magnetic susceptibility of the rock matrix related to magnetic fabric studies non ferromagnetic minerals constitute what is called the rock matrix whose susceptibility k1 is directly accessible using only high magnetic fields measurements on minerals and a wide range of rock types show that k1 is mainly due to paramagnetism and hardly exceeds 10 3 si with an anisotropy degree p less than 1 35 different methods to estimate the role of the matrix component in low field susceptibility k and its anisotropy include petrological and chemical analysis evolution of p vs k low temperature studies and comparison of k with remanent magnetizations

anisotropic behavior of grip ices and flow in central greenland abstract mechanical tests have been performed on strongly textured ice samples coming from a wide range of depths from 1328 down to 2868 m of the greenland ice core project grip for different sample orientations with respect to the prescribed stress in this way two directional viscosities corresponding to the easy glide and to the hard glide orientations were determined along the core the viscoplastic anisotropy gradually increases down to a depth of 2600 m and slightly decreases below revealing a clear correlation between rheology and texture the experimental mechanical response compares well with that predicted by the viscoplastic self consistent vpsc model the vpsc model is also applied to ice samples that exhibit an axisymmetric texture to show in more detail the sensitivity of the rheology to specific texture parameters this leads to a number of recommendations for future mechanical tests on anisotropic samples a large scale ice flow model is finally used to estimate the influence of ice anisotropy on the flow along the grip gisp2 flow line the particular mechanical behavior of deep grip ices in the stress regime corresponding to an ice divide leads to deformation rates that are highly sensitive to bedrock topography and texture pattern this feature is likely to favour the formation of stratigraphic disturbances in deep ice layers as observed in the last 300 m of both grip and gisp2 cores

internal reflecting horizons in spitsbergen glaciers a single pronounced internal reflecting horizon has been observed on radio echo sounding from over 30 glaciers in spitsbergen they are often present along the entire length of the glacier remaining at a fairly constant depth 100 200 m below the ice surface echo strength data from radio echo sounding have been used to obtain reflection coefficients for these horizons of between 15 and 25 db combined with results of ice core studies the possible causes of this internal layer are investigated the presence of water is found to be the most likely explanation indicating the existence at depth of a layer of temperate ice

the role of folding and foliation development in the genesis of medial moraines examples from svalbard glaciers abstract the structure and debris content of four glaciers in nw spitsbergen svalbard are described to test the hypothesis that medial moraines are the product of folding accompanied by the development of longitudinal foliation using clast orientation clast shape and clast lithological data combined with measurements of fold axes and foliation in ice this hypothesis is shown to be applicable to small valley glaciers with multiple basins feeding a narrow tongue a common situation in svalbard the glaciers examined show stratification commonly incorporating angular rockfall derived debris folded to varying degrees along flow parallel axes throughout the tongue an axial plane foliation is commonly associated with this folding and attains its greatest strength at flow unit boundaries the debris takes a medium level to high level transport path through the glacier emerging at point sources defined by gently dipping fold hinges near the snout as more of the folded debris layer melts out downglac

microfabric and microshear evolution in deformed till abstract the bed deformation model of glacier movement requires that ice masses shear their beds to high strains for homogeneous basal tills that can result from such deformation however there are no sedimentological criteria for estimating strain magnitude thus a large ring shear device was used to shear two basal tills and their particle fabrics and microshears were studied at various strains up to 108 sand particle fabrics in a longitudinal flow plane became steady at shear strains of 7 39 steady fabrics were moderately strong with s1 eigenvalues of 0 71 0 74 these values are smaller than those from experiments with pebble sized particles perhaps reflecting the greater probability of sand particles colliding with larger grains fabrics parallel to flow strengthened progressively indicating that particles did not reorient with their long axes transverse to flow r1 and r2 microshears developed and persisted to the highest strains attained r1 shears became more pervasive with strain and oriented closer to the shear plane movement along both sets of shears provides an explanation for the plunge of the steady state sand fabric approximately 10 upglacier these experiments provide a basis for estimating till deformation magnitude but results will require cautious application to the geologic record where deformation processes may be ambiguous

large scale erosion rates from in situ produced cosmogenic nuclides in european river sediments abstract we have calculated long term erosion rates of 20 100 mm kyr from quartz contained 10be in the bedload of middle european rivers for catchments ranging from 102 to 105 km2 these rates average over 10 40 kyr and agree broadly with rock uplift incision and exhumation rates historic soil erosion rates and erosion rates calculated from the measured sediment loads of the same rivers moreover our new erosion rate estimates correlate well with lithology and relief however in the regen neckar loire and meuse catchments cosmogenic nuclide derived erosion rates are consistently 1 5 4 times greater than the equivalent rates derived from measured river loads this may be due to the systematic under representation of high magnitude low frequency transport events in the gauging records which cover less than a century alternatively the discrepancy may derive from spatially non uniform erosion and preferential tapping of deeper sections of the irradiation profile a third explanation relates the high cosmogenic nuclide derived erosion rates to inheritance of an elevated pleistocene erosion signal uncertainties associated with the cosmogenic nuclide derived erosion rate estimates are not greater than the potential errors in conventional estimates therefore the cosmogenic nuclide approach is an effective tool for rapid catchment wide assessment of time integrated rates of bedrock weathering and erosion and we anticipate its fruitful application to the quaternary sedimentary record

dynamic implications of discontinuous recrystallization in cold basal ice taylor glacier antarctica crystallographic investigations have been conducted of cold 17 c debris bearing ice from the base of an antarctic outlet glacier taylor glacier the 4 m thick sequence studied has been retrieved from a 20 m long tunnel dug from the glacier snout and has been analyzed with an automatic ice fabric analyzer aifa the top and bottom of the sequence consists of clean meteoric ice englacial facies whereas alternating debris rich and clean bubbly ice layers are found in the middle part stratified facies ice from the englacial facies displays a polygonal texture and a strong c axis clustering toward the vertical denoting recrystallization through subgrain rotation sgr in contrast clean ice from the stratified facies shows sgr fabrics which are delimited at the contact with debris rich layers by large interlocking grains organized in ribbons these two distinct textures within the stratified facies are associated with looser c axis patterns at the scale of single thin sections which is interpreted as resulting from migration recrystallization mr the change from sgr to mr trends marks a clear increase in grain boundary and nucleation kinetics hence the term discontinuous recrystallization and may be associated with strain localization at rheological interfaces during basal ice genesis analogies with bottom ice from deep polar ice sheets where temperature is commonly higher than at the studied site are highlighted two recrystallization scenarios are proposed accounting for the development of both types of fabrics it is shown that by controlling the repartition of stress and strain energy within basal ice the rheology of debris bearing ice layers plays a decisive role in recrystallization dynamics at structural interfaces we also demonstrate how the same recrystallization regimes may occur in cold glaciers and temperate ice sheets provided that strain accumulation has been high enough in the former this challenges the common belief that migration fabrics observed in bottom ice from deep ice sheets are exclusive to warm stagnant annealed ice copyright 2008 by the american geophysical union

the localization of strain and c axis evolution in anisotropic ice using a series of combined compression simple shear experiments it has been possible to investigate strain distributions and crystal orientation fabrics related to varying layer orientation in ice a variety of flattening strains accompanied by layer buckling simple shear and the development of a lenticular layering are produced in anisotropic ice masses in samples where the creep curve has only just reached a minimum strain rate the c axis preferred orientation is similar to that in the starting material with specific c axis concentrations affected by the extent of preserved host grains at shear strains where 1 it was found that the c axis preferred orientations were highly variable depending on the magnitude of strain strain distribution and upon the modification and degree of rotation of initial c axis preferred orientation however once recrystallization dominates in high strain zones 1 there is a rapid development of an asymmetric two maxima fabric with little evidence of any contributions from inherited fabric elements the final c axis pattern is asymmetric with respect to the direction of shortening with a strong maximum at 80 to the shear zone with a sense of asymmetry in the direction of the shear and a secondary maximum lying at 50 to the plane of shearing

