submarine landforms and shallow acoustic stratigraphy of a 400 km long fjord shelf slope transect kangerlussuaq margin east greenland kangerlussuaq fjord is a relatively uniform steep walled basin whose floor has an almost smooth surface debris is supplied mainly from icebergs from the fast flowing kangerlussuaq glacier sedimentation after iceberg release from multi year sea ice is mainly by rain out of fine grained englacial debris streamlined glacial lineations and drumlins were produced at the sedimentary bed of an ice sheet that expanded into kangerlussuaq trough at the last glacial maximum lgm bedrock channels and crescentic overdeepenings indicate warm based ice and free water beneath parts of the former ice sheet cross cutting iceberg scour marks which characterise outer kangerlussuaq shelf were produced not only during deglaciation but also occasionally through the holocene by deep keeled icebergs from further north in east greenland the outward convex contours of the shelf edge and slope beyond kangerlussuaq trough and debris flows on the slope suggest a glacier influenced high latitude fan the distribution of streamlined subglacial landforms demonstrates that the greenland ice sheet extended throughout kangerlussuaq fjord and reached at least 200 km across the shelf in kangerlussuaq trough at the lgm streamlined landform orientation indicates ice flow from the interior of greenland down the axis of kangerlussuaq trough there is little evidence for discrete sedimentary depocentres in the trough implying that ice probably retreated rapidly from the outer and mid shelf during deglaciation

late quaternary glacial interglacial changes in sediment composition at the east greenland continental margin and their paleoceanographic implications stable isotope records and sedimentological and organic geochemical investigations of marine sediments from the east greenland sea at 70 n provide important information about glacial interglacial variations of paleoenvironments through the last 225 kyr the oxygen isotope records established on the planktonic foraminifer n pachyderma sin show some excursions from the global climate pattern probably due to local regional overprint by meltwater supply the cold low saline east greenland current and fluctuations in sea ice covering were a crucial element controlling the carbonate production in the subsurface surface water column in the east greenland sea over the last 225 kyr the beginning of termination ia is ams 14c dated at about 15 8 kyr b p and interpreted as a greenland ice sheet meltwater signal the stage 2 3 boundary is dated at about 25 kyr b p the timing of the onset of the last deglacial meltwater event is about 800 years earlier than that of the barents shelf ice sheet meltwater signal recorded in the fram strait several major pulses of increased supply of coarse grained terrigenous material by glacio marine processes occurred during the last 225 kyr the supply of coarse grained ice rafted debris at the east greenland continental slope reached maximum values during the last glacial maximum stage 2 weichselian 15 19 kyr b p the drastic climatic change and the gradual retreat of continental ice masses glaciers during the last deglaciation termination i are clearly documented in the marine sedimentary sequences from shelf and upper slope environments this process resulted in distinctly decreased supply and deposition of ice rafted debris in the open shelf upper slope environments during termination i the sea ice cover also decreased causing an increase in surface water productivity indicated by increased organic carbon and biogenic opal deposition

structures and way up criteria in migmatites with application to the velay dome french massif central abstract a detailed structural analysis has been applied to the migmatite sequence of the southern limb of the velay dome france in an attempt to study the structural implications of the occurrence of metatexites and diatexites metatexites have a plastic behaviour presumably dominated by the solid skeleton which largely takes up the deformation in contrast diatexites have a predominantly viscous behaviour because the solid framework has been disrupted and they are dominated by the granitic melt rather than the solid component therefore the metatexite to diatexite transition has a major rheological and tectonic importance because it separates regions that record plastic stable deformation from regions that record viscous unstable deformation where gravitational instabilities may develop mesoscopic syn migmatitic way up criteria which assume that melt migration is partly driven by gravity give evidence for a southward overturning of the velay dome this overturning is supported by sedimentary and plutonic way up criteria seen in schists and granitic plutons respectively the n s asymmetry of the velay dome is related to late variscan asymmetric extension on a regional scale consistency of data provides further evidence that syn migmatization way up criteria are regionally reliable structural tools applicable to other migmatitic terrains

thickness and extent of the subglacial till layer beneath an antarctic paleo ice stream fast flowing ice streams and outlet glaciers currently account for as much as 90 of the discharge from the antarctic and greenland ice sheets although the deformation of subglacial material has been proposed as the mechanism for this rapid motion such sediment is usually hidden under several kilometers of ice marine geophysical records have allowed reconstruction of the three dimensional thickness of the sedimentary bed beneath a large antarctic paleo ice stream for the first time fast flow is indicated by streamlined seafloor lineations that form the surface of a layer of low shear strength unsorted sediment averaging 4 6 m thick rapid motion of the paleo ice stream was a result of subglacial deformation within this layer

the last glaciation and deglaciation of the northeast greenland continental shelf revealed by hydro acoustic data about 16 of the greenland ice sheet drains in the area of the northeast greenland shelf between 76 n and 80 5 n via marine terminating glaciers most of it is via the northeast greenland ice stream the largest ice stream of greenland during ice ages the ice sheet extended onto the continental shelf and modern day cross shelf troughs were filled by ice streams in this study high resolution hydro acoustic data acquired during three decades of research were jointly investigated to reveal the past glacial conditions our data shows that westwind trough and norske trough were filled by fast flowing ice streams that extended to the shelf edge during the last glacial maximum in between the cross shelf troughs ice domes resided on shallow banks that may have contributed about a decimetre to global sea level most probably these ice domes initiated fast ice flow through sinks in the inter trough area in westwind trough ice sheet retreat to the inner shelf after the last glacial maximum was intermittent in contrast in norske trough the ice sheet retreat appears relatively rapid with no evidences for phases of grounding line stabilization probably during the younger dryas the ice sheet readvanced to a mid shelf position in both troughs documented by grounding zone wedges during this time a thick ice shelf was present in norske trough releasing tabular icebergs ice sheet retreat from the mid shelf to the coastline during holocene deglaciation was rapid

the limitations of three dimensional kinematic vorticity analysis abstract the kinematic vorticity number wk can be calculated for three dimensional as well as two dimensional geologic deformations for steady state deformations wk can be correlated to and analyzed in terms of finite strains the analysis shows that assumptions commonly made for two dimensional deformations are not applicable to three dimensional deformations a single wk describes an infinite number of three dimensional deformations further even knowledge of flow apophyses orientation instantaneous stretching axes orientation and or wk are not sufficient to describe deformation three dimensional deformations also require knowledge of the deformation type or boundary conditions of deformation e g transpression hence in addition to being difficult to estimate the value of knowing wk for three dimensional deformations is greatly reduced compared with plane strain the most useful methods of determining wk from naturally deformed rocks are presented

pervasive granitoid magma transfer through the lower middle crust during non coaxial compressional deformation granitic magmas migrated through early proterozoic middle lower crust at mt hay central australia via a diverse network of narrow structurally controlled channelways during a period of progressive w sw directed thrusting d1a d1d they utilized existing folds boudins and shear zones or created new channels by magmatic fracture either parallel to layering or rarely in irregular arrays the magmas rose obliquely parallel to the plunging 50 60 regional elongation direction which was defined by coaxial folds boudin necks and a strong mineral elongation lineation megacrystic charnockitic magmas migrated through metre scale conduits during d1a d1b but leucosomes were generally restricted to smaller centimetre scale structures that existed throughout the entire deformation history thus d1a d1b leucosomes were potential feeders of in situ partial melts to the adjacent larger conduits of charnockite magma thereby providing a pervasive interconnected network that allowed efficient migration of all magma types during the early stages of thrusting the upper middle crust of the anmatjira reynolds range area contains abundant megacrystic granitoid sheets that are of similar age and geochemistry to those at mt hay they are considered to have formed as syntectonic intrusions emplaced during w sw directed thrusting as at mt hay suggesting that granitic magmas formed near the base of the continental crust passed through the mid lower crustal level 25 30 km exposed at mt hay and accumulated in batholithic proportions at shallower crustal levels 12 20 km such as the anmatjira reynolds range area the observations imply that granitoid magmas in the deep crust are capable of pervasive migration through the crust during major compressive noncoaxial shear deformation localization of magmas by sequentially developed narrow compressive structures suggests that dilatancy followed successive foliation forming events a situation that can occur during steady state deformation if the effective confining pressures are low which would be a result of high and possibly variable rates of magma influx the inferred rapid melt segregation and migration during deformation suggest that large chambers do not form until magma reaches neutral buoyancy in the middle to upper continental crust

low gradient outlet glaciers ice streams drained the laurentide ice sheet observations of geomorphic features dated with cosmogenic nuclides are used to determine the configuration of glaciation in cumberland sound baffin island during the last glacial maximum cumberland sound which was a major drainage of the laurentide ice sheet at its northeastern arctic canada margin contained low gradient ice with a surface slope of 0 038 and a driving stress between 2 and 7 kpa these values are similar to those observed for the ice streams that dominate the dynamics of the west antarctic ice sheet today the presence of low gradient ice in cumberland sound and other major outlets may similarly have influenced laurentide ice sheet behavior and would account for the iceberg discharge episodes that produced the heinrich event debris layers observed in north atlantic ocean sediments

