textural characteristics and genesis of the aeolian sediments in the kuwaiti desert several types of aeolian deposits have been recognized in kuwait a smooth sand sheets that resemble desert floor sand b immobile sands that include rugged vegetated sand sheets and wadi fill deposits and c mobile sands that form active sand sheets and sand dunes simple size frequency curves illustrate the genetic relationship between the various aeolian sediment types the four size parameters namely mean size sorting skewness and kurtosis were calculated scatter plot diagrams of sorting versus mean size and sorting versus kurtosis are effective in differentiating smooth sand sheet deposits from dune sands active sand sheet deposits can also be recognized because they are usually located between the two end members smooth sand sheets and dune sands size parameters change with location regardless of their types coarsening and positive skewness usually increase downwind mineralogical and textural characteristics of the aeolian deposits in kuwait revealed that they are mostly derived from the lower mesopotamian muddy flood plain deposits the sand fraction of the al dibdibba gravelly deposits and the disintegrated material from calcretic and gypcretic duricrusts distribution of depositional and deflational areas indicates that the northern desert of kuwait is characterized by a positive sand budget whereas the southern desert has a negative sand budget

strength of single crystal orthopyroxene under lithospheric conditions creep strength of oriented orthopyroxene single crystals was investigated via shear deformation experiments under lithospheric conditions p pressure 1 3 gpa and t temperature 973 1 373 k for the a orientation shear direction 001 on 100 plane the samples have transformed completely to clinoenstatite and much of the deformation occurred after transformation in contrast for the b orientation shear direction 001 on 010 plane samples remained orthoenstatite and deformation occurred through dislocation motion in orthoenstatite the strength of orthopyroxene with these orientations is smaller than for olivine aggregates under all experimental conditions flow of the b orientation samples is described by a power law and the pre exponential constant the apparent activation energy and the stress exponent are determined to be a 10 9 5 s 1 mpa 4 2 q 114 kj mol and n 4 2 however for the a orientation the results cannot be fit by a single flow law and we obtained the following a 108 9 s 1 mpa 3 0 q 459 kj mol and n 3 0 at high temperatures 1 173 k and a 10 27 4 s 1 mpa 14 3 q 296 kj mol and n 14 3 at low temperatures 1 173 k the stress exponent for the low temperature regime is high suggesting that deformation involves some processes where the activation energy decreases with stress such as the peierls mechanism our study shows that orthopyroxene with these orientations is significantly weaker than olivine under the lithospheric conditions suggesting that orthopyroxene may reduce the strength of the lithosphere although the extent to which orthopyroxene weakens the lithosphere depends on its orientation and connectivity

shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

soil aggregate stability within morphologically diverse areas abstract knowledge of spatial distribution of soil aggregate stability as an indicator of soil degradation vulnerability and its possible prediction are required for many scientific and practical environmental studies the goal of our study was to provide a model for predicting soil aggregate stability within morphologically diverse areas where soil properties have been affected by soil material redistribution due to erosion the study was performed on a study site 6 ha area in the loess region of southern moravia czech republic haplic chernozem which is an original dominant soil unit has been transformed into different soil units eroded phases of chernozem regosol colluvial chernozem and colluvial soil 36 sampling spots were selected in order to represent diverse soils the following soil properties were measured oxidable organic carbon content cox caco3 content phh2o phkcl soil particle density s bulk density d porosity p actual field soil water content field content of iron and manganese in ammonium oxalate extract feo and mno and dithionite citrate extract fed and mnd and mass specific magnetic susceptibility lf and hf the aggregate stability was assessed using various tests to study different disruption mechanisms terrain attributes were derived from a digital elevation model in general the lowest soil aggregate stability was observed on steep slopes which were highly impacted by soil erosion the highest aggregate stability was measured on soils sampled at relatively flat upper parts which were less influenced by erosion processes higher stability was also obtained on toe slopes where the sedimentation of previously eroded soil material occurred the simple correlations revealed that characteristics resulting from the tests studying aggregate slaking due to the compression of the entrapped air water stable aggregate index and coefficient of vulnerability from fast wetting test were positively impacted by the cox p feo mno fed mnd lf and hf values and negatively by the d value the soil aggregate stability was also negatively influenced by the plan and total terrain curvatures i e larger aggregate stability was measured at concave parts in comparison with that at convex parts almost no statistically significant relationships were found in the case of the tests evaluating either aggregate disintegration caused by the micro cracking due to the different swelling or by the physico chemical dispersion due to the osmotic stress or the mechanical aggregate breakdown the multiple linear regressions resulted in the model for estimating the wsa index using the cox content total terrain curvature and actual field soil water content field in this model the cox content positively and the total terrain curvature and field value negatively influenced the value of the wsa index since cox was positively related with iron content and thus also with the magnetic susceptibility the alternative model was proposed for less costly and time consuming wsa estimation the wsa index may be predicted by combining the mass specific magnetic susceptibility lf and hf total terrain curvature and actual field soil water content field

crystallographic preferred orientation of olivine in the earth s deep upper mantle abstract simple shear deformation experiments of olivine were conducted at pressures of 11 1 12 7 gpa and temperatures of 1670 1770 k c type olivine fabric developed by the 1 0 0 0 0 1 slip system and similar fabrics were dominantly developed in a wide range of water content coh 49 19 242 ppm h si in olivine because the seismic fast a axes are perpendicular to the shear plane only for c type fabric a positive negative transition of vsh vsv difference in velocity between horizontally and vertically polarized shear waves inevitably occurs when c type fabric is developed by a pressure induced fabric transition the positive negative transition of vsh vsv at 200 km depth reported in global one dimensional models is known to be well explained by the horizontal flow of olivine with a low water content

pressure effect on forsterite dislocation slip systems implications for upper mantle lpo and low viscosity zone abstract in order to better constrained the effect of pressure p on olivine dislocation slip system activities deformation experiments were carried out in a deformation dia apparatus d dia on pure forsterite fo100 single crystals at p 5 7 gpa temperature t 1675 k differential stress and specimen strain rates e were monitored in situ by synchrotron x ray diffraction and radiography respectively two compression directions were tested promoting either 1 0 0 slip or 0 0 1 slip in 0 1 0 crystallographic plane comparison of the obtained high p rheological data with room p data previously reported by darot and gueguen 1981 shows that 1 0 0 slip is strongly inhibited by pressure while 0 0 1 slip is virtually p insensitive this translates in creep power laws into a high activation volume v a 15 3 cm 3 mol for 1 0 0 0 1 0 slip system and v c 0 1 2 cm 3 mol for 0 0 1 0 1 0 slip system using these laws along geotherms at natural condition shows that the 1 0 0 slip 0 0 1 slip transition may occur at 200 km depth in the upper mantle and be responsible for the observed lattice preferred orientation lpo transition a rheological law for polycrystalline forsterite is deduced from the single crystal rheological laws assuming that individual grains are randomly oriented in the aggregate applying the aggregate law within a 2d geodynamic model of upper mantle couette flow suggests that the pressure dependence of olivine dislocation slip activities may partly explain the low viscosity zone lvz observed underneath oceanic plates

the significant role of inorganic matters in preservation and stability of soil organic carbon in the baoji and luochuan loess paleosol profiles central china abstract the preservation and stability mechanisms of soil organic carbon soc are the important factors to evaluate the capacity of soil carbon pool and the potential of sustainable utilization to understand the preservation time and mechanisms of soc under burial conditions in the present study the distributions of total organic carbon toc and stable organic carbon stoc and their correlations with the contents of clays and clay minerals and different forms of iron oxides were investigated in the baoji and luochuan loess paleosol profiles four facts were observed as the followings 1 the labile soc almost was decomposed and the mostly stable soc was preserved in the loess and paleosol after 375 kyr since their formation stoc could be preserved at least 762 kyr in both loess and paleosol under burial condition 2 the toc was positively correlated with clay contents with correlation coefficients of 0 72 baoji and 0 63 luochuan 3 the toc stoc mineral protected organic carbon moc and recalcitrant organic carbon roc were positively correlated with kaolinite with correlation coefficients of 0 93 0 72 0 52 0 81 baoji and 0 78 0 58 0 50 0 49 luochuan respectively both with neither illite nor vermiculite 4 the toc was highly correlated with complex iron fe p with correlation coefficients of 0 90 baoji and 0 82 luochuan so with amorphous oxides of iron fe o as well among them fe o mainly affected by sorption and fe p by complexation on soc preservation whereas kaolinite had both chemical and physical effects the values of coefficients further highlight that the contributions of inorganic matters to the fixation of organic carbon were ranked to an order of kaolinite fe p fe o

