contradictory correlations of paleoproterozoic glacial deposits local regional or global controls abstract there is little agreement among recently proposed correlation schemes for paleoproterozoic glaciogenic rocks scattered thinly around the globe correlations are hindered by the dearth of tight geochronological control on the ages of glacial deposits which are notoriously difficult to date most attempts at global correlation are based on comparison to the huronian supergroup in canada which contains three glaciogenic formations although the two lower glacial units were deposited in a rift setting and have a restricted distribution even in north america they have been used in attempts to effect international correlations the gowganda formation together with correlatives comprises the thickest and by far the most widespread paleoproterozoic glacial deposits in north america glacial deposits are also reported from south africa western australia and elsewhere where they appear to fall within the same time period as the huronian supergroup 2 45 2 2 ga but attempts to correlate individual diamictite bearing units to the three huronian glaciogenic formations have proved difficult none of the huronian glacial formations has been precisely dated until such geochronological data are available it is practical and prudent to recognize that during the early paleoproterozoic as in the cryogenian the earth was susceptible to glaciations that could be triggered by a variety of local events including uplift related to rifting and compressional orogeny within the 250 million years of the huronian glacial event 2 45 2 2 ga it is commonly assumed that ice sheet fluctuations were globally synchronous but this has not been demonstrated

stress measurements of deforming olivine at high pressure abstract rheological properties of mantle minerals are critical for understanding the dynamics of the earth s deep interior due to limitations in experimental technique previous quantitative studies of the rheological properties of mantle minerals are limited to either low pressure or low temperature the present understanding of mantle flow is mostly inferred from the extrapolation of relatively low pressure data to mantle high pressure conditions however the effect of pressure represented by activation volume on the rheological properties of olivine is still controversial therefore deformation experiments carried out at mantle pressures are necessary to understand and model mantle flow here we report an experimental study of plastic deformation of san carlos olivine mg fe 2 sio 4 under upper mantle conditions macroscopic differential stress and strain rates have been measured in situ in a large volume high pressure apparatus using newly developed techniques the differential stress at high temperature and high pressure that we measured is significantly lower than that estimated by many currently accepted olivine flow laws we document the first in situ experimental differential stress results in a multi anvil press our results give direct evidence for a relatively small activation volume less than 10 5 m 3 mol 1 this shows that the effect of pressure on dislocation creep is small

laboratory and seismological observations of lower mantle isotropy we have carried out seismological and mineral physics investigations to identify the extent and origin of elastic anisotropy in the lower mantle based on observations of shear wave splitting we conclude that the lower mantle is effectively isotropic this result is surprising since the lower mantle is composed of elastically anisotropic minerals silicate perovskite mgo sio 2 that have been extensively strained by geologic processes to reconcile the seismic observations we have measured texture development during deformation and high temperature recrystallization of mantle silicates we show that these experiments provide a direct explanation for the shear wave splitting measurements the lower mantle appears isotropic because silicate perovskite maintains an isotropic texture during deformation and recrystallization

spatial resolution of time reversal arrays in shallow water a series of time reversal experiments was performed in shallow water including a range dependent slope environment time reversal arrays implemented with center frequencies of 445 and 3500 hz achieved sharp focal regions up to ranges of 30 and 13 km respectively in 110 130 m shallow water in this paper resolution expressions are derived using an image method to describe the focal sizes achieved with time reversal arrays in various ocean environments analysis for the measured data indicates that the focal size approaches the diffraction limit of an array for given waveguide conditions i e waveguide geometry and attenuation the measured focal size has implications for the maximum achievable resolution of linear matched field processing which is a computational implementation of the time reversal process

garnet yield strength at high pressures and implications for upper mantle and transition zone rheology 1 garnet helps control the mechanical behavior of the earth s crust mantle and transition zone here measurements are presented suggesting that garnet long considered to be a high viscosity phase is actually weaker than the other dominant components in the transition zone the mechanical behavior of garnet at high pressures was examined using radial diffraction techniques in the diamond anvil cell the yield strength of grossular garnet was inferred from synchrotron x ray measurements of differential lattice strains the differential stress was found to increase from 1 3 0 6 gpa at a hydrostatic pressure 5 8 1 1 gpa to 4 1 0 4 gpa at 15 7 1 0 gpa where it was level to 19 gpa the strength results are consistent with inferred strength values for majorite garnet from measurements in the diamond cell normal geometry bolstering the idea that garnet structured materials may all have similar strengths in this low temperature high differential stress regime garnet is shown to be significantly weaker than anhydrous ringwoodite and to have a strength similar to hydrous ringwoodite this result suggests that the presence of water in the transition zone may not be required to explain a weak rheology and therefore models of transition zone behavior built assuming that garnet is the high strength phase may need to be revised

effects on post glacial rebound from the hard rheology in the transition zone summary we analyse the influences of a viscosity increase in the transition zone between 420 and 670 km on the geophysical signatures induced by post glacial rebound ranging from the perturbations in the earth s rotation to the short wavelength features associated with the migration of the peripheral bulge a self gravitating model is adopted consisting of an elastic lithosphere a three layer viscoelastic mantle and an inviscid core the horizontal displacements and velocities and the stress pattern are extremely sensitive to the viscosity increase and to the chemical stratification of the transition zone the hardening of the upper and the chemical density jumps in mantle below the 420 discontinuity induces a channel effect which contaminates the horizontal deformation both in the near field and in the far field from the ice sheets these findings indicate that intraplate geodetic data can be used to put bounds on the viscosity increase in the transition zone and on the amount of chemical stratification in the mantle the stress field induced in the lithosphere by the pleistocenic ice sheet disintegration is a very sensitive function of mantle viscosity stratification the existence of seismic activity along passive continental margins of previously glaciated areas requires a substantial viscosity increase in the mantle with the viscosity of the transition zone acting as a controlling parameter a viscously stratified mantle is responsible for a delayed upward migration of stress in the lithosphere which can account for the seismicity today

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

the effect of strain energy on growth rates during the olivine spinel transformation and implications for olivine metastability in subducting slabs we have studied growth kinetics during the transformation of mg1 8fe0 2sio4 san carlos olivine to its high pressure polymorphs wadsleyite phase and ringwoodite phase at 800 1200 c and nominal pressures of 16 20 gpa in experiments in which a large 500 600 m olivine single crystal contained in a matrix of either fine grained olivine or nacl was transformed reaction rims of wadsleyite ringwoodite form on the margins of the single crystal by incoherent grain boundary nucleation and interface controlled growth contrary to theoretical expectations the growth rate of these reaction rims decreases sharply as a function of time for instance at 1100 c and 18 gpa growth ceases on an experimental timescale after the rim width reaches 20 25 m in order to explain this observation we develop an elastic model based on the theory of a misfitting inclusion comparing the results of this model with the experimental data suggests that elastic strain energy which develops because of the large volume decrease associated with the transformation is responsible for the decreasing growth rates on the other hand experimental and theoretical results suggest that elastic strain energy is relatively unimportant when grains of the product phase are randomly dispersed and distinct reaction rims do not form this is the case when the nucleation rate is low the growth rate is fast and the reactant olivine is fine grained on a geological timescale in subducting lithosphere where the grain size of the olivine is large the growth rates of grain boundary nucleated reaction rims are likely to be controlled by viscoelastic relaxation therefore current kinetic models of olivine metastability in subducting slabs which are based on simple extrapolations of experimental data and on the assumption that the growth rate is constant at fixed temperature and pressure need to be reevaluated

sedimentology dating and palaeoclimatic interpretation of a 76 3 ka record from lago grande di monticchio southern italy abstract detailed sedimentological and microstratigraphical investigations of lacustrine sediments from lago grande di monticchio southern italy provide a time scale based on rates of sedimentation obtained from annual laminations although not annually laminated throughout a time scale covering the last 76 3 ka is available by interpolation of sedimentation rates results of this chronology agree with radiocarbon dates and with ar ar dates on tephra layers from the same lacustrine sediment sequence occurrence of varves in combination with physical and geochemical properties are related to variations in the trophic state of the lake and to changing runoff both controlled by palaeoclimatic conditions water depth was relatively shallow with dominating bioturbation and only few varves preserved for most of the pleniglacial a marked increase in precipitation and related runoff characterises the early last glacial maximum when water depth reached a maximum and clastic varves were deposited subsequently water depth was shallower again a drastic change towards nutrient rich conditions occurred at the pleistocene holocene transition during the holocene organic varves were deposited two periods with higher organic productivity were recognised prior to the last glacial maximum around 50 ka and 73 ka these intervals are interpreted as the result of interstadial conditions the oldes of the two events shows laminations comparable to those at the glacial holocene transition the periods with increased minerogenic influx may be characterised as stadial conditions lasting from 14 40 ka and from 50 73 ka

deformation of olivine spinel aggregates in the system mg ni 2geo4 deformed to high strain in torsion implications for upper mantle anisotropy 1 deformation of mantle phases to high strain is fundamental for understanding mantle dynamics however it is technically challenging to perform deformation experiments yielding accurate mechanical results at the high pressures required for certain mantle phases such as wadsleyite and ringwoodite deformation experiments on analog materials of such high pressure phases can provide insight into the deformation properties of the mantle the mg ni 2geo4 system undergoes the olivine spinel transformation at much lower pressures than the silicate counterpart hence the high strain deformation properties of an olivine spinel rock can be studied at experimentally tractable pressures and temperatures experiments were conducted in a gas medium deformation apparatus at constant angular velocity a temperature of 1473 k and 300 mpa confining pressure at these conditions with a mg mg ni ratio of 0 9 the resulting aggregate comprised 70 olivine 30 spinel and 1 orthopyroxene the applied strain rate ranged from 10 4 to 10 5 s 1 yielding shear stresses supported by the samples of between 100 and 250 mpa samples were deformed to a range of shear strains between 1 and 7 in order to study the microstructural development during high strain deformation in these experiments the spinel phase did not deform to produce a crystallographic preferred orientation cpo the olivine phase produced a cpo which is consistent with slip on the 001 hk0 slip system if this slip system is dominant in olivine below 250 300 km in the upper mantle the observed seismic anisotropy can be explained without resorting to changes in flow regime or deformation mechanism at this depth

