global p wave tomography on the effect of various mantle and core phases abstract in this work many global tomographic inversions and resolution tests are carried out to investigate the influence of various mantle and core phase data from the international seismological center isc data set on the determination of 3d velocity structure of the earth s interior our results show that when only the direct p data are used the resolution is good for most of the mantle except for the oceanic regions down to about 1000 km depth and for most of the d layer and pp rays can provide a better constraint on the structure down to the middle mantle in particular for the upper mantle under the oceans pcp can enhance the ray sampling of the middle and lower mantle around the pacific rim and europe while pdiff can help improve the spatial resolution in the lowermost mantle the outer core phases pkp pkikp and pkkp can improve the resolution in the lowermost mantle of the southern hemisphere and under oceanic regions when finer blocks or grid nodes are adopted to determine a high resolution model pp data are very useful for improving the upper mantle structure the resulting model inferred from all phases not only displays the general features contained in the previous global tomographic models but also reveals some new features for example the image of the hawaiian mantle plume is improved notably over the previous studies it is imaged as a continuous low velocity anomaly beneath the hawaiian hotspot from the core mantle boundary cmb to the surface implying that the hawaiian mantle plume indeed originates from the cmb low velocity anomalies along some mid oceanic ridges extend down to about 600 km depth our results suggested that later seismic phases are of great importance in better understanding the structure and dynamics of the earth s interior

impact of bedrock surface topography on spatial distribution of quaternary sediments and on the flow pattern of late weichselian glaciers on the east european craton russian plain this paper presents the first high quality maps of the bedrock surface topography and the quaternary sediment distribution for the late weichselian glaciation area on the east european craton eec and the reasoning behind the estimated sediment distribution geographic information system gis based three dimensional model of the bedrock surface topography the current digital terrain model dtm and the ice flow pattern of the last glaciation provided the main source data for the analyses and interpretations in the current continental area of the western eec which was covered by the last scandinavian ice sheet sis the bedrock lies overwhelmingly 85 above the contemporary sea level with an average altitude of 54 m above sea level the topography of the bedrock surface is very well reflected in the current topography and an 80 m difference in the ice bed surface altitudes results in the topographic focusing of the ice flow and thus all late weichselian ice streams were accommodated to the depressions in the subglacial surface such depressions are characterised by a thin 10 m layer of sediment with one till bed just 50 m upslope from the bottom of the depressions the thickness of the sediment increases two to threefold and comprises five or more till layers indicating relief enhancement through subsequent glaciations the average calculated thickness of the quaternary deposits in the study area was 49 m and the total volume of sediments was 20 280 km3

self organization in the thermomechanical flow of ice sheets this paper uses a three dimensional thermomechanical ice sheet model to investigate the initiation and evolution of ice streams within ice sheets ice streams are distinguished within general ice sheet flow by their relatively fast of the order of kilometers per year velocity and are thought to be crucial in determining the response of ice sheets to climatic change we show that streaming can arise solely as a consequence of internal feedback between ice sheet flow and temperature inhomogeneities in topography and or geology are not therefore necessary conditions for ice stream development this spatial patterning replaces the large scale surging reported from previous modeling work in one horizontal dimension we suggest that surging is restricted to situations where ice flow is channelized e g valley glaciers and that it is replaced by streaming where horizontal ice flow is unconstrained ice sheets the ice streams in some of our experiments show switching behavior reminiscent of that known from the siple coast ice streams west antarctica the cause appears to be the competition for ice discharge and drainage basin area between adjacent ice streams this may lead to the gradual reduction of a stream s discharge below the threshold necessary to generate enough frictional heat to maintain warm viscous ice at the ice stream bed in this case the stream will stagnate abruptly which then prompts localized ice thickening and the initiation of a new stream in a different location

a slab in depth three dimensional geometry and evolution of the indo australian plate geoscientists use plate tectonics to explain many aspects of both continental evolution and evolution of the planet as a whole the subduction of material at convergent plate boundaries forms a fundamental component to the theory of plate tectonics plates continents subduction zones and spreading centers all exhibit motion and geometric evolution so to try and resolve the past geometries of the planet geologists have utilized plate tectonic reconstructions here we present a three dimensional image of the subducted indo australian plate below southeast asia and show that the geometry of the subducted slab at depth is intimately related to the geometric evolution of se asia over the past 50 ma including the collision of india with the asian continent we show how the once semicontinuous subducting indo australian plate has been segmented during collision between india australian and the subduction margin to the north thus we have found that the geometry of the subducted plate should form a key component to the interpretation of the evolution of earth s surface as complexities and evolution of the subducted plate are manifest in the evolution of the overriding plate

global mantle heterogeneity and its influence on teleseismic regional tomography abstract a new global p wave tomography model is determined using a flexible grid parameterization this new model better reveals the mantle structure under the polar regions than the previous tomographic models the subducting slabs are generally imaged clearly as high velocity high v zones the young slabs are still subducting in the upper mantle and the mantle transition zone mtz whereas the old and ancient slabs are either stagnant in the mtz or have subducted down to the lower mantle even reaching the core mantle boundary low velocity low v anomalies are generally revealed in the mantle under the hotspot regions it seems that a variety of mantle upwelling plumes exist some strong plumes are visible in the whole mantle under the long living hotspots such as those in south central pacific africa hawaii and iceland whereas weak plumes are visible in only some depth range under the minor hotspots under the intraplate volcanoes in east eurasia bering sea and west alaska significant low v anomalies are revealed in the upper mantle which may reflect hot and wet upwelling associated with corner flows in the big mantle wedge bmw above the stagnant pacific slab in the mtz and perhaps deep slab dehydration as well the subduction triggered magmatism in the bmw may be a new class of mantle plumes we also used the new global model to investigate the influence of whole mantle heterogeneity on the determination of upper mantle tomography under japan with a teleseismic inversion method the results show that the mantle heterogeneities outside the target volume of regional tomography can cause significant changes 0 2 0 4 s to the observed relative travel time residuals of a distant earthquake the pattern of regional tomography remains the same even after correcting for the whole mantle heterogeneity but there are some changes in the amplitude of velocity anomalies in the regional tomography hence it is necessary to correct for the mantle heterogeneities outside the target volume so as to obtain a better regional tomography

mantle flow in subduction systems the subslab flow field and implications for mantle dynamics 1 the character of the mantle flow field in subduction zones remains poorly understood despite its importance for our understanding of subduction dynamics in particular little attention has been paid to mantle flow beneath subducting slabs in order to identify processes that make first order contributions to the global pattern of subslab mantle flow we have compiled shear wave splitting measurements from subduction zones worldwide from previously published studies and estimated average splitting parameters for the subslab region globally the subslab region is overwhelmingly dominated by trench parallel fast splitting directions we tested for relationships between splitting delay time a measure of the strength of anisotropy and parameters that are indicators of tectonic processes such as trench migration velocity convergence velocity and obliquity age of subducting lithosphere and slab dip curvature seismicity and thickness we used several different plate motion models to describe plate and trench motions and evaluated the differences among the models we find that only one parameter namely the trench migration velocity in a pacific hot spot reference frame appears to correlate well with the strength of the subslab splitting signal this finding supports a model in which the mantle beneath subducting slabs is dominated by three dimensional flow induced by trench migration we explore several implications of our model for various aspects of mantle dynamics including the choice of a suitable reference frame s for mantle flow the existence of a thin decoupling zone between slabs and the subslab mantle and consequences for mass transfer between the upper and lower mantle

were deforming subglacial beds beneath past ice sheets really widespread abstract the concept of widespread large strain deformation of subglacial unconsolidated unfrozen sediments during pleistocene glaciations has inconsistencies in the geological record numerous properties of tills and related sediments are difficult to reconcile with pervasive strains as predicted by the deforming bed theory while accepting glacier bed deformation as a geological process occurring under certain circumstances we propose that it was much less widespread than believed by some instead we believe that basal sliding and englacial transport are major ice movement and debris transport mechanisms

variations in basal conditions on rutford ice stream west antarctica seismic reflection data from two lines on rutford ice stream are presented and are compared with data already published from a third line on the ice stream the amplitude and phase of the reflections have been used to investigate the properties of the sub ice material multiple reflections on long record length data allowed calibration of the reflection coefficient at the ice bed interface and determination of the acoustic impedance of the bed material the characteristics of the bed material vary both along and across the ice stream the average acoustic impedance of the bed material across the glacier at the upstream line is 3 88 x 106 kg m 2s 1 this decreases to 3 19 x 106kg m 2s 1 52 km further downstream these values are within the range which is typical of soft sediments using acoustic impedance as an indicator of subglacial porosity some areas of the ice stream bed are interpreted as dilatant water saturated sediments undergoing pervasive deformation in other areas the bed is not deforming and basal sliding may be a more important process the proportion of the ice stream width over which bed deformation occurs increases downstream

