rayleigh wave tomography beneath intraplate volcanic ridges in the south pacific sources recorded over a 12 month period provides resolution of phase velocities at periods upto100s theaveragephasevelocitiesforthestudyarearevealapronouncedlow velocity zone reaching a minimum shear velocity of 3 95 km s the negative velocity gradient defining the base of the lithosphere observed at 40 15 km abruptly reverses at 70 km depth the underlying positive gradient changes slope at 125 km we attribute these changes in gradient to the onset of incipient partial melting of upwelling mantle in thepresenceofwaterat125km followedbyincreasedmeltproductionat70kmthatleadsto dehydration of the residual matrix and migration of melt to the surface spreading center rayleigh wave tomography shows that there are anomalously low shear velocities extending to at least 50 km depth beneath the sojourn ridge and the hotu matua volcanic complex with relatively high velocities between these volcanic chains these observations are not consistent with passive models for the origin of the volcanic ridges involving lithospheric extension or thermoelastic cracking dynamic models invoking flow in the asthenosphere in the form of small scale convection or viscous fingering instabilities may explain the observed pattern of seismic velocity anomalies

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

toroidal mantle flow through the western u s slab window the circular pattern of anisotropic fast axis orientations of split sks arrivals observed in the western u s cannot be attributed reasonably to either preexisting lithospheric fabric or to asthenospheric strain related to global scale plate motion a plume origin for this pattern accounts more successfully for the anisotropy field but little evidence exists for an active plume beneath central nevada we suggest that mantle flow around the edge of the sinking gorda juan de fuca slab is responsible for creating the observed anisotropy seismic images and kinematic reconstructions of gorda juan de fuca plate subduction have the southern edge of this plate extending from the mendocino triple junction to beneath central nevada and flow models of narrow subducted slabs produce a strong toroidal flow field around the edge of the slab consistent with the observed pattern of anisotropy this flow may enhance uplift extension and magmatism of the northern basin and range while inhibiting extension of the southern basin and range

shallow mantle anisotropy beneath the juan de fuca plate the anisotropic fabric of the oceanic mantle lithosphere is often assumed to parallel paleo relative plate motion rpm however we find evidence that this assumption is invalid beneath the juan de fuca jdf plate using travel times of seismic energy propagating through the topmost mantle we find that the fast direction of p wave propagation is rotated 18 3 counterclockwise to the paleo spreading direction and strikes between pacific jdf relative and jdf absolute plate motion apm the mean mantle velocity is 7 85 0 02 km s with 4 6 0 4 anisotropy synthesis of the plate averaged pn anisotropy signal with measurements of pn anisotropy beneath the jdf ridge and sks splits across the jdf plate suggest that the anisotropic structure of the topmost mantle continues to evolve away from the spreading center to more closely align with apm we infer that the oceanic mantle lithosphere may record the influence of both paleo rpm and paleo apm

changes in seismic anisotropy shed light on the nature of the gutenberg discontinuity the boundary between the lithosphere and asthenosphere is associated with a platewide high seismic velocity lid overlying lowered velocities consistent with thermal models seismic body waves also intermittently detect a sharp velocity reduction at similar depths the gutenberg g discontinuity which cannot be explained by temperature alone we compared an anisotropic tomography model with detections of the g to evaluate their context and relation to the lithosphere asthenosphere boundary lab we find that the g is primarily associated with vertical changes in azimuthal anisotropy and lies above a thermally controlled lab implying that the two are not equivalent interfaces the origin of the g is a result of frozen in lithospheric structures regional compositional variations of the mantle or dynamically perturbed lab

oriented grain growth and modification of frozen anisotropy in the lithospheric mantle abstract seismic anisotropy throughout the oceanic lithosphere is often assumed to be generated by fossilized texture formed during deformation at asthenospheric temperatures close to the ridge here we investigate the effect of high temperature and high pressure static annealing on the texture of previously deformed olivine aggregates to simulate residence of deformed peridotite in the lithosphere our experiments indicate that the orientation and magnitude of crystallographic preferred orientation cpo will evolve due to the preferential growth of grains with low dislocation densities these observations suggest that texture and stored elastic strain energy promote a style of grain growth that modifies the cpo of a deformed aggregate we demonstrate that these microstructural changes alter the orientation distributions and magnitudes of seismic wave velocities and anisotropy therefore static annealing may complicate the inference of past deformation kinematics from seismic anisotropy in the lithosphere

the role of oceanic transform faults in seafloor spreading a global perspective from seismic anisotropy mantle anisotropy beneath mid ocean ridges and oceanic transforms is key to our understanding of seafloor spreading and underlying dynamics of divergent plate boundaries observations are sparse however given the remoteness of the oceans and the difficulties of seismic instrumentation to overcome this we utilize the global distribution of seismicity along transform faults to measure shear wave splitting of over 550 direct s phases recorded at 56 carefully selected seismic stations worldwide applying this source side splitting technique allows for characterization of the upper mantle seismic anisotropy and therefore the pattern of mantle flow directly beneath seismically active transform faults the majority of the results 60 return nulls no splitting while the non null measurements display clear azimuthal dependency this is best simply explained by anisotropy with a near vertical symmetry axis consistent with mantle upwelling beneath oceanic transforms as suggested by numerical models it appears therefore that the long term stability of seafloor spreading may be associated with widespread mantle upwelling beneath the transforms creating warm and weak faults that localize strain to the plate boundary

radial seismic anisotropy as a constraint for upper mantle rheology abstract seismic shear waves that are polarized horizontally sh generally travel faster in the upper mantle than those that are polarized vertically sv and deformation of rocks under dislocation creep has been invoked to explain such radial anisotropy convective flow of the upper mantle may thus be constrained by modeling the textures that progressively form by lattice preferred orientation lpo of intrinsically anisotropic grains while azimuthal anisotropy has been studied in detail the radial kind has previously only been considered in semi quantitative models here we show that radial anisotropy averages as well as radial and azimuthal anomaly patterns can be explained to a large extent by mantle flow if lateral viscosity variations are taken into account we construct a geodynamic reference model which includes lpo formation based on mineral physics and flow computed using laboratory derived olivine rheology previously identified anomalous v sv regions beneath the east pacific rise and relatively fast v sh regions within the pacific basin at 150 km depth can be linked to mantle upwellings and shearing in the asthenosphere respectively continental anisotropy at shallow 50 km depth is under predicted and these deviations are in quantitative agreement with the expected signature of frozen in stochastically oriented anisotropy from past tectonic episodes we also consider two end member models of lpo formation for wet and dry conditions for the asthenosphere 150 km allowing for lateral variations in volatile content the residual signal can be much reduced and the inferred volatile patterns underneath the pacific appear related to plume activity in deeper layers 250 km anisotropy indicates that small scale convection disrupts plate scale shear underneath old oceanic lithosphere we suggest that studying deviations from comprehensive geodynamic reference models or residual anisotropy can provide new insights into the nature and dynamics of the asthenosphere

