a reply in comments by g kaser on the use of planimetric surface area in glacier mass balance calculations a potential source of errors by jacobsen and theakstone

flood geomorphology of arthurs rock gulch colorado paleoflood history abstract episodic late quaternary flooding is recorded by bouldery deposits and slackwater sediments along arthurs rock gulch an ephemeral stream west of fort collins colorado flood deposits consist of individual granodiorite and pegmatite boulders boulder bars and coarse overbank sediment that rest on erosional terrace segments along the channel we identified evidence for at least five flood in the lower two thirds of the 1 84 km 2 drainage basin flood deposits are differentiated by their position above the active channel weathering characteristics degree of boulder burial by colluvium amount of lichen cover and position with respect to terrace and colluvial deposits age estimates for the flood deposits are based on radiocarbon dating tree ring analyses and relative age criteria from four sites in the basin at least two floods occurred in the last 300 years a third flood is at least 5000 years old but likely younger than 10 000 yr bp and the two oldest floods occurred at least 40 000 years bp

earthquake swarms precursory to moderate to great earthquakes in the northeast india region abstract the northeast india region has seen ten m 7 1 2 earthquakes since 1897 including two great earthquakes of m 8 7 in 1897 and 1950 the last m 7 1 2 earthquake occurred on august 17 1952 with the exception of three earthquakes others are found to be associated with periods of background normal seismicity precursory swarms quiescence and mainshocks and the associated foreshocks and aftershocks the dataset has been critically examined for completeness considering the current capabilities for defining locations and also considering other parameters regression equations relating the mainshock magnitude mm the average magnitude of the largest two events in the swarm mp and the time interval mp between the beginning of the swarm and the mainshock have been developed these are mm 1 37mp 1 41 and mm 3log10tp 3 27 it is important to recognize precursory swarms and quiescence in real time indeed we believe we have recognized one such sequence in the vicinity of the arakan yoma fold belt on the basis of mp and tp values the lateral extent of swarm activity and the fact that no event of m 6 has occurred since 1975 in the preparation zone defined by the 1963 1965 swarm we estimate that an m 8 earthquake could occur at any time in the area bounded by 21 and 25 1 2 n and 93 and 96 e the focal depth of this impending earthquake is estimated to be 100 40 km in view of the focal depths of the other events in the swarm

mass balance of rhonegletscher during 1882 83 1986 87 les bilans de masse effectues sur le glacier des sources du rhone alpes suisses pendant differentes periodes 1884 85 1908 09 1979 80 et 1981 82 sont presentes

earthquake prediction in northeast india a review a multidimensional geophysical approach is made to study earthquake precursors in northeast india resistivity gravity and seismological precursor anomalies for the two recent damaging earthquakes one on december 30 1984 magnitude 5 8 and the other on august 6 1988 magnitude 7 5 are very encouraging to intensify these studies in the region improved methods of magnetic and geohydrological precursor studies are suggested

seismotectonics in northeast india a stress analysis of focal mechanism solutions of earthquakes and its kinematic implications summary in northeast india three major plates interact along two convergent boundaries the himalayas and the indo burma ranges which meet at the assam syntaxis to clarify this tectonic interaction and the underlying dynamics we determine the regional seismotectonic stress from the stress inversion of 285 double couple focal mechanism solutions of earthquakes with an average magnitude of 5 we then compare the reconstructed stress regimes with the available information about geodetically determined relative displacements north south compression in a direction consistent with india eurasia convergence prevails in the whole area from the eastern himalayas to the bengal basin through the shillong mikir massif and the upper assam valley e w extension in tibet is related to this n s india eurasia convergence not only does the major n s compression affect the outer segments of the indo burma ranges it also extends into the descending slab of indian lithosphere below these ranges although stresses at depth are controlled by bending of the slab beneath the burmese arc the existence of widespread n s compression in the bengal basin far away from the himalayan front is compatible with the previously proposed convergence between a shillong mikir assam valley block and the indian craton e w compression inside this block supports the hypothesis of a component of eastward extrusion stress inversion of focal mechanism solutions in the indo burma ranges reveals a complex stress pattern the burmese arc and its underlying lithosphere experience nearly arcperpendicular extension with ese wnw trends in the northernmost ne trending segment and ene wsw trends in the main n s arc segment such extensional stress documented from many arcs is likely a response to pull from and bending of the subducting plate at the same time the indo burma ranges are under compression as a result of oblique convergence between the sunda and indian plates the maximum compressive stress rotates from ne sw across the inner and northern arc to e w near the bengal basin this rotation is consistent with the deformation partitioning reflected in the rotation of relative displacement vectors from a ssw directed sunda burma motion to a wsw directed burma india motion as a consequence of this partitioning the major belt parallel fault zones show a variety of movements across the main n s arc segment from right lateral slip in the inner ranges to oblique reverse dextral slip in the outer ranges and pure thrusting in the westernmost foreland belt