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

high temperature elasticity of iron bearing olivines the first high temperature data on the nine adiabatic elastic moduli for iron bearing olivine are reported these measurements are on two single crystal specimens of natural olivine at ambient pressure and from room temperature to a maximum of 1500 k the two specimens contain 8 and 9 modal percent fayalite which required the oxygen fugacity be controlled at high temperature to preserve their chemical stability the rectangular parallelepiped resonance apparatus was adapted to buffer the specimens from the atmosphere with a mixture of co and co2 gas a small increase 1 2 gpa in the adiabatic bulk modulus of each specimen over that of end member forsterite was found the data are high quality to extreme temperatures with good agreement found when comparing the temperature derivatives of the elastic moduli of the two specimens neither specimen exhibits measurable nonlinear temperature dependence in the computed isotropic bulk and shear moduli which is in contrast to published forsterite data the temperature derivatives of the isotropic bulk modulus ks are 1 69 1 80 10 2 gpa k 1 for the two olivine specimens and the shear modulus g derivatives are 1 38 1 36 10 2 gpa k 1 these derivatives are only slightly larger in magnitude than ks t p 1 56 10 2 and g t p 1 30 10 2 gpa k 1 found previously for iron bearing olivine over a very small temperature range there are also no significant differences between the temperature derivatives found here and the average derivatives of end member forsterite from data retrieved over a slightly larger temperature range several dimensionless parameters have been calculated from these results and are discussed in view of systematics which bear on high pressure phases in earth s transition zone one result from these systematics related to the seismic velocities in the earth and especially the shear wave velocities is that an olivine content of less than 50 is implied at the 400 km discontinuity if earth s upper mantle is isochemical furthermore the substitution of almost 10 iron for magnesium at the forsterite end of the olivine solid solution series has little effect on the dimensionless parameters or on the temperature derivatives of the elastic moduli at high temperature

b type olivine fabric in the mantle wedge insights from high resolution non newtonian subduction zone models several hypotheses have been proposed to explain trench parallel shear wave splitting in the mantle wedge of subduction zones these include 3 d flow effects parallel melt filled cracks and b type olivine fabric we predict the distribution of btype and other fabrics with high resolution thermal and stress models of subduction zones a composite viscous rheology is used that incorporates wet diffusion creep wet dislocation creep and a stress limiting approximated peierls creep law rheological parameters for dislocation and diffusion creep are based on deformation experiments the effects of variable viscous coupling between the slab and mantle are explored using kinematic boundary conditions that change along the top of the slab two end member models are observed which depend on the depth of partial viscous coupling between the slab and mantle 1 deep partial coupling which gives rise to cold high stress conditions in the forearc mantle and high temperature low stress conditions in the arc and back arc mantle 2 full viscous coupling at the base of a 40 km conducting lid which is characterized by high temperature and low stress the case with deep partial coupling which produces a better match with attenuation tomography and heat flow shows a large region with suitable conditions for b type fabric in the forearc mantle and a rapid transition toward the back arc to conditions that are more suitable for a c or e type fabrics full viscous coupling gives rise to high temperature low stress conditions unsuitable for b type fabric our modeling predicts that the best candidates for regions with b type fabric are the forearc mantle and a cold 10 15 km layer above the slab d 2005 elsevier b v all rights reserved

anisotropy and mantle flow in the chile argentina subduction zone from shear wave splitting analysis 1 we examine shear wave splitting in teleseismic phases to observe seismic anisotropy in the south american subduction zone data is from the charge network which traversed chile and western argentina across two transects between 30 s and 36 s beneath the southern and northwestern parts of the network fast polarization direction j is consistently trench parallel while in the northeast j is trench normal the transition between these two zones is gradual we infer that anisotropy sampled by teleseismic phases is localized within or below the subducting slab we explain our observations with a model in which eastward nazca entrained asthenospheric flow is deflected by retrograde motion of the subducting nazca plate resulting southward flow through this area produces n s j observed in the south and northwest e w j result from interaction of this flow with the local slab geometry producing eastward mantle flow under the actively flattening part of the slab index terms 7203 seismology body wave propagation 7218 seismology lithosphere and upper mantle 8123 tectonophysics dynamics seismotectonics 8150 tectonophysics plate boundary general 3040 9360 information related to geographic region south america citation anderson m l g zandt e triep m fouch and s beck 2004 anisotropy and mantle flow in the chile argentina subduction zone from shear wave splitting analysis geophys res lett 31 l23608 doi 10 1029 2004gl020906

sedimentological characteristics of the surficial sediments of the kuwaiti marine environment northern arabian gulf the textural characteristics carbonate content and the coarse fraction components of the recent bottom sediments of the marine environment off kuwait are described and the faunal sediment associations discussed the sediments were subdivided into seven textural classes namely sand silty sand muddy sand sandy silt sandy mud silt and mud most of the study area is covered with muddy sediments whereas sandy deposits are restricted to the rocky bottoms near the southern flat of kuwait bay the southern coast of kuwait and around the islands and bathymetric highs the textural classes carbonate contents and faunal types of the coarse fraction were used to construct a biolithofacies map of the marine bottom sediments off kuwait in which nine facies are identified the sedimentological characteristics of the recent marine bottom sediments off kuwait reflect the interaction between autochthonous calcareous fragments mostly of biogenic origin lime rock fragments derived from beachrocks and submerged reef flats and allochthonous terrigenous detritus transported to the area mainly by dust storms the kuwaiti offshore area is generally a low energy depositional environment with little sediment transport

seismicity and shape of the subducted nazca plate an updated compilation of earthquake locations and focal mechanism solutions from the international seismological centre and preliminary determination of earthquakes is the basis of a comprehensive study of the geometry of the wadati benioff zone beneath western south america the new data support previous mapping of a sharp flexure rather than a tear in the subducted nazca plate beneath southern peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with 30 dip at latitude 33 s in contrast the transition from 30 slab dip beneath bolivia to a nearly horizontal dip in the region between 28 s 32 s is more gradual occurring over several hundred kilometers of along strike distance between 20 s and 32 s this southward flattening corresponds to a broadening of a horizontal benchlike part of the subducted plate formed between 100 and 125 km depth the transition in continental tectonic style near 27 s 28 s from a wide volcanically active plateau to a narrow nonvolcanic cordillera appears not to be associated with the main slab flattening which begins to the north of these latitudes but with a more abrupt change in curvature of the subducted slab from convex upward to concave upward immediately below the plate boundary interface the concept of gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the south american plate we hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin focal mechanism solutions for intermediate and deep earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths

relative motion of the nazca farallon and south american plates since late cretaceous time by combining reconstructions of the south american and african plates the african and antarctic plates the antarctic and pacific plates and the pacific and nazca plates we calculated the relative positions and history of convergence of the nazca and south american plates despite variations in convergence rates along the andes periods of rapid convergence averaging more than 100 mm a between the times of anomalies 21 49 5 ma and 18 42 ma and since anomaly 7 26 ma coincide with two phases of relatively intense tectonic activity in the peruvian andes known as the late eocene incaic and mio pliocene quechua phases the periods of relatively slow convergence 50 to 55 30 mm a at the latitude of peru and less farther south between the times of anomalies 30 31 68 5 ma and 21 and between those of anomalies 13 36 ma and 7 correlate with periods during which tectonic activity was relatively quiescent thus these reconstructions provide quantitative evidence for a correlation of the intensity of tectonic activity in the overriding plate at subduction zones with variations in the convergence rate

