quaternary glaciation of the himalayan tibetan orogen glacial geological evidence from throughout the himalayan tibetan orogen is examined to determine the timing and extent of late quaternary glaciation in this region and its relation to similar changes on a global scale the evidence summarised here supports the existence of expanded ice caps and extensive valley glacier systems throughout the region during the late quaternary however it cannot yet be determined whether the timing of the extent of maximum glaciation was synchronous throughout the entire region or whether the response was more varied the lack of organic material needed for radiocarbon dating has hindered past progress in glacial reconstruction however application of optically stimulated luminescence and terrestrial cosmogenic radionuclide methods has recently expanded the number of chronologies throughout the region limits to the precision and accuracy available with these methods and more importantly geological uncertainty imposed by processes of moraine formation and alteration both conspire to limit the time resolution on which correlations can be made to milankovitch timescales several ka in order to put all studies on a common scale well dated sites have been re evaluated and all the published terrestrial cosmogenic nuclide ages for moraine boulders and glacially eroded surfaces in the himalayan tibetan orogen have been recalculated locally detailed studies indicate that there are considerable variations in the extent of glaciation from one region to the next during a glaciation glaciers throughout monsoon influenced tibet the himalaya and the transhimalaya are likely synchronous both with climate change resulting from oscillations in the south asian monsoon and with northern hemisphere cooling cycles in contrast glaciers in pamir in the far western regions of the himalayan tibet orogen advanced asynchronously relative to the other regions that are monsoon influenced regions and appear to be mainly in phase with the northern hemisphere cooling cycles broad patterns of local and regional variability based on equilibrium line altitudes have yet to be fully assessed but have the potential to help define changes in climatic gradients over time copyright 2008 john wiley sons ltd

a continuum mechanical formulation for shallow polythermal ice sheets this paper is concerned with a new theoretical approach to model grounded ice sheets in three dimensions these are considered as polythermal i e there will be regions with temperatures below the

cracking the lid sill fed dikes are the likely feeders of flood basalt eruptions although subparallel swarms of dikes are thought to be the primary feeders to voluminous volcanic eruptions increasing recognition of volumetrically significant sill complexes suggests that they too play an important role in magma ascent through the shallow crust however the extent to which sills and interconnected sill fed dikes actually transport magma to the earth s surface in many large igneous provinces lip is presently unclear by analyzing field relationships and dimensions of intrusions of the ferrar lip in south victoria land antarctica we show that sill fed dikes were the likely feeders for voluminous flood basalt eruptions these intrusions are small but numerous with cumulative dimensions equivalent to a feeder network 308 000 km long and 1 8 m wide due to the tremendous aerial extent of this intrusive network each individual dike feeder segment would only be required to actively feed magma for 2 to 3 days on average to erupt the 70 000 km3 of flood lavas represented by the kirkpatrick basalts the ferrar intrusions form a broadly distributed array of small moderately dipping dikes 2 km long 1 8 m wide 56 mean dip exhibiting almost any orientation this sill fed dike network contrasts with dike swarms conventionally depicted to feed flood basalt provinces and has the appearance of a variably cracked lid atop a sill complex the cracked lid model may apply to a range of shallow feeder systems 4 km depth intruding sedimentary basins where the effects of far field tectonic stresses are negligible and sill intrusions exert the dominant control on dike orientations we conclude that sill inflation and resulting deformation of surrounding host rock plays a critical role in the ascent of magma in shallow volcanic systems that span the full spectrum of eruptive volumes

vegetation changes and associated climate variations during the past 38 000 years reconstructed from the shaamar eolian paleosol section northern mongolia abstract this study presents a pollen based reconstruction of the vegetation changes and associated climate variations at the shaamar eolian paleosol section in the northern mongolian plateau the semi quantitative reconstructions of paleovegetation types and temperature and moisture indices show that the vegetation in the shaamar area was a taiga forest dominated by picea abies and pinus sibirica type between 38 000 and 30 000 cal bp when the reconstructed moisture level was the highest of the past 38 000 years the vegetation during the ensuing period from 30 000 to 23 000 cal bp was mainly a steppe dominated by artemisia and poaceae under low temperature and relatively low moisture conditions the vegetation during the period from 23 000 to 10 000 cal bp varied from a taiga forest through steppe dominated by poaceae taiga forest and forest steppe to a taiga forest the climate was cold and relatively dry in the early phase 23 000 to 20 000 cal bp and warmer and somewhat drier in the later phase 19 000 to 11 000 cal bp the pollen assemblages and the vegetation zonal indices suggest that the vegetation was a forest steppe during the period from 10 000 to 3000 cal bp under rising temperature and low moisture conditions a taiga forest has dominated the landscape during the past 3000 years under cooling and wetting conditions comparison of our pollen record with other paleoclimatic records suggests that the climate changes in the northern mongolian plateau have been controlled or modulated by ocean atmospheric coupling dynamics in the north atlantic region

cosmogenic beryllium 10 and neon 21 dating of late pleistocene glaciations in nyalam monsoonal himalayas we present cosmogenic 10be and 21ne chronologies from 21 erratic boulders on three moraine sequences in nyalam county monsoonal himalayas southern margin of the tibetan plateau the surface exposure ages provide evidence for at least two distinct glacial advances during the late stage of the last glacial cycle and for one or more significantly older glaciations the distribution of cosmogenic ages from the three ridges of the old moraine sequence is inconsistent with their stratigraphic order because exposure periods of the erratics prior to deposition on the moraine surface is shown to be small the chronology stratigraphy mismatch suggests that the cosmogenic ages do not date moraine deposition but most likely significant moraine ridge denudation and related boulder exhumation after initial deposition of the moraines during the penultimate glacial cycle or earlier the surface exposure ages based on various currently accepted production rate scaling protocols yield age differences of up to 35 reflecting the poor knowledge of terrestrial cosmogenic production rates at low latitude high altitude sites even within this conservative uncertainty range our results do not yield evidence for late pleistocene glaciations in monsoonal tibet to be asynchronous to those in mid latitudes on both hemispheres there is an urgent need to improve the knowledge of terrestrial cosmogenic nuclide production rates and their scaling to low latitude regions to fully exploit the climate information archived in tropical moraine sequences

extent and spatial distribution of pleistocene glaciations in eastern tibet as a contribution to the controversial discussion regarding the nature and extent of the pleistocene glaciation on the tibetan plateau additional observations and results were obtained during several chinese german joint expeditions in the investigation area of the eastern and central tibetan plateau it will be shown that the maximum extent of the last glaciation was limited to the glaciation of isolated mountain groups or smaller plateau glaciations these glaciations are characterized by glacial erosional features such as u shaped valleys in the inner mountain massifs and cirques in the higher summit areas accumulation forms such as terminal and lateral moraines less common than basal moraines are marked by granitic and basaltic erratics using two examples from the eastern margin of the tibetan plateau nianbaoyeze shan and northern chola shan the extent of pleistocene glaciations can be mapped by the distribution of terminal and lateral moraines and erratic granite boulders on the slopes of the surrounding schists two to three main pleistocene glaciations can be separated using weathering criteria paleosols and the overlying stratum on the moraines and associated terrace sequences overlying stratum consist of aeolian sandy silt or solifluction debris the younger moraines are found to be characterized by large erratics were therefore named big boulder moraine and can be found over the investigated area the ice free areas are characterized by pleistocene terraces along the main rivers loess deposits with sand and ice wedges at higher elevations e g the basin of zoige and paleosols in the basins below 3500 m a s l e g aba basin or garze basin the hypothesis of an extensive plateau glaciation could not be verified for this region a rising snowline equilibrium line altitudes elas is revealed in two cross sections during the last glacial maximum lgm at the eastern and northern margin of the tibetan plateau snowlines migrated from about 4000 m in the east minshan to 4300 m in the nianbaoyeze and to about 4700 m in the west bayan har shan a rise was also recorded from about 3800 m in the north central qilian shan up to a range of 4300 4950 m different exposures in the kunlun shan to approximately 5200 m in the tanggula shan in the center of the tibetan plateau the general increase of modern and lgm snowlines or elas towards the plateau corresponds with a decrease of precipitation from east to west and from north to south respectively precipitation was more pronounced at the eastern margin during glacial times than compared to present

retracted the past valley glacier network in the himalayas and the tibetan ice sheet during the last glacial period and its glacial isostatic eustatic and climatic consequences

alteration induced changes of magnetic fabric as exemplified by dykes of the koolau volcanic range abstract we studied 93 samples from 8 basaltic dykes of the koolau volcanic range on the island of oahu hawaii usa to determine the influence of hydrothermal alteration on the magnetic fabric as determined by anisotropy of magnetic susceptibility ams measurements rock magnetic as well as microscopic investigations show that only 25 of the samples have retained their original magnetomineralogical composition of unaltered ti poor titanomagnetite the remaining samples have undergone hydrothermal alteration which transformed the primary magnetic phase into a granular intergrowth of titanomagnetite titanomaghemite and hematite in both sample groups this magnetic phase occurs in coarse tens of microns irregularly shaped particles as well as interstitial clusters of smaller