anomalous behaviour of precursor resistivity in shillong area ne india summary daily measurement of apparent resistivity from 1984 may to 1985 october in shillong plateau india has been carried out for an earthquake precursor study the resistivity changes which occur 7 to 10 days before earthquakes do not necessarily satisfy the dilatancy hypothesis which of late has been subjected to criticisms of not being a generalized phenomenological explanation of precursor resistivity variation prior to earthquakes resistivity may increase or decrease or may not even show any change depending on the orientation of measuring electrodes the elastic nature of the geological formation where observations are made and the direction of tectonic stress

discharges of turbid water during mini surges of variegated glacier alaska u s a discharges of water sediment and dissolved impurities from variegated glacier ala ska were monitored in the early summers of 1980 and 1981 during the occurrence of mini surg es sea sonal trends weat her related eve nts and diurnal variat ions similar to behavior of other temperat e gl acier streams were found the prin cipal effect in the stream associated with mini surge occurrence was a brief discharge of extremely turbi d water the turbidity is assumed to be introduced into the basal hydraulic system by initiation of the fast motion of a mini surge at a time and locat ion on the upper glacier known from oth er meas ure ments the mean water velocity in the hydraulic system over the int erven ing distance is thereby determined 0 3 m s i the mean water velocity together with the water discharge 16 m3 s 1 at the terminus places con straints on the distribution of water velocity u and total cros s sectional area at of the flow paths along the gl acier base this leads to the con clusion that within the zone of mini surge occurrence in its unpe rturbed state u is about 0 1 m s 1 or possibly less at is about 10 2 m2 or possibly more and it must be divided into a very large number of small passag eways be blocked by constrict ions or both the total water cro ss se ction corresponds to a layer 0 1 2 m thick when spr ead unifo rmly over the glacier width the water velocity is clos e to or less than the propa gation velo city of the mini s urges bet ween the zone affected by mini surges and the stream a dyna mically less active lower section of the glacier is proba bly underlain by a small number of conduits in which the water velocity may be very high 2 m s i water di scharge follo wing the mini surges puts an upper limit on water storage changes ass ociated with the anomalous ice motion

hydrodynamic interpretation of a boulder berm and associated debris torrent deposits abstract the hydrodynamic environment associated with a boulder berm is reconstructed from the geometry and sedimentology of the berm and associated debris torrent deposits berms may result from a variety of hydrodynamic scenarios but in this instance the berm was associated with flow separation downstream of an abrupt change in channel morphology sediment fabric analyses are summarized particularly as eigenvalues and these data compared with the fabric of debris flows and clear water gravel deposits although data are few the eigenvalue results can be interpreted consistently as reflecting a transition between the end members of a sediment fabric continuum the value of detailed studies of berm geometry and sedimentology is discussed with reference to their hydraulic significance this latter aspect is important for palaeo discharge reconstruction of both newtonian and non newtonian flows

solute dynamics of meltwater of gangotri glacier garhwal himalaya india the present study investigates solute dynamics of meltwater of gangotri glacier system in terms of association of different chemical compounds with the geology of the area in the meltwater the presence of cations varied as c mg2 c ca2 c na c k while order of concentration of anions has been c hco3 c so42 c cl c no3 in years 2003 and 2004 the magnesium and calcium are found as the dominant cations along with bicarbonate and sulphate as dominant anions the high ratios of c ca2 mg2 total cations and c ca2 mg2 c na k indicate that the meltwater chemistry of the gangotri glacier system catchment is mostly controlled by carbonate weathering attempts are made to develop rating curves for discharge and different cations sporadic rise in discharge without corresponding rise in concentration of most of cations is responsible for their loose correlation in a compound valley glacier like gangotri glacier

stable imbrication and delicate unstable settings in coastal boulder deposits indicators for tsunami dislocation abstract medium sized and coarse debris pebbles and boulders can exhibit patterns of setting particularly if they are elongated or tabular these patterns such as imbrication indicate certain transport processes mostly as to direction and character many years of observation and measuring of coarse coastal deposits are the basis for the conclusions presented here on environmental transport processes their character and their energy beside imbrication special emphasis is given to unstable or delicate settings of large boulders that evidently have remained in these positions from hundreds to thousands of years this deserves an explanation to how they arrived at these positions and why they have not changed into a more stable position given an environment of constant energy input along coastlines the discussion lays emphasis on whether both settings of mega boulders can be indicative of tsunami transport processes

