subglacial deformation associated with fast ice flow from the columbia glacier alaska abstract the columbia glacier is a large well studied tidewater glacier which has one of the highest non surging modern glacier flow rates previous studies from ice drilling have tentatively suggested the presence of a deforming bed this study examined the recently deglaciated since 1974 foreland and concluded there were features indicative of deforming bed conditions i e streamlined subglacial bedforms squeeze type push moraines and crevasse diapirs it was also argued that the elongation ratio and height of streamlined subglacial bedforms can be used as a proxy for shear strain and thickness of the deforming layer respectively and when these two parameters are combined together they produce a complex but predictable eigenvalue pattern in the tills comprising the landforms

styles of subglacial glaciotectonic deformation within the context of the anglian ice sheet a substantial proportion of the glacial sediments of east anglia england are chaotic and highly deformed this has been attributed to a direct glaciotectonic deformation or b slumping of glaciomarine sediments we show that interpretation a leads to a classification of subglacial glaciotectonic deposits into sites of constructional deformation where little or no erosion of the preglacial deposits has occurred and styles of deformation are preserved moving up in the sequence and excavational deformation where substantial erosion of preglacial sediments has occurred and deformational styles are superimposed on each other by using glaciological and geotechnical theory we show that the expression of these deformational styles in east anglia is a natural response to the stress fields imposed by the ice sheet and that in consequence their spatial relationships are explicable the depth of deformation is related to the geotechnical properties of till and substrate the stress and effective stress fields and found to be consistent with observations

deep earth structure major developments in earth structure in the last four years have been concentrated in the description of the earth s lateral heterogeneity the regions that are heterogenous and the per cent variation of velocity and density in each region most studies find that lateral variation is concentrated in the upper 400 and lower 200 km of the mantle a radially symmetric earth model has been defined that represents the best average fit to seismic data in a broad frequency band sampling many regions p and s velocity is found to increase in zones of 50 km or less at 400 and 650 to 700 km depth the model is transversely anisotropic in the upper mantle it possesses a vertical axis of symmetry such that the elastic constants are different for vertical propagation than for horizontal and intermediate angles of propagation the real earth generally exhibits azimuthal anisotropy as well but the azimuthal anisotropy cannot be resolved by a global average of data the nature and magnitude of the anisotropy agrees with that found in ultramafic samples of the upper mantle in attenuation models of intrinsic attenuation have included the dispersive properties of intrinsic anelasticity and constructed relaxation models consistent with an observed frequency dependence of q in the body wave band there has been progress in mapping the scattering properties of the lithosphere attenuation due to scattering in the crust and lithosphere has been shown to have strong effects on the amplitudes of seismic waves at local and teleseismic distances

differential shear wave attenuation and its lateral variation in the north atlantic region because the attenuation of seismic waves is sensitive to variations in temperature and to partial melting mapping seismic q in the upper mantle is an important tool for assessing the processes responsible for lateral heterogeneity utilizing a digital data base of over 150 seismograms and a spectral ratio technique we have measured ss s differential attenuation in the north atlantic region differential attenuation is positively correlated with ss s travel time residual and both differential attenuation and travel time residual decrease with increasing seafloor age in an earlier paper we showed that the age dependence of ss s travel time residual can be explaned entirely by the cooling of the oceanic lithosphere i e contributions from the asthenosphere or from a mantle melt fraction are not required the assumption that plate cooling also dominates the variation of differential attenuation with age permits the derivation of an empirical relation between q 1 and temperature for the oceanic thermal lithosphere i e the depth interval 0 125 km however both the absolute values and the depth distribution of q we obtain under this assumption are at variance with the results of surface wave attenuation studies and the variation of q 1 with temperature that we derive is not as strongly dependent on temperature as indicated by some laboratory measurements we therefore develop models for q in which lateral variations include contributions from the asthenospheric low q zone as well as from lithospheric cooling the q models obtained under this alternative assumption are in good agreement with those obtained from surface wave studies and are therefore preferred over those models with lateral variations confined to the upper 125 km systematic long wavelength 1000 7000 km variations in differential attenuation corrected for seafloor age are evident along the axis of the mid atlantic ridge these variations can be qualitatively correlated with long wavelength variations in ss s differential travel time residuals and are attributed to along axis differences in upper mantle temperature