thermal evolution and exhumation in obliquely convergent transpressive orogens most p t t path models to date have considered a linear erosion rate for exhumation from burial depth related to isostatic readjustment of crustal thickness a few have discussed extension enhanced exhumation erosional exhumation can only restore lower crustal rocks from the thickened mountain root to their previous original depth in the pre collisional crust one major assumption of all models to date is that the compressive forces responsible for crustal thickening cease before elevation and erosion begins however compression is often still active even if the crustal thickening has stopped further compression of the thickened and strongly deformed orogenic root is responsible for forceful exhumation extrusion of softened rocks upwards hence the rate of exhumation is related to the rate of convergence of colliding plates extrusional exhumation can elevate buried rocks to any depth depending on the action of fault shear systems in the extrusional exhumation models examined here the ascending rocks cool faster because they approach the zero temperature surface conditions more rapidly than by isostatic erosion the exhumation rate is also governed by the angle between the plate boundary and the displacement vector implying that the convergent plate boundaries are regarded as complex transpressive systems in which the degree of obliquity can be expressed by the ratio of pure to simple shear components a low ratio of pure simple shear typical for wrench dominated plate boundaries implies long distance horizontal transport from the buried position in the original orogen and longer periods during which metamorphic heating occurs a high ratio of pure simple shear characteristic for frontal like convergence implies a rapid exhumation without significant heating thus granulites barrovian type and blueschist facies metamorphism are characterised by increasing angle of obliquity 10 30 90 respectively the high temperature limit at low ratio of pure simple shear is the geotherm t and lies at temperatures that would commonly be taken to indicate that addition of mantle heat was required to generate hot geotherms and p t t paths wrench faulting over 100 s of kilometres at reasonable slow convergence rates could lead to extensive dehydration melting and granulite formation in large scale collisional orogens

experimental deformation of partially molten westerly granite under fluid absent conditions with implications for the extraction of granitic magmas the mechanical behavior of partially molten westerly granite was investigated in the temperature range 800 1100 c 250 mpa confining pressure by means of constant strain rate creep and stress relaxation tests the only water in the samples came from the breakdown of hydrous phases biotite minor chlorite and muscovite and alteration products of feldspars thus the amount of melt was controlled by the test temperature and ranged from 3 at 800 c to 50 at 1100 c over that temperature range strength decreased from 500 mpa to less than 1 mpa and a preliminary constitutive flow law for the partially molten rock was obtained to allow extrapolation to low strain rates the comparative viscosity of the melt alone was estimated at 950 and 1000 c from the distance it could be made to penetrate into a porous sand under a known pressure gradient under all conditions deformation of the matrix of solid grains was by brittle fracture only samples containing up to 10 vol melt failed with the formation of a shear fault zone at higher melt fractions melt filled pores collapsed by shear enhanced compaction squeezing the melt into axial cracks above 40 vol melt unfractured solid grains were carried about passively in the flowing liquid there was no evidence of a rheologically critical melt percentage in this system by analogy with the uniaxial compaction of water saturated soils a simple model is erected to describe a two stage process for the extraction of granitic melts from their protoliths with the aid of nonhydrostatic stress shear enhanced compaction is inferred to drive melt into a network of melt filled veins whereupon porous flow through the high permeability vein network allows rapid drainage of melt to higher crustal levels

strain partitioning during partial melting and crystallizing felsic magmas abstract the rheology of partly molten felsic systems changes dramatically during the onset of melting and through crystallization solid to liquid and liquid to solid transitions are not rheologically identical contradicting the concept of a unique rheological critical melt percentage we use four thresholds separating the successive stages according to the melt fraction for the melting transition 1 the liquid percolation threshold connects the melt pockets at about 8 of melt thus allowing local magma displacement at the metric scale and 2 for greater melt percentages 20 25 a melt escape threshold corresponds to the onset of transport over large distances of the melt and part of the residual solid phase leading ultimately to granitic bodies for the solidification transition 3 the first formation of a rigid framework of crystals which can accumulate stress although the melt can still flow corresponds to the rigid percolation threshold 55 solids and 4 the system becomes totally locked at about 72 75 solid which corresponds to the particle locking threshold the present paper focuses on strain partitioning induced by the coexistence of two contrasting rheological phases between these thresholds on a variety of scales non coaxial deformation is preferentially accommodated by the lower viscosity melt which increases the vorticity in the melt phase and reinforces asymmetries in the flow pattern strain partitioning also increases the scale length of the weakest phase during melting and magma ascent whereas strain partitioning decreases the scale length during crystallizing strain partitioning also affects the migration of melt by raising the particle locking threshold and lowering the melt escape threshold for three dimensional deformations such as transpression the direction of melt segregation e g horizontal may differ from the direction of melt migration e g vertical

the physiography of high arctic cross shelf troughs abstract a comprehensive inventory of high arctic cross shelf troughs is compiled from international bathymetric chart of the arctic ocean ibcao bathymetric data v 3 0 the location of 75 cross shelf troughs is presented alongside a synthesis of their key physiographic characteristics and available glacial geological evidence the troughs are interpreted to have been occupied intermittently and eroded by marine terminating ice streams that traversed the shelf during at least one and often many quaternary full glacial periods considerable variation in cross shelf trough physiography exists in the high arctic trough lengths range between 35 and 1400 km widths from 12 to 260 km and maximum depths from 200 to 1000 m the longest cross shelf troughs extend through inter island channels on the beaufort sea queen elizabeth islands and barents kara sea margins the gradient of the upper slope beyond high arctic glacial troughs which ranges between 0 3 and 13 is shown to have a negative relationship with palaeo ice stream drainage basin area and trough length glacial sedimentary depocentres or trough mouth fans tmfs are inferred on the basis of bathymetric and where available seismic evidence to exist beyond the majority of high arctic cross shelf troughs evidence of shelf progradation or tmf development is absent from the slope beyond several high arctic troughs probably as a result of limited sediment supply to the margin and or sediment by passing of the upper slope on the south greenland continental margin it is likely that steep slopes prevented the development of significant glacial sedimentary depocentres beyond cross shelf troughs or that glacigenic debris has been removed from the upper slope by submarine slope failure

bedform signature of a west antarctic palaeo ice stream reveals a multi temporal record of flow and substrate control abstract the presence of a complex bedform arrangement on the sea floor of the continental shelf in the western amundsen sea embayment west antarctica indicates a multi temporal record of flow related to the activity of one or more ice streams in the past mapping and division of the bedforms into distinct landform assemblages reveals their time transgressive history which implies that bedforms can neither be considered part of a single downflow continuum nor a direct proxy for palaeo ice velocity as suggested previously a main control on the bedform imprint is the geology of the shelf which is divided broadly between rough bedrock on the inner shelf and smooth dipping sedimentary strata on the middle to outer shelf inner shelf bedform variability is well preserved revealing information about local complex basal ice conditions meltwater flow and ice dynamics over time these details which are not apparent at the scale of regional morphological studies indicate that past ice streams flowed across the entire shelf at times and often had onset zones that lay within the interior of the antarctic ice sheet today in contrast highly elongated subglacial bedforms on sedimentary strata of the middle to outer shelf represent a timeslice snapshot of the last activity of ice stream flow and may be a truer representation of fast palaeo ice flow in these locations a revised model for ice streams on the shelf captures complicated multi temporal bedform patterns associated with an antarctic palaeo ice stream for the first time and confirms a strong substrate control on a major ice stream system that drained the west antarctic ice sheet during the late quaternary

spatial and temporal patterns of iceberg rafting ird along the east greenland margin ca 68 n over the last 14 cal ka twelve 1 2 m 10 cm diameter gravity cores collected in 1988 and 1991 from the continental shelf and fjords of east greenland near kangerlussuaq fjord trough ca 68 n 32 w have distinct changes in lithofacies and in the quantity of iceberg rafted ird sediments these changes are readily observed in x radiographs of the split cores we quantify the ird contribution through grain size analyses and counting the number of clasts 2 mm from the x radiographs chronological control is provided by acclerated mass spectroscopy 14c dates on foraminifera during deglaciation after 14 cal ka there was one interval of ird accumulation ca 12 13 cal ka followed by a brief return to ird conditions centred at 9 cal ka thereafter a prominent feature of most cores on the shelf is an increase in ird accumulation that started ca 5 6 cal ka and which has increased toward the present indicators of iceberg rafting such as the net sand flux and numbers of clasts 2 mm ka 1 follow a power law distribution when graphed against distance from the present east greenland coast a measure of the position of the glacier margins the form of the relationship indicates that there is a dramatic decrease in the supply of sediment from the fjords to the shelf these relationships are used to estimate changes in the location of the ice margin during the late quaternary based on a site on the east greenland slope denmark strait and to discuss factors that can negate such a simple transfer function 1997 by john wiley sons ltd

