interaction among alluvial cover bed roughness and incision rate in purely bedrock and alluvial bedrock channel a major control on bedrock incision is the interaction between alluvial cover and erosive mobile grains the extent of alluvial cover is typically predicted as a function of relative sediment flux sediment supply rate over bed load transport capacity qbs qbc yet little is known about how the bed roughness affects the alluvial cover we performed field experiments with various flow discharges sediment supply rates grain sizes and bed surface topographies we then developed physically based models for estimating the threshold of sediment movement and the extent of alluvial cover so as to include the effect of roughness change the results for the threshold of sediment movement and the extent of alluvial cover obtained from our models show reasonable agreement with the results of the field experiments we explored the sensitivity of the models to variations in sediment supply and bedrock relative roughness bedrock hydraulic roughness height over grain size ksb d the results suggest the following 1 a larger relative roughness yields a greater dimensionless critical shear stress required for initial sediment motion 2 at a given sediment supply rate the extent of alluvial cover is larger when the relative roughness is larger 3 when the sediment supply rate and the relative roughness are small throughput bed load moves over and can abrade a purely bedrock channel with no alluvial cover and 4 the critical value of sediment supply rate below which throughput bed load transport occurs increases with decreasing relative roughness the experimental results and analysis provide a framework for treating the a incisional morphodynamics of purely bedrock rivers by throughput bed load with no alluvial cover b incisional alluvial morphodynamics of mixed bedrock alluvial rivers and c purely alluvial morphodynamics as well as the transition between these states

influence of subglacial drainage system evolution on glacier surface motion haut glacier d arolla switzerland 1 the relationship between the evolution of subglacial drainage system morphology and spatial patterns of glacier surface velocity was investigated using dye tracing experiments and ground surveying throughout the 1995 melt season at haut glacier d arolla switzerland with the onset of high and variable melt season discharges subglacial drainage changed from a predominantly distributed system to a predominantly channelized system the change occurred later farther up glacier during the period of drainage evolution the glacier was subjected to three periods of rapidly rising meltwater discharge the magnitude and spatial pattern of the glacier s velocity response differed between these periods and can be explained in terms of the impact of the evolving drainage system morphology on the amplitude and spatial distribution of basal hydrological forcing increasing discharge through a distributed drainage system caused widespread basal forcing and high glacier velocity increasing discharge through incipient channels below moulins not yet connected to the main channel caused more localized basal forcing and slightly increased glacier velocity increasing discharge through a fully channelized drainage system caused no significant basal forcing and glacier velocity was not significantly different from the annual deformation flow pattern empirical orthogonal function analysis of flow patterns defined two distinct spatial modes of surface velocity which corresponded closely with the drainage system morphologies inferred to be present during each event the relative importance of these modes changed through the melt season suggesting a temporal change in the spatial pattern of hydrologically induced basal forcing

a recipe for microcontinent formation accreted slivers of continental margins are common in the geologic record but the processes that lead to their formation are poorly understood we observe an association of plume related microcontinent isolation and subsequent long term asymmetries in oceanic crustal accretion based on four recent examples the seychelles in the indian ocean jan mayen in the norwegian greenland sea and the east tasman plateau and the gilbert seamount complex in the tasman sea these microcontinents formed by rerifting of a young continental margin 25 m y old in the vicinity of a mantle plume stem followed by asymmetric seafloor spreading two dimensional numerical stochastic basin modeling suggests that a yield strength minimum along the landward edge of a rifted margin thermally enhanced by heating from a mantle plume may cause a spreading ridge to jump onto this zone of weakness this action isolates a passive margin segment the association of large igneous provinces and microcontinents should be useful for identifying similar events in the geological record

upper mantle anisotropy beneath the african iris and geoscope stations summary upper mantle anisotropy beneath the african iris and geoscope stations is investigated through the measurements of splitting of teleseismic shear waves such as sks skks and pks phases seismic anisotropy data are interesting on their own as a measure of upper mantle active or frozen deformation beneath a given station but each station is of potential interest since it can be used to retrieve source side seismic anisotropy at remote sites if one is able to perform station side anisotropy correction we performed systematic investigations of teleseismic shear wave splitting at 15 stations from the iris and geoscope global seismic networks which are located on both the oceanic and the continental parts of the african plate anisotropy is generally well observed at continental stations the patterns we present generally show much more complexity than the results previously published from smaller data sets despite this complexity the splitting parameters generally appear in several places to contain a signature of the regional geodynamic setting rift structures archaean craton panafrican belt although a deeper source of anisotropy asthenospheric may be present at the oceanic stations anisotropy measurements are much more difficult to perform because the signal is generally of poor quality msey in the seychelles indian ocean is the exception and displays a clear correlation of the azimuth of the fast split shear wave with the trend of the absolute plate motion as defined by hotspot tracks

