an experimental study of the relationship between joint spacing and layer thickness abstract two methods for measuring joint spacing are described and compared the area method is a constant for a given outcrop area and is not affected by joint distribution within that area in contrast the line method depends on the location of the linear traverse two kinds of joint sets are distinguished on bedding surfaces 1 a poorly developed set represents the early stages of development when typical joint lengths are less than typical spacing 2 a well developed set represents later stages when lengths are much greater than spacing the area method is applicable to both poorly and well developed sets whereas the line method produces inconsistent results for poorly developed joint sets surface textures on many joints indicate point fracture origins and propagation parallel to bedding whereas most numerical model studies of spacing assume linear origins and propagation perpendicular to bedding the laboratory experiments described in this paper do not suffer from these restrictions an important concept confirmed during these experiments is fracture saturation when the applied strain reaches a certain value fracture spacing stops evolving and remains nearly constant these fracture sets are well developed spacing at saturation is a function of layer thickness but is independent of strain whereas spacing before saturation varies strongly with applied strain thus plotting spacing vs thickness and comparing the slopes of lines fit to such data for poorly developed joint sets in different layers is unlikely to be a diagnostic test for differences in material properties on the other hand spacing may be a sensitive indicator of strain for layers with poorly developed joint sets assessing fracture saturation is a lirst order consideration when gathering spacing data

changes in ice dynamics and mass balance of the antarctic ice sheet the concept that the antarctic ice sheet changes with eternal slowness has been challenged by recent observations from satellites pronounced regional warming in the antarctic peninsula triggered ice shelf collapse which led to a 10 fold increase in glacier flow and rapid ice sheet retreat this chain of events illustrated the vulnerability of ice shelves to climate warming and their buffering role on the mass balance of antarctica in west antarctica the pine island bay sector is draining far more ice into the ocean than is stored upstream from snow accumulation this sector could raise sea level by 1m and trigger widespread retreat of ice in west antarctica pine island glacier accelerated 38 since 1975 and most of the speed up took place over the last decade its neighbour thwaites glacier is widening up and may double its width when its weakened eastern ice shelf breaks up widespread acceleration in this sector may be caused by glacier ungrounding from ice shelf melting by an ocean that has recently warmed by 0 3 degrees c in contrast glaciers buffered from oceanic change by large ice shelves have only small contributions to sea level in east antarctica many glaciers are close to a state of mass balance but sectors grounded well below sea level such as cook ice shelf ninnis mertz frost and totten glaciers are thinning and losing mass hence east antarctica is not immune to changes

rapid disintegration of the wordie ice shelf in response to atmospheric warming the breaking up of ice shelves around the antarctic peninsula has been cited1 as a sign that a dangerous warming is beginning in antarctica here we present satellite images showing the disintegration of the wordie ice shelf which lies off the west coast of the antarctic peninsula fig 1 fracture either in the form of surface crevasses or rifts extending to the bottom of the ice shelf has been responsible for iceberg calving and weakening the central region of the ice shelf these fracture processes which led to retreat of the ice front were apparently enhanced by the presence of increased amounts of melt water resulting from a warming trend recorded in mean annual air temperatures in marguerite bay if this warming trend continues other nearby ice shelves on the antarctic peninsula may be at risk but substantial additional warming would be required before similar processes could initiate breakup of the ross and filchner ronne ice shelves which help stabilize the west antarctic ice sheet

fringe cracks and plumose structures in layered rocks stepping senses and their implications for palaeostress interpretation abstract many joint surfaces show en echelon arrays of microfractures fringe cracks f joints twist hackles along fringe zones in continuity with plumose structures in layered rocks joints with two fringe zones showing opposite stepping senses at the top and bottom type a are more frequent than those showing a single stepping sense type p consistent with such an arrangement their curved advancing fronts show continuous 180 radial patterns of steps showing a spiral staircase geometry in double plumose structures diverging from an initiation point different combinations of symmetry elements allow us to define four basic stepping types type pa single stepping sense on the full joint surface type aa opposite stepping senses at both fringes type pp opposite stepping senses at both sides of the starting point type ap four alternate quadrants with opposite stepping most of these types have been observed in limestone beds of the mequinenza area ebro basin spain and glamorgan coast south wales according to our observations the varied stepping arrangements are independent of changes in orientation within the parent joint the latter showing more persistent strikes than fringe cracks all these features suggest that fringe crack arrays in continuity with plumose structures are related to local stress redistribution at the advancing front and as a general rule do not record remote stresses after careful observation and discrimination only those en echelon cracks consistently stepped and showing no geometric continuity with plume barbs should be included in palaeostress analysis

ice dammed lateral lake and epishelf lake insights into holocene dynamics of marguerite trough ice stream and george vi ice shelf alexander island antarctic peninsula abstract we present new data regarding the past dynamics of marguerite trough ice stream george vi ice shelf and valley glaciers from ablation point massif on alexander island antarctic peninsula this ice free oasis preserves a geological record of ice stream lateral moraines ice dammed lakes ice shelf moraines and valley glacier moraines which we dated using cosmogenic nuclide ages we provide one of the first detailed sediment landform assemblage descriptions of epishelf lake shorelines marguerite trough ice stream imprinted lateral moraines against eastern alexander island at 120 m at ablation point massif during deglaciation lateral lakes formed in the ablation and moutonnee valleys dammed against the ice stream in george vi sound exposure ages from boulders on these shorelines yielded ages of 13 9 to 9 7 ka following recession of the ice stream george vi ice shelf formed in george vi sound an epishelf lake formed at 15 20 m asl in ablation and moutonnee valleys dated from 9 4 to 4 6 ka suggesting that the lake was stable and persistent for some 5000 years lake level lowering occurred after this with the lake level at 12 m at 3 1 0 4 ka and at 5 m asl today a readvance of the valley glaciers on alexander island at 4 4 0 7 ka is recorded by valley glacier moraines overlying epishelf lake sediments we speculate that the glacier readvance which occurred during a period of warmth may have been caused by a dynamic response of the glaciers to a lowering in surface elevation of george vi ice shelf