when an inverse fabric is not inverse an integrated ams spo study in morb like dykes terra nova 23 49 55 2011 abstract magma flow direction is recorded in the macroscopic mineral petrofabric and magnetic fabric of the morb like rooi rand dyke swarm the bulk magnetic fabric is sub parallel to the dyke plane resulting from preferred orientation of grains during magma flow this is referred to as type a fabric a study of mineral shape preferred orientation reveals an additional fabric which is orthogonal to the average dyke orientation in 30 of the data this fabric is carried by both plagioclase and opaque grains and would typically be interpreted as inverse it is referred to here as type b fabric however because it is carried by the shape preferred orientation of macroscopic grains and is in most cases not coaxial to the anisotropy of magnetic susceptibility fabric it is interpreted as having been acquired during increased grain interaction during late stage magma flow associated with decreasing magma pressure

frozen bed fennoscandian and laurentide ice sheets during the last glacial maximum the areal extents of the laurentide and fennoscandian ice sheets during the last glacial maximum about 20 000 years ago are well known1 but thickness estimates range widely from high domed2 to thin3 with large implications for our reconstruction of the climate system regarding for example northern hemisphere atmospheric circulation and global sea levels this uncertainty stems from difficulties in determining the basal temperatures of the ice sheets and the shear strength of subglacial materials4 a knowledge of which would better constrain reconstructions of ice sheet thickness here we show that in the absence of direct data the occurrence of ribbed moraines in modern landscapes can be used to determine the former spatial distribution of frozen and thawed bed conditions we argue that ribbed moraines were formed by brittle fracture of subglacial sediments induced by the excessive stress at the boundary between frozen and thawed bed conditions resulting from the across boundary difference in basal ice velocity maps of glacial landforms from aerial photographs of canada and scandinavia reveal a concentration of ribbed moraines around the ice sheet retreat centres of quebec keewatin newfoundland and west central fennoscandia together with the evidence from relict landscapes that mark glacial areas with frozen bed conditions the distribution of ribbed moraines on both continents suggest that a large area of the laurentide and fennoscandian ice sheets was frozen based and therefore high domed and stable during the last glacial maximum

is this magnetic fabric normal a review and case studies in volcanic formations abstract when studying weakly anisotropic rocks non standard relationships between magnetic and expected petrofabric axes are sometimes observed in particular the so called inverse or intermediate fabrics a review of possible explanations for these effects is presented and leads to the conclusion that a majority of abnormal magnetic fabrics often encountered in magnetite rich volcanic rocks are due to abnormal preferred orientations or distributions of magnetite grains through case studies in dikes from skye island yemen and the iberian peninsula and magma and ash flows from the ethiopian traps and southern corsica the various possible origins of such fabrics are examined it is possible to retrieve reliable magma flow directions from these sites provided proper data selection is performed leading to non unique interpretations

an alternative emplacement model for the classic ardnamurchan cone sheet swarm nw scotland involving lateral magma supply via regional dykes abstract sub volcanic intrusive networks of which cone sheets are recognised as a major constituent partially control volcano growth and eruption style the accepted cone sheet model is that these confocally dipping intrusions originate from an unexposed central magma chamber through dip parallel magma flow however the emplacement mechanism of cone sheets remains poorly understood the 58 ma cone sheet swarm of ardnamurchan nw scotland offers an excellent opportunity to further resolve the emplacement dynamics of cone sheets through studying magma flow and their importance in volcanic edifice construction structural measurements and anisotropy of magnetic susceptibility ams analyses have constrained a lateral magma flow pattern consistently oriented nw se in the majority of the ardnamurchan cone sheets field observations also highlight the importance of host rock structure and interference between competing local and regional stress fields in controlling intrusion geometry our observations suggest cone sheet formation may be linked to laterally propagating nw se trending regional dykes sourced from laterally adjacent magmatic systems likely the palaeogene mull central complex which are deflected around the central complex by stress field interference implicitly edifice construction and potential eruption precursors observed at a volcano may instigate or result from magmatic activity in laterally adjacent volcanic systems

the magnetic zone axis a new element of magnetic fabric for the interpretation of the magnetic lineation abstract the magnetic zone axis is the common direction of the intersection of the magnetic foliations of different samples it is easily determined using tensors from the minimum susceptibility axes it provides useful information for the interpretation of the magnetic fabric particularly of the maximum axis a bootstrap method reveals small local disturbance of the magnetic foliation it yields also the confidence zone of the magnetic zone axis

towards a gis assessment of numerical ice sheet model performance using geomorphological data a major difficulty in assimilating geomorphological information with ice sheet models is the lack of a consistent methodology to systematically compare model output and field data as an initial step in establishing a quantitative comparison methodology automated proximity and conformity analysis apca and automated flow direction analysis afda have been developed to assess the level of correspondence between modelled ice extent and ice marginal features such as end moraines as well as between modelled basal flow directions and palaeo flow direction indicators such as glacial lineations to illustrate the potential of such an approach an ensemble suite of 40 numerical simulations of the fennoscandian ice sheet were compared to end moraines of the last glacial maximum and the younger dryas and to glacial lineations in northern sweden using apca and afda model experiments evaluated in this manner were ranked according to level of correspondence such an approach holds considerable promise for optimizing the parameter space and coherence of ice flow models by automated quantitative assessment of multiple ensemble experiments against a database of geological or glaciological evidence

landscape analysis of the huang he headwaters ne tibetan plateau patterns of glacial and fluvial erosion the large scale geomorphology of the huang he yellow river headwaters centered around the bayan har shan 5267 m asl in the northeastern part of the tibetan plateau is dominated by an uplifted

magnetic and shape fabrics of magnetite in simple shear flows abstract the magnetite fabrics measured by anisotropy of magnetic susceptibility ams and by shape preferred orientation spo optical methods are classically used as flow kinematics indicators in lava flows the development of magnetite fabrics during simple shear strains 20 was performed using a suspension of 1 volume fraction of multidomain magnetite randomly contained in a mixture of silicone and wax we measured ams fabric and spo ellipsoids by calculating a quadratic shape tensor from oriented thin sections for jeffery 1922 fabrics are usable to determine the flow kinematics including the amount of applied finite strain for 8 fabric elements foliation and lineation are stabilised closely parallel to the flow plane and the shear direction respectively two and three dimensional numerical simulations using measured aspect ratios of magnetite point out that the large scattering of aspect ratios and the initial orientation distribution of particles are together responsible for a wide ranging loss of periodicity the stable ams and spo fabrics observed at large strains in experiments are the result of these primary fabric properties combined to collisions between particles and possibly their complex three dimensional shapes in addition the constant angular relationship observed at large strains between fabrics and flow components is related to the transient collisions consequently the determination of the lava flow kinematics by using fabric properties measured either by ams or by spo analyses should be indubitably associated to a detailed study of the three dimensional shape of the solid carriers regularly shaped populations of low elongated particles will be capable to produce cyclic to oscillating fabrics while the fabric of elongated particles will be more sensitive to the shape parameters and collisions ultimately favouring stable fabrics at large strains

lithospheric structures and precambrian terrane boundaries in northeastern botswana revealed through magnetotelluric profiling as part of the southern african magnetotelluric experiment 1 within the framework of the southern african magnetotelluric experiment a focused study was undertaken to gain improved knowledge of the lithospheric geometries and structures of the westerly extension of the zimbabwe craton zim into botswana with the overarching aim of increasing our understanding of southern african tectonics the area of interest is located in northeastern botswana where kalahari sands cover most of the geological terranes and very little is known about lithospheric structures and thicknesses some of the regional scale terrane boundary locations defined based on potential field data are not sufficiently accurate for local scale studies investigation of the nnw sse orientated 600 km long zim line profile crossing the zimbabwe craton magondi mobile belt and ghanzi chobe belt showed that the zimbabwe craton is characterized by thick 220 km resistive lithosphere consistent with geochemical and geothermal estimates from kimberlite samples of the nearby orapa and letlhakane pipes 175 km west of the profile the lithospheric mantle of the ghanzi chobe belt is resistive but its lithosphere is only about 180 km thick at crustal depths a northward dipping boundary between the ghanzi chobe and the magondi belts is identified and two middle to lower crustal conductors are discovered in the magondi belt the crustal terrane boundary between the magondi and ghanzi chobe belts is found to be located further to the north and the southwestern boundary of the zimbabwe craton might be further to the west than previously inferred from the regional potential field data

