strength and water weakening of mantle minerals olivine wadsleyite and ringwoodite rheological properties of olivine and its high pressure polymorphs both dry and hydrous have been studied by monitoring x ray diffraction peak broadening as a function of pressure temperature and time the measurements were carried out up to a pressure of 10 gpa and a temperature of 600 c for olivine and wadsleyite and a pressure of 20 gpa and a temperature of 1000 c for ringwoodite the observations indicate that olivine is much weaker than the other two minerals for this range of conditions and furthermore water weakens olivine dramatically but only slightly weakens wadsleyite and ringwoodite when the temperature increases from 25 c to 400 c the yield strength in the olivine drops by 39 without water and 62 with water however there is hardly any change in strength for wet or dry wadsleyite and ringwoodite

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

shear deformation of dry polycrystalline olivine under deep upper mantle conditions using a rotational drickamer apparatus rda abstract shear deformation experiments on dry hot pressed polycrystalline san carlos olivine have been conducted at 4 9 9 6 gpa 1300 1870 k and strain rates of 0 6 7 4 10 5 s 1 using a rotational drickamer apparatus rda at a synchrotron facility the stress was measured from the orientational dependence of lattice spacing for the 1 3 0 1 3 1 1 1 2 1 2 2 1 4 0 and 2 4 1 planes as well as from the dislocation densities based on the mechanical and microstructural observations we infer that deformation occurs by power law creep involving dislocation glide and climb under high temperature conditions whereas deformation is due to exponential creep through the peierls mechanism at relatively low temperatures in both regimes the strength of olivine at steady state deformation at these pressures is much larger than those at lower pressures the activation volumes v is estimated to be 15 20 10 6 m 3 mol for the power law creep combined with the previous results under wet conditions we present the recommended values of flow law parameters on olivine for power law creep constrained by high pressure deformation experiments

robust characteristics method for modelling multiphase visco elasto plastic thermo mechanical problems abstract we have extended our previous 2d method gerya t v yuen d a 2003 characteristics based marker in cell method with conservative finite differences schemes for modeling geological flows with strongly variable transport properties phys earth planet interiors 140 295 320 which is a combination of conservative finite differences with marker in cell techniques to include the effects of visco elasto plastic rheology self gravitation and a self consistently derived evolving curvilinear planetary surface this code is called i2elvis and can solve a new class of computationally challenging problems in geodynamics such as shear localization with large strains crustal intrusion emplacement of magmas bending of realistic visco elasto plastic plates and core formation by vigorous shell tectonics activities related to a global rayleigh taylor instability of a metal layer formed around silicate rich lower density primordial core during planetary accretion we discuss in detail the computational strategy required the rheological constraints to be satisfied at each time step and spatial location we show analytical benchmarks and examples drawn from comparing between numerical and analogue experiments in structural geology subducting slab bending with a visco elasto plastic rheology and equilibrium spherical configurations from self gravitation we have also tested possibilities of future applications by addressing 3d geometries based on multigrid method and including inertial effects in the momentum equation with tracers in order to simulate meteoritic impact events and eventually earthquake instabilities

an image of the columbia plateau from inversion of high resolution seismic data the authors use a method of traveltime inversion of high resolution seismic data to provide the first reliable images of internal details of the columbia river basalt group crbg the subsurface basalt sediment interface and the deeper sediment basement interface velocity structure within the basalts delineated on the order of 1 km horizontally and 0 2 km vertically is constrained to within plus minus 0 1 km s for most of the seismic profile over 5 000 observed traveltimes fit their model with an rms error of 0 018 s the maximum depth of penetration of the basalt diving waves truncated by underlying low velocity sediments provides a reliable estimate of the depth to the base of the basalt which agrees with well log measurements to within 0 05 km 165 ft the authors use image blurring calculated from the resolution matrix to estimate the aspect ratio of images velocity anomaly widths to true widths for velocity features within the basalt from their calculations of image blurring they interpret low velocity zones lvz within the basalts at boylston mountain and the whiskey dick anticline to have widths of 4 5 and 3 km respectively within the upper 1 5 km of the model at greater depth the widths of these imaged lvzs thin tomore approximately 2 km or less they interpret these linear subparallel low velocity zones imaged adjacent to anticlines of the yakima fold belt to be brecciated fault zones these fault zones dip to the south at angles between 15 to 45 degrees less

