compositional and metamorphic controls on velocity and reflectivity in the continental crust an example from the grenville province of eastern quebec we present velocity and density data for a suite of gabbros and metagabbros over a range of metamorphic grades from subgreenschist to the eclogite facies most of these rocks have a mesocratic protolith with overlapping bulk chemistry ranges providing an opportunity to investigate the systematics of mineralogy and physical properties across a wide range of metamorphic grades with minimal compositional variation in addition more mafic gabbros in the eclogite facies were also investigated in order to take into account variations in the extent of transformation of gabbro to eclogite controlled by bulk chemistry the physical properties data are used as the basis for a first order crustal model composed of granitic anorthositic and gabbroic components the objective of constructing the crustal model is to evaluate the relative contribution of compositional variation and metamorphism to the velocity density and reflectivity structure of the continental crust the model suggests that in the upper crust if fracture porosity is neglected composition dominates the velocity and density structure while in the middle and lower crust composition and metamorphism play equally important roles simulations designed to predict potential reflection coefficient size distributions for the various bulk chemistries and mineralogies in the crustal model indicate step increases in the potential for large reflection coefficients that are dominantly controlled by metamorphic grade step increases in reflectivity are predicted between the lower and upper amphibolite facies and between incompletely and completely eclogitized metagabbros in the eclogite facies we also consider several alternative mixing models which account for the effects of one of the three lithologic components occurring as isolated bodies rather than perfectly mixed with the other two components these models indicate a reduction of the potential for large reflection coefficients by as much as 50 emphasizing the importance of the degree of mixing for crustal reflectivity

variscan accretionary complex of northwest iberia terrane correlation and succession of tectonothermal events the allochthonous terranes of northwest iberia can be correlated with specific paleogeographic realms of the continental masses and intervening oceans involved in the variscan collision assuming that the existing ophiolites represent the suture formed by the closure of the rheic ocean the units in the footwall to the suture correspond to the outer edge of the gondwana continental margin and the units in the hanging wall are interpreted as fragments of the conjugate margin represented by the meguma terrane this correlation establishes a precise link between circum atlantic terranes and makes it possible to draw a relatively simple scenario of the successive tectonothermal events recorded following the amalgamation of avalon to laurentia the remaining outboard terranes underwent a progressive accretion to this continent that ended with the collision between laurentia and gondwana

opposite p t t paths of hercynian metamorphism between the upper units of the cabo ortegal complex and their substratum northwest of the iberian massif abstract in the cabo ortegal region two metamorphic groups can be distinguished which have followed opposite p t paths in their hercynian metamorphism the upper group comprises the three uppermost units of the allochthonous complex with mafic and related lithologies of cabo ortegal the lower metamorphic group includes the two basal units of the complex and the lithological sequence outcropping in the ollo de sapo anticlinorium the lithologies of the upper group were affected by high p and medium high t eo hercynian metamorphism during a subduction episode related to the onset of the hercynian orogenic cycle as a result of such metamorphism the basic rocks were transformed into b type eclogites and also into high p mafic granulites the advance of the hercynian collision produced the progressive uplift of the subducted lithologies and finally their emplacement as large thrust sheets over the lower metamorphic group this final evolution is clearly reflected in the p t t paths of the high p lithologies which show pronounced decompressions with only minor cooling the evolution of the lower metamorphic group does not reflect high p events the characteristic regime is a barrovian type medium p metamorphism showing p t t paths with a progressive compression during heating at least partially the burial of the rock association was due to the overthrusting of a thick allochthonous pile this is represented by the upper metamorphic group

