p except for relicts of chromite all the original minerals are replaced by calcite barite an illite type mineral and some minor to rare phases carbonate fluorapatite quartz titanium dioxide chalcopyrite three different members of the cobaltite gersdorffite series including a phase with a composition close to coni ass 2 rammelsbergite safflorite nickeline maucherite orcelite native nickel anomalous bornite galena pyrite sphalerite and two cu phases cu1 66s and cu4fes4 the secondary mineralogy varies rapidly at a mm scale indicating non equilibrium conditions during the alteration by the time the mud was cemented to limestone most of the original minerals might have been transformed to secondary phases during subsequent stages of diagenesis the temperature remained within the 0 50 c range for millions of years probably explaining the formation of the phases of unusual compositions a temperature rise to 200 300 c chiefly due to regional heating during the caledonian orogeny in silurian devonian time might have modified the secondary mineralogy

palaeozoic continental type gabbros in the gran paradiso nappe western alps italy early alpine eclogitization and geochemistry abstract palaeozoic gabbros intruded by late variscan granitoids occur in the southern part of the gran paradiso nappe western alps they are the first gabbroic rocks described in the variscan basement of the internal pennine nappes both gabbros and granitoids sustained the early alpine high pressure metamorphism at t around 500 c and p between 12 and 20 kbar and garnet ca na amphiboles omphacite talc paragonite assemblages developed in the gabbros in little deformed metagabbros the complete eclogitic recrystallization and the relatively advanced compositional homogenization of some eclogitic minerals were probably favoured by fluid infiltration however the microstructural re organization failed and the pre alpine magmatic domains survived in response to the lack of pervasive deformation the little deformed metagabbros are bounded by glaucophane bearing ductile shear zones formed during the early post eclogitic path major and trace element abundances and ree patterns of the gabbros indicate derivation from a basaltic sub alkaline melt with a continental affinity although a hiatus exists between the gabbros and the associated gran paradiso granitoids the ratios between some hygromagmatophile elements and the normalized patterns for some elements suggest a few affinities between the two lithologies these rocks probably formed during the same magmatic cycle and the granitoids originated either from anatexis of continental crust during gabbro intrusion or more likely by fractionation and interaction between the early basaltic melt and the sialic crust

petrological constraints and pt path of devonian collapse tectonics within the scandian mountain belt western gneiss region norway in norway post caledonian extension controlled the formation and sedimentation of middle devonian sedimentary basins these collapse basins were deposited along a low angle extensional detachment fault on top of caledonian high grade metamorphic rocks of the western gneiss region this paper concerns the petrological analysis of eclogite and high pressure micaschist collected within the extensional shear zone study of gneisses and micaschists surrounding these high pressure rocks allows description of the complete pt path which characterized this area application of appropriate geothermometers and geobarometers confirms that the retrograde evolution following high pressure conditions ll 15kbar at 500 600 c was characterized by nearly isothermal decompression followed by amphibolite facies metamorphism the shape of the pt path is compared with theoretical models of thermal evolution of continental orogenic crust during uplift we can observe an increase in temperature during the first stages of decompression smaller than predicted by the models considering the evidence for extensional tectonic in the study area we propose to explain such a small increase in temperature during the decompression by the intervention of extension which accelerates the collapse process and reduces the late orogenic heating of the caledonian thickened crust

compositional variation and the origin of deep crustal reflections abstract identifying the sources of crustal reflections is essential for deriving more geological information from deep crustal reflection profiles theoretical and model studies help place constraints on the role of compositional variation in producing deep crustal reflections analysis of laboratory derived velocities and densities of rock types typical for the continental crust indicates that reflection coefficients are generally small but significantly 17 of the possible reflection coefficients have magnitudes between 0 1 and 0 2 comparison between reflection coefficients derived from well logs and reflections observed in associated seismic profiles indicates that constructive interference associated with geological layering is at least as important as the magnitude of the reflection coefficients for producing detectable reflections constructive interference can increase reflection amplitude by two to three times but is limited to a relatively narrow range of layer thickness for a typical 10 40 hz seismic wavelet and typical crustal velocities of about 6 km s constructive interference occurs for layer thickness ranging between about 35 and 80 m layers thinner than 35 m interfere destructively if reflections result from compositional variation seismic models of hypothetical and observed geologic relations provide analogs for interpreting complex reflection patterns observed in deep crustal reflection profiles such models show reflection patterns similar to those observed in the reflection profiles the models indicate that the reflections could originate in the complexly deformed and intruded terranes that are common in the crystalline crust and it may not be necessary to appeal to unobserved phenomena such as special lamellae or fluid filled fractures to explain the reflections

from granulites to eclogites in the sesia zone italian western alps a record of the opening and closure of the piedmont ocean the sesia zone italian western alps offers one of the best preserved examples of pre alpine basement reactivated under eclogite facies conditions during the alpine orogenesis a detailed mineralogical study of eclogitized acid and basic granulites and related amphibolites is presented in these rare weak to undeformed rocks microstructural investigations allow three main metamorphic stages to be distinguished a a medium to low p granulite stage giving rise to the development of orthopyroxene garnet plagioclase brown amphibole ilmenite biotite in basic granulites and garnet k feldspar plagioclase cordierite sillimanite biotite ilmenite in acid granulites b a post granulite re equilibration associated with the development of shear zones producing discrete amphibolitization of the basic granulites and widespread development of biotite sillimanite cordierite spinel in the acid rocks c an eo alpine eclogite stage giving rise to the crystallization of high p and low t assemblages in an effort to quantify this evolution independent well calibrated thermobarometers were applied to basic and acid rocks for the granulite event p t estimates are 7 9 kbar and 700 800 c and for subsequent retrograde evolution p t was 4 5 kbar and 600 c for the eo alpine eclogite metamorphism pressure and temperature conditions were 14 16 kbar and 550 c the inferred p t path is consistent with an uplift of continental crust produced by crustal thinning prior to the subduction of the continental rocks in the light of the available geochronological constraints we propose to relate the pre alpine granulite and post granulite retrograde evolution to the permo jurassic extensional regime the complex granulite eclogite transition is thus regarded as a record of the opening and of the closure of the piedmont ocean

garnets recording deep crustal earthquakes syn eclogite facies pseudotachylytes of the bergen arcs western norway were formed during co seismic faulting at a depth of 60 km during caledonian continent collision concurrent with fluid induced partial eclogitization of precambrian anorthositic granulites the pseudotachylytes contain the high pressure assemblage omphacite garnet and kyanite and formed in the metastable granulites at the border of eclogitized areas rock volumes corresponding to outcrops of 100 m2 in area were shattered by pseudotachylytes associated with ultramylonites cataclasites and shear zones the granulite facies garnets unzoned and inclusion free away from the pseudotachylyte veins display planar and curved fractures brecciation and melting with decreasing distance from the veins sieve textured garnet developed by the disintegration of granulite facies garnet crystallization of eclogite facies mineral inclusions and welding together of garnet fragments fractured granulite facies garnets trend to more almandine rich compositions with variable grossular contents in the pseudotachylyte veins a second generation of minute 10 200 m euhedral or dendritic garnets has a higher femg ratio than the granulite facies garnet and a wide range in grossular content grs11 grs89 suggesting rapid growth and cooling fracturing of garnet and its preferential disappearance in the pseudotachylyte show that garnet has a relatively low fracture toughness seismic faulting caused enhanced reaction rates during ongoing fluid induced eclogitization of the deep crust by increasing the surface to volume ratio of garnet and its dislocation density the sudden volume reduction associated with eclogitization of metastable crust may have nucleated further faulting and pseudotachylyte formation