lateritization cycles and their relation to the formation and quality of kaolin deposits in south kerala india abstract laterite occurs extensively over the crystalline and sedimentary rocks in the midland and lowland areas of south kerala india two lateritization cycles are identified in this area large good quality kaolin deposits composed mostly of kaolinite are characteristic of the sedimentary sequence in south kerala these deposits were formed on deposition of the weathering materials of the khondalites towards the first cycle of lateritization after deposition and uplift of the sedimentary rocks another lateritization cycle affected these as well as the khondalites during pre quaternary times with the formation of a planation surface at 25 125 m above sea level having thick laterite profiles the laterite profiles over the kaolin deposits show higher concentration of fe oxides mostly in the form of hematite and titania compared to their concentration in the kaolins higher contents of cr and ni are also characteristic of the laterite over kaolin deposits recrystallization of the kaolinite appearance of al fe and si amorphous phases in the kaolin clays and partial removal of fe and ti from them are attributed to the second lateritization cycle

an evaluation of electromagnetic methods in the presence of geologic noise an important element of electromagnetic em prospecting is survey design numerical modeling algorithms may be used to calculate signal to geologic noise s n ratios to compare different survey configurations and measured responses quantitatively our models consist of a prismatic three dimensional 3 d target in a conductive half space which may contain an overburden conductor the models are energized by a time varying current transmitted in a loop of wire the signal is the scattered or anomalous response of the target while the geologic noise is either the response of the half space or the anomalous response of the overburden conductor for typical loop sizes in exploration the coincident loop configuration has a relatively high s n ratio and thus a relatively high capability to resolve the target in the case of half space noise measurements made with the horizontal loop moving coil configuration can be just as effective if the coil separation is one and one half to two times the depth of buria

experience with the em 60 electromagnetic system for geothermal exploration in nevada lawrence berkeley laboratory lbl conducted controlled source electromagnetic em surveys at three geothermal prospects in northern nevada over 40 soundings were made in panther canyon grass valley near winnemucca soda lakes near fallon and mccoy west of austin to test and demonstrate the applicability of lbl s em 60 system to geothermal exploration the em 60 is a frequency domain system using three component magnetic detection typically 65 a is applied to an 100 m diameter four turn horizontal loop generating a dipole moment 10 sup 6 mks over the frequency range 10 sup 3 to 10 sup 3 hz with such a source loop soundings were made at transmitter receiver separations of up to 4 km providing a maximum depth of penetration of 4 km

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

seismic anisotropy beneath the northern apennines italy and its tectonic implications we examined shear wave splitting in sks and s phases from 22 teleseisms at 10 temporary stations on a transect across the northern apenninic arc the array near 43 n spans from corsica island across the tyrrhenian region and the apenninic belt to the adriatic coast we applied particle motion covariance matrix decomposition and cross correlation methods to estimate the polarization direction of the fast split shear wave and the delay time between split phases t most of the analyzed shear waves show clear evidence of splitting the in the adriatic foreland and in the apennines are approximately parallel to the strike of the mountain belt nw se the largest t correspond to the highest elevations suggesting that anisotropy is related to the compressional tectonics which built the apennines and that this tectonic compression involved at least the entire lithosphere in the tyrrhenian area we observe oriented about e w suggesting a reorientation of the mantle fabric due to astenospheric flow responsible for the e w post orogenic extension observed at the surface

electromagnetic mapping of electrical conductivity beneath the columbia basalts sedimentary rocks beneath the columbia river basalt group are recognized as having potential for oil and gas production but the overlying layered basalts effectively mask seismic reflections from the underlying sediments four electromagnetic em methods have been applied on profiles crossing boylston ridge a typical east west trending anticline of the yakima fold belt in an attempt to map the resistivity interface between the basalts and the sediments and to map variations in structure and resistivity within the sediments the em surveys detected strong variations in resistivity within the basalts and in particular the continuous magnetotelluric array profiling emap revealed resistivity lows beneath the surface anticlines these low resistivity zones probably coincide with fracturing in the core of the anticlines and they appear to correlate well with similar zones of low seismic velocity observed on a nearby seismic profile the controlled source em surveys in loop transient long offset transient and variable offset frequency domain were designed in anticipation of relatively uniform high resistivity basalts and were found to have been seriously distorted by the intrabasalt conductors discovered in the field in particular the resistivity sections derived from 1d inversions were found to be inconsistent and misleading the emap survey provided the most information about themore subsurface resistivity distribution and was certainly the most cost effective however both controlled source and emap surveys call for accurate 2d or 3d inversion to accommodate the geological objectives of this project references 18 less