palaeoglaciology of an ice sheet through a glacial cycle the european ice sheet through the weichselian abstract satellite images provide unique means of identifying large scale flow generated lineations produced by former ice sheets they can be interpreted to reconstruct the major elements which make up the integrated large scale structure of ice sheets ice divides ice streams interstream ridges ice shelves calving bays the evolving palaeoglaciological structure of the european ice sheet during its decay from the last glacial maximum lgm is reconstructed by reference to these components and in the context of a new map showing isochrons of retreat during the retreat phase in particular the time dependent dynamic evolution of the ice sheet and the pattern of ice stream development are reconstructed crossing lineations are widespread the older ones are suggested to have formed during molten bed phases of ice sheet growth and preserved by frozen bed conditions during the glacial maximum particularly in areas which lay during deglaciation beneath ice divides and inter ice stream ridges both areas of slow flow and possibly frozen bed conditions four phases of growth a1 to a4 and five phases of decay r1 to r5 are used to describe the major climatically and dynamically determined stages in the evolution of the ice sheet through the last glacial cycle the growth and decay patterns are quite different and associated with major shifts in the ice divide reflecting growth from the fennoscandian mountains and decay away from marine influenced margins these patterns were determined by the locations of nucleation areas spatial patterns of climate and calving at marine margins the prevalence of streaming within the retreating ice sheet suggests that the mean elevation of the ice sheet was lower than predicted from glaciological models which do not include streaming and that this might reconcile glaciological models and earth rheology models which infer paleao ice sheet thickness by inverting sea level data

terrestrial cosmogenic nuclide surface exposure dating of the oldest glacial successions in the himalayan orogen ladakh range northern india terrestrial cosmogenic nuclide surface exposure dating of moraine boulders and alluvial fan sediments define the timing of five glacial advances over at least the last five glacial cycles in the ladakh range of the transhimalaya the glacial stages that have been identified are the indus valley glacial stage dated at older than 430 ka the leh glacial stage occurring in the penultimate glacial cycle or older the kar glacial stage occurring during the early part of the last glacial cycle the bazgo glacial stage at its maximum during the middle of the last glacial cycle and the early holocene khalling glacial stage the exposure ages of the indus valley moraines are the oldest observed to date throughout the himalayan orogen we observe a pattern of progressively more restricted glaciation during the last five glacial cycles likely indicating a progressive reduction in the moisture supply necessary to sustain glaciation a possible explanation is that uplift of himalayan ranges to the south and or of the karakoram mountains to the west of the region may have effectively blocked moisture supply by the south asian summer monsoon and mid latitude westerlies respectively alternatively this pattern of glaciation may reflect a trend of progressively less extensive glaciation in mountain regions that has been observed globally throughout the pleistocene

magma flow revealed by magnetic fabric in the okavango giant dyke swarm karoo igneous province northern botswana to determine the magma flow direction of the giant 179 ma okavango dyke swarm of northern botswana we measured the anisotropy of magnetic susceptibility ams of 23 dykes dykes are located in two sections shashe and thune rivers which are about 300 km and 400 km from the presumed magma source respectively the nuanetsi triple point we collected samples from the margins of the dykes in order to use the imbrication of magnetic foliation to determine magma flow direction about half of the magnetic fabric in the dykes is inverse i e with the magnetic foliation perpendicular to the dyke plane lateral flow to the west and vertical flow is in evidence in the shashe section however the overall analysis of normal and inverse magnetic fabric data supports that lateral flow to the west was dominant in the shashe section across the thune section a poorly defined imbricated magnetic foliation also suggests lateral flow to the west

the eismint benchmarks for testing ice sheet models we present a series of benchmark experiments designed for testing and comparing numerical ice sheet models following the outcome of two eismintworkshops organized to intercompare large scale ice sheet models currently in operation model benchmark experiments are described for ice sheets underfixed and moving margin conditions these address both steady state and time dependent behavior under schematic boundary conditions and with prescribedphysics a comparison was made of each model s prediction of basic geophysical variables such as ice thickness velocity and temperature consensusachieved in the model intercomparison provides reference solutions against which the accuracy and consistency of ice sheet modeling codes can be assessed

magnetic fabric anisotropy of magnetic susceptibility constraints on emplacement mechanism of clastic dikes clastic dikes are generally classified into neptunian and injected dikes using the magnetic fabrics ams anisotropy of magnetic susceptibility we attempt to classify the clastic dikes in the late cretaceous dadaepo basin se korea and interpret their emplacement mechanisms the neptunian dikes exhibit a typical oblate sedimentary fabric which makes a sharp contrast with the injected dikes the fabrics of the injected dikes are greatly influenced by current conditions and transportation types of filling materials based on the ams fabrics they are classified into four types 1 vp vertical flow parallel type is formed by imbrication of long axis of grains in low to moderate energy vertical flow of a newtonian fluid and characterized by a bilateral symmetry of fabrics across the dike 2 vt vertical flow transverse fabric type results from grain rolling in vertical high energy flow and is characterized by subvertical k2 and subhorizontal k1 axes on the dike plane 3 hp horizontal flow parallel type is formed by imbrication of long axis of grains in horizontal low to moderate energy flow resulting in subvertical k3 and subhorizontal k1 and k2 axes 4 ht horizontal flow transverse type is formed by grain rolling in horizontal high energy flow resulting in streaked k2 k3 on the dike plane and horizontally clustered k1 axes the ams fabrics of each type can be a significant indicator for flow direction based on abundant ams fabrics formed by high energy current coexistence of paleoseismic structures and tectonic setting of the basin earthquake induced liquefaction is the most plausible trigger for the dike formation

flow directions in dikes from anisotropy of magnetic susceptibility data the bootstrap way one of the first applications of anisotropy of magnetic susceptibility ams was an attempt to determine flow directions from mafic dikes khan 1962 since the seminal work of knight and walker 1988 defining the expected behavior of ams in response to magma flow there has been increasing interest in using ams for this purpose here we present a quantitative method for interpretation of ams data from dikes using a parametric bootstrap first dikes must be sampled with at least five and preferrably more samples from within 10 cm of the dike margin the distributions of the eigenvalues and eigenvectors of the ams tensor are delineated by calculating eigenparameters of many bootstrapped paradata sets we generate paradata sets by first selecting a sample at random then calculating a replacement set of data by drawing tensor elements from normal distributions with the mean and standard deviation of the entire site the bounds containing 95 of the eigenparameters of the bootstrapped data serve as confidence limits for the parameter of interest classification of dikes proceeds as follows sites whose maximum and intermediate eigenvalues could not be distinguished are deemed uninterpretable in addition sites with principal eigenvectors with angles 45 away from the dike margin inverse or with markedly different directions on either side of the dike scissored are excluded the remaining dikes are classified as having unique flow direction information if the principal eigenvectors from at least one side are distinct from the dike plane based on the distribution of the bootstrapped principal eigenvectors if neither side has principal eigenvectors distinct from the dike plane the dikes are classified as having lineation information only a study comprising 251 dikes from the troodos ophiolite has 151 sites with directional data 38 sites with lineations only 7 inverse sites 5 scissored sites and 55 sites not fitting into any other category the flow directions interpreted from the data were generally southerly toward a fossil transform zone

thermo mechanically coupled ice sheet response cold polythermal temperate classical mixture concepts are the appropriate vehicle for describing the dynamics of ice masses containing some water we review and derive respectively the theoretical formulations of cold polythermal and temperate ice masses emphasize the peculiarities of the model equations and point to difficulties that were encountered with the proposed models the focus is both on the adequate physical motivation of the models and the consistency of their mathematical representation

late cenozoic uplift of mountain ranges and global climate change chicken or egg the high altitudes of most mountain ranges have commonly been ascribed to late cenozoic uplift without reference to when crustal thickening and other tectonic processes occurred deep incision and recent denudation of these mountain ranges abundant late cenozoic coarse sediment near them and palaeobotanical evidence for warmer climates where high mountain climates today are relatively cold have traditionally been interpreted as evidence for recent uplift an alternative cause of these phenomena is late cenozoic global climate change towards lower temperatures increased alpine glaciation a stormier climate and perturbations to humidity vegetative cover and precipitation

reconstruction of the 2 4 million km2 late pleistocene ice sheet on the tibetan plateau and its impact on the global climate during 20 expeditions new data were obtained on the maximum extent of glaciation in tibet and the surrounding mountains evidence was found of moraines at altitudes as low as 460 m on the southern flank of the himalayas and 2300 m on the northern slope of the tibetan plateau in the qilian shan on the northern slopes of the karakorum aghil and kuen lun moraines occurred as far down as 1900 m in southern tibet radiographic analyses of erratics document former ice thicknesses of at least 1200 m glacial polishing and knobs in the himalayas and karakorum are proof of glaciers as thick as 1200 2000 m on the basis of this evidence a 1100 1600 m lower equilibrium ela line was reconstructed for the ice age which would mean 2 2 4 million km2 of ice covering almost all of tibet since the equilibrium ela line was far below the average altitude of tibet on mount everest and k2 radiation was measured up to 6650 m yielding values of 1200 1300 w m2 because of the subtropical latitude and the high altitude solar radiation in tibet is 3 4 times greater than the energy intercepted between 60 and 70 n or s with an area of 2 2 4 million km2 and an albedo of 90 the tibetan ice sheet caused the same heat loss on the earth as a 6 9 6 million km2 sized ice sheet at 60 70 n because of its proximity to the present day equilibrium ela line tibet must have undergone large scale glaciation earlier than other areas being subject to intensive radiation the tibetan ice must have performed an amplifying function during the onset of the ice age