isotopic and ionic variations in glacier river water during three contrasting ablation seasons the significance of the baseflow component of glacier river discharge in summer varies with geographical location altitude glacier geometry and glacier size baseflow is maintained by meltwater generated above the transient equilibrium line and by water released from temporary storage on in or beneath the glacier at the norwegian glacier austre okstindbreen where precipitation is generally high throughout the year and the summers are cool and wet observations in three successive but contrasting years have shown that na ion concentrations in the glacier river water are influenced strongly by the amount of snowmelt this itself depends on the preceding winter conditions which determine the amount of accumulation and on the current summer s weather the efficiency of the glacier s drainage systems depends on the general progress of summer ablation the speed with which the systems develop influences ion provision from subglacial sources ca 2 ion concentrations are largely determined by subglacial conditions oxygen isotope variations in glacier river water reflect the relative contributions made to total discharge by snow meltwater and other sources the composition of the snow cover which is a function of winter temperatures has a strong influence ice meltwater has low isotopic variability but the isotopic composition of rainfall varies markedly a simple model of mixing of englacial and subglacial waters each of a constant composition cannot be applied to a high latitude glacier of the size and altitudinal range of austre okstindbreen

temporal changes of glacier hydrological systems indicated by isotopic and related observations at austre okstindbreen okstindan norway 1976 87

sediment concentration versus water discharge during single hydrologic events in rivers relations between sediment concentration c and water discharge q for a hydrologic event such as a flood are studied qualitatively by analyzing smoothed temporal graphs discharge and concentration vs time in terms of mode spread and skewness comparing cq ratios at a given discharge on the rising and falling limbs of the discharge hydrograph provides a consistent reliable method for categorizing c q relations five common classes of such relations are single valued straight or curved clockwise loop counterclockwise loop single valued plus a loop and figure eight temporal graph mode and skewness influence the type of relation whereas temporal graph spread affects the details of the particular c q relation its graphical breadth shape orientation and plotted location field examples of the various types of relations are given including varieties that heretofore have received little attention such as the figure eight explanations for each type of c q relation are discussed

subglacial hydrology and sediment transport at bondhusbreen southwest norway tunnels driven in rock under the glacier bondhusbreen in norway and thence upward to the ice rock interface provide a rare opportunity to study a subglacial drainage system early in the melt season suspended sediment discharges in the subglacial water channels are large and channels apparently change position these effects are inferred to be a consequence of increased separation of ice from the bed due to increased water pressure water pressures are believed to be high at this time of year because water channels having closed by plastic deformation during the winter when discharges are low are too small to handle the increasing spring discharges water thus backs up in the channel system calculations suggest that further changes in channel position occur as discharges increase because the potential energy released by the water is able to melt ice in the walls of channels faster than it is replaced by inward flow of ice channels can thus trend diagonally across the direction of glacier flow although such changes may occur at any time during the melt season in response to variations in discharge there appears to be a seasonal evolution of the drainage pattern under bondhusbreen that is repeated each year these shifting channel patterns make the engineering task of locating and diverting water beneath glaciers more difficult than anticipated

spatial and temporal variations of oxygen isotopes in snowpacks and glacial runoff in different types of glacial area in western china in order to improve understanding of spatial and temporal variations of stable isotopes in atmospheric precipitation snow cover and glacier meltwater in different regions of china samples were collected for isotopic analysis in four areas yulong mountain yunnan himalaya temperate glacier area samdain kangsang mountain nyainqentanglha shan subpolar glacier area the headwater area of the urumqi river tien shan subpolar glacier area and muztag mountain pamirs polar glacier area sampling was undertaken in both summer and winter between 2000 and 2003 the d 18 o values show a temperature altitude effect in new winter snow on yulong mountain reflecting the condensation and fractionation processes associated with the winter monsoon but a different more complex pattern in residual snow deposited during the summer monsoon this old snow is influenced by the precipitation amount effect solar radiation and evaporation and the water content of the snowpack the summer precipitation at samdain kangsang mountain is associated with the summer monsoon there is a marked precipitation amount effect during the long passage of the southwest india monsoon from the distant moisture source to samdain kangsang mountain and the summer precipitation is strongly depleted of the heavy isotope above 6000 m the high radiation flux causes much evaporation from the snow surface the associated 18 o enrichment of the snow is reflected in a reverse altitude effect the d 18 o values in the summer snowpack of the tien shan and muztag mountain decrease with increasing altitude and decreasing air temperature indicating a temperature altitude effect post depositional processes cause isotopic changes during the transformation of snow firn ice to meltwater the effects are much stronger at temperate than at polar glaciers moreover changes in the isotopic signal at both temperate and polar glaciers can result from evaporation sublimation ablation and drifting