weertman regelation multiple refreezing events and the isotopic evolution of the basal ice layer a simple model is developed to s imulate the isotopic fractionation which accompanies weertman regelation at the bed of temperate based glaciers the fractionation equations of jouzel and souchez 1 982 are applied to multiple refreezing events over measured glacier bed profiles and mass balance is maintained as the basal ice and meltwater produced at one bedrock hummock enter the next s imulation results indicate that undeformed regelation ice layers are on the order of millimetres to centimetres thick often being completely melted at the stoss face of certain hummocks and exceptionally reaching a thickness in excess of 1 0 cm neither the internal morphology nor the isotopic composition of these layers is constant but both vary down glacier in accordance with bedrock configuration a glacier wide fractionation process is identified whereby heavy isotopes are preferentially removed fro m the basal meltwater film and incorporated into the basal ice this process might go some way to explaining the anomalously light isotopic composition measured in base flow waters leaving some glaciers vertical isotope profiles through undeformed basal ice layers are reconstructed and show that significant isotopic excursions can occur at a scale of millimeteres while the range of isotopic compositions within such m ulti layered regelation ice is greater than that which would occur in ice produced by a single refreezing event in circumstances where the regelation system is disrupted by removal of film waters into a network of linked cavities i t is found that the remaining basal ice may be significantly enriched in heavy isotopes relative to the composition of the initial mass inputs to the system heavy isotope enrichment of this m agnitude and consideration of the thickness of the basal ice layers concerned may explain the absence of recorded basal ice samples heavy enough to h ave been formed in equilibrium with subglacial precipitates sampled at one of the sites glacier de tsanfleuron and reported in an earlier paper

tectonic processes in a surge type glacier the 1982 1983 surge of variegated glacier involved the development growth and downglacier propagation of a velocity peak associated with rapid basal sliding facilitated by high subglacial water pressures passage of the velocity peak through the glacier was preceded by an episode of longitudinal shortening and followed by an episode of elongation the deformation history of the glacier ice was dependent upon location relative to the surge nucleus and the final position reached by the propagating velocity peak ice above the surge nucleus experienced continuous and cumulative elongation ice below the final position of the velocity peak experienced continuous and cumulative shortening ice between these two points experienced shortening followed by elongation and low cumulative strain the large scale pattern of ice structure development reflects these deformation histories surging is equivalent to thrust sheet emplacement by a combination of gravity gliding over a weakened basal layer and push from behind with the gravity driven motion of the surging part of the glacier providing the push which allows the surge front to propagate the relationships established between the deformation history of surging glaciers and the development of ice structures may facilitate the interpretation of structures in thrust sheets

pliocene pleistocene incision of the green river kentucky determined from radioactive decay of cosmogenic 26al and 10be in mammoth cave sediments cosmogenic 26al and 10be in sediments washed into mammoth cave kentucky record the history of 3 5 m y of water table position governed by incision and aggradation of the green river a tributary of the ohio river upper levels of the cave formed during a period of slow river incision and were later filled with sediment due to river aggradation at 2 3 2 4 ma a brief surge of river incision ca 2 ma was followed by river stability and cave passage formation at a lower level rapid incision through 15 m of bedrock ca 1 5 ma was prompted by repositioning of the ohio river to its present course along an ice sheet margin renewed incision ca 1 2 ma and aggradation at 0 7 0 8 ma correlate with major ice advances in the ohio river basin measurements of 26al and 10be also indicate that sandstone capped uplands have maintained slow erosion rates of 2 7 m m y for the past 3 5 m y despite accelerated pleistocene river incision rates of 30 m m y

former dynamic behaviour of a cold based valley glacier on svalbard revealed by basal ice and structural glaciology investigations large numbers of small valley glaciers on svalbard were thicker and more extensive during the little ice age lia demonstrated by prominent ice cored moraines up to several kilometres beyond present day margins the majority of these glaciers have since experienced a long period of strongly negative mass balance during the 20th century and are now largely frozen to their beds indicating they are likely to have undergone a thermal transition from a polythermal to a cold based regime we present evidence for such a switch by reconstructing the former flow dynamics and thermal regime of tellbreen a small cold based valley glacier in central spitsbergen based on its basal sequence and glaciological structures within the basal sequence the underlying matrix supported diamict is interpreted as saturated subglacial traction till which has frozen at the bed indicating that the thermal switch has resulted in a cessation of subglacial sediment deformation due to freezing of the former deforming layer this is overlain by debris poor dispersed facies ice interpreted to have formed through strain induced metamorphism of englacial ice the sequential development of structures includes arcuate fracture traces interpreted as shear planes formed in a compressive transpressive stress regime and fracture traces interpreted as healed extensional crevasses the formation of these sediment ice facies and structures is indicative of dynamic warm based flow most likely during the lia when the glacier was significantly thicker

magnetic fabrics of the douglas till of the superior lobe exploring bed deformation kinematics abstract the superior lobe of the laurentide ice sheet during its last advance into northwestern wisconsin may have moved and transported sediment primarily through deep widespread shear of its bed to high strains 100 to study the kinematics of this deformation we measured the anisotropy of magnetic susceptibility of 3750 intact samples of its basal till the douglas member of the miller creek formation collected from eight profiles at 0 2 m depth increments a benchmark for interpretations was provided by results of ring shear experiments conducted on the douglas till which provided fabric characteristics for the case of horizontal simple shear orientations of maximum susceptibility k1 were controlled by preferred orientations of silt sized magnetite grains and were similar to sand grain orientations measured in thin sections strengths of fabrics formed by orientations of k1 indicate that most of the till sampled was deformed but to variable strains deformation averaged over the 20 km spanned by the study approximated horizontal simple shear however large differences in k1 fabric azimuths 45 over lateral distances of meters to tens of meters indicate highly heterogeneous bed deformation perhaps focused in anastomosing zones with associated divergent and convergent till shear this interpretation is supported by orientations of principal susceptibilities that imply in many cases either that shear planes were steeply dipping particularly transverse to the shearing direction or that there were major components of pure shear variations in k1 fabric azimuth with depth indicate that most of the till thickness did not shear simultaneously rather till accumulated at the bed as shear direction changed in response to temporally shifting zones of shallow deformation

control of regional sea level by surface uplift and subsidence caused by magmatic underplating of earth s crust magmatic underplating of the crust is a common feature of major basalt provinces the emplacement of magma within the lithosphere leads to surface uplift with a magnitude of the order of 10 of the thickness of the underplated material the composition of associated igneous rock samples suggests that much of the underplated material is made of gabbro this gabbro is denser than the magma from which it crystallizes so uplift of earth s surface caused by the emplacement of magma at the crust mantle boundary must be followed by subsidence as the magma solidifies this subsidence is equal to approximately half the original uplift and takes place within 0 1 m y of injection of the magma the association of magmatic underplating with surface uplift followed by subsidence provides a mechanism for changes of sea level including previously unexplained episodic highstands of sea level

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

scale invariant sheath folds in salt sediments and shear zones abstract sheath folds are developed in a broad spectrum of geological environments in which material flow occurs including gravity driven surficial deformation in ignimbrites unconsolidated sediments and salt together with deeper level ductile shear zones in metamorphic rocks this study represents the first geometric comparison of sheath folds in these different settings across a wide range of scales elliptical closures defining eye folds represent y z cross sections through highly curvilinear sheath folds our analysis of the published literature coupled with field observations reveals remarkably similar ellipticities r yz for sheath folds in metamorphic shear zones r yz 4 23 salt r yz 4 29 sediment slumps r yz 4 34 glaciotectonites r yz 4 48 and ignimbrites r yz 4 34 nested eye folds across this range of materials n 1800 reveal distinct and consistent differences in ellipticity from the outer r yz to the inner most r y z elliptical rings of individual sheath folds the variation in ratios from outer to inner rings r r yz r y z in gravity driven surficial flows typically displays a relative increase in ellipticity to define cats eye folds r r yz that are remarkably constant r 2 0 99 across 9 orders of magnitude sheath y axes range from 0 1 mm to 75 km our findings lead us to conclude that the geometric properties of sheath folds are scale invariant and primarily controlled by the type and amount of strain with r also reflecting the rheological significance of layering associated with original buckle fold mechanisms the scaling pattern of sheath folds reflects the length scales of the precursor buckle folds and width of deformation zones across a broad range of materials and environments with continued deformation the layering marking the original folds may become increasingly passive to define sheath folds these empirical relationships suggest sheath folding is a fundamental mode of viscous response across a broad spectrum of materials strain rates and scales encompassing a variety of deformation settings

the evolution of a submarine landform record following recent and multiple surges of tunabreen glacier svalbard this study focuses on the glacial landform record associated with recent surge events of tunabreen a calving tidewater glacier in tempelfjorden spitsbergen submarine geomorphology and recent te