megasource time domain electromagnetic sounding methods the colorado school of mines time domain electromagnetic em sounding system makes use of a grounded length of cable powered with high amplitude current square waves to generate an em field for probing the earth the vertical component of magnetic induction is detected at a sounding site located at a relatively large distance compared to the desired depth of investigation with a source moment of a million ampere meters or greater offset distances of several tens of kilometers can be achieved easily providing depths of investigation of up to 10 km the recorded induction field versus time curves are routinely interpreted by comparison with computer generated theoretical curves for a layered earth megasource em surveys have been carried out at the geysers in northern california and near yakima in central washington providing apparently meaningful information on the electrical structure in these areas at depths as great as 10 km

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

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

seismic anisotropy beneath the northern apennines italy mantle flow or lithosphere fabric shear wave splitting estimates from recordings of 10 portable seismographic stations during the first year of the retreat seismic deployment in combination with broadband data from the italian national seismic network are associated with seismic anisotropy within the upper mantle beneath the northern apennines anisotropic parameters derived from both shear wave splitting and p travel time residuals vary geographically and depend on event back azimuth reflecting complexity in the underlying mantle strain field variations of the splitting time delays and fast polarization seem to exclude a 2 d sublithosphere corner flow associated with the apennines subduction as the main source of the inferred anisotropy the anisotropic signal may be generated by a frozen in fabric of the adriatic and tyrrhenian lithosphere domains or by flow variations induced by episodic and fragmentary slab rollback

hydrolytic weakening of olivine at mantle pressure evidence of 100 010 slip system softening from single crystal deformation experiments abstract to date most of the experiments performed to quantify the hydrolytic weakening of olivine single crystals have been performed at low confining pressure and low water fugacities in order to determine if the degree of hydrolytic weakening changes at higher water fugacities higher water contents we have performed a series of high pressure p and high temperature t deformation experiments in hydrous condition on san carlos olivine fo92 and forsterite fo100 single crystals in a deformation dia apparatus d dia deformation was carried out in axial compression along the 1 1 0 c crystallographic reference frame direction in order to activate 1 0 0 0 1 0 dislocation slip system alone at p ranging from 4 to 7 gpa and t 1200 c water was supplied during the experiments through dehydration of a talc sleeve talc enstatite coesite h 2 o around the sample while the differential stress and strain rate of the specimen were calculated from in situ x ray diffraction and time resolved imaging respectively run products were investigated using fourier transform infrared ftir spectroscopy and optical electron microscopy sem and tem structurally bound hydroxyl content c oh derived from absorbance in ftir spectra based on the paterson 1982 relation were typically between 290 and 720 ppm h si in the run products and the majority of these hydroxyls were observed as distinct peaks in the range of wavenumber from 3620 to 3500 cm 1 tem reveals that the 1 0 0 0 1 0 slip system was effectively activated and responsible for crystal deformation and that dislocation glide was significantly assisted by recovering processes such as dislocation climb and dipole annihilation combining the experimental data from this study and mackwell et al 1985 we estimate the activation volume v 110 c for san carlos olivine in order to produce a reasonable water fugacity exponent r in the flow law and obtain v 110 c 12 1 cm 3 mol for r 0 and v 110 c 17 3 cm 3 mol for r 1 2 comparisons of the strengths of the wet single crystals to the strengths of dry single crystals calculated from the rheological laws bai et al 1991 darot and gueguen 1981 raterron et al 2009 indicate that the wet single crystal cylinders were between 1 3 and 1 6 times weaker than their dry counterpart at same conditions which is consistent with previous studies of 1 0 0 0 1 0 olivine rheology performed at low confining pressure 300 mpa

