a persistent and dynamic east greenland ice sheet over the past 7 5 million years 10be and 26al isotopic evidence in quartz sand from the seafloor shows that a dynamic east greenland ice sheet has existed for the past 7 5 million years understanding the growth and stability of the greenland ice sheet is a long standing priority in palaeoclimate research and one that is at the heart of modern discussion of sea level rise paul bierman and colleagues present marine records of cosmogenic isotopes and conclude that an ice sheet in east greenland was continuously eroding sediments over the past 7 5 million years although the method could miss transient deglaciations and cannot firmly distinguish between remnant and larger ice sheets the results are consistent with model simulations of a waxing and waning but continuously present regional ice sheet climate models show that ice sheet melt will dominate sea level rise over the coming centuries but our understanding of ice sheet variations before the last interglacial 125 000 years ago remains fragmentary this is because terrestrial deposits of ancient glacial and interglacial periods1 2 3 are overrun and eroded by more recent glacial advances and are therefore usually rare isolated and poorly dated4 in contrast material shed almost continuously from continents is preserved as marine sediment that can be analysed to infer the time varying state of major ice sheets here we show that the east greenland ice sheet existed over the past 7 5 million years as indicated by beryllium and aluminium isotopes 10be and 26al in quartz sand removed by deep ongoing glacial erosion on land and deposited offshore in the marine sedimentary record5 6 during the early pleistocene epoch ice cover in east greenland was dynamic in contrast east greenland was mostly ice covered during the mid to late pleistocene the isotope record we present is consistent with distinct signatures of changes in ice sheet behaviour coincident with major climate transitions although our data are continuous they are from low deposition rate sites and sourced only from east greenland consequently the signal of extensive deglaciation during short intense interglacials could be missed or blurred and we cannot distinguish between a remnant ice sheet in the east greenland highlands and a diminished continent wide ice sheet a clearer constraint on the behaviour of the ice sheet during past and ultimately future interglacial warmth could be produced by 10be and 26al records from a coring site with a higher deposition rate nonetheless our analysis challenges the possibility of complete and extended deglaciation over the past several million years

topographic characteristics of laboratory induced shear fractures the topography of laboratory induced shear fracture surfaces of westerly granite was studied three types of fracture surfaces were examined 1 a fresh fracture from the shear failure of an intact sample under polyaxial loading 2 40 mpa 3 15 mpa 2 a shear fracture subjected to frictional sliding of 100 m under polyaxial loading 3 a shear fracture subjected to frictional sliding of 800 m under conventional triaxial loading 1 2 3 40 mpa both sliding distances are within the range of the grain size of westerly granite the results are represented by a power spectral method

cold based laurentide ice covered new england s highest summits during the last glacial maximum to better understand glacial history and process in new england northeastern united states a mountainous area overrun by the laurentide ice sheet we measured three cosmogenic nuclides in nine upland samples the concentrations of 10be and 26al in some samples collected near the summits of katahdin maine and mount washington and little haystack mountain new hampshire are 2 10 times higher than expected for a single exposure period considering field evidence indicating that continental ice covered all new england peaks during the last glacial maximum in situ 14c exposure ages from the summits are much younger suggesting that high elevation sampling sites were ice covered before and during the last glacial maximum field and isotopic data are consistent with new england summits being covered in part by cold based continental ice that did not erode much rock the contrast in erosion rates between stable summits and deeply eroded valleys likely contributes to the development and maintenance of northern appalachian topography

age of the pineo ridge system implications for behavior of the laurentide ice sheet in eastern maine u s a during the last deglaciation abstract the laurentide ice sheet was a major driver of global sea level change during the last deglaciation and may have impacted both atmospheric and oceanic circulation an understanding of past changes in the ice sheet is important for constraining its interaction with other components of the climate system here we present the geologic context and chronology for ice sheet fluctuations in eastern maine adjacent to the north atlantic ocean thought to be a key player in the termination of the last ice age retreat of the laurentide ice sheet through coastal maine first produced a series of lobate grounding line moraines followed by deposition of the prominent pineo ridge system which crosscut the earlier moraine set and which is characterized by extensive ice contact deltas closely spaced parallel moraines and association with eskers our new 10 be surface exposure ages indicate that the pineo ridge system which extends for more than 100 km in eastern maine and atlantic canada dates to 15 3 ka 800 years older than recent estimates our data are in accord with inboard minimum limiting radiocarbon ages of terrestrial materials which indicate deglaciation as early as 15 3 ka as well as of marine shells that are as old as 15 0 ka both the deglaciation that produced the lobate moraines and the short lived readvance that led to the pineo ridge system occurred during heinrich stadial 1 given that faunal and isotopic evidence indicates that the ocean remained cold during deglaciation of coastal maine we infer that ice recession was due to rising summer air temperatures that gave way briefly to cooling to allow minor readvance glacial deposits north of the pineo ridge system display evidence of ice stagnation and downwasting suggesting rapid ice retreat following deposition of the delta moraine complex coincident with the onset of the bolling

a new extraction technique and production rate estimate for in situ cosmogenic 14c in quartz the potential of in situ cosmogenic 14c in situ 14c for surficial process studies is widely recognized yet a reliable means of isolating it has proved difficult to develop we present here a new method for extracting in situ 14c from quartz that overcomes difficulties encountered with earlier techniques yielding more reliable production rate estimates comparison of 14c thermal release patterns from surficial and deeply shielded quartz samples lifton 1997 demonstrated that contaminant 14c is released at or below 500 c and that 14c released between 500 and 1500 c is essentially all in situ produced the new technique builds on this key result using resistance heating of samples in the presence of a lithium metaborate libo2 flux and collection of all evolved carbon as co2 between 500 c and 1100 to 1200 c our improved method has four distinct advantages over other extraction methods 1 we can identify and quantitatively eliminate atmospheric organic 14c contamination 2 we can identify the in situ 14c component unambiguously without assumptions of 14co 14co2 production proportions within the rock or equilibria on extraction 3 in situ 14c is reliably extracted from quartz at lower temperatures and in less time than earlier methods and 4 blank 14c levels are consistently low 2 3 0 1 105 14c atoms 1 our new extraction procedures should thus enable researchers to use in situ 14c in diverse applications without reservation we developed our new procedures using samples of wave cut quartzite benches from the well dated bonneville 17 4 0 3 cal ky and provo 16 8 0 3 cal ky shorelines of pleistocene lake bonneville utah and from underlying deeply shielded locations in situ 14c was extracted from quartz separated from 2 bonneville shoreline samples 6 aliquots and 1 provo shoreline sample 2 aliquots results demonstrate that our new procedures can effectively isolate the in situ 14c fraction with replicate analytical precision better than 2 1 n 5 while remaining consistent with earlier results this level of precision and accuracy is comparable to or exceeds those currently obtainable with in situ cosmogenic 10be 26al 3he 21ne and 36cl resulting weighted mean in situ 14c site production rates for the bonneville and provo shorelines are 52 9 1 7 14c atoms g sio2 y and 48 7 2 8 14c atoms g sio2 y 1 respectively consistent with earlier production rate estimates current and previously published in situ 14c site production rate estimates were then scaled to sea level and high geomagnetic latitude using the latitude altitude scaling models of lal 1991 and dunai 2000 results indicate that both models yield sea level high latitude production rates consistent with independent estimates our new in situ 14c data yield integrated late quaternary production rate estimates at sea level and high latitude of 15 1 0 5 14c atoms g sio2 y using the lal 1991 model and 15 8 0 5 14c atoms g sio2 y with that of dunai 2000 until significant uncertainties in these models are addressed however we prefer the value from the widely used lal 1991 model as our best estimate of the integrated late quaternary production rate for in situ 14c