debris entrainment and landform genesis during tidewater glacier surges the englacial entrainment of basal debris during surges presents an opportunity to investigate processes acting at the glacier bed the subsequent melt out of debris rich englacial structures during the quiescent phase produces geometrical ridge networks on glacier forelands that are diagnostic of surge activity we investigate the link between debris entrainment and proglacial geomorphology by analyzing basal ice englacial structures and ridge networks exposed at the margins of tunabreen a tidewater surge type glacier in svalbard the basal ice facies display clear evidence for brittle and ductile tectonic deformation resulting in overall thickening of the basal ice sequence the formation of debris poor dispersed facies ice is the result of strain induced metamorphism of meteoric ice near the bed debris rich englacial structures display a variety of characteristics and morphologies and are interpreted to represent the incorporation and elevation of subglacial till via the squeezing of till into basal crevasses and hydrofracture exploitation of thrust faults reoriented crevasse squeezes and preexisting fractures these structures are observed to melt out and form embryonic geometrical ridge networks at the base of a terrestrially grounded ice cliff ridge networks are also located at the terrestrial margins of tunabreen neighboring von postbreen and in a submarine position within tempelfjorden analysis of network characteristics allows these ridges to be linked to different formational mechanisms of their parent debris rich englacial structures this in turn provides an insight into variations in the dominant tectonic stress regimes acting across the glacier during surges

anomalous depth dependency of the stress field in the 2007 noto hanto japan earthquake potential involvement of a deep fluid reservoir 1 we have elucidated depth variations in the stress field associated with the 2007 noto hanto japan earthquake by stress tensor inversion using high quality aftershock data obtained by a dense seismic network aftershocks that occurred above 4 km in depth indicated a strike slip stress regime by contrast aftershocks in deeper parts indicated a thrust faulting stress regime this depth variation in the stress regime correlates well with that in the slip direction derived from a finite source model using geodetic data furthermore the maximum principal stress 1 axis was stably oriented approximately w20 n down to the depth of the mainshock hypocenter largely in agreement with the regional stress field but below that depth the 1 axis had no definite orientation indicating horizontally isotropic stress one likely cause of these drastic changes in the stress regime with depth is the buoyant force of a fluid reservoir localized beneath the seismogenic zone

determining glacier flow direction from till fabrics abstract field and experimental data indicate that fabrics of sheared basal tills based on particle orientations and on anisotropy of magnetic susceptibility are inclined relative to the macroscopic plane of shear as a result if this plane is tilted with a transverse component one of the longest standing axioms of glacial geology that particles in basal till are preferentially oriented parallel to the direction of shear is not strictly true the resultant correction to the shearing direction increases with the transverse and longitudinal components of shear plane inclination and with the magnitude of the fabric inclination relative to the shear plane azimuthal corrections calculated herein range from being negligible to a few tens of degrees for reasonable ranges of shear plane tilt the past success of fabric azimuths as indicators of flow direction despite the absence of this correction likely reflects the clustering of shear planes about the horizontal in basal tills however on the flanks of accreting subglacial bedforms shear plane attitudes may well conform to the local bedform slope as illustrated with hypothetical fabrics of drumlins and flutes azimuthal corrections can in that case be sufficiently large and systematic to seriously affect interpretations of bedform genesis

quantifying quartz enrichment and its consequences for cosmogenic measurements of erosion rates from alluvial sediment and regolith abstract in situ cosmogenic 26 al and 10 be record the residence time of quartz grains near the earth s surface and thus can be used to measure whole catchment erosion rates averaged over millennial time scales quartz is enriched in hillslope regolith by the dissolution of more soluble minerals thus its residence time will be longer than the regolith average it has been noted that this introduces a bias into erosion rate estimates derived from cosmogenic nuclide concentrations in regolith or alluvium geomorphology 27 1999 131 but the magnitude of this bias has not previously been measured the enrichment of quartz in regolith and the resulting bias in cosmogenic erosion rate estimates can be quantified using concentrations of immobile elements such as zirconium in bedrock and regolith here we show that the erosion rate bias introduced by regolith dissolution is less than 12 across 22 granitic catchments that span a wide range of temperate climates except in extreme weathering environments biases due to regolith dissolution will be a small component of the overall uncertainty in cosmogenic erosion rate measurements

rotating micro structures in antarctic cold basal ice implications for glacier flow and its interpretation structural analyses were conducted in the basal zone of an antarctic glacier the studied basal ice sequence was retrieved from a 20 m long subglacial tunnel dug at the margin of the glacier and is at the temperature of 17 c for the first time rotating clast systems embedded within debris rich ice were thin sectioned using specially designed cutting techniques the observed structures reflect the occurrence of pervasive shearing at the base of the glacier and can be used as shear sense indicators in addition some of these structures provide evidence for the presence of thin liquid films at the time of formation despite the marked freezing temperature of the ice it is showed here that cautious analysis of deformation structures present in debris bearing ice may bring insights not only into the flow dynamics of the embedding matrix but also into the behaviour of the interstitial fluid network at the base of cold glaciers and ice sheets

paleowind directions from the magnetic fabric of loess profiles in central alaska abstract two parallel profiles of alternating loess and paleosol horizons from the halfway house site west of fairbanks in central alaska were evaluated in terms of their potential for preserving paleowind directions combined the profiles span specmap marine oxygen isotope stages 1 6 approximately the last 150 ka our analytical tools consisted of measuring the sample s anisotropy of magnetic susceptibility and characterizing the magnetic mineralogy by multiple approaches both primary eolian and secondary reworked magnetic fabrics are recognized along certain intervals of both profiles primary eolian fabrics display a horizontal magnetic foliation and a well defined magnetic lineation corresponding to the paleodirection of sediment transport and deposition a systematic change in transport direction is observed where long term prevailing winds appear to shift from a nw se to n s direction during glacial and interglacial periods respectively

ice core evidence for the thickness and character of clear facies basal ice at glacier de tsanfleuron switzerland five ice cores have been retrieved from a transect close to the terminus of glacier de tsanfleuron switzerland the cores extend from the ice surface to the glacier bed and are 3 5 44 8 m long stratigraphic logging based on bubble size and density reveals the presence of a highly metamorphosed basal ice layer about 10 m thick from which all traces of bubble rich ice have been removed this bubble poor ice which corresponds closely with clear facies ice observed in cavities beneath numerous temperate based glaciers contrasts with the overlying bubble rich or bubble foliated englacial ice and the underlying debris rich and bubble free dispersed facies basal ice down core patterns in major ion composition stable isotope composition and total gas content and composition are generally consistent with formation of clear facies ice by deformation related metamorphism of bubbly englacial ice in addition isotopic data suggest that storage of downward percolating meltwaters occurs close to the upper surface of the clear facies ice layer perhaps reflecting a local variation in ice permeability across the transition from englacial to clear facies ice enrichment in crustally derived ionic species is noted in the lowermost decimetres of the debris free clear facies ice that immediately overlies debris rich dispersed facies basal ice this ionic enrichment in debris free ice is interpreted in terms of active inter granular meltwater flow within some decimetres of the glacier bed

magnetic characterisation of folded aeolian sandstones interpretation of magnetic fabrics in diamagnetic rocks abstract this study provides an original example of exploitation of anisotropy of magnetic susceptibility ams for rocks with weak magnetic susceptibility within the upper weber sandstone at split mountain utah 430 cores from 31 sites were collected for magnetic characterization the magnetic susceptibility ranges from 10 to 10 si indicating a mostly diamagnetic matrix with degree of anisotropy up to 1 6 specific treatment of magnetic susceptibility allows using diamagnetic data the fabrics are fairly clustered and triaxial sedimentary magnetic fabrics show a foliation plane parallel to the lamina of the sand dunes without defined lineation apart from sedimentary fabrics 70 magnetic fabric patterns averaged for sites distributed on the anticline are well defined in sub groups related to the major structural domains of the anticline the fracture network at split mountain is composed of a dominant n120 set and a secondary n035 set a scenario of strain record is proposed based on the correlation of 1 fracture sets orientation 2 diagenetic cementation 3 paleostresses and 4 distribution of magnetic susceptibility anisotropy following the sevier orogeny and n120 fracture set emplacement the n035 fracture network and ams signal were recorded during the laramide layer parallel shortening phase with local deviation along pre existing structures and recorded a partitioning of the strain during early folding with a maximum horizontal stress axis perpendicular to the fold bounding faults within the fold