detection of upper mantle flow associated with the african superplume a continental scale low seismic velocity anomaly in the mid to lower mantle beneath africa is a robust feature of global tomographic models assuming the low velocities are associated with warm less dense material the african seismic anomaly has been ascribed to a long lived thermal upwelling from the lower mantle such a large scale upwelling should also affect the regional horizontal flow field in the upper mantle to test this model we compare seismic anisotropy inferred from shear wave splitting measurements with instantaneous flow calculations that incorporate mantle density structure inferred from seismic tomography we calculate splitting parameters at 13 ocean island stations surrounding africa splitting measurements from island stations are ideal for interpreting anisotropy induced by asthenospheric flow because they lack a thick overlying lithosphere that may also contribute to the observed anisotropy we tested for a possible lithospheric contribution by comparing the splitting measurements with the fossil spreading directions we find that although the fossil lithospheric fabric closely matches the observed fast polarization directions at stations 500 km from a ridge axis they are a poor fit to the data at stations located farther off axis thus we conclude that far from a ridge axis the observed anisotropy is dominated by asthenospheric flow to test for an active component of mantle upwelling we considered several models with varying assumptions about the velocity at the base of the asthenosphere that it is 1 stationary below plates moving in the no net rotation nnr or hotspot reference frames 2 driven by plate motions at the earth s surface or 3 driven by a combination of plate motion and mantle density heterogeneity inferred from either seismic tomography or the history of subduction we find that the best fitting flow field is generated by plate motions and density heterogeneity associated with large scale upwelling originating in the lower mantle beneath southern africa and is manifest as a radial pattern of flow at the base of the asthenosphere this model provides a significantly better fit to the observed anisotropy than a model in which mantle flow is driven through a passive response to subduction the resulting sub asthenospheric flow field is estimated to have velocities of 0 3 cm year and an asthenospheric viscosity of 3 1019 pa s is found to be most consistent with the regional anisotropy geoid height and dynamic topography

subglacial drainage eskers and deforming beds beneath the laurentide and eurasian ice sheets glaciological theory predicts that the subglacial drainage network at the base of gently sloping ice sheets resting on deforming sediment should consist of many wide shallow probably braided canals distributed along the ice sediment interface rather than an arborescent network of relatively few large tunnels as would develop over a rigid substrate a corollary prediction examined here is that eskers which form in large subglacial tunnels should be rare where subglacial bed deformation occurred but they may be relatively common where the bed was rigid bed deformation would be most likely where subglacial till was relatively continuous fine grained and of low permeability that is in regions where till is derived primarily from underlying sedimentary bedrock but unlikely where discontinuous coarse grained high permeability till was derived from underlying crystalline bedrock the observed distribution of eskers in areas covered by the laurentide and eurasian british scandinavian and barents sea ice sheets during the last glaciation shows that most eskers occur over crystalline bedrock overlain by discontinuous high permeability till but are rare or absent over sedimentary bedrock overlain by fine grained low permeability till thus matching reasonably well our prediction glaciological theory and geologic evidence indicate that esker systems on a subcontinental scale are time transgressive sedimentological evidence for a canal drainage system appears to be present in fine grained tills where eskers are largely absent

eskers in a complete wet based glacial system in the phlegra montes region mars although glacial landsystems produced under warm wet based conditions are very common on earth even here observations of subglacial landforms such as eskers emerging from extant glaciers are rare this paper describes a system of sinuous ridges emerging from the in situ but now degraded piedmont terminus of a late amazonian aged 150 ma glacier like form in the southern phlegra montes region of mars we believe this to be the first identification of martian eskers that can be directly linked to their parent glacier together with their contextual landform assemblage the eskers are indicative of significant glacial meltwater production and subglacial routing however although the eskers are evidence of a wet based regime the confinement of the glacial system to a well defined regionally significant graben and the absence of eskers elsewhere in the region is interpreted as evidence of sub glacial melting as a response to locally enhanced geothermal heat flux rather than climate induced warming these observations offer important new insights to the forcing of glacial dynamic and melting behaviour on mars by factors other than climate

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

a theoretical framework for the morphodynamics of bedrock channels 1 bedrock rivers are characterized by conditions of insufficient sediment supply wherein the amount of sediment supplied to the channel is less than the channel s sediment transport capacity consequently bedrock channels generally exhibit discontinuous sediment cover which current approaches to the morphodynamics of alluvial rivers are unable to handle to fill this gap we present a theoretical framework in which local sediment transport rates are proportional to the areal concentration of sediment available for transport on the bed and the sediment continuity equation is reformulated to account for temporal changes in areal concentration of sediment on nonalluviated surfaces we then perform a linear stability analysis that shows that an initially uniform distribution of the areal concentration of sediment on the bed is unstable to small perturbations this suggests that the sediment cover in bedrock channels tends to concentrate into regions which may eventually be alluviated in general agreement with experimental observations