measurement of the melting curve of mg0 9fe0 1sio3 at lower mantle conditions and its geophysical implications an experimental determination of the melting of mg0 9fe0 1sio3 under lower mantle conditions is presented samples were compressed and laser heated nd yag in a mao bell type diamond cell the temperature fields generated in the sample are measured using a simple tomographic technique and spectroradiometer mg0 9fe0 1sio3 in the perovskite structure melts at 3000 300 k independently of pressure over the range of 22 65 gpa the melting curve of mg0 9fe0 1sio3 represents an experimental upper bound upon the geotherm from 700 to 1500 km this melting curve corresponds to a homologous temperature of 0 70 or 0 85 for one or two layer mantle convection respectively this is consistent with either one or two layer mantle convection and a lower mantle of constant viscosity based upon homologous temperature arguments

towards self consistent modeling of the martian dichotomy the influence of one ridge convection on crustal thickness distribution in order to find an explanation for the origin of the martian crustal dichotomy a number of recent papers have examined the effect of layered viscosity on the evolution of a degree 1 mantle convection e g roberts and zhong roberts j h zhong s 2006 j geophys res 111 e06013 and yoshida and kageyama yoshida m kageyama a 2006 j geophys res 111 doi 10 1029 2005jb003905 b03412 it was found that a mid mantle viscosity jump combined with highly temperature and depth dependent rheology are effective in developing a degree 1 convection within a short timescale such a layered viscosity profile could be justified by martian mineralogy however the effect of a degree 1 convective planform on the crustal thickness distribution has not yet been demonstrated it is not obvious whether a thinner crust due to sublithospheric erosion and crustal thinning or a thicker crust due to enhanced crustal production would form above the hemisphere of mantle upwelling also the general shape of the dichotomy which is not strictly hemispherical has not yet been fully investigated here we propose a model of the crustal patterns produced by numerical simulations of martian mantle convection using the finite volume multigrid code stagyy tackley p j 2008 phys earth planet int 107 7 18 doi 10 1016 j pepi 2008 08 005 a self consistent treatment of melting crustal formation and chemical differentiation has been added to models of three dimensional thermal convection this allows us to obtain global maps of the crustal thickness distribution as it evolves with time the obtained results demonstrate that it is indeed possible to form a crustal dichotomy as a consequence of near degree 1 mantle convection early in mars history we find that some of the observed patterns show intriguing first order similarities to the elliptical shape of the martian dichotomy in all models the region of thick crust is located over the region of mantle upwelling which itself is a ridge like structure spread over roughly one half of the planet a planform we describe as one ridge convection

relative strength of the pyrope majorite solid solution and the flow law of majorite containing garnets even though the garnet phase is the second most abundant phase in the upper mantle and transition zone no previous studies have directly measured the effect of majorite content on the strength of garnet under mantle conditions here we report the results of constant strain rate and stress relaxation experiments on garnets in the pyrope majorite solid solution which constrain the strength of majoritic containing garnets relative to pyrope as a function of majorite content and temperature we find that at temperatures below 650 degrees c both pure pyrope and majoritic garnets have the same strength conversely above 650 degrees c we find that majoritic garnets are initially stronger than pure pyrope but weaken with increasing temperature and majorite content and with significant majorite contents are weaker than pyrope above approximately 800 degrees c we develop a flow law for the entire pyrope majorite solid solution as a function of temperature and majorite content c 2010 elsevier b v all rights reserved

array invariant based ranging of a source of opportunity the feasibility of tracking a ship radiating random and anisotropic noise is investigated using ray based blind deconvolution rbd and array invariant ai with a vertical array in shallow water this work is motivated by a recent report byun verlinden and sabra j acoust soc am 141 797 807 2017 that rbd can be applied to ships of opportunity to estimate the green s function subsequently the ai developed for robust source range estimation in shallow water can be applied to the estimated green s function via rbd exploiting multipath arrivals separated in beam angle and travel time in this letter a combination of the rbd and ai is demonstrated to localize and track a ship of opportunity 200 900 hz to within a 5 standard deviation of the relative range error along a track at ranges of 1 8 3 4 km using a 16 element 56 m long vertical array in approximately 100 m deep shallow water

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

the effects of shallow rheological boundaries in the upper mantle on inducing shorter time scales of diapiric flows the authors report on a study of pulsating diapiric flow in the upper mantle they study the effect of introducing new phase boundaries in the mantle on such flow basic models have an interface at a depth of 670 km in the rheological properties the authors study the effect of introducing another phase boundary at a depth close to 400 km

high temperature creep of olivine single crystals 2 dislocation structures dislocation structures and densities in experimentally deformed single crystals of san carlos olivine were examined using the oxidation decoration technique the high temperature deformation experiments were conducted at various temperatures t differential stresses and oxygen fugacities o2 the samples were buffered against either orthopyroxene opx or magnesiowustite mw solid state powders and compressed along one of the three 45 orientations in samples that were deformed and subsequently quenched under load seven distinct dislocation structures were observed a for samples compressed parallel to 110 c at 16 50 mpa and o2 10 6atm two different dislocation arrangements were identified for opx buffered samples at 1400 t 1475 c the dislocation structure was composed of curved screw segments that had usually cross slipped and straight edge segments of comparable length at 1300 t 1350 c for opx buffered samples and at 1350 t 1500 c for mw buffered samples the dislocation structure is dominated by arrays of very long edge or mixed dislocations with only a few screw dislocations b for both opx buffered and mw buffered samples compressed parallel to 101 c at 1340 t 1400 c and 30 60 mpa the dislocation configurations are characterized by zigzag near edge dislocations at o2 10 5atm and by straight cross slipped screw dislocations plus pinned edge dislocations at o2 10 9atm c for samples compressed parallel to 011 c at t 1400 c and 70 150mpa the primary dislocations change from long straight edge dislocations for o2 10 4atm and opx buffer to a combination of straight edge dislocations and polygon shaped half loops for o2 10 4atm and mw buffer to gently curved near edge dislocations for o2 10 9atm for both solid state buffers abundant long straight screw dislocations were present in each case for the 011 c samples all of the above variations in dislocation structure are paralleled by changes in the measured power law equation that describes the behavior of creep mechanisms of olivine thus the different dislocation structures are associated with different rate controlling creep mechanisms for all of the dislocation structures the dislocation density increases with differential stress according to the relation 1 4 dislocation structures in samples that were deformed and then statically annealed have also been studied for both 110 c and 011 c samples deformed over a wide range of experimental conditions the dislocation structures on the slip planes generally consist of short curved edge or mixed dislocation segments in 101 1 samples deformed at low o2 50 mpa and 1400 c the dislocation structure was much less altered by annealing than the dislocation structures in 110 c and 011 c samples the density of dislocations decreased much less for these 101 c samples than for the 110 c and 011 c samples due to climb of the edge segments the straight screw segments found in quenched samples became curved and many low angle 100 tilt boundaries connected by straight 100 screw dislocations developed during the annealing process

plate motion and dragging of the upper mantle lateral variations of lithospheric thickness and their implications for intraplate deformation the impact of lateral variations in the thickness of the lithosphere on surface topography horizontal intraplate deformation and stress accumulation is studied for plates that drift with respect to the highly viscous lower mantle and the transition zone the lithosphere and upper mantle are de scribed by a viscoelastic maxwell rheology within the frame work of a finite element scheme which allows the modeling of heterogeneous lithospheric structures in 2d vertical cross sections the geophysical signatures are found to be extremely sensitive to lateral viscosity contrasts which interact with the upper mantle flow this mechanism can contribute to a certain extent and in concert with the other driving forces of plate tec tonics to the evolution of back arc basins to the explanation of the largest angle of subduction in west dipping slabs and to the initiation of subduction of an oceanic lithosphere undemeath a stable continental one

a mineral physics test of a pyrolite mantle an homogeneous upper mantle chemical composition defined by the pyrolite model is compatible with the observed seismic velocities such a test is now possible for depths to 670 km as a result of new mineral elasticity and phase equilibria data three specific criteria for comparing the mineralogical velocity model with that of the mantle include i the velocity jump at the 400 km discontinuity ii the velocity increase between 400 and 670 km and iii the absolute value of velocity in the 300 to 400 km region shallower than the discontinuity the first criterion is sensitive to the olivine content of the mantle and the pressure derivatives of the elastic properties of the high pressure phase the application of this criterion suggests a mantle with 40 65 olivine by volume the second criterion is sensitive to the details of the pyroxene to garnet transformation and the relative abundances of the pyroxenes and garnet the third criterion depends on details of this transformation as well as iron content and the temperature of the mantle

deformation across the mantle transition zone a theoretical mineral physics view abstract the dynamics of the earth s mantle is still poorly constrained due to the lack of understanding the transfer of matter between the upper and the lower mantle and their convective vigor the transition zone tz might play a crucial role as the interface connecting the upper to the lower mantle here we examine the rheology of the main tz minerals wadsleyite ringwoodite and majorite garnet based on a mineral physics approach using the results of lattice friction modeling and dislocation glide mobilities together with the available data on self diffusion in the tz minerals we quantify their plastic deformation by diffusion and dislocation creep from theoretical plasticity models we show that pure climb creep is expected to contribute to the plasticity of the tz without the need of significant diffusion related hydrolytic weakening matching well the geophysical observations our model results predict that crystallographic preferred orientations cpo might only develop along with stress concentrations as present around cold subducting slabs which can be locally weaker than the surrounding tz despite their lower temperatures