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

shear wave anisotropy in the upper mantle beneath the nazca plate in northern chile data from the projecto de investigacion sismologica de la cordillera occidental pisco seismic network and from six broadband seismographs that were operating in northern chile were used to investigate the mantle in the convergent boundary zone between nazca plate and the south american continent for the presence of anisotropy broadband data as well as long period filtered data of teleseismic sks and pks phases were analyzed for the presence of shear wave splitting as a possible indicator for seismic anisotropy in the mantle beneath the pisco network measurable shear wave splitting was observed with maximum delay times between the slow and fast split wave of the order of 1 s splitting of s waves from intermediate depth events located directly beneath the pisco network in the descending nazca plate is generally associated with small delay times of the order of 0 1 s a value typical for the continental crust near vertical scs reflections from two deep earthquakes in argentina and one nearby intermediate depth earthquake have similar splitting parameters as the sks phases this means that the anisotropic zone causing the splitting of the core phases can be constrained to the pacific mantle underlying the subducting nazca plate it probably does not extend deeper than about 260 km the majority of the anisotropy directions inferred from the core phases are parallel to the absolute plate motion apm direction of the nazca plate which is about n80 e at some stations however the fast polarization direction is pointing n160 e nearly parallel to the strike of the trench and the andes which would be compatible with the trench parallel flow model for south america proposed by russo and silver 1994 this direction is observed over an approximately 100 km wide band to the west of the active volcanic zone it may represent either a second anisotropy regime in the mantle a small scale diversion of slab entrained mantle flow or a relatively small area where slab entrainment of mantle flow is reduced or ceases to exist the large number of observed apm parallel fast directions suggests however that the mantle beneath the descending nazca plate in northern chile deforms mainly as the result of slab entrained mantle flow the large variations of anisotropy directions in the andean subduction zone indicate that asthenospheric flow in the pacific mantle has a complex pattern which may vary over scale lengths of a few hundred kilometers and which may be governed by slab morphology

mantle flow at a slab edge seismic anisotropy in the kamchatka region the junction of the aleutian island and the kamchatka peninsula defines a sharp turn in the boundary of the pacific and north american plates terminating the subduction zones of the northwest pacific the regional pattern of shear wave birefringence near the junction indicates that trench parallel strain follows the seismogenic benioff zone but rotates to trench normal beyond the slab edge asthenospheric mantle is inferred to flow around and beneath the disrupted slab edge and may influence the shallowing dip of the benioff zone at the aleutian junction

flow behavior and microstructures of hydrous olivine aggregates at upper mantle pressures and temperatures deformation experiments on olivine aggregates were performed under hydrous conditions using a deformation dia apparatus combined with synchrotron in situ x ray observations at pressures of 1 5 9 8 gpa temperatures of 1223 1800 k and strain rates ranging from 0 8 10 5 to 7 5 10 5 s 1 the pressure and strain rate dependencies of the plasticity of hydrous olivine may be described by an activation volume of 17 6 cm3 mol 1 and a stress exponent of 3 2 0 6 at temperatures of 1323 1423 k a comparison between previous data sets and our results at a normalized temperature and a strain rate showed that the creep strength of hydrous olivine deformed at 1323 1423 k is much weaker than that for the dislocation creep of water saturated olivine and is similar to that for diffusional creep and dislocation accommodated grain boundary sliding while dislocation microstructures showing the 001 slip or the 001 100 slip system were developed at temperatures of 1633 1800 k a much stronger pressure effect on creep strength was observed for olivine with an activation volume of 27 7 cm3 mol 1 assuming a stress exponent of 3 5 water fugacity exponent of 1 2 and activation energy of 520 kj mol 1 i e power law dislocation creep of hydrous olivine because of the weak pressure dependence of the rheology of hydrous olivine at lower temperatures water weakening of olivine could be effective in the deeper and colder part of earth s upper mantle

shear wave splitting and implications for mantle flow beneath the melt region of the east pacific rise shear wave splitting across the fast spreading east pacific rise has been measured from records of sks and skks phases on the ocean bottom seismometers of the mantle electromagnetic and tomography melt experiment the direction of fast shear wave polarization is aligned parallel to the spreading direction delay times between fast and slow shear waves are asymmetric across the rise and off axis values on the pacific plate are twice those on the nazca plate splitting on the pacific plate may reflect anisotropy associated with spreading induced flow above a depth of about 100 km as well as a deeper contribution from warm asthenospheric return flow from the pacific superswell region

textural characteristics of dust fallout and potential effect on public health in kuwait city and suburbs the textural and mineralogical characteristics of the dust fallout along with the potential effect on human health were investigated at 12 locations in kuwait city and suburbs during march 2006 february 2007 mineralogically the dust comprises mostly calcite and quartz with grain size ranging from 1 to 25 m chemically elements such as ca fe mg and al are the dominant ones with the highest amount of average percentage concentration 12 89 for ca results showed that humans in the 12 sites are being exposed to different ranges of average percentage concentrations of minerals such as calcite 15 00 51 80 quartz 26 40 66 9 albite 6 30 17 30 dolomite 1 20 10 20 and gypsum 0 00 6 4 where site k3 recorded the lowest average concentrations for calcite dolomite and gypsum and the highest for quartz and albite the elements ca fe mg al k zn na cr and cd recorded ranges of 5 845 12 890 0 946 3 141 0 752 2 513 0 545 2 261 0 059 0 307 0 004 0 792 0 003 0 018 0 001 0 005 and 0 0004 respectively inhalation of such concentrations of calcite particles may cause coughing sneezing and nasal irritation and in cases of chronic exposure to excessive oral doses of calcite may produce alkalosis and hypercalcemia quartz can have potentially serious respiratory effects following long term exposure 1 year or more which is the case here in our study comparing with international and local standards it is highly likely that harmful and hazardous effects on the human body are being experienced by long term exposure to dust fallout in kuwait

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

shear wave anisotropy beneath the volcanic front in south kyushu area japan development of c type olivine cpo under h2o rich conditions 1 shear wave splitting from local intermediate depth earthquakes is investigated to detect the anisotropic structure in the mantle wedge beneath the south kyushu area japan we observed shear wave splitting with nee sww to nww see polarization directions and delay times of 0 04 0 63 s trench normal shear wave polarization anisotropy with a delay time greater than 0 3 s probably overlaps in the high vp vs region at a depth of 100 150 km beneath the volcanic front model calculations for the cause of the anisotropy suggest that the development of c type olivine crystallographic preferred orientation cpo with a trench parallel b axis concentration and a trench normal a axis concentration best reproduces observations compared to a b or e type olivine cpos and antigorite cpo we conclude that a thick anisotropic layer approximately 50 km is formed by concentration of interstitial fluid in peridotite we briefly discuss the mechanism carrying water to the deep mantle wedge a possible explanation is that interstitial fluids in the mantle wedge are effectively released because of a decrease in dihedral angles of olivine fluid interfaces at 150 km depth our results imply a close relation between the high density of explosive volcanoes in the south kyushu area and the underlying water rich mantle

distribution of seismic anisotropy in the subduction zone beneath the wellington region new zealand summary shear wave splitting measurements from s arrivals of local earthquakes recorded at the incorporated research institutions for seismology iris broadband sensor snzo are used to determine a basic anisotropic structure for the subduction zone in the wellington region with the use of high frequency filters fast anisotropic polarization w and splitting time dt measurements typical of crustal anisotropy are evident but the larger splitting expected from the mantle is often not resolved the small splitting seen agrees well with the results of previous studies concerning shallow crustal anisotropy with the use of lower frequency filters measurements more consistent with mantle anisotropy are made anisotropy of 4 4 0 9 per cent with a fast polarization of 29 38 is calculated for the subducting slab from 20 to 70 km depth using this result in addition to the results of previous studies a model is proposed the model requires a frequency dependent anisotropy of less than 1 4 per cent when measured with a period of 2 s to be present in the sub slab mantle separate from this population a band of events in northern cook strait with an 86 10 fast polarization is seen this is at about 40 from the strike of the hikurangi margin and suggests a source of shear strain 40 removed from that found in the majority of the region the cause of this is probably a deformation in the subducting slab in this region as it moves towards a greater incline to the south