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

recent basal melting of a mid latitude glacier on mars evidence for past basal melting of young late amazonian debris covered glaciers in mars mid latitudes is extremely rare thus it is widely thought that these viscous flow features vffs have been perennially frozen to their beds we identify an instance of recent localized wet based mid latitude glaciation evidenced by a candidate esker emerging from a vff in a tectonic rift in tempe terra eskers are sedimentary ridges deposited in ice walled meltwater conduits and are indicative of glacial melting we compare the candidate esker to terrestrial analogues present a geomorphic map of landforms in the rift and develop a landsystem model to explain their formation we propose that the candidate esker formed during a transient phase of wet based glaciation we then consider the similarity between the geologic setting of the new candidate esker and that of the only other candidate esker to be identified in association with an existing mid latitude vff both are within tectonic graben rifts proximal to volcanic provinces finally we calculate potential basal temperatures for a range of vff thicknesses driving stresses mean annual surface temperatures and geothermal heat fluxes which unlike previous studies include the possible role of internal strain heating strain heating can form an important additional heat source especially in flow convergence zones or where ice is warmer due to elevated surface temperatures or geothermal heat flux elevated geothermal heat flux within rifts perhaps combined with locally elevated strain heating may have permitted wet based glaciation during the late amazonian when cold climates precluded more extensive wet based glaciation on mars

velocity discharge relationships derived from dye tracer experiments in glacial meltwaters implications for subglacial flow conditions repeated dye tracer tests were undertaken from individual moulins at haut glacier d arolla switzerland over a number of diurnal discharge cycles during the summers of 1989 1991 it was hoped to use the concepts of at a station hydraulic geometry to infer flow conditions in subglacial channels from the form of the velocity discharge relationships derived from these tests the results however displayed both clockwise and anticlockwise velocity discharge hysteresis in addition to the simple power function relationship assumed in the hydraulic geometry approach clockwise hysteresis seems to indicate that a moulin drains into a small tributary channel rather than directly into an arterial channel and that discharges in the two channels vary out of phase with each other anticlockwise hysteresis is accompanied by strong diurnal variations in the value of dispersivity derived from the dye breakthrough curve and is best explained by hydraulic damming of moulins or sub englacial passageways despite the complex velocity discharge relationships observed some indication of subglacial flow conditions may be obtained if tributary channels comprise only a small fraction of the drainage path and power function velocity discharge relationships are derived from dye injections conducted during periods when the supraglacial discharge entering the moulin and the bulk discharge vary in phase analyses based on this premise suggest that both open and closed channel flow occur beneath haut glacier d arolla and that flow conditions are highly variable at and between sites

mesozoic tectono stratigraphic evolution of the seychelles microcontinent abstract the seychelles region developed as a unique continental sliver through the three phases of rift tectonism that since the late permian created the western indian ocean the history of rifting in seychelles can perhaps be traced back to 700 ma when the granites that form the central islands were emplaced the low 18 o of the high temperature minerals within these slightly alkaline granites on mahe indicate an intraplate origin and the fracturing of the lithosphere consequent to their emplacement created a locus of weakness for future rift tectonism to exploit thermally induced extensional rifting initiated during the carboniferous and rekindled in the late permian 225 ma eventually severed super gondwana in the middle jurassic 160 ma into east and west gondwana a proto indian ocean the somali basin which separated east and west gondwana ceased to expand in the early cretaceous 115 ma this rift drift event is recorded in seychelles by block faulted sequences of distal fluviatile through near shore to open marine deposits along its western margin and possibly at farquhar within east gondwana transform rifting then separated madagascar and seychelles india in the late cretaceous at 84 ma a spreading ridge developed between these blocks forming the mascarene basin as a new arm of the evolving indian ocean a thick progradational wedge of sediment initially alluvial followed by lacustro marine and eventually open marine resulted from this event along the southwest seychelles margin finally hotspot related thermally induced rifting causing block faulting and fluviatile to marine deposition eventually separated seychelles from india at the cretaceous tertiary boundary this separation coincided with the deccan volcanic event and the development of the carlsberg spreading ridge which replaced the mascarene spreading ridge and initiated the opening of the northwestern indian ocean basin sedimentation on the seychelles microcontinent throughout the tertiary produced predominantly shelf carbonates

macroforms large bedforms and rhythmic sedimentary sequences in subglacial eskers south central ontario implications for esker genesis and meltwater regime abstract eskers of south central ontario were deposited in closed subglacial conduits which were continuous main conduits this interpretation is supported by the intimate association of eskers with an anastomosing network of tunnel channels relatively continuous esker ridges minimal post formational disturbance of esker sediments intercalation of till and stratified sand and gravel diapiric folding at an esker core low variability in palaeocurrent direction and upslope flow paths down esker trends in clast lithology roundness and sphericity indicate continuous conduits with sediment supply by subglacial deformation of adjacent material into the conduits melt out of sediment from the conduit walls and fluvial resedimentation esker ridge morphology is attributed to synchronous erosion transportation and deposition along the main conduits such that ridge discontinuities are primarily explained as zones of non deposition during esker formation composite pseudoanticlinal and oblique accretion avalanche bed macroforms are identified gravel facies within these macroforms were deposited from fluidal flows or hyperconcentrated dispersions progradation of macroforms or migration of constituent large bedforms through the main conduits may have temporarily blocked constricted portions of those conduits and acted as possible internal autogenic controls on sediment availability and flow resistance the location of macroforms within conduits was primarily controlled by conduit geometry and sediment availability and later by feedback between macroform and conduit geometries sand and gravel units alternate rhythmically in vertical section rhythmicity is interpreted as a response to episodic flood flows controlled by seasonal changes in water supply but not necessarily representing annual events fans beads anabranched reaches and extended hummocky deposits are intimately associated with the main esker ridges they are interpreted in terms of subglacial cavities and localized flotation zones connected to the main conduits during flood events in phase wave structures are the products of hydraulic jumps in hyperconcentrated flood flows at flow expansions into swells within the main conduits into cavities connected to the main conduits or at a grounding line esker ridges record only the most powerful flood events whereas fans and beads record flow events in finer detail being primarily depositional reservoirs laterally fining deposits record the waning stages of conduit operation