absolute plate motions relative to deep mantle plumes abstract advances in whole waveform seismic tomography have revealed the presence of broad mantle plumes rooted at the base of the earth s mantle beneath major hotspots hotspot tracks associated with these deep mantle plumes provide ideal constraints for inverting absolute plate motions as well as testing the fixed hotspot hypothesis in this paper 27 observed hotspot trends associated with 24 deep mantle plumes are used together with the morvel model for relative plate motions to determine an absolute plate motion model in terms of a maximum likelihood optimization for angular data fitting combined with an outlier data detection procedure based on statistical tests the obtained t25m model fits 25 observed trends of globally distributed hotspot tracks to the statistically required level while the other two hotspot trend data comores on somalia and iceland on eurasia are identified as outliers which are significantly incompatible with other data for most hotspots with rate data available t25m predicts plate velocities significantly lower than the observed rates of hotspot volcanic migration which cannot be fully explained by biased errors in observed rate data instead the apparent hotspot motions derived by subtracting the observed hotspot migration velocities from the t25m plate velocities exhibit a combined pattern of being opposite to plate velocities and moving towards mid ocean ridges the newly estimated net rotation of the lithosphere is statistically compatible with three recent estimates but differs significantly from 30 of 33 prior estimates

stratigraphical and morphological evidence for pingo genesis in the cerberus plains rootless debris cones or pseudocraters occur in platy patterned ground throughout the cerberus plains of mars and are thought to represent the products of explosive magma ice interaction lanagan et al 2001 geophys res lett 28 2365 2368 fagents et al 2002 in smellie j l chapman m g eds volcano ice interaction on earth and mars in geol soc spec publ vol 202 pp 295 317 requiring lava and water interspersed they are central to theories of multiple magmatic and aqueous flood events burr et al 2002 icarus 159 53 73 berman d c hartmann w k 2002 icarus 159 1 17 and widespread sheet volcanism keszthelyi et al 2000 j geophys res 105 15027 15049 in the region during the late amazonian a region reported to have been occupied by water bodies ranging from lakes to oceans scott et al 1995 map of mars showing channels and possible paleolake basins usgs miscellaneous investigations series map i 2461 1 30 000 000 the nature of the platy substrate is the subject of debate with evidence given for lava keszthelyi et al 2000 j geophys res 105 15027 15049 plescia j b 2003 icarus 164 79 95 and ice brakenridge g r 1993 lunar planet sci xxiv part 1 175 176 rice et al 2002 lunar planet sci xxxiii abstract 2026 murray et al 2005 nature 434 352 355 the superposition relationships of cones and platy deposits in the channels of the athabasca valles precludes a magmatic origin indicating later formation as permafrost mounds or pingos with implications for geologically recent flood volcanism age constraints on young surfaces and recent climate change on mars

active deformation of the gorda plate constraining deformation models with new geophysical data the gorda plate the southernmost fragment of the larger juan de fuca plate system is an example of a nonrigidly deforming tectonic accommodation zone or buffer plate absorbing deformation and allowing the surrounding larger plates to act in a more rigid fashion here we present a new structural analysis of the plate based on full plate bathy metric coverage augmented by seismic reflection data and earthquake moment tensors and locations we interpret internal deformation of the gorda plate as an asymmetrical flexural slip buckle with a vertical axis utilizing reactivation of spreading ridge fabric normal faults as strike slip faults newly formed second generation faults crosscutting the structural grain overprint the reactivated structures the spreading fabric faults finally begin a second phase of extension as the plate approaches the subduction zone this model based on fault constraints has allowed investigation of ridge plate subduction interac tions and suggests that spreading rate variations along the gorda ridge may be con trolled by internal deformation of the plate rather than the reverse as previously hypothesized

origin of azimuthal seismic anisotropy in oceanic plates and mantle abstract seismic anisotropy is ubiquitous in the earth s mantle but strongest in its thermo mechanical boundary layers azimuthal anisotropy in the oceanic lithosphere and asthenosphere can be imaged by surface waves and should be particularly straightforward to relate to well understood plate kinematics and large scale mantle flow however previous studies have come to mixed conclusions as to the depth extent of the applicability of paleo spreading and mantle flow models of anisotropy and no simple globally valid relationships exist here we show that lattice preferred orientation lpo inferred from mantle flow computations produces a plausible global background model for asthenospheric anisotropy underneath oceanic lithosphere the same is not true for absolute plate motion apm models a 200 km thick layer where the flow model lpo matches observations from tomography lies just below the 1200 c isotherm of a half space cooling model indicating strong temperature dependence of the processes that control the development of azimuthal anisotropy we infer that the depth extent of shear and hence the thickness of a relatively strong oceanic lithosphere can be mapped this way these findings for the background model and ocean basin specific deviations from the half space cooling pattern are found in all of the three recent and independent tomographic models considered further exploration of deviations from the background model may be useful for general studies of oceanic plate formation and dynamics as well as regional scale tectonic analyses

shear attenuation beneath the juan de fuca plate implications for mantle flow and dehydration abstract at mid ocean ridge spreading centers the separation of lithospheric plates induces upwelling and pressure release melting in the upper mantle many questions remain about the associated flow pattern melt distribution and the interaction of spreading centers with hotspots based on the propagation of rayleigh waves across arrays of ocean bottom seismometers we present shear wave attenuation and velocity models in the vicinity of the juan de fuca plate with the best resolution to date of any spreading center there is strong attenuation centered at depths of 70 80 km just below the expected dry solidus the top of the attenuating region is somewhat deeper than predicted for the thermal effects of passive mantle upwelling beneath the spreading center the shear velocity structure shows the lowest velocities west of the spreading center particularly near axial seamount and high velocities east of the axis extending to a greater depth than predicted by the passive flow model together these observations support a model in which buoyant upwelling and melt extraction west of the spreading center first depletes and dehydrates the mantle above the dry solidus and then the associated downwelling carries depleted melt free residual mantle downward beneath the juan de fuca plate the shear velocities and attenuation can be well matched with a model in which the dominant changes in the physical properties occur just before the onset of melting

mars global surveyor observations of martian fretted terrain the martian fretted terrain between latitudes 30 and 50 n and between 315 and 360 w has been reexamined in light of new mars orbiter camera moc and mars orbiter laser altimeter mola data from mars global surveyor much of the terrain in the 30 50 latitude belt in both hemispheres has a characteristic stippled or pitted texture at moc 1 5 m scale the texture appears to result from partial removal of a formerly smooth thin deposit as a result of sublimation and deflation a complex history of deposition and exhumation is indicated by remnants of a former thicker cover of layered deposits in some hollows and on some slopes particularly those facing the pole are smooth textured deposits outlined by an outward facing escarpment throughout the study area are numerous escarpments with debris flows at their base the escarpments typically have slopes in the 20 30 range at the base of the escarpment is commonly a deposit with striae oriented at right angles to the escarpment outside this deposit is the main debris apron with a surface that typically slopes 2 3 and complex surface textures suggestive of compression sublimation and deflation the presence of undeformed impact craters indicates that the debris flows are no longer forming fretted valleys contain lineated fill and are poorly graded they likely form from fluvial valleys that were initially like those elsewhere on the planet but were subsequently widened and filled by the same mass wasting processes that formed the debris aprons slope reversals indicate that downvalley flow of the lineated fill is minor the ubiquitous presence of breaks in slope formed by mass wasting and the complex surface textures that result from mass wasting deflation and sublimation decreases the recognizability of the shorelines formerly proposed for this area

