the climate memory of an arctic polythermal glacier knowledge of glacier equilibrium line altitude ela changes and trends in time is essentialfor future predictions of glacier volumes we present a novel method for determining trends in elachange at mccall glacier alaska usa over the last 50 years based on mapping of the cold temperatetransition surface cts marking the limit between cold and temperate ice of a polythermal glacier latent heat release from percolating meltwater and precipitation keeps the ice column temperate in theaccumulation area a change from accumulation to ablation zone reduces this heat release leadinglocally to glacier ice cooling by mapping the cts along the whole glacier length using radio echosounding and employing a thermodynamic model the timing of the cooling was determined fromwhich past elas were constructed these are in accord with mass balance measurements carried out onmccall glacier since the 1950s we show that with a warming climate mccall glacier tends to cool ina counter intuitive way

observed and modelled ice temperature and velocity along the main flowline of east rongbuk glacier qomolangma mount everest himalaya knowledge of present day ice temperature and velocity is important in order to determine how fast a glacier will respond to present and future climate change we apply a two dimensional higher order thermomechanical flowband model to simulate present day ice temperature and velocity along the main flowline of east rongbuk glacier qomolangma mount everest himalaya we use recent 2005 11 observational data to validate the numerical model modelled and observed ice surface velocities exhibit good agreement modelled ice temperatures agree well with observed values in two shallow boreholes that are similar to 18 m deep the model suggests that the ablation zone of east rongbuk glacier km 4 to km 10 from the glacier head is underlain by temperate ice

a numerical study of plane ice sheet flow l ecoulement bidimensionnel stationnaire d une nappe de glace reposant sur le sol est analyse numeriquement au moyen du modele simplifie de morland et hutter la glace se comporte comme un fluide visqueux non lineaire dependant de la temperature la nappe de glace est supposee longue et mince

late cretaceous intraplate silicic volcanic rocks from the lake chad region an extension of the cameroon volcanic line silicic volcanic rocks at hadjer el khamis near lake chad are considered to be an extension of the cameroon volcanic line cvl but their petrogenetic association is uncertain the silicic rocks are divided into peraluminous and peralkaline groups with both rock types chemically similar to within plate granitoids in situ u pb zircon dating yielded a mean 206pb 238u age of 74 4 1 3 ma indicating the magmas erupted 10 million years before the next oldest cvl rocks i e 66 ma the sr isotopes i e isr 0 7021 0 7037 show a relatively wide range but the nd isotopes i e 143nd 144ndi 0 51268 0 51271 are uniform and indicate that the rocks were derived from a moderately depleted mantle source thermodynamic modeling shows that the silicic rocks likely formed by fractional crystallization of a mafic parental magma but that the peraluminous rocks were affected by low temperature alteration processes the silicic rocks are more isotopically similar to late cretaceous basalts identified within the late cretaceous basins i e 143nd 144ndi 0 51245 0 51285 of chad than the uncontaminated cvl rocks i e 143nd 144ndi 0 51270 0 51300 the age and isotopic compositions suggest the silicic volcanic rocks of the lake chad region are related to late cretaceous extensional volcanism in the termit basin it is unlikely that the silicic volcanic rocks are petrogenetically related to the cvl but it is possible that magmatism was structurally controlled by suture zones that formed during the opening of the central atlantic ocean and or the pan african orogeny

a level set method for modeling the evolution of glacier geometry a level set method is proposed for modeling the evolution of a glacier surface subject to a prescribed mass balance this leads to a simple and versatile approach for computing the evolution of glaciers the description of vertical fronts and overriding phenomena presents no difficulties topological changes are handled naturally and steady state solutions can be calculated without integration over time a numerical algorithm is put forth as a means of solving the proposed model of glacier surface evolution it is evaluated by comparing different numerical solutions of the model with analytical and published numerical solutions the level set method appears to be a reliable approach for dealing with different glaciological problems

theoretical contemporary observational and palaeo perspectives on ice sheet hydrology processes and products publisher elsevier articletitle theoretical contemporary observational and palaeo perspectives on ice sheet hydrology processes and products journaltitle earth science reviews articlelink http dx doi org 10 1016 j earscirev 2016 01 010 content type article copyright copyright 2016 elsevier b v all rights reserved

diamictic sediments within high arctic lake sediment cores evidence for lake ice rafting along the lateral glacial margin sediment cores from six small lake basins in the canadian high arctic reveal a gravel rich 30 by weight to gravel poor 2 diamict facies underlying massive post glacial clayey silt ten other lakes contain a second diamict facies within what are interpreted to be glaciolacustrine sedimentary assemblages the sedimentology clast fabrics and fossil remains diatoms ostracodes and chironomid head capsules within both diamict facies suggest that these deposits are not tills clast fabrics yielded low s1 0 41 0 57 and high s3 0 09 0 22 eigenvalues placing them within the range of ice rafted diamictons and glacigenic sediment flows the high percentage of clast dip angles 45 15 61 random clast azimuth and lower diamict contacts conformable to underlying current bedded sediment favours an origin as a rain out or settling deposit samples of the matrix and scrapings of clasts from the diamicts revealed a diatom assemblage dominated by littoral and planktonic forms such as are found in the littoral regions of the lakes today this contrasts sharply with the assemblages within the overlying clayey silt in which benthic forms predominate clasts are thus interpreted to have been rafted from the littoral areas of the lake the process proposed to explain this is rafting by the lake ice cover in a glacial marginal environment early season meltwater impounded along the lateral margin of retreating cold based glaciers would buoyantly lift the lake ice cover and any adfrozen lake sediment higher lake levels and increased areal extent of seasonal freeze on between the lake ice cover and the lake bed would allow the redeposition of littoral sediments to the benthic regions through greater lateral shifting of the ice cover as it broke up incision by meltwater streams into the lateral glacial margins would later isolate the lake allowing seasonal warming of lake water enough to support the growth and maturation of the ostracode and chironomid species found as fossils within the diamicts

hydrochemistry as an indicator of subglacial drainage system structure a comparison of alpine and sub polar environments the anion compositions so 2 4 hco 3 and cl of runoff from the haut glacier d arolla switzerland and austre broggerbreen svalbard are compared to assess whether or not variations in water chemistry with discharge are consistent with current understanding of the subglacial drainage structure of warm and polythermal based glaciers these glacial catchments have very different bedrocks and the subglacial drainage structures are also believed to be different yet the range of anion concentrations show considerable overlap for so 2 4 and hco 3 concentrations of cl are higher at austre broggerbreen because of the maritime location of the glacier correcting so 2 4 for the snowpack component reveals that the variation in non snowpack so 2 4 with discharge and with hco 3 is similar to that observed at the haut glacier d arolla hence if we assume that the provenance of the non snowpack so 2 4 is the same in both glacial drainage systems a distributed drainage system also contributes to runoff at austre broggerbreen we have no independent means of testing the assumption at present the lower concentrations of non snowpack so 2 4 at austre broggerbreen may suggest that a smaller proportion of runoff originates from a distributed drainage system than at the haut glacier d arolla

modelling the long term mass balance and firn evolution of glaciers around kongsfjorden svalbard we analyse the long term 1961 2012 distributed surface mass balance and firn evolution of the kongsvegen and holtedahlfonna glacier systems in northwestern svalbard we couple a surface energy balance model to a firn model with forcing provided from regional climate model output in situ observational data are used to calibrate model parameters and validate the output the simulated area averaged surface mass balance for 1961 2012 is slightly positive 0 08 m w e a 1 which only fractionally compensates for mass loss by calving refreezing of percolating water in spring summer 0 13 m w e a 1 and stored water in fall winter 0 18 m w e a 1 provides a buffer for runoff internal accumulation i e refreezing below the previous year s summer surface in the accumulation zone peaks up to 0 22 m w e a 1 and is unaccounted for by stake observations superimposed ice formation in the lower accumulation zone ranges as high as 0 25 m w e a 1 a comparison of the periods 1961 99 and 2000 12 reveals 21 higher annual melt rates since 2000 and a 31 increase in runoff which can only in part be ascribed to recent warmer and drier conditions in response to firn line retreat both albedo lowering snow ice albedo feedback and lower refreezing rates refreezing feedback further amplified runoff

links between short term velocity variations and the subglacial hydrology of a predominantly cold polythermal glacier the surface velocity of a predominantly cold polythermal glacier john evans glacier ellesmere island canada varies significantly on both seasonal and shorter time scales seasonal variations reflect the penetration of supraglacial water to the glacier bed through significant thicknesses of cold ice shorter term events are associated with periods of rapidly increasing water inputs to the subglacial drainage system earlyseason short term events immediately follow the establishment of a drainage connection between glacier surface and glacier bed and coincide with the onset of subglacial outflow at the terminus a mid season short term event occurred as surface melting resumed following cold weather and may have been facilitated by partial closure of subglacial channels during this cold period there is a close association between the timing and spatial distribution of horizontal and vertical velocity anomalies the temporal pattern of surface water input to the glacier and the formation seasonal evolution and distribution of subglacial drainage pathways these factors presumably control the occurrence of highwater pressure events and water storage at the glacier bed the observed coupling between surface water inputs and glacier velocity may allow predominantly cold polythermal glaciers to respond rapidly to climate induced changes in surface melting

geodynamic evolution of the pan african belt in central africa with special reference to cameroon the pan african belt in central africa has benefited from the many petrographic structural and geochronological studies in the recent years that have improved our understanding of the belt however those studies have also produced various and often divergent evolutionary models for the belt some of which do not even involve well defined cratons following a review of the available data in cameroon we propose a model of continent continent collision that involved the congo craton and the north central cameroon active margin showing archean to paleoproterozoic inheritances this model is based among others on i the prominent role of the congo craton as demonstrated by the regional extension of external nappes on its northern edge and the concomitant exhumation of the 620 ma granulitic rocks believed to have formed at the root of the collision zone and ii the late development of a strike slip fault system in central cameroon as the result of horizontal movement following the multistage collision

proglacial sediment dynamics from daily to seasonal scales in a glaciated alpine catchment bossons glacier mont blanc massif france the sediment yields of alpine catchments are commonly determined from streamload measurements made some distance downstream from glaciers however this approach indiscriminately integrates erosion processes occurring in both the glacial and proglacial areas a specific method is required to ascertain the respective inputs from i subglacial and supraglacial sediments ii proglacial hillslopes and iii proglacial alluvial areas or sandurs this issue is addressed here by combining high resolution monitoring 2 min of suspended sediment concentrations at different locations within a catchment with discharge gauging and precipitation data this methodological framework is applied to two proglacial streams draining the bossons glacier mont blanc massif france the bossons and crosette streams for the bossons stream discharge and suspended load data were acquired from june to october 2013 at 1 15 and 1 5 km from the glacial terminus respectively upstream and downstream from a small valley sandur these hydro sedimentary data are compared with the crosette stream dataset acquired at the outlet of the bossons glacier subglacial drainage system a fourfold analysis focusing on seasonal changes in streamload and discharge multilinear regression modelling evaluation of the sandur flux balance and probabilistic uncertainty assessment is used to determine the catchment sediment budget and to explain the proglacial sediment dynamics the seasonal fluctuation of the sediment signal observed is related to the gradual closing of the subglacial drainage network and to the role of the proglacial area in the sediment cascade the proglacial hillslopes appear to be disconnected from the main channel and the valley sandur acts as a hydrodynamic sediment buffer both daily and seasonally our findings show that an understanding of proglacial sediment dynamics can help in evaluating paraglacial adjustment and subglacial erosion processes