evidence of midmantle anisotropy from shear wave splitting and the influence of shear coupled p waves 1 until recently the midmantle region of the earth the transition zone and the uppermost lower mantle has been considered isotropic recent work by wookey et al 2002 presented evidence of anisotropy in the midmantle region near the tonga kermadec subduction zone inferred from shear wave splitting in teleseismic s phases at australian seismic stations this data set is revisited to examine the possibility of contamination by shear coupled p waves using reflectivity modeling we explore the effects of these phases on the measurement of shear wave splitting contamination of such measurements is demonstrated for short epicentral distances 30 wave field decomposition is a simple technique which can be used to separate these phases from the main s wave arrival the tonga australia data set is reprocessed after wave field decomposition shear wave splitting ranging between 0 7 and 6 2 s is observed a range comparable to that observed by wookey et al 2002 as before the polarization of the fast shear wave is observed to be predominantly horizontal however the magnitudes of splitting for several events at short epicentral distances are significantly reduced suggesting some influence of shear coupled p waves in the original analysis the results are compared with possible models of mantle anisotropy and the results can be best explained by significant anisotropy in the midmantle region the best candidate location for this anisotropy is in the uppermost lower mantle and scenarios involving the alignment of lower mantle or subducted materials by regional dynamic processes are suggested to account for this

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

late wisconsin ice surface profile calculated from esker paths and types katahdin esker system maine abstract values of the gradient of the former ice surface can be inferred at points along a flow line from deviations of esker paths or transitions in esker type and numerically integrated to give the profile a profile calculated in this way shows that during formation of the katahdin esker system about 12 700 yr ago the ice thickness at distances of 10 20 50 100 and 140 km from the terminus which is about two thirds of the distance to the ice divide was approximately 200 300 600 750 and 900 m the terminal reach was computed by assuming an unknown uniform basal drag and matching the profile to its known elevation at the terminus and known gradient 10 km upglacier correction for isostatic rebound based on the elevations of contemporaneous deltas and of the marine limit proved unnecessary because the tilt due to the difference in uplift at the two ends of the profile is only 0 1 m km 1 with other plausible assumptions as to sea levels in the past elevations of the marine limit or exact location of the terminus the profile could be as much as roughly 100 m higher it hits mount katahdin about 500 m below its summit which is at 1600 m in agreement with the geological evidence farther west the steepening of the upper part of the profile suggests that the mountain dammed and diverted the ice basal drag computed from the profile varies from about 20 kpa 0 2 bar near the terminus to 30 kpa 0 3 bar at 100 km to 70 kpa 0 7 bar at 140 km the relatively low values away from the influence of mount katahdin agree with independent evidence from deep sea cores of substantial late wisconsin ice sheet thinning without comparable areal reduction the method has potential for application over wide areas that were occupied by the laurentide and scandinavian ice sheets

global azimuthal seismic anisotropy and the unique plate motion deformation of australia differences in the thickness of the high velocity lid underlying continents as imaged by seismic tomography have fuelled a long debate on the origin of the roots of continents1 2 3 4 5 some of these differences may be reconciled by observations of radial anisotropy between 250 and 300 km depth with horizontally polarized shear waves travelling faster than vertically polarized ones2 this azimuthally averaged anisotropy could arise from present day deformation at the base of the plate as has been found for shallower depths beneath ocean basins6 such deformation would also produce significant azimuthal variation owing to the preferred alignment of highly anisotropic minerals7 here we report global observations of surface wave azimuthal anisotropy which indicate that only the continental portion of the australian plate displays significant azimuthal anisotropy and strong correlation with present day plate motion in the depth range 175 300 km beneath other continents azimuthal anisotropy is only weakly correlated with plate motion and its depth location is similar to that found beneath oceans we infer that the fast moving australian plate contains the only continental region with a sufficiently large deformation at its base to be transformed into azimuthal anisotropy simple shear leading to anisotropy with a plunging axis of symmetry may explain the smaller azimuthal anisotropy beneath other continents