geochemical and sedimentologic variations in inter annually laminated sediments from santa monica basin abstract laminated sediments were recently discovered in the center of santa monica basin california continental borderland and extend from the present day to at least the little ice age ca 1600 a d unlike the annual varves observed in santa barbara basin the light dark lamination couplets observed in 210 pb dated cores from the center of santa monica basin occur at 3 to 6 year intervals very similar in length to the el nino southern oscillation enso phenomena the mechanisms that control the temporal patterns of laminations in this basin are unknown potential causal mechanisms of lamination formation in santa monica basin include 1 changes in the flux of terrigenous vs marine organic carbon and detritus 2 surface water processes which affect marine productivity and organic carbon flux to the sediments and 3 deep water processes which affect oxygen renewal and preservation of organic matter within the basin a box core from the central portion of santa monica basin was analyzed using a suite of geochemical sedimentary and micropaleontologic techniques in order to a investigate which if any of these mechanisms has contributed to the occurrence of laminations and b reconstruct proxy records of climate change samples were extracted from this core at 1 mm increments and provide a temporal resolution of 1 3 years per sample a resolution which is comparable in length to the thickness of individual laminae within the core geochemical and micropaleontologic analyses of core samples provide paleoclimatic and paleoceanographic records which are comparable to and extend beyond historical regional and global climate records in addition to direct comparisons between these records spectral analysis was used to investigate whether there were similar patterns of variability that would implicate dominant forcing functions in the historic and proxy records herein we present preliminary results of records from ca 1860 to the present our results suggest that organic carbon fluxes are the dominant factors that affect lamination variability input of organic carbon is primarily a function of primary productivity and to a lesser extent a function of percentage of terrestrial input thus 1 and 2 are both mechanisms of lamination formation stability of 3 only allows preservation of the record of these variations variations in southern california rainfall appear to be linked to decadal scale variations in downcore density profiles and thus cause variations in the thickness and character of the laminations in the deepest portions of this basin thus filtered density records can be used to reconstruct general paleo rainfall trends which compare well with historic rainfall records variations in the isotopic composition of total organic carbon toc are compared to historic enso and climate records and primarily reflect changes in the relative contribution of marine organic carbon to the basin variations in the carbon isotopic composition of n pachyderma mirror changes in the sedimentary 13 c toc record from santa monica basin and suggest that primary productivity is the primary control on the flux of organic carbon to the sediments and exerts a strong influence on the interannual variability in lamination composition variations in paleo sea surface temperatures generated from the oxygen isotopic composition of n pachyderma correlate well with historical sst records and suggest potential usage as a proxy for general paleosst patterns oxygen sensitive assemblages of benthic foraminifera bolivina argentea and b spissa have been used to identify potential large scale flushing events and construct a historical record of variations in deep water oxygen levels spectral analysis of regional historic records indicate dominance of short term enso length and long term decadal patterns of climate variability the short term enso signal is not dominant in the santa monica proxy records however strong decadal length signals are identified in all of the proxy records and suggest that the dominant forcing mechanisms in santa monica basin may be decadal in length

glaciomarine sedimentation in the gowganda formation huronian northern ontario abstract the gowganda formation which outcrops north of lake huron is a world famous example of a glacial deposit it was interpreted by lindsey as continental in origin in the north cobalt area and marine in the south near whitefish falls however drill cores from near the northern limit of the outcrop consist entirely of resedimented glacial diamictites ice rafted deposits and pelagic marine shelf sediment no true tills sediments deposited by grounded ice are present in these cores or in outcrop near elliot lake reinterpretation of the diamictites and sandstones is based on recent improvements in our understanding of sediment gravity flows the conglomerates are debris flows and the sandstones include liquefied fluidized and grain flow deposits and thin bedded turbidites these ediments show crude upward fining cyclicity or grading and probably were deposited on submarine outwash fans the presence of marine deposits so far north and the absence of true tills calls for a reappraisal of gowganda sedimentology by means of modern sedimentological basin analysis techniques a depositional model based on an ice shelf is proposed

dynamic models of subduction geophysical and geological evidence in the tyrrhenian sea summary predictions based on a 2 d finite element model for subduction underneath the calabrian arc in southern italy are compared with a variety of geophysical and geological data such as the present day stress pattern within the slab uplift from the elevation of marine terraces in calabria and subsidence in the tyrrhenian marsili basin from odp leg 107 we model the behaviour of the slab driven by slab pull in agreement with the present tectonic style in this part of the mediterranean as suggested by several investigators the model accounts for the crustal lithospheric and mantle structures in a vertical cross section perpendicular to the calabrian subduction zone the shape of the slab is constrained on the basis of new tomographic images in the southern tyrrhenian sea which were obtained from the regional seismic stations of the istituto nazionale di geofisica while the rheological properties of the mantle are taken from global dynamic models density contrasts between the subducted slab and the surrounding mantle based on petrological models drive the flow in our viscoelastic model stress values displacements and vertical velocities at the surface are sampled at different times after loading until dynamic equilibrium is reached our estimates are appropriate for a time window of 100 kyr the validity of our comparison with the geological record is based on the assumption that the tectonic configuration in the past was not substantially different from that of the present day two families of models with unlocked and locked subduction faults are considered the unlocked models allow for roll back of the trench of about 20 mm yr in agreement with some geological estimates the same family of models predicts uplift of the calabrian arc of about 1 mm yr l and subsidence in the marsili basin of 1 2 mm yr in agreement with geological surveys the deviatoric stress obtained from the unlocked model is consistent with the continuous distribution of deep seismicity in the southern tyrrhenian sea with minor concentration within the lithospheric wedge locked models fail to reproduce these geophysical and geological observations predictions derived from a detached slab model are not consistent with the continuous hypocentral distribution of deep seismicity in the southern tyrrhenian sea deformation at the surface and the stress patterns at depth for a detached slab differ substantially from those of a continuous plate dynamic topography and horizontal motions are reduced when compared with the continuous plate with deviatoric stresses concentrated within the relict slab our results indicate that subduction is a major tectonic process in the southern tyrrhenian sea

viscoelastic responses of a hard transition zone effects on postglacial uplifts and rotational signatures abstract recent studies from laboratory s karato 1989 and monte carlo inversion y ricard et al 1989 have argued that the transition zone between 400 and 670 km depth may be stiffer than the lower mantle and the upper mantle above it using a five layer viscoelastic spherical model we have calculated the transient displacements of postglacial rebound the induced polar motions and the temporal variations of the geopotential up to degree eight of the zonal coefficients we have compared two models a one with two viscoelastic layers separated at 670 km and b the other with three layers in which a hard garnet layer lies between the upper and lower mantle forward modelling shows that it may be possible to discern the presence of a hard garnet layer with a viscosity of at least ten times greater than the upper mantle on the basis of uplift data near the center of the former laurentide ice sheet and from polar wander and j 2 data temporal variations of higher gravity harmonics such as j 6 and j 8 can potentially place even tighter constraints on the rheological properties of the hard transition zone a lower mantle viscosity between 2 and 4 10 22 pa s is generally preferred in models with a garnet layer which may be as large as 50 times more viscous than the upper mantle

horizontal motions due to post glacial rebound north american horizontal post glacial rebound velocities are computed for three five layered incompressible spherical maxwell earth models using a simple glacial loading cycle centered on hudson bay the reference earth model has a 120 km thick elastic lithosphere and upper and lower mantle viscosities of 1021 pa s and 2 1022 pa s respectively the second model has a lower viscosity upper mantle 5 1020 pa s and a higher viscosity lower mantle 1023 pa s and the third has a 250 km thick lithosphere the reference model and the low viscosity upper mantle model calculations are in good agreement with uplift and gravity data although they may somewhat overestimate the present day central uplift rate the thick lithosphere model fares less well because it fails to exhibit the well pronounced peripheral bulge sinking that is required by eastern north america tide gauge data the reference model generally has the largest horizontal rebound velocities reaching a maximum of 4 mm yr towards hudson bay at 2400 km from the load center the effect of a thick lithosphere or of a low viscosity upper mantle is to reduce present day horizontal velocities especially in the formerly glaciated region the sensitivity of horizontal motions to lithospheric thickness and upper mantle viscosity may assist in discriminating between different models of the earth s rheology once sufficient high quality geodetic data exist

interpretation of sks waves using samples from the subcontinental lithosphere abstract the seismic properties of five south african kimberlite nodules have been calculated from petrofabric measurements these garnet lherzolite and harzburgite composition nodules 56 80 olivine are known from previous studies to have originated at depths of 120 170 km in the subcontinental lithosphere their in situ seismic properties have been calculated by extrapolating the elastic constants to the appropriate temperature 900 1050 c and pressure 3 0 3 5 gpa conditions the average of the five samples has a maximum s wave anisotropy v s of 3 7 which is sufficiently high to explain previously reported teleseismic sks delay times t in continental shield areas of up to 1 7 s the biggest delay time is observed when the foliation is vertical the lineation horizontal and the fastest s wave polarized parallel to the lineation i e an orientation for transcurrent motion delay times of over 1 s can only be generated by this orientation a strong mantle crust mechanical coupling is suggested for such situations the potential use of seismic anisotropy as an indicator of strain in the lithosphere has been investigated using experimental and simulated fabric data for olivine the most abundant phase 70 in the upper mantle the v p and v s seismic anisotropy increases with fabric strength and finite strain recrystallization tends to reduce fabric strength and seismic anisotropy resulting in saturation values for experimental and simulated fabrics which correspond to approximately 50 60 strain or 8 9 v s for pure olivine aggregates a survey of 25 naturally deformed peridotites of oceanic origin suggests an average maximum s wave anisotropy for the olivine component of 9 or about 8 for a lherzolite or harzburgite rock composition when the orthopyroxene component is taken into account the ophiolite samples are twice as anisotropic as the kimberlite nodules if an average anisotropy value is representative of the lithosphere then the sks delay times represent variations in anisotropic layer thickness for delay times over 1 s as the orientation is constrained to be foliation vertical and lineation horizontal the magnitude of s wave anisotropy is less sensitive than v p to variations in olivine volume fraction in range 50 100 and to deformation or fabric intensity further suggesting that s wave anisotropy is particularly apt for the determination of the anisotropic lithosphere thickness for smaller delay times there is some trade off between structural orientation and thickness

reply to the comments of s karato on petrofabrics and seismic properties of garnet peridotites from the uhp sulu terrane china by xu et al tectonophysics 421 2006 111 127 abstract we thank karato for his interest in our recent paper in tectonophysics xu z q wang q ji s c chen j zeng l s yang j s chen f y liang f h wenk h r 2006 petrofabrics and seismic properties of garnet peridotite from the uhp sulu terrane china implications for olivine deformation mechanism in a cold and dry subducting continental slab tectonophysics 421 111 127 however karato is incorrect to take water or water related species herein referred to generically as water as an almost unique cause for any olivine fabric transition in nature no matter what geological conditions are in this reply we will first give a succinct introduction to the context of the ongoing debate and then provide some additional arguments in favour of our conclusion that the olivine c type fabric of the zhimafang garnet bearing peridotites was formed under the conditions of high pressure low temperature and low water content during the subduction of cold and dry supracrustal rocks of the yangtze plate the map of olivine slip systems as a function of differential stress and water content drawn by karato and coworkers cannot be reliably extrapolated to earth s interior until relevant scaling laws have been established the role of high water content on the fabric transitions is more likely to modify recrystallization kinetics than to switch the dominant slip systems