hydrological discharges and motion of fels and black rapids glaciers alaska u s a implications for the structure of their drainage systems characteristics of the hydrology and motion of black rapids and fels glaciers alaska were observed from 1986 to 1989 hydrological measurements included stage electrical conductivity and suspended sediment concentration in the discharge stream of each glacier and were made at 0 5 1 h intervals continuously through most of the melt seasons variations in the glacier speed were monitored through the full year at a number of locations along the length of each glacier using time lapse photography 1 d time resolution strain meters 0 5 1 h resolution and seismometers set up to count acoustic emissions both glaciers show similar seasonal diurnal and short term event changes in hydrological discharges and ice speed the hydrological behavior is analyzed in terms of a fast sub system composed of surface streams moulins and large tunnels with discharge that responds rapidly and a slow sub system composed of heterogeneous small passageways through the ice and distributed over the bed that maintain approximately uniform discharge over a day the timing and amplitude of water discharge in the diurnal cycle indicate that roughly 10 40 of the water is routed directly into the fast system the remaining 90 60 of the water enters the slow system dilution of the solute discharged from the slow system by the variable discharge in the fast system results in changes in water discharge and solute concentration that are approximately equal in relative amplitude and inversely related a small time lag from discharge maximum minimum to solute minimum maximum suggests that the fast system is confined to roughly the lowermost 30 40 of the full glacier length the residence time of water in the fast system is short compared to 1 d the slow system contains both short and long residence time passages characteristics of the diurnal cycles are somewhat variable through the melt season but no systematic evolutionary patterns were discerned even though large changes in the mean discharges of water and solutes occur which suggests parallel evolution of the variables that control the response of the fast system events were characterized by contemporaneous increases in suspended sediment concentration in the discharge water and distinct changes in straining on the glaciers events caused by increases in melt or precipitation related to weather and events related to release from reservoirs internal to the glaciers could be distinguished based on the changes in electrical conductivity of the discharge water the correlated changes in sediment discharge and motion of the glaciers indicate that the events were associated with temporary modifications of the slow passages distributed over the bed that allowed enhanced sliding and access of basal water flow to erosion products hydrological differences between black rapids and fels glaciers can be explained by differences in the size of the glaciers if there is a difference in bed structure that explains the difference in dynamics surge black rapids glacier versus non surge fels glacier it does not affect the hydrological parameters that were observed

pebble population ellipsoid a three dimensional algebraic description of sedimentary fabric orientation and shape of clasts can be approximated by an ellipsoid such ellipsoids are easily transformed into a fixed geographic coordinate system and provide a 3 x 3 matrix description of the pebble these matrices can be summed and an average matrix determined eigenvector calculations produce the principal axes of the population ellipsoid paleocurrent interpretations based on these ellipsoids agree with other geological data other potential uses include the calculation of average clast dimension in any direction the calculation of volume changes associated with erosion and the characterization of various sedimentary environments deformed clasts are analyzed to illustrate the application of the population ellipse to strain studies

reversal of the middle upper songhua river in the late early pleistocene northeast china abstract the study of drainage evolution in the songnen plain northeast china is essential for understanding the interactions between tectonics topography climate drainage evolution in the region despite this it remains unclear whether the paleo songhua river upper middle reaches of the songhua river once flowed westward into the songnen plain and what forcings influenced the drainage development of the songnen plain in this contribution we provide insight into drainage development on the songnen plain using an integrated approach of magnetic susceptibility geochemical elemental and sr nd isotopic and heavy mineral composition recovered from a 102 m sediment core sediments extracted from the eastern edge of the songnen plain with sedimentological and topographic archives our results show that a dramatic provenance shift occurred at the depth of 62 2 m 0 94 ma in the core sediments it is proposed that the songhua river was separated by the jia yi jiamushi yilan divide into two segments before 0 94 ma the west flowing proto songhua river and the east flowing proto sanjiang river lower reach of the songhua river however the proto songhua river reversed direction at 0 94 ma when the jia yi divide was cut through by headward erosion of the proto sanjiang river marking the birth of the songhua river as a single eastward flowing river system tectonics is suggested as the first order control on the evolution of the songhua river before 1 20 ma whereas climate changes may be responsible after 1 20 ma

