two dimensional modelling of subduction zone anisotropy with application to southwestern japan summary we present a series of 2 d numerical models of viscous flow in the mantle wedge induced by a subducting lithospheric plate we use a kinematically defined slab geometry approximating the subduction of the philippine sea plate beneath eurasia through finite element modelling we explore the effects of different rheological and thermal constraints e g a low viscosity region in the wedge corner power law versus newtonian rheology the inclusion of thermal buoyancy forces and a temperature dependent viscosity law on the velocity and finite strain field in the mantle wedge from the numerical flow models we construct models of anisotropy in the wedge by calculating the evolution of the finite strain ellipse and combining its geometry with appropriate elastic constants for effective transversely isotropic mantle material we then predict shear wave splitting for stations located above the model domain using expressions derived from anisotropic perturbation theory and compare the predictions to 500 previously published shear wave splitting measurements from seventeen stations of the broad band f net array located in southwestern japan although the use of different model parameters can have a substantial effect on the character of the finite strain field the effect on the average predicted splitting parameters is small however the variations with backazimuth and ray parameter of individual splitting intensity measurements at a given station for different models are often different and rigorous analysis of details in the splitting patterns allows us to discriminate among different rheological models for flow in the mantle wedge the splitting observed in southwestern japan agrees well with the predictions of trench perpendicular flow in the mantle wedge along with b type olivine fabric dominating in a region from the wedge corner to about 125 km from the trench

deep earthquakes in subducting slabs hosted in highly anisotropic rock fabric analysis of deep subduction zone earthquakes those at depths greater than 60 km reveals the physical and chemical properties of a descending oceanic lithosphere at mantle depths over the past five decades it has been observed that a large fraction of deep earthquakes has non double couple non dc seismic radiation patterns in contrast shallow earthquakes tend to have dc radiation patterns due to mechanisms of shear faulting these observations have been used to argue that deep earthquakes rupture differently from shallow earthquakes here we show that the observed global distribution of non dc deep earthquakes could be caused by shear faulting mechanisms but in a highly anisotropic laminated rock fabric that surrounds the deep earthquakes within subducted slabs for intermediate depth earthquakes 60 300 km we found a large shear wave anisotropy of 25 possibly caused by laminated fabric or aligned melt pockets oriented parallel to the slab interface which provides new supporting evidence for the metamorphic dehydration reactions in slabs however at deep focus depths 300 km the putative metastable phase change mechanism alone cannot explain the seismic anisotropy instead our results and those from recent experiments suggest materials such as magnesite or perhaps carbonatite melt may play a role in generating deep focus earthquakes radiation patterns for deep earthquakes could be a result of shear faulting mechanisms similar to those for shallow earthquakes but in highly anisotropic rock fabric suggest seismic analyses

seismic anisotropy tracing plate dynamics in the mantle elastic anisotropy is present where the speed of a seismic wave depends on its direction in earth9s mantle elastic anisotropy is induced by minerals that are preferentially oriented in a directional flow or deformation earthquakes generate two seismic wave types compressional p and shear s waves whose coupling in anisotropic rocks leads to scattering birefringence and waves with hybrid polarizations this varied behavior is helping geophysicists explore rock textures within earth9s mantle and crust map present day upper mantle convection and study the formation of lithospheric plates and the accretion of continents in earth history

antigorite induced seismic anisotropy and implications for deformation in subduction zones and the tibetan plateau the present study which is a follow up of the journal of geophysical research paper by ji et al 2013a provides a new calibration for both seismic and fabric properties of antigorite serpentinites comparisons of the laboratory velocities of antigorite serpentinites measured at high pressures with crystallographic preferred orientation data measured using electron backscatter diffraction techniques demonstrate that seismic anisotropy in high t serpentinite which is essentially controlled by the antigorite c axis fabric is independent on the operating slip system but strongly dependent on the regime and magnitude of finite strain experienced by the rock extrapolation of the experimental data with both pressure and temperature suggests that vp anisotropy decreases but shear wave splitting vs and vp vs increase with increasing pressure in either cold or hot subduction zones for a cold steeply subducting slab antigorite is most likely deformed by nearly coaxial flattening or trench parallel movements forming trench parallel seismic anisotropy for a hot shallowly subducting slab however antigorite is most likely deformed by simple shear or transpression trench normal seismic anisotropy can be observed when the subducting dip angle is smaller than 30 the geophysical characteristics of the tibetan plateau such as strong heterogeneity in vp vs and attenuation shear wave splitting and electric conductivity may be explained by the presence of strongly deformed serpentinites in lithospheric shear zones reactivated along former suture zones between amalgamated blocks hydrated zones of subducting lithospheric mantle and the crust mantle boundary if the temperature is below 700 c in the region of interest

olivine antigorite orientation relationships microstructures phase boundary misorientations and the effect of cracks in the seismic properties of serpentinites antigorite bearing rocks are thought to contribute significantly to the seismic properties in the mantle wedge of subduction zones here we present a detailed study of the microstructures and seismic properties in a sample of antigorite olivine schist previously studied by kern et al 1997 2015 we have measured crystallographic orientations and calculated the seismic properties in three orthogonal thin sections microstructures indicate that deformation is localized in the bands with high antigorite fractions resulting in strong crystallographic preferred orientations cpos with point maxima of poles to 100 parallel to lineation and poles to 001 to the foliation normal olivine cpo suggests deformation under high temperature and low stress with a 100 fiber texture the cpo strength varies with grain size but is strong even in fine grained antigorite and larger grains tend to display higher internal misorientation orientation relationships between olivine and antigorite are evident in phase boundary misorientation analysis 100 ol 001 atg being more frequent than 001 ol 010 atg two new orientation relationships between olivine and antigorite have been documented seismic velocities decrease while anisotropy increases with increasing antigorite modal content antigorite grain shape has a weak effect on seismic velocities but it is important on anisotropy comparison between cpo derived seismic velocities using voigt reuss hill averages and geometric mean only showed good agreement in 1 3 of experimental velocities if the crack porosity of 1 63 measured experimentally at 600 mpa was used in the self consistent model with two crack orientations with planes normal to z and y good match with all experimental velocities was achieved the self consistent model implies important crack porosity in the foliation plane at 600 mpa that reduces vp normal to the foliation by 0 3 km s

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

seismic signatures of a hydrated mantle wedge from antigorite crystal preferred orientation cpo abstract we report electron backscatter diffraction ebsd measurements of antigorite crystal preferred orientations cpos from 7 samples of antigorite schist from serpentinite melanges adjacent to the motagua fault system in central guatemala the cpos range from diffuse girdle to point distributions of 001 atg girdle distributions of 001 atg are not expected from deformation theory or experiments suggesting that they are a result of growth rather than deformation and may thus be topotactically related to the cpo of the olivine from which the antigorite grew the calculated seismic anisotropy ranges from 6 to 28 in v p and 5 to 33 in v s and is highest for samples with a point maximum of 001 atg for all samples the minimum v p corresponds to the pole to 001 atg and the maximum v p occurs within the foliation for samples with a clearly defined foliation trench parallel shear wave splitting observations for subduction zones can best be explained by a combination of olivine b type cpo and antigorite oriented with 001 parallel to the foliation only a relatively thin 20 km zone of hydrated mantle is required to explain the observed splitting times

frequency dependent shear wave splitting beneath the japan and izu bonin subduction zones abstract despite its importance for our understanding of physical processes associated with subduction the geometry of mantle flow in subduction zones remains poorly understood particularly in the mantle wedge above subducting slabs constraints on mantle flow and deformation can be obtained by measurements of shear wave splitting a valuable tool used to characterize the geometry and strength of seismic anisotropy a complete characterization of shear wave splitting is particularly important for understanding the mantle wedge beneath japan which overlies multiple subduction zones with complex slab morphologies previous studies indicate that the upper mantle beneath japan exhibits complicated anisotropy that manifests itself in complex splitting patterns to characterize better the geometry of mantle anisotropy beneath japan we analyzed direct s waves from local earthquakes originating in the subducting slabs for evidence of shear wave splitting using data from 54 broadband seismic stations in japan s f net array in addition both local s and teleseismic sks phases were examined using data from four f net stations in the izu bonin arc in order to characterize any frequency dependence of splitting parameters that may indicate the presence of complex anisotropy we carried out our splitting analysis in two different frequency bands 0 02 0 125 hz and 0 125 0 5 hz our measurements indicate that shear wave splitting due to upper mantle anisotropy beneath japan is highly complex and exhibits both dramatic spatial variations and a strong dependence on frequency

