the ktb apatite fission track profiles building on a firm foundation abstract deep boreholes serve as natural laboratories for testing thermochronometers under geological conditions the kontinentale tiefbohrung ktb is an interesting candidate because the geological evidence suggests that approximate isothermal holding since the last documented exhumation in the late cretaceous to palaeocene is a reasonable assumption for the thermal histories of the ktb samples we report 30 new apatite fission track ages and 50 new mean confined track lengths determined on cores from the 4 km deep pilot hole the and external detector ages are consistent with the population ages from earlier studies and together define a clear age profile the mean track lengths from this and earlier studies reveal the effects of experimental factors the measured age and length profiles are compared with the predictions of 24 annealing models for isothermal holding there are clear discrepancies between the measured and calculated profiles down to 1 5 km depth the measured mean track lengths are shorter than the predicted the balance of methodological evidence indicates that this is due to seasoning i e a shortening of the fossil confined tracks without attendant age reduction from 2 5 to 4 0 km depth the mean track lengths are longer than the predictions this suggests that the bias model that weights the probabilities of observing tracks of different length and which is based on experiments relating surface track densities to mean track lengths is not appropriate for confined tracks experimental and methodological factors are sometimes difficult to disentangle but present a sufficient margin for there to be no need to go against the independent geological evidence unknown geological events cannot be ruled out but their existence cannot be inferred from the fission track data alone much less can the nature or magnitude of such events be specified

the instability theory of drumlin formation and its explanation of their varied composition and internal structure despite their importance in understanding glaciological processes and constraining large scale flow patterns in palaeo glaciology there is little consensus as to how drumlins are formed attempts to solve the drumlin problem often fail to address how they are created from an initially flat surface in the absence of obvious cores or obstacles this is a key strength of the instability theory which has been described in a suite of physically based mathematical models and proposes that the coupled flow of ice and till causes spontaneous formation of relief in the till surface encouragingly model predictions of bedform height and length are consistent with observations and furthermore the theory has been applied to a range of subglacial bedforms and not just drumlins however it has yet to confront the myriad observations relating to the composition and internal structure of drumlins and this could be seen as a major deficiency this paper is a first attempt to assess whether the instability theory is compatible with the incredible diversity of sediments and structures found within drumlins we summarise the underlying principles of the theory and then describe and attempt to explain the main types of drumlin composition e g bedrock till glaciofluvial sediments and combinations thereof contrary to a view which suggests that the presence of some sedimentary sequences e g horizontally stratified cores is inconsistent with the theory we suggest that one would actually expect a diverse range of constituents depending on the inheritance of sediments that pre date drumlin formation the duration and variability of ice flow and the balance between erosion and deposition till continuity at the ice bed interface we conclude that the instability theory is compatible with and potentially strengthened by what is known about drumlin composition and as such offers the most complete and promising solution to the drumlin problem to date

assemblages of submarine landforms produced by tidewater glaciers in svalbard 1 high resolution swath bathymetry from the marine margins of several svalbard tidewater glaciers shows an assemblage of submarine landforms that is probably linked to glacier surging these landforms are essentially unmodified since their initial deposition over the past hundred years or so because they have not been subjected to subaerial erosion or periglacial activity swath images comprise an assemblage of superimposed landforms allowing reconstruction of relative age of deposition 1 large transverse ridges interpreted as recessional moraines overridden by a subsequent ice advance 2 a series of curvilinear streamlined bedforms orientated parallel to former ice flow interpreted as lineations formed subglacially during rapid advance 3 large terminal ridges marking the farthest extent of ice at the last advance with flow lobes immediately beyond interpreted as submarine debris flows 4 a series of interconnected rhombohedral ridges interpreted as a product of soft sediment squeezing into crevasses formed at the glacier bed probably formed during immediate post surge stagnation and 5 a series of fairly evenly spaced small transverse ridges interpreted as push moraines produced annually at tidewater glacier termini during retreat a simple descriptive landsystem model for tidewater glaciers of probable surge type is derived from these observations we also show that megascale glacial lineations can form not only beneath large ice streams but are also produced beneath surging tidewater glaciers lying on deforming sedimentary beds

genesis of streamlined landforms and flow history of the green bay lobe wisconsin usa abstract the distribution of streamlined landforms in southeastern wisconsin suggests that drumlins and flutes formed during several phases of the green bay lobe between 18 and 14 000 bp the largest group of drumlins formed during a still stand of the ice margin during the johnstown phase presumably about 18 16 000 bp flutes and smaller drumlins are superimposed on larger forms and larger drumlins are remolded this indicates that drumlin modification continued during retreat three fields containing smaller drumlins formed after ice re advanced a short distance or stabilized during the green lake rush lake and st anna phases about 16 14 000 bp the drumlin forming process included erosion and deformation of pre existing ice marginal and proglacial sediments drumlins associated with the johnstown phase show an increase in length up glacier this is probably the result of variations in flow velocity length of time of drumlin formation and sediment availability flutes and small drumlins are associated with retreat moraines and formed near a thin retreating ice margin ice surface profile reconstructions suggest that margins were relatively steep during the johnstown milton green lake and early rush lake phases during the lake mills and late rush lake phases when flutes were forming the ice surface slopes were lower and margins were retreating during retreat minor sublobes developed some perhaps resulting from surges into small proglacial lake basins thus steep ice margins and driving stresses on the order of 15 25 kpa were typical during drumlin formation the formation of flutes and the remolding of larger drumlins into smaller forms took place beneath gently sloping ice of retreating margins associated with lower driving stresses

lithology sedimentology and genesis of the wadena drumlin field minnesota u s a abstract studies on both the regional and small scales of the lithology sedimentology and stratigraphy of the wadena drumlin field in central minnesota u s a provide insights to the processes that operated within the wadena lobe during the drumlin forming phase horizontal changes in the down glacier direction in the nature of the drumlin till include an increase in the carbonate content decrease in grain size and magnetite content and variations in fine fraction mineralogy these changes also occur with depth at individual sites it is inferred that these patterns reflect progressive incorporation and mixing of the substrate which for much of the drumlin field is a till of northwestern origin with the till already in transit within the base of the wadena lobe derived from the northeast factors contributing to enhanced erosion within the wadena drumlin field include increased vertical strain rates over a major subglacial highland the presence of deformable bed material and a lowered ice profile which permitted elevated rates of bed deformation the patterns of mixing are generally consistent with substrate incorporation either by regelation or in the context of a deforming bed although neither of these processes adequately explain all of the observed features of the drumlin till where localized departures from the generalized pattern exist it can be shown that these are the result of either syn or post depositional processes primarily winnowing or the influence of a different substrate additional data are presented including macrofabric measurements and detailed site descriptions that are consistent with an accretionary progressive build up of the drumlin forms

size and shape characteristics of drumlins derived from a large sample and associated scaling laws abstract ice sheets flowing across a sedimentary bed usually produce a landscape of blister like landforms streamlined in the direction of the ice flow and with each bump of the order of 102 to 103 m in length and 101 m in relief such landforms known as drumlins have mystified investigators for over a hundred years a satisfactory explanation for their formation and thus an appreciation of their glaciological significance has remained elusive a recent advance has been in numerical modelling of the land forming process in anticipation of future modelling endeavours this paper is motivated by the requirement for robust data on drumlin size and shape for model testing from a systematic programme of drumlin mapping from digital elevation models and satellite images of britain and ireland we used a geographic information system to compile a range of statistics on length l width w and elongation ratio e where e l w for a large sample mean l is found to be 629 m n 58 983 mean w is 209 m and mean e is 2 9 n 37 043 most drumlins are between 250 and 1000 metres in length between 120 and 300 metres in width and between 1 7 and 4 1 times as long as they are wide analysis of such data and plots of drumlin width against length reveals some new insights all frequency distributions are unimodal from which we infer that the geomorphological label of drumlin is fair in that this is a true single population of landforms rather than an amalgam of different landform types drumlin size shows a clear minimum bound of around 100 m horizontal maybe drumlins are generated at many scales and this is the minimum or this value may be an indication of the fundamental scale of bump generation proto drumlins prior to them growing and elongating a relationship between drumlin width and length is found with r2 0 48 and that is approximately w 7 l 1 2 when measured in metres a surprising and sharply defined line bounds the data cloud plotted in e w l space and records a scale dependent maximum elongation limit approximated by emax l1 3 when l measured in metres for a given length for some reason as yet unknown drumlins do not exceed the elongation ratio defined by this scaling law we also report and compare our statistics to an amalgamated sample 25 907 drumlins of measures derived from around 50 published investigations any theory must be able to explain the drumlin statistics and fundamental scaling properties reported herein and they thus provide powerful tests for drumlin modelling

durango apatite fission track dating using length based age corrections and neutron fluence measurements by natural thorium thin films and natural u doped glasses calibrated through natural uranium thin films abstract in this work we present an approach for neutron fluence measurements based on natural thorium thin films and natural uranium doped glasses calibrated through natural uranium thin films to be used for dating with the fission track method ftm this neutron dosimetry approach allows the employment of ftm even when dating is carried out using low neutron thermalization facilities besides it makes possible the determination of the th u ratio of the mineral to be dated durango apatite which is often employed in ftm as an age standard was analyzed this apatite presented a fairly high th u ratio 29 9 1 7 th fissions were 18 12 and 10 of the total for irradiations where thermal to fast neutron flux ratios were 2 4 4 4 and 5 2 respectively these results show that th fission must be considered for this apatite when not well thermalized irradiation facilities are used the ratio between spontaneous and induced track length ls li close to 0 89 indicates a certain amount of rejuvenation of the age of durango apatite therefore its apparent age should be corrected the application of a technique based on track length measurements produced a corrected age of 29 7 1 1 ma consistent with the independent reference age of this apatite 31 4 0 5 ma this result represents a support for viability of the neutron dosimetry approach studied in this work for ftm