enso and sunspot cycles in varved eocene oil shales from image analysis abstract on the premise that sequential changes in varve thickness offer a proxy for climatic variations we investigated varve thickness in three core segments from the distal lacustrine oil shales tipton and laney members of the green river formation by means of an image analysis program of two strong bimodal periodicities one at 4 8 5 6 years is interpreted as an el nino type enso phenomenon of atmospheric dynamics while the other at 10 4 14 7 years is interpreted as the sunspot cycle originally recognized in this formation by bradley 1929 1931 weaker periodicities may exist at ca 8 and 33 years the latter also recognized by bradley taken in conjunction with the work of bradley 1929 1931 and of crowley et al 1986 this suggests that some but not all of the oil shale of the green river formation is truly varved and can be used to infer climatic time series automated instrumental scans have greatly extended the possibilities of varve studies as approaches to climatic variations in time but careful selection of facies in their geological framework is necessary

blind deconvolution of shipping sources in an ocean waveguide this paper investigates the applicability of a ray based blind deconvolution rbd method for underwater acoustic sources of opportunity such as ships recorded on a receiver array the rbd relies on first estimating the unknown phase of the random source by beamforming along a well resolved ray path and then matched filtering each received signal using the knowledge of this random phase to estimate the full channel impulse responses cirs between the unknown source and the array elements up to an arbitrary time shift as well as recovering the radiated signal by the random source the performance of this rbd is investigated using both numerical simulation and experimental recordings of shipping noise in the frequency band 300 800 hz for ranges up to several kilometers the ray amplitudes of the estimated cirs are shown to be consistent with known bottom properties in the area furthermore cirs obtained for an arbitrarily selected shipping track are used as data derived replicas to perform broadband matched field processing to locate another shipping source recorded at a later time in the vicinity of the selected track

paleoclimatology and provenance of the glaciogenic gowganda formation paleoproterozoic ontario canada a chemostratigraphic approach the gowganda formation dated as about 2 3 ga is widely but not universally interpreted as glaciogenic and our chemostratigraphic study supports such an interpretation the bulk compositions of diamictite matrix materials and associated argillites are considered to have resulted from the mixture of two detrital components one is glacial flour similar in composition to the unweathered archean upper crust whereas the other is chemically weathered detritus resembling average proterozoic shale in composition the first component has a chemical index of alteration cia value of 50 reflecting the abundance of feldspar and dearth of aluminous clay minerals the second component has a cia value of 70 and was produced primarily in weathering profiles by chemical weathering typical of temperate climatic regimes diamictite matrix materials 92 samples have an average cia value of 57 the argillites have higher values the average is 62 97 samples the lower cia values of the diamictites reflect a higher proportion of glacial flour suggesting a stronger influence of frigid climatic conditions than that which prevailed during deposition of the argillites systematic trends in cia values are developed across diamictite argillite boundaries and within a thick argillite between two major diamictite units these values reflect climatic variations with maximum amelioration during deposition of the middle section of the argillite rare earth element data and th sc and ti al ratios from the diamictite matrix materials all suggest a provenance that included a large supracrustal component 45 together with lesser amounts of tonalite trondhjemite granite and granite the proportion of supracrustal materials such as volcanic rocks and shales may have been exaggerated by selective comminution during glacial transport the matrix materials of the gowganda diamictites are severely depleted in ca and proportionally enriched in na relative to the source the anorthitic component of plagioclase has been selectively replaced by an albitic component during diagenesis and metamorphism a chemostratigraphic approach provides high resolution data for reconstruction of paleoclimatic conditions and valuable information concerning provenance and metasomatic processes

predicted glide systems and crystal preferred orientations of polycrystalline silicate mg perovskite at high pressure implications for the seismic anisotropy in the lower mantle abstract we use first first principle methods and the peierls nabarro model to evaluate the resistance to glide characterized by the peierls stress of glide systems for end member mgsio 3 perovskite at mantle pressures 010 100 is the easiest glide system in mg perovskite at all pressures peierls stresses increase systematically with pressure for all systems except 001 010 indicating the importance of lattice friction at lower mantle pressures the ratio of the maximum peierls stress for each system relative to the 010 100 value defines their critical resolved shear stress crss these crss are used in a visco plastic self consistent homogenization model to predict the evolution of crystal preferred orientations cpo during deformation of polycrystalline mg perovskite in axial compression 100 tends to align with the compression direction in agreement with in situ observations in axial compression experiments in simple shear 010 concentrates near the shear direction and 100 although more dispersed tends to align near the shear plane consistent with the dominant activity of the easier 010 100 system the calculated seismic anisotropy for a 100 mg perovskite aggregate using the cpo in simple shear and the elastic constants of mgsio 3 perovskite at lower mantle pressures and temperatures is weak 3 for p waves with and 2 for s waves and decreases with increasing temperature and pressure p waves show the fastest propagation parallel to the lineation and s waves fast polarization is in the foliation at 38 gpa and normal to the lineation at 88 gpa this weak anisotropy is consistent with global seismological observations of a nearly isotropic lower mantle there are however two regions where strain induced mg perovskite cpo could contribute to anisotropy a low temperature regions in the uppermost lower mantle where the predicted s wave polarization anisotropy may attain 1 6 with a fast polarization parallel to the foliation b in high high temperature domains in the d layer where mg perovskite may be the major stable phase leading to polarization of fast s waves normal to the lineation for propagation directions at high angle to the lineation and an apparent isotropy for all other propagation directions

deep focus earthquakes and recycling of water into the earth s mantle for more than 50 years observations of earthquakes to depths of 100 to 650 kilometers inside earth have been enigmatic at these depths rocks are expected to deform by ductile flow rather than brittle fracturing or frictional sliding on fault surfaces laboratory experiments and detailed calculations of the pressures and temperatures in seismically active subduction zones indicate that this deep focus seismicity could originate from dehydration and high pressure structural instabilities occurring in the hydrated part of the lithosphere that sinks into the upper mantle thus seismologists may be mapping the recirculation of water from the oceans back into the deep interior of our planet

olivine flow mechanisms at 8 gpa abstract the mechanisms responsible for high temperature olivine deformation are investigated at a pressure of 8 gpa and temperatures up to 1780 k san carlos olivine specimens of different average grain sizes 0 5 and 5 m were deformed simultaneously between hard alumina pistons during relaxation experiments these experiments are carried out in a multi anvil high pressure apparatus coupled with synchrotron x ray radiation the different grain size specimens experienced identical p t stress condition at any given time a new method for measuring strains and strain rates 10 6 s 1 of specimens at high pressure is documented this method uses time resolved in situ x ray imaging and an image analysis computation the microstructures of run products recovered after being quenched at different temperatures were characterized by transmission electron microscopy tem we find that high temperature olivine flow is grain size insensitive at 8 gpa which suggests that dislocation creep dominates olivine deformation at high pressure this result is confirmed by the tem investigation of our deformed specimens in which we find evidences of the activation of olivine dislocation slip systems specimen microstructures are consistent with dynamic recrystallization as an assisting process in olivine deformation during the high pressure experiments extrapolation of our results to the low stress level and large grain size expected in the mantle suggests that dislocation creep assisted by dynamic recrystallization may also dominate natural olivine deformation in the upper mantle

ultradeep ultramafic mantle xenoliths transmission electron microscopy preliminary results abstract the defect microstructure of ultradeep garnets from the jagersfontein kimberlite pipe south africa was investigated by transmission electron microscopy these crystals exhibit a high density of dislocations most of which form well organized and regularly spaced subgrain boundaries free dislocations are also detected most of them have a burgers vector 1 2 111 shortest lattice repeat but a few 100 burgers vectors are also characterized especially in junctions small needle shaped pyroxene exsolutions are also detected they exsolved from the si rich garnets during assent to the earth s surface their nucleation and growth required bulk diffusion and this process may have affected the original dislocation microstructure it is believed however that the observed dislocation pattern still reflects the original deformation regime of garnets at very high pressure and temperature and this dislocation pattern strongly suggests that garnets were highly ductile finally a number of precipitates of a dense amorphous phase of pure carbon are also detected they might result from the amorphization of small diamonds exsolved from the garnet as the pressure was progressively released

on transient rheology and glacial isostasy theoretical predictions from analytical self gravitating layered earth models with maxwell burgers body and standard linear solid rheologies are employed to explore the extent to which transient creep may have influenced the inference of long term mantle viscosity from analysis of the various signatures derived from the process of glacial isostatic adjustment the compatibility of the various models is checked by their ability to fit simultaneously the relative sea level free air gravity and the rotational data we conclude that transient creep cannot operate throughout the entire mantle on account of the extreme sensitivity of the uplift and gravity data to any significant departures of the steady upper mantle viscosity from 1021 pa s transient rheology would most likely be unimportant in the upper mantle restricting transient rheology to the lower mantle we find that models with an adiabatic density jump at 670 km depth are not satisfactory because of their inability to produce sufficiently large gravity anomalies and polar wander speeds our calculations show that the preferred models with transient creep in the lower mantle are chemically stratified with a nonadiabatic density change of around 10 the long term viscosities in the lower mantle associated with compatible transient rheological models lie between 5 1022 and 1023 pa s these values are consistent with viscosity estimates derived from correlation of long wavelength geoid anomalies and the density perturbations in the deep mantle

mantle heterogeneities geoid and plate motion a monte carlo inversion seismic tomography in both the upper and the lower mantle as well as subducting oceanic slabs defined by seismicity has been translated into density heterogeneities to generate models of mantle circulation these models can predict both the surface velocities and the geoid which can be compared with plate tectonics and gravity data a given model is specified by 6 parameters related to the viscosities of 3 mantle layers and the absolute amplitudes of density variations in the upper and lower mantle as well as in the slabs the values of these parameters are chosen at random within an acceptable range each model is submitted to an appropriate test comparing observations with predictions the results of the most successful models selected by this monte carlo inversion are displayed they yield preferred mantle viscosity structures exhibiting large variations at depth with a physical interface between upper and lower mantle i e with the possibility for the circulation to penetrate the 650 km discontinuity two classes of viscosity profiles stand out the first one implies a regular increase of the viscosity in the sublithospheric mantle with reasonable values for the density parameters the second one is unexpected in the sense that it predicts a very stiff bottom for the upper mantle it also requires vanishingly small amplitudes for the upper mantle density heterogeneities defined by tomography which would thus have to be of lithological rather than thermal origin with a chemical interface at 650 km the outcome is very similar the same two classes of viscosity structures do yield a satisfactory geoid prediction however only the class of models with a stiff layer at midmantle depths predicts acceptable surface velocities altogether the best models out of some 60 000 which have been tested only explain one third of the geoid and two thirds of the surface divergence for spherical harmonic degrees 1 to 6 nevertheless the main features of these two observed patterns are present in the computed maps and 4 out of 6 correlation coefficients lie close to the 90 confidence level this is true for the geoid as well as for the surface divergence of the displacement velocity however as the internal viscosity structure has been assumed to have spherical symmetry the rotational component of the surface velocities cannot be predicted