seismic wide angle constraints on the crust of the southern urals a wide angle seismic reflection refraction data set was acquired during spring 1995 across the southern urals to characterize the lithosphere beneath this paleozoic orogen the wide angle reflectivity features a strong frequency dependence while the lower crustal reflectivity is in the range of 6 15 hz the pmp is characterized by frequencies below 6 hz after detailed frequency filtering the seismic phases constrain a new average p wave velocity crustal model that consists of an upper layer of 5 0 6 0 km s which correlates with the surface geology 5 7 km depths at which the velocities increase to 6 2 6 3 km s 10 30 km depths at which on average the crust is characterized by velocities of 6 6 km s and finally the lower crust from 30 35 km down to the moho which has velocities ranging from 6 8 to 7 4 km s two different s wave velocity models one for the n s and one for the e w were derived from the analysis of the horizontal component recordings crustal sections of poisson s ratio and anisotropy were calculated from the velocity models the poisson s ratio increases in the lower crust at both sides of the root zone a localized 2 3 anisotropy zone is imaged within the lower crust beneath the terranes east of the root this feature is supported by time differences in the sms phase and by the particle motion diagrams which reveal two polarized directions of motion velocities are higher in the central part of the orogen than for the siberian and eastern plates these seismic recordings support a 50 56 km crustal thickness beneath the central part of the orogen in contrast to moho depths of 45 km documented at the edges of the transect the lateral variation of the pmp phase in frequency content and in waveform can be taken as evidence of different genetic origins of the moho in the southern urals

composition of the crust beneath southeastern china derived from an integrated geophysical data set 1 the nonuniqueness in determining the composition of the crust can be reduced by comparing the data for p wave velocity vp s wave velocity vs vp vs ratio mass density and lame impedances to data determined from laboratory measurements of these physical variables on a variety of crustal rock samples the composition of the crust along the tunxi wenzhou transect southeastern china is presented as a model based on a complete set of geophysical data involving seismic p wave and s wave velocity density gravity heat flow and temperature surveys which allow us to place tighter constraints on possible crustal models the yangtze and the cathaysia blocks have a crustal structure characterized by remarkable differences between the upper middle and lower crust integrated geophysics data are here interpreted to indicate 1 an average composition of granite gneiss for the upper crust with presence of mica quartz schist felsic granulite paragranulite and granite granodiorite beneath the yangtze block and basalt down to the upper part of the middle crust just over the jiangshan shaoxing fault followed laterally by granite gneiss beneath the songyang qintiang and wenping depressions 2 granite granodiorite and biotite gneiss for the upper half of the middle crust with presence of mica quartz schist beneath the cathaysia block 3 gabbro and in less proportion basalt for the lower part of the middle crust and 4 amphibolite for the lower crust with presence of mafic garnet granulite just over the jiangshan shaoxing fault and beneath the wenping depression

structural assemblage of high pressure mantle and crustal rocks in a subduction channel cabo ortegal nw spain 1 high pressure metamorphic fold nappes and ductile thrusts occur in the cabo ortegal upper allochthons they involve arc root lithospheric mantle peridotites high pressure volcanic arc granulites normal mid oceanic ridge basalt eclogites and shallow crustal volcanosedimentary high pressure gneiss units confluence of units from such disparate tectonic settings occurred at an eo hercynian convergent plate margin their internal deformation juxtaposition and high pressure synmetamorphic reworking resulted from kinematically simple but intense and polyphase ductile deformations fold nappes exhibit a remarkable sheath like form and contain mineral and stretching lineations defined by high pressure mineral assemblages subparallel to fold axes we argue that these structures outline the internal organization of high pressure sheets at deep tectonic realms of an oblique subduction collision orogenic channel

synthetic seismic reflection profile through the ivrea zone serie dei laghi continental crustal section northwestern italy a geologic cross section restored to its original horizontal orientation in permian triassic time has been constructed for the middle and lower continental crustal rocks of the ivrea verbano zone and the adjacent serie dei laghi of northwestern italy seismic p wave velocities of a representative suite of rock samples were measured to high pressure and high temperature conditions a synthetic seismic reflection profile 76 km long and 30 km thick was computed to compare what can be deduced from the seismic profile with what is known in much more detail from geologic mapping imaged features correspond closely to those seen on many present day profiles and the broad features of the tectonic evolution would be correctly interpreted but important recumbent fold structures would be missed and relationships between intrusive bodies and their country rocks would be unclear