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

branched glycerol dialkyl glycerol tetraethers in arctic lake sediments sources and implications for paleothermometry at high latitudes branched glycerol dialkyl glycerol tetraethers brgdgts are analyzed in different lakes of the mackenzie canadian arctic and kolyma siberian arctic river basins to evaluate their sources and the implications for brgdgt based paleothermometry in high latitude lakes the comparison of brgdgt distributions and concentrations in the lakes with those in river suspended particulate matter riverbank sediments and permafrost material indicates that brgdgts in arctic lake sediments have mixed sources in contrast to global observations distributional offsets between brgdgts in arctic lakes and elsewhere in the catchment are minor likely due to the extreme seasonality and short window of biological production at high latitudes consequently both soil and lake calibrated brgdgt based temperature proxies return sensible temperature estimates even though the mean air temperature mat in the arctic is below the calibration range the original soil calibrated mbt cbt methylation of branched tetraethers cyclisation of branched tetraethers proxy generates mats similar to those in the studied river basins whereas using the recently revised mbt cbt calibration overestimates mat the application of the two global lake calibrations generating summer air temperatures sat and mat respectively illustrates the influence of seasonality on the production of brgdgts in lakes as the latter overestimates actual mat whereas the sat based lake calibration accounts for this influence and consequently returns more accurate temperatures our results in principle support the application of brgdgt based temperature proxies in high latitude lakes in order to obtain long term paleotemperature records for the arctic although the calibration and associated transfer function have to be selected with care

resolving three dimensional anisotropic structure with shear wave splitting tomography summary shear wave splitting observations are a commonly used tool for inferring anisotropy and flow within the earth s interior here we present the development and validation of a new technique for imaging anisotropy in the upper mantle using local events shear wave splitting tomography swst the mantle is parametrized as a 3 d block model of crystallographic orientations with the elastic properties of olivine and orthopyroxene and both orthorhombic and hexagonal symmetries are tested to efficiently forward calculate splitting the christoffel equation is used to progressively split the horizontal components of a synthetic wavelet in each block of the model and predicted shear wave splitting parameters are obtained with an eigenvalue minimization technique numerically calculated partial derivatives are utilized in a linearized damped least squares inversion to solve for a best fitting model of crystallographic orientations to account for the non linear properties of shear wave splitting the inversion is applied iteratively and partial derivatives are recalculated after each iteration a starting model that incorporates information from predicted splitting parameters is found by spatially averaging fast directions and the ratio of observed to predicted splitting times models from inversions utilizing this average starting model reach lower misfit levels than do inversions with a random or uniform starting model modelling results using synthetic data from several anisotropic structures i e sharp lateral and vertical variations in anisotropy both within an idealized and a real nicaragua costa rica subduction zone illustrate the capabilities and limitations of swst with a station spacing of 25 km in an idealized subduction zone containing uniformly spaced events down to 225 km both the azimuth and dip of crystallographic axes are resolvable to a depth of 100 150 km and lateral heterogeneities in anisotropy on a scale of 50 km at arc and forearc distances from the trench are retrieved spatial resolution of anisotropy at scales of 75 km is possible further into the backarc above 150 km depth the geometry of stations and observed seismicity in the nicaragua costa rica subduction zone yields partial to good resolution at scales of 50 75 km beneath the forearc arc and limited regions of the backarc down to 100 km and resolution at coarser scales is possible in wider regions beneath the backarc given the distributions of seismic sources within many subduction zones and the advances in broad band seismic array deployments this new method offers a powerful means with which to constrain the orientation of anisotropic fabric in the upper mantle

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

seismic azimuthal anisotropy in the oceanic lithosphere and asthenosphere from broadband surface wave analysis of obs array records at 60 ma seafloor we analyzed seismic ambient noise and teleseismic waveforms of nine broadband ocean bottom seismometers deployed at a 60 ma seafloor in the southeastward of tahiti island the south pacific by the tomographic investigation by seafloor array experiment for the society hotspot project we first obtained one dimensional shear wave velocity model beneath the array from average phase velocities of rayleigh waves at a broadband period range of 5 200 s the obtained model shows a large velocity reduction at depths between 40 and 80 km where the lithosphere asthenosphere boundary might exist we then estimated shear wave azimuthal anisotropy at depths of 20 100 km by measuring azimuthal dependence of phase velocities of rayleigh waves the obtained model shows peak to peak intensity of the azimuthal anisotropy of 2 4 with the fastest azimuth of nw se direction both in the lithosphere and asthenosphere this result suggests that the ancient flow frozen in the lithosphere is not perpendicular to the strike of the ancient mid ocean ridge but is roughly parallel to the ancient plate motion at depths of 20 60 km the fastest azimuths in the current asthenosphere are subparallel to current plate motion at depths of 60 100 km additional shear wave splitting analysis revealed possible perturbations of flow in the mantle by the hot spot activities and implied the presence of azimuthal anisotropy in the asthenosphere down to a depth of 190 210 km

seismic evidence for rotating mantle flow around subducting slab edge associated with oceanic microplate capture tectonic plate reorganization at a subduction zone edge is a fundamental process that controls oceanic plate fragmentation and capture however the various factors responsible for these processes remain elusive we characterize seismic anisotropy of the upper mantle in the explorer region at the northern limit of the cascadia subduction zone from teleseismic shear wave splitting measurements our results show that the mantle flow field beneath the explorer slab is rotating anticlockwise from the convergence parallel motion between the juan de fuca and the north america plates re aligning itself with the transcurrent motion between the pacific and north america plates we propose that oceanic microplate fragmentation is driven by slab stretching thus reorganizing the mantle flow around the slab edge and further contributing to slab weakening and increase in buoyancy eventually leading to cessation of subduction and microplate capture

segmentation of mid ocean ridges attributed to oblique mantle divergence mantle flow beneath mid ocean ridges was thought to respond passively to plate motions analysis of seismic data from ridges reveals a skew between the directions of plate motion and mantle flow implying mantle flow may stress the plates