pleistocene glaciation and the earth s precession constant we predict time dependent variations in the earth s precession constant arising from the ice and ocean mass fluctuations associated with the late pleistocene glacial cycles our predictions incorporate contributions from both the surface mass load redistribution and the adjustment of the solid earth in this regard we adopt spherically symmetrical self gravitating maxwell viscoelastic earth models and obtain results for a large suite of radial viscosity profiles these profiles include a set obtained from published inferences based on post glacial relative sea level and uplift histories as well as a set intended to sample the sensitivity of the results to variations in the viscosity within a number of major subregions of the mantle e g the transition zone the upper mantle and the lower mantle a more detailed measure of this sensitivity is also obtained by computing frechet kernels for the predictions we construct an ice model which incorporates the ice 3g model for the final deglaciation event and which is constrained to yield a eustatic sea level variation which matches observed fluctuations in oxygen isotope records over the last 800 kyr in all cases the ocean mass redistribution is constrained to be gravitationally self consistent the computed frechet kernels indicate that the predictions are most sensitive to variations in viscosity in the deepest regions of the mantle indeed in some cases the sensitivity peaks at the core mantle boundary both positive and negative perturbations to the precession constant are predicted with the maximum peak to peak relative variation being 0 20 per cent for the published viscosity models and 0 32 per cent for all other models furthermore the mean relative perturbation in the precession constant with respect to the present day value is found to reach 0 08 per cent for the published viscosity models and 0 20 per cent for other models recent solutions for the earth s precession obliquity and insolation parameters laskar joutel boudin 1993 indicate a passage through resonance associated with a perturbation of jupiter and saturn in the case when the mean relative perturbation in the precession constant is 0 15 per cent we find that this threshold is not achieved for any of the published viscosity models however it is reached for example for earth models with lower mantle viscosities which exceed 30 50 1021 pa s or models characterized by large two orders of magnitude jumps in viscosity at mid lower mantle depths

modern spectral climate patterns in rhythmically deposited argillites of the gowganda formation early proterozoic southern ontario canada abstract rhythmically deposited argillites of the gowganda formation ca 2 0 2 5 ga probably formed in a glacial setting drop stones and layered sedimentary couplets in the rock presumably indicate formation in a lacustrine environment with repeating freeze thaw cycles it is plausible that temporal variations in the thickness of sedimentary layers are related to interannual climatic variability e g average seasonal temperature could have influenced melting and the amount of sediment source material carried to the lake a sequence of layer couplet thickness measurements was made from high resolution digitized photographs taken at an outcrop in southern ontario canada the frequency spectrum of thickness measurements displays patterns that resemble some aspects of modern climate coherent periodic modes in the thickness spectrum appear at 9 9 10 7 layer couplets and at 14 3 layer couplets it is unlikely that these coherent modes result from random processes modern instrument records of regional temperature and rainfall display similar spectral patterns with some datasets showing significant modes near 14 yr in both parameters rainfall and temperature could have affected sedimentary layering in the gowganda argillite sequence and climate modulation of couplet thickness emerges as the most likely explanation of the observed layering pattern if this interpretation is correct the layer couplets represent predominantly annual accumulations of sediment i e they are varves and the thickness spectrum provides a glimpse of early proterozoic climatic variability the presence of interannual climate patterns is not unanticipated but field evidence presented here may be of some value in developing a climate theory for the early proterozoic

north atlantic oscillation dynamics recorded in greenland ice cores carefully selected ice core data from greenland can be used to reconstruct an annual proxy north atlantic oscillation nao index this index for the past 350 years indicates that the nao is an intermittent climate oscillation with temporally active coherent and passive incoherent phases no indication for a single persistent multiannual nao frequency is found in active phases most of the energy is located in the frequency band with periods less than about 15 years in addition variability on time scales of 80 to 90 years has been observed since the mid 19th century

upper mantle seismic anisotropy and lithospheric decoupling the lithosphere underlying stable continental shield regions is thought to extend to a depth of 200 km compressional wave velocity models derived from a recent approximately ns refraction profile in northern australia include a pronounced high velocity zone at a depth of around 200 250 km conventional petrological interpretations in terms of mineralogical or compositional changes cannot readily explain this feature calculated velocities for the garnet pyrolite model using a derived geotherm fit the velocities above and below this high velocity zone but cannot account for the high velocity 8 6 km s 1 zone itself this feature is explained in terms of velocity anisotropy which might reflect mechanical decoupling of the continental lithosphere from the underlying mantle

lehmann discontinuity as the base of an anisotropic layer beneath continents long period surface wave r 1 g 1 body wave s ss sss and scs reverberation data have been inverted to obtain anisotropic structures along seismic corridors that sample australia and the western pacific these models support the proposal that the lehmann discontinuity beneath stable continents represents a transition from an anisotropic lithosphere to a more isotropic material in the lower part of the continental tectosphere

constraints on seismic velocity anomalies beneath the siberian craton from xenoliths and petrophysics high seismic vp velocity anomalies 8 7 9 0km s 1 have long been known about in regions of the uppermost mantle of the siberian craton often in association with kimberlite fields laboratory measurement of seismic properties of five xenoliths three peridotites and two eclogites from the udachnaya kimberlite under confining pressures up to 600 mpa were extrapolated to uppermost mantle p t conditions of 1500 mpa and 500 c however none of the velocities are high enough to explain the observations eclogites or peridotites are commonly considered to be the source of anomalous high velocities we prefer a peridotitic source to an eclogitic source due to the unusual chemistry and regional uniformity of eclogitic garnets required maximum velocity limitations on laboratory measurements of seismic properties of natural eclogites and purported abundance of eclogites in the lithosphere alternatively a highly depleted peridotite such as dunite or harzburgite can produce velocities high enough to match observations olivine petrofabrics in most peridotites including the three peridotites used in this study are great enough to produce the observed high velocities provided olivine petrofabrics are continuous enough and correctly oriented to be seismically detectable and the modal proportion of olivine is high there have been suggestions by other authors that the siberian upper mantle is highly depleted and that a lithosphere scale shear zone exists which may have acted to organize fabrics into segments large enough for detection anomalously high vp vs velocity ratios of greater than 1 8 are expected parallel to the olivine 100 maxima required to be present in a high velocity olivine dominated upper mantle vp vs velocity ratios can serve as a means of inferring large scale anisotropy when limited seismic data are available as in siberia 2006 elsevier b v all rights reserved

tem observations of several spinel garnet assemblies toward the rheology of the transition zone in order to better identify the mineral phase which controls the rheology of the transition zone between 410 and 660 km depth transmission electron microscopy observations were made on several coexisting spinel garnet assemblies alkremite xenolith pyrope rich mgo 1 1al2o3 spinel assembly deformed at 1173k 800 mpa in a griggs apparatus mg fe 3 al mg si 2si3o12 majorite mg fe 2sio4 spinel assembly synthesized in a laser heated diamond anvil cell it was found that garnet crystals systematically remain undeformed while spinel crystals are plastically deformed these results are in accord with the assumption that the rheology of majorite is stronger than the rheology of spinel in the conditions of the transition zone

high pressure deformation in two phase aggregates abstract we investigate the rheological behavior of multi phase aggregates at high pressure and high temperature using synchrotron x ray radiation as the probing tool we are able to quantify the stress state of individual phases within the aggregates this method provides fundamental information in interpreting the behavior of two phase multi phase mixtures which contribute to our understanding of the deformation process at deep earth conditions we choose mgal2o4 spinel and mgo periclase as our model materials mixtures of various volume proportions were deformed in a multi anvil high pressure deformation apparatus at pressure of 5 gpa and elevated temperatures stress is determined from x ray diffraction providing a measure of stress in each individual phase of the mixture in situ during the deformation macroscopic strain is determined from x ray imaging we compare the steady state strength of various mixtures at 1000 c and 800 c and at the strain rate in the range of 1 8 to 8 8 10 5 s 1 our data indicate that the weak phase mgo is responsible for most of the accumulated strains while the strong phase spinel is supporting most of the stress when the volume proportion is 75 spinel and 25 mgo the intermediate compositions 40 60 are much weaker than either of the end members while the grain sizes for the intermediate compositions submicrons are much smaller than the end members 5 10 m we conclude that a change in flow mechanism resulting from these smaller grains is responsible for the low strength of the intermediate composition mixtures this study demonstrates an approach of using synchrotron x rays to study the deformation behaviors of multi phase aggregates at high pressure and high temperature