upper mantle structure and geodynamics of the sumatra subduction zone from 3 d teleseismic p wave tomography abstract teleseismic waveforms from a passive array of 63 seismic stations are applied to image the 3 d p wave velocity structure of the mantle in the sumatra subduction zone in this study 102 teleseisms with good signal to noise ratios were selected in total 1627 p wave relative arrivals have been extracted with high accuracy based on multi channel cross correlation and adaptive stacking methods with this dataset we conducted the 3 d tomographic inversions for the 3 d p wave velocity structure of the upper mantle in the sumatra subduction zone the tomographic results suggest significant lateral variations of p wave velocity in the mantle the trench parallel low velocity features in the upper mantle reveal that there may be a trench parallel flow associated with 3 d mantle flow beneath the subducting slab the subducting indo australian plate is also clearly imaged as a continuous high velocity anomalous zone in the mantle along the sumatra island beneath southern sumatra there could have a lateral detachment of high velocity anomaly from 560 km to the deep part of the slab which may imply the slab tearing if it is true we could infer that this tear in the slab may be terminated around 17 5 ma the advancing speed of the indo australian plate increasing from north sumatra to south sumatra plays a key role in the slab tear which suggests likely that the northeastward subducting of the indo australian plate along the sunda trench and the southeastward clockwise motion of the sunda shelf may form a torque tracing the sumatra island that facilitated the clockwise rotation of the fast moving plate with the oblique subduction of the indo australian plate the asymmetric distribution of stresses along this torque on either side may induce the slab tearing

sharpening the tomographic image of the subducting slab below sumatra the andaman islands and burma summary taking advantage of the increased ray coverage due to seismicity following the 2004 december and 2005 march great earthquakes an improved iterative regional global tomographic method was applied to the sumatra andaman and adjacent regions to better constrain the 3 d mantle velocity heterogeneity in the region velocity and hypocentral parameters were iteratively perturbed to sharpen the image of the subducted slab several iterations were performed and the effects of source mislocation were considered in the iterative process an issue usually neglected in global tomography we find that source relocation between iterations increases the amplitudes of slab anomalies and sharpens the definition of slab geometry beyond what can be achieved by a fixed source iterative inversion alone in addition extensive restoration tests of synthetic data were conducted that emphasize enhancements obtained by our iterative process these tests show significant increases in amplitude and decreased smearing of synthetic slab features thus when applied to the real data similar enhancements are inferred in the resulting model which better illustrates the complex slab geometry in the upper mantle and transition zone regions along the sumatra andaman and burma subduction zones

low gradient outlet glaciers ice streams drained the laurentide ice sheet observations of geomorphic features dated with cosmogenic nuclides are used to determine the configuration of glaciation in cumberland sound baffin island during the last glacial maximum cumberland sound which was a major drainage of the laurentide ice sheet at its northeastern arctic canada margin contained low gradient ice with a surface slope of 0 038 and a driving stress between 2 and 7 kpa these values are similar to those observed for the ice streams that dominate the dynamics of the west antarctic ice sheet today the presence of low gradient ice in cumberland sound and other major outlets may similarly have influenced laurentide ice sheet behavior and would account for the iceberg discharge episodes that produced the heinrich event debris layers observed in north atlantic ocean sediments

slab dehydration and mantle upwelling in the vicinity of the sumatra subduction zone evidence from receiver function imaging of mantle transition zone discontinuities

physical conditions at the base of a fast moving antarctic ice stream boreholes drilled to the bottom of ice stream b in the west antarctic ice sheet reveal that the base of the ice stream is at the melting point and the basal water pressure is within about 1 6 bars of the ice overburden pressure these conditions allow the rapid ice streaming motion to occur by basal sliding or by shear deformation of unconsolidated sediments that underlie the ice in a layer at least 2 meters thick the mechanics of ice streaming plays a role in the response of the ice sheet to climatic change

moho depth variations in the taiwan orogen from joint inversion of seismic arrival time and bouguer gravity data abstract the joint inversion of different geophysical datasets is an effective means to reduce the non uniqueness and improve the reliability of geophysical inversion in this study seismic arrival time and bouguer gravity datasets are jointly inverted to obtain an image of 3 d velocity structures in the taiwan orogen the model obtained from joint inversion fits the arrival time observations at least as well as when inverted individually and the gravity observations are much better fit when included in the inversion implying a reduction in ambiguity by simultaneously modeling the disparate datasets moho depth variations estimated by the 3 d p wave velocity model suggest a maximum moho depth of 56 km located beneath the backbone central range and the trend of the moho is largely consistent with the topography of the central range with eastward asymmetric crustal thickening the root beneath the central range appears to be smaller in lateral extent than previously imaged and the velocity gradients into the uppermost mantle are significantly higher the lack of evidence for a significant amount of eurasian crust in the mantle supports geodynamical models of accreted rather than consumed continental crust in a collisional environment

numerical modeling of advective transport of saturated deforming sediment beneath the lake michigan lobe laurentide ice sheet abstract we investigated the dynamics of advective transport of saturated fine grained deforming sediment beneath the late wisconsinan lake michigan lobe lml of the laurentide ice sheet using a numerical flowline model sediment rheology is modeled using a general constitutive law for mass movement of fine grained material geotechnical tests on selected tills deposited by the lml in northern illinois suggest an effective viscosity of about 5 109 pa s for fully saturated sediment and mild nonlinearity in the stress strain relation numerical calculations of sediment flux based on conservative parameters and assumptions yield estimates on the order of 100 m3 y meter width of the lobe suggesting that advective transport could account directly for at least 25 of the total sediment flux estimated on the basis of field evidence less restrictive parameters and assumptions would increase sediment flux attributable to both advective transport as well as other ice velocity dependent sediment transport processes modeling results also suggest that maximum advective transport rates might be associated with viscosities up to an order of magnitude higher than the viscosity exhibited by fully saturated sediment these results thus suggest that subglacial sediment deformation could account for a substantial portion of total sediment flux in areas with fine grained glacial till and that transport rates are strongly sensitive to the subglacial hydrologic processes that control basal water pressure and sediment viscosity

geologic and topographic controls on fast flow in the laurentide and cordilleran ice sheets ice streams are fast flowing arteries which play a vital role in the dynamics and mass balance of present day ice sheets although not fully understood fast flow dynamics are intimately coupled with geologic topographic thermal and hydrologic conditions of the underlying bed these are difficult observables beneath contemporary ice sheets hindering elucidation of the processes which govern ice streaxn behavior for past ice sheets the problem is antithetic geologic evidence of former ice streams exists but spatim and temporal histories are uncertain however detailed knowledge of bed geology and topography is available in many places we take advantage of this information to compile terrain characteristics relevant to fast flow dynamics in the laurentide and cordilleran ice sheets using seed points where fast flowing wisconsinan ice has been geologically inferred discriminant analysis of a suite of north american geologic and topographic properties yields a concise measure of ice bed coupling strength our analysis suggests that the interior plains and continental shelf regions of north america have low basal coupling relative to areas of variable relief or exposed bedrock in the cordillera and on the canadian shield we conclude that the interior plains and continental shelves are both topographically and geologically predisposed to large scale basal flows i e ice streaxns or surge lobes this result holds independent of whether the mechanism of fast flow is sediment deformation or decoupled sliding over the bed

continental scale transport of sediments by the baltic ice stream elucidated by coupled grain size and nd provenance analyses abstract we introduce a methodology for determining the transport distance of subglacially comminuted and entrained sediments we pilot this method on sediments from the terminal margin of the baltic ice stream the largest ice stream of the fennoscandian ice sheet during the last glacial maximum a strong correlation r2 0 83 between the end and latitudes of circum baltic river sediments enables us to use end as a calibrated measure of distance the proportion of subglacially transported sediments in a sample is estimated from grain size ratios in the silt fraction