alpha recoil track dating of quaternary volcanics like fission track dating alpha recoil track art dating is based on the accumulation of nuclear particles that are released by natural radioactivity and produce etchable tracks in solids arts are formed during the alpha decay of uranium and thorium as well as of their daughter nuclei when emitting an alpha particle the heavy remaining nucleus recoils 30 40 nm leaving behind a trail of radiation damage through etching the art tracks become visible with interference phase contrast microscopy under the presupposition that all tracks are preserved since the formation of a sample their total number is a measure for the sample s age the technique has already been proposed more than three decades ago huang w h walker r m 1967 fossil alpha particle recoil tracks a new method of age determination science 155 1103 1106 but due to inherent difficulties it was hardly applied or made any substantial progress since then the present paper describes a methodology by which reliable art ages of dark mica can be obtained the technique is applicable to tiny 0 5 mm single mica flakes in the 102 to 106 years age range it is tested on quaternary volcanics from the eifel region germany alpha recoil dating has a great potential for quaternary chronometry and tephrochronology

miocene range growth along the altyn tagh fault insights from apatite fission track and u th he thermochronometry in the western danghenan shan china the left lateral strike slip altyn tagh fault that defines the northern margin of the tibetan plateau plays a crucial role in accommodating the cenozoic deformation related to the growth of plateau however the slip history along the fault remains highly debated here we report new 14 16 ma apatite fission track aft and 9 11 ma apatite u th he ahe data in the western danghenan shan north tibet age elevation relationships and aft ahe age differences suggest a period of rapid exhumation with an average rate of 0 1 0 3 km ma from 16 to 9 ma for this area thermal history modeling indicates that this was preceded by accelerated exhumation between the late oligocene and middle miocene 15 ma a northward increase in aft ages and asymmetric topography across the western danghenan shan indicate that the uplift and exhumation are mainly controlled by the thrust fault along the southern flank of the western danghenan shan as the thrust fault is a branch of the altyn tagh fault the rapid exhumation probably represents onset of the transition along the altyn tagh fault from left slip motion to crustal shortening in the dangnenan shan region our findings show that the middle miocene deformation is not only recorded in the middle and northern qilian shan but also in the southwestern portion of the qilian shan which favors a synchronous middle miocene deformation model for the entire qilian shan

standardless fission track ages of the iugs age standards abstract standardless fission track ages for four zircon two titanite and three apatite age standards most of them recommended by the fission track working group of the iugs subcommission on geochronology were redetermined the age calculations used the decay constant for spontaneous fission of 238u f 8 5 0 1 10 17 a 1 recommended by the iupac the ratio of etchable to latent track length in both the dated minerals zircon titanite and apatite and mica detectors and the 187 track counts with the external detector and population methods we found the resulting standardless fission track ages are in line with results from more recent high precision geochronology including zircon u pb dating these results confirm that standardless fission track dating of zircon titanite and apatite with the external detector method is feasible as well as dating of apatite with the population method and that calibration can be based on the zircon u pb ages of the current standard samples on the other hand there is no room for an age correction for partial annealing of the fossil tracks based on their mean confined track lengths of the apatite titanite and zircon age standards

statistics for la icp ms based fission track dating abstract despite the conceptual elegance and simplicity of the external detector method edm for fission track dating an increasing number of laboratories are switching to la icp ms as a means of measuring the uranium content of apatite zircon and sphene la icp ms based fission track laft dating offers shorter turnaround times removes the need to handle radioactive materials and hydrofluoric acid and facilitates double dating with the u pb method this paper aims to bring the statistical treatment of laft data on an equal footing with the edm by formulating four different analytical protocols depending on the accuracy and reproducibility of the uranium measurements under the absolute dating approach the spontaneous track densities and uranium concentrations are directly plugged into the fundamental fission track age equation assuming that both of these measured quantities have been determined accurately and that the fission decay constant and equivalent isotopic track length are known the zeta calibration approach avoids making these assumptions by normalisation to a reference material of known age uranium zoning has a detrimental effect on the accuracy of laft ages this effect can be removed by counting only those fission tracks located within the laser ablation pit alternatively the uranium heterogeneity may be quantified by fitting multiple ablation spots in some or all the analysed grains using a lognormal distributional assumption for the uranium concentration laft dating is arguably less well suited than the edm to young and u poor samples that lack sufficient spontaneous fission tracks to reveal visual evidence for uranium zoning such samples occasionally contain no fission tracks at all resulting in infinite analytical uncertainties this paper introduces a pragmatic solution to this problem in which the uranium measurement are converted to virtual induced fission track counts to produce strictly positive ages with finite standard errors with the age equation and zero track strategy in place laft ages can be subjected to more sophisticated statistical analysis using a logarithmic transformation these ages can be visualised on radial plots and deconvolved into finite and continuous mixtures the methods introduced in this paper have been implemented in a software package called isoplotr that is available free of charge at http isoplotr london geochron com

high resolution seismic stratigraphy of late pleistocene glacial lake iroquois and its holocene successor oneida lake new york abstract oneida lake new york is the remnant of glacial lake iroquois a large proglacial lake that delivered fresh water to the atlantic ocean during the last deglaciation the formation of glacial lake iroquois and its subsequent drainage into the atlantic ocean via the mohawk valley was a significant shift in the routing of laurentide ice sheet meltwater to the east instead of south via the allegheny or susquehanna rivers catastrophic drainage of glacial lake iroquois into the atlantic ocean via the champlain valley is interpreted as the meltwater pulse responsible for the intra allerod cold stadial therefore understanding the evolution of glacial lake iroquois has significant implications for understanding late pleistocene paleoclimate high resolution chirp seismic reflection data provides insight into the evolution of glacial lake iroquois and oneida lake three seismic units image distinct stages of the oneida basin unit 1 is interpreted as proglacial lake deposits that overlie glacial till unit 2 is interpreted as sediments deposited when the oneida basin became isolated from glacial lake iroquois and unit 3 is interpreted as lacustrine sediments of the modern lake distally sourced turbidites possibly triggered by seismic activity or ice sheet meltwater pulses are represented as reflection free acoustic facies that infill topographic lows and range in thickness from 1 5 m within otherwise conformable proglacial lake deposits local slump deposits imaged at the boundary between unit 1 and 2 were likely triggered by the drainage of glacial lake iroquois wave cut terraces indicative of a low stand on the upper bounding surface of unit 2 are likely the result of drier conditions during the holocene hypsithermal furthermore preservation of this low stand suggests a rapid rise in lake level possibly the result of the same transition to a wetter climate responsible for the nipissing transgression observed in the laurentian great lakes

tectonothermal evolution of the neoproterozoic grampian and appin groups southwestern monadhliath mountains scotland metasedimentary rocks of the neoproterozoic grampian and appin groups exposed in the glen roy district monadhliath mountains scotland were strongly folded during an initial progressive tectonothermal event of the caledonian orogeny the attitude and style of the first folds varies from open upright ne trending structures in the west e g the appin synform to recumbent nw facing nappes in the east e g the treig synform these early nappes occur only south of the corrieyairack complex and their amplitude decreases towards the nw ductility contrasts induced sliding such as that locally recorded along the grampian appin group lithostratigraphical boundary subsequently the early structures were deformed by tight upright d2 folds nw thrusting along the eilrig shear zone during d2 is interpreted as occurring in response to a transfer of bulk transport to deeper structural levels relative to d1 prograde metamorphism accompanied deformation with the thermal peak in the glen roy district t 500 600 c p 7 0 8 0 kbar coinciding with d2 folding it is suggested that the regional variation in the style of deformation was controlled at least in part by pre existing major structures most notably fundamental transverse basement lineaments temporal changes in the style of deformation occurred in response to changing crustal architecture during progressive deformation the study demonstrates the diachronous nature of deformation and also illustrates how its is possible to preserve areas with simple deformation histories in the centre of a major orogenic belt

confined fission track length measurement of zircon assessment of factors affecting the paleotemperature estimate abstract we carried out a series of laboratory annealing experiments in order to understand the factors affecting observed confined track lengths in thermally annealed zircon the variation of confined fission track lengths according to both experimental and analytical conditions was determined on zircons from nisatai dacite it was found that etching temperature track etching criteria measurement criteria of horizontal confined tracks hct s and the orientation of hct s significantly control the observed track lengths if samples are highly annealed i e 600 c for 1 hr annealing the first three factors may be related to the presence of gap zones in an annealed track whilst the fourth is caused by anisotropic annealing and etching characteristics for reliable thermal history reconstruction therefore it should be the minimum requirement to document explicitly the experimental and analytical conditions of these factors in zircon fission track analysis