glacier stress patterns and sediment transfer associated with the formation of superimposed flutes abstract glacial flutes superimposed on megaflutes in the foreland of austre okstindbreen north norway provide information suitable for the identification of the areas of erosion and deposition during the formation of the superimposed flutes and the calculation of the volumes of sediment involved in this process detailed analysis of the geometry particle size and clast fabric of the sediments involved in these landforms indicate that material was transferred to an incremental zone from an adjacent erosional trough along with additional material derived directly from the glacier sediment transfer is restricted to an upper till with a thickness of about 0 40 m but deformation imposed during the formation of the superimposed flutes can be traced to a depth of around 0 75 m in a lower sheared silty sand stress patterns revealed by clast fabric analysis in the superimposed flutes and adjacent parts of the megaflute indicate compressive flow at the proximal end of the bedform in the lee of an apparently obstructing boulder convergent extending flow in the central part of the bedform with a characteristic herringbone pattern and divergent extending flow at the terminal zone of the feature the convergent divergent pattern is considered to be a response to ice flow cells caused by high pressure over the troughs between the flutes and low pressure over the flute crestline these ice flow cells are considered to be initiated by factors such as upstanding boulders and bedrock knobs clusters of boulders within the diamicton and lateral and longitudinal oscillations developed in response to identifiable perturbations first order estimates of the rates of sediment transfer involved in the formation of the superimposed flutes vary between 0 0077 m3 m 2 a 1 and 0 38 m3 m 2 a 1 these figures have been applied in an exploratory fashion to megaflutes and drumlins from elsewhere in order to estimate the time required for glacier bedforms to develop and test the reliability of the figures derived the most meaningful results suggest that glacier bedforms are created within a period of between 4 and 400 years irrespective of their size this suggests that glacier bedforms develop rapidly and are maintained in a condition of steady state that reflects the balance between glacier energy and resisting bed material

influence of late cenozoic mountain building on ocean geochemical cycles in a steady state ocean input fluxes of dissolved salts to the sea must be balanced in mass and isotopic value by output fluxes for the elements strontium calcium and carbon rivers provide the primary input whereas marine biogenic sedimentation dominates removal dissolved fluxes in rivers are related to rates of continental weathering which in turn are strongly dependent on rates of uplift the largest dissolved fluxes today arise in the himalayan and andean mountain ranges and the tibetan plateau during the past 5 m y uplift rates in these areas have increased significantly this suggests that weathering rates and river fluxes may have increased also the oceanic records of carbonate sedimentation level of the calcite compensation depth and delta sup 13 c and delta sup 87 sr in biogenic sediments are consistent with a global increase in river fluxes since the late miocene the cooling of global climate over the past few million years may be linked to a decrease in atmospheric co sub 2 driven by enhanced continental weathering in these tectonically active regions

sediment deformation beneath glaciers rheology and geological consequences experiments beneath breidamerkurjokull in iceland have led to development of flow laws for the subglacial till relating strain rate to shear stress and effective pressure and assuming either bingham fluid or nonlinearly viscous fluid behavior water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment which inhibits sliding at the ice sediment interface where water pressures are equal or near to ice pressures infiltration does not occur and sliding may result a one dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation this predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice bed interface or unstable states where zero or negative effective pressures are predicted time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds it is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed it is suggested that unstable deformation at zero or negative effective stress leads to piping in subglacial sediments at the glacier terminus and the growth of sediment floored subglacial tunnels their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation where they discharge large water volumes subglacial sediments flow laterally toward them producing tunnel valleys this sediment is then removed by water flowing along the axial tunnel tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas

upper mantle attenuation and velocity structure from measurements of differential s phases summary although much progress has been made in determining the 3 d distribution of seismic wave velocities in the earth substantially less is known about the 3 d distribution of intrinsic attenuation in this study variations in attenuation and shear velocity of the earth s mantle are constrained using measurements of differential traveltime and attenuation the data are broad band displacement sh seismograms filtered to have energy in the period range 8 20 s the seismograms are obtained from over 600 globally distributed earthquakes of magnitude mw 5 5 or greater differential traveltimes and differential t values from multiple s phases are estimated by a waveform fitting method resulting in approximately 4300 measurements for ss s in the distance range 50u 105u and 1000 measurements for sss ss in the distance range 90u 179u each measurement consists of a differential traveltime and a corresponding differential t the differential traveltimes and t values are inverted to obtain models of the lateral variation of shear velocity and lateral variation of qm where qm 1 qm the models explain the data well but have limited depth resolution the velocity models show good correlation with previous studies in particular low velocities are observed underlying mid oceanic ridges and convergent margins and high velocities are found for continental regions the qm models show shield regions to be less attenuating than prem with ridges appearing as highly attenuating features the models have limited depth resolution and to address this problem we also present a shear velocity model obtained from the combination of body wave and surface wave data sets

regional reconstruction of subglacial hydrology and glaciodynamic behaviour along the southern margin of the cordilleran ice sheet in british columbia canada and northern washington state usa abstract subglacial landsystems in and around okanagan valley british columbia canada are investigated in order to evaluate landscape development subglacial hydrology and cordilleran ice sheet dynamics along its southern margin major landscape elements include drumlin swarms and tunnel valleys drumlins are composed of bedrock diamicton and glaciofluvial sediments their form truncates the substrate tunnel valleys of various scales km to 100s km length incised into bedrock and sediment exhibit convex longitudinal profiles and truncate drumlin swarms okanagan valley is the largest tunnel valley in the area and is eroded 300 m below sea level over 600 m of late wisconsin age sediments consisting of a fining up sequence of cobble gravel sand and silt fill okanagan valley landform substrate relationships landform associations and sedimentary sequences are incompatible with prevailing explanations of landsystem development centred mainly on deforming beds they are best explained by meltwater erosion and deposition during ice sheet underbursts during the late wisconsin glaciation okanagan valley functioned as part of a subglacial lake spanning multiple connected valleys few 100s km of southern british columbia subglacial lake development started either as glaciers advanced over a pre existing sub aerial lake catch lake or by incremental production and storage of basal meltwater high geothermal heat flux geothermal springs and or subglacial volcanic eruptions contributed to ice melt and may have triggered along with priming from supraglacial lakes subglacial lake drainage during the underburst s sheetflows eroded drumlins in corridors and channelized flows eroded tunnel valleys progressive flow channelization focused flows toward major bedrock valleys in okanagan valley most of the pre glacial and early glacial sediment fill was removed a fining up sequence of boulder gravel and sand was deposited during waning stages of the underburst s and bedrock drumlins in okanagan valley were enhanced or wholly formed by this underburst s subglacial lake development and drainage had an impact on ice sheet geometry and ice volumes the prevailing conceptual model for growth and decay of the cis suggests significantly thicker ice in valleys compared to plateaus subglacial lake development created a reversal of this ice sheet geometry where grounded ice on plateaus thickened while floating valley ice remained thinner due to melting and enhanced sliding with significant transfer of ice toward the ice sheet margin subglacial lake drainage may have hastened deglaciation by melting ice lowering ice surface elevations and causing lid fracture this paper highlights the importance of ice sheet hydrology its control on ice flow dynamics distribution and volume in continental ice masses

differential shear wave attenuation δt across the east pacific rise ss phases from earthquakes on fracture zones near the easter island cordillera and the west chile rise which are recorded in the united states have reflection points on either side of the east pacific rise epr near the equator the east west records from seven wwssn stations of seven events in this region were used to obtain spectral amplitudes of horizontally polarized s and ss waves ss to s amplitude ratios were formed and differential attenuation t computed within the frequency band 0 01 to 0 11 hz the values of t vary between 0 1 sec and 35 8 sec for the 23 station event pairs used however the change in t with distance from the axis of the epr does not reflect the smooth variation expected using a model of a simple cooling slab

