kinematics of crustal stacking and dispersion in the south eastern bohemian massif the south eastern bohemian massif consolidated during the late variscan orogeny by the oblique collision of two continental crustal blocks after closure of an oceanic realm one microcontinent comprises portions which are now distributed among moravian and moldanubian units and which are characterized by late proterozoic tectonothermal events especially by granitoid intrusions the other microcontinent includes the gfohl gneiss and granulites gfohl nappe of probable early palaeozoic protolith ages both continental blocks are separated by an ophiolite like assemblage which is preserved in portions of the raabs unit

milankovitch forcing of fluctuations in the level of tropical lakes from 18 to 0 kyr bp an atmospheric general circulation model has been used to simulate the climates of january and july at 3 000 yr intervals from 15 kyr bp onwards the model simulates a strengthened monsoon circulation and increased precipitation in the northern hemisphere tropics culminating at 9 6 kyr bp the computed hydrological budgets were used to estimate the area of closed lakes between 8 9 and 26 6 n giving with minor exceptions results in good agreement with geological evidence

the influence of changing orbital parameters and surface boundary conditions on climate simulations for the past 18 000 years abstract general circulation model experiments at 3000 year intervals for the past 18 000 years were made to estimate the magnitude timing and pattern of the climatic response to prescribed changes of orbital parameters date of perihelion axial tilt eccentricity and glacial age lower boundary conditions ice sheets land albedo sea ice and sea surface temperature the experiments used the community climate model ccm of the national center for atmospheric research ncar the response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial age boundary conditions the continental interior of eurasia was 2 4 k warmer in summer and summer monsoon precipitation of north africa south asia was increased by 10 20 between 12 000 and 6000 yr bp before present when perihelion occurred during northern summer rather than in winter as now and the earth s axial tilt was larger than now southern hemisphe

january and july simulations with a spectral general circulation model abstract we describe the results of january and july simulations carded out with a nine level spectral model employing a rhomboidal truncation at wavenumber 15 sea surface temperature sea ice distribution and solar zenith angle are held constant in each simulation the model includes interactive clouds and radiative processes after ramanathan et al 1983 selected fields are shown which highlight the model s strengths and weaknesses the latitude height distribution of the zonal wind is successfully simulated the model captures the separation between the wintertime westerly jets in the troposphere and stratosphere and thus simulates the sign reversal in the vertical wind shear across the jet axis in the upper troposphere in addition to the zonal wind we show also the zonally averaged temperature meridional wind and vertical velocity regional distributions of sea level pressure surface air temperature precipitation and a number of other fields defined at various pressure levels are compared in

re evaluation of polyphase kinematic and 40 ar 39 ar cooling history of moldanubian hot nappe at the eastern margin of the bohemian massif a structural and geochronological 40ar 39ar study was performed in kilometre scale middle and lower crustal lens shaped domains dominated by a preserved subvertical foliation surrounded by horizontally foliated migmatites these domains occur within the moldanubian nappe overlying the brunia microcontinent at the eastern margin of the european variscides three main deformation phases were recognized subvertical s2 fabric trending nw se in lower crustal rocks and ne sw in mid crustal rocks it is reworked by ht mt horizontal fabric s3 along margins of crustal domains and in surrounding migmatites s3 bears a prolate ne lineation parallel to the s2 s3 intersection in the lower crustal domain in the middle crustal units l3 is weak connected to oblate strain and trends ne sw parallel to the s2 s3 intersection d4 non coaxial shear deformation is mainly localized at the boundary between the moldanubian nappe and brunia and bears strong top to the nne shear criteria in order to constrain kinematics of the d3 deformation strain modelling was performed to show that the moldanubian hot nappe was frontally thrust over the brunia indentor the renewed d4 tangential movement only heterogeneously reactivates the horizontal s3 this evolution is recorded in 40ar 39ar amphibole cooling ages which show two statistically significant carboniferous peaks at 342 and 332 ma which are also reflected by published detrital muscovite 40ar 39ar ages in the adjacent foreland basin this geochronological record is correlated with progressive erosion of the topographically elevated upper crustal part of the moldanubian nappe during d3 frontal thrusting followed by greenschist facies d4 transpressive reactivation and subsequent erosion of high grade parts of the nappe

tectonic evolution of the european variscan belt constrained by palaeomagnetic structural and anisotropy of magnetic susceptibility data from the northern vosges magmatic arc eastern france geochronology structural and anisotropy of magnetic susceptibility data from the northern vosges batholith which belongs to the northern vosges mid german crystalline rise arc show contrasting emplacement modes of southern granodiorites and northern granites the ene wsw trending fabrics of granodiorites 346 334 ma are parallel to the metamorphic cleavage affecting the host rocks developed during regional compression the nnw sse trending fabrics of younger granitoids c 330 ma reveal an extensional emplacement mode associated with a normal shear zone separating the two magmatic suites palaeomagnetism shows that the switch from a compressive to an extensional regime coincides with a regional counterclockwise rotation the 330 325 ma extension is further supported by palaeomagnetic and seismic data indicating southeastward tilt of the whole batholith finally the system is rotated clockwise without any structural overprint data from the early carboniferous northern vosges magmatic arc and remote bohemian massif suggest that this evolution is valid for the whole eastern branch of the european variscan belt for which the following model is proposed 1 late devonian early carboniferous east west shortening of the variscides above the rhenohercynian subduction zone 2 axial nnw sse shortening of the assembled variscan orogen associated with 3 internal sinistral rotation of inherited rhenohercynian transform faults and shortening of intervening blocks 4 east west extension and dextral external rotation of blocks between dextrally reactivated transforms supplementary materials analytical procedures for geochronology magnetism anisotropy of magnetic susceptibility and palaeomagnetism are available at www geolsoc org uk sup18635

intercalibration of the hb3gr 40ar 39ar dating standard abstract the 40 ar 39 ar dating technique is based on the knowledge of the age of neutron fluence monitors standards recent investigations have improved the accuracy and precision of the ages of most of the phanerozoic aged standards e g fish canyon tuff sanidine fcs alder creek sanidine ga1550 biotite and lp 6 biotite however no specific study has been undertaken on the older standards i e hb3gr hornblende and nl 25 hornblende generally used to date precambrian high ca k and or meteoritic rocks in this study we show that hb3gr hornblende is relatively homogenous in age composition ca k and atmospheric contamination at the single grain level the mean standard deviation of the 40 ar 39 ar k f value derived from this study is 0 49 comparable to the most homogeneous standards the intercalibration factor which allows direct comparison between standards between hb3gr and fcs is r fcs hb3gr 51 945 0 167 using an age of 28 02 ma for fcs the age of hb3gr derived from the r value is 1073 6 5 3 ma 1 internal error only and 8 8 ma including all sources of error this age is indistinguishable within uncertainty from the k ar age previously reported at 1072 11 ma turner g huneke j c podosek f a wasserburg g j 1971 40 ar 39 ar ages and cosmic ray exposure ages of apollo 14 samples earth planet sci lett 12 19 35 the r value determined in this study can also be used to intercalibrate fcs if we consider the k ar date of 1072 ma as a reference age for hb3gr we derive an age of 27 95 0 19 ma 1 internal error only for fcs which is in agreement with the previous determinations altogether this shows that hb3gr is a suitable standard for 40 ar 39 ar geochronology

vertical extrusion and horizontal channel flow of orogenic lower crust key exhumation mechanisms in large hot orogens a large database of structural geochronological and petrological data combined with a bouguer anomaly map is used to develop a two stage exhumation model of deep seated rocks in the eastern sector of the variscan belt an early sub vertical fabric developed in the orogenic lower and middle crust during intracrustal folding followed by the vertical extrusion of the lower crustal rocks these events were responsible for exhumation of the orogenic lower crust from depths equivalent to 18 20 kbar to depths equivalent to 8 10 kbar and for coeval burial of upper crustal rocks to depths equivalent to 8 9 kbar following the folding and vertical extrusion event sub horizontal fabrics developed at medium to low pressure in the orogenic lower and middle crust during vertical shortening fabrics that record the early vertical extrusion originated between 350 and 340 ma during building of an orogenic root in response to se directed saxothuringian continental subduction fabrics that record the later sub horizontal exhumation event relate to an eastern promontory of the brunia continent indenting into the rheologically weaker rocks of the orogenic root indentation initiated thrusting or flow of the orogenic crust over the brunia continent in a north directed sub horizontal channel this sub horizontal flow operated between 330 and 325 ma and was responsible for a heterogeneous mixing of blocks and boudins of lower and middle crustal rocks and for their progressive thermal re equilibration the erosion depth as well as the degree of reworking decreases from south to north pointing to an outflow of lower crustal material to the surface which was subsequently eroded and deposited in a foreland basin indentation by the brunia continental promontory was highly noncoaxial with respect to the se oriented saxothuringian continental subduction in the early visean suggesting a major switch of plate configuration during the middle to late visean