laboratory measurements of p wave and s wave velocities across a surface analog of the continental crust mantle boundary cabo ortegal spain the paleozoic cabo ortegal complex of nw spain provides an exposed analog of the continental crust mantle transition it is composed of an overturned section that at its base begins with felsic gneisses followed upward by eclogites intermediate and mafic granulties and ultramafic rocks peak metamorphic conditions reached c 800 c and 1 7 gpa in the middle to late devonian fourteen samples were analysed for p wave and s wave velocities as well as density at the high pressure lab at dalhousie university canada seismic velocities were measured at pressures of 10 to 600 mpa at a temperature of 20 c when possible measurements were made parallel and perpendicular to banding and parallel to the lineation the major element composition of each sample was measured by xrf at the university of barcelona spain samples display a broad range of p wave and s wave velocities 6 2 to 8 2 km s and 3 2 to 4 6 km s at 600 mpa respectively that generally increase with density 2 7 to 3 4 g cm3 and reflect an overall increase from middle to lower crustal velocities in the felsic gneisses and intermediate to mafic granulites to mantle velocities in the eclogites and ultramafic rocks the seismic moho p wave velocity 7 6 km s is reached at the mappable contact between the gneisses and the eclogite whereas the compositional moho or crust mantle transition occurs at the transitional contact between the mafic granulites and peridotites between 200 and 600 mpa p wave anisotropy ranges from between 2 and 8 whereas s wave anisotropy ranges from 1 to around 8 according to rock type poisson s ratios calculated from the laboratory measurements are within the range of those determined from field experiments elsewhere p wave reflection coefficients between the various lithologies range from 0 21 to 0 08 these laboratory data provide a calibration for the physical properties and the nature of reflectivity of the in situ lower continental crust and upper mantle transition

seismic reflection coefficients from mantle fault zones summary several bright reflections from structures within the mantle can be seen on birps deep seismic reflection profiles we have calculated apparent reflection coefficients for the brightest of these events and obtain values around 0 1 it is not possible to produce such large reflections by either compositional layering or seismic anisotropy if olivine and pyroxene are the only significant minerals in the mantle these large reflections can be produced by a mafic layer or a partially hydrated layer within normal peridotite the brightest reflections seem to be best explained as major faults or shear zones within the mantle

an interpretation of wide angle compressional and shear wave data in southwest germany poisson s ratio and petrological implications shear wave velocity especially when used with compressional wave velocity is an important constraint on estimates of the composition and physical state of the continental lithosphere we present shear velocity s models derived from high quality s wave data along three profiles in southwest germany which combine with existing p wave velocity p models to give poisson s ratio in the crust and uppermost mantle we then construct a laterally varying petrological model of the crust by comparing our p and s models with published laboratory measured rock velocities in general poisson s ratio is 0 25 in the uppermost crust 0 22 0 23 in the mid crust but reaching a low of 0 15 in the black forest low p zone and varies laterally from 0 24 to 0 30 in the lower crust the average poisson s ratio of the whole crust is everywhere about 0 25 the laminated signature of the lower crust prominent on reflection and p wave refraction data is absent on the s wave refraction data implying a strong high low alternation of poisson s ratio in the lower crust this suggests that the lower crustal laminae represent mafic silicic compositional layering for example basic sills in an originally more quartz rich lower crust rather than alternating zones of high and low pore pressure there is no evidence for a shear wave low velocity zone lvz in the black forest mid crust despite the presence of a strong p wave lvz this suggests high quartz content and the presence of pore fluids at low not high pore pressure a clear p to s converted moho reflection pm s is visible on several shots indicating that the moho is at least locally a first order discontinuity no shear wave refractions from the upper mantle are present even where strong p wave refractions are observed possibly indicating an increase of poisson s ratio with depth in the upper mantle the petrological modeling indicates that the upper and middle crust are probably of granitic composition that the mid crustal low p zones have high quartz content and that the lower crust probably consists of granulites of laterally varying bulk composition from granodioritic to gabbroic the lower crust is more mafic in areas of high reflection density on seismic reflection profiles suggesting that the laminations are caused by mafic intrusions into the lower crust either prior to the variscan orogeny or during tertiary volcanic activity

