microstructures petrofabrics and seismic properties of ultra high pressure eclogites from sulu region china implications for rheology of subducted continental crust and origin of mantle reflections ultra high pressure uhp eclogites from sulu region china represent mafic components of the continental crust which were first subducted to mantle depths greater than 100 km and then exhumed to the earth s surface detailed investigation of microstructures chemical compositions petrofabrics and seismic properties of the uhp eclogites can provide important information on the operating deformation mechanisms and rheology of subducted continental crust and on the origin of seismic reflections within the upper mantle we present here results from field optical and tem observations electron back scattered diffraction ebsd measurements and numerical computations of the seismic properties of uhp eclogites collected from fresh surface outcrops at the drill site maobei donghai county jiangsu province of the chinese continental scientific drilling program ccsd two types of eclogites have been distinguished type 1 coarse grained eclogites deformed by recovery accommodated dislocation creep at the peak metamorphic conditions and type 2 fine grained eclogites which are composed of reworked type 1 materials during recrystallizationaccommodated dislocation creep in shear zones which were active during the exhumation of the uhp metamorphic rocks both garnet and omphacite in these eclogites deformed plastically and the flow strength contrast between these two constituent minerals is apparently much less than an order of magnitude under the uhp metamorphic conditions plasticity of eclogites under uhp conditions can effectively facilitate channeled flow along the interplate shear zone the

crystallographic fabrics of omphacite rutile and quartz in vendee eclogites armorican massif france consequences for deformation mechanisms and regimes abstract this study aims at further understanding of the mechanisms how lattice preferred orientations lpo develop during deformation in the main eclogite minerals microstructures and textures of deformed eclogites from the les essarts complex western france were investigated using optical microscopy and electron backscatter diffraction ebsd in the scanning electron microscope microfabric analyses of eclogite facies minerals are used to identify their deformation mechanisms which define the rheology at high pressure metamorphic conditions mechanisms of intracrystalline deformation by dislocation movement dislocation creep result usually in a non linear flow law typically power law while diffusive processes diffusion creep correspond to linear flow laws general microstructural observations may suggest intracrystalline deformation dislocation creep of omphacite the omphacite lpo vary between s and l type and correlate with oblate or prolate grain shape fabrics respectively until now these lpo types have not been understood by plasticity models based on dislocation glide on the known slip systems in clinopyroxene an alternative interpretation is given in terms of anisotropic growth and dissolution with grain boundary diffusion as the rate controlling process there are further indications suggesting diffusion creep with concomitant anisotropic growth and dissolution as a main deformation mechanism in omphacite in omphacite around a hollow garnet crystallographic and shape fabrics align with the c 001 axes parallel to the grain elongations defining the mineral lineation which rotates locally with the inferred flow direction in this part the grain sizes of omphacite and rutile are larger than in the surrounding matrix the geometry of both the shape and crystallographic fabrics is interpreted to represent the local stress regime directions and ratios of the principal stresses the lpo of rutile duplicate the lpo of omphacite and a similar distinction between s and l type was used rutile deformation mechanisms probably involve dislocation creep as well as diffusion creep quartz mainly occurs as an interstitial phase with weak lpo patterns interpreted as random no representative obliquity of the lpo in omphacite nor rutile with respect to foliation and lineation was observed to be used as potential shear sense criteria however the rutile lpo was slightly rotated relative to the omphacite lpo consistently in most samples the results suggest that diffusion processes are strongly involved in the deformation of eclogites a linear flow law should be taken into account in tectonic models where eclogites are incorporated

resurfacing of deeply buried oceanic crust in naga hills ophiolite north east india petrofabric microstructure and seismic properties abstract we have studied the microstructure texture and seismic anisotropy of a foliated blueschist eclogite rock in the naga hills ophiolite nho complex north east india the rock is composed essentially of omphacite sodic amphibole glaucophane sodic calcic amphibole barroisite phengite and garnet and represents oceanic crust that experienced neo tethyan subduction and metamorphism before it resurfaced and accreted in the ophiolite omphacite and glaucophane show weak shape preferred orientation spo while phengite displays a strong spo the crystallographic preferred orientation cpo of omphacite is characterized by the 001 axes gridle within the foliation and the 010 poles concentrated sub perpendicular to the foliation for glaucophane the 001 axis aligns parallel to lineation and the 100 axis and 110 pole plunge perpendicular to foliation these cpos correspond to sl type fabrics related to plane strain deformation geometry based on our analysis we propose that the observed texture had developed from plastic deformation through dislocation creep employing the textural data we have presented the seismic anisotropies of the individual minerals blueschist eclogite domains and discussed their contributions in the bulk rock anisotropy the calculated bulk rock seismic anisotropies avp and avs of the rock are 12 8 and 8 1 respectively which correlate with the observed seismic anisotropy of the subducting oceanic crust

seismic properties of lawsonite eclogites from the southern motagua fault zone guatemala abstract we present new data on the crystal preferred orientation cpo and seismic properties of omphacite and lawsonite in extremely fresh eclogite from the southern motagua fault zone guatemala to discuss the seismic anisotropy of subducting oceanic crust the cpo of omphacite is characterized by 010 001 and it shows p wave seismic anisotropies avp of 1 4 3 2 and s wave seismic anisotropies avs of 1 4 2 7 lawsonite exhibits 001 planes parallel to the foliation and 010 axes parallel to the lineation and seismic anisotropies of 1 7 6 6 avp and 3 4 14 7 avs the seismic anisotropy of a rock mass consisting solely of omphacite and lawsonite is 1 2 4 1 avp and 1 8 6 8 avs for events that propagate more or less parallel to the maximum extension direction x the fast s wave velocity vs polarization is parallel to the z in the y z section rotated from the x z section causing trench normal seismic anisotropy for orthogonal subduction based on the high modal abundance and strong fabric of lawsonite the avs of eclogites is estimated as 11 7 in the case that lawsonite makes up 75 of the rock mass on this basis we suggest that lawsonite in both blueschist and eclogite may play important roles in the formation of complex pattern of seismic anisotropy observed in ne japan weak trench parallel anisotropy in the forearc basin domains and trench normal anisotropy in the backarc region

petrofabrics of high pressure rocks exhumed at the slab mantle interface from the point of no return in a subduction zone sivrihisar turkey the highest pressure recorded by metamorphic rocks exhumed from oceanic subduction zones is 2 5 gpa corresponding to the maximum decoupling depth mdd 80 10 km identified in active subduction zones beyond the mdd the point of no return exhumation is unlikely the sivrihisar massif turkey is a coherent terrane of lawsonite eclogite and blueschist facies rocks in which assemblages and fabrics record p t fluid deformation conditions during exhumation from 80 to 45 km crystallographic fabrics and other features of high pressure metasedimentary and metabasaltic rocks record transitions during exhumation in quartzite microstructures and crystallographic fabrics record deformation in the dislocation creep regime including dynamic recrystallization during decompression and a transition from prism slip to activation of rhomb and basal slip that may be related to a decrease in water fugacity during decompression 2 5 to 1 5 gpa phengite lawsonite and omphacite or glaucophane in quartzite and metabasalt remained stable during deformation and omphacite developed an l type crystallographic fabric in marble aragonite developed columnar textures with strong crystallographic fabrics that persisted during partial to complete dynamic recrystallization that was likely achieved in the stability field of aragonite p 1 2 gpa results of kinematic vorticity analysis based on lawsonite shape fabrics are consistent with shear criteria in quartzite and metabasalt and indicate a large component of coaxial deformation in the exhuming channel beneath a simple shear dominated interface this large coaxial component may have multiplied the exhuming power of the subduction channel and forced deeply subducted rocks to flow back from the point of no return

shear wave properties and poisson s ratios of ultrahigh pressure metamorphic rocks from the dabie sulu orogenic belt china implications for crustal composition 1 shear wave velocities vs anisotropy and shear wave splitting have been measured at pressures up to 600 800 mpa for ultrahigh pressure uhp metamorphic rocks from the dabie sulu orogenic belt china with a focus on three types of eclogites type 1 eclogites are coarse grained unaltered samples showing high densities and high vs values 4 85 0 06 km s at 600 mpa type 2 eclogites are fine grained sheared samples with intermediate vs values 4 53 0 04 km s at 600 mpa and type 3 eclogites are overprinted by amphibolite facies metamorphism and display low vs values 4 33 0 09 km s at 600 mpa the compositional layering and retrograde metamorphism can result in significant anisotropy and shear wave splitting in eclogites suggesting their plausible contribution to seismic anisotropy in the lower crust upper mantle and particularly in subducted slabs integrating our p and s wave velocity results with reliable data from previous studies we estimated the pressure and temperature derivatives of vp vs and poisson s ratios for common rock types in the uhp metamorphic belt the geometric means were used as a mixture rule to invert the lithological and chemical compositions of the layered crust from seismic refraction velocities the inferred crustal composition suggests that the eclogite bearing uhp rocks are tectonic slices of crust that have been thrust along a series of shear zones during the continental collision between the north china and yangtze cratons over a normal uhp free middle lower crust with overall intermediate composition

shear wave polarization anisotropy and subduction induced flow in the mantle wedge of northeastern japan abstract shear wave polarization anisotropy in the central part of northeastern ne japan was investigated we analyzed s phases of 293 intermediate depth earthquakes recorded at 77 stations and obtained 1286 splitting parameters the obtained results show the systematic variation in splitting parameters across the arc the leading shear wave polarization directions fast direction obtained at stations in the western side of the study area are oriented nearly e w which is sub parallel to the direction of relative plate motion between the pacific plate and the north american okhotsk plate in contrast fast directions obtained at stations in the eastern side of the study area are oriented approximately n s stations in the western side have a larger average delay time compared to those in the eastern side these variations suggest that the nature of anisotropy is quite different between the eastern and the western sides both experimental studies on the deformation of olivine and numerical simulations of plate driven flow predict that the lattice preferred orientation of olivine causes the fast direction sub parallel to the flow direction for the conditions in the mantle wedge of ne japan we thus infer that the lattice preferred orientation of olivine generated by flow induced strain is the most likely candidate for anisotropy that produces splitting with e w fast direction three possible causes of anisotropy in the eastern side are considered deformation of water rich olivine in the mantle wedge trench parallel flow in the mantle wedge due to along strike dip variations in the slab and anisotropy in the crust and the slab we could not however determine which causes are dominant further systematic study is required to reveal the cause of anisotropy in the eastern side

deformation and recrystallization mechanisms in naturally deformed omphacites from the sesia lanzo zone geophysical consequences buatier m van r rmund h l m drury m r and lardeaux j m 1991 deformation and recrystallization mechanisms in naturally deformed omphacites from the sesia lanzo zone geophysical consequences teetonophysrc c 195 11 27 naturally deformed omphacites clinopyroxenes from the sesia lanzo zone western italian alps have been studied using light and transmission electron microscopy techniques deformation and recrystallization of omphacite took place at 5 50 600 c and minimum pressures of 14 16 kbar tem reveals isolated free dislocations loops nodal points tiltwalls complex dislocation networks minor twins stacking faults and antiphase domain boundaries the activated slip systems are l z llo llo l llo and al assisted by climb in nodal points and complex dislocation networks dislocations with burgers vector l 2 112 are present in low strain domains most of the subgrain transitional and new grain boundaries subparallel to wl develop from 110 tiltwalls which accommodate lattice bending produced by the l 2 110 110 slip system a component of the misorientation across such newly formed sub grain boundaries is however also formed by sub rain boundary migration grain rotations in the intermediate and high strain domains are more complex but can be explained by the activated slip systems the omphacite microstructures indicate that initial strain induct rec stallisation occurred by a rotation mechanism where single crystals become polycrystals by the development of numerous high angle grain boundaries at a subsequent more advanced stage migration recrystallization is also activated and a different kind of recrystallization mechanism occurs which involves the discontinuous development of new strain free grains these grains are derived from the extremum of the orientations produced by heterogeneous strain and slip system activity the evident ductility of omphacite during low to intermediate eclogite metamorphism is inferred to be facilitated by hydrolitic weakening effects as such the plastic behaviour of omphacite should be incorporated in geophysical flow models dealing with subducted oceanic and or lithosperic plate margins