numerical simulations of texture development and associated rheological anisotropy in regions of complex mantle flow 1 the development of lattice preferred orientations lpo within olivine aggregates under flow in the upper mantle leads to seismic and rheological or viscoplastic anisotropies we compare predictions from different micromechanical models applying several commonly used theoretical descriptions to an upwelling flow scenario representing a typical oceanic spreading center significant differences are obtained between models in terms of lpo and associated rheology in particular in regions where the flow direction changes rapidly with superior predictions for the recently proposed second order approach this highlights the limitations of ad hoc formulations lpo induced rheological anisotropy may have a large effect on actual flow patterns with 1 2 orders of magnitude variation in directional viscosities compared to the isotropic case citation castelnau o d k blackman and t w becker 2009 numerical simulations of texture development and associated rheological anisotropy in regions of complex mantle flow geophys res lett 36 l12304

seismic observations of large scale deformation at the bottom of fast moving plates abstract we present a new tomographic model of azimuthal anisotropy in the upper mantle dr2012 and discuss in details the geodynamical causes of this anisotropy our model improves upon dkp2005 seismic model debayle et al 2005 through a larger dataset expanded by a factor 3 7 and a new approach which allows us to better extract fundamental and higher mode information our results confirm that on average azimuthal anisotropy is only significant in the uppermost 200 250 km of the upper mantle where it decreases regularly with depth we do not see a significant difference in the amplitude of anisotropy beneath fast oceanic plates slow oceanic plates or continents the anisotropy projected onto the direction of present plate motion shows a very specific relation with the plate velocity it peaks in the asthenosphere around 150 km depth it is very weak for plate velocities smaller than 3 cm yr 1 increases significantly between 3 and 5 cm yr 1 and saturates for plate velocities larger than 5 cm yr 1 plate scale present day deformation is remarkably well and uniformly recorded beneath the fastest moving plates india coco nazca australia philippine sea and pacific plates beneath slower plates plate motion parallel anisotropy is only observed locally which suggests that the mantle flow below these plates is not controlled by the lithospheric motion a minimum plate velocity of around 4 cm yr 1 is necessary for a plate to organize the flow in its underlying asthenosphere the correlation of oceanic anisotropy with the actual plate motion in the shallow lithosphere is very weak a better correlation is obtained with the fossil accretion velocity recorded by the gradient of local seafloor age the transition between frozen in and active anisotropy occurs across the typical age isotherm that defines the bottom of the thermal lithosphere around 1100 c under fast continents mostly under australia and india the present day velocity orients also the anisotropy in a depth range around 150 200 km depth which is not deeper than what is observed under oceans

reconciling mantle attenuation temperature relationships from seismology petrology and laboratory measurements seismic attenuation measurements provide a powerful tool for sampling mantle properties laboratory experiments provide calibrations at seismic frequencies and mantle temperatures for dry melt free rocks but require 102 103 extrapolations in grain size to mantle conditions also the effects of water and melt are not well understood at the same time body wave attenuation measured from dense broadband arrays provides reliable estimates of shear wave attenuation qs 1 affording an opportunity for calibration we reanalyze seismic data sets that sample arc and back arc mantle in central america the marianas and the lau basin confirming very high attenuation qs 25 80 at 1 hz and depths of 50 100 km at each of these sites independent petrological studies constrain the temperature and water content where basaltic magmas last equilibrated with the mantle 1300 1450 c the qs measurements correlate inversely with the petrologically inferred temperatures as expected however dry attenuation models predict qs too high by a factor of 1 5 5 modifying models to include effects of h2o and rheology dependent grain size shows that the effects of water enhanced dissipation and water enhanced grain growth nearly cancel so h2o effects are modest therefore high h2o in the arc source region cannot explain the low qs nor in the back arc where lavas show modest water content most likely the high attenuation reflects the presence of melt and some models of melt effects come close to reproducing observations overall body wave qs can be reconciled with petrologic and laboratory inferences of mantle conditions if melt has a strong influence beneath arcs and back arcs

seismic structure of the upper mantle in a central pacific corridor the seismic structure of the tonga hawaii corridor has been investigated by combining two data sets revenaugh and jordan s reflectivity profile from scs reverberations which provides travel times to and impedance contrasts across the major mantle discontinuities and 1500 new observations of frequency dependent phase delays for the three component s ss and sss body waves and the r1 and g1 surface waves which constrain the velocity structure within this layered framework the shear waves turning in the upper mantle showed significant splitting of the sh and sv components indicative of shallow polarization anisotropy the data set was inverted in conjunction with attenuation and mineralogical constraints to obtain a complete spherically symmetric radially anisotropic structure the final model pa5 is characterized by a high velocity anisotropic lid bounded at 68 km depth by a large negative g discontinuity a low velocity anisotropic layer below g extending to a small l discontinuity at 166 km an isotropic steep gradient region between 166 km and 415 km and transition zone discontinuities at 415 507 and 651 km the depth of the radial anisotropy in pa5 is shallower than in most previous studies based on surface waves and higher modes the average value of radial shear anisotropy in the lid 3 7 is consistent with the magnitude expected from the spreading controlled models of olivine orientation while anisotropy in the low velocity zone which is required by our data set could be induced either by paleostrains that took place near the ridge crest or by shearing in the asthenosphere as a result of present day plate motions on the basis of recent work by hirth and kohlstedt we suggest that the g discontinuity is caused by a rapid increase in the water content of mantle minerals with depth marking the fossilized lower boundary of the melt separation zone active during crust formation the high gradient zone between 200 and 400 km is a characteristic feature of convecting oceanic upper mantle and is probably controlled by a steady decrease in the homologous temperature over this depth interval the average shear velocity gradient in the transition zone is lower than in most previous seismic models in better agreement with the predictions for a pyrolitic composition

timescales for the evolution of seismic anisotropy in mantle flow 1 we study systematically the relationship between olivine lattice preferred orientation and the mantle flow field that produces it using the plastic flow recrystallization model of kaminski and ribe 2001 in this model a polycrystal responds to an imposed deformation rate tensor by simultaneous intracrystalline slip and dynamic recrystallization by nucleation and grain boundary migration numerical solutions for the mean orientation of the a axes of an initially isotropic aggregate deformed uniformly with a characteristic strain rate show that the lattice preferred orientation evolves in three stages 1 for small times recrystallization is not yet active and the average a axis follows the long axis of the finite strain ellipsoid 2 for intermediate times the fabric is controlled by grain boundary migration and the average a axis rotates toward the orientation corresponding to the maximum resolved shear stress on the softest slip system 3 for the fabric is controlled by plastic deformation and average a axis rotates toward the orientation of the long axis of the finite strain ellipsoid corresponding to an infinite deformation the infinite strain axis in more realistic nonuniform flows lattice preferred orientation evolution depends on a dimensionless grain orientation lag parameter x defined locally as the ratio of the intrinsic lattice preferred orientation adjustment timescale to the timescale for changes of the infinite strain axis along path lines in the flow explicit numerical calculation of the lattice preferred orientation evolution in simple fluid dynamical models for ridges and for plume ridge interaction shows that the average a axis aligns with the flow direction only in those parts of the flow field where 1 calculation of provides a simple way to evaluate the likely distribution of lattice preferred orientation in a candidate flow field at low numerical cost