the use of planimetric surface area in glacier mass balance calculations a potential source of errors large errors in the estimation of glacier ablation and accumulation may arise from the uncritical use of planimetric surface areas in volume calculations based on specific point values three dimensional digital terrain models of glacier surfaces show that the actual surface area is likely to be substantially larger than the planimetric area with high resolution digital terrain models approximations of the true surface area may be as much as 20 larger the errors are sufficient for questions to be raised about some calculations of water storage in glaciers because incorrect surface area values may result in ablation being underestimated substantially errors may also be introduced into calculations of radiation energy inputs to a crevassed glacier surface

radon content of groundwater as an earthquake precursor evaluation of worldwide data and physical basis the properties of a worldwide data set of 91 radon 222 rn anomalies the frequency of occurrence the precursor time interval and the distribution of peak amplitudes are correlated with earthquake data such as the respective magnitude and epicentral distance these anomalies were reported as precursors to earthquakes in the united states ussr china japan and iceland although the data set is incomplete and limited by experimental deficiencies several consistent patterns emerge radon anomalies from different tectonic regions show similar patterns the radon anomalies occur at greater epicentral distances for earthquakes of the larger magnitude anomalies preceding large earthquakes m 6 are frequently observed at a distance of 100 to 500 km these distances are larger than several times the rupture dimensions of the future earthquakes the time from the onset of an anomaly to the time of the earthquake the precursor time increases with magnitude but decreases with distance between epicenter and radon station in addition radon anomalies are observed more frequently prior to large earthquakes than prior to small ones indicating that the preparation zone increases in size as magnitude increases the peak amplitude does not scale with magnitude but forms a consistent pattern with epicentral distance in that the larger the earthquake magnitude the farther away the largest amplitudes tend to occur the preparation zone of the earthquake where the anomalies occur forms an almost continuous annulus that expands with time away from the future rupture zone the outer radius of this annulus scales with the earthquake magnitude model calculations indicate that strain fields of at most 10 6 to 10 8 strain caused the radon anomalies if these strains are divided by the appropriate precursor time minimum strain rates from 10 7 day 1 to 10 10 day 1 are obtained such small strains and strain rates suggest that in most cases neither mechanical crack growth induced by dilatancy nor mechanical coupling between pore pressure and the rock matrix caused the anomalies large changes in the orientation of the local strain field however could occur and affect the local stress intensity factor since changes in the stress intensity factor can result in stress corrosion the occurrence of radon anomalies is attributed to slow crack growth controlled by stress corrosion in a rock matrix saturated by groundwater

subglacial hydrology for an ice sheet resting on a deformable aquifer glissements et deformation de la calotte de glace en relation avec le materiau de l aquifere et la densite des chenaux de drainage

microseismicity and source mechanism study shillong plateau northeast india microearthquakes recorded by two surveys in the shillong plateau during december 1982 july 1983 and january february 1984 respectively have been analyzed for seismicity and tectonic study in the region of 1 000 events recorded during the surveys 180 events with reliable p and s readings which are observed within 50 km radius of the network are relocated by the homogeneous station method it is found that the events are mostly confined within a depth range of 10 to 35 km the seismicity in general may be correlated with the major geological structure lineaments in the area high rates of seismicity are observed in the shillong and tura areas on the other hand an aseismic zone is delineated to the east of shillong composite source mechanisms of two groups of microearthquakes show thrust fault mechanisms one group of microearthquakes 1982 to 1983 in the shillong area shows pure thrust faulting with a se nw compressive stress p axis whereas the other group of microearthquakes 1984 in the tura area shows thrust faulting with a strike slip component and a ene wsw compressive stress p axis the stress orientations are compatible with the known tectonics of the region collision of the indian plate with the eurasian plate at the himalaya and subduction of the indian plate beneath the burmese plate at the arakan yoma range

background seismicity rates and precursory seismic quiescence imperial valley california the problem of recognizing precursory seismicity rate changes consists of two steps first a background or normal rate must be determined and second anomalous rates must be recognized in this paper we address the first of these steps determining a background seismicity rate two problems must be dealt with if a meaningful background rate is to be determined first dependent events must be recognized and removed from consideration and second detection or reporting changes must be accounted for a number of techniques are available for recognizing dependent events we used the one developed by mcnally 1976 in this work the technique we used for recgonizing detection and reporting changes was developed by habermann 1983 we demonstrate these techniques on seismicity from the imperial valley region of california after a reasonable background rate is determined we examine seismicity rates in this region prior to the imperial valley earthquake of 15 october 1979 the source area of the earthquake shows a possible precursory quiescence which started during july 1979 the time of this rate decrease is close to the time of a change in the seismic instrumentation in the imperial valley which makes it difficult to determine whether the decrease is real or related to detection several other seismicity rate changes observed in the imperial valley appear to be related to changes in the operation of the network there these include quiescent periods which occur only if magnitude cutoffs are applied to the data and periods of low reporting of events with m 2 5 these observations suggest that magnitudes were not reported for some events during several time periods all of these observations indicate that determination of detection histories is crucial for meaningful seismicity studies