slow senonian and fast palaeocene early eocene uplift of the granitoids in the central eastern pontides turkey apatite fission track results abstract dating and forward modelling of the fission track data of apatite samples from the dereli ebinkarahisar region south of giresun in the eastern turkish pontides provides quantitative data on the regional tectonics resulting from the closure of neo thetys and the collision of eurasia and gondwana the age vs elevation profiles identified senonian 80 7 3 2 to 62 4 2 5 ma slow uplift and denudation interpreted as the result of the diapiric ascent of subduction related plutons above the neo tethyan subduction zone beneath the eurasian continent this was followed by rapid differential uplift during the palaeocene early eocene 57 4 2 4 to 47 8 2 4 ma which juxtaposed granitoid units of different ages compositions and emplacement levels in the crust and is thought to be related to the collision between the pontide eurasian and anatolide gondwana basements the modelling results must be interpreted with caution but appear to indicate a period of mio pliocene ca 5 ma reheating related to volcanism associated with the westward escape of the anatolian plate and uplift from the pliocene ca 3 5 ma up to the present

size shape and spatial arrangement of mega scale glacial lineations from a large and diverse dataset mega scale glacial lineations msgls are a characteristic landform on ice stream beds solving the puzzle of their formation is key to understanding how ice interacts with its bed and how this in turn influences the dynamics of ice streams however a comprehensive and detailed characterization of this landform s size shape and spatial arrangement which might serve to test and refine formational theories is largely lacking this paper presents a detailed morphometric analysis and comparison of 4043 msgls from eight palaeo ice stream settings three offshore norway and antarctica four onshore canada and one from under a modern ice stream in west antarctica the length of msgls is lower than previously suggested mode 1000 2000 m median 2892 m and they initiate and terminate at various locations on an ice stream bed their spatial arrangement reveals a pattern that is characterized by an exceptional parallel conformity 80 of all mapped msgls have an azimuth within 5 from the mean values and a fairly constant lateral spacing mode 200 300 m median 330 m which we interpret as an indication that msgls are a spatially self organized phenomenon results show that size shape and spatial arrangement of msgls are consistent both within and also generally between different ice stream beds we suggest this results from a common mechanism of formation which is largely insensitive to local factors although the elongation of msgls mode 6 8 median 12 2 is typically higher than features described as drumlins these values and those of their width mode 100 200 m median 268 m overlap which suggests the two landforms are part of a morphological continuum and may share a similar origin we compare their morphometry with explicit predictions made by the groove ploughing and rilling instability theories of msgl formation although the latter was most compatible neither is fully supported by observations 2014 the authors earth surface processes and landforms published by john wiley sons ltd

on the calibration of fission track annealing models to obtain thermal histories from fission track length data the rates at which track lengths are reduced must be estimated the usual method is to use empirical equations calibrated via laboratory analogues a major concern is the extrapolation of the equations to predict temperatures on the geological time scale which is far outside the range of the calibration data using objective statistical criteria it is shown that the inclusion of geological data from age standards and deep boreholes is an important measure to ensure that model predictions of geological annealing rates are at least close to being correct it is shown that high temperature geological data should be the preferential target in the search for new field evidence a discussion on the use and limitations of geological data and their consequences is presented

the cookie cutter a method for obtaining a quantitative 3d description of glacial bedforms recording the position and attributes of subglacial bedforms particularly drumlins is paramount in determining the extent and dynamics of former ice sheets this paper presents a method of deriving 3d properties of glacier bedforms drumlins in this case in order to quantify bedform dimensions and acquire information necessary for further investigations such as calculating amounts of sediment moved subglacially and hence interactions between a glacier and a deformable bed the method developed here is a semi automated technique called cookie cutter after the baking implement process this method is applied to 5 m spatial resolution dem data and is based on manual geomorphological mapping of the dem images which forms the judgmental part of the process the mapped bedforms are then processed individually using an automated technique which is described in detail with worked examples from western central scotland which was last glaciated during the last glacial maximum and the younger dryas the advantages and potential sources of error are discussed and results from the sample area are used to compare the volumes of drumlins in an area of lgm glaciation with those in an area glaciated during the younger dryas

the relationship between track shortening and fission track age reduction in apatite combined influences of inherent instability annealing anisotropy length bias and system calibration abstract the relationship between reduced track density 0 and reduced mean confined track length l l 0 during annealing has been studied for both spontaneous and induced tracks in four apatite samples with different chemical composition compositional range and origin and with spontaneous tracks giving mean confined track lengths varying from 14 5 m down to 12 3 m in all four apatites induced tracks show a relationship between l l 0 and 0 composed of two regions at higher values the two appear to be equal while at lower values l l 0 is greater than 0 this can be understood qualitatively in terms of the anisotropy of annealing and the bias against revelation of short tracks data from three apatites of uniform composition show near identical relationships in a sample showing a range of apatite compositions the point of divergence from the 1 1 relationship is somewhat higher than in the uniform composition apatites because individual grains anneal at different rates and the presence of highly annealed grains affects the track density more strongly than the mean track length spontaneous tracks show similar relationships to those observed for induced tracks in each sample when the data are renormalised to allow for the natural annealing present in the spontaneous tracks previous studies which led to the idea of length reduction without density reduction for spontaneous tracks did not renormalise density data as well as length data thereby distorting the apparent relationship data for the multi compositional apatite sample where annealing has taken place in geological conditions also agrees with the laboratory annealing data for both spontaneous and induced tracks this consistency suggests that a single process is responsible in all three cases implying that a laboratory based description of annealing can be applied to geological conditions with confidence recognition that spontaneous tracks in apatite are commonly annealed suggests that in general the fission track age should be interpreted as a reflection of both the time over which tracks have been retained and the distribution of etchable track lengths since the latter is a manifestation of the thermal history the fission track age should be interpreted in similar terms

erratum to metamictisation of natural zircon accumulation versus thermal annealing of radioactivity induced damage annealing of the radiation damage generated in decay events is a common phenomenon in natural zircons we have studied relationships between radiation damage age and radionuclide content of natural zircons on a micro scale the degree of metamictisation was estimated by means of confocal laser raman spectroscopic measurements raman band broadening in well ordered to moderately metamict zircons that have not experienced significant healing of the radiation damage since the time of their growth shows a nearly linear dependence on the fluence this dependence is proposed as a calibration line for examining unknown zircons in order to consider complete or incomplete damage storage which may contribute to the understanding of the low t thermal history of their host rocks seven examples for the application are presented radiation damage may anneal heterogeneously with preferred re ordering in more damaged micro areas results suggest that single point defects do not anneal continuously over geologic periods of time but are in contrast a comparably stable type of radiation damage in contrast to recrystallisation processes structural recovery due to simple thermal annealing is not necessarily connected with loss of radiogenic pb the most prominent example for this are concordant zircons from sri lanka although they are excellent standards for u pb dating sri lankan zircons should not be used as standards for structural radiation damage the experimental work is completed by monte carlo simulations of decay events in zircon

morphometry and core type of streamlined bedforms in southern sweden from high resolution lidar abstract this paper generates a new data set of streamlined glacial bedforms in southern sweden which is used both to test conclusions from previous work on streamlined bedform morphometrics and to advance our knowledge of streamlined bedforms on the scandinavian shield the data set consists of streamlined glacial bedforms in southeast sweden mapped from the new lidar derived swedish national height model which has a pixel resolution of 2 0 m with a vertical resolution of 1 cm we have mapped 10 311 features of the mapped features 135 are known to have an unconsolidated sediment core 2120 a bedrock component and 8055 whose core composition is unknown the extracted morphological variables are then subjected to a univariate and bivariate analysis we find that the extracted characteristics broadly fit into the lower end of the modal and median value spectrum of similar bedforms from around the world the distribution of the variables is found to be log normal to a first order approximation the covariant relationships between height and length width and area are examined after the variables have been log transformed and are found to be significant if not particularly strong rock cored features are found to have a longer modal length than soft cored features which suggests that reconstructions of past flow velocities from streamlined landforms need to closely consider core type additionally we find no support for a derived scaling law for streamlined features by plotting length width and elongation ratio against one another

standardless fission track dating of the durango apatite age standard abstract five dating strategies were used for determining the standardless fission track age of the durango apatite these use the same fossil track densities but differ in the manner in which the induced track densities are determined a conventional age calculation without correction for experimental factors gives inconsistent ages with method related differences 15 correcting for these factors brings the ages in line with each other and with the reference age but leaves no room for a partial annealing correction based on the confined track lengths three further reasons suggest that a length correction is not appropriate 1 the evidence for length based corrections is inconclusive 2 the plateau age of the durango apatite is consistent with its apparent fission track age to within 1 3 the calculated effective etchable length of the fossil tracks agrees within error with that of the induced tracks both are further consistent with the measured mean length of confined induced tracks the circumstance that the u th he ages of the accepted and proposed apatite age standards are consistent with their reference ages leaves no margin for a lowered fission track age resulting from partial annealing although the case of the durango apatite itself is inconclusive because of its exceptional crystal size it is conjectured that the shortening of the fossil tacks in the durango apatite is due to a lowering of the track etch rate over time in this case annealing equations fitted to induced track data underestimate the extent of confined track length reduction in geological samples