contribution of crystallographic preferred orientation to seismic anisotropy across a surface analog of the continental moho at cabo ortegal spain the lithostratigraphic sequence of the upper allochthon of the cabo ortegal complex in northwestern spain provides an excel lent analog for the direct study of petrophysical properties of the continental moho transition the various lithologies present were sampled for velocity measurements on minicores and determination of seismic veloc ities through microstructural analyses with emphasis on the crustal part of the sequence here we present data from electron backscatter diffraction ebsd to determine the crystallographic preferred orientation of all major rock forming minerals the orientation data are then combined with the elastic properties densities and modal fractions of the mineral phases to calculate p wave and s wave velocities calculated velocities coincide very well with direct measurements made on minicores of the same samples in the high pressure laboratory the threedimensional velocity patterns show the signifi cant contribution of the crystallographic preferred orientation of certain constituent minerals on the bulk properties of the rock the bulk anisotropic signal is dominated by highly anisotropic phases that are susceptible to both shape and crystal preferred orientation factors of particular importance are small modal fractions of micas in gneisses and amphiboles and clinopyroxenes in eclogites and high pressure granulites in the lower crustal rocks the direction of maximum p wave velocities although contained in the foliation plane does not coincide with the orientation of the mineral and stretching lineation and therefore cannot be used as an indicator of the main strain direction in rocks the maximum seismic birefringence is often contained within the foliation plane but very rarely coincides with the lineation our data set also illustrates the strong effect of the breakdown reactions of clinopyroxene and the appearance of plagioclase on the petrophysical properties of mafi c lower crustal rocks while it does not enhance anisotropy it does produce a drop in velocity and as a consequence enhances the refl ectivity of the contact between lower crustal rocks and ultra mafi cs from the mantle

omphacite textures in eclogites of the tauern window implications for the exhumation of the eclogite zone eastern alps abstract we discuss the relationship of omphacite lattice preferred orientation lpo patterns in eclogite and kinematic indicators in the matrix surrounding the eclogite during the very rapid exhumation of the deeply buried eclogite zone in the tauern window of the eastern alps lpo patterns are presented from profiles parallel to the eclogite stretching lineation across the eclogite zone the omphacite textures show symmetric patterns across the entire eclogite zone only in few cases slight asymmetries are observed the patterns are characterized by an alignment of 110 with the foliation and a maximum of 001 close to the lineation and are indicative of deformation by intracrystalline glide on the 010 001 and 110 001 slip systems the patterns have close resemblance to s type lpo patterns and probably developed during the burial and accretion of the eclogites we argue that the lpo patterns are not related to deformation associated with the very rapid exhumation of the eclogite zone we conclude that exhumation related deformation was concentrated in the metasedimentary rocks that surround the eclogites allowing the eclogites to preserve their burial lpo signature during the exhumation process kinematic indicators show top n thrusting at the base of the eclogite zone and sinistral strike slip faulting at the top most of the kinematic indicators record blueschist and greenschist facies metamorphic conditions however because of the very rapid exhumation of the eclogite zone with rates exceeding 36 km myr 1 the kinematic indicators are interpreted to reflect the entire exhumation process from eclogite facies conditions we discuss a model according to which the eclogite zone was exhumed in an oblique extrusion wedge

rheology of omphacite at high temperature and pressure and significance of its lattice preferred orientations abstract we have investigated the rheology of hot pressed polycrystalline omphacite di 58 jd 42 space group c2 c at strain rates of 10 4 10 5 s temperatures of 1300 1500 k and a pressure of 3 gpa using a 5 gpa griggs type deformation apparatus the rheological constitutive equation of omphacite is determined with a stress exponent of 3 5 0 2 and an activation energy of 310 50 kj mol our study shows that 1 the creep strength of omphacite falls between those of diopside and jadeite 2 experimental omphacite microfabrics are indistinguishable from natural ones s type fabric and its mesoscopic foliation develop under conditions of axially symmetric shortening whereas l type fabric and its mesoscopic lineation develop under conditions of general strain with l parallel to the greatest elongation 3 as in naturally deformed omphacite the deformation substructure is characterized by the dominant slip systems 110 1 2 110 110 001 and 100 001 we conclude that both s and l type omphacite fabrics are produced by these slip systems fabric differences arise solely from variation of the geometry and orientation of the finite strain ellipsoid we also confirm the previous suggestion that na bearing pyroxenes are significantly weaker than na free pyroxenes

first record and timing of uhp metamorphism from zircon in the xitieshan terrane implications for the evolution of the entire north qaidam metamorphic belt the xitieshan terrane is one of four metamorphic terranes in the north qaidam metamorphic belt which is an early paleozoic high pressure to ultrahigh pressure hp uhp metamorphic belt in nw china however conclusive evidence and precise timing of uhp metamorphism in the xitieshan terrane have not been well documented in this study we report an integrated study of zircon grains from an amphibolite in the xitieshan terrane in terms of mineral inclusions trace elements and u pb age systematics the first record of coesite is reported as an inclusion in a metamorphic zircon which provides unambiguous evidence for the uhp metamorphism of the xitieshan terrane the metamorphic zircon domains have weak or no zoning very low th u ratios insignificant eu anomalies and flat hree patterns zircon grains contain inclusions of garnet omphacite and rutile in addition to the coesite inclusion these inclusions indicate that the metamorphic zircon grains formed under uhp metamorphic conditions the metamorphic zircon grains were dated by the sims and la icpms methods and yield weighted mean ages of 432 14 and 441 9 ma respectively combined with previous results the hp uhp metamorphism of the xitieshan terrane may have lasted 460 440 ma with the peak uhp metamorphism at 441 9 ma a compilation of the reported geochronological data reveals that all four terranes of the north qaidam metamorphic belt might have experienced coeval uhp metamorphism during the early paleozoic 420 450 ma and thus may have suffered a coherent subduction uhp metamorphism and exhumation cycle

relationships between eclogite facies mineral assemblages deformation microstructures and seismic properties in the yuka terrane north qaidam ultrahigh pressure metamorphic belt nw china to understand the relationships between eclogite facies mineral assemblages deformation microstructures and the seismic properties of subducting oceanic crust eclogites from the yuka terrane no

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

elasticity of glaucophane seismic velocities and anisotropy of the subducted oceanic crust abstract upon subduction the oceanic crust transforms to blueschists and eclogites with seismic properties that gradually become similar to those of the surrounding mantle in order to evaluate the anisotropy of blueschists and glaucophane bearing eclogites the elastic constants of glaucophane single crystal plates from the sesia lanzo zone aosta valley western alps were measured using brillouin spectroscopy at ambient conditions the mean p and s wave velocities are 7 8 and 4 6 km s 1 respectively and the anisotropy is high 38 1 av p and 27 3 av s glaucophane develops strong lpo characterized by the 001 axes concentrated sub parallel to the lineation and the 110 poles concentrated sub perpendicular to the foliation in both blueschist and eclogite rocks the measured lpo is in good agreement with viscoplastic self consistent numerical models seismic properties of glaucophane bearing blueschist and eclogite are calculated by combining measured lpo and the single crystal elastic moduli of glaucophane with the other main mineral constituents of the rock mostly epidote for blueschist and omphacite and garnet for eclogite blueschists present stronger anisotropies av p 16 1 and av s 10 3 than eclogites av p 2 9 and av s 1 7 the shear wave splitting and resulting delay times for a 7 km thick layer of eclogite or blueschist are low for the eclogite

the dislocation substructures of naturally deformed omphacites abstract a transmission electron microscopy study of naturally deformed omphacite has revealed twins antiphase boundaries and a variety of dislocation substructures consisting of curved free dislocations loops isolated nodal points and networks both free dislocations and dislocation boundaries contain nodes that involve dislocations with the following burgers vectors 001 1 2 110 and 1 2 112 this implies a reaction of the type 001 1 2 110 1 2 112 by analogy with quartz and olivine the dislocation substructures in the omphacites are considered to be indicative of dislocation creep mechanisms and responsible for most of the optical defect structures using microstructural and geochemical data the metamorphic conditions during the creep deformation are estimated to be minimum 8 kbar 550 c maximum 15 kbar 625 c

trench parallel flow beneath the nazca plate from seismic anisotropy shear wave splitting of s and sks phases reveals the anisotropy and strain field of the mantle beneath the subducting nazca plate cocos plate and the caribbean region these observations can be used to test models of mantle flow two dimensional entrained mantle flow beneath the subducting nazca slab is not consistent with the data rather there is evidence for horizontal trench parallel flow in the mantle beneath the nazca plate along much of the andean subduction zone trench parallel flow is attributale utable to retrograde motion of the slab the decoupling of the slab and underlying mantle and a partial barrier to flow at depth resulting in lateral mantle flow beneath the slab such flow facilitates the transfer of material from the shrinking mantle reservoir beneath the pacific basin to the growing mantle reservoir beneath the atlantic basin trenchparallel flow may explain the eastward motions of the caribbean and scotia sea plates the anomalously shallow bathymetry of the eastern nazca plate the long wavelength geoid high over western south america and it may contribute to the high elevation and intense deformation of the central andes

seismic properties of subducting oceanic crust constraints from natural lawsonite bearing blueschist and eclogite in sivrihisar massif turkey abstract investigating the seismic properties of natural lawsonite lws bearing blueschist and eclogite is particularly important for constraining the seismic interpretation of subducting oceanic crust based on seismological observations to achieve this end we analyzed in detail the mineral fabrics and seismic properties of foliated lws blueschist and lws eclogites from sivrihisar massif in turkey in both blueschists and eclogites the lawsonite fabric is characterized by three different patterns 0 0 1 axes aligning sub normal to foliation and 0 1 0 axes aligning sub parallel to lineation normal type 0 0 1 axes aligning sub parallel to lineation and 1 0 0 axes aligning sub normal to foliation with a girdle sub normal to lineation abnormal type and 0 0 1 axes aligning both sub normal to foliation and sub parallel to lineation 0 1 0 axes aligning sub parallel to lineation and 1 0 0 axes aligning sub normal to foliation transitional pattern in contrast glaucophane and omphacite mostly present consistent axial fabrics with the 0 0 1 axes aligning to lineation these mineral fabrics produce whole rock seismic anisotropies with similar patterns however the variations in seismic anisotropies are mainly controlled by the rock type to a lesser extent are determined by the lawsonite fabric type and to only a small extent are affected by mineral fabric strength despite the constructive abnormal type lawsonite fabric on whole rock seismic anisotropies because of their weaker mineral fabric strength or deformation degree the abnormal type lws blueschist still exhibit comparatively lower seismic anisotropies than those normal type lws blueschist from other localities based on the calculated seismic anisotropies and velocities we estimated that when oceanic crust transforms from lws blueschist to lws eclogite with increasing subduction depth 1 p wave and max s wave polarization anisotropies reduce about 70 and 40 respectively and 2 variations of vp and vs contrasts relative to mantle peridotites are about 7 to 3 and 8 to 6 respectively these results corroborate the important roles of lws bearing blueschist and eclogite in interpreting the existence and gradual weakening of low velocity layers in subducting oceanic crust during the subduction process

deformation inside a paleosubduction channel insights from microstructures and crystallographic preferred orientations of eclogites and metasediments from the tauern window austria the eclogite zone of the tauern window is an exhumed subduction channel comprising eclogites with different grades of retrogression in a matrix of high pressure metasediments the rocks were exposed to 600 c and 20 25 kbars and then retrogressed during their exhumation first under blueschist facies and later under amphibolite facies metamorphism to gain insights into the deformation within the subduction channel during subduction and exhumation both fresh and retrogressed eclogites as well as the surrounding metasediments were investigated with respect to their deformation microstructures and crystallographic preferred orientations cpos pristine and retrogressed eclogites show grain boundary migration and subgrain rotation recrystallization microstructures in omphacite a misorientation axes analysis reveals the activity of complementary deformation mechanisms including grain boundary sliding and dislocation creep the omphacite cpos of the eclogites correspond to dominant sl fabrics characteristic of plane strain deformation though there are local variations towards flattening or constriction within the paleosubduction channel the glaucophane cpos in retrogressed eclogites match those of omphacite suggesting that a constant strain geometry persisted during exhumation at blueschist facies conditions plastic deformation of the host high pressure metasediments outlasted that of the eclogites as indicated by white mica fabrics and quartz cpo the latter is consistently asymmetric pointing to the operation of non coaxial deformation the microstructures and cpo data indicate a continuous plastic deformation cycle with eclogite and blueschist facies metamorphism related to subduction and exhumation of the different rock units