sliding of till over bedrock scratching polishing comminution and kinematic wave theory the observed ratio of removal of material by i scratches gouged by large clasts and ii polishing by debris suggests that the latter is the dominant form of bedrock erosion we examine this with a multi stage breakdown model of glacial debris which includes active erosion by fines these models suggest that a mother clast can through the action of its daughter debris erode several times its own volume of bedrock till sliding over bedrock is likely to be a potent source of scouring by proposing a till bedrock sliding law we investigate the kinematics of sliding till bodies ice flow naturally tends to thin and extend till cover even in the absence of longitudinal gradients in the applied stress thicker till cover has an increased effective pressure at its base a lower sliding velocity and for larger thicknesses a decrease in sediment flux with thickness this implies backward moving kinematic waves and shocks it is suggested that this is related to the blunt upstream faces of drumlins and that drumlinization can be a consequence of debris sliding over bedrock

athabasca glacier canada a field example of subglacial ice and till erosion the athabasca glacier resting on a rigid bed provides an excellent example of subglacial ice and till erosion the presence of a thin mobile till layer is shown by the presence of flutes saturated till layer push moraines and ploughed boulders cross cutting striations v shaped striations and reversed stoss and lee clasts are indicative of clasts rotating within this layer as the till moves it erodes the bedrock and clasts within it a combination of erosion by ice and till produces stoss and lee clasts and generates striations on flutes and embedded clasts as well as eroding the bedrock into a continuum of smoothed rounded and streamlined forms

paleocurrent and fabric analyses of fluvial conglomerates of the paleogene noda group northeast japan abstract clast supported fluvial conglomerates of the paleogene noda group northeast japan consist of two contrasting formations the lower minato formation comprising dominantly acidic volcanic gravels and the conformably overlying kuki formation of mesozoic basement gravels crudely horizontally stratified or massive sheets of conglomerates facies gm comprise most of the sediments of both the formations and show no abrupt lateral changes of facies debris flow deposits are absent from the sediments of the group paleocurrent analysis from gravel imbrication of facies gm suggests that sediments of the minato formation were deposited by eastward paleoflow and had multiple sources in the western volcanic range such characteristics of the formation seem to show contrasting sedimentological aspects when compared with a typical alluvial fan or a fan delta sediments of the lower part of the group were formed as a coalescing braid delta elsewhere denudation of lower mesozoic basement rocks in the northern source area provided sources for the uppermost part of the group kuki formation fabric and structure of the deposits give some clues to understand the complexities of small scale fluvial topographies symmetrical or asymmetrical concave up gravel alignments of the conglomerate facies gt suggest a small scale channel dissecting obliquely into a broad longitudinal bar of facies gm elsewhere dip orientations of imbricated gravels of facies gm coincide well with the maximum dip direction of cross bedding within the overlying sand bar facies sp however the facies change from gm to sp is remarkable features of gravel imbrication provide useful information to understand bounding surface problems

global lateral variations of shear wave attenuation in the upper mantle we analyze several thousand high quality globally recorded ss s differential waveforms to constrain the lateral variation of shear wave attenuation q in the upper mantle we use a multitaper frequency domain technique to measure attenuation parameterized by a t operator and implement a robust estimation technique to compute t and its variance the differential waveform technique minimizes the effect of factors such as finite source duration and structural complexity near the source and receiver so the differential ss s waveforms are mainly sensitive to the shear attenuation in the upper mantle under the ss bounce point we use seismograms recorded at ranges of 45 to 100 and compute the ss s differential t from the broadening of the ss waveform relative to the hilbert transform of the s waveform a careful choice of fitting windows allows us to reduce the biasing effects of interfering phases which can affect t by up to 0 5 s the t residuals with respect to preliminary reference earth model prem vary by 1 5 s with an average of 0 24 s our study suggests an average q value of 112 most of the lateral variations of q are within 30 of this value in the top 400 km of the mantle slightly lower than the prem value of 128 there is a qualitative correlation of t residual with tectonic region with distinctly higher attenuation observed under young oceans compared to platforms and shields also the lateral variations of the residuals are similar in trend to those observed in studies of the attenuation of scs multiples at long wavelengths the qp map shows a modest correlation with shear wave attenuation maps computed from surface wave analyses and with the patterns of lateral variations of shear velocities at certain upper mantle depths predicted by the model s16b30 the correlation with the velocity model is highest at 300 500 km depth indicating that there may be a contribution to long wavelength attenuation from relatively deep regions formal inversion for an upper mantle q model shows that while lateral resolution is quite good depth resolution is poor as might be expected better depth resolution must await combined body wave and surface wave inversions