experimental deformation of olivine single crystals at mantle pressures and temperatures abstract deformation experiments were carried out in a deformation dia high pressure apparatus d dia on oriented san carlos olivine single crystals at pressure p ranging from 3 5 to 8 5 gpa temperature t from 1373 to 1673 k and in poor water condition oxygen fugacity fo2 was maintained within the olivine stability field and contact with enstatite powder ensured an orthopyroxene activity aopx 1 two compression directions were tested promoting either 1 0 0 slip alone or 0 0 1 slip alone in 0 1 0 crystallographic plane here called respectively a slip and c slip constant applied stress and specimen strain rates e were monitored in situ using time resolved x ray synchrotron diffraction and radiography respectively transmission electron microscopy tem investigation of run products revealed that dislocation creep was responsible for sample deformation comparison of the obtained high p deformation data with the data obtained at room p by bai et al bai q mackwell s l kohlstedt d l 1991 high temperature creep of olivine single crystals 1 mechanical results for buffered samples journal of geophysical research 96 2441 2463 on identical materials deformed at comparable t fo2 aopx conditions allowed quantifying the p effect on a slip and c slip rheological laws a slip transition with increasing pressure from dominant a slip to dominant c slip is documented a slip appears sensitive to pressure which translates into the high activation volume v a 12 4 c m 3 mol in the corresponding rheological law while pressure has little effect on c slip with v c 3 4 c m 3 mol these results may explain the discrepancy between olivine low p and high p deformation data which has been debated in the literature for more than a decade

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

deformation of olivine under mantle conditions an in situ high pressure high temperature study using monochromatic synchrotron radiation polycrystalline samples of san carlos olivine were deformed at high pressure 2 8 7 8 gpa high temperature 1153 to 1670 k and strain rates between 7 10 sup 6 and 3 10 sup 5 s sup 1 using the d dia apparatus stress and strain were measured in situ using monochromatic x rays diffraction and imaging respectively based on the evolution of lattice strains with total bulk strain and texture development we identified three deformation regimes one at confining pressures below 3 4 gpa one above 4 gpa both below 1600 k and one involving growth of diffracting domains associated with mechanical softening above approx 1600 k the softening is interpreted as enhanced grain boundary migration and recovery below 1600 k elasto plastic self consistent analysis suggests that below 3 4 gpa deformation in olivine occurs with large contribution from the so called a slip system 100 010 above approx 4 gpa the contribution of the a slip decreases relative to that of the c slip 001 010 this conclusion is further supported by texture refinements thus for polycrystalline olivine the evolution in slip systems found by previous studies may be progressive starting from as low as 3 4 gpa and up to 8 gpa during such a gradual change activation volumes measured on polycrystalline olivine cannot be linked to a particular slip systemmore straightforwardly the quest for the activation volume of olivine at high pressure should cease at the expense of detailed work on the flow mechanisms implied such evolution in slip systems should also affect the interpretation of seismic anisotropy data in terms of upper mantle flow between 120 and 300 km depth less

the columbia river basalts between 17 million and 6 million years ago 200 000 square kilometers of the american northwest were flooded by basaltic lava that erupted through fissures in the crust up to 150 kilometers long larger individual eruptions covered over a third of the columbia plateau in a few days the lavas represent partial melts of the earth s mantle that were only slightly modified by near surface upper crustal processes the abundant chemical and mineralogical data now available offer an opportunity to study mantle composition and the processes involved in the evolution of the earth s crust