sediment provenance and dispersal in a deep marine foreland basin the lower carboniferous culm basin czech republic the lower carboniferous culm basin of the czech republic represents the most easterly and southerly part of the european variscan foreland basin system the nne ssw trending basin is exposed along the eastern flank of the bohemian massif it contains up to 7 5 km of deep marine sediment deposited as an axial turbidite system sourced from south southwest of the basin heavy mineral data and clast types indicate that for much of the visean sediment was sourced from a predominantly low grade metamorphic terrane and visean limestones in the latest visean a progressive change in source material is recorded by an increase in high grade metamorphic detritus studies of variscan nappe pile emplacement along the eastern flank of the bohemian massif allow an assessment of the relationship between basin development and nappe emplacement basin initiation coincided with underthrusting of high grade metamorphic moldanubian nappes at 340 ma sedimentation was synchronous with nappe emplacement throughout the visean which resulted in uplift and erosion of a low grade metamorphic terrane moldanubian nappes were exhumed at approximately 330 ma when high grade metamorphic detritus entered the basin two phases of northward directed sediment progradation are recognized sediment progradation is considered to be related to changes in drainage basin size and or climatic fluctuations within the variscan orogen since sediment is inferred to have been supplied to the basin axially rather than laterally and subsidence curves show no significant change in subsidence rates in contrast to many models of foreland basin sediment distribution filling of the culm basin was largely independent of tectonic activity adjacent to the basin margin and or of subsidence rate changes sedimentation in the culm basin commenced 10 15 ma earlier than the rest of the variscan foreland and records the first impact of the northwardly propagating variscan orogen in northern europe

on the pulling power of ice streams gravity wants to pull an ice sheet to the center of the earth but cannot because the earth s crust is in the way so ice is pushed out sideways instead or is it the ice sheet sees nothing preventing i t from spreading out except air which is much less massive than ice therefore does not ice rush forward to fill this relative vacuum does not the relative vacuum suck ice into i t because nature abhors a vacuum if so the ice sheet is not only pulled downward by gravity i t is also pulled outward by the relative vacuum this pulling outward will be most rapid where the ice sheet encounters least resistance the least resistance exists along the bed of ice streams where ice bed coupling is reduced by a basal water layer especially if the ice stream becomes afloat and the floating p art is relatively unconfined around its perimeter and unpinned to the sea floor ice streams are t herefore fast currents of ice that develop near the margins of an ice sheet where t hese conditions exist because of these condit ions ice streams pull ice out of ice sheets and have pulling power equal to the longitudinal gravitational pulling force multiplied by the ice stream velocity t hese boundary conditions beneath and beyond ice streams can be quantified by a basal buoyancy factor that provides a life cycle classification of ice streams into inception growth mature declining and terminal stages during which ice streams disintegrate the ice sheet surface profiles of ice streams are diagnostic of the stage in a life cycle and hence of the vitality of the ice sheet

contrasting tectono metamorphic evolution of orogenic lower crust in the bohemian massif a numerical model abstract the bohemian massif located at the eastern margin of the european variscan belt is characterised by an exceptional accumulation of felsic high pressure granulites the petrological structural and geochronological studies of this region revealed systematic differences between the tectonometamorphic evolution of the southern moldanubian and northern west sudetes parts of the orogen two contrasting tectonic scenarios have been proposed gravity driven vertical mass exchanges followed by continental indentation in the moldanubian domain and crustal scale folding leading to gneiss dome formation in the west sudetes we present a numerical model in order to correlate the apparent differences between these two regions with the variations in the dynamics of the modelled system we model two colliding blocks an orogenic root where a felsic lower crust is overlain by a mafic layer and a middle crust and a continental indentor we examine the role of the rate of convergence of the two blocks radiogenic heat production within the felsic lower crust and efficiency of erosion the prograde part of the metamorphic evolution is controlled by the rate of convergence and the peak temperature depends on the heat production the retrograde evolution is controlled mostly by erosional processes in the models where the material is weakened due to the heating in the felsic lower crust the gravitational instability of the mafic and felsic layers causes their complete vertical exchange followed by a flow above the indentor in colder and or faster models the thickening is dominated by the buckling of the mafic layer these two styles of deformation i e gravity dominated and fold dominated models correspond to the structures observed in the moldanubian and the west sudetes moreover the calculated pressure temperature paths of the felsic lower crust are in agreement with available data

contrasting styles of deformation during progressive nappe stacking at the southeastern margin of the bohemian massif thaya dome abstract variscan nappe stacking at the southeastern margin of the bohemian massif is a polyphase process implying a progressive modification of the deformation regime from thrusting to wrenching associated change of finite strain axes orientations and progressive downwards migration of thrust or wrench planes a first high temperature deformation phase d 1 is related to nappe stacking of moldanubian units in a nw se direction it is recorded only in these units d 2 corresponds to the piling of the moldanubian units over the moravian ones producing internal nappe stacking of the latter and a first medium temperature mylonitization of thaya basement rocks the main feature of this stage is a switch in finite strain axes orientation and northeastward transpressional shearing strain partitioning is important within the moravian nappes and implies alternation of plane strain oblate and constrictional fabrics d 3 is also a phase connected with strong non coaxial ne directed shearing it is concentrated in zones of weakness and causes reactivation of older structures the structural pattern strongly depends on the position in the nappe pile strike slip tectonics dominate near the resistant eastern cadomian basement border foliation parallel extensional reactivation producing large scale folds is typical in moravian nappes while extensional tectonics prevail in the uppermost gfohl nappe large scale strain partitioning during d 2 stacking of moravian nappes and the extension of the upper part of the nappe pile during d 3 reactivation are mechanically controlled by a lateral ramp basement geometry along the bruno vistulian cadomian foreland

middle holocene dry period in the northern midwestern united states lake levels and pollen stratigraphy abstract four cores from lake mendota in the yahara river valley of south central wisconsin provide pollen charcoal and sediment stratigraphic evidence of a middle holocene dry period in the northern midwest the lake level was lower between about 6500 and 3500 yr b p indicating that runoff from the upper yahara river basin may have ceased during the drier seasons of the year derived estimates of precipitation between 6500 and 3500 yr b p indicate a decrease of about 10 down to about 700 mm from the present precipitation value of 800 mm a warming between 6500 and 3500 yr b p is also interpreted based on a change of vegetation from rich mesophytic forest before 6500 yr b p to quercus savanna with increased charcoal abundances after that time after 3500 yr b p a closed quercus forest and decreased charcoal suggest a cooler and wetter climate the qualitative paleoecological interpretations quantitative precipitation reconstructions based on pollen from the region including the lake mendota data and estimates of hydrologic budget based on lake level changes all show indications of a dry and warm middle holocene evidence from a large regional array of sites also supports these interpretations

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

first finding of microdiamond coesite and other uhp phases in felsic granulites in the moldanubian zone implications for deep subduction and a revised geodynamic model for variscan orogeny in the bohemian massif abstract heavy mineral fractions and polished thin sections from felsic granulites from the moldanubian zone of the bohemian massif were thoroughly studied by means of raman microspectroscopy combined with optical microscopy and scanning electron microscopy the following phases were identified among others as inclusions in robust minerals such as garnet and zircon diamond characterized by an intense narrow peak at 1332 cm 1 was found in two inclusions in zircon they have a size of 5 m coesite identified by its very characteristic peak at 520 cm 1 was found in an inclusion in garnet together with quartz coesite has been almost completely transformed into quartz only minor coesite nano domains remain kumdykolite the orthorhombic polymorph of naalsi 3 o 8 characterized by strong peaks at 220 456 and 492 cm 1 occurs either as single crystals or as a part of multiphase inclusions in garnet and in zircon along with other mineral phases such as k feldspar phengite rutile moissanite sic exhibiting the characteristic raman bands at 767 788 and 969 cm 1 occurs as inclusions in garnet diamond and coesite are considered to have formed at the peak ultrahigh pressure metamorphic uhpm conditions kumdykolite has been proposed to be a metastable phase formed during rapid cooling from high temperature moissanite points to extremely reduced conditions during subduction to great depths the finding of uhp phases in felsic granulites in the moldanubian zone is clear evidence for subduction of crustal materials to mantle depths the garnet hosting the uhp phase inclusions usually preserves prograde compositional zoning this in combination with the uhpm mineral inclusions suggests that the felsic material should have passed uhp metamorphism at a low temperature gradient isothermal decompression the commonly accepted model at temperatures of 850 950 c would have substantially modified and homogenized the garnet composition eliminating any compositional zoning therefore the uhpm felsic rocks could have experienced only a short term granulite facies metamorphism after they were exhumed to crustal levels

some remarks on fabric overprints and constrictional ams fabrics in igneous rocks during the ascent emplacement and post emplacement deformation of igneous rocks two or more phases of deformation that overprint each other are often depicted these overprints when magnetic minerals are present are recorded in magnetic fabric in this contribution overprints are studied by means of numerical modeling following several basic scenarios common to igneous rocks biotite and amphibole that occur often together in igneous rocks are considered as carriers of the anisotropy of magnetic susceptibility modeling shows that 1 a constrictional fabric with a low degree of anisotropy as commonly recorded in magmatic rocks may result from a deformation overprint and not necessarily from an extensional transtensional regime and 2 that the constrictional ams fabrics originates from orthogonal superimposition of a deformation event on an ams fabric inherited from earlier magma emplacement history therefore the interpretation of a constrictional fabric must be performed with caution numerical modeling may provide a suitable help in strengthening the interpretation of real magnetic fabric data