vp vs ratio along the voring margin ne atlantic derived from obs data implications on lithology and stress field a total of 13 regional ocean bottom seismograph obs profiles with an accumulated length of 2207 km acquired on the voring margin ne atlantic have been travel time modelled with regards to s waves the vp vs ratios are found to decrease with depth through the tertiary layers which is attributed to increased compaction and consolidation of the rocks the vp vs ratio in the intra campanian to mid campanian layer 1 75 1 8 in the central voring basin is significantly lower than for the layers above and beneath suggesting higher sand shale ratio this layer was confirmed by drilling to represent a layer of sandstone this mid cretaceous anomaly is also present in the northern voring basin as well as on the southern lofoten margin further north the vp vs ratio in the extrusive rocks on the voring plateau is estimated to be 1 85 conformable with mafic basaltic rocks landward of the continent ocean transition cot the vp vs ratio in the layer beneath the volcanics is estimated to be 1 67 1 75 these low values suggest that this layer represents sedimentary rocks and that the sand shale ratio might be relatively high here the vp vs ratio in the crystalline basement is estimated to be 1 67 1 75 in the basin and on the landward part of the voring plateau indicating the presence of granitic granodioritic continental crust in the lower crust the vp vs ratio in the basin decreases uniformly from southwest to northeast from 1 85 1 9 to 1 68 1 73 suggesting a gradual change from mafic gabbroic to felsic granodioritic lower crust significant 3 5 azimuthal s wave anisotropy is observed for several sedimentary layers as well as in the lower crust all these observations can be explained by invoking the presence of liquid filled microcracks aligned vertically along the direction of the present day maximum compressive stress nw se

uplift of deep crust during orogenic extensional collapse a model based on field studies in the sogn sunnfjord region of western norway the western gneiss region wgr in norway experienced high pressure metamorphism during silurian devonian continent continent collision the eclogite bearing lower crust is separated from the middle and upper crust by major detachment zones formed during extensional collapse of the orogen formation of the devonian basins is related to the extension the footwall of the detachment zones comprises three structural and metamorphic zones the upper zone zone 1 is characterized by penetrative homogeneous down to the west simple shear developed under retrograde greenschist facies metamorphism zone 2 suffered inhomogeneous simple shear of the same polarity petrography and mineral chemistry data from the lower zone zone 3 show a record of initial eclogite facies metamorphism at 600 c and 16 kbar which was decompressed almost isothermally to amphibolite facies conditions at 550 c and 10 12 kbar both the eclogite and amphibolite facies metamorphism developed in a regime of pure shear with vertical shortening the rapid decompression records an approach of approximately 20 km to the surface related to uplift that was probably the result of the removal of a thickened thermal boundary layer in the mantle lithosphere the pure shear regime which developed initially in the lower crust was truncated by zones of simple shear as the lower crust was uplifted to middle and upper crustal levels extension by simple shear in the upper crust was rooted in the lower crust where extension occurred by pure shear the shear zones in zones 1 and 2 did not penetrate the pure shear regime of the lower crust a considerable amount of tectonic stripping of the orogenic welt predates deposition of the devonian sedimentary basins

high pressure metamorphism during the llano orogeny inferred from proterozoic eclogite remnants mineral chemistries and textures in mafic garnet pyroxene rock within the lost creek gneiss in the purdy hill quadrangle mason county texas record three distinct p t events two of them at pressures higher than any previously documented for these grenville age rocks the earliest metamorphic episode produced an eclogitic assemblage of pyrope rich almandine sodian augite orthopyroxene pargasitic amphibole some orthopyroxene cores record pressures greater than 15 kbar even for the most conservative estimated temperature of 500 c garnet and orthopyroxene rims and matrix clinopyroxene and amphibole record a second event at 750 c and 8 kbar symplectites of clinopyroxene plagioclase and coronas around garnet of plagioclase amphibole magnetite formed in response to reheating and hydration associated with postkinematic intrusion at the amphibolite facies conditions 600 650 c 3 5 5 kbar commonly recorded in the region recognition of early high pressure metamorphic episodes markedly strengthens the correlation between these rocks and other grenville age exposures along the continental margin and provides petrologic support for models invoking proterozoic subduction accompanying the llano orogeny

initiation of crustal scale thrusts triggered by metamorphic reactions at depth insights from a comparison between the himalayas and scandinavian caledonides active eclogitization has recently been inferred at depth beneath the himalaya from geophysical observations and the mechanical consequences of eclogitization can be observed in the field in the eroded and extended nappe stack of the scandinavian caledonides there proterozoic metastable granulites and igneous protoliths underwent partial eclogitization during the collision of baltica with laurentia the reaction began in pseudotachylites and veins and eventually formed a connected network of eclogite facies shear zones that localized deformation and weakened the lower crust of baltica during the collision with laurentia all these features can be compared with the seismic activity of the indian lower crust its strength loss beneath the himalayan ranges and its delayed density increase regarding its penetration in the eclogite facies the caledonian bergen arc eclogites and the himalayan ama drime eclogites are both derived from continental crust in both cases these eclogites were formed contemporaneously with the activation of the main thrusts responsible for the construction of the orogenic wedges the main central thrust in the himalayas and the main thrust below the jotun nappe complex in the caledonides the similarities in these two orogens which compare both in size and structure highlight the importance of eclogitization at depth as a mechanism for weakening of the lower crust and for decoupling of the crust and lithospheric mantle in collision zones

eclogite facies shear zones deep crustal reflectors strongly foliated eclogite facies rocks in 30 150 m thick shear zones of caledonian age occur within a grenvillian garnet granulite facies gabbro anorthosite terrain in the bergen arcs of norway the predominant eclogite facies mineral assemblages in the shear zones are omphacite garnet zoisite kyanite in gabbroic anorthosite and omphacite garnet in gabbro eclogite facies rocks in shear zones are generally fine grained alternating omphacite garnet and kyanite clinozoisite rich layers define gneissic layering a strong shape preferred orientation of omphacite kyanite and white mica phengitic muscovite and or paragonite define the foliation the anorthositic eclogites show omphacite b axis maxima approximately normal to the foliation and c axis girdles within the foliation plane p wave velocities v determined at confining pressures to 600 mpa for samples from eciogite facies shear zones range from 8 3 to 8 5 km s l and anisotropy ranges from 1 to 7 the few samples with more pronounced anisotropy tend to be approximately transversely isotropic with minimum velocities for propagation directions normal to foliation and maximum velocities for propagation directions parallel to foliation the fast propagation direction lies within the c axis girdles parallel to foliation and the slow propagation direction is parallel to the b axis concentration normal to foliation in samples for which omphacite crystallographic preferred orientation was determined v for the granulite facies protoliths average about 7 5 km s high calculated reflection coefficients for these shear zones 0 04 0 14 indicate that they are excellent candidates for deep crustal reflectors in portions of crust that experienced high pressure conditions but escaped thermal reactivation