mantle flow geometry from ridge to trench beneath the gorda juan de fuca plate system shallow mantle flow could be induced by the motions of overriding tectonic plates or by deeper mantle convection analysis of mantle flow patterns in the pacific northwest shows that flow aligns with the motions of the largest oceanic plates

ridge asymmetry and deep aqueous alteration at the trench observed from rayleigh wave tomography of the juan de fuca plate using rayleigh wave tomography of noise removed ocean bottom seismometer obs data from the cascadia initiative we illuminate the structure of the upper mantle beneath the juan de fuca plate beneath the juan de fuca ridge there is strong asymmetry with a pronounced low velocity zone in the 25 65 km depth range extending to the west from the spreading axis this anomaly has velocities low enough to indicate the presence of melt the asymmetry in velocity structure and the much greater abundance of seamounts on the west flank of the ridge suggest that dynamic buoyant upwelling is important perhaps triggered by thermal or compositional anomalies beneath axial seamount in contrast there is no evidence for asymmetry in the axial zone or lower than expected velocities beneath the gorda ridge on the eastern flank of the juan de fuca ridge the shear velocity in the 25 65 depth range is higher than expected the lithosphere appears to be colder and thicker than predicted by standard plate cooling models perhaps caused by the downwelling counterpart of the upwelling on the west side of the ridge close to the trench there is a sharp decrease in shear velocity we interpret this as aqueous alteration caused by hydrothermal circulation through deep normal faults associated with bending of the plate beneath the astoria and nitinat fans where abyssal plain sediment is thickest the velocity decrease is much smaller which is consistent with a thick sediment cap that prevents hydrothermal alteration of the plate

a surface wave analysis of seismic anisotropy beneath eastern north america summary the nature of upper mantle seismic anisotropy beneath central and eastern north america has been evaluated using the phase velocities of surface waves traversing the length of the missouri to massachusetts moma broad band array frequency dependent phase delays of fundamental mode love and rayleigh waves measured across the array using a cross correlation procedure require the presence of anisotropy in the upper mantle 2 d radially anisotropic structures obtained via linearized inversion of these data contain shear anisotropy with a magnitude of 3 per cent between the moho and at least 180 km depth models in which the mantle lithosphere is isotropic cannot satisfy the data this anisotropy is approximately constant across the tectonic transition from the craton to the atlantic margin forward models consisting of a shallow lithospheric layer of azimuthal anisotropy derived from previous shear wave splitting observations fail to satisfy the surface wave delay times the combined surface wave and splitting results suggest the presence of two layers of anisotropy a lithospheric layer that produces the phase delay differences observed in love and rayleigh waves but is transparent to vertically propagating shear body waves and a deeper presumably asthenospheric layer that generates the shear wave splitting one plausible model is that the lithosphere is characterized by vertically heterogeneous anisotropic fabric which would produce minimal splitting in vertical shear waves such fabric has been hypothesized for regions of weak and complex splitting such as south africa and australia the results here imply that it may be appropriate for north america as well

sharad sounding radar on the mars reconnaissance orbiter 1 sharad shallow radar is a sounding radar provided by agenzia spaziale italiana asi as a facility instrument on the mars reconnaissance orbiter mission its 20 mhz center frequency and 10 mhz bandwidth complement the lower frequency relatively narrower bandwidth capability of the marsis sounding radar a joint italian u s team has guided the experiment development and is responsible for data analysis and interpretation the radar transmits signals at a 700 hz pulse repetition frequency prf and collects reflections from both the surface and near subsurface of mars vertical and horizontal resolutions are respectively 15 m free space and 3 6 km cross track by 0 3 1 km along track the scientific objective of sharad is to map in selected locales dielectric interfaces to at least several hundred meters depth in the martian subsurface and to interpret these results in terms of the occurrence and distribution of expected materials including competent rock soil water and ice a signal to noise ratio of 50 db for a specular surface return is achieved with 10 w of radiated power by using range and azimuth focusing in ground data processing preprocessed data as well as range and azimuth focused data will be formatted according to planetary data system pds standards and be made available from the asi science data center asdc and from the geosciences node of the planetary data system pds important targets for sharad include the polar layered deposits sedimentary stacks especially in terra meridiani buried channel systems buried impact craters volcanic complexes and shallow ice deposits in equilibrium with the atmosphere

western u s seismic anisotropy revealing complex mantle dynamics abstract the origin of the complex pattern of sks splitting over the western united states u s remains a long lasting debate where a model that simultaneously matches the various sks features is still lacking here we present a series of quantitative geodynamic models with data assimilation that systematically evaluate the influence of different lithospheric and mantle structures on mantle flow and seismic anisotropy these tests reveal a configuration of mantle deformation more complex than ever envisioned before in particular we find that both lithospheric thickness variations and toroidal flows around the juan de fuca slab modulate flow locally but their co existence enhances large scale mantle deformation below the western u s the ancient farallon slab below the east coast pulls the western u s upper mantle eastward spanning the regionally extensive circular pattern of sks splitting the prominent e w oriented anisotropy pattern within the pacific northwest reflects the existence of sustaining eastward intrusion of the hot pacific oceanic mantle to beneath the continental interior from within slab tears below oregon to under the snake river plain and the yellowstone caldera this work provides an independent support to the formation of intra plate volcanism due to intruding shallow hot mantle instead of a rising mantle plume

thermal structure radial anisotropy and dynamics of oceanic boundary layers defining the oceanic lithosphere as a thermal boundary layer allows to explain to first order age dependent bathymetry and isotropic wave speeds in contrast ss precursors and receiver functions suggest a subhorizontal interface within this layer on top of a radially anisotropic zone comparing a suite of geodynamic scenarios against surface wave dispersion data and seismic discontinuities we find that only weak age dependency of the radially anisotropic zone is compatible with observations we show that this zone is confined from below by a second weaker seismic interface while observed azimuthal anisotropy is consistent with lattice preferred orientation of olivine due to asthenospheric flow underneath the lithosphere radial anisotropy requires additional contributions perhaps from petrological fabrics or melt ponding this implies that seismic reflectors previously associated with the base of the lithosphere are instead associated with preserved structures embedded in it they carry information about plate formation but have little control on plate deformation

tomography reveals buoyant asthenosphere accumulating beneath the juan de fuca plate the boundary between earth s strong lithospheric plates and the underlying mantle asthenosphere corresponds to an abrupt seismic velocity decrease and electrical conductivity increase with depth perhaps indicating a thin weak layer that may strongly influence plate motion dynamics the behavior of such a layer at subduction zones remains unexplored we present a tomographic model derived from on and offshore seismic experiments that reveals a strong low velocity feature beneath the subducting juan de fuca slab along the entire cascadia subduction zone through simple geodynamic arguments we propose that this low velocity feature is the accumulation of material from a thin weak buoyant layer present beneath the entire oceanic lithosphere the presence of this feature could have major implications for our understanding of the asthenosphere and subduction zone dynamics