structural constraints on reworking in the western ghats granulite belt india and the antarctic analogue abstract the granulite belt along the west coast of india in kerala tamil nadu is particularly important for the study of proterozoic events in the pre drift east gondwana super continent in antarctica structural petrological and geochronological lines of evidence points to a proterozoic reworking of archaean granulites in the western ghats granulite belt the late proterozoic pan african event is overwhelmingly dominant though some late archaean to early proterozoic granulites have been reported structural evidence presented here points to a late proterozoic reworking resulting in a break or discontinuity in the tectonic evolution of this granulite belt thus relicts or vestiges of probable late archaean to early proterozoic granulites are recognised in the largely pan african granulite belt of the western ghats

holocene climatic change in the northern midwest pollen derived estimates mapping of holocene pollen data in the midwestern united states has revealed several broadscale vegetational changes that can be interpreted in climatic terms these changes include 1 the early holocene northward movement of the spruce dominated forest and its later southward movement after 3000 yr b p and 2 the eastward movement of the prairie forest border into southwestern wisconsin by 8000 yr b p and its subsequent westward retreat after 6000 yr b p when certain basic assumptions are met multiple regression models can be derived from modern pollen and climate data and used to transform the pollen record of these vegetational changes into quantitative estimates of temperature or precipitation to maximize the reliability of the regression equations we followed a sequence of procedures that minimize violations of the assumptions that underlie regression analysis reconstructions of precipitation during the holocene indicated that from 9000 to 6000 yr b p precipitation decreased by 10 to 25 over much of the midwest while mean july temperature increased by 0 5 to 2 0 c at 6000 yr b p precipitation was less than 80 of its modern values over parts of wisconsin and minnesota after 6000 yr b p precipitation generally increased while mean july temperature decreased in the north and increased in the south the time of the maximum temperature varies within the midwest and is earlier in the north and later in the south

late extensional modification of a compressional orogen evidence from eastern ghats granulite belt india and its analogue in other gondwana fragments

synconvergent ductile flow in variable strength continental crust numerical models with application to the western grenville orogen 1 we present results from numerical models for a convergent orogen with laterally variable lower crustal strength representing a simplified orogenic system in which a strong craton flanked by progressively weaker terranes collides with another continent with progressive convergence crustal thickening and thermal relaxation lower crust becomes decoupled from upper and middle crust forming a ductile orogenic infrastructure beneath a stronger superstructure collision with strong external crust results in uplift and expulsion of ductile nappes from the orogenic core creating allochthonous terranes overlying a lower crustal indentor the extent of transport and exhumation of lower crustal nappes over the indentor reflects the amount of convergence and the erosion rate the western grenville orogen displays across strike variations in age tectonic history and protolith association suggesting a systematic variation in precollision crustal strength the laurentian craton margin and accreted terranes were variably reworked at synorogenic depths of 25 35 km during the ottawan orogeny deformation propagated from younger monocyclic rocks in the southeast into older polycyclic rocks flanking the craton on the northwest a comparison between numerical model results and crustal scale cross sections from the georgian bay and montreal val d or transects shows close correspondence between crustal structure and model geometry this indicates that the models produce geologically realistic results and provides a context for interpreting the tectonic evolution of the western grenville orogen contrasts between the results of homogeneous channel flow models and the present ductile nappe models suggest that the effects of different styles of ductile flow can be distinguished in the geologic record

chronology of late paleozoic tectonothermal activity in the southeastern bohemian massif austria moldanubian and moravo silesian zones 40ar 39ar mineral age controls abstract new 40ar 39ar hornblende and muscovite ages from the moldanubian and moravian zones of the southeastern bohemian massif indicate rapid cooling associated with continental underplating during early carboniferous plate collision and closure of the moravo silesian foredeep 40ar 39ar ages have been determined for hornblende concentrates from 1 amphibolite from the moldanubian nappe complex including the dobra gneiss rehberg amphibolite and loosdorf complex all part of the variegated unit 2 micaschist structurally overlying the bittesch gneiss and 3 amphibolite within the bittesch gneiss moravian zone the five hornblende concentrates display variably discordant 40ar 39ar age spectra because of contamination with extraneous argon components however 36ar 40ar vs 39ar 40ar isotope correlations are well defined for the five hornblende concentrates mswd the hornblende and muscovite ages are interpreted to date post variscan metamorphic cooling through ca 500 c and ca 400 c respectively no record of pre variscan thermal events is preserved in either the hornblende or muscovite argon systems however the widespread occurrence of extraneous argon components suggest a pre variscan stage of tectonothermal development the near concordancy of the hornblende and muscovite cooling ages suggest relatively rapid cooling through the contrasting argon closure temperatures and indicate that relatively high crustal levels had likely been attained during the early carboniferous

new constraints on antarctic plate motion and deformation from gps data four years 1995 1998 of continuous global positioning system gps data recorded by 11 permanent international gps service igs stations in antarctica and surrounding regions have been processed using an optimized method for regional geodetic networks the 4 year time series has allowed us to extract nonlinear variations and constant horizontal velocities for all the stations in december 1997 we have set up a permanent gps station at dumont d urville in terre adelie the resulting data along with those of the scientific committee on antarctic research scar campaigns 1995 and 1996 are included in our processing a coseismic displacement 1 to 2 cm is detected in the horizontal components of this station we relate this to the march 25 1998 mw 8 1 balleny islands earthquake for which a dislocation model yields prediction in the right direction for the six antarctic stations the horizontal rates are very consistent with the rigid plate rotation all the residual velocities are negligible less than 2 mm yr except at o higgins in the head of the peninsula 7 mm yr toward the continent the position of the rotation pole 62 0 n 146 7 w and its rate 0 264 ma are significantly different from the nnr nuvel la predictions for the antarctic tectonic plate but are consistent with the australian relative motion the high horizontal residual in the peninsula area where iceberg calving rates and ice shelf disintegration have recently increased can be interpreted as the horizontal component of the elastic crustal response to the load variations

ductile extrusion of the higher himalayan crystalline in bhutan evidence from quartz microfabrics quartz textures measured from deformed quartz tectonites within the lesser himalaya and higher himalaya crystalline of bhutan show similar patterns orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense the asymmetry of c axis and a axis patterns consistently indicates top to the south shearing the obliquity of the texture and the inferred finite strain plane strain to moderately constrictional suggest the strain regime had a combination of rotational and irrotational strain path in most of the samples from the bhutan himalaya the inferred deformation mechanisms suggest moderate to high temperature conditions of deformation that produced the observed crystallographic preferred orientation much higher temperature of deformation is indicated in the quartz veins from a leucogranite the observed ductile deformation is pervasively developed in the rocks throughout the investigated area the intensity of deformation increases only slightly in the vicinity of the main central thrust simultaneous southward shearing within a large part of the higher himalaya crystalline near and above the main central thrust and normal faulting across the south tibetan detachment is explained by the tectonically induced extrusion of a ductily deforming wedge the process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes channel flow accounts for some observed phenomena in the himalayan orogen such as inverted metamorphic sequences near the main central thrust not related to an inversion of isotherms and the syntectonic emplacement of leucogranites into the extruding wedge locally leading to an inversion of isotherms due to heat advection

high pressure temperature metamorphism in the st leonhard granulite massif austria evidence from intermediate pyroxene bearing granulites the st leonhard granulite massif lower austria is one of the small occurrences of high pressure granulite found in the gfohl unit at the highest tectono stratigraphic level of the moldanubian zone although predominantly composed of extremely deformed acidic garnet kyanite bearing rocks thin conformable layers of intermediate garnet clinopyroxene bearing granulites are seen pressure temperature estimates for the peak metamorphic assemblage of garnet clinopyroxene ternary feldspar quartz in these rocks are 15 19 kbar 950 1050 c a close coherence between results obtained from a combination of independent geothermobarometers and those derived from an internally consistent thermobarometric method indicate the retention of high pressure temperature equilibrium mineral compositions even though there is a wealth of petrographic evidence for significant post peak metamorphic decompression pressure temperature estimates for the orthopyroxene bearing intermediate pressure decompression stage obtained from discrete reaction textures are 8 12 kbar and 800 900 c post decompressive cooling from 800 to 500 c at ca 5 8 kbar is recorded by the final amphibolite facies biotite bearing assemblage together with petrological constraints from the enclosing acid granulites

evolution of nappes in the eastern margin of the bohemian massif a kinematic interpretation the entire pile of nappes in the eastern margin of the bohemian massif is characterized by two stages of variscan nappe emplacement each exhibiting a different kinematic and metamorphic evolution

estimates of holocene precipitation for rajasthan india based on pollen and lake level data abstract a pollen profile obtained from lake sediments at lunkaransar rajasthan in northwest india was used along with a pollen climate calibration function to estimate past precipitation between 10 500 and 3500 yr b p the estimated precipitation was about 500 mm yr or about 200 mm yr above the modern value a model was used for the energy and hydrologic budget of a lake basin and lake at sambhar located 240 km se of lunkaransar to calculate that a 200 mm yr increase in rainfall above the modern amount would have raised the lake level about 20 m above the modern level topographic charts and satellite imagery provided some evidence in support of an enlarged paleolake of that elevation but field exploration would be required to confirm the size and date of a former lake after about 3500 yr b p the lunkaransar profile indicated a desiccated lake bed because no pollen was preserved the pollen climate calibration function was of no use for estimating the amount of the precipitation decline a reduction of precipitation of about 200 mm yr below the modern amount was estimated from the energy and hydrologic budget for paleolake sambhar by assuming that the lake was one tenth of its present size during the dry interval