microstructural evidence for magma confluence and reusage of magma pathways implications for magma hybridization karakoram shear zone in nw india in the karakoram shear zone ladakh nw india miocene leucogranitic dykes form an extensive varied and complex network linking an anatectic terrane exposed in the pangong range with leucogranites of the karakoram batholith mineral paragenesis of the heterogeneous anatectic source rocks suggests melting has resulted from water influx into rocks at upper amphibolite facies conditions and microstructures suggest anatexis was contemporaneous with shearing the network is characterized by continuous and interconnected dykes with only rare cross cutting relationships forming swarms and chaotic injection complexes where magmatic rocks cover up to 50 of the outcrop area despite this volume of magma the system did not lose continuity suggesting that it did not flow en masse and that the magma network was not all liquid simultaneously leucogranites in this network including leucosomes in migmatites carry an isotopic signature intermediate between the two main anatectic rocks in the source suggesting efficient homogenization of the magmatic products here we describe a number of microscopic features of these magmatic rocks which suggests that several pulses of magma used the same pathways giving rise to textural and chemical disequilibrium features these include i narrow tortuous corridors of fine grained minerals cutting across or lining the boundaries of larger grains interpreted to be remnants of magma filled cracks cutting across a pre existing magmatic rock ii corrosion of early formed grains at the contact with fine grained material iii compositional zoning of early formed plagioclase and k feldspar grains and quartz overgrowths documented by cathodoluminescence imaging iv incipient development of rapakivi and anti rapakivi textures and iv different crystallographic preferred orientation of early formed quartz and fine grained quartz mapping of the fine grained corridors interpreted to represent late melt channels reveal an interlinked network broadly following the s c fabric defined by pre existing magmatic grains we conclude that early formed dykes provided a pathway exploited intermittently or continuously by new magma batches new influxes of magma opened narrow channels and migrated through a microscopic network following predominantly grain boundaries along an s c fabric related to syn magmatic shearing a mixed isotopic signature resulted not from the mixing of magmas but from the micro scale interaction between new magma batches and previously crystallized magmatic rocks through local equilibration

precambrian tectonics and the physicochemical evolution of the continental crust i the gabbro eclogite transition revisited abstract we report the first occurrence of retrograde eclogite shear zones cutting proterozoic granulites from australia the musgrave range granulites were metamorphosed at about 12 kbar pressure and 850 900 c the granulites then cooled isobarically and developed a variety of lower temperature mineral reaction coronas typical of garnet granulites these rocks are cut by a major shear zone the davenport shear in which lower temperature fine grained eclogite developed this demonstrates that mafic rocks in the deep crust can cool down into the thermodynamic stability field of eclogite without undergoing transformation to eclogite we agree with harte et al 1981 that rocks lacking a fluid phase in stable tectonic regions are unlikely to maintain local equilibrium at ambient geotherm conditions until depths well within the upper mantle are reached it is proposed that many isobaric high pressure granulites could have cooled down into the eclogite field before their later excavation to the surface and did not transform to an eclogite mineralogy the inability of the lower crust to transform into eclogite by retrograde reaction has important implications for the seismic petrologic and geochemical character of the lower crust the continental moho cannot be caused by the gabbro eclogite transition preservation of relatively low density high temperature granulite and gabbroic mineral assemblages in the lower continental crust prohibits major recycling of lower crust back into the mantle once it is removed from active plate boundaries however in island arcs with magmatically thickened crust mafic rocks at a depth of 60 to 70 km will be in the eclogite stability field at high enough temperatures 800 900 c for eclogite to rapidly form and sink back into the mantle this could result in a net accreted crust of broadly andesitic composition in contrast later growth of continental crust by basalt underplating in stabilised shield regions will not result in subsequent eclogite formation and lower crustal recycling back into the mantle will not occur thus estimates of the average composition of the continental crust which are based on a mass balance of plate margin basaltic input from the mantle will not be adequate

deep crustal mineral assemblages and potential for crustal rocks below the moho in the scottish caledonides summary the scottish caledonian lower crust is strongly seismically reflective beneath the moho at about 30km very few reflections are seen on seismic records with the exception of a strong reflector at about 50km depth the rocks between the moho and this strong reflector have been interpreted as crustal but with mantle velocities and densities we report tests of this in which the densities and seismic velocities of likely crustal precursors are assessed through determination of the stable mineral assemblages along geotherms appropriate to the region likely crustal precursors were selected from suites of exhumed deep crustal rocks and deep crustal xenoliths and from a variety of basaltic rocks mineral assemblages were estimated using a variant of the lowest free energy search procedure of wood 1987 with account taken of nonideal mixing in solid solutions geotherms were calculated from the heatdow heatproduction relationship for the scottish caledonides using the methods of pollack chapman 1977 seismic velocities were estimated from empirical density velocity relationships and from the velocities of individual mineral species weighted for modal proportions

trace element rich brines in eclogitic veins implications for fluid composition and transport during subduction primary and pseudosecondary fluid inclusions occur in oscillatory and sector zoned omphacite in eclogitic veins from the monviso ophiolitic complex in the western alps the inclusions contain aqueous brines and daughter crystals of halite sylvite calcite dolomite albite anhydrite and or gypsum barite baddeleyite rutile sphene fe oxides pyrite and monazite this daughter mineral suite indicates high solubilites of na k ca mg fe zr ti p ba ce la th and s species and provides direct evidence for transport of high fieldstrenght large ion lithophile and light rare earth elements as dissolved species during subduction fluid inclusion heterogeneities preserved within and between adjacent grains in the veins however suggest that the scale of fluid equilibration was small a crack seal geometry in some of the veins implies that fluid release in pulses rather than steady flow controlled mineral deposition and growth in the veins from these observations we develop a model of fluid release and entrapment in which pulses of fluid are associated in time with increments of shear and tensile failure the rate of fluid release and the reduction in porosity both depend on the rate of plastic flow vein fluids may initially be derived from decreptitation of early fluid inclusions in the host eclogites small scale fluid heterogeneities implied by the fluid inclusions in the veins are best interpreted in terms of limited fluid flow and hence limited metasomatism we conclude that element recycling into the mantle wedge during subduction will depend at least as strongly on fluid transport mechanisms as on element solubilities in the fluid phase at monviso despite evidence for high trace element solubilities in saline brines the elements were not removed from the downgoing slab prior to teaching depths of 40 km