constraining landscape history and glacial erosivity using paired cosmogenic nuclides in upernavik northwest greenland high latitude landscape evolution processes have the potential to preserve old relict surfaces through burial by cold based nonerosive glacial ice to investigate landscape history and age in the high arctic we analyzed in situ cosmogenic 10 be and 26 al in 33 rocks from upernavik northwest greenland we sampled adjacent bedrock boulder pairs along a 100 km transect at elevations up to 1000 m above sea level bedrock samples gave significantly older apparent exposure ages than corresponding boulder samples and minimum limiting ages increased with elevation two isotope calculations 26 al 10 be on 20 of the 33 samples yielded minimum limiting exposure durations up to 112 k y minimum limiting burial durations up to 900 k y and minimum limiting total histories up to 990 k y the prevalence of 10 be and 26 al inherited from previous periods of exposure especially in bedrock samples at high elevation indicates that these areas record long and complex surface exposure histories including significant periods of burial with little subglacial erosion the long total histories suggest that these high elevation surfaces were largely preserved beneath cold based nonerosive ice or snowfields for at least the latter half of the quaternary because of high concentrations of inherited nuclides only the six youngest boulder samples appear to record the timing of ice retreat these six samples suggest deglaciation of the upernavik coast at 11 3 0 5 ka average 1 standard deviation there is no difference in deglaciation age along the 100 km sample transect indicating that the ice marginal position retreated rapidly at rates of 120 m yr 1

roughness of fault surfaces over nine decades of length scales 1 we report on the topographic roughness measurements of five exhumed faults and thirteen surface earthquake ruptures over a large range of scales from 50 m to 50 km we used three scanner devices lidar laser profilometer white light interferometer spanning complementary scale ranges from 50 m to 10 m to measure the 3 d topography of the same objects i e five exhumed slip surfaces vuache sillingy bolu corona heights dixie valley magnola a consistent geometrical property i e self affinity emerges as the morphology of the slip surfaces shows at first order a linear behavior on a log log plot where axes are fault roughness and spatial length scale covering five decades of length scales the observed fault roughness is scale dependent with an anisotropic self affine behavior described by four parameters two power law exponents h constant among all the faults studied but slightly anisotropic h 0 58 0 07 in the slip direction and h 0 81 0 04 perpendicular to it and two pre factors showing variability over the faults studied for larger scales between 200 m and 50 km we have analyzed the 2 d roughness of the surface rupture of thirteen major continental earthquakes these ruptures show geometrical properties consistent with the slip perpendicular behavior of the smaller scale measurements our analysis suggests that the inherent non alignment between the exposed traces and the along or normal slip direction results in sampling the slip perpendicular geometry although a data gap exists between the scanned fault scarps and rupture traces the measurements are consistent within the error bars with a single geometrical description i e consistent dimensionality over nine decades of length scales

shear localization in transformation induced faulting first order similarities to brittle shear failure abstract transformation induced faulting a leading candidate for the mechanism of deep focus earthquakes leads to catastrophic shear failure in laboratory specimens by the growth and coalescence of microscopic mode i anticracks or packets of denser microcrystalline transformed material we have studied this phenomenon in mg 2 geo 4 olivine undergoing a polymorphic phase transformation to its denser spinel phase at 2 7 gpa and 1200 k under applied stress in order to better understand the details of anticrack growth and coalescence which leads to localized shear zones and macroscopic failure here we report observations that address the early stages of shear localization preserved in one experiment of this series are microstructures typical of anticrack nucleation and growth as well as those involved in the self organization of these features during the shear localization process we have analyzed sem and optical images of individual anticrack reaction interfaces and coalesced structures from this experiment and another experiment conducted under similar conditions but with a different strain history we have isolated the reaction boundaries in these images and have performed fractal analysis of these boundary lines we find that the fractal dimension of isolated uncoalesced anticrack structures is consistently higher than that of structures formed from coalescence and coarsening of fine grained reaction products this result is consistent with the varying effects of stress and thermal feedback at these interfaces and also represents an interesting parallel with the fractal nature of brittle cracks and fault surfaces we also measured the length of the long axis of uncoalesced anticrack structures found in the experiment arrested at failure and find a roughly log normal distribution of sizes very similar to the distribution of microcrack lengths reported in experiments involving brittle shear failure microcracks and microanticracks both also commonly nucleate on grain boundaries intracrystalline flaws and phase boundaries shear zones faults formed by either of these mechanisms also regularly form at 30 to maximum compression from this set of observations we conclude that the general phenomenon of shear failure creates similar features in both systems perhaps in any system which generates a shear instability despite the major differences in the detailed microphysics because of the detailed and unambiguous measurements that are possible with images of transformation induced faulting experiments further studies of this faulting mechanism may provide new insights into brittle failure and shear localization processes

characteristic features of local breakdown near a crack tip in the transition zone from nucleation to unstable rupture during stick slip shear failure abstract characteristic features of local breakdown near a propagating crack tip during slip failure nucleation and its transition process to unstable rupture under mode ii conditions are experimentally studied using a rock sample with a simulated fault the size of which is large compared with the size of the breakdown zone in addition distinctive features in the nucleation zone are compared with those in the zone of steady dynamic rupture propagation it is found that the local breakdown stress drop the local dynamic stress drop the local stress increase immediately before the slip weakening process the dimensionless parameter s defined by das and aki and the shear fracture energy are functions of crack length in the transition zone from nucleation to unstable rupture i e they all increase with crack growth in the nucleation zone this is contrasted with the observations in the zone of steady dynamic rupture propagation where these physical parameters do not depend on crack length an increase in the magnitudes of these parameters with crack growth in the nucleation zone results in increasing resistance to crack extension with crack growth in the nucleation zone the reason for the increase in crack growth resistance in this zone is that a slip failure nucleus is formed at a point or zone where the local strength and the shear fracture energy are at a minimum this basic physical information can provide the key to earthquake prediction since the nucleation itself is a precursory phenomenon the ratio of the size of the breakdown zone to the crack length decreases abruptly beyond the critical crack length rupture velocity v accelerates with crack growth and reaches the shear wave velocity or super shear and v is found to be expressed empirically as a function of crack length in the nucleation zone a positive correlation is found between s and v in the nucleation zone v increases with increasing s however no such correlation was observed in the zone of steady dynamic rupture propagation where v seems to have reached the shear wave velocity of super shear for s 0 5 this is consistent with das and aki s numerical result the interrelationships between shear stress slip displacement slip velocity and acceleration near the crack tip during the breakdown in the nucleation zone are revealed and they are compared with those in the zone of dynamic rupture propagation the present experiments suggest that the relationship between shear stress and slip displacement is more fundamental during the breakdown process than the relationship between shear stress and slip velocity a model is presented to describe the breakdown process in the nucleation zone