analysis of pyrrhotite properties accompanying recrystallization of metamorphosed nickel sulfide ore from kambalda western australia serial sectioning was carried out on a single sample of oriented pyrrhotite pentlandite ore from the durkin shoot at kambalda this permitted the sequential investigation of a number of pyrrhotite properties including grain morphologies triple junction angles grain lattice orientation microhardness peak broadening in x ray diffractograms and approximate fe s ratios of the monoclinic pyrrhotite by microprobe analysis the present study confirms the presence of previously identified flattened tabular f and equant polyhedral e pyrrhotite morphologies and establishes that the e type has recrystallized from the f variety the latter is composed of subgrain aggregates with c axes that tend to be oriented perpendicular to the pyrrhotite foliation formation of the e type pyrrhotite from the f variety involved subgrain rotations subgrain coalescence and grain growth this was accompanied by definite reductions in values for triple junction angles and in microhardness as well as apparent reductions i

the eastward shift of the walker circulation in response to global warming and its relationship to enso variability this study investigates the global warming response of the walker circulation and the other zonal circulation cells represented by the zonal stream function in cmip3 and cmip5 climate models the changes in the mean state are presented as well as the changes in the modes of variability the mean zonal circulation weakens in the multi model ensembles nearly everywhere along the equator under both the rcp4 5 and sres a1b scenarios over the pacific the walker circulation also shows a significant eastward shift these changes in the mean circulation are very similar to the leading mode of interannual variability in the tropical zonal circulation cells which is dominated by el nino southern oscillation variability during an el nino event the circulation weakens and the rising branch over the maritime continent shifts to the east in comparison to neutral conditions vice versa for a la nina event two thirds of the global warming forced trend of the walker circulation can be explained by a long term trend in this interannual variability pattern i e a shift towards more el nino like conditions in the multi model mean under global warming further interannual variability in the zonal circulation exhibits an asymmetry between el nino and la nina events el nino anomalies are located more to the east compared with la nina anomalies consistent with this asymmetry we find a shift to the east of the dominant mode of variability of zonal stream function under global warming all these results vary among the individual models but the multi model ensembles of cmip3 and cmip5 show in nearly all aspects very similar results which underline the robustness of these results the observed data era interim reanalysis from 1979 to 2012 shows a westward shift and strengthening of the walker circulation this is opposite to what the results in the cmip models reveal however 75 of the trend of the walker circulation can again be explained by a shift of the dominant mode of variability but here towards more la nina like conditions thus in both climate change projections and observations the long term trends of the walker circulation seem to follow to a large part the pre existing dominant mode of internal variability

multi genetic origin of the continental moho insights from lithoprobe this paper reviews four hypotheses for the origin of the continental crust mantle boundary and discusses seismic parameters with which these hypotheses might be tested the relict moho hypothesis posits that the oceanic moho is preserved during continental assembly the magmatic underplating hypothesis posits formation of a new moho by episodic emplacement of sill like intrusive bodies the metamorphic or metasomatic front hypothesis posits that the moho is overprinted by a phase transformation and the regional decollement hypothesis posits that the moho behaves as a structural detachment these hypotheses are not mutually exclusive and examples from the canadian lithoprobe program suggest that all four may be applicable in different regions of north america comparison of seismic images from a fossil subduction zone with modern subduction at cascadia suggests that serpentinization of the forearc mantle a previously unrecognized mechanism for overprinting and erasing the reflection moho may have occurred in the proterozoic

a single devonian subduction event for the hp ht metamorphism of the cabo ortegal complex within the iberian massif abstract ion microprobe data are reported for zircons from ultramafic mafic and metasedimentary high pressure hp rocks of the cabo ortegal allochthonous complex galicia nw spain the new data suggest that the protoliths of eclogites morb type and mg and al rich and mafic granulites have formed in early ordovician times 495 11 469 9 and 478 12 ma respectively involving oceanic and or continental mantle sources we have found no evidence for a hp metamorphic event at 480 ma and for the previously proposed second hp event in the silurian instead a single subduction induced pt t loop with a climax around 390 ma early devonian can be deduced from the relevant domains of zircons from the following rocks two different types of eclogites two hp paragneisses one garnet pyroxenite and one spinel harzburgite rapid cooling and exhumation minimum rates of ca 90 c ma and ca 5 8 km ma are derived for the time immediately after the peak conditions after cooling to ca 500 c slower cooling and exhumation rates ca 15 18 c ma and ca 0 18 km ma are probably artefacts due to an episode of reheating at ca 350 ma this interpretation is supported by the scatter of various mineral ages and the presence of synkinematic granitoids a subduction related acadian early devonian hp metamorphism followed by an early carboniferous collision can thus be demonstrated for the first time also for the iberian part of the european variscides