electromagnetic sounding in the columbia basin yakima washington a controlled source electromagnetic csem survey was conducted near yakima washington to map the thickness and resistivity of a thick volcanic sequence overlying a sedimentary section and to infer structure in the sediments the survey was conducted with a frequency domain system employing loop transmitters 400 500 m on a side and three component squid magnetometer receivers separated from the loop by 1 to 5 km data collected along a 30 km profile orthogonal to regional strike were interpreted initially with 1 d layered models which were then pieced together to make a geoelectric section induction logs in a 5000 m exploration hole at one end of the profile agree very well with the csem soundings made around the hole the geoelectric section reveals a smoothly varying thickness of volcanics with a pronounced anticlinal structure approximately concordant with a surface topographic ridge to assess the validity of inferring lateral structure from 1 d interpretations we made scale models of an anticlinal structure and of a surface inhomogeneity and conducted csem measurements over the scale models layered model inversions of these data show that the anticlinal structure and its location are very well determined by 1 d inversion but its height and width are not accurately determined csem sounding over the surface inhomogeneity model shows that this feature does not significantly degrade the interpretation of a deep target layer in this setting a geoelectric section made up from 1 d interpretations provides good qualitative measures of subsurface structure and also provides excellent starting models for detailed 2 d or 3 d modeling

kaolin deposits at melthonnakkal and pallipuram within trivandrum block southern india abstract the kaolin deposits at melthonnakkal and pallipuram mines form part of the warkalli formation belonging to the tertiary sequence in southern kerala and occur at the boundary between the tertiary sequence and precambrian granulite facies metapelites khondalites the sedimentary clays are composed mainly of kaolinite quartz and gibbsite xrd and sem studies have revealed that kaolinite is well crystallized variety and the platy crystals are scarcely broken in the sedimentary clays these sedimentary kaolins are considered to have been formed by intense tropical weathering of the khondalites and subsequently transported and deposited with high organic input into lakes near the weathering crust over the basement rock besides the surficial parts of the sedimentary deposits are extensively lateritized with the formation of goethite and hematite by quaternary tropical weathering processes

forward modeling of the development of seismic anisotropy in the upper mantle development of seismic anisotropy in response to upper mantle flow is approached through an integrated numerical model this model allows to predict the splitting parameters for a shear wave propagating across an upper mantle which deformed in response to a given geodynamic process it consists of 1 thermo mechanical modeling of the finite strain field 2 modeling olivine lattice preferred orientation lpo generated by this strain field 3 calculation of the 3 d elastic properties associated with this lpo and 4 estimation of the shear wave splitting parameters the time lag between the fast and slow split shear wave arrivals dt and the polarization azimuth of the fast wave modeled olivine lpo are constrained relative to lpo measured in naturally and experimentally deformed peridotites comparison of predicted shear wave splitting parameters with seismological data allows us to quantify the possible contribution of the modeled upper mantle flow to the measured splitting values and hence to constrain the interpretation of shear wave splitting data in terms of upper mantle flow we use this forward model to investigate the seismic anisotropy generated in ocean basins by a velocity gradient between the plate and the deep mantle fast shear wave polarizations calculated assuming a constant plate motion are in good agreement with both the sks polarization and the fast propagation direction for p and rayleigh waves observed in the pacific and indian oceans suggesting that away from mid ocean ridges seismic anisotropy in oceanic basins primarily results from asthenospheric deformation by resistive drag beneath the plate delay times are however overestimated this may be ascribed to a stronger strain localization in nature or to partial erosion of the anisotropic layer by hotspots indeed hotspot activity may explain the short length scale variations of dt in the southern pacific finally twolayer models that simulate a change in pacific plate motion as suggested by the bend in the hawaii emperor chain fail to reproduce the observed shear wave splitting this is consistent with previous suggestions that the emperor chain track may not faithfully record the pacific plate absolute motion before 43 ma 1998 elsevier science b v all rights reserved