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

seismic anisotropy and mantle flow from the great basin to the great plains western united states shear wave splitting and p sks and s travel time residuals are calculated for teleseismic arrivals recorded on the colorado plateau great basin program for array seismic studies of the continental lithosphere passcal portable broadband seismic deployment and for permanent stations in the western united states little shear wave splitting is observed for broadband recordings in the northern colorado plateau the rocky mountains or the central great basin the transition between the colorado plateau and the great basin is marked by moderate shear wave splitting 1 0 s and unusually late teleseismic phase arrivals this suggests material with a higher content of mantle melt or volatiles than regions to either side splitting in the transition between the colorado plateau and great basin is part of a pattern of fast polarizations that align in a semicircle surrounding a central great basin region of null no splitting measurements away from the california plate boundary splitting to the north and south of our study region aligns roughly parallel to the absolute plate motion of the north american plate no simple spatial relation of splitting with geological and geophysical features such as mountain ranges velocity anomalies gravity magnetics or heat flow is evident in most of the western united states however splitting in the great basin is compatible with asthenospheric flow the smallest shear wave splitting delay times coincide with the eureka low in heat flow also having low s velocity at 300 km depth we suggest that the circumferential pattern of fast polarization directions ringing a central region of nulls in the great basin is caused by mantle flow by the interaction of mantle up welling and the absolute motion of the north american plate

upper mantle anisotropy beneath the seychelles microcontinent 1 the seychelles plateau is a prime example of a microcontinent yet mechanisms for its creation and evolution are poorly understood recently acquired teleseismic data from a deployment of 26 stations on 18 islands in the seychelles are analyzed to study upper mantle seismic anisotropy using sks splitting results strong microseismic noise is attenuated using a polarization filter results show significant variation in time delays t 0 4 2 4 s and smooth variations in orientation 15 69 where is the polarization of the fast shear wave the splitting results cannot be explained by simple asthenospheric flow associated with absolute plate motions recent work has suggested that anisotropy measurements for oceanic islands surrounding africa can be explained by mantle flow due to plate motion in combination with density driven flow associated with the african superswell such a mechanism explains our results only if there are lateral variations in the viscosity of the mantle it has been suggested that the seychelles are underlain by a mantle plume predictions of flow induced anisotropy from plume lithosphere interaction can explain our results with a plume possibly impinging beneath the plateau finally we consider lithospheric anisotropy associated with rifting processes that formed the seychelles the large variation in the magnitude of shear wave splitting over short distances suggests a shallow source of anisotropy fast directions align parallel to an area of transform faulting in the amirantes farther from this area the orientation of anisotropy aligns in similar directions as plate motions this supports suggestions of transpressive deformation during the opening of the mascarene basin

polarization filtering for automatic picking of seismic data and improved converted phase detection summary data adaptive polarization filtering is used to improve the detection of converted seismic phases both direct waves and mode converted ps and sp arrivals may be more easily picked on the filtered records an autopicking routine is applied that cuts the polarization filtered traces according to the modelled traveltime of each phase through an initial structure use of forward modelled source receiver times reduces the likelihood of an automatic pick being incorrectly made on spurious spikes in the polarization filtered trace it is therefore a realistic way of automatically picking multiphase data sets or more generally linearly polarized phases where low signal to noise ratios may be encountered the method is suitable for any three component seismic data and is here applied to local earthquakes recorded in north island new zealand intermediate energy is observed between the direct p and s arrivals due to phase conversion at the interface between the indo australian and subducting pacific plates the amplitudes of these converted arrivals are often too low for them to be identified above the p wave coda but polarization filtering of the records enables the yield of converted phase picks to be greatly increased

the angular velocities of the plates and the velocity of earth s centre from space geodesy summary using space geodetic observations from four techniques gps vlbi slr and doris we simultaneously estimate the angular velocities of 11 major plates and the velocity of earth s centre we call this set of relative plate angular velocities geodvel for geodesy velocity plate angular velocities depend on the estimate of the velocity of earth s centre and on the assignment of sites to plates most geodetic estimates of the angular velocities of the plates are determined assuming that earth s centre is fixed in an international terrestrial reference frame itrf and are therefore subject to errors in the estimate of the velocity of earth s centre in itrf2005 and itrf2000 earth s centre is the centre of mass of earth oceans and atmosphere cm the velocity of cm is estimated by slr observation of lageos s orbit herein we define earth s centre to be the centre of mass of solid earth ce we determine the velocity of ce by assuming that the portions of plate interiors not near the late pleistocene ice sheets move laterally as if they were part of a rigid spherical cap the geodvel estimate of the velocity of ce is likely nearer the true velocity of cm than are the itrf2005 and itrf2000 estimates because 1 no phenomena can sustain a significant velocity between cm and ce 2 the plates are indeed nearly rigid aside from vertical motion and 3 the velocity of cm differs between itrf2005 and itrf2000 by an unacceptably large speed of 1 8 mm yr 1 the velocity of earth s centre in geodvel lies between that of itrf2000 and that of itrf2005 with the distance from itrf2005 being about twice that from itrf2000 because the geodvel estimates of uncertainties in plate angular velocities account for uncertainty in the velocity of earth s centre they are more realistic than prior estimates of uncertainties geodvel differs significantly from all prior global sets of relative plate angular velocities determined from space geodesy for example the 95 per cent confidence limits for the angular velocities of geodvel exclude those of revel sella et al for 34 of the 36 plate pairs that can be formed between any two of the nine plates with the best constrained motion the median angular velocity vector difference between geodvel and revel is 0 028 myr 1 which is up to 3 1 mm yr 1 on earth s surface geodvel differs the least from the geodetic angular velocities that altamimi et al determine from itrf2005 geodvel s 95 per cent confidence limits exclude 11 of 36 angular velocities of altamimi et al and the median difference is 0 015 myr 1 geodvel differs significantly from nearly all relative plate angular velocities averaged over the past few million years including those of nuvel 1a the difference of geodvel from updated 3 2 myr angular velocities is statistically significant for all but two of 36 angular velocities with a median difference of 0 063 myr 1 across spreading centres eight have slowed down while only two have sped up we conclude that plate angular velocities over the past few decades differ significantly from the corresponding angular velocity averaged over the past 3 2 myr