geochemical weathering at the bed of haut glacier d arolla switzerland a new model waters were sampled from 17 boreholes at haut glacier d arolla during the 1993 and 1994 ablation seasons three types of concentrated subglacial water were identified based on the relative proportions of ca 2c h co 3 and so4 2 to si type a waters are the most solute rich and have the lowest relative proportion of si they are believed to form in hydrologically inefficient areas of a distributed drainage system most solute is obtained from coupled sulphide oxidation and carbonate dissolution so cd it is possible that there is a subglacial source of o2 perhaps from gas bubbles released during regelation because the high so4 2 levels found up to 1200 eq l are greater than could be achieved if sulphides are oxidized by oxygen in saturated water at 0 c c 414 eq l a more likely alternative is that sulphide is oxidized by fe 3c in anoxic environments if this is the case exchange reactions involving fe iii and fe ii from silicates are possible these have the potential to generate relatively high concentrations of hco3 with respect to so4 2 formation of secondary weathering products such as clays may explain the low si concentrations of type a waters type b waters were the most frequently sampled subglacial water they are believed to be representative of waters flowing in more efficient parts of a distributed drainage system residence time and reaction kinetics help determine the solute composition of these waters the initial water rock reactions are carbonate and silicate hydrolysis and there is exchange of divalent cations from solution for monovalent cations held on surface exchange sites hydrolysis is followed by so cd the so4 2 concentrations usually are 414 eq l although some range up to 580 eq l which suggests that elements of the distributed drainage system may become anoxic type c waters were the most dilute yet they were very turbid their chemical composition is characterized by low so4 2 h co3 ratios and high ph type c waters were usually artefacts of the borehole chemical weathering environment true type c waters are believed to flow through sulphide poor basal debris particularly in the channel marginal zone the composition of bulk runoff was most similar to diluted type b waters at high discharge and was similar to a mixture of type b and c waters at lower discharge these observations suggest that some supraglacial meltwaters input to the bed are stored temporarily in the channel marginal zone during rising discharge and are released during declining flow little of the subglacial chemical weathering we infer is associated with the sequestration of atmospheric co2 t he progression of reactions is from carbonate and silicate hydrolysis through sulphide oxidation by first oxygen and then fe iii which drives further carbonate and silicate weathering a crude estimate of the ratio of carbonate to silicate weathering following hydrolysis is 4 1 we speculate that microbial oxidation of organic carbon also may occur both sulphide oxidation and microbial oxidation of organic carbon are likely to drive the bed towards suboxic conditions hence we believe that subglacial chemical weathering does not sequester significant quantities of atmospheric co2 and that one of the key controls on the rate and magnitude of solute acquisition is microbial activity which catalyses the reduction of fe iii and the oxidation of fes2 copyright 2002 john wiley sons ltd

pebble orientation on large experimental debris flow deposits abstract replicable pronounced orientation of discoid pebbles 8 mm embedded on surfaces of large 10 m3 experimental debris flow deposits reveals that strongly aligned imbricate fabric can develop rapidly over short distances in mass flows pebble long axes aligned subparallel to deposit margins as well as subparallel to margins of surge waves arrested within the deposits pebble alignment exhibited modes both parallel to a p the primary flow direction intermediate axes dipped preferentially inward from surge wave margins b i orientation repetitive development of margin parallel imbricate fabric distributed across deposit surfaces provides compelling evidence that deposits formed dominantly through progressive incremental accretion rather than through simple en masse emplacement pronounced fabric along deposit and arrested surge wave margins reflects significant grain interaction along flow margins this sedimentological evidence for significant marginal grain interaction complements theoretical analyses iverson 1997 and other experimental data major 1996 iverson 1997 that indicate that resistance along flow margins is an important factor affecting debris flow deposition the fabric on the experimental deposits demonstrates that debris flows can develop strongly imbricate particle orientation that mimics fabric developed during fluvial deposition particle shape and local stress fields appear to have more control over fabric development than does general depositional process other criteria in addition to particle orientation are needed to discriminate mass flow from fluvial gravel deposits and to unravel depositional history