glaciotectonic deformation within a flute from the isfallsglaciaren sweden the internal structure of a glacial flute in an area of fluted moraine outside the glacier isfallsglaciaren in the kebnekaise massif lapland northern sweden has been observed along a series of sections the till in the flute is underlain by glaciofluvial or glaciolacustrine sediment that has been subject to extensive glaciotectonic deformation the flute forming process is discussed and it is suggested that the flute was initiated beneath the warm based part of the glacier by the sediment moving into the low pressure area in the lee side of a clast associated with a deforming bed it is suggested that deformation increased both down the lee and outwards from the centre line these results were compared with fabric data from two other flute sites from norway where similar deformation patterns were observed it is suggested that because the deformation pattern in the flutes formed beneath both cold toe and warm based glaciers are similar then the warm based deforming bed model is valid for both followed by either passive freezing by the frozen toe moving over them or active freezing by the release of pressure from the surrounding ice causing the water saturated till in the cavity to refreeze to the basal ice

clasts with stoss lee form in lodgement tills a discussion abstr act cla sls modifi ed b y glacial erosion a rc d escribed rrom lodge ment till s in front of the g lacier m yrd alsjokull south i cel and many cl asts show modifica ti on of their lower surfaces in th e sa me way as th e ir upper o nes h o wever the lower surfaces have a smooth ed do wn g lacier face and a trunca ted up g lacier fa ce which is th e o pposite o ri entatio n to that o f th e upper surfaces this so call ed d ouble sloss i ee form is interp re ted as a res ponse to basa l trans port over a brading ma teri als fo ll owing depos ition of th e clast a nd succeeded by g lacia l erosion it is sugges ted th a t clasts with a double stoss iee rorm are a diagnos ti c criteri on fo r subg lacial depos ition by lodgement furlh ermore the distributi o n and orienta tion of cl as ts with a stoss i ee form was in ves ti gated o n a ground mora ine surface 17 3 of2199 cl as ts with a n a ax is di a meter 30 cm had a stoss iee fo rm the proportion of clasts with th eir smoo th ed e nds facing up glacier within 22 5 of the ice fl o w directi on was 72 7 thus th e preferred stoss sid e ori enta tion is closely rela ted to th e ice mo vement a nd indica tes th e direc ti on from which th e ice came only 11 7 of bo ulders with a divergent stoss sid e ori ent a ti on a re located in conn ecti o n with annual mo rain es it is sugges ted th a t such bould ers have mainl y been re de posit ed beneath the ice a nd not at the ice rront by minor adva nces of the gl acier

till fabric associated with deformable beds this paper investigates the development of till fabric associated with glaciers overlying deformable beds first the till fabrics from both modern and ancient soft bed and hard bed subglacial environments are investigated and then soft bedded sites with specific strain histories are examined the results indicate that lodgement tills had a strong fabric whilst there is a wide range of much weaker fabric strengths associated with soft bed tills it is suggested from a theoretical study of soft bed till deposition that longitudinal and vertical processes in the subglacial deforming layer are important in fabric development field sites were chosen to isolate these two factors and fabrics investigated in areas of known relative shear strain and known relative thickness of the deforming layer the results indicate that 1 for a fixed deforming layer fabric strength does increase with increasing deformation except at very high strains where the development of a transverse fabric can lead to a reduction in the overall fabric strain and 2 lodgement tills and soft bed tills associated with a thin deforming layer have a high fabric strength whilst soft bed tills associated with a thick deforming layer have a low fabric strength where the deforming layer is thin clasts are oriented parallel with the glacier direction either by lodging against a hard bed or ploughing against a soft bed however where the deforming layer is thicker and shear strains are high melt out processes encourage a parallel fabric but the increased shear strain and changes in the deforming layer thickness cause the clasts to develop both a parallel and transverse fabric these processes combine to reduce the overall fabric strength till fabric studies are a further sedimentological technique to separate the depositional processes associated with a homogeneous till

fabric signature of subglacial till deformation breidamerkurjokull iceland the foreland of breidamerkurjokull iceland is the only locality where tills known to have undergone subglacial deformation are exposed till on the foreland has a two tiered structure consisting of a dilatant upper horizon c 0 5 m thick and a compact lower till these horizons correspond to the ductile deforming a horizon and the brittle ductile b horizon observed below the glacier by g s boulton and co workers the relationship between known strain history and a variety of macrofabric elements is examined for these two genetic facies of deformation till the upper horizon exhibits variable a axis fabrics and abundant evidence for clast re alignment reflecting ductile flow and rapid clast response to transient strains in contrast the lower horizon has consistently well organized a axis fabrics with a narrow range of dip values recording clast rotation into parallel with strain axes during brittle or brittle ductile shear the data indicate that till strain history imparts identifiable macrofabric signatures providing important analogues to guide the interpretation of pleistocene tills

characterization of pebble fabrics in modern terrestrial glacigenic sediments pebble fabric data are available from several facies of glacigenic sediments deposited by modern glaciers where sedimentary processes can be observed or inferred with relatively little ambiguity over 100 samples from contemporary environments illustrate fabrics characterizing melt out till deformed and undeformed lodgement till sediment flow deposits and ice slope colluvium lodgement till fabric variability is related to the two layer structure of these sediments a structureless friable upper layer with low shear strength and high consolidation coefficient overlying a very compact material of horizontal platy structure fabric strength assessed by eigenvalue analysis is weaker and pebble dip is more dispersed in the upper structureless horizon stronger fabrics in the lower platy horizon may be primary depositional fabrics which are destroyed by subglacial shearing to give weaker fabrics in the upper horizon alternatively upper horizon fabrics may be characteristic of all recently deposited lodgement tills with stronger fabrics developing at depth by dewatering and consolidation there is a general reduction in fabric strength and an increase in particle dip associated with the transition from melt out tills through undeformed and deformed lodgement tills to sediment flow deposits and ice slope colluvium there is however considerable overlap in the fabric strengths characteristic of sediment flow deposits and deformed lodgement tills fabric data from modern glacial sedimentary facies are used to assist in interpreting the mode of deposition of some quaternary glacial sediments relatively strong fabrics characteristic of melt out tills and undeformed lodgement tills are more likely to be diagnostic of genesis than weaker fabrics associated with deformed sediments

the sharpsburg kentucky earthquake 27 july 1980 main shock parameters and isoseismal maps abstract an earthquake having an average body wave magnitude of 5 1 occurred on sunday 27 july 1980 near sharpsburg kentucky the earthquake was widely felt throughout the eastern united states and had a maximum modified mercalli intensity of vii in the epicentral region the total felt area was approximately 673 000 km 2 the well constrained focal mechanism based on 128 p wave first motions combined with other geological and seismological evidence indicates a fault plane striking n42 e dipping 50 e with a slip vector 184 of the strike this is a right lateral strike slip event with a small component of thrust isoseismal data strongly suggest a northeast directed rupture the strike is parallel to the trend of the west hickman creek fault zone but 30 km to the east of any known faults

the interpretation and classification of subglacially deformed materials abstract a general classification of subglacially deformed materials is proposed based on sediment properties and their relationship to styles of subglacial strain deformation till is defined as homogenized usually diamictic material formed by glacially induced shear of subsole materials three types are recognized type a formed by pervasive ductile deformation type b formed by brittle shear and comminution till produced by the reduction of void space by in situ crushing and abrasion the term glacitectonite is adopted for materials that have undergone subglacial shear but retain some of the structural characteristics of the parent material original structures may be truncated by glacitectonic fabric elements type a or distorted but not truncated type b the principles of strain and material response particle orientation mechanisms fabric development and drainage conditions are reviewed examples of deformation tills and glacitectonites from breidamerkurjokull iceland slettmarkbreen norway loch lomond scotland and east yorkshire england are used to illustrate the nature of subglacially deformed materials and their relationship to former subglacial strain and drainage conditions the geologic evidence suggests a continuum of bed strengths can be recognized in deforming substrata ranging from high strength low strain type b deformation till and comminution till to low strength high strain type a deformation till

subglacial comminution in the deforming bed inferences from sem analysis this study investigates the variations in erosional processes beneath briksdalsbreen a norwegian valley glacier with a thin coarse grained deforming bed the subglacial zone was investigated in situ close to the central glacial flow line site a and close to the valley wall site b via boreholes the till was collected using two different sediment samplers and the bed was examined using a borehole video camera in order to evaluate how representative borehole sampling was of the subglacial environment the results from in situ subglacial samples were compared with random samples taken from an exposed subglacial surface in the glacier foreland as well as a sample taken from the site after glacier retreat the results indicated that the texture of the exposed subglacial surface was similar to in situ samples from subglacial site b close to the valley wall and that the samples collected with the small sediment sampler had the closest fit sem analysis revealed eroded quartz grains at both sites as a result of rotation abrasion and fracture the samples from close to the valley wall were very poorly sorted with evidence for rounding and preservation of pre weathered surfaces which suggest that a relatively simple pattern of erosion from crushing to rotation dominated however the till from the centre of the glacier was better sorted more angular and with few pre weathered surfaces we suggest that this results from a more complex glacial erosional history associated with greater strain and longer residence time within the deforming layer when compared with other deformation tills it was confirmed that there is a relationship between grain size and erosional style clay rich tills show little comminution fine sand rich tills generate a significant silt component whilst the coarse sand rich tills at briksdalsbreen showed complex deformational styles but no significant silt component

upper mantle q structure beneath the east pacific rise a model of q as a function of depth has been determined for the upper mantle beneath the east pacific rise the model is appropriate for shear waves with periods from roughly 15 to 30 s the amplitudes of multibounce sh phases which propagate within the upper mantle were used to constrain the q structure the multibounce phase amplitudes were measured with respect to the s wave amplitude for each seismogram the data consist of s ss sss and ssss waves for distances from 30 to 80 the sources are earthquakes which occurred on transform faults associated with the east pacific rise and the seismograms were recorded by world wide standard seismograph network and canadian seismic network stations in western north america mantle beneath oceanic crust less than 15 ma and tectonically active continental crust is sampled since the s wave data primarily sample the deep mantle the resulting upper mantle q model depends on the q structure assumed for the lower mantle we assumed a constant q of 312 for the mantle beneath 650 km depth with this assumption the upper mantle phase amplitudes require a q of about 70 in the upper 150 km of the mantle from 150 to 400 km depth q increases to 180 the q in the transition zone from 400 to 650 km depth was found to be 180 significantly lower than the lower mantle q the resulting q model is consistent with several surface wave studies of mantle attenuation beneath the pacific however our results predict a qscs 210 which is significantly higher than many direct measurements of scs attenuation indicate furthermore the large range in measured qscs is inconsistent with lateral q variations being confined to the upper mantle if the region we have studied represents the low q extreme of upper mantle we conclude that large lateral variations in q exist in the lower mantle

the sharpsburg kentucky earthquake of 27 july 1980 the sharpsburg kentucky earthquake was the second largest earthquake to have occurred in the united states east of the continental divide in the past 20 yr having a seismic moment of 4 1 1023 dyne cm a surface wave focal mechanism study defines a nodal plane striking n30 e dipping 50 se and a nearly vertical nodal plane striking n60 w p wave first motion data indicate right lateral motion on the nodal plane striking n30 e with the pressure axes oriented east west these angles can be varied by 10 without affecting the fit to the surface wave data the surface wave solution is reinforced by a modeling of long period seismograms at regional distances the p pp and sp polarities and amplitudes from the short period vertical component array stack at norsar are used together with six unambiguous short period p wave first motions recorded in north america to test whether it is possible to constrain focal mechanism solutions with such data these solutions are compatible with the surface wave solution waveform modeling of the norsar data suggests a source pulse duration of 1 0 sec and constrains the depth to 12 0 km to match mb estimates from norsar and canadian stations t for teleseismic p must be 0 7 and 0 5 respectively when the synthetics are scaled using the surface wave seismic moment in spite of extensive coverage of the epicentral zone fewer than 70 aftershocks were recorded the largest aftershock and an mblg 2 2 aftershock locations suggest that the nodal plane striking n30 e is the fault plane an aftershock area of 30 to 50 km2 implies a stress drop of 2 8 to 6 bars and a dislocation of 2 0 to 3 4 cm because of the variety of studies performed this earthquake is presently the best studied eastern north american seismic event with well constrained estimates of focal depth focal mechanism and seismic moment and indications of the duration of the source time function and upper mantle p wave t

fluted moraine formation and till genesis below a temperate valley glacier slettmarkbreen jotunheimen southern norway clast fabric and morphological data have been used to determine the origin of fluted subglacial tills exposed by recent retreat of the slettmarkbreen glacier norway a new method for the interpretation of clast fabric data allows aspects of the strain and depositional history of the till to be reconstructed the till formed by a combination of lodgement and subsole deformation by slip along discrete shear planes lodgement was dominant for the larger size fractions 125 mm while the smaller material was more susceptible to deformation the fluted till surface reflects the tendency for the till matrix to deform into regions of low confining pressure in the lee of lodged boulders downglacier components of till flow are thought to have resulted in significant sediment transfer towards the margin

deformation of till beneath ice stream b west antarctica the behaviour and possible instability of the west antarctic ice sheet depend fundamentally on the dynamics of the large ice streams which drain it model calculations show that most ice stream velocity arises at the bed1 2 and radar sounding has shown the bed to be wet3 but the basal boundary condition is not well understood seismic evidence from the upstream b camp upb on the siple coast of west antarctica4 shows that the ice stream there rests on a layer of unconsolidated sediment averaging 5 or 6 m thick in which the water pressure is only 50 kpa less than the overburden pressure because this thin layer occurs well inland beneath an active ice sheet and rests on a surface showing flutes4 characteristic of glacial erosion5 we presume that it is glacial till we propose here that deformation within the till is the primary mechanism by which the ice stream moves and we discuss implications of this hypothesis

microscopic evidence of subglacial deformation abstract subglacial deformation is by now a well established process however recognition of this process in pleistocene tills is not always clear studies of thin sections of pleistocene tills show that there are a variety of microscopic features caused by deformation this paper gives a review of microscopic evidence for subglacial deformation and starts with a case where deformation could be established macroscopically then traces the microscopic features through planar and rotational movements which are described and separated accordingly attention is also given to features caused by crushing of grains and sediment dewatering these studies indicate that almost all basal tills show evidence of subglacial deformation rather than sedimentary derivation this implies that if we want to understand subglacial processes such tills should be described in tectonic rather than sedimentary terminology

subglacial observations from oksfjordjokelen north norway stepped bedrock topography at the snout of a small outlet glacier from oksfjordjokelen north norway produces an extensive subglacial cavity system which stretches some 70m across and 100m up glacier giving access beneath ice 50 m thick inside the cavity regelation ice clean glacier ice and deforming basal ice have been observed samples were taken and basal debris concentrations at the glacier sole were found to vary between 0 005 and 15 38 per cent by volume the basal ice velocity has been determined using a linear variable differential transformer attached to an analogue recorder and also by means of measured displacements of ice crews and clasts embedded in the basal ice velocities were found to differ both spatially and temporally from a maximum of 2 55 mm h1 to a minimum of 0 3 mm h 1 the measurements and observations which have been related to present theory show how spatially averaged values for a number of variables could lead to inaccuracies in predicted erosion values certainly at a local scale on the exposed foreland jointcontrolled lee side faces provide evidence for extensive subglacial plucking here taken to mean the removal of preloosened bed material and or material resulting from bed failure indeed in the cavity the early stages of removal of joint controlled blocks by ice deformation along joints have been observed the importance of debris rich basal ice is shown in the formation of large striations up to 500cm 16cm 2cm present on the foreland

deformation till in the great lakes region implications for rapid flow along the south central margin of the laurentide ice sheet structural and lithologic data indicate that while deposited under actively moving ice considerable portions of three muddy calcareous subglacial tills in the great lakes region probably experienced some component of ductile deformation viscous till flow and ductile shear are invoked to explain a combination of features such as recumbent isoclinal folds unlithified sediment clasts mixed ostracode shells reversed stone lee ends girdle and transverse fabrics irregular stone pavements fine striae following stone curves and inconsistent stone striae and a axes deforming fine textured subglacial till is considered as a subhorizontal shear zone rheologically layered with associated structures in descending order ductile e g isoclinal folds brittle ductile e g fissility and brittle e g till wedges rheology would be controlled mainly by till pore water content matrix texture and stone content spatial and temporal superposition of rheologies and subglacial processes probably occur