direct observations of evolving subglacial drainage beneath the greenland ice sheet simultaneous observations of moulins and boreholes in western greenland show that water delivery to the base of the ice sheet via moulins affects short term ice velocity fluctuations but not late season ice velocity decelerations increased meltwater delivery to the base of the greenland ice sheet will increase the ice sheet velocity accelerating its inevitable rush towards the ocean and subsequent sea level rise or will it debate on this topic is at the forefront of cryospheric research but has been hampered by the lack of simultaneous observations of hydraulic head in moulins vertical shafts that deliver water to the base of the ice sheet and boreholes which monitor basal water pressure lauren andrews and colleagues now provide these observations from a small region in western greenland and reveal that water delivery by moulins into channelized basal flow is indeed linked to short term fluctuations in ice velocity late season decelerations in ice velocity however seem to be controlled by changes in unchannelized flow rather than any shifts in the moulin system seasonal acceleration of the greenland ice sheet is influenced by the dynamic response of the subglacial hydrologic system to variability in meltwater delivery to the bed1 2 via crevasses and moulins vertical conduits connecting supraglacial water to the bed of the ice sheet as the melt season progresses the subglacial hydrologic system drains supraglacial meltwater more efficiently1 2 3 4 decreasing basal water pressure4 and moderating the ice velocity response to surface melting1 2 however limited direct observations of subglacial water pressure4 5 6 7 mean that the spatiotemporal evolution of the subglacial hydrologic system remains poorly understood here we show that ice velocity is well correlated with moulin hydraulic head but is out of phase with that of nearby 0 3 2 kilometres away boreholes indicating that moulins connect to an efficient channelized component of the subglacial hydrologic system which exerts the primary control on diurnal and multi day changes in ice velocity our simultaneous measurements of moulin and borehole hydraulic head and ice velocity in the paakitsoq region of western greenland show that decreasing trends in ice velocity during the latter part of the melt season cannot be explained by changes in the ability of moulin connected channels to convey supraglacial melt instead these observations suggest that decreasing late season ice velocity may be caused by changes in connectivity in unchannelized regions of the subglacial hydrologic system understanding this spatiotemporal variability in subglacial pressures is increasingly important because melt season dynamics affect ice velocity beyond the conclusion of the melt season8 9 10

deglacial meltwater drainage and glaciodynamics inferences from laurentide eskers canada abstract this paper evaluates current knowledge of laurentide eskers in canada in the light of developments in glacier hydrology and glacial sedimentology questions regarding the morpho sedimentary relations of eskers the synchroneity and operation of r channel systems the role of supraglacial meltwater input and proglacial water bodies the controls on esker pattern and the glaciodynamic condition of the ice sheet at the time of esker formation are discussed a morphologic classification of eskers is proposed five types of eskers are identified and investigated type i eskers likely formed in extensive synchronous dendritic r channel networks under regionally stagnant ice that terminated in standing water type ii eskers likely formed in short subaqueously terminating r channels or reentrants close to an ice front or grounding line that may have actively retreated during esker sedimentation type iii eskers plausibly formed in short r channels that drained either to interior lakes in or tunnel channels under regionally stagnant ice type iv eskers may have formed as time transgressive segments in short subaerially terminating r channels or reentrants that developed close to the ice margin as the ice front underwent stagnation zone retreat or downwasted and backwasted regionally stagnant ice however formation in synchronous r channels cannot be discounted on the basis of reported observations type v eskers may have formed in h channels that terminated subaerially the spatial distribution of these esker types is discussed the factors that determined laurentide r channel pattern and operation were likely a complex combination of i supraglacial meltwater discharge ii the number and location of sink holes iii the ice surface slope thickness and velocity and iv the permeability topography and rigidity of the bed these factors cause and respond to changes in ice dynamics and thermal regime over the glacial cycle

shear wave splitting to test mantle deformation models around hawaii seismic anisotropy allows us to study mantle deformation and it can thus help to constrain mantle flow in the vicinity of hotspots hypotheses for the cause of seismic anisotropy in this environment include the parabolic asthenospheric flow paf model radial flow from a mantle plume impinging on a moving lithosphere is dragged by the plate in the direction of absolute plate motion in map view this gives a parabolic pattern of flow opening in the direction of plate motion we present new shear wave splitting observations from land and ocean stations around the hawaiian islands that can be explained by the parabolic flow model the observations suggest asthenospheric anisotropy under the hawaiian islands which may be explained if dislocation creep persists to deeper depths there than in other regions perhaps due to the higher temperatures near hotspots