the origin of tectonic lineation in extensional basins combined neutron texture and magnetic analyses on undeformed clays abstract in extensional sedimentary basins fine grained sediments that appear undeformed at the outcrop scale can carry a magnetic fabric consistent with the regional deformation pattern the origin of the magnetic lineation which is often found in extensional basins is not yet well understood in clays from extensional basins in southern italy the magnetic lineation is tectonically controlled and oriented perpendicular to the main normal faults a combined analysis of magnetic and mineral fabrics was made to gain insight into the processes that lead to a lineation in extensional settings low field high field and low temperature susceptibility measurements were used to distinguish the ferrimagnetic and paramagnetic contributions to the magnetic susceptibility and its anisotropy the magnetic anisotropy of the sediments is predominantly carried by paramagnetic phyllosilicates neutron texture analysis was used to evaluate the spatial distribution of chlorite basal planes results demonstrate that the orientation of the magnetic lineation is related to the spatial distribution of chlorite lying parallel to the common axis of differently oriented basal planes a quantitative correlation between the magnetic and rock fabric was made comparing the low and high field magnetic anisotropy ams hfa to the theoretical anisotropy calculated from the chlorite preferred orientation a good linear correlation is found between the degree of theoretical anisotropy and the ams and hfa results show that the integrated approach of magnetic and mineral fabric investigations represents a valid alternative tool for detecting grain scale and regional deformation patterns in weakly deformed extensional basins where macroscopic evidence of deformation is often not visible

origin and development of textures and fabrics in basal ice at summit central greenland abstract detailed textural and crystallographic characteristics of the bottom 6 m of the grip core summit central greenland are presented the results are discussed in the light of present day fabric analogues ice fabrics are shown to retain the mark from the entrainment of locally formed ice by the growing ice sheet this mark is only partly overprinted by the present day stress configuration at the ice divide these results corroborate the model put forward by souchez et al 1 on stable isotopes and gas composition grounds in which the basal silty ice appears as a remnant of a growing stage of the ice sheet during this stage which probably represents the original build up ice of local origin formed at the ground surface was overridden by the main ice sheet and incorporated into its base preservation of remnants of ancient fabric patterns and occurrence of annealing recrystallization fabrics just above the first layer of silty ice are used amongst other arguments to demonstrate that this basal ice is not presently subject to large cumulative strains the implications for flow and dating models are discussed

the structural evolution of sheath folds a case study from cap de creus abstract it has long been recognised that within zones of intense non coaxial deformation fold hinges may rotate progressively towards the transport direction ultimately resulting in highly curvilinear sheath folds however there is a surprising lack of detailed and systematic field analysis of such evolving sheath folds this case study therefore focuses on the sequential development of cm scale curvilinear folds in the greenschist facies el llimac shear zone cap de creus spain this simple shear dominated dextral shear zone displays superb three dimensional exposures of sheath folds defined by mylonitic quartz bands within phyllonite increasing amounts of fold hinge curvature are marked by hinge segments rotating into sub parallelism with the mineral lineation lm whilst the acute angle between the axial planar hinge girdle and foliation also displays a sequential reduction although lm bisects the noses of sheath folds it is also clearly folded and wrapped around the sheath hinges lm typically preserves a larger angle with the fold hinge on the lower limb l compared to the upper u limb l u suggesting that lm failed to achieve a steady orientation on the lower limb adjacent sheath fold hinges forming fold pairs may display the same sense of hinge arcing to define synthetic curvature or alternatively opposing directions of antithetic curvature such patterns reflect original buckle fold geometries coupled with the direction of shearing the ratio of long short fold limbs decreases with increasing hinge curvilinearity indicating sheath folds developed via stretching of the short limb rather than migrating or rolling hinge models this study unequivocally demonstrates that both hinges of fold pairs become curvilinear with sheaths closing in the transport direction recording greater hinge line curvilinearity compared to adjacent return hinges this may provide a useful guide to bulk shear sense

deformation of the batestown till of the lake michigan lobe laurentide ice sheet deep pervasive shear deformation of the bed to high strains 100 may have been primarily responsible for flow and sediment transport of the lake michigan lobe of the laurentide ice sheet to test this hypothesis we sampled at 0 2 m increments a basal till from one advance of the lobe batestown till along vertical profiles and measured fabrics due to both anisotropy of magnetic susceptibility and sand grain preferred orientation unlike past fabric studies interpretations were guided by results of laboratory experiments in which this till was deformed in simple shear to high strains fabric strengths indicate that more than half of the till sampled has a 5 probability of having been sheared to moderate strains 7 30 secular changes in fabric azimuth over the thickness of the till probably due to changing ice flow direction as the lobe receded indicate that the bed accreted with time and that the depth of deformation of the bed did not exceed a few decimeters orientations of principal magnetic susceptibilities show that the state of strain was commonly complex deviating from bed parallel simple shear deformation is inferred to have been focused in shallow temporally variable patches during till deposition from ice

is there a single surge mechanism contrasts in dynamics between glacier surges in svalbard and other regions 1 during the 1990s monacobreen a 40 km long tidewater glacier in svalbard underwent a major surge we mapped the surge dynamics using ers synthetic aperture radar images differential dual azimuth interferometry and intensity correlation tracking a series of 11 three dimensional 3 d velocity maps covering the period 1991 1997 show a months long initiation and years long termination to the surge with no indication of a surge front travelling downglacier during the surge the front of the glacier advanced 2 km the velocity and derived strain rate increased by more than an order of magnitude and maximum ice flow rates measured during 1994 were 5 m d 1 the spatial pattern of both velocity and strain rate was remarkably consistent and must therefore be controlled by spatially fixed processes operating at the glacier bed we combine these results with those published in the literature to construct a typical svalbard glacier surge cycle and compare this to surge dynamics of glaciers from other cluster regions especially those of variegated glacier in alaska the strong contrast in dynamics suggests that there exist at least two distinct surge mechanisms

switch of flow direction in an antarctic ice stream fast flowing ice streams transport ice from the interior of west antarctica to the ocean and fluctuations in their activity control the mass balance of the ice sheet the mass balance of the ross sea sector of the west antarctic ice sheet is now positive that is it is growing mainly because one of the ice streams ice stream c slowed down about 150 years ago1 here we present evidence from both surface measurements and remote sensing that demonstrates the highly dynamic nature of the ross drainage system we show that the flow in an area that once discharged into ice stream c has changed direction now draining into the whillans ice stream formerly ice stream b this switch in flow direction is a result of continuing thinning of the whillans ice stream and recent thickening of ice stream c further abrupt reorganization of the activity and configuration of the ice streams over short timescales is to be expected in the future as the surface topography of the ice sheet responds to the combined effects of internal dynamics and long term climate change we suggest that caution is needed when using observations of short term mass changes to draw conclusions about the large scale mass balance of the ice sheet

the experimental basis for interpreting particle and magnetic fabrics of sheared till particle fabrics of basal tills may allow testing of the bed deformation model of glacier flow which requires high bed shear strains 100 field studies however have not yielded a systematic relationship between shear strain magnitude and fabric development to isolate this relationship four basal tills and viscous putty were sheared in a ring shear device to strains as high as 714 fabric was characterized within a zone of shear deformation using the long axis orientations of fine gravel and sand particles and the anisotropy of magnetic susceptibility ams of small 5 8 cm3 intact samples results indicate that till particles rotate toward the plane of shearing with long axis orientations that become tightly clustered in the direction of shear 0 78 s1 0 94 for three dimensional data these strong steady state fabrics are attained at shear strains of 7 30 with no evidence of fabric weakening with further strain regardless of the specific till or particle size fraction under consideration these results do not support the jeffery model of particle rotation which correctly describes particle rotation in the viscous putty but not in the tills owing to fluid mechanical assumptions of the model that are violated in till the sensitivity of fabric development to shear strain magnitude indicates that for most till units where shear strain magnitude is poorly known attributing fabric variations to spatial differences in other variables such as till thickness or water content will be inherently speculative attributing fabric characteristics to particular basal till facies is uncertain because shear strain magnitude is unlikely to be closely correlated to till facies weak or spatially variable fabrics in the absence of post depositional disturbance or major deviations from unidirectional simple shear indicate that till has not been pervasively sheared to the high strains required by the bed deformation model strong flow parallel fabrics are a necessary but insufficient criterion for confirming the model copyright 2008 john wiley sons ltd

bedrock incision rock uplift and threshold hillslopes in the northwestern himalayas the topography of tectonically active mountain ranges reflects a poorly understood competition between bedrock uplift and erosion dating of abandoned river cut surfaces in the northwestern himalayas reveals that the indus river incises through the bedrock at extremely high rates 2 12 mm yr 1 in the surrounding mountains the average angles of hillslopes are steep and essentially independent of erosion rate suggesting control by a common threshold process in this rapidly deforming region an equilibrium is maintained between bedrock uplift and river incision with landsliding allowing hillslopes to adjust efficiently to rapid river down cutting