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

array invariant based source localization in shallow water using a sparse vertical array the array invariant proposed for robust source localization in shallow water exploits the beam time migration of broadband signals the approach requires minimal knowledge of the environment but involves plane wave beamforming using a short aperture vertical array in stratified acoustic waveguides in this paper the array invariant approach is extended to a large aperture vertical array that is sparse with significant variation of the sound speed across the aperture for conventional beamforming the extension is feasible because the array invariant in shallow water utilizes surface bottom reflected arrivals that behave like those in ideal waveguides robust source range estimation is demonstrated using data from a 16 element 56 m long vertical array at various ranges 1 5 3 5 km from a broadband source 0 5 2 khz in approximately 100 m deep shallow water

high frequency source localization in a shallow ocean sound channel using frequency difference matched field processing matched field processing mfp is an established technique for locating remote acoustic sources in known environments unfortunately environment to propagation model mismatch prevents successful application of mfp in many circumstances especially those involving high frequency signals for beamforming applications this problem was found to be mitigated through the use of a nonlinear array signal processing technique called frequency difference beamforming abadi et al 2012 building on that work this nonlinear technique was extended to bartlett mfp where ambiguity surfaces were calculated at frequencies two orders of magnitude lower than the propagated signal where the detrimental effects of environmental mismatch are much reduced previous work determined that this technique has the ability to localize high frequency broadband sources in a shallow ocean environment with a sparse vertical array using both simulated and experimental propagation data using simulations the performance of this technique with horizontal arrays and adaptive signal processing techniques was investigated results for signals with frequencies from 10 khz to 30 khz that propagated in a 100 m deep shallow ocean sound channel with a downward refracting sound speed profile will be shown for source array ranges of one to several kilometers sponsored by the office of naval research

effects of phase transions and possible compositional changes on the seismological structure near 650 km depth if the lower mantle is enriched in fe si relative to the upper mantle the compositional boundary is expected to be an interface with a density jump of 5 2 and with depth varying laterally by 100 300 km but with changes in seismic velocities that are nearly unobservable 0 to 1 0 2 5 discontinuities in compressional vp and shear vs wave velocities in contrast the polymorphic transformation of transition zone minerals to the perovskite bearing assemblage of the lower mantle is estimated to produce a jump of 5 10 percent in density vp and vs near 650 km depth and the depth of the seismological discontinuity due to the phase transition is relatively independent 50 150 km variation of temperature or horizontal position if present a compositional boundary is not in general expected to occur at the identical depth 650 km as the phase transition to the perovskitebearing assemblage although the depths of the two may be linked

climatic cycles recorded in glacially influenced rhythmites of the gowganda formation huronian supergroup abstract the gowganda formation of the 2 45 2 2 ga huronian supergroup contains glacially induced varve like rhythmites that potentially preserve a detailed record of climatic conditions during the paleoproterozoic era four rhythmic couplet thickness records were measured at two outcrops near wharncliffe ontario for the purpose of time series analysis the couplets which range from 1 to 32 mm thick are composed of alternating layers of siltstone and claystone time series analysis of the couplet thickness records using the mtm toolkit of mann and lees 1996 consistently revealed periodicities in the range of 2 2 2 9 couplets per cycle which is consistent with climatic cycles such as the quasi biennial oscillation qbo and the el nino southern oscillation enso observed in modern times this periodicity suggests that the rhythmic couplets represent annual deposits i e varves evidence for the presence of cycles at 3 0 4 9 couplets 6 6 6 9 couplets 8 8 9 2 couplets 22 8 couplets and 30 1 31 0 couplets were also observed in some couplet thickness records however the presence of these longer term cycles was inconsistent from site to site

petrofabrics and seismic properties of garnet peridotite from the uhp sulu terrane china implications for olivine deformation mechanism in a cold and dry subducting continental slab abstract lattice preferred orientations lpo of olivine diopside enstatite and garnet from the zhimafang garnet peridotite body in the sulu ultrahigh pressure uhp metamorphic terrane china were measured using the electron backscatter diffraction ebsd technique the peridotite was captured from a mantle wedge immediately adjacent the subducted yangtze slab and then experienced the uhp metamorphism at 750 950 c and 4 7 gpa the olivine lpo is characterized by the 001 axis close to the stretching lineation and the 100 plane subparallel to the foliation indicating the prevailing of 100 001 slip enstatite lpo displays the dominance of 100 001 slip diopside developed complex lpo patterns that are difficult to explain using a single slip system of 100 001 garnet is almost randomly oriented due to its low volume fractions cubic symmetry and the presence of numerous slip systems calculated seismic properties of the peridotite yield a maximum p wave velocity normal to the foliation and a minimum along the foliation with anisotropy up to 8 in strongly sheared samples the s wave velocity pattern is complex but the fast polarization plane generally normal to the foliation the inferred shear sense from the olivine lpo is top to se in contrary to exhumation induced top to nw thrusting recorded in the quartz lpo implying that the olivine lpo formed at early uhp metamorphic conditions the olivine crystals have relatively low water contents 141 475 h 106 si indicating a fluid deficient environment for the lpo formation the present study suggests that a combination of low temperature and uhp plays a much more important role than the water content to promote the activation of 100 001 slip in olivine

early proterozoic ice proximal glaciomarine deposition the lower gowganda formation at cobalt ontario canada mapping at 1 2 000 scale in the cobalt area has allowed detailed interpretation of the glaciogenic deposystem for the basal 180 m of the coleman member lowermost unit of the gowganda formation huronian supergroup deposition at cobalt occurred on the subsiding shelf of a south facing passive margin the unconformity that separates subhorizontal coleman member lithofacies from underlying archean rocks exhibits a subdued topography and less than 100 m of relief evidence of plucking and lee side quarrying of this surface support southerly movement of a grounded ice sheet massive basal diamictites were deposited subglacially as extensive primary basal till and as restricted lee side till that infilled paleotopographic lows and covered preglacial regolith coarsening upward rhythmite sandstone clast supported diamictite associations were deposited as prograding subaqueous outwash fans in front of a slowly retreating wet based tidewater ice sheet grounded in 50 to 300 m of water rhythmite characteristics indicate a distal change from underflow to interflow overflow sediment transport supporting an ice contact marine basin interpretation differences in dropstone abundance in the rhythmites suggest variable rates of ice front calving and iceberg transport interfan areas received sediment from melt water and from abundant sediment gravity flows off over steepened fan slopes and collapsing ice marginal banks an upper diamictite association contains dropstone diamictite poorly laminated mudstones diamictite with random clast orientations and thick diamictite sequences characteristic of deposition from sediment gravity flows these features suggest transition from a grounded to a partially floating ice sheet before deposition of the uppermost coleman member strata preserved at cobalt

elasticity and strength of hydrous ringwoodite at high pressure oh bearing hydrous ringwoodite compressed non hydrostatically in a diamond anvil cell supports a differential stress that increases from 2 9 to 4 5 gpa over the pressure range of 6 7 13 2 gpa at room temperature this result suggests a significant water weakening effect when compared with results from similar experiments on the anhydrous counterpart kaver and duffy geophys res lett 28 2001 2691 2694 the elastic anisotropy 2c44 c11 c12 of hydrous ringwoodite is measured to be 0 87 7 throughout this pressure range similar to measured values for anhydrous ringwoodite kaver and duffy geophys res lett 28 2001 2691 2694 this lattice anisotropy cannot be explained by anelastic effects such as faulting and twinning within the structure these results suggest that hydrous minerals in the upper mantle and transition zone may have higher ductile strain rates for a fixed shear stress at high temperature resulting in stronger preferred lattice orientation this in turn may be seismically detectable which opens the possibility of using seismic anisotropy as a marker for local volatile containing areas within the upper mantle and transition zone

solar activity expressed in a modern varve thickness sequence annually laminated sediments varves form in particular depositional settings e g where seasonal climate produces fluctuations in runoff volume variations in runoff affect the amount and type

solar signature in sedimentary cycles from the late precambrian elatina formation australia climatic cyclicity is recorded by regular variations in the thickness of varves in the elatina formation a late precambrian periglacial lake deposit in south australia earlier conclusions that the climatic cycles reflect solar variability are strongly supported by a complexity of periods revealed through study of drill cores of the varved sequence the new data promise to contribute significantly to our understanding of solar processes

experimental constraints on the strength of the lithospheric mantle 1 to provide a better understanding of rheological properties of mantle rocks under lithospheric conditions we carried out a series of experiments on the creep behavior of polycrystalline olivine at high pressures 4 9 gpa relatively low temperatures 673 t 1273 k and anhydrous conditions using a deformation dia differential stress and sample displacement were monitored in situ using synchrotron x ray diffraction and radiography respectively experimental results were fit to the low temperature plasticity flow law on the basis of this analysis the low temperature plasticity of olivine deformed under anhydrous conditions is well constrained by our data with a peierls stress of p 5 9 0 2 gpa a zero stress activation energy of ek 0 320 50 kj mol 1 and ap 1 4 10 7 s 1 mpa 2 compared with published results for high temperature creep of olivine a transition from low temperature plasticity to high temperature creep occurs at 1300 k for a strain rate of 10 5 s 1 for a geological strain rate of 10 14 s 1 extrapolation of our low temperature flow law to 873 k the cutoff temperature for earthquakes in the mantle yields a strength of 600 mpa the low temperature high stress flow law for olivine in this study provides a solid basis for modeling tectonic processes occurring within earth s lithosphere

new developments in deformation experiments at high pressure although the importance of rheological properties in controlling the dynamics and evolution of the whole mantle of earth is well recognized experimental studies of rheological properties and deformation induced microstructures have mostly been limited to low pressure conditions this is mainly a result of technical limitations in conducting quantitative rheological experiments under high pressure conditions a combination of factors is changing this situation increased resolution of composition and configuration of earth s interior has created a greater demand for well resolved laboratory measurement of the effects of pressure on the behavior of materials higher strength materials have become readily available for containing high pressure research devices and new analytical capabilities in particular very bright synchrotron x ray sources are now readily available to high pressure researchers one of the biggest issues in global geodynamics is the style of mantle convection and the nature of chemical differentiation associated with convectional mass transport although evidence for deep mantle circulation has recently been found through seismic tomography e g van der hilst et al 1997 complications in convection style have also been noted they include 1 significant modifications of flow geometry across the mantle transition zone as seen from high resolution tomographic studies fukao et al 1992 masters et al 2000 van der hilst et al 1991 and 2 complicated patterns of flow in the deep lower mantle 1500 2500 km perhaps caused by chemical heterogeneity kellogg et al 1999 van der hilst and karason 1999 these studies indicate that while large scale circulation involving the whole mantle no doubt occurs significant deviations from simple flow geometry are also present two mineral properties have strong influence on convection 1 density and 2 viscosity rheology contrasts in the past the effects of density contrast have been emphasized honda et al 1993 kellogg et al 1999 tackley et al 1993 and the influence of

differential contraction of subducted lithosphere layers generates deep earthquakes abstract the origin of forces for generating deep earthquakes remains elusive we propose a new mechanical model that involves constant release of shear strain accumulated between the crust and mantle lithosphere caused by differential volume changes during phase transformations as a slab sinks this generates increasing down dip compression inside the slab with depth consistent with the global distribution of deep earthquake focal mechanisms using experimentally calibrated slab rheology we show that the estimated distribution of slab internal stress agrees well with the depth frequency relationship of deep earthquakes in both cold and warm slabs with the peak in seismicity at 600 km depth corresponding to an induced stress maximum this mechanism of residual stress within a cold slab also provides a solution to deep earthquakes occurring beneath former subduction zones such as western tonga and spain we further suggest that this model may reconcile existing geodynamic paradoxes concerning the absolute strength of slabs discrepancies on mantle rock strength between laboratory and geophysical inversions as well as the competition between earthquake generating stresses and slab pull forces