structural anisotropy of normal fault surfaces abstract precise description of natural fault surfaces is indispensable to understanding the geometry mechanics and fluid transport properties of faults profiles of fault surfaces in the wasatch fault zone and oquirrh mountains utah are measured at 30 increments within the fault plane to determine the directional anisotropy of surface roughness at wavelengths between 10 3 m and 30 m and then compared with profiles of larger scale fault surfaces surface anisotropy and an increasing ratio of surface amplitude to wavelength are consistent with self affine fault topography at wavelengths between 1 mm and approximately 5 km fractal dimension of surface profiles generally decreases systematically as the angle to the slip direction increases directional anisotropy is described by an azimuthal scaling function k sin 0 or af af max 1 sin 1 where and af are the amplitude to wavelength ratio and anisotropy factor respectively at azimuth measured clockwise relative to slip direction within the fault surface and 0 is the amplitude to wavelength ratio parallel to slip direction k 90 0 is an anisotropy coefficient and increases systematically with spatial wavelength on the fault surface characterization of natural fault surfaces provides parameters such as fractal dimension d intercept log c of power spectra profile variance and variation in anisotropy factor af which are needed to generate fractal models of natural fault surfaces using spectral synthesis we generate sample models which illustrate the differences between fault surfaces characterized by constant versus azimuthally varying fractal dimension the latter model surfaces contain low amplitude corrugations superimposed on elongate ridges which parallel slip direction this surface texture resembles that of natural fault surfaces that refract across lithologic layering or are cut by secondary faults such as r and r shears

seismic history of a bedrock fault scarp using quantitative morphology together with multiple dating methods a case study of the luoyunshan piedmont fault southwestern shanxi rift china abstract the study of bedrock fault scarps is a useful approach to identify paleo earthquakes and acquire the faulting history in bedrock areas where traditional trench techniques are not applicable here we report a study on the luoyunshan piedmont fault lyspf southwestern shanxi rift china although several studies have analyzed the lyspf its faulting behavior is still under debate for example the tectonic geomorphology fails to determine the frequency of past slips and the elapsed time to address these problems we investigated two bedrock fault surfaces luoyuncun northern segment and weicun southern segment using quantitative morphology analyses together with multiple quaternary dating techniques quantitative morphology analysis shows that both bedrock fault surfaces have the characteristics of vertical segmentation this kind of segmentation indicates that the fault surfaces are exhumed intermittently likely owing to periodic faulting earthquakes therefore earthquake events can be identified as morphological segments and the corresponding co seismic displacement can be determined by the height of these segments according to the results from the quantitative morphology and dating techniques four seismic events with a co seismic vertical displacement of 0 9 m 0 82 m 1 64 to 2 38 m and 2 30 to 2 54 m on the northern segment occurred during the late pleistocene whereas two seismic events with a co seismic vertical displacement of 1 33 m and 1 to 1 08 m are identified on the southern segment occurred during the holocene combined with the results of knickpoint series and trench techniques of the lyspf eight paleo earthquake events were identified since 45 ka with an average recurrence interval of 5 8 kyr the more complete paleo earthquake sequence also showed the possibility that the variation of the slip rate along the lyspf depends on the segment and that the displacement produced by the linfen m 7 earthquake of 649 ce has been recorded at the wc fault surface

evolution of slip surface roughness through shear a significant part of displacement in fault zones occurs along discrete shear surfaces the evolution of fault surface topography is studied here in direct shear laboratory experiments matching tensile fracture surfaces were sheared under imposed constant normal stress and sliding velocity the roughness evolution was analyzed using measurements of surface topography before and after slip we show that shearing reduces the initial surface roughness at all measurement scales at all wavelengths the roughness ratio between initial and final roughness increases as a function of the slip distance for a given test the roughness ratio increases with wavelength up to a few millimeters beyond which the ratio becomes wavelength independent at this region the roughness measured after slip follows a power law similar to that of the initial tensile fracture surface we interpret this geometrical evolution as a consequence of the deformation stage of interlocked asperities which is followed by shear induced dilation

variation in glacial erosion near the southern margin of the laurentide ice sheet south central wisconsin usa implications for cosmogenic dating of glacial terrains we measured the abundance of cosmo genic 10 be and 26 al in 22 samples collected from five striated granite metarhyolite and quartzite outcrops in south central wiscon sin that were covered by the late wisconsin laurentide ice sheet in two outcrops mea sured nuclide abundances are consistent with the existing radiocarbon chronology of ice retreat in three outcrops nuclide abun dances were up to eight times higher than predicted by the radiocarbon chronology at these three sites several thousand years of ice flow eroded only centimeters to deci meters of rock allowing a significant quan tity of nuclides 10 5 two samples carry the equivalent of 150 000 yr of surface exposure even though they were covered by ice during the last glacial maximum advance samples highest on the landscape or in plucked ar eas have less inheritance than those from the lee sides of large hills or lower in the landscape three quartzite samples collect ed 10 km up ice from the margin contain three to four times the expected nuclide abundance 10 5 to 10 6 atoms per gram of quartz in contrast eight other quartzite and granite samples from two outcrops 50 km up ice from the former margin contain only 10 5 atoms of 10 be per gram of quartz consistent with late pleistocene ex posure and little if any nuclide inheri tance this relationship between glacial erosion and distance from the former ter minus is consistent with a marginal zone of minimal subglacial erosion the ice was ei ther frozen to its bed there or the ice thick ness and duration of ice cover were less near the terminus these data together with simple model ing of nuclide production by deeply pene trating muons suggest that many meters of rock must be removed to reduce inheri tance to negligible levels 1000 yr in con tinental terrains with low long term erosion rates our results indicate that cosmogenic dating of exposed bedrock surfaces near former ice margins or in areas where ice was frozen to the bed may be uncertain and in some cases impossible because nu clides are inherited from prior periods of cosmic ray exposure unfortunately striat ed glacial outcrops that can be used to dem onstrate most easily the assumption of no postglacial erosion are also the most likely to have undergone little glacial erosion this finding suggests that cosmogenic nuclide production rates based on glacially striated surfaces may include cosmogenic nuclides inherited from prior exposure

roughness of natural fault surfaces the roughness of fault surfaces is important in the mechanics of fault slip and could play a role in determining whether sliding occurs via earthquakes or fault creep we have made preliminary measurements of the power spectral density of several fault surfaces over the wavelength range from 10 5 to 1 m using field and laboratory scale profilimeters the fault surfaces are strongly anisotropic profiles parallel to the slip direction have amplitudes about one order of magnitude lower than those perpendicular to the slip direction over most of the wavelength interval measured fault roughness perpendicular to the slip direction is similar to the roughness of natural joints the greater smoothness in the slip direction presumably represents wear resulting from slip combining our data with previous measurements of the roughness spectrum of the san andreas fault shows that fault surfaces are fractal over nearly eleven orders of magnitude in wavelength