the biotite isograd central pyrenees a deformation controlled reaction the biotite isograd reaction in cambro ordovician pelites from the garonne dome in the central pyrenees involves the production of biotite at the expense of chlorite and the gradual reduction in the celadonite si content of individual white micas the mineral assemblages in the biotite bearing rocks in the melles area retain abundant evidence of chemical disequilibrium due to the sluggish nature of major element diffusion within the white micas the progress of the isograd reaction is strongly controlled by the progressive development of regional variscan fabrics and chemical exchange in the white micas in these metamorphic conditions is only possible during active deformation chemical resetting of the white micas takes place via the development of compositional zoning in the deforming micas this probably occurs as a consequence of the introduction of defects and dislocations causing more efficient diffusion within parts of individual grains in the absence of deformation this biotite isograd reaction would take place at significantly higher temperature and be controlled by the relatively high closure temperature of major element diffusion in white micas thus assigning thermal significance to such continuous isograd reactions is impossible without independent constraints kinetic factors such as deformation may be the dominant influence in many cases not the thermodynamic controls

influence of radiation damage on titanite he diffusion kinetics titanite is a common u th bearing accessory mineral that previous study suggested has a u th he closure temperature tc of 210 175 c although it has been recognized that radiation damage influences the he retentivity of apatite and zircon this effect has not been documented in titanite we acquired 51 the analyses from 11 archean basement samples across the kaapvaal craton south africa the the dates range from 20 to 1200 ma and are negatively correlated with their 10 970 ppm span of effective uranium concentration eu raman spectra acquired for 4 of these samples display peak broadening with increasing eu 4he diffusion experiments on the same 4 samples suggest tc values that vary by 175 c estimated alpha dose versus tc for our results and published the diffusion data document tc values of 150 210 c at alpha doses 50 1016 g with a sharp decrease in he retentivity above this threshold this damage diffusivity pattern is similar to zircon but the damage level at which the reduction in retentivity occurs appears to be lower for titanite because titanite typically has substantially lower eu than zircon for the same protracted thermal history the dates are likely to record higher temperature portions of the history than zircon u th he dates the results demonstrate that the dating can access a much larger temperature range and potentially be used to decipher more detailed time temperature paths than previously thought

sediment rich meltwater plumes and ice proximal fans at the margins of modern and ancient tidewater glaciers observations and modelling turbid meltwater plumes and ice proximal fans occur where subglacial streams reach the grounded marine margins of modern and ancient tidewater glaciers however the spacing and temporal stability of these subglacial channels is poorly understood this has significant implications for understanding the geometry and distribution of quaternary and ancient ice proximal fans that can form important aquifers and hydrocarbon reservoirs remote sensing and numerical modelling techniques are applied to the 200 km long marine margin of a svalbard ice cap austfonna to quantify turbid meltwater plume distribution and predict its temporal stability results are combined with observations from geophysical data close to the modern ice front to refine existing depositional models for ice proximal fans plumes are spaced ca 3 km apart and their distribution along the ice front is stable over decades numerical modelling also predicts the drainage pattern and meltwater discharge beneath the ice cap modelled water routing patterns are in reasonable agreement with satellite mapped plume locations however glacial retreat of several kilometres over the past 40 years has limited build up of significant ice proximal fans a single fan and moraine ridge is noted from marine geophysical surveys closer to the ice front there are smaller recessional moraines and polygonal sediment lobes but no identifiable fans schematic models of ice proximal deposits represent varying glacier terminus stability i stable terminus where meltwater sedimentation produces an ice proximal fan ii quasi stable terminus where glacier readvance pushes or thrusts up ice proximal deposits into a morainal bank and iii retreating terminus with short still stands allowing only small sediment lobes to build up at melt stream portals these modern investigations are complemented with outcrop and subsurface observations and numerical modelling of an ancient ordovician glacial system thick turbidite successions and large fans in the late ordovician suggest either high magnitude events or sustained high discharge consistent with a relatively mild palaeo glacial setting for the former north african ice sheet

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

till deposition by glacier submarginal incremental thickening macro and micro scale sedimentological analyses of recently deposited tills and complex push squeeze moraines on the forelands of icelandic glaciers and in a stacked till sequence at the former younger dryas margin of the loch lomond glacier lobe in scotland are used to assess the depositional processes involved in glacier submarginal emplacement of sediment where subglacial meltwater is unable to flush out subglacial sediment or construct thick debris rich basal ice by cumulative freeze on processes glacier submarginal processes are dictated by seasonal cycles of refreezing and melt out of tills advected from up ice by a combination of lodgement deformation and ice keel and clast ploughing although individual till layers may display typical a and b horizon deformation characteristics the spatially and temporally variable mosaic of subglacial processes will overprint sedimentary and structural signatures on till sequences to the extent that they would be almost impossible to classify genetically in the ancient sediment record at the macro scale icelandic tills display moderately strong clast fabrics that conform to the ice flow directions documented by surface flutings very strong fabrics typify unequivocally lodged clasts despite previous interpretations of these tills as subglacial deforming layers micro morphological analysis reveals that shearing played only a partial role in the emplacement of till matrixes and water escape and sediment flowage features are widespread a model of submarginal incremental thickening is presented as an explanation of these data involving till slab emplacement over several seasonal cycles each cycle involves 1 late summer subglacial lodgement bedrock and sediment plucking subglacial deformation and ice keel ploughing 2 early winter freeze on of subglacial sediment to the thin outer snout 3 late winter readvance and failure along a decollement plane within the till resulting in the carriage of till onto the proximal side of the previous year s push moraine 4 early summer melt out of the till slab initiating porewater migration water escape and sediment flow and extrusion repeated reworking of the thin end of submarginal till wedges produces overprinted strain signatures and clast pavements

lidar based volume assessment of the origin of the wadena drumlin field minnesota usa abstract the wadena drumlin field wdf 7500 km 2 in west central minnesota usa is bordered along its outer extremity by the till cored alexandria moraine marking the furthest extent of the southwesterly flowing wadena ice lobe at c 15 000 kyr bp newly available high resolution light detection and ranging lidar data reveal new information regarding the number morphology and extent of streamlined bedforms in the wdf in addition a newly developed quantitative methodology based on relief curvature analysis of lidar elevation based raster data is used to evaluate sediment volumes represented by the wdf and its bounding end moraine these data are used to evaluate models for the origin of drumlins high resolution lidar based mapping doubles the streamlined footprint of the wadena lobe to 16 500 km 2 increases the number of bedforms from 2000 to 6000 and most significantly reclassifies large numbers of bedforms mapped previously as drumlins as mega scale glacial lineations msgls indicating that the wadena ice lobe experienced fast ice flow the total volume of sediment in the alexandria moraine is 71 110 km 3 that in the drumlins and msgls is 2 83 km 3 and the volume of swales between these bedforms is 74 51 km 3 the moraine volume is equivalent to a till layer 6 8 m thick across the entire bed of the wadena lobe suggesting drumlinization and moraine formation were accompanied by widespread lowering of the bed this supports the hypothesis that drumlins and msgls are residual erosional features carved from a pre existing till swales represent missing sediment that was eroded subglacially and advected downglacier to build the alexandria moraine during fast ice flow alternatively the relatively small volume of sediment represented by subglacial bedforms indicates they could have formed rapidly by depositional processes

automated drumlin shape and volume estimation using high resolution lidar imagery curvature based relief separation a test from the wadena drumlin field minnesota abstract resolving the origin s of drumlins and related megaridges in areas of megascale glacial lineations msgl left by paleo ice sheets is critical to understanding how ancient ice sheets interacted with their sediment beds msgl is now linked with fast flowing ice streams but there is a broad range of erosional and depositional models further progress is reliant on constraining fluxes of subglacial sediment at the ice sheet base which in turn is dependent on morphological data such as landform shape and elongation and most importantly landform volume past practice in determining shape has employed a broad range of geomorphological methods from strictly visualisation techniques to more complex semi automated and automated drumlin extraction methods this paper reviews and builds on currently available visualisation semi automated and automated extraction methods and presents a new curvature based relief separation cbrs technique for drumlin mapping this uses curvature analysis to generate a base level from which topography can be normalized and drumlin volume can be derived this methodology is tested using a high resolution 3 m lidar elevation dataset from the wadena drumlin field minnesota usa which was constructed by the wadena lobe of the laurentide ice sheet ca 20 000 years ago and which as a whole contains 2000 drumlins across an area of 7500 km 2 this analysis demonstrates that cbrs provides an objective and robust procedure for automated drumlin extraction there is strong agreement with manually selected landforms but the method is also capable of resolving features that were not detectable manually thereby considerably expanding the known population of streamlined landforms cbrs provides an effective automatic method for visualisation of large areas of the streamlined beds of former ice sheets and for modelling sediment fluxes below ice sheets