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

effect of temperature on the b to c type olivine fabric transition and implication for flow pattern in subduction zones abstract the effect of temperature on the b to c type fabric transition in olivine was investigated by large strain shear deformation experiments a new series of deformation experiments was conducted at 1273 1373 k 2 0 gpa and strain rates ranging from 3 8 10 4 to 3 2 10 5 s 1 under water saturated conditions and combined with previous results at 1473 k the influence of temperature on deformation fabrics was investigated two types of olivine fabrics were identified one in which the olivine 0 0 1 axis is subparallel to the shear direction and the 0 1 0 plane is parallel to the shear plane b type and another one in which the 0 0 1 axis is parallel to the shear direction and the 1 0 0 plane is parallel to the shear plane c type the b type olivine fabric occurs at higher stress than the c type but the stress magnitude at which the b to c type fabric transition occurs decreases significantly with decreasing temperature the temperature dependence of the transition stress is parameterized using a range of exponential flow law that is constrained by the experimental study when the experimental results are extrapolated to lower temperatures the fabric transition between b and c type is predicted to occur at 1 mpa and 1000 k this prediction is consistent with the olivine fabrics in naturally deformed peridotites found in plate convergent regions the present study indicates that when some amounts of water 200 ppm h si are present the b type fabric will dominate in low temperature regions of the upper mantle whereas the c type or e type fabric will dominate at high temperatures the complex seismic anisotropy in subduction zones can be explained by changes in the dominant type of olivine fabrics caused by spatial variations in physical and chemical variables such as temperature and water content rather than by complexities in mantle wedge flow pattern

change of olivine a axis alignment induced by water origin of seismic anisotropy in subduction zones abstract the effects of water on the crystallographic preferred orientation cpo of olivine aggregates were investigated through simple shear deformation experiments under asthenospheric upper mantle conditions pressure 2 1 5 2 gpa temperature 1490 1830 k using a deformation dia apparatus formation of the a type olivine fabric developed by the 010 100 slip system was observed under water depleted conditions coh 1000 ppm h si in comparison with fabrics of anhydrous olivine 111 ppm h si those of olivine having higher water contents 625 ppm h si in olivine showed stronger anisotropic properties e g p wave anisotropy s wave polarization anisotropy and the ratio of horizontally and vertically polarized shear waves the water induced olivine cpo transition from a type to b type like fabric accompanies a change in the alignment of the seismic fast a axes resulting in flow parallel and flow perpendicular shear wave splitting under water depleted and water rich conditions respectively the rotation of the fast direction of shear wave splitting across an arc which is observed in many subduction zones is well explained by the likely bimodal water distribution along the mantle wedge i e water rich in fore arc and water depleted in back arc regions and the developments of two different types of olivine fabrics i e b type like fabric in fore arc and a type fabric in back arc regions

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

towards an orbital time scale for chinese loess deposits abstract in this study we analyzed the grain size of the baoji loess soil sequence at 10 cm intervals results show that grain size variations are very sensitive to loess soil alternations with loess units being much coarser than soils such a change in loess soil grains is interpreted as the result of the glacial interglacial cyclic variations in the intensity of the winter monsoon winds out of siberia the grain size record is thus employed as a proxy indicator of the winter monsoon circulation and tuned to the orbital records calculated recently by berger and loutre 1991 under the control of major magnetic reversals the tuning is independent of any correlation with 18o signals in the deep sea sediments the resulting grain size time scale is tightly constrained as suggested by the following facts 1 the filtered obliquity and precession components from the grain size data on the orbital time scale closely match the theoretical orbital records 2 ages of the major magnetic reversals estimated from the grain size time scale are in good agreement with the k ar dated ages 3 there is close coherence between the baoji grain size time series and the orbital variations at the orbital frequency bands over the entire 0 2 5 ma period and 4 the grain size record on the orbital time scale shows a close similarity to the orbitally tuned dsdp site 607 and odp site 677 18o records examination of time dependent characteristics of the grain size record suggests that there are two major shifts of dominant periodicities in the long term monsoonal cycles one occurring at about 0 8 ma bp and the other around 1 7 ma bp

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

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

large scale trench normal mantle flow beneath central south america abstract we investigate the anisotropic properties of the fore arc region of the central andean margin between 17 25 s by analyzing shear wave splitting from teleseismic and local earthquakes from the nazca slab with partly over ten years of recording time the data set is uniquely suited to address the long standing debate about the mantle flow field at the south american margin and in particular whether the flow field beneath the slab is parallel or perpendicular to the trench our measurements suggest two anisotropic layers located within the crust and mantle beneath the stations respectively the teleseismic measurements show a moderate change of fast polarizations from north to south along the trench ranging from parallel to subparallel to the absolute plate motion and are oriented mostly perpendicular to the trench shear wave splitting measurements from local earthquakes show fast polarizations roughly aligned trench parallel but exhibit short scale variations which are indicative of a relatively shallow origin comparisons between fast polarization directions from local earthquakes and the strike of the local fault systems yield a good agreement to infer the parameters of the lower anisotropic layer we employ an inversion of the teleseismic waveforms based on two layer models where the anisotropy of the upper crustal layer is constrained by the results from the local splitting the waveform inversion yields a mantle layer that is best characterized by a fast axis parallel to the absolute plate motion which is more or less perpendicular to the trench this orientation is likely caused by a combination of the fossil crystallographic preferred orientation of olivine within the slab and entrained mantle flow beneath the slab the anisotropy within the crust of the overriding continental plate is explained by the shape preferred orientation of micro cracks in relation to local fault zones which are oriented parallel to the overall strike of the andean range our results do not provide any evidence for a significant contribution of trench parallel mantle flow beneath the subducting slab

in situ observation of crystallographic preferred orientation of deforming olivine at high pressure and high temperature abstract simple shear deformation experiments on polycrystalline olivine and olivine single crystal were conducted at pressures of 1 3 3 8 gpa and temperatures of 1223 1573 k to understand the achievement of steady state fabric strength and the process of dynamic recrystallization development of crystallographic preferred orientation cpo of olivine was evaluated from two dimensional x ray diffraction patterns and shear strain was measured from x ray radiographs the steady state fabric strength of the a type fabric was achieved within total shear strain of 2 at strains higher than 1 an increase in concentration of the 0 1 0 axes mainly contributes to an increase in fabric strength at strains higher than 2 the magnitude of v sh v sv i e ratio of horizontally and vertically polarized shear wave velocities scarcely increased in most of the runs the v sh v sv of peridotite 70 vol olivine 30 vol minor phases having the steady state a type olivine fabric coincides with that of recent global one dimensional models under the assumption of horizontal flow suggesting that the seismic anisotropy observed in the shallow upper mantle is mostly explained by the development of a type olivine fabric experimental results on the deformation of single crystal olivine showed that the cpo of olivine is influenced by the initial orientation of the starting single crystal because strain is concentrated in the recrystallized areas and the relic of the starting single crystal remains in the upper mantle the old cpo of olivine developed in the past may affect the olivine cpo developed in the present

estimation of dust fallout rates in the northern arabian gulf the penetration of dust clouds over a sea surface is modelled as a sediment transport diffusion problem the model is used to determine the steady state concentration field of two dimensional dust clouds during offishore dust storms the rates of dust deposition into the sea can then be calculated from the obtained concentration fields the model was applied to offshore dust storms blowing over the arabian gulf yielding estimated dust sedimentation rates of 0 8 mm yr 1 this is substantially higher by several orders of magnitude than dust fallout estimates in most seas of the world the calculated fallout rates are in line with the results of existing field measurements