global upper mantle structure from finite frequency surface wave tomography 1 we report global shear wave velocity structure and radial anisotropy in the upper mantle obtained using finite frequency surface wave tomography based upon complete three dimensional born sensitivity kernels because wavefront healing effects are properly taken into account finite frequency surface wave tomography improves the resolution of small scale mantle heterogeneities especially for deep anomalies that are constrained by the longest period surface waves in our finite frequency model ffsw1 the globally averaged radial anisotropy shows a transition from positive sh sv to negative anisotropy sv sh at about 220 km consistent with a change in the dominant mantle circulation pattern from predominantly horizontal flow at shallow depths to vertical flow at greater depths the radial anisotropy beneath cratons and the old pacific plate agrees well with previous studies however our model exhibits a strong negative radial anisotropy at depths greater than 120 km beneath mid ocean ridges a feature that is not present in previous upper mantle models more interestingly the depth extent of the ridge anomalies is distinctly different beneath fast and slow spreading centers anomalies beneath fast spreading centers are stronger but the strength decreases rapidly below 250 km in contrast beneath slow spreading centers such as the northern mid atlantic ridge and the red sea anomalies extend down at least to the top of the transition zone the different depth extent of the ridge anomalies suggests that the primary driving force of slow spreading seafloor may be different from that of fast spreading seafloor and that active upwelling beneath slow spreading ridges may play a major role in the opening of the seafloor

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

flow law for ringwoodite at subduction zone conditions the plastic properties of ringwoodite a high pressure modification of olivine are experimentally determined at the pressure of 20 gpa and temperatures up to 1350 c stress relaxation experiments have been carried out on fine grained samples using a two stage t cup multiple anvil high pressure apparatus with synchrotron generated x rays we observe the presence of a significant weakening of the shear strength with a decrease from 2 5 to 0 2 gpa between 600 and 1000 c in this region the activation energy u is about 275 kj mol and it decreases with increasing stress reaching a value of about 200 kj mol when the shear stress attains 2 5 gpa we propose that the plasticity is associated with vigorous dislocation glide controlled by short range lattice resistance forces at lower temperatures the shear strength is mildly dependent on temperature reaching 3 gpa at room temperature while above 1000 c the flow may be governed by a power law relation with n about 3 5 2003 elsevier science b v all rights reserved

modeling dislocation glide in mg2sio4 ringwoodite towards rheology under transition zone conditions abstract deformation resulting from thermally activated plastic slip is modeled in mg2sio4 ringwoodite at 20 gpa for a wide range of temperatures the model relies on the structures of the rate controlling 1 2 1 1 0 screw dislocations which have been modeled using the peierls nabarro galerkin method these calculations are parametrized by density functional theory calculations of surfaces of the 0 0 1 1 1 0 and 1 1 1 planes at finite temperatures dislocation mobility is controlled by kink pair nucleation on the thermally activated 1 4 1 1 0 partial screw dislocations as they occur in ringwoodite single slip critical resolved shear stresses crss corresponding to this mechanism are deduced from orowan s equation the results are found to be in reasonably good agreement with experimental data at 20 gpa which show high effective flow stresses under laboratory conditions finally the crss s are calculated for typical mantle strain rates of 10 16 s 1 at appropriate temperatures expected in the lower transition zone results show that dislocation glide remains difficult and that lattice friction is not yet negligible in ringwoodite under natural conditions

deformation of mg0 9 fe0 1 sio3 perovskite aggregates up to 32 gpa room temperature investigations on the shear stress and deformation mechanisms of mg0 9fe0 1 sio3 perovskite are performed in situ up to 32 gpa using radial x ray diffraction and the diamond anvil cell as a deformation apparatus the uniaxial stress supported by the perovskite aggregate is found to increase continuously with pressure up to 10 9 1 9 gpa at 32 1 gpa our measurements show no development of significant lattice preferred orientations in the sample which indicates that deformation by dislocation glide is not the dominant deformation mechanism under these conditions assuming that the underlying cause for seismic anisotropy in the deep earth is elastic anisotropy combined with lattice preferred orientation our results indicate that silicate perovskite deformed under the conditions of this experiment would not be the source of seismic anisotropy c 2003 elsevier science b v all rights reserved

the viscosity structure of the mantle during 1991 1994 the four year period covered by this report there has been a shift in emphasis away from one dimensional radial mantle viscosity models and toward an understanding of the three dimensional radial and lateral structure of mantle viscosity our understanding of mantle viscosity comes primarily from two sources studying the creep properties of mantle minerals under appropriate pressures and temperatures and using theoretical flow models to predict surface observables such as plate velocities gravity geoid and heat flow laboratory measurements of deformation indicate that the rheology of upper mantle minerals such as olivine mg fe 2sio4 is a strong function of temperatue grain size and stress cf karato and wu 1993 the deformation of minerals under mantle conditions generally follows a flow law of the form e a nd mexp q qrt 1 where is the deformation rate is the deviatoric stress is the shear modulus d is the grain size of the rock q is the activation energy for the deformation mechanism t is the temperature in kelvin r is the gas constant and a is a constant viscosity is defined as e 2 therefore deformation is directly related to viscosity substituting equation 1 into equation 2 an effective viscosity eff a n 1dmexp qrt 3 can be defined for temperature changes of 100 degrees k the viscosity changes by an order of magnitude at constant stress cf karato and wu 1993 changes of deviatoric stress by a factor of 2 change the viscosity by an order of magnitude cf karato and wu 1993 other factors such as partial pressure of oxygen and water content may also have important effects

plastic deformation of garnets systematics and implications for the rheology of the mantle transition zone abstract plastic properties of materials with garnet structure have been studied under wide temperature conditions ranging from room temperature to 95 of the melting temperatures using uniaxial compression and hot microhardness tests garnets studied include single crystals of oxide garnets y 3 al 5 o 12 gd 3 ga 5 o 12 and y 3 fe 5 o 12 and silicate garnets various solid solutions including grossular almandine andradite pyrope spessartine and uvarovite both uniaxial compression and hot hardness tests indicate that there is a general trend in the plasticity of garnets when the data are compared at normalized conditions t t m and and that the resistance to plastic deformation in garnets is significantly higher than most of the other minerals in the earth s mantle based on both stress dip tests and microstructural observations it is proposed that the creep strength of garnet is largely controlled by the resistance to dislocation glide rather than by recovery processes this conclusion is consistent with the high peierls stress inferred from the hot hardness tests the high peierls stress in garnets is presumably due to the large unit cell i e long burgers vectors and or the bcc packing which are common to all garnets we postulate therefore that the present results can be applied to the strength of high pressure garnet majorite and suggest that garnet rich layers in the earth such as subducted oceanic crust in the transition zone or a possibly garnet rich bottom part of the transition zone will be considerably stronger than surrounding regions

brecciation of clasts in diamictites of the gowganda formation ontario canada igneous boulders in the gowganda formation were brecciated prior to the induration of the diamictites in which they occur the breccias evidently represent the results of the maximum stresses imposed on a lodgment till in the final stages of emplacement the stresses necessary to fracture granitoid clasts are likely to occur at the base of glaciers of unremarkable size and could easily be imposed by small glaciers if the pore pressure in the underlying till dropped as a result of sudden leakage of pore fluid

high frequency 8 16 khz model based source localization matched field or model based processing has now been widely demonstrated for improving source localization and detection in ocean waveguides most of the processing approaches become increasingly sensitive to fluctuations or uncertainties as the frequency increases as a result there has been very limited work above 1 khz and there is a perception that above several kilohertz the technique cannot be applied we have conducted acoustic communications experiments in a variety of shallow water sites around coastal areas of the united states these experiments show that a clear multipath structure is readily observed even in the 8 16 khz band furthermore it is shown that model based processing can then be exploited to localize sources at these high frequencies out to ranges of several kilometers

the strength of mantle silicates at high pressures and room temperature implications for the viscosity of the mantle the rheological properties of the upper mantle important in convective and tectonic processes have long been studied through low pressure creep experiments on olivine1 5 in comparison our understanding of deformation in the deep earth is incomplete because there are few experimental constraints on the rheology of the transition zone 400 670 km depth and the lower mantle estimates of the viscosity at these depths have had to rely on geophysical models of for example post glacial uplift the geoid and changes in the earth s rate of rotation6 13 recently we developed techniques for measuring solid state creep at mantle pressures14 and here we use these methods to make direct laboratory measurements of the strength of key mantle minerals we study mg fe 2sio4 olivine representative of about 60 of the upper mantle mg fe 2sio4 spinel which constitutes a large fraction of the transition zone and mg fe sio3 perovskite and perovskite mg fe o magnesiowiistite assemblages which comprise most of the lower mantle all at room temperature and pressures of up to 60 gpa our results suggest that the transition zone may form a layer of relatively high viscosity or strength between the upper and lower mantle a simple theological model shows that this behaviour may be compatible with current geophysical observations

pressure sensitivity of olivine slip systems and seismic anisotropy of earth s upper mantle the mineral olivine dominates the composition of the earth s upper mantle and hence controls its mechanical behaviour and seismic anisotropy experiments at high temperature and moderate pressure and extensive data on naturally deformed mantle rocks have led to the conclusion that olivine at upper mantle conditions deforms essentially by dislocation creep with dominant 100 slip the resulting crystal preferred orientation has been used extensively to explain the strong seismic anisotropy observed down to 250 km depth the rapid decrease of anisotropy below this depth has been interpreted as marking the transition from dislocation to diffusion creep in the upper mantle but new high pressure experiments suggest that dislocation creep also dominates in the lower part of the upper mantle but with a different slip direction here we show that this high pressure dislocation creep produces crystal preferred orientations resulting in extremely low seismic anisotropy consistent with seismological observations below 250 km depth these results raise new questions about the mechanical state of the lower part of the upper mantle and its coupling with layers both above and below