dissolution and replacement creep a significant deformation mechanism in mid crustal rocks abstract zoning patterns and zoning truncations in metamorphic minerals in a granodioritic orthogneiss indicate that strain and s c fabrics in these rocks were produced by dissolution precipitation and replacement processes even at epidote amphibolite facies metamorphic conditions the metamorphic fabric is defined by alternating layers and folia dominated by quartz feldspars and biotite epidote zoning patterns in most metamorphic plagioclase orthoclase epidote and sphene are truncated at boundaries normal to the shortening direction suggesting dissolution interfaces of relict igneous orthoclase phenocrysts that face the shortening direction are embayed and replaced by biotite epidote and myrmekitic intergrowths of plagioclase and quartz metamorphic plagioclase grains are also replaced by epidote we interpret these microstructures to reflect strain enhanced dissolution the cores of many grains show asymmetric overgrowths with at least two generations of beards all oriented on the ends of grains that face the extension direction we interpret these textures to reflect precipitation of components dissolved by deformation enhanced dissolution while biotite and quartz probably deformed by dislocation creep the overall deformation was accommodated by dissolution perpendicular to the shortening direction and precipitation parallel to it these chemical processes must have been activated at lower stresses than the dislocation creep predicted from extrapolations of data from experiments in dry rocks thus wet crust is likely to be weaker than calculated from these experimental studies

meiofauna towards the south sandwich trench 750 6300m focus on nematodes abstract meiobenthos excluding foraminifers from the south sandwich trench was studied in sediment samples collected by multiple core muc or multiple grab mg sampling stations were chosen along a depth transect that covered the continental slope and abyssal plain towards the trench water depths from 747 to 6319 m total abundance and biomass of meiobenthos ranged between 354 and 1675 ind 10 cm 2 and between 16 3 and 80 8 g c 10 cm 2 respectively standing stock decreased though not linearly with increasing water depth biomasses and densities were situated above the world ocean s regression line of meiobenthic stocks against water depth and higher on the trench floor 354 930 ind 10 cm 2 18 7 g c 10 cm 2 at 6300 m compared to most other oceans trench regions this observation confirms earlier results from the antarctic continental margins that antarctic meiofauna is characterised by dense populations in which nematodes predominate over the other taxonomic groups 87 98 of total abundance taxonomic investigations at genus level revealed that chromadoridae monhysteridae desmodoridae desmoscolecidae xyalidae and cyatholaimidae were dominant most of the nematode genera in total 94 occurred in all depths with only a slight distinction between the shallow daptonema dichromadora and molgolaimus preferentially at 750 2300 m and deep with tricoma preferentially from 3000 m on the shallow communities were comparable to those from the weddell sea continental margin the sediments at 2300 6300 m harboured in big lines similar compositions as abyssal and trench communities worldwide two main morph clusters were distinguished among the south sandwich trench nematodes plump nematodes with average l w 9 belonging to the taxonomic similar selective deposit feeding genera of the desmoscolecida and comparable slender nematodes l w 26 biomass size class 4 to 2 such as monhystera acantholaimus daptonema dichromadora molgolaimus microlaimus and some species from paracanthonchus mixed among mainly selective and epistratum feeding modes they formed the bulk of the trench community the results did not fit well with the general decrease in standing stock with increasing water depth and distance from the continental shelf similarly nematode size length from 124 to 2991 m biomass from 0 023 to 0 042 g dwt did not show clear bathymetric trends the lack of the usual deep sea trends of structural characteristics of meiofauna in general density total biomass taxon composition and functional attributes of nematodes morphometrics biomass trophic guild structure and maturity index is explained by the particularly complexity of the sedimentary environment of the south sandwich trench which was the result of physical processes turbidites bioturbation activity by invertebrate taxa and food supply along the depth transect

microstructures and petro fabrics of lawsonite blueschist in the north qilian suture zone nw china implications for seismic anisotropy of subducting oceanic crust abstract we conducted a detailed study on the microstructures and petro fabrics of massive and foliated lawsonite blueschist lbs in north qilian suture zone nw china the lattice preferred orientation lpo of glaucophane and lawsonite in foliated lawsonite blueschist lbs is considered to be dominantly formed by the deformation mechanism of dislocation creep and rigid body rotation respectively the lpo of glaucophane is mainly characterized by the 001 axis aligning parallel to lineation and the 100 axis and 110 pole plunging perpendicular to foliation in contrast the lpo of lawsonite features the maximum 010 axis concentrated close to lineation and the 001 axis strongly clustered normal to foliation the preferred orientation of 010 axis of lawsonite parallel to lineation is supported by a two dimensional numerical modeling using the finite volume method fvm the mineral lpos are much stronger in foliated lbs than in massive lbs in addition a kinematic vorticity analysis suggests that both pure shear dominant wm 0 18 0 26 and simple shear dominant wm 0 86 0 93 deformation regimes are present in foliated lbs the 001 axis and 010 pole of glaucophane and the 100 and 010 axes of lawsonite tend to distribute in a foliation parallel girdle in the pure shear dominant samples but simple shear dominant samples display more lineation parallel concentrations of a 001 axis of glaucophane and a 010 axis of lawsonite because the whole rock seismic anisotropies in foliated lbs are significantly higher than those in massive lbs and a counteracting effect on seismic anisotropies occurs between glaucophane and lawsonite the delay time of fast s wave polarization anisotropy induced by an actual subducting oceanic crust with a high subducting angle 45 60 is expected to range from 0 03 to 0 09 s lower bound for massive lbs and from 0 1 to 0 3 s upper bound for foliated epidote blueschist

metamorphic evolution of the coesite bearing ultrahigh pressure terrane in the north qaidam northern tibet nw china widespread evidence for ultrahigh pressure uhp metamorphism is reported in the dulan eclogite bearing terrane the north qaidam altun hp uhp belt northern tibet this includes 1 coesite and associated uhp mineral inclusions in zircon separates from paragneiss and eclogite identified by laser raman spectroscopy 2 inclusions of quartz pseudomorphs after coesite and polycrystalline k feldspar quartz in eclogitic garnet and omphacite and 3 densely oriented sio2 lamellae in omphacitic clinopyroxene these lines of evidence demonstrate that the dulan region is a uhp metamorphic terrane in the north dulan belt ndb eclogites are characterized by the peak assemblage grt omp rt phn coe pseudomorph and retrograde symplectites of cpx ab and hbl pl the peak conditions of the ndb eclogites are p 2 9 3 2 gpa and t 631 687 c the eclogite shows a near isothermal decompression p t path suggesting a fast exhumation in the south dulan belt sdb three metamorphic stages are recognized in eclogites 1 a peak eclogite facies stage with the assemblage grt omp ky rt phn at p 2 9 3 3 gpa and t 729 746 c 2 a high pressure granulite facies stage with grt cpx jd 30 pl an24 29 scp at p 1 9 2 0 gpa t 873 948 c and 3 an amphibolite facies stage with the assemblage hbl pl ep czo at p 0 7 0 9 gpa and t 660 695 c the clockwise p t path of the sdb eclogites is different from the near isothermal decompression p t path from the ndb eclogites which suggests that the sdb was exhumed to a stable crustal depth at a slower rate in essence these two sub belts formed in different tectonic settings they both subducted to mantle depths of around 100 km but were exhumed to the earth s surface separately along different paths this uhp terrane plays an important role in understanding continental collision in north western china

cataclastic deformation mechanism for the development of core mantle structures in amphibole core mantle structures in mineral grains have typically been interpreted as having formed during crystal plastic deformation optical scanning electron and transmission electron microscope investigations of what appear to be core mantle structures in amphibole from the cheyenne belt southeastern wyoming indicate that subgrainlike structures herein subgrains formed as a result of cataclastic rather than crystal plastic deformation optical and scanning electron microscope evidence to support a cataclastic origin include 1 the angular shape of subgrains with boundaries commonly parallel to 110 cleavage 2 the wide variety of subgrain sizes 3 the compositional zonation of the amphibole subgrains with actinolitic hornblende interiors overgrown by magnesio hornblende and 4 the observation that some of the undulatory extinction in porphyroclast cores is due to slip along cleavage planes whether dislocations observed in the amphibole grains were formed prior to cataclastic deformation work hardening or formed in response to crack propagation is unknown the implication of these observations is that core mantle structures in other minerals may at least in some cases have also formed during cataclastic deformation rather than during recovery and or recrystallization therefore optical identification of core mantle structures is not a priori evidence for a crystal plastic deformation mechanism

constriction due to subduction evidence for slab pull in the marianske lazně complex central european variscides pervasive l and l s tectonites of the marianske lazn eclogites include pencil fabrics constrictional folds and mullions that result from the first deformation increment observed petrological and microstructural data suggest that these structures formed by constrictional flow at deep subduction levels after or even during the eclogite facies stage and before the subsequent high pressure granulite facies overprint constriction at mantle depths is explained by taking into account the geometry and seismic properties of recent subduction zones here constrictional strain and associated l s tectonites should be expected below the rheologycontrolled slab bend at about 40 km depth where dip parallel extension and a reduced viscosity of the downgoing slab are compatible with a coaxial flow regime

magnitude and symmetry of seismic anisotropy in mica and amphibole bearing metamorphic rocks and implications for tectonic interpretation of seismic data from the southeast tibetan plateau we calibrated the magnitude and symmetry of seismic anisotropy for 132 mica or amphibole bearing metamorphic rocks to constrain their departures from transverse isotropy ti which is usually assumed in the interpretation of seismic data the average bulk vp anisotropy at 600 mpa for the chlorite schists mica schists phyllites sillimanite mica schists and amphibole schists examined is 12 0 12 8 12 8 17 0 and 12 9 respectively most of the schists show vp anisotropy in the foliation plane which averages 2 4 for phyllites 3 3 for mica schists 4 1 for chlorite schists 6 8 for sillimanite mica schists and 5 2 for amphibole schists this departure from ti is due to the presence of amphibole sillimanite and quartz amphibole and sillimanite develop strong crystallographic preferred orientations with the fast c axes parallel to the lineation forming orthorhombic anisotropy with vp x vp y vp z effects of quartz are complicated depending on its volume fraction and prevailing slip system most of the mica or amphibole bearing schists and mylonites are approximately transversely isotropic in terms of s wave velocities and splitting although their p wave properties may display orthorhombic symmetry the results provide insight for the interpretation of seismic data from the southeast tibetan plateau the n s to nw se polarized crustal anisotropy in the sibumasu and indochina blocks is caused by subvertically foliated mica and amphibole bearing rocks deformed by predominantly compressional folding and subordinate strike slip shear these blocks have been rotated clockwise 70 90 around the east himalayan syntaxis without finite eastward or southeastward extrusion in responding to progressive indentation of india into asia

