volume loss from antarctic ice shelves is accelerating the floating ice shelves surrounding the antarctic ice sheet restrain the grounded ice sheet flow thinning of an ice shelf reduces this effect leading to an increase in ice discharge to the ocean using 18 years of continuous satellite radar altimeter observations we have computed decadal scale changes in ice shelf thickness around the antarctic continent overall average ice shelf volume change accelerated from negligible loss at 25 64 cubic kilometers per year for 1994 2003 to rapid loss of 310 74 cubic kilometers per year for 2003 2012 west antarctic losses increased by 70 in the past decade and earlier volume gain by east antarctic ice shelves ceased in the amundsen and bellingshausen regions some ice shelves have lost up to 18 of their thickness in less than two decades

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

applying the tilt depth and contact depth methods to the magnetic anomalies of thin dykes a new method for the calculation of the depth location and dip of thin dykes from pole reduced magnetic data is introduced the depth can be obtained by measuring the distance between chosen values of a tilt angle that is based upon the ratio of the magnetic field and its hilbert transform over the dyke alternatively it can be obtained from the horizontal derivative of the ratio of the hilbert transform of the field to the field itself over the dyke the latter method also allows the dip of the dyke to be estimated from the gradient of the depth estimates

structural overprinting criteria determined from regional aeromagnetic data an example from the hill end trough east gondwana abstract determining overprinting relationships in the geological record has underpinned our understanding of the evolution of the earth and has provided the context in which to interpret geoscientific datasets geological overprinting relationships are generally determined from outcrop relationships when outcrop is absent or limited overprinting relationships using geological observations is often ineffective or ambiguous we show how aeromagnetic data can be used to inform superposition of geological structures using a combination of folded stratigraphy faults both high angle and low angle and plutons from the well exposed hill end trough in the phanerozoic eastern lachlan orogen overprinting relationships reveal episodic inversion of the late silurian to early devonian back arc basin succession preserved within the hill end trough occurred during superimposed shortening events the first event resulted in the development of regional folds associated with the tabberabberan orogeny and the second event resulted in the development of overprinting north south and north northwest trending faults during the early carboniferous kanimblan orogeny inversion structures are overprinted by a carboniferous granite which is overprinted by previously unrecognised northeast and northwest conjugate faults which we interpret to be related to the permo triassic hunter bowen orogeny

larsen ice shelf has progressively thinned the retreat and collapse of antarctic peninsula ice shelves in tandem with a regional atmospheric warming has fueled speculation as to how these events may be related satellite radar altimeter measurements show that between 1992 and 2001 the larsen ice shelf lowered by up to 0 27 0 11 meters per year the lowering is explained by increased summer meltwater and the loss of basal ice through melting enhanced ocean driven melting may provide a simple link between regional climate warming and the successive disintegration of sections of the larsen ice shelf

interpretation of magnetic phase anomalies over 2d tabular bodies in this study phase angle inverse tangent of the ratio of the horizontal to vertical gradients of magnetic anomalies profile over two dimensional tabular bodies has been subjected to detailed analysis for determining the source parameters distances between certain characteristic positions on this phase curve are related to the parameters of two dimensional tabular magnetic sources in this paper i have derived the mathematical expressions for these relations it has been demonstrated here that for locating the origin of the 2d tabular source knowledge on the type of the model contact sheet dyke and fault is not necessary a procedure is evolved to determine the location depth width and magnetization angle of the 2d sources from the mathematical expressions the method is tested on real field data the effect of the overlapping bodies is also discussed with two synthetic examples the interpretation technique is developed for contact sheet dike and inclined fault bodies

the evolving instability of the remnant larsen b ice shelf and its tributary glaciers 2015 elsevier b v following the 2002 disintegration of the northern and central parts of the larsen b ice shelf the tributary glaciers of the southern surviving part initially appeared relatively unchanged and hence assumed to be buttressed sufficiently by the remnant ice shelf here we modify this perception with observations from icebridge altimetry and insar inferred ice flow speeds our analyses show that the surfaces of leppard and flask glaciers directly upstream from their grounding lines lowered by 15 to 20 m in the period 2002 2011 the thinning appears to be dynamic as the flow of both glaciers and the remnant ice shelf accelerated in the same period flask glacier started accelerating even before the 2002 disintegration increasing its flow speed by 55 between 1997 and 2012 starbuck glacier meanwhile did not change much we hypothesize that the different evolutions of the three glaciers are related to their dissimilar bed topographies and degrees of grounding we apply numerical modeling and data assimilation that show these changes to be accompanied by a reduction in the buttressing afforded by the remnant ice shelf a weakening of the shear zones between its flow units and an increase in its fracture the fast flowing northwestern part of the remnant ice shelf exhibits increasing fragmentation while the stagnant southeastern part seems to be prone to the formation of large rifts some of which we show have delimited successive calving events a large rift only 12 km downstream from the grounding line is currently traversing the stagnant part of the ice shelf defining the likely front of the next large calving event we propose that the flow acceleration ice front retreat and enhanced fracture of the remnant larsen b ice shelf presage its approaching demise

ice shelf thickness over larsen c antarctica derived from satellite altimetry 1 satellite radar altimetry can be used to infer the thickness of floating ice shelves around antarctica under the assumption of hydrostatic equilibrium ice shelf thickness is an essential parameter in mass budget calculations and is one of the more poorly characterised using data from the ers 1 radar altimeter recorded in 1994 5 we calculate the thickness of larsen c ice shelf on the antarctic peninsula the surface elevation was determined to an accuracy of 2 3 4 35 m as compared to elevations from the laser altimeter onboard icesat using a model for firn depth and density we created a 1 km grid of ice shelf thickness for larsen c the accuracy of the ice thickness retrieval was determined from independent airborne radio echo sounding data the results indicated a bias of 0 22 m and random error of 36 7 m which is equivalent to 12 7 of the mean thickness for this ice shelf

antarctic ice shelf thickness from satellite radar altimetry ice shelf thickness is an important boundary condition for ice sheet and sub ice shelf cavity modelling it is required near the grounding line to calculate the ice flux used to determine ice sheet mass balance by comparison with the upstream accumulation in this mass budget approach the accuracy of the ice thickness is one of the limiting factors in the calculation we present a satellite retrieval of the ice thickness for all antarctic ice shelves using satellite radar altimeter data from the geodetic phases of the european remote sensing satellite ers 1 during 1994 95 supplemented by icesat data for regions south of the ers 1 latitudinal limit surface elevations derived from these instruments are interpolated on to regular grids using kriging and converted to ice thicknesses using a modelled firn density correction the availability of a new spatial variable firn density correction significantly reduces the error in ice thickness as this was previously the dominant error source comparison to airborne data shows good agreement particularly when compared to soar casertz data on the largest ice shelves biases range from 13 0 m for areas where the assumption of hydrostatic equilibrium breaks down to 53 4 m in regions where marine ice may be present

subsurface investigation of the neogene mygdonian basin greece using magnetic data a high resolution ground and marine magnetic survey was executed to determine the structure of the subsurface and the thickness of the sedimentary cover in the mygdonian basin a spacing of approximately 250 m or 500 m between measurement stations was selected to cover an area of 15 km 22 km edge detectors such as total horizontal derivative thdr analytic signal as tilt derivative tdr enhanced total horizontal gradient of tilt derivative ethdr were applied to map the subsurface structure depth was estimated by power spectrum analysis tilt derivative source parameter imaging spi and 2d forward modeling techniques spectral analysis and spi suggest a depth to the basement ranging from near surface to 600 m for some selected locations depth was also calculated using the tdr technique suggesting depths from 160 to 400 m 2d forward magnetic modeling using existing boreholes as constraints was carried out along four selected profiles and confirmed the presence of alternative horsts and grabens formed by parallel normal faults the dominant structural trends inferred from thdr as tdr and ethdr are n s nw se ne sw and e w this corresponds with the known structural trends in the area finally a detailed structural map showing the magnetic blocks and the structural architecture of the mygdonian basin was drawn up by collating all of the results

tilt depth method a simple depth estimation method using first order magnetic derivatives aeromagnetic data are routinely presented as contour or color shaded maps of the total magnetic intensity tmi an interpreter s task is to identify features anomalies contained within the map and qualitatively and or quantitatively interpret them into geologic structures at depth if the map contains anomalies that have large magnetic intensities the bodies might be considered to have large magnetizations or to be at shallow depths small amplitude anomalies superimposed on these anomalies could be masked or even missed by an interpreter thus the task of the interpreter is to use the spectral content of the anomalies to try and resolve these ambiguities part of this process is also to obtain estimates of the depth and shape of the body causing the anomalies

changes in ice dynamics elevation and mass discharge of dinsmoor bombardier edgeworth glacier system antarctic peninsula abstract the northern antarctic peninsula is one of the fastest changing regions on earth the disintegration of the larsen a ice shelf in 1995 caused tributary glaciers to adjust by speeding up surface lowering and overall increased ice mass discharge in this study we investigate the temporal variation of these changes at the dinsmoor bombardier edgeworth glacier system by analyzing dense time series from various spaceborne and airborne earth observation missions precollapse ice shelf conditions and subsequent adjustments through 2014 were covered our results show a response of the glacier system some months after the breakup reaching maximum surface velocities at the glacier front of up to 8 8 m d in 1999 and a subsequent decrease to 1 5 m d in 2014 using a dense time series of interferometrically derived tandem x digital elevation models and photogrammetric data an exponential function was fitted for the decrease in surface elevation elevation changes in areas below 1000 m a s l amounted to at least 130 15 m between 1995 and 2014 with change rates of 3 15 m a between 2003 and 2008 current change rates 2010 2014 are in the range of 1 7 m a mass imbalances were computed with different scenarios of boundary conditions the most plausible results amount to 40 7 3 9 gt the contribution to sea level rise was estimated to be 18 8 1 8 gt corresponding to a 0 052 0 005 mm sea level equivalent for the period 1995 2014 our analysis and scenario considerations revealed that major uncertainties still exist due to insufficiently accurate ice thickness information the second largest uncertainty in the computations was the glacier surface mass balance which is still poorly known our time series analysis facilitates an improved comparison with grace data and as input to modeling of glacio isostatic uplift in this region the study contributed to a better understanding of how glacier systems adjust to ice shelf disintegration

depth to curie temperature across the central red sea from magnetic data using the de fractal method abstract the central red sea rift is considered to be an embryonic ocean it is characterised by high heat flow with more than 90 of the heat flow measurements exceeding the world mean and high values extending to the coasts providing good prospects for geothermal energy resources in this study we aim to map the depth to the curie isotherm 580 c in the central red sea based on magnetic data a modified spectral analysis technique the de fractal spectral depth method is developed and used to estimate the top and bottom boundaries of the magnetised layer we use a mathematical relationship between the observed power spectrum due to fractal magnetisation and an equivalent random magnetisation power spectrum the de fractal approach removes the effect of fractal magnetisation from the observed power spectrum and estimates the parameters of depth to top and depth to bottom of the magnetised layer using iterative forward modelling of the power spectrum we applied the de fractal approach to 12 windows of magnetic data along a profile across the central red sea from onshore sudan to onshore saudi arabia the results indicate variable magnetic bottom depths ranging from 8 4 km in the rift axis to about 18 9 km in the marginal areas comparison of these depths with published moho depths based on seismic refraction constrained 3d inversion of gravity data showed that the magnetic bottom in the rift area corresponds closely to the moho whereas in the margins it is considerably shallower than the moho forward modelling of heat flow data suggests that depth to the curie isotherm in the centre of the rift is also close to the moho depth thus curie isotherm depths estimated from magnetic data may well be imaging the depth to the curie temperature along the whole profile geotherms constrained by the interpreted curie isotherm depths have subsequently been calculated at three points across the rift indicating the variation in the likely temperature profile with depth

the integration of gravity magnetic and seismic data in delineating the sedimentary basins of northern sinai and deducing their structural controls abstract the sinai peninsula is a part of the sinai sub plate that located between the southeast nubian arabian shield and the southeastern mediterranean northward the main objectives of this investigation are to deduce the main sedimentary basin and its subdivisions identify the subsurface structural framework that affects the study area and determine the thickness of sedimentary cover of the basement surface the total intensity magnetic map bouguer gravity map and seismic data were used to achieve the study aims structural interpretation of the gravity and magnetic data were done by applying advanced processing techniques these techniques include reduce to the pole rtp power spectrum tile derivative and analytical signal techniques were applied on gravity and magnetic data two dimensional gravity and magnetic modeling and interpretation of seismic sections were done to determine the thickness of sedimentary cover of the study area the integration of our interpretation suggests that the northern sinai area consists of elongated troughs that contain many high structural trends four major structural trends have been identified that reflecting the influence of district regional tectonic movements these trends are 1 ne sw trend 2 nnw sse trend 3 ene wsw trend and 4 wnw ese trend there are also many minor trends e w nw se and n s structural trends the main sedimentary basin of north sinai is divided into four sub basins 1 northern maghara 2 northeastern sinai 3 northwestern sinai and 4 central sinai basin the sedimentary cover ranges between 2 km and 7 km in the northern part of the study area

intrusion of warm surface water beneath the mcmurdo ice shelf antarctica 1 a 6 month temperature record collected below mcmurdo ice shelf in 2011 2012 shows the temporal and spatial structure of the summertime warm water signal that penetrates beneath the ice shelf the strength and duration of the warm water intrusion suggest an annual melt rate at windless bight of 0 71 m yr a ross sea numerical model demonstrates a seasonal warm water pathway leading from the west side of the ross sea polynya rsp toward mcmurdo sound the warm water enters mcmurdo sound subducts beneath the ice shelf and causes accelerated summer melting temperature data were recorded using distributed temperature sensing fiber optics which gives a vertical temperature profile at a 1 m vertical resolution this study constitutes one of the first successful implementations of this technology in polar regions

the tuareg shield terranes revisited and extended towards the northern gondwana margin magnetic and gravimetric constraints abstract the trans saharan belt is one of the most important orogenic systems constitutive of the pan african cycle which at the end of the neoproterozoic led to the formation of the gondwana supercontinent it is marked by the opening and closing of oceanic domains collision of continental blocks and the deformation of thick synorogenic sedimentary basins it extends from north to south over a distance of 3000 km in africa including the nigerian shield and the tuareg shield as well as their counterparts beneath the phanerozoic oil rich north and south saharan sedimentary basins in this study we take advantage of potential field methods magnetism and gravity to analyze the crustal scale structures of the tuareg shield terranes and to track these pan african structures below the sedimentary basins offering a new 1000 km extent the map interpretations are based on the classical potential field transforms and two dimensional forward modeling we have identified geophysical units and first order bounding lineaments essentially defined owing to magnetic and gravimetric anomaly signatures in particular we are able to highlight curved terminations which in the trans saharan context have been still poorly documented we provide for the first time a rheological map showing a categorization of contrasted basement units from the south of the tuareg shield up to the atlas belt these units highlight the contrasted rheological behavior of the tuareg tectonostratigraphic terranes during i the northerly pan african tectonic escape characteristic of the trans saharan belt and ii the north sahara basin development especially during intraplate reworking tied to the variscan event the discovery of a relatively rigid e w oriented unit to the south of the atlas system and on which the escaping pan african terranes were blocked offers a new perspective on the structural framework of the north gondwana margin it will help to understand how occurred the rendezvous of the n s oriented pan african terranes and the e w oriented cadomian peri gondwanan terranes

strong surface melting preceded collapse of antarctic peninsula ice shelf 1 during the austral summer of 2001 02 melting at the surface of larsen ice shelf in the antarctic peninsula was three times greater than the average of five previous summers this exceptional melt event lasted for three months and was followed by the collapse of larsen b ice shelf during which 3 200 km2 of ice shelf surface was lost the strong melting was caused by a persistent atmospheric circulation anomaly which depleted sea ice concentrations in front of larsen ice shelf and transported warm air to the ice shelf throughout the 2001 02 summer this supports the theory that large meltwater fluxes accelerate the retreat of antarctic peninsula ice shelves

glacier acceleration and thinning after ice shelf collapse in the larsen b embayment antarctica 1 ice velocities derived from five landsat 7 images acquired between january 2000 and february 2003 show a two to six fold increase in centerline speed of four glaciers flowing into the now collapsed section of the larsen b ice shelf satellite laser altimetry from icesat indicates the surface of hektoria glacier lowered by up to 38 6 m in a six month period beginning one year after the break up in march 2002 smaller elevation losses are observed for crane and jorum glaciers over a later 5 month period two glaciers south of the collapse area flask and leppard show little change in speed or elevation seasonal variations in speed preceding the large post collapse velocity increases suggest that both summer melt percolation and changes in the stress field due to shelf removal play a major role in glacier dynamics

foehn winds link climate driven warming to ice shelf evolution in antarctica rapid warming of the antarctic peninsula over the past several decades has led to extensive surface melting on its eastern side and the disintegration of the prince gustav larsen a and larsen b ice shelves the warming trend has been attributed to strengthening of circumpolar westerlies resulting from a positive trend in the southern annular mode sam which is thought to promote more frequent warm dry downsloping foehn winds along the lee or eastern side of the peninsula we examined variability in foehn frequency and its relationship to temperature and patterns of synoptic scale circulation using a multidecadal meteorological record from the argentine station matienzo located between the larsen a and b embayments this record was further augmented with a network of six weather stations installed under the u s nsf larsen ice shelf system antarctica project significant warming was observed in all seasons at matienzo with the largest seasonal increase occurring in austral winter 3 71 c between 1962 1972 and 1999 2010 frequency and duration of foehn events were found to strongly influence regional temperature variability over hourly to seasonal time scales surface temperature and foehn winds were also sensitive to climate variability with both variables exhibiting strong positive correlations with the sam index concomitant positive trends in foehn frequency temperature and sam are present during austral summer with sustained foehn events consistently associated with surface melting across the ice sheet and ice shelves these observations support the notion that increased foehn frequency played a critical role in precipitating the collapse of the larsen b ice shelf

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

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

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

estimation of icesat intercampaign elevation biases from comparison of lidar data in east antarctica 1 the ice cloud and land elevation satellite icesat laser campaign elevation biases are estimated from an analysis of icesat and airborne land vegetation and ice sensor lidar intrasensor and intersensor elevation differences from two regions of the antarctic ice sheet that experienced minimal surface elevation change elevation observations are corrected for a combination of accumulation melting and firn densification processes and glacial isostatic adjustment icesat elevations are adjusted for the saturation and gaussian centroid corrections relative to laser campaign l3i biases are found to be less than 8 cm except for campaign l2e at 14 72 cm corresponding to the time of a significant accumulation anomaly in east antarctica the intercampaign bias trend estimated from intersensor elevation differences computed over campaigns l2a to l2f september 2003 to october 2009 excluding l2e is 1 04 0 48 cm yr the intercampaign bias trend represents a correction to icesat derived antarctica mass balance of 117 53 gt yr

a gradient ratio method for the semi automatic interpretation of gravity map data sets the application of semi automatic interpretation techniques to potential field data can be of significant assistance to a geophysicist this paper generalizes the magnetic vertical contact model tilt depth method to gravity data using a vertical cylinder and buried sphere models the method computes the ratio of the vertical to the total horizontal derivative of data and then identifies circular contours within it given the radius of the contour and the contour value itself the depth to the source can be determined the method is applied both to synthetic and gravity data from south africa the matlab source code can be obtained from the author upon request

from ice shelf tributary to tidewater glacier continued rapid recession acceleration and thinning of rohss glacier following the 1995 collapse of the prince gustav ice shelf antarctic peninsula glasser n f scambos t a bohlander j truffer m pettit e davies b j 2011 journal of glaciology 57 203 397 406

the air content of larsen ice shelf 1 the air content of glacial firn determines the effect and attribution of observed changes in ice surface elevation but is currently measurable only using labor intensive ground based techniques here a novel method is presented for using radar sounding measurements to decompose the total thickness of floating ice shelves into thicknesses of solid ice and firn air or firn water the method is applied to a 1997 98 airborne survey of larsen ice shelf revealing large spatial gradients in air content that are consistent with existing measurements and local meteorology the gradients appear to be governed by meltwater induced firn densification we find sufficient air in larsen c ice shelf for increased densification to account for its previously observed surface lowering and the rate of lowering superficially agrees with published trends in melting this does not preclude a contribution to the lowering from oceanic basal melting but a modern repeat of the survey could conclusively distinguish atmosphere led from ocean led change the technique also holds promise for the calibration of firn density models derivation of ice thickness from surface elevation measurements and calculation of the sea level contribution of changes in grounded ice discharge

accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf acceleration exceeds 27 km 3 per year and ice is thinning at rates of tens of meters per year we attribute this abrupt evolution of the glaciers to the removal of the buttressing ice shelf the magnitude of the glacier changes illustrates the importance of ice shelves on ice sheet mass balance and contribution to sea level change index terms 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 3349 meteorology and atmospheric dynamics polar meteorology 6924 radio science interferometry 9310 information related to geographic region antarctica citation rignot e g casassa p gogineni w krabill a rivera and r thomas 2004 accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf geophys res lett 31 l18401 doi 10 1029 2004gl020697

icesat antarctic elevation data preliminary precision and accuracy assessment 1 since first light on february 20th 2003 nasa s ice cloud and land elevation satellite icesat has derived surface elevations from 86 n to 86 s latitude these unique altimetry data have been acquired in a series of observation periods in repeated track patterns using all three geoscience laser altimeter system glas lasers here we focus on antarctic ice sheet elevation data that were obtained in 2003 2004 we present preliminary precision and accuracy assessments of selected elevation data and discuss factors impacting elevation change detection we show that for low slope and clear sky conditions the precision of gla12 laser 2a release 21 data is 2 1 cm and the relative accuracy of icesat elevations is 14 cm based on crossover differences

2001 2009 elevation and mass losses in the larsen a and b embayments antarctic peninsula we investigate the elevation and mass balance response of tributary glaciers following the loss of the larsen a and b ice shelves antarctic peninsula in 1995 and 2002 respectively our study uses modis imagery to track ice extent and aster and spot5 digital elevation models dems plus atm and icesat laser altimetry to track elevation changes spanning the period 2001 09 the measured larsen b tributary glaciers hektoria green evans punchbowl jorum and crane lost up to 160 m in elevation during 2001 06 and thinning continued into 2009 elevation changes were small for the more southerly flask and leppard glaciers which are still constrained by a larsen b ice shelf remnant in the northern embayment continued thinning of 3 m a 1 on drygalski glacier 14 years after the larsen a ice shelf disintegrated suggests that mass losses for the exposed larsen b tributaries will continue for years into the future grounded ice volume losses exceed 13 km 3 for crane glacier and 30 km 3 for the hektoria green evans glaciers the combined mean loss rate for 2001 06 is at least 11 2 gt a 1 our values differ significantly from published mass budget based estimates for these embayments but are a reasonable fraction of grace derived rates for the region 40 gt a 1

new developments of the magnetic tilt depth method to improve structural mapping of sedimentary basins this paper interprets aeromagnetic data for a deep basin section of the karroo rift in south east tanzania we use a novel integrated approach involving the application of advanced derivatives to define structure and the tilt depth method to determine and map the depth to basement in the latter case we use the result of both reduced to pole and reduced to equator data to help constrain the shape and depth of the basin we show that for a reduced to pole aeromagnetic data set the generalized form of the local phase called the tilt derivative is an effective means of providing an initial first pass mapping of a sedimentary basin in terms of its fault structure dip direction of faults and depth to basement since the amplitude of the tilt derivative does not contain information on the strength of the geomagnetic field nor magnetization other than inclination of the causative body the susceptibility contrast across faults contacts is derived from the analytic signal derivative we also investigate how effective the tilt derivative and tilt depth method are for structural and depth to basement mapping in regions close to the magnetic equator where the reduction to pole transform is often unstable this is done using the same tanzania data set transformed to the pole and the equator we find that the tilt derivative applied to the reduction to equator data cannot be used to map the structure because of the effects of magnetic anisotropy which results in the magnetic response of structures varying with strike azimuth to overcome this anisotropy problem the analytic signal and or local wavenumber derivatives which are for all practical purposes independent of inclination provide the best means of defining the major structural trends we also find that the tilt depth method provides coherent depth to basement estimates for both reduced to pole and reduced to equator data for the deep basin sections of the karroo rift there is a sparsity of tilt depth results from both the reduced to pole and reduced to equator data sets however each set of results has a different spatial coverage so when combined they provide a better spatial sampling of the long wavelength magnetic character of the basin and thus improve the constraints on the minimum curvature gridding method to map the shape and depth of the basin

high resolution near bottom vector magnetic anomalies over raven hydrothermal field endeavour segment juan de fuca ridge high resolution near bottom vector magnetic data were collected by remotely operated vehicle jason over the raven hydrothermal vent field 47 57 3 n 129 5 75 w located north of main endeavour vent field on the endeavour segment of the juan de fuca ridge the survey was part of a comprehensive heat flow study of the raven site using innovative thermal blanket technology to map the heat flux and crustal fluid pathways around a solitary hydrothermal vent field raven hydrothermal activity is presently located along the western axial valley wall while additional inactive hydrothermal deposits are found to the nw on the upper rift valley wall magnetic inversion results show discrete areas of reduced magnetization associated with both active and inactive hydrothermal vent deposits that also show high conductive heat flow higher spatial variability in the heat flow patterns compared to the magnetization is consistent with the heat flow reflecting the currently active but ephemeral thermal environment of fluid flow while crustal magnetization is representative of the static time averaged effect of hydrothermal alteration a general nw to se trend in reduced magnetization across the raven area correlates closely with the distribution of hydrothermal deposits and heat flux patterns and suggests that the fluid circulation system at depth is likely controlled by local crustal structure and magma chamber geometry magnetic gradient tensor components computed from vector magnetic data improve the resolution of the magnetic anomaly source and indicate that the hydrothermally altered zone directly beneath the raven site is approximately 15 106 m3 in volume

geomagnetic field anomalies over the lombok island region an attempt to understand the local tectonic changes during the last years several investigations on the earthquakes and related tectonic structures along the java trench have been conducted in this study we focus on the lombok island west nusa tenggara indonesia which lies between the centres of the highest intensity of contiguous negative positive geomagnetic anomalies in this region the geological and tectonic structures however are not known in detail for this island whereas a better knowledge of these conditions could lead to better estimate the potential earthquake risks and thus a possible improvement of the existing early warning system we have performed a ground based geomagnetic survey at 56 stations in the lombok island during october november 2006 for a detailed mapping and interpretation of geomagnetic anomalies related to the geological and tectonic characteristics the 2d and 3d magnetic maps show a general geomagnetic anomaly pattern in the lombok island which consists of repeated contiguous negative positive anomalies two forward models have been proposed for a profile connecting a strongest apparent dipolar structure the first model assumes a uniformly magnetized sphere as the source of the anomaly and could be interpreted as a specific local structure composed by a quite large magnetic body the second model considers several rocks with different susceptibilities and magnetizations and could be interpreted as a discontinuity in the geological structures this model agrees with the local geological surface conditions and the known large scale regional tectonic structure therefore it is used to interpret our results in terms of tectonic characteristics which suggests the potential existence of a new tectonic element e g a local normal fault in this region

icesat profiles of tabular iceberg margins and iceberg breakup at low latitudes 1 icesat elevation profiles of tabular iceberg margins and the ronne ice shelf edge reveal shapes indicative of two types of bending forces icebergs and shelf fronts in sea ice covered areas have broad 1000 m wide rounded 0 6 m high berms and outer edges that slope down several meters toward the water bergs in warmer water have 2 to 5m ramparts with 1500 m wide edge parallel moats inboard of the edge this latter pattern was first revealed in images from international space station iss showing edge parallel melt ponds on one iceberg just prior to its disintegration model results indicate the patterns are caused by hydrostatic and lithostatic forces acting on the ice face berm profiles arise from differences between ice and water pressure along the face rampart moat profiles result from waterline erosion creating a submerged bench of ice that lifts the ice edge we use the results to discuss iceberg breakup at low latitudes

implementation of magnetic gravity and resistivity data in identifying groundwater occurrences in el qaa plain area southern sinai egypt abstract el qaa plain is one of the areas that have been proved to be promising as to its soil and groundwater resources in the southwestern part of sinai this study was carried out to study the lateral and vertical variations in the subsurface lithologic properties in el qaa plain area and delineating the subsurface structure that affecting el qaa plain depression gravity magnetic data fifty one 51 vertical electrical sounding ves s and digital elevation model dem map were used two dimensional density modeling analytical signal tilt derivatives euler deconvolution techniques and interpretation of the resistivity data were applied on potential data this study concluded that el qaa plain is a ne sw depression confined by two normal faults and subdivided into five sedimentary units rock fragments and boulders derived to the plain from the surrounding highlands are the main components of the first surface unit that characterized by high resistivity values this layer is overly another unit composed of gravelly sand with thickness ranging between 10 and 140 m the third unit is composed mainly of sand with intercalations of clay with thickness ranging from 2 to 152 m a reefal limestone unit is a fourth unit interpreted from this study the fifth unit is composed of clay the water bearing formations in el qaa plain area are located principally in the second and the third layers finally the depth to the basement surface ranges from 400 m to more than 1 5 km below sea level also there are many minor structural trends interpreted from this study and affecting the el qaa plain are striking in n s e w and ne sw directions

ice loss processes in the seal nunataks ice shelf region from satellite altimetry and imagery the seal nunataks ice shelf snis 743 km 2 in 2013 is an unofficial name for a remnant area between the former larsen a and larsen b ice shelves off the northeastern antarctic peninsula analyses using landsat 7 etm and terra aster images from 2001 to 13 and icesat altimetry from 2003 to 09 show it has retreated and thinned following the larsen a 1995 and larsen b 2002 disintegrations despite some regional cooling and more fast ice since 2008 snis continues to lose ice along its margins and may be losing contact with some nunataks detailed analysis of data from four icesat tracks indicates that ice shelf thinning rates range between 1 9 and 2 7 m a 1 and generally increase from west to east an icesat repeat track crossing the adjacent robertson island shows a mean elevation loss of 1 8 m a 1 two tracks crossing the snis s remaining tributary rogosh glacier show sub meter elevation losses comparing shelf remnant and grounded ice thinning rates implies that basal ocean melting augments snis thinning by 1 m a 1 a rate that is consistent with other estimates of ocean driven shelf thinning in the region

interpretation of tensor gravity data using an adaptive tilt angle method abstract full tensor gravity gradiometry ftg data are routinely used in exploration programmes to evaluate and explore geological complexities hosting hydrocarbon and mineral resources ftg data are typically used to map a host structure and locate target responses of interest using a myriad of imaging techniques identified anomalies of interest are then examined using 2d and 3d forward and inverse modelling methods for depth estimation however such methods tend to be time consuming and reliant on an independent constraint for clarification this paper presents a semi automatic method to interpret ftg data using an adaptive tilt angle approach the present method uses only the three vertical tensor components of the ftg data tzx tzy and tzz with a scale value that is related to the nature of the source point anomaly or linear anomaly with this adaptation it is possible to estimate the location and depth of simple buried gravity sources such as point masses line masses and vertical and horizontal thin sheets provided that these sources exist in isolation and that the ftg data have been sufficiently filtered to minimize the influence of noise computation times are fast producing plausible results of single solution depth estimates that relate directly to anomalies for thick sheets the method can resolve the thickness of these layers assuming the depth to the top is known from drilling or other independent geophysical data we demonstrate the practical utility of the method using examples of ftg data acquired over the vinton salt dome louisiana usa and basalt flows in the faeroe shetland basin uk a major benefit of the method is the ability to quickly construct depth maps such results are used to produce best estimate initial depth to source maps that can act as initial models for any detailed quantitative modelling exercises using 2d 3d forward inverse modelling techniques

remote sensing and geophysical investigations of moghra lake in the qattara depression western desert egypt abstract the western desert covers two thirds of the land area of egypt and occupies one of the driest regions of the sahara seven depressions within the desert siwa qattara fayum bahariya farafra dakhla and kharga may represent parts of old drainage systems with deflation extensive erosion and possibly some tectonic activity oases with freshwater exist in these depressions geological and geophysical investigations in the qattara depression indicate the presence of buried fluvial channels with southeast to northwest flow directions from the highland areas the origin of these fluvial systems as well as the origin of the depressions themselves is still unresolved and many ideas have been suggested moghra lake at the northeastern tip of the qattara basin may be a remnant of a larger paleolake including the mouth of a paleo river we present here the results of our recent work in this area using alos palsar radar remote sensing data which indicated the presence of buried channels that may have fed the larger moghra paleolake ground penetrating radar gpr data along 2d profiles were acquired the migrated gpr sections identified a major paleochannel with numerous minor channels at its margins gpr interpretations are verified by field observations trenching and stratigraphic information from outcrop data potential field analyses identify possible aquifers that are controlled by regional structures density contrasts within the sedimentary units physical boundaries of uplifted basement blocks and depths to causative sources were also identified this work contributes to the reconstruction of paleodrainage of this region and helps in understanding processes involved in the formation of the qattara depression

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

the application of spatial derivatives to non potential field data interpretation source body edge detection is a common feature in the processing and interpretation of potential field data sets a wide range of spatial derivatives is available to enhance the information contained in the basic data here the ability of these procedures to assist with the mapping interpretation of non potential field data is considered the study uses airborne electromagnetic conductivity data but also provides a general context for other conductivity resistivity data provided the non potential field nature of active and thus spatially focused measurements is acknowledged the study discusses and demonstrates the application of a range of common spatial derivative procedures including the analytic signal and upward continuation to both magnetic and conductivity data the ability of the tilt derivative to provide enhanced mapping of conductivity data is considered in detail tilt and its associated functions are formed by taking combinations of vertical and horizontal derivatives of the data set theoretical forward modelling studies are first carried out to assess the performance of the tilt derivative in relation to the detection and definition of concealed conductivity structure the tilt derivative embodies automatic gain control that normalizes the detection and definition of both weak and strong conductivity gradients across an appropriate subsurface depth range the use of high order spatial derivatives inevitably results in a degree of noise cultural perturbation amplification that is survey and technique specific both of these aspects are considered using practical case studies of jointly obtained magnetic and conductivity data at a variety of spatial scales

antarctic ice sheet loss driven by basal melting of ice shelves accurate prediction of global sea level rise requires that we understand the cause of recent widespread and intensifying glacier acceleration along antarctic ice sheet coastal margins atmospheric and oceanic forcing have the potential to reduce the thickness and extent of floating ice shelves potentially limiting their ability to buttress the flow of grounded tributary glaciers indeed recent ice shelf collapse led to retreat and acceleration of several glaciers on the antarctic peninsula but the extent and magnitude of ice shelf thickness change the underlying causes of such change and its link to glacier flow rate are so poorly understood that its future impact on the ice sheets cannot yet be predicted here we use satellite laser altimetry and modelling of the surface firn layer to reveal the circum antarctic pattern of ice shelf thinning through increased basal melt we deduce that this increased melt is the primary control of antarctic ice sheet loss through a reduction in buttressing of the adjacent ice sheet leading to accelerated glacier flow the highest thinning rates occur where warm water at depth can access thick ice shelves via submarine troughs crossing the continental shelf wind forcing could explain the dominant patterns of both basal melting and the surface melting and collapse of antarctic ice shelves through ocean upwelling in the amundsen and bellingshausen seas and atmospheric warming on the antarctic peninsula this implies that climate forcing through changing winds influences antarctic ice sheet mass balance and hence global sea level on annual to decadal timescales

mass loss of larsen b tributary glaciers antarctic peninsula unabated since 2002 ice mass loss continues at a high rate among the large glacier tributaries of the larsen b ice shelf following its disintegration in 2002 we evaluate recent mass loss by mapping elevation changes between 2006 and 2010 11 using differencing of digital elevation models dems the measurement accuracy of these elevation changes is confirmed by a null test subtracting dems acquired within a few weeks the overall 2006 2010 11 mass loss rate 9 0 2 1 gt a 1 is similar to the 2001 02 2006 rate 8 8 1 6 gt a 1 derived using dem differencing and laser altimetry this unchanged overall loss masks a varying pattern of thinning and ice loss for individual glacier basins on crane glacier the thinning pulse initially greatest near the calving front is now broadening and migrating upstream the largest losses are now observed for the hektoria green glacier basin having increased by 33 since 2006 our method has enabled us to resolve large residual uncertainties in the larsen b sector and confirm its state of ongoing rapid mass loss

recent elevation changes of svalbard glaciers derived from icesat laser altimetry abstract we have tested three methods for estimating 2003 2008 elevation changes of svalbard glaciers from multi temporal icesat laser altimetry a linear interpolation of crossover points between ascending and descending tracks b projection of near repeat tracks onto common locations using digital elevation models dems and c least squares fitting of rigid planes to segments of repeat track data assuming a constant elevation change rate the two repeat track methods yield similar results and compare well to the more accurate but sparsely sampled crossover points most glacier regions in svalbard have experienced low elevation thinning combined with high elevation balance or thickening during 2003 2008 the geodetic mass balance excluding calving front retreat or advance of svalbard s 34 600 km2 glaciers is estimated to be 4 3 1 4 gt y 1 corresponding to an area averaged water equivalent w e balance of 0 12 0 04 m w e y 1 the largest ice losses have occurred in the west and south while northeastern spitsbergen and the austfonna ice cap have gained mass winter and summer elevation changes derived from the same methods indicate that the spatial gradient in mass balance is mainly due to a larger summer season thinning in the west and the south than in the northeast our findings are consistent with in situ mass balance measurements from the same period confirming that repeat track satellite altimetry can be a valuable tool for monitoring short term elevation changes of arctic glaciers

the automatic determination of the location depth and dip of contacts from aeromagnetic data abstracta simple new method termed the contact depth method for the determination of the depth location and dip of contacts from pole reduced magnetic data was evaluated the depth was obtained by computing the horizontal derivative of the tangent of the tilt angle of the magnetic field over the contact although it is based upon the tilt depth method it does not require the distance between contour lines to be measured and it additionally allows the dip of the contact to be estimated from the gradient of the depth estimates the horizontal location of the contact is that of the zero value of the tilt angle the method uses second and third order derivatives of the magnetic field to obtain the parameters of the contact so it is sensitive to noise when tested on synthetic data and on aeromagnetic data from southern africa the method gave plausible results

structural and stratigraphic controls on magmatic volcanogenic and shear zone hosted mineralization in the chapais chibougamau mining camp northeastern abitibi canada the structure and stratigraphy of the chapais chibougamau mining camp in the archean abitibi greenstone belt of the superior craton quebec canada is reevaluated from new aeromagnetic data geochronology and field studies to provide a framework for syngenetic and epigenetic mineralization mineralization in the chapais chibougamau area is synvolcanic volcanogenic massive sulfide vms synmagmatic fe ti v ni cu au ag cu zn pb veins chibougamau type cu au veins and shear zone hosted including opemiska cu au veins subtype vms deposits occur within felsic volcanic rocks of tholeiitic affinity and mafic to felsic volcanic rocks of transitional to calc alkaline affinity at the top of three volcanic cycles of the roy group vms deposits and early polymetallic veins au ag cu zn pb are associated with nnw to nne striking synvolcanic faults fe ti v deposits occur within the layered zone of the lac dore complex especially where it thickens in areas of interpreted synmagmatic faults subeconomic magmatic ni cu deposits occur at the contacts of mafic ultramafic or tonalitic intrusions chibougamau type cu au veins formed through magmatic hydrothermal processes they are cut by dikes that predate regional d2 deformation but are located within and are deformed by nw and ne trending d2 shear zones shear zone hosted au deposits are developed preferentially within regional e w trending deformation corridors and along nne striking sinistral shear zones while opemiska type cu au veins occur within regional overturned anticlines in mafic sills of the cummings complex u pb dating of detrital zircons provides maximum depositional ages for metasedimentary rocks of the blondeau formation of the roy group and the stella formation of the opemisca group which unconformably overlies the roy group the blondeau formation is 2721 3 ma which is coeval to the age of the windfall member in the urban barry belt both show similar exploration potential as they host early polymetallic au ag cu zn pb veins the stella formation is 2704 2 ma and is unaffected by first generation folds thus providing a time constraint for the earliest folding event d1 this new age also indicates that sedimentation in the chapais chibougamau area occurred earlier than for the timiskaming type assemblages in the southern abitibi greenstone belt but the structural setting and presence of alkaline intrusions suggests similar potential for syenite associated au deposits in the stella formation

rapid bedrock uplift in the antarctic peninsula explained by viscoelastic response to recent ice unloading abstract since 1995 several ice shelves in the northern antarctic peninsula have collapsed and triggered ice mass unloading invoking a solid earth response that has been recorded at continuous gps cgps stations a previous attempt to model the observation of rapid uplift following the 2002 breakup of larsen b ice shelf was limited by incomplete knowledge of the pattern of ice unloading and possibly the assumption of an elastic only mechanism we make use of a new high resolution dataset of ice elevation change that captures ice mass loss north of 66 s to first show that non linear uplift of the palmer cgps station since 2002 cannot be explained by elastic deformation alone we apply a viscoelastic model with linear maxwell rheology to predict uplift since 1995 and test the fit to the palmer cgps time series finding a well constrained upper mantle viscosity but less sensitivity to lithospheric thickness we further constrain the best fitting earth model by including six cgps stations deployed after 2009 the larissa network with vertical velocities in the range 1 7 to 14 9 mm yr this results in a best fitting earth model with lithospheric thickness of 100 140 km and upper mantle viscosity of 6 10 17 2 10 18 pa s much lower than previously suggested for this region combining the larissa time series with the palmer cgps time series offers a rare opportunity to study the time evolution of the low viscosity solid earth response to a well captured ice unloading event

icesat observations of arctic sea ice a first look 1 analysis of near coincident icesat and radarsat imagery shows that the retrieved elevations from the laser altimeter are sensitive to new openings containing thin ice or open water in the sea ice cover as well as to surface relief of old and first year ice the precision of the elevation estimates measured over relatively flat sea ice is 2 cm using the thickness of thin ice in recent openings to estimate sea level references we obtain the sea ice freeboard along the altimeter tracks this step is necessitated by the large uncertainties in the sea surface topography compared to that required for accurate determination of freeboard unknown snow depth introduces the largest uncertainty in the conversion of freeboard to ice thickness surface roughness is also derived for the first time from the variability of successive elevation estimates along the altimeter track overall these icesat measurements provide an unprecedented view of the arctic ocean ice cover at length scales at and above the spatial dimension of the altimeter footprint of 70 m

prospectivity analysis of orogenic gold deposits in saqez sardasht goldfield zagros orogen iran abstract diverse deposit types or mineral systems form by diverse geological processes so translation of knowledge about the controls of mineralization acquired from the 4d geological modeling into 2d spatial predictor maps is a major challenge for prospectivity analysis in this regard mathematical functions have been used to model the conceptual or perceived spatial relationships between geological variables and targeted type or system of mineralization in this paper due to the different models of spatial relationships between predictors and mineral deposits we investigated the performance of different fuzzification functions to quantify the relationships we demonstrated that various types of relationships between exploration features and a mineralization type sought could be quantified using different fuzzification functions for prospectivity analysis we illustrated the process of the prospectivity analysis by using a data set of orogenic gold deposits in saqez sardasht goldfield iran prospectivity modeling of orogenic gold mineralization in the study area showed that the ne sw trending targets have priority for further prospecting of the deposits

calving fluxes and basal melt rates of antarctic ice shelves iceberg calving has been assumed to be the dominant cause of mass loss for the antarctic ice sheet with previous estimates of the calving flux exceeding 2 000 gigatonnes per year1 2 more recently the importance of melting by the ocean has been demonstrated close to the grounding line and near the calving front3 4 5 so far however no study has reliably quantified the calving flux and the basal mass balance the balance between accretion and ablation at the ice shelf base for the whole of antarctica the distribution of fresh water in the southern ocean and its partitioning between the liquid and solid phases is therefore poorly constrained here we estimate the mass balance components for all ice shelves in antarctica using satellite measurements of calving flux and grounding line flux modelled ice shelf snow accumulation rates6 and a regional scaling that accounts for unsurveyed areas we obtain a total calving flux of 1 321 144 gigatonnes per year and a total basal mass balance of 1 454 174 gigatonnes per year this means that about half of the ice sheet surface mass gain is lost through oceanic erosion before reaching the ice front and the calving flux is about 34 per cent less than previous estimates derived from iceberg tracking1 2 7 in addition the fraction of mass loss due to basal processes varies from about 10 to 90 per cent between ice shelves we find a significant positive correlation between basal mass loss and surface elevation change for ice shelves experiencing surface lowering8 and enhanced discharge9 we suggest that basal mass loss is a valuable metric for predicting future ice shelf vulnerability to oceanic forcing

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