strain and fabric analyses based on porphyroclast interaction abstract two dimensional computer models of the interaction of elliptical clasts in a variety of kinematic conditions are used to study how clast imbrication and shape fabric development relate to strain and strain history the models provide templates for a known clast density and clast ellipticity that correlate clast imbrication angle of collision and clast shape fabric to strain and kinematics of deformed rocks modeling also shows that the mode of clast interaction plays a significant role in the accumulation of imbricated clasts and the development of shape fabric upon collision between clasts the trains formed can remain fixed or rotate if they rotate imbricated clasts can become independent when they are no longer forced together by the flow based upon the mode of clast and train rotation using the march rotation of passive lines and the jeffery rotation of rigid particles as end members three models are defined in an attempt to simulate the conditions of clast interaction under varying conditions the march fixed train march rotating train and jeffery rotating train models simulate solid state intermediate and magmatic conditions respectively modeling shows that the two march models produce similar results but the jeffery model of train rotation always produces a lower proportion of imbricated clasts and a much weaker shape fabric application of the modeling to the study of a syntectonic granite in the sierra nevada suggests that deformation was dominated by simple shear some k feldspar imbrication and fabric development took place initially in the magmatic stage and culminated in the solid state to produce a large proportion of imbricated k feldspar porphyroclasts which define a strong shape fabric this type of analysis provides a rationale to determine syn vs post emplacement deformation in the granite

late quaternary sedimentation in kejser franz joseph fjord and the continental margin of east greenland the marine sedimentary record in kejser franz joseph fjord and on the east greenland continental margin contains a history of late quaternary glaciation and sedimentation evidence suggests that a middle shelf moraine represents the maximum shelfward extent of the greenland ice sheet during the last glacial maximum on the upper slope coarse grained sediments are derived from the release of significant quantities of iceberg rafted debris ird and subsequent remobilization by subaqueous mass flows the middle lower slope is characterized by hemipelagic sedimentation with lower quantities of ird dropstone mud and sandy mud punctuated episodically by deposition of diamicton and graded sand gravel facies by subaqueous debris flows and turbidity currents derived from the mass failure of upper slope sediments the downslope decrease of ird reflects either the action of the east greenland current egc confining icebergs to the upper slope or to the more ice proximal setting of the upper slope relative to the lgm ice margin sediment gravity flows on the slope are likely to have fed into the east greenland channel system contributing to its formation in conjunction with the cascade of dense brines down the slope following sea ice formation across the shelf deglaciation commenced after 15 300 14c years as indicated by meltwater derived light oxygen isotope ratios an abrupt decrease in both ird deposition and delivery of coarse grained debris to the slope at this time supports ice recession with icebergs confined to the shelf by the egc glacier ice had abandoned the middle shelf before 13 000 14c years with ice loss through iceberg calving and deposition of diamicton continued retreat of glacier ice from the inner shelf and through the fjord is marked by a transition from subglacial till bedrock in acoustic records to ice proximal meltwater derived laminated mud to ice distal bioturbated mud ice abandoned the inner shelf before 9100 14c years and probably stabilized in fosters bugt at 10 000 14c years distinct oxygen isotope minima on the inner shelf indicate meltwater production during ice retreat the outer fjord was free of ice before 7440 14c years glacier retreat through the mid outer fjord was punctuated by topographically controlled stillstands where ice proximal sediment was fed into fjord basins the dominance of fine grained commonly laminated facies during deglaciation supports ablation controlled ice mass loss glacimarine sedimentation within the holocene middle outer fjord system is dominated by sediment gravity flow and suspension settling from meltwater plumes suspension sediments comprise mainly mud facies indicating significant meltwater deposition that overwhelms debris release from icebergs in this east greenland fjord system the relatively widespread occurrence of fine grained lithofacies in east greenland fjords suggests that meltwater sedimentation can be significant in polar glacimarine environments the ice distal continental margin is characterized by meltwater sedimentation in the inner shelf deep iceberg scouring over shallow shelf regions winnowing and erosion by the east greenland current on the middle outer shelf and hemipelagic sedimentation on the continental slope

ice stream retreat following the lgm and onset of the west greenland current in uummannaq trough west greenland the deglacial history and oceanography of uummannaq trough central west greenland continental shelf was investigated using foraminiferal sedimentological and bathymetric records together with a radiocarbon chronology providing a timeline for the retreat of glacial ice after the last glacial maximum lgm to map ice stream retreat data were collected from cores from the outer jr175 vc45 and jr175 vc43 and inner jr175 vc42 uummannaq trough a large ice stream fed by confluent glaciers draining the interior of the greenland ice sheet extended across the outer shelf during the lgm and was in retreat by 15 0 cal kyr bp foraminiferal data indicate that the warm west greenland current wgc was established prior to 14 0 cal kyr bp which is the hitherto earliest record of atlantic water found on the west greenland shelf for each of the cores foraminifera indicate that ice sheet retreat was followed quickly by incursion of the wgc suggesting that the warm water may have enhanced ice retreat prior to the younger dryas cold event the radiocarbon chronology indicates that the ice sheet retreated to the mid shelf where it subsequently stabilised and formed a large grounding zone wedge gzw after the younger dryas around 11 5 cal kyr bp the ice retreated rapidly from the gzw and into the fjords

greenland ice sheet retreat history in the northeast baffin bay based on high resolution bathymetry new swath bathymetric data acquired in 2010 and 2015 indicate a variety of glacial landforms in cross shelf troughs of the melville bay northeast baffin bay these landforms reveal that at their maximum extent ice streams in the troughs crossed the shelf all the way to the shelf edge moraines grounding zone wedges gzws and subglacial till lobes on the continental shelf define a pattern of variable ice stream retreat in the individual troughs on the outer shelf in the northern cross shelf trough ice stream retreat was slow compared to more episodic retreat in the central at least one stabilization on the outer shelf and southern cross shelf trough re advances at the shelf edge and fast retreat thereafter large gzws on the mid to inner shelf of the troughs indicate periods of grounding zone stabilization according to glacial landforms the final retreat across the inner shelf before 8 41 ka bp was episodic to slow furthermore evidence has been found for localized ice domes with minor ice streams on inter trough banks the glacial landforms in melville bay thus indicate the varying and discontinuous ice sheet retreat history across the northwest greenland continental shelf

bedmachine v3 complete bed topography and ocean bathymetry mapping of greenland from multibeam echo sounding combined with mass conservation greenland s bed topography is a primary control on ice flow grounding line migration calving dynamics and subglacial drainage moreover fjord bathymetry regulates the penetration of warm atlantic water aw that rapidly melts and undercuts greenland s marine terminating glaciers here we present a new compilation of greenland bed topography that assimilates seafloor bathymetry and ice thickness data through a mass conservation approach a new 150 m horizontal resolution bed topography bathymetric map of greenland is constructed with seamless transitions at the ice ocean interface yielding major improvements over previous data sets particularly in the marine terminating sectors of northwest and southeast greenland our map reveals that the total sea level potential of the greenland ice sheet is 7 42 0 05 m which is 7 cm greater than previous estimates furthermore it explains recent calving front response of numerous outlet glaciers and reveals new pathways by which aw can access glaciers with marine based basins thereby highlighting sectors of greenland that are most vulnerable to future oceanic forcing

shear criteria in granite and migmatite deformed in the magmatic and solid states microstructural criteria for the determination of the sense of shear in rocks homogeneously deformed in the partially melted state are similar to those which apply to solid state deformation sense of shear determination is either direct deduced from the sense of rotation of markers or indirect involving the obliquity between the shear and foliation planes or between the successive foliations imprinted at different stages of progressive deformation this study is a by product of the detailed structural and microstructural investigation of a high grade metamorphic rock pile variscan vosges massif france which underwent subhorizontal shearing during partial melting and further solidification depending on the rock chemistry on the position in the pile and the relative timing of progressive deformation layered migmatites and homogeneous granites were variously deformed in the partially melted and solid states the sense of shear obtained from these rock types using the criteria presented here consistently gives a top to sw direction

formation of mega scale glacial lineations observed beneath a west antarctic ice stream most discharge from large ice sheets takes place through fast flowing ice streams and their speed is strongly modulated by interactions between the ice and the underlying sediments seismic surveys and investigations through boreholes have revealed a spatial association between fast ice flow and saturated deformable sediments nevertheless our knowledge of the morphology of the interface between ice and sediments is still limited resulting in only rudimentary understanding of the basal boundary conditions beneath ice streams and the generation of subglacial bedforms here we present radar data from the bed of a west antarctic ice stream that reveal the presence of mega scale glacial lineations we combine these data with previously published seismic data and show that these lineations develop in areas of dilatant deforming till and are part of a dynamic sedimentary system that undergoes significant change by erosion and deposition on decadal timescales we find that the mega scale glacial lineations are indistinguishable from those found on beds of palaeo ice streams providing conclusive evidence for the hypothesis that highly elongate bedforms are a characteristic of fast flow regions in ice sheets