closely spaced fractures in layered rocks initiation mechanism and propagation kinematics spacings of opening mode fractures joints and veins in layered sedimentary rocks often scale with the layer thickness field observations reveal that the ratio of fracture spacing to the thickness of the fractured layer s tf ranges from less than 0 1 to greater than 10 there is a critical spacing to layer thickness ratio that defines the condition of fracture saturation and explains the observed spacing ratios between 0 8 and 1 2 values of s tf 1 2 are explained as the results of the fracturing process having not reached the saturation level to explain ratios of s tf 0 8 we study the possibility for further fracture infilling by considering flaw distributions between adjacent fractures loaded by extension of the layer results show that infilling fractures grow more easily from flaws located near the interface than from those in the middle of the fractured layer the propagation of a flaw located in the middle of the fractured layer is unstable but for the flaw to propagate toward the interfaces its height has to be greater than a critical size this critical size decreases with increasing s tf the propagation behavior of a flaw with one of its tips at the interface depends on s tf the propagation is unstable when s tf is greater than a critical value when s tf is less than this critical value the propagation is first unstable then stable and then unstable again an infilling fracture can cut through the fractured layer only if s tf is greater than another critical value otherwise the infilling fracture can only partially cut the fractured layer for models with the same elastic constants for the fractured layer and the neighboring layers this critical value is 0 546 and the minimum spacing to layer thickness ratio of fractures formed by the in filling process under extension is 0 273

retreat of northern margins of george vi and wilkins ice shelves antarctic peninsula the george vi and wilkins ice shelves are considered at risk of disinteg ration due to a regional atmospheric warming trend on the antarctic peninsula retreat of the northern margin of the george vi ice shelf has been observed previously but the wilkins ice shelf was thought to be stable we investigated the positions of the northern fronts of these shelves from the literature and looked for changes on 1974 landsat and 1992 and 1995 european remote sensing satellite ers synthetic aperture radar images our investigation shows that the northern george vi ice shelf lost a total of 906 km 2 between 1974 and 1992 and an additional 87 km2 by 1995 the northern margin of the wilkins ice shelf lost 796 km2 between 1990 and 1992 and another 564 km2 between 1992 and 1995 armadas of tabular icebergs were visible in front of this shelf in the ers images these two ice shelves mark the southern most documented conspicuous retreat of ice shelf margins

honeycomb development on alexander island glacial history of george vi sound and palaeoclimatic implications two step cliffs mars oasis w antarctica abstract analysis of three generations of glacial deposits and of a range of geomorphic features including widespread honeycombs and tafonis at two step cliffs mars oasis 71 52 s 68 15 w provides new insights into the geomorphological evolution of west antarctica with special respect to alveolar weathering at two step terrace indicators of the inherited character of cavernous weathering were found such as 97 non flaking and varnished backwalls and 80 tafoni floors that are till covered and or sealed by lithobiontic coatings based on the ne predominant aspect of the alveolized boulder faces tafoni initiation is attributed to coastal salt spray weathering by halite coming from the george vi sound during the 6 5 ka bp open water period the present day activity of these inherited cavities is restricted to roof flaking attributed to a combination of processes involving thermal stresses this 6 5 ka bp phase of coastal alveolization is the first step of a six stage holocene geomorphological scenario that includes alternatively phases of glacial advance or stationing and phases of vegetal colonization and or rock weathering and aeolian abrasion on the deglaciated outcrops this geomorphic scenario is tentatively correlated with the available palaeoenvironmental record in the antarctic peninsula region with two potential geomorphic indicators of the holocene optimum being identified 1 clusters of centimetric honeycombs facing the sound marine optimum at 6 5 ka bp 2 salmon pink lithobiontic coatings preserved inside cavities and at the boulder surface terrestrial optimum at 4 3 ka bp

long melt seasons on ice shelves of the antarctic peninsula an analysis using satellite based microwave emission measurements we have examined the record of melt season duration on the antarctic peninsula using two techniques for detecting the presence of a melt signal in microwave emission time series covering the period 1978 2000 we have obtained similar estimates of melt season length using the cross polarized gradient ratio xpgr technique and calibrations previously applied in greenland and a technique which detects the jump in emission caused by melt without using a sensor and frequency dependent threshold value the close correspondence between results from the two techniques on peninsula ice shelves suggests that the xpgr analysis can be used over the length of the time series the results show that the long melt seasons of 1992 93 and several later years were exceptional occurrences on the northern parts of the larsen ice shelf these melt seasons were followed by disintegration events supporting a possible cause and effect relationship

deformation and failure of the ice bridge on the wilkins ice shelf antarctica a narrow bridge of floating ice that connected the wilkins ice shelf antarctica to two confining islands eventually collapsed in early april 2009 in the month preceding the collapse we observed deformation of the ice bridge by means of satellite imagery and from an in situ gps station terrasar x images acquired in stripmap mode were used to compile a time series the ice bridge bent most strongly in its narrowest part westerly while the northern end near charcot island shifted in a northeasterly direction in the south the ice bridge experienced compressive strain parallel to its long axis gps position data were acquired a little south of the narrowest part of the ice bridge from 19 january 2009 analysis of these data showed both cyclic and monotonic components of motion meteorological data and re analysis of the output of weather prediction models indicated that easterly winds were responsible for the cyclic motion component in particular wind stress on the rough ice melange that occupied the area to the east exerted significant pressure on the ice bridge the collapse of the ice bridge began with crack formation in the southern section parallel to the long axis of the ice bridge and led to shattering of the southern part ultimately the narrowest part only 900 m wide ruptured the formation of many small icebergs released energy of 125 106 j

a recent sea ice retreat west of the antarctic peninsula satellite passive microwave data show a record decrease in sea ice extent in the bellingshausen sea from mid 1988 through early 1991 the change coincides with more southerly surface winds increased cyclonic activity and rising surface air temperatures which reached historic highs along the west coast of the antarctic peninsula in 1989 preceded by high ice cover in 1986 87 the retreat was most evident during summer in the formerly perennial sea ice field over the continental shelf ocean heat storage probably contributed to the persistence and coastal propagation of this anomaly

surface melting on antarctic peninsula ice shelves detected by passive microwave sensors a 13 year time series of spaceborne passive microwave radiance measurements over the antarctic peninsula ice shelves reveals a systematic increase in the duration of the summer melt season combined with data from meteorological stations on the antarctic peninsula the annual motion and long term trends of the 0 c isotherm can be monitored

joint initiation in bedded clastic rocks abstract two general types of sedimentary structures serve as initiation points ip for cross fold joints in siltstone beds of the ithaca formation appalachian plateau new york planar bed boundaries and geometrically complex features such as bedforms trace fossils and soft sediment deformation structures the relative abundance of each ip type varies with the stratigraphic position of the jointed body when they are mechanically isolated by thicker shales siltstone bodies have a higher percentage of ip associated with complex features this association supports the hypothesis that sedimentary structures serve to concentrate stress thus becoming preferred sites for primary joint initiation secondary joint initiation occurs at planar bed boundaries through interaction with pre existing joints in adjacent siltstone beds that are mechanically coupled to form composite joints the propagation path of joints passing through planar bed boundaries of composite joints can be traced backward to primary ip at complex sedimentary structures modeling suggests that when beds are mechanically coupled the modest joint tip stress concentration across a shale layer e g

the retreat of jones ice shelf antarctic peninsula in recent decades several ice shelves along the antarctic peninsula have diminished in size as a result of climate warming using aerial photographic satellite and survey data we document a similar retreat of jones ice shelf which was another small ice shelf on the west coast of the antarctic peninsula this ice shelf was roughly stable between 1947 and 1969 but in the early 1970s it began to retreat and had completely disappeared by early 2003 jones ice shelf has two ice fronts only a few kilometres apart and its retreat provides a unique opportunity to examine how different ice fronts retreat when subjected to similar climate forcing we mapped the retreat of both the east and west ice fronts of jones ice shelf and found that although individual episodes of retreat may be related to particularly warm summers the overall progress of retreat of the two ice fronts has been rather different this suggests that in this case the course of retreat is controlled by the geometry of the embayment and location of pinning points as well as climatic events

how fracture systems affect permeability development in shallow water carbonate rocks an example from the gargano peninsula italy abstract fracture networks control the permeability of many reservoirs since the fracture patterns of petroleum reservoirs in situ are difficult to study in detail field analogues are very important for understanding their fracture related permeability here we present the results of a study of the fracture system of carbonate rocks of lower cretaceous age in a quarry associated with the damage zone and fault core of a major fault zone on the gargano peninsula in south italy we measured the attitude of 1541 fractures and faults along several vertical and horizontal scan lines there are two main fracture sets one strikes between e w and ese wnw the other nne ssw a total of 675 fracture spacing measurements indicate log normal spacing distributions with an arithmetic mean fracture spacing of 0 29 m and a median of 0 15 m the data plotted on a log log plot suggest three main spacing subpopulations each of which follows approximately a power law with different fractal dimensions subpopulation 1 where the spacing ranges from 1 to 10 cm and the straight line slope d fractal dimension is 0 20 represents fractures confined to laminated carbonate mudstones multilayers that form the microbial mat deposits of a peritidal cycle subpopulation 2 where the spacing ranges from 11 to 55 cm and d is 0 77 represents fractures confined to thicker layers forming a part of a peritidal cycle the contacts of which are marked by stylolites subpopulation 3 where the spacing ranges from 56 to 243 cm and d is 2 81 represents fractures that dissect comparatively thick units of an entire peritidal cycle for the spacing the minimum coefficient of variation c v defined as standard deviation divided by the mean is 1 00 essentially randomly spaced fractures while its maximum c v is 1 62 suggesting that some fractures form clusters some clusters being denser than others the clusters composed of fractures with varying attitudes and therefore commonly intersecting are likely to contribute significantly to the overall permeability of the carbonate rock fracture aperture opening data n 324 also show a log normal size distribution with a mean opening of 1 01 cm and median of 0 29 cm log log plots indicate that a part of this data groups into two subpopulations i and ii each of which follows approximately a power law the straight line slope d the fractal dimension of subpopulation i is 0 46 whereas that of subpopulation ii is 1 49 we present boundary element models showing that laminated carbonate mudstones and their contacts modify the local stress fields so as to encourage fracture offset and commonly arrest our results also show that when a fluid driven subpopulation 2 fracture approaches subpopulation 1 fractures the induced tensile stresses may result in the opening up of many of the subpopulation 1 fractures directly above the tip of the subpopulation 2 fractures if in addition the contacts between the multilayers are weak they also tend to open up thus generating a large interconnected cluster of vertical fractures and horizontal contacts the results suggest that the tensile stresses induced by a comparatively large fluid driven subpopulation 2 fracture may contribute to the formation of an interconnected cluster of subpopulation 1 fractures and associated contacts thereby significantly increasing the permeability of the carbonate rock

numerical simulations of the ice flow dynamics of george vi ice shelf antarctica a diagnostic dynamic thermodynamic ice shelf model is applied to the george vi ice shelf situated in the bellinghausen sea antarctica the george vi ice shelf has a peculiar flow geometry which sets it apart from other ice shelves inflow occurs along the two longest and almost parallel sides whereas outflow occurs on the two ice fronts that are relatively short and situated at opposite ends of the ice shelf two data sources were used to derive the ice thickness distribution conventional radio echo sounding from the british antarctic survey was combined with thickness inferred from surface elevation obtained by the nasa glas satellite system assuming hydrostatic equilibrium we simulate the present ice flow over the ice shelf that results from the ice thickness distribution the inflow at the grounding line and the flow rate factor the high spatial resolution of the ice thickness distribution leads to very detailed simulations the flow field has some extraordinary elements e g the stagnation point characteristics resulting from the unusual ice shelf geometry

stratigraphic controls on vertical fracture patterns in silurian dolomite northeastern wisconsin vertical opening mode fractures are mapped on quarry walls to assess the stratigraphic controls on fracture patterns in the relatively undeformed silurian dolomite of northeastern wisconsin our two stage study uses maps of vertical fractures to assess the effectiveness of various types of stratigraphic horizons e g organic partings or cycle bounding mud horizons in terminating opening mode fractures first the mechanical stratigraphy of the exposures is interpreted from the observed fracture pattern both visual inspection and a newly developed quantitative method are employed to identify effective mechanical interfaces the two methods show similar results confirming the validity of qualitative visual inspection the second stage of our study stochastically predicts mechanical stratigraphy and subsequent fracture pattern from empirical relationships between the observed sedimentary stratigraphy and the interpreted mechanical stratigraphy for example 63 of cycle bounding mud horizons within the inner middle and middle shelf facies associations serve as mechanical interfaces these empirical percentages are input to a monte carlo analysis of 50 stochastic realizations of mechanical stratigraphy comparisons of the stochastically predicted and interpreted mechanical stratigraphy yield errors ranging from 13 to 33 this method yields far better results than assuming that all stratigraphic horizons act as mechanical interfaces the methodology presented in this article demonstrates an improved prediction of fracture pattern within relatively undeformed strata from both complete characterization of sedimentary stratigraphy and understanding mechanical controls on fracturing

ice shelf disintegration by plate bending and hydro fracture satellite observations and model results of the 2008 wilkins ice shelf break ups abstract satellite remote sensing observations of three break up events in 2008 for the wilkins ice shelf 28 february to 6 march 27 may to 31 may and 28 june to mid july provide unprecedented detail of ice shelf calving during rapid break up the observations reveal that the wilkins break ups occur through a distinctive type of shelf calving which we term disintegration as well as more typical rifting and calving here we focus on the disintegration process which is characterized by repeated rapid fracturing that creates narrow ice edge parallel blocks with subsequent block toppling and fragmentation forming an expanding iceberg and ice rubble mass we use these data to develop and test a model of floating ice plate disintegration in which ice plate bending stresses at the ice front arising from buoyancy forces can lead to runaway calving when free mobile water is available high resolution satellite images and laser altimetry of the first break up event provide details of fracture spacings ice thicknesses and plate bending profiles that agree well with our model predictions we suggest that surface or near surface meltwater is the main pre condition for disintegration and that hydro fracture is the main mechanism brine layers from near waterline brine infiltration can support a similar process but this is less effective unless regional ice stress patterns contribute to the net stress available at the crack tip for fracturing a combination of brine enhanced fracturing and changing internal net extensional stresses was the likely mechanism behind the latter two wilkins events

transformation induced jointing as a gauge for interfacial slip and rock strength various theoretical and numerical models have been proposed in order to explain joint formation and spacing in layered rock series however most of these models assume that the interfaces between the rock layers are perfectly welded i e no slip occurs and that all the layers are subjected to the same remote strain due to various processes e g tectonic processes other factors may also induce extensional strain in rocks e g phase transformations however such processes may induce different amounts of strain on the layers in a rock series leading to a strain mismatch between these layers in this paper we present a 1 d finite difference linear elastic model which allows joint formation within the middle layer in a three layer rock series and is induced by a strain mismatch between the fractured central layer and the surrounding matrix furthermore the central layer in our model is not necessarily welded to the matrix layers and is allowed to slip along the interfaces between these layers if the shear strength of the material at the interface is reached we find that the final fracture spacing to layer thickness ratio s tf in such layered systems is directly proportional to the ratio of the tensile and shear strength of the material changes in the material properties such as the shear modulus or young s modulus do not affect these results a natural analog of joint formation driven by phase transformations is found in the orthopyroxenite dykes of the leka ophiolite complex loc norway joint formation in orthopyroxenite dykes results from serpentinization driven expansion of the surrounding dunite matrix detailed field studies and measurements 583 sample points yield s tf ratios between 0 1 and 1 0 with a mean value of 0 45 0 20 we demonstrate that the strain mismatch driven joint formation associated with interfacial slip explains the low s tf ratios obtained from field measurements and may also help us constrain rock strength

circulation and melting beneath george vi ice shelf antarctica introduction george vi ice shelf sandwiched between the western coast of palmer land and the eastern coast of alexander island is the largest and most studied of the west antarctic peninsula ice shelves it covers an area of approximately 25 000 km2 and is underlain by circumpolar deep water cdw with temperatures in excess of 1oc giving rise to rapid basal melting bishop and walton 1981 lennon et al 1982 the maximum ice thickness of about 500 m occurs about 70 km from the southern ice front where a ridge of thick ice extends across george vi sound near 70ow see figure 1 effectively dividing the upper water column into northern and southern regions the northern ice front which faces marguerite bay appears to be near the geographical limit of ice shelf viability and has undergone a gradual retreat in recent decades lucchitta and rosanova 1998 a timeframe over which much of the nearby wordie ice shelf disintegrated doake and vaughan 1991 there is extensive surface melting over the northern parts of the ice shelf and much of the ice column near the northern ice front appears to be temperate paren and cooper 1986 conditions in the south where the ice front faces into ronne entrance are colder and the ice front position appears to be steady the vast majority of the flow into the ice shelf comes from palmer land but basal melting is sufficient to remove most of this so that the ice is derived almost exclusively from local accumulation by the time it reaches the ice fronts potter et al 1984 there is some evidence at the margins of the ice shelf to suggest that it may have disappeared completely during the early holocene before reforming sugden and clapperton 1981 bentley et al 2005

climate since ad 1510 on dyer plateau antarctic peninsula evidence for recent climate change a 480 year record of the oxygen isotope ratios dust content chemical species and net accumulation from ice cores drilled in 1989 90 on dyer plateau in the antarctic peninsula is presented the continuous analyses of small sub annual samples reveal well preserved annual variations in both sulfate content and 18o thus allowing an excellent time scale to be established this history reveals a recent pronounced warming in which the last two decades have been among the warmest in the last five centuries furthermore unlike in east antarctica on dyer plateau conditions appear to have been fairly normal from ad 1500 to 1850 with cooler conditions from 1850 to 1930 and a warming trend dominating since 1930 reconstructed annual layer thicknesses suggest an increase in net accumulation beginning early in the 19th century and continuing to the present this intuitive conflict between increasing net accumulation and depleted 18o cooler climate in the 19th century appears widespread in the peninsula region and challenges our understanding of the physical relationships among moisture sources air temperatures and snow accumulation the complex meteorological regime in the antarctic peninsula region complicates meaningful interpretation of proxy indicators and results in a strong imprint of local high frequency processes upon the larger scale climate picture

mechanism for joint saturation in mechanically layered rocks an example from southern israel abstract over 700 joint spacings were measured along a 190 m scanline within a single limestone bed of the turonian gerofit formation southern israel by investigating joints belonging to a single systematic set confined to a bed of uniform thickness one can factor out the major influences of bed thickness lithology and mechanical properties on joint set development joint spacing distributions and fracture spacing ratios fsr vary dramatically along the length of the scanline and correlate directly with throughgoing fracture zones belonging to the same tectonic stress regime regions cross cut by fracture zones display narrower median joint spacings fsr of 1 3 and unimodal joint spacing distributions where fracture zones are absent joint spacings have wider medians fsr of 0 8 and multimodal distributions because fracture zones most likely represent areas of high strain and laboratory experiments demonstrate a systematic decrease in fracture spacing with increasing strain regions of the bed cross cut by fracture zones are likely at a more advanced stage of joint set development and hence closer to saturation than other regions joint saturation in the gerofit formation may be achieved by a shift in scale from single layer to multi layer jointing as the development of throughgoing joints prevents the formation of additional bedding confined joints

crustal accretion and dynamic feedback on mantle melting of a ridge centred plume the iceland case abstract a major consequence of the interaction of a plume with an oceanic ridge is the enhanced melt production and associated crust generation in the case of iceland crustal thickness as large as 20 to 40 km has been reported crustal seismic velocities are high and have to be explained by thermal or chemical effects in the first part of the paper we address the question whether extraction of melt out of the plume beneath a slowly spreading ridge and deposition of extracted basalt volumes at the surface produces a dynamic feedback mechanism on mantle melting to study this question we solve the convection equations for a ridge centred plume with non newtonian rheology including melting melt extraction associated with deposition of cold crust at the surface of the model and using a simplified approach for compaction the assumption of cold crust is justified if the thickness of each deposited basaltic layer is less than roughly 1 km depending on the buoyancy flux of the plume crustal thicknesses between 10 and 40 km are modelled showing characteristic dipping structures resembling the rift ward dipping basaltic layers of east and western iceland comparing the resulting crustal thickness and magma generation rate with models in which the dynamic effect of crust deposition has been suppressed indicates that melt generation beneath a slowly spreading ridge is considerably damped by the dynamic feedback mechanism if the plume buoyancy flux exceeds 400 to 600 kg s based on the observed crustal thickness of iceland our models predict a plume buoyancy flux of 1140 kg s in the second part we study the accretion of the icelandic crust by a thermo mechanical model in more detail based on the navier stokes the heat transport and the mass conservation equations including volumetric sources hot 1200 c molten crustal material is injected into the newly forming crust with a constant rate at different crustal source regions a deep widespread emplacement of dykes and sills including crustal underplating b magma chambers at shallow to mid crustal level and c surface extrusions and intrusions in fissure swarms at shallow depth connected to volcanic centres we identify the material from the different source regions by a marker approach varying the relative dominance of these source regions characteristic crustal structures evolve showing shallow dipping upper crustal layers with dip angles between 10 and 15 the thermal structure of the crust varies between cold crust shallow source region dominating and hot crust deep source region dominating we use observations of maximum depth of seismicity to constrain the depth of the 650 c isotherm and seismological inferences on the lower crust to constrain temperatures in that region the best agreement with our models is achieved for crust formation dominated by deep dykes and underplating with a considerable influence of magma chamber accretion

retreating glacier fronts on the antarctic peninsula over the past half century the continued retreat of ice shelves on the antarctic peninsula has been widely attributed to recent atmospheric warming but there is little published work describing changes in glacier margin positions we present trends in 244 marine glacier fronts on the peninsula and associated islands over the past 61 years of these glaciers 87 have retreated and a clear boundary between mean advance and retreat has migrated progressively southward the pattern is broadly compatible with retreat driven by atmospheric warming but the rapidity of the migration suggests that this may not be the sole driver of glacier retreat in this region

rapid collapse of northern larsen ice shelf antarctica in january 1995 4200 square kilometers of the northern larsen ice shelf antarctic peninsula broke away radar images from the ers 1 satellite complemented by field observations showed that the two northernmost sections of the ice shelf fractured and disintegrated almost completely within a few days this breakup followed a period of steady retreat that coincided with a regional trend of atmospheric warming the observations imply that after an ice shelf retreats beyond a critical limit it may collapse rapidly as a result of perturbated mass balance

structure and sedimentology of george vi ice shelf antarctic peninsula implications for ice sheet dynamics and landform development collapse of antarctic ice shelves in response to a warming climate is well documented but its legacy in terms of depositional landforms is little known this paper uses remote sensing structural glaciological and sedimentological data to evaluate the evolution of the c 25000 km 2 george vi ice shelf sw antarctic peninsula the ice shelf occupies a north south trending tectonic rift between alexander island and palmer land and is nourished mainly by ice streams from the latter region the structure of the ice shelf is dominated by inherited foliation and fractures and with velocity data indicates a largely compressive flow regime the formation of a moraine complex at the margin of the ice shelf is controlled by debris entrained within foliation and folds this englacial debris is of basal origin and includes both local mesozoic sedimentary and volcanic lithologies and exotic crystalline rocks from palmer land folding of basal ice to a high level in the source glaciers on palmer land is required to bring the debris to the surface these results have implications for understanding flow dynamics of ice shelves under compressive flow and debris entrainment and moraine formation associated with palaeo ice shelves supplementary material photographs of ice shelf morphology ice facies and ice structure detailed descriptions of ice facies including foliation logs supporting evidence for interpreting sedimentary facies complete dataset of sedimentological data including triangular plots of clast shape clast roundness histograms particle size distribution of sand size and lower and pie chart of local clasts versus exotic clasts from palmer land and a table summarizing the characteristics of representative clasts in the ice shelf moraine based on thin section analysis with indication of their provenance are available at http www geolsoc org uk sup18831

finite element analysis of the stress distribution around a pressurized crack in a layered elastic medium implications for the spacing of fluid driven joints in bedded sedimentary rock bedding perpendicular joints confined to individual beds in interbedded sedimentary rocks commonly exhibit spacings which are proportional to the thickness of the jointed bed and which vary according to lithology or structural position the mechanical explanation for this relationship is well understood when the joints are driven by far field crack normal tensile stresses but poorly understood for cracks driven by elevated fluid pressures where the crack driving stress is the difference between the crack normal compression and the fluid pressure in the crack through a series of finite element numerical models we investigate how various parameters influence the driving stress distribution around pressurized cracks in layered media and thereby identify factors influencing the spacing of fluid driven joints for the situation we modeled we observe that 1 crack driving stress is reduced in the vicinity of pressurized joints and that the extent of the stress reduction depends on the contrast in elastic properties between the layers and 2 crack driving stress distribution depends on the ambient pore pressure during jointing these results indicate the spacing of fluid driven joints should depend on lithology and pore pressure

development of radar glacier zones on the king george island ice cap antarctica during austral summer 1996 97 as observed in ers 2 sar data

mechanical stratigraphic controls on fracture patterns within carbonates and implications for groundwater flow abstract groundwater flow in low matrix permeability carbonate rocks is largely controlled by fracture networks the stratigraphic features that control fracture initiation and termination within a sequence of sedimentary rock strata define the mechanical stratigraphy of the sequence we investigate the effectiveness of various types of stratigraphic horizons in terminating opening mode fractures in two different carbonate rock sequences a relatively homogeneous dolomite sequence in door county wi and an interbedded chalk and marl sequence within the austin chalk tx additionally we present analog and numerical modeling results that delineate the specific mechanisms that facilitate fracture termination the combination of model results and empirical relationships between observed sedimentary features and mechanical stratigraphy shows 1 fractures terminate at weak contacts e g thin organic layers shallowly buried contacts or thick fine grained units adjacent to thin fractured beds 2 fractures propagate across strong contacts e g intracycle contacts between different lithology and thin fine grained units adjacent to thick fractured beds and 3 fractures step over at moderate strength contacts we use these guidelines to predict fracture network from sedimentary stratigraphy by qualitatively assessing the mechanical stratigraphy of a portion of the relatively complex cretaceous shelf margin sequence at sant corneli spain this predictive demonstration illustrates the utility of assessing the mechanical stratigraphy of subsurface strata within which fractures are not directly observable we conclude that for a variety of carbonate mechanical stratigraphic sequences dominant fluid flow characteristics such as horizontal high flow zones and flow compartmentalization can be evaluated using fracture spacing and connectivity within fracture networks that is predicted from sedimentary stratigraphy although the resulting heterogeneous flow networks do not rely on every fracture present they are highly dependent on the mechanical stratigraphy

experimental investigation of mechanical behavior of bedded rock salt containing inclined interlayer abstract uniaxial and triaxial compression tests have been carried out on specimens of rock salt interlayers and composite rock salt containing inclined interlayers in combination with failure mechanism analysis of the stratified rock structures the variation and impact factors of bonding stresses between the layers have been investigated and the fracture properties and failure mechanisms of inclined bedded rock salt have been revealed the research demonstrated that the inclined interlayer influences the deformation and fracture properties of rock salt due to the composite mechanical characteristics cracks primarily initiate near the interface region and then propagate to the interlayer and the rock salt adjacent to it the thickness of the interlayer affects the entire performance of the composite specimen and governs the development of the local fractures macro cracks tend to form more easily in the test specimens that contain a medium thick interlayer than in the ones containing only a thin interlayer increasing confining pressure gradually changes the overall mechanical behaviors from brittle to more ductile at higher confining pressure the ductile behavior is characterized by differences of strain hardening behaviors and also the geometry of cracks at different locations in the interlayer as compared to the results of uniaxial compression tests

glaciological and oceanographic calculations of the mass balance and oxygen isotope ratio of a melting ice shelf on a etudie l alimentation du george vi ice shelf en relation avec l accumulation le velage et la fonte on releve la relation temperature taux d isotope de l oxygene ces donnees sont couplees a des donnees oceanologiques sur les eaux chaudes et son interaction avec le shelf

oceanic controls on the mass balance of wilkins ice shelf antarctica 0 8 m a 1 driven by a mean basal melt rate of wb 1 3 0 4 m a 1 interannual variability was large associated with changes in both surface mass accumulation and wb basal melt rate declined significantly around 2000 from 1 8 0 4 m a 1 for 1992 2000 to 0 75 0 55 m a 1 for 2001 2008 the latter value corresponding to approximately steady state ice shelf mass observations of ocean temperature t obtained during 2007 2009 by instrumented seals reveal a cold deep halo of winter water ww t 1 6 c surrounding wis the base of the ww in the halo is 170 m approximately the mean ice draft for wis we hypothesize that the transition in wb in 2000 was caused by a small perturbation 10 20 m in the relative depths of the ice base and the bottom of the ww layer in the halo we conclude that basal melting of thin ice shelves like wis is very sensitive to upper ocean and coastal processes that act on shorter time and space scales than those affecting basal melting of thicker west antarctic ice shelves such as george vi and pine island glacier

extreme sediment and ice discharge from marine based ice streams new evidence from the north sea a major problem for understanding the dynamics of ice streams has been a lack of precise data on ice streaming longevity and sediment transport efficacy here we present the first well constrained data on sediment flux from a paleoice stream this has been achieved by computing the volume of sediment deposited as debris flows on the fan located at the outlet of the norwegian channel ice stream and converting to a flux measurement by accounting for the duration of streaming in this episode between 20 and 19 ka during the last glacial stage in this period the ice stream delivered an average 1 1 gt of sediment per year equivalent to 8000 m 3 yr 1 per meter width of ice stream front the calculated flux is an order of magnitude higher than most previous estimates for other paleoice streams and is comparable to the present sediment flux from the world9s largest rivers the short period of debris flow deposition suggests that the norwegian channel ice stream underwent rapid on off switching with punctuated iceberg delivery to the north atlantic as a consequence

magmatic lithospheric heating and weakening during continental rifting a simple scaling law a 2 d thermomechanical rifting model and the east african rift system 1 continental rifting is accompanied by lithospheric thinning and decompressional melting after extraction melt is intruded at shallower depth thereby heating and weakening the lithosphere in a feedback mechanism this weakening may assist rifting and melt production a one dimensional kinematic lithospheric thinning model is developed including decompressional melting and intrusional magma deposition the intrusional heating effect is determined as a function of thinning rate and amount melting parameters potential temperature and the depth range of emplacement the temperature increases approximately proportionally to the square root of the thinning rate and to the square of the supersolidus potential temperature simple scaling laws are derived allowing predicting these effects and the surface heat flux for arbitrary scenarios two dimensional thermomechanical extension models are carried out for a multicomponent crust mantle two phase melt matrix system with a rheology based on laboratory data including magmatic weakening in good agreement with the 1 d kinematic models it is found that the lithosphere may heat up by several 100 k this heating enhances viscous weakening by one order of magnitude or more in a feedback mechanism rifting is dynamically enforced leading to a significant increase of rift induced melt generation including the effect of lateral focusing of magma toward the rift axis the laws are applied to different segments of the east african rift system the amount of intrusional heating increases with maturity of the rift from o 10 k to up to 200 k or 400 k at the afar rift depending on the depth range of the magmatic emplacement

mechanisms controlling rupture shape during subcritical growth of joints in layered rocks during joint propagation the orientation of plume lines on joint faces allows for mapping the path taken by the crack tip line during rupture growth rupture shapes in layered clastic sediments of the devonian catskill delta finger lakes district new york indicate joint growth through three stages where velocity of the tip line v tl varies as a function of the crack tip stress intensity k i the initial stage of growth is characterized by a rupture of approximately circular or elliptical shape that expands from an initiation fl aw with a velocity v tl in some cases primary growth involves self correction where an elliptical rupture redistributes the crack tip stress so that the rupture returns to a more stable circular shape the crack tip line eventually propagates to a bedding interface where properties of either a plastic or a noncohesive bedding interface cause v tl 0 along a portion of the tip line the onset of secondary growth occurs when the rupture tip line intersects both bedding interfaces and splits into two discontinuous segments that propagate synchronously as a single coherent rupture nonuniform nonsystematic v tl eventually interferes with the rupture s ability to grow coherently and thus leads to the transition to tertiary growth characterized by the detachment of the coherent rupture into one or more independently propagating noncoherent tip lines throughout all three stages of rupture growth the k i dependent v tl points to subcritical propagation variation from a smooth to rough plume morphology is consistent with propagation through region i of the subcritical regime with a transition at v tl 10 5 m sec rupture velocity may enter region ii of the subcritical regime but such a broad range of k i must be crossed to reach region iii and quasistatic behavior at the critical stress intensity k ic that region ii acts as a barrier through which few joints pass thus greatly limiting the number of postcritical joints in the catskill delta if not in the crust of the earth

recent atmospheric warming and retreat of ice shelves on the antarctic peninsula in 1978 mercer1 discussed the probable effects of climate warming on the antarctic ice sheet predicting that one sign of a warming trend in this region would be the retreat of ice shelves on the antarctic peninsula analyses of 50 year meteorological records have since revealed atmospheric warming on the antarctic peninsula2 3 and a number of ice shelves have retreated4 8 here we present time series of observations of the areal extent of nine ice shelves on the antarctic peninsula showing that five northerly ones have retreated dramatically in the past fifty years while those further south show no clear trend comparison with airtemperature data shows that the pattern and magnitude of ice shelf retreat is consistent with the existence of an abrupt thermal limit on iceshelf viability the isotherm associated with this limit having been driven south by the atmospheric warming ice shelves therefore appear to be sensitive indicators of climate change

pattern of retreat and disintegration of larsen b ice shelf antarctic peninsula the retreat of the larsen b ice shelf antarctic peninsula and the collapse of its northern section are analyzed using satellite images acquired between january 1995 and may 2003 over 1 week during march 2002 after a period of steady retreat since 1995 2300 km 2 of the ice shelf broke up into many small icebergs this rapid collapse occurred at the end of an exceptionally warm summer and after a multi year period of decreasing surface net mass balance ice thinning flow acceleration and widening of rifts the ice shelf area decreased from 11 512 km 2 in january 1995 to 3463 km 2 in march 2002 and 2667 km 2 in april 2003 ers synthetic aperture radar sar images were used to identify ice shelf zones with different surface morphology which generated icebergs of different sizes and shapes the pattern of retreat and break up similar to that of larsen a in 1995 suggests that fracturing enhanced by abundant surface melt played a key role in addition the recent changes of grounded and residual floating ice north of larsen b are analyzed by means of envisat advanced synthetic aperture radar asar images up to summer 2003 showing significant loss of grounded ice upstream of those ice shelf sections which disintegrated in 1995 and 2002

a synthesis of remote sensing data on wilkins ice shelf antarctica wilkins ice shelf has an area of 16000 km2 and lies off the west coast of the antarctic peninsula bounded by alexander latady charcot and rothschild islands several ice shelves including wilkins exist close to a climatic limit of viability the recent disintegration of the neighbouring wordie ice shelf has been linked to atmopsheric warming observed on the antarctic peninsula the limit of ice shelf viability thus appears to have migrated south should this continue the question arises how long will wilkins ice shelf survive compared with the other ice shelves on the antarctic peninsula few surface glaciological data have been collected on wilkins ice shelf we compare contrast and combine a variety of remotely sensed data the recently declassified geosat geodetic mission altimetry landsat mss and tm imagery and radio echo sounding data res to study its structure and mass balance regime we find that this shelf has an unusual mass balance regime and relies heavily for sustenance on in situ accumulation its response to a continued atmospheric warming may be significantly different from that of wordie ice shelf wordie ice shelf was fed by several dynamic outlet glaciers which accelerated the disintegration process when the ice shelf fractured wilkins ice shelf by contrast is almost stagnant and is expected to respond by normal calving at the ice front changes in the accumulation rate or basal melt rate may however dominate any dynamic effect over the last two decades the ice front positions have remained stable

discrete element modeling of boudinage insights on rock rheology matrix flow and evolution of geometry 1 we use discrete element model simulations to model the full boudinage process from initial fracturing of intact material to post fracture flow of material into gaps between fragments and to investigate the role which the material properties of the weak and strong layers play in this process the models are deformed in 2d plane strain under a range of confining stresses in coaxial bulk flow results show that the material properties i e mohr coulomb or quasi viscous for the matrix and elastic brittle for the competent layer lead to the development of natural looking boudin morphologies and deformation patterns in the matrix the details of the matrix rheology only have a minor influence on the morphology of the boudins by varying the material properties of the competent layer between fully brittle and semi ductile we obtain a wide range of deformation patterns ranging from pinch and swell structures to a variety of boudin types including drawn shear band and straight sided torn boudins in a number of models we observe rotation of the boudin blocks despite the applied deformation being purely coaxial these rotations are generally related to asymmetrical rhombic boudin shapes some features observed in natural boudins such as concave block faces or the formation of veins between fragments are not modeled because pore fluids are not yet included in our model

structure of southeastern antarctic peninsula ice shelves and ice tongues from synthetic aperture radar imagery examination of synthetic aperture radar data collected over the southeastern antarctic peninsula shows that features sometimes mapped as ice shelves are more likely composed of numerous ice tongues interspersed within a matrix of fast ice and icebergs the tongues are formed by the seaward extension of numerous small mountain glaciers that drain from the antarctic peninsula once afloat the tongues intermingle with a matrix of fast ice and brash examination of 1997 radarsat 1 image mosaics shows that southeastern antarctic peninsula composite ice shelves covered an area of about 3500 km 2 like ice tongues around the rest of antarctica these features are highly fragmented and likely to be susceptible to mechanical failure one such composite shelf located between new bedford and wright inlets was observed to decrease in area by 1200km 2 between 1997 and 2000

evidence for a non linear relationship between fracture spacing and layer thickness in experiments extension fractures were generated in rigid layers of plaster of paris resting on a viscous substratum pitch the experimental results predict a non linear relationship between the spacing of fractures in uniform brittle layers and layer thickness for fractures generated by the tractional force of embeddins weak rocks we derive an equation which relates the critical fracture spacing c with layer thickness b tensile strength of the layer material viscosity of the embedding medium ratio o and bulk strain rate eb the equation shows that the spacing increases as a function of the square root of the layer thickness the theory also predicts that the fracture spacing depends on the strain rate when the embedding weak medium is viscous

how much magma is required to rift a continent article i nfo article history many continental rifts are associated with and preceded by the formation of large igneous provinces partial melting of a large region of anomalously hot upwelling mantle isrequired to explain these huge volumes of pre and syn rift basaltic magma the uplift associated with a shallow region of hot mantle is often invoked as a cause of the rift push force that may drive the observed extension the rift push force is likely to be an order of magnitude toosmall todrive tectonicstretching of normal continentallithosphere but may besufficient to drive opening of lithosphere cutting dikes intrusion of a sequence of such dikes along a narrow zone may provide enough heat to significantly weaken the lithosphere here we use a newly developed 2d numerical approach in which magma filled dikes open in continental lithosphere in response to an evolving model stress field and the advection and diffusion of heat is tracked to investigate the relation between the volume of magma and the amount of lithospheric weakening the available rifting force lithospheric structure and parameters controlling magma flux are varied in the numerical simulations for continental lithosphere 60 km thick hl h1200 c heat diffusion is unimportant and only a small amount of magma is required 10 20 m y similar to magmatic rifts like the red sea

fracture patterns in nonplane strain boudinage insights from 3 d discrete element models 1 we use 3 d discrete element method simulations to model the evolution of boudin structures in a layered material under nonplane strain conditions as the models are shortened perpendicular to the layer orientation they are extended at different rates in the two layer parallel directions the particular emphasis of the study is on the orientation of fractures between the boudin blocks the results show that the fracture orientation distribution is closely connected to the ratio of the two layer parallel extension rates the anisotropy of the fracture orientation distribution increases systematically from no anisotropy at isotropic layer parallel extension to a highly anisotropic distribution in case of uniaxial extension we also observe an evolution of the anisotropy of fracture orientation distribution with increasing deformation in each individual model from a high initial anisotropy towards a value characteristic for the ratio of the layer parallel extension rates the observations about the relation between the strain ratios and the fracture patterns do have the potential to serve as the basis for a new method to analyze strains in naturally boudinaged rocks

linear to non linear relationship between vein spacing and layer thickness in centimetre to decimetre scale siliciclastic multilayers from the high ardenne slate belt belgium germany abstract typical spacing distributions have frequently been described for fractures in thin sedimentary layers 40 cm vein spacing tends to increase to a maximum value becoming more or less independent of layer thickness the resemblance with fracture spacing suggests that in an unfractured rock vein saturation can occur high fluid pressures are responsible for vein nucleation but the stress state around the initial veins controls the spacing pattern subsequently in a vein saturated rock or the existing veins will thicken by the process of crack sealing or a new cross cutting vein generation will develop in case the regional stress field changes relatively with respect to the existing veins

refinements of shear lag model and its applications abstract since it was developed by cox cox h l 1952 the elasticity and strength of paper and other fibrous materials br j appl phys 3 72 79 the shear lag model slm or fiber loading theory has been widely used by material scientists and structural geologists as a powerful analytical method for analyzing mechanical interactions between hard and soft phases layers in composites and two phase rocks we found that several approximations in previous mechanical derivations of the model can be improved in terms of mechanical equilibrium and boundary conditions our refinements make the model more rigorous and elaborate applications of the improved model include 1 prediction of the axial tensile stress in strong inclusions embedded in a continuous weak matrix and consequently interpretation of extension fracture boundinage 2 modeling of the distribution of shear stresses in the weak matrix surrounding the strong inclusions and accordingly interpretation of the variations of dislocation density and of recrystallized grain size in composites and polyphase rocks and 3 prediction of mechanical properties of industrial composites and two phase rocks the analytical results of the improved model agree approximately with those obtained from both experimental investigations and numerical calculations

strain accommodated by brittle failure in adjacent units of the monterey formation u s a scale effects and evidence for uniform displacement boundary conditions abstract extensional strain accommodated by brittle deformation was measured in adjacent mudstone and dolostone units at arroyo burro beach california the dolostone failed in effective tension whereas the mudstone failed in shear both in response to the same extensional tectonic event in the mudstone unit initial bed length and fault displacement methods document extensional strains of 6 6 0 3 and 6 0 0 3 respectively upon adjusting the displacement estimates according to theoretical fault displacement population analysis the corrected strain becomes 9 7 0 3 for the mudstone measurements of vein apertures in the dolostone document extensional strain that varies according to scale with outcrop vein scanlines indicating a strain of 3 4 0 1 and thin section scanlines yielding a strain of 5 8 0 2 applying theoretical fault displacement population analysis to vein apertures in dolostone shows that small veins below the detection limit of outcrop surveys contribute significantly to fracture related strain within the dolostone this difference in extensional strain may arise because veins measured in outcrop extend across the entire bed height and thus are controlled by the dolostone mechanical layer thickness whereas microscopic veins measured in thin section terminate without regard to a bounding layer the corrected dolostone strain becomes 10 2 1 0 matching the revised strain calculated in the adjacent mudstone unit and indicating uniform displacement boundary conditions for the two markedly different lithologies the 10 extensional strain at arroyo burro indicates significant strike parallel nw se extension accommodated by brittle failure during development of the western transverse ranges fold and thrust belt

eruptive environment of volcanism on brabant island evidence for thin wet based ice in northern antarctic peninsula during the late quaternary terrestrial volcanism occurred extensively on brabant island northern antarctic peninsula during the late quaternary 200 ka probably entirely late pleistocene two compositionally distinct volcanic sequences formed three large shield volcanoes the volcanoes were constructed in association with a 150 m thick non ice stream glacial cover although it was likely to be thicker few hundred metres where it extended onto the continental shelf the glacial cover on the volcanoes was probably mainly permeable snow firn and fractured crevassed ice it was wet based erosive and sediment forming indicating either a temperate or perhaps more likely sub polar thermal regime and would have been more extensive than that present today such a thin ice sheet would have had a low surface gradient during periods of extension across the continental shelf this is the first direct evidence for the thickness and thermal regime of the terrestrial ice sheet in the antarctic peninsula during any quaternary period prior to the last glacial maximum

the link between climate warming and break up of ice shelves in the antarctic peninsula a review of in situ and remote sensing data covering the ice shelves of the antarctic peninsula provides a series of characteristics closely associated with rapid shelf retreat deeply embayed ice fronts calving of myriad small elongate bergs in punctuated events increasing flow speed and the presence of melt ponds on the ice shelf surface in the vicinity of the break ups as climate has warmed in the antarctic peninsula region melt season duration and the extent of ponding have increased most break up events have occurred during longer melt seasons suggesting that meltwater itself not just warming is responsible regions that show melting without pond formation are relatively unchanged melt ponds thus appear to be a robust harbinger of ice shelf retreat we use these observations to guide a model of ice shelf flow and the effects of meltwater crevasses present in a region of surface ponding will likely fill to the brim with water we hypothesize building on weertman 1973 hughes 1983 and van der veen 1998 that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat a thermodynamic finite element model is used to evaluate ice flow and the strain field and simple extensions of this model are used to investigate crack propagation by meltwater the model results support the hypothesis