plastic flow of pyrope at mantle pressure and temperature despite the abundance of garnet in deforming regions of the earth such as subduction zones its rheological properties are not well defined by laboratory measurements here we report measurements of steady state plastic properties of pyrope in its stability field temperature up to 1573 k pressure up to 6 8 gpa strain rate approx 10 5 s 1 using a deformation dia apparatus d dia coupled with synchrotron radiation synthetic pyrope py100 and natural pyrope py70alm16gr14 are both studied in a dry environment transmission electron microscopy tem investigation of the run products indicates that dislocation glide assisted by climb within grains and dynamic recrystallization for grain boundary strain accommodation is the dominant deformation process in pyrope both synthetic and natural pyropes stress and strain rate data as measured in situ by x ray diffraction and imaging are best fitted with the single flow law

mantle rheology viscomagnetic coupling at the core mantle boundary and differential rotation of the core induced by pleistocenic deglaciation summary the influence of the viscomagnetic coupling at the core mantle boundary cmb on the differential rotation of the core and non tidal acceleration induced by the pleistocenic deglaciation is investigated for stratified mantle models with steadystate or transient rheologies for a realistic modelling of the viscomagnetic coupling the time dependent viscoelastic topography at the cmb induces a westward drift of the core with respect to the mantle that can be correlated to the zonal component of the secular drift of the geomagnetic field starting from a reference model based on the maxwell rheology and a viscosity profile inferred from j2 data and non tidal acceleration with 1021 pas and 5 x 1021 pas for upper and lower mantle respectively we study first the effects of the hardening of the upper mantle in the transition zone between 420 and 670 km depth this hardening is responsible for a 30 per cent reduction in the westward drift with respect to a uniform upper mantle the impact of a viscosity decrease on the top of the lower mantle is considered next the softening of the lower mantle beneath the 670 km discontinuity counteracts the effects of the transition zone enhancing the differential rotation of the core the d layer is also implemented in order to analyse the influence of the decoupling at the bottom of the mantle responsible for faster relaxation of the topography if lower mantle viscosity is increased beyond the threshold of 10 pas we obtain a differential rotation of the core in the opposite direction or an eastward drift we also analyse the impact of a burgers rheology for the lower mantle to simulate transient effects for this rheology steady state viscosities in the lower mantle higher than pa s in agreement with estimates from long wavelength geoid anomalies and recent findings from true polar wander allow a westward drift of the core non tidal acceleration is generally less affected by rheological variations than the differential rotation changes in the non tidal acceleration induced by the various rheological models are on the other hand comparable with the error bounds in the observed values

the gowganda formation in the southern part of the huronian outcrop belt ontario canada stratigraphy depositional environments and regional tectonic significance abstract the early proterozoic huronian supergroup of the north shore of lake huron comprises a lower part that is largely of continental origin and a more widespread dominantly marine upper succession the gowganda is the lowest formation of the upper huronian in the southern part of the huronian outcrop belt it is in conformable but commonly erosional contact with the underlying serpent formation the basal part of the gowganda formation unit 1 is stratified diamictites boulder and cobble orthoconglomerates and subordinate thin turbiditic sandstones these rocks are largely resedimented but glaciogenic the basal diamictite complex is overlain by unit 2 a thick argillite formed in deep water during a period of glacial retreat unit 3 begins with a discontinuous glaciomarinite this is overlain by resedimented boulder and cobble conglomerates that pass up into sandy turbidites down to basin movements on syn sedimentary north trending normal faults led to deposition of coarsening upward wedges of eastward transported debris there is a thick stratified diamictite complex at the top of unit 3 in the eastern part of the study area it includes a 50 m thick argillite the diamictite complex is overlain by a series of coarsening upward sequences formed by prograding deltas these comprise unit 4 of the gowganda formation the lowest deltaic sequence includes a thick diamictite in the east and a more widespread thin diamictite in its upper part the glaciogenic and resedimented nature of the rocks at the base of the gowganda formation suggests that basin deepening was tectonic rather than due to eustatic sea level rise glaciation in the huronian supergroup may have been initiated by uplift of rift shoulders and propagated by influx of marine waters into the interior of the continent the regional extent of the gowganda formation its unconformable relationship with older huronian rocks in the north and its largely resedimented nature all indicate an episode of regional subsidence possibly after continental separation similar tectonic and climatic events have been postulated for early proterozoic rocks of southeastern wyoming and possibly elsewhere lending support to earlier correlations based mainly on stratigraphic similarities glaciogenic diamictites are present above the breakup unconformity in many early and late proterozoic successions suggesting a possible causative relationship

on the lehmann discontinuity the lehmann discontinuity 200km in the upper mantle is proposed to result from a change in the preferred orientation of olivine as a result of a change in deformation mechanism in the shallow upper mantle deformation is due to dislocation creep and results in an anisotropic structure whereas in the deeper upper mantle diffusion creep becomes dominant due to a smaller activation volume resulting in an isotropic structure for anisotropy due to dislocation glide corresponding to a horizontal flow the resultant anisotropy will yield vsh vsv and vph vpv thus for waves propagating nearly vertically e g scs waves there will be discontinuous jumps in the s and p wave velocities at this discontinuity in this model the depth of the lehmann discontinuity depends on the deformation conditions such as stress level geotherm water fugacity and grain size giving a natural explanation for its observed variation with tectonic setting

pleistocene deglaciation and the global gravity field the present day global geoid and free air gravity anomaly signals induced by pleistocene deglaciation have been computed along with their secular variations a revised green function has been derived for the free air gravity anomaly and we find that an earth model with a lower mantle viscosity moderately larger than the upper mantle value satisfies observations of the anomaly over both hudson s bay and fennoscandia using this preferred earth model the predicted global geoid anomaly map is characterized by peak negative values of 36 7 9 4 and 21 9 m over hudson s bay northern europe and antarctica respectively and by a smooth small amplitude 2 5 m upwarping over the major ocean basins although the field has an order of magnitude less power than the observed there is a significant spatial correlation between the two inferences of mantle viscosity based upon comparisons between the observed geoid and the computed deglaciation induced anomaly are hampered by the large contribution from the convective circulation in the earth s mantle we have subtracted a predicted convection signal based on seismic tomography from the observed field however the residual geoid thus obtained is still too inaccurate to allow a useful comparison in contrast a filtered version of the observed geoid over canada which retains harmonic coefficients in the degree range 10 22 provides a relatively uncontaminated by convection datum that is indeed satisfied by the preferred earth model secular variations in the earth s gravitational field are naturally dominated by the signal from pleistocene deglaciation in this respect we compute zonal harmonics in the expansion of the present day rate of change of the geopotential and in particular we predict amplitudes comparable to that for the previously analysed j2 for j4 j5 j7 and j9 all of which reflect in large part the response due to the melting of the massive laurentide ice sheet this analysis suggests that measurement of these higher order secular variations of the zonal harmonics based upon analysis of long time series of lageos ranging data could provide very useful additional constraints on the radial variation of mantle viscosity these constraints along with all others described in this paper are shown to be relatively insensitive to assumptions concerning the deflection during the glacial isostatic adjustment process of the 400 and 670 km density discontinuities within the mantle

deformation of olivine at subduction zone conditions determined from in situ measurements with synchrotron radiation abstract we report measurements of the deformation stress for san carlos olivine at pressures of 3 5 gpa temperatures of 25 1150 c and strain rates of 10 7 10 5 s 1 we determine a deformation stress of approximately 2 5 gpa that is relatively temperature and strain rate independent in the temperature range of 400 900 c the deformation experiments have been carried out on a deformation dia d dia apparatus sam85 at x17b2 nsls powder samples are used in these experiments enstatite mgsio 3 3 5 total quality of sample is used as the buffer to control the activity of silica ni foil is used in some experiments to buffer the oxygen fugacity water content is confirmed by ir spectra of the recovered samples samples are compressed at room temperature and are then annealed at 1200 c for at least 2 h before deformation the total plastic and elastic strains macroscopic are derived from the direct measurements of the images taken by x ray radiograph technique the differential stresses are derived from the diffraction determined elastic strains in the regime of 25 400 c there is a small decrease of stress at steady state as temperature increases in the regime of 400 c to the transition temperature the differential stress at steady state 2 5 gpa is relatively insensitive to the changes of temperature and strain rate however it drastically decreases to about 1 gpa and becomes temperature dependent above the transition temperature and thereafter the transition temperature is near 900 c above the transition temperature the flow agrees with power law creep measurements of previous investigations the anisotropy of differential stress in individual planes indicates that the deformation of olivine at low temperature is dominated by 0 0 1 1 0 0 accounting to a slower strain rate in the natural system the transition temperature for the olivine in the slab is most likely in the range of 570 660 c

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

solar variance recorded in lacustrine deposits from the devonian and proterozoic of scotland abstract continental lacustrine laminites deposited in low latitudes have been examined from the devonian and torridonian mesoproterozoic strata of northern scotland to investigate the consistency of sunspot periodicity through geological time the thicknesses of 2448 annual laminae were measured from the middle devonian c 388 ma achanarras limestone member lower caithness flagstone group and of 2821 laminae from the mesoproterozoic 1 2 ga poll a9mhuilt member of the stoer group time series analysis resolved periodicities of 13 8 and 13 9 years in the complete devonian and mesoproterozoic datasets respectively when sedimentological variance was considered and the data were examined as evolutive spectra those sections consisting of the most robust data consistently yielded periodicities between 8 6 and 13 7 years for the devonian section and 9 2 and 10 6 years for the mesoproterozoic section these correspond well to schwabe cycles evidence for the presence of the longer hale cycles 20 years was also recognized in the devonian time series the sunspot periodicities identified within the devonian and mesoproterozoic show little variance from published examples from the quaternary and eocene and therefore provide evidence for the consistency of solar processes and solar atmospheric interactions for the past 1 2 ga

kinetics of melting in peridotite from volume strain measurements abstract partial melting in the earth s mantle involves a combination of processes such as ionic diffusion and solid liquid transition these processes yield an array of characteristic times for the melting kinetics both the magnitude and time constant of melt induced volume strain affect seismic velocity attenuation and buoyancy in situ monitoring of samples during melting are enabled by multi anvil high pressure devices coupled with synchrotron x ray radiation in this study the volume strain induced in a sample from melting is measured as a function of time using x ray images we find a doubling of the effective thermal expansion for only 2 melt both from our data and from thermodynamic models of peridotite the characteristic time of melting is determined to be less than one second these findings have a direct impact on the effect of melting on seismic velocities