an application of eigenvalue methods to structural domain analysis the subdivision of a geologic map into structural domains involves the location of regions in which the rock fabric has certain geometrical characteristics typically foliation data should share a common axis the location of such domains involves working interactively with map and equal area projections of the data set a tedious and often subjective process eigenvector methods can quantify this type of analysis a set of four eigenvalue based indexes assists in discriminating among fabric distributions particularly between strong and weak cylindrical distributions these indexes form the basis of a triangular diagram for distinguishing among point girdle random and cylindrical fabrics a domain search proceeds by subdividing the data set and attempting to maximize the total cylindricity or other characteristic the method has been applied to a set of foliation data from a fold nappe in the western gneiss region of the norwegian caledonides the resulting domains define a late fold that refolds the recumbent nappe lineation and fold axis data from the area support the results of the domain search this method may be valuable in other areas of complex geometry

southward extrusion of tibetan crust and its effect on himalayan tectonics the tibetan plateau is a storehouse of excess gravitational potential energy accumulated through crustal thickening during india asia collision and the contrast in potential energy between the plateau and its surroundings strongly influences the modern tectonics of south asia the distribution of potential energy anomalies across the region derived from geopotential models indicates that the himalayan front is the optimal location for focused dissipation of excess energy stored in the plateau the modern pattern of deformation and erosion in the himalaya provides an efficient mechanism for such dissipation and a review of the neogene geological evolution of southern tibet and the himalaya shows that this mechanism has been operational for at least the past 20 million years this persistence of deformational and erosional style suggests to us that orogens like other complex systems can evolve toward steady state configurations maintained by the continuous flow of energy the capacity of erogenic systems to self organize into temporally persistent structural and erosional patterns suggests that the tectonic history of a mountain range may depend on local energetics as much as it does on far field plate interactions

crustal flow modes in large hot orogens abstract crustal scale channel flow numerical models support recent interpretations of himalayan tibetan tectonics proposing that gravitationally driven channel flows of low viscosity melt weakened middle crust can explain both outward growth of the tibetan plateau and ductile extrusion of the greater himalayan sequence we broaden the numerical model investigation to explore three flow modes homogeneous channel flow involving laterally homogeneous crust heterogeneous channel flow involving laterally heterogeneous lower crust that is expelled and incorporated into the mid crustal channel flow and the hot fold nappes style of flow in which mid lower crust is forcibly expelled outward over a lower crustal indentor to create fold nappes that are inserted into the mid crust the three flow modes are members of a continuum in which the homogeneous mode is driven by gravitational forces but requires very weak channel material the hot fold nappe mode is driven tectonically by for example collision with a strong crustal indentor and can occur in crust that is subcritical for homogeneous flows the heterogeneous mode combines tectonic and gravitationally driven flows preliminary results also demonstrate the existence and behaviour of mid crustal channels during advancing and retreating dynamical mantle lithosphere subduction an orogen temperature magnitude t m diagram is proposed and the positions of orogens in t m space that may exhibit the flow modes are described together with the characteristic positions of a range of other orogen types

thick skinned versus thin skinned thrusting rheology controlled thrust propagation in the variscan collisional belt the southeastern bohemian massif czech republic austria the variscan nappe assembly within the southeastern bohemian massif includes 1 crystalline nappes which were transported from initial granulite to amphibolite facies conditions to uppermost crustal levels 2 nappe assembly of footwall units under very low to low grade metamorphic conditions and 3 frontal thin skinned imbrication of flysch sequences which were deposited within a deep and narrow foreland basin during loading of the lower plate thick skinned thrusting of hinterland tectonic units resulted in an inverted metamorphic zonation and contrasts with the thin skinned tectonic character of foreland units 40ar 39ar hornblende and muscovite plateau ages from hanging wall units reflect complete rejuvenation of intracrystalline argon isotopic systems during late variscan thrusting between 340 ma and 325 ma the 40ar 39ar mineral ages together with previously published 40ar 39ar and sm nd data record systematically decreasing variscan mineral ages structurally downward into footwall units this is explained by rapid exhumation and cooling synchronously with thrusting along crustal scale ramps during foreland directed nappe propagation the location of decollement zones within specific structural levels was apparently controlled by crustal scale rheological con trasts late variscan cooling ages contrast with cadomian 40ar 39ar hornblende and muscovite ages recorded within the structural basement immediately below the major variscan thrust plane where intracrystalline isotopic systems were only partially rejuvenated during variscan thrusting these record a sequence of cadomian magmatic and metamorphic events in foreland units including 1 pre intrusive metamorphism and deformation under amphibolite facies metamorphic conditions 610 ma 2 greisen formation during a cadomian collisional event 600 ma and 3 intrusion of calcalkaline granitoid bodies with i type affinities 590 ma

siple coast ice streams in a general antarctic ice sheet model abstract in an earlier paper by verbitsky and saltzman a vertically integrated high resolution nonlinearly viscous nonisothermal ice sheet model was presented to calculate the present day equilibrium regime of the antarctic ice sheet steady state solutions for the ice topography and thermodynamics represented by the extent of the areas of basal melting were shown to be in good agreement with both observations and results obtained from other three dimensional thermodynamical equations the solution for the basal temperature field of the west antarctic siple coast produced areas at the pressure melting point separated by strips of frozen to bed ice the structure of which looks very similar to ice streams a e since the possible response of the siple coast basal temperature pattern to global warming and to associated changes in the snowfall rate is not obvious a special sensitivity study was conducted results of such a study suggest that increased precipitation rate and associated intensification

the neogene east antarctic ice sheet a dynamic or stable feature abstract this paper is a review of the history and behaviour of the east antarctic ice sheet during the late neogene and is a response to the publication of sugden et al 1993a the case for a stable east antarctic ice sheet in geografiska annaler 75a 4 the antarctic ice sheet is a critical factor in the global climate and oceanographic system knowledge of its historical development is critical to understanding global palaeoceanographic and palaeoclimatic systems and their development the webb harwood hypothesis proposes that the ice sheet while being present in antarctica for more than 40 ma was until recently ca 2 5 ma a dynamic feature waxing and waning under a more temperate climatic and glaciologic regime the stabilistic hypothesis proposes that the current ice sheet has existed in its present cold and polar form for at least the last 14 ma the case for a stable east antarctic ice sheet is the clearest statement of the stabilistic argument to date the authors find support for their viewpoint by presenting evidence from several geologic data bases that does not support their own null meltdown hypothesisr which they deem to be equivalent to the webb harwood hypothesis thereby they conclude supporting the stabilistic hypothesis however their conclusions are weakened by failure to reject their null hypothesis conflict between the meltdown and webb harwood hypotheses infraction of geological laws and poor application of stratigraphic principles several issues are highlighted here as important for direction of future research and discussion concerning late cenozoic antarctic glacio climatic history

the last pluvial climatic episodes in the deserts of southwestern north america long term variations in the effective moisture of deserts are linked both to glacial interglacial climatic cycles and through the milankovitch theory to orbitally induced changes in solar radiation intensity1 4 for southwestern north america comparison of palaeobotanical data with model predictions of palaeoclimate reveals two distinct pluvial regimes during the late wisconsin and early holocene 23 8 kyr bp southward displacement of the aleutian low pressure centre and strengthened westerlies from 40 to 30 n were important features of the full glacial 18 kyr bp pluvial maximum increased cyclonic storms and weakened monsoons led to a winter precipitation regime a feature seen both in the fossil record5 6 and climate models7 8 dominantly zonal flow during this period contrasts with the latest wisconsin and early holocene 12 8 kyr bp when meridional circulation transported maritime tropical air into the desert interior we now show that plant macrofossil assemblages from packrat neotoma spp middens9 in the mojave desert provide evidence for increased temperatures and summer rainfall during this period10 12 the anomalously late persistence to 8 kyr bp of woodland in low latitude low elevation deserts of the south west13 is attributed to enhanced monsoonal circulation3

major and trace elements in pyrope almandine garnets as sediment provenance indicators of the lower carboniferous culm sediments drahany uplands bohemian massif abstract the lower carboniferous culm basin is part of the european variscan foreland basin garnet is an abundant heavy mineral at all stratigraphic levels of the culm sediments the protivanov rozstani and myslejovice formation from base to top this paper compares the major element electron microprobe and trace element la icp ms compositions of garnet for the purpose of studying the provenance of the culm sediments the polymict garnet assemblages ranging from the protivanov to lower part of the myslejovice formation goniatite zones pe go pass into oligomict ones dominated by pyrope almandine garnets uppermost part of the myslejovice formation goniatite zones go go as the predominant low grossular pyrope almandines from oligomict garnet assemblages are very uniform and homogeneous in major element composition we studied their trace element composition and zoning pyrope almandine garnets are poor in lree and show enrichment in hree the chondrite normalized patterns are almost flat from dy to lu and show a significant negative eu anomaly typical for granulite grade garnets major element compositions of detrital low grossular pyrope almandines can only be matched with some granulites and garnetiferous felsic gneisses of the bohemian massif garnets from granulites cropping out at the present day erosion level of the bohemian massif have usually higher ca and or lower mg contents the major and trace element compositions and zoning patterns of pyrope almandine garnets from the upper part of the myslejovice formation compare well with garnets from the miroslav granulites and small moldanubian granulite bodies west of the t ebic massif the moldanubian granulites are commonly associated with mantle derived peridotites and eclogites the presence of detrital cr spinels with high cr and low mg single grains of cr spinels and fine grained kelyphitic intergrowths of cr spinel with chlorite comparable with spinel types from moldanubian peridotites could indicate the joint occurrence of peridotites serpentinized peridotites and granulites at the erosion level of the bohemian massif during the deposition of the uppermost culm sediments 325 ma while the older culm sediments polymict garnet assemblages represent the accumulation of clastic material from the northern and north western areas moravo silesian and lugian zone the uppermost culm sediments oligomict garnet assemblages contain material from western and south western areas moldanubian zone and moravian nappe the general influx of clastic material changed as a result of clockwise rotation of the eastern part of the variscan belt during this time span 340 325 ma