the lebombo monocline and associated feeder dyke swarm diagnostic of a successful and highly volcanic rifted margin abstract as part of the karoo large igneous province lip the 700 km long and 182 ma old lebombo monocline outcrops inland of a younger mozambique lowland cover that separates it from the indian ocean detailed mapping of a particularly well exposed olifant river traverse across this monocline correlates basaltic and rhyodacitic lava units within the lebombo group to at least three hosted generations of intrusions within the monocline parallel northern lebombo dyke swarm implying that a much denser feeder dyke swarm is covered by younger flows measured attitudes and thicknesses of up to 197 lavas and 791 dykes statistically quantify a structural cross section from which a properly weighted lava log of nearly 5 km basalts inter bedded with an overlying 3 5 km of rhyodacites is constructed the geological section is compatible with a slightly deeper sub lava section through the innermost 30 km s of a relatively narrow east greenland margin to which the lebombo monocline can be rigorously compared in both cases field relationships are consistent with early tectonic extension being overwhelmed by dyke dilation within margin parallel swarm segments that may have been injected laterally from central magma chambers thus it is argued that the lebombo monocline is part of a successfully rifted margin that furthermore belongs to a distinct class of highly volcanic and relatively narrow margins which only form proximal to classical triple rift lip centers where an elevated magma flux sustains a magmatic mode of extension for the karoo lip this conforms with 1 a tight gondwana reconstruction of africa with antarctica 2 successful breakup during intense 182 ma emplacement of the lebombo group 3 triple rift formation together with a short lived 181 178 ma okavango dyke swarm and 4 anomalously thick igneous crust formation during continued 2 5 cm yr separation into a loose 155 ma plate configuration of africa and antarctica

dike surface lineations as magma flow indicators within the sheeted dike complex of the troodos ophiolite cyprus mesoscopic flow lineations and anisotropy of magnetic susceptibility ams have been measured for dikes within the cretaceous age troodos ophiolite with the goal of comparing the direction of initial magma flow through dike conduits immediately following crack propagation with that of flow of subsequent magma emplaced during later stages of dike growth dike margin indicators of flow include cusp axes and elongate vesicles found high in the ophiolite pseudostratigraphy and ridge and groove structures termed hot slickenlines found throughout the complex a unique flow direction is determined where elongate vesicles near dike margins display imbrication with respect to the margin significant changes in vesicle elongation directions across dikes likely indicate either changes in magma flow direction after dike propagation or back flow of magma during the waning stages of intrusion surface lineations generally lie subparallel to the direction of flow inferred from ams determinations on cores within 5 cm of dike margins surface lineations also lie subparallel to the long axis e1 of the orientation ellipsoid defined by long axes of groundmass plagioclase phenocrysts measured in sections from ams cores correlation of surface lineations with interior indicators of flow ams plagioclase trachytic texture indicate that the surface features are good proxies for grain scale magma flow directions during dike propagation in troodos dikes orientations of surface flow features in the dikes of the troodos ophiolite indicate an approximately equal mix of subhorizontal to near vertical magma flow contradicting the paradigm of primarily vertical flow of magma beneath continuous axial magma chambers at oceanic spreading centers our data are consistent with a model of magma emplacement both vertically and horizontally away from isolated magma chambers beneath axial volcanoes spaced along a ridge crest

magma flow directions of shallow dykes from the east greenland volcanic margin inferred from magnetic fabric studies abstract the role played by plume generated crustal magmatic complexes in the segmentation of volcanic margins is highlighted by a preliminary study of magma flow directions in shallow intrusives from the east greenland volcanic margin we investigate the magmatic texture using anisotropy of magnetic susceptibility for eight dykes of tholeitic affinity belonging to a dyke swarm associated with the tertiary opening of the north atlantic ocean the thickness of the sampled dykes ranges from 3 to 37 m the dykes are of doleritic texture and contain up to 12 of opaque minerals and 35 of plagioclase laths dykes showing a magnetic foliation plane within the dyke plane i e a magnetic fabric usually attributed to magmatic processes represent 40 of the samples from the studied swarm these dykes have a low degree of anisotropy and their ellipsoid of magnetic susceptibility is strongly oblate inverse magnetic fabrics where the maximum principal susceptibility axis lies near the pole to the dyke account for 60 of the data we interpret these inverse fabrics either as alteration minerals with highly prolate ellipsoids of magnetic susceptibility or primary titanomagnetites with oblate ellipsoid of low anisotropy the flow direction is inferred for normal fabric dykes using the mirror imbrication of magnetic lineation analysis of thin sections shows a good agreement between magnetic fabric directions and phenocryst preferred orientations the inferred flow directions are predominantly horizontal throwing a new light on volcanic margin development

shallow intrusive directions of sheeted dikes in the troodos ophiolite anisotropy of magnetic susceptibility and structural data sheeted dikes play a central role in the formation of oceanic crust it is commonly assumed that sheeted dikes intrude vertically upward from elongated mid ocean ridge mor magma chambers but there are no direct observational data bearing on this hypothesis this assumption contrasts with the intrusive behavior of subaerial volcanoes where magmas rise into shallow central magma chambers that laterally feed vertically oriented dikes the authors have studied intrusive directions of sheeted dikes in a structural analogue to oceanic crust the troodos ophiolite structural and magnetic fabric data of 65 dikes provide consistent results and suggest a broad distribution of shallow 20 degree to nearly vertical upward magma transport directions these data suggest that horizontal emplacement has to be considered for sheeted dikes at mors implying more centralized mor plumbing systems than previously thought such plumbing systems provide ample opportunity for complex mixing fractionation and contamination of mor lavas in magma chambers and tabular magma storage volumes whether the mor magma supply is linear or centralized also has a fundamental effect on crustal accretion processes and the geometry of hydrothermal convection systems

quaternary glaciation of mount everest abstract the quaternary glacial history of the rongbuk valley on the northern slopes of mount everest is examined using field mapping geomorphic and sedimentological methods and optically stimulated luminescence osl and 10 be terrestrial cosmogenic nuclide tcn dating six major sets of moraines are present representing significant glacier advances or still stands these date to 330 ka tingri moraine 41 ka dzakar moraine 24 27 ka jilong moraine 14 17 ka rongbuk moraine 8 2 ka samdupo moraines and 1 6 ka xarlungnama moraine and each is assigned to a distinct glacial stage named after the moraine the samdupo glacial stage is subdivided into samdupo i 6 8 7 7 ka and samdupo ii 2 4 ka comparison with osl and tcn defined ages on moraines on the southern slopes of mount everest in the khumbu himal show that glaciations across the everest massif were broadly synchronous however unlike the khumbu himal no early holocene glacier advance is recognized in the rongbuk valley this suggests that the khumbu himal may have received increased monsoon precipitation in the early holocene to help increase positive glacier mass balances while the rongbuk valley was too sheltered to receive monsoon moisture during this time and glaciers could not advance comparison of equilibrium line altitude depressions for glacial stages across mount everest reveals asymmetric patterns of glacier retreat that likely reflects greater glacier sensitivity to climate change on the northern slopes possibly due to precipitation starvation

magnetic fabric and inferred flow direction of dikes conesheets and sill swarms isle of skye scotland abstract we have measured the ams of 16 dikes 35 conesheets and three sills associated with the cuillin hills magmatic center of the isle of skye in the inner hebrides of scotland and of nine dikes located in the regional dike swarm of skye sixty three intrusives totalling 734 samples were studied to determine the plumbing of the skye volcanic system low field susceptibility versus temperature k t identified three different mineral phases in the area namely ti rich magnetite pyrrhotite and titanomaghemite the petrofabrics of the 63 intrusives yielded coherent flow azimuths regardless of their time of emplacement three main types of magnetic fabric a c were found fabric type a plane kmax kint parallel to the dike plane represents the magma flow direction within the intrusives and is the dominant fabric 55 of all the intrusives within the cuillin hills magmatic center and its regional dike swarm the kmax inclinations show that 55 of the intrusives were fed by horizontal to sub horizontal ams inclination of the kmax axis is equal or less than 30 magma fluxes and the rest of them were fed by inclined to vertical fluxes horizontal magma flow means lateral magma injection inside fractures and becomes more probable as the source is located further away

evidence from magnetic fabric for the flow pattern of magma in the mackenzie giant radiating dyke swarm continental flood basalts and their associated swarms of mafic dykes are thought to result from melting in mantle plumes1 3 but the processes by which the resulting magma makes its way from mantle to crust remain obscure here we use magnetic anisotropy measurements to reconstruct the pattern of magma flow in the proterozoic mackenzie dyke swarm our results indicate that magma was injected vertically within 500 km of the focal point of the dyke swarm and then travelled horizontally at all further distances out to at least 2 100 km the sharp transition from vertical to horizontal flow at between 500 and 600 km may correspond to the outer boundary of melt generation in the mantle plume believed to be responsible for the mackenzie igneous events