deformation fabrics of the cima di gagnone peridotite massif central alps switzerland evidence of deformation at low temperatures in the presence of water we report a new observation of the olivine b type lattice preferred orientation lpo from the garnet peridotite at cima di gagnone switzerland the olivine b type fabric forms at low temperatures and or high stress in the presence of water and is of particular interest because it may be used to explain the trench parallel shear wave splitting that is often observed at subduction zones in conjunction with the olivine b type fabric we have found strong orthopyroxene lpo that is identical to those formed under water free conditions this suggests that water may not have a significant effect on orthopyroxene fabric from the olivine microstruc ture we determine that a stress of 22 8 mpa was applied during the deformation event that formed the olivine lpo using an olivine flow law and assuming geological strain rates we determine the temperature of deformation to be 800 175c this does not preclude an ultra deep origin for the ultramafic rocks at cima di gagnone but indicates that much of the deformation recorded in the microstructure occurred at modest temperatures

dehydration breakdown of antigorite and the formation of b type olivine cpo abstract peridotite formed by contact metamorphism and dehydration breakdown of an antigorite schist from the happo area central japan shows a strong olivine crystallographic preferred orientation ol cpo the lack of mesoscale deformation structures associated with the intrusion and the lack of microstructural evidence for plastic deformation of neoblastic grains suggest that olivine cpo in this area did not form as a result of solid state deformation instead the good correspondence between the original antigorite orientation and the orientation of the newly formed olivine implies the cpo formed by topotactic growth of the olivine after antigorite ol cpo is likely to develop by a similar process in subduction zone environments where foliated serpentinite is dragged down to depths where antigorite is no longer stable the happo ol cpo has a strong a axis concentration perpendicular to the lineation and within the foliation commonly referred to as b type ol cpo seismic fast directions parallel to the ocean trench are observed in many convergent margins and are consistent with the presence of b type ol cpo in the mantle wedge of these regions experimental work has shown that b type cpo can form by dislocation creep under hydrous conditions at relatively high stresses there are however several discrepancies between the characteristics of natural and laboratory samples with b type ol cpo 1 the formation conditions stress and temperature of some natural examples with b type cpo fall outside those predicted by experiments 2 in deformation experiments slip in the crystallographic c axis direction is important but has not been observed in natural examples of b type cpo 3 experimental work suggests the presence of h2o and either high shear stress or relatively low temperatures are essential for the formation of b type cpo these conditions are most likely to be achieved close to subduction boundaries but these regions are also associated with serpentinization which prevents strong olivine cpo patterns from forming we show b type ol cpo can form as a result of static topotactic growth of olivine after high temperature breakdown of foliated serpentinite these results resolve the discrepancies between experimental and natural examples of b type cpo and show the need to rethink the formation process of olivine cpo in convergent margins topotactic growth of olivine after antigorite can account for the inferred distribution of b type ol cpo in the mantle wedge more successfully than dislocation creep

shear wave splitting beneath the galapagos archipelago shear wave splitting measurements in the galapagos archipelago show a rapid change from consistently oriented anisotropy to no measurable anisotropy at the western edge of the archipelago delay times are 0 4 0 9 s and fast polarization directions are 81 109 e these directions are consistent with anisotropy resulting from shear of the asthenosphere by the overlying plate there is no indication of fossil lithospheric anisotropy in the plate spreading direction in contrast beneath the center of the archipelago the upper mantle is isotropic or weakly anisotropic the isotropic region coincides approximately with a volume of anomalously low upper mantle velocities suggesting that the presence of melt may weaken the effects of fabric on anisotropy or that melt preferred orientation generates a vertical fast polarization direction alternatively the complex flow field associated with a near ridge hotspot may result in apparent isotropy

microfossil biostratigraphy of prograding neogene platform margin carbonates bahamas age constraints and alternatives abstract benthic and planktic foraminifera and calcareous nannofossils were recovered in shallow water carbonate rock cores from two continuous boreholes drilled 7 5 km apart on the west platform margin of the great bahama bank the microfossils define six biostratigraphic units in each hole one unit in each hole represents a correlative condensed section seven foraminiferal biozones are recognized in 11 of the units between the holes middle miocene globorotalia fohsi robusta zone n12 late miocene g acostaensis zone n16 and g humerosa zone n17 early pliocene g margaritae evoluta subzone n19 late pliocene g exilis subzone n21 and tentatively g tosaensis tosaensis zone n21 and early pleistocene g crassaformis viola subzone n22 the twelfth unit is inferred to be of g crassaformis viola subzone n22 age the oldest unit is onshore the youngest is offshore as presently interpreted the nannofossil and foraminiferal zonations are partially correlative although the microfossils unequivocally constrain the series ages of the sediments the incompleteness of the fossil record allows for alternative biozonal age models within the series the miocene and pliocene biozones are common to both holes but the greatest similarities between the holes are the significant mixing of middle and late miocene and late miocene early pliocene faunas the greatly condensed intervals at the miocene pliocene boundary and the early pliocene influx of deep water benthic and pelagic foraminifera of particular importance is the tentative recognition of late pliocene g tosaensis tosaensis zone n21 in one borehole subsequent data not available to this phase of the study indicate that much of the zone is likely missing its absence will lend support to speculations of a regional unconformity in the bahamas the microfossils indicate that 1 several transgressions occurred from the middle miocene to at least the earliest pleistocene 11 5 0 46 ma during which banktop derived sediments accumulating at the margin prograded the platform seaward 2 a condensed interval on the bank top may represent a late miocene lowstand a period of sediment bypassing or a lack of accommodation space 3 the slope received thin layers of pelagic sediments in a condensed interval during the late miocene and early pliocene while the bank top accommodated early pliocene 4 2 3 4 ma deep water indicators prior to a likely period of exposure 2 35 1 89 ma 4 two cycles of banktop sediment production and starvation occurred during the pliocene 5 the pliocene transgression was punctuated by stillstands or low amplitude reversals during which parts of biozones did not accumulate and 6 the sediments containing the most complete microfossil datum record are the thin pelagic strata that mark interruptions in the regular shedding of transgressive deposits from the platform sedimentation rate patterns varied but were generally higher offshore than onshore

systematic variations in magmatic sulphide chemistry from mid ocean ridges back arc basins and island arcs abstract immiscible sulphide liquids preserved as magmatic sulphide globules are hosted in igneous rocks of highly variable composition formed during magmatic processes at different tectonic settings here we report on compositional in situ data of magmatic sulphides from mid ocean ridge back arc and island arc magmatic systems iron ni rich monosulphide solid solutions mss that mainly consist of pyrrhotite and pentlandite are the dominant phases in magmatic sulphide globules in subduction related magmas mss are most abundant in relatively evolved melts and characterised by low ni and high fe contents as well as low ni cu ratios 1 thus mss chemistry varies systematically in lavas from the different tectonic settings in terms of their fe ni distribution and ni cu ratio the observed mineralogical and chemical variations between mss from the different settings reflect early s saturation accompanied by olivine fractionation along mid ocean ridges compared to later stage s saturation in arc systems associated with fe ti oxide fractionation these systematics are probably related to the relatively oxidised character of subduction zone magmas opposed to more reduced mid ocean ridge melts large ni cu variations in mss from the same locality and in individual samples suggest that s saturation is a multistage or continuous process leading to inclusions of magmatic sulphides that represent different fractionation stages in the ascending magma

