3 μm spectrophotometric survey of m and e class asteroids many asteroids show absorption features diagnostic of hydrated minerals in the 3 m region of the spectrum reflectance studies in this region can determine the hydration state of surface minerals and by inference the thermal histories of bodies we have made spectrophotometric observations of eight e class m class x class and p class asteroids in december 1991 and eight e class and m class asteroids in august 1993 using the infrared telescope facility on mauna kea hawaii in 1991 we discovered a 3 m absorption band on the e class asteroid 44 nysa and the p class asteroid 50 virginia and found a possible absorption on the e class asteroid 64 angelina we confirmed the presence of a 3 m absorption band on 44 nysa and on the m class asteroid 92 undina t d jones et al 1990 icarus 88 172 192 and detected this feature on the m class asteroid 201 penelope in 1993 these data demonstrate that high albedo and medium albedo objects such as e and m class asteroids can be hydrated and need not be interpreted as primarily of igneous origin we revise the interpreted mineralogy of some e and m class asteroids as a result

the formation of supraglacial debris covers by primary dispersal from transverse englacial debris bands glaciological controls on debris cover formation are investigated from the perspective of primary dispersal of supraglacial debris across a melting ice surface this involves the migration of angled debris septa outcrops across a melting thinning glacier ablation zone three measures of a glacier s ability to evacuate supraglacial debris are outlined 1 a concentration factor describing the focusing of englacial debris into specific supraglacial mass loads 2 the rate of migration of a septum outcrop relative to the local ice surface and 3 a downstream velocity differential between a slower septum outcrop and the faster ice surface velocity measures 1 and 2 are inversely related while measure 3 increases down glacier to explain why slow moving thinning ice rapidly becomes debris covered data from glacier d estelette italian alps are used to illustrate these processes and to explore the potential for debris cover formation and growth in different glaciological environments the transition from a clean transport dominated to a debris covered ablation dominated glacier is explained by the melting out of more closely spaced debris septa in combination with the geometric nteractions of angled septa and ice surface in a field of reducing flow and increasing ablation the growth and shrinkage of debris covers are most sensitive to glaciological changes at glaciers with gently dipping debris bearing foliation but less sensitive at high compression glaciers whose termini are constrained by moraine dams and other forms of obstruc tion these findings show that a variety of debris covered glacier types will show a spectrum of response characteristics to negative mass balance copyright 2013 john wiley sons ltd

degradation of glacial deposits quantified with cosmogenic nuclides quartermain mountains antarctica many glacial deposits in the quartermain mountains antarctica present two apparent contradictions regarding the degradation of unconsolidated deposits the glacial deposits are up to millions of years old yet they have maintained their meter scale morphology despite the fact that bedrock and regolith erosion rates in the quartermain mountains have been measured at 0 1 4 0 m ma 1 additionally ground ice persists in some miocene aged soils in the quartermain mountains even though modeled and measured sublimation rates of ice in antarctic soils suggest that without any recharge mechanisms ground ice should sublimate in the upper few meters of soil on the order of 103 to 105 years this paper presents results from using the concentration of cosmogenic nuclides beryllium 10 10be and aluminum 26 26al in bulk sediment samples from depth profiles of three glacial deposits in the quartermain mountains the measured nuclide concentrations are lower than expected for the known ages of the deposits erosion alone does not always explain these concentrations and deflation of the tills by the sublimation of ice coupled with erosion of the overlying till can explain some of the nuclide concentration profiles the degradation rates that best match the data range 0 7 12 m ma 1 for sublimation of ice with initial debris concentrations ranging 12 45 and erosion of the overlying till at rates of 0 4 1 2 m ma 1 overturning of the tills by cryoturbation vertical mixing or soil creep is not indicated by the cosmogenic nuclide profiles and degradation appears to be limited to within a few centimeters of the surface erosion of these tills without vertical mixing may partially explain how some glacial deposits in the quartermain mountains maintain their morphology and contain ground ice close to the surface for millions of years copyright 2010 john wiley sons ltd