crustal shear wave velocity structure beneath sumatra from receiver function modeling abstract we estimated the shear wave velocity structure and vp vs ratio of the crust beneath the sumatra region by inverting stacked receiver functions from five three component broadband seismic stations located in diverse geologic setting using a well known non linear direct search approach neighborhood algorithm na inversion results show significant variation of sediment layer thicknesses from 1 km beneath the backarc basin station bkni and pmbi to 3 7 km beneath the coastal part of sumatra region station lhmi and mnai and nias island station gsi average sediment layer shear velocity vss beneath all the stations is observed to be less 1 35 km s and their corresponding vp vs ratio is very high 2 2 3 0 crustal thickness beneath sumatra region varies between 27 and 35 km with exception of 19 km beneath nias island with average crustal vs 3 1 3 4 km s vp vs 1 8 it is well known that thick sediments with low vs and high vp vs amplify seismic waves even from a small magnitude earthquake which can cause huge damage in the zone this study can provide the useful information of the crust for the sumatra region since sumatra is an earthquake prone zone which suffered the strong shaking of great andaman sumatra earthquake this study can also be helpful for seismic hazard assessment

palaeo ice streams abstract the location and behaviour of ice streams is one of the most important controls on ice sheet configuration and stability in order to reconstruct former ice sheets we need to know ice stream location and timing once identified the beds of palaeo ice streams provide an unprecedented opportunity to glean information about their basal environment something that remains very difficult under contemporary ice streams this paper represents the first synthesis and discussion of palaeo ice stream research from a variety of former ice sheets and includes new insights relating their configuration and activity to the evidence they leave behind the ambiguous use of the term ice stream is addressed and four possible configurations for ice streams are presented diagnostic landforms and landform assemblages that we would expect an ice stream to produce are discussed and the temporal context of bedform generation is described as either a rubber stamped or smudged bedform imprint resulting from isochronous or time transgressive landform generation in particular we focus on the configuration of terrestrially terminating ice streams for which there are no modern analogues technological advances in marine geophysics are helping to generate a convergence of interest between antarctic glaciology and palaeo glaciology it is suggested that investigations around the fringes of contemporary ice sheets i e west antarctica can provide evidence that directly links the geomorphological record of palaeo ice streams with their contemporary counterparts in addition computational advances and modelling adaptations have permitted the incorporation of ice streams within ice sheet models we argue that data from palaeo ice stream beds are invaluable to ice sheet stream modelling experiments and will help us understand ice stream operation and the linkages between climate perturbations and both palaeo and contemporary ice sheets

4 d evolution of se asia s mantle from geological reconstructions and seismic tomography abstract how the collision between india and asia is related to processes deeper in the mantle is unclear here we compare geological reconstructions of block motions within asia since 50 ma with the tomographically imaged three dimensional 3 d morphology of subducted lithosphere to obtain insight into the spatiotemporal evolution of mantle structure past positions of the convergent margin show remarkable similarities with slab geometry at specific depths the striking change in slab geometry from a linear structure beneath 1100 km to an increasingly distorted shape at depths of less than 700 km results from collision the slab contours match the progressive deformation of asia s margin including india s indentation and sundaland s extrusion ever since the onset of collision the indian plate appears to have overridden its own sinking mantle and it does not seem at present to underthrust tibet significantly north of the zangbo suture if correct this observation would provide further evidence against models of plateau build up involving indian lithosphere the tomographic images beneath india confirm that asian deformation has absorbed at least 1500 km of convergence since collision began from the match between the southeastward motion of sundaland between 40 and 20 ma and the principal change in slab structure between 700 and 1100 km depths we infer that lateral advection in the mantle is small and that the sinking rate beneath sunda was 2 cm yr in the lower mantle and 5 cm yr above the transition zone

imaging continental breakup using teleseismic body waves the woodlark rift papua new guinea this study images the upper mantle beneath the d entrecasteax islands papua new guinea providing insight into mantle deformation beneath a highly rifted continent adjacent to propagating spreading centers differential travel times from p and s wave teleseisms recorded during the 2010 2011 cdpapua passive seismic experiment are used to invert for separate vp and vs velocity models of the continental rift a low velocity structure marks the e w axis of the rift correlating with the thinnest crust high heat flow and a linear trend of volcanoes this slow region extends 250 km along strike from the oceanic spreading centers demonstrating significant mantle extension ahead of seafloor breakup the rift remains narrow to depth indicating localization of extension perhaps as a result of mantle hydration a high vp structure at depths of 90 120 km beneath the north of the array is more than 6 5 faster than the rift axis and contains well located intermediate depth earthquakes these independent observations place firm constraints on the lateral thermal contrast at depth between the rift axis and cold lithosphere to the north that may be related to recent subduction although the polarity of subduction cannot be resolved this geometry is gravitationally unstable downwelling or small scale convection could have facilitated rifting and rapid lithospheric removal although this may require a wet mantle to be realistic on the required time scales the high v structure agrees with the maximum p t conditions recorded by young ultra high pressure rocks exposed on the rift axis and may be implicated in their genesis

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

rupture process of the 2004 sumatra andaman earthquake the 26 december 2004 sumatra andaman earthquake initiated slowly with small slip and a slow rupture speed for the first 40 to 60 seconds then the rupture expanded at a speed of about 2 5 kilometers per second toward the north northwest extending 1200 to 1300 kilometers along the andaman trough peak displacements reached 15 meters along a 600 kilometer segment of the plate boundary offshore of northwestern sumatra and the southern nicobar islands slip was less in the northern 400 to 500 kilometers of the aftershock zone and at least some slip in that region may have occurred on a time scale beyond the seismic band

teleseismic double difference relocation of earthquakes along the sumatra andaman subduction zone using a 3 d model 1 we have extended the double difference seismic tomography method to teleseismic distances with 3 d ray tracing conducted through nested regional global velocity models and applied the method to relocate seismicity from the sumatra andaman region before and after the great earthquakes of 2004 and 2005 we tested the algorithm s accuracy using both independent local data and an alternate relocation method and found good agreement between the results the use of depth phases differential times and a realistic 3 d velocity model improves the accuracy and precision of epicenters and focal depths systematically shifting them perpendicular to the trench and shallower respectively the relocations refine the location of the megathrust and other faults the patterns of aftershocks and their relation to slip during the two great earthquakes in addition the relocations reveal several discrete features not readily discernible in the scatter of teleseismic catalogs including an arcuate narrow band of earthquakes presumed to define the updip rupture limit in the 2005 event and a lineation at depth tracing the subduction of the investigator fracture zone when viewed in conjunction with tomography results the geometry and structural features of the subduction zone are revealed in unprecedented detail

macro scale bed roughness of the siple coast ice streams in west antarctica subglacial bed conditions are key to understanding ice stream behaviour and evolution with bed roughness re ecting substrate composition and ow resistance here we present an analysis of bed roughness in the siple coast region of west antarctica from airborne radio echo sounding data the ice streams are associated generally with low bed roughness values which decrease downstream the bed of the slow owing ice stream c 10 000 km2 is also characterized by being smooth at all scales wavelengths ranging from 5 km to in excess of 40 km furthermore the bed is smooth either side of ice stream c this suggests the location of the ice stream is controlled by internal ice sheet dynamics rather than by bed morphology if the ice stream were encouraged to migrate laterally when active there would be little resistance offered by the subglacial morphology other inter ice stream regions are rough however indicating a subglacial topographic in uence on ice stream position bed roughness increases up ow of ice streams which unless the bed is modi ed may limit the inland migration of these systems copyright 2004 john wiley sons ltd

tomographic image of the southern california mantle p wave teleseismic travel time delays recorded by the southern california array are inverted by the method of backprojection tomography to obtain images of variations in the p wave velocity structure to a depth of 750 km two major upper mantle features arc resolved one beneath the transverse ranges region and another beneath the salton trough region the transverse ranges feature appears as a curtainlike east trending high velocity anomaly this feature is 60 km thick extends most deeply on its eastern end to 250 km and attains a maximum velocity 3 greater than average southern california mantle of the same depth the salton trough feature which is not as well resolved as the transverse ranges feature is composed of low velocities in the upper 70 100 km these p wave velocities arc depressed 3 4 compared to average southern california mantle tests of the inversion indicate the major aspects of the imaged structure are authentic

geomorphological criteria for identifying pleistocene ice streams ice streams are critical regulatory mechanisms in contemporary ice sheets it has been inferred that they also had a significant effect on the dynamics of former ice sheets subsequently many people have invoked their widespread occurrence from a variety of formerly glaciated areas hypothesised locations however have often outweighed meaningful evidence this paper addresses the problem using the characteristics of contemporary ice streams as a basis for their identification from former ice sheet beds a convergence of knowledge gained from contemporary ice stream research coupled with theories of glacial geomorphology allows several geomorphological criteria to be identified as suggestive signatures of ice stream activity it is envisaged that the geomorphological criteria developed here will introduce a more objective approach to the study of former ice streams the criteria are used to construct conceptual land system models of the beds of former ice streams and it is hoped such models can provide an observational template upon which hypotheses of former ice streams can be better based