age migrations of anorogenic ring complexes in northern nigeria forty rb sr isotopic measurements on minerals and whole rock samples from 11 anorogenic complexes in northern nigeria complement the evidence for systematic age variations along a series of en echelon ene wsw and n s lineaments following two major fracture directions of extensional deformation in the precambrian basement the new ages presented in this paper confirm our earlier discoveries of a general southerly age trend but substantiate for the first time that major local migrations of magmatic activity were concentrated along at least two ene wsw zones from the overlap in analytical error the age patterns also suggest that contemporaneous anorogenic magmatism may have been locally derived from several simultaneous high level magma chambers connected to a common deeper source different models that have been proposed for the origin of linear mid plate magmatic activity are critically reviewed in our opinion magma generation was most likely initiated in the asthenosphere lithosphere decoupling zone or

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

permian alkaline undersaturated and carbonatite province and rifting along the west african craton recent work1 2 carried out in north east mali which showed the pan african orogeny 600 myr to be the result of a collision between the passive margin of the west african craton and the active margin of a continental mass situated to the east has also outlined an entire province of alkaline undersaturated ring complexes and carbonatites the first to be discovered in west africa excluding morocco3 4 fig 1 this province is quite distinct petrologically and structurally from the iforas alkaline younger granite province situated in the pan african domain east of the suture which has yielded rb sr whole rock ages of between 560 and 520 myr ref 5 the undersaturated province is located along a well defined rift on the edge of the west african craton which is totally devoid of pan african magmatism we report here that the permian radiometric age obtained for one of these intrusions shows the existence of a hitherto unsuspected important tectono magmatic event in this part of africa which may have far reaching structural implications as well as economic interest

mechanisms of englacial conduit formation and their implications for subglacial recharge abstract ideas about the character and evolution of englacial drainage systems have been deeply influenced by the theoretical model developed by shreve 1972 movement of water in glaciers journal of glaciology 11 62 205 214 the shreve model is based on three main assumptions 1 englacial drainage is in steady state 2 englacial water will flow along the steepest hydraulic gradient within the glacier and 3 pressure head equals the pressure of the surrounding ice minus a small component due to melting of the walls the shreve model has been widely adopted as a fundamental component of englacial drainage theory there is no evidence however that the model provides a realistic picture of actual glacial drainage systems to evaluate shreve s theory we used speleological techniques to directly survey englacial conduits we mapped a total of 8 25 km of passage in 27 distinct englacial conduits in temperate polythermal cold based and debris covered glaciers between 2005 and 2008 new information reported here is supplemented by published data on 40 other englacial conduits located worldwide and surveyed to ice depths of 176 m using speleological techniques in all cases englacial drainage systems consisted of a single unbranching conduit englacial conduit morphologies were found to be intimately linked to the orientation of a glacier s principal stresses or the presence of pre existing lines of high hydraulic conductivity if a sufficient supply of water is available hydrofracturing forms vertical conduits in zones of longitudinal extension and subhorizontal conduits where longitudinal stresses are compressive on unfractured glacier surfaces relatively shallow subhorizontal conduits with migrating nickpoints form by cut and closure provided channel incision is significantly faster than surface lowering conduits can also form along permeable debris filled crevasse traces that connect supraglacial lake basins of different potential our results suggest that shreve type englacial drainage systems do not exist and implies that englacial conduits can only penetrate through thick ice to recharge the bed where supraglacial water bodies either intersect or are advected through zones of acceleration

an investigation into the mechanisms controlling seasonal speedup events at a high arctic glacier bingham r g hubbard a l nienow p w sharp m j 2008 an investigation into the mechanisms controlling seasonal speedup events at a high arctic glacier journal of geophysical research earth surface 113 f2 f02006 sponsorship nerc nserc the university of alberta circumpolar institute and northern science training programme the geological society of america

a cut and closure origin for englacial conduits in uncrevassed regions of polythermal glaciers on uncrevassed regions of polythermal glaciers englacial conduits can form by incision of supraglacial stream channels followed by roof closure the origin and evolution of examples in longyearbreen svalbard and khumbu glacier nepal were determined by speleological survey the development of perennial incised channels requires that incision is significantly faster than glacier surface ablation and thus will be favoured by high meltwater discharges in combination with cool climatic conditions or thick debris cover incised canyons can become blocked by drifted winter snow refrozen meltwater ice rafting from non local sources allochthonous breccias and roof collapses autochthonous breccias conduit closure can also occur in response to ice creep particularly at depth following isolation from the surface englacial conduits continue to evolve by vadose incision down to local base level in the case of longyearbreen incision allowed the channel to reach the glacier bed but on khumbu glacier deep incision is prevented because an effectively impermeable terminal moraine provides a high base level for the glacier drainage system during our period of observations deeper parts of the longyearbreen conduit became blocked by a combination of ice accumulation and creep causing the stream course to be re routed to higher levels on that glacier incision blockage and upward re routing are cyclic we conclude that cut and closure is the dominant mechanism of englacial conduit formation on uncrevassed regions of polythermal glaciers

vertical distribution of water within the polythermal storglaciaren sweden 1 knowledge of water content and its distribution in polythermal glaciers is required to model their flow and thermal state however observations of water content variations with depth in polythermal glaciers are scarce water content can be estimated from radio wave speed because they depend on one another we obtained continuous profiles of radio wave speed variations with depth from zero offset radar profiles collected in boreholes approximately 80 m deep in the upper ablation area of storglaciaren northern sweden these profiles show that the microcrystalline water system in the temperate ice is relatively homogeneous the overall hydrothermal structure at this location is composed of a 20 m thick upper layer of cold water free ice underlain by a temperate ice layer whose average water content is 0 6 0 3 these results are corroborated by surface radar and thermistor measurements which show that the depth of the cold temperate transition is 21 m and the calculated water content at that transition is 0 6 0 1 these findings imply that the whole temperate ice layer is from 3 to 4 times softer than the cold ice and consequently that realistic ice flow models of polythermal glaciers should include the effect of water content on viscosity

late permian mafic rocks identified within the doba basin of southern chad and their relationship to the boundary of the saharan metacraton the doba gabbro was collected from an exploration well through the cretaceous doba basin of southern chad the gabbro is composed mostly of plagioclase clinopyroxene and fe ti oxide minerals and displays cumulus mineral textures whole rock 40 ar 39 ar step heating geochronology yielded a late permian plateau age of 257 1 ma the major and trace elemental geochemistry shows that the gabbro is tholeiitic in composition and has trace element ratios i e la yb n 7 sm yb pm 3 4 nb y 1 zr y 5 indicative of a basaltic melt derived from a garnet bearing mantle source the moderately enriched sr nd isotopes i e i sr 0 70495 to 0 70839 nd t 1 0 to 1 3 fall within the mantle array i e oib like and are similar to other late permian plutonic rocks of north central africa i e i sr 0 7040 to 0 7070 the enriched isotopic composition of the doba gabbro contrasts with the more depleted compositions of the spatially associated neoproterozoic post pan african within plate granites the contrasting nd isotope composition between the older within plate granites and the younger doba gabbro indicates that different mantle sources produced the rocks and thus may mark the southern boundary of the saharan metacraton

a model for polythermal ice incorporating gravity driven moisture transport the flow of ice sheets and glaciers dissipates significant amounts of heat which can result in the formation of temperate ice a binary mixture of ice and small amounts of melt water that exists at the melting point many ice masses are polythermal in the sense that they contain cold ice below the melting point as well as temperate ice temperature and melt water or moisture content conversely affect the flow of these ice masses through their effect on ice viscosity and sliding behaviour ice flow models therefore require a component that can solve for temperature and moisture content and determine the free boundary between the cold and temperate subdomains we present such a model based on the theory of compacting partial melts by contrast with other models we describe gravity and pressure gradient driven drainage of moisture while maintaining a divergence free ice flow at leading order we also derive the relevant boundary conditions at the free cold temperate boundary and find that the boundary behaves differently depending on whether ice enters or exits the temperate region the paper also describes a number of test cases used to compare with a numerical solution and investigates asymptotic solutions applicable to the limit of small compaction pressure gradients in the temperate ice regions a simplified enthalpy gradient model is finally proposed which captures most of the behaviour of the full model in this limit

on the thermal regime of an arctic valley glacier a study of white glacier axel heiberg island n w t canada from 1974 to 1981 a total of 32 bore holes was drilled on white glacier and vertical ice temperature profiles measured the data obtained allowed the construction of three longitudinal and four transverse profiles of the two dimensional temperature distributions thus an extensive layer of temperate or near temperate ice was discovered close to the bedrock in the lowest part of the glacier tongue it was also found that the temperature distribution cannot be in a steady state since there is a temperature minimum 100 150 m below the glacier surface in the accumulation area a simple numerical model calculation shows that this minimum can be mostly explained by the general climatic warming since 1880 the 10 m temperatures show diffuse relations to climatic conditions and balance zones a simple scheme for extrapolating surface temperatures is discussed

ice flow across a warm based cold based transition at a glacier margin where polythermal glaciers have frozen margins that buttress otherwise temperate based sliding ice longitudinal compression can strongly influence ice flow trajectory and consequently sediment transport paths past efforts to model flow in the vicinity of a basal thermal transition btt have generally relied on simplified boundary conditions or rheological idealizations making these model results difficult to apply to real glacier termini herein we present results of numerical simulations using a power law rheology and with boundary conditions that better represent the frozen margin model results indicate that a transition to a non sliding frozen margin causes a decline in surface velocity made possible by upward ice flow implying either enhanced ablation for steady state simulations or the formation of a surface bulge permitting ice loss by ablation combined with numerical smoothing of the basal slip transition subdues basal stress concentrations and thereby inhibits development of structural discontinuities such as thrust faults upward ice flow is accommodated by vertical extension up glacier of the btt this strain regime can potentially account for key structural features in polythermal glacier termini without appealing to thrusting

late holocene changes in character and behaviour of land terminating glaciers on james ross island antarctica virtually no information is available on the response of land terminating antarctic peninsula glaciers to climate change on a centennial timescale this paper analyses the topography geomorphology and sedimentology of prominent moraines on james ross island antarctica to determine geometric changes and to interpret glacier behaviour the moraines are very likely due to a late holocene phase of advance and featured 1 shearing and thrusting within the snout 2 shearing and deformation of basal sediment 3 more supraglacial debris than at present and 4 short distances of sediment transport retreat of 100m and thinning of 15 20m has produced a loss of 0 1km3 of ice the pattern of surface lowering is asymmetric these geometrical changes are suggested most simply to be due to a net negative mass balance caused by a drier climate comparisons of the moraines with the current glaciological surface structure of the glaciers permits speculation of a transition from a polythermal to a cold based thermal regime small land terminating glaciers in the northern antarctic peninsula region could be cooling despite a warming climate

a model computation of moisture content in polythermal glaciers measurements in boreholes and internal radar horizons confirm the existence of polythermal structures in polar and subpolar glaciers and ice sheets the occurrence of an intraglacial phase boundary makes it necessary to consider two layers and transition conditions between cold and temperate ice the governing field equations and constitutive relations for a two layer model are stated with emphasis on boundary and transition conditions a reduced plane flow model is introduced by using dimensionless variables and performing a scale analysis according to the stated aim of this paper a parallel sided slab model is suitable for rough analyses of borehole measurements its governing equations are deduced and their analytical and numerical treatment is discussed sensitivity tests reveal a strong dependence of moisture content profiles on the numerical value of the water diffusivity coefficient and especially on strain heating

implications of nb u th u and sm nd in plume magmas for the relationship between continental and oceanic crust formation and the development of the depleted mantle abstract the nb u and th u of the primitive mantle are 34 and 4 04 respectively which compare with 9 7 and 3 96 for the continental crust extraction of continental crust from the mantle therefore has a profound influence on its nb u but little influence on its th u conversely extraction of midocean ridge type basalts lowers the th u of the mantle residue but has little influence on its nb u as a consequence variations in th u and nb u with sm nd can be used to evaluate the relative importance of continental and basaltic crust extraction in the formation of the depleted sm nd enriched mantle reservoir this study evaluates nb u th u and sm nd variations in suites of komatiites picrites and their associated basalts of various ages to determine whether basalt and or continental crust have been extracted from their source region emphasis is placed on komatiites and picrites because they formed at high degrees of partial melting and are expected to have nb u th u and sm nd that are essentially the same as the mantle that melted to produce them the results show that all of the studied suites with the exception of the barberton have had both continental crust and basaltic crust extracted from their mantle source region the high sm nd of the gorgona and munro komatiites require the elevated ratios seen in these suites to be due primarily to extraction of basaltic crust from their source regions whereas basaltic and continental crust extraction are of subequal importance in the source regions of the yilgarn and belingwe komatiites the sm nd of modern midocean ridge basalts lies above the crustal extraction curve on a plot of sm nd against nb u which requires the upper mantle to have had both basaltic and continental crust extracted from it it is suggested that the extraction of the basaltic reservoir from the mantle occurs at midocean ridges and that the basaltic crust together with its complementary depleted mantle residue is subducted to the core mantle boundary when the two components reach thermal equilibrium with their surroundings the lighter depleted component separates from the denser basaltic component both are eventually returned to the upper mantle but the lighter depleted component has a shorter residence time in the lower mantle than the denser basaltic component if the difference in the recycling times for the basaltic and depleted components is 1 0 to 1 5 ga a basaltic reservoir is created in the lower mantle equivalent to the amount of basalt that is subducted in 1 0 to 1 5 ga and that reservoir is isolated from the upper mantle it is this reservoir that is responsible for the sm nd ratio of the upper mantle lying above the trend predicted by extraction of continental crust on the plot of sm nd against nb u

metacraton nature genesis and behavior article i nfo in this paper we show with examples that cratons involved in intercontinental collisions in a lower plate position are often affected by orogenic events leading to the transformation of their margins in some cases craton interiors can also be shaped by intense collisional processes leading to the generation of intra cratonic orogenic belts we propose to call these events metacratonization and the resulting lithospheric tract metacraton metacratons can appear similar to typical orogenic belts i e active margin transformed by collisional processes but are actually sharply different their main distinctive characteristics not all are present in each metacraton are 1 absence of pre collisional events 2 absence of lithospheric thickening high pressure metamorphism being generated by subduction leading to high gradient in strain and meta morphic intensity 3 preservation of allochthonous pre collisional oceanic terranes 4 abundant post collisional magmatism associated with shear zones but not with lithospheric thickening 5 presence of high temperature low pressure metamorphism associated with post collisional magmatism 6 intraconti nental orogenic belts unrelated to subduction and oceanic basin closures reactivation of the rigid but fractured metacratonic lithosphere will cause doming asthenospheric volcanism emplacement and mineral izations due to repetitive mineral enrichments this paper provides several geological cases exemplifying these different metacratonic features in scandinavia sahara central africa and elsewhere a special focus is given to the saharan metacraton because it is where the term metacraton originated and it is a vastly ex panded tract of continental crust 5 000 000 km 2

a type granites from the pan african orogenic belt in south western chad constrained using geochemistry sr nd isotopes and u pb geochronology abstract the zabili granitic pluton sw chad exposed in the mayo kebbi massif is dominated by a coarse grained hornblende biotite granite grading into a fine grained biotite granite along its southern margin petrologic micrographic intergrowth of quartz and alkali feldspars granophyric microstructures the presence of fluorite and bastnaesite as accessory minerals and geochemical data high silica alkalis and fe mg depletions in cao mgo tio 2 high ga nb zr ga al ree depletions in ba sr eu and compatible elements indicate that this pluton consists of a type granites crystallized from hot apatite and zircon saturation temperatures ranging from 744 c to 923 c extremely differentiated magmas u pb zircon geochronology indicates that the magmas crystallized at 567 10 ma and reveals the presence of older neoproterozoic xenocrystic zircons at 668 5 ma in both facies within the fine grained biotite granite discordant zircons with u pb and pb pb ages ranging from neoproterozoic to archaean are also reported the 668 5 ma old zircons are considered to derive from country rocks while discordant zircons characterized by angular shapes internal fractures and inherited cores are likely to represent multi sources detrital crystals that have recorded at least one metamorphic event old pre neoproterozoic zircons are reported for the first time for rocks of the mayo kebbi massif and they attest to the contribution of an old basement likely to be the eastern nigeria basement and or the congo craton involved in a collisional event with a juvenile neoproterozoic crust prior to the emplacement of the zabili granitic pluton initial end values calculated for the zabili pluton range from 2 6 to 7 0 the highest value recorded by one sample from the coarse grained hornblende biotite granite being close to the one of the depleted mantle at 570 ma end 7 4 combining geochronology nd isotopes composition and geochemical modeling leads us to suggest the following model for the origin of the zabili granitic pluton i contribution of juvenile magmas or partial melting of a juvenile basaltic protolith characterized by a short crustal residence time ii interaction of granitic magmas with older continental materials as suggested by the presence of pre neoproterozoic zircons and lower initial end values of the fine grained biotite granite and iii fractional crystallization of feldspars and ferromagnesians to produce the observed geochemical features of sample gab b from which an initial end value of 7 0 has been calculated

an enthalpy formulation for glaciers and ice sheets polythermal conditions are ubiquitous among glaciers from small valley glaciers to ice sheets conventional temperature based cold ice models of such ice masses cannot account for that portion of the internal energy which is latent heat of liquid water within temperate ice so such schemes are not energy conserving when temperate ice is present temperature and liquid water fraction are however functions of a single enthalpy variable a small enthalpy change in cold ice is a change in temperature while a small enthalpy change in temperate ice is a change in liquid water fraction the unified enthalpy formulation described here models the mass and energy balance for the three dimensional ice fluid for the surface runoff layer and for the subglacial hydrology layer together in a single energy conserving theoretical framework it is implemented in the parallel ice sheet model results for the greenland ice sheet are compared with those from a cold ice scheme this paper is intended to be an accessible foundation for enthalpy formulations in glaciology

petroleum geology of cretaceous tertiary rift basins in niger chad and central african republic this overview of the petroleum geology of rift basins in niger chad and central african republic car is based on exploration work by exxon and partners in the years 1969 1991 the work included 50 000 km of modern reflection seismic 53 exploration wells 1 000 000 km2 of aeromagnetic coverage and about 10 500 km of gravity profiles the results outline ten cretaceous and tertiary rift basins which constitute a major part of the west and central african rift system wcars the rift basins derive from a multiphased geologic history dating from the pan african approximately 750 550 ma to the holocene main rifting was probably caused by the stretching and subsidence of african crustal blocks during the breakup of gondwana in the early cretaceous and by r activated plate movements in the early tertiary wcars in the study area is divided into the west african rift subsystem was and the central african rift subsystem cas was basins in niger and chad are chiefly extensional and are filled by up to 13 000 m of lower cretaceous to holocene continental and marine clastics the basins contain five oil 19 43 api and two oil and gas accumulations in upper cretaceous and eocene sandstone reservoirs the hydrocarbons are sourced and sealed by upper cretaceous and eocene marine and lacustrine shales the most common structural styles and hydrocarbon traps usually are associated with normal fault blocks cas rift basins in chad and car are extensional and transtensional and are filled by up to 7500 m of chiefly lower cretaceous continental clastics the basins contain eight oil 15 39 api and one oil and gas discovery in lower and upper cretaceous sandstone reservoirs the hydrocarbons are sourced by lower cretaceous shales and sealed by interbedded lacustrine and flood plain shales structural styles range from simple fault blocks through complex flower structures the main hydrocarbon traps are in contractional anticlines geological conditions favor the discovery of potentially commercial volumes of oil in wcars basins of niger chad and car

the last glaciation of east central ellesmere island nunavut ice dynamics deglacial chronology and sea level change during the last glacial maximum of east central ellesmere island trunk glaciers inundated the landscape entering the smith sound ice stream accelerator mass spectrometry ams dates on individual shell fragments in till indicate that the ice advanced after 19 ka bp the geomorphic and sedimentary signatures left by the trunk glaciers indicate that the glaciers were polythermal the configuration and chronology of this ice is relevant to the reconstruction of ice core records from northwestern greenland the history of iceberg rafting of clastic sediments to northern baffin bay the reopening of the seaway between the arctic ocean and baffin bay and the regional variability of arctic paleoenvironments deglaciation began with the separation of ellesmere island and greenland ice at fiord mouths 8 8 5 ka bp ice reached fiord heads between 6 5 and 4 4 ka bp trunk glacier retreat from the fiords of east central ellesmere island occurred up to 3000 years later than in west coast fiords this later retrea

the permo jurassic alkaline province of tadhak mali geology geochronology and tectonic significance the tadhak alkaline complexes located along the eastern edge of the west african craton in northeastern mali intrude the 2 ga old basement or the overlying pan african nappes and are covered by cretaceous sediments this magmatic activity was accompanied by doming and rifting northnortheast trending normal faults define two crustal segments exhibiting variously sized 1 km to 17 km intrusive complexes these comprise eroded undersaturated ring complexes with occasional carbonatites in the west and high level phonolitic plugs with shallow intrusive nepheline syenites in the east new rb sr isotopic data indicate 100 ma of within plate magmatic activity in the area without any obvious age progression in addition to the adrar tadhak complex 262 7 ma the following intrusions have been dated tirkine 215 11 ma anezrouf 184 14 ma and tidjerazraze in imanal 161 5 ma a clear break thus exists with the cambrian alkaline granite province of the iforas located on the opposite side of the pan african suture 87sr 86sr initial ratios are similar throughout between 0 7044 and 0 7049 which indicates a bulk earth isotopic composition the conclusion of exclusively mantle origin dupal composition previously demonstrated for the adrar tadhak complex can now be extended to the other complexes this implies repeated tapping of a single mantle source throughout the life of the province the described magmatic and tectonic permo jurassic activity has been triggered by intraplate stresses focussed along the pre existing pan african suture zone together with similar features in europe and africa the permo jurassic tadhak alkaline province heralds the progressive break up of pangaea and the opening of the atlantic ocean

initial field observations on qaanaaq ice cap northwestern greenland to study the glaciological processes controlling the mass budget of greenland s peripheral glaciers and ice caps field measurements were carried out on qaanaaq ice cap a 20 km long ice cap in northwestern greenland in the summer of 2012 we measured surface melt rate ice flow velocity and ice thickness along a survey route spanning the ice margin 200 m a s l to the ice cap summit 1110 m a s l melt rates in the ablation area were clearly influenced by dark materials covering the ice surface where degree day factors varied from 5 44 mm w e k 1 d 1 on a clean surface to 8 26 mm w e k 1 d 1 in the dark regions ice velocity showed diurnal variations indicating the presence of surface meltwater induced basal sliding mean ice thickness along the survey route was 120 m with a maximum thickness of 165 m ice velocity and temperature fields were computed using a thermomechanically coupled numerical glacier model modelled ice temperature obtained by imposing estimated annual mean air temperature as the surface boundary condition was substantially lower than implied by the observed ice velocity this result suggests that the ice dynamics and thermodynamics of the ice cap are significantly influenced by heat transfer from meltwater and changing ice geometry

late quaternary glacial history of lake hazen basin and eastern hazen plateau northern ellesmere island nunavut canada the glacial history of the broad interior of northeastern ellesmere island is first documented here studies of glacial geomorphology and marine and lacustrine sedimentology indicate that the regio

sensitivity of barnes ice cap baffin island canada to climate state and internal dynamics barnes ice cap is a remnant of the laurentide ice sheet which covered much of northern north america during the last glacial maximum barnes reached a quasi equilibrium state 2000 years ago and has remained similar in size since then with a small increase during the little ice age in this study we combine historical observations 1960 1980 with more recent satellite and airborne data 1995 2010 to drive a mass balance model coupled to a transient thermomechanical model with an adaptive mesh geometry the model is used to characterize the current state of the ice cap and to investigate its stability as a function of climate and its own internal dynamics on millennial time scales we show that ice flow is influenced by adjustment of an unsteady shape by gently sloping bedrock and by contrasting viscosities between the pleistocene and holocene ice on shorter time scales barnes is affected by surge activity sensitivity tests reveal that barnes experienced climate conditions which enabled its stability 2000 to 3000 years ago but will disappear under current climate conditions in the next millennium

localized basal motion of a polythermal arctic glacier mccall glacier alaska usa we analyzed the ice flow of mccall glacier alaska usa by numerical glacier modeling and radio echo sounding res model experiments were carried out with a higher order numerical ice flow model and results were validated with measurements of annual ice velocities and compared with previous estimates of ice flow dynamics during the 2003 summer campaign detailed res measure ments were carried out along the central flowline of the ablation area with a 5 mhz central frequency ice penetrating radar where 10 m ice temperatures are approximately 7 5 c the bed reflection power brp beneath this central flowline abruptly increases at one location area followed by a slow decrease down glacier the model experiments show that basal sliding 50 is necessary to match the observed annual mean surface velocities in the area that is characterized by high brp values however when thermomechanical effects are taken into account a temperate basal ice layer is apparent in the ablation area which locally softens the ice and can explain to a certain extent the anomalous flow field the model results confirm that the present temperature field is a remnant of a larger glacier geometry that was near steady state before the onset of enhanced surface thinning in the 1970s

three fe ti oxide ore bearing gabbro granitoid complexes in the panxi region of the permian emeishan large igneous province sw china the permian 260 ma emeishan large igneous province of sw china contains three nearly identical gabbro granitoid complexes that host giant fe ti oxide deposits the fe ti oxide deposits are within the lower portions of evolved layered gabbroic intrusions and are spatially and temporally associated with a type granitic plutons the 264 3 ma taihe layered gabbroic intrusion hosts a large magmatic fe ti oxide deposit and is coeval with the taihe peralkaline a type granitic pluton which is dated at 261 2 ma within the a type granitic pluton are microgranular enclaves which have compositions intermediate between the gabbro and host granite primitive mantle normalized incompatible element plots show corresponding reciprocal patterns between the mafic and felsic rocks the chondrite normalized ree patterns show eu anomalies changing from positive eu eu 1 5 to 5 9 in the gabbroic intrusion to negative in the enclaves eu eu 0 4 to 0 6 and granites eu eu 0 2 to 0 5 whole rock end t values of the gabbroic intrusion end t 2 5 to 3 3 are similar to those of the enclaves end t 1 0 to 2 0 and granite end t 1 5 to 1 9 whereas the zircon ehf t values of the gabbro ehf t 8 1 0 8 are indistinguishable from those of the granites ehf t 9 2 1 0 suggesting that the parent magmas for all rock types originated from the same mantle source geochemical modeling indicates that the gabbros and granites can be generated by fractional crystallization of a common parental magma similar to high ti emeishan flood basalt the compositional jump from the gabbro to the enclaves is attributed to the crystallization of fe ti oxide minerals the results of this study and other studies suggest that the magmatic conditions for example pressure composition fo2 which led to the formation of at least three fe ti oxide bearing gabbro granitoid complexes were relatively common during the development of the emeishan large igneous province

factors influencing the basal temperatures of a high arctic polythermal glacier a number of glaciers in the canadian high arctic are composed primarily of cold ice but the ice at or near their beds reaches the pressure melting point pmp in the ablation zone past modelling studies have suggested that the basal temperatures of some of these glaciers reach the pmp where they should not indicating that they are not in thermal equilibrium with present day surface air temperatures to investigate the possible reasons for thermal disequilibria in such glaciers a two dimensional ice temperature model was used to simulate the inferred thermal characteristics of john evans glacier ellesmere island results indicate that while surface refreezing and historical ice thickness changes have had a warming effect upon basal ice temperatures supraglacial meltwater reaching the glacier bed provides the single most critical heat source for explaining the apparent thermal disequilibrium between present day inferred ice bed temperatures and those modelled under present day boundary conditions

discriminating glacier thermal and dynamic regimes in the sedimentary record abstract this paper provides a description and evaluation of the sedimentary facies and environments associated with a range of glacier thermal and dynamic regimes with additional consideration given to the tectonic context new and previously published data are evaluated together and are presented from modern terrestrial and marine glacial sedimentary environments in order to identify a set of criteria that can be used to discriminate between different glacier thermal regimes and dynamic styles in the sedimentary record sedimentological data are presented from a total of 28 glaciers in 11 geographical areas that represent a wide range of contemporary thermal dynamic and topographic regimes in the context of landsystems representatives from terrestrial environments include temperate glaciers in the european alps patagonia new zealand the cordillera blanca peru cold glaciers in the dry valleys of antarctica and the antarctic peninsula region and polythermal valley glaciers in svalbard northern sweden the yukon and the khumbu himal nepal the glaciomarine environment is illustrated by data from cold and polythermal glacier margins on the east antarctic continental shelf and from a polythermal tidewater glacier in svalbard along with general observations from temperate glaciers in alaska these data show that temperate glacial systems particularly in high relief areas are dominated by rockfall and avalanche processes although sediments are largely reworked by glaciofluvial processes debris in polythermal glaciers is both thermally and topographically influenced in areas of moderate relief debris is mainly of basal glacial origin and the resulting facies association is dominated by diamicton in high relief areas such as the himalaya the debris load in polythermal glaciers is dominated by rockfall and avalanche inputs resulting in extensive accumulations of sandy boulder gravel cold glaciers are dominated by basal debris entrainment but sediments are little modified from the source materials which are typically sandy boulder gravel from older till and sand from glaciofluvial glaciolacustrine and aeolian sources similar facies associations but with different facies geometry and thickness occur in equivalent glaciomarine settings application of these concepts can aid the interpretation of glacier thermal regime and hence palaeoclimate in quaternary and ancient glacial systems

origin of the cameroon line of volcano capped swells abstract swells of the cameroon line a 1000 km long line of 10 volcano capped swells resemble other volcano capped topographic and bathymetric swells on the african plate however individual swells of the cameroon line are about 10 times smaller in area and distinct in being arranged in a straight line half on the continent and half on the ocean floor these peculiarities are here interpreted as related to an underlying mantle plume the 711 plume at lat 7 n long 11 5 e and to the location of this plume area on the bisector of a right angled bend in the continental margin i suggest that circumstances that led to the development of the cameroon line included 1 the stress field in the neighborhood of the right angled bend in the continental margin which favored extension normal to the bisector and 2 formation of a zone of extension aligned with the bisector that joined a point in the lithosphere over the 711 plume to the continental margin the right angled bend in the continental margin h

cold temperate transition surface and permafrost base cts pb as an environmental axis in glacier permafrost relationship based on research carried out on the storglaciaren and its forefield northern sweden here we present empirical ground penetrating radar gpr and electroresistivity tomography data ert to verify the cold temperate transition surface permafrost base cts pb axis theoretical model the data were collected from storglaciaren in tarfala northern sweden and its forefield the gpr results show a material relation between the glacial ice and the sediments incorporated in the glacier and a geophysical relation between the cold ice and the temperate ice layers clearly identifying lateral glacier margins is difficult as periglacial and glacial environments frequently overlap in this case we identified areas showing permafrost aggradation already under the glacier particularly where the cts is replaced by the pb surface this structure appears as a result of the influence of a cold climate over both the glacial and periglacial environments the results show how these surfaces form a specific continuous environmental axis thus both glacial and periglacial areas can be treated uniformly as a specific continuum in the geophysical sense similarly other examples previously described also allow identifying a continuation of permafrost from the periglacial environment onto the glacial base in addition the ert results show the presence of double layered periglacial permafrost possibly suggesting a past climatic fluctuation in the study area

improved discrimination of subglacial and periglacial erosion using 10be concentration measurements in subglacial and supraglacial sediment load of the bossons glacier mont blanc massif france deciphering the complex interplays between climate uplift and erosion is not straightforward and estimating present day erosion rates can provide useful insights glaciers are thought to be powerful erosional agents but most published glacial erosion rates combine periglacial subglacial and proglacial erosion processes within a glaciated catchment sediments found in subglacial streams originate either from glacial erosion of substratum or from the rock walls above the glacier that contribute to the supraglacial load terrestrial cosmogenic nuclides tcn are produced by interactions between cosmic ray particles and element targets at the surface of the earth but their concentration becomes negligible under 15 m of ice measuring tcn concentrations in quartz sand sampled in subglacial streams and in supraglacial channels is statistically compliant with stochastic processes e g rockfalls and may be used to discriminate subglacial and periglacial erosion results for two subglacial streams of the bossons glacier mont blanc massif france show that the proportion of sediments originating from glacially eroded bedrock is not constant it varies from 50 to 90 n 6 the difference between the two streams is probably linked to the presence or absence of supraglacial channels and sinkholes which are common features of alpine glaciers therefore most of the published mean catchment glacial erosion rates should not be directly interpreted as subglacial erosion rates in the case of catchments with efficient periglacial erosion and particularly rockfalls the proportion of sediments in the subglacial stream originating from the supraglacial load could be considerable and the subglacial erosion rate overestimated here we estimate warm based subglacial and periglacial erosion rates to be of the same order of magnitude 0 39 0 33 and 0 29 0 17 mm a 1 respectively

spatial pattern and stability of the cold surface layer of storglaciaren sweden the mechanisms controlling the spatial distribution and temporal fluctuations of the thermal structure in polythermal glaciers have to date been poorly investigated and are not fully understood