observations of martian gullies and constraints on potential formation mechanisms ii the northern hemisphere abstract the formation process es responsible for creating the observed geologically recent gully features on mars has remained the subject of intense debate since their discovery we present new data and analysis of northern hemisphere gullies from mars global surveyor data which is used to test the various proposed mechanisms of gully formation we located 137 mars orbiter camera moc images in the northern hemisphere that contain clear evidence of gully landforms and analyzed these images in combination with mars orbiter laser altimeter mola and thermal emission spectrometer tes data to provide quantitative measurements of numerous gully characteristics parameters we measured include apparent source depth and distribution vertical and horizontal dimensions slopes orientations and present day characteristics that affect local ground temperatures northern hemisphere gullies are clustered in arcadia planitia tempe terra acidalia planitia and utopia planitia these gullies form in craters 84 knobby terrain 4 valleys 3 other unknown terrains 9 and are found on all slope orientations although the majority of gullies are equator facing most gullies 63 are associated with competent rock strata 26 are not associated with strata and 11 are ambiguous assuming thermal conductivities derived from tes measurements as well as modeled surface temperatures we find that 95 of the gully alcove bases with adequate data coverage lie at depths where subsurface temperatures are greater than 273 k and 5 of the alcove bases lie within the solid water regime the average alcove length is 470 m and the average channel length is 690 m based on a comparison of measured gully features with predictions from the various models of gully formation we find that models involving carbon dioxide melting ground ice in the upper few meters of the soil dry landslide and surface snowmelt are the least likely to describe the formation of the martian gullies although some discrepancies still exist between prediction and observation the shallow and deep aquifer models remain as the most plausible theories interior processes involving subsurface fluid sources are generally favored over exogenic processes such as wind and snowfall for explaining the origin of the martian gullies these findings gleaned from the northern hemisphere data are in general agreement with analyses of gullies in the southern hemisphere heldmann j l mellon m t 2004 icarus 168 285 304

mantle anisotropy beneath the earth s mid ocean ridges observations of seismic anisotropy at oceanic spreading centres offer insights into mid ocean ridge processes and the formation of new plates here remote observations of seismic anisotropy beneath mid ocean ridges are made using measurements of source side shear wave splitting over 100 high quality measurements are made using earthquakes that occur near mid ocean ridges and transform faults but are observed at teleseismic distances in general for off axis ridge events the polarisation of fast shear waves phi prime prime is approximately parallel to the spreading direction nearer the ridge lesssim 50 km phi prime prime becomes more scattered and is often ridge parallel delay times delta t tend to increase from 1 s near the ridge axis to sim 3 s further away slow spreading regions gakkel and southwest indian ridges show smaller amounts of splitting than faster spreading centres at transform zones the pattern is more complex coverage beneath the east pacific rise is especially good and we observe a systematic increase in delay times in s wave splitting measurements compared to previous sks splitting observations made at ocean bottom seismometers one compatible explanation is the presence of horizontally aligned connected layers of melt at depth with an inclusion density which increases near the top of the layer this is also compatible with other observations of the lab discontinuity and surface wave derived measurements of radial anisotropy

confidence intervals for motion and deformation of the juan de fuca plate finite rotation parameters for 17 isochrons from anomalies 1 4a for the juan de fuca pacific plate pair are reported comparison of confidence intervals for plate rotations derived by different techniques indicates that a single plate misfit criterion significantly underestimates the confidence region defining the confidence interval by a threshold level of misfit determined by an f statistic gives similar results to the technique of bootstrap resampling for internally consistent data bootstrap resampling offers far greater robustness in the case of inconsistent data precision of about 1 km in the direction normal to isochrons is available for the younger anomalies this precision allows confident recognition of 2 5 km of eastward deformation of the pacific plate 50 100 km south of the active sovanco transform slight deformation of the northern juan de fuca plate adjacent to the nootka fault zone is also apparent east of the northern gorda ridge younger 3 ma anomalies do not show measurable deformation near the blanco fracture zone but older anomalies are up to 30 km east of their position predicted by rigid behavior of the juan de fuca plate reconstruction of the overlap geometry of past propagators shows no evidence that the extent of overlap has ever exceeded 70 km even with offset of 75 km in one case and 140 km in another new techniques for testing for intervals of constant plate motion require at least two changes in instantaneous motion at 99 confidence a clockwise change in motion direction at about 5 ma and an increase in the gradient of spreading rate since 2 ma two other changes in gradient since 5 ma are required at 95 confidence

three dimensional seismic structure of the mid atlantic ridge 35 n evidence for focused melt supply and lower crustal dike injection 1 we gathered seismic refraction and wide angle reflection data from several active source experiments that occurred along the mid atlantic ridge near 35 n and constructed three dimensional anisotropic tomographic images of the crust and upper mantle velocity structure and crustal thickness the tomographic images reveal anomalously thick crust 8 9 km and a low velocity bull s eye from 4 to 10 km depth beneath the center of the ridge segment the velocity anomaly is indicative of high temperatures and a small amount of melt up to 5 and likely represents the current magma plumbing system for melts ascending from the mantle in addition at the segment center seismic anisotropy in the lower crust indicates that the crust is composed of partially molten dikes that are surrounded by regions of hot rock with little or no melt fraction our results indicate that mantle melts are focused at mantle depths to the segment center and that melt is delivered to the crust via dikes in the lower crust our results also indicate that the segment ends are colder receive a reduced magma supply and undergo significantly greater tectonic stretching than the segment center

deformation of the so called gorda plate a new compilation of magnetic anomaly identifications recent side scan sonar data and other data provide constraints on the mechanism of deformation of the area commonly known as the gorda plate a sharp boundary between the rigid juan de fuca plate and deformed material trends southeast from the northern part of the gorda ridge in material older than 2 3 ma the boundary is gradational and trends more to the east previous observations that the anomalies east of the ridge are shorter than their counterparts on the pacific plate are confirmed there is no evidence for subduction or obduction along the mendocino transform adjacent to the ridge the deformation is interpreted to be distributed nw se right lateral simple shear possibly controlled by ductile deformation of the lower crust deformation of this sense is capable of producing the observed shortening the anomalies away from the ridge apparently in crust of age 2 3 ma there must be a transition to deformation on observed ne sw left lateral faults parallel to the original spreading fabric