relative sea level change kjove land scoresby sund east greenland implications for seasonality in younger dryas time the age of the milne land stade mls moraines in the scoresby sund region of east greenland is key for testing the hypothesis that strong seasonality marked abrupt cooling events such as the younger dryas registered in greenland ice cores the relevant chronology is based on 69 radiocarbon dates of shells from raised beaches and deltas related to marine inundation and isostatic rebound that accompanied glacier retreat from the moraines taken together these dates form the basis for a relative sea level curve that shows very high rates of emergence typical of recently deglaciated regions upward extrapolation of this curve suggests that the marine limit 134 m a s l dates to about 12 400 cal yr b p the shorelines that mark the marine limit lie within areas that were ice covered when the glaciers were at the outer limit of the mls advance hence the moraines that form the outer limit must antedate 12 400 cal yr b p and probably are from earliest younger dryas or allerod time at holger danskes briller an inner moraine grades to a massive ice contact delta now at 101 m a s l which is dated to 11 000 11 300 cal yr b p the age of the outer mls moraines along with constraints on the maximum possible younger dryas ice extent and snowline lowering are consistent with the idea that east greenland climate experienced strong seasonality during younger dryas time

be 10 ages from central east greenland constrain the extent of the greenland ice sheet during the last glacial maximum traditional ice sheet reconstructions have suggested two distinctly different ice sheet regimes along the east greenland continental margin during the last glacial maximum lgm ice to the shelf break south of scoresby sund and ice extending no further than to the inner shelf at and north of scoresby sund we report new 10be ages from erratic boulders perched at 250 m a s l on the kap brewster peninsula at the mouth of scoresby sund the average 10be ages calculated with an assumed maximum erosion rate of 1 cm ka and no erosion respectively 17 3 2 3 ka and 15 1 1 7 ka overlap with a period of increased sediment input to the scoresby sund fan 19 15 ka the results presented here suggest that ice reached at least 250 m a s l at the mouth of scoresby sund during the lgm and add to a growing body of evidence indicating that lgm ice extended onto the outer shelf in northeast greenland

the influence of deformation on the formation of axial planar leucosomes and the segregation of small melt bodies within the migmatitic napperby gneiss central australia in the napperby gneiss central australia a distinctive group of elongate leucosomes occurs along the axial planes of mesoscopic crenulations that have harmonic two dimensional kink band like geometries the leucosomes truncate a fabric defined by biotite quartz k feldspar plagioclase sillimanite and are chiefly composed of k feldspar and quartz with or without plagioclase they are characterized by accessory garnet or aggregates of intergrown ilmenite and magnetite produced during the melting reactions bi plag q sill g ksp melt and bi plag q ru ilm mt ksp melt the absence of garnet and intergrown ilmenitemagnetite aggregates in the gneiss surrounding the crenulation zones indicates that these melting reactions preferentially occurred in the deformation zones in spite of a homogeneous distribution of reactant phases in the gneiss this means that strain partitioning and partial melting are coupled processes in which the garnet and ilmenite magnetite producing melt reactions initially occurred in high strain zones where the melting reaction activation energy was first overstepped this was a consequence of an additional strain induced matrix free energy component on top of the temperature effect once reaction product nucleation had occurred the deformation zones became the sites for continued melting which was energetically favoured over nucleating garnet and ilmenite magnetite elsewhere the melt reactions producing garnet and ilmenite magnetite resulted in the formation of grain supported leucosomes with a higher melt component than the surrounding gneiss and the leucosomes were therefore relatively incompetent in a differential stress field diffusional processes will cause elongation of the leucosome in a plane normal to 1 parallel to the axial planes of folds if sufficient melting has occurred an interconnected leucosome network will result creating a potentially important buoyancy driven melt drainage system in the napperby gneiss interconnected leucosome bearing crenulation zones are mostly vertical which is an ideal orientation for buoyancy driven diffusive melt migration however melt segregations remained small due to a melt impermeable lithological boundary overlying the gneiss

experimental and theoretical constraints on melt distribution in crustal sources the effect of crystalline anisotropy on melt interconnectivity in partially molten systems the equilibrium distribution of melt at the grain scale is governed by the principle of interfacial energy minimization in ideal sources i e partially molten rocks that are monomineralic have single valued solid liquid and solid solid interfacial energies and are subject to hydrostatic stress the wetting angle is known to be a unique characteristic which specifies the melt configuration for a given melt fraction crustal rocks cannot be modelled as ideal sources because of their polymineralic nature the moderate to high anisotropy of interfacial energies which characterizes common refractory minerals and the possible presence of a crystallographic preferred orientation that partially molten crustal rocks depart from ideal sources is documented by a series of high p high t experiments illustrating the textural relationships of biotite and amphibole with silicic melts the melt distributions observed in these experiments differ significantly from those expected in ideal sources 1 crystal melt interfaces are commonly planar rational faces rather than smoothly curved irrational surfaces and 2 the concept of a unique wetting angle does not hold as shown in the biotite silicic melt system these textural features demonstrate that anisotropy of crystal melt interfacial energy is a factor of primary importance in modelling the grain scale distribution of partial melts the petrological implications of our study are the following 1 1 at high degrees of anisotropy and low melt fractions melt is predicted to form isolated plane faced pockets at grain corners the overall shape of these pockets and therefore the value of the connectivity threshold c are expected to be very sensitive to the ratio of solid solid to solid liquid interfacial energies ss sl c is the melt fraction at which melt interconnectivity is established melt pockets with low volume to surface ratio and low but non constant wetting angles should prevail at high ss sl resulting in very low values of c 1 to a few vol higher values of c a high volume to surface ratio of melt pockets and high wetting angles are expected at low ss sl 2 2 the wetting angle at hornblende hornblende melt junctions at 1200 mpa 975 c is 25 a review of existing data indicates that quartz melt and feldspar melt wetting angles are also low to moderate 12 60 a very low value of c should therefore be the general rule during crustal anatexis in particular a connectivity threshold lower than 3 4 vol is predicted for partially molten amphibolite 3 3 in biotite rich rock types such as melanosomes in migmatites the combination of a pronounced crystalline anisotropy and a marked preferred orientation of mica flakes leads to a very low permeability normal to layering biotite rich melanosomes should therefore impede chemical interactions between neighbouring leucosomes and mesosomes

holocene evolution of the western greenland ice sheet assessing geophysical ice sheet models with geological reconstructions of ice margin change abstract geophysical ice sheet models are used to predict future ice sheet dimensions and in turn these projections help estimate the magnitude of eustatic sea level rise before models can confidently predict ice sheet behavior they must be validated by being able to duplicate the geological record of ice sheet change here we review geological records of greenland ice sheet gris change with emphasis on the warmer than present middle holocene and compare these records to published studies that numerically simulate gris behavior through the holocene geological records are concentrated in west and southwest greenland which are also the regions where the gris margin likely experienced the greatest distance of inland retreat during the middle holocene several records spanning from melville bugt to jakobshavn isfjord in western greenland indicate the gris achieved its minimum extent between 5 and 3 ka and farther south in the kangerlussuaq region new data presented here indicate the ice margin reached its minimum extent between 4 2 and 1 8 ka in the narsarsuaq region in southern greenland the gris likely achieved its minimum configuration between 7 and 4 ka we highlight key similarities and discrepancies between these reconstructions and model results and finally we suggest that despite some degree of inland retreat the west and southwest gris margin remained relatively stable and close to its current position through the holocene thermal maximum

shear zone controlled magma emplacement or magma assisted nucleation of shear zones insights from northeast brazil abstract in the borborema province of northeast brazil neoproterozoic granitoids and large scale transcurrent shear zones are spatially associated suggesting a genetic link between magma bodies and shear zones in some cases magma emplacement was clearly favored by shear zone activity but for several plutons this model is not satisfactory in these plutons pre full crystallization strike slip deformation evidenced by parallelism of magmatic foliations and lineations with the solid state mylonitic fabric and by a transition from magmatic to solid state flow is restricted to the vicinity of the shear zones evidence of shear zone activity prior to magma emplacement is lacking and the magmatic foliation away from the shear zones is in most cases shallowly dipping and concordant with the slightly older gently dipping regional gneissic foliation field and anisotropy of magnetic susceptibility mapping together with petrographic and geochemical studies performed in one of the magmatic complexes of the borborema province have revealed a structure and a magmatic fabric incompatible with the shear zone controlled emplacement model away from the shear zones this complex has retained a stratification inherited from the mixing of crystal poor magmas of contrasting composition and a magmatic fabric characterized by low to moderate dip magmatic foliations bearing a nw trending lineation which contrast with vertical foliations bearing ne or e w trending stretching lineations in the shear zones and indicates that crystallization started prior to shear zone development based on evidence that magma emplacement predated strike slip shearing and on information about the transition from magmatic to solid state deformation observed in the studied plutons we suggest that incompletely solidified plutons within the crust represent rheological heterogeneities that may induce strain localization and favor shear zone nucleation we propose that in the studied cases deformation first concentrated within the plutons where shear zones nucleated and grew subsequently as the solid phase component increased the crystal mush began to behave as a solid and the shear zones propagated into the country rocks

the role of strain localization in the segregation and ascent of anatectic melts namaqualand south africa granulite facies gneisses of the late proterozoic okiep copper district of the namaqua province in south africa preserve evidence of a range of anatectic melt features that reflect the initial stages of segregation and ascent of crustally derived magmas during high grade metamorphism these melt bodies include both in situ and sharply transgressive subvertical pipe like bodies that show vertical dimensions of several hundreds of metres and horizontal dimensions of tens to hundreds of metres migmatite bodies are spatially closely linked with narrow upright zones characterized by intense high strain fabrics locally referred to as steep structures they display a progressive textural development from diktyonitic textures in in situ bodies via schollen and raft textures to largely homogeneous intrusive granites that have migrated vertically for distances of several hundreds of metres or more the intimate association of the migmatite bodies with the steep structures reflects strain induced melt segregation facilitated by increased permeabilities in these ductile deformation zones and migration of melts into sites of strain incompatibility at the intersections between the regional subhorizontal gneissosity and the superimposed high strain zones subsequent melt migration was focused along the network of subvertical structural anisotropies provided by the steep structures melt migration was controlled by a combination of buoyancy shear enhanced melt compaction during ongoing deformation melt compaction due to the settling of wall rock fragments from higher stratigraphiclevels and subordinate brittle fracturing the unusual geometry of the steep structures and the intensely heterogeneous nature of the strain and the absence of similar strain features and voluminous melt bodies elsewhere in the granulite facies terrane suggest a positive feed back mechanism between melt generation and strain localization in steep structures the structural development of the migmatite bodies illustrates that the efficiency of melt segregation and migration in a mid crustal segment is dependent not only on the fertility of its lithologies but also on its deformational style

mechanisms of pluton emplacement and structural evolution of a 2 1 ga juvenile continental crust the birimian of southwestern niger abstract the 2 1 ga birimian terranes of southwestern niger consist of a granite greenstone association the structures of the greenstone belts result from the interference between pluton emplacement and regional transcurrent deformation sinistral n s strike slip faults ne sw trending schistosity the regional cleavage trend is observed to change in the vicinity of elliptical plutons cleavage trajectories bend around their contacts resulting in triple junctions at the pluton extremities and narrow aureoles of intense penetrative schistosity and high grade amphibolitefacies metamorphism are developed around them structural features of plutons suggest syntectonic emplacement and age data indicate that all plutons were emplaced between 2 16 and 2 10 ga the plutons are assembled in two distinct types of batholith elongated either parallel or perpendicular to the inferred regional shortening direction a batholith parallel to the inferred regional shortening direction dolbel batholith consists of km sized subcircular alkali granite plutons except for the southernmost one which is of a larger size and exhibits an elliptical shape with a nw se short axis parallel to the shortening direction crosscutting relationships indicate a progressively northward emplacement trend for those plutons internal fabrics generally correspond to layering and magmatic flow fabrics and accessorily to schistosity and shear zones which represent late stage plastic deformation restricted to the pluton margin batholiths which are perpendicular to the inferred regional shortening direction are composed of large elliptical plutons 500 to 5000 km 2 in area consisting mainly of granodiorite and covering 70 of the region the plutons never show crosscutting relationships but are moulded against each other their internal fabric corresponds to a solid state foliation it either dips towards the centre of the pluton funnel or teardrop shaped intrusions or is subhorizontal in the centre and vertical at the margin dome shaped intrusions the foliation and the geometry of the largest pluton tera pluton were acquired during pluton ascent and inflation whereas the geometry of later shear zones indicates that deformation in the pluton was then passively acquired at the final emplacement level due to regional deformation the contrast in both the orientation and structural features of the batholiths reflects distinct emplacement mechanisms the small alkali granite plutons resulted from magma injection within nw se fractures parallel to the inferred regional shortening direction whereas the granodiorite plutons are assumed to be emplaced as diapirs forceful intrusions with long axes perpendicular to the shortening direction these two intrusion mechanisms for plutons emplaced during the same deformation event and at the same crustal level are considered to reflect a local change in the rheological behaviour of the crustal segment studied in connection with the thermal effect induced by the large granodioritic plutons this vertical tectonic regime appears to be typical of palaeoproterozoic juvenile continental crust areas and related to an interplay between emplacement of tremendous volumes of granodioritic magma and a regional transcurrent deformation

ice sheet grounding line dynamics steady states stability and hysteresis 1 the ice sheet ice shelf transition zone plays an important role in controlling marine ice sheet dynamics as it determines the rate at which ice flows out of the grounded part of the ice sheet together with accumulation this outflow is the main control on the mass balance of the grounded sheet in this paper we verify the results of a boundary layer theory for ice flux in the transition zone against numerical solutions that are able to resolve the transition zone very close agreement is obtained and grid refinement in the transition zone is identified as a critical component in obtaining reliable numerical results the boundary layer theory confirms that ice flux through the grounding line in a two dimensional sheet shelf system increases sharply with ice thickness at the grounding line this result is then applied to the large scale dynamics of a marine ice sheet our principal results are that 1 marine ice sheets do not exhibit neutral equilibrium but have well defined discrete equilibrium profiles 2 steady grounding lines cannot be stable on reverse bed slopes and 3 marine ice sheets with overdeepened beds can undergo hysteresis under variations in sea level accumulation rate basal slipperiness and ice viscosity this hysteretic behavior can in principle explain the retreat of the west antarctic ice sheet following the last glacial maximum and may play a role in the dynamics of heinrich events

chronology of the last recession of the greenland ice sheet a new deglaciation chronology for the ice free parts of greenland the continental shelf and eastern ellesmere island canada is proposed the chronology is based on a new compilation of all published radiocarbon dates from greenland and includes crucial new material from southern northeastern and northwestern greenland although each date provides only a minimum age for the local deglaciation some of the dates come from species that indicate ice proximal glaciomarine conditions and thus may be connected with the actual ice recession in addition to shell dates dates from marine algae lake sediments peat terrestrial plants and driftwood also are included only offshore and in the far south have secure late glacial sediments been found other previous reports of late glacial sediments older than 11 5 cal kyr bp from onshore parts of greenland need to be confirmed most of the present ice free parts of greenland and nares strait between greenland and ellesmere island were not deglaciated until the early holocene copyright c 2002 john wiley sons ltd less

freshwater forcing from the greenland ice sheet during the younger dryas evidence from southeastern greenland shelf cores abstract we present evidence for freshwater forcing from the greenland ice sheet during the last deglaciation explore its potential involvement in the younger dryas cooling and investigate the role of atlantic intermediate water in the retreat of the greenland ice sheet paleoceanographic conditions and ice sheet retreat history are interpreted from ird stable isotopes and foraminiferal assemblages in sediment cores retrieved along the length of the kangerlussuaq trough a cross shelf trough on the southeast greenland shelf the vedde ash 11 980 cal yr bp which is a marker bed within the younger dryas and the saksunarvatn tephra 10 180 cal yr bp an early holocene marker horizon have been located in these cores the tephra markers and calibrated 14c dates on foraminifers and molluscs provide the basis for the age models in the cores the stable isotope records indicate a 0 5 depletion of the 18o between 13 4 and 12 8 cal ka bp a pronounced light isotope event exceeding 1 5 on the inner shelf and 0 8 on the outer shelf occurs at the beginning of the younger dryas cold period 12 8 cal ka bp and ends by 12 cal ka bp immediately before deposition of the vedde ash the light isotope spikes are interpreted as glacial meltwater spikes associated with retreat of the adjacent greenland ice sheet associated with the light isotope events are episodic high percentages of the atlantic intermediate water benthic foraminiferal species cassidulina neoteretis which continue from the bolling allerod into the younger dryas high percentages of this species suggest that atlantic intermediate water entered the kangerlussuaq trough during the allerod and the younger dryas via the irminger current as it does today we conclude that advection of atlantic intermediate water into the kangerlussuaq trough south of the denmark strait promoted melting and retreat of the marine portions of the greenland ice sheet explaining the observed glacial meltwater spikes

antarctic palaeo ice streams we review the geomorphological sedimentological and chronological evidence for palaeo ice streams on the continental shelf of antarctica and use this information to investigate basal conditions and processes and to identify factors controlling grounding line retreat a comprehensive circum antarctic inventory of known palaeo ice streams their basal characteristics and minimum ages for their retreat following the last glacial maximum lgm is also provided antarctic palaeo ice streams are identified by a set of diagnostic landforms that nonetheless display considerable spatial variability due to the influence of substrate flow velocity and subglacial processes during the lgm palaeo ice streams extended via bathymetric troughs to the shelf edge of the antarctic peninsula and west antarctica and typically to the mid outer shelf of east antarctica the retreat history of the antarctic ice sheet since the lgm is characterised by considerable asynchroneity with individual ice streams exhibiting different retreat histories this variability allows antarctic palaeo ice streams to be classified into discrete retreat styles and the controls on grounding line retreat to be investigated such analysis highlights the important impact of internal factors on ice stream dynamics such as bed characteristics and slope and drainage basin size whilst grounding line retreat may be triggered and to some extent paced by external atmospheric and oceanic forcing the individual characteristics of each ice stream will modulate the precise timing and rate of retreat through time

an extensive and dynamic ice sheet on the west greenland shelf during the last glacial cycle considerable uncertainty surrounds the extent and timing of the advance and retreat of the greenland ice sheet gis on the continental shelf bordering baffin bay during the last glacial cycle here we use marine geophysical and geological data to show that fast flowing ice sheet outlets including the ancestral jakobshavn isbrae expanded several hundred kilometers to the shelf edge during the last glaciation ca 20 ka retreat of these outlets was asynchronous initial retreat from the shelf edge was underway by 14 880 calibrated cal yr b p in uummannaq trough radiocarbon dates from the adjacent disko trough and adjoining trough mouth fan imply later deglaciation of jakobshavn isbrae and significantly an extensive readvance and rapid retreat of this outlet during the younger dryas stadial yd this is notable because it is the first evidence of a major advance of the gis during the yd on the west greenland shelf although the short duration suggests that it may have been out of phase with yd temperatures

the origin of massive diamicton facies by iceberg rafting and scouring scoresby sund east greenland almost 90 of 39 m of core material recovered from scoresby sund and the adjacent east greenland shelf is massive diamicton interpreted to be formed predominantly by the release of iceberg rafted debris and reworking by iceberg scouring there is also likely to be a contribution from suspension settling of fines derived from glaciofluvial sources model calculations suggest that the 14c derived holocene sedimentation rate of 0 1 0 3 m 1000 yr 1 in scoresby sund can be accounted for mainly by iceberg rafting of debris a further 4 of core material is of gravel or coarse sand lenses interpreted to reflect iceberg dumping of debris intensive iceberg scouring which reworks sea floor sediments is observed on acoustic records from over 30 000 km2 of the scoresby sund fiord system and the adjacent east greenland shelf 69 72 n and 75 n the rate of iceberg production from greenland ice sheet outlet glaciers and iceberg drift tracks on the shelf suggests that iceberg rafting and scouring may be important over a significant proportion of the 500 000 km2 area above the shelf break the relatively extensive modern occurrence of massive diamicton formed by iceberg rafting and scouring together with suspension settling of fines suggests that it may also be a significant facies in the glacier influenced geological record the recognition in the geological record of the massive diamicton facies described above may also indicate the former presence of fast flowing ice sheet outlet glaciers

paleo ocean redox environments of the upper ordovician wufeng and the first member in lower silurian longmaxi formations in the jiaoshiba area sichuan basin a study of paleo ocean redox environments is important for understanding the deposition of black shale and has practical implications for shale gas exploration here we selected a total of 52 shal

history of a stable ice margin east greenland during the middle and upper pleistocene abstract the east greenland record of glaciation and environmental change is based on a combination of onshore and offshore studies glaciological work on contemporary outlet glaciers provides background for interpreting the glaciation styles of the past the scoresby sund fjord system the single largest outlet of the inland ice in east greenland and the adjacent continental shelf and slope were the main working areas the regionality of these results was tested with onshore and offshore work on sites further to the north in the east greenland fjord zone the record spans the period since isotope stage 7 ca 240 ka the most extensive glaciation phases were the lollandselv and scoresby sund glaciations isotope stages 7 and 6 the langelandselv interglaciation isotope substage 5e comprises shallow marine sediments at more than 20 sites onshore and cibicides wuellerstorfii carrying sediments on the continental slope summer temperatures were 2 3 warmer than the holocene optimum and there was vigorous advection of warm atlantic water into the fjords the warm period seems to be accompanied by a eustatic sea level rise of ca 20 m during the weichselian there were three major glacier advances the aucellaelv jyllandselv and flakkerhuk stades isotope substages 5d 5b and stages 3 2 all three advances mark a shift from one topographically determined stable position to another in each phase the glaciers occupied almost the same space in the fjord basins and had their fronts on the inner shelf but the glacial regime and duration was different the aucellaelv stade lasted ca 10 ka and the regime resembled that of present day outlet glaciers 600 km further to the north the flakkerhuk stade lasted for ca 50 ka but left only thin and sporadic till as well as glaciolacustrine sediments in ice dammed river valleys the regime was polar similar to present day glaciers in antarctica its culmination the last glacial maximum in the period between 22 and 14 ka is shown by a maximum in ird deposition on the continental slope after 12 ka and before 10 ka the outer fjord basins in the southern fjord zone had been abandoned by the glaciers and at ca 9 5 ka the last resurgence the milne land stade had ended

partitioning of non coaxiality in deforming layered rock masses abstract partitioning of non coaxiality and other flow parameters in deforming layered rock masses with nearly incompressible newtonian rheology is examined in two dimensions by the finite element method the results show that layers with viscosity contrast in suitable orientation deform by different and even by a reverse sense of non coaxiality this remarkable partitioning of non coaxiality is due to a large rotation rate of instantaneous stretching axes in a heterogeneous body as a result of geometry changing through deformation rather than through rotation of material lines

the permeability controlled accumulation of primary magma in a planetary mantle abstract the initial accumulation of magma within a partially molten planetary mantle occurs in three stages within a mantle with evenly dispersed minerals the first melt is formed as small drops at grain corners and edges by increasing degree of partial melting the drops of interstitial liquid become interconnected whereafter the partially molten mantle becomes permeable at the onset of permeability the interstitial melt begins to accumulate in horizontal layers of melt situated tens of metres apart these layers form the source of magma for the magma chambers that supply magma during eruptions

the mortagne granite pluton france emplaced by pull apart along a shear zone structural and gravimetric arguments and regional implication the emplacement mechanism and kinematics and the 3 d geometry of the variscan granite pluton of mortagne are interpreted using systematic measurements of the granite structure allied with micro structural observations from a detailed gravimetric study it is proposed that magma infilled a pull apart structure which formed along a jog of a northwest southeast trending sinistral zone of shear a second episode of shearing under a dextral regime and independent of emplacement was responsible for perigranitic tectonites these successive transcurrent opposite movements already documented for variscan times in western france are discussed concerning the age of the mortagne pluton 313 15 m y

timing and significance of glacially influenced mass wasting in the submarine channels of the greenland basin the most extensive systems of submarine channels on the continental margins of the norwegian greenland sea occur in the greenland basin 72 75jn geophysical records show that these channels are up to 100 m deep 4 km wide and extend for about 300 km from the middle continental slope off northeast greenland to the abyssal depths of the basin mass wasting deposits in the form of debris flows and turbidity currents as well as hemipelagic sediments dominate sediment cores recovered from the channels radiocarbon dates indicate that debris flow and turbidity current activity in the channels had ceased by 13 000 years bp and sedimentation rates show a corresponding order of magnitude decrease after this time masswasting in the submarine channels of the greenland basin therefore relates to full glacial and deglacial conditions this implies a more extensive late weichselian ice sheet in northeast greenland than traditionally thought the ice sheet extended onto the outer continental shelf and may have reached the shelf edge delivering debris and sediment laden meltwater onto the upper slope channel formation is most likely the product of turbidity current activity during successive glaciations of the northeast greenland continental shelf holocene sediments from the channel systems are predominantly of hemipelagic origin and formed in a low energy ice distal environment the low gradient 1j channels of the greenland basin are therefore a glacially influenced sedimentary system contrasts in slope morphology and sediment architecture between the greenland basin and trough mouth fans elsewhere in the norwegian greenland sea are not the result of variations in slope gradient rather these contrasts reflect the interplay between bedrock lithology of the continental shelf rates of sediment delivery sediment texture meltwater volume and relative size of ice sheet drainage basins

irregular glacial interstadials recorded in a new greenland ice core the greenland ice sheet offers the most favourable conditions in the northern hemisphere for obtaining high resolution continuous time series of climate related parameters profiles of 18o 16o ratio along three previous deep greenland ice cores1 3 seemed to reveal irregular but well defined episodes of relatively mild climate conditions interstadials during the mid and late parts of the last glaciation but there has been some doubt as to whether the shifts in oxygen isotope ratio were genuine representations of changes in climate rather than artefacts due to disturbed stratification here we present results from a new deep ice core drilled at the summit of the greenland ice sheet where the depositional environ ment and the flow pattern of the ice are close to ideal for core recovery and analysis the results reproduce the previous findings to such a degree that the existence of the interstadial episodes can no longer be in doubt according to a preliminary timescale based on stratigraphic studies the interstadials lasted from 500 to 2 000 years and their irregular occurrence suggests complexity in the behaviour of the north atlantic ocean circulation

the deformation matrix for simultaneous simple shearing pure shearing and volume change and its application to transpression transtension tectonics simultaneous simple shearing and pure shearing with or without additional volume change can be combined into a single upper triangular deformation matrix the off diagonal term is named the effective shear strain and is a simple function of the pure shearing and simple shearing components a three dimensional deformation matrix for the simultaneous combination of coaxial deformation with or without additional volume change and up to three simple shearing systems with mutually orthogonal shear planes is also presented by using this matrix one can easily extract the various properties of incremental as well as finite strain and the progressive as well as finite rotation of passive markers during deformation the case of transpression transtension is revised using the unified deformation matrix the orientation of the major axis of the strain ellipsoid 1 is always horizontal if the deformation is transtensional switches from horizontal to vertical during transpressional wrenching 1 wk 0 81 for constant vorticity deformations and is always vertical for highly transpressional deformations wk 0 81 for transpression material lines initially rotate towards the horizontal shearing direction but generally turn to rotate towards the vertical axis after a certain strain for transtension all material lines rotate towards a direction in the horizontal plane which is oblique to the shearing direction

iceberg scouring in scoresby sund and on the east greenland continental shelf modern and relict scours produced by iceberg keels have been reported from a number of high latitude continental shelves in the northern hemisphere and antarctica but observations are largely absent from the east greenland shelf about 18 km3 yr 1 of icebergs are calved into the scoresby sund fjord system east greenland these icebergs have submarine keels with drafts up to 550 m modern iceberg contacts with the sea floor are inferred from calculated keel depths and stationary icebergs identified on sequential landsat imagery acoustic profiles parasound 3 5 khz of the sea floor show that scouring is a significant process in water depths 550 m iceberg scouring intensity varies inversely with water depth the most intense scouring occurs at depths of 300 400 m iceberg scours were observed on over 25 000 km2 of the east greenland shelf at 69 72 n and 75 n scours are being actively produced on the modern shelf but relict scours probably dating back to the late weichselian are also present the rate of iceberg production from greenland ice sheet outlet glaciers and iceberg drift tracks on the shelf suggests that iceberg scouring is important over a significant proportion of the 500 000 km2 area above the shelf break around greenland

chemical exchange in migmatites during cooling abstract the chemistry of the leucosome components in limited size migmatitic associations is generally believed to reflect the composition of the partial melt that could have been extracted to yield an anatectic magma several well documented studies of migmatites indicate that the distributions of some trace elements in leucosome melanosome pairs are inconsistent with equilibrium partial melting but are rather in agreement with chemical reequilibration during cooling in migmatites the liquid fraction does not leave the system this may induce an alteration of the chemical composition of the whole migmatite system by extensive chemical exchange between the liquid and the solid residue during cooling if this is the case the leucosome and melanosome components are not representative of the chemical composition of the liquid and solid residue respectively at the time of partial melting a rough evaluation of the residences time of the melt fractions in cooling in situ migmatitic bodies yields minimum estimates of 1 to 10 ma for such time spans the characteristics distances of diffusion distances of diffusion induced chemical transfer exceed the sizes of individual mineral grains which is consistent with a re equilibration process the release of latent heat of crystallization in cooling migmatitic systems and the presence of an interstitial liquid phase are critical in determining the extent and scale of chemical exchange the mechanism involves 1 reducing the cooling rate and 2 promoting the efficiency of chemical diffusion through the liquid phase thus some doubt is cast on the ability of small scale migmatitic systems to provide relevant information about chemical mass balance during partial crustal melting

a 10be chronology of lateglacial and holocene mountain glaciation in the scoresby sund region east greenland implications for seasonality during lateglacial time abstract thirty eight new cosmogenic 10 be exposure ages from the scoresby sund region of east greenland indicate that prominent moraine sets deposited by mountain glaciers date from 780 to 310 yr approximately during the little ice age from 11 660 to 10 630 yr at the end of the younger dryas cold interval or during preboreal time and from 13 010 to 11 630 yr during lateglacial time equilibrium line altitudes elas interpreted from lateglacial to early holocene moraines indicate summertime cooling between 3 9 and 6 6 c relative to today s value much less than the extreme younger dryas cooling registered by greenland ice cores mean annual temperatures of 15 c colder than today s value this apparent discrepancy between paleotemperature records supports the contention that younger dryas cooling was primarily a wintertime phenomenon 10 be ages of lateglacial and holocene moraines show that mountain glaciers during the little ice age were more extensive than at any other time since the early holocene epoch in addition 10 be ages of lateglacial moraines show extensive reworking of boulders with cosmogenic nuclides inherited from prior periods of exposure consistent with our geomorphic observations and cosmogenic exposure dating studies in other arctic regions

ice sheet grounding zone wedges gzws on high latitude continental margins abstract grounding zone wedges gzws are asymmetric sedimentary depocentres which form through the rapid accumulation of glacigenic debris along a line source at the grounding zone of marine terminating ice sheets during still stands in ice sheet retreat gzws form largely through the delivery of deforming subglacial sediments the presence of gzws in the geological record indicates an episodic style of ice retreat punctuated by still stands in grounding zone position moraine ridges and ice proximal fans may also build up at the grounding zone during still stands of the ice margin but these require either considerable vertical accommodation space or sediment derived from point sourced subglacial meltwater streams by contrast gzws form mainly where floating ice shelves constrain vertical accommodation space immediately beyond the grounding zone an inventory of gzws is compiled from available studies of bathymetric and acoustic data from high latitude continental margins the locations and dimensions of gzws from the arctic and antarctic alongside a synthesis of their key architectural and geomorphic characteristics are presented gzws are only observed within cross shelf troughs and major fjord systems which are the former locations of ice streams and fast flowing outlet glaciers typical high latitude gzws are less than 15 km in along flow direction and 15 to 100 m thick gzws possess a transparent to chaotic acoustic character which reflects the delivery of diamictic subglacial debris many gzws contain seaward dipping reflections which indicate sediment progradation and wedge growth through continued delivery of basal sediments gzw formation is inferred to require high rates of sediment delivery to a fast flowing ice margin that is relatively stable for probably decades to centuries although the long term stability of the grounding zone is controlled by ice sheet mass balance the precise location of any still stands is influenced strongly by the geometry of the continental shelf the majority of high latitude gzws occur at vertical or lateral pinning points which encourage grounding zone stabilisation through increasing basal and lateral drag and reducing mass flow across the grounding zone

deglaciation chronology sea level changes and environmental changes from holocene lake sediments of germania havn so sabine o northeast greenland abstract germania havn so is located at the outermost coast of northeastern greenland according to radiocarbon dating the lake basin was deglaciated in the early holocene around 11 000 cal yr bp at that time the lake was a marine bay but the lake was isolated soon after deglaciation at 10 600 cal yr bp the marine fauna was species poor indicating harsh conditions with a high sedimentation rate and lowered salinity due to glacial meltwater supply the pioneer vegetation around the lake was dominated by mosses and herbs deposition of relatively coarse sediments during the early holocene indicates erosion of the newly deglaciated terrain remains of the first woody plant salix herbacea appear at 7600 cal yr bp and remains of other woody plants salix arctica dryas octopetala cassiope tetragona and empetrum nigrum appear around one millennium later declining concentrations of d octopetala and the caddis fly apatania zonella in the late holocene probably imply falling summer temperatures only moderate changes in the granulometric and geochemical record during the holocene indicate relatively stable environmental settings in the lake which can probably be explained by its location at the outer coast and the buffering effect of the neighboring ocean

holocene glacimarine sedimentation inner scoresby sund east greenland the influence of fast flowing ice sheet outlet glaciers abstract holocene glacimarine sedimentation associated with fast flowing outlet glaciers draining the greenland ice sheet is investigated using sedimentary and acoustic data from inner scoresby sund east greenland sedimentation in inner scoresby sund is dominated by three processes which are influenced by differences in proximity to fast flowing outlet glaciers extent of glacier ice cover and fjord bathymetry 1 sediment gravity flow principally in the form of turbidity currents and debris flows 2 suspension sedimentation from turbid meltwater plumes and 3 iceberg rafting these processes result in texturally and sedimentologically heterogeneous lithofacies proportionally fine grained muds laminated stratified and massive facies dominate cores recovered from inner scoresby sund accounting for 80 of the total whereas diamict facies account for only 15 abundant fine grained muds demonstrate that meltwater flux and sedimentation is significant in this high arctic glacimarine environment in settings proximal to fast flowing outlet glaciers with increasing distance from these glacier termini muds are replaced progressively by iceberg rafted coarse grained sediment the dominance of this iceberg rafted sediment in outer scoresby sund reflects both its more distal location from fast flowing glacier termini and the high calving flux associated with these ice masses laminated muds deposited by turbidity currents and suspension settling from overflow plumes in inner scoresby sund are similar to lithofacies produced in temperate and subpolar glacimarine systems this implies a similarity in sedimentation processes and resulting facies across a wide spectrum of climatically glaciologically fast flowing and non fast flowing ice masses and oceanographically variable glacimarine settings recognition of laminated fine grained facies in the geological record therefore does not necessarily indicate a temperate palaeo glacial setting however the predominance of iceberg rafted diamict facies in ice distal sedimentary records suggests the former presence of relatively cold environmental conditions in which iceberg sedimentation played a dominant role

deglaciation of a major palaeo ice stream in disko trough west greenland recent work has confirmed that grounded ice reached the shelf break in central west greenland during the last glacial maximum lgm here we use a combination of marine sediment core data including glacimarine lithofacies and ird proxy records and geomorphological and acoustic facies evidence to examine the nature of and controls on the retreat of a major outlet of the western sector of the greenland ice sheet gris across the shelf retreat of this outlet which contained the ancestral jakobshavns isbrae ice stream from the outer shelf in disko trough was rapid and progressed predominantly through iceberg calving however minor pauses in retreat tens of years occurred on the middle shelf at a trough narrowing forming subtle grounding zone wedges by 12 1 cal kyr bp ice had retreated to a basalt escarpment and shallow banks on the inner continental shelf where it was pinned and stabilised for at least 100 years during this time the ice margin appears to have formed a calving bay over the trough and melting became an important mechanism of ice mass loss fine grained sediments muds were deposited alternately with ird rich sediments diamictons forming a characteristic deglacial lithofacies that may be related to seasonal climatic cycles high influxes of subglacial meltwater emanating from the nearby ice margins deposited muddy sediments during the warmer summer months whereas winters were dominated by iceberg calving leading to the deposition of the diamictons this is the first example of this glacimarine lithofacies from a continental shelf setting and we suggest that the calving bay configuration of the ice margin plus the switching between calving and melting as ablation mechanisms facilitated its deposition by channelling meltwater and icebergs through the inner trough the occurrence of a major stillstand on the inner shelf in disko trough demonstrates that the ice dynamical response to local topography was a crucial control on the behaviour of a major outlet in this sector of the gris during retreat

the partitioning of deformation in flowing rock masses abstract most geological structures owe their development to heterogeneous and or non steady flow flow partitioning refers to the division of the instantaneous velocity field into components related to translation strain and rotation flow partitioning during the development of a geological structure usually changes with time and different histories of flow are involved at different points in the developing structure heterogeneity leads to the development of flow domains with different characteristics notably the degree of non coaxiality of deformation and different partitionings of the rotational component of flow into shear induced vorticity and spin during non steady flow there is often continual repartitioning of vorticity between the spin and shear induced components spin is an important aspect of flow leading to the development of structure this paper discusses the partitioning of deformation in a flowing rock mass coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries non coaxial flow is favoured by suitably oriented material anisotropy and or the presence of well defined boundary constraints involving parallel relative displacements a developing anisotropy e g a crystallographic fabric in a shear zone can induce a change from coaxial to non coaxial flow other effects are related to the scale of approximation for example bulk shear induced vorticity can be locally converted to spin flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response the factors that affect flow partitioning e g spin versus non coaxial deformation are therefore of interest

warm water pathways toward nioghalvfjerdsfjorden glacier northeast greenland nioghalvfjerdsfjorden glacier 79ng is the largest of three marine terminating outlet glaciers draining the northeast greenland ice stream to understand how atlantic waters supply waters in the cavity beneath the floating 79ng we analyze historic and recent bathymetric hydrographic and velocity observations obtained on the northeast greenland continental shelf the bathymetry is characterized by a trough system consisting of the westwind trough and the norske trough in the northern and southern part of the continental shelf respectively atlantic waters recirculating in fram strait cross the shelf break and enter the trough system at its southeastern inlet toward the inner shelf warm atlantic intermediate water aiw present below 200 m in the norske trough shows large contributions of the recirculating atlantic water we found that the bathymetry is sufficiently deep to provide a direct subsurface pathway for warm aiw between the shelf break and the 79ng cavity via the norske trough likewise based on the hydrographic data we show that the norske trough supplies aiw warmer than 1 c to the 79ng which is not present in the westwind trough our moored and lowered velocity measurements indicate that a boundary current carries warm aiw along the northeastern slope of norske trough toward the 79ng we suggest that anomalies in atlantic water temperatures in fram strait could reach 79ng within less than 1 5 years thereby modifying the glacier s basal melt rates

the role of seasonality in abrupt climate change abstract a case is made that seasonality switches dominated by wintertime were instrumental in abrupt climate changes in the north atlantic region during the last glaciation and into the holocene the primary evidence comes from mismatches between mean annual temperatures from greenland ice cores in comparison with snowline changes in east greenland northern europe and north america the most likely explanation is a shutdown or reduction in strength of the conveyor this allows the spread of winter sea ice across the north atlantic thus causing the northern region to experience much colder winters because they mimic the greenland temperature rather than the snowline signal changes in the atlantic intertropical convergence zone and the asian monsoon may also share a winter linkage with greenland thus the paleoclimate record is consistent with the notion that a huge continental sector of the northern hemisphere stretching from greenland to asia was close to an extreme winter threshold during much of the last glaciation winter climate crossed this threshold repeatedly with marked changes in seasonality that may well have amplified and propagated a signal of abrupt change throughout the hemisphere and into the tropics

mega scale glacial lineations and cross cutting ice flow landforms landsat images reveal a previously unsuspected large scale pattern of streamlining within drift that is assumed to reflect former phases of ice flow such a glacial grain can be regarded as a landform assemblage comprised of a number of components drumlins and megaflutes form part of the pattern but in addition there are two previously undocumented ice moulded landform elements streamlined lineations of much greater proportions referred to as mega scale glacial lineations and a distinctive cross cutting topology within the grain the ice moulded landform assemblage is described and illustrated with reference to examples from canada possible modes of genesis of such landforms are discussed and their glaciological implications outlined the discovery of this pattern indicates the pervasive nature of subglacial deformation of sediment and demands a radical re interpretation of ice sheet dynamics

evidence for the asynchronous retreat of large outlet glaciers in southeast greenland at the end of the last glaciation recent rapid changes in the marine terminating sectors of the greenland ice sheet gris have prompted concerns about the future stability of the ice sheet long term records of ice sheet behaviour provide valuable context to assess the magnitude of current change and may help resolve the mechanisms driving deglaciation we report 23 new 10be exposure ages which constrain the deglacial history of two large fjord systems in southeast se greenland we compare our chronologies with existing data from the centre of the sector to examine the timing and style of deglaciation at a regional scale glacial erratic 10be exposure ages demonstrate that kangerdlugssuaq fjord deglaciated at 11 8 ka at the end of the younger dryas yd 12 8 11 6 ka retreat at kangerdlugssuaq fjord coincided with known incursion of the warm irminger current ic onto the continental shelf this is inferred to have initiated retreat comparison with recently published results from sermilik fjord and new 10be ages from bernstorffs fjord indicates deglaciation occurred 1 ka later in the south of the se region sermilik fjord 10 9 ka and bernstorffs fjord 10 4 ka deglaciated later retreat likely occurred in response to dramatic climatic amelioration at the termination of the yd stadial we suggest that the disparate timing of deglaciation across the se region may be primarily explained by the varying influence of the warm ic glaciers in southern se greenland were isolated from warm atlantic waters during the yd by complex shelf bathymetry in all fjord settings ice retreat was rapid and persistent consistent with the absence of geomorphological evidence for stillstand or readvance events ice retreat was accompanied by rapid thinning and likely continued to well within present day ice sheet margins glacial erratic 10be age determinations and geomorphological observations show no evidence for holocene readvance events prior to the little ice age lia

migmatite structures in the central gneiss complex boca de quadra alaska migmatite structures in the coast plutonic metamorphic complex are well exposed in the inlet of boca de quadra southeast alaska two types of anatectic migmatites are present patch migmatites formed by in situ melting and subsequent crystallization of melt diktyonitic migmatites comprise a discontinuous veined network of leucocratic material in which leucosomes enclose boudins of host rock the margins of these boudins show the development of both melanosomes and shear band fabrics strain analysis of diktyonitic melanosomes indicates that these regions have undergone volume decreases of 20 27 this volume decrease is attributed to melt extraction into the adjacent fracture filling leucosomes thus diktyonitic migmatites formed by shear induced segregation of partial melt whereas in patch migmatites the lack of shear stresses inhibited melt segregation the variable structural style of anatectic migmatites in boca de quadra is not related to host rock composition but may be due to differences in the amount of differential stress during migmatization these in turn may be controlled by host rock strength and or diachroneity of migmatization and deformation determination of volume changes during migmatization using strain analysis is potentially capable of discriminating intrusive and anatectic migmatites and consequently of documenting melt segregation and subsequent migration across crustal levels

substantial export of suspended sediment to the global oceans from glacial erosion in greenland approximately 8 of the fluvial suspended sediment exported to the world s oceans comes from rivers draining the greenland ice sheet according to an analysis of satellite imagery furthermore the export is dominated by areas where subglacial erosion is high

effect of mechanical interactions on the development of shape preferred orientations a two dimensional experimental approach abstract modifications in the development of preferred orientations of rigid particles due to mechanical interactions between particles are studied experimentally in two dimensions experiments in both pure shear and simple shear were performed and the results analysed using an easy and fast new automatic method for increased concentration of rigid particles the rotation of individual particles may be slowed down or even stopped due to collisions or disturbance of the flow in the matrix caused by neighbours in coaxial flow the fabric intensity is consequently reduced in simple shear flow the fabric evolution is no longer cyclic the intensity is on average weaker and the fabric axis rotates asymptotically toward the shear plane in the case of preferred orientations of particles having a low aspect ratio 2 5 the fabric rotates through the shear plane then undergoes a reverse rotation and rotates toward the shear plane again in the light of these experimental results we emphasize that quantification of finite strain based on such fabrics may lead to significant underestimates and calculated values should be taken as minimum estimates the kinematic significance of concentrated rigid particle preferred orientations in rocks is modified by interactions between particles techniques for qualitative or quantitative kinematic analysis may become unreliable for simple shear flows the concentrated population fabrics show an angular behaviour close to that of passive markers they tend to align parallel to the shear plane in simple shear flow