automated analysis of imbrication and flow direction in alluvial sediments using laser scan data abstract an automated method for analysis of imbrication and flow direction in alluvial sediments is described that uses digital elevation models dems derived from three dimensional laser scanning of dry gravel river beds a global analysis of the dem is used that does not depend on location and analysis of individual grains the method is more convenient and quantitative than are existing techniques methods are described for determining the suitability of particular data sets for analysis by this method the method is applied to gravel river beds on the south island of new zealand

oxygen isotopes in glacier river water austre okstindbreen okstindan norway there is no discernible river discharge from austre okstindbreen norway in winter any water formed by basal melting is likely to be stored in isolated cavities or sediments at the bed in summer a baseflow component of discharge relatively depleted of 18 o is diluted by water more enriched in 18 o glacier ice meltwater with a high 18 o value passes rapidly through channelized systems and emerges in the glacier river within a few hours headward extension of the systems in late summer may tap water stored within the glacier since the previous summer s close down stored water also is released during early melt season events when the low capacity drainage systems cannot accommodate inputs the high 18 o value indicates that the probable source of the released water is glacier ice or low altitude snowfall stratigraphic variations of 18 o values resulting from changing weather conditions characterize the accumulating winter snowpack during the melt season the mean 18 o value of the residual pack rises the water leaving it depleted of 18 o passes slowly through the glacier smoothing out variations of supply 18 o values of rain vary between and within events and their effect on glacier river 18 o values is unpredictable

sediment entrainment by a soft bedded glacier a model based on regelation into the bed experimental results indicate that wet based soft bedded glaciers may penetrate their substrates by regelation melting and refreezing and thereby entrain sediment in principle there should be a steady depth of penetration at which the downward regelation speed driven by the interfacial effective pressure equals the basal melt rate herein the magnitude and distribution of penetration are estimated for the case of a glacier resting on a deformable bed with channelized basal drainage the distance between channels and the distribution of effective pressure across the bed are calculated and special attention is paid to the interdependence of the basal melt rate and effective pressure a major uncertainty is whether fine grained tills may impede or prevent regelation as a result of high surface tension at the ice water interface predicted penetration depths range from millimetres to a few decimetres and thus dirty basal ice layers of such thicknesses might be expected predicted distances between channels agree well with those indicated by borehole measurements at ice stream b west antarctica effective pressures and hence penetration depths increase toward channels therefore the edges of interfluves bounded by anastomosing subglacial channels should be eroded preferentially this motivates the testable hypothesis that such erosion contributes to the formation of streamlined landforms such as drumlins copyright 2000 john wiley sons ltd

cobble imbrication as a sensitive indicator of subtle local changes in river flow direction imbrication of cobbles in pleistocene terrace gravels along the shoshone river near powell wyoming shows significant changes in orientation from a lower set to an upper set within a gravel layer the lower gravels show a mean orientation of 126 with spreads from s 38 to s 56 the mean orientation of the upper gravels is 070 with spreads from s 19 to s 26 change from the lower to the upper deposit is abrupt with no noticeable variability near the contact in fact the contact can easily be overlooked without close scrutiny of the cobble fabric this suggests that cobble imbrication may be a sensitive indicator of subtle changes in the flow direction of a braided stream because the earlier fabric was not disturbed by deposition of the younger unit there is every reason to believe that such fabrics can and will be preserved even in the consolidated rock record and can be used for paleoenvironmental interpretations

is shillong region northeast india undergoing a dilatancy stage precursory to a large earthquake abstract gupta h k and singh v p 1982 is shillong region northeast india undergoing a dilatancy stage precursory to a large earthquake in a l hales and z suzuki editors earthquake prediction tectonophysics 85 31 33

fabric analysis of a desert debris slope bell mountain california the fabric of coarse particles mantling a debris slope on bell mountain california is investigated to identify the dominant process moving such particles this slope is typical of debris slopes in the mojave desert underlain by closely jointed and or mechanically weak rocks samples of rods and disks analyzed from five sites ranging in gradient from 11 17 to 33 17 are found to display essentially the same fabric particles tend to be aligned downslope and to lie flat on the ground surface there is no evidence of imbrication signifying sliding or creep nor of transverse modes indicating rolling it is concluded that the fabric is probably produced by hydraulic action and that this process is mainly responsible for moving coarse particles on gradients up to 33 on debris slopes underlain by closely jointed and or mechanically weak rocks