reconciling records of ice streaming and ice margin retreat to produce a palaeogeographic reconstruction of the deglaciation of the laurentide ice sheet abstract this paper reconstructs the deglaciation of the laurentide ice sheet lis including the innuitian ice sheet from the last glacial maximum lgm with a particular focus on the spatial and temporal variations in ice streaming and the associated changes in flow patterns and ice divides we build on a recent inventory of laurentide ice streams and use an existing ice margin chronology to produce the first detailed transient reconstruction of the ice stream drainage network in the lis which we depict in a series of palaeogeographic maps results show that the drainage network at the lgm was similar to modern day antarctica the majority of the ice streams were marine terminating and topographically controlled and many of these continued to function late into the deglaciation until the ice sheet lost its marine margin ice streams with a terrestrial ice margin in the west and south were more transient and ice flow directions changed with the build up peak phase and collapse of the cordilleran laurentide ice saddle the south eastern marine margin in atlantic canada started to retreat relatively early and some of the ice streams in this region switched off at or shortly after the lgm in contrast the ice streams draining towards the north western and north eastern marine margins in the beaufort sea and in baffin bay appear to have remained stable throughout most of the late glacial and some of them continued to function until after the younger dryas yd the yd influenced the dynamics of the deglaciation but there remains uncertainty about the response of the ice sheet in several sectors we tentatively ascribe the switching on of some major ice streams during this period e g m clintock channel ice stream at the north west margin but for other large ice streams whose timing partially overlaps with the yd the drivers are less clear and ice dynamical processes rather than effects of climate and surface mass balance are viewed as more likely drivers retreat rates markedly increased after the yd and the ice sheet became limited to the canadian shield this hard bed substrate brought a change in the character of ice streaming which became less frequent but generated much broader terrestrial ice streams the final collapse of the ice sheet saw a series of small ephemeral ice streams that resulted from the rapidly changing ice sheet geometry in and around hudson bay our reconstruction indicates that the lis underwent a transition from a topographically controlled ice drainage network at the lgm to an ice drainage network characterised by less frequent broad ice streams during the later stages of deglaciation these deglacial ice streams are mostly interpreted as a reaction to localised ice dynamical forcing flotation and calving of the ice front in glacial lakes and transgressing sea basal de coupling due to large amount of meltwater reaching the bed debuttressing due to rapid changes in ice sheet geometry rather than as conveyors of excess mass from the accumulation area of the ice sheet at an ice sheet scale the ice stream drainage network became less widespread and less efficient with the decreasing size of the deglaciating ice sheet the final elimination of which was mostly driven by surface melt

genesis of boston harbor drumlins massachusetts abstract drumlins in boston harbor are composed of two superposed silty sandy diamicton units and subordinate sand and gravel the lower compact undifferentiated diamicton is interpreted as a pre wisconsin till based on truncated weathering profiles preserved below an erosion surface the upper less compact diamicton contains numerous discontinuous and poorly sorted lenses and stringers of sand and sandy gravel that tend to parallel the erosion surface the upper diamicton and subordinate sandy gravel lenses are interpreted to date from the late wisconsin glaciation along the exposed sections of the long island drumlin complex the preserved weathering profile on the lower diamicton is thicker beneath the drumlin axes and is either thin or missing in the interdrumlin areas this suggests that glacial erosion and streamlining of pre wisconsin deposits and their weathering profile took place after the weathering profile developed and before subsequent deposition of the overlying diamicton unit the latter drapes the erosional drumlin form without contributing substantially to relief of the drumlin form

l apatite de durango mexique analyse d un mineral standard pour la datation par traces de fission abstract durango apatite is analyzed by four analysts using three different dating techniques both the annealing technique and the external detector technique are applied to basal and prismatic sections cut from single crystals the population technique is applied to mounts of randomly oriented grains provided an individual determination of the geometry factor is performed all ages are indistinguishable from the reference age of 31 4 0 5 ma there is no noticeable influence of either the dating technique or the crystallographic orientation of the dated section on the ft age based on the projected lengths of surface tracks spontaneous track length reduction varies from 4 to 10 mean 6 depending on the analyst two simple corrections are applied to eliminate the net effect of the missing short tracks on the mean projected length after correction the amount of shortening of the spontaneous tracks is found to be slightly larger mean 8 but is still dependent on the analyst 4 to 12 the mean is in fairly good agreement with the value obtained from confined track length measurements 9 finally an isochronal 1 h plateau annealing procedure was applied to a separate set of basal and prismatic sections cut from the same single crystals annealing temperatures up to 300 c do not affect the ft age erratic ages at 350 c are ascribed to the occurrence of unetchable gaps it is argued that the absence of a distinct plateau up to temperatures where it could be expected from theoretical considerations indicates that the ft age of durango apatite needs no correction

on the densities of etchable fission tracks in a mineral and co irradiated external detector with reference to fission track dating of minerals abstract the ratio r of the densities of etchable fission tracks in a mineral and co irradiated external detector is shown to depend on the length ratios of the latent l f l f and etched fission tracks l e l e in the two media calculations of l f for apatite titanite zircon and l f for muscovite external detectors allow a precise determination of r and confirm that neglecting r could constitute the most significant source of systematic error in fission track dating with the external detector method the ratios of the measured fission track densities in an external and internal surface of apatite and a muscovite external detector are calculated from r and the track counting efficiencies q the fact that the results are consistent with the experiment supports the accuracy of r and of the q factors the external detector ages of a basal and prismatic section of durango apatite are consistent with its reference age without the need for a length correction a critical examination of the uncertainties related to the thermal neutron fluence measurements fission decay constant calculated r value independently determined q factors and the reference age of the standard demonstrates that the assumption that a reduction of the confined track length of the spontaneous tracks in durango apatite necessitates a proportional correction of its apparent fission track age is the element most in question published u th he ages lend support to the conclusion that such a correction is not required the significance of r and of the ratio q of the q factors for the induced and spontaneous track counts leads us to propose a formalisation of the age equation in which these are included as well as a formal length correction l which is however not set equal to the ratio of the confined track lengths of the induced and spontaneous tracks the accuracy of the thermal neutron fluence and the decay constant leads us to propose a possible deconvolution of the calibration factor if for suitable age standards l 1 the proposed deconvoluted calibration factor 0 is a function of r and q which can be determined in ways that are independent of the reference ages of the standards this can either be interpreted as a multiple calibration or as establishing the fission track method as an independent dating method

advantages and challenges of automated apatite fission track counting abstract fission track thermochronometry data are often a core element of modern tectonic and denudation studies soon after the development of the fission track method interest emerged for the development of automated counting procedures to replace the time consuming labor of counting fission tracks under the microscope automated track counting became feasible in recent years with increasing improvements in computer software and hardware and the development of microscopes and stage systems one such example examined in this study is the commercial automated fission track counting procedure from autoscan systems pty and the university of melbourne thermochronology group we conducted experiments that are designed to reliably and consistently test the ability of this fully automated counting system to recognize fission tracks in apatite and a muscovite external detector fission tracks were analyzed in a series of samples with a step wise increase in sample complexity in addition to the automated counting procedure the same samples were also analyzed using conventional counting procedures we found for all samples that the fully automated fission track counting procedure using the autoscan system yields a larger scatter in the measured fission track densities compared to conventional manual track counting this scatter typically resulted from the false identification of tracks regardless of the image filtering procedure used or sample coating large differences between track densities analyzed with the automated counting persisted between different grains analyzed in one sample as well as between different samples as a result of these differences a manual correction of the fully automated fission track counts is necessary for each individual surface area and grain counted this manual correction procedure significantly increases the time required to analyze a sample with the automated counting procedure compared to the conventional approach

cenozoic exhumation and deformation of northeastern tibet and the qinling is tibetan lower crustal flow diverging around the sichuan basin apatite fission track thermochronology data elucidate the cooling exhumation history of the qinling qin mountains which contain a paleozoic mesozoic orogenic collage north of the sichuan basin and northeast of the tibetan plateau in particular we examine the extent to which the qinling were affected by the rising plateau the northern and eastern qinling show continuous cooling and slow exhumation since the cretaceous in contrast in the southwestern qinling rapid cooling initiated at 9 4 ma a few million years later than in the eastern tibetan plateau a compilation of major cenozoic faults in the eastern tibetan plateau and the qinling and their kinematic and dynamic characterization shows that deformation in the qinling has predominantly been strike slip active sinistral and dextral strike slip faults delineate an area of eastward rock flow and bound the area of rapid late cenozoic cooling outlined by apatite fission track thermochronology these data can be interpreted to indicate that lower crustal flow has been diverted around the longmen shan and beneath the southwestern qinling causing active plateau uplift in this area alternatively northeastern tibet may be growing eastward faster in the western qinling than the entire south china block is extruding to the east

constraints of new apatite fission track ages on the tectonic pattern and geomorphic development of the northern margin of the tibetan plateau abstract the northern margin of the qilian shan is the northernmost edge of the tibetan plateau the deformation timing and geomorphic development along the northern margin of the qilian shan are critical to understanding the dynamics of plateau growth although previous studies have suggested that thrusting along the middle and westernmost sections of the northern margin of the qilian shan began in the middle miocene the 600 km mountain range front may have experienced differential thrusting and exhumation history we collected 11 apatite fission track samples along an elevation transect to the south of jiayuguan to unravel the cooling history and erosion variation of the northern margin of western section of the qilian shan the apatite fission track ages fission track lengths and thermal modeling suggest a period of rapid exhumation initiated at 15 10 ma along the northern margin of the qilian shan the new fission track ages can be divided into three domains with different mean ages as the distribution of the fission track ages is tightly confined by tectonic structures among them we infer that tectonics plays the first order control on erosion variations meanwhile the strong linkage between erosion rates and glacier distribution may indicate that glacial buzzsaw exerts an important role in mountain building in the northern qilian shan additionally combining this result with published data we suggest that the northern margin of the qilian shan experienced a phase of synchronous deformation in the middle miocene