the structure and timing of lateral escape during the scandian orogeny a combined strain and geochronological investigation in finnmark arctic norwegian caledonides abstract determining the timing duration and mechanism of tectonic events within an orogenic cycle such as ocean subduction continent continent collision or gravitational collapse is challenging especially in ancient orogenic belts variations in the tectonic transport direction however can be used as a guide to these stages of orogeny while thrust sheets within the caledonian allochthon in north norway were emplaced broadly eastwards perpendicular to the trend of the orogen many features indicate material transport in other orientations one dominant feature of the mageroy nappe sitting above and infolded with the kalak nappe complex is the development of a strong n s lineation orthogonal to the main transport direction strain measurements in part determined by a new method are used in the context of the regional structural data to identify the critical stage in orogeny when compressional forces are balanced by orogen parallel lateral escape quantitative 3 d strain estimation in the mageroy nappe indicates prolate deformation with c 50 horizontal shortening parallel to the thrusting direction e w and c 200 extension along the orogenic strike n s with c 30 vertical shortening temporal constraint on this fabric is provided by ar ar isotopic analysis of undeformed white mica in cross cutting granitic pegmatites these data show that prolate deformation occurred before the white mica cooling age of 416 4 ma while the previously determined depositional age of the hellefjord schist indicates that it occurred after 438 4 ma a granitic pegmatite that intruded the hellefjord schist after an initial deformation phase but during or prior to a later deformation has been dated at 431 2 ma by u pb zircon a previous lower age constraint on this deformation of 428 5 ma is given by metamorphic zircon overgrowths on fractured grains these results constrain the continental collision between baltica and laurentia in finnmark to the interval c 431 428 ma placed in a regional context these results indicate that lateral escape was orthogonal to the thrusting direction and occurred during the continent continent collision stage in the scandian orogeny before gravitationally driven collapse

first cosmogenic ages for glacial deposits from the plata range 33 s new inferences for quaternary landscape evolution in the central andes this paper presents the first numerical cosmogenic ages of glacial deposits from the eastern slope of the plata peak 6000 in the arid central andes 32 s the moraines are distributed along three different valleys vallecitos angostura and las mulas in the first only one moraine was identified and no blocks could be sampled for dating the other two have evidence of two glacial advances angostura i and angostura ii the advances loma de los morteritos i and ii were recognised in the las mulas valley in both cases first and second advances are preserved at a similar altitude surface boulders on the top of moraines were sampled for be10 obtained ages are all similar and late pleistocene in age they fit very well with local stratigraphy and regional paleoclimate evidence the loma de los morteritos moraine ii located at 3000 masl dated to 25 000 years ages of 8 000 and 12 000 years were found for the angostura ii deposit 3300 masl an age was obtained for the outwash deposit identified along the el salto valley 69 000 years associated with a late pleistocene drift quantitative datings presented here improve knowledge about the glacial chronology of the plata range and quaternary stratigraphy of the central andes even though they represent a maximum age for these glacial advances new findings support the occurrence of two late pleistocene glacial advances and an older one in the blanco river valley adjusted ages around 8 000 and 25 000 years for younger stage could be evidence of the last glacial maximum lgm period for this region

nd isotopic evidence for exotic detritus in the kalak nappe complex north norwegian caledonides samples from the kalak nappe complex north norwegian caledonides have yielded the following ranges of nd depleted mantle model ages orthogneiss 2 8 ga paragneisses 1 2 1 8 ga and soroy succession metasediments 1 2 2 0 ga the presence in the complex of sedimentary detritus derived from young

origin and 10be surface exposure dating of a coarse debris accumulation in the hrubý jesenik mountains central europe abstract coarse debris accumulations cover large areas in high latitude and mountain environments but their age and origin remain mostly unknown here we reassess the morphology of a prominent coarse debris accumulation in the sudetes mountain range in central europe and constrain its timing with 10be surface exposure data our observations indicate that the landform in the south eastern slope of the su ridge appears consistent with the morphology of a block slope the oldest minimum age of 150 1 4 8 ka obtained for the tor at the upper limit of the block slope indicates the disintegration of the ridge and surface lowering under periglacial conditions prior to the last glacial interglacial cycle four exposure ages retrieved for block samples range from 84 3 3 8 to 26 8 2 6 ka providing evidence for block emplacement during the last cold period the exposure ages extend existing chronological dataset for peri glacial landforms in the sudetes and constrain the timing of cold environments in the area prior the last glacial maximum the ages are also consistent with surface exposure data reported for block fields and block slopes in western europe and confirm the formation of similar landforms over the last glacial period in the sudetes

exploring the ingredients required to successfully model the placement generation and evolution of ice streams in the british irish ice sheet abstract ice stream evolution is a major uncertainty in projections of the future of the greenland and antarctic ice sheets accurate simulation of ice stream evolution requires an understanding of a number of ingredients that control the location and behaviour of ice stream flow here we test the influence of geothermal heat flux grid resolution and bed hydrology on simulated ice streaming the palaeo record provides snapshots of ice stream evolution with a particularly well constrained ice sheet being the british irish ice sheet biis we implement a new basal sliding scheme coupled with thermo mechanics into the bisicles ice sheet model to simulate the evolution of the biis ice streams we find that the simulated location and spacing of ice streams matches well with the empirical reconstructions of ice stream flow in terms of position and direction when simple bed hydrology is included we show that the new basal sliding scheme allows the accurate simulation for the majority of biis ice streams the extensive empirical record of the biis has allowed the testing of model inputs and has helped demonstrate the skill of the ice sheet model in simulating the evolution of the location spacing and migration of ice streams through millennia simulated ice streams also prompt new empirical mapping of features indicative of streaming in the north channel region ice sheet model development has allowed accurate simulation of the palaeo record and allows for improved modelling of future ice stream behaviour

weathering history of an exposed bedrock fault surface interpreted from its topography abstract morphologic features of bedrock fault scarps are underutilized in studying faulting and weathering history partly because of a lack of accurate quantitative parameters for topography the study employs ground based lidar to measure five patches at different levels on the same fault surface and then calculates roughness in the form of power spectral density in directions parallel and perpendicular to the slip the power spectral density and spatial frequency typically follow a power law for each fault patch showing approximately linear relationships in a log log plot however due to additional power introduced by weathering all spectral curves especially those parallel to the slip can be divided into two segments lower frequency wavelengths of several centimeters several meters and higher frequency wavelengths of several centimeters and below domains this shows that the topographic features at different spatial scales are dominated by different mechanical processes faulting abrasion in the lower frequency domain and the weathering process in the higher frequency domain moreover we develop two parameters to quantify the degree of weathering of a fault outcrop which is significant to describe the evolution of the fault scarp and infer the date of faulting under calibration

felsenmeer persistence under non erosive ice in the torngat and kaumajet mountains quebec and labrador as determined by soil weathering and cosmogenic nuclide exposure dating soil analyses and terrestrial cosmogenic nuclide exposure dating are combined and a conceptual model proposed to explain altitudinal weathering contrasts in high latitude highlands we show that summits in the torngat and kaumajet mountains were covered by ice during the last glacial maximum and that their felsenmeer cover probably survived multiple glaciation events for similar lithologies soils on felsenmeer covered summits are signigicantly more weathered than those below the felsenmeer limit displaying higher concentrations of crystalline iron amorphous aluminium and silicium extracted with oxalate secondary minerals such as gibbsite and kaolinite occur in felsenmeer soils whereas those formed in till lacked these secondary minerals 10be and 26al exposure ages for nine of ten samples from high elevation tors and autochthonous felsenmeer blocks range from 73 6 to 157 15 ka by contrast ages of 11 4 1 0 and 11 7 1 0 ka are measured for bedrock in the much lower saglek zone indicating extensive 3 m glacial erosion of this zone during late wisconsinan glaciation 26al 10be ratios demonstrate that exposure of the high elevation surfaces was interrupted during at least one cosmic ray shielding event by either ice or till cover in either case late wisconsinan glaciers could not have extensively eroded these surfaces five erratics dated above the saglek zone including one in the felsenmeer zone have exposure ages ranging from 11 6 1 0 to 13 6 0 7 ka this indicates that valley and high elevation ice persisted through the younger dryas chron and provides further evidence that the highlands were not nunataks during the late wisconsinan period