paleoglaciological reconstructions for the tibetan plateau during the last glacial cycle evaluating numerical ice sheet simulations driven by gcm ensembles abstract the tibetan plateau is a topographic feature of extraordinary dimension and has an important impact on regional and global climate however the glacial history of the tibetan plateau is more poorly constrained than that of most other formerly glaciated regions such as in north america and eurasia on the basis of some field evidence it has been hypothesized that the tibetan plateau was covered by an ice sheet during the last glacial maximum lgm abundant field and chronological evidence for a predominance of local valley glaciation during the past 300 000 calendar years that is 300 ka coupled to an absence of glacial landforms and sediments in extensive areas of the plateau now refute this concept this furthermore calls into question previous ice sheet modeling attempts which generally arrive at ice volumes considerably larger than allowed for by field evidence surprisingly the robustness of such numerical ice sheet model results has not been widely queried despite potentially important climate ramifications we simulated the growth and decay of ice on the tibetan plateau during the last 125 ka in response to a large ensemble of climate forcings 90 members derived from global circulation models gcms using a similar 3d thermomechanical ice sheet model as employed in previous studies the numerical results include as extreme end members as an ice free tibetan plateau and a plateau scale ice sheet comparable in volume to the contemporary greenland ice sheet we further demonstrate that numerical simulations that acceptably conform to published reconstructions of quaternary ice extent on the tibetan plateau cannot be achieved with the employed stand alone ice sheet model when merely forced by paleoclimates derived from currently available gcms progress is however expected if future investigations employ ice sheet models with higher resolution bidirectional ice sheet atmosphere feedbacks improved treatment of the surface mass balance and regional climate data and climate reconstructions

thick dyke emplacement and internal flow a structural and magnetic fabric study of the deep seated dolerite dyke of foum zguid southern morocco 1 knowledge on forced magma injection and magma flow in dykes is crucial for the understanding of how magmas migrate through the crust to the earth s surface because many questions still persist we used the long thick and deep seated foum zguid dyke morocco to investigate dyke emplacement and internal flow by means of magnetic methods structural analysis petrography and scanning electron microscopy we also investigated how the host rocks accommodated the intrusion regarding internal flow 1 important variations of the rock magnetic properties and magnetic fabric occur with distance from dyke wall 2 anisotropy of anhysteretic remanent magnetization reveals that anisotropy of magnetic susceptibility ams results mainly from the superposition of subfabrics with distinct coercivities and that the imbrication between magnetic foliation and dyke plane is more reliable to deduce flow than the orientation of the ams maximum principal axis and 3 a dominant upward flow near the margins can be inferred the magnetic fabric closest to the dyke wall likely records magma flow best due to fast cooling whereas in the core the magnetic properties have been affected by high temperature exsolution and metasomatic effects due to slow cooling regarding dyke emplacement this study shows that the thick forceful intrusion induced deformation by homogeneous flattening and or folding of the host sedimentary strata dewatering related to heat as recorded by thick quartz veins bordering the dyke in some localities may have also helped accommodating dyke intrusion the spatial arrangement of quartz veins and their geometrical relationship with the dyke indicate a preintrusive to synintrusive sinistral component of strike slip

quaternary glaciation of tibet the geological evidence the question of the number and extent of pleistocene glaciations on the qinghai xizang tibet plateau remains contentious there has been considerable research activity in tibet by chinese scientists since the late 1950s and their principal findings including data on glacial and periglacial geomorphology sedimentology tectonics stratigraphy and lake history have been used to reconstruct ice margins and palaeosnowlines three and in places four distinct glaciations have been recognised the most extensive occurring in the later middle pleistocene valley and piedmont glacial systems with some mountain ice caps and a small ice sheet on the upper reaches of the huang he have been recognised but there is no evidence of a single tibetan ice sheet as the plateau and the himalaya karakoram underwent accelerating uplift through the quaternary there occurred progressive desiccation in the interior as the influx of indian ocean moisture was constrained equilibrium line elevations in the last glacial maximum were 4000 m in the south east and northeast plateau margins rising to 5500 m in the northwest interior the heat island provided by the plateau the progressive reduction in precipitation most marked during the glacials and the strong southeast to northwest precipitation gradients produced a glacier distribution pattern dominated by the trans tibetan mountain ranges tectonics and periglacial including aeolian processes have played an equal role with glaciation in generating the plateau s quaternary sedimentary succession

artefacts of isotropic inversion applied to magnetotelluric data from an anisotropic earth summary 2 d isotropic approaches are standard for magnetotelluric mt data modelling and inversion unfortunately the real subsurface structure is not isotropic everywhere and one should be aware of the possible consequences of applying an isotropic inversion to data from an anisotropic earth the work presented herein was motivated by discovering an unusual mid to lower crust conductor that appeared to be bent downward into the lithospheric mantle compared to the neighbouring terranes when applying routine 2 d isotropic inversion one major difference between the terranes is the presence of the okavango giant mafic dyke swarm northeastern botswana beneath which the conductor is imaged to be in the lithospheric mantle rather than the lower crust the limited width of the dykes makes them an anisotropic feature rather than a normal 2 d structure at mt scale to examine the possible effects of the dykes synthetic data were generated from an 1 d model accounting for the dyke swarm by using an anisotropic block and then inverted isotropically the synthetic tests showed that the normal 2 d decomposition and strike analysis techniques are not removing these large scale anisotropic effects and that an isotropic inversion result obtained in the presence of an anisotropic structure has to be treated with caution the comparison of the synthetic data with the presented case history strongly suggests that the lithospheric depths of the conductor is an artefact and it is most likely located in the lower crust as everywhere else in the area focused anisotropic inversion of the dyke swarm area supports the assumption of a crustal anisotropic structure related to the okavango dyke swarm that affects the isotropic inversion results

holocene late pleistocene climatic ice core records from qinghai tibetan plateau three ice cores to bedrock from the dunde ice cap on the north central qinghai tibetan plateau of china provide a detailed record of holocene and wisconsin wurm late glacial stage lgs climate changes in the subtropics the records reveal that lgs conditions were apparently colder wetter and dustier than holocene conditions the lgs part of the cores is characterized by more negative 18 o ratios increased dust content decreased soluble aerosol concentrations and reduced ice crystal sizes than the holocene part these changes occurred rapidly 10 000 years ago in addition the last 60 years were apparently one of the warmest periods in the entire record equalling levels of the holocene maximum between 6000 and 8000 years ago

emplacement of magma in eastern iceland dikes insights from magnetic fabric and rock magnetic analyses abstract the anisotropy of magnetic susceptibility ams coupled with mineral magnetic identification has been investigated in about 640 samples from 27 basaltic dikes of tertiary age in eastern iceland the dikes are a few meters thick and they have been sampled all across three types of magnetic fabrics with well defined mean principal axes of anisotropy are recognized in the first one the maximum axis is vertical and the minimum axis coincides with the pole of the dike plan geometrically normal fabric the second fabric shows opposite orientation geometrically inverse fabric the third type is when these two fabrics coexist with the normal one at the margins and the inverse one in the center of the dike mixed fabric no relationship is observed between the type of fabric and the width of the dikes each fabric is carried by clear magnetic assemblages made of low x 0 15 ti content fine grained magnetites for the normal one and high ti content around tm60 coarse magnetite grains for the inverse fabric upon heating the magnetic properties and the orientation of the anisotropy ellipsoid characterizing the inverse fabric in the mixed dikes change and evolve toward the normal type these two geometrically normal and inverse fabrics are interpreted as primary ones representing different physico chemical conditions during the emplacement of the magma the ams results indicate that during the volcanic events occurring about 10 myr ago the extrusion of the magma was vertical at the depth of the studied sites in eastern iceland

pre rift tectonic scenario of the eo cretaceous gondwana break up along se brazil sw africa insights from tholeiitic mafic dyke swarms

fabric analysis using the intercept method abstract the intercept method is a convenient yet powerful method for the numerical analysis of fabrics for geological applications it can be used when a set of objects e g pores grains or aggregates of a specific mineral lineaments etc with unique characteristics can be identified in an image the intercept method analyses boundaries of objects as a population of lines how intercepts are definined is therefore important in order to establish the significance of the intercept counts when the method is applied to digital images the subdivision of the information into square pixels must also be considered we apply a linear filter which minimizes the effects of grid anisotropy on the counting of intercepts analysis of the resulting rose of intercept counts by a fourier series considerably facilitates the interpretation of the data a rose of directions can readily be derived for the phase boundaries if a population of objects can be assumed to have started with an originally isotropic orientation and to have been deformed passively the fourier components of the rose of intercepts also give the sectional strain the fourier components also permit an objective test of the validity of describing a fabric as resulting from a homogeneous strain on an initially isotropic section two computer programs for the counting and analysis of intercepts on digital images are presented and applied in four examples which illustrate the usefulness of the intercept method for magmatic fabric measurement strain analysis and lineament analysis we also illustrate the importance of fourier series decomposition of rose of intercept counts for fabric interpretations intercept results are also shown to be compatible with those derived by other methods eigenvector surfor inertia tensor autocorrelation methods