stable isotope geochronology of the australian regolith australian regolith profiles can be assigned to one of three post palaeozoic age groups on the basis of the oxygen isotope composition of authigenic clay minerals developed in the profile systematic variations in the isotopic composition of meteoric waters and therefore of the authigenic regolith minerals that formed in equilibrium with them are a result of the continent s drift from high to low latitudes and changes in global climate residual clays collected in situ from regolith profiles of post mid tertiary age have 18o values between 17 5 and 21 3 with the exception of samples from northern australia north of approximately 20 s which have anomalously low values because meteoric waters in northern australia are largely derived from monsoonal rain the high 18o values of post mid tertiary residual clays are consistent with australia s northward drift to low warm latitudes during this time compared with the much higher latitudes it occupied for most of its post palaeozoic history late tertiary residual clays have 18o values that are indistinguishable from all other post mid tertiary clays indicating that further subdivision of the sample groups is not feasible on a continent wide scale comparison with hypothetical modern clays in equilibrium with modern meteoric waters confirms that there has been very little change in the isotopic composition of meteoric waters in australia since the mid tertiary pre mid tertiary clays have lower 18o values between 10 0 and 17 5 which are consistently lower than post mid tertiary clays 18o values of less than 14 to 15 are thought to reflect weathering events during much earlier geologic periods perhaps early or mid mesozoic when australia was at high latitude and the australian climate was humid and cool to cold several workers have documented the existence of very old regolith profiles and land surfaces in australia and support for the above hypothesis comes from three analyses of clays from profiles of stratigraphically demonstrable pre late mesozoic age which have 18o values between 10 2 and 12 3 regolith profiles composed of low 18o clays 15 are widespread in australia and it is possible that a much greater part of the modern landscape than previously recognized developed in the early or mid mesozoic the 18o values of transported i e sedimentary kaolinites reveals that the major source for many tertiary kaolinitic sediments was this postulated pre late mesozoic low 18o regolith the stableisotope evidence that much of the australian regolith formed in comparatively cold conditions suggests that contrary to some traditional interpretations lateritization and deep weathering phenomena are not solely the result of weathering in tropical or sub tropical climates the hydrogen isotope composition of the clays range from 115 to 50 however it is inferred that the majority of clays have undergone some post formational hydrogen isotope exchange which renders their d composition unsuitable for dating purposes

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

grain boundary sliding in san carlos olivine flow law parameters and crystallographic preferred orientation 1 we performed triaxial compressive creep experiments on aggregates of san carlos olivine to develop a flow law and to examine microstructural development in the dislocation accommodated grain boundary sliding regime gbs each experiment included load and temperature steps to determine both the stress exponent and the activation energy grain boundary maps created with electron backscatter diffraction data were used to quantify grain size distributions for each sample inversion of the resulting data produced the following flow law for gbs gbs 104 8 0 8 2 9 0 3 d0 7 0 1 exp 445 20 kj mol 1 rt with d and gbs in units of mpa m and s 1 respectively although relatively weak crystallographic preferred orientations cpos have 010 maxima parallel to the compression direction along with 100 and 001 girdles perpendicular to the compression direction cpos and subgrain boundary misorientation axes suggest that the 010 100 slip system contributes significantly to deformation we propose that these experimental results are best modeled by a deformation mechanism in which strain is accomplished primarily through grain boundary sliding accommodated by the motion of dislocations extrapolation of our flow laws to mantle conditions suggests that gbs is likely to be the dominant deformation mechanism in both lithospheric shear zones and asthenospheric flow and therefore strong upper mantle seismic anisotropy can not be attributed solely to the dominance of dislocation creep

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

the role of water in the deformation of dunite specimens of two natural dunites anita bay of 100 m grain size and aheirn of 900 m grain size have been deformed in experiments at temperatures between 1200 and 1400 c and constant strain rates between 10 4 and 10 6 s 1 in a gas medium deformation apparatus at 300 mpa confining pressure prior to these experiments the specimens were given a heat treatment in a controlled oxygen fugacity furnace at 1200 c for 60 hours to dehydrate the trace layer silicate minerals present and drive off adsorbed water the predried specimens are much stronger than specimens tested under wet conditions in an earlier study the predried specimens also do not show the apparent grain size dependence seen under wet conditions infrared absorption measurements on the present specimens and those of the earlier study indicate that for a given rock flow stress can be correlated with the intensity of a residual water related broad absorption the addition of water to a predried specimen results in both a mechanical reweakening and an intensifying of the broadband infrared component the weakening of the dunites under wet conditions is probably strongly influenced by the presence of small amounts of a water related species in the grain boundaries but intragranular weakening may also be contributing it is suggested that the concentration of the water related species in the grain boundaries is governed by its concentration in the small melt fraction present the amounts of water that have been found to be of importance for the strength of these dunites are of the order of 0 01 wt amounts much less than most estimates of the water content of the earth s undepleted upper mantle such water may have an important role in determining upper mantle flow behavior stress strain curves for all experiments are available with entire manuscript on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 008 2 50 payment must accompany order

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