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

numerical modeling of subglacial sediment deformation implications for the behavior of the lake michigan lobe laurentide ice sheet we apply a numerical model of the late wisconsin circa 20 000 years b p lake michigan lobe lml laurentide ice sheet to investigate how fine grained subglacial sediment might influence lobe behavior particularly rapid millennial scale marginal oscillations observed in the geologic record over the canadian shield we assume a rigid bed hard bedded basal boundary condition in areas overlain by fine grained sediment soft bedded the base of the ice is coupled to a deformable sediment layer using a rate dependent stress strain law geotechnical tests of clay rich till deposited by the lml provide control for sediment rheologic parameters simulated cross sectional profiles are consistent with reconstructions from geologic evidence time dependent simulations suggest that a soft bedded lobe could have reached steady state in about 20 000 years or less in contrast to 50 000 to 60 000 years for an otherwise identical hard bedded lobe a soft bedded lobe with sediment viscosity at the experimentally determined value is about twice as responsive to millennial scale shifts in accumulation or ablation as a nonsliding hard bedded lobe but in both cases the response is slower than that indicated by the geologic record results suggest that while strong millennial scale changes in accumulation and ablation can produce responses in hard bedded or soft bedded ice that are consistent with the geologic record changes in subglacial sediment viscosity even relatively modest changes whether independent or in conjunction with climate change might more readily account for millennial and submillennial scale fluctuations of the lobe margin these observations do not exclude a role for sliding but they do provide some perspective from which to evaluate relative contributions of the various processes that influence lobe behavior

glacial geomorphology of terrestrial terminating fast flow lobes ice stream margins in the southwest laurentide ice sheet glacial geomorphological mapping of southern alberta canada reveals landform assemblages that are diagnostic of terrestrial terminating ice streams fast flowing outlet glaciers with lobate snouts spatial variability in features that comprise the landform assemblages reflects changes in a palaeo ice stream activity switch on off and b snout basal thermal regimes associated with climate sensitive steady state flow palaeo ice stream tracks reveal distinct inset sequences of fan shaped flowsets indicative of receding lobate ice stream margins former ice lobe margins are demarcated by a major often glacially overridden transverse moraine ridges commonly comprising glacitectonically thrust bedrock and b minor closely spaced recessional push moraines and hummocky moraine arcs details of these landform types are well exhibited around the former southern margins of the central alberta ice stream where larger scale more intensive mapping identifies a complex glacial geomorphology comprising minor transverse ridges mtr types 1 3 hummocky terrain ht types 1 3 flutings and meltwater channels spillways the mtr type 1 constitute the summit corrugation patterns of glacitectonic thrust moraines or major transverse ridges and have been glacially overrun and moderately streamlined the mtr type 2 sequences are recessional push moraines similar to those developing at modern active temperate glacier snouts the mtr type 3 document moraine construction by incremental stagnation because they occur in association with hummocky terrain the close association of hummocky terrain with push moraine assemblages indicates that they are the products of supraglacial controlled deposition on a polythermal ice sheet margin where the ht type 3 hummocks represent former ice walled lake plains the ice sheet marginal thermal regime switches indicated by the spatially variable landform assemblages in southern alberta are consistent with palaeoglaciological reconstructions proposed for other ice stream fast flow lobes of the southern laurentide ice sheet where alternate cold polythermal and temperate marginal conditions associated with climate sensitive steady state flow sequentially gave way to more dynamic streaming and surging activity

further investigation of the initial fission track length and geometry factor in apatite fission track thermochronology the external detector method edm is a widely used technique in fission track thermochronology ftt in which two different minerals are concomitantly employed spontaneous tracks are observed in apatite and induced ones in the muscovite external detector they show intrinsic differences in detection and etching properties that should be taken into account in this work new geometry factor values g in apatite were obtained by directly measuring the ed is ratios and independently determined gqr ed is values through the measurement of projected lengths five mounts two of which were large area prismatic sections and three samples composed of random orientation pieces have been used to determine the g values a side effect of applying edm is that the value of the initial confined induced fission track l 0 is not measured in routine analyses the l 0 value is an important parameter to quantify with good confidence the degree of annealing of the spontaneous fission tracks in unknown age samples and is essential for accurate thermal history modeling the impact of using arbitrary l 0 values on the inference of sample thermal history is investigated and discussed the measurement of the l 0 value for each sample to be dated using an extra irradiated apatite mount is proposed this extra mount can be also used for determining the g value as an extension of the ed is ratio method eight apatite samples from crystalline basement with grains at random orientation were used to determine the g values the results found are statistically in agreement with the values found for apatite samples from durango mexico measured in prismatic section and also measured at random orientation there was no observable variation in efficiency regarding crystal orientation showing that it is relatively safe using non prismatic grains especially in samples with paucity of grains as it is the case of most basin samples implications for the calibration and for the calibration of the direct spectrometer based fission track dating are also discussed

an extended episode of early mesoproterozoic metamorphic fluid flow in the reynolds range central australia the products of metamorphic fluid flow are preserved in zones within the marbles and metamorphosed semipelites of the upper calcsilicate unit in the granulite portion of the late palaeoproterozoic reynolds range group northern arunta block central australia the zones of retrogression characterized by minerals such as wollastonite grossular and clinohumite local resetting of oxygen isotopic compositions and local major element metasomatism were channelways for water rich fluids derived from granulite facies metapelites u th pb isotopic ages measured by the shrimp ion microprobe on zircon and monazite from a granulite facies semipelite an early semiconcordant aluminous quartz rich fluid flow segregation and a late discordant quartz rich segregation record some of the extended thermal history of the area zircon cores from the semipelite show its likely protolith to be an igneous rock 1812 11 ma old itself derived from a source containing zircon as old as 2 2 ga low th u overgrowths on the zircon grew during granulite facies metamorphism at 1594 6 ma monazite cooled to its blocking temperature at 1576 8 ma zircon cores from the semiconcordant segregation are dominantly 2 3 ga old indicating that the source of the fluids was not the particular metamorphosed semipelite studied two generations of low th u overgrowths on the zircon give indistinguishable ages for the older and younger of 1589 8 and 1582 8 ma respectively the monazite age is the same 1576 12 ma zircon from the late discordant segregation gave 1568 4 ma fluid flow occurred for at least 18 3 ma and ended 26 3 ma after the peak of metamorphism suggesting a very slow cooling rate of 3 c ma 1 the last regional high grade metamorphism in the reynolds range occurred at 1 6 ga not 1 78 ga as previously thought the high grade event at 1 78 ga is a separate event that affected only the basement to the reynolds range group

confined fission tracks in ion irradiated and step etched prismatic sections of durango apatite apatite fission track analysis retraces the thermal histories of geological samples under 120 c but low numbers of confined tracks often lead to weak constraints on the modelling results ion irradiation and increasing etching time are effective means to increase the number of etched confined tracks our results support a linear dependence of the number of intersected tracks on the lengths and densities of both the ion and fission tracks the effectiveness of ion irradiation also depends on the etched track geometries i e on the beam orientation and etching conditions knife blade shaped ion tracks parallel to the apatite c axis are more efficient than funnel shaped tracks perpendicular to c but add to the angular bias in favour of high angles to c the irradiation conditions do not affect the mean track length at a given angle to c the number of confined tracks increases in linear proportion to etching time above a certain threshold which we interpret as the time required to etch a track to measurable dimensions past this threshold the rate of confined track revelation is rapid the high rate of increase is itself a direct consequence of the fact that confined tracks are not measurable if etched for less than a certain minimum length of time etching time does not affect the angular bias but it does increase the mean track lengths at all angles the average rate of increase of the fossil track lengths exceeds that of the induced tracks at the 2 level under some conditions ion irradiation leads to blocking of fission tracks and the development of semi tracks the elliptical model of mean horizontal confined track length vs angle to the c axis in prismatic apatite sections is over constrained with respect to the known symmetries and does not provide the best possible fit to the dataset some observations suggest a fundamental difference between fossil and induced tracks 2007 elsevier b v all rights reserved

cretaceous cenozoic history of the southern tan lu fault zone apatite fission track and structural constraints from the dabie shan eastern china apatite fission track aft and structural data outline the late cretaceouscenozoic history of the southern tan lu fault zone tlfz one of asia s major faults the triassic jurassic dabie orogen earth s largest track of ultrahigh pressure rock exposure and its foreland the yangtze foreland fold thrust belt the fission track analyses utilized the independent z and n methods for age determination which yielded within error identical ages ages from triassic jurassic syn orogenic foreland sediments are younger than their depositional age and thus were reset a group of ages records rapid cooling following shallow emplacement of granitoids of the widespread latest jurassicearly cretaceous yanshanian magmatism most ages are 90 to 55 ma and document cooling following reheating at 110 90 ma the time when the basement units of the dabie shan were last at 200 jc this cooling coincides with rifting marked by the late cretaceouseocene red bed deposition in eastern china during this period the dabie basements units exhumed in the footwall of the tan lu fault with the qianshan basin in the hanging wall the associated stress field is transtensional nw trending principal extension direction the youngest fission track ages and temperature time path modeling point to enhanced cooling in the footwall of the tan lu and associated faults at 45f10 ma the related stress field is transtensional with the principal extension direction changing trend from nw to w it may be the far field expression of the india asia collision superposed on the back arc extension setting in eastern china a regional unconformity at f25 ma marks an upper bound for the inversion of the late cretaceouseocene rift structures during the neogene further subsidence in the eastern china basins was accommodated by sub horizontal ne sw extension and followed by the presently active nw se extension the tan lu fault along the eastern edge of the dabie shan had normal