s p conversion from the transition zone beneath tonga and the nature of the 670 km discontinuity summary using more than 300 recordings of deep tonga earthquakes from the warramunga seismic array central australia we have searched for short period s p conversions from the 670 km discontinuity arriving in the p wave codas the array seismograms were stacked and plotted in sections to optimize observation of s670p yielding the following results for the northern part of tonga 17 3 18 4 s s670p phases are observed with apparent conversion depths between about 660 and 680 km for the central part 20 2 22 3 s s670p is not clearly observed for the southern part 23 1 25 7 s s670p is observed for conversion depths between about 660 and 700 km intermittent occurrence of s p conversion may result from different characteristic source radiation patterns among the three sections overall uncertainties in conversion depths resulting from unmodelled lateral velocity variations and hypocentral location errors are perhaps a few tens of kilometres the s p rays must travel either within or very close to the subducted slab because of the source receiver slab dip geometry so we conclude that large 50 km deformations of the 670 km discontinuity do not occur beneath tonga our observations appear to be inconsistent with a chemical discontinuity at 670 km but are easily explained if the 670 km discontinuity is a strongly pressure dependent and perhaps moderately endothermic phase change in both the slab and surrounding mantle

deciphering boulder mobility and erosion from cosmogenic nuclide exposure dating large boulders are prominent features in many geomorphic systems and are frequently targeted for cosmogenic exposure dating presently there are little data or theory predicting exposure age erosion rate and mobilization frequency of boulders in environments such as channels talus slopes or moraines here we explore the potential for cosmogenic isotope analysis to constrain the transport and erosion history of boulders through a series of numerical experiments we model the statistical evolution of nuclide concentrations around the surface of boulders stable boulders have distinctive radial distributions of surface concentration in comparison to those that are periodically mobile and this can be used to establish boulder stability mean nuclide accumulation rates around the surface of an eroding boulder increase when the radius is smaller than approximately 1 5 e folding lengths 1 2 m of neutron flux intensity whereupon nuclide accumulation on the underside of the boulder becomes non negligible 10 model results for cases of no cosmogenic inheritance and uniform erosion indicate the normalized standard deviation of nuclide surface concentration systematically decreases with increasing number of boulder mobilization events this may be used to constrain the minimum number of times a boulder has moved for up to approximately four events or distinguish between rarely and frequently mobilized boulders using non dimensional scaling relations between surface concentration statistics boulder size and time we propose methods to estimate the minimum age frequency of movement and erosion rate of mobile boulders with application to a range of geomorphic problems

direct evidence for the undulation of the 660 km discontinuity beneath tonga comparison of japan and california array data short period seismograms of tonga deep earthquakes recorded by japanese and californian seismic networks are stacked to identify the s p converted wave associated with the 660 km discontinuity the travel time difference between this s p converted wave and the direct p wave is used to constrain the depth of the 660 km discontinuity analysis of a total of 29 events produced a detailed topographical map of the discontinuity beneath the tonga subduction zone two events which exhibit clear s p conversions in both japan and california data are selected to show directly the depth variations of the 660 km discontinuity adjacent to the subducting slab the s p conversion points on the ray paths to japan are observed to be approximately 10 to 30 km deeper than the conversion points on those to california which represents direct evidence for a slab induced depression of the 660 km discontinuity

glaciological response to distal tephra fallout from the 1947 eruption of hekla south iceland glacier responses to tephra deposition are shown to be highly variable where wind transported eruption plumes produce narrow distal fallout zones with steep lateral thickness gradients significant but short lived advances of faster flowing glaciers can be triggered by deposition from modest eruptions the 1947 eruption of hekla south iceland covered nearby glaciers with variable thicknesses of tephra causing dramatic spatial variations in ablation rate relative snow ablation increased by an estimated 80 at the eastern margin of eyjafjallajokull but decreased by 54 at the western margin relative ice ablation increased by only 4 in the east but decreased by 75 at the western margin only 15 km distant the effect on mass balance therefore depends on tephra distribution as well as on the nature of the glacier surface on gigjokull retardation of ablationwas greatest at the terminus associated with an anomalous advance of 328 m between 1947 and 1954 other glaciers in the fallout zone show either no recognizable response or accelerated retreat advances will be short lived due to the rapid redistribution of supraglacial tephra in a maritime climate and the potential for discriminating between volcanically and climatically forced advances in the glacio geomorphological record is low the long term glaciological effect of volcanism is to create thin low albedo covers after reworking which make conditions less favourable for glaciation