inferring upper mantle flow from seismic anisotropy an experimental perspective abstract patterns of mantle flow are most directly inferred from observations of seismic anisotropy which is mainly caused by the crystallographic preferred orientation cpo of olivine the most abundant mineral in the upper mantle cpo is generated by high temperature ductile deformation which often yields predictable relationships between the elastic or seismic properties of a material and the kinematics of flow over the last 15 years there has been a wealth of new data describing the how olivine cpo forms and evolves as a function of deformation conditions and strain magnitude in this review we explore the relationships between deformation the evolution of cpo and the development of seismic anisotropy from the perspective of experimental rock mechanics we first review the experimental basis for the study of olivine cpo evolution from the formative studies in the early nineteen sixties through recent advances we then review some emerging complications to the study of cpo evolution such as the long lived transient cpos that arise from changes in deformation kinematics mechanisms and conditions finally we discuss the origins of seismic anisotropy and the challenges of interpreting seismic anisotropy in terms of mantle flow

savani a variable resolution whole mantle model of anisotropic shear velocity variations based on multiple data sets we present a tomographic model of radially anisotropic shear velocity variations in the earth s mantle based on a new compilation of previously published data sets and a variable block parameterization adapted to local raypath density we employ ray theoretical sensitivity functions to relate surface wave and body wave data with radially anisotropic velocity perturbations our database includes surface wave phase delays from fundamental modes up to the sixth overtone measured at periods between 25 and 350 s as well as cross correlation traveltimes of major body wave phases before inversion we apply crustal corrections using the crustal model crust2 0 and we account for azimuthal anisotropy in the upper mantle using ray theoretical corrections based on a global model of azimuthal anisotropy while being well correlated with earlier models at long spatial wavelength our preferred solution savani additionally delineates a number of previously unidentified structures due to its improved resolution in areas of dense coverage this is because the density of the inverse grid ranges between 1 25 in well sampled and 5 in poorly sampled regions allowing us to resolve regional structure better than it is typically the case in global s wave tomography our model highlights i a distinct ocean continent anisotropic signature in the uppermost mantle ii an oceanic peak in above average 1 which is shallower than in previous models and thus in better agreement with estimates of lithosphere thickness and iii a long wavelength pattern of 1 associated with the large low shear velocity provinces in the lowermost mantle

seismic anisotropy in the upper mantle 2 predictions for current plate boundary flow models 1 the anisotropic seismic structure due to flow induced mineral alignment is investigated for a series of models designed to simulate deformation in the upper mantle within a few hundred kilometers of a plate boundary the orientation distributions of olivine enstatite aggregates evolve along streamlines of each flow model based on each grains plastic response to the local stress strain field the effective elastic tensor for these textured aggregates provides predictions of p wave anisotropy and shear wave splitting throughout the model space p and s travel time delay patterns and fast shear wave polarization angles are found to vary significantly with incidence angle for a given model comparison of predicted fast p direction for our method versus a finite strain based estimate shows that agreement is acceptable for much of the model space but notable differences occur in regions up to several tens of kilometers in size two dimensional models of spreading center flow are presented for slow and fast rates and for several cases in which the ridge migrates over the deeper mantle the effect of flow in the third dimension is addressed in a few calculations for one comparison of flow in the mantle wedge at a subduction zone the introduction of trench parallel flow causes significant changes in the predicted patterns of p wave anisotropy magnitude more than orientation and sks splitting

age independent seismic anisotropy under oceanic plates explained by strain history in the asthenosphere the depth of the oceanic lithosphere asthenosphere boundary lab as inferred from shear wave velocities increases with lithospheric age in agreement with models of cooling oceanic lithosphere on the other hand the distribution of radial anisotropy under oceanic plates is almost age independent in particular radial anisotropy shows a maximum positive gradient at a depth of 70 km which if used as a proxy indicates an age independent lab depth these contrasting observations have fueled a controversy on the seismological signature of the lab to better understand the discrepancy between these observations we model the development of lattice preferred orientation lpo in upper mantle crystal aggregates and predict the seismic anisotropy produced by plate driven mid ocean ridge flows the model accounts for the progressive cooling of the lithosphere with age and can incorporate both diffusion and dislocation creep deformation mechanisms we find that an age independent distribution of radial anisotropy is the natural consequence of these simple flows the depth and strength of anisotropy is further controlled by the deformation regime dislocation or diffusion creep experienced by crystals during their ascent towards and subsequent motion away from the ridge axis comparison to surface wave tomography models yield constraints on rheological parameters such as the activation volume although not excluded additional mechanisms proposed to explain some geophysical signatures of the lab such as the presence of partial melt or changes in water content are not required to explain the radial anisotropy proxy our prediction that the age independent radial anisotropy proxy marks the transition to flow induced asthenospheric anisotropy provides a way to reconcile thermal mechanical and seismological views of the lab

an assessment of evidence for pingos on mars using hirise pingos are small hills with cores of ice formed by injection and freezing of pressurized water the possibility of pingos on mars is of particular interest because of the associated implications for liquid water we have systematically searched for candidate pingos using images from the high resolution imaging science experiment hirise camera since pingos are expected to develop surface fractures due to extension of the frozen ground over the ice core we have searched for fractured features and identified a variety of mounds these features are confined to the martian mid latitudes in the bands where gullies are also most common the observed fractured mounds have a variety of morphologies and are likely of multiple origins isolated fractured mounds found on the floors of gullied craters in the southern hemisphere match the general morphologic characteristics of terrestrial pingos and are the best candidates for martian pingos but there is currently no direct evidence for presence of ice cores and it is difficult to produce the necessary water volumes so these features should still be interpreted with caution other fractured mounds appear more likely to be erosional remnants of an unusual mantling layer or possibly thermokarst structures flat topped mounds in utopia have some characteristics fracture pattern and latitudinal distribution consistent with pingos but differ in other aspects such as shape and setting while we do not rule out a pingo origin we prefer an erosional model for these enigmatic features

development of shape and lattice preferred orientations application to the seismic anisotropy of the lower crust abstract we review the physical basis of the development of fabrics in plastic and viscous flow and illustrate the typical fabrics formed by these processes in the main rock forming silicates of the lower crust feldspar quartz pyroxene and amphibole the orientation process in plastic deformation where a single slip is dominant is recalled and the role of the constraint of neighbouring grains is emphasized the fabric development of anisometric crystals in viscous flow is discussed as a function of the main controlling parameters shear strain aspect ratio and interference between crystals the same sense of fabric asymmetry is introduced by plastic and viscous flow between the flow plane and the shape preferred orientation and hence coherent kinematic analysis can be undertaken in both modes of flow in order to assess the role of such fabrics in the seismic laminations of the lower continental crust we have calculated the seismic p wave properties of typical fabrics for hypothetical monomineralic and polymineralic rocks the calculations show that the strongest anisotropies develop in monomineralic rock with values between 5 and 16 compared with 5 and 8 for typical rock compositions the strongest anisotropies for layered monomineralic rocks generated by fabrics is only 6 compared to the 14 suggested by model studies of the observed seismic laminations we suggest that other effects such as compositional layering and or constructive interference of seismic waves are responsible for augmenting the apparent anisotropy