lithoprobe reflection studies of archean and proterozoic crust in canada abstract lithoprobe canada s national collaborative earth science research project established to develop a comprehensive understanding of the evolution of the north american continent is a multidisciplinary program spearheaded by seismic reflection studies five recently recorded seismic lines discussed in this paper are located in precambrian regions the mesoproterozoic grenville province the paleoproterozoic trans hudson orogen tho the paleoproterozoic archean basement of alberta and the archean superior province data acquired across the grenville orogen in eastern quebec show strong reflectivity throughout the crust upper crustal reflections can be correlated with exposed structural elements including extensional shear zones and packages of deformed high pressure rocks eclogites in a marine survey across the grenville orogen off southeastern labrador seismic images show variably dipping reflections and a structural high associated with a major gravity anomaly data acquired across central alberta show crustal scale thrust stacking and imbrication of the archean hearne craton to the east across the trans hudson orogen images of similar collisional features are observed geochronologic constraints indicate contemporaneity of tectonic activity between the two regions at 1 8 ga suggesting that collisional tectonic activity was coeval over a broad crustal region ca 1000 km across strike in the superior province seismic data across a collision zone involving the northern abitibi greenstone belt and the arc related opatica plutonic belt show spectacular crustal reflectivity and dipping reflections that extend 8 s 30 km into the mantle the latter feature is interpreted as representing a relict 2 69 ga old suture associated with subduction providing the first direct evidence that plate tectonics was active in the late archean these five examples supported by other lithoprobe results refute a number of generalizations about crustal reflectivity that have been made in the past and illustrate how reflection studies combined with other geoscience studies can lead to a better understanding of precambrian tectonics reflectivity persists throughout the crust there is no general separation into a poorly reflective upper crust and a reflective lower crust crustal reflectivity in archean and proterozoic regions is as pervasive as that in areas of more recent tectonism the precambrian reflection moho is generally well defined but shows a range of characteristics relative ages of reflectors can be discerned and tectonic significance can be attached to characteristic features of the crustal reflectivity

structural development and petrofabrics of eclogite facies shear zones bergen arcs western norway implications for deep crustal deformational processes caledonian eclogite facies shear zones developed from grenvillian garnet granulite facies anorthosites and gabbros in the bergen arcs of western norway allow direct investigation of the relations between macroscopic structures and crystallographic preferred orientation cpo in lower continental crust field relations on the island of holsnoy show that the eclogites formed locally from granulite facies rocks by progressive development of 1 eclogite adjacent to fractures 2 eclogite in discrete shear zones 2 m thick 3 eclogite breccia consisting of 80 well foliated eclogite that wraps around rotated granulite blocks and 4 anastomosing subparallel eclogite facies shear zones 30 100 m thick continuous over distances 1 km within the granulite terrane these shear zones deformed under eclogite facies conditions at an estimated temperature of 670 50 c and a minimum pressure of 1460 mpa which corresponds to depths of 55 km in the continental crust detailed investigation of the major shear zones shows the development of a strong foliation defined by the shape preferred orientation of omphacite and by alternating segregations of omphacite garnet rich and kyanite zoisite rich layers a consistent lineation throughout the shear zones is defined by elongate aggregates of garnet and omphacite the cpo of omphacite determined from five axis universal stage measurements shows a strong b axis maximum normal to foliation and a c axis girdle within the foliation plane with weak maxima parallel to the lineation direction these patterns are consistent with deformation of omphacite by slip parallel to 001 and suggest glide along 010 the lineation and cpo data reveal a consistent sense of shear zone movement although the displacement was small localized faulting of high grade rocks accompanied by fluid infiltration can be an important mode of failure in the lower continental crust field relations show that granulite facies rocks can exist in a metastable state under eclogite facies conditions and imply that the lower crust can host differing metamorphic facies at the same depth deformation of granulite and partial conversion to eclogite such as is exposed on holsnoy island may be an orogenic scale process in the lowermost crust of collisional orogens

eclogitized gabbros from the eastern grenville province textures metamorphic context and implications undeformed lenses of olivine orthopyroxene gabbro belonging to the 1429 ma shabogamo intrusive suite parautochthonous belt of the eastern grenville province host the first occurrence of eclogite mineral assemblages reported from the grenville province these rocks preserve igneous textures overprinted by eclogite mineralogy and by textures related to subsequent unloading high pressure signatures include omphacite pseudomorphs after augite plagioclase replacement by garnet kyanite corundum and orthopyroxene omphacite garnet coronas between olivine and plagioclase transformations related to decompression include replacement of garnet kyanite corundum by spinel plagioclase growth of plagioclase collars between clinopyroxene and garnet and de jadeitization of omphacite clinopyroxenes with numerous plagioclase inclusions and garnets surrounded by clinopyroxene plagioclase symplectites in granoblastic mafic rocks from the same area are also evidence of decompression from high pressures these

crustal structure of the baltic shield based on off fennolora refraction data abstract since the early 1960s deep seismic sounding experiments have been carried out on the baltic shield in this study we will mainly concentrate on the results obtained from two international profiles sveka and baltic carried out in finland in 1981 and 1982 results from these profiles are shown and discussed and compared with those obtained from the fennolora and from the other recent refraction profiles of the baltic shield in fennoscandia according to the results from sveka and baltic and average crustal velocity is 6 6 6 7 km s which is rather high several distinct reflection boundaries have been found within the crust in the lower part of the crust a high velocity layer with a p wave velocity of 7 0 7 5 km s has been found in some cases in addition the results indicate that the crustal structure has a clear block like character different blocks being separated from each other by deep fractures the crustal thickness in the baltic shield is about 45 km on average whereas around the ladoga bothnian bay zone in central finland it is about 10 km thicker than this thus there is a large scale depression in the moho boundary in the central part of the baltic shield

mineral fluid melt interactions in high pressure shear zones in the bergen arcs nappe complex caledonides of w norway implications for the fluid regime in caledonian eclogite facies metamorphism abstract felsic extension veins consisting of plagioclase ab 91 phengite quartz clinozoisite and minor accessory calcite occur in the central parts of caledonian eclogite facies shear zones at holsnoy island in the bergen arcs nappe complex the veins have the field and textural characteristics of syntectonic intrusions and have crystallized at pressures and temperatures compatible with the peak of eclogite facies metamorphism ca 700 c 16 19 kbar the quartz contains early primary fluid inclusions of a aqueous brine 31 34 wt nacl and b a co 2 n 2 mixture with x n 2 5 mole percent the two fluids were present simultaneously and are interpreted as immiscible at the pt conditions of vein formation later generations of fluid inclusions comprise nitrogen rich co 2 n 2 mixtures and aqueous fluids with low salt contents the latter were trapped at a late stage of the cooling and uplift history of the nappe complex at the temperature of the peak of eclogite facies metamorphism at holsnoy ca 700 c introduction of a fluid phase with high water activity caused partial melting of the surrounding mafic to intermediate lithologies the muscovite granite extension veins represent the crystallization products of anatectic melts migrating through the shear zones at eclogite facies conditions

elasticity of a grossular pyrope almandine garnet the single crystal elastic properties of a natural grossular rich garnet solid solution have been measured at ambient conditions using brillouin spectroscopy this garnet sample is unusual for a grossular in that it contains appreciable pyrope and almandine components with a composition of gr48 4py27 6alm23 4sp0 6 it is thus suitable for investigating the variation of elastic properties with composition in ca rich garnet solid solutions that are stable only at high pressure the values obtained for the bulk modulus k 170 4 6 and for the shear modulus 101 6 3 of the garnet examined in this study vary by less than 1 from those obtained from a weighted sum of the end member moduli this suggests that any composition in the grossular pyrope almandine system which comprise the majority of upper mantle garnets is accurately estimated with this simple approach more complex elasticity composition variations observed for the grossular andradite garnets may be specific to garnet solid solutions where the cation substitutions occur at six coordinated crystallographic sites