upper mantle discontinuity structure in the region of the tonga subduction zone we study the mantle structure below the southwest pacific in order to examine the geometry of the tonga slab at depth and its interaction with the 410 and 660 km discontinuities hereafter called the 410 and the 660 we utilize data from stations of both the lau basin ocean bottom seismogram experiment and island stations of the southwest pacific seismic experiment the tectonic complexity of this region containing both the tonga subduction zone and the associated lau back arc spreading center make it an ideal area to investigate the upper mantle discontinuities using a high resolution technique such as common conversion point stacking of receiver functions we produce a high resolution image of the upper mantle near the tonga subduction zone to show the interaction between the discontinuities and the subducting slab our results show the 410 uplifted by 30 km near the tonga slab and the 660 depressed by 20 to 30 km as expected for thermally controlled olivine phase transitions

mantle layering from scs reverberations 2 the transition zone this is the second paper in a four part sequence that examines the nature of mantle layering using the scsn phases and internal reflections observed within the reverberative interval of sh polarized seismograms mantle reflectivity profiles for 18 seismic corridors sampling portions of indonesia australia and the western and central pacific are constructed from low frequency 25 mhz scs reverberations i e scsn type phases reflected once from internal mantle discontinuities modeling the reflectivity profiles using synthetic seismograms yields precise path specific estimates of the travel times to the 410 km and 660 km discontinuities and their reflection coefficients the data are consistent with an olivine dominated model of the transition zone marked by two major phase transitions having average apparent depths of 414 2 km and 660 2 km and impedance contrasts of 0 046 0 010 and 0 072 0 010 respectively the interpath variations observed in the reverberation travel times imply that the topography on the two discontinuities is of the order of 20 km peak to peak and is negatively correlated consistent with an endothermic transition at 660 km whose clapeyron slope magnitude is comparable to that of the 410 km exothermic transition in the case of the 660 km discontinuity we have been able to supplement these long wavelength 1500 5000 km observations with an estimate of intrapath topography inferred from the frequency dependence of the 660 km reflection coefficient which gives peak to peak variations less than 40 km on scale lengths of 500 1500 km for the 660 km discontinuity the observed topography is significantly less than the dynamic topography expected for a simple compositional interface in regions of subduction however we do observed an intriguing negative correlation between the apparent depth and the reflection coefficient of the 660 km discontinuity which may involve small compositional variations plausible explanations for this correlation include heating of an initially cool chemical boundary layer gravitationally trapped above an endothermic phase transition loss of reflected energy due to local curvature or roughness of the reflector and or extreme topography occurring on a small compositional component to the discontinuity 40 of the total impedance contrast by stripping the 410 km and 660 km peaks from the composite path reflectivity profile we identify three minor reflectors at mean depths of 520 710 and 900 km the shallowest may mark the phase spinel phase transition the 710 km discontinuity can be attributed to the ilmenite gamet perovskite phase transition although changes in perovskite symmetry can potentially provide explanations for both the 710 km and 900 km features

upper mantle structure of the southwest pacific from regional waveform inversion we determine the upper mantle seismological structure of the southwest pacific by inverting complete regional seismograms recorded by the southwest pacific seismic experiment a deployment of 12 broadband seismographs in the fiji tonga and niue islands we first present a method to invert entire seismic waveforms for a laterally homogeneous velocity structure partial derivatives are computed using an efficient reflectivity code for differential seismograms randall 1994 and a conjugate gradient method is used to perform the nonlinear inversion band pass filtered 10 55 mhz vertical and radial component seismograms extending from the p arrival to the surface waves are used to constrain the structure waveforms are selected to provide regional distance 400 1500 km propagation paths predominantly within one of the tectonic regions of the southwest pacific source depths ranging from 10 to 240 km allow the deeper regions of the model to be resolved with relatively short path lengths results show extremely large lateral variability in the upper mantle structure of the southwest pacific the inactive south fiji basin shows upper mantle velocities much lower than those previously observed for the pacific lithosphere of a similar age the north fiji and lau basins containing active back arc spreading systems show upper mantle shear velocities as slow as 3 8 km s 1 the exceptionally low seismic velocities in the active back arc regions may represent the slowest upper mantle velocities in the world the velocity difference between the active back arc basins and the old pacific lithosphere immediately east of the tonga trench is maximum at 30 90 km depth with a variation of 18 indicating partial melting in this depth range velocity heterogeneity of 4 extends to a depth of 170 km velocity structure below 220 km depth is poorly resolved but there is some evidence of a 2 difference extending to depths of 350 km