anisotropy beneath a highly extended continental rift we have employed shear wave splitting techniques to image anisotropy beneath the d entrecasteaux islands in southeastern papua new guinea our results provide a detailed picture of the extending continent that lies immediately ahead of a propagating mid ocean ridge tip we image the transition from continental to oceanic extension a dense shear wave splitting data set from a 2010 to 2011 passive source seismic deployment is analyzed using single and multichannel methods splitting delay times of 1 1 5 s are observed and fast axes of anisotropy trending n s parallel to rifting direction predominate the results this trend is linked to lattice preferred orientation of olivine primarily in the shallow convecting mantle driven by up to 200 km of n s continental extension ahead of the westward propagating woodlark rift this pattern differs from several other continental rifts that evince rift strike parallel fast axes and is evident despite the complex recent tectonic history we contend that across most of this rift the unusually high rate and magnitude of extension has been sufficient to produce a regime change to a mid ocean ridge like mantle fabric stations in the south of our array show more complex splitting that might be related to melt or to complex inherited structure at the edge of the extended region

modeling olivine cpo evolution with complex deformation histories implications for the interpretation of seismic anisotropy in the mantle relating seismic anisotropy to mantle flow requires detailed understanding of the development and evolution of olivine crystallographic preferred orientation cpo recent experimental and field studies have shown that olivine cpo evolution depends strongly on the integrated deformation history which may lead to differences in how the corresponding seismic anisotropy should be interpreted in this study two widely used numerical models for cpo evolution d rex and vpsc are evaluated to further examine the effect of deformation history on olivine texture and seismic anisotropy building on previous experimental work models are initiated with several different cpos to simulate unique deformation histories significantly models initiated with a preexisting cpo evolve differently than the cpos generated without preexisting texture moreover the cpo in each model evolves differently as a function of strain numerical simulations are compared to laboratory experiments by boneh and skemer 2014 in general the d rex and vpsc models are able to reproduce the experimentally observed cpos although the models significantly over estimate the strength of the cpo and in some instances produce different cpo from what is observed experimentally based on comparison with experiments recommended parameters for d rex are m 10 5 and 0 3 and for vpsc 10 100 numerical modeling confirms that cpo evolution in olivine is highly sensitive to the details of the initial cpo even at strains greater than 2 these observations imply that there is a long transient interval of cpo realignment which must be considered carefully in the modeling or interpretation of seismic anisotropy in complex tectonic settings

mantle dynamics and seismic anisotropy abstract observations of seismic anisotropy yield some of the most direct constraints available on both past and present day deformation in the earth s mantle insight into the character of mantle flow can also be gained from the geodynamical modeling of mantle processes on both global and regional scales we highlight recent progress toward understanding mantle flow from both observations and modeling and discuss outstanding problems and avenues for progress particularly in the integration of seismological and geodynamical constraints to understand seismic anisotropy and the deformation that produces it to first order the predictions of upper mantle anisotropy made by global mantle circulation models match seismological observations well beneath the ocean basins but the fit is poorer in regions of greater tectonic complexity such as beneath continental interiors and within subduction systems in many regions of the upper mantle models of anisotropy derived from surface waves are seemingly inconsistent with shear wave splitting observations which suggests that our understanding of complex anisotropic regions remains incomplete observations of anisotropy in the d layer hold promise for improving our understanding of dynamic processes in the deep earth but much progress remains to be made in characterizing anisotropic structure and relating it to the geometry of flow geochemical heterogeneity or phase transitions major outstanding problems related to understanding mantle anisotropy remain particularly regarding the deformation and evolution of continents the nature of the asthenosphere subduction zone geodynamics and the thermo chemical state of the lowermost mantle however we expect that new seismological deployments and closer integration of observations with geodynamical models will yield rapid progress in these areas

identifying and removing tilt noise from low frequency 0 1 hz seafloor vertical seismic data low frequency 0 1 hz vertical component seismic noise can be reduced by 25 db or more at seafloor seismic stations by subtracting the coherent signals derived from 1 horizontal seismic observations associated with tilt noise and 2 pressure measurements related to infragravity waves the reduction in effective noise levels is largest for the poorest stations sites with soft sediments high currents shallow water or a poorly leveled seismometer the importance of precise leveling is evident in our measurements low frequency background vertical seismic spectra measured on a seafloor seismometer leveled to within 1 10 4 radians 0 006 degrees are up to 20 db quieter than on a nearby seismometer leveled to within 3 10 3 radians 0 2 degrees the noise on the less precisely leveled sensor increases with decreasing frequency and is correlated with ocean tides indicating that it is caused by tilting due to seafloor currents flowing across the instrument at low frequencies this tilting generates a seismic signal by changing the gravitational attraction on the geophones as they rotate with respect to the earth s gravitational field the effect is much stronger on the horizontal components than on the vertical allowing significant reduction in vertical component noise by subtracting the coherent horizontal component noise this technique reduces the low frequency vertical noise on the less precisely leveled seismometer to below the noise level on the precisely leveled seismometer the same technique can also be used to remove background noise due to the seafloor pressure field up to 25 db noise reduction near 0 02 hz and possibly due to other parameters such as temperature variations

reinterpretation of pingos in antarctica abstract in 1983 pingos were reported in the vestfold hills antarctica the features form dome shaped hills up to 4 m high and 12 m in diameter and occur on an ice cored moraine their form structure and sedimentary processes observed on the surface of the moraine together with a consideration of the environmental requirements for pingo growth preclude their origin as pingos they appear to be residual landforms that have formed on the surface of the moraine as the ice core has been destroyed by ablation and thermal erosion the previous interpretation of the features has relied on the shape of the hills and their occurrence in a similar geologic setting to pingos in the northern hemisphere the interpretation appears to have misinterpreted tension cracks and coherent slumping of sediment as dilation cracks and as evidence for the extrusion of a central sediment plug pingos on supraglacial debris have not been described from any other locations and it seems that their formation is inconsistent with the ablation of the ice core of moraines

the effect of deformation history on the evolution of olivine cpo abstract olivine crystallographic preferred orientation cpo is the primary cause of seismic anisotropy in the upper mantle in tectonic environments with complex flow patterns for example corner flow near mid ocean ridges or subducting slabs the interpretation of seismic anisotropy may be complicated by evolving thermochemical deformation conditions and the integrated deformation history to understand how deformation history influences cpo evolution deformation experiments were conducted on samples of aheim dunite which has a strong pre existing texture experiments were performed in a triaxial geometry using a griggs apparatus at p 1 gpa t 1473 k up to a maximum strain of 0 7 to simulate different deformation histories samples were deformed in three different configurations with the pre existing foliation perpendicular oblique and parallel to the shortening axis of deformation distinct patterns of cpo development are observed for each experimental configuration likewise texture strength symmetry and orientation evolved differently in each set of experiments these data are interpreted as evidence that cpo did not reach steady state and that achieving steady state texture requires larger strains than previously thought it is concluded that the integrated deformation history plays a significant role in cpo evolution and the consequent interpretation of seismic anisotropy in earth s mantle