constitutive behavior and stability of frictional sliding of granite an understanding of the frictional sliding on faults that can lead to earthquakes requires a knowledge of both constitutive behavior of the sliding surfaces and its mechanical interaction with the loading system we have determined the constitutive parameters for frictional sliding of initially bare surfaces of westerly granite using a recently developed high pressure rotary shear apparatus that allows long distances of sliding and therefore a greater assurance of attaining steady state behavior from experiments conducted at room temperature and normal stresses of 27 84 mpa several important results have been found 1 a gouge layer 100 to 200 m thick was developed from the initially bare rock surfaces after 18 to 70 mm of sliding 2 the steady state frictional resistance attained after about 10 mm of sliding is proportional to the negative of the logarithm of the sliding velocity 3 abrup changes in the velocity of sliding result in initial changes in the frictional resistance which have the same sign as the velocity change and are followed by a gradual decay to a new steady state value over a characteristic distance of sliding this velocity weakening behavior is essentially identical with that found by several previous workers on the same material at lower normal stress 4 our results are well described by a two state variable constitutive law the values of the constitutive parameters are quite similar to those found previously at low normal stress but the characteristic distance is about an order of magnitude smaller than that found at 10 mpa normal stress with thicker layers of coarser gouge 5 we have approximated our results with a one state variable constitutive law and compared the results with the predictions of existing nonlinear stability analysis in addition we have extended the stability analysis to systems possessing two state variables with such formulations good agreement is found between the experimentally observed and theoretically predicted transitions between stable and unstable sliding these results allow a better understanding of the instabilities that lead to earthquakes

rapid late pleistocene incision of atlantic passive margin river gorges the direct and secondary effects of rapidly changing climate caused large rivers draining the atlantic passive margin to incise quickly into bedrock beginning about 35 000 years ago measured in samples from bedrock fluvial terraces 10 beryllium shows that both the susquehanna and potomac rivers incised 10 to 20 meter deep gorges along steep convex lower reaches during the last glacial cycle this short lived pulse of unusually rapid down cutting ended by 13 000 to 14 000 years ago the timing and rate of downcutting are similar on the glaciated susquehanna and unglaciated potomac rivers indicating that regional changes not simply glacial meltwater initiated incision

cosmogenic nuclide ages for new england coastal moraines martha s vineyard and cape cod massachusetts usa abstract cosmogenic nuclide exposure ages for 13 glacially transported boulders atop the martha s vineyard moraine ma usa indicate that the southeastern margin of the laurentide ice sheet reached its maximum extent during the last glaciation 23 200 500 yr ago another 10 age determinations from the younger buzzards bay moraine near woods hole ma indicate that this moraine complex was formed 18 800 400 yr ago these ages correlate approximately with the terminations of cooling cycles recorded in greenland ice cores and coeval ice rafting events suggesting that the marginal position of this sector of the ice sheet was tightly coupled to north atlantic climate during the last glacial maximum

preservation of arctic landscapes overridden by cold based ice sheets for nearly 40 years a massive well preserved glaciomarine delta more than 54 000 years old and ancillary landforms have formed the cornerstone of models positing limited ice sheet extent in arctic canada during the late wisconsinan we present exposure ages for large boulders on the delta surface which coupled with preservation of relict landforms demonstrate that the region was covered by minimally erosive cold based ice during the late wisconsinan our data suggest that surficial features commonly used to define the pattern of late wisconsinan ice movement cannot be used on their own to constrain late wisconsinan ice sheet margins in arctic regions

complexities of rock fracture and rock friction from deformation of westerly granite a series of rock friction experiments has been carried out to study the complexities in rock fracture and rock friction intact westerly granite samples were loaded to shear failure in a laboratory polyaxial loading apparatus the resultant fractured samples were reloaded to cause frictional sliding both polyaxial loading 1 2 3 0 and equal confining condition 1 2 3 0 were used the deformation processes were monitored by macroscopic axial stress strain optical holography and ultrasonic velocity measurements

age of the berlin moraine complex new hampshire usa and implications for ice sheet dynamics and climate during termination 1 at its late pleistocene maximum the laurentide ice sheet was the largest ice mass on earth and a key player in the modulation of global climate and sea level at the same time this temperate ice sheet was itself sensitive to climate and high magnitude fluctuations in ice extent reconstructed from relict glacial deposits reflect past changes in atmospheric temperature here we present a cosmogenic 10be surface exposure chronology for the berlin moraines in the white mountains of northern new hampshire usa which supports the model that deglaciation of new england was interrupted by a pronounced advance of ice during the bolling allerod together with recalculated 10be ages from the southern new england coast the expanded white mountains moraine chronology also brackets the timing of ice sheet retreat in this sector of the laurentide in conjunction with existing chronological data the moraine ages presented here suggest that deglaciation was widespread during heinrich stadial 1 event 18 14 7 ka despite apparently cold marine conditions in the adjacent north atlantic as part of the white mountains moraine system the berlin chronology also places a new terrestrial constraint on the former glacial configuration during the marine incursion of the st lawrence river valley north of the white mountains

late glacial fluctuations of the laurentide ice sheet in the white mountains of maine and new hampshire u s a abstract prominent moraines deposited by the laurentide ice sheet in northern new england document readvances or stillstands of the ice margin during overall deglaciation however until now the paucity of direct chronologies over much of the region has precluded meaningful assessment of the mechanisms that drove these events or of the complex relationships between ice sheet dynamics and climate as a step towards addressing this problem we present a cosmogenic 10be surface exposure chronology from the androscoggin moraine complex located in the white mountains of western maine and northern new hampshire as well as four recalculated ages from the nearby littleton bethlehem moraine seven internally consistent 10be ages from the androscoggin terminal moraines indicate that advance culminated 13 2 0 8 ka in close agreement with the mean age of the neighboring littleton bethlehem complex together these two datasets indicate stabilization or advance of the ice sheet margin in northern new england at 14 13 ka during the allerod greenland interstadial i

the new north american varve chronology a precise record of southeastern laurentide ice sheet deglaciation and climate 18 2 12 5 kyr bp and correlations with greenland ice core records new glacial varve records from long cores combined with records from key surface exposures and new radiocarbon ages have allowed the correction consolidation expansion and calibration of ernst antevs9 original new england varve chronology nevc in the connecticut valley of new england u s a the varve records have been reformulated with corrections and a new numbering system as the new north american varve chronology navc which is a continuous 5659 yr varve sequence that spans most of the last deglaciation 18 200 12 500 yr bp in the northeastern united states rates of ice recession for separate intervals terminated by abrupt glacial stillstands and readvances have been determined for western new england ice recession history is coupled to varve thickness changes that depict changes in meltwater production in the connecticut valley and show the relationship of changes in ablation rate summer climate variation to glacial readvances and periods of halted and rapid ice recession up to 300 m yr comparison of varve thickness records to greenland ice core climate records show that after 15 000 yr bp climate changes of sub century and longer scales recorded in both records appear identical and synchronous after 15 000 yr bp therefore there was a link between north atlantic climate and marginal processes of the southeastern sector of the laurentide ice sheet lis prior to 15 000 yr bp when the lis was closer to an equilibrium condition retreat rates were generally lower and changes in varve thickness and ablation were more subtle but can still be linked to ice sheet activity only weak relationships between varve thickness changes and greenland climate are evident suggesting that changes in the southeastern lis during this time may have been significantly influenced by climate patterns unique to the north american continent or ice dynamics