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

the use of laboratory velocity data for estimating temperature and partial melt fraction in the low velocity zone comparison with heat flow and electrical conductivity studies peridotite is widely considered to be the dominant component of the upper most mantle seismic velocities of a dry peridotite determined in the laboratory at high pressure and hypersolidus temperature show a rapid decrease with increasing temperature the strong temperature dependence of velocities can be used to estimate temperature and melt fraction in the low velocity zone of the earth the laboratory results show that the pressure dependence of both velocity and melt fraction appears to be well accounted for by the pressure dependence of the solidus temperature of the peridotite i e homologous temperature dependence this observation allows us to extrapolate laboratory results to higher pressures greater depths with some confidence requiring only a knowledge of the solidus as a function of pressure using the ratios of lithospheric to asthenospheric velocities temperature and melt fraction in the asthenosphere the low velocity zone can be determined from laboratory velocity data examples of thermal structures of the upper mantle beneath oceanic plates are presented we choose three locations of geophysical interest iceland plateau pacific ocean and philippine sea where reliable seismic velocities have been determined in the upper mantle from surface wave studies a large amount of partial melt 5 vol and a higher temperature than the dry peridotite solidus are inferred in 0 5 m y asthenosphere under the iceland plateau and the active marginal basin of the east philippine sea the partial melt zone appears to extend deeper 100 km and to greater ages 20 m y in the pacific ocean region than under the slowly spreading iceland plateau partial melting is not expected in the asthenosphere older than 5 m y under the iceland plateau even though the volume fraction of partial melt is not so large 3 vol beneath the pacific plate the melting zone may extend to a distance of above 2000 km from the ridge many seamounts observed in the pacific ocean may result from this vast region of melting under the plate we suggest a linear relation between the width of melting zone and the plate velocity the laboratory results are also applied to other low velocity zones it is suggested that a velocity drop up to about 6 in the asthenosphere does not necessarily require the existence of partial melt but can be explained by high subsolidus temperature this implies that partial melt may not exist generally but only in limited areas in the low velocity zone comparison of seismic results with theoretical models may overestimate the temperature and melt fraction if the velocity drop at subsolidus temperature is neglected temperature and melt fraction obtained in this study are discussed together with results from heat flow and electrical studies almost the same temperatures as inferred from seismic velocity and a dry peridotite solidus are calculated from heat flow data indicating that the upper mantle under mid ocean ridges may be fairly dry even a water or co2 undersaturated solidus 0 1 wt does not give a temperature consistent with the lithospheric geotherm and laboratory velocity results it is noted that electrical conductivity studies generally yield lower temperatures and larger melt fractions than this study unknown pressure frequency and impurity effects on laboratory conductivities of partially molten mantle rocks are suggested as possible reasons for this discrepancy

new type of olivine fabric from deformation experiments at modest water content and low stress a new type of olivine fabric was found by high strain simple shear deformation experiments in the presence of trace amounts of water at 0 5 2 2 gpa and 1470 1570 k in this fabric called e type fabric the olivine 100 axis is subparallel to the shear direction and the 001 plane is parallel to the shear plane this geometry suggests that the 100 001 slip system makes the dominant contribution to total strain this fabric is dominant at a modest water content 200 coh 1000 h 10 6 si at low stresses and high temperatures some mylonites from peridotite massifs show this type of olivine fabric which suggests the presence of water during the shear localization the seismic anisotropy caused by this fabric is qualitatively similar to that by dry fabric a type but the magnitudes of anisotropy are different between the two types for horizontal flow the amplitude of vsh vsv anisotropy is weaker but the amplitude of shear wave splitting is stronger for the e type fabric than for the a type dry fabric seismic anisotropy in the oceanic upper mantle may be due to the olivine e type fabric

subslab anisotropy in the andaman subduction zone controlled by slab dip abstract the mechanisms that govern anisotropy in subduction zones are debated in this study we present evidence of subslab mantle crustal and mantle wedge anisotropy in the andaman nicobar subduction zone using 52 teleseismic and 103 local shear wave splitting sws measurements the local sws measurements reveal a dominant trench parallel anisotropy in the crust that can be attributed to spo associated with arc parallel fault planes of normal earthquakes and strike slip faults parallel to the trench 3d flow from the subslab mantle possibly caused by a tear in the subducting slab is a plausible mechanism for anisotropy in the mantle wedge synthesis of the present results with the previous ones from the andaman sumatra java arc reveals a clear relation between the fast polarization azimuths in a ray coordinate system and slab dip the sub slab anisotropy is trench parallel in steeply subducting 40 in java to trench normal in gently subducting segments in the andaman and sumatra regions

weathering mineralogical evolution and soil organic matter along a holocene soil toposequence developed on carbonate rich materials a toposequence of holocene soils located between 1100 2400 m asl in the italian alps served as the basis for the following analyses the weathering of limestone and dolomite the calculation of mass balances understanding the formation of pedogenic fe and al the determination of soil mineral and clay mineral reactions and transformation and the measurement of accumulation and stabilisation mechanisms of soil organic matter leaching of carbonates is most intense at the lower elevations although calcite and dolomite have a higher solubility at low temperatures the pco2 in the soil is higher at lower elevations and weathering is driven mainly by carbonic acids at higher elevations organic acids appear to determine the mineral transformations and weathering reactions to a greater extent this suggests that two very different weathering regimes carbonic and organic acid weathering exist along the toposequence the transformation of mica into vermiculite is the main process in both the clay and fine earth fraction weathering of silicate minerals started even before the carbonates had been completely removed from the soils the transformation mechanisms of silicate minerals in the a and o horizon at higher elevations was at least as intensive as that at the climatically warmer sites the neoformation of pedogenetic clays at climatically cooler sites was slightly greater than that at the warmer sites however the formation rate of secondary fe and al phases was more pronounced at lower elevation which means that this process seemed to be driven dominantly by carbonic acid weathering of primary minerals soil organic matter som abundance in the mineral soil is nearly 15 kg m2 at all sites and surprisingly no climate driven effect could be detected in general the preservation and stabilisation of som was due to poorly crystalline al and fe phases and vermiculite regardless of some variations in the composition of the parent material varying calcite dolomite ratio

rheology of fine grained forsterite aggregate at deep upper mantle conditions high pressure and high temperature deformation experiments on fine grained synthetic dunite forsterite aggregate were conducted to determine the dominant deformation mechanism in the deep upper mantle the sintered starting material has 90 forsterite 10 enstatite and an average grain size of 1 m deformation experiments were performed using a deformation dia apparatus at pressures of 3 03 5 36 gpa temperatures of 1473 1573 k and uniaxial strain rates of 0 91 10 5 to 18 6 10 5 s 1 at dry circumstances 50 h 106si the steady state flow stress was determined at each deformation condition derived stress strain rate data is analyzed together with that reported from similar but low pressure deformation experiments using flow law equations for diffusion creep stress exponent of n 1 grain size exponent of p 2 and for dislocation accommodated grain boundary sliding gbs disl n 3 p 1 the activation volume for diffusion creep v dif and for gbs disl v gbs of dunite is determined to be 8 2 0 9 and 7 5 1 0 cm3 mol respectively calculations based on these results suggest that both diffusion creep and dislocation creep play an important role for material flow at typical deformation conditions in the earth s asthenospheric upper mantle whereas the contribution of gbs disl is very limited and dislocation creep is the dominant deformation mechanism during the deformation of olivine in sheared peridotite xenolith though these conclusions are not definitive these are the first results on potential deformation mechanisms of forsterite aggregate based on extrapolation in the pressure temperature stress and grain size space

aeolian processes and sand encroachment problems in kuwait abstract the surface of kuwait is carved in a calcretized clastic sequence of miocene pleistocene age mostly covered by a thin blanket of recent aeolian deposits fast rate of development and increase of human activities contributed to denudation of the vegetative cover and exposure of dry loose sediments to wind action at present kuwait is intensively subjected to aeolian actions manifested by frequent occurrences of sand and dust storms this dictated the need for regional understanding of the magnitude of the aeolian processes their behavior with man made establishments and their long and short term impacts a comparison between the nature and distribution of recent surface sediments in kuwait in 1979 and 1990 shows significant changes that reflect the work of aeolian processes during this period the direct impact of the enhancement of aeolian action is represented by the increase in the rate of sediment transport transportation of fine grained particles in suspension frequently occurs during dust storms usually initiated in southern iraq the particles migrate over kuwait as thick dust clouds and finally settle in the northern part of the arabian gulf bedload transport of sand or saltation frequently occurs during summer whenever wind speed reaches 5 4 m s the average annual sand drift rate in kuwait is about 20 m 3 m width 1 yr 1 the majority of the drift occurring during the period may august to the southeast bedform transport or sand dune movement is also recognized in kuwait sand dunes mostly occur as small barchans of about 3 m height the measured rate of movement of these barchans varies with height a barchan of average height usually moves about 20 m in nine monthes january august the remarkable high rate of sand transport and the increase in development activities in the desert areas are responsible for the enhancement of the sand encroachment problems in kuwait almost all installations roads and farms are significantly affected by the accumulation of considerable amounts of aeolian sand these problems have pronounced adverse environmental and economic impacts

b type olivine fabric induced by low temperature dissolution creep during serpentinization and deformation in mantle wedge the b type olivine fabric i e the 010 axes subnormal to foliation and the 001 axes subparallel to the lineation has been regarded as an important olivine fabric for interpreting global trench parallel s wave polarization in fore arc regions however strong serpentinization and cold temperature environment in the mantle wedge should inhibit development of the b type olivine fabric that requires high temperature to activate solid state plastic deformation here we report fabrics of olivine and antigorite generated at low temperatures 300 370 c during serpentinization in a fossil mantle wedge of the val malenco area central alps olivine in the serpentine matrix develops a pronounced b type fabric while antigorite in the same matrix displays a strong crystallographic preferred orientation cpo with the 001 planes and the 010 axes subparallel to foliation and lineation respectively the following evidence leads to the conclusion that the b type olivine fabric results from dissolution creep assisted by grain boundary sliding gbs and grain rotation rather than solid state plastic deformation 1 serpentinization took place at low temperatures and a fluid enriched environment ideal for dissolution precipitation creep 2 the voids and zigzag boundaries along the interface between antigorite and olivine suggest a fluid dissolution reaction 3 the primary coarse olivine develops a nearly random fabric indicating the b type fabrics in the fine grained olivine may not be inherited fabrics these results document for the first time the b type olivine cpo formed by dissolution creep at low temperatures during serpentinization and provide a mechanism to reconcile petrofabric observations with geophysical observations of trench parallel fast s wave seismic anisotropy in fore arc mantle wedge regions

decelerating nazca south america and nazca pacific plate motions space geodetic estimates of the rate of nazca south america convergence and nazca pacific spreading averaging over several years show that present day rates are significantly slower than the 3 million year average nuvel 1a model the implied rates of deceleration are consistent with longer term trends extending back to at least 20 ma about the time of initiation of andes growth and may reflect consequences of ongoing subduction and construction of the andes e g increased friction and viscous drag on the subducted slab as the leading edge of south america thickens

the influence of plate motions on three dimensional back arc mantle flow and shear wave splitting both the polarization direction of the fast shear waves and the types of deformation within overriding plates vary between the back arc basins of western pacific subduction zones the goal of this study is to test the possibility that motions of the overriding plates may control the patterns of seismic anisotropy and therefore the observed shear wave splitting we calculated three dimensional models of viscous asthenospheric flow driven by the motions of the subducting slab and overriding plates shear wave splitting was calculated for polymineralic anisotropy within the back arc mantle wedge assuming that the anisotropy was created by flow induced strain predicted splitting may differ substantially depending on whether anisotropy is computed directly using polycrystalline plasticity models or is based on the orientation of finite strain a parameter study shows that both finite strain and textural anisotropy developed within three dimensional plate coupled asthenospheric flow models are very heterogeneous when back arc shearing occurs within the overriding plate therefore predicted shear wave splitting varies strongly as a function of plate motion boundary conditions and with ray path through the back arc asthenosphere flow models incorporating plate motion boundary conditions for the tonga southern kuril and eastern aleutian subduction zones produce splitting parameters consistent with observations from each region trench parallel flow generated by small variations in the dip of the subducting plate may be important in explaining observed fast directions of anisotropy sampled within the innermost corner of the mantle wedge

seismic anisotropy and mantle flow seismic anisotropy in the mantle is primarily due to deformation induced lattice preferred orientation lpo of olivine crystals the aim of this paper is to better understand how such lpo is produced by flow and to learn whether observations of seismic anisotropy can be used to infer the local direction of flow within the mantle the basis of the work is a two dimensional theory which describes how lpo evolves during an arbitrary time dependent deformation ribe 1989 i investigate the evolution of lpo beneath ocean ridges above subducted slabs and in thermal convective flows the ocean ridge model predicts that the seismic anisotropy produced at a fast spreading ridge should be greater in magnitude than that produced at a slow spreading ridge and that the fast axis should be more nearly horizontal the subduction zone model predicts that strong seismic anisotropy should be produced above subducted slabs with the fast axis aligned with the slab regardless of the subduction angle the evolution of lpo in typical convective flows is extremely complex and depends strongly on such factors as the mode of heating and the surface boundary conditions in general there is no simple relation between the lpo and the local flow direction in any of the models considered the attempt to use seismic anisotropy to map flow patterns on a global scale is therefore probably unwarranted however the lpo does become aligned with the flow direction when the flow is a progressive simple shear such a flow occurs in limited portions of the mantle beneath the lithosphere at a fast spreading ridge above subducted slabs and in boundary layers beneath rigid surfaces such as continents observations of seismic anisotropy may therefore indicate the sense of shear at shallow depths in the mantle

shear wave anisotropy beneath the andes from the banjo seda and pisco experiments we present the results of a detailed shear wave splitting analysis of data collected by three temporary broadband deployments located in central western south america the broadband andean joint experiment banjo a 1000 km long east west line at 20 s and the projecto de investigacion sismologica de la cordillera occidental pisco and seismic exploration of the deep altiplano seda deployed several hunderd kilometers north and south of this line we determined the splitting parameters fast polarization direction and t splitting delay time for waves that sample the above and below slab regions teleseismic ks and s scs waves from local deep focus events as well as s waves from intermediate focus events that sample only the above slab region all but one of the ks stacks for the banjo stations show e w fast directions with t varying between 0 4 and 1 5 s however for ks recorded at most of the seda and pisco stations and for local deep focus s events north and south of banjo there is a rotation of to a more nearly trench parallel direction the splitting parameters for above slab paths determined from events around 200 km deep to western stations yield small delay times 0 3 s and n s fast polarization directions assuming the anisotropy is limited to the top 400 km of the mantle olivine stability field these data suggest the following spatial distribution of anisotropy for the above slab component as one goes from east where ks reflects the above slab component to west changes from e w to n s and delay times are substantially reduced this change may mark the transition from the brazilian craton to actively deforming e w shortening andean mantle we see no evidence for the strain field expected for either corner flow or shear in the mantle wedge associated with relative plate motion the small delay times for above slab paths in the west require the existence of significant spatially varying below slab anisotropy to explain the ks results the implied anisotropic pattern below the slab is not easily explained by a simple model of slab entrained shear flow beneath the plate instead flow induced by the retrograde motion of the slab in combination with local structural variations may provide a better explanation

mantle anisotropy beneath northwest pacific subduction zones to assess the location strength and orientation of seismic anisotropy in the southern kuril japan and izu bonin subduction zones we analyzed shear wave splitting in local s phases and teleseismic sks phases fast directions from these phases are roughly parallel to the direction of absolute pacific plate motion wnw in izu bonin roughly parallel to the strike of the trench nne in central honshu and roughly n in the southern kuril back arc in the vicinity of sakhalin island assuming that the orientation and strength of anisotropy does not vary with depth modeling of splitting times from local s and teleseismic phases recorded at sakhalin island requires that the maximum depth of anisotropy lies between 480 km and 950 km in contrast splitting times for local s phases from events in izu bonin rule out anisotropy in the transition zone all the data are consistent with a model in which the lower transition zone 520 660 km and lower mantle are largely isotropic and in which anisotropy occurs intermittently in the upper transition zone 410 520 km possibly due to preferred orientation of spinel assuming that splitting in the upper mantle is produced by preferred orientation of olivine the observed fast directions indicate that the geometry of back arc strain varies systematically between subduction zones the relationship of fast directions and plate motions suggests that the subducting slab exerts significant control on back arc flow but that flow correlated with upper plate deformation must also exist

a laboratory model of subduction zone anisotropy abstract measurements of split seismic shear waves near pacific subduction zones show that fast polarization directions are primarily trench parallel in the seaward side mantle beneath a subducting slab and variable in the mantle wedge beneath back arc basins it is generally accepted that the azimuth of the fast polarization direction defines the average orientation of the crystallographic a axis in olivine here we use laboratory experiments to determine the orientation of the olivine a axis in the upper mantle in the vicinity of a subducting slab a laboratory analog for olivine a axis motion during creep deformation is developed using small cylinders whiskers suspended in a viscous fluid subduction beneath a stationary overriding plate variable dip angles and realistic rollback and down dip plate kinematics are examined plate rollback produces strong plate parallel whisker alignments for all dip angles examined alignment in the wedge occurs in three layers a layer of dip parallel alignment above the downgoing plate a layer of subhorizontal alignment beneath the overriding plate and a core of intermediate orientations the amount of alignment beneath the overriding plate and the wedge core decreases with increasing dip angle our results indicate that olivine a axis orientation in the seaward side mantle is controlled by the amount of slab rollback and that orientations in the mantle wedge depend upon slab dip angle

upper mantle seismic anisotropy resulting from pressure induced slip transition in olivine seismic anisotropy in the oceanic lithosphere results from flow induced crystallographic preferred orientation of dry olivine during lithosphere creation recent experiments however showed that high water activity changes the flow mechanisms of olivine and hence the crystallographic preferred orientation better explaining the seismic anisotropy in the mantle wedge above subduction zones whether changes in the crystallographic preferred orientation of olivine are unique to the effects of water has become controversial and is critical to resolve here we report low stress high strain experiments on typical dry mantle rock at high pressures and temperatures showing that at 3 gpa pressure induces the same profound transition in olivine crystallographic preferred orientation that is produced by high water activity at lower pressure one important consequence for global tectonics is that alignment of fast seismic waves parallel to trenches beneath subducting slabs probably reflects entrainment of sub lithospheric mantle in the direction of subduction rather than trench parallel flow as interpreted at present from the large variety of crystallographic preferred orientations in olivine in both experiments and natural rocks we infer that in addition to the pressure induced changes in olivine slip systems implied here there are probably further changes in slip systems at higher pressure and temperature seismic anisotropy in earth s oceanic lithosphere and in the mantle wedge above subduction zones is associated with crystallographic preferred orientations of olivine experiments at high pressure and temperature suggest that a pressure of 3 gpa can induce the same changes in the crystal structure of olivine as high water activity at lower pressures

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

deep structure of the northeastern japan arc and its implications for crustal deformation and shallow seismic activity abstract seismic tomography studies in the northeastern japan arc have revealed the existence of an inclined sheet like seismic low velocity and high attenuation zone in the mantle wedge at depths shallower than about 150 km this sheet like low velocity high attenuation zone is oriented sub parallel to the subducted slab and is considered to correspond to the upwelling flow portion of the subduction induced convection the low velocity high attenuation zone reaches the moho immediately beneath the volcanic front or the ou backbone range running through the middle of the arc nearly parallel to the trench axis which suggests that the volcanic front is formed by this hot upwelling flow aqueous fluids supplied by the subducted slab are probably transported upward through this upwelling flow to reach shallow levels beneath the backbone range where they are expelled from solidified magma and migrate further upward the existence of aqueous fluids may weaken the surrounding crustal rocks resulting in local contractive deformation and uplift along the backbone range under the compressional stress field of the volcanic arc a strain rate distribution map generated from gps data reveals a notable concentration of east west contraction along the backbone range consistent with this interpretation shallow inland earthquakes are also concentrated in the upper crust of this locally large contraction deformation zone based on these observations a simple model is proposed to explain the deformation pattern of the crust and the characteristic shallow seismic activity beneath the northeastern japan arc

can slab melting be caused by flat subduction slab melting has been suggested as a likely source of adakitic arc magmas i e andesitic and dacitic magmas strongly depleted in y and heavy rare earth elements existing numerical and petrologic models however restrict partial melting to very young 5 ma oceanic crust typically at 60 80 km depth paradoxically most of the known pliocene quaternary adakite occurrences are related to subduction of 10 45 ma lithosphere which should not be able to melt under normal subduction zone thermal gradients we propose an unusual mode of subduction known as flat subduction occurring in 10 of the world s convergent margins that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust of the 10 known flat subduction regions worldwide eight are linked to present or recent 6 ma occurrences of adakitic magmas observations from chile ecuador and costa rica suggest a three stage evolution 1 steep subduction produces a narrow calc alkaline arc typically 300 km from the trench above the asthenospheric wedge 2 once flat subduction begins the lower plate travels several hundred kilometers at nearly the same depth thus remaining in a pressure temperature window allowing slab melting over this broad distance and 3 once flat subduction continues for several million years the asthenospheric wedge disappears and a volcanic gap results as in modern day central chile or peru the proposed hypothesis which reconciles thermal models with geochemical observations has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins

crustal origin of trench parallel shear wave fast polarizations in the central andes abstract in this study sks and local s phases are analyzed to investigate variations of shear wave splitting parameters along two dense seismic profiles across the central andean altiplano and puna plateaus in contrast to previous observations the vast majority of the measurements reveal fast polarizations sub parallel to the subduction direction of the nazca plate with delay times between 0 3 and 1 2 s local phases show larger variations of fast polarizations and exhibit delay times ranging between 0 1 and 1 1 s two 70 km and 100 km wide sections along the altiplano profile exhibit larger delay times and are characterized by fast polarizations oriented sub parallel to major fault zones based on finite difference wavefield calculations for anisotropic subduction zone models we demonstrate that the observations are best explained by fossil slab anisotropy with fast symmetry axes oriented sub parallel to the slab movement in combination with a significant component of crustal anisotropy of nearly trench parallel fast axis orientation from the modeling we exclude a sub lithospheric origin of the observed strong anomalies due to the short scale variations of the fast polarizations instead our results indicate that anisotropy in the central andes generally reflects the direction of plate motion while the observed trench parallel fast polarizations likely originate in the continental crust above the subducting slab

high pressure and temperature fabric transitions in olivine and variations in upper mantle seismic anisotropy the effects of pressure on crystallographic preferred orientation cpo of olivine aggregates were investigated through simple shear deformation experiments at pressures between 2 1 and 7 6 gpa and temperatures of 1493 1673 k under dry conditions using a deformation dia apparatus and the variations in seismic anisotropy were evaluated under the earth s upper mantle conditions we found that the monotonic decrease in seismic anisotropy with depth is caused by the pressure dependency of the seismic properties of a type developed by the 010 100 100 slip system olivine fabric while the rapid decrease is caused by the fabric transition from a type to b c type by the hk0 001 001 slip systems at 7 6 gpa and 1673 k moreover an alternative transition from a type fabric to b type like fabric by the 010 001 001 slip system occurs at 7 6 gpa and lower temperature these two temperature dependent fabric transitions occurring at 7 6 gpa result in low seismic anisotropy with vsh vsv the ratio of horizontally and vertically polarized shear waves 1 at low temperatures i e old continental mantle conditions and vsh vsv 1 at high temperatures i e oceanic mantle conditions at greater depths consistent with seismological observations thus the variations of cpo with pressure and temperature in olivine under dry conditions can explain the seismic anisotropy signatures observed in the upper mantle without invoking other mechanisms