basal glacier ice and massive ground ice different scientists same science whilst glaciologists and permafrost researchers investigate ice bodies using similar techniques there has been surprisingly little collaboration between the two communities this paper examines the potential benefits of interdisciplinary research into the formation of basal ice beneath glaciers and the origin of massive ice in glaciated permafrost regions active collaboration in these areas has already improved our understanding of the formation of basal ice beneath cold based glaciers the critical role played by basal freezing in controlling the dynamic behaviour of stagnating ice streams and the significance of glacier permafrost interactions at the margins of pleistocene ice sheets however in order to promote future collaboration certain obstacles need to be overcome the contrasting ice classification schemes employed by glaciologists and permafrost scientists for example need to be unified in order to allow detailed comparisons of ice rich sequences in both environments this could in turn enable exciting research advances most notably by facilitating the identification of preserved remnants of pleistocene ice sheets within permafrost regions that provide a potentially invaluable and currently largely untapped source of palaeoglaciological information

debris entrainment by basal freeze on and thrusting during the 1995 1998 surge of kuannersuit glacier on disko island west greenland kuannersuit glacier a valley glacier on disko island in west greenland experienced a major surge from 1995 to 1998 where the glacier advanced 10 5 km and produced a similar to 65 m thick stacked sequence of debris rich basal ice and meteoric glacier ice the aim of this study is to describe the tectonic evolution of large englacial thrusts and the processes of basal ice formation using a multiproxy approach including structural glaciology stable isotope composition delta o 18 and delta d sedimentology and ground penetrating radar we argue that the major debris layers that can be traced in the terminal zone represent englacial thrusts that were formed early during the surge thrust overthrow was at least 200 300 m and this lead to a 30 m thick repetition of basal ice at the ice margin it is assumed that the englacial thrusting was initiated at the transition between warm ice from the interior and the cold snout the basal debris rich ice was mainly formed after the thrusting phase two sub facies of stratified basal ice have been identified a lower massive ice facies s m composed of frozen diamict enriched with heavy stable isotopes overlain by laminated ice facies s l consisting of millimetre thick lamina of alternating debris poor and debris rich ice we interpret the stratified basal ice as a continuum formed mainly by freeze on processes and localized regelation first laminated basal ice is formed and as meltwater is depleted more sediment is entrained and finally the glacier freezes to the base and massive diamict is frozen on the increased ability to entrain sediments may partly be associated with higher basal freezing rates enhanced by loss of frictional heat from cessation of fast flow and conductive cooling through a thin heavily crevassed ice during the final phase of the glacier surge the dispersed basal ice facies d was mainly formed by secondary processes where fine grained sediment is mobilized in the vein system of ice our results have important implications for understanding the significance of basal ice formation and englacial thrusting beneath fast flowing glaciers and it provides new information about the development of landforms during a glacier surge copyright c 2010 john wiley sons ltd less

distribution anisotropy the cause of ams in igneous rocks understanding of ams in pristine basic igneous rocks in terms of flow orientation of ellipsoidal magnetite grains is thwarted by the fact that these grains 1 typically are irregularly equant or skeletal rather than distinctly ellipsoidal and 2 usually crystallize interstitially and late after magma flow has ceased however the distribution of such grains will be relatively anisotropic if they grew in residual liquid volumes within a preferredly oriented by flow silicate template this constraint may impose a slight preferred shape orientation to the irregular magnetite grains during their growth but in addition we suggest it is the anisotropic magnetic interaction between the grains which is the basic cause of ams experiments involving the casting of magnetite epoxy mixtures in various foliated or lineated glass templates have yielded samples with the expected ams symmetry i e k3 perpendicular to glass plates or k1 parallel to glass rods slicing of near isotropic synthetic blank and natural rock cores and interleaving with glass slides showed a progressive increase in anisotropy of the blank from 0 4 to 2 and the natural rock from 0 9 to 2 9 in both the k3 axis migrates to be perpendicular to the glass slides

magnetic anisotropy of hematite natural crystals increasing low field strength experiments hematite is a very abundant mineral in natural rock samples despite being one of the most important carriers of remanent magnetization its magnetic anisotropy is not well understood partially due to its high coercivity and complex behavior in particular the field intensity beyond which the rayleigh relation no longer holds varies from one crystal to another this field threshold is usually less than the field used in most commercial instruments the nonlinear behavior of low field susceptibility may thus hinder the magnetic fabric analysis we have carried out an intensive study of the low field bulk susceptibility and anisotropy of magnetic susceptibility ams at increasing low fields in the range of 2 450 a m effective value in a collection of hematite natural crystals standard rock magnetic properties x ray diffraction and mass spectrometry have also been determined in order to discover the parameters influencing the low field susceptibility variations with field the ams principal directions the shape of the ams ellipsoid and the degree of anisotropy are the parameters that can vary with different applied fields it has been found that there is no correlation between magnetic properties like coercivity or saturation magnetization and the range in which the rayleigh approximation is valid however there seems to be a correlation with the peak width determined from x ray diffraction suggesting that the rayleigh region in hematite crystals is related to the spatial orientation of the physical domains within the basal plane

thermal structure of svalbard glaciers and implications for thermal switch models of glacier surging switches between cold and warm based conditions have long been invoked to explain surges of high arctic glaciers here we compile existing and new data on the thermal regime of six glaciers in svalbard to test the applicability of thermal switch models two of the large glaciers of our sample are water terminating while one is land terminating all three have a well known surge history they have a thick basal layer of temperate ice superimposed by cold ice a cold terminus forms during quiescence but is mechanically removed by calving on tidewater glaciers the other three glaciers are relatively small and are either entirely cold or have a diminishing warm core all three bear evidence of former warm based thermal regimes and in two cases surge like behavior during the little ice age in svalbard therefore three types of glaciers have switched from slow to fast flow 1 small glaciers that underwent thermal cycles during and following the lia switches between cold and warm based conditions 2 large terrestrial glaciers which remain warm based throughout the entire surge cycle but develop cold termini during quiescence and 3 large tidewater glaciers that remain warm based throughout the surge cycle our results demonstrate that thermal switching cannot explain the surges of large glaciers in svalbard we apply the concept of enthalpy cycling to the spectrum of surge and surge like behavior displayed by these glaciers and demonstrate that all svalbard surge type glaciers can be understood within a single conceptual framework

dating soils and alluvium with cosmogenic 21ne depth profiles case studies from the pajarito plateau new mexico usa abstract 21 ne depth profiles measured from amalgamated samples of sand sized quartz from the quaternary bandelier tuff were used to estimate inheritance corrected exposure ages in stream terraces and an alluvial fan on the pajarito plateau stable alluvium and associated soils display a downward decreasing type i nuclide profile pattern bioturbated colluvium with uniform type iii nuclide concentrations has buried these alluvial surfaces another possible profile type with downward increasing nuclide concentrations type ii is described but was not found on the pajarito plateau we present mathematical models for each of the profile patterns relating nuclide concentration to inheritance time averaged bulk density and either exposure time or burial rate curve fits of type i profiles are used to estimate the age of alluvial landforms a burial correction technique for deposits buried by younger colluvium is also presented inheritance and burial corrected 21 ne surface exposure ages of 144 17 ka and 63 8 ka for two pajarito plateau stream terraces agree within error with independent soil profile development index dates of 160 ka and 68 78 ka a soil overlying the 50 60 ka el cajete pumice yields an inheritance corrected 21 ne exposure age of 44 11 ka that also agrees closely with a 14 c age on soil charcoal of 41 ka these results demonstrate that reliable cosmogenic 21 ne exposure ages can be obtained for fine grained alluvial surfaces with the depth profile technique

magnetic fabric of sheared till a strain indicator for evaluating the bed deformation model of glacier flow wet based portions of ice sheets may move primarily by shearing their till beds resulting in high sediment fluxes and the development of subglacial landforms this model of glacier movement which requires high bed shear strains can be tested using till microstructural characteristics that evolve during till deformation here we examine the development of magnetic fabric using a ring shear device to deform two wisconsin age basal tills to shear strains as high as 70 hysteresis experiments and the dependence of magnetic susceptibility of these tills on temperature demonstrate that anisotropy of magnetic susceptibility ams develops during shear due to the rotation of primarily magnetite particles that are silt sized or smaller at moderate shear strains 6 25 principal axes of maximum magnetic susceptibility develop a strong fabric s 1 eignevalues of 0 83 0 96 without further strengthening at higher strains during deformation directions of maximum susceptibility cluster strongly in the direction of shear and plunge up glacier consistent with the behavior of pebbles and sand particles studied in earlier experiments in contrast the magnitude of ams does not vary systematically with strain and is small relative to its variability among samples this is because most magnetite grains are contained as inclusions in larger particles and hence do not align during shear although processes other than pervasive bed deformation may result in strong flow parallel fabrics ams fabrics provide a rapid and objective means of identifying basal tills that have not been sheared sufficiently to be compatible with the bed deformation model