arc parallel shear deformation and escape flow in the mantle wedge of the central america subduction zone evidence from p wave anisotropy he p wave anisotropy of the crust and uppermost mantle has been determined in two separate areas of the continental margin of central america covering offshore and onshore parts of southern nicaragua and northern and central costa rica our study is based on traveltime data from local earthquakes recorded with three combined onshore and offshore seismic networks that had been deployed in three half year intervals between 2002 and 2006 the p wave traveltime data have been tomographically inverted in terms of an elliptical anisotropic p wave velocity distribution in the upper crust beneath costa rica the seismic anisotropy is of the order of 2 5 and locally strongly variable in orientation corresponding to the complicated tectonic structure in contrast the upper mantle beneath both study regions shows stronger anisotropy of the order of 5 10 and a more coherent orientation of the axis of fast p wave velocity that varies systematically on regional scales the upper mantle of the incoming oceanic plate is characterized by a trench normal orientation of the fast p wave velocity axis presumably corresponding to mineral alignment lattice preferred orientation lpo in transport direction this pattern is corroded in the uppermost part of the subduction zone possibly by the influence of bending related trench parallel faults and serpentinization that can overprint or annihilate the lpo anisotropy the upper mantle of the overriding plate and mantle wedge shows a clear trench parallel orientation of the fast p wave velocity axis the anisotropy is stronger in nicaragua than in southern central costa rica factor of 2 orders of magnitude the development of a stronger anisotropy in the nicaragua could be driven by a change in the stress regime from compressional in southern costa rica to transpressional in nicaragua corresponding to a change from near orthogonal subduction in the se to oblique subduction and slab retreat in the nw from a comparison with s sks wave gps and geochemical observations follows that the most likely explanation for the observed pattern of p wave anisotropy in the mantle wedge is lpo caused by a trench parallel shear deformation and or nw oriented escape flow originating in the compressional zone near the cocos ridge collision area

subduction related antigorite cpo patterns from forearc mantle in the sanbagawa belt southwest japan antigorite atg is stable throughout large parts of the wedge mantle of most subduction zones atg shows strong acoustic anisotropy and crystallographic preferred orientation cpo patterns of this mineral may contribute significantly to seismic anisotropy in convergent margins atg cpo patterns from the higashi akaishi ha forearc mantle body of southwest japan adds to the data set suggesting the most common atg cpo pattern has a c axis perpendicular to the foliation and a b axis parallel to the stretching lineation statistical analysis using the eigenvector method of atg cpo from two mutually perpendicular directions in the same sample yz section and xz section shows no significant differences implying sample preparation has no significant affect on the resulting atg cpo reuss uniform stress averages of anisotropy for the higashi akaishi samples are approximately treble the values for voigt uniform strain averages when comparing calculated anisotropy of hydrated mantle peridotite samplesdsuch as the higashi akaishi unitdwith observed s wave delay times in convergent margins the appropriate averaging method needs to be considered

on topotaxy and compaction during antigorite and chlorite dehydration an experimental and natural study dehydration reactions result in minerals replacement and a transient fluid filled porosity these reactions involve interface coupled dissolution precipitation and might therefore lead to fixed crystallographic orientation relations between reactant protolith and product phases i e topotaxy we investigate these two phenomena in the dehydration of a foliated antigorite atg serpentinite by comparing the crystallographic preferred orientation cpo developed by olivine ol orthopyroxene opx and chlorite chl during high pressure antigorite and chlorite dehydration in piston cylinder experiments and in natural samples recording the dehydration of antigorite cerro del almirez betic cordillera spain experiments were performed under undrained conditions resulting in fluid filled porosity and in strong cpo of the prograde minerals controlled by the pre existing antigorite cpo in the reactant foliated serpentinite the orientation of aol opx and c text chl is parallel to c text atg from the protolith the cerro del almirez samples show similar locally well developed topotactic relations between orthopyroxene chlorite and antigorite but the product cpos are weaker and more complex at the thin section scale in contrast to the experiments olivine from natural samples shows a weak correlation between bol and the former c text atg we relate the strengthening of local topotactic relations and the weakening of the inherited cpo at a larger scale in natural samples to compaction and associated fluid migration microstructural features that might be related to compaction in the natural samples include 1 smooth bending of the former foliation 2 gradual crystallographic misorientation up to 16 of prismatic orthopyroxene due to buckling by dislocation creep 3 inversion of enstatite to low clinoenstatite p21 c along lamellae and 4 brittle fracturing of prismatic orthopyroxene enclosed by plastically deformed chlorite the coexistence of orthopyroxene buckling and clinoenstatite lamellae enables estimating the local strain rates and shear stresses generated during compaction an lower bound for the strain rates in the order of 10 12 to 10 13 s 1 and shear stresses of 60 70 mpa are estimated based on creep data lower shear stresses 20 40 mpa are retrieved using a theoretical approach these data point to slow compaction and fluid extraction in nature if the system is not perturbed by external forces with rates only marginally higher than the viscoplastic deformation of the solid matrix

an olivine fabric database an overview of upper mantle fabrics and seismic anisotropy we present a unique database of 110 olivine petrofabrics and their calculated seismic properties the samples come from a variety of the upper mantle geodynamic environments ophiolites subduction zones and kimberlites with a wide range of micro structures a phenomenological relationship is established between p and s wave seismic anisotropy and the degree of crystal alignment fabric strength seismic anisotropy increases rapidly at low fabric strength before reaching a near saturation level of 15 to 20 for p waves and 10 to 15 for s waves despite a large variation in the symmetry of fabric patterns the average seismic properties of the different fabric micro structural and geodynamic settings have similar anisotropies in both magnitude and symmetry hence it would seem possible to determine some measure of fabric strength from seismic anisotropy if the dimensions of the anisotropic region are known but not geodynamic environment or details of the petrofabric pattern a simple pattern of seismic anisotropy characterises the average sample of the database which has the following features the polarisation plane of the fastest s wave is parallel or sub parallel to the foliation plane the maximum shear wave splitting is parallel to the y structural direction in foliation plane and normal to the lineation the maximum of the p wave velocity is parallel to the high concentration of 100 axes which is sub parallel to the lineation the 100 orientation distribution has the greatest influence on the p wave seismic anisotropy the 100 and 001 orientation distributions have the greatest influence on the symmetry of s wave anisotropy although the magnitude of anisotropy is influenced by the distribution of all three principal axes although the database only contains olivine petrofabrics this statistical study clearly shows that seismic anisotropy can be used to deduce the orientation of the structural frame in the upper mantle 1998 elsevier science b v all rights reserved

natural examples of olivine lattice preferred orientation patterns with a flow normal a axis maximum tectonic plate motion is thought to cause solid state plastic flow within the underlying upper mantle and accordingly lead to the development of a lattice preferred orientation of the constituent olivine crystals the mechanical anisotropy that results from such preferred orientation typically produces a direction of maximum seismic wave velocity parallel to the plate motion direction this has been explained by the existence of an olivine preferred orientation with an a axis maximum parallel to the induced mantle flow direction in subduction zones however the olivine a axes have been inferred to be arranged roughly perpendicular to plate motion which has usually been ascribed to localized complex mantle flow patterns recent experimental work suggests an alternative explanation under conditions of high water activity a b type olivine preferred orientation may form with the a axis maximum perpendicular to the flow direction natural examples of such b type preferred orientation are however almost entirely unknown here we document widespread b type olivine preferred orientation patterns from a subduction type metamorphic belt in southwest japan and show that these patterns developed in the presence of water our discovery implies that mantle flow above subduction zones may be much simpler than has generally been thought