detailed structure and sharpness of upper mantle discontinuities in the tonga subduction zone from regional broadband arrays 1 recordings of deep tonga earthquakes from two arrays of 12 broadband seismographs each in the fiji and tonga islands are stacked and searched for reflections and conversions from upper mantle discontinuities in the tonga subduction zone the arrays operated as part of the seismic arrays in fiji and tonga saft experiment from july 2001 to august 2002 in comparison with the commonly used teleseismic approaches the short path lengths for the local data provide smaller fresnel zones and high frequency content for precise mapping of discontinuity topography and sharpness to enhance the low amplitude discontinuity phases s410p p660p and s660p deconvolved seismograms from each event array pair are aligned on the maximum amplitude of the direct p wave and subsequently slant stacked for the 410 km discontinuity the results show no systematic variations in depth with distance to the cold slab the 660 km discontinuity varies between 656 and 714 km in depth for the southern and central parts of the subduction zone the largest depths occur in the core of the tonga slab for the northern part two separate depressions of the 660 km discontinuity are observed these anomalies are interpreted as being induced by the active steeply subducting tonga deep zone and a subhorizontally lying remnant of subducted lithosphere from the fossil vityaz trench respectively interpreting the deflections of the 660 km discontinuity in terms of local temperatures implies a thermal anomaly of 800 k to 1200 k at 660 km depth except for the southern region where it may thicken the width of the depressed 660 km discontinuity region implies that the tonga slab seems to penetrate the 660 km discontinuity with little deformation waveform modeling suggests that both the 410 and 660 km discontinuities are sharp the 660 km discontinuity is at most 2 km thick in many parts of the region and a first order discontinuity cannot be precluded the 410 km discontinuity thickness shows somewhat more variability and ranges from 2 to 10 km outside the slab and is at most 10 km thick within the slab this suggests that the subduction process does not produce dramatic effects on the sharpness of the discontinuities

what controls dead ice melting under different climate conditions a discussion abstract in the geological record hummocky dead ice moraines represent the final product of the melt out of dead ice processes and rates of dead ice melting in ice cored moraines and at debris covered glaciers are commonly believed to be governed by climate and debris cover properties here backwasting rates from 14 dead ice areas are assessed in relation to mean annual air temperature mean summer air temperature mean annual precipitation mean summer precipitation and annual sum of positive degree days the highest correlation was found between backwasting rate and mean annual air temperature however the correlation between melt rates and climate parameters is low stressing that processes and topography play a major role in governing the rates of backwasting the rates of backwasting from modern glacial environments should serve as input to de icing models for ancient dead ice areas in order to assess the mode and duration of deposition a challenge for future explorations of dead ice environments is to obtain long term records of field based monitoring of melt progression furthermore many modern satellite borne sensors have high potentials for recordings of multi temporal digital elevation models dems for detection and quantification of changes in dead ice environments in recent years high accuracy dems from airborne laser scanning altimetry lidar are emerging as an additional data source however time series of high resolution aerial photographs remain essential for both visual inspection and high resolution stereographic dem production