dynamics of tidewater surge type glaciers in northwest svalbard the evolution of ice dynamics through surges of four tidewater terminating glaciers in northwest svalbard is investigated by remote sensing a 20 year time series of glacier surface flow speeds and frontal positions is presented covering the recent surges of monacobreen comfortlessbreen blomstrandbreen and fjortende julibreen surface flow speeds were derived using feature tracking between pairs of ers sar and alos palsar images while frontal positions were taken from the same imagery as well as more frequent but lower spatial resolution envisat wide swath mode images during all four surges increased ice flow caused the tidewater margin to advance while the calving flux was initially reduced to near zero due to compressive stresses limiting crevasse propagation as ice speed decreased the terminus continued to advance until the glacier s speed had returned to its pre surge flow rate only at this time did the terminus start to retreat and peak iceberg calving flux was established we conclude that terminus advance closely tracks glacier speed up that there is little mass loss through calving during the most active phase of the surge and that seasonal cycles of terminus positions diminish during the active surge phase

anisotropy of diamagnetic susceptibility in thassos marble a comparison between measured and modeled data abstract a study of shear zones within the calcite marble complex of the island of thassos greece shows that the low field anisotropy of magnetic susceptibility ams technique can be successfully applied to diamagnetic rocks for characterizing rock fabrics the strain path involves both an early pure shear stage and a simple shear overprint that is documented by a transition from triaxial neutral to uniaxial prolate shapes of ams ellipsoids the maximum susceptibility is oriented perpendicular to the rock foliation reflecting the preferred orientation of calcite c axes in the protolith as well as in the mylonites for three samples that represent different types of calcite fabrics the ams was recalculated from neutron and electron backscatter diffraction textural data a comparison of the measured and modeled data shows a good coincidence for the orientation of the principal ams axes and for the recalculated anisotropy data both measured and modeled data sets reflect the change from neutral to distinct prolate ellipsoids during progressive deformation

magnetic fabrics in the basal ice of a surge type glacier 1 anisotropy of magnetic susceptibility ams has been shown to provide specific useful information regarding the kinematics of deformation within subglacially deformed sediments here we present results from debris rich basal glacier ice to examine deformation associated with glacier motion basal ice samples were collected from tunabreen a polythermal surge type glacier in svalbard the magnetic fabrics recorded show strong correlation with structures within the ice such as sheath folds and macroscopic stretching lineations thermomagnetic low temperature susceptibility varying field susceptibility and isothermal remanent magnetism acquisition experiments reveal that the debris rich basal ice samples have a susceptibility and anisotropy dominated by paramagnetic phases within the detrital sediment sediment grains entrained within the basal ice are inferred to have rotated into a preferential alignment during deformation associated with flow of the glacier an up glacier directed plunge of magnetic lineations and subtle deviation from bulk glacier flow at the margins highlight the importance of noncoaxial strain during surge propagation the results suggest that ams can be used as an ice petrofabric indicator for investigations of glacier deformation and interactions with the bed

a mechanism of basal ice layer formation involving major ice fabric changes an opportunity to analyse the mechanism for the formation of a basal ice layer was provided by glacier de tsanfleuron in the swiss alps this glacier has some lateral subglacial cavities in which a basal ice layer forms on the bedrock floor and is subsequently incorporated at the base of the glacier petrographic and crystallographic analyses of the different types of ice have provided a means of investigating the successive stages of the process these analyses show major structural changes which provide field confirmation of the deformation mechanisms studied by several authors in laboratory experiments

denudation rates determined from the accumulation of in situ produced 10be in the luquillo experimental forest puerto rico we present a simple method for estimation of long term mean denudation rates using in situ produced cosmogenic 10be in fluvial sediments procedures are discussed to account for the effects of soil bioturbation mass wasting and attenuation of cosmic rays by biomass and by local topography our analyses of 10be in quartz from bedrock outcrops soils mass wasting sites and riverine sediment from the icacos river basin in the luquillo experimental forest puerto rico are used to characterize denudation for major landform elements in that basin the 10be concentration of a discharge weighted average of size classes of river sediment corresponds to a long term average denudation of 43 m ma 1 consistent with mass balance results

ropy flow structures a neglected indicator of magma flow direction in sills and dikes ropy flow structures that form in dikes and sills are very similar to ropy lava structures pahoehoe but are of intrusive origin they are unusual among magma flow indicators in that they provide a frozen record of the magma flow our observations suggest that they occur in large vesicles which form as a result of local repeated pressure drops during propagation and emplacement of the magma adiabatic gas expansion causes a ductile rim to form around a vesicle this rim is then sheared by the underlying flow anisotropy of magnetic susceptibility data from the contact zones confirm the flow data from the ropy flow structures but the results from deeper within the body both locally and regionally yield different yet consistent flow orientations this discrepancy suggests that a distinction can be made between the magma flow close to the contact and the deeper regional flow patterns in sills and dikes

experimental work on the effect of pre existing anisotropy on fabric development in glaciers abstract localization of deformation in ice is known to be important at all scales in deforming glaciers however relatively little is known of the significance of shear localization and the influence of fabric development in anisotropic ice at microscopic scales c axis preferred orientation and the inclination of the primary layering in anisotropic ice masses during both plane strain compression and combined simple shear compression have been examined a series of creep tests in the temperature range of 5 to 1 c over a range of shortening strains varying from 10 to 40 and compressive stresses ranging from 0 to 0 7 mpa have been undertaken significant variations in the strain rate and microstructural development have been observed in the plane strain compression experiments that reflects the varying orientations of the anisotropy and its relationship to easy glide directions in the ice mass in the unconfined combined compression and shear experiments minimum shear stress rates vary between the variously oriented anisotropic ice masses and deviate from the normal power flow law for isotropic ice where annealing occurs such ice masses preserve the pre existing c axis fabric and hence may reflect a contribution from both the recrystallized and inherited ice components

deformation and magnetic fabrics in ductile shear zones a review abstract the anisotropy of magnetic susceptibility ams is a well established petrofabric tool for indicating relative strain and microstructural character and has been validated on various rock types and different structural settings the magnetic susceptibility of a rock k depends primarily on the nature and abundance of magnetic minerals the physical arrangement and lattice preferred orientation of these magnetic minerals give rise to magnetic anisotropy the ams scalar parameters most commonly used to constrain strain include the corrected degree of anisotropy p 1 a proxy for fabric intensity and the shape factor 1 t 1 an indicator of the magnetic fabric symmetry prolate vs oblate a number of studies have shown that a positive correlation generally exists between p and strain thus the ams shows a great potential as a tool for examining deformation in geologic structures characterized by large strain gradients such as shear zones however a number of caveats exist i the increase of p with strain cannot be solely attributed to deformation because p also increases with k regardless of deformation ii strain across shear zones is typically heterogeneous and is often localized in units of different lithology thus making the separation of the lithological and strain controls on ams difficult also deformation is commonly accompanied by mineral segregation or fluid rock interaction that induces changes in magnetic mineralogy iii even if the undeformed lithology was uniform across a shear zone variations in strain rate or temperature may result in different deformation mechanisms hence the relationship between p and strain depends strongly on both the mineral carriers of ams and on deformation mechanisms and iv the ams is unable to resolve composite fabrics such as those resulting from s c structures where minerals on the c and s planes respectively contribute to ams

spatially averaged long term erosion rates measured from in situ produced cosmogenic nuclides in alluvial sediment spatially averaged erosion rates of small catchments can be accurately inferred from the concentrations of cosmogenic nuclides in stream sediment here we test this technique at two catchments by comparing erosion rates inferred from cosmogenic nuclides with rates of alluvial fan deposition over the past 16 000 years these two independent estimates agree within one standard error demonstrating that cosmogenic nuclide signatures of stream sediment can be used to measure spatially averaged long term erosion rates using this technique we show that long term erosion rates are an exponential function of average hillslope gradient at these sites

the structural geology of a surge type glacier abstract the range of deformation structures found in surge type variegated glacier alaska is similar to that found in many thin skinned thrust belts and includes foliations folds dip slip and strike slip faults thrust faults and fractures the development of structural relationships and the overall structural assemblage can be related to the deformation histories of quiescent and surge phases of motion the surge phase dominates the formation of brittle structures because of the large stresses and strain rates associated with it the relationship between downglacier verging overturned folds with hinge lines transverse to flow and a family of transverse arcuate thrust faults is characteristic of structural development in the lower part of the glacier during the surge longitudinal foliation develops mainly from sedimentary stratification in the upper part of the glacier in a marginal transpressive deformation regime during quiescent phase flow from broad accumulation basins the structural relationships in this surge type glacier may aid in the interpretation of structures in other gravity driven tectonic settings

strain patterns in glacitectonically thrusted sediments and conditions during thrusting abstract glacitectonically thrusted blocks of unlithified sediment are commonly inferred to have been frozen during thrusting with implications for paleoclimate and glacier dynamics we inferred strain patterns within thrusted blocks of glaciolacustrine mud in the rubjerg knude and mons klint glacitectonic complexes of coastal denmark to try to assess whether the mud was frozen during thrusting fabrics based on the anisotropy of magnetic susceptibility ams of the muds are interpreted with results of ring shear experiments and models of ams fabric development during layer parallel shortening lps in intact distal stratigraphy fabrics indicate moderate lps fabrics where thrusts have disrupted more proximal stratigraphy indicate rotation of mud sheets during thrusting with insufficient penetrative strain to reset the lps fabric the exception is within decimeters of faults at rubjerg knude where shear overprinted the lps fabric although these data indicate that mud sheets were mostly rigid during thrusting this is not necessarily evidence of permafrost critical taper theory indicates that even a gentle slope of the proglacial surface away from the glacier margin would have allowed slip without internal deformation of thrust sheets requiring only minimal pore water pressures

the basal ice layer of glaciers and ice sheets abstract in many glaciers and ice sheets there is a basal ice layer bil in which the ice is conditioned primarily by processes operating at the bed the bil is chemically and physically distinctive and is characterised by a component of basally derived sediment the bil is a rheological control on ice sheet dynamics an indicator of subglacial conditions and processes an agent of subglacial geologic processes the source of a substantial proportion of glacial sediments a limit to the downward extension of the climate record from deep ice cores if debris characteristics of the bil are preserved in glacial sediments former glacier conditions can be inferred analysis of stable isotopes gas and cation composition ice crystallography debris composition and structural glaciology has elucidated many mechanisms for bil formation common elements can be identified from a range of locations key issues are entrainment of ice and debris from the bed formation of new ice metamorphism of existing ice in the bil and thickening of the bil by deformation a distinction can be made between basal ice formed by accretion of material from the bed stratified facies and basal ice formed by metamorphic processes within the ice close to the bed dispersed facies

freezing rate effects on the physical characteristics of basal ice formed by net adfreezing a number of theoretical and empirical studies have indicated that many individual characteristics of ice formed by processes of net basal adfreezing may be sensitive to the rate of propagation of the freezing front through the reservoir concerned the effects of freezing rate on the the stable isotope chemistry and crystallography of ice in addition to the disposition and character of included debris and gas are reported unidirectional freezing through a cylindrical reservoir containing various water sediment mixtures has been conducted in the laboratory and the resulting cores analysed for debris and gas disposition and ice crystal size and fabric the data lend support to inferences drawn from studies concerned with specific ice properties and an idealized suite of characteristics is developed which may be diagnostic of basal ice formed by net adfreezing

glacitectonism subglacial and glacilacustrine processes during a neoproterozoic panglaciation north east svalbard

paleoerosion rates from cosmogenic 10be in a 1 3 ma terrace sequence response of the river meuse to changes in climate and rock uplift abstract river borne quartz carries a cosmogenic nuclide memory that is a function of the catchment wide erosion rate this record may be preserved in fluvial deposits such as river terraces if the age of a terrace is independently known and transport time in the river system is relatively short then the upstream erosion rate at the time of terrace deposition can be determined we have used cosmogenic nuclides to date river terraces in the lower meuse catchment the netherlands and to obtain a 1 3 ma record of paleoerosion rates in a 104 km2 drainage basin comprising the ardennes mountains paleoerosion rates were uniform within the range of 25 35 mm ka from 1 3 to 0 7 ma after 0 7 ma erosion rates have increased progressively to late pleistocene values of around 80 mm ka around 0 7 ma both climatic and tectonic boundary conditions changed the amplitude and duration of climate cycles increased significantly resulting in long periods of sustained low temperatures in the meuse catchment in additio

anisotropy of magnetic susceptibility ams and diamagnetic fabrics in the durness limestone nw scotland anisotropy of magnetic susceptibility ams and diamagnetic fabrics in the durness limestone nw scotland

evaluating magnetic lineations ams in deformed rocks magnetic lineation in rocks is given by a cluster of the principal axes of maximum susceptibility kmax of the anisotropy of magnetic susceptibility ams tensor in deformed rocks magnetic lineations are generally considered to be the result of either bedding and cleavage intersection or they parallel the tectonic extension direction in high strain zones our ams determinations based on a variety of samples that were taken from mudstones slates and schists from the pyrenees and appalachians show that strain is not the only factor controlling the development of magnetic lineation we find that the development and extent to which the magnetic lineation parallels the tectonic extension direction depends on both the original ams tensor which in turn depends on the lithology and the deformation intensity rocks having a weak pre deformational fabric will develop magnetic lineations that more readily will track the tectonic extension

depositional controls on the magnetic characteristics of lodgement tills and other glacial diamict facies genetic interpretation of massive unstructured diamict and diamictite facies is a commonly encountered problem faced by sedimentologists data are presented concerning the magnetic characteristics namely anisotropy of magnetic susceptibility ams and natural remanent magnetism nrm of facies deposited by 1 lodgement processes at a glacier base lodgement till and 2 subaqueously by pelagic mud deposition and ice rafting rain out diamicts lodgement tills have an nrm that is distorted around the geomagnetic pole position to form a girdle approximately 90 in length either transverse or parallel to ice flow direction this distension appears to be the result of subglacial shear processes because other diamicts deposited passively by melt out below stagnant ice and modelled by a laboratory experiment show a nondeformed nrm clustering around the geomagnetic pole the ams data show that lodgement tills have only a weakly orientated magnetic microfabric glaciolacustrine rain out diamicts show

oblique lineations in orthogneisses and supracrustal rocks vertical partitioning of strain in a hot crust eastern borborema province ne brazil detailed structural work conducted at the eastern area of the neoproterozoic brasiliano zpan african borborema province northeastern brazil has shown two orientations of stretching lineations with ese trend in supracrustal rocks and ne trend in underlying orthogneisses in the metasedimentary sequence numerous kinematic indicators showing a top to the northwest sense of shear denote a welldeveloped non coaxial deformation in the orthogneisses lineations formed dominantly during coaxial deformation although a component of ne directed shear is locally observed the two lineations were produced under similar high temperature metamorphic conditions and are interpreted as the result of a protracted nw se contractional strain regime where i subhorizontal non coaxial shear with superimposed flattening led to an initial phase of nw directed thrusting ii flattening strains mainly accumulated in the orthogneisses with progressive deformation leading to a lineation oblique to the transport direction iii the subhorizontal fabric in basement and cover rocks was refolded by overturned folds and then iv cross cut by conjugate ene striking dextral and nne striking sinistral shear zones contemporaneous with ne trending upright folds it is proposed that vertical partitioning of strain between basement and cover may explain the presence of oblique lineations in this orogenic belt that did not go through a final stage of extensional collapse q 2005 elsevier ltd all rights reserved

explicit treatment of inheritance in dating depositional surfaces using in situ 10be and 26al we describe a new strategy for dating depositional landscape surfaces using in situ produced cosmogenic radionuclides crns that removes the complication of nuclide inheritance by clasts prior to depositon two amalgamated samples each consisting of 30 clasts one from the surface and one from a fixed depth in the subsurface constrain this crn inheritance and date the surface the inheritance may be used to estimate minimum exhumation rates and maximum transport times within the geomorphic system we test the technique using sup 10 be and sup 26 al to date the third fr3 of five terraces along the fremont river utah and the third wr3 of 15 along the wind river wyoming whereas effective ages based solely upon the surface samples yield 118 138 ka fr3 and 93 ka wr3 the subsurface samples reveal that inheritance accounts for 22 wr3 to 43 fr3 of the total crn concentration taking this into account yeilds terrace ages of nearly 61 81 ka for fr3 and nearly 67 76 ka for wr3 we explore the dependence of age estimates on the accumulation history of the terrace silt caps 22 refs 3 figs 1 tab

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

basal ice facies a review and unifying approach abstract over the past 30 years numerous basal ice facies have been identified named and classified however the resulting facies descriptions and names are inconsistent and no single scheme encompasses all of the different ice types that exist at different ice masses in this paper we review and critique existing basal ice facies names descriptions and classification schemes and propose a new non genetic approach that has the capacity to name and describe all basal ice types we define six fundamental basal cryofacies and a further 12 composite cryofacies which can all be defined on the basis of a cursory evaluation of debris disposition and concentration more detailed cryofacies description is based on characterizing three sets of ice properties that can also be estimated visually in the field a the thickness of the basal ice facies and its constituent sub layers b the concentration and texture of debris included within any or all of those layers and c the concentration and size of bubbles included within any or all of those layers we also propose a shorthand method for the presentation of this descriptive information here codes for the layer thicknesses are presented as standard text and codes for the included debris characteristics and included bubble characteristics are presented as superscripts and subscripts respectively sub layers are characterized similarly but within a nested sequence of brackets we evaluate the effectiveness of these two schemes by using them to rename and reclassify several existing basal ice facies results indicate that the schemes are robust and that they provide a coherent non genetic framework for the effective naming and description of basal cryofacies

estimating rates of denudation using cosmogenic isotope abundances in sediment we propose as a testable hypothesis a basin scale approach for interpreting the abundance of in situ produced cosmogenic isotopes an approach which considers explicitly both the isotope and sediment flux through a drainage basin unlike most existing models which are appropriate for evaluating in situ produced cosmogenic isotope abundance at discrete points on earth s surface our model is designed for interpreting isotope abundance in sediment because sediment is a mixture of materials in favourable cases derived from throughout a drainage basin we suggest that measured isotope abundances may reflect spatially averaged rates of erosion we investigate the assumptions and behaviour of our model and conclude that it could provide geomorphologists with a relatively simple means by which to constrain the rate of landscape evolution if a basin is in isotopic steady state and if sampled sediments are well mixed

dating fluvial terraces with and profiles application to the wind river wyoming abstract fluvial strath terraces provide a record of river incision and the timing of climatic perturbations to the fluvial system dating depositional surfaces like terraces that are older than the range of 14 c however is difficult we employ a cosmogenic radionuclide crn profile technique that addresses a major problem of crn dating on such surfaces nuclide inheritance by measuring 10 be and 26 al profiles we constrain the exposure age and the mean crn inheritance for the deposit the crn profile also yields a self check on the assumptions underlying the method we report our attempts to date terraces along the wind river wy like many sequences of western north american fluvial terraces these are inferred to reflect oscillation between glacial and interglacial conditions in the headwaters previous dating of some of these terraces and the associated terraces and glacial deposits makes this a unique location to compare dating methods dates from five sites along the bull lake glacial correlative terrace wr 3 are 118 125 ka which agrees with dates on bull lake age moraines and independent age estimates on the terrace and is consistent with the model of terrace glacial relationship crn inheritance is significant and highly variable requiring it be considered despite the additional sampling complexity assuming all inheritance in wr 3 deposits arises during exhumation in the headwaters we obtain minimum mean rates of exhumation of 13 130 m my for the source rocks alternatively assuming the crns are inherited during clast transport the time of fluvial transport from source to terrace is 10 ka it increases downstream and is lower for sand than cobbles the crn ages for older terraces wr 7 300 ka and wr 15 510 ka are lower by 50 than previous estimates based on tephrochronology the most plausible explanation is eolian deflation of a once thicker loess cover on the terrace surfaces mean thicknesses of loess of 0 5 1 5 m are required to reconcile these concentrations of crn with the previous estimates of age difficulty in dating the older terraces emphasizes that geologic caution independent estimates of age and multiple sample sites should still be part of dating depositional surfaces with crns even when employing the inheritance correction technique

isothermal remanent magnetization of greenland ice preliminary results the magnetic mineral content of wind transported dust should reflect atmospheric transport dynamics and conditions in its source areas and could thus be used as an environmental proxy to test the feasibility of determining the magnetic mineral content in polar ice isothermal remanent magnetization irm was measured on a small suite of greenland ice samples of holocene interglacial and last glacial maximum lgm age although the extremely low dust concentrations limit weak field susceptibility measurements all samples contained an easily measurable concentration of magnetic minerals that can be estimated using irm intensity provided that special precautions are used irm experiments at liquid nitrogen temperatures indicate ice magnetic properties which are consistent with that expected from varying concentrations of magnetite or maghemite interestingly the holocene ice samples tend to have higher magnetic concentrations despite having much lower total polar dust contents than the few lgm ice samples tested thus far

low field variation of magnetic susceptibility as a tool for magnetic mineralogy of rocks abstract low field variation of magnetic susceptibility was investigated on a collection of several hundreds specimens of various minerals and rocks using the kly 4s kappabridge the measurement is fully automated being executed in 21 distinct fields ranging from 2 to 450 a m in one frequency of 875 hz the measurement is rapid 7 min per specimen so that large collections of specimens can be investigated the results can be processed both graphically and mathematically for the latter processing parameters of two kinds were introduced one characterizes the susceptibility change with field the other one characterizes the field in which the susceptibility no longer obeys the rayleigh law and starts becoming more complex the results were evaluated statistically remarkable differences were revealed between individual minerals and between some rock types for example the field variation of susceptibility of pyrrhotite is in general an order of magnitude larger than that of titanomagnetite the susceptibility increase in pyrrhotite starts at the field an order of magnitude lower than that of titanomagnetite low field variation of susceptibility then appears as an interesting phenomeon that helps in the identification of magnetic minerals and in some cases also in assessing the compositional variation of them

the geometry and topology of natural sheath folds a new tool for structural analysis abstract curvilinear sheath folds are classically depicted as displaying symmetrical geometries about two orthogonal mirror planes centred along the x y axial surface and the x z medial culmination depression surface which bisects the fold nose however 10 000 geometric analyses of minor folds and fabrics formed during ductile thrusting in the caledonides of northern scotland reveals that major dome and basin sheath folds can display distinct and predictable asymmetries across both axial and medial surfaces the strain is typically heterogeneous so that structural fabrics and younging evidence are preserved within sheath folds at varying stages of development this allows an analysis of the evolution of such structures from tongue folds to more extreme tubular forms geometric relationships between measured orientations of fold hinges axial planes extension lineations and foliations are compared on fabric topology plots ftps which provide an effective tool for monitoring planar and linear fabric rotations with increasing progressive non coaxial deformation they consistently display systematic variation from regions of lower to higher strain on passing from upper to lower fold limbs across major axial surfaces and on crossing medial surfaces from short to long hinge line segments axial and medial surfaces effectively therefore divide major sheath folds into quadrants with different amounts senses and combinations of planar and linear fabric rotation within each domain such heterogeneous deformation implies that models of intense non coaxial deformation uniformly affecting pre existing folds may overestimate bulk displacement and shear strain variable fold hinge line rotation about medial surfaces also provides an effective mechanism for the closure of major sheaths which may otherwise project for unfeasible distances in the x direction bedding cleavage intersections are developed at greater angles to the transport direction than fold hinges which they transect in a consistent and predictable sense thereby confirming the direction of fold rotation even in areas which lack information on fold facing in cross section asymmetric tear drop eyes indicate the sense of fold hinge line vergence whilst fold limb and hinge line asymmetry may be combined on 3 d vergence ranking and rotation grids which allow location and relative strain states of minor structures to be accurately predicted within the overall sheath framework systematic fabric analysis on ftps may be applied to the investigation of ductile deformation across a broad range of scenarios

structure and changing dynamics of a polythermal valley glacier on a centennial timescale midre lovenbreen svalbard hambrey michael murray t glasser n f hubbard a 2005 structure and changing dynamics of a polythermal valley glacier on a centennial timescale midre lovenbreen svalbard journal of geophysical research 110 pp 1 19 rae2008

the mid pleistocene climate transition onset of 100 ka cycle lags ice volume build up by 280 ka the mid pleistocene climate transition mpt is the complex climatic change which brought the late pleistocene ice ages we explore the mpt in the time and frequency domains by new methods of time series analysis high resolution oxygen isotope records reveal that the ice volume related increase in 18o mean amplitude 0 29 0 05 1 transition midpoint 922 12 ka duration 40 9 ka significantly preceded the abrupt increase in the amplitude of the 100 ka cycle at 641 9 ka this finding can be quantitatively simulated using a simple ice bedrock model in which due to the additional ice the calving threshold is exceeded the simulated calving events prior to 650 ka are separated by 77 ka whereas after 650 ka they occur pseudo periodically with a mean period of nearly 100 ka the cause for the delay of the 100 ka calving cycle was not a slow bedrock relaxation rather the coincidental combination of insolation existing ice mass and bedrock depression