thermodynamics of water solubility and partitioning when it was realized that nominally anhydrous minerals could be a major storage site of water in the mantle smyth 1987 bell and rossman 1992 it also became rather obvious that many mantle samples probably have lost most of their water during ascent e g ingrin and skogby 2000 while analyses of natural samples therefore in many circumstances only provide a lower limit of the actual water content in the mantle measurements of water solubility give an upper limit of the amount of water that might be stored in a mineral experimental measurements of water solubility have therefore naturally evolved in the last decades as a major tool for understanding the water storage capacity of the mantle closely related to studies of water solubility are studies of water partitioning among minerals if the water solubility in two minerals is known under given conditions the partition coefficient is just the ratio of the water solubilities under the same conditions provided that the composition of the fluid coexisting with the two minerals is the same since the pioneering work of bai and kohlstedt 1992 and of kohlstedt et al 1996 the water solubility in all major upper mantle minerals has been studied as well as the partitioning of water between the minerals of the lower mantle and the transition zone solubility studies by themselves do not directly give the water content in the mantle they constrain the storage capacity of the mantle i e the maximum amount of water that may be stored if the mantle were water saturated however as most of the mantle is very likely water undersaturated the actual water content is probably far below the storage capacity estimates of the actual abundance and distribution of water in the mantle may be obtained by combining solubility and partitioning data with direct

development of a type olivine fabric in water rich deep upper mantle abstract water controls the activity of slip systems in olivine resulting in various types of olivine crystallographic preferred orientation i e fabric in mantle rocks the a type olivine fabric is the most commonly observed olivine fabric in natural peridotites development of a type olivine fabric developed by the 010 100 slip system is known to be limited to the water poor conditions of the shallow upper mantle 2130 ppm h si in olivine while b type fabric by the 010 001 slip system was observed under moderately wet conditions 750 2130 ppm h si developments of c type by the 100 001 slip system fabric was limited to water poor conditions v sh v sv the ratio of horizontally and vertically polarized shear waves with depth in the global one dimensional models is well explained by the olivine fabrics developed in the horizontal flow of a water poor mantle only a type olivine fabric can explain the vertical mantle flow which associates the seismic anisotropy of v sh v sv 200 km depth a strong anomaly of v sh v sv

lateral variations in lowermost mantle shear wave anisotropy beneath the north pacific and alaska s waves recorded by long period world wide standardized seismograph network and broadband stations in north america for deep northwest pacific subduction zone earthquakes provide evidence for anisotropy in the lowermost mantle shear velocity structure beneath the north pacific and alaska systematic delays of up to 4 s are observed between longitudinal components sv and transverse components sh of motion for core reflected scs waves as well as for core grazing and diffracted s waves the absence of significant splitting for s waves that have turning points more than a few hundred kilometers above the core mantle boundary indicates that anisotropy is localized within the d region the lowermost portion of the mantle sv sh differential arrival times for both scs and sdiff along with path length estimates assuming a 250 km thick d region indicate spatial variations in the strength of shear wave anisotropy the strongest anisotropy 1 1 5 is found in the eastern part of the study area with systematic reduction in magnitude toward the west a transverse isotropy model can explain the data with the velocity structure for horizontally polarized waves vsh having a 2 3 discontinuous shear velocity increase at the top of d as proposed in earlier studies of the region and a similar structure for s wave particle motion in the direction normal to the core mantle boundary vsv but with the velocity jump at the top of d and the velocity within d being reduced from that for vsh by 0 5 1 5 km s large uncertainties exist for velocity gradients above and below the velocity jump but the requirement of a reduced vsv relative to vsh in d is clear synthetic waveforms calculated by using separate isotropic structures for sh and sv match the observations well and constrain the basic anisotropic structure because the shear waves all traverse the region with near grazing geometries the study area exhibits strong lateral variations in lower mantle shear velocity structure and variable thickness of the d layer topography of the d layer is not well resolved because of trade off with volumetric heterogeneity

pressure induced slip system transition in forsterite single crystal rheological properties at mantle pressure and temperature deformation experiments were carried out in a deformation dia high pressure apparatus d dia on oriented mg2sio4 olivine fo100 single crystals at pressure p ranging from 2 1 to 7 5 gpa in the temperature t range 1373 1677 k and in dry conditions these experiments were designed to investigate the effect of pressure on olivine dislocation slip system activities responsible for the lattice preferred orientations observed in the upper mantle two compression directions were tested promoting either 100 slip alone or 001 slip alone in 010 crystallographic plane constant applied stress sigma and specimen strain rates formula were monitored in situ using time resolved x ray synchrotron diffraction and radiography respectively transmission electron microscopy tem investigation of the run products reveals that dislocation creep assisted by dislocation climb and cross slip was responsible for sample deformation a slip transition with increasing pressure from a dominant 100 slip to a dominant 001 slip is documented extrapolation of the obtained rheological laws to upper mantle p t and sigma conditions suggests that 001 slip activity becomes comparable to 100 slip activity in the deep upper mantle while 001 slip is mostly dominant in subduction zones these results provide alternative explanations for the seismic anisotropy attenuation observed in the upper mantle and for the puzzling seismic anisotropy anomaliesmore commonly observed in subduction zones less

lowermost mantle anisotropy beneath the pacific imaging the source of the hawaiian plume we utilized recordings of seismic shear phases provided by several north american broadband seismometer arrays to provide unique constraints on shear wave anisotropy beneath the northern and central pacific ocean using a new analysis method that reduces measurement errors and enables the analysis of a larger number of available waveforms we examined relative travel times of teleseismic s and sdiff that sample a large area of lowermost mantle structure the results of this study provide evidence for small scale lateral and depth variations in shear wave anisotropy for a broad region of the lowermost mantle beneath the pacific ocean in particular we image a localized zone of anomalously strong anisotropy whose strength increases toward the top of dq beneath hawaii our results combined with a previous study of vp vsh ratios indicate that ancient subducted slab material may be responsible for observations beneath the northern pacific while lenses or layers of core mantle boundary reaction products or partial melt oriented by horizontal inflow of mantle material to the hawaiian plume source can explain observations beneath the central pacific fl 2001 elsevier science b v all rights reserved

a strongly negative shear velocity gradient and lateral variability in the lowermost mantle beneath the pacific a new approach for constraining the seismic shear velocity structure above the core mantle boundary is introduced whereby sh sks differential travel times amplitude ratios of sv sks and sdiff waveshapes are simultaneously modeled this procedure is applied to the lower mantle beneath the central pacific using da ta from numerous deep focus southwest pacific earthquakes recorded in north america we analyze 90 broadband and 248 digitized analog recordings for this source receiver geometry sh sks times are highly variable and up to 10 s larger than standard reference model predictions indicating the presence of laterally varying low shear velocities in the study area the travel times however do not constrain the depth extent or velocity gradient of the low velocity region sv sks amplitude ratios and sh waveforms are sensitive to the radial shear velocity profile and when analyzed simultaneously with sh sks times rnveal up to 3 shear velocity reductions restricted to the lowermost 190 50 km of the mantle our preferred model for the central eastern pacific region ml has a strong negative gradient with 0 5 reduction in velocity relative to the preliminary reference earth model prem at 2700 km depth and 3 reduction at 2891 km depth and slight velocity reductions from 2000 to 2700 km depth 0 0 5 lower than prem significant small scale 100 500 km shear velocity heterogeneity 0 5 1 is required to explain scatter in the differential times and amplitude ratios