fabric and kinematic study of the bites orthogneiss southwestern moravia result of large scale northeastward shearing parallel to the moldanubian moravian boundary abstract the body of the bites orthogneiss bo in southwestern moravia was affected by ductile shear deformation of probably late variscan age north to northeast directed shearing is indicated by the orientation of sheath fold axes s c fabric strong asymmetrical c axis preferred orientation and ne dipping shear bands deformational structures stretching lineation mylonitic foliation and sheath folds associated with thrust wrench tectonics are connected with lower amphibolite to green schist facies metamorphism a strain study shows complex fabric development from the core to the boundaries of the bo thrust sheet fabrics in the core of the bo body are characterized by oblate strain ellipsoids low strain intensity and quartz c axis fabrics that can be approximated to small circle girdles centred around the z axis of finite strain strain ellipsoids near the boundaries of the bo thrust sheet are mostly prolate with high strain intensity and quartz diagrams show single or incomplete crossed girdles highly asymmetrical with respect to the structural framework the coaxial strain history in the core and the noncoaxial history which prevails at the boundaries of the bo thrust sheet are interpreted as a result of different strain paths in the framework of a crustal scale shear zone

chronological constraints on the pre orogenic history burial and exhumation of deep seated rocks along the eastern margin of the variscan orogen bohemian massif czech republic key lithological units of the high grade eastern margin of the bohemian massif were dated using the u pb and pb pb methods on zircons in order to establish a chronological framework for the geodynamic evolution of the variscan orogenic root the protolith ages for metagranitoids orthogneisses and granulites of thickened lower and middle crust reveal the existence of magmatic activity that occurred over a 100 million year time interval from cambro ordovician to early devonian times probably related to discontinuous intracontinental rifting of neoproterozoic crust our geochronological data suggest that the eastern part of the orogenic root represents thermally softened and rifted neoproterozoic crust preserved farther to the east as the brunia microcontinent zircon ages for felsic granulites high grade gneisses of the lower crust and of a syn convergence granodioritic intrusion in the upper crust indicate that thickening and exhumation of the crust occurred during a narrow time interval between 370 and 340 ma exhumation of the lower crust to mid crustal levels was a localized process that occurred at 340 ma and was associated with crustal scale folding in the internal part of the root as well as orogenic channel flow along the eastern collisional margin both types of exhumation mechanisms were driven by deep level wedging indentation of the easterly brunia continent followed by deposition of heavy minerals and pebbles derived from high pressure rocks in the adjacent foreland basin final orogenic development was characterized by ne sw dextral transpressive shearing parallel to the brunia margin as well as dextral trans tension associated with activity along the elbe lithospheric fault these processes affected the marginal parts of the orogenic root and were accompanied by 330 to 325 ma old syntectonic granitoid intrusions along reactivated lithotectonic boundaries rotation of the assembled orogenic belt accompanied by lithospheric faulting driven by westerly subduction roll back may be the most plausible model to explain late deformation of the orogenic root

late palaeozoic palaeomagnetic and tectonic constraints for amalgamation of pangea supercontinent in the european variscan belt abstract this paper provides a review of the evolution of palaeomagnetic directions and sequence of tectonic events in the european variscan belt during late palaeozoic times these data are correlated with palaeomagnetic records from gondwana and baltica in order to provide a large scale geodynamic framework during the carboniferous to permian the early carboniferous palaeomagnetic records reported mainly from the rhenohercynian and saxothuringian magmatic arcs indicate a 70 anticlockwise rotation while late carboniferous to permian magnetic directions from various rocks of western and central europe are consistent with 120 clockwise rotation of the eastern and central parts of the variscan belt our review of the chronology of tectonic events is based on a robust database of geochronologically constrained deformation metamorphic and magmatic events which allow discretizing the 80 ma long orogenic evolution into five principal events in this scenario the variscan belt can be regarded as a linear sub plate isolated from gondwana and laurussia by the rhenohercynian and palaeotethysian oceans during devonian times this linear composite belt was segmented by transform faults and boundaries in the late devonian to early carboniferous times 360 335 ma during progressive e w closure of rhenohercynian ocean synchronously with collision between saxothuringian and moldanubian blocks subsequent relocation of subduction to the northern boundary palaeotethysian ocean was responsible for n s shortening almost orthogonal to the ancient sub plate n s elongation at around 335 325 ma this deformation resulted in dextral reactivation of transform boundaries associated with anticlockwise rotation of intermittent blocks at the end of this rotation the faults were parallelized to the teysseire tornquist zone the southern margin of baltica while the lozenge shaped blocks of the former variscan sub plate were further shortened during continuous contraction this evolution can fully explain the 70 anticlockwise rotation from cn3 to cn2 palaeomagnetic directions which are regarded as successive carboniferous magnetizations subsequently the variscan belt suffered a giant transtensional event from 325 to 310 ma that was related to the development of extensional syn magmatic core complexes over the whole belt and significant dextral reactivation of earlier nw se trending transform faults this extensional event was associated with important tilts recorded by cp magnetic overprint resulting from a major thermally induced remagnetization during this event new sets of sinistral transfer nne ssw trending faults originated that partly reactivated boundaries of the principal tectonic zones blocks delimited by second order sinistral and first order dextral faults then rotated in a clockwise manner by 80 the whole system subsequently suffered a period of nne ssw shortening that affected the variscan belt namely along the former laurussian and former variscan sub plate contact in the north and in the south where the giant cantabrian orocline developed at around 310 300 ma due to hard collision with gondwana this deformation is associated with the clockwise rotation of laurussia together with the accreted northern sector of the variscan belt and the anticlockwise of gondwana this clockwise 30 rotation is achieved by a1 remagnetizations while the southern part of iberia suffered anticlockwise rotation the final stage of rapid clockwise rotation affecting the northern limb of the iberian arc including corsica and sardinia is attributed to giant n s extension affecting the whole variscan belt at the onset of permo triassic opening of the tethys ocean this complex evolution is regarded as a result of the readjustment of inhomogeneous and thermally and mechanically instable mobile space in between reorganizing the gondwana and laurussia megaplates and the opening of the palaeotethys ocean during the final stages of formation of the pangea supercontinent finally the palaeomagnetically constrained rotations and tectonic evolution of the variscan belt are explained using a pinned model of internal and external rotations of blocks driven by activation of dextral shear zones by n s compression and e w transtension in particular

formation and disintegration of the antarctic ice sheet a model of the antarctic ice sheet has been used to simulate the ice sheet in warmer climates in order to investigate what kind of ice sheet geometries one canreasonably expect under what kind of climatic conditions and to find out which physical mechanisms may be involved to explain them the results of theseexperiments reveal the large stability of in particular the east antarctic ice sheet it would require a temperature rise of between 17 and 20k above presentlevels to remove this ice sheet from the subglacial basins in the interior of the continent and of 25k to melt the antarctic ice sheet completely down for atemperature rise below 5k the model actually predicts a larger antarctic ice sheet than today as a result of increased snowfall whereas the west antarctic icesheet was found not to survive temperatures more than 8 10k above present values furthermore basal temperature conditions in these experiments point tothe problems involved to raise the base to the pressure melting point over the large areas necessary to consider the possibility of sliding instability theseresults bear on a lively debate regarding the late cenozoic glacial history of antarctica in particular based on these findings it appears difficult to reconcile ahighly variable east antarctic ice sheet up to the pliocene with the modest warmings recorded in for instance the deep sea records for the late neogene

polykinematic foreland basins initiated during orthogonal convergence and terminated by orogen oblique strike slip faulting an example from the northeastern variscan belt abstract an extensive early carboniferous foreland basin the moravosilesian culm basin mscb delineates the northeastern margin of the bohemian massif variscan orogenic belt the basin overlays a complex suture between the outer brunovistulia microplate forming a promontory previously accreted to laurussia and inner gondwana derived units that form the remainder of the bohemian massif our structural paleomagnetic and magnetic anisotropy data from the eastern portion of the mscb indicate that its depositional architecture internal fabric and deformation by folding and faulting record a major switch from orthogonal convergence to orogen oblique strike slip shearing first the brunovistulia microcontinent was subducted frontally towards w during 346 335 ma followed by a shift of the subduction vector towards ssw at around 335 330 ma from 330 ma onwards the underthrusting had ceased and the wnw ese dextral elbe shear zone related to the westward movement of gondwana with respect to laurussia controlled deformation of this region displacement of the northerly eastern sudetes block towards ese produced regional wnw ese pure shear shortening against the brunovistulia indentor and resulted in an overall doubly vergent structure of the mscb simultaneously the southerly moldanubian unit was translated towards wnw away from the brunovistulian microplate and likely contributed to an extensional regime related to orogenic collapse progressing outward from the internal domains of the bohemian massif on a larger scale we suggest that the mscb is an illustrative example of a polykinematic foreland basin where deposition syn to post sedimentary deformation and thus geodynamic processes changed significantly during basin evolution starting off with microplate impingement into a continental margin reentrant and terminated by strike slip shearing our data also do not support previous models for the tectonic development of the northeastern termination of the variscan belt which suggested either a large magnitude vertical axis oroclinal rotation of the foreland or its translation along an orogen perpendicular strike slip fault

behavior of the east antarctic ice sheet as deduced from a coupled gcm ice sheet model while the possible instability of the west antarctic ice sheet has been widely recognized for some time as a potential source of sea level rise in an enhanced greenhouse warming the stability of the east antarctic ice sheet has only recently become the subject of such a conjecture we approach an aspect of this issue using a numerical experiment with an atmospheric general circulation model gcm coupled to a 3 dimensional ice sheet model response of the ice sheet model to the external forcing generated by an atmospheric gcm due to doubling the co2 concentration does not show any appreciable changes in the horizontal extent of the ice sheet due to normal creep and topographic instabilities suggesting that an antarctic collapse resulting from these factors is unlikely the mechanics of basal sliding non isothermal effects and ice shelves are as yet too poorly understood to make quantitative estimates of possible instabilities due to these processes

origin of migmatites by deformation enhanced melt infiltration of orthogneiss a new model based on quantitative microstructural analysis a detailed field study reveals a gradual transition from high grade solid state banded orthogneiss via stromatic migmatite and schlieren migmatite to irregular foliation parallel bodies of nebulitic migmatite within the eastern part of the gfohl unit moldanubian domain bohemian massif the orthogneiss to nebulitic migmatite sequence is characterized by progressive destruction of well equilibrated banded microstructure by crystallization of new interstitial phases kfs pl and qtz along feldspar boundaries and by resorption of relict feldspar and biotite the grain size of all felsic phases decreases continuously whereas the population density of new phases increases the new phases preferentially nucleate along high energy like like boundaries causing the development of a regular distribution of individual phases this evolutionary trend is accompanied by a decrease in grain shape preferred orientation of all felsic phases to explain these data a new petrogenetic model is proposed for the origin of felsic migmatites by melt infiltration from an external source into banded orthogneiss during deformation in this model infiltrating melt passes pervasively along grain boundaries through the whole rock volume and changes completely its macro and microscopic appearance it is suggested that the individual migmatite types represent different degrees of equilibration between the host rock and migrating melt during exhumation the melt topology mimicked by feldspar in banded orthogneiss forms elongate pockets oriented at a high angle to the compositional banding indicating that the melt distribution was controlled by the deformation of the solid framework the microstructure exhibits features compatible with a combination of dislocation creep and grain boundary sliding deformation mechanisms the migmatite microstructures developed by granular flow accompanied by melt enhanced diffusion and or melt flow however an ams study and quartz microfabrics suggest that the amount of melt present did not exceed a critical threshold during the deformation to allow free movements of grains

temporal variability of microparticle properties in polar ice sheets abstract four recent ice core studies reveal a consistently recurring temporal correlation between increased microparticle concentrations and lower global temperatures more negative 13 o 16 o ratios a continuous 900 year record of particle deposition from the 101 m south pole core was obtained by analyzing 6218 samples the concentration of insoluble particles with diameters 0 63 m increases substantially between a d 1450 and 1850 a period of slightly reduced global temperatures encompassing the latest neoglacial or little ice age there is evidence suggesting that some of the additional material may be volcanic although further substantiation is required the microparticle analyses of selected sections from three deep cores coupled with the respective 18 o measurements reveal that in all three cores the last glacial or late wisconsin ice contained great quantities of microparticles the ratio of the average microparticle concentration in wisconsin sections to that in holocene sections is 6 for the 905 m dome c antarctica core 3 for the 2164 m byrd station antarctica core and 12 for the 1387 m camp century greenland core microparticle increases of this magnitude can not be accounted for merely by a reduction in net accumulation these data suggest that the global atmosphere was heavily laden with suspended particulates near the end of the last major glaciation

monsoon climate of the early holocene climate experiment with the earth s orbital parameters for 9000 years ago values for the precession and obliquity of the earth 9000 years ago indicate that the global average solar radiation for july 9000 years ago was 7 percent greater than at present when the estimated solar radiation values are used in a low resulation climate model the model simulates an intensified continent scale monsoon circulation this result agrees with paleoclimatic evidence from africa arabia and india that monsoon rains were stronger between 10 000 and 5000 years ago than they are today

pressure temperature evolution of metapelitic granulites in a polymetamorphic terrane the rauer group east antarctica distinctive lithological associations and geological relationships and initial geochronological results indicate the presence of an areally extensive region of reworked archaean basement containing polymetamorphic granulites in the rauer group east antarctica structurally early metapelites from within this reworked region preserve complex and varied metamorphic histories which largely pre date and bear no relation to a late proterozoic metamorphism generally recognized in this part of east antarctica in particular magnesian metapelite rafts from long point record extreme peak p t conditions of 10 12 kbar and 100 1050 c and an initial decompression to 8 kbar at temperatures of greater than 900 c initial garnet orthopyroxene sillimanite assemblages contain the most magnesian and pyrope rich garnets xmg 0 71 yet found in granulite facies rocks a high temperature decompressional p t history is consistent with reaction textures in which the phase assemblages produced through garnet breakdown vary systematically with the initial garnet xmg composition reflecting the intersection of different divariant reactions in rocks of varied composition as pressures decreased this history is thought to relate to archaean events whereas a lower temperature c 750 800 c decompression to 5 kbar reflects late proterozoic reworking of these relict assemblages the major late proterozoic c 1000 ma granulite facies metamorphism is recorded in a suite of younger fe rich metapelites and associated paragneisses in which syn to post deformational decompression through 2 4 kbar from maximum recorded p t conditions of 7 9 kbar and 800 850 c is constrained by geothermobarometry and reaction textures this p t evolution is thought to reflect rapid tectonic collapse of crust previously thickened through collision

evidence from north west canada for an early holocene milankovitch thermal maximum according to the milankovitch theory of global climatic change maximum summer solar radiation at high latitudes of the northern hemisphere occurred at 10 000 yr bp refs 1 2 in particular it predicts summer solstice radiation greater by 9 10 preliminary climate simulation experiments with these increased values of radiation confirm that high latitude land surfaces received maximal insolation at 10 000 yr refs 3 4 paradoxically however the large volume of fossil pollen and other evidence from north america indicates a maximum of holocene warmth at 7 000 6 000 yr ref 5 and a recent review of the evidence from new england suggests that the warming began at 9 000 and ended at 5 000 yr but also stresses the difficulties of interpretation in terms of climate change6 we summarize here data from sites in the north west corner of mainland canada fig 1 that directly support the milankovitch hypothesis

cenozoic glaciation in the southern hemisphere with the end of glaciation in gondwana in early permian time a prolonged interval began when the earth was without ice sheets though not necessarily free of mountain glaciers not until sometime during the tertiary era did ice sheets build up once more first in antarctica and later in the northern hemisphere the fundamental cause of the cooling that led to the formation of the antarctic ice sheet and eventually to major glaciation in lower latitudes is generally held to have been the increasing thermal isolation of antarctica as australia and later south america separated from it allowing zonal oceanic circulation to develop rifting between australia and antarctica began between 1 10 and 90 m y ago cande mutter 1982 but not until much later probably sometime between 30 and 25 m y ago had it progressed far enough for deep circulation to begin as the south tasman rise cleared north victoria land kennett et a1 1974 kennett 1977 weissel et ai 1977 however antarctica did not reach its present high level of thermal isolation until south america had separated from it enabling the antarctic circumpolar current to become established savin et ai 1975 according to barker burrell 1977 1982 coherent spreading between the antarctic peninsula and tierra del fuego started about 29 m y ago and deep circulation began 23 5 2 5 m y ago thereafter increasing in volume the initial ice fields and ice caps that later expanded to become the

vostok ice core provides 160 000 year record of atmospheric co2 direct evidence of past atmospheric co2 changes has been extended to the past 160 000 years from the vostok ice core these changes are most notably an inherent phenomenon of change between glacial and interglacial periods besides this major 100 000 year cycle the co2 record seems to exhibit a cyclic change with a period of some 21 000 years

evolution of moldanubian rocks in austria review and synthesis the moldanubian zone in austria comprises three major lithological units despite general agreement that nappe tectonics contributed to its current structure the number and position of tectonic boundaries or continental pieces that were involved in its evolution as well as the age extent and position of oceanic sutures are disputed recent models ascribe the moldanubian tectonostratigraphic structure to its oblique n to ne directed collision with moravia only the rocks of the moldanubian bunte series and gfohl unit experienced a common intensive overprint in the range 700 800 c and 8 11 kbar textural evidence suggests that this overprint was attained during nearly isothermal decompression so the rocks experienced higher pressures prior to this overprint these conditions constrain a continent continent collision environment that contributed to the formation of the moldanubian granulites the estimated metamorphic temperatures are close to tmax during this hercynian high t overprint the minerals underwent extensive diffusion controlled homogenization of elements the early stages of retrogression of these units were characterized by isobaric cooling at c 6 kbar in the range 650 500 c that is related to the oblique collision of the moldanubian and moravian zones cooling to c 400 c is demonstrated by unstrained diasporized corundum inclusions in garnet of common moldanubian granulites the available age data including cooling ages from metamorphic rocks show a very wide variation between 490 and 280 ma that depends on sample characteristics and the dating method used they demonstrate clearly however that the metamorphic overprint is hercynian the possibility that the large variation in ages reflects homogenization resetting and closure of the isotopic systems attained at different sample and method specific times is discussed age data varying between c 370 and c 346 ma tentatively date different stages during the hercynian high t decompression the majority of zircon and monazite u pb ages as well as the hornblende and muscovite ar ar cooling ages cluster between c 345 and c 326 ma and date the effective closure conditions and the onset of rapid nearly isobaric cooling the continent continent collision that formed the granulites pre dates c 370 ma the intra moldanubian nappe stacking pre dates thrusting of the moldanubian zone over the moravian zone the range c 340 335 ma is the lower limit for completion of tectonic activity in the moldanubian zone the moldanubian series are post tectonically intruded by granitoids of the southern bohemian pluton recent age determinations and geochemical evidence suggest that the formation of the early granitoid types took place in the lower crust in connection with the hercynian high grade overprint the moldanubian monotone series in austria is separated from the other moldanubian units by a conspicuous tectonic horizon it also differs from them by its characteristic high t low p overprint which is best demonstrated by a widespread cordierite gneiss

the impact of cold north atlantic sea surface temperatures on climate implications for the younger dryas cooling 11 10 k the sensitivity of global climate to colder north atlantic sea surface temperatures is in vestigated with the use of the giss general circulation model north atlantic ocean temperatures 18 000 b p resembling those prevalent during the younger dryas were incorporated into the model of the present climate and also into an experiment using orbital parameters and land ice characteristic of 11 000 b p the results show that with both 11 000 b p and present conditions the colder ocean temperatures produce cooling over western and central europe in good agreement with younger dryas paleoclimatic evidence cooling also occurs over extreme eastern north america although the precise magnitude and location depends upon the specification of ocean temperature change in the western atlantic despite the presence of increased land ice and colder ocean temperatures the younger dryas summer air temperatures at northern hemisphere midlatitudes in the model are warmer than those of today due to changes in the orbital parameters chiefly precession and atmospheric subsidence at the perimeter of the ice sheets

pre variscan geological events in the austrian part of the bohemian massif deduced from u pb zircon ages in an attempt to elucidate the pre variscan evolution history of the various geological units in the austrian part of the bohemian massif we have analysed zircons from 12 rocks mainly orthogneisses by means of shrimp conventional multi grain and single grain u pb isotope dilution mass spectrometry two of the orthogneisses studied represent cadomian metagranitoids that formed at ca 610 ma spitz gneiss and ca 580 ma bittesch gneiss a metagranite from the thaya batholith also gave a cadomian zircon age 567 5 ma traces of neoproterozoic zircon growth were also identified in several other samples underlining the great importance of the cadomian orogeny for the evolution of crust in the southern bohemian massif however important magmatic events also occurred in the early palaeozoic a sample of the gfohl gneiss was recognised as a 488 6 ma old granite a tonalite gneiss from the realm of the south bohemian batholith was dated at 456 3 ma and zircon cores in a moldanubian metagranitic granulite gave similar ages of 440 450 ma this ordovician phase of magmatism in the moldanubian unit is tentatively interpreted as related to the rifting and drift of south armorica from the african gondwana margin the oldest inherited zircons in a migmatite from the south bohemian batholith yielded an age of ca 2 6 ga and many zircon cores in both moravian and moldanubian meta granitoid rocks gave ages around 2 0 ga however rocks from the moldanubian unit show a striking lack of zircon ages between 1 8 and 1 0 ga reflecting an ancestry from armorica and the north african part of gondwana respectively whereas the moravian bittesch gneiss contains many inherited zircons with mesoproterozoic and early palaeoproterozoic ages of ca 1 2 1 5 and 1 65 1 8 ga indicating a derivation from the south american part of gondwana

excess40ar in metamorphic rocks from broken hill new south wales implications for40ar 39ar age spectra and the thermal history of the region 40ar 39ar age spectrum analyses of samples from broken hill new south wales indicate that the region has experienced a complex thermal history following high grade metamorphism 1660 ma ago the terrain cooled slowly 3 c ma 1 until about 1570 ma ago when the temperature fell below about 500 c following granitoid emplacement 1500 ma ago the region remained relatively cold until affected by a thermal pulse 520 40ma ago causing temperatures to rise to 350 c in some places during this event accumulated40ar was released from minerals causing a significant ar partial pressure to develop laboratory ar solubility data combined with the40ar 39ar age spectra gives a local estimate of this partial pressure of 10 4atm the region finally cooled below 100 c about 280 ma ago 40ar 39ar age spectrum analyses of hornblende plagioclase and clinopyroxene containing excess40ar are characterized by saddle shaped age spectra detailed analysis of plagioclase samples reveals a complex diffusion behaviour which is controlled by exsolution structures this effect in conjunction with the presumed different lattice occupancy of excess40ar with respect to radiogenic40ar appears to be responsible for the saddle shaped age spectra

anatomy of a diffuse cryptic suture zone an example from the bohemian massif european variscides the fate of the lower plate during continental collision can be examined in deeply eroded orogens such as the late paleozoic variscan belt in continental europe in particular the bohemian massif at its eastern extremity preserves well the evolution of an andean type orogen involved in continental collision this process included relamination of subducted light felsic material rich in radioactive elements underneath a dense mafic lower crust of the upper plate this led to gravity driven overturns and overprinting of the original suture by a broad zone of mixed upper and lower plate materials in the studied example this zone of interaction repeatedly reappears within the orogen forming a so called diffuse cryptic suture zone

terrestrial conditions at the last glacial maximum and climap sea surface temperature estimates are they consistent climap 1981 seasonal reconstruction of the earth s surface at the last glacial maximum geological society of america map and chart series mc 36 boundary conditions were used as inputs to the giss general circulation model and the last glacial maximum lgm climate was simulated for six model years the simulation was compared with snow line depression and pollen inferred temperature data at low latitudes specifically for hawaii colombia east africa and new guinea the model does not produced as much cooling at low latitudes as is implied by the terrestrial evidence an alternative experiment in which the climap sea surface temperatures were uniformly lowered by 2 c produces a better fit to the land data although in hawaii model temperatures are still too warm the relatively warm climap tropical sea surface temperatures also provide for only a slight decrease in the hydrologic cycle in the model in contrast to both evidence of lgm tropical aridity and the results of the experiment with colder ocean temperatures with the climap sea surface temperatures the lgm global annual mean surface air temperature is 3 6 c colder than at present if the ocean temperatures were allowed to cool in conformity with the model s radiation balance the lgm simulation would be 5 6 c colder than today and in better agreement with the tropical land evidence

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

pollen studies in the junin area central peruvian andes laguna junin is a large lake 675 km long but only 4 m deep located in the grass covered altiplano at an altitude of 4 100 m northeast of lima pollen analysis of a 30 m core indicates that from about 12 000 yr ago until some time after 3 000 yr ago about 30 of the pollen was carried in by easterly winds from the east andean forests which today are at least 600 m lower in elevation contemporaneous pollen deposition in lakes on moraines of the last glaciation west of the junin plain was dominated by local grassland types and had little contribution from this easterly source at some time after 3 000 yr ago perhaps about 1 200 yr ago the pollen influx to laguna junin from this distant forest source greatly diminished perhaps because of deforestation by increased human populations between about 12 000 and 24 000 yr ago in the junin core the sparse pollen concentration was dominated by grassland types supplemented by polylepis acaena alnus and compositae blown in from the sub puna shrublands that may have occupied much of the east andean slopes in place of the humid forest that occurs there today the silty sediment in this section of the core is interpreted as outwash from piedmont glaciers that invaded the plain especially from the cordillera to the west these glaciers left a distinctive pattern of moraines and outwash fans which served to dam tine lake basin an unconformity in the sediment covers the time from about 24 000 to 39 000 yr ago when the lake was probably dry before this time the silty sediment and the dominance of grassland and shrubland pollen types indicates an earlier interval of glaciation recorded on the landscape by an older set of moraines and outwash fans which initially dammed the lake basin

conversion of the magnetic susceptibility tensor into the orientation tensor in some rocks abstract the preferred orientation of minerals in a rock with a structural or sedimentary fabric can be referred to by the orientation tensor this paper describes the determination of the orientation tensor of the crystallographic c axes from the rock magnetic anisotropy and the mineral anisotropy degree in rocks in which the magnetic anisotropy is dominantly carried by one mineral with uniaxial magnetic anisotropy e g phyllosilicates pyrrhotite hematite the orientation tensor of biotite c axes in four samples of the bites orthogneiss determined in this way is similar to that determined independently through the universal stage measurement of biotite leaves in thin sections and is obtained much more rapidly by the magnetic method