deep mantle serpentinization in subduction zones insight from in situ b isotopes in slab and mantle wedge serpentinites abstract serpentinization occurs both within the seafloor when seawater percolates through fractures to alter underlying peridotite and in subduction zones when aqueous fluids from the dehydration of the slab are released within the subduction channel the main difference between these two environments is therefore the source of serpentinizing fluid however distinguishing between the various types of serpentinites encountered in suture zones is difficult essentially because of the lack of significant differences in their mineralogy and major and trace element chemistry eight samples from corsica and the western alps representing slab serpentinites i e serpentinites embedded within the subducting slab which are fragments of former abyssal serpentinites and seven samples from guatemala and nicaragua representing mantle wedge serpentinites i e serpentinites that form in situ above the subducting slab by hydration of the overlying mantle wedge have been selected to study their fluid mobile elements ree concentrations and boron b isotopic signatures the trace element results do not allow a conclusion about the source of fluid s but the b isotopic signatures do permit identification of the different fluid sources responsible for the serpentinization the 11b values of slab serpentinites are significantly positive ranging from 12 to 34 in the same range as present day abyssal serpentinites 5 5 to 40 5 which are altered by seawater therefore these positive signatures indicate serpentinization by seawater derived fluid the b isotopic signature acquired during oceanic residency is therefore preserved even after subduction processes subjected the serpentinites to eclogite facies conditions by contrast 11b in samples from the mantle wedge range from 14 to 10 supporting the interpretation of interactions with fluids derived from the progressive dehydration of the slab which releases fluids with slightly positive and negative 11b it indicates that mantle wedge serpentinite isotopic signature is acquired via the infiltration of subducted crust derived metamorphic fluids during the subduction processes

effect of water and stress on the lattice preferred orientation of olivine the influence of water and stress on the lattice preferred orientation lpo of olivine aggregates was investigated through large strain shear deformation experiments at high pressures and temperatures p 0 5 2 1 gpa t 1470 1570 k under both waterpoor and water rich conditions the specimens are hot pressed synthetic olivine aggregates or single crystals of olivine water was supplied to the sample by decomposition of a mixture of talc and brucite deformation experiments were conducted up to shear strain 6 using the griggs apparatus where water fugacity was up to 13 gpa at the pressure of 2 gpa the water content in olivine saturated with water increases with increasing pressure and the solubility of water in olivine at p 0 5 2 gpa was 400 1200 ppm h si several new types of lpo in olivine are found depending on water content and stress samples deformed in waterpoor conditions show a conventional lpo of olivine where the olivine 100 axis is subparallel to the shear direction the 010 plane subparallel to the shear plane type a however we identified three new types type b c and e of lpo of olivine depending on the water content and stress the type b lpo of olivine which was found at relatively high stress and or under moderate to high water content conditions is characterized by the olivine 001 axis subparallel to the shear direction the 010 plane subparallel to the shear plane the type c lpo which was found at low stress and under water rich conditions is characterized by the olivine 001 axis subparallel to the shear direction the 100 plane subparallel to the shear plane the type e lpo which was found under low stress and moderate water content is characterized by the olivine 100 axis subparallel to the shear direction the 001 plane subparallel to the shear plane observations by transmission electron microscopy tem and scanning electron microscopy sem show that the dislocations in water poor samples type a are curved and both b 100 and b 001 dislocations have a similar population numerous subgrains are seen in water poor samples in backscattered electron images in contrast water rich samples both type b and type c contain mostly b 001 dislocations and dislocations are straight and sub grain boundaries are rare compared to those in water poor samples these observations suggest that 1 dominant slip systems in olivine change with water fugacity and stress and 2 grain boundary migration is enhanced in the presence of water seismic anisotropy corresponding to the fabrics under water rich condition is significantly different from that under water poor condition consequently the relationship between seismic anisotropy and flow geometry in water rich regions is expected to be

quantitative analysis of quaternary nannoplankton in the subtropical northwestern pacific ocean abstract time progressive changes of nannoflora and biometry of coccoliths for the last 1 3 m y were investigated for a piston core recovered from the west mariana ridge the species compositions were quantitatively investigated under an electron microscope and six assemblages assemblages a through f were identified significant changes in relative abundances of various types of genus gephyrocapsa are the main cause for the stratigraphic change of the flora a dominant occurrence of gephyrocapsa protohuxleyi var a which can be identified only under an electron microscope was observed for the first time in a thin sequence which corresponds to the lowest part of the brunhes epoch 0 75 0 62 ma because of this characteristic occurrence assemblage d was further subdivided into three subassemblages the dominant occurrence of small gephyrocapsa which is well known in the small gephyrocapsa zone of the equatorial pacific ocean was not observed but the almost complete absence of medium to large gephyrocapsa during approximately 200k years of the middle pleistocene was confirmed a stratigraphically restricted occurrence of a large form of reticulogenestra was also confirmed by the biostratigraphic study under a light microscope and the last and first occurrences of the largest form reticulofenestra sp a 6 5 m were identified at 0 81 and 0 98 ma respectively in addition to these four events precise ages for another five well known evolutionary events were also calibrated these ages are generally in good agreement with previously reported ages from other world oceans and diachronism was not evident for most of these evolutionary events

elasticity of antigorite seismic detection of serpentinites and anisotropy in subduction zones abstract serpentinization of the mantle wedge is an important process that influences the seismic and mechanical properties in subduction zones seismic detection of serpentines relies on the knowledge of elastic properties of serpentinites which thus far has not been possible in the absence of single crystal elastic properties of antigorite the elastic constants of antigorite the dominant serpentine at high pressure in subduction zones were measured using brillouin spectroscopy under ambient conditions in addition antigorite lattice preferred orientations lpo were determined using an electron back scattering diffraction ebsd technique isotropic aggregate velocities are significantly lower than those of peridotites to allow seismic detection of serpentinites from tomography the isotropic v p v s ratio is 1 76 in the voigt reuss hill average not very different from that of 1 73 in peridotite but may vary between 1 70 and 1 86 between the voigt and reuss bonds antigorite and deformed serpentinites have a very high seismic anisotropy and remarkably low velocities along particular directions v p varies between 8 9 km s 1 and 5 6 km s 1 46 anisotropy and 8 3 km s 1 and 5 8 km s 1 37 and v s between 5 1 km s 1 and 2 5 km s 1 66 and 4 7 km s 1 and 2 9 km s 1 50 for the single crystal and aggregate respectively the v p v s ratio and shear wave splitting also vary with orientation between 1 2 and 3 4 and 1 3 and 2 8 for the single crystal and aggregate respectively thus deformed serpentinites can present seismic velocities similar to peridotites for wave propagation parallel to the foliation or lower than crustal rocks for wave propagation perpendicular to the foliation these properties can be used to detect serpentinite quantify the amount of serpentinization and to discuss relationships between seismic anisotropy and deformation in the mantle wedge regions of high v p v s ratios and extremely low velocities in the mantle wedge of subduction zones down to about 6 and 3 km s 1 for v p and v s respectively are difficult to explain without strong preferred orientation of serpentine local variations of anisotropy may result from kilometer scale folding of serpentinites shear wave splittings up to 1 1 5 s can be explained with moderately thick 10 20 km serpentinite bodies

metamorphic reworking of a high pressure low temperature melange along the motagua fault guatemala a record of neocomian and maastrichtian transpressional tectonics abstract the guatemala suture zone is a major east west left lateral strike slip boundary that separates the north american and caribbean plates in guatemala the motagua fault the central active strand of the suture zone underwent two major collisional events within a system otherwise dominated by strike slip motion the first event is recorded by high pressure low temperature hp lt eclogites and related rocks that occur within serpentinites both north and south of the motagua fault lawsonite eclogites south of the fault are not significantly retrograded and give 40ar 39ar ages of 125 116 ma and sm nd mineral isochrons of 144 132 ma eclogites north of the fault give similar sm nd isochron ages 131 126 ma but otherwise differ in that they are strongly overprinted by a lower pressure assemblage and along with associated hp lt rocks give much younger 40ar 39ar ages of 88 55 ma indicating a later amphibolite facies metamorphic event we propose therefore that all serpentinite hosted eclogites along the motagua fault formed at essentially the same time in different parts of a laterally extensive lower cretaceous forearc subduction system but subsequently underwent different histories the southern assemblages were thrust southwards present coordinates immediately after hp metamorphism whereas the northern association was retrograded during a later collision that thrust it northward at ca 70 ma they were subsequently juxtaposed opposite each other by major strike slip motion this model implies that the hp rocks on opposing sides of the motagua fault evolved along a plate boundary that underwent both dip slip and strike slip motion throughout the late cretaceous as a result of oblique convergence the juxtaposition of a convergent and strike slip system means that hp lt rocks within serpentinites can be found at depth along much of the modern guatemala suture zone and its eastward extension into the northern caribbean both sets of assemblages were exhumed relatively recently by the uplift of mountain ranges on both sides of the fault caused by movement along a restraining bend recent exhumation explains the apparently lack of offset of surface outcrops along a major strike slip fault

olivine 100 normal to foliation lattice preferred orientation in prograde garnet peridotite formed at high h2o activity cima di gagnone central alps automated electron backscattered diffraction ebsd was applied using a scanning electron microscope to obtain lattice preferred orientation lpo data for olivine in garnet peridotites of the central alps as a reference frame the lpos of enstatite were also investigated in the garnet peridotite at cima di gagnone cdg a weak foliation carrying a distinct lineation is present the lineation is characterized by elongated enstatite olivine and poikiloblastic garnet olivine shows a very unusual lpo with 100 normal to foliation and 001 parallel to lineation achsenverteilungsanalyse ava maps demonstrate that 001 of olivine grains corresponds quite well to their maximum length axes which are preferentially parallel to the lineation numerous planar hydrous defects within 001 planes of olivine are marked by palisades of ilmenite rods and show a preferred orientation normal to lineation calculated p wave velocities for cdg are fastest 8 32 km s 1 normal to foliation with a relatively low anisotropy 2 9 compared to mantle peridotites with the usual 010 100 lpo where the fastest vp direction is towards the lineation the relationship between flow geometry and seismic anisotropy is significantly different at cdg several mechanisms for the formation of the lpo type at cdg are considered with glide possible on 100 001 of olivine on the basis of field data as well as petrographic and petrologic evidence it has been demonstrated that the cdg garnet peridotite formed by prograde metamorphism from a hydrous protolith at pressures and temperatures of about 3 0 gpa and 750 c respectively the cdg lpo is interpreted to have formed during hydrous subduction zone metamorphism the same interpretation may hold for the previously investigated olivine lpo at alpe arami which is similar to that at the nearby cdg the observed anomalous lpo is no proof for ultradeep 3 0 gpa conditions

crystallographic preferred orientation of talc determined by an improved ebsd procedure for sheet silicates implications for anisotropy at the slab mantle interface due to si metasomatism

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

evidence of high water content in the deep upper mantle inferred from deformation microstructures deep upper mantle rocks from the norwegian caledonides show evidence for large strain deformation in both olivine and garnet under varying water contents using microstructural observations including lattice preferred orientation of olivine and subgrain boundaries of majoritic garnet we infer the following deformation history at depths exceeding 150 km large strain deformation occurred at low stress 10 mpa and modest temperature 1300 k involving high water content 1000 h 10 6 si in olivine this was followed by low strain deformation at lower water content 200 1000 h 10 6 si and modest stress 40 mpa in the shallower parts these observations show that the deep upper mantle in this region had a considerably higher water content than the upper mantle near mid ocean ridges

sequential deformation from serpentinite mylonite to metasomatic rocks along the sashu fault sw japan the deformation of serpentinites along the sashu fault saganoseki peninsula sw japan involved both mylonitization and brecciation the brecciation was accompanied by metasomatism and the metasomatized rocks record additional ductile deformation serpentinite mylonite is composed mainly of antigorite with minor magnetite and cr spinel foliation and lineation in this rock type are defined by the shape preferred orientation of antigorite and the alignment of fine grained magnetite and carbonate minerals the antigorite also shows a lattice preferred orientation lpo as measured with a universal stage with c axes oriented near perpendicular to the foliation and b axes oriented parallel to the lineation brecciation is concentrated in the reaction zone along the boundary between serpentinites and surrounding country rocks talc and chlorite schists developed in this zone as a result of associated metasomatism and deformation consequently they contain composite planar fabrics and drag folds the breccias themselves are composed of serpentinite fragments cut by veins of calcite and talc analyses of fluid inclusions in calcite from the breccia indicate that brecciation occurred at 200 300 c at depths of 3 8 11 8 km metasomatic reactions particularly the formation of talc contributed to weakening of this serpentinite bearing fault zone

olivine fabric transitions and shear wave anisotropy in the ryukyu subduction system recent shear wave splitting measurements from the fore arc region of the ryukyu subduction system show large magnitude 0 3 1 6 s trenchparallel splitting in both local and teleseismic phases the similarity of splitting parameters associated with shallow local s and teleseismic phases suggests that the source of anisotropy is located in the fore arc mantle one explanation for this pattern of shear wave splitting involves a transition from commonly observed high temperature olivine fabrics with flow parallel seismically fast directions to a flow normal b type olivine fabric in the cold fore arc mantle of the ryukyu wedge we test the b type fabric hypothesis by comparing observed splitting parameters to those predicted from geodynamic models that incorporate olivine fabric development the distribution of olivine fabric is calculated with high resolution thermomechanical models of the ryukyu subduction zone that include realistic slab geometry and an experimentally based wet olivine rheology we conclude that b type fabric can explain the magnitude and trench parallel orientation of deep local s phases that sample the core of the forearc mantle however our calculations show that b type fabric alone cannot account for large magnitude trench parallel splitting associated with teleseismic phases that sample the shallow tip of the fore arc mantle alternative models for trench parallel teleseismic splitting in the shallow tip of the fore arc mantle involve the addition of crustal or slab anisotropy and highly anisotropic foliated antigorite serpentinite 2008 elsevier b v all rights reserved

seismic anisotropy of oceanic upper mantle shear wave splitting methodologies and observations we develop methodologies to obtain accurate measurements of shear wave splitting and apply these techniques to examine the pattern of oceanic upper mantle anisotropy to obtain high quality estimates of receiver splitting at island stations we devise a stacking method that finds the optimum splitting parameters and 95 error region for teleseismic shear wave phases from a suite of earthquake events to obtain additional measurements in oceanic regions we develop techniques to measure splitting parameters and errors for ss phases that sample upper mantle anisotropy at their bounce points however we find the data are often of low resolution and anomalous characteristics are sometimes found that make splitting difficult to interpret ten second splitting of ss is observed across the banjo seismic array but we cannot unambiguously attribute this signal to mantle anisotropy at the bounce the receiver splitting methods are used to assess the adequacy of a two anisotropic layer model for the pacific region with fast polarization azimuth in the lithosphere oriented in the fossil spreading direction and in the asthenosphere oriented in the absolute plate motion direction this model has been proposed to explain surface wave data in the pacific ocean but our splitting results demonstrate that oceanic anisotropy patterns are more heterogeneous than would be predicted while the island splitting measurements could reflect the influence of individual hotspot upwellings hotspot effects do not appear to be universally dominant we propose that splitting observations alternatively indicate broad scale differences in the underlying character of oceanic upper mantle anisotropy associated with coherant patters of lithospheric structure and asthenospheric flow in particular splitting surface wave models and regional studies all support a model where lithospheric anisotropy throughout the south pacific has been erased or reoriented toward the absolute plate motion direction whereas more limited observations in the north pacific indicate that the fossil lithospheric signature appears to be preserved

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

arc parallel flow within the mantle wedge evidence from the accreted talkeetna arc south central alaska 1 residual mantle exposures in the accreted talkeetna arc alaska provide the first rock analog for the arc parallel flow that is inferred from seismic anisotropy at several modern arcs the peridotites exposed at the base of the jurassic talkeetna arc have a moho parallel foliation and indicate dislocation creep of olivine at temperatures of 1000 to 1100 c slip occurred chiefly on the 001 100 slip system which has only rarely been observed to be the dominant slip system in olivine stretching lineations and olivine 100 slip directions are subparallel to the long axis of the talkeetna arc for over 200 km indicating that mantle flow was parallel to the arc axis the alignment of the olivine 100 axes yields a calculated s wave anisotropy with the fast polarization direction parallel to the arc thus 1 the fast polarization directions observed parallel to some modern arcs now have an exposed geological analog 2 arc parallel fast polarization directions can be caused by anisotropic peridotites and do not require the presence of fracture zones fluid filled pockets or glide on the 010 001 h2o induced slip system 3 seismic anisotropy beneath modern arcs may be due to slip on 001 100 with a horizontal foliation rather than slip on 010 100 with a vertical foliation and 4 the observed dominance of the 001 100 slip system may be due to high h2o concentrations suggesting that strain in the oceanic upper mantle may be accommodated dominantly by 001 100 olivine slip

plastic deformation and development of antigorite crystal preferred orientation in high pressure serpentinites we have inferred the deformation mechanisms of antigorite by high resolution ebsd mapping of samples from cerro del almirez ultramafic massif betic cordillera se spain textural relations and phase diagram calculations constrain the foliation development conditions to the subduction prograde path at up to 600 630 c and 1 6 1 9 gpa deformation was followed by static annealing of antigorite at ca 680 c the crystal preferred orientation cpo of antigorite is characterised by a strong alignment of 001 poles normal to the foliation plane and a weaker but clear parallelism between 100 axes and the macroscopic lineation defined by the elongation of magnetite aggregates analysis of misorientations across subgrains shows predominance of 010 rotation axis consistent with activation of the 100 001 slip system however tilt subgrain boundaries subparallel to 100 probably formed by edge dislocations of this system are subsidiary most subgrain boundaries are subparallel to 001 planes they are interpreted as 001 twins wherein continuing viscoplastic deformation resulted in a slight increase of the misorientation between the twins modelling of the evolution of the antigorite cpo using a lower bound approach and considering different deformation regimes and sets of basal and non basal slip systems has shown that intensities of 100 and 010 maxima reflect the relative strength of the antigorite 100 001 and 010 001 systems activation of other basal or non basal slip systems does not change significantly the cpo patterns but results in less concentrated cpo 3d transpression models better reproduce the cpo of natural antigorite serpentinites we propose that the widespread occurrence of strong cpo in high pressure antigorite serpentinite is consistent with deformation by dislocation creep with dominant glide on hk0 001 together with the activation of twinning implying that a power law rheology would better account for the mechanical behaviour of antigorite serpentinite deep in the subduction channel and mantle wedge

stress strain and b type olivine fabric in the fore arc mantle sensitivity tests using high resolution steady state subduction zone models 1 shear wave splitting observations at several subduction systems show trench parallel fast directions in the fore arc mantle the presence of b type olivine fabric in the mantle wedge may provide an explanation for this pattern of anisotropy under low temperature and hydrated conditions sensitivity tests are shown that provide insights into the distribution and magnitude of b type fabric using two dimensional high resolution kinematic dynamic subduction zone models these models include a wet olivine rheology and a simulated aseismic creep zone in the fore arc mantle with parameterized viscous coupling between the slab and mantle wedge the calculated thermal structure is in reasonable agreement with heat flow observations and q tomography for small amounts of coupling and moderate amounts of shear heating in the seismogenic and aseismic creep zones these small amounts of viscous coupling give rise to a slowly flowing fore arc mantle with thermal stress and strain conditions suitable for significant b type fabric development the minimum shear stress necessary to drive geologically significant flow in the predicted b type region is sensitive to the magnitude of shear heating along the aseismic creep zone and ranges from less than 100 to 300 mpa the minimum time required to generate sufficient finite strain for fabric development in the b type region is around 10 myr depending on the amount of coupling and shear heating the predicted distribution of olivine fabric is consistent with a pattern of shear wave anisotropy with trench parallel fast polarization directions above the fore arc and a rapid rotation to trench perpendicular in the arc and back arc

the influence of dolomite on the plastic flow of calcite rheological microstructural and chemical evolution during large strain torsion experiments abstract aggregates composed of calcite and dolomite have been deformed and annealed at high temperature 700 800 c and high pressure 300 mpa to investigate the mutual effects of deformation and chemical reaction strain partitioning fabric development and phase segregation in a polyphase rock samples deformed in torsion displayed an almost newtonian creep with stress exponent of 1 7 0 3 specimens were deformed up to a maximum shear strain of 10 and developed a clear texture documented by electron back scatter diffraction measurements in both calcite and dolomite both the texture and the foliation created during the large strain experiments display a monoclinic symmetry closely reproducing that of naturally deformed carbonate marbles with the preferred orientation of the c axes inclined against the sense of shear detailed microstructural investigation shows that grains are internally strain free and that grain boundaries are preferentially aligned in straight segments implying that deformation occurred through a combination of grain boundary sliding and diffusion processes accommodated by dislocation activity chemical investigations performed on both statically annealed and torsion samples indicate that deformation may have an important influence on the phase equilibrium of a reacting system calcite grains deformed with excess of dolomite with respect to the equilibrium were able to incorporate higher amounts of mg within the calcite lattice than the static samples as documented by electron probe microanalysis moreover the above analysis documented that reaction rate was faster in torsion than in static experiments at the same conditions of pressure and temperature

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

an automated strategy for calculation of phase diagram sections and retrieval of rock properties as a function of physical conditions we formulate an algorithm for the calculation of stable phase relations of a system with constrained bulk composition as a function of its environmental variables the basis of this algorithm is the approximate representation of the free energy composition surfaces of solution phases by inscribed polyhedra this representation leads to discretization of high variance phase fields into a continuous mesh of smaller polygonal fields within which the composition and physical properties of the phases are uniquely determined the resulting phase diagram sections are useful for understanding the phase relations of complex metamorphic systems and for applications in which it is necessary to establish the variations in rock properties such as density seismic velocities and volatile content through a metamorphic cycle the algorithm has been implemented within a computer program that is general with respect to both the choice of variables and the number of components and phases possible in a system and is independent of the structure of the equations of state used to describe the phases of the system

paleoenvironmental changes during sapropel 19 i cycle 90 deposition evidences from geochemical mineralogical and micropaleontological proxies in the mid pleistocene montalbano jonico land section southern italy an integrated micropaleontological geochemical and mineralogical study has been performed across the mid pleistocene sapropel 19 i cycle 90 from the montalbano jonico land section southern italy to reconstruct the paleoenvironmental conditions at time of its formation the sapropel interval is characterized by two oxygen depletion phases phase a and c interrupted by a temporary re oxygenation interval phase b the beginning and the end of sapropel deposition are dated at 957 0 81 kyr and 950 0 86 kyr respectively the duration of the interruption is estimated to 0 350 0 32 kyr the multiproxy approach highlights that deposition of sapropel 19 reflects a period of enhanced freshwater runoff induced by a wetter climate as a consequence of a more efficient fluvial erosion a higher terrigenous input mostly ascribable to a southern apennines source and an increased turbidity of surface waters accompanied most of sapropel deposition biotic and abiotic proxies document that different paleoenvironmental conditions occur through phases a c the beginning of phase a is characterized by warm on land paleoclimate as well as warm and oligotrophic surface water conditions during the upper part of phase a temperature starts decreasing and surface waters appear more productive this change probably represents the prelude to cooler and drier conditions characterizing phase b which displays a river supply reduction and an eolian input increase sahara dust during phase c the restored depleted oxygen environment at the bottom sediments is clearly coupled with the re establishment of humid conditions and increased river supply at the same time enhanced mixing of water column a cooler paleoclimate and increased productivity of surface waters are recorded the latter likely favored by the enhanced mixing of water column and also increased delivery of land derived nutrients the end of phase c is marked by a restored normal run off enhanced productivity in surface waters and low oxygen conditions at the bottom sediments persist slightly above phase c the overall results suggest that the onset of sapropel deposition is related to water stratification that caused low oxygen exchanges with the sea bottom although enhanced productivity characterizes most of the sapropel deposition it was not the primary factor triggering sapropel deposition

global mantle flow and the development of seismic anisotropy differences between the oceanic and continental upper mantle 1 viscous shear in the asthenosphere accommodates relative motion between earth s surface plates and underlying mantle generating lattice preferred orientation lpo in olivine aggregates and a seismically anisotropic fabric because this fabric develops with the evolving mantle flow field observations of seismic anisotropy can constrain asthenospheric flow patterns if the contribution of fossil lithospheric anisotropy is small we use global viscous mantle flow models to characterize the relationship between asthenospheric deformation and lpo and compare the predicted pattern of anisotropy to a global compilation of observed shear wave splitting measurements for asthenosphere 500 km from plate boundaries simple shear rotates the lpo toward the infinite strain axis isa the lpo after infinite deformation faster than the isa changes along flow lines thus we expect the isa to approximate lpo throughout most of the asthenosphere greatly simplifying lpo predictions because strain integration along flow lines is unnecessary approximating lpo with the isa and assuming a type fabric olivine a axis parallel to isa we find that mantle flow driven by both plate motions and mantle density heterogeneity successfully predicts oceanic anisotropy average misfit 13 continental anisotropy is less well fit average misfit 41 but lateral variations in lithospheric thickness improve the fit in some continental areas this suggests that asthenospheric anisotropy contributes to shear wave splitting for both continents and oceans but is overlain by a stronger layer of lithospheric anisotropy for continents the contribution of the oceanic lithosphere is likely smaller because it is thinner younger and less deformed than its continental counterpart

deformation mechanisms and rheology of serpentines in experiments and in nature microstructures in serpentine samples recovered from deformation experiments performed at high pressure 1 8 gpa high temperature 150 500 degrees c and laboratory strain rates 4 10 4 10 6 s 1 were studied using transmission electron microscopy on thin sections prepared by focused ion beam lizardite crystals deform by easy glide along the basal planes associated with microkink this mechanism is associated with a plastic strength of similar to 100mpa that defines the upper bound of lizardite strength in natural conditions antigorite crystals deform essentially by conjugated slip along 101 and 10 1 over bar planes observed in sections close to 010 this conjugate system results in an apparent global slip system akin to 100 001 in all samples delamination and comminution produce fine grained interconnected regions at grain boundaries because intracrystalline deformation mechanisms are insufficient to satisfy the von mises criterion the deformation laws of lizardite plastic flow and antigorite strain rate dependent differ because of their differing intracrystalline deformation mechanisms subgrain boundary geometry in natural samples and 100 001 crystal preferred orientations indicate the activation of slip systems similar to those observed in experimentally deformed samples suggesting that the strain rate dependent rheology of antigorite derived from experiments applies to subduction zone conditions delamination of antigorite crystals of a few tens to hundreds of nanometers is not observed in natural samples from subduction zones suggesting plastic deformation of serpentinites at natural low strain rates and stresses is likely accompanied by recrystallization through dissolution precipitation processes that act as a low temperature equivalent of dynamic recrystallization through diffusion at high temperature

spatial variations in antigorite fabric across a serpentinite subduction channel insights from the ohmachi seamount izu bonin frontal arc abstract we conducted a microstructural study of samples from a natural serpentinite shear zone in the ohmachi seamount izu bonin frontal arc the serpentinite samples consist mainly of columnar antigorite grains that show marked variations in texture from two approximately orthogonal sets of grains interpenetrating to aligned schistose forms because the two types of grains have similar compositions these textural differences are interpreted to reflect the existence of a strain gradient toward a plate interface in a subduction zone the crystal preferred orientation cpo of antigorite with interpenetrating texture is almost randomly oriented whereas in the case of schistose texture the cpo shows a typical 010 001 pattern we also found that with increasing intensity of schistosity the polarization plane of v s 1 for antigorite grains becomes aligned parallel to the flow plane consistent with a plane oriented normal to the maximum concentration of slow antigorite c axes this configuration results in seismic anisotropy that is approximately five times higher than that for olivine grains these findings indicate that if a serpentinite layer on the plate interface attains large bulk shear strains 2 the resultant alignment of antigorite grains within the layer strongly influences the orientation and magnitude of seismic anisotropy in the mantle wedge depending on the dip angle of the subducting slab

serpentine mineral replacements of natural olivine and their seismic implications oceanic lizardite versus subduction related antigorite we report on microstructural data obtained by optical microscopy and transmission electron microscopy concerning the crystallographic relationships of serpentine minerals with their host olivine in two contrasting situations in the first case mesh textured lizardite liz is developed in a standard 60 serpentinized oceanic harzburgite from the oman ophiolite where olivine converts to columnar lizardite the joined columns are perpendicular to the basal plane 001 liz corresponding to the pseudofibres observed optically the plane 001 liz is locally parallel to the narrow boundary ol liz thus column orientations register the interface of serpentinization the ol liz relationships are not strictly topotactic but reflect preferred cracking orientations in olivine parallel to 010 ol in the second case antigorite atg develops in a rare sample of antigorite schist in a kimberlite from moses rock colorado plateau representative of a suprasubduction zone mantle wedge high resolution transmission electron microscopy hrtem images along 010 atg show domains of very regular modulation with a 43 center dot 5 a wavelength m 17 where m is the number of silicate tetrahedra along the wave with few defects indicative of hp ht antigorite and also heavily kinked regions as fingerprints of strong tectonic shear tem imaging and electron diffraction patterns reveal two topotactic relationships between antigorite and olivine 100 atg 010 ol and atg ol the planes in contact are 001 atg 100 ol and 001 atg 010 ol respectively the 010 atg 001 ol and antigorite lamellae are parallel to the forsterite b axis in both cases the topography of olivine serpentine interfaces is controlled by open fluid pathways along microcracks oriented according to the anisotropy of the olivine aggregate in the cases studied the serpentine aggregate exhibits a preferred orientation inherited from that of the peridotite these results have some relevance to the seismic anisotropy of serpentinized mantle anisotropy of propagation of seismic waves as a result of the olivine fabric is maintained and reinforced with the development of lizardite conversely the development of antigorite produces a trench parallel fast s wave polarization and an anisotropy that is lowered at low degrees of serpentinization and then increased with increasing serpentinization

unusual mantle poisson s ratio subduction and crustal structure in central alaska 1 subduction zones alter the upper plate in a variety of ways including metasomatism via slab derived fluids and accretion of exotic terranes these processes should produce distinctive seismic signatures as seismic velocities are sensitive to compositional variations a new method is developed to estimates depths to interfaces and poisson s ratio vp vs ratio for dipping layered structures by stacking teleseismic receiver functions it also estimates crustal vp vs and thickness using this method we analyze a broadband data set from the central alaska subduction zone to seek evidence for mantle wedge composition crustal thickness varies from 27 to 45 km and crustal vp vs indicates an intermediate to mafic composition expected for these accreted island arc terranes inversions for mantle structure confirm the presence of a 15 20 km thick low velocity zone atop of the downgoing plate to 130 km depth perhaps subducted crust of an exotic terrane vp vs in the mantle wedge allows for 15 15 serpentinization where the slab is 80 km deep and the wedge is cold where the slab is deeper vp vs for the mantle wedge is unusually low 1 7 this value is lower than predicted for any common mantle mineral and may indicate significant quantities of quartz provided that past physical properties measurements are not grossly in error these results suggest that the mantle wedge in subduction zones can occasionally differ substantially in bulk composition from normal subcrustal mantle perhaps because collisions of exotic terranes cause tectonic mixing of crustal material into the mantle

interpreting magnetic fabrics in amphibole bearing rocks abstract if amphibole is a major constituent in a rock its magnetic fabric can be largely controlled by the crystallographic preferred orientation of amphibole this study describes the para magnetic anisotropy in two amphibolites both containing ca 70 hornblende with rather strong crystallographic preferred orientation both amphibolites display a significant and well defined anisotropy of magnetic susceptibility with the minimum susceptibility approximately normal to foliation however in one amphibolite the maximum susceptibility is parallel to the lineation whereas in the other it is not this seemingly inconsistent observation can be explained by the intrinsic susceptibility anisotropy of single crystals of hornblende and their texture in the rocks numerical models show how the principal susceptibility axes relate to macroscopic foliation and lineation for point and fiber textures this study underlines the potential of using magnetic anisotropy to obtain information about mineral fabrics in mafic rocks at the same time it highlights the necessity for taking into account single crystal properties of the mineral s responsible for the anisotropy and their crystallographic preferred orientation when interpreting magnetic fabrics

minimizing and quantifying mis indexing in electron backscatter diffraction ebsd determinations of antigorite crystal directions antigorite is common in hydrated mantle domains and commonly shows strong alignment either due to deformation or growth in a preferred orientation the alignment of antigorite imparts a strong anisotropy to the host rock affecting physical properties a quantitative analysis of how antigorite affects these properties requires a reliable measurement of the crystallographic preferred orientation of antigorite and ebsd analysis is the most widely used technique potential problems include i mis indexing which can add significant uncertainties to the results and ii sample preparation which may affect the measured cpo in particular for automated mapping combining results derived from x ray goniometry ebsd and u stage techniques with fib tem analysis leads to the following conclusions concerning these two issues i there is a significant issue with mis indexing a and b axes due to rotational similarities about the c axis similar problems may also affect the c axes measurements but this is less significant than the a and b axes when data are filtered using lower mad values filtering using mad values of 0 7 can significantly change the resulting cpo ii sample preparation can also affect the measured cpo sections prepared parallel to the foliation show only minor differences with mad values of 2 0 mis indexing problems can be minimized by using an mad value of 0 7 and analysing thin sections cut parallel to the foliation

anisotropy patterns in the subducting lithosphere and in the mantle wedge a case study the southern italy subduction system 1 in this study we present a collection of high quality s wave splitting measurements in the southern italy subduction system we analyzed 76 deep earthquakes located within the descending slab using the method of silver and chan 1991 to determine the splitting parameters and t the local deep earthquakes allow us to analyze raypaths primarily sampling the slab and the wedge above it mainland calabria is an outcropping forearc enabling us to sample rays that propagate up the slab s wave splitting parameters show a complex pattern of anisotropy with variable fast directions and with delay times ranging from 0 1 s to 2 2 s we compared local s wave splitting data with sks results at the same stations and we found that the average t is very different 1 8 s for sks and 0 5 s for s we found consistency between averaged s fast directions and the sks splitting measurements including a pattern of toroidal mantle flow at the sw edge of the slab the s wave splitting parameters show frequency dependent behavior that we attribute to the presence of small scale anisotropic heterogeneities comparison of s splitting measurements to p wave velocity anomaly at 100 200 km depth shows that where the rays primarily sample the slab the delay times are small in contrast where s rays sample the mantle wedge the delay times are quite high this t pattern depicts the slab as a weakly anisotropic region and suggests that the main source of anisotropy in the subduction zone is the surrounding asthenosphere

trench parallel anisotropy produced by serpentine deformation in the hydrated mantle wedge although seismic anisotropy in the upper mantle is generally attributed to the crystal preferred orientation of olivine the strong trench parallel anisotropy observed in several subduction systems is difficult to explain in terms of olivine anisotropy even if the entire mantle wedge were to act as an anisotropic source using high pressure deformation experiments ikuo katayama and colleagues show that the crystal preferred orientation of serpentine the main hydrous mineral in the upper mantle can produce the strong trench parallel seismic anisotropy observed in such subduction systems although seismic anisotropy in the upper mantle is generally attributed to the crystal preferred orientation of olivine the strong trench parallel anisotropy observed in several subduction systems is difficult to explain in terms of olivine anisotropy using high pressure deformation experiments it is now shown that the crystal preferred orientation of serpentine the main hydrous mineral in the upper mantle can produce the strong trench parallel seismic anisotropy observed in such subduction systems seismic anisotropy is a powerful tool for detecting the geometry and style of deformation in the earth s interior as it primarily reflects the deformation induced preferred orientation of anisotropic crystals1 2 although seismic anisotropy in the upper mantle is generally attributed to the crystal preferred orientation of olivine3 the strong trench parallel anisotropy delay time of one to two seconds observed in several subduction systems4 5 is difficult to explain in terms of olivine anisotropy even if the entire mantle wedge were to act as an anisotropic source here we show that the crystal preferred orientation of serpentine the main hydrous mineral in the upper mantle can produce the strong trench parallel seismic anisotropy observed in subduction systems high pressure deformation experiments reveal that the serpentine c axis tends to rotate to an orientation normal to the shear plane during deformation consequently seismic velocity propagating normal to the shear plane plate interface is much slower than that in other directions the seismic anisotropy estimated for deformed serpentine aggregates is an order of magnitude greater than that for olivine6 and therefore the alignment of serpentine in the hydrated mantle wedge results in a strong trench parallel seismic anisotropy in the case of a steeply subducting slab this hypothesis is also consistent with the presence of a hydrous phase in the mantle wedge as inferred from anomalously low seismic wave velocities7

an inverted continental moho and serpentinization of the forearc mantle volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back arc region but the forearc mantle in the corner of the wedge is thought to be significantly cooler here we explore the structure of the mantle wedge in the southern cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers we find very low shear wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an inverted continental moho which reverts to normal polarity seaward of the cascade arc this observation provides compelling evidence for a highly hydrated and serpentinized forearc region consistent with thermal and petrological models of the forearc mantle wedge this serpentinized material is thought to have low strength and may therefore control the down dip rupture limit of great thrust earthquakes as well as the nature of large scale flow in the mantle wedge

seismic anisotropy produced by serpentine in mantle wedge abstract trench parallel seismic anisotropy has been observed in many subduction zones in the upper mantle in this study the crystal preferred orientation cpo and seismic anisotropy of natural serpentine from val malenco in northern italy and punta bettolina in western italy are studied it is found that the 001 axes are aligned subnormal to the foliation but the 010 axes of the serpentine are aligned subparallel to the lineation which is significantly different from that produced in a recent high pressure experiment cpos of serpentine found herein can be used to explain the trench parallel seismic anisotropy in a cold mantle wedge for serpentine deformed not only at angles greater than 45 but also at low angles such as in horizontal shear from the surface indicating the broader implications of the cpo of serpentine for interpreting seismic anisotropy than previously thought it is also found that seismic anisotropy caused by the cpo of serpentine depends on the degree of serpentinization and flow geometry the seismic anisotropy increases with antigorite volume fraction the seismic anisotropy of shear waves is large i e 23 36 for samples with an antigorite volume fraction of 40 93 current data suggest that the trench parallel seismic anisotropy with a delay time of 1 2 s in the forearc mantle wedge can be attributed to the cpo of antigorite having 010 axes aligned subparallel to the flow direction and 001 axes subnormal to the flow plane

global prevalence of double benioff zones double benioff zones provide opportunities for insight into seismogenesis because the underlying mechanism must explain two layers of deep earthquakes and the separation between them we characterize layer separation inside subducting plates with a coordinate rotation to calculate the slab normal distribution of earthquakes benchmark tests on well established examples confirm that layer separation is accurately quantified with global seismicity catalogs alone global analysis reveals double benioff zones in 30 segments including all 16 subduction zones investigated with varying subducting plate ages and stress orientations which implies that they are inherent in subducting plates layer separation increases with age and is more consistent with dehydration of antigorite than chlorite

frequency dependent shear wave splitting and heterogeneous anisotropic structure beneath the gulf of california region abstract the gulf of california region has undergone a major evolution over the past 15 ma since subduction of the now extinct farallon plate ceased and transform motion began leading to the initiation of rifting and the opening of the gulf of california 5 ma ago the character of dynamic processes in the upper mantle beneath the region remains poorly understood but constraints on seismic anisotropy in the upper mantle can shed light on contemporary mantle flow processes in order to characterize more fully the anisotropic structure of the upper mantle beneath the region i present measurements of sks splitting at 14 broadband stations of the nars baja array at periods between 8 and 50 s the measured splitting parameters t at nars baja stations exhibit a high degree of complexity with dramatic lateral variations significant backazimuthal variations at individual stations and a large number of well constrained null measurements over a range of backazimuths the dependence of splitting parameters on frequency is evaluated by applying a series of bandpass filters to high signal to noise sks arrivals i find that while fast directions are generally insensitive to the sks frequency content delay times are frequency dependent at several stations the measurements presented in this study indicate that the anisotropic structure of the upper mantle beneath the gulf of california region is complex and exhibits a high degree of both lateral and vertical heterogeneity the observed splitting is consistent with a scenario in which a large scale reorganization of the mantle flow field has been inhibited by the presence of a stalled fragment of oceanic lithosphere associated with farallon subduction the pattern of mantle flow appears to be controlled by more localized processes likely including small scale buoyant upwelling associated with partial melting beneath the central part of the gulf of california rift