dating buried glacier ice using cosmogenic 3he in surface clasts theory and application to mullins glacier antarctica abstract we develop a modeling framework to describe the accumulation of terrestrial cosmogenic 3 he in antarctic debris covered glaciers the framework helps quantify the expected range in cosmogenic nuclide inventories for measured clasts at the surface of supraglacial debris we first delineate the physical factors that impact clast movement within and on top of debris covered glaciers including the effects of 1 ice ablation 2 erosion at the debris surface and 3 stochastic geomorphic processes that impact clast movement within and on top of supraglacial debris we then explicitly calculate the impact of each process in altering the total inventory of cosmogenic nuclides in surface clasts assuming basic elements of ice dynamics and debris entrainment are known the model results provide an estimate for the total accumulation of cosmogenic nuclides as well as the expected range in nuclide inventories for any clast at the surface of debris covered glaciers because the values are quantified the approach can be applied to help evaluate the robustness of existing and future cosmogenic datasets applied to these systems as a test we applied our model framework towards mullins glacier a cold based debris covered alpine glacier in the dry valleys of antarctica our simulated values for cosmogenic nuclide inventories compare well with those previously measured from fifteen surface cobbles along mullins glacier 3 he both in terms of expected ranges and absolute values and suggest that our model framework adequately incorporates most of the complicating factors that impact cosmogenic datasets for cold based debris covered glaciers relating these cosmogenic nuclide inventories to ice ages the results show that ice within mullins glacier increases non linearly ranging from 12 ka to 220 ka in areas of active flow to 1 6 ma in areas of slow moving to stagnant ice

seismic evidence for a thinner mantle transition zone beneath the south pacific superswell broadband seismograms recorded by the seismic stations deployed on oceanic islands in the south pacific for two deep earthquakes in 1998 are used to investigate the mantle transition zone structure beneath the south pacific where a large scale hot plume might ascend from the core mantle boundary cmb by stacking s waves reflected at the mantle transition zone discontinuities we find clear signals associated with the 410 km and 660 km discontinuities the transition zone thickness which is constrained from the travel time difference between the reflected waves from the 410 km and 660 km discontinuities is observed to be approximately 15 km thinner beneath the south pacific superswell than that beneath other regions the result suggests that the transition zone temperature beneath french polynesia is approximately 100 200 k higher than the surrounding mantle

ice ablation and meteorological conditions on the debris covered area of baltoro glacier karakoram pakistan during the recent italian expedition k2 2004 50 years later june july 2004 on baltoro glacier karakoram pakistan glaciological field experiments were carried out on the debris covered area of this high elevation glacier the aim was to investigate the ice ablation and its relations with debris thermal properties and meteorological conditions ablation measurements along the glacier up to about 5000 m and within a dedicated test field were combined with meteorological data from two automatic weather stations located at urdukas 4022 m a s l and at k2 base camp 5033 m a s l in addition temperature measurements of the debris cover at different depth levels along the glacier allowed the calculation of debris surface temperature and of the debris thermal resistance r using the air temperature the local mean lapse rate 0 00758 cm 1 and the measured ablation the degree day factors k at different locations on the glacier were calculated the ice ablation rates were related to debris thickness and elevation they are typically on the order of 4 cm d 1 during the observation period however it was found that the surface topography slope aspect has an influence on the total ablation similar to that of the debris thickness thermal resistance of the debris cover and its distribution over the glacier were estimated finally a best guess estimate of the total meltwater production was calculated from available climate data

pervasive seismic wave reflectivity and metasomatism of the tonga mantle wedge subduction zones play critical roles in the recycling of oceanic lithosphere and the generation of continental crust seismic imaging can reveal structures associated with key dynamic processes occurring in the upper mantle wedge above the sinking oceanic slab three dimensional images of reflecting interfaces throughout the upper mantle wedge above the subducting tonga slab were obtained by migration of teleseismic recordings of underside p and s wave reflections laterally continuous weak reflectors with tens of kilometers of topography were detected at depths near 90 125 200 250 300 330 390 410 and 450 kilometers p and s wave impedances decreased at the 330 kilometer and 450 kilometer reflectors and s wave impedance decreased near 200 kilometers in the vicinity of the slab and near 390 kilometers just above the global 410 kilometer increase the pervasive seismic reflectivity results from phase transitions and compositional zonation associated with extensive metasomatism involving slab derived fluids rising through the wedge