paleoclimatic estimates from water and energy budgets of east african lakes abstract the turkana nakuru elmenteita and naivasha basins in the rift valley of east africa experienced high water levels during the period of 10 000 7000 yr b p analyses of the modern hydrologic and energy budgets for these basins along with that of lake victoria were used to infer the amount of precipitation that would have been required to maintain the enlarged paleolakes of the early holocene precipitation must have been at least 150 300 mm yr 15 to 35 above the modern average the precipitation estimates were fairly consistent among the various basins but no quantitative estimate was made for the additional precipitation required to account for overflow from the rift valley lake basins discharge from the lake victoria basin around 1880 a d was considerably above the more recent average and the increased discharge into the white nile for that period might have been similar to that of the early holocene a sensitivity analysis showed that temperature changes were probably not too important for changing the hydrologic energy budget changes of albedo bowen ratio and cloudiness were likely to have been of greater importance

clasts of variscan high grade rocks within upper visean conglomerates constraints on exhumation history from petrology and u pb chronology in this paper u pb zircon monazite and rutile data for crystalline rocks deposited as clasts in the upper visean conglomerates at the eastern margin of the bohemian massif are reported u pb data of spherical zircon from three different granulite clasts yielded a mean age of 339 0 0 7 ma 2 while oval and spherical grains of another granulite pebble define a slightly younger date of 337 1 1 1 ma these ages are interpreted as dating granulite facies metamorphism thermochronology and the derived pressure temperature p t path of the granulite pebbles reflect two stage exhumation of the granulites near to isothermal decompression from at least 44 km to mid crustal depths of around 22 km was followed by a near isobaric cooling stage based on reaction textures and geothermobarometry minimum average exhumation rate corresponds to 2 8 4 3 mm year 1 the extensive medium pressure high temperature overprint on granulite assemblages is dated by u pb in monazite at c 333 ma this thermal event probably has a close link to generation and emplacement of voluminous moldanubian granites including the cordierite granite present in clasts this granite was emplaced at mid crustal levels at 331 3 ma u pb monazite whereas the u pb zircon ages record only a previous magmatic event at c 378 ma eclogites and garnet peridotites normally associated with high pressure granulites are absent in the clasts but exotic subvolcanic and volcanic members of the ultrapotassic igneous rock series durbachites of the bohemian massif have been found in the clasts it is therefore assumed that the clasts deposited in the upper visean conglomerates sampled a structurally higher tectonic unit than the one that corresponds to the present denudation level of the moldanubicum of the bohemian massif the strong medium temperature overprint on granulites dated at c 333 ma is attributed to the relatively small size of the entirely eroded bodies compared with the presently exposed granulites

coupling an agcm with an ism to investigate the ice sheets mass balance at the last glacial maximum the aim of this paper is to investigate the consistency between the ice sheets reconstruction at the lgm and the climate simulated by agcm in particular to investigate whether the laurentian and the fennoscandian ice sheets have an equilibrated mass balance we use two complementary approaches first we analyze the changes of snow and water budgets over the ice sheets only using the model results of lgm runs second we use a thermomechanical ice sheet model ritz et al 1997 forced with the agcm simulated climate to perform long term runs at finer spatial resolution analyzing the results obtained with both approaches we show that the ice sheet mass balance computed directly from agcm results may be biaised moreover we show using two versions of the lmd agcm that the ice sheet mass balance computed by the ism is drastically sensitive to the summer surface temperatures simulated by the agcm

anticlockwise and clockwise rotations of the eastern variscides accommodated by dextral lithospheric wrenching palaeomagnetic and structural evidence new palaeomagnetic data from lower carboniferous granitoids of the orogenic root of the eastern variscan belt moldanubian domain show a polyphase palaeomagnetic record comparison of the data with existing palaeomagnetic measurements from lower palaeozoic sequences of the bohemian massif demonstrates a carboniferous remagnetization of these latter units all the data presented suggest that the saxothuringian basement the moldanubian orogenic root system as well as the eastern neoproterozoic brunovistulian basement were already assembled during the early carboniferous the whole variscan belt subsequently rotated in a clockwise direction during mid late carboniferous times structural and geochronological data indicate that this rotation was accompanied by large scale dextral wrenching along nw se trending lithospheric faults in a first stage the blocks limited by wrench faults rotated anticlockwise in bookshelf manner the data presented rule out the existing model of oroclinal bending of the rhenohercynian zone at the eastern termination of the variscan belt

ice core sample measurements give atmospheric co 2 content during the past 40 000 yr recent measurements1 2 on ice samples from camp century greenland 77 10 n 61 08 w byrd station antarctica 80 01 s 110 31 w and dome c 74 40 s 125 10 e suggest that during the late part of the last glaciation the atmospheric co2 concentration was significantly lower than during the holocene further investigation of this natural increase of the atmospheric co2 concentration in the past should aid our understanding of the climatic implications of the man made co2 increase since the beginning of industrialization3 here we report new and precise measurements of the co2 concentration of the air occluded in bubbles of ice samples from camp century and byrd station using a new dry extraction technique the extracted gases were analysed with an ir laser spectrometer irls samples from 22 different depths were analysed from each core the samples are distributed over a depth interval corresponding approximately to the past 40 000 yr in addition results for ice samples from selected depth horizons from a colder region north central greenland 74 37 n 39 36 w and from a warmer region dye 3 greenland 65 11 n 43 50 w are given based on these results we estimate the trend of the atmospheric co2 concentration during the past 40 000 yr

coherent response of arabian sea upwelling and pollen transport to late quaternary monsoonal winds the indian summer monsoon causes large seasonal changes in the environments of both eastern africa and the arabian sea here we compare time series 140 kyr long of selected pollen types and foraminiferal upwelling faunas preserved in marine sediments to identify the frequencies of their variability and the coherence and phase of the terrestrial and marine responses to monsoonal circulation during interglacial intervals the pollen and upwelling records of the western arabian sea indicate stronger monsoons and are both coherent and in phase at periodicities near the precession of the earth s rotational axis 23 kyr we conclude that the strength of the monsoonal southwesterlies accounts for this high coherence of marine and terrestrial records during interglacial intervals low correlation during glacial phases indicates weaker monsoons and some decoupling of these two records these data indicate that two distinct modes of monsoon variability may exist an interglacial mode with strong monsoons and effective coupling of terrestrial and marine records and a glacial mode with both weaker monsoons and decreased linkage between arabian sea pollen and upwelling records

a new variscan deformation map of england and wales collections of bedding orientations from subareas of 550 km 2 are used to define bedding fabrics from the variscan fold belt and foreland in england and wales the bedding fabrics constructed from data taken from published geological maps 1 50 000 scale are analysed in terms of the scheidegger watson orientation tensor the calculated eigenvalues of the orientation tensor define the average structural trends including the best fit fold axis for each subarea the eigenvectors of the tensor allow the original sedimentary fabrics to be distinguished from those modified by deformation the progressive tectonic modification of these fabrics leads to the concept of fabric evolution paths and the extent of the modification is expressed by a deformation parameter although it is emphasised that this parameter is not solely a function of finite strain magnitude its variation is represented on a map which brings out different degrees of variscan tectonic modification of bedding fabrics this map delineates a variscan bedding fabric front corresponding to the northern boundary of a zone of pervasive and strong deformation in the foreland this new analysis allows the variation of variscan trends to be mapped and the significance of these trends to be quantified in terms of deformation magnitude 1999 elsevier science b v all rights reserved

simulation of the climate of 18 000 years bp results for the north american north atlantic european sector and comparison with the geologic record of north america abstract the large ice sheets in north america and europe and the extensive sea ice cover in the north atlantic at the time of the last glacial maximum must have greatly modified the surface temperature patterns and in turn the location and intensity of the surface winds and jet streams a general circulation model was used to simulate the january and july patterns of temperature precipitation and wind for 18 ka bp boundary conditions for the model consisting of ice sheet location and height sea ice location and sea surface temperature were prescribed from climap 1981 the model results are illustrated and described for the north american north atlantic european sector the jet stream splits around the north american ice sheet and the southern branch strengthens considerably compared to present over the southern portion of the united states the sea ice margin of the north atlantic and the southern edge of the european ice sheet geologic evidence principally from north america of wind temperature and moisture conditions is assessed from sand dune and loess records estimates of snowline depression pollen records and lake level studies the geologic evidence is generally compatible with the model simulation

the co 2 induced thickening thinning of the greenland and antarctic ice sheets as simulated by a gcm ccmi and an ice sheet model scaling analysis shows that the mean thickness of an ice sheet depends on the product of two poorly known quantities the ice viscosity and the net snow accumulation rate we adjust the viscosity of an ice sheet in order to get a consistent value of this product for the present day ice sheet volume and area given the net snow accumulation rate calculated by an atmospheric general circulation model gcm we then hold this artificial rheology constant in further numerical experiments we hope that in doing so we can partially compensate for systematic gcm errors in simulating the snow accumulation rate and therefore thickening thinning of ice sheets will depend mostly on the tendency in the net accumulation change rather than on its absolute value using this approach the response of the greenland and antarctic ice sheets to doubling co2 concentration is simulated and the horizontal distribution of possible thickening thinning of polar ice obtained we find that initially the region of thickening ice is close to the area of increased snowfall rate but later it significantly changes under the influence of internal ice flow dynamics the sea level changes predicted by our experiments agree with some empirical estimates the sensitivity experiment with assigned basal sliding does not show significant changes in the large scale ice topography meaning for example that there is no indication of a possible disintegration of the west antarctic ice sheet at the same time the regional thickening thinning of ice and consequently the sea level change depends strongly on processes at the ice sheet bottom