vertical seismic profile results from the kola superdeep borehole russia multi offset vertical seismic profiles vsps from the kola superdeep borehole sg 3 as part of a larger seismic study of the kola region conducted during the spring of 1992 sample the dipping pechenga complex from 2175 m to 6000 m and contribute to the understanding of reflectivity in crystalline and precambrian environments from the surface to 6000 m the sg 3 borehole penetrates interlayered proterozoic metavolcanic and metasedimentary units and a mylonitic shear zone ranging from greenschist to amphibolite metamorphic grade respectively the kola vsps display a 6 velocity decrease which coincides to a mylonitic shear zone located between 4500 m and 5100 m within the sg 3 borehole seismic interfaces are identified by mode converted energy ps and sp transmissions and reflections in addition to primary seismic phases the vsp shear wave energy is generated at or near the source by vertical vibrators p wave and s wave reflections are generally detected from the same reflecting horizons but increases in relative s wave and sp reflection amplitudes originate at 1900 m 3800 m 4500 m and 5100 m depths these depths coincide with zones of elevated vo vs and may support the presence of free pore fluid which is reported from initial drilling for the proterozoic lithologies sampled by the vsp reflection events result from five mylonitic shear zones and three lithologic contrasts

deep structures and breakup along volcanic rifted margins insights from integrated studies along the outer voring basin norway three dimensional multichannel seismic survey and well data allows a deeper insight into rifting mechanisms and crustal structure of the norwegian volcanic rifted margin a surprising high velocity lower crustal dome marked by a strong amplitude reflection t reflection is described below the north gjallar ridge ngr near the volcanic plateau formed during the late paleocene early eocene breakup faulting along the ngr occurs during early campanian paleocene and is controlled by the crustal dome the t reflection was previously related to either the top of mafic underplated crust or soft lower crust updomed during the continental breakup the influence and timing of the crustal dome on the basin structure shows clearly that the dome predates the continental breakup based on the structural relationship and the results of the modelling it is therefore concluded that the t reflection does not necessarily originate from an anomalous tertiary magmatic event related to the breakup underplated material but could be also partly attributed to pre breakup mafic material or and inherited high pressure granulite eclogite rocks in the continental domain q 2004 elsevier ltd all rights reserved

reflections from a mylonitized zone in central sweden dipping reflections imaged on a deep seismic reflection profile acquired within the baltic shield can be indirectly linked to strain induced acoustic impedance contrasts within a regional brittle ductile shear zone the singo shear zone measurements made at various scales and frequencies estimate rock velocity within the shear zone and correlate well with the ductile component of strain previously determined by structural analysis velocity measurements include some made on cored field samples at confining pressures of 0 4 gpa in a laboratory 10 mhz sensor frequencies and some made in situ using ultrasonic 54 khz sensors on exposures within the shear zone laboratory measurements show a 10 change in seismic anisotropy and a contrast of 0 5 km s 1 in bulk rock velocity between granitoid protolith and highly strained ultramylonite the in situ velocity measurements demonstrate that the velocity variations persist over considerable volumes including zones where strain and associated volume loss of silica remain elevated without mylonitization synthetic seismograms generated using a one dimensional velocity model of the shear zone show that velocity variations observed in the field were large enough in magnitude and of sufficient scale to produce reflections observable on the deep seismic reflection profile collected using 6 25 hz frequencies ray trace two dimensional forward modeling of a high density wide angle reflection section demonstrates that deep dipping reflections could result from strain induced acoustic impedance contrasts within the singo or similar nearby shear zones

crustal structure along the egt profile across the tornquist fan interpreted from seismic gravity and magnetic data abstract the tornquist fan is a northwestward widening splay of late carboniferous to early permian fault zones in the danish area which emanates from the teisseyre tornquist zone in the baltic sea it is located over an area of deep sedimentary basins around denmark in the southwestern corner of the former baltica plate this paper presents an integrated model of seismic velocity density and magnetic properties along the egt profile across the tornquist fan between southern sweden and northern germany the model shows distinct features that may be related to the main tectonic events in the area the caledonian crustal suture between baltica and eastern avalonia is interpreted from changes in physical parameters at all crustal levels most noticeable as an abrupt lateral change in seismic velocity in the lowest crust at moho level from 6 9 km s in baltica to only 6 4 km s in eastern avalonia thick sedimentary successions beneath the mesozoic sequence are interpreted as palaeozoic sedimentary rocks which may have been deposited on the southern margin of the baltica plate in a deep caledonian foreland basin and in late carboniferous early permian rifts and extensional basins rift related magmatism is indicated by the presence of zones of high velocity density and magnetic susceptibility as well as strong remanent magnetization thermal subsidence following the emplacement of these bodies may be the origin of regional subsidence of the danish and north german basins of late paleozoic and mesozoic age late cretaceous to early cenozoic inversion of basin structures is detected at the sorgenfrei tornquist zone stz in the transition between shield and basin areas the stz coincides with a pronounced change in crustal thickness and the southwestern edge of a major magmatic intra crustal body significant abrupt changes in moho depth exist along the profile 26 30 km beneath the deep basins ca 35 km beneath the ringkobing fyn basement high and 48 km beneath the baltic shield around the sorgenfrei tornquist inversion zone the basement in the tornquist fan area is of proterozoic age the moho topography indicates strong thinning of the crust during its phanerozoic evolution the model supports the hypothesis that existing tectonic structures and weakness zones are continuously being reactivated during subsequent tectonic activity

chemical microstructural changes in eclogite facies shear zones monviso western alps north italy as indicators of strain history and the mechanism and scale of mass transfer abstract variations of eclogite facies mineral chemistry are related to the textural characteristics of shear zones in fe ti metagabbros from the monviso meta ophiolitic complex in the italian western alps three different kinds of microstructural domains are described 1 small lenses of low strain rocks with preserved igneous textures 2 larger high strain domains where deformation caused pervasive mylonitic foliation s1 and a stretching mineral lineation l1 and 3 two types of eclogite facies veins the latter can be subdivided into veins composed of only na pyroxenes filling small tension gashes and dilatation fractures and veins composed of a na pyroxene garnet rutile apatite assemblage occupying large fractures that randomly cross cut the mylonites in low strain domains and mylonites the na pyroxenes are omphacites and the garnets are almandine rich in some mylonite layers containing phengite and blue amphibole crosscutting s1 chloromelanite iron rich omphacites exsolved from omphacite in type 1 veins the pyroxene is either omphacite or concentric and sector zoned omphacite chloromelanite in type 2 veins pyroxenes are omphacite and or concentric and sector zoned omphacite chloromelanite and or zoned kosmochlor chromium rich omphacite omphacites associated garnets are richer in pyrope component than those from low and high strain domains possible origins of both sector and concentric zoning and exsolution are discussed it is emphasized that a miscibility gap between the omphacite and acmite end members partly controlled the omphacite chloromelanite distribution we define a chemical vector coat constrains the strain history in eclogite facies rocks and give indications of the scale and mechanism of mass transfer involved during foliation development and fracturing p t conditions are calculated for each microstructural domain the garnet pyroxene thermometer yields scattered temperature values these values relate to the dependence of temperature estimates on the amount of fe3 in the pyroxene and possibly on non ideal substitution in omphacite acmite solid solution

thrust tectonics deep structure and crustal subduction in the alps and himalayas the structural restoration of collision orogenic belts onto crustal templates provides important insights into the tectonic evolution deep structure and amounts of plate convergence after the initial contact between two continental masses balanced cross sections have been constructed parallel to the local displacement directions across the western alps and western himalayas and demonstrate very large amounts of crustal shortening above intra crustal detachments to achieve a balance substantial volumes of lower crust must have been subducted beneath the two tectonic hinterlands a model of eclogite metamorphism is invoked to facilitate this subduction and to explain the varying isostatic responses of the alpine and himalayan hinterlands patterns of eclogite metamorphism are controlled by the geometry of thrust profiles on a lithospheric scale the development of crust mantle detachments being of crucial importance such a profile is proposed for the himalayas suggesting that relatively small volumes of the footwall crust succumbed to eclogite metamorphism in the alps however a steeper thrust profile apparently developed emplacing mantle onto crust causing wholescale eclogite metamorphism in the underlying franco swiss crust the resultant density increase would provide a mechanism of isostatic collapse and flexural subsidence in the po plain region

the problem of the moho in the alps abstract the tertiary alps are the product of continental collision and material balance estimates require the subduction of continental crust and the underlying moho the present moho in the alpine region consists of three parts that of the european plate me that of the adriatic african plate ma and a new moho m2 believed to have developed in the subducted continental crust by high pressure metamorphism at 40 60 km depth the fate of m2 however is uncertain some of it is apparently destroyed subsequently as some eclogite facies and even highest p coesite bearing rocks somehow find their way to the surface where they form exotic bodies from microscopic to nappe size on the other hand the continental moho exposed in the ivrea body tells a different story here continental granulite facies rocks are intruded by ultramafic and mafic masses this suggests an origin in an extensional environment possibly in subduction related gravitational collapse basins such successor basins succeed the basins of the original tethys that had been largely consumed by the late cretaceous they puncture the 1000 2000 km wide belt of africa europa collisional interaction e g the aegean the tyrrhenian and the pannonian basins although in some ways similar to the back arc basins of the pacific these mostly intracontinental basins differ in important aspects such as their small size in these basins asthenospheric upwelling replaces m2 by a new moho m3 and the eclogitic rocks underlying m2 by the ultramafic mafic intrusions of the ivrea type material balance considerations suggest that most of the eclogues remain part of the mantle for some time on the other hand recent deep reflection seismic surveys in several places indicate delamination of a part of the lower crust and the uppermost mantle with the formation of imbrications and duplexes some of which are wedged into the middle crust of the opposite plate they may be the first stage in the formation of granulite and eclogite nappes

shear deformation and eclogite formation within granulite facies anorthosites of the bergen arcs western norway the anorthosites of the bergen arcs contain two distinct metamorphic mineral assemblages which are stable under different p t conditions a granulite facies assemblage which equilibrated at t 800 900 c p 10 kbar is defined by the minerals plagioclase al rich diopside cpx i garnet gnt i scapolite orthopyroxene dark brown hornblende this assemblage is linked by corona structures to a primary magmatic mineralogy where coexistence of olivine and plagioclase limits the maximum crystallization pressure of the anorthosites to 8 kbar at 1200 c the anorthosites are transected by a series of fine grained shear zones in which the granulite facies assemblage is replaced by an eclogite mineralogy defined by omphacite cpx ii jd40 50 garnet kyanite epidote clinozoisite phengite quartz amphibole with plagioclase sometimes remaining in the marginal parts mineral compositions indicate equilibration conditions for the eclogite paragenesis of t 700 800 c p 16 19 kbar a series of textural and mineralogical changes which can be observed as the strain increases into the shear zones demonstrates that the eclogite mineralogy is formed at the expense of the granulite facies minerals during the early stages of shear zone development the eclogite mineralogy defines a strong mylonitic fabric which again is truncated by centimetres thick secondary shear zones containing clasts of both granulite and eclogite minerals these latter fine grained ultramylonites may be connected to uplift of this deep seated complex the eclogite shear zones are highly oblique to the granulite structures indicating that the eclogite shear zones formed under a new stress regime rb sr dating shows that the granulite facies assemblage formed in sveconorwegian time or earlier while minerals from shear zones define ages around 400 ma indicating a caledonian formation of the eclogites this work demonstrates the critical role of deformation in triggering metamorphic reactions whether the shear stress also was important in stabilizing the eclogite assemblages is not clear from the available data

40ar 39ar mineral dates related to devonian extension in the southwestern scandinavian caledonides abstract middle devonian sedimentary basins exposed in west central norway formed during post caledonian extension they developed along west dipping low angle normal faults which effected ductile shearing of pre caledonian basement units of the western gneiss region wgr kinematic indicators suggest that the west vergent ductile deformation overprinted older east vergent caledonian fabric elements ductile extensional deformation led to development of a 2 km thick shear zone beneath the devonian basins five amphibole and nine muscovite concentrates from rocks within the shear zone and from unaffected gneisses of the western gneiss region have been analyzed with 40ar 39ar incremental release dating techniques in an attempt to calibrate the timing of extensional events well defined plateau ages are recorded by muscovite concentrates within the shear zone amphiboles display disturbed plateau ages clearly related to extraneous argon outside the shear zone muscovite records caledonian ages results suggest that a late tectonometamorphic event affected the rocks within the shear zone this event which not exceed ca 500 c completely rejuvenated muscovite argon systems and perturbed argon distribution within amphiboles this metamorphism is correlated with the extensional deformation which effected exhumation of the caledonian high pressure rocks and controlled formation of devonian basins

poisson s ratio of eclogite the role of retrogression available laboratory measurements of combined p and s wave velocities of eclogites from the dabie sulu ultrahigh pressure metamorphic belt and data from manghnani et al j gcophys res 79 1974 5427 5446 show clear positive correlations between poisson s ratio and the intrinsic water content h 2 o for samples with sio 2 56 with the correlation coefficient r of 0 63 0 72 there are no significant correlations r less than or equal to0 43 between poisson s ratio and other major oxides the positive correlations are interpreted to result from hydration of eclogites during their exhumation and exposure at the surface our best estimates of the average poisson s ratios for unaltered quartz bearing eclogites with h 2 o 1 0 are 0 24 0 25 0 01 which is identical to that calculated from single crystal elastic properties but lower than the averages 0 27 0 02 of measurements compiled by rudnick and fountain rev geophys 33 1995 267 309 and christensen j geophys res 101 1996 3139 3156 accordingly unaltered quartz bearing eclogites have poisson s ratios distinct from mafic granulites 0 28 0 01 and marginally different from ultramafic rocks 0 26 0 02 c 2001 elsevier science b v all rights reserved

crustal structure of the southeastern grenville province northern new york state and eastern ontario the grenville province exposes an oblique cross section through mid lower crustal lithologies that were pervasively deformed and subjected to regional thermal overprinting during the grenvillian orogeny 1 1 ga the southeastern grenville province is divided into two subterranes by the carthage colton mylonite zone a 110 km long lineament characterized by intense ductile shear and igneous intrusion which separates the amphibolite facies metasediments of the central metasedimentary belt to the west from the granulite facies metaplutonics of the central granulite terrane to the east the recognition of distinct lithotectonic domains separated by zones of intense ductile shear in the grenville province raises questions concerning the deep structure of these subterranes and in particular the means by which the mid lower crustal rocks exposed in the grenville province were emplaced seismic refraction wide angle reflection data were acquired to investigate the deep structural interrelationships within the southeastern grenville province a travel time inversion for velocity and interface depth was applied to the seismic data together with constraints from amplitude modeling to produce a seismic velocity model of the crust in the southeastern grenville province in the central metasedimentary belt the upper crust is characterized by velocities in the range 6 3 6 4 km s and a poisson s ratio of 0 26 0 01 which are attributed to quartzofeldspathic rocks farther east in the central granulite terrane upper crustal velocities of 6 55 km s and a poisson s ratio of 0 28 0 01 are associated with the marcy anorthosite the seismic homogeneity of the upper crust in the region of the carthage colton mylonite zone suggests that this boundary is a shallow feature limited to the upper 2 3 km of the crust the deep crustal structure of the southeastern grenville province is characterized by two discrete and laterally discontinuous seismic interfaces in the central metasedimentary belt the top of the lower crust is delineated by an eastward dipping interface at 24 28 km depth in the central granulite terrane prominent en echelon reflections referred to as the tahawus complex form a gently arched dome at 17 22 km depth interpretation of the tahawus complex as a zone of layered mafic cumulates is supported by its high velocity 7 1 km s and poisson s ratio 0 27 0 02 the lower crust is characterized by a velocity of 7 0 7 2 km s and an anomalously high poisson s ratio of 0 30 0 02 which are representative of pyroxene garnet granulites in contrast velocities of 6 8 7 0 km s are modeled beneath the central granulite terrane and appear to signify a lateral change in composition the moho lies at 44 45 km depth and is characterized by pronounced en echelon reflection segments suggesting compositional interlayering around the crust mantle boundary the velocity of the upper mantle is 8 0 8 2 km s an anomalous upper mantle layer with a reversed velocity of 8 6 km s dips eastward from 50 to 60 km depth beneath the southeastern grenville province our results indicate that remnants of magmatic intrusions that mobilized and thickened the crust during the grenvillian orogeny are preserved in the mid lower crust as a layered cumulate body tahawus complex and in the upper mantle as an eclogitic lens possibly delaminated from the overthickened crust during uplift of the southeastern grenville province

kerguelen plateau crustal structure and basin formation from seismic and gravity data we use multichannel seismic data gravity and subsidence modeling in conjunction with plate reconstructions to evaluate the crustal origin and composition of the kerguelen plateau predominantly oceanic crust of the southern and parts of the central kerguelen plateau appears to include continental fragments related to the breakup of india and antarctica these fragments may have been metamorphosed during emplacement of the main plateau the upper crust is basaltic the middle crust is intrusive mafic rock and intruded continental crust and the lower crust is a plagioclase rich metamorphic rock the labuan basin crust is predominantly oceanic with stranded kerguelen plateau fault blocks high density lower crust in the labuan basin is probably composed of serpentinized peridotites formed during slow rifting and spreading plate reconstruction models indicate opening between eastern broken ridge and southern kerguelen plateau at 90 ma heralding the formation of the labuan basin and diamantina zone crustal attenuation and slow accretion of oceanic crust continued until the australian and antarctic plates separated at ci8 time 40 ma plate reconstructions of the free air gravity field indicate that the naturaliste plateau fits against antarctica and that elan bank and india were juxtaposed until 110 ma both naturaliste plateau and elan bank are probable microcontinents a 1 km positive residual depth anomaly in the oceanic basins adjacent to the plateau along with the positive geoid anomaly centered beneath the northern kerguelen plateau imply that the lithosphere is partially dynamically supported by an upwelling hot asthenosphere of the kerguelen hot spot

the granulite eclogite facies transition a comparison of experimental work and a natural occurrence in the bergen arcs western norway a unique segment of the deeper parts of the caledonian continent continent collision zone is exposed in one of the thrust sheets in the bergen arcs of western norway it demonstrates that precambrian granulite facies rocks underwent fluid controlled eclogitization on a regional scale the transition from granulite to eclogite facies is sharp it occurs over distances of centimetres to metres and follows shear zones and infiltration fronts major anastomosing shear zones typically 50 to 100 m thick with well banded eclogite facies rocks crosscut the lithological boundaries they can be followed continuously for several kilometres breccias where metre size blocks of granulite float in an eclogite matrix outcrop over areas of 0 5 km2 areas comprising more than 80 eclogite occupy several square kilometres this mixture of metamorphic facies indicates the metastable occurrence of granulite facies rocks in the lower crust of modern and ancient orogenic belts in contrast to earlier experimental work such field relations imply that the granulite eclogite facies boundary in nature can be sharp and independent of lithological boundaries a partly eclogitized lower crust of the kind exposed in the bergen arcs can explain the 7 2 7 8 km s vp layer present in many crust mantle transition zones anastomosing shear zones and areas of almost complete eclogitization represent potential lower crustal reflectors

crustal structures in the central eastern alpine sector a revision of the available dss data abstract in the alpine sector east of the giudicarie line the investigation of crustal structure is less developed than in the western and central sectors because seismic near vertical reflection nvr profiles are not yet available however it is felt that a revision by a complete homogeneous reinterpretation of several refraction wide angle reflection profiles or deep seismic soundings dss recorded during different phases of the early exploration will help in defining a model of the deep crust and of the moho boundary in the whole area from the central to the eastern alps six interpretative cross sections are proposed as the final output of the seismic modelling as well as a map of the crust mantle boundary the main results of this investigation are the following the lower crust and the moho beneath the northern alps show a fairly homogeneous structure the seismic velocity of the european lower crust is anomalously low while in the north the southward dip of the bottom of the european crust is of about 12 in the center of the investigated area its dip is reduced to about 6 in this zone of flatter moho a complex crust mantle transition is observed the northern strip of the adriatic crust appears highly fragmented into small blocks each block dipping in different directions n ne nw as far as the relationship between the surface tectonic features and the deep crustal structure is concerned it is found that while the edge of the adriatic moho is fairly coincident with the periadriatic lineament in its western sector tonale line it appears to be strongly displaced in the eastern one pustertal line also in the eastern alps in agreement with the interpretation of the european geotraverse crossing eastern swiss alps and lombardian southern alps a wedge of the adriatic lower crust overrides the more ductile european crust protruding northwards beyond the edge of the adriatic moho these results can help to better investigate the complex interactions between the european and adriatic plates as well as to study the relationships between deep and surface structures

numerical models of ductile rebound of crustal roots beneath mountain belts crustal roots formed beneath mountain belts are gravitationally unstable structures which rebound when the lateral forces that created them cease or decrease significantly relative to gravity crustal roots do not rebound as a rigid body but undergo intensive internal deformation during their rebound and cause intensive deformation within the ductile lower crust 2 d numerical models are used to investigate the style and intensity of this deformation and the role that the viscosities of the upper crust and mantle lithosphere play in the process of root rebound numerical models of root rebound show three main features which may be of general application first with a low viscosity lower crust the rheology of the mantle lithosphere governs the rate of root rebound second the amount of dynamic uplift caused by root rebound depends strongly on the rheologies of both the upper crust and mantle lithosphere and third redistribution of the rebounding root mass causes pure and simple shear within the lower crust and produces subhorizontal planar fabrics which may give the lower crust its reflective character on many seismic images

seismic structure of the continental crust based on rock velocity measurements from the kapuskasing uplift the michipicoten greenstone belt mgb wawa gneiss terrane wgt and kapuskasing structural zone ksz in ontario canada are regarded as a 25 km thick partial cross section through the crust of the superior province we measured compressional wave velocities vp at confining pressures up to 600 mpa for representative rock samples from these terranes vp for mgb is variable 6 1 7 km s and depends on composition and anisotropy compressional wave velocities for low anisotropy quartzofeldspathic gneisses and intrusive rocks from the wgt and the ksz show little variation over a wide range of silica content but mafic gneisses and anorthositic rocks show a wide velocity range over a narrower silica range a trend corroborated by data for other high grade rocks a model of the continental crust based on the laboratory data shows three megalayers with differing seismic characteristics the shallowest levels mgb are seismically heterogeneous with high average velocities 6 6 km s at 100 mpa because of the dominance of metabasalts the balance of the upper crust wgt is seismically homogeneous as it is dominated by a variety of isotropic quartzofeldspathic rocks with average velocities of 6 35 km s at 100 mpa and 6 6 km s at 600 mpa the uppermost lower crust ksz is layered seismically heterogeneous and transversely isotropic the average velocity at 600 mpa is 6 84 km s the boundary between wgt and ksz is marked by a velocity increase of between 0 2 and 0 3 km s an increase similar to that commonly associated with midcrustal discontinuities i e conrad in shield areas here this boundary corresponds to the amphibolite granulite facies transition and to a change from a tonalitic middle crust to a lithologically heterogeneous uppermost lower crust refraction results show an anomalous high velocity zone in the upper crust under the ksz that on the basis of laboratory velocity data can be correlated with ksz lithologies these data indicate that ksz rocks can be traced to the west to depths as great as 20 km levels deeper than 20 km not exposed at the surface in the ksz are characterized by velocities greater than 7 0 km s and may be dominated by mafic and anorthositic lithologies or by more garnet rich rocks the velocity model of the superior province crust based on geological reconstruction and laboratory velocity data is in general agreement with the velocity structure to depths of about 25 km determined for the crust below the mgb from refraction experiments the velocity structure of this model is also generally similar to the velocity structure of shield areas

plagioclase breakdown and regeneration reactions in grenville kyanite eclogite at glenelg nw scotland a rare type of eclogite at glenelg contains finegrained white streaks about 1 mm across consisting of kyanite oligoclase quartz kfeldspar garnet and omphacite bordering the streaks are relatively enriched in grossular and jadeite components respectively the streaks are interpreted as former plagioclase which reacted with neighbouring garnet and omphacite during a period of shearing and compression p t conditions during the reaction are estimated as 740 c and 17 kbar subsequent reaction manifest by narrow rims of oligoclase on kyanite new growth of garnet and resorption of omphacite is interpreted as an effect of near isothermal decompression by about 2 kbar the compressive phase is attributed to dextral transpression oblique convergence along the grenville front in scotland

eclogite formation and dynamics of crustal roots under continental collision zones field observations on deep parts of the root zone of the caledonian orogen are combined with the geophysical data available for recent continental collision zones such as the alps to provide a model for dynamics of crustal root zones rocks descending into the root zone are locally infiltrated by fluids which cause eclogitization along shear zones and fluid pathways the eclogite facies rocks vary in density from 3 1 to 3 6 g cm 3 reflecting the compositional variation in their protoliths where the amount of highly ductile eclogitic material reaches about 40 the descending crustal segment loses its coherence and disintegrates into a breccia where blocks of granulite facies rocks float in a matrix of eclogite material a density stratified root zone develops where light unreacted or partly eclogitized material floats on an ocean dominated by eclogite facies rocks the top of which may correspond to the moho catastrophic decent of this eclogitic layer may 1 transport blocks of light material to depths of 90 km which upon further disintegration due to eclogitization are released and return by buoyancy forces to the crust 2 result in extension and uplift of the partly eclogitized lighter material

subduction interface processes recorded by eclogite facies shear zones monviso w alps abstract the monviso ophiolite lago superiore unit constitutes a well preserved almost continuous upper fragment of oceanic lithosphere subducted at c 80 km depth thereby providing a unique opportunity to study mechanical coupling processes and meter scale fluid rock interactions occurring at such depths in present day subduction zones it is made of i a variably thick 50 500 m section of eclogitized basaltic crust associated with minor calcschist lenses overlying a 100 400 m thick metagabbroic body and of ii a c 1 km thick serpentinite sole we herein focus on the three major eclogite facies shear zones found at the top of the unit at the boundary between basalts and gabbros and between gabbros and serpentinites respectively strain localization occurred at lithological interfaces irrespective of material strength while ductile deformation dominates along the shear zones local brittle behavior is demonstrated by the existence of numerous eclogite breccias of fe ti metagabbros and widespread garnet fractures possibly linked with intermediate depth eclogite facies micro seismicity these m to hm sized fragments of fe ti metagabbros were later sheared and disseminated within serpentinite schists along the gabbro serpentinite boundary lower shear zone lsz pervasive and focused fluid flow is attested in the lsz by significant alteration of bulk rock compositions weakening of the rocks and widespread crystallization of hydrous parageneses by contrast the intermediate shear zone isz shows evidence for more restricted short range fluid flow the activity of both the isz and lsz ceased during early lawsonite eclogite facies exhumation when deformation localized deeper within the serpentinite sole allowing for the detachment and preservation of this large ophiolitic fragment

seismic structure across the caledonian deformation front along mona lisa profile 1 in the southeastern north sea abstract seismic data from the mona lisa marine and onshore north sea acquisition for lithospheric seismic analysis project in the southeastern north sea image the caledonian deformation front cdf which is the collisional suture between baltica to the north and east and avalonia to the south and west the n s trending mona lisa normal incidence reflection profile 1 was recorded to 26 s twt coincident wide angle data were acquired on nine ocean bottom hydrophones and several onshore mobile seismographs along and off the profile the model of compressional seismic velocity shows three different crustal types a a typical three layered shield type crust below the ringkobing fyn high to the north b a highly complex transitional crust in the central part and c a two layered crust of caledonian origin to the south sharp and strong normal incidence and wide angle reflections from moho were recorded south of the caledonian deformation front in contrast to less distinct reflections further north s dipping crustal reflections from 4 to 11 s twt over 70 km horizontal distance terminate at moho and coincide with a change in the seismic velocity structure this indicates northward obduction of avalonian crust non migrated normal incidence seismic sections show crossing weak n dipping and stronger s dipping reflections to 20 s twt from the uppermost mantle we propose a tectonic model where the closure of the tornquist sea took place along a n dipping subduction zone which was later overprinted by a late or post caledonian s dipping shear zone sub moho velocities are 7 8 7 9 km s under 34 35 km thick baltica crust and 8 1 8 3 km s under 25 26 km thick caledonian crust the sub horizontal moho across the caledonian collision zone implies late or post caledonian re equilibration of the seismological moho we interpret the low velocity upper mantle 7 8 8 1 km s to the north as former baltica lower crust in eclogite facies after pressure induced metamorphism as a result of lithospheric flexure during the caledonian orogeny these rocks today appear as upper mantle that was uplifted to their present position during the middle devonian collapse of the north german polish caledonides