topographic characteristics of rupture surface associated with the 12 may 2008 wenchuan earthquake it is very important to describe accurately the topography of rupture surfaces for our understanding of seismic faulting because the topographic characteristics of the rupture include much information about the earthquake and fault mechanics two fresh rupture surfaces of the m w 7 9 2008 wenchuan earthquake referred to as the bajiaomiao surface and the shaba surface have been measured by scanning with a 3d portable laser scanner the acquired sets of digital elevation model dem data are analyzed using power spectral and root mean square rms roughness the fresh rupture surfaces exhibit self affine behavior and both the power spectral density and rms roughness have a power law relationship with the length of profiles the roughness of the surface parallel to the slip direction is generally less than the roughness perpendicular to the slip direction in a log log plot of power spectral density versus spatial frequency there is an obvious inflection that divides the spatial frequency into a lower frequency domain and a higher frequency domain the wavelength corresponding to the inflection is 7 mm in the direction parallel to the slip for the bajiaomiao surface in both patch i and patch ii smaller than that in the direction perpendicular to the slip 10 mm in patch i and 9 mm in patch ii and 8 mm in the direction parallel to the slip but larger than that in the direction perpendicular to the slip 6 mm the slope of least squares fitting line to the rms roughness curve in a log log plot is the h exponent also called the hurst exponent which depends on the direction of the profile and describes the morphological anisotropy of the fault surface the slip directions indicated by the minimum h values 85 and 75 on the bajiaomiao surface and 45 on the shaba surface are equal to the slip directions observed in the field a secondary minimum h value 160 in the shaba rupture surface reveals a set of concealed striations produced by an earthquake prior to the wenchuan earthquake moreover the h value of the topographic profile perpendicular to the slip on the bajiaomiao surface is slightly larger than 0 8 whereas it is quite smaller than 0 8 on the shaba surface probably indicating reverse faulting on the bajiaomiao surface and normal faulting on the shaba surface through linear fitting between the slopes of power spectral density and the slopes of rms roughness h in the whole length of profile a relationship can be gain 1 22 1 72 h it does not adhere to theory relationship strictly 1 2 h probably caused by the noise in the data the multifractal of the rupture surface and the analysis methods of roughness

landscape preservation under fennoscandian ice sheets determined from in situ produced 10be and 26al some areas within ice sheet boundaries retain pre existing landforms and thus either remained as ice free islands nunataks during glaciation or were preserved under ice differentiating between these alternatives has significant implications for paleoenvironment ice sheet surface elevation and ice volume reconstructions in the northern swedish mountains in situ cosmogenic 10be and 26al concentrations from glacial erratics on relict surfaces as well as glacially eroded bedrock adjacent to these surfaces provide consistent last deglaciation exposure ages 8 13 kyr confirming ice sheet overriding as opposed to ice free conditions however these ages contrast with exposure ages of 34 61 kyr on bedrock surfaces in these same relict areas demonstrating that relict areas were preserved with little erosion through multiple glacial cycles based on the difference in radioactive decay between 26al and 10be the measured nuclide concentration in one of these bedrock surfaces suggests that it remained largely unmodified for a minimum period of 845 418 461 kyr these results indicate that relict areas need to be accounted for as frozen bed patches in basal boundary conditions for ice sheet models and in landscape development models subglacial preservation also implies that source areas for glacial sediments in ocean cores are considerably smaller than the total area covered by ice sheets these relict areas also have significance as potential long term subglacial biologic refugia

a millennial perspective on arctic warming from 14c in quartz and plants emerging from beneath ice caps 1 observational records show that the area of ice caps on northern baffin island arctic canada has diminished by more than 50 since 1958 fifty 14c dates on dead vegetation emerging beneath receding ice margins document the persistence of some of these ice caps since at least 350 ad in situ cosmogenic 14c in rock surfaces and 14c in plant macrofossils from lake sediment cores demonstrate that the plateau remained ice free through the middle holocene but has supported ice caps for more than 2000 of the past 2800 years the rapid disappearance of these ice caps over the past century despite decreasing summer insolation further demonstrates the unusual character of 20th century warmth widespread ice cap expansion 1280 ad early in the little ice age and intensified expansion 1450 ad coincide with peak stratospheric volcanic aerosol loading and reduced solar luminosity suggesting that these mechanisms may have initiated ice cap growth subsequently maintained by strong positive feedbacks

glacial history and landscape evolution of southern cumberland peninsula baffin island canada constrained by cosmogenic 10be and 26al since the first application of cosmogenic nuclides to the study of glacial history and processes in 1990 increasing numbers of studies have used a variety of cosmogenic isotopes to quantify the exposure age and erosion rate of glaciated landscapes however obtaining chronological data from glaciated landscapes once covered by cold based nonerosive ice is challenging because these surfaces violate assumptions associated with simple cosmogenic exposure dating nonerosive glacial ice fails to completely remove nuclides produced during previous periods of exposure leaving behind rock surfaces with complex multigenetic nuclide inventories here we constrain the glacial history landscape evolution and efficiency of subglacial erosion in the pangnirtung fiord region of southern baffin island using over 300 paired analyses of in situ cosmogenic 10be and 26al simple exposure ages are 6 3 160 ka for 10be n 152 and 4 3 124 ka for 26al n 153 paired bedrock boulder samples have discordant ages simple exposure ages generally increase with elevation 10be and 26al ages for the same sample disagree and both boulders and bedrock yield multimodal age distributions all patterns suggestive of limited subglacial erosion measured 26al 10be ratios indicate that about one third of the samples in the data set experienced at least one period of pre holocene exposure followed by burial with limited erosion modeled two isotope minimum limiting exposure durations are as high as hundreds of thousands of years and minimum limiting burial durations range up to millions of years implying that parts of southern baffin island have been preserved beneath nonerosive glacial ice for much if not all of the quaternary a subset of the samples contains few nuclides inherited from prior periods of exposure and is thus useful for constraining the chronology of the most recent deglaciation using these samples we infer that deglaciation of most of the landscape occurred ca 11 7 ka and that the duval moraines a prominent feature of the last deglaciation formed ca 11 2 ka

characterization of fault roughness at various scales implications of three dimensional high resolution topography measurements accurate description of the topography of active fault surfaces represents an important geophysical issue because this topography is strongly related to the stress distribution along fault planes and therefore to processes implicated in earthquake nucleation propagation and arrest to date due to technical limitations studies of natural fault roughness either performed using laboratory of field profilometers were obtained mainly from 1 d profiles with the recent development of light detection and ranging lidar apparatus it is now possible to measure accurately the 3 d topography of rough surfaces with a comparable resolution in all directions both at field and laboratory scales in the present study we have investigated the scaling properties including possible anisotropy properties of several outcrops of two natural fault surfaces vuache strike slip fault france and magnola normal fault italy in limestones at the field scale digital elevation models of the fault roughness were obtained over surfaces of 0 25 m2 to 600 m2 with a height resolution ranging from 0 5 mm to 20 mm at the laboratory scale the 3 d geometry was measured on two slip planes using a laser profilometer with a spatial resolution of 20 m and a height resolution less than 1 m