annealing kinetics of kr tracks in monazite implications for fission track modelling article this paper reports data related to the potential of the mineral monazite for reconstructing the low temperature thermal histories of geological samples from confined fission track length measurements the data are derived from isothermal annealing experiments on 300 mev 86 kr tracks simplified versions of the

converging ice streams a new paradigm for reconstructions of the laurentide ice sheet in southern ontario and deposition of the oak ridges moraine geomorphometric analyses of high resolution 5 m digital elevation datasets shows that the greater toronto area and its immediate environs is built across the former till beds of two fast flowing ice streams the newly named the simcoe ice stream sis and the halton ice stream ois within the last laurentide ice sheet lis these formed during regional deglaciation sometime around 13 000 ybp and are directly analogous to fast flowing 3 km yr 1 ice streams in today s antarctic and greenland ice sheets their subglacial footprint in southern ontario consists of flow sets of highly elongated flow parallel till ridges up to 4 km in length and 100 m in width megascale glacial lineations msgls that converge on the oak ridges moraine orm from the north sis and south his orm is southern ontario s largest glacial landform extending from the niagara escarpment to kingston over some 160 km and was deposited as a series of en echelon glaciolacustrine fan deltas in a complexly evolving glaciolacustr

testing techniques to quantify drumlin height and volume synthetic dems as a diagnostic tool glacial bedform height h and volume v likely preserve important information about the behaviour of former ice sheets however large systematic errors exist in the measurement of h and v three semi automated methods to isolate drumlins from other components of the landscape e g trees hills as portrayed by nextmap have recently been devised however it is unclear which is most accurate this paper undertakes the first quantitative comparison of such readily implementable methods illustrating the use of statistically representative synthetic landscapes as a diagnostic tool from this analysis guidelines for quantifying the 3d attributes of drumlins are proposed specifically to avoid obtaining incorrect estimates caused by substantial systematic biases interpreters should currently take three steps declutter the digital elevation model for estimating h but not v remove height data within the drumlin then interpolate across the resultant hole to estimate a basal surface using delaunay triangulation results are demonstrated through analysis of drumlins in an area in western central scotland the guidance arguably represents the best current advice for subglacial bedforms in general highlighting the need for more studies into the quality of mapped data using synthetic landscapes copyright 2014 john wiley sons ltd

annealing kinetics of fission tracks in zircon an experimental study a series of laboratory annealing experiments on zircon fission tracks has been carried out under heating conditions of 350 750 c for 10 1 103 hr i e 4 5 min 40 days variation in the confined lengths of spontaneous fission tracks was determined using zircon grains from nisatai dacite the fading contours of normalized mean track length r on the arrhenius diagram showed as sets of straight lines we performed a series of model fittings called the parallel and fanning models in order to describe the decrease in r with increasing temperature or heating time the lowest temperature limit of the zircon partial annealing zone zpaz was defined as r 0 95 and the highest as r 0 4 which approximately corresponded to the total fading of surface tracks extrapolation of the results of the laboratory experiments to the geological timescale gives for a heating duration of 106 yr estimated values of the zpaz of 210 320 c 60 c 2 with the parallel model 190 350 c 50 c 2 with the fanning model critical temperature t0 and 170 390 c 50 c 2 with the fanning model t0 the temperatures of the zpaz decrease by 20 c for an annealing duration that is an order of magnitude longer because the estimated closure temperature of zircon fission track analysis approximately corresponds to the middle of the zpaz these results support the previously estimated closure temperature of 240 c by varying the etching time it was revealed that significant removal of radiation damage occurs at r 0 93

an integrated microtextural and chemical approach to zircon geochronology refining the archaean history of the napier complex east antarctica integrated textural and chemical characterisation of zircon is used to refine the u pb geochronology of the archaean ultra high temperature napier complex east antarctica scanning electron microscope characterisation of zircon and the rare earth element compositions of zircon garnet and orthopyroxene are integrated to place zircon growth in an assemblage context thereby providing tighter constraints on the timing of magmatic and metamorphic events data indicate that magmatism occurred in the central and northern napier complex at ca 2 990 ma a regional relatively low pressure metamorphic event occurred at ca 2 850 2 840 ma mineral ree data from garnet bearing orthogneiss indicate that ca 2 490 2 485 ma u pb zircon ages provide an absolute minimum age for the ultrahigh temperature uht foliation preserved in this rock internal zircon zoning relationships and estimated zircon garnet dree values from paragneiss suggest that an absolute minimum age of ultra high temperature metamorphism is ca 2 510 ma but that it is more likely to be older than ca 2 545 ma we suggest that the high proportion of published zircon u pb data with ages between ca 2 490 2 450 ma reflects late post peak zircon growth and does not date the timing of peak uht metamorphism

submarine landforms and the reconstruction of fast flowing ice streams within a large quaternary ice sheet the 2500 km long norwegian svalbard margin 57 80 n morphological interpretation of regional and detailed bathymetric data sets on the 2500 km long norwegian shelf from the north sea 57 n to svalbard 80 n has revealed a dynamic ice flow pattern along the western marginof the scandinavian and barents svalbard ice sheets about 20 cross shelf troughs with megascale glacial lineations msgl elongate ridges and grooves oriented parallel to trough long axes are interpreted as former pathways for fast flowing ice streams studies of large scale margin morphology and seismic profiles have identified large submarine fans at the mouths of several major cross shelf troughs less dynamic ice probably existed on shallower banks the two largest paleo ice streams were the norwegian channel ice stream and bear island trough ice stream each 150 200 km wide at the mouth the lengths of individual msgl vary from hundreds of meters to several tens of kilometers and the distance between ridges varies from 0 1 to 3 km msgl amplitudes reach 15 m but are commonly 10 m the onset of msgl and hence fast ice flow is generally close to the outer coast at the border zone between crystalline rocks and softer sedimentary rocks transverse submarine ridges on various scales commonly parallel to the shelf edge reflect either the maximum ice sheet position or the recessional pattern of the ice sheet lateral ice stream moraines several tens of kilometers long have also been mapped along the sides of several cross shelf troughs identifying the border zone between fast ice flow and stagnant or slow flowing ice on intervening banks

behaviour of zircon in high grade metamorphic rocks evidence from hf isotopes trace elements and textural studies hf isotopic data of minerals in a mafic pyroxene granulite from the southern bohemian massif together with their major and trace element composition and petrological observations were used to decipher the metamorphic history and behaviour of zircon in the granulite the hf isotopic composition in the minerals was used to estimate whether the decompression reaction namely the consumption of garnet and rutile could have provided zr for the formation of newly grown metamorphic zircon the age of the decompression reaction indicated by the evolution of hf isotopes in garnet and orthopyroxene is between 333 and 331 ma i e ca 7 ma younger than the available u pb zircon ages from the moldanubian granulites and than the newly obtained 343 2 ma laser ablation icp ms u pb age of zircons the combination of bulk and in situ hf isotopic data major and trace element composition and petrological modeling of p t evolution revealed that the formation of zircons can not be related to the decompression phase of the evolution of the mafic granulites

the influence of high u th inclusions on the u th pb systematics of almandine pyrope garnet results of a combined bulk dissolution stepwise leaching and sem study abstract the isotope systematics of garnet are increasingly being used to define prograde thermal histories in metamorphic belts this approach to geochronology has the distinct advantage that time information is directly related to the pressure temperature deformation history recorded by the same mineral one potential problem with the technique however is that the inventories of radiogenic isotopes apparently measured in garnet separates may be located in inclusions rather than the garnet lattice itself inclusions of old refractory minerals such as zircon may bias the ages to the extent that they are rendered meaningless in terms of dating garnet growth establishing pt histories and constraining tectonic mechanisms in this contribution we present u th pb data obtained on bulk dissolutions of garnet from the himalaya as well as the results of a detailed sem and u th pb stepwise leaching study in order to further clarify these issues our experiment has the advantage that the age difference between the garnets and potential inclusions is very large so that the influence of the latter is highlighted the u th pb data from the bulk dissolutions are highly complex and yield apparent ages ranging from 1000 ma to 30 ma the stepwise leaching study demonstrates that this complexity results from small inclusions of allanite and zircon detected with the sem the bulk of the radiogenic 206pb in these samples was released only by treatment at high pressure in a bomb in addition the 208pb 206pb ratio of the radiogenic pb released in earlier leach steps is inconsistent with the available constraints on the th u ratio of garnet lattices as measured by sims in this case the u pb ages of these garnets is dependent purely on the age of the included high u th phases and the degree to which they have isotopically re equilibrated with the whole rock during the subsequent metamorphism that grew garnet simple mass balance arguments show that in this particular instance very small amounts of these inclusions are required amounts that escape detection during conventional sample preparation these data also have implications for current thinking on the closure temperature for the sm nd and u pb systems in garnet