deglaciation of the northern hemisphere at the onset of the eemian and holocene high resolution tims u series dating of sea level events obtained from coral reef complexes suggests that global deglaciation from the saale penultimate glacial to the eem interglacial marine 18o stages 6 5 may have occurred earlier in relation to milankovitch insolation forcing than that from the wisconsinan glacial to the holocene interglacial marine 18o stages 2 1 however the interpretation of these data has been problematic because of the possibility of isotope exchange in order to investigate whether these different lead lag relations between milankovitch forcing and ice volume are feasible from the point of view of large scale ice sheet dynamics and thermodynamics the three dimensional polythermal ice sheet model sicopolis simulation code for polythermal ice sheets is applied to the entire northern hemisphere which gives the major contribution to global ice volume changes due to the relative stability of the antarctic ice sheet and simulations through the last two climatic cycles are conducted the simulations cover the interval from 250 kyr bp until today and are driven by surface temperature reconstructions of deep ice cores grip vostok and simple parameterizations for the change of precipitation with time discussion of the results is focused on the saale eem and the wisconsinan holocene transitions the amount and rate of deglaciation are in good agreement with the specmap record for both cases and the evidence of the data for an early start of the eem interglacial is supported

local and regional controls on the lateral emplacement of the ben hiant dolerite intrusion ardnamurchan nw scotland abstract sub volcanic and related satellite intrusions record the delivery storage and accommodation of magma before eruption however when several volcanic centres are in close proximity the relationship between centre and satellites may be ambiguous here we examine the structure of the ben hiant dolerite satellite intrusion discuss its relationship with the ardnamurchan central complex located 2 km to the northwest and explore the possibility of a genetic connection with the next nearest centre the mull central complex 35 km to the southeast structural field observations and anisotropy of magnetic susceptibility ams fabric analyses reveal that the ben hiant dolerite was emplaced as a series of lobes in a sequence of stacked sheets the ams fabric data further indicate that the ben hiant dolerite intruded laterally and may have been sourced from either the ardnamurchan central complex or the mull central complex testing either model and discriminating between these two possible source reservoirs remains an outstanding challenge however the potential implications of the lateral movement of magma over many 10 s of kilometres between upper crustal magmatic centres suggests hidden complexities associated with volcanic plumbing systems that must be tested structurally as well as geochemically

global ice volume variations through the last glacial cycle simulated by a 3 d ice dynamical model abstract a coupled ice sheet ice shelf bedrock model was run at 20 km resolution to simulate the evolution of global ice cover during the last glacial cycle the mass balance model uses monthly mean temperature and precipitation as input and incorporates the albedo mass balance feedback the model is forced by the grip and vostok derived temperature records and by sea level and solar radiation variations which are distributed spatially and seasonally model simulations show that at the last glacial maximum lgm a volume of ice representing about 65 m of sea level equivalent is stored on the eurasian continent whereas north america stores 42 m the other regions greenland tibet south america and antarctica contribute about 17 m to global sea level lowering at the lgm compared to present day sensitivity experiments show that the ice volumes of the eurasian and north american ice sheets are particularly sensitive to summer temperature changes finally the timing and mechanism of grounded ice formation and expansion for the last glacial were studied for the eurasian and north american continents ice sheet formation begins at 118 kyr bp in the barents and kara sea region over baffin island and in the high cordilleran range following a 4 c cooling alternating regions of ice sheet expansion by direct snowfall ice flow and grounding of ice shelves can be identified these relate to specific geomorphological traces and may as such help in model validation

on the problem of quaternary glaciations and the extent and patterns of pleistocene ice cover in the qinghai xizang tibet plateau abstract data accumulated during investigations since the 1960s indicate that up to four pleistocene glaciations can be recognized in the qinghai xizang tibet plateau and the adjacent himalaya mountains in order of decreasing age these are the xixabangma nyanyaxungla guxiang and baiyu glacial episodes the history of pleistocene glacial evolution in various areas throughout the plateau differs and the pattern is very complex throughout the early pleistocene the elevation of the qinghai xizang plateau was approximately 2000 m above sea level during the xixabangma glacial episode recorded at higher elevations the climate and topographical conditions on the plateau were largely unfavourable for the development of glaciers present investigations have revealed that the oldest till is only found at isolated localities on the plateau glaciofluvial or alluvial deposits such as the gongba conglomerate dating from 2 4 to 0 73 ma bp and the fluvial lacustrine deposits of the qiantang group dating from 2 4 to 1 7 ma bp are present rather than glacial till therefore the postulate of a large early pleistocene ice sheet proposed previously has been disproves during the most recent glaciation uplift of the plateau and the himalayas continued the majority of the qinghai xizang plateau reached 4000 m a s l and the great himalaya were approximately 6000 m a s l the mountains formed a barrier against the incursion of the indian monsoon so that precipitation decreased sharply on the northern flank of the himalayas and the climate to the north became cold and dry all of the glaciers on the qinghai xizang plateau underwent extensive retreat thus during the pleistocene no unified glacial cover existed over the qinghai xizang plateau similarly the source area of the hwang he yellow river did not support a local ice sheet

magnetic and plagioclase linear fabric discrepancy in dykes a new way to define the flow vector using magnetic foliation in basaltic dykes the magnetic lineation k1 maximum magnetic susceptibility axis is generally taken to indicate the flow direction during solidification of the magma this assumption was tested in tertiary basaltic dykes from greenland displaying independent evidence of subhorizontal flow the digital processing of microphotographs from thin sections cut in k1 k2 planes yields the preferred linear orientation of plagioclase which apparently marks the magma flow lineation in up to 60 of cases the angular separation between k1 and the assumed flow direction is greater than 45 this suggests that the uncorroborated use of magnetic lineations in dykes is risky a simple geometrical method is proposed to infer the flow vector from ams in dykes based solely on magnetic foliations

rock texture and magnetic lineation in dykes a simple analytical model abstract we develop two simple models for simulating the combination of magnetic sub fabrics related to magma flow in dykes the basic assumptions are i the petrofabric is representative of the flow fabric and ii the petrofabric is composed of s c type structures related to flow the first model consists of summing the magnetic tensors of two identical sub fabrics differing solely by their relative rotation this model accounts for the possible change of the macroscopic magnetic lineation from a flow related fabric to a lineation situated at the geometric intersection between the two sub fabrics such a result is obtained in the case of oblate to highly oblate sub fabric ellipsoids the second model integrates the effect of very oblate grains of variable orientations into calculating the shape controlled magnetic tensor of each sub fabric and emphasizes the possible under estimation of fabric superposition due to microscopic disordering the magma fluxes along the east greenland volcanic margin are illustrated by the flow pattern within the major dyke swarm the magmatic flow vectors inferred from the imbrication of magnetic foliation at the dyke margins are primarily horizontal the classic use of magnetic lineation can lead to contradictory results giving flow vectors perpendicular to the flow directions the magnetic lineation is situated close to the zone axis of magnetic foliation planes over a wide range of scales throughout the dyke swarm suggesting that the contradiction may arise from the association of several textural domains at the sample scale forward modelling of macroscopic magnetic fabrics using the first model yields good agreement with the measured magnetic fabric of the east greenland dykes our results which are applicable to strained sedimentary rocks highlight the possible misuse of the magnetic lineation due to combination of magnetic textures the exchange between a microscopic lineation i e mineralogical lineation and a macroscopic lineation i e intersection lineation is particularly expected for dykes that generally bear oblate magnetic textures

distribution anisotropy the cause of ams in igneous rocks understanding of ams in pristine basic igneous rocks in terms of flow orientation of ellipsoidal magnetite grains is thwarted by the fact that these grains 1 typically are irregularly equant or skeletal rather than distinctly ellipsoidal and 2 usually crystallize interstitially and late after magma flow has ceased however the distribution of such grains will be relatively anisotropic if they grew in residual liquid volumes within a preferredly oriented by flow silicate template this constraint may impose a slight preferred shape orientation to the irregular magnetite grains during their growth but in addition we suggest it is the anisotropic magnetic interaction between the grains which is the basic cause of ams experiments involving the casting of magnetite epoxy mixtures in various foliated or lineated glass templates have yielded samples with the expected ams symmetry i e k3 perpendicular to glass plates or k1 parallel to glass rods slicing of near isotropic synthetic blank and natural rock cores and interleaving with glass slides showed a progressive increase in anisotropy of the blank from 0 4 to 2 and the natural rock from 0 9 to 2 9 in both the k3 axis migrates to be perpendicular to the glass slides

magma flow directions inferred from field evidence and magnetic fabric studies of the streitishvarf composite dike in east iceland abstract anisotropy of magnetic susceptibility ams and rock magnetic studies have been made on three outcrops separated by 12 km along strike nne ssw on the streitishvarf composite dike in east iceland samples for this study have been collected from the inner quartz porphyry part of the dike which show clear field evidence of a lateral flow component from north to south at one of the sites this flow component is consistent with margin ams results from all three sites the quartz porphyry has a substantial bulk magnetic susceptibility 10 2 si mainly carried by magnetically soft titanium poor titanomagnetite mdf 15 mt the ferrimagnetic grains yield a characteristic remanent magnetization in all three sites which gives a virtual geomagnetic pole at latitude 52 6 s and longitude 319 6 e the degree of anisotropy is low p j 1 033 and the magnetic fabrics shifts from oblate to prolate shapes depending on dike margin and outcrop the magnetic fabric has been interpreted according to the imbrication model using the minor susceptibility axis as shear plane indicator the absolute directions given by the minor susceptibility are then quantified using vector algebra the magma flow is indicated as an upward directed flow flowing from north to south with an inclination between 30 and 64 with a 95 confidence ellipse of 3 9 a model for the intrusion of the streitishvarf dike has been constructed where a magma pocket with felsic magma is punctured by a mafic dike enabling the felsic magma to rise and extend to the south within the pathway created the results of this study confirm the applicability of ams in studies of magma flow directions in igneous dikes of felsic composition

formation of an archaean continent about 30 of the earth is covered by continents but only about 10 small kernels of these continentsae known as archaean cratonsae are continental fragments formed before 2 5 gyr ago the kaapvaal craton of south africa which formed and stabilized between 3 7 and 2 7 gyr ago is one of the oldest reasonably sized examples of these continental fragments it consists of a mosaic of subdomains that have been welded together by processes similar to those of modern day plate tectonics the earliest subdomains may have owed their origin to the onset of efficient recycling from the earth s hydrosphere into the mantle

characterisation of grain size shape and orientation of plagioclase in the rooi rand dyke swarm south africa abstract magmatic type a fabric co exists with dyke orthogonal type b fabric in both the plagioclase and opaque grain fractions in dykes of the rooi rand dyke swarm rrds we present new data from the rrds pertaining to the size shape texture and orientation of plagioclase texturally the samples range from intersertal to sub ophitic and phenocrystic plagioclase phyric more than 90 of plagioclase grains are

using magma flow indicators to infer flow dynamics in sills abstract fabrics from anisotropy of magnetic susceptibility ams analyses and shape preferred orientation spo of plagioclase are compared with field structures such as bridge structures intrusive steps and magma lobes formed during magma intrusion in jurassic sills this is to constrain magma flow directions in the sills of the karoo igneous province along the kwazulu natal north coast and to show how accurately certain structures predict a magma flow sense thus improving the understanding of the karoo sub volcanic dynamics the ams fabrics are derived from magnetite grains and are well constrained however the spo results are commonly steeply inclined poorly constrained and differ to the ams fabrics both techniques resulted in asymmetrical fabrics successful relationships were established between the ams fabric and the long axes of the magma flow indicators implying adequate magma flow prediction however where numerous sill segments merge either in the form of magma lobes or bridge structures the coalescence process creates a new fabric between the segments preserving late stage magma migration between the merged segments overprinting the initial magma flow direction

magnetic fabric and its significance in the 1400 ma mealy diabase dykes of labrador canada an anistropy of magnetic susceptibility ams study of the relatively fresh mealy diabase dyke swarm indicates a correlation between the ams and the deformational effects in the rocks several ams components were revealed during experimental work on 211 samples that had been heated to 640 c during a paleomagnetic study and 22 fresh samples progressively treated in alternating fields seventeen sites with slight or no deformation have a magnetic fabric composed of two major components the stable component attributed to a primary shape anisotropy has minimum and maximum principal susceptibility directions preferentially oriented vertically and along dyke strike n 60 e respectively this configuration is consistent with an origin due to a post emplacement stress field whereby horizontal extension acted on a vertical sheet of partially crystallized magma the orientation of this component with respect to the present horizontal suggests that the dykes have not been tilted since their emplacement the unstable component removed by fields up to 250 mt is attributed to induced domain anisotropy produced by mild tectonic stress during the grenville orogeny the approximate orientation of its principal components as found by the method of substitutional ellipsoids suggests an east west directed maximum stress four other sites with noticeable deformation have magnetic fabric that could not be reliably analyzed but which may be largely composed of domain and stress induced anisotropies developed during the grenville orogeny

effect of orogeny plate motion and land sea distribution on eurasian climate change over the past 30 million years the eurasian climates of today 10 million and 3o million years ago are simulated using an atmospheric general circulation model that incorporates realistic continental geography and epicontinental sea distributions the resulting climates compare well with various palaeoclimate records the retreat of the paratethys an epicontinental sea shifts the central asian climate from temperate to continental conditions and plays as important a role as uplift of the himalayan tibetan plateau in driving the asian monsoon changes

temperature accumulation and ice sheet elevation in central greenland through the last deglacial transition we present a combined heat and ice flow model constrained by measurements of temperature in the greenland ice sheet project 2 gisp2 borehole and by the gisp2 18o record and depth age scale which determines a history of temperature accumulation rate and ice sheet elevation for the past 50 000 years in central greenland important results are that the temperature increase from average glacial to holocene conditions was large approximately 15 c with a 20 c warming from late glacial to holocene that the average accumulation rate during the last glacial maximum between 15 and 30 kyr b p was 5 5 to 7 cm yr 1 approximately 25 of the modern accumulation rate that long term 500 1000 years averaged accumulation rate and temperature have been inversely correlated during the most recent 7 millennia of the holocene and that the greenland ice sheet probably thickened during the deglacial transition the inverse correlation of accumulation rate and temperature in the mid and late holocene suggests that the greenland ice sheet is more prone to volume reduction in a warmed climate than previously thought and demonstrates that accumulation rate is not a reliable proxy for temperature the elevation history of the ice sheet is poorly constrained by the model and independent evidence is needed we also present a simple estimate of the response time for thinning of the interior region of an ice sheet due to retreat of its margins this was approximately 1900 years for central greenland during deglaciation

the limited influence of glaciations in tibet on global climate over the past 170 000 yr extensive ice cover on the tibetan plateau would significantly influence earth s climate in general and the asian monsoon system in particular but extent and timing of quaternary glaciations in tibet remain highly controversial we dated erratics on top of moraines in the climatic key areas of central and east tibet using cosmogenic 10be 26al and 21ne consistent exposure ages obtained by various nuclides indicate a continuous period of exposure since the deposition of the samples our data imply that glacial advances were restricted to a few 10 km during the last 170 kyr in central tibet and during the peak of the last glaciation 24 13 kyr ago in eastern tibet advances of tibetan glaciers were much less prominent than elsewhere in the northern hemisphere most likely due to very arid conditions and high sublimation rates a proposed ice dome covering the entire plateau can be excluded thus albedo increase of tibet most likely did trigger neither northern hemisphere ice ages nor paleomonsoon changes during the last two glacial cycles the glacial advance during marine isotope stage 2 in east tibet and the absence of significant glacial events during the holocene suggest a relation of snowline lowering in east tibet to north atlantic cooling events rather than to periods of high precipitation by an intensified monsoon

interconnected sills and inclined sheet intrusions control shallow magma transport in the ferrar large igneous province antarctica field observations and structural data from intrusive complexes at allan hills and terra cotta mountain south victoria land antarctica demonstrate that interconnected sills and inclined sheets transported magma through the shallow subsurface these sills and sheets represent the upper crustal top 4 km plumbing system of the 183 ma ferrar large igneous province the sheets are short in length

observed degree day factors and their spatial variation on glaciers in western china the degree day factor ddf is an important parameter for the degree day model which is a widely used method for ice and snowmelt computation spatial variations of the ddf greatly affect the accuracy of snow and ice melt modelling this study analyzes the spatial variability of ddfs obtained from observed glaciers in different regions of western china the results clearly show that the ddf for a single glacier is subject to significant small scale variations and the factor for maritime glaciers is higher than that for subcontinental and extremely continental glaciers in western china the factors increase gradually from northwest to southeast in general the regional patterns of ddfs are detectable on the glaciers due to the unique climatic environment and heat budget of the tibetan plateau and surrounding regions low ddfs can be expected for cold dry areas whereas high ddfs can be expected for warm wet areas in western china depending on spatial variation of the characteristics of ddfs and the meteorological data we can provide gridded degree day models for non monitored glaciers to reconstruct gridded historical glacier mass balance series in western china

magma flow directions in dikes of the koolau complex oahu determined from magnetic fabric studies we compared the magnetic fabric with the macroscopic surface lineation direction previously assumed to be parallel with magma flow direction for 35 dikes of the koolau complex oahu anisotropy of magnetic susceptibility ams was determined for 530 samples from 59 dikes including a sill and a pluglike intrusive body and two filled lava tubes and statistically significant ams clusters were found in all but five dikes for the 35 lineated dikes the mean maximum ams direction 1 coincides with the macroscopic lineations to within 25 in 18 and to within 15 in eight the ams ellipsoid shape in about half of the samples is prolate cigar shaped and in about half is oblate pancake shaped we interpret the ams to be determined by shape anisotropy of ferromagnetic grains the mean total anisotropy is 2 2 and total anisotropy h ranges from 0 24 to as high as 14 12 the principal ams axes for samples from a dike are tightly grouped on equal area plots and for most of the dikes the minimum ams axes plot near to the pole of the dike our ams measurements enable us to infer absolute magma flow directions for half the dikes studied the maximum ams axes of samples from one side of the dike form a tight cluster on an equal area plot and those from the other side of the dike form another tight cluster the two clusters are close together separated by 10 30 for narrow dikes 100 cm wide and are farther apart 30 60 for wide dikes and are symmetrically disposed on either side of the plane of the dike we infer that these paired clusters are caused by imbrication of nonequant ferromagnetic crystals against each margin of the dike during deposition in a velocity gradient and hence indicate the flow azimuth i e the absolute magma flow direction nearly half of the dikes measured 25 gave paired ams plots in 12 the magma flowed up at low oblique angles 5 30 from the horizontal in five it flowed down at 5 20 in four it flowed horizontally 5 in three it flowed up at a steep angle 50 75 and in one it flowed down at a steep angle the magma flow in general was rarely vertical or horizontal

simulations of the northern hemisphere through the last glacial interglacial cycle with a vertically integrated and a three dimensional thermomechanical ice sheet model coupled to a climate model

magma flow pattern in dykes of the azores revealed by anisotropy of magnetic susceptibility the localization of magma melting areas at the lithosphere bottom in extensional volcanic domains is poorly understood large polygenetic volcanoes of long duration and their associated magma chambers suggest that melting at depth may be focused at specific points within the mantle to validate the hypothesis that the magma feeding a mafic crust comes from permanent localized crustal reservoirs it is necessary to map the fossilized magma flow within the crustal planar intrusions using the ams we obtain magmatic flow vectors from 34 alkaline basaltic dykes from sao jorge sao miguel and santa maria islands in the azores archipelago a hot spot related triple junction the dykes contain titanomagnetite showing a wide spectrum of solid solution ranging from ti rich to ti poor compositions with vestiges of maghemitization most of the dykes exhibit a normal magnetic fabric the orientation of the magnetic lineation k1 axis is more variable than that of the k3 axis which is generally well grouped the dykes of sao jorge and sao miguel show a predominance of subhorizontal magmatic flows in santa maria the deduced flow pattern is less systematic changing from subhorizontal in the southern part of the island to oblique in north these results suggest that the ascent of magma beneath the islands of azores is predominantly over localized melting sources and then collected within shallow magma chambers according to this concept dykes in the upper levels of the crust propagate laterally away from these magma chambers thus feeding the lava flows observed at the surface

application of a polythermal three dimensional ice sheet model to the greenland ice sheet response to steady state and transient climate scenarios abstract steady state and transient climate change computations are performed with the author s three dimensional polythermal ice sheet model simulation code for polythermal ice sheets for the greenland ice sheet the distinctive feature of this model is the detailed consideration of the basal temperate ice layer in which the water content and its impact on the ice viscosity are computed its transition surface to the cold ice region is accounted for by continuum mechanical jump conditions on this interface the simulations presented include steady states subject to a range of physical parameters and two different climates present and glacial conditions as well as three types of transient scenarios namely i sinusoidal milankovic period forcing ii paleoclimatic forcing from the greenland ice core project core reconstruction and iii future greenhouse warming forcing

solidification and turbulence non laminar during magma ascent insights from 2d and 3d analyses of bubbles and minerals in an etnean dyke

zircon geochronology of archaean felsic sequences in the zimbabwe craton a revision of greenstone stratigraphy and a model for crustal growth u pb ion microprobe shrimp work on zircon populations from 13 zimbabwean archaean felsic rocks are presented and interpreted samples were extracted from felsic volcanic sequences from most of the major greenstone belts and represent the first zircon geochronological data from within the greenstone belts themselves the data demonstrate a late archaean volcanicity spanning 250 ma which began at least 2900 ma ago and ended at 2650 ma the intrusion of extensive granitoid sills of the chilimanzi suite at c 2 6 ga marks the widespread stabilization of the craton based on the new zircon data and a re evaluation of published mapping a new stratigraphic subdivision is presented for the late 3 3 ga fragment of archaean crust in zimbabwe these data suggest that the early archaean crust is restricted to the tokwe segment in the south of the country since even the oldest of the felsic volcanics 2 90 ga lower belingwean have zircons which are 50 ma older it is suggested that remnants of earlier cyclic greenstone granitoid events must underlie the present craton and that all of the currently exposed greenstone belts of zimbabwe were developed on sialic crust

climatic and topographic controls on the style and timing of late quaternary glaciation throughout tibet and the himalaya defined by 10be cosmogenic radionuclide surface exposure dating abstract temporal and spatial changes in glacier cover throughout the late quaternary in tibet and the bordering mountains are poorly defined because of the inaccessibility and vastness of the region and the lack of numerical dating to help reconstruct the timing and extent of glaciation throughout tibet and the bordering mountains we use geomorphic mapping and 10 be cosmogenic radionuclide crn surface dating in study areas in southeastern gonga shan southern karola pass and central western nyainqentanggulha shan and tanggula shan tibet and we compare these with recently determined numerical chronologies in other parts of the plateau and its borderlands each of the study regions receives its precipitation mainly during the south asian summer monsoon when it falls as snow at high altitudes gonga shan receives the most precipitation 2000 mm a 1 while near the margins of monsoon influence the karola pass receives moderate amounts of precipitation 500 600 mm a 1 and in the interior of the plateau little precipitation falls on the western nyainqentanggulha shan 300 mm a 1 and the tanggula shan 400 700 mm a 1 the higher precipitation values for the tanggula shan are due to strong orographic effects in each region at least three sets of moraines and associated landforms are preserved providing evidence for multiple glaciations the 10 be crn surface exposure dating shows that the formation of moraines in gonga shan occurred during the early mid holocene neoglacial and little ice age on the karola pass during the lateglacial early holocene and neoglacial in the nyainqentanggulha shan date during the early part of the last glacial cycle global last glacial maximum and lateglacial and on the tanggula shan during the penultimate glacial cycle and the early part of the last glacial cycle the oldest moraine succession in each of these regions varies from the early holocene gonga shan lateglacial karola pass early last glacial western nyainqentanggulha shan and penultimate glacial cycle tanggula shan we believe that the regional patterns and timing of glaciation reflect temporal and spatial variability in the south asian monsoon and in particular in regional precipitation gradients in zones of greater aridity the extent of glaciation has become increasingly restricted throughout the late quaternary leading to the preservation of old 100 ka glacial landforms in contrast in regions that are very strongly influenced by the monsoon 1600 mm a 1 the preservation potential of pre lateglacial moraine successions is generally extremely poor this is possibly because lateglacial and holocene glacial advances may have been more extensive than early glaciations and hence may have destroyed any landform or sedimentary evidence of earlier glaciations furthermore the intense denudation mainly by fluvial and mass movement processes which characterize these wetter environments results in rapid erosion and re sedimentation of glacial and associated landforms which also contributes to their poor preservation potential

geologically and geomorphologically constrained numerical model of laurentide ice sheet inception and build up abstract the locations shapes and masses of evolving ice sheets during interglacial to glacial transitions remain elusive but need to be clarified for identification of the climatic feedbacks that amplified low amplitude insolation changes to become the major global climatic shift of the last glacial maximum the best way to explore probable spatial patterns of ice sheet inception and build up is through ice sheet modelling and validating model output against geomorphological and geological data we have modelled the 120 55 kyr bp evolution of the laurentide ice sheet focusing on the locations outline flow patterns and basal thermal regimes of its precursor ice centres for this we employed a time dependent mass balance driven finite element model forced with grip ice core oxygen isotope values calibrated to temperature the model achieves a good fit to relict ice flow systems in keewatin the interior plains and the hudson bay lowland thus our model indicates that the first coherent ice centre in north america was situated over baffin island and the melville and boothia peninsulas and expanded onto mainland canada at 90 75 kyr bp a previously unknown 200 km long moraine zone near the thlewiaza river keewatin may demarcate the marginal position of this early central arctic ice sheet cordilleran and keewatin ice merged and the hudson bay lowland was inundated from the east from quebec by 65 kyr bp

magnetic anisotropy produced by magma flow theoretical model and experimental data from ferrar dolerite sills antarctica summary volcanic rocks forming sills dykes or lava flows may display a magnetic anisotropy derived from the viscous flow during their emplacement we model a sill as a steadystate flow of a bingham fluid driven by a pressure gradient in a horizontal conduit the magma velocity as a function of depth is calculated from the motion and constitutive equations vorticity and strain rate are determined for a reference system moving with the fluid the angular velocity and the orientation of an ellipsoidal magnetic grain immersed in the fluid are calculated as functions of time or strain magnetic susceptibility is then calculated for a large number of grains with a uniform distribution of initial orientations it is shown that the magnetic lineation oscillates in the vertical plane through the magma flow direction and that the magnetic foliation plane changes periodically from horizontal to vertical the results are compared with the magnetic fabric of ferrar dolerite sills victoria land east antarctica derived from low field susceptibility measurements

significance of magnetic and petrofabric in karoo feeder dykes northern lebombo abstract the orientation of magnetic and petrofabric in 14 mafic dykes of the northern lebombo dyke swarm nlds has been determined using the anisotropy of magnetic susceptibility ams and mineral shape preferred orientation spo in order to constrain the magma flