rise in frequency of surface melting at siple dome through the holocene evidence for increasing marine influence on the climate of west antarctica 1 a new melt layer history from siple dome west antarctica indicates notable late holocene summertime warming visual stratigraphic analyses of the 1004 m ice core identified 62 years with melt layers melting events began around 11 7 ka followed by a period of no melting from 8 8 6 6 ka melt layer frequency increased from 6 6 ka to the present with the 1000 year average melt layer frequency reaching a maximum of 2 at 0 8 ka we use our millennial scale archive of melt events as a unique seasonal paleothermometer to elucidate changes in west antarctic holocene summer climate our calibration suggests the change in melt frequency from 0 to 2 may represent a summer temperature increase of 2 c from the middle to late holocene this temperature change cannot be explained entirely by local change in ice elevation or summer insolation and is in contrast to east antarctic climate records which show peak warmth in the early holocene followed by stable or decreasing temperature we interpret the rise in melt frequency as evidence of an increasing marine influence on the ross sea sector of west antarctica although the surface elevation of siple dome has not changed greatly the continued lateral retreat of the west antarctic ice sheet from its last glacial maximum configuration across the outer continental shelf and the delayed drawdown in ice thickness from the adjacent coastal marie byrd land region in conjunction with periods of increased cyclogenesis perhaps related to variations in enso would allow a moderated maritime climate to more easily reach west antarctica

extensive liquid meltwater storage in firn within the greenland ice sheet surface melt water from the greenland ice sheet can become trapped in firn delaying its journey to the sea radar and ice core observations provide direct evidence of a perennial aquifer in the firn layer in southern greenland that represents a potentially significant contribution to the greenland mass budget

bed length does not remain constant during deformation recognition and why it matters abstract we apply multiple balancing restoration methods to three examples of fault bend folds exhibiting increasing levels of complexity and uncertainty three methods the chamberlin depth to detachment calculation direct measurement of fault displacement and flexural slip restoration balancing assume that bed lengths and thicknesses blt remain constant during deformation the area depth strain ads method allows bed lengths and thicknesses to vary during deformation for a kinematic model the agreement among methods is exact to within measurement error for an experimental sand model the disagreement among methods is substantial the ads relationship shows that the sand model has significant layer parallel shortening and an area increase of 4 a previously published interpretation of a seismically imaged fault bend fold from the rosario oil field venezuela is nearly line length balanced but the ads relationship indicates small but significant anomalies including an area deficit for the deeper stratigraphic levels a revised interpretation with a more internally consistent ads relationship suggests that much of the footwall uplift is real and not a velocity pull up our comparisons of the results of the various balancing restoration techniques show the resolving power of the ads method to detect sub resolution changes in bed lengths and thicknesses and to identify footwall structures overlooked by the constant blt methods

geometric analysis of fold development in overthrust terranes abstract fault bend folding fault propagation folding and detachment or decollement folding are three distinct scenarios for fold thrust interaction in overthrust terranes simple kink hinge models are used to determine the geometric associations implicit in each scenario bedding maintains constant thickness in the models except in the forelimb of the fold the forelimb is allowed to thicken or thin without limit the models address individual folds and the calculated fold geometries are balanced structures each mode of fold thrust interaction has a distinct set of geometric relationships final fold geometry is adequate in itself to discern many fault bend folds this is not the case for fault propagation and detachment folds these two fold forms have very similar geometric relationships some knowledge of the nature of the underlying thrust or decollement zone is usually needed to distinguish between them the geometry of a fold is altered in a predictable fashion by transport through an upper ramp hinge and by fault parallel shearing of the structure the shearing results in a tighter fold whereas transport through the ramp hinge produces a broader fold the viability of the geometric analysis technique is demonstrated through its application to a pair of detachment folds from the canadian cordillera the geometric analysis is also used to evaluate cross sections through subsurface structures in an example from the turner valley oil field the analysis indicates how the interpretation should be altered so as to balance the cross section the analysis reveals hidden assumptions and specific inconsistencies in structural interpretations

palaeotemperatures still exist in the greenland ice sheet the temperature distribution through the greenland ice sheet at the dye 3 borehole is a record of the past climatic changes in the arctic the numerical model of the temperature distribution now presented reproduces the observed temperature distribution within 0 03 k and shows that the basal ice is still cooled 5 k by the cold ice age climate the results suggest a mean ice age temperature of 32 2 c which is 12 k colder than the present temperature and a precipitation rate 50 25 of the present rate calculations of a more detailed temperature history through the present inter glacial period reveal evidence of the ad 1920 50 maximum the little ice age and the atlantic period

variations in melt layer frequency in the gisp2 ice core implications for holocene summer temperatures in central greenland the rare melt features in the gisp2 central greenland deep ice core have decreased in frequency over the most recent 7000 years calibration of this change in melt frequency against modern spatial variation of melt frequency and temperature in central greenland and against modern temporal variability of temperatures in central greenland indicates that mean mid summer temperatures have cooled over the most recent 7000 years probably by slightly more than l c if variability of summer temperatures has not changed comparison to grip isotopic records from central greenland and to the melt record from the agassiz ice cap arctic canada suggests some seasonal and regional coherence for this cooling signal as well as for a cold event about 8000 8500 bp

chironomids record terrestrial temperature changes throughout arctic interglacials of the past 200 000 yr quaternary interglacial periods provide glimpses of a warmer arctic and useful perspectives on possible future conditions but records of arctic terrestrial conditions over multiple interglacial periods are rare here we take advantage of a site in the canadian arctic where lacustrine sediments representing the past three interglacial periods are preserved in an extant lake we use subfossil insects chironomids preserved in this exceptional sedimentary archive to derive temperature reconstructions through the holocene up to a d 2005 through the last interglacial sensu stricto marine isotope stage or mis 5e and a portion of the penultimate interglacial mis 7 chironomid inferred temperatures are warmest for the early holocene and mis 5e two periods with enhanced northern hemisphere insolation forcing relative to today twentieth century warming at this site apparently caused the recent extirpation of cold stenothermous chironomid taxa assemblages from mis 5e have close analogs in modern training set data as determined by squared chord distance and mis 5e species assemblages are very similar to holocene assemblages at this site mis 7 sediments record summer temperatures similar to those of the mid to late holocene followed by a descent into glacial conditions even mis 7 chironomid assemblages dating back 200 000 yr have close modern analogs these lake sediments also provide direct evidence for a period of regional deglaciation between mis 5e and the holocene most likely mis 5a to our knowledge the data presented here represent the longest paleotemperature record thus far generated using chironomids the existence of close modern analogs for ancient chironomid assemblages at lake cf8 suggests that this method can provide useful paleotemperature estimates extending back hundreds of millennia

climatic signal of ice melt features in southern greenland the stratigraphie record of melt features in intermediate depth polar ice cores has provided valuable data on past summer climate1 3 the thermal drilling technique used in previous studies precluded extending this record beyond a few hundred years recently however a 901 m deep ice core was mechanically drilled in southern greenland the excellent core quality and improved techniques for measuring melt features have resulted in an 2 200 yr record with consistently high quality data we report here that the temporal variations in melt feature abundance expressed as annual melt per cent amp provide proxy data on summer climate that can complement other records extending back thousands of years the amp profile indicates that relatively warm summers prevailed from about ad 950 until ad 1500 culminating in the thirteenth and fourteenth centuries the periods from about 300 to 150 bc ad 350 to 550 and ad 1550 to 1800 had relatively little summer melting

differentiating bubble free layers from melt layers in ice cores using noble gases melt layers are clear indicators of extreme summer warmth on polar ice caps the visual identification of refrozen meltwater as clear bubble free layers cannot be used to study some past warm periods because in deeper ice bubbles are lost to clathrate formation we present here a reliable method to detect melt events based on the analysis of kr ar and xe ar ratios in ice cores and apply it to the detection of melt in clathrate ice from the eemian at neem greenland additionally melt layers in ice cores can compromise the integrity of the gas record by dissolving soluble gases or by altering gas transport in the firn which affects the gas chronology we find that the easily visible 1 mm thick bubble free layers in the wais divide ice core do not contain sufficient melt to alter the gas composition in the core and do not cause artifacts or discontinuities in the gas chronology the presence of these layers during winter and the absence of anomalies in soluble gases suggests that these layers can be formed by processes other than refreezing of meltwater consequently the absence of bubbles in thin crusts is not in itself proof of a melt event

characterization and formation of melt layers in polar snow observations and experiments from west antarctica author posting international glaciological society 2005 this article is posted here by permission of international glaciological society for personal use not for redistribution the definitive version was published in journal of glaciology 51 2005 307 312 doi 10 3189 172756505781829395

the role of strain in area constant detachment folding abstract the classical constant area relationship for balancing cross sections is reformulated to accommodate layer parallel strain leading to a general equation for the layer parallel strain in a fold formed above a stratigraphically fixed detachment horizon the strain required for area balance can be calculated from a knowledge of the fold shape and the depth to detachment the strains predicted from a simple kinematic model are realistic in the order of 10 30 layer parallel shortening the strain may be penetrative or occur as small scale disharmonic folds and faults the tip top anticline a fold at the leading edge of the wyoming thrust belt is used to illustrate the applicability of this reformulation the computed layer parallel strains in the well controlled upper portion of the structure are less than 5 because most of the shortening is accommodated by second order folds and faults

eemian interglacial reconstructed from a greenland folded ice core efforts to extract a greenland ice core with a complete record of the eemian interglacial 130 000 to 115 000 years ago have until now been unsuccessful the response of the greenland ice sheet to the warmer than present climate of the eemian has thus remained unclear here we present the new north greenland eemian ice drilling neem ice core and show only a modest ice sheet response to the strong warming in the early eemian we reconstructed the eemian record from folded ice using globally homogeneous parameters known from dated greenland and antarctic ice core records on the basis of water stable isotopes neem surface temperatures after the onset of the eemian 126 000 years ago peaked at 8 4 degrees celsius above the mean of the past millennium followed by a gradual cooling that was probably driven by the decreasing summer insolation between 128 000 and 122 000 years ago the thickness of the northwest greenland ice sheet decreased by 400 250 metres reaching surface elevations 122 000 years ago of 130 300 metres lower than the present extensive surface melt occurred at the neem site during the eemian a phenomenon witnessed when melt layers formed again at neem during the exceptional heat of july 2012 with additional warming surface melt might become more common in the future

melt features in ice cores from site j southern greenland some implications for summer climate since ad 1550 horizontal and vertical distributions of melt features ice layers were examined using two ice cores 206 6 and 101 5 m deep 1 m apart from site j 66 51 9 n 46 15 9 w 2030 m a s l the temperature at 10 m was 16 3 c we observed 2804 melt features with a total thickness of 30 32 m in the 206 6 m core corresponding to 16 4 by volume of the ice equivalent core length horizontal distribution of melt features was examined by correlating melt feature thicknesses in the two cores the correlation coefficient was 0 71 n 514 for each melt feature in the two cores it was maximum for data passed through 5 and 40 year low pass filters a significant relationship p 0 005 n 36 was obtained for the vertical distribution of melt features and the june temperature on the west coast of greenland jakobshavn using this june temperatures at jakobshavn since 1550 were estimated there are three periods 1685 1705 1835 70 and 1933 45 during which mean june temperatures clearly decreased when they were estimated to he 0 1 0 4 and 0 2 c lower than the average for the whole period 1550 1989 the first two cold periods have been identified in melt features of the dye 3 and devon island ice cores and in a tree ring profile from yukon territory canada melt feature percentages in the site j ice core have increased since about 1945 probably reflecting summer temperature warming on the ice sheet

using noble gas ratios to determine the origin of ground ice abstract argon krypton and xenon have different solubilities in water meaning their ratios in water are different from those in atmospheric air this characteristic is used in a novel method to distinguish between ice bodies which originate from the compaction of snow i e buried snow banks glacial ice vs ice which forms from the freezing of groundwater i e pingo ice ice which forms from the compaction of snow has gas ratios similar to atmospheric air while ice which forms from the freezing of liquid water is expected to have gas ratios similar to air equilibrated water this analysis has been conducted using a spike dilution noble gas line with gas extraction conducted on line samples were mixed with an aliquot of rare noble gases while being melted then extracted gases are purified and cryogenically separated samples have been analysed from glacial ice buried snow bank ice intrusive ice wedge ice cave ice and two unknown ice bodies ice bodies which have formed from different processes have different gas ratios relative to their formation processes

ice layer and surface crust permeability in a seasonal snow pack ice layers in snowpacks are thought to retard or obstruct gas and melt water flow through the pack yet there has been little published information on the permeability of ice layers in seasonal snow in this paper investigations of ice layer permeabilities are reported for prominent ice layers 1 to 3 mm thick in a seasonal snowpack for distinct fairly solid ice layers 1 to 3 mm thick permeabilities ranged from 1 to 19 x 10 10 m 2 these values are significantly lower than values for the surrounding snow within 24 h early in the melt season the permeability of the surface crust increased 160 from approximately 70 to 120 x 10 10 m 2 this increase in permeability was accompanied by a small increase in grain size and a decrease in albedo the snow air interface can exhibit dramatic changes in transport properties over relatively short periods of time even without precipitation events these changes driven by the surface meteorology affect transport properties such as permeability as well as albedo

variability in the surface temperature and melt extent of the greenland ice sheet from modis 1 satellite derived moderate resolution imaging spectroradiometer modis ice surface temperature ist of the greenland ice sheet shows a positive trend and two major melt events from 2000 to present ist increased by 0 55 0 44 c decade with the greatest increase 0 95 0 44 c decade found in northwestern greenland where coastal temperatures and mass loss are also increasing and outlet glaciers are accelerating ist shows the highest rates of increase during summer 1 35 0 47 c decade and winter 1 30 1 53 c decade followed by spring 0 60 0 98 c decade in contrast a decrease in ist was found in the autumn 1 49 1 20 c decade the ist trends in this work are not statistically significant with the exception of the trend in northwestern greenland major surface melt covering 80 or more of the ice sheet occurred during the 2002 and 2012 melt seasons where clear sky measurements show a maximum melt of 87 and 95 of the ice sheet surface respectively in 2002 most of the extraordinary melt was ephemeral whereas in 2012 the ice sheet not only experienced more total melt but melt was more persistent and the 2012 summer was the warmest in the modis record 6 38 3 98 c our data show that major melt events may not be particularly rare during the present period of ice sheet warming

simulating arctic climate warmth and icefield retreat in the last interglaciation in the future arctic warming and the melting of polar glaciers will be considerable but the magnitude of both is uncertain we used a global climate model a dynamic ice sheet model and paleoclimatic data to evaluate northern hemisphere high latitude warming and its impact on arctic icefields during the last interglaciation our simulated climate matches paleoclimatic observations of past warming and the combination of physically based climate and ice sheet modeling with ice core constraints indicate that the greenland ice sheet and other circum arctic ice fields likely contributed 2 2 to 3 4 meters of sea level rise during the last interglaciation

the extreme melt across the greenland ice sheet in 2012 1 the discovery of the 2012 extreme melt event across almost the entire surface of the greenland ice sheet is presented data from three different satellite sensors including the oceansat 2 scatterometer the moderate resolution imaging spectroradiometer and the special sensor microwave imager sounder are combined to obtain composite melt maps representing the most complete melt conditions detectable across the ice sheet satellite observations reveal that melt occurred at or near the surface of the greenland ice sheet across 98 6 of its entire extent on 12 july 2012 including the usually cold polar areas at high altitudes like summit in the dry snow facies of the ice sheet this melt event coincided with an anomalous ridge of warm air that became stagnant over greenland as seen in melt occurrences from multiple ice core records at summit reported in the published literature such a melt event is rare with the last significant one occurring in 1889 and the next previous one around seven centuries earlier in the medieval warm period given its rarity the 2012 extreme melt across greenland provides an exceptional opportunity for new studies in broad interdisciplinary geophysical research

structural style and kinematic analysis of folding in the southern dezful embayment oilfields sw iran abstract compression of a region containing multiple detachment layers produces large scale disharmonic folding where individual competent units separated by detachment fold independently within the succession here we present a case study of the kinematic evolution of three subsurface structures the karanj paranj and parsi oilfields bounded by multiple detachments located in the southern dezful embayment a major petroleum province of the zagros fold thrust belt in sw iran interpretation of seismic sections and the construction of corresponding balanced cross sections reveals different geometries along the folds and the complete decoupling of these structures from the surface geology this is controlled by the reactivation of basement faults and flow of the gachsaran incompetent units miocene as the upper detachment and the kazhdumi upper cretaceous pabdeh gurpi and dashtak formations as the middle detachments respectively detachment folding with limb rotation started in the middle miocene and continued with the migration of the basal detachment horizon equivalent hormuz series into the core of the anticlines flow of both the basal and upper detachments continued until no more material is available whereupon the shortening is accommodated by fault development the development of a decoupled disharmonic folding style was controlled by the upper detachment surface through migration from the crests of the anticlines to the synclinal areas and by the reactivation of the basement faults from nw to se part of the area the total shortening amounts are changed between 21 and 12

estimating the detachment depth in cross sections involving detachment folds this paper evaluates four different methods that have been proposed for the estimation of the detachment depth beneath detachment folds guidelines are presented in order to use the most suitable method in a particular region deformed by detachment folds these guidelines are constructed considering the assumptions of each method the influence of different parameters on the estimation of the detachment depth folding mechanisms cross sectional area variations position and orientation of the regional datum number of stratigraphic horizons bed length and thickness dip and position of the bounding lines initial thickness of the ductile unit single anticlines or fold trains and the available data ductile unit thickness regional datum detachment depth moreover a new simple method based on two previous methods is presented the advantages of this new method are that the calculations are extremely simple and it uses information from more than one stratigraphic horizon therefore it does not depend strongly on the accuracy of the data at one level the precision of the proposed methods is tested by their application to a number of natural and experimental single anticlines and fold trains it appears that better predictions are obtained when analysing a complete detachment fold train than when dealing with a single detachment anticline the reason might be that ductile material flow beneath the folds along the cross sectional plane is taken into account when dealing with a long enough cross section we also suggest some hypotheses concerning what the shape and dimensions of the detachment folds indicate about the detachment depth in those cases where insufficient data are available to apply the methods proposed

constraining ventilation during deepwater formation using deep ocean measurements of the dissolved gas ratios 40ar 36ar n2 ar and kr ar equilibrium saturation and for d 40 ar a value of 1 188 0 055 relative to air these data are used to constrain high latitude ventilation processes in the framework of three box and seven box ocean models for the three box model tracer data we constrain the appropriate surface area of the high latitude region in both models to be 3 6 2 5 1 7 of ocean surface area and the bubble air injection rate to be 22 7 8 8 7 3 mol air m 2 yr 1 results for the seven box model were similar with a high latitude area of 3 3 2 2 1 3 our results provide geochemical support for suggestions that the effective area of high latitude ventilation is much smaller than the region of elevated preformed nutrients and demonstrate that noble gases strongly constrain the ocean solubility pump reducinghigh latitude surfaceareaweakens theco2solubility pumpintheboxmodels and limits communication between the atmosphere and deep ocean these tracers should be usefulconstraintsonhigh latitudeventilationandthestrengthofthesolubilitypumpinmore complex ocean general circulation models

glacier regime on the northern slope of the himalaya xixibangma glaciers abstract the mass energy exchange components and ice thickness of the xixibangma glacier massif were evaluated during the chinese russian glaciological expedition to the northern himalaya in 1991 long term data from two chinese meteorological stations were used to analyze the glacier climatic regime in the surrounding area it was determined that solar radiation income during the summer autumn monsoon is half that theoretically possible because of cloudiness melting of the glaciers on the northern slopes of the himalaya is driven by solar radiation up to 86 continuous cold katabatic winds decrease turbulent exchange resulting in only a minor contribution from sensible heat transfer to the heat balance of glaciers strong katabatic winds suppress precipitation but favor evaporation reducing glacier melting even in the ablation zone evaporation is very important in the heat balance consuming up to 57 of the heat and so limiting glacier melting autumn ice formation occurs as refrozen melt water in a thin snow layer on the ice surface the mean mass balance index for the period from 1956 to 1990 of the xixibangma glacier was negative amounting to 34 kg m 2

oceanic forcing of penultimate deglacial and last interglacial sea level rise sea level histories during the two most recent deglacial interglacial intervals show substantial differences1 3 despite both periods undergoing similar changes in global mean temperature4 5 and forcing from greenhouse gases6 although the last interglaciation lig experienced stronger boreal summer insolation forcing than the present interglaciation7 understanding why lig global mean sea level may have been six to nine metres higher than today has proven particularly challenging2 extensive areas of polar ice sheets were grounded below sea level during both glacial and interglacial periods with grounding lines and fringing ice shelves extending onto continental shelves8 this suggests that oceanic forcing by subsurface warming may also have contributed to ice sheet loss9 12 analogous to ongoing changes in the antarctic13 14 and greenland15 ice sheets such forcing would have been especially effective during glacial periods when the atlantic meridional overturning circulation amoc experienced large variations on millennial timescales16 with a reduction of the amoc causing subsurface warming throughout much of the atlantic basin9 12 17 here we show that greater subsurface warming induced by the longer period of reduced amoc during the penultimate deglaciation can explain the more rapid sea level rise compared with the last deglaciation this greater forcing also contributed to excess loss from the greenland and antarctic ice sheets during the lig causing global mean sea level to rise at least four metres above modern levels when accounting for the combined influences of penultimate and lig deglaciation on glacial isostatic adjustment this excess loss of polar ice during the lig can explain much of the relative sea level recorded by fossil coral reefs and speleothems at intermediate and far field sites a reduction in the strength of the atlantic meridional overturning circulation initiated during the penultimate deglaciation led to excess polar ice losses contributing to higher sea levels during the last interglacial period

retention of greenland runoff by refreezing implications for projected future sea level change melting produced at the surface of subfreezing permeable firn does not freely percolate but instead refreezes near its point of origin raising the firn temperature and decreasing the pore space if sufficient meltwater is introduced the firn will warm to 0 c and subsequent water will fill the remaining pore space without freezing only after the residual water content of the firn is exceeded will water flow through the firn and possibly escape as runoff this process prevents summer melt on high arctic glaciers and ice caps from escaping in its entirety and it must be accounted for in modeling glacier runoff on the basis of surface energy balance a model is presented here which describes in a simple way the transient process of infiltration refreezing and runoff in a future warming climate the model is applied to greenland for which predictions of runoff induced sea level rise that do not consider the refreezing process could be as much as 5 0 cm too high over 150 years

trace element and physical response to melt percolation in summit greenland snow surface melt on a glacier can perturb the glaciochemical record beyond the natural variability while the centre of the greenland ice sheet is usually devoid of surface melt many higharctic and alpine ice cores document frequent summertime melt events current hypotheses interpreting melt affected ice core chemistry rely on preferential elution of certain major ions however the precise nature of chemistry alteration is unknown because it is difficult to distinguish natural variability from melt effects in a perennially melt affected site we use eight trace element snow chemistry records recovered from summit greenland to study spatial variability and melt effects on insoluble trace chemistry and physical stratigraphy due to artificially introduced meltwater differences between non melt and melt affected chemistry were significantly greater than the spatial variability in chemistry represented by nearest neighbour pairs melt perturbed trace elements particularly rare earth elements retained their seasonal stratigraphies suggesting that trace elements may serve as robust chemical indicators for annual layers even in melt affected study areas results suggest trace element transport via meltwater percolation will deposit eluted material down pit in refrozen areas below the nearest surface chemistry peak in our experiments snow chemistry analyses are more sensitive to melt perturbations than density changes or unprocessed near infrared digital imagery

interglacial and holocene temperature reconstructions based on midge remains in sediments of two lakes from baffin island nunavut arctic canada abstract midge remains insecta diptera chironomidae and chaoboridae from two upland lake basins on the cumberland peninsula of baffin island were analyzed and the results used in a transfer function to reconstruct paleotemperatures this upland region remained ice free throughout the last glacial cycle and the two lake basins retain sediments deposited during the previous interglacial period as well as during the holocene period but they did not accumulate sediment during most of the glacial period midge remains are abundant and well preserved at both sites midge inferred summer surface water temperatures and mean july air temperatures were estimated using an inference model built on modern samples spanning a geographic range extending from devon island canada to maine usa data from twenty nine new surface samples from baffin island were added to an existing inference model the new weighted averaging wa model for summer surface water temperature yields r 2 jack 0 88 and rmsep 2 22 c for summer water temperatures and r 2 jack 0 88 and rmsep 1 53 c for mean july air temperatures reconstructions at both sites indicate that summer temperatures during the last interglacial were higher than at any time in the holocene and 5 to 10 c higher than present peak holocene temperatures occurred in the first half of the period and have decreased since about the mid holocene

atmospheric co2 over the last 1000 years a high resolution record from the west antarctic ice sheet wais divide ice core 1 we report a decadally resolved record of atmospheric co2 concentration for the last 1000 years obtained from the west antarctic ice sheet wais divide shallow ice core the most prominent feature of the pre industrial period is a rapid 7 ppm decrease of co2 in a span of 20 50 years at 1600 a d this observation confirms the timing of an abrupt atmospheric co2 decrease of 10 ppm observed for that time period in the law dome ice core co2 records but the true magnitude of the decrease remains unclear atmospheric co2 variations over the time period 1000 1800 a d are statistically correlated with northern hemispheric climate and tropical indo pacific sea surface temperature however the exact relationship between co2 and climate remains elusive due to regional climate variations and or uneven geographical data density of paleoclimate records we observe small differences of 0 2 0 6 ppm among the high precision co2 records from the law dome epica dronning maud land and wais divide antarctic ice cores however those records share common trends of co2 change on centennial to multicentennial time scales and clearly show that atmospheric co2 has been increasing above preindustrial levels since 1850 a d

excess area and depth to detachment a plot of the excess area in cross section vs the depth to a reference level for several stratigraphic horizons in a compressional fold produces a curve that gives the depth to detachment the curve is a straight line for detachment folds the slope of the line is the displacement on the lower detachment and the depth intercept where excess area zero is the depth to detachment for a fault bend fold the result is a line that abruptly changes slope at the position corresponding to the upper detachment the difference in slope between the two parts of the line is the displacement on the upper detachment added layer parallel shear produces a curve on the graph tested on detachment folds from the jura mountains the method accurately predicts the depth to detachment he tip top field anticline in the wyoming thrust belt is found to be interpretable as either a detachment anticline with a basal detachment close to the top of the basement or as a fault bend fold with a basal detachment at the base of the triassic the excess area diagram is useful for testing a cross section for balance determining the structural style detachment anticline vs fault bend fold and for examining alternatives in the construction of cross sections

geometric constraints derived from the law of conservation of volume and applied to evolutionary models for detachment folding this paper presents geometric arguments for modifying the conventional concept for the evolution of concentric fold trains above nonductile detachment zones the common geometric model for concentric folding above a detachment invokes a wave train with fixed fold axes regularly spaced along the fold panel limb lengths from crest to trough remain constant as the limb dips increase to uplift the anticlinal crests while the synclinal troughs slide parallel to the detachment plane the basic depth to detachment calculation which derives from the law of conservation of volume requires that the area of uplift above regional for a concentric anticline be equal to the arithmetic product of the shortening and the depth to detachment this imposes a geometric constraint with which a constant limb length anticline growing by increasing limb dip above a fixed detachment plane cannot comply unless in the early stages mobile material in the detachment zone flows from the synclinal troughs to the anticlinal crests and then in the later stages the flow direction reverses relatively few detachment zones contain a significant amount of mobile salt or shale therefore in most concentric detachment fold trains a basic geometric incompatibility exists between the conventional geometric model and themore law of conservation of volume the conflict can be resolved by an evolutionary fold model wherein the anticlinal fold limbs are short at the inception of folding and grow longer as dips increase and the fold grows in this model the anticlinal axes generally are fixed and the synclinal axes active which has structural and economic implications for the distribution of permeability and syntectonic sediments as well as for the migration and entrapment of hydrocarbons 18 figs less

greenland ice sheet contribution to sea level rise buffered by meltwater storage in firn observations of firn structure and meltwater retention on the greenland ice sheet s percolation zone a region of the ice sheet that is perennially covered by snow and firn quantify the capacity of the firn to store future surface meltwater and to delay expansion of the area contributing to sea level rise the extent and the fate of surface melt on the greenland ice sheet are important factors contributing to sea level change the pore spaces in firn a type of partly compacted snow can be filled by meltwater and thus act as a temporary reservoir here joel harper et al show that the greenland ice sheet parts of which are permanently covered by snow and firn can buffer several hundred gigatonnes of meltwater for several decades although this buffering capacity is only temporary and cannot be easily regained it has the potential to delay the pace of near term sea level rise surface melt on the greenland ice sheet has shown increasing trends in areal extent and duration since the beginning of the satellite era1 2 3 records for melt were broken in 20054 20075 20106 and 20127 much of the increased surface melt is occurring in the percolation zone a region of the accumulation area that is perennially covered by snow and firn partly compacted snow the fate of melt water in the percolation zone is poorly constrained some may travel away from its point of origin and eventually influence the ice sheet s flow dynamics and mass balance and the global sea level whereas some may simply infiltrate into cold snow or firn and refreeze with none of these effects here we quantify the existing water storage capacity of the percolation zone of the greenland ice sheet and show the potential for hundreds of gigatonnes of meltwater storage we collected in situ observations of firn structure and meltwater retention along a roughly 85 kilometre long transect of the melting accumulation area our data show that repeated infiltration events in which melt water penetrates deeply more than 10 metres eventually fill all pore space with water as future surface melt intensifies under arctic warming a fraction of melt water that would otherwise contribute to sea level rise will fill existing pore space of the percolation zone we estimate the lower and upper bounds of this storage sink to be 322 44 gigatonnes and gigatonnes respectively furthermore we find that decades are required to fill this pore space under a range of plausible future climate conditions hence routing of surface melt water into filling the pore space of the firn column will delay expansion of the area contributing to sea level rise although once the pore space is filled it cannot quickly be regenerated

strain partitioning in the helvetic thrust belt of eastern switzerland from the leading edge to the internal zone abstract the strain and strain partitioning in the helvetic thrust belt are analysed in order to determine their variation across the belt from the external thin skinned portion to the internal thick skinned portion and in order to begin the detailed mechanical analysis of the entire fold and thrust belt coarse grained limestones of jurassic and cretaceous age were chosen for analysis because they can be traced from the external to the internal zones the strain within a bed is partitioned into three components transgranular pressure solution microfaults veins twinning granular and intragranular exclusive of twinning crystal plasticity associated with recrystallization transgranular strain was very important in the early history of the helvetic nappes giving way to twinning at a later stage under tectonic burial twinning strain was early in the infrahelvetic complex giving way to intracrystalline deformation at the later stage of burial the transition from transgranular to intragranular dominated strain occurred at a vitrinite reflectance value of 3 5 significantly below the greenschist grade of metamorphism in a large scale fold at the leading edge of the helvetic nappes transgranular strains were associated with the buckling stage of the fold and twinning occurred during fold tightening in more internal parts of the helvetic nappes twinning strains are also late with regard to transgranular strains but here their pattern corresponds to the regional finite strain pattern in the internal zone the infrahelvetic complex twinning seems to have been an early event and records an initial layer parallel shortening followed by regional internal deformation with intragranular strains

formation of ice layers by infiltration and refreezing of meltwater melt layer frequency and magnitude in polar and sub polar ice cores have been interpreted as measures of past summer temperature and calibrations have been proposed relating frequency of occurrence of ice layers in ice cores to past summer temperatures but observations in the percolation facies in greenland and an analysis of the combined processes of meltwater infiltration and refreezing of water in snow indicate that in addition to unusually high rates of meltwater input formation of ice layers will also be facilitated by unusually cold initial conditions or early onset of melt uniform warming of both summer and winter conditions has the opposite effect and suppresses ice layer formation in favor of uniform wetting and refreezing of the snowpack numerical modeling of infiltration and refreezing at a stratigraphic fine to coarse transition allows quantification of the effects of significant parameters initial temperature grain size and density contrast across the stratigraphic transition water input rate and minimum impermeable layer thickness calculations are made to distinguish threshold values of parameters at which infiltration progresses faster than refreezing resulting in a break through of water across the stratigraphic transition from values leading to the formation of an ice layer when refreezing progresses faster than infiltration

physical properties of the wais divide ice core the wais west antarctic ice sheet divide deep ice core was recently completed to a total depth of 3405 m ending 50 m above the bed investigation of the visual stratigraphy and grain characteristics indicates that the ice column at the drilling location is undisturbed by any large scale overturning or discontinuity the climate record developed from this core is therefore likely to be continuous and robust measured grain growth rates recrystallization characteristics and grain size response at climate transitions fit within current understanding significant impurity control on grain size is indicated from correlation analysis between impurity loading and grain size bubble number densities and bubble sizes and shapes are presented through the full extent of the bubbly ice where bubble elongation is observed the direction of elongation is preferentially parallel to the trace of the basal 0001 plane preferred crystallographic orientation of grains is present in the shallowest samples measured and increases with depth progressing to a vertical girdle pattern that tightens to a vertical single maximum fabric this single maximum fabric switches into multiple maxima as the grain size increases rapidly in the deepest warmest ice a strong dependence of the fabric on the impurity mediated grain size is apparent in the deepest samples

a method for precise measurement of argon 40 36 and krypton argon ratios in trapped air in polar ice with applications to past firn thickness and abrupt climate change in greenland and at siple dome antarctica we describe a method for measuring the 40ar 36ar ratio and the 84kr 36ar ratio in air from bubbles trapped in ice cores these ratios can provide constraints on the past thickness of the firn layer at the ice core site and on the magnitude of past rapid temperature variations when combined with measured 15n 14n both variables contribute to paleoclimatic studies and ultimately to the understanding of the controls on earth s climate the overall precision of the 40ar 36ar method 1 standard error of the mean is 0 012 for a sample analyzed in duplicate corresponding to 0 6 m in reconstructed firn thickness we use conventional dynamic isotope ratio mass spectrometry with minor modifications and special gas handling techniques designed to avoid fractionation about 100 g of ice is used for a duplicate pair of analyses an example of the technique applied to the gisp2 ice core yields an estimate of 11 3k of abrupt warming at the end of the last glacial period 15 000 years ago the krypton argon ratio can provide a diagnostic of argon leakage out of the bubbles which may happen naturally during bubble close off or artifactually if samples are warmed near the freezing point during core retrieval or storage argon leakage may fractionate the remaining 40ar 36ar ratio by 0 007 per change in 84kr 36ar introducing a possible bias in reconstructed firn thickness of about 2 m if thermal diffusion is not accounted for or 6 m if thermal diffusion effects are quantified with measured 15n 14n reproducibility of 84kr 36ar measured in air is about 0 2 1 standard error of the mean but is about 1 for ice core samples ice core samples are systematically enriched in 84kr 36ar relative to atmosphere by 5 probably reflecting preferential size dependent exclusion of the smaller argon atom during bubble entrapment recent results from the siple dome ice core reveal two climate events during the last deglaciation including an 18 m reduction in firn thickness associated with an abrupt warming at sometime between 18 and 22 kyr bp and a partial or total removal of the firn during an ablation event at 15 3 kyr bp

precision and accuracy of the calcite strain gage technique the twinned calcite strain gage technique allows the determination of strain magnitude and direction in mildly deformed sedimentary rocks in that the measurements required for the calculation are rather time consuming it is important to know the minimum number required to obtain a precision that is comparable with the accuracy of the result this paper documents a series of tests on three different limestones having different textures and porosities in order to determine the precision and accuracy of the technique as a function of the number of measurements and the number of perpendicular thin sections the tests show that for most purposes the best measurement strategy will be to make 25 measurements on each of two perpendicular thin sections the best determined principal strain axis is the one having a magnitude that is opposite in sign from the other two over the range of zero to 11 shortening the magnitude of the odd signed strain axis will than have a precision and accuracy of about 11 in magnitude and a precision and accuracy of about 6 to 8 in orientation differences in texture and porosity do not appear to have significant effects upon precision and accuracy

onset of deglacial warming in west antarctica driven by local orbital forcing an annually resolved ice core record from west antarctica indicates that warming driven by local insolation resulting from sea ice decline began in that region about 2 000 years before warming in east antarctica reconciling two alternative explanations for deglacial warming in the southern hemisphere

lateral variations of strain in experimental forced folds abstract packages of rock layers deformed under confining pressure by the uplift and rotation of a steel block forcing assembly translate both towards and away from the margin of the principal uplifted block the resulting asymmetric forced folds and especially their long planar limbs exhibit along layer variations in strain alternations of layer elongation and contraction occur along profiles extending away from the antiform synform couplet layer normal strains are mostly nil so the longitudinal strains largely equate to volume strains in these plane strain models spaced anomalies in outcrops indicating either increased cementation or erosional weakness may suggest that similar processes operate in nature to produce variations in damage caused during the flexural slip process two non exclusive explanations are offered to account for the patterns of strain observed in the experiments 1 they may be caused by decaying wavetrains of small scale flexural deflections and their local strain patterns related to the bending of the major forced folds or 2 they may be caused by a patchy development of layer parallel slip and the consequent spatial variability in displacements

a method to measure kr n2 ratios in air bubbles trapped in ice cores and its application in reconstructing past mean ocean temperature 1 we describe a new method for precise measurement of kr n2 ratios in air bubbles trapped in ice cores and the first reconstruction of atmospheric kr n2 during the last glacial maximum lgm 20 000 years ago after gravitational correction the kr n2 record in ice cores should represent the atmospheric ratio which in turn should reflect past ocean temperature change due to the dependence of gas solubility on temperature the increase in krypton inventory in the glacial ocean due to higher gas solubility in colder water causes a decrease in the atmospheric inventory of krypton assuming kr and n2 inventories in the ocean atmosphere system are conserved we use a mass balance model to estimate a mean ocean temperature change between the lgm and today we measured kr n2 in air bubbles in greenland gisp2 ice from the late holocene and lgm using the present atmosphere as a standard the late holocene kr n2 means from two sets of measurements are not different from zero 0 07 0 30 and 0 14 0 93 as expected from the relatively constant climate of the last millennium the mean kr n2 in air bubbles from the lgm is 1 34 0 37 using the mass balance model we estimate that the mean temperature change between the lgm ocean and today s ocean was 2 7 0 6 c although this error is large compared to the observed change this finding is consistent with most previous estimates of lgm deep ocean temperature based on foraminiferal 18o and sediment pore water 18o and chlorinity

predicting reservoir scale faults with area balance application to growth stratigraphy a reservoir that appears to lack faults at one scale of resolution or at one sampling density may nevertheless contain faults that are below the resolution of the observations the area depth relationship from a balanced cross section is shown to contain the necessary information for predicting the sub resolution fault heave existing area depth theory is extended to include growth units allowing structural length and thickness changes to be separated from the depositional changes the technique is validated with a numerical model of a growth full graben and a sand box model of a half graben then field tested in the gilbertown graben a growth structure within the regional peripheral fault trend along the northern margin of the gulf of mexico a cross section developed from wells alone is used to infer the abundance of sub resolution faults by the area balance technique a small but significant amount of sub resolution extension is predicted and then confirmed with a highresolution seismic line q 2003 elsevier science ltd all rights reserved

analysis and modeling of melt water refreezing in dry snow a dynamic zone of thermal disequilibrium is described which separates wet snow at 0 c from dry sub freezing snow the dynamic zone tends to be eliminated by thermal equilibrium through freezing but is sustained and propagated into the sub freezing snow by water flow from the wet snow the width of the dynamic zone is controlled by the rate of water inflow and by the rate of freezing of water on to sub freezing ice grains which is in turn controlled by the ice water geometry two ice water geometries are investigated isolated ice spheres and capillary tubes of ice into which water is pulled by capillary suction the rate of freezing of water is calculated for the two models for various initial dimensions and temperatures equilibrium times are short typically about 0 5 s but depend on the assumed geometry which is poorly constrained by existing data equilibration times and freeze on mass fluxes are calculated for a variety of general conditions these results can be used in numerical models of wetting front propagation into cold snow

kinematic model of detachment folding including limb rotation fixed hinges and layer parallel strain abstract detachment anticlines formed above a stratigraphically fixed detachment horizon and without thinning in the syncline will in general require layer parallel shortening a kinematic model for detachment folds is presented that has planar limbs and is area balanced but requires bed length changes that can be expressed as layer parallel strain the fold grows primarily by limb rotation not by limb lengthening at constant dip the kinematic evolution of the geometry and strain is examined fold geometry is well predicted and the strain estimated in applications of the model to two map scale detachment folds a triangle zone from the front of the wyoming thrust belt and a duplex cored anticline from the appalachian plateau

application of the ads method to predict a hidden basal detachment nw borneo fold and thrust belt abstract the nw borneo margin is characterized by a complex deepwater fold and thrust belt despite previous studies the definition of a univocal detachment level for folding and thrusting is still lacking the area depth strain ads method can be used to determine the location for a detachment in areas lacking data and to balance geological cross sections this study applies the ads method to the central part of the nw borneo fold and thrust belt to predict a structurally conclusive detachment level in an area lacking a seismic detachment reflection seismic interpretations were completed after the ads determination of the basal detachment providing input for a 2d sequential restoration that delivered values on shortening distribution and shortening rate the kinematic and mechanic analyses presented document that the central part of the nw borneo fold and thrust belt is affected by both near and far field stresses and that the far field crustal shortening component becomes more important northward this work demonstrates that the ads method can be effectively applied in fold thrust belt settings with limited information on the detachment supports in a quantitative way the tectonic and stratigraphic interpretation of seismic reflection data and provides a robust structural base for the restoration of balanced cross sections including the reconstruction of syn kinematically eroded stratigraphy

evaluation analysis and prediction of geologic structures abstract balanced cross sections claim to be better because they apply a rigorous set of rules to develop the conceptual model of the structures present in an area balanced cross sections can be further improved and become more useful to understanding real physical problems by collection of additional data such as seismic reflection surveys collection of additional stratigraphic data or collection of rock fabric information the additional information validates the initial model and provides details on deformation conditions and on local rock responses to the deformation although individual cross sections are two dimensional the objective of evaluation and analysis of deformed regions should be three dimensional whenever possible to recognize the challenges of the real world subsurface system analysis derived from the hydrologic community emphasizes conceptual model development through model verification validation uncertainty quantification benchmarking and meta analysis their approach includes many steps informally used by the structural geology community but in a much more explicit way newer geological applications of structural geology would benefit from this more rigorous approach for designing and doing performance predictions as technological needs become more socially sensitive such as for carbon storage sites new areas of energy exploration in higher population density areas or for nuclear waste storage facilities

a high precision method for measurement of paleoatmospheric co2 in small polar ice samples we describe a high precision method now in use in our laboratory for measuring the co 2 mixing ratio of ancient air trapped in polar ice cores occluded air in ice samples weighing 8 15 g is liberated by crushing with steel pins at 35 c and trapped at 263 c in a cryogenic cold trap co 2 in the extracted air is analyzed using gas chromatography replicate measurements for several samples of high quality ice from the siple dome and taylor dome antarctic ice cores have pooled standard deviations of 0 9 ppm this high precision technique is directly applicable to high temporal resolution studies for detection of small co 2 variations for example co 2 variations of a few parts per million on millennial to decadal scales

extreme firn metamorphism impact of decades of vapor transport on near surface firn at a low accumulation glazed site on the east antarctic plateau snow and firn properties control the transport of vapor gases and water between the atmosphere and the underlying strata an understanding of this transport and the properties that control it is important for predicting air snow transfer of chemical species and for interpreting ice cores remote sensing images of east antarctica show large areas of alternating light and dark bands these low amplitude long wavelength features have glazed downwind faces and rough upwind faces and are called megadunes the first linked measurements of the permeability and the associated microstructure for a glazed area within a well defined megadune area are reported in this paper permeability and density were measured along with grain scale properties derived from digital image processing of preserved thick sections at this cold low accumulation glazed site a clear layering pattern exists in the top meter the firn density ranges from 0 24 to 0 50 g cm 3 permeability measurements range from 50 x 10 10 to 200 x 10 10 2 several times greater than corresponding profiles from warmer higher accumulation sites like siple dome antarctica it is shown that buoyancy driven natural convection may be important in post depositional processes in very cold low accumulation sites like this

new insights into deformation mechanisms in the gravitationally driven niger delta deep water fold and thrust belt we use two and three dimensional seismic reflection data from the deep water niger delta fold and thrust belt to document evidence for two discrete postfaulting deformation mechanisms an early phase of thrust propagation folding is followed by folding caused by thickness changes within the basal shale detachment unit the later phase of folding is caused by a lateral redistribution of the strata within the basal detachment unit this example of late deformation occurred over a 4 5 m y period as a result of the displacement of approximately 590 km3 370 mi3 of the underlying strata within the detachment unit in another deformation event in the basal detachment unit about 160 km3 100 mi3 moved laterally creating a synform in the overburden and parallel onlap fill indicative of the relatively rapid creation of accommodation space on the basis of seismic reflection data from the delta and a consideration of the volumes and rates of movement of sedimentary rock we conclude that the poorly imaged succession commonly referred to as mobile shale cannot deform solely by ductile mechanisms as interpretations of shale tectonic provinces have commonly suggested but instead by brittle processes that involve thickening by thrust faulting and subseismic accommodation structures processes such as liquefaction where a complete loss of shear strength is observed had a minimal function therefore the term mobile shale in this setting is widely exaggerated an awareness of postfaulting deformation mechanisms will be important for the successful exploration of gravitationally driven fold and thrust belts

area length and thickness conservation dogma or reality abstract the basic assumption of quantitative structural geology is the preservation of material during deformation however the hypothesis of volume conservation alone does not help to predict past or future geometries and so this assumption is usually translated into bed length in 2d or area in 3d and thickness conservation when subsurface data are missing geologists may extrapolate surface data to depth using the kink band approach these extrapolations preserving both thicknesses and dips lead to geometries which are restorable but often erroneous due to both disharmonic deformation and internal deformation of layers first the bolivian sub andean zone case is presented to highlight the evolution of the concepts on which balancing is based and the important role played by a decoupling level in enhancing disharmony second analogue models are analyzed to test the validity of the balancing techniques chamberlin s excess area approach is shown to be on average valid however neither the length nor the thicknesses are preserved we propose that in real cases the length preservation hypothesis during shortening could also be a wrong assumption if the data are good enough to image the decollement level the chamberlin excess area method could be used to compute the bed length changes

co 2 diffusion in polar ice observations from naturally formed co 2 spikes in the siple dome antarctica ice core this is the publisher s final pdf the published article is copyrighted by the international glaciological society and can be found at http www igsoc org journal

fractionation of gases in polar ice during bubble close off new constraints from firn air ne kr and xe observations gas ratios in air withdrawn from polar firn snowpack show systematic enrichments of ne n2 o 2 n2 and ar n2 in the firn ice transition region where bubbles are closing off air from the bubbles in polar ice is correspondingly depleted in these ratios after accounting for gravitational effects gas in the bubbles becomes fractionated during the process of bubble close off and fractionation may continue as ice cores are stored prior to analysis we present results from firn air studies at south pole and siple dome antarctica which add ne kr and xe measurements to the suite of observations ne o2 and ar appear to be preferentially excluded from the shrinking and occluding bubbles and these gases therefore accumulate in the residual firn air creating a progressive enrichment with time and depth in firn air early sealing of gases by thin horizontal impermeable layers into a nondiffusive zone or lock in zone greatly enhances this enrichment a simple model of the bubble close off fractionation and lock in zone enrichment fits the data adequately the model presumes that fractionation is caused by selective permeation of gas through the ice lattice from slightly overpressured bubbles the effect appears to be size dependent because ne o2 and ar have smaller effective molecular diameters than n2 and fractionation increases strongly with decreasing size ne is fractionated 34 2 times more than o2 in south pole firn air and reaches an enrichment of 90 in the deepest sample the large atoms kr and xe do not appear to be fractionated by this process despite the large size difference between the two gases suggesting a threshold atomic diameter of 3 6a above which the probability becomes very small that the gas will escape from the bubble these findings have implications for ice core and firn air studies that use gas ratios to infer paleotemperature chronology and past atmospheric composition

fault related folding a review of kinematic models and their application abstract folding that is directly related to fault activity is an important deformation feature that occurs all over the world in mountain belts accretionary wedges fold and thrust belts and intra plate settings in either strike slip compressional or extensional regimes due to their widespread occurrence knowledge about the development of these structures is important to a broad spectrum of geoscience sub disciplines such as structural geology seismology geomorphology petroleum geology and quaternary geology fault related folding has been analyzed intensively over the last 140 years for the sake of this review we divide the folds according to the way the faults and the folds form that is into detachment fault bend and fault propagation folds all fault related folds are caused by changes in fault parameters the simplest method to produce folds is to transport material along faults that have stepped flat ramp flat geometries fault bend fold alternatively the slip can decrease along the length of the fault and depending on whether the fault remains within a detachment layer or steps up through mechanical stratigraphy either a detachment fold or a fault propagation fold is formed respectively detachment folding was first investigated in the early 20th century whereas the full significance of fault propagation folds was recognized quite late in the 1980s seminal work on fault related folding was carried out in the 1930s but quantitative kinematic models have only been available in the last 30 years these models are extremely valuable because they allow a comprehensive understanding of the evolution of fault related folds and lead to more accurate predictions of the sub surface structure from the mid 1990s onwards numerical simulations have been used to identify how fault parameters such as dip and fault bend angle propagation to slip ratio and shape of the trishear zone influence the geometry of the related folding this is directly applicable to the analysis of the shape of anticlines produced however this does not mean that fold geometry is uniquely related to fault geometry on the contrary different kinematic approaches can lead to a similar fold shape

mechanisms of thrust propagation some examples and implications for the analysis of overthrust terranes abstract evidence is presented for the existence of three fundamental mechanisms by which thrust sheets move the mechanisms are 1 failure of a stiff layer to form ramp flat geometry imbricated thrust sheets 2 detachment of a layer by folding decollement thrust sheets 3 differential layer parallel shortening lps thrust sheets the mechanisms are independent but may operate interactively to form families of hybrid thrust sheets since lps thrust sheets have not been previously described or documented data from the new york plateau deformed almost exclusively by differential lps is presented to demonstrate the physical characteristics of this thrust mechanism information from the plateau indicates that finite strain behavior closely reflects the geometric boundary conditions and is independent of temperature and depth of burial finite strain data are used to construct a set of iso strain maps in both the deformed and undeformed states the iso strain maps are in turn used to determine the displacement field for the thrust sheet the displacement field allows visualization of the effects of both lagrangian and eulerian transformations on an initially orthogonal grid utilizing sections from a number of overthrust belts it is shown that the three mechanisms occur universally although in various proportions lps is regionally developed throughout the central and northern appalachians the presence of the lps thrust sheets probably accounts for the failure of structural cross sections to bed length balance in these blind thrust terranes finally a series of examples drawn from the field as well as the literature are used to illustrate both the characteristics of hybrid thrust sheet families as well as to indicate how knowledge of their behavior can be utilized in developing strategies for the construction and balancing of structural cross sections

geometry of thin skinned tectonic systems in relation to detachment layer thickness in sedimentary basins characterizing features of both extensional and compressional thin skinned systems may be controlled by the thickness of the underlying mobile material within extensional fault arrays detachment layer thickness in conjunction with detachment dip controls variations in fault spacing and polarity because rigid fault blocks are only free to rotate as long as there is enough underlying mobile material to redistribute between the fault block keels different styles of extensional fault array range from single polarity arrays with closely spaced faults thin detachment to arrays where the fault blocks rotate until the fault blocks separate from one another thick detachment departure from this relationship depends on the amount of internal deformation that can be sustained by the domino fault blocks within contractional zones detachment folds occur where the amount of mobile material can fill the evolving fold core via redistribution thrusts occur where there is insufficient mobile material to sustain detachment fold growth the variations in extensional and contractional style that are described here in relation to mobile layer thickness can be combined to give an idealized model of regional variation in structural style in relation to regional variation in stratigraphy applying this model to a salt basin an allochthon in the area of thickest salt may be characterized by updip raft tectonics and downdip detachment fold belts this style would be laterally transitional toward areas of thinner or lower net salt to updip domino fault arrays and downdip thrust systems