rapid subglacial streamlined bedform formation at a calving bay margin using the lidar derived swedish national height model we have identified previously undescribed shallow streamlined glacial bedforms small scale drumlins on the narke plain in south central sweden these drumlins could only be detected with high resolution lidar due to both their subtle size and forest cover in this area the ice margin receded in a subaqueous environment with a proglacial water depth in the order of 100 m during the last deglaciation as indicated by the configuration of marginally formed de geer moraine ridges draping the drumlinoids the receding ice margin formed deeply indented calving bays these were located around subaqueous outlets of the subglacial melt water drainage with their apex position marked geomorphologically by beaded esker ridges the mapped small scale drumlins are aligned perpendicular to the reconstructed ice sheet margin and suggest formation along flow lines adjusted to the configuration of these calving embayments as they propagated up flow with ice margin retreat based on these geometric relationships we argue that the emplacement of the drumlins was near marginal 7 7 1 km from the margin on a short timescale 5 35 years less

micro zircon origin and evolution during metamorphism the size and distribution of zircon within a garnet mica schist from the scottish highlands were assessed using scanning electron microscopy the study reveals that abundant 0 2 3 0 m sized zircon is preferentially concentrated within garnet and biotite porphyroblasts the micro zircon has grown during regional metamorphism and represents 90 of the total number of zircon in the schist it is texturally distinct from a few larger detrital zircon grains in the schist that commonly preserve evidence of dissolution and more rarely small metamorphic outgrowths the sequential incorporation of zircon in porphyroblasts allows prograde changes in the morphology of the zircon population to be identified zircon is reactive and soluble and responds to medium grade metamorphism in a series of dissolution and crystallization events linked to possible changes in fluid composition deformation also has a significant influence on the distribution of zircon allowing inclusions previously trapped within biotite to react about 8 106 micro zircon occur as inclusions within a typical individual 5 mm garnet porphyroblast and their presence must be considered prior to trace element or isotopic analysis of such metamorphic phases

formation of mega scale glacial lineations on the dubawnt lake ice stream bed 1 size shape and spacing from a large remote sensing dataset abstract mega scale glacial lineations msgls are the largest flow parallel bedforms produced by ice sheets and are formed beneath rapidly flowing ice streams knowledge of their characteristics and genesis is likely to result in an improved understanding of the rate at which ice and sediment are discharged by ice sheets but there is little consensus as to how they are formed and there are few quantitative datasets of their characteristics with which to formulate or test hypotheses this paper presents the results of a remote sensing survey of 46 000 bedforms on the dubawnt lake palaeo ice stream bed focussing on a central transect of 17 038 that includes highly elongate bedforms previously described as msgls within this transect lineations exceed 10 km in length max 20 km and 23 have elongation ratios 10 1 max 149 1 highly elongate features are interspersed with much shorter drumlin like features but longer bedforms are typically narrower suggesting that their length develops more quickly than or at the expense of their width bedforms are broadly symmetrical in plan form and have a preferred lateral spacing of 50 250 m which implies a regular rather than random pattern of corrugations comparison with drumlins reveals that the more attenuated msgls simply extend the tail of the distribution of data rather than plotting as a separate population taken together this supports the idea of a subglacial bedform continuum primarily controlled by ice velocity but existing hypotheses of msgl formation are either not supported or are insufficiently developed to explain our observations rather we conclude that under conditions of rapid ice flow msgls attain their great length relatively quickly decades through a probable combination of subglacial deformation which attenuates ridges and erosional processes that removes material from between them

ice streams of the late wisconsin cordilleran ice sheet in western north america abstract the late wisconsin cordilleran ice sheet cis of western north america is thought to have reached its maximum extent 2 5 106 km2 as late at c 14 5 ka most 80 of the ice sheet s bed consists of high mountains but its core zone sited on plateaux of the intermontane belt of british columbia and coterminous parts of the usa shows broad swaths of subglacially streamlined rock and sediment broad scale mapping from new digital imagery data identifies three subglacial bed types 1 hard beds of variably streamlined bedrock 2 drumlinized soft beds of deformation till reworked from antecedent sediment and 3 mixed beds of variably streamlined bedrock protruding through drumlinized sediment drumlins on soft beds appear to be erosional features cut into till and antecedent sediments and identify the catchment areas of paleo ice streams expressed downglacier as flow sets of megascale glacial lineations msgls grooved and cloned drumlins appear to record the transition from drumlins to msgls the location of paleo ice streams reflects topographic funneling of ice from plateau surfaces through outlet valleys and a soft bed that sustained fast flow rock cut msgls are also present locally on the floors of outlet valleys cis disintegrated in

residual relief separation digital elevation model enhancement for geomorphological mapping geomorphologically mapped data form a primary set of observations that can be used to infer former environmental conditions thus objective and consistent mapping of landforms from remotely sensed data e g satellite imagery digital elevation models dems is paramount for reconstructing palaeoenvironments this paper proposes a technique residual relief separation to enhance landforms in dems prior to visualization and digital mapping this is applied to a 600 km2 region surrounding lough gara ireland where drumlins 200 m wide overlie a regional relief of hills 10 km wide here residual relief separation uses this difference in width scale regional relief is approximated by a 1 km wide median filter then subtracted to leave the drumlins in a residual topography in a second step the residual relief is normalized to allow for amplitude variations in the drumlins across the area 5 40 m high finally visualization uses a simple black to white colour scale for height although not numerically outperforming other visualization techniques this method performs equally well and as the data are not illuminated there is no azimuthal bias additional benefits include the relatively simple calculation intuitive visual comprehension no emphasis of noise and the possibility of using any desired visualization technique after the landscape has been topographically manipulated

subglacial till formation sedimentary characteristics and classification we review the major subglacial till forming processes as presently understood by glacial researchers and define the parameters within which tills are produced and reconcile them with sedimentary end members processes of deformation flow sliding lodgement and ploughing coexist at the base of temperate glacier ice and act to mobilize and transport sediment and deposit it as various end members ranging from glacitectonically folded and faulted stratified material to texturally homogeneous diamicton the dominance of any one subglacial process varies both spatially and temporally giving rise to the possibility that a till or complex till sequence contains a superimposed signature of former transportation deposition at the ice bed interface we recommend that while glacial geologists and geomorphologists should be able to recognize the sedimentary imprints of various subglacial processes genetic fingerprinting of subglacial tills should be less process specific and till classification must reflect the range of products encompassed by the subglacial till production continuum glacial geologists can presently unequivocally identify a glacitectonite rock or sediment that has been deformed by subglacial shearing deformation but retains some of the structural characteristics of the parent material b subglacial traction till sediment deposited by a glacier sole either sliding over and or deforming its bed the sediment having been released directly from the ice by pressure melting and or liberated from the substrate and then disaggregated and completely or largely homogenised by shearing and support the theoretical case for c melt out till sediment released by the melting of stagnant or slowly moving debris rich glacier ice and directly deposited without subsequent transport or deformation because observations on contemporary glaciers reveal that their beds are most likely to be mosaics of deformation and sliding and warm based and cold based conditions the patterns of which change temporally and spatially it is extremely unlikely that subglacial till end members in the geological record will be anything but hybrids produced by the range of processes operative in the subglacial traction zone

independent fission track ages ϕ ages of proposed and accepted apatite age standards and a comparison of ϕ z ζ and ζ0 ages implications for method calibration abstract the results of 94 determinations of the independent fission track ages ages of proposed and accepted apatite age standards show that the ages of the accepted standards are consistent with their reference ages but that those of the proposed age standards are not the fct and dur data have been used to calculate z and 0 calibration factors the 0 values agree with independent measurements and with calculated values the z and 0 ages t t z t and t 0 of 71 samples dated in the course of geological investigations are on the whole consistent although systematic deviations from the mean t m exceed the statistical uncertainties these are however 2 and traceable to statistical variation including counting statistics and an extra poisson component resulting from somewhat inconsistent track identification criteria the overall agreement between t z t and t 0 implies that measurements of s i d and are reproducible but says nothing about their intrinsic accuracies the fact that t also agrees with t z t and t 0 t t m 1 000 0 001 is inconsistent with the 10 length reduction of confined spontaneous tracks in the standards the same applies to the t t ref ratio mean 1 009 0 010 it is concluded that the agreement between t and t ref and between t and t z t and t 0 is an indication of the fact that the calculated ages are indeed accurate this implies that the 10 reduction of the spontaneous track lengths in fct and dur has no measurable effect on their fission track ages to within the

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

geomorphological reconstruction of the labrador sector of the laurentide ice sheet abstract the labrador sector formed one of three major domes of the laurentide ice sheet during the last wisconsinan glaciation reconstruction of its evolution is important because it modulated atmospheric and oceanic circulation over 1 100 ka time scales this paper reports new geomorphological evidence of ice flow which is used to reconstruct glimpses of the behaviour of the labrador sector during its evolution flow evidence indicates a southern position 51 n of the labrador ice divide presumed to be the usual location when the ice sheet was close to its maximum extent a previously inferred dramatic surge of labrador ice across hudson strait and onto baffin island at 9 9 9 4 ka is verified seemingly contradictory evidence regarding deglaciation in northern quebec has been analysed and a reconstruction developed that solves this problem deglacial retreat of the labrador ice sheet was highly asymmetric with rapid retreat of the southern margin northwards migration of the centre of mass by 500 km and slow retreat along the northern margin as it shrank its thermal regime changed from cold centred to entirely cold based and it finally fragmented into many residual ice caps and glaciers this dynamic and asymmetric behaviour may be due to destabilisation of the labrador ice sheet triggered by heinrich event ice streaming through hudson strait