depression of the 660 km discontinuity beneath the tonga slab determined from near vertical scs reverberations the occurrence of the march 9 1994 mw 7 6 deep tonga earthquake during a temporary deployment of broadband seismographs provides an excellent opportunity to study the seismic discontinuity structure in the vicinity of a subducting slab near vertical scs reverberations from the 410 520 and 660 discontinuities are visible at 10 stations in the south pacific at frequencies between 0 003 0 033 hz without stacking we use a regional velocity model and ray theory to model the interval from scs to scs3 a grid search is then performed on the depth and impedance contrast at each of the seismic discontinuities to optimize the fit to both the underside and topside reflections stacked records indicate prominent discontinuities at 400 km and 665 km depth beneath and to the west of the slab and 410 km 500 km and 640 km to the east of the slab analysis of individual records suggests a local depression of 30 km in the 660 and a broadening of the transition zone directly beneath and to the west of the tonga slab in a region of possible near horizontal slab these results are consistent with a large cool region just to the west of the intersection of the tonga slab with the 660

implications of slab mineralogy for subduction dynamics phase relations among mantle minerals are perturbed by the thermal environment of subducting slabs both under equilibrium and disequilibrium metastable conditions such perturbations yield anomalies not only in seismic velocities but also in density the buoyancy forces arising from these density anomalies may exert several important effects they contribute to the stress field within the slab in a fashion consistent with observed patterns of seismicity they may affect subduction rates both by inducing time dependent velocity changes under equilibrium conditions and by imposing velocity limits through a thermal feedback loop under disequilibrium conditions they may affect slab morphology possibly inhibiting penetration of slabs into the lower mantle and allowing temporary stagnation of deflected or detached slabs latent heat release from phase transitions under disequilibrium conditions in slabs can yield isobaric superheating which may generate adiabatic shear instabilities capable of triggering deep seismicity 2001 elsevier science b v all rights reserved

contributions and unrealized potential contributions of cosmogenic nuclide exposure dating to glacier chronology 1990 2010 abstract this paper reviews the application of cosmogenic nuclide exposure dating to glacier chronology exposure dating of glacial landforms has made an outsize impact on this field because the technique filled an obvious need that had already been recognized by glacial geologists by now hundreds of studies have used cosmogenic nuclide exposure dating to date glacial deposits and in fact it is rare to find a study of glacial geology or glacier chronology or any paleoclimate synthesis that makes use of such studies that does not involve exposure dating these developments have resulted in major contributions to glacier chronology and paleoclimate in particular i reconstructing antarctic ice sheet change ii establishing the chronology of late pleistocene and holocene glacier change in mountain regions where it was previously unknown iii establishing the broad chronological outlines of mountain glaciations prior to the last glacial maximum and iv gaining insight into subglacial erosional processes through the observation that many glaciated surfaces preserve cosmogenic nuclide inventories from long past ice free periods as well as the present one an important potential future contribution will be the application of the large data set of exposure dated glacier chronologies to better understand global and regional climate dynamics during lateglacial and holocene millennial scale climate changes however this contribution cannot be realized without significant progress in two areas i understanding and accounting for geologic processes that cause apparent exposure ages on glacial landforms to differ from the true age of the landform and ii minimizing systematic uncertainties in exposure ages that stem from cosmogenic nuclide production rate estimates and scaling schemes at present there exists an enormous data set of exposure ages on glacial deposits but these data cannot be used to their full potential in paleoclimate syntheses due to an inadequate understanding of geologic scatter and production rate uncertainties the intent of this paper is to highlight this situation and suggest some strategies for realizing this potential

two chloride sources in soils of the mcmurdo dry valleys antarctica 1 sulfate nitrate and chloride are major water soluble anions in soils of the mcmurdo dry valleys mdv antarctica we measured the chloride stable isotope composition of 40 soil samples from 16 different soil profiles the soil profiles come from three different microclimate zones in the mdv a coastal thaw zone which fosters a seasonally saturated active layer a stable upland zone in which air and soil temperatures are too cold to foster active layers and or significant snowmelt and an intermediate inland mixed zone results show that the total water soluble chloride content is low in both the coastal and the upland zones but highly variable in the inland mixed zone average 770 to 11520 ppm individual soil 37cl values range from 2 10 to 0 56 with profile average 37cl values broadly decreasing from the coast toward upland valleys vertically the 37cl increases with depth in soil profiles developed on basal tills but decreases on sublimation tills we propose that the spatial cl and 37cl patterns are the result of continuous influx and leaching of two major chloride sources an airborne sea salt chloride ssc with a 37cl value 0 0 and a secondary atmospheric chloride sac with a 37cl value 2 10 our model suggests that sublimation tills have their cl input from two boundaries a till atmosphere interface at the top and a buried ice till interface at the bottom differing distinctively from basal tills where there is no salt contribution from the bottom our model also suggests a higher influx of ssc relative to sac in the mdv in the past overall characterizing the sources budgets and spatial vertical patterns of chloride constitutes a first step toward unraveling climate tectonic and landscape changes archived in mdv soil profiles