lateral variation in slab orientation beneath toba caldera northern sumatra the investigator fracture zone ifz subducts beneath toba caldera the earth s largest quaternary caldera in northern sumatra suggesting a possible relationship between them locations of sub crustal earthquakes based on arrival times of p and s waves at a seismograph network surrounding toba reveal the geometry of the subducted slab and the ifz beneath toba a vertical tear of less than 20 km in the slab across the ifz as previously suggested cannot be ruled out but the large scale geometry of the slab is dominated by a broad bend of slab contours parallel to the concaveseaward indentation of the trench the slab shape is probably a response to the trench curvature can explain the change in trend of the volcanic arc near toba and may cause shallowing of the forearc basin near nias island the decrease in radius of curvature of the slab contours is not accompanied by an observable decrease in dip angle possibly resulting in lateral compressive stress in the slab the high rate of seismicity along the subducted investigator fracture zone that intersects the slab obliquely to its plunge direction is uncommon at subducted fracture zones and is likely caused by such lateral stress in the slab

upper mantle tomography and dynamics beneath the north china craton 1 a high resolution tomographic model of the upper mantle beneath the north china craton ncc is determined using a large number of precisely hand picked teleseismic p wave arrival times the results are generally consistent with previous results but high quality arrivals provide new insights into the dynamics beneath the ncc obviously north south trending low velocity low v zones are revealed down to 300 400 km depth under the shanxi rift and tanlu fault zone while a north south trending high velocity high v zone representing the remainder of detached lithosphere is visible down to 200 km depth under the western portion of eastern ncc high v anomalies representing the detached lithosphere are detected at 200 400 km depth under central and eastern ncc under the ordos block high v anomalies are visible above 400 km depth indicating intact lithosphere broad high v anomalies representing the stagnant pacific slab are imaged with a low v anomaly from datong volcano to the edge of bohai sea in the mantle transition zone beneath eastern and central ncc suggesting that the pacific slab has subducted to central ncc but with a gap a continuously y shaped low v structure is clearly imaged under datong volcano and bohai sea from the lower mantle through this gap in the mantle transition zone to the upper mantle indicating the existence of a lower mantle plume these results suggest that in addition to the subduction of the pacific plate the plume has also played an important role in lithospheric destruction by thermal erosion of the asthenosphere and detachment of the lithosphere beneath the ncc

ice streams in the laurentide ice sheet identification characteristics and comparison to modern ice sheets this paper presents a comprehensive review and synthesis of ice streams in the laurentide ice sheet lis based on a new mapping inventory that includes previously hypothesised ice streams and includes a concerted effort to search for others from across the entire ice sheet bed the inventory includes 117 ice streams which have been identified based on a variety of evidence including their bedform imprint large scale geomorphology topography till properties and ice rafted debris in ocean sediment records despite uncertainty in identifying ice streams in hard bedrock areas it is unlikely that any major ice streams have been missed during the last glacial maximum laurentide ice streams formed a drainage pattern that bears close resemblance to the present day velocity patterns in modern ice sheets large ice streams had extensive onset zones and were fed by multiple tributaries and where ice drained through regions of high relief the spacing of ice streams shows a degree of spatial self organisation which has hitherto not been recognised topography exerted a primary control on the location of ice streams but there were large areas along the western and southern margin of the ice sheet where the bed was composed of weaker sedimentary bedrock and where networks of ice streams switched direction repeatedly and probably over short time scales as the ice sheet retreated onto its low relief interior several ice streams show no correspondence with topography or underlying geology perhaps facilitated by localised build up of pressurised subglacial meltwater they differed from most other ice stream tracks in having much lower length to width ratios and have no modern analogues there have been very few attempts to date the initiation and cessation of ice streams but it is clear that ice streams switched on and off during deglaciation rather than maintaining the same trajectory as the ice margin retreated we provide a first order estimate of changes in ice stream activity during deglaciation and show that around 30 of the margin was drained by ice streams at the lgm similar to that for present day antarctic ice sheets but this decreases to 15 and 12 at 12 cal ka bp and 10 cal ka bp respectively the extent to which these changes in the ice stream drainage network represent a simple and predictable readjustment to a changing mass balance driven by climate or internal ice dynamical feedbacks unrelated to climate or both is largely unknown and represents a key area for future work to address

flow dynamics and till genesis associated with a marine based antarctic palaeo ice stream geophysical and geological data indicate that during the last glacial cycle a palaeo ice stream drained the antarctic peninsula ice sheet apis through marguerite bay to the edge of the continental shelf via a bathymetric trough marguerite trough mega scale glacial lineations msgl in outer marguerite trough are formed in an acoustically transparent sediment unit consisting of a soft deformation till the association of msgl with deformation till indicates that bed deformation occurred beneath the ice stream and the thickness of the subglacial deforming layer was on the order of several metres msgl within outer marguerite trough are related to a pre existing thick sequence of soft sediments on the outer shelf and are inferred to have formed by a combination of subglacial sediment deformation by attenuation from a point source and groove ploughing morphologically the msgl differ in several respects from those documented elsewhere and this variation suggests a polygenetic origin for this subglacial landform an initial advance of the apis across the marguerite bay continental shelf at the lgm deposited a stiff subglacial till following this advance the ice margin within marguerite trough retreated 70 100 km before stabilising and depositing a grounding zone wedge an ice stream then developed in the trough and extended to the shelf edge forming a soft deformation till development of the ice stream may have been a glacio dynamic response to regional deglaciation subsequent retreat of the marguerite trough ice stream was underway by 13 490 uncorrected 14c years before present based on the geophysical and core data ice stream retreat appears to have been rapid and it contrasts with reconstructions of palaeo ice streams from other locations in antarctica this implies marked regional variations in antarctic ice stream dynamics during late quaternary deglaciation

shallow magma chamber under the wudalianchi volcanic field unveiled by seismic imaging with dense array the wudalianchi volcano field wdf is a typical intraplate volcano in northeast china with generation mechanism not yet well understood as its last eruption was around 300 years ago the present risk for volcano eruption is of particular public interest we have carried out a high resolution ambient noise tomography to investigate the location of magma chambers beneath the volcanic cones with a dense seismic array of 43 seismometers and 6 km spatial interval significant low velocity anomalies up to 10 are found at 7 13 km depth under the weishan volcano consistent with the pronounced high electrical conductivity anomalies from previous magnetotelluric survey we propose these extremely low velocity anomalies can be interpreted as partial melting in a shallow magma chamber with volume at least 200 km3 which may be responsible for most of the recent volcanic eruptions in wdf therefore this magma chamber may pose a serious hazard for northeast china

detrital carbonate rich sediments northwestern labrador sea implications for ice sheet dynamics and iceberg rafting heinrich events in the north atlantic much of the bed of the central and eastern sectors of the laurentide ice sheet was underlain by paleozoic carbonates we propose that pulses of detrital carbonate rich sediments in two cores from the northwestern labrador sea reflect episodes when an ice stream from the hudson strait extended to the shelf break and delivered sediment onto the slope and deep sea plain atomic mass spectroscopy 14 c dating of planktonic foraminifera in cores hu75 009 iv 55 2 4 km water depth and hu87 033 009 1 4 km water depth and 150 km north northwest indicates that the youngest event occurred between ca 14 and 15 ka and another occurred between 19 and 21 ka the two carbonate intervals in northwestern labrador sea cores are coeval with heinrich events 1 and 2 in the eastern north atlantic lat 45 50 n long 20 w where they are associated with an increase in lithic fragments and a drastic reduction in the numbers of foraminifera these changes have been linked with the massive production of icebergs associated with ice sheet surges our evidence indicates that heinrich events 1 and 2 are associated with the dynamics of the hudson strait ice stream and denote considerable glaciological instability

structure and evolution of lithospheric slab beneath the sunda arc indonesia tomographic imaging reveals seismic anomalies beneath the sunda island arc indonesia that suggest that the lithospheric slab penetrates to a depth of at least 1500 kilometers the sunda slab forms the eastern end of a deep anomaly associated with the past subduction of the plate underlying the mesozoic tethys ocean in accord with previous studies the lithospheric slab was imaged as a continuous feature from the surface to the lower mantle below java with a local deflection where the slab continues into the lower mantle the deep slab seems to be detached from the upper mantle slab beneath sumatra this complex slab structure is related to the tertiary evolution of southeastern asia and the indian ocean region

tertiary diachronic extrusion and deformation of western indochina structural and 40ar 39ar evidence from nw thailand the wang chao and three pagodas fault zones cut the western part of the indochina block and run parallel to the red river fault evidence of intense ductile left lateral shear is found in the lansang gneisses which form a 5 km wide elongated core along the wang chao fault zone dating by 40 ar 39 ar shows that such deformation probably terminated around 30 5 ma the wang chao and three pagodas faults offset the north striking lower mesozoic metamorphic and magmatic belt of northern thailand 40 ar 39 ar results suggest that this belt suffered rapid cooling in the tertiary probably around 23 ma these results imply that the extrusion of the southwestern part of indochina occurred in the upper eocene lower oligocene it probably induced rifting in some basins of the gulf of thailand and in the malay and mekong basins in the oligo miocene the continuing penetration of india into asia culminated with the extrusion of all of indochina along the ailao shan red river fault this occurred concurrently with the onset of e w extension more to the south plotting in a geographical reference frame the diachronic time spans of movement on left lateral faults east and southeast of tibet implies that the northward movement of the indian indenter successively initiated new strike slip faults located farther and farther north along its path

teleseismic tomography of the upper mantle beneath the southern lachlan orogen australia with the goal of better understanding the deep structure and tectonic setting of the lachlan orogen 50 short period seismic stations were deployed across the southern end of the great dividing range in victoria southeast australia between 2005 and 2006 to record distant earthquakes a total of 7452 relative pwave arrival time residuals from 169 teleseismic events have been extracted from the continuous records using an adaptive stacking technique which exploits the coherency of global phases across the array these residuals are mapped as three dimensional perturbations in p wavespeed in the upper mantle beneath the array using a recently developed iterative non linear tomographic procedure which combines a grid based eikonal solver and a subspace inversion technique the capability of the new scheme to include interface geometry is utilised in order to investigate the effects of a priori moho topography on the resolution of upper mantle structure the resultant images show a pattern of p wavespeed anomalies that lacks a predominant orientation and therefore does not favour a purely w e subduction accretion model for the formation of the lachlan orogen one of the main features of the three dimensional model is a zone of elevated wavespeed beneath the northern end of the array which extends to a depth of approximately 150 km and contrasts with significantly lower wavespeeds to the south this anomaly which does not appear to be an artifact of relative arrival time residual contributions from the adjoining mountainous terrane may reflect the presence of a substantial piece of proterozoic lithosphere incorporated within the phanerozoic subduction accretion setting of the lachlan orogen another key feature of the solution model is a zone of relatively low velocity beneath the newer volcanic province northwest of melbourne which extends from the crust to a depth of approximately 200 km this is likely to represent the signature of elevated temperatures associated with a diffuse mantle source for the quaternary volcanism in victoria 2008 elsevier b v all rights reserved

landform and sediment imprints of fast glacier flow in the southwest laurentide ice sheet evidence for former fast glacier flow ice streaming in the southwest laurentide ice sheet is identified on the basis of regional glacial geomorphology and sedimentology highlighting the depositional processes associated with the margin of a terrestrial terminating ice stream preliminary mapping from a digital elevation model of alberta identifies corridors of smoothed topography and corridor parallel streamlined landforms megaflutes to mega lineations that display high levels of spatial coherency ridges that lie transverse to the dominant streamlining patterns are interpreted as a series of minor recessional push moraines b thrust block moraines or composite ridges hill hole pairs constructed during readvances surges and c overridden moraines cupola hills apparently of thrust origin together these landforms demarcate the beds and margins of former fast ice flow trunks or ice streams that terminated as lobate forms localised cross cutting and or misalignment of flow sets indicates temporal separation and the overprinting of ice streams lobes the fast flow tracks are separated by areas of interlobate or inter stream terrain in which moraines have been constructed at the margins of neighbouring competing ice streams outlet glaciers this inter stream terrain was covered by more sluggish non streaming ice during full glacial conditions thin tills at the centres of the fast flow corridors in many places unconformably overlying stratified sediments suggest that widespread till deformation may have been subordinate to basal sliding in driving fast ice flow but the general thickening of tills towards the lobate terminal margins of ice streams outlet glaciers is consistent with subglacial deformation theory in this area of relatively low relief we speculate that fast glacier flow or streaming was highly dynamic and transitory sometimes with fast flowing trunks topographically fixed in their onset zones and with the terminus migrating laterally the occurrence of minor push moraines and flutings and associated landforms because of their similarity to modern active temperate glacial landsystems are interpreted as indicative of ice lobe marginal oscillations possibly in response to seasonal climatic forcing in locations where meltwater was more effectively drained from the glacier bed further north the occurrence of surging glacier landsystems suggests that persistent fast glacier flow gave way to more transitory surging possibly in response to the decreasing size of ice reservoir areas in dispersal centres and also locally facilitated by ice bed decoupling and drawdown initiated by the development of ice dammed lakes copyright 2008 john wiley sons ltd

asperity of the 2013 lushan earthquake in the eastern margin of tibetan plateau from seismic tomography and aftershock relocation summary the mw 6 6 lushan earthquake occurred on 2013 april 20 in the eastern margin of tibetan plateau and was about 50km away from the 2008 mw 7 9 wenchuan earthquake rupture zone by using arrival times from portable and permanent seismic stations local earthquake tomography and aftershock relocation were conducted by using 41955 first p and 42687 first s arrivals low vp vs high vp and vs anomaly are found in the source region which is well consistent with the aftershock distributions the ruptured fault plane might be a blind thrust and is well defined by the vp vs structure and aftershock seismicity high velocity and lowvp vs anomalies may suggest the mafic or ultramafic materials thrust from mid lower crust and define the asperity that concentrated stresses responsible for seismogenesis in contrast low vs anomalyisfoundinthegapbetweenthewenchuanandlushanearthquakes suggesting of ductile rocks with low stress

numerical reconstruction of a soft bedded laurentide ice sheet during the last glacial maximum we used a numerical ice sheet model to reconstruct the north american laurentide ice sheet during the last glacial maximum our model simulates ice sheet conditions that can be specified experimentally as either a rigid substrate hard bed or a wet deformable till soft bed basal sliding is excluded we use geologic records of former basal ice sheet processes to prescribe the distribution of hard and soft beds our reconstruction of the laurentide ice sheet is significantly lower in ice surface height and contains less ice volume than the climap maximum reconstruction in contrast our reconstruction agrees well with the ice 4g reconstruction both in height and volume because the ice 4g reconstruction is based on the inversion of relative sea level data whereas our reconstruction is based on glacial geology and ice mechanics this agreement suggests that soft beds provide a glaciological mechanism to explain the shape and volume of the laurentide ice sheet that is most consistent with observations of relative sea level change and other geodynamic considerations

distribution of glacial geomorphic features on the antarctic continental shelf and correlation with substrate implications for ice behavior surveys were conducted seaward of all the major drainage outlets of the antarctic ice sheet from the pennell coast north victoria land to marguerite bay antarctic peninsula the results show that the ice sheet extended onto the outer shelf glacial troughs occur offshore of all major glacial outlets where the substrate is crystalline bedrock ice flow tended to follow the structural grain of the bedrock deposited little sediment and eroded the underlying bedrock where ice flowed over relatively soft more easily eroded sedimentary strata the direction of ice flow was more directly offshore and depositional features characterize the sea floor in these areas the signature of the grounded ice consists of till deposits and large scale geomorphic features drumlins occur within the region of contact between crystalline and sedimentary substrates the different geological substrates are interpreted to have exerted a fundamental control on the behavior of past ice sheets the troughs in the areas of bedrock composed of sedimentary substrate are interpreted to have been occupied by relatively fast flowing ice ice streams and the troughs in the areas of crystalline substrate are interpreted to have been occupied by slower moving ice the area between these two zones was characterized by ice acceleration and is marked by drumlins

slab1 0 a three dimensional model of global subduction zone geometries 1 we describe and present a new model of global subduction zone geometries called slab1 0 an extension of previous efforts to constrain the two dimensional non planar geometry of subduction zones around the focus of large earthquakes slab1 0 describes the detailed non planar three dimensional geometry of approximately 85 of subduction zones worldwide while the model focuses on the detailed form of each slab from their trenches through the seismogenic zone where it combines data sets from active source and passive seismology it also continues to the limits of their seismic extent in the upper mid mantle providing a uniform approach to the definition of the entire seismically active slab geometry examples are shown for two well constrained global locations models for many other regions are available and can be freely downloaded in several formats from our new slab1 0 website http on doi gov d9arbs we describe improvements in our two dimensional geometry constraint inversion including the use of average active source seismic data profiles in the shallow trench regions where data are otherwise lacking derived from the interpolation between other active source seismic data along strike in the same subduction zone we include several analyses of the uncertainty and robustness of our three dimensional interpolation methods in addition we use the filtered subduction related earthquake data sets compiled to build slab1 0 in a reassessment of previous analyses of the deep limit of the thrust interface seismogenic zone for all subduction zones included in our global model thus far concluding that the width of these seismogenic zones is on average 30 larger than previous studies have suggested

tethyan subducted slabs under india abstract tomographic imaging of the mantle under tibet india and the adjacent indian ocean reveals several zones of relatively high p wave velocities at various depths under the hindu kush region in northeastern afghanistan and southern tajikistan a regional northward dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the indian plate under northern pakistan this same slab shows a roll over structure with the deeper portion overturned and dipping southward as can also be seen in the distribution of earthquake hypocenters farther east southeast e g in the vicinity of nepal a well resolved anomaly below 450 km depth is connected to the slab under the hindu kush but seems to be separated from the lithosphere above 350 km these upper mantle anomalies are interpreted as the remnants of delaminated sub continental lithosphere that went down when greater india continued to converge northward with asia after 45 ma the deeper high velocity anomalies under the indian sub continent appear clearly separated from the shallower ones as well as from each other and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the indian plate they occur at depths between 1000 and 2300 km and occasionally descend down to the core mantle boundary the anomalies form three parallel wnw ese striking zones we interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the neo tethys ocean closed between india and tibet in the cretaceous and earliest tertiary the northern deep mantle zone under northern afghanistan the himalayas and the lhasa block in southern tibet may represent the last subducted remnant of the paleo tethys ocean which is thought to have closed before the hauterivian stage of the early cretaceous the middle zone continues southeastward as a rather straight high velocity zone towards sumatra where it becomes convex southward and parallel to the subduction zone under the sunda arc comparison of this straight middle zone near india with the shallower upper 600 1000 km northern zone which displays a cusp like shape near the yunnan sw china syntaxis of the eastern himalayas supports the notion that the shallow northern zone represents later subduction than the deeper middle zone the suggestion of a counterclockwise rotation 20 of the indian plate during tertiary indentation of asia is supported by these features the present day latitudes of 5 35 n of the deep slabs under india and adjacent areas correspond to the approximate paleolatitudes of the cretaceous subduction zones the slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey which implies that lateral movements in the deeper mantle were not large

dynamics of the north american ice sheet complex during its inception and build up to the last glacial maximum the north american ice sheet complex played a major role in global sea level fluctuations during the late quaternary but our knowledge of its dynamics is based mostly on its demise from the last glacial maximum lgm a period characterised by non linear behaviour in the form of punctuated ice margin recession episodic ice streaming and major shifts in the location of ice divides in comparison knowledge of the pre lgm ice complex is poorly constrained largely because of the fragmentary nature of the evidence relating to ice sheet build up in this paper we explore the inception and growth of ice 120 20 ka using a glacial systems model which has been calibrated against a large and diverse set of data relating to the deglacial interval we make use of calibration data prior to the lgm but its scarcity introduces greater uncertainty which is partly alleviated by our large ensemble analysis results suggest that following the last interglaciation oxygen isotope stage ois 5e the ice complex initiated over the north eastern canadian arctic and in the cordillera within a few thousand years it then underwent rapid growth to an ois 5 maximum at 110 ka 5d and covered 70 of the area occupied by the lgm ice cover although only 30 by volume an ois 5 minimum is modelled at 80 ka 5a before a second phase of rapid growth at the start of ois 4 which culminated in a large ice complex at 65 ka almost as large as at the lgm subsequent deglaciation was rapid maximum modelled sea level contribution of 16 cm per century and resulted in an ois 3 minimum between ca 55 60 ka thereafter the ice complex grew towards its lgm configuration interrupted by several phases of successively less significant mass loss our results support and extend previous inferences based on geological evidence and reinforce the notion of a highly dynamic pre lgm ice complex e g with episodes of 10 s m of eustatic sea level equivalent in 50 but even the thin ois 5 ice sheets exhibit fast flow features several 1000 m a 1 in major topographic troughs notwithstanding the severe limitations imposed by the use of the shallow ice approximation we note that most fast flow features generated prior to the lgm correspond to the location of known ice streams during deglaciation i e in major topographic troughs and over soft sediments at the southern and western margins moreover the modelled flux of these ice streams sensu lato appears to be non linearly scaled to ice sheet volume i e there is no evidence that decay phases were associated with significantly increased ice stream activity this hypothesis requires testing using a model with higher order physics and future modelling would also benefit from additional pre lgm constraints e g dated ice free margin positions to help reduce and quantify uncertainties

seismic anisotropy in the sumatra subduction zone an important tool for understanding deformation occurring within a subduction zone is the measurement of seismic anisotropy through observations of shear wave splitting sws in sumatra two temporary seismic networks were deployed between december 2007 and february 2009 covering the fore arc between the fore arc islands to the back arc we use sks and local sws measurements to determine the type amount and location of anisotropy local sws measurements from the fore arc islands exhibit trench parallel fast directions which can be attributed to shape preferred orientation of cracks fractures in the overriding sediments in the sumatran fault region the predominant fast direction is fault trench parallel while in the back arc region it is trench perpendicular the trench perpendicular measurements exhibit a positive correlation between delay time and raypath length in the mantle wedge while the fault parallel measurements are similar to the fault parallel fast directions observed for two crustal events at the sumatran fault this suggests that there are two layers of anisotropy one due to entrained flow within the mantle wedge and a second layer within the overriding crust due to the shear strain caused by the sumatran fault sks splitting results show a nnw sse fast direction with delay times of 0 8 3 0 s the fast directions are approximately parallel to the absolute plate motion of the subducting indo australian plate the small delay times exhibited by the local sws 0 05 0 45 s in combination with the large sks delay times suggest that the anisotropy generating the teleseismic sws is dominated by entrained flow in the asthenosphere below the slab

laurentide ice streaming on the canadian shield a conflict with the soft bedded ice stream paradigm elucidating the controls on the location and vigor of ice streams is crucial to understanding the processes that lead to fast disintegration of ice flows and ice sheets in the former north american laurentide ice sheet ice stream occurrence appears to have been governed by topographic troughs or areas of soft sediment geology this paper reports robust evidence of a major paleo ice stream over the northwestern canadian shield an area previously assumed to be incompatible with fast ice flow because of the low relief and relatively hard bedrock a coherent pattern of subglacial bedforms drumlins and megascale glacial lineations demarcates the ice stream flow set which exhibits a convergent onset zone a narrow main trunk with abrupt lateral margins and a lobate terminus variations in bedform elongation ratio within the flow set match theoretical expectations of ice velocity in the center of the ice stream extremely parallel megascale glacial lineations tens of kilometers long with elongation ratios in excess of 40 1 attest to a single episode of rapid ice flow we conclude that while bed properties are likely to be influential in determining the occurrence and vigor of ice streams contrary to established views widespread soft bed geology is not an essential requirement for those ice streams without topographic control we speculate that the ice stream acted as a release valve on ice sheet mass balance and was initiated by the presence of a proglacial lake that destabilized the ice sheet margin and propagated fast ice flow through a series of thermomechanical feedbacks involving ice flow and temperature

receiver function structure beneath four seismic stations in the sumatra region abstract we estimated the velocity structure beneath four three component broad band seismic stations in the sumatra region by the joint inversion of teleseismic receiver functions and surface wave group velocities the stations part of geofon network ge are located in diverse geologic settings including backarc basins forearc basins and the forearc ridge the stations are distributed roughly trench parallel providing samples of conditions along the length of sumatra 143 receiver functions were computed by employing an iterative time domain deconvolution in order to minimize acausal noise the teleseismic observations at each station were grouped by back azimuth and ray parameter and then stacked to increase the signal to noise ratios surface wave group velocity dispersion was measured for paths across the sunda block from events to the northeast of the stations p wave velocity profiles were derived by inverting the stacks and group velocity data using a linearized time domain inversion scheme with an interpolated crust2 0 global model for input the results differ significantly from crust2 0 and reflect the diversity of geologic environments within which the stations are located crustal thicknesses beneath the stations range from 16 0 km at the forearc to 30 0 km at a backarc basin these results add to the available velocity and crustal thickness data for the sumatra region

deep structure of japan subduction zone as derived from local regional and teleseismic events we have determined a detailed three dimensional p wave velocity structure of the japan subduction zone to 500 km depth by inverting local regional and teleseismic data simultaneously we used 45 318 p wave arrivals from 1241 shallow and deep earthquakes which occurred in and around the japan islands the arrival times are recorded by the japan university seismic network which covers the entire japan islands densely and uniformly we also used 4211 travel time residuals from 100 teleseismic events which are read from seismograms recorded by seismic stations in northeastern japan in comparison with the previous results obtained from only local and regional events the present result for the area around the lower plate boundary and the mantle below the plate is determined more reliably because of the addition of 7035 data from 100 teleseismic events and 41 very deep earthquakes in the crust and uppermost mantle low velocity zones are clearly visible beneath active volcanoes in the mantle wedge the low velocity zones generally parallel with the slab and exist continuously to a depth of about 200 km which is consistent with the petrological geochemical and geodynamic studies we consider that the existence of volcanism related low velocity anomalies in the mantle wedge is a general seismological characteristic of subduction zones in light of all the available tomographic results for many subduction zones in the world the pacific slab beneath japan is imaged more clearly than in previous studies as a high velocity zone with a thickness of 80 90 km and a p wave velocity 4 6 higher than the normal mantle lower velocity anomalies are found in the mantle below the slab

reconstruction of palaeo ice sheets the use of geomorphological data the article discusses the nature of the glacial inversion problem which is defined as the extraction of time slice ice sheet flow patterns from the patchy and partly overprinted landform record present in former ice sheet areas a coherent inversion model for derivation of flow patterns and interior ice sheet configuration from geomorphological data is presented glacial landscapes are classified according to the three criteria of internal age gradients presence or absence of meltwater traces aligned to flow traces and basal condition frozen bed thawed bed inferred from morphology the inversion model uses landscapes classified accordingly spatially delineated into fans as input data relative chronologies at fan intersections are used to sort fans in a relative age stack that can be linked to stratigraphic dating information

unstable behavior of the laurentide ice sheet over deforming sediment and its implications for climate change geologic records of fluctuations of the laurentide ice sheet margin following the most recent glacial maximum ca 20 000 14c yr b p identify fundamental differences in ice sheet behavior depending on subglacial bed conditions rapid and irregular icemargin fluctuations occurred only over areas of deforming sediment indicating nonclimatic forcing controlled by the inherent instability of coupled ice sheet deforming sediment dynamics in contrast largely uninterrupted ice margin retreat with no evidence of significant readvance occurred over rigid bed areas indicating stable behavior unstable ice sheet behavior was most pronounced from 15 000 until 10 000 14c yr b p by which time most of the ice margin had retreated onto a rigid bed unstable ice sheet behavior would have been an integral component in controlling variable fluxes of icebergs and meltwater as well as meltwater routing to the north atlantic thus affecting thermohaline circulation the abrupt climate oscillations in the north atlantic region that ended at 10 000 14c yr b p may thus have their origin in the inherently unstable behavior of the laurentide ice sheet overriding deforming sediment

precambrian tectonics of the south indian shield inferred from geophysical data geophysical parameters such as magsat landsat gravity deep seismic sounding seismicity and heat flow data have identified cratonic and mobile units in the precambrian continental crust of the south indian shield landsat imagery and gravity field data suggest that the shield is divided into four geophysical domains from north to south namely 1 the deccan trap covered gneissic basement and the gneiss terrain around hyderabad 2 the dharwar craton subdivided into its eastern and western blocks 3 the granulite terrains of kerala karnataka and tamil nadu and 4 the eastern ghats granulite belt the smooth transition in the gravity field from the dharwar greenstone terrain to the deccan trap covered area to the north indicates the northward continuity of the granite gneiss basement however the trap covered region lacks any significant short wavelength anomalies of the type associated with the dharwar greenstone belts indicating the probable absence of any such large scale high density bel

complex slab subduction beneath northern sumatra 1 new data provided by the 2004 2005 sumatra andaman great earthquake sequences allow us to image with improved detail the p wave velocity structure beneath sumatra and adjacent regions below northern sumatra we find that the slab is folded at depth exhibiting geometry similar to that of the volcanic arc and the trench at the surface we speculate that this fold plays a major role in the segmentation of the sumatra megathrust and may impede rupture propagation in the region north of sumatra significant slab material in the mantle transition zone is imaged for the first time and we infer the presence of a major tear between the upper mantle and transition zone there

mantle structure and seismotectonics of the sunda and banda arcs abstract we have examined the mantle structure and seismotectonic features of the sunda and banda arcs indonesia based on the p wave tomographic images focal mechanism solutions gravity anomaly and heat flow data on the basis of slab morphology and seismicity we can divide the arcs into three parts the western sunda eastern sunda and banda arc the slab like tomographic image penetrates to a depth of about 500 km below the western sunda arc where seismicity does not exceed a depth of 250 km in the eastern sunda arc where a seismic gap exists between 300 and 500 km depths the slab appears to be continuous and to penetrate into the lower mantle beneath the banda arc with seismicity down to a depth of about 650 km the slab dips gently and does not appear to penetrate into the lower mantle the positive gravity anomaly shows a systematic pattern namely the anomaly along the eastern sunda arc is larger than that in the western sunda and the banda arcs along the back arc side of the sunda and banda arcs the heat flow decreases from the west to the east seismic strain release from the shallow earthquakes calculated from the cmt solutions show the strain axes to be oblique to the structural trends the cmt solutions show that the eastern sunda arc is characterized by normal earthquakes along the trench and back arc thrusting earthquakes north of the volcanic line in the western sunda and the eastern sunda arcs earthquakes of the down dip extension type dominate the slab down to a depth of 200 km while down dip compression earthquakes occur below 500 km depth in the banda arc deep earthquakes show down dip extension to a depth of 500 km below this depth the state of stress is not clearly defined

tomographic image of the crust and upper mantle beneath the western tien shan from the manas broadband deployment possible evidence for lithospheric delamination abstract we combine teleseismic p arrival times from the recent manas deployment of broadband sensors with p and s arrival times from local events recorded by the genghis deployment and analog observations from the kyrgyz institute of seismology to generate a high resolution 20 km image of elastic wavespeeds in the crust and upper mantle beneath the western tien shan the total data set consists of 29 006 p and 21 491 s arrivals from 2176 local events recorded at 144 stations along with 5202 p arrivals from 263 teleseismic events recorded at 40 stations the most significant feature in our image of the mantle beneath the tien shan is a pair of large elongated high wavespeed regions dipping in opposite directions from the near surface to depths of at least 400 km these regions appear to be continuous and extend upwards to bounding range fronts where the tarim basin is being overthrust by the kokshal range on the south side and the kazach shield underthrusts the kyrgyz range on the north side while it is tempting to interpret these high wavespeed anomalies as evidence for contemporary subduction of continental lithosphere such a scenario is difficult to reconcile with both the timing of the orogen and the size of the wavespeed anomaly we suggest instead that they represent downwelling side limbs of a lithospheric delamination beneath the central part of the tien shan possibly by siphoning of the bordering continental lithosphere as the central part descends

uppermost mantle structure of the eastern margin of the tibetan plateau from interstation pn traveltime difference tomography abstract tomographic inversion with interstation pn differential traveltime data provides a reliable velocity image of the uppermost mantle across the longmenshan fault belt which borders the eastern tibetan plateau and sichuan basin low pn velocities pn velocities 8 1 km s appear beneath the sichuan basin to the east suggesting a relatively weak upper mantle beneath eastern tibetan plateau and a strong upper mantle beneath the sichuan basin the pn velocities are consistent with the proposition that the rigid and stable lithosphere of the sichuan basin may act as a barrier to the eastward escape flow of the ductile crust of the eastern tibetan plateau consistency of pn velocities with the smoothed topography crustal thickness and bouguer gravity implies that the dynamic processes of the lithospheric mantle should be responsible for the crustal thickening and relief of uplift in the eastern tibetan plateau however the longmeshan fault belt appears to be separated into two segments by the huya and leidong faults as manifested by significant discrepancies in topography gradient seismic activity rupture process and aftershock distribution of wenchuan earthquake this segmentation is also supported by difference in pn velocities along the strike of the fault belt suggesting different tectonic environments in the crust and lithospheric mantle understanding of the mechanism for the segmentation requires further investigation

mega scale glacial lineations and cross cutting ice flow landforms landsat images reveal a previously unsuspected large scale pattern of streamlining within drift that is assumed to reflect former phases of ice flow such a glacial grain can be regarded as a landform assemblage comprised of a number of components drumlins and megaflutes form part of the pattern but in addition there are two previously undocumented ice moulded landform elements streamlined lineations of much greater proportions referred to as mega scale glacial lineations and a distinctive cross cutting topology within the grain the ice moulded landform assemblage is described and illustrated with reference to examples from canada possible modes of genesis of such landforms are discussed and their glaciological implications outlined the discovery of this pattern indicates the pervasive nature of subglacial deformation of sediment and demands a radical re interpretation of ice sheet dynamics