cold subduction and the formation of lawsonite eclogite constraints from prograde evolution of eclogitized pillow lava from corsica a new discovery of lawsonite eclogite is presented from the lancone glaucophanites within the schistes lustres nappe at defile du lancone in alpine corsica the fine grained eclogitized pillow lava and inter pillow matrix are extremely fresh showing very little evidence of retrograde alteration peak assemblages in both the massive pillows and weakly foliated inter pillow matrix consist of zoned idiomorphic mg poor 0 8 wt mgo garnet omphacite lawsonite chlorite titanite a local overprint by the lower grade assemblage glaucophane albite with partial resorption of omphacite and garnet is locally observed garnet porphyroblasts in the massive pillows are mn rich and show a regular prograde growth type zoning with a mn rich core in the inter pillow matrix garnet is less manganiferous and shows a mutual variation in ca and fe with fe enrichment toward the rim some garnet from this rock type shows complex zoning patterns indicating a coalescence of several smaller crystallites matrix omphacite in both rock types is zoned with a rimward increase in xjd locally with cores of relict augite numerous inclusions of clinopyroxene lawsonite chlorite and titanite are encapsulated within garnet in both rock types and albite quartz and hornblende are also found included in garnet from the inter pillow matrix inclusions of clinopyroxene commonly have augitic cores and omphacitic rims the inter pillow matrix contains cross cutting omphacite rich veinlets with zoned omphacite si rich phengite si 3 54 apfu ferroglaucophane actinolite and hematite these veinlets are seen fracturing idiomorphic garnet apparently without any secondary effects pseudosections of matrix compositions for the massive pillows the inter pillow matrix and the cross cutting veinlets indicate similar p t conditions with maximum pressures of 1 9 2 6 gpa at temperatures of 335 420 c the inclusion suite found in garnet from the inter pillow matrix apparently formed at pressures below 0 6 0 7 gpa retrogression during initial decompression of the studied rocks is only very local late veinlets of albite glaucophane without breakdown of lawsonite indicate that the rocks remained in a cold environment during exhumation resulting in a hairpin shaped p t path

very low temperature record of the subduction process a review of worldwide lawsonite eclogites abstract lawsonite eclogites preserve a record of very low temperature conditions in subduction zones all occur at active margin settings typically characterized by accretionary complexes lithologies and as tectonic blocks within serpentinite matrix melange peak lawsonite eclogite facies mineral assemblages garnet omphacite lawsonite rutile typically occur in prograde zoned garnet porphyroblasts their matrix is commonly overprinted by higher temperature epidote bearing assemblages greenschist or amphibolite facies conditions erase former lawsonite eclogite relics various pseudomorphs after lawsonite occur particularly in some blueschist eclogite transitional facies rocks coesite bearing lawsonite eclogite xenoliths in kimberlitic pipes and lawsonite pseudomorphs in some relatively low temperature ultrahigh pressure eclogites are known using inclusion assemblages in garnet lawsonite eclogites can be classified into two types l type such as those from guatemala and british columbia contain garnet porphyroblasts that grew only within the lawsonite stability field and e type such as from the dominican republic record maximum temperature in the epidote stability field formation and preservation of lawsonite eclogites requires cold subduction to mantle depths and rapid exhumation the earliest occurrences of lawsonite eclogite facies mineral assemblages are early paleozoic in spitsbergen and the new england fold belt of australia this suggests that since the phanerozoic secular cooling of earth and subduction zone thermal structures evolved the necessary high pressure temperature conditions buoyancy of serpentinite and oblique convergence with a major strike slip component may facilitate the exhumation of lawsonite eclogites from mantle depths

constriction during exhumation evidence from eclogite microstructures the eclogite zone tauern window eastern alps austria represents one of only a few examples of high pressure units providing both the prograde and the retrograde metamorphic evolution the eclogites are associated with rocks originating from continental and transitional crust that also have been affected by high pressure metamorphism eclogites that formed along the prograde path indicate metamorphic conditions of 17 20 kbar 1700 2000 mpa at 550 580 c the peak assemblages formed at 21 25 kbar 2100 2500 mpa at 600 620 c omphacite microstructures in particular shape fabrics and crystallographic preferred orientations indicate that the final phases of the prograde evolution were characterized by flattening strain the evolution at the pressure peak and along the exhumation path shows a constrictional strain geometry this change is interpreted to have been controlled by the force balance between slab pull related to an oceanic slab and the buoyancy of subducted adjacent continental crustal material including the eclogites at a certain lithospheric level where the subducted continental rocks were entirely surrounded by high density lower crustal and upper mantle material the buoyancy forces exceeded the slab pull forces this force change resulted in the buoyancy driven extrusion of continental material between two lithospheric plates the part of the subducted slab that remained in buoyant equilibrium was therefore affected by constriction moreover this process may have resulted in breakoff of the subducted oceanic slab if so the cessation of slab pull resulted in accelerated buoyancy driven extrusion of the continental material extrusion of these high pressure sheets would have been accompanied by constriction subparallel to the subduction channel as well

shear wave anisotropy in the crust mantle wedge and subducting pacific slab under northeast japan to study the anisotropic structure beneath northeast ne japan we made 4366 shear wave splitting measurements using high quality seismograms of many earthquakes occurring in the crust and the subducting pacific slab our results provide important new information on the s wave anisotropy in the upper crust lower crust mantle wedge and subducting pacific slab in the upper crust the anisotropy is mainly caused by the stress aligned fluid saturated microcracks the measured delay times dts increase to 0 10 s at 10 11 km depth the fast velocity directions fvds are parallel to either the tectonic stress or the strike of active faults the maximum dts for the low frequency earthquakes near the moho are 0 15 0 17 s suggesting strong anisotropy at the base of the crust or in the uppermost mantle the measurements for the intermediate depth earthquakes in the pacific slab show dominant e w trench normal fvds in the back arc area and n s trench parallel fvds in the fore arc area the trench normal fvds in the back arc area are caused by the corner flow in the mantle wedge as a result of the subduction of the pacific plate the maximum dts for the slab earthquakes reach 0 30 0 32 s at 100 km depth but only half of the total dts are produced in the mantle wedge the small dts in the mantle wedge may result from an isotropic or weak anisotropic zone in the middle of the mantle wedge in the fore arc the dominant trench parallel fvds for the slab earthquakes are consistent with those for the upper crust earthquakes and 80 of the total dts can be accounted for by the anisotropy in the crust in the subducting pacific slab the trench parallel fvds may reflect either the original fossil anisotropy in the pacific plate when the plate was produced in the mid ocean ridge or the preferred orientations of the crystals and cracks in the upper part of the subducting slab

garnet omphacite phengite thermobarometry of eclogites from the coesite bearing unit of the southern dora maira massif western alps abstract using relevant geothermobarometric methods p t data were collected for the reconstruction of the metamorphic evolution of 34 eclogite samples taken from small lenses and boudins within the ultrahigh pressure uhp metamorphic coesite bearing brossasco isasca unit biu of the dora maira massif the mineral phases used clinopyroxene garnet phengite or growth zones thereof were identified as being coexistent for different stages of metamorphism on the basis of careful petrographic studies of several published geothermobarometers the garnet clinopyroxene thermometer of powell powell r 1985 regression diagnostics and robust regression in geothermometer geobarometer calibration the garnet clinopyroxene geothermometer revisited j metamorph geol 3 pp 231 243 combined with the garnet clinopyroxene phengite barometer after waters and martin waters d martin h n 1993 the garnet clinopyroxene phengite barometer terra abstr 5 pp 410 411 was chosen here because it provided the most reliable results nevertheless the scatter of p t data points for the prograde stage i peak metamorphic stage ii and retrograde stage iii development of the eclogites is still considerable among the many possible reasons for this inconsistency discussed a partial lack of equilibration of some of the eclogites during their metamorphic history should be taken into account despite the data scatter an average p t path could be estimated which includes the following coordinates for stage i 15 kbar 500 c 25 kbar 570 c 32 kbar 650 c for stage ii 36 kbar 720 c and for stage iii 24 kbar 680 c and 14 kbar 620 c this is in fair agreement with p t paths derived earlier for other rock types of the biu on the basis of other geothermobarometers

tectonic history of the altyn tagh fault system in northern tibet inferred from cenozoic sedimentation the active left slip altyn tagh fault defines the northern edge of the tibetan plateau to determine its deformation history we conducted integrated research on cenozoic stratigraphic sections in the southern part of the tarim basin fission track ages of detrital apatites existing biostratigraphic data and magnetostratigraphic analysis were used to establish chronostratigraphy whereas composition of sandstone and coarse clastic sedimentary rocks was used to determine the unroofing history of the source region much of the detrital grains in our measured sections can be correlated with uplifted sides of major thrusts or transpressional faults implying a temporal link between sedimentation and deformation the results of our studies together with existing stratigraphic data from the qaidam basin and the hexi corridor suggest that crustal thickening in northern tibet began prior to 46 ma for the western kunlun shan thrust belt at ca 49 ma for the qimen tagh and north qaidam thrust systems bounding the north and south margins of the qaidam basin and prior to ca 33 ma for the nan shan thrust belt these ages suggest that deformation front reached northern tibet only 10 5 m y after the initial collision of india with asia at 65 55 ma because the aforementioned thrust systems are either termination structures or branching faults of the altyn tagh left slip system the altyn tagh fault must have been active since ca 49 ma the altyn tagh range between the tarim basin and the altyn tagh fault has been a long lived topographic high since at least the early oligocene or possibly late eocene this range has shed sediments into both the tarim and qaidam basins while being offset by the altyn tagh fault its continuous motion has made the range act as a sliding door which eventually closed the outlets of westward flowing drainages in the qaidam basin this process has caused large amounts of oligocene miocene sediments to be trapped in the qaidam basin the estimated total slip of 470 70 km and the initiation age of 49 ma yield an average slip rate along the altyn tagh fault of 9 2 mm yr remarkably similar to the rates determined by gps global positioning system surveys this result implies that geologic deformation rates are steady state over millions of years during continental collision

a complex pattern of mantle flow in the lau backarc shear wave splitting analysis of local events recorded on land and on the ocean floor in the tonga arc and lau backarc indicate a complex pattern of azimuthal anisotropy that cannot be explained by mantle flow coupled to the downgoing plate these observations suggest that the direction of mantle flow rotates from convergence parallel in the fiji plateau to north south beneath the lau basin and arc parallel beneath the tonga arc these results correlate with helium isotopes that map mantle flow of the samoan plume into the lau basin through an opening tear in the pacific plate

seismic anisotropy beneath the shumagin islands segment of the aleutian alaska subduction zone lithospheric strain and mantle flow in subduction zone back arcs reflect the fundamental dynamics of lithospheric subduction in this study we inferred possible strain geometries in the mantle beneath the eastern aleutians by mapping seismic anisotropy with shear wave splitting we analyzed s phases of local events recorded by the shumagin seismic network and obtained splitting parameters fast direction and splitting time t and their 95 confidence regions to avoid complexity from converted phases data were restricted to paths at incidence angles less than 32 and to events deeper than 50 km the s phases sample the forearc and back arc of the shumagin segment from 159 7 w to 163 8 w at depths up to 216 km in our data set of 84 local events 44 of 127 s phases yielded constrained splitting parameters with consistent fast directions that are approximately parallel to the volcanic arc roughly 60 the remaining 83 s phases produced null measurements and the fast directions inferred from these phase polarizations agree with the fast axis alignment in the constrained measurements splitting times vary from 0 1 to 0 35 s and show an overall increase with hypocentral depth observed splitting parameters are well matched by predicted values for olivine rich mantle wedge models with 1 sv anisotropy where the olivine b axis is arc orthogonal and the a axis is vertical no arc parallel strain or arcparallel no vertical strain these results are consistent with mantle wedge strain models in which arc normal compression is accompanied by arc parallel or vertical shearing or extension

deformation fabrics of natural blueschists and implications for seismic anisotropy in subducting oceanic crust abstract investigations of microstructures are crucial if we are to understand the seismic anisotropy of subducting oceanic crust and here we report on our systematic fabric analyses of glaucophane lawsonite and epidote in naturally deformed blueschists from the diablo range and franciscan complex in california and the hida mountains in japan glaucophanes in the analyzed samples consist of very fine grains that are well aligned along the foliation and have high aspect ratios and strong crystal preferred orientations cpos characterized by a 1 0 0 0 0 1 pattern these characteristics together with a bimodal distribution of grain sizes from some samples possibly indicate the occurrence of dynamic recrystallization for glaucophane although lawsonite and epidote display high aspect ratios and a strong cpo of 0 0 1 0 1 0 the occurrence of straight grain boundaries and euhedral crystals indicates that rigid body rotation was the dominant deformation mechanism the p wave av p and s wave av s seismic anisotropies of glaucophane av p 20 4 av s 11 5 and epidote av p 9 0 av s 8 0 are typical of the crust consequently the fastest propagation of p waves is parallel to the 0 0 1 maxima and the polarization of s waves parallel to the foliation can form a trench parallel seismic anisotropy owing to the slowest v s polarization being normal to the subducting slab the seismic anisotropy of lawsonite av p 9 6 av s 19 9 is characterized by the fast propagation of p waves subnormal to the lawsonite 0 0 1 maxima and polarization of s waves perpendicular to the foliation and lineation which can generate a trench normal anisotropy the av s of lawsonite blueschist 5 6 9 2 is weak compared with that of epidote blueschist 8 4 11 1 calculations of the thickness of the anisotropic layer indicate that glaucophane and lawsonite contribute to the trench parallel and trench normal seismic anisotropy beneath ne japan but not to that beneath the ryukyu arc our results demonstrate therefore that lawsonite has a strong influence on seismic velocities in the oceanic crust and that lawsonite might be the cause of complex anisotropic patterns in subduction zones