fractal geometry in the san andreas fault system it has been noted that the spatial distribution of earthquakes and the mode of strain release in the san andreas fault system is related to the complexity of fault geometry because of their rough appearance over many length scales faults can be regarded as fractal surfaces direct estimates of fractal dimension d of portions of the san andreas fault system between the northern gabilan range and the salton sea including the postulated extent of the great 1857 fort tejon earthquake are obtained from measured fault lengths analogous to the lengths of coastlines as discussed by mandelbrot regions characterized by complicated fault geometry are associated with larger values of d based on fault traces mapped at a scale of 1 750 000 d is 1 3 for this reach of the fault defined as a 30 km wide band about a main fault trace for that part near parkfield which could be associated with the nucleation of the 1857 earthquake d is 1 1 at this same scale d is 1 4 for the san andreas and related faults near san bernardino where the 1857 rupture stopped compared to 1 2 for the san andreas san juan fault segments near the point of arrest of the 1966 parkfield earthquake at finer map scales 1 24 000 and 1 62 500 critical lengths of 500 m and 1 km are identified which might relate to the extent of off san andreas fault offsets the critical lengths also suggest that fault geometry is not self similar if this fractal geometry persists through the seismic cycle it may be possible to use a quantitative measure of complexity to explain the occurrence of great and characteristic earthquakes along a given reach of fault

carbonate isotope chemostratigraphy suggests revisions to the geological history of the west finnmark caledonides northern norway strontium and carbon isotopic data from amphibolite facies marbles of the falkenes formation soroy succession structurally upper kalak nappe complex in west finnmark northern norway indicate that the marbles were deposited between 760 and 710 ma marbles of similar age have previously been identified only in the uppermost allochthon in north central norway where they are considered to have a laurentian ancestry a similar origin and tectonostratigraphic position appears likely for the falkenes formation in contrast structurally lower units within the kalak nappe complex appear to have baltican affinities this dichotomy requires that commonly held ideas regarding the tectonostratigraphy of the west finnmark caledonides be revised combined with recent u pb dating of zircon and monazite from rocks associated with the marbles the isotopic data suggest that the soroy succession of the kalak nappe complex is not a continuous depositional sequence as previously thought but rather consists of a number of disparate and probably unrelated thrust sheets assembled during late silurian scandian orogenesis this work shows that the west finnmark caledonides share a generally similar tectonometamorphic history with certain other parts of the scandinavian caledonides and contributes to already existing data that may allow chemostratigraphic correlation and reconstruction of the entire caledonian belt

mid pleistocene cosmogenic minimum age limits for pre wisconsinan glacial surfaces in southwestern minnesota and southern baffin island a multiple nuclide approach abstract paired 10 be and 26 al analyses n 14 indicate that pre wisconsinan glaciated bedrock surfaces near the northern baffin island and southern minnesota paleo margins of the laurentide ice sheet have long and complex histories of cosmic ray exposure including significant periods of partial or complete shielding from cosmic rays using the ratio 26 al 10 be we calculate that striated outcrops of sioux quartzite in southwestern minnesota southern margin were last overrun by ice at least 500 000 years ago weathered bedrock tors on the once glaciated uplands of baffin island northern margin are eroding no faster than 1 1 m myr 1 the equivalent of at least 450 000 years of surface and near surface exposure our data demonstrate that exposure ages and erosion rates calculated from single nuclides can underestimate surface stability dramatically because any intermittent burial and the resultant lowering of nuclide production rates and nuclide abundances will remain undetected

preglacial surface remnants and quaternary glacial regimes in northwestern sweden abstract we present a detailed map of the distribution of preglacial surface remnants in the kebnekaise region of northwestern sweden in this mountain area we discern four important large scale geomorphological units each representing a specific set of erosional agents and formative conditions these are i intact preglacial surface remnants characterized by gentle slopes round summits wide shallow valleys and an absence of rock basins ii preglacial surface remnants showing signs of minor glacial erosion and deposition iii glacially scoured surfaces including glacial troughs iv deep fluvial valleys cut into the preglacial surface the pattern of glacial erosion is explained as the result of three specific modes of glaciation known to have existed during the last 120 000 years and inferred to have repeatedly prevailed during the last 2 75 million years cirque glaciation mountain ice sheets and fennoscandian ice sheets a deep ocean oxygen isotope record of foraminifera from the north atlantic dsdp 607 was used to infer the temporal extent of these modes of glaciation during the last 2 75 million years we interpret the preglacial landscape preservation and the pattern of glacial erosion in terms of the configuration the basal thermal regime and the duration of such glaciation events the average subglacial thermal regime of both ice sheet types was frozen on the uplands and melting in the main valleys where outlet glaciers and ice streams formed the pre glacial landscape is best preserved at intermediate elevations low enough not to have been covered by cirque glaciers and apparently high enough not to have experienced melted bed conditions and subglacial erosion during ice sheet overriding events in a narrow high relief zone along the elevation axis interglacial fluvial erosion was morphologically important the absence of glacial erosion on uplands in this zone allowed fluvial erosion to commence on the same locations during each ice free interval in contrast no persistent fluvial valley pattern could develop in zones subjected to repeated glacial scouring and hence derangement of fluvial patterns

fractal analysis applied to characteristic segments of the san andreas fault fractal theory is applied in a quantitative analysis of the san andreas fault saf geometry the method which directly measures the increase in total fault length with a decrease in ruler size gives the fractal dimension d and scaling properties for the chosen length band 0 5 1000 km a physical interpretation of d is that it measures the irregularity of the fault trace in the selected band the fault is subdivided into six segments of distinctive seismic behavior a main fault trace which shows either maximum coseismic slip or creeping was selected for analysis with three alternative branches examined for the saf system south of san bernardino branches of the fault trace are not considered in this work fractal dimensions calculated for the different segments range from 1 0008 to 1 0191 and are different from 1 0 to the 95 confidence interval these small changes reflect the overall smoothness of the main fault trace significant variations in d among segments indicate heterogeneities in the fault smoothness along strike d also changes significantly from the short length band to the long length band where the demarcation point ranges from 1 to 2 km the short length band has larger d values a slight correlation is obtained between the fractal dimension of the main trace and the extent of subparallel faulting this indicates some correspondence between the main fault trace irregularity and the complexity of subsidiary fault traces in plan view

cosmogenic nuclide evidence for minimal erosion across two subglacial sliding boundaries of the late glacial fennoscandian ice sheet abstract the existence of sliding and frozen bed areas under ice sheets is significant in understanding basal thermal regimes patterns of erosion and landform development and in constraining boundary conditions for the reconstructions of ice sheets recognition of subglacial boundaries between sliding and frozen bed areas for former ice sheets is typically based on distinct morphological contrasts between areas with glacial landform assemblages and relict areas showing little alteration of pre existing features some of these boundaries especially on continental shield areas however are clearly visible from air photos but have minimal topographic expression understanding the chronology and erosional development of such boundaries is important to provide insight into the pattern and persistence of basal conditions under ice sheets geomorphic evidence and cosmogenic radionuclide concentrations of bedrock outcrops on either side of two sliding boundaries on ultevis and arvestuottar low relief upland plateaus in northern sweden are consistent with negligible erosion in relict landscape frozen bed areas due to the last glaciation but also indicate insignificant erosion in the sliding areas such a pattern and magnitude of landscape modification indicates that sliding was short lived in these areas likely as a transient phase during deglaciation these sites demonstrate that short periods of sliding are in some cases sufficient to produce landscapes that are recognized as glacial from air photos thus regions of sliding identified on shield areas must be viewed as the cumulative total area that has experienced sliding at any time during a glaciation the actual extent of sliding areas during any single ice sheet phase is presumably considerably less than this cumulative total which has important implications for establishing appropriate basal boundary conditions for ice sheet reconstructions