kinematic thermal and petrological model of the central alps lepontine metamorphism in the upper crust and eclogitisation of the lower crust abstract seismic and seismological studies as well as gravimetric models indicate that a slab of european lithospheric mantle and lower crust is currently underthrust below the apulian crust we assume a simple kinematic model in which the lower and upper subducted european crusts are decoupled along a decollement the lower crust goes into subduction without deformation the upper crust deforms by pure shear with a horizontal compressional axis the total erosional flux is adjusted to balance upper crust input so that the belt keeps the same geometry different distributions of erosion being used the computed temperature field is steady state if the kinematic model applies during a minimum time of 40 myr for a convergence rate of 8 mm yr equilibrium mineral assemblages and densities are determined from the computed p t conditions for a granodioritic chemical composition of the upper crust and an andesitic composition of the lower crust assuming local isostasy the density model fits the average topographic profile across the central alps the p t t paths obtained for the part of the upper crust initially at depths 10 to 16 km are compatible with the medium pressure oligocene metamorphism in the lepontine dome the peak calculated temperature for the deepest non subducted crustal rocks is 600 c for a pressure of 0 8 gpa near the lower limit of high pressure amphibolites we thus propose that the lepontine metamorphism corresponds to the steady state thermal regime however either faster erosion rates in the internal part of the belt or tectonic denudation are required for exhumation of the deeper portion of the belt the computed temperature field implies eclogitisation of the lower crust at a depth of 55 to 60 km we conclude that the moho limiting the deepest part of the root may correspond to the eclogitisation phase change lower crust eclogites have a density comparable to or higher than that of the mantle depending on their chemical composition 3 37 for andesitic eclogites 3 56 for gabbroic eclogites thus andesitic eclogites may stay in gravitational equilibrium in the mantle below the root whereas gabbroic eclogites are gravitationally unstable and should sink

high pressure high temperature seismic velocity structure of the midcrustal and lower crustal rocks of the ivrea verbano zone and serie dei laghi nw italy the ivrea verbano zone and the adjacent serie dei laghi in the inner arc of the western alps nw italy display an upended cross section through much of the thickness of the continental crust as it existed in that region in triassic jurassic time we collected a suite of oriented rock samples that represent most of the volume of the rocks of the region p and s wave velocity measurements were made in three orthogonal directions related to the mesoscopic fabric of the rocks at room temperature and up to 550 mpa confining pressure combined high temperature and high pressure measurements were made up to 700 c on a subset of the samples the lithologic units were divided into 23 different principal rock types and we present velocity data averaged in terms of these groups vertical and horizontal velocity and anisotropy sections are computed from the measured velocity data based on a restored geologic section these show that vp and vs both increase and anisotropy decreases systematically with depth of burial reflecting variations in rock type and metamorphic grade with depth vp and vs anisotropy arises mainly because of crystallographic preferred orientation or mineralogical banding with the slowest direction tending to be normal to the foliation or banding the vp velocity sections were used to compute synthetic seismic reflection profiles for the region when it lay at the bottom of the continental crust these correspond well with contemporary deep reflection profiles for regions of comparable tectonic evolution

development of shape and lattice preferred orientations application to the seismic anisotropy of the lower crust abstract we review the physical basis of the development of fabrics in plastic and viscous flow and illustrate the typical fabrics formed by these processes in the main rock forming silicates of the lower crust feldspar quartz pyroxene and amphibole the orientation process in plastic deformation where a single slip is dominant is recalled and the role of the constraint of neighbouring grains is emphasized the fabric development of anisometric crystals in viscous flow is discussed as a function of the main controlling parameters shear strain aspect ratio and interference between crystals the same sense of fabric asymmetry is introduced by plastic and viscous flow between the flow plane and the shape preferred orientation and hence coherent kinematic analysis can be undertaken in both modes of flow in order to assess the role of such fabrics in the seismic laminations of the lower continental crust we have calculated the seismic p wave properties of typical fabrics for hypothetical monomineralic and polymineralic rocks the calculations show that the strongest anisotropies develop in monomineralic rock with values between 5 and 16 compared with 5 and 8 for typical rock compositions the strongest anisotropies for layered monomineralic rocks generated by fabrics is only 6 compared to the 14 suggested by model studies of the observed seismic laminations we suggest that other effects such as compositional layering and or constructive interference of seismic waves are responsible for augmenting the apparent anisotropy