the role of sediment supply in esker formation and ice tunnel evolution abstract meltwater is an important part of the glacier system as it can directly influence ice sheet dynamics although it is important that ice sheet models incorporate accurate information about subglacial meltwater processes the relative inaccessibility of contemporary ice sheet beds makes direct investigation challenging former ice sheet beds contain a wealth of meltwater landforms such as eskers that if accurately interpreted can provide detailed insight into the hydrology of former ice sheets eskers are the casts of ice walled channels and are a common landform within the footprint of the last laurentide and cordilleran ice sheets in south western alberta esker distribution suggests that both water and sediment supply may have been important controls the longest esker ridge segments are located within meltwater valleys partially filled by glaciofluvial sediments whereas the shortest esker ridge segments are located in areas dominated by clast poor till through detailed esker ridge planform and crest type mapping and near surface geophysics we reveal morpho sedimentary relationships that suggest esker sedimentation was dynamic but that esker distribution and architecture were primarily governed by sediment supply through comparison of these data with data from eskers elsewhere we suggest three formative scenarios 1 where sediment supply and flow powers were high coarse sediment loads result in rapid deposition and rates of thermo mechanical ice tunnel growth is exceeded by the rate of ice tunnel closure due to sediment infilling high sedimentation rates reduce ice tunnel cross sectional area cause an increase in meltwater flow velocity and force ice tunnel growth thus ice tunnel growth is fastest where sedimentation rate is highest this positive feedback results in a non uniform ice tunnel geometry and favours macroform development and non uniform ridge geometry 2 where sediment supply is limited but flow power high the rate of sedimentation is less than the rate of thermo mechanical ice tunnel growth here the ice tunnel enlarges faster than it fills with sediment and its evolution is independent of sedimentation resulting in more uniform ice tunnel geometry in these cases esker architecture is dominated by extensive vertical accretion of tabular units and ridge geometry is more uniform 3 where sediment is truly supply limited the sedimentation rate is negligible regardless of water supply and like scenario 2 ice tunnel growth is independent of sediment deposition forming a relatively uniform ice tunnel or eroding the bed because meltwater flows transport few gravel clasts the ice tunnel is not completely filled with gravel and instead saturated and pressurized diamicton or bedrock if deformable from beneath the surrounding ice is squeezed into the relatively low pressure ice tunnel during waning flow or after ice tunnel shutdown resulting in deformation of limited gravels deposited within the ice tunnel and a landform cored with diamicton or deformed bedrock and with a relatively uniform ridge geometry our data demonstrate that an esker map is a minimum map of ice walled channel location and that continued detailed investigation of morpho sedimentary relationships is essential to gaining a complete picture of esker forming processes validating the morpho sedimentary relationships identified in south western alberta and other areas with a larger data set may allow improved remote predictive esker mapping over larger areas and inferences to be made about spatial and temporal variations in esker depositional environments and ice tunnel evolution

links between proglacial stream suspended sediment dynamics glacier hydrology and glacier motion at midtdalsbreen norway two hourly suspended sediment concentration variations observed during the summer of 1987 in the proglacial stream draining midtdalsbreen norway are modelled using multiple regression and time series techniques suspended sediment fluctuations are influenced by stream discharge variations diurnal hysteresis effects medium term sediment supply and transport variations and the recent suspended sediment concentration history of the stream they do not appear to be influenced by seasonal exhaustion or rainfall variations possible reasons for this are discussed large positive residuals from the fitted models are major pulses of suspended sediment unrelated to discharge variations these sediment flushes correlate with periods of enhanced glacier motion they cannot be explained by enhanced sediment production by subglacial erosion but are probably due to the tapping of subglacially stored sediment during sudden changes in the hydraulics and or configuration of the subglacial hydrological system seasonal changes in the lag between glacier motion peaks and suspended sediment flushes suggest that the subglacial hydrological system evolves over the summer from a distributed to a more channelized configuration

bedload hysteresis in a glacier fed mountain river sediment transport during flood events often reveals hysteretic patterns because flow discharge can peak before counterclockwise hysteresis or after clockwise hysteresis the peak of bedload hysteresis in sediment transport has been used in the literature to infer the degree of sediment availability counterclockwise and clockwise hysteresis have been in fact interpreted as limited and unlimited sediment supply conditions respectively hysteresis has been mainly explored for the case of suspended sediment transport but it was rarely reported for bedload transport in mountain streams this work focuses on the temporal variability of bedload transport in an alpine catchment saldur basin 18 6 km2 italian alps where bedload transport was monitored by means of an acoustic pipe sensor which detects the acoustic vibrations induced by particles hitting a 0 5m long steel pipe runoff dynamics are dominated by snowmelt in late spring early summer mostly by glacier melt in late summer early autumn and by a combination of the snow and glacier melt in mid summer the results indicate that hysteretic patterns during daily discharge fluctuations are predominantly clockwise during the snowmelt period likely due to the ready availability of unpacked sediments within the channel or through bank erosion in the lower part of the basin on the contrary counterclockwise hysteresis tend to be more frequent during late glacier melting period possibly due to the time lag needed for sediment provided by the glacial and peri glacial area to be transported to the monitoring section however intense rainfall events occurring during the glacier melt period generated predominantly clockwise hysteresis thus indicating the activation of different sediment sources these results indicate that runoff generation processes play a crucial role on sediment supply and temporal availability in mountain streams