analysis of multiple cosmogenic nuclides constrains laurentide ice sheet history and process on mt mansfield vermont s highest peak abstract constraining glacial history and process on mt mansfield the highest peak in vermont 1339 m a s l provides insight into how the laurentide ice sheet shaped the underlying landscape when latest pleistocene ice retreated and how upland and lowland glacial histories relate here we quantify in situ cosmogenic 10be in 20 bedrock and boulder surfaces as well as in situ cosmogenic 14c in three of those surfaces to assess subglacial erosion and exposure history isotopic concentrations indicate that mt mansfield s lower elevations 400 1200 m a s l were deeply eroded by at least several meters during the last glaciation and then deglaciated rapidly 10be ages across this elevation span are indistinguishable and average 13 9 0 6 ka n 15 suggesting that 800 m of ice thinning occurred within at most about a millennium conversely the higher elevations 1200 m a s l preserve a more complex geomorphic history mt mansfield s summit surfaces contain 10be from previous periods of exposure indicating that the mountaintop landscapes were likely preserved beneath cold based weakly erosive glacial ice exposure ages from the shorter lived isotope 14c are younger 9 7 and 11 7 ka suggesting that mt mansfield s summit was covered until the early holocene perhaps by snowfields ice carapaces and or till our findings in context of previous work suggest that thinning laurentide ice flowed through the valleys for at most hundreds of years following deglaciation of the uplands but that the summit remained shielded by ice or sediment for millennia after the valleys became ice free

dating sediment burial with in situ produced cosmogenic nuclides theory techniques and limitations abstract dating sediment burial over million year time scales is crucial in many areas of the earth sciences and archeology but is often difficult using traditional techniques sediment burial can be dated by the radioactive decay of cosmogenic nuclides provided that the sediment was exposed to cosmic rays prior to burial dating calculations are straightforward if sediment is buried deeply and rapidly enough to prevent cosmogenic nuclide production after burial however the analysis can be complicated by postburial production if sediment is insufficiently shielded from secondary cosmic ray nucleons and muons this paper discusses how buried sediments can be dated over timescales up to 5 myr using 26 al and 10 be in quartz

cosmogenic nuclide age estimate for laurentide ice sheet recession from the terminal moraine new jersey usa and constraints on latest pleistocene ice sheet history

cosmogenic radionuclides from fiord landscapes support differential erosion by overriding ice sheets the interpretation of differentially weathered mountainous areas along the fringes of pleistocene ice sheets is fundamental for determining ice sheet behavior and thickness during the last glaciation two existing interpretations are either that highly weathered uplands remained as nunataks while freshly eroded troughs held outlet glaciers during the last glaciation or that uplands and lowlands were equally covered by ice but that it was differentially erosive as a function of its spatially variable basal thermal regime cosmogenic radionuclide measurements from 33 bedrock samples and 27 upland erratics from differentially weathered fi ord landscapes on northeastern baffi n island shed light on laurentide ice sheet lis dynamics and thickness tors on weathered upland surfaces have minimum 10 be ages between ca 50 and ca 170 ka n 12 whereas the majority of erratics perched in the uplands range from ca 10 to ca 13 ka n 14 indicating that the whole landscape was glaciated during the last glacial maximum lgm glacially sculpted bedrock n 7 near sea level refl ects the age of deglaciation and increasing amounts of isotopic inheritance in highelevation sculpted bedrock n 3 higher elevation intermediately weathered bedrock n 11 and highest elevation intensely weathered bedrock refl ect the weakening of erosive power as the ice sheet transitioned from fi ords to interfi ord plateaus cold based glaciation of uplands was contemporaneous with warm based glaciation in fi ords suggesting the presence of ice streams 26 al and 10 be concentrations measured in 10 bedrock samples indicate that tors have experienced a complex history of alternating periods of exposure burial and limited glacial erosion the paired isotope data reveal a minimum duration of burial of 80 500 k y presumably by cold based ice and indicate that the tors have an age of at least 150 580 ka these data together with other reconstructions of ice streams in the eastern canadian arctic suggest that ice streams were a large infl uence on northeastern lis dynamics throughout the late quaternary thus the northeastern lis was sensitively tied with north atlantic thermohaline circulation and abrupt climate change

simple mathematical model of a rough fracture a simple mathematical model of rough walled fractures in rock is described which requires the specification of only three main parameters the fractal dimension the rms roughness at a reference length scale and a length scale describing the degree of mismatch between the two fracture surfaces fractured samples collected from natural joints and laboratory specimens have been profiled to determine the range of these three parameters in nature it is shown how this surface roughness model can be implemented on a computer allowing future detailed study of the mechanical and transport properties of single fractures and the scale dependence of these properties

cosmogenic exposure age evidence for rapid laurentide deglaciation of the katahdin area west central maine usa 16 to 15 ka abstract katahdin the highest peak in maine and part of the second highest mountain range in new england provides an opportunity to assess the timing and style of continental ice sheet surface lowering during deglaciation we collected 14 samples from boulders on the adjacent basin ponds moraine from bedrock and boulders on the upper part of the mountain and from boulders in the surrounding area to estimate the age at which they were exposed by deglaciation of the laurentide ice sheet measurements of in situ produced 10be which are consistent with measurements of 26al indicate that the katahdin edifice became exposed from under ice by 15 3 2 1 ka n 6 an age indistinguishable from the adjacent basin ponds moraine 16 1 1 2 ka n 5 a boulder in the lowlands several km south of the moraine dates to 14 5 0 8 ka and a boulder deposited at pineo ridge about 170 km se of katahdin dates to 17 5 1 1 ka these data show that samples collected over an elevation range of 1 6 km and a distance of 170 km all have exposure ages that are indistinguishable within uncertainties together these data suggest that the laurentide ice sheet surface dropped rapidly and the ice sheet margin retreated quickly across maine between about 16 and 15 ka perhaps influenced by calving of the marine based ice sheet in the st lawrence lowlands to the north and the penobscot basin to the south

alkaline rocks and carbonatites of northwestern russia and northern norway linked wilson cycle records extending over two billion years 1 tectonic analysis reveals a previously unidentified link between alkaline rocks and carbonatites arcs forming the kola alkaline and carbonatite province kacp in arctic russia and arcs of the seiland igneous province sip in the northern caledonides of finnmark norway we here conclude that the rocks of the two provinces have been derived by decompression melting of deformed alkaline rocks and carbonatites darcs under rifts and within the underlying lithospheric mantle preserved within the paleoproterozoic lapland kola suture zone lksz and its extension beneath the allochthons of the norwegian caledonides the lksz projects under the caledonide allochthons to the postulated site of eruption of the arcs of the sip in an intra continental rift at a place where an ocean was soon to open we suggest that the sip arcs formed in this intra continental rift by decompression melting involving darcs that had lain in the underlying lithospheric mantle within the along strike extension of the lksz for more than one billion years thrusting and metamorphism during the scandian collision between laurentia and baltica transformed the sip arcs into darcs that occupy the immediate footwall to the suture zone between laurentia and baltica at the collision the sip rocks were carried south southeastwards from their then location on the outermost continental margin of baltica to their current location in the highest level of a sandstone dominated nappe complex the arcs and darcs of kola and finnmark provide independent evidence of the operation of two complete wilson cycles of the opening and the closing of oceans in the region