crustal p wave velocity and velocity ratio study in northeast india by a microearthquake survey the upper crustal 20 km p wave velocity beneath the shillong plateau and nowgong area has been studied by the time distance plot method thep arrival data of the shallow 20 km microearthquakes from three temporary networks are used and the average velocity is found to be 5 55 km s the velocity ratio v p v s for the upper crust 0 20 km as well as for the lower crust 21 40 km are determined by the wadati plot method and station by station method the average value obtained by the two methods is compatible thev p v s ranges between 1 74 to 1 76 a generalized seismic velocity model of the area is suggested by this study which has been very useful for microearthquake location

palaeoglaciation of bayan har shan northeastern tibetan plateau glacial geology indicates maximum extents limited to ice cap and ice field scales key locations within an extensive area of the northeastern tibetan plateau centred on bayan har shan have been mapped to distinguish glacial from non glacial deposits prior work suggests palaeo

regional study of the anomalous change in apparent resistivity before the tangshan earthquake m 7 8 1976 in china electrical resistivity measurements have been conducted as a possible means for obtaining precursory earthquake information before five great earthquakes m 7 h 25 km in china the apparent resistivity a showed systematic variations within a region 200 km from the epicenters in particular 9 stations in the tangshan tianjin beijing region prior to the tangshan earthquake m 7 8 h 11 km 27 july 1976 showed a consistent decrease of apparent resistivity around the epicenter with a maximum resistivity change of 6 and a period of variation of 2 3 years simultaneous water table observations in this region showed a declining water table and ground surface observations indicated a slight 5 mm uplift in the epicenter region relative to its surroundings

investigations of glacier hydrological systems using dye tracer techniques observations at pasterzengletscher austria a l aide de traceurs colores on a mis en evidence une division laterale du drainage dans le bas de la langue qui n est pas determinee par la direction des crevasses obliques la reapparition de l eau dans les differents moulins est complexe bien qu elle reste constante dans le temps

subglacial till a physical framework for its properties and processes the continuum theory of mixtures provides a framework around which present knowledge of subglacial till can be organized the proposed framework is provisional in two senses the constitutive description of till will require improvement and not all potentially significant processes have been included till is treated as a three component mixture of water clasts coarse grained solids and fine grained matrix solids these components obey mass balance equations and their behavior is subject to constitutive relations that describe such processes as darcian water transport fine grained sediment transport consolidation shear deformation dilatancy and comminution five state variables uniquely determine the compressibility permeability shear strength and other physical properties numerical solutions of the state evolutions equations illustrate that the properties of till are highly responsive to changes in the subglacial environment

hydrogeological implications of glacial landscape evolution at skeiðararsandur se iceland abstract patterns of groundwater recharge and flow in present day and palaeo glacial environments are governed by both geomorphology and subsurface sedimentology this paper assesses the hydrogeological implications of glacial landscape evolution and develops a conceptual model of glacial landscape evolution and hydrogeological interaction the ice marginal zone of skei ararsandur se iceland was selected as a case study of a temperate actively receding glacier landsystem that is affected by both surging behaviour and periodic glacier outburst floods jokulhlaups the groundwater table elevation was measured approximately every 2 5 days throughout two six week summer field seasons in 2000 and 2001 and once in march 2001 using a network of shallow piezometers and hydraulic conductivity was calculated based on grain size distribution analysis of sampled aquifer materials the hydraulic conductivity of the near surface aquifer at the study site spans three orders of magnitude and is highest and most varied in areas inundated by the november 1996 jokulhlaup the shallow water table in the study area responds dynamically to precipitation events with the greatest fluctuation in an enclosed moraine lake basin with a perched aquifer underlain by ice and in a zone of outwash fans proximal to the ice margin the seasonal decline in water table is greatest at the ice margin reflecting the reduced winter recharge from the stagnant ice zone landsystems with morainic relief are associated with localised moraine precipitation fed groundwater systems these localised groundwater systems are superimposed on the intermediate regional scale groundwater systems that discharge further down sandur and offshore and are dominated by the gently sloping geomorphologically uniform outwash plain groundwater fed lakes formed in depressions from ice blocks deposited during jokulhlaups may provide important ecological niches the investigations at this site demonstrate that glacial outburst floods and glacier margin fluctuations produce a suite of sediments landforms and environments that affect the hydrogeological system

combined measurements of subglacial water pressure and surface velocity of findelengletscher switzerland conclusions about drainage system and sliding mechanism interpretation des mesures de pression dans les forages et des vitesses superficielles la comparaison avec les modeles suggere que l on a neglige le frottement du aux debris dans les modeles et qu il n existe jamais de chenaux cylindriques rectilignes