omphacite fabric variation in the cabo ortegal eclogite nw spain relationships with strain symmetry during high pressure deformation petrostructural study of cabo ortegal eclogite tectonites provides an insight into the relationships between omphacite lattice preferred orientation and the fundamental deformation parameters in particular strain symmetry microstructural evidence is consistent with a tectonic origin of eclogite fabrics by coeval pyroxene plastic deformation and recrystallization under eclogite facies metamorphic conditions omphacite shape fabrics and garnet spatial distributions are anisotropic and representative of the fields of either apparent flattening or constriction omphacite lattice preferred orientations are dominated by 010 axis maxima normal to the foliation s type or flattening fabrics by 001 axis maxima parallel to the lineation l type or constriction or correspond to other fabric patterns described in the literature omphacite crystallographic fabrics are consistently asymmetric with respect to the structural framework defined by foliation and lineation this suggests that non coaxial deformation components either simple or general shear accompanied eclogite fabric development in addition to any flattening or constriction strain paths and permit shear sense determination these petrostructural features were acquired in the course of eo hercynian ductile deformation probably during deep subduction under 1 5 1 7 gpa confining pressure temperatures above 600 700 c low differential stress 15 20 mpa and slow strain rate 10 12 10 15s 1

shear wave splitting from local events beneath the ryukyu arc trench parallel anisotropy in the mantle wedge we present shear wave splitting measurements from local slab earthquakes at eight seismic stations of the japanese f net array located in the ryukyu arc we obtained high quality splitting measurements for 70 event station pairs and found that the majority of the measured fast directions were parallel to the strike of the trench and perpendicular to the convergence direction splitting times for individual measurements ranged from 0 25 to 2 s most values were between 0 75 and 1 25 s both the fast directions and the split times were similar to results for teleseismic s k ks and s wave splitting at the same stations which suggests that the anisotropy is located in the mantle wedge above the slab we considered several mantle deformation scenarios that would result in predominantly trench parallel fast directions and concluded that for the ryukyu subduction system the most likely explanation for the observations is corner flow in the mantle wedge combined with b type olivine fabric in this model the flow direction in the wedge is perpendicular to the trench but the fast axes of olivine crystals tend to align perpendicular to the flow direction resulting in trench parallel shear wave splitting

fabrics produced mimetically during static metamorphism in retrogressed eclogites from the zermatt saas zone western italian alps abstract lattice preferred orientations lpos are commonly interpreted to form by dislocation creep consequently they are used to infer deformation at the metamorphic grade at which the minerals were stable especially if those minerals show a shape fabric here we show that lpos can occur through mimicry of a pre existing lpo so they formed statically not during deformation omphacite and glaucophane lpos occur in eclogite facies rocks from the zermatt saas unit of the northwest italian alps barroisite grew during greenschist facies retrogression and has an lpo controlled significantly by the eclogite facies omphacite and glaucophane lpos rather than directly by deformation using spatially resolved lattice orientation data from the three key minerals collected using electron backscatter diffraction we deploy a new technique of interphase misorientation distribution analysis to prove this barroisite lpo develops by mimicry of omphacite via a particular lattice orientation relationship and by direct topotactic and epitactic replacement of glaucophane lpo in turn influenced anisotropic grain growth resulting in a barroisite grain shape fabric thus regional retrogression during exhumation of the zermatt saas high pressure rocks was in large part static rather than dynamic as previously interpreted in general the possibility of mimetic fabrics forming during metamorphic reactions must be borne in mind when interpreting direct structural observations and seismic anisotropy data in terms of deformation in both crust and mantle

experimental deformation of muscovite abstract the strength and mechanical anisotropy of muscovite have been investigated by shortening single crystals at 45 0 and 90 to the basal plane 001 at temperatures t from 20 to 400 c confining pressures pc from 10 to 400 mpa and strain rates e from 2 5 10 7 to 2 3 10 4 s 1 samples deformed as 45 to 001 exhibit smooth undulatory extinction in thin section associated with dislocation glide with few well developed low angle kink boundaries basal shear strengths depend on confining pressure at pc e a exp a exp q rt results from temperature and strain rate stepping experiments performed at pc 200 mpa yield an activation energy q of 47 19 kj mol 1 and an exponential constant a of 0 5 0 2 mpa 1 samples loaded parallel to the basal plane shorten by the development of sharply defined kink bands strengths associated with kink band formation are higher than those associated with glide in samples compressed at 45 to 001 and exhibit significant pressure dependence 0 8 up to confining pressures of 200 mpa kink bands are generated at the ends of samples where contact is made with the pistons samples loaded at 90 to 001 are much stronger than samples of all other orientations and deformation is accomplished by fracture fracture strengths exhibit a strong dependence on confining pressure 0 9 similar in magnitude to other silicates that are brittle at the same conditions by comparison with biotite muscovite is weaker in shear on 001 at the experimental conditions tested with an activation energy for glide q that is significantly smaller than that for glide in biotite

p wave velocities of polymineralic rocks comparison of theory and experiment and test of elastic mixture rules abstract we compare the p wave velocities vp of 696 dry samples measured at pressures up to 0 6 1 0 gpa with values calculated from the volume fraction and room pressure elastic constants of each constituent mineral using 16 averaging methods the exceptional large number of samples covers almost all common lithologic types of igneous and metamorphic rocks the calculated vp data are in good agreement with laboratory values measured at about 300 mpa even though elastic constants of only 22 common minerals are used in the computation the mean vp of a polymineralic rock is exclusively controlled by the volume fractions of its constituent minerals while grain shape and crystallographic preferred orientations anisotropy and other perturbations have minimum effects the mean vp can be fairly well predicted as long as a relevant mixture rule is used and the volume fraction is accurately determined for each mineral however none of the mixture rules can simultaneously produce the best fit to measured p wave velocities for all the lithologic types one method may work well for one lithology but poor for other lithology even for a given lithology an averaging method may yield good agreement at moderate pressure but poor agreement at high pressure applications of an inappropriate mixture rule will potentially cause the misinterpretation of the crust and mantle vp data in terms of mineralogical compositions and structures

dynamic recrystallization of garnet and related diffusion processes abstract electron backscatter diffraction and compositional data of permian metapegmatite garnet from the koralpe basement eastern alps austria show the evolution of distinct intracrystalline shear zones in c 10 mm sized almandine spessartine garnet orientation maps reveal continuous distortion of the crystal lattice new garnet grains grain size 20 30 m within intragranular deformation zones oriented subparallel to the main foliation of the metapegmatite have similar crystal orientations to the host garnet whereas recrystallized grains along fractures or along growth heterogeneities show a much weaker crystallographic preferred orientation cpo needles of kyanite as well as inclusions of apatite xenotime and rutile occur along the deformation zones together with the ca variation across the deformation zones these inclusions give evidence of material diffusion related to crystal plastic deformation of garnet the individual garnet porphyroclasts record the evolution of recrystallization microfabrics new grains once formed by subgrain rotation recrystallization are able to deform partly by diffusion accommodated grain boundary sliding resulting in a weakening of the cpo whereas magmatic garnet growth had taken place at lp conditions in equilibrium with andalusite the occurrence of kyanite in some deformation zones triggered by the localized crystal plasticity of garnet indicates that the deformation was related to the cretaceous hp metamorphic event

crystallographic preferred orientations in eclogites a review abstract during the last decades processes taking place in subduction zones have been a topic of extensive debate there are many models on a tectonic scale but knowledge of deformation at microscopic levels is essential for an understanding of the processes involved crystallographic preferred orientation cpo investigations of minerals in eclogites yield the possibility to study deformation in the crustal part of subducted oceanic lithosphere as well as subducted continental units containing basic intrusions which are frequently exhumed during continental collision from cpo data of omphacite and garnet as well as other constituent minerals in eclogites conclusions on stress and strain at depth in currently active subduction zones can be drawn as the pressure temperature and strain path of the now exhumed rocks can be unraveled this contribution provides an overview of cpo studies of eclogites concerning slip systems deformation strain and other possible cpo forming mechanisms in all major constituent mineral phases in eclogites and the implications of these microstructural data on subduction zone processes are discussed nevertheless there are still many open questions and future research is essential

p wave velocity and anisotropy of lawsonite and epidote blueschists constraints on water transportation along subducting oceanic crust p wave velocity vp and the anisotropy of lawsonite and epidote blueschists were measured up to 1 0 gpa and 400 c using the ultrasonic pulse transmission technique the slowest vp in the direction normal to foliation is similar between lawsonite and epidote blueschists 7 0 7 2 km s at 1 0 gpa and room temperature while the fastest vp in the direction parallel to lineation markedly differs between lawsonite blueschists 7 4 7 6 km s at 1 0 gpa and room temperature and epidote blueschist 7 9 km s at 1 0 gpa and room temperature crystallographic orientation measurements for main constituent min erals revealed that both epidote 0 1 0 axes fastest vp direction in epidote single crystal and amphibole 0 0 1 axes fastest vp direction in amphibole single crystal are preferentially oriented parallel to lin eation to enhance vp anisotropy of the epidote blueschist in contrast lawsonite 0 0 1 axes fastest vp direction in lawsonite single crystal are oriented subnormal to foliation whereas amphibole 0 0 1 axes are oriented subparallel to lineation so that relatively weak vp anisotropy was observed in the lawsonite blueschist our experimental results in conjunction with recent seismological observations suggest that the vp of the subducting oceanic crust at 50 km has been observed in several subducting slabs however the high vp values at 50 km depth are difficult to be explained by blueschists this indicates that the blueschist would be at least partially transformed to hydrous mineral bearing eclogite at 50 km depth in subducting oceanic crusts

plastic deformation and development of clinopyroxene lattice preferred orientations in eclogites abstract we use an anisotropic viscoplastic self consistent vpsc model to simulate the development of omphacite lattice preferred orientations lpos in response to deformation by dislocation glide in these simulations we consider slip systems identified either in naturally deformed omphacite or in experimentally deformed diopside simulated lpos reproduce very well the characteristic omphacite lpo pattern in naturally deformed eclogites a strong concentration of 001 axes sub parallel to the lineation and of 010 poles sub perpendicular to the foliation these models reconcile the interpretation of omphacite lpos in eclogites and tem observations of naturally deformed omphacite since they show that omphacite lpos in naturally deformed eclogites may develop by dislocation glide on 1 2 110 110 001 110 and 001 100 we also investigate the effect of the strain regime on the omphacite lpo development the simulations show that changes in deformation regime lead to second order variations in omphacite lpo patterns similar to those observed in eclogites such as an asymmetry of the lpo relative to the structural frame a stronger concentration of the 001 axes relative to the 010 pole concentration or a dispersion of 001 axes in the foliation plane this suggests that omphacite lpo patterns may carry information on the deformation regime simple shear transtension active during the high pressure events

physical properties of ultrahigh pressure metamorphic rocks from the sulu terrain eastern central china implications for the seismic structure at the donghai ccsd drilling site about 30 samples representing major lithologies of sulu ultrahigh pressure uhp metamorphic rocks were collected from surface exposures and exploration wells and compressional vp and shear wave vs velocities and their directional dependence anisotropy were determined over a range of constant confining pressures up to 600 mpa and temperatures ranging from 20 to 600 degreesc samples range in composition from acidic to ultramafic p and s wave velocities measured at 600 mpa vary from 5 08 to 8 64 km s and 2 34 to 4 93 km s respectively densities are in the range from 2 60 to 3 68 g cm 3 to make a direct tie between seismic measurements refraction and reflection and subsurface lithologies the experimental velocity data corresponding to shallow depths were used to calculate velocity profiles for the different lithologies and profiles of reflection coefficients at possible lithologic interfaces across the projected 5000 m chinese continental scientific drilling program ccsd crustal segment comparison of calculated in situ velocities with respective intrinsic velocities suggests that the in situ velocities at shallow depths are lowered by an increased abundance of open microcracks the strongly reflective zone beneath the donghai drill site can be explained by the impedance contrasts between the different lithologies contacts between eclogite peridotite and felsic rocks gt gneiss granitic gneiss in particular may give rise to strong seismic reflections in addition shear induced lattice preferred orientation lpo related seismic anisotropy can increase reflectivity for the explanation of the high velocity bodies 6 4 km s around 1000 m and below 3200 m depth large proportions of eclogite peridotite about 40 and 30 vol respectively are needed c 2002 elsevier science b v all rights reserved

textures of eclogites and blueschists from syros island greece inferences for elastic anisotropy of subducted oceanic crust many blueschists and eclogites are inferred to have formed from oceanic basalts in subducted slabs knowledge of their elastic behaviour is essential for reconstructing the internal structure of subduction zones the cycladic blueschist unit exposed on syros island greece contains rocks belonging to an exhumed tertiary subduction complex they were possibly part of a subduction channel a shear zone above the subducting slab in which exhumation is possible during subduction intense plastic deformation forming crystallographic preferred orientations cpo accompanied blueschist and eclogite metamorphism cpo of the constituent minerals in the collected samples was determined by time of flight neutron diffraction two samples are foliated fine grained blueschists with strong cpo rich in glaucophane zoisite and phengite two coarser grained eclogite samples rich in omphacite and clinozoisite or glaucophane have weaker cpo vp and vs anisotropies were computed from the orientation distribution function and single crystal elastic constants all samples show velocity maxima parallel to the mineral lineation and minima normal to the foliation providing important constraints on orientations of seismic anisotropy in subduction channels vp anisotropies are up to three times higher 6 5 12 in the blueschists than in the eclogites 3 4 pointing to a potentially important lithological control of elastic anisotropy in subducted oceanic crust

pressure dependence and anisotropy of p wave velocities in ultrahigh pressure metamorphic rocks from the dabie sulu orogenic belt china implications for seismic properties of subducted slabs and origin of mantle reflections the compressional wave velocities vp pressure derivatives vpv and anisotropy a of three types of eclogites and country rocks from the dabie sulu ultrahigh pressure uhp metamorphic belt china have been measured under confining pressures up to 800 mpa type 1 eclogites which are coarse grained and subjected to almost no retrograde metamorphism experienced recovery accommodated dislocation creep at peak metamorphic conditions in the diamond stability field type 2 eclogites are fine grained reworked type 1 materials that experienced recrystallizationaccommodated dislocation creep under quartz coesite boundary conditions during the early stage of exhumation type 3 eclogites are retrogressed samples that were overprinted by significant amphibolite facies metamorphism during a late stage of exhumation within the crust type 1 eclogites are richer in al2o3 and mgo but poorer in sio2 and na2o than type 2 and type 3 eclogites anisotropy of type 1 and type 2 eclogites is generally low b4 because volumetrically important garnet is elastically quasi isotropic while type 3 eclogites can exhibit high anisotropy n10 due to the presence of strongly anisotropic retrograde minerals such as amphibole plagioclase and mica the transition of the pressure dependence of velocity from the poroelastic to elastic regimes occurs at a critical pressure pc which depends mainly on the density and distribution of microcracks and in turn on the exhumation history of rocks the vp pressure relationship can be expressed by vp a lnp 2 blnp c pvpc and vp v0 dp pzpc where p is the confining

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

deep water sedimentary environments of the northwestern weddell sea and south sandwich islands antarctica abstract two transects have been sampled using short cores multi and box seabed photography video sequences and sediment profile images across the northwestern weddell sea and south sandwich forearc antarctica a total of 12 core stations were examined for sediment structure texture and composition to determine their depositional history four of the core stations from the weddell continental slope abyssal plain and south sandwich forearc and trench were further examined for trace metal analysis ba u mn and pb for sediment source and palaeoproductivity and 210pb for age profiles and depth of mixing by bioturbation the 3055 m station at the base of the weddell continental slope recovered laminated muddy silty contourites with a surface layer of hemipelagites with abundant ice rafted debris possibly originating from the 1995 larsen a and 2002 larsen b ice shelf disintegration this is interpreted as a moderate energy depositional environment influenced by weddell sea bottom water along slope current reworking sediment sediment sources to this station include the direct input of clastic material from ice with variation in sediment sources from in situ current winnowing and along slope transport reflected in the mn li and ba li ratios the 4760 m station on the weddell abyssal plain recovered hemipelagites interpreted as a low energy depositional environment with sediment supplied both from direct ice rafting and pelagic settling during productive seasonally ice free periods the 210pb profiles indicate a sedimentation rate of 0 024 cm year 1 with the 206 207pb isotope ratios suggesting a principle sediment source from ice rafting the 2292 and 6326 m stations from the slope and floor of the south sandwich forearc recovered pelagic and turbiditic sediments this is interpreted as a high energy down slope depositional environment with high surface biogenic productivity and sediment supply leading to localised down slope mass movements on the slope and floor high levels of mn li and elevated u li ratios demonstrate a strong concentration of mn in organic poor sediment the mn redoxcline is present at 10 cm depth the high ba li 500 ppm in the forearc floor core reflects some limited enhanced productivity source sediment for the forearc may in part be derived from the northwest weddell abyssal plain based on the 206 207pb isotope ratios reflecting transport by weddell sea bottom water

ebsd measured lattice preferred orientations and seismic properties of eclogites we investigated the deformation mechanisms and the seismic properties of 10 eclogite samples from different localities alps norway mali and eastern china through the analysis of their microstructures and lattice preferred orientations lpo these samples are representative of various types and intensity of deformation under eclogitic metamorphic conditions omphacite and garnet lpo were determined from electron backscatter diffraction ebsd technique garnet appears to be almost randomly oriented whereas omphacite develops strong lpo characterized by the 001 axes concentrated sub parallel to the lineation and the 010 poles concentrated sub perpendicular to the foliation in order to analyze the deformation mechanisms that produced such omphacite lpo we compare our observations to lpo simulated by viscoplastic self consistent numerical models a good fit to the measured lpo is obtained for models in which the dominant slip systems are 1 2h110i 11 0 001 110 and 001 100 dominant activation of these slip systems is in agreement with tem studies of naturally deformed omphacite seismic properties of eclogite are calculated by combining the measured lpo and the single crystal elastic constants of omphacite and garnet although eclogite seismic anisotropies are very weak less than 3 for both p and s wave they are generally characterized by a maximum p wave velocity sub parallel to the lineation and by a minimum velocity approximately normal to foliation the mean p and s wave velocities are high respectively 8 6 and 4 9 km s the s wave anisotropy pattern displays complex relationships with the structural frame but the fast polarization plane generally tends to be parallel to the foliation calculated reflection coefficients show that an eclogite crust interface is generally a good reflector rc 0 1 whereas an eclogite body embedded in the upper mantle would be hardly detectable

garnet zoning and reaction textures in overprinted eclogites bohemian massif european variscides a record of their thermal history during exhumation eclogites from different parts of the variscan basement of the bohemian massif show a variation in temperatures recorded at peak pressures as well as during subsequent exhumation these differences are reflected in the extent of modification of pre syn and post eclogite facies garnet compositions and by the types of reactions taking place at garnet rims munchberg massif eclogites formed at conditions of 600 650 c 20 kbar were mainly overprinted in the amphibolite facies field and contain garnets with an old homogeneous core and often well preserved idiomorphic growth zoning to mg rich rims in the marianske lazn complex the eclogite facies stage 640 715 c 15 kbar many eclogites show well developed granulite facies overprints with widespread orthopyroxene growth in garnet and omphacite domains spinel and or sapphirine after kyanite and in some cases even a low pressure high temperature overprint that produced fe rich olivine the latter rocks clearly endured higher temperatures during breakdown but remarkably strong growth zoning and sharp overgrowth features are preserved in some cases diffusion modeling shows that the preservation of zoning in garnet of rocks that have experienced granulite facies temperatures is only possible if the time spent at these conditions is short 1 ma this time scale is inconsistent with a smooth p t t path as expected from thermal modeling for erosion or extension driven exhumation and implies that these variscan rocks experienced short lived thermal events during their exhumation history

microfabric characteristics and rheological significance of ultra high pressure metamorphosed jadeite quartzite and eclogite from shuanghe dabie mountains china quantitative analysis of the structural evolution of jadeite quartzite a rare ultra high pressure uhp rock type from the dabie mountains of eastern china sheds light on the formation and evolution of uhp orogenic belts worldwide geological mapping of the shuanghe area where jadeite quartzites crop out was carried out to determine the spatial relationships between different uhp rocks within this orogen the deformation mechanisms of jadeite quartzite geodynamical parameters stress strain strain rate and microstructure including lattice preferred orientation lpo were determined from six jadeite quartzite samples from the shuanghe area lpos of clinopyroxene jadeite and omphacite garnet rutile and quartz from these jadeite quartzite samples are compared with those of three eclogites preserving different degrees of deformation from the shuanghe area microstructural lpos of jadeite omphacite garnet rutile and quartz were determined using electron backscattered diffraction ebsd analysis quartz fabrics were largely recrystallized during late low grade stages of deformation whereas garnet shows no strong lpo patterns rutile fabrics show a weak ls fabric along 001 jadeite and omphacite show the strongest eclogite facies lpo patterns suggesting that they may provide important information about mantle deformation patterns and control the rheology of deeply subducted continental crust microstructural data show that the jadeite lpo patterns are similar to those of omphacite and vary between l and s types which correlate with prolate and oblate grain shape fabrics spo quartz lpos are monoclinic microstructural analysis using tem shows that the dominant slip systems of jadeite in one sample are 100 001 110 001 and 1 1 0 1 2 110 while in another sample no dislocations are observed abundant dislocations in quartz were accommodated by the dominant slip system 0001 1120 indicating basal glide and represents regional shearing during the exhumation process this suggests that dislocation creep is the dominant fundamental deformation mechanism in jadeite under uhp conditions the protoliths of jadeite quartzite metasedimentary rocks from the northern passive continental margin of the yangtze craton experienced the same deep subduction and were deformed under similar rheological conditions as other uhp eclogite marble and paragneiss experimental uhp deformation of quartzo feldspathic gneiss with a chemical composition similar to the bulk continental crust has shown that the formation of a jadeite stishovite rock is associated with a density increase of the host rock similar to the eclogite conversion from basaltic protoliths the resulting rock can be denser than the surrounding mantle pyrolite up to depths of 660 km 24 gpa thus processes of deep continental subduction may be better understood through understanding the rheology and mechanical behaviour of jadeite jadeite quartzites such as those from the shuanghe may be exhumed remnants of deeply subducted slabs of continental crust other parts of which subducted past thedepth of no return and remain in the deep mantle

lawsonite vorticity and subduction kinematics the exceptional preservation of lawsonite in eclogite and blueschist in a subduction complex sivrihisar turkey allows evaluation of high pressure h p deformation and exhumation using vorticity analysis of a h p index mineral the sivrihisar h p rocks were deformed during h p metamorphism and initial decompression by oblique normal and or dextral shearing beneath serpentinized peridotite we evaluated kinematic vorticity w m pure shear 0 w m simple shear 1 using the shape fabric of lawsonite crystals in blueschist layers and meter scale eclogite pods pods record dominant simple shear w m 0 8 in their eclogitic core and at pod margins partially transformed to blueschist map scale blueschist layers record a large component of pure shear w m 0 4 0 6 consistent with oblique extrusion at depth in the subduction zone and local simple shear w m 0 8 at the fault contact with serpentinite a combination of oblique unroofing from beneath a weak serpentinized hanging wall and pure shear extrusion in the subduction channel accounts for rapid exhumation along a low geothermal gradient and the rare preservation of lawsonite eclogite during exhumation to the earth9s surface

structure and rifting evolution of the northern newfoundland basin from erable multichannel seismic reflection profiles across the southeastern margin of flemish cap we present the results from processing and interpreting five lines from the 1992 erable multichannel seismic reflection experiment extending from the southeastern margin of flemish cap into the northern newfoundland basin these profiles reveal significant along strike variations in the rifting styles experienced by flemish cap in the southwest a 100 km wide transition zone is identified between thinned continental crust and thin oceanic crust similar to the conjugate galicia bank and iberian margins this transition zone contains a section of deep basement adjacent to a series of shallower ridges and is interpreted as exhumed serpentinized mantle along strike towards the northeast this transition zone pinches out completely within 100 km and is replaced by thin oceanic crust directly adjacent to thinned continental crust by interpreting nearby seismic profiles and profiles on the conjugate margins using the same classification criteria we construct regional maps of the distribution of crustal domains on both sides of the north atlantic these maps reveal significant variations in rifting style on the conjugate margins and along strike of each margin and also highlight the role of ancient transfer zones in compartmentalizing these rifting variations into four distinct regions we propose that the limited localization of shallow topographically high serpentinized peridotite ridges on the newfoundland iberia and flemish cap galicia bank conjugate margins was directly related to an increase in the rate of extension following the separation of flemish cap and galicia bank which exhumed deeper less serpentinized mantle

eclogite as a seismic marker in subduction channels seismic velocities anisotropy and petrofabric of cabo ortegal eclogite tectonites spain high pressure sheath like fold nappes made of normal mid ocean ridge basalt n morb eclogites high pressure volcanic arc granulites arc root lithospheric mantle peridotites and shallow crustal volcanosedimentary gneisses were amalgamated at deep realms of an eohercynian orogenic channel and currently crop out in the cabo ortegal complex compressional seismic wave velocity measurements determined in eclogite at confining pressures of 600 mpa range from 8 4 to 7 3 km s whereas anisotropy varies between 0 7 and 3 2 shear wave velocities range from 4 8 to 4 2 km s and the respective anisotropies vary between 0 1 and 3 4 the poisson9s ratios are low 0 25 0 265 independent of the degree of retrogression or deformation seismic velocities and anisotropy are related to the fabric of omphacite and calculated eclogite anisotropy is not symmetrical with respect to the tectonic fabrics calculated seismic reflection coefficients induced by lithological structural and rock petrofabric variations of eclogites show that these can be identified as reflectors imprinting a particular penetrative and dipping fabric to discrete seismic profile segments of paleosubduction settings

p wave velocities anisotropy and hysteresis in ultrahigh pressure metamorphic rocks as a function of confining pressure 1 the chinese continental drilling project ccsd has drilled to a depth of 5100 m at maobei n34 40 e118 67 donghai county jiangsu province in the eastern segment of the dabie sulu ultrahigh pressure uhp metamorphic terrane the borehole which penetrated through all of the high velocity layers and seismic reflectors observed within the uppermost crust on seismic refraction and reflection profiles reveals the main lithologies to be coesite bearing felsic gneisses metabasic rocks i e amphibolite retrogressed and non retrogressed eclogites and ultramafic rocks i e garnet peridotite and serpentinite p wave velocities anisotropy and hysteresis of 31 typical ccsd core samples and 35 representative surface samples collected from the sulu uhp belt were measured at hydrostatic confining pressures up to 800 mpa the velocity pressure curves can be well described by a four parameter exponential equation derived from theory v p v0 dp b0 exp kp where v0 is the projected velocity at zero pressure if pores cracks were absent d is the intrinsic pressure derivative of velocity in the linear elastic regime b0 is the initial velocity drop caused by the presence of pores cracks at zero pressure and k is the decay constant of the velocity drop in the nonlinear poro elastic regime the seismic hysteresis is caused by irreversible changes in grain contact increases in microcrack aspect ratios and reduction of void space during the pressurization depressurization cycle the statistical properties of p wave velocities in the uhp rocks provide an important set of basic information for the interpretation of field seismic data from the root zones of continental convergent orogenic belts and modern and ancient subduction zones

a new calibration of seismic velocities anisotropy fabrics and elastic moduli of amphibole rich rocks 1 a large portion of the middle to lower crust beneath the continents and oceanic island arcs consists of amphibolites dominated by hornblende and plagioclase we have measured p and s wave velocities vp and vs and anisotropy of 17 amphibole rich rock samples containing 34 80 vol amphibole at hydrostatic pressures p up to 650 mpa combined petrophysical and geochemical analyses provide a new calibration for mean density average major element contents mean vp p and vs p coefficients intrinsic vp and vs anisotropy poisson s ratios the logarithmic ratio rs p and elastic moduli of amphibole rich rocks the vp values decrease with increasing sio2 and na2o k2o contents but increase with increasing mgo and cao contents the maximum 0 38 0 40 km s and minimum s wave birefringence values occur generally in the propagation direction parallel to y and normal to foliation respectively amphibole plays a critical role in the formation of seismic anisotropy whereas the presence of plagioclase quartz pyroxene and garnet diminishes the anisotropy induced by amphibole crystallographic preferred orientations cpos the cpo variations cause different anisotropy patterns illustrated in the flinn diagram of vp x vp y vp y vp z plots the results make it possible to distinguish in terms of seismic properties the amphibolites from other categories of lithology such as granite granodiorite diorite gabbro diabase felsic gneiss mafic gneiss eclogite and peridotite within the earth s crust hence amphibole aligned by dislocation creep anisotropic growth or rigid body rotation is the most important contributor to the seismic anisotropy of the deep crust beneath the continents and oceanic island arcs which contains rather little phyllosilicates such as mica or chlorite

southern ocean deep sea biodiversity from patterns to processes abstract the southern ocean is characterized by a narrow and deep shelf an almost isothermal water column and a large area of deep sea surrounding antarctica however knowledge of the deep sea faunal composition particularly in the southern ocean is still scarce in comparison with shelf and upper slope environment for that reason a deep sea project was devoted to investigate this little known area of the southern ocean andeep antarctic benthic deep sea biodiversity colonisation history and recent community patterns took place in 2002 2005 and provided first insights into the biodiversity and biogeography of southern ocean benthic animals from meio to megafauna the results with the very general patterns are outlined here based on the knowledge on biodiversity patterns gained through andeep a follow up project andeep systco system coupling was established in the international polar year in order to investigate the processes driving the biodiversity pattern observed this expedition took place in 2007 2008 and only preliminary data can be presented at this stage given that the material was available for only a couple of months since the return of r v polarstern some key results identified after the systco expedition are presented

thermobarometry of phengite bearing eclogites in the dabie mountains of central china pressure temperature conditions for formation of the peak metamorphic mineral assemblages in phengite bearing eclogites from dabieshan have been assessed through a consideration of fe2 mg2 partitioning between garnet omphacite and garnet phengite pairs and of the reaction equilibrium celadonite pyrope grossular muscovite diopside which incorporates an evaluation of the extent of the strongly pressure dependent inverse tschermak s molecule substitution in the phengites for the latter equilibrium the calibration and recommended activity composition models indicated by waters martin 1993 have been employed and importantly yield results consistent with petrographic evidence for the stability at peak conditions of coesite in certain samples and quartz in others confirmation that in some phengite eclogite samples peak silicate mineral assemblages have equilibrated at confining pressures sufficient for the stability of coesite and in some cases even diamond rather negates previous suggestions that coesite may have been stabilized in only very localized possibly just intracrystalline domains inherent difficulties in the evaluation of peak metamorphic temperatures from fe2 mg2 partitioning between mineral phases due to uncertainties over fe3 fe2 ratios in the minerals especially omphacites and to re equilibration during extensive retrograde overprinting in some samples are also assessed and discussed our results indicate the existence in south central dabieshan of phengite eclogites with markedly different equilibration conditions within two structurally distinct tectonometamorphic terranes thus our data do not support earlier contentions that south central dabieshan comprises a structurally coherent continental crust terrane with a regional p t gradient signalling previous deepest level subduction in the north instead we recognize the central dabie ultra high pressure coesite eclogite bearing terrane to be structurally overlain by a southern dabie high pressure quartz eclogite bearing terrane at a major southerly dipping shear zone along which late orogenic extensional collapse appears to have eliminated at least 20 km of crustal section

development of shape and lattice preferred orientations of amphibole grains during initial cataclastic deformation and subsequent deformation by dissolution precipitation creep in amphibolites from the ryoke metamorphic belt sw japan amphibolites from the ryoke metamorphic belt sw japan were deformed initially by cataclasis and subsequently by dissolution precipitation creep initial cataclastic deformation produced a rather weak shape preferred orientation spo of brown amphibole grains with small aspect ratios as well as a poorly developed amphibole lattice preferred orientation lpo with na a 100 axes scattered subnormal to the foliation and ng or c 001 axes scattered around the lineation during later deformation by dissolution precipitation creep preferential dissolution at grain boundaries subparallel to the foliation and simultaneous compaction normal to the foliation have likely produced a distinct spo of elongate brown amphibole grains subparallel to the foliation as well as their lpo such that their ng or c axes are scattered around the lineation while na a and nb b 010 are spread along a girdle normal to the lineation also during this deformation green amphibole precipitated as isolated grains or in pressure shadow regions around brown amphibole grains nucleation and anisotropic growth of isolated green amphibole grains according to the orientations of the principal stress directions produced an lpo of these grains such that their na a are oriented normal to foliation nb b within the foliation normal to the lineation and ng or c axes are parallel to the lineation in addition there is an associated spo growth of green amphibole in pressure shadow regions around brown amphibole grains occurs either syntaxially or anisotropically according to the orientations of the principal stress directions q 2003 elsevier ltd all rights reserved

eclogite inclusions from subducted metaigneous continental crust malpica tui allochthonous complex nw spain petrofabric geochronology and calculated seismic properties this study describes the strain geometry crystal plastic deformational features isotopic age of metamorphism and calculated seismic properties of two medium temperature eclogite types from the malpica tui allochthonous complex of variscan nw iberia the eclogite types are eclogites with coronitic garnets and eclogites with a planolinear fabric both of them were buried deformed and recrystallized under maximum p and t of 2 6 gpa and 610 640 c and subsequently exhumed in a late devonian subduction channel the metamorphic peak of the subduction exhumation cycle occurred ca 375 ma ago omphacite petrofabric ties eclogites with coronitic garnet to non coaxial constrictional strain and eclogites with planolinear fabrics to non coaxial flattening strain and stretching along the lineation we also used omphacite crystallographic preferred orientations to calculate and constrain the seismic properties of the eclogites the slight variations in petrophysical properties observed are interpreted to result from variations in the strain regime recorded by pristine eclogites or from variations in the modal proportions of the constituent high pressure minerals we foresee that eclogite in subduction metamorphic complexes might be either seismically undetectable or detected as planar features with high impedance contrasts relative to their host rocks

deformation induced clinopyroxene fabrics from eclogites abstract clinopyroxene omphacite microfabric studies in eclogites are useful in characterizing the deformation that has affected deeper parts of subduction zones we present the microfabrics and tem structures of four samples representative of the various deformation types known in eclogites the 001 axis tends to be parallel to the lineation while the 010 plane is close to the foliation but these general features undergo strong variations in relation to the shape fabric in strongly lineated samples 010 tends to be spread out in a great circle normal to the lineation whereas 001 is dispersed in the foliation in the case of strongly foliated samples the slip systems identified by tem are 001 100 001 110 and 1 2 110 110 operation of dislocation creep is testified by the presence of dislocations and dislocation walls but this process alone cannot explain all the observed features particularly some aspects of the crystallographic preferred orientation hence other deformation mechanisms must be involved these may be dynamic migration recrystallization by grain boundary migration as well as mass transfer processes this study illustrates that not every crystallographic mineral preferred orientation can be explained by dislocation glide and or twinning processes alone in addition it also demonstrates that eclogite microfabrics can be used to unravel deformation parameters in subduction and or collision zones

crystal preferred orientations of garnet comparison between numerical simulations and electron back scattered diffraction ebsd measurements in naturally deformed eclogites observations of dislocations sub grains and elongated crystal shapes support plastic deformation of garnet in laboratory experiments and naturally deformed eclogites to evaluate the crystal preferred orientations cpo of garnet formed in axial shortening pure shear and simple shear we performed numerical simulations of cpo development during plastic flow using the visco plastic self consistent model as input for the models we use the slip systems determined by transmission electron microscopy using experimentally deformed specimens although in garnet 66 slip systems are available slip on the k111l 110 system provides over 86 of the total strain in the simulations characteristic cpo distributions are produced for the three deformation paths with the cpo being strongest for axial shortening and weakest for simple shear compared with low symmetry minerals the pole figure densities of garnet which has cubic symmetry are weak k100l axes tend to align with the shortening direction in all three deformation modes the simulations are compared with cpo of naturally deformed garnet from nine eclogite samples from the alps norway and mali which contain 20 40 garnet all samples show weak garnet cpo only two samples have a cpo pattern similar to the simulations for simple shear no samples are similar to simulations for axial strain or pure shear the presence of other weaker minerals such as omphacite and quartz with volume fractions higher than garnet probably prevented garnet from becoming highly strained and developing characteristic cpos in these eclogites higher volume fractions of garnet and higher temperature conditions may however allow the development of garnet cpo in the mantle transition zone particularly within subducted oceanic material morb q 2004 published by elsevier ltd

does cation ordering in omphacite influence development of lattice preferred orientation three mantle eclogite xenoliths from the robert victor mine in south africa were studied the calculated pt conditions are ps4 5 gpa and t between 950 and 1145 8c the clinopyroxene has a jadeite component between 0 19 and 0 52 in situ at these compositions and temperatures clinopyroxene omphacite has the high temperature c2 c structure which has perfect cation disordering the lattice preferred orientation lpo of clinopyroxene in the samples of different microstructure and composition has been measured by ebsd the lpo for all three samples is characterized by a strong alignment of the 001 axes with the lineation typical of the l type or ls type patterns for omphacite these data disagree with a cation ordering model for dislocation slip which predicts a s type lpo pattern with the 001 axes forming a girdle in the foliation plane numerical simulation using the vpsc model has been undertaken to show the influence of the h110i 1 10 slip system on the lpo development in axially symmetric compression and extension and pure and simple shear the activity of the h110i 1 10 system should be very sensitive to changes in the cation ordering as the clinopyroxene goes towards the low temperature fully ordered p2 n structure increasing the critical resolved shear stress on this system from 1 to 10 resulted in a decrease of the slip activity of 80 but produced no significant changes to the lpo the most important slip systems for lpo development are 001 100 100 010 and 001 010 in all simulations the ls index is introduced to quantify the symmetry of omphacite lpo the index is based on the eigenvalues of the 010 and 001 pole figures and has a value of one for the end member l type zero for the end member s type and intermediate values for ls types the naturally deformed omphacite lpos have ls indexes of between 0 61 and 0 85 confirming they are ls types but closer to the l type end member in the vpsc models the s type end member develops in axially symmetric compression ls indexz0 whereas the l type end member develops in axially symmetric extension ls indexz1 in simple and pure shear the ls type fabric develops with ls index of between 0 55 and 0 60 the vpsc simulations show that the ls index is nearly constant with increasing strain for a constant macroscopic velocity gradient which prescribes the constant strain path we conclude that development of s and l type lpos in omphacite is controlled by the strain path and that the ls index is a useful quantitative indicator of fabric symmetry q 2005 elsevier ltd all rights reserved

a low variance mineral assemblage with talc and phengite in an eclogite from the saxonian erzgebirge central europe and its p t evolution a light coloured fine grained eclogite sample from near the village of hammerunterwiesenthal in the erzgebirge nw bohemian massif preserves the low variance mineral assemblage of garnet omphacite phengite talc amphibole clinozoisite quartz rutile and accessory phases porphyroblasts of amphibole clinozoisite and phengite formed during a late stage iii of metamorphism paragonite joined the assemblage late in this stage iiib the chemical zonation of the minerals was carefully studied various geothermobarometric methods were applied especially involving phengite and talc the constrained p t path for the eclogite starts at about 480 c and 25 kbar stage ib followed by a significant temperature rise stage ii at slightly increasing pressure at the peak p t conditions of 720 c and 27 kbar blastesis of amphibole clinozoisite and phengite was caused by infiltrating hydrous fluids the resulting density reduction may have allowed buoyant uplift of the eclogite subsequently significant cooling occurred at high pressures stage iiib is characterized by p t conditions around 520 c and 18 kbar at reduced water activities this unusual late p t evolution might explain the freshness of the eclogite including the preservation of chemical zonation on the micrometre scale

the misorientation index development of a new method for calculating the strength of lattice preferred orientation abstract using orientation data from experimentally deformed olivine we explore some practical problems with the j index a commonly applied measure of fabric strength we show that the j index is highly dependent on several factors including the number of discrete data in the orientation distribution function odf and arbitrary numerical parameters specified for its calculation because of this non uniqueness we conclude that the j index is difficult to interpret and should only be applied with caution as an alternative to the j index we propose a new measure of fabric strength that is based on the distribution of uncorrelated misorientation angles this m index is shown to be insensitive to the parameters specified for its calculation for typical deformed olivine samples we show that 150 discrete data are adequate to quantify fabric strength using the m index technique the m index correlates well with seismic anisotropy particularly for materials of the same fabric type therefore we conclude that the m index technique is well suited for the quantification of fabric strength and the comparison of like materials

mapping p wave anisotropy of the honshu arc from japan trench to the back arc abstract we determined a 3 d p wave anisotropic tomography beneath the entire honshu arc using about 448 000 high quality p wave arrival times from 18 335 local earthquakes that occurred beneath the northeast ne japan land area and the fore arc area under the pacific ocean our results show that low velocity low v zones exist beneath the active arc volcanoes in the crust and in the central portion of the mantle wedge above the subducting pacific slab the low v anomalies are related to the arc magmatism low v zones are also revealed in the fore arc area which are probably caused by large volumes of water releasing upwards from dehydration of the subducting oceanic crust and sediments the anisotropic amplitude in the upper crust is weaker than that in other portions under ne japan in the mantle wedge the fast velocity direction fvd is generally trench normal in back arc area which may reflect that the olivine a axis aligns with the transport direction induced by the slab driven corner flow the fvd becomes trench parallel in the central portion of the fore arc mantle wedge which is possibly induced by the olivine of b type fabric in the slab driven corner flow the fvd shows trench parallel in the low v zones in the fore arc mantle wedge close to the upper boundary of the pacific slab which may reflect the b type olivine fabric dominating in those areas or it may be induced by the dextral shearing of the overlying crust the trench parallel fvd is also revealed beneath the volcanic front indicating complex 3 d mantle flows in the mantle wedge the fvd in the subducting pacific slab is mostly trench parallel which may reflect one or more of the following possibilities 1 the original fossil anisotropy of the pacific plate formed at the mid ocean ridge 2 cracks within the slab and 3 the olivine fabric transition due to the changes in water content stress and temperature

shear wave splitting beneath the southwestern kurile arc and northeastern japan arc a new insight into mantle return flow 1 shear wave splitting in the southwestern part of the kurile arc and the northeastern ne japan arc is investigated using the waveforms from local earthquakes for both arcs observed shear wave splitting shows clear evidence for a striking rotation of fast direction across the arc suggesting the different feature of anisotropy between the fore arc and back arc sides trench parallel fast directions are observed in the fore arc side which is consistent with the anisotropy expected from the deformation of the b type olivine fabric as well as the anisotropy in the slab fast directions observed in the back arc side show approximately e w and ese wnw in the ne japan arc and n s in the southwestern kurile arc which are characterized by the local maximum dip direction of the subducted pacific plate not by the direction of plate convergence this relationship suggests that the direction of the mantle return flow is governed by the local slab geometry

u pb zircon geochronology of coesite bearing eclogites from the southern dulan area of the north qaidam uhp terrane northwestern china spatially and temporally extensive uhp metamorphism during continental subduction coesite bearing eclogites from 100 km 2 in the southern dulan area north qaidam mountains nqm of western china contain zircon that records protolith crystallization and ultra high pressure uhp metamorphism sensitive high resolution ion microprobe mass spectrometer and laser ablation inductively coupled plasma mass spectrometry u pb analyses from cathodoluminescence cl dark zircon cores in a coesite bearing eclogite yield an upper intercept age of 838 50 ma and oscillatory zoned cores in a kyanite bearing eclogite gave a weighted mean 206 pb 238 u age of 832 20 ma these zircon cores yield steep heavy rare earth element hree slopes and negative eu anomalies that suggest a magmatic origin thus c 835 ma is interpreted as the eclogite protolith age unzoned cl grey or bright zircon and zircon rims from four samples yield weighted mean ages of 430 4 438 2 446 10 and 446 3 ma flat hree patterns without eu anomalies and contain inclusions of garnet omphacite rutile phengite and rare coesite these ages are interpreted to record 16 5 myr of uhp metamorphism these new uhp ages overlap the age range of both eclogite and paragneiss from the northern dulan area suggesting that all uhp rock types in the dulan area belong to the same tectonic unit our results are consistent with slow continental subduction but do not match oceanic subduction and diapiric exhumation uhp model predictions these new data suggest that similar to eclogites in other hp uhp units of the nqm and south altyn tagh protoliths of the eclogites in the dulan area formed in a continental setting during the neoproterozoic and then subducted to mantle depth together with continental materials during the early palaeozoic

seismic anisotropy in the izu bonin subduction system 1 we evaluated shear wave splitting in local s and teleseismic ss phases from earthquakes that sample the izu bonin subduction system backarc to examine the dynamics of slab deformation and mantle flow splitting times for local s phases range from 0 6 to 1 7 s and 0 9 to 3 2 s for teleseismic ss phases fast polarization directions range from nw se to ene wsw and suggest complex mantle deformation across the region mantle flow induced by both plate motion and trench parallel flow likely combine with fossil slab fabric to control the shear wave splitting observations in the southern portion of izu bonin strong variations in fast polarization directions beneath northern izu bonin suggest a diversion to trench parallel mantle flow behind the philippine sea plate this study demonstrates that many regions of subduction previously inaccessible by conventional shear wave splitting techniques may now be feasibly examined through the use of new complementary shear wave splitting data sets

fluid flow metasomatism and amphibole deformation in an imbricated ophiolite north cascades washington abstract metasomatic tremolite rich mylonites are widespread in imbricate thrust slices of ultramafic rocks of the ophiolitic ingalls complex in washington state protoliths for these amphibolite facies mylonites were peridotite and serpentinite abundant syntectonic tremolite veins in the ultramafites record narrowly channelized flow of infiltrating fluids whereas metasomatic mylonite zones record more pervasive flow fluids were probably released mainly by prograde devolatization reactions within serpentinite and mafic ophiolitic rocks that experienced earlier hydrothermal metamorphism olivine apparently deformed by dislocation creep in the mylonites in the tremolite rich rocks locally preserved amphibole porphyroclasts deformed mainly by microfracturing acicular tremolites which dominate the mylonites form syntectonic overgrowths on porphyroclasts and probably record diffusive mass transfer which may have accompanied cataclasis acicular tremolites subsequently were folded and define both post crystalline crenulations and polygonal arcs fluid flow deformation and metamorphism were apparently complexly interrelated in the imbricate zone thrusts juxtaposed contrasting rock types that were sources and sinks for fluids and shear zones focused fluid flow metamorphism probably facilitated deformation through the release of fluids during dehydration reactions high fluid pressure may have led to hydraulic fracturing and may have controlled strain softening in the tremolitic mylonite zones as it favored microcracking and diffusive mass transfer over dislocation creep infiltrating metasomatic fluids probably play an important role in the evolution of shear zones in many ultramafic bodies during medium grade metamorphism