modeling sediment transport in ice walled subglacial channels and its implications for esker formation and proglacial sediment yields sediment yields from glacierized basins are used to quantify erosion rates on seasonal to decadal timescales as well as conditions at the glacier bed and eskers hold valuable information about past subglacial hydraulic conditions in their spatial organization geometry and sedimentary structures ultimately eskers are a record of past glacio fluvial sediment transport but there is currently no physical model for this process we develop a 1 d model of morphodynamics in semicircular bedrock floored subglacial channels we adapt a sediment conservation law developed for mixed alluvial bedrock conditions to subglacial channels channel evolution is a function of melt opening by viscous heat dissipation from flowing water and creep closure of the overlying ice to which we add the closure or enlargement due to sediment deposition or removal respectively we apply the model to an idealized land terminating glacier and find that temporary sediment accumulation in the vicinity of the terminus or the formation of an incipient esker is inherent to the dynamics of the channelized water flow the alluviation of the bed combined with the pressurized channel flow produces unexpected patterns of sediment evacuation we show that the direction of hysteresis between sediment and water discharge is not necessarily linked to a supply or transport limited system as has been hypothesized for proglacial sediment yields we also find that the deposition of an incipient esker is a function of a compromise between water discharge and sediment supply but perhaps more importantly ice surface slope and the temporal pattern of water delivery to the bed

three dimensional modeling of plume lithosphere interaction in order to understand better the dynamics of hot spots such as hawaii we present a three dimensional numerical model for the interaction of a thermal plume with a moving lithosphere the model domain is a rectangular box filled with fluid whose viscosity depends upon temperature and pressure the lithosphere is represented by a layer of cold highly viscous fluid moving with an imposed horizontal velocity u in the x direction and a thermal plume is generated by a circular temperature anomaly on the bottom of the box the steady flow is determined numerically using a hybrid spectral finite difference technique the flow is characterized by a stagnation streamline of width y x1 5 that represents the edge of the spreading plume material we illustrate the detailed behavior of the model using the example of the hawaiian plume our best fitting hawaiian model is obtained by adjusting the plume buoyancy flux b until the predicted topography anomaly matches the observed hawaiian swell topography we find b 4100 kg s 1 which implies a plume radius of 90 km for an assumed plume mantle temperature contrast of 300 c the predicted topography is supported primarily by density anomalies beneath the lithosphere and cannot be explained by lithospheric erosion we therefore conclude that the classical lithospheric reheating model is unable to account for hotspot swells the horizontal flux of buoyancy associated with the swell exceeds b by up to 80 suggesting that current estimates of b for mantle plumes are too high empirically derived scaling laws for the width of the stagnation streamline and for the topography anomaly exhibit power law dependencies on b and u that agree well with those predicted by the refracted plume model of olson 1990 the principal weakness of the model is that the predicted geoid topography ratio of 0 010 for the hawaiian swell is about twice the observed value of 0 004 0 006

coupled changes in sand grain size and sand transport driven by changes in the upstream supply of sand in the colorado river relative importance of changes in bed sand grain size and bed sand area sand transport in the colorado river in marble and grand canyons was naturally limited by the upstream supply of sand prior to the 1963 closure of glen canyon dam the river exhibited the following four effects of sand supply limitation 1 hysteresis in sediment concentration 2 hysteresis in sediment grain size coupled to the hysteresis in sediment concentration 3 production of inversely graded flood deposits and 4 development or modification of a lag between the time of a flood peak and the time of either maximum or minimum depending on reach geometry bed elevation construction and operation of the dam has enhanced the degree to which the first two of these four effects are evident and has not affected the degree to which the last two effects of sand supply limitation are evident in the colorado river in marble and grand canyons the first three of the effects involve coupled changes in suspended sand concentration and grain size that are controlled by changes in the upstream supply of sand during tributary floods sand on the bed of the colorado river fines this causes the suspended sand to fine and the suspended sand concentration to increase even when the discharge of water remains constant subsequently the bed is winnowed of finer sand the suspended sand coarsens and the suspended sand concentration decreases independently of discharge also associated with these changes in sand supply are changes in the fraction of the bed that is covered by sand thus suspended sand concentration in the colorado river is likely regulated by both changes in the bed sand grain size and changes in the bed sand area a physically based flow and suspended sediment transport model is developed tested and applied to data from the colorado river to evaluate the relative importance of changes in the bed sand grain size and changes in the bed sand area in regulating suspended sand concentration although the model was developed using approximations for steady uniform flow and other simplifications that are not met in the colorado river the results nevertheless support the idea that changes in bed sand grain size are much more important than changes in bed sand area in regulating the concentration of suspended sand published by elsevier b v