amphibole and lower crustal seismic properties abstract measurements of seismic anisotropy frequently offer insights of the deformation kinematics in situ within the earth in the continental crust seismic anisotropy is commonly ascribed to regionally aligned lattice preferred orientation lpo of mica crystals however determinations of composition suggest that the deep continental crust contains rather little mica in this contribution the role of amphibole a far more plausible constituent especially for deep continental crust of basic composition is assessed a field analogue approach is adopted where the lpo are measured correlated with the strain state of the rocks and used to calculate the seismic properties the seismic properties may be up scaled through the specimen using the modal proportions of the constituent mineral phases to examine the role of composition in controlling the intensity of seismic anisotropy seismic properties are computed using an array of modal compositions so called rock recipes the case history comes from the central block of the lewisian of nw scotland where a regional basic dyke swarm the scourie dykes is locally deformed within amphibolite facies shear zones conditions that are representative of the deep crust shear strain profiles across a 100 m wide shear zone using deflected banding in the host gneisses yield values of 0

geochemical and geochronological cross section of the deep variscan crust the cabo ortegal high pressure nappe northwestern spain the high pressure rocks of the cabo ortegal nappe and related massifs are represented by several tectonometamorphic units which underwent separate histories before regional overthrusting and amphibolite facies recrystallization at 380 ma the occurrence of final cooling is recorded by 40ar 39ar and rb sr mica ages of up to 350 ma u pb zircon dating and sm nd and rb sr isotope data together with ree and major and trace element geochemistry provide constraints on the timing of earlier tectonothermal events and the nature of the protoliths the earliest tectonothermal event occurred at 490 480 ma and corresponds to the emplacement of lree enriched magmas of probable calc alkaline affinity and their metamorphism in the granulite facies this occurred at approximately the same time as the eclogite facies metamorphism of morb like protholiths associated with basic to intermediate granulites together this suggests formation of syn accretion granulites in an active margin setting a second high grade metamorphic event is recorded at 420 ma in metasedimentary formations associated with eclogite like assemblages garnet omphacite plagioclase this event is possibly related to a collisional plate tectonic setting in a back arc basin the age and origin of the associated ultrabasic rocks remains unclear but a late pyroxenite vein has yielded an emplacement age of 390 ma

deformation high pressure metamorphism and exhumation of ultramafic rocks in a deep subduction collision setting cabo ortegal nw spain abstract new petrological and microstructural data from various metaperidotite boudins within large ductileshear zones in the cabo ortegal allochthonous complex in nw spain have important implications forthetectonicmodelsofthearea theperidotites myloniticgarnetharzburgite ti clinohumiteandmagnes ite olivine orthopyroxenite contain mineral assemblages that equilibrated at high to ultra high pressuremetamorphic conditions as well as microstructures of tectonic origin formed at temperatures well above800 c olivine and orthopyroxene fabrics resulted from ow at high temperature 1000 c and solid statenon coaxialplastic owatintermediatetemperature 800 900 c flowcauseddynamicrecrystalliz ationandformationofmoderatetostronglatticepreferredorientationsunderlowtomoderatediverentialstresses and strain rates characteristic of upper mantle and deep crustal deformation the microstructuresand textural relationships suggest that the mylonitic garnet harzburgite represents mantle fragments withlithospheric and asthenospheric imprints whereas the olivine orthopyroxenite resulted from serpentiniteburial to depths where it acquired a characteristic high ultra high pressure metamorphic signature bothtypes of ultrama tes converged to a common sitein a subduction zone that was later incorporated duringcontinental collision to the nw iberian massif as exotic allochthonous complexes that record structuraland metamorphic evidence of the earliest phases of the hercynian orogeny key words cabo ortegal metaperidotite petrofabric rheology ti clinohumite