the role of thermal stress fatigue in the breakdown of rock in cold regions abstract many geomorphological studies have ignored the role of thermal stress fatigue engineers have long maintained that thermal stress is an effective process physicists and the ceramics industry have provided a theoretical foundation to explain the nature and mode of operation of thermal breakdown recent geomorphological studies have shown the importance of thermal stress in cold regions by means of high frequency rock temperature data which can identify rates of temperature change in excess of 2 c min 1 the occurrence of weathered material in cold but dry regions can be explained by thermal stress shock differential radiation receipts on slopes of varying orientation and gradient can exert a very strong effect

cold based debris covered glaciers evaluating their potential as climate archives through studies of ground penetrating radar and surface morphology we describe the morphology and internal structure of mullins and friedman glaciers two cold based debris covered alpine glaciers that occur in neighboring valleys in the mcmurdo dry valleys antarctica both glaciers are overlain by a single dry supraglacial debris layer 8 75 cm thick each mantling debris layer is marked with near identical patterns of arcuate ridges and steps as well as corresponding changes in bulk grain size meter scale surface topography and thermal contraction crack polygons results from 24 km of ground penetrating radar data show that the ice within the uppermost 1 2 km of mullins and friedman glaciers is essentially free of englacial debris 1 by volume but thereafter is interspersed with bands of englacial debris each 3 m thick and dipping up glacier that intersect the ice surface at all major surface ridges and steps the similarity in number and pattern of englacial debris bands and corresponding surface ridges and steps across both glaciers along with model results and observations that call for negligible basal entrainment is best explained by episodic environmental change at valley headwalls our working hypothesis is that layers of englacial debris originate as supraglacial lags that form in ice accumulation areas during times of reduced net ice accumulation following renewed net ice accumulation these lags are subsequently buried by snow and ice flow englacially and intersect the ice surface to impart distinctive changes in the texture of supraglacial debris and topographic relief the implication is that the englacial structure and surface morphology of these cold based debris covered glaciers preserves a consistent record of climate and environmental change

dynamics of double subduction numerical modeling abstract double subduction is a geodynamic process in which two plates following each other are synchronously subducted double subductions are known for both modern izu bonin marianas and ryukyu arcs and ancient west himalaya collision zone plate tectonics however our knowledge about this process is limited to conceptual schemes and some restricted analogue experiments in order to fill this gap we performed 2d numerical experiments using a coupled petrological thermomechanical approach based on finite differences and marker in cell techniques combined with thermodynamic database for the mantle we investigated the influence of convergence rate intermediate plate length activation volume of the mantle dislocation creep and age of the lithosphere based on these experiments we conclude that a subduction rates at two zones running in parallel differ and vary in time even when the total convergence rate remains constant supremacy of either subduction zone depends on physical parameters such as i relative rates of the plates ii slab ages and iii length of the middle plate b subduction dynamics of the double subduction system involves several processes unknown in simple subduction systems such as i eduction i e un subduction ii subduction re initiation iii subduction flip triggered by shallow slab breakoff and iv turn over of detached slabs to up side down attitudes c simulated tomographic structures related to slab propagation account for both penetration and non penetration of the 660 km discontinuity non penetration is favored by i low convergence rate ii faster relative movement of the overriding plate iii young age of the subducting slab and iv up side down turn over of detached slab

seismic evidence against a mantle chemical discontinuity near 660 km depth beneath izu bonin we analyze p wave codas from 12 deep izu bonin earthquakes recorded by hundreds of western united states seismograph stations for evidence of small amplitude phases caused by near source mantle discontinuities for nearly every event the dominant phase in the coda is the result of an s to p conversion from a nearly horizontal discontinuity ranging in depth from 650 to 690 km this is interpreted as a thermally depressed spinel to perovskite and magnesiowustite phase transition if the 660 km seismic discontinuity is also associated with a change in chemistry it would be dynamically depressed by a subducting slab we consistently see that there is no nearly horizontal discontinuity between 700 and 1000 km with shear wave velocity contrast exceeding 1 this observation places constraints on the properties of a postulated chemical discontinuity separating the upper and lower mantles

mechanism of slab thickening near 670 km under indonesia we examine a new data set of reliably relocated earthquakes deeper than 400 km under indonesia developed by schoffel and das 1999 the accuracy of this data allows us to demonstrate that the slab thickens shortens and weakens before penetrating below 670 km by shearing along conjugate fault planes on the upper and lower portions of the seismic zone we also show that the earthquakes in the lower portion of the wadati benioff zone width are generally fewer and smaller than those in the central portions with only 8 of the total moment released in the period 1976 1996 being due to earthquakes on the lower side

role of debris cover in the thermal physics of glaciers modele de fonte de la glace en presence de debris morainiques on en deduit une technique de noircissement de la surface des glaciers en vue d augmenter le debit d eau

calculating ice melt beneath a debris layer using meteorological data generalized numerical models of sub debris ice ablation are preferable to empirical approaches for predicting runoff and glacier response to climate change as empirical methods are site specific and strongly dependent upon the conditions prevailing during the measurement period we present a modified surface energy balance model to calculate melt beneath a surface debris layer from daily mean meteorological variables despite numerous simplifications the model performs well and modelled melt rates give a good match to observed melt rates suggesting that this model can produce reliable estimates of ablation rate beneath debris layers several decimetres thick this is a useful improvement on previous models which are inappropriate for thick debris cover

thickness of the mantle transition zone beneath the south pacific as inferred from analyses of scs reverberated and ps converted waves abstract we study the thickness of the mantle transition zone beneath the south pacific in order to determine temperature anomalies associated with presumed hot plumes in the south pacific superswell the scs reverberation method is used to determine the average thickness between an event station pair and the velocity spectrum stacking vss method is used to determine the thickness near each station the thickness obtained by the scs reverberation method shows that the average thickness beneath the superswell is less than the globally averaged thickness by 6 km while the average thickness beneath the tonga fiji subduction is greater than the global average by 10 km the vss method shows nearly normal thickness beneath stations used in the superswell region we conclude that substantial thinning of the transition zone associated with presumed hot plumes occurs locally beneath the society hot spot as revealed by our previous ss precursor study and is not widespread beneath the whole area of the superswell

whole mantle shear structure beneath the east pacific rise we model broadband seismograms containing triplicated s s 2 and s 3 along with scs to produce a pure path one dimensional model extending from the crust to the core mantle boundary beneath the east pacific rise we simultaneously model all body wave shapes and amplitudes thereby eliminating depth velocity ambiguities the data consist of western north american broadband recordings of east pacific rise epr affiliate transform events that form a continuous record section out to 82 and sample nearly the entire east pacific rise the best fitting synthetics contain attenuation and small changes in lithospheric thickness needed to correct for variation in bounce point ages the 660 km discontinuity is particularly well resolved and requires a steep gradient 4 extending down to 745 km we find no discernible variation in apparent depths of the 405 and 660 km discontinuities over ridge orthogonal distances on the order of 1000 km or 20 ma lithosphere body waveform comparisons indicate that we can resolve discontinuity depths to less than 10 km providing an upper limit to transition zone topography these depth estimates in conjunction with the fan shot nature of the ray paths lower the detection limit from s 2 precursor analysis of the lateral length scale over which short wavelength topographic variation could occur and indicate the sub epr transition zone and upper mantle are remarkably homogeneous the lower mantle beneath the east pacific rise is well modeled by prem with the greatest variation occurring in scs reflecting strong heterogeneity along the core mantle boundary together these observations require that the east pacific rise spreading ridge cannot be actively supplied from the local lower mantle and that tomographically imaged lateral variation beneath the ridge likely reflects lateral smearing of outlying velocity gradients dynamically the transition zone therefore appears vertically decoupled from the overlying east pacific rise spreading system