influence of subglacial geology on the position of a west antarctic ice stream from seismic observations ice streams drain much of the interior west antarctic ice sheet and buffer the main ice reservoir from oceanic influences1 2 the slow flowing interior feeds the floating ross ice shelf with ice via fast flowing ice streams3 that are believed to modulate sea level change through their control of inland ice storage understanding ice stream behaviour and predicting the response to climate change4 requires a better knowledge of the subglacial geology5 6 it is known that a thawed ice bed and high pressure basal water are necessary but not sufficient conditions to cause ice streaming7 8 moreover it has been hypothesized that a soft sedimentary bed is also required because of its intrinsic low frictional resistance to flow9 and owing to its high erodibility so as to generate till that can deform and lubricate ice motion10 11 or to bury rough features and smooth the bed for sliding here we use seismic observations to provide evidence that one margin of the upglacier part of an ice stream is directly above the boundary of a basin with such sedimentary fill the ice stream is within the basin and the ice outside the basin is slow flowing the basin fill presents an order of magnitude lower frictional resistance to ice flow than the subglacial material outside the basin we conclude that the ice stream position is dependent on subglacial geology

mantle structure beneath indonesia inferred from high resolution tomographic imaging we investigated mantle structure beneath the indonesian region by means of tomographic inversions of traveltime residuals of direct p and the surface reflected depth phases pp and pwp the hypocentres and phase data used in the inversions were derived from the reprocessing of data reported to international data centres which include data from the australian skippy project we used more than 6 0 10 data from more than 50 000 globally distributed earthquakes recorded at a subset of nearly 3000 seismological stations worldwide the use of depth phases improved the sampling of the upper mantle structure in particular beneath the back arc regions the mapping of distant aspherical mantle structure into the study region was minimized by combining a high resolution regional with a low resolution global inversion the inversion was linearized around the ak135 reference velocity model and a combination of minimum norm and gradient damping was applied to constrain the solution inversion of different subsets of the data and test inversions of synthetic data indicate that resolution is sufficient to substantiate the conclusions pertinent to the complex large scale structure of the subducted slab beneath the indonesian archipelago the tomographic images confirm previous evidence for regional slab penetration into the upper part of the lower mantle but suggest a more complex trajectory of mantle flow beneath indonesia the implied lateral variation in the shape of the upper mantle slab is generally in good agreement with inferences from seismicity the data reveal a seismic anomaly beneath the sunda arc which is detected to at least 1500 km in depth and forms the eastern end of a deep anomaly associated with the past subduction of lithosphere of the mesozoic tethys ocean below java the lithospheric slab is probably continuous from the earth s surface to the lower mantle but beneath sumatra the deep slab seems to be detached from the seismogenic slab the high resolution images suggest that the subducted slab is deflected in the transition zone beneath the banda arc and that the descending slabs form a spoon shaped structure north of the banda sea we detect the two opposing subducted slabs of the molucca sea plate this is consistent with previous studies but our images suggest that the westward subduction has a significantly steeper dip than the eastward subduction slab structure beneath the banda and molucca seas is explained in the context of the continent arc collision between australia and the southeast indonesian island arc and the arc arc collision between sangihe and halmahera inversion of traveltime residuals of direct s phases a novelty for this region largely confirms the inferences from the p wave data even though less detail is resolved

surface form of the southern laurentide ice sheet and its implications to ice sheet dynamics reconstructions of the ice surface morphology of several lobes of the southern laurentide ice sheet reinforce previous arguments that this sector of the ice sheet was thin and low sloping driving stresses estimated from the geometry of the reconstructed ice surfaces are 0 7 4 3 kpa for the 14 ka des moines lobe 0 9 1 2 kpa for the 14 ka james lobe 0 9 1 7 kpa for the 18 20 ka lake michigan lobe 1 8 2 9 kpa for the 15 18 ka chippewa sublobe and 17 22 kpa for the 15 18 ka green bay lobe previous estimates of rates of ice margin advance 450 2 000 m yr indicate moderate to fast ice velocities for the ice lobes reconstructed driving stresses and velocity estimates of the des moines james and lake michigan lobes are analogous to the distal ends ice plains of low sloping 0 4 x 10 3 but fast moving 500 m yr west antarctic ice streams whose dynamics have been attributed to sliding and or subglacial sediment deformation by pervasive shear these reconstructions support recent models of the laurentide ice sheet which include movement by sliding or by subglacial sediment deformation along its southern western and northwestern sectors evidence for either mechanism should be represented in the sedimentologic and geomorphic records thin ice in these regions indicates that the laurentide ice sheet contained less ice volume and represented less of an orographic obstacle to atmospheric circulation than has been considered in models of the ice sheet on a rigid bed with steep profiles

the tekapo glacier new zealand during the last glacial maximum an active temperate glacier influenced by intermittent surge activity abstract quaternary glaciations have created impressive landform assemblages that can be used to understand palaeo glacier extent character and behaviour and hence past global and local glacier forcings however in the southern hemisphere and especially in new zealand the quaternary glacial landform record is relatively poorly investigated with regard to glaciological properties in this study a 1 m digital elevation model dem was generated from airborne lidar data and supplemented with aerial imagery and field observations to analyse the exceptionally well preserved glacial geomorphology surrounding lake tekapo new zealand we describe a rich suite of last glacial maximum lgm and recessional ice marginal subglacial supraglacial glaciofluvial and glaciolacustrine landform assemblages these represent two landsystems comprising i fluted till surfaces with low relief push moraine ridges and ii crevasse squeeze ridges zig zag eskers and attenuated lineations the former landsystem records the behaviour of an active temperate glacier and the latter landsystem which is superimposed upon and inset within the former strongly suggests intermittent surge phases the two landsystem signatures indicate a sequential change in ice marginal dynamics during recession that was likely to have been partially non climatically driven overall we present the first evidence of surge type glacier behaviour in new zealand

irregular oscillations of the west antarctic ice sheet model simulations of the west antarctic ice sheet suggest that sporadic perhaps chaotic collapse complete mobilization of the ice sheet occurred throughout the past one million years the irregular behaviour is due to the slow equilibration time of the distribution of basal till which lubricates ice sheet motion this nonlinear response means that predictions of future collapse of the ice sheet in response to global warming must take into account its past history and in particular whether the present basal till distribution predisposes the ice sheet towards rapid change

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle

p wave tomography and origin of the changbai intraplate volcano in northeast asia abstract we present the first seismic image of the upper mantle beneath the active intraplate changbai volcano in northeast asia determined by teleseismic travel time tomography the data are measured at a new seismic network consisting of 19 portable stations and 3 permanent stations our results show a columnar low velocity anomaly extending to 400 km depth with a p wave velocity reduction of up to 3 high velocity anomalies are visible in the mantle transition zone and deep focus earthquakes occur at depths of 500 600 km under the region suggesting that the subducting pacific slab is stagnant in the transition zone as imaged clearly by global tomography these results suggest that the intraplate changbai volcano is not a hotspot like hawaii but a kind of back arc volcano related to the deep subduction and stagnancy of the pacific slab under northeast asia

laurentide glacial landscapes the role of ice streams glacial landforms of the north american prairie can be divided into two suites that result from different styles of ice flow 1 a lowland suite of level to streamlined till consistent with formation beneath ice streams and 2 an upland and lobe margin suite of thick hummocky till and glacial thrust blocks consistent with formation at ice stream and ice lobe margins southern laurentide ice lobes hypothetically functioned as outlets of ice streams broad branching lowlands bounded by escarpments mark the stable positions of the ice streams that fed the lobes if the lobes and ice streams were similar to modern ice streams their fast flow was facilitated by high subglacial water pressure favorable geology and topography in the midcontinent encouraged nonuniform ice flow and controlled the location of ice streams and outlet lobes

freshwater forcing of abrupt climate change during the last glaciation large millennial scale fluctuations of the southern margin of the north american laurentide ice sheet occurred during the last deglaciation when the margin was located between about 43 and 49 n fluctuations of the ice margin triggered episodic increases in the flux of freshwater to the north atlantic by rerouting continental runoff from the mississippi river drainage to the hudson or st lawrence rivers we found that periods of increased freshwater flow to the north atlantic occurred at the same time as reductions in the formation of north atlantic deep water thus providing a mechanism for observed climate variability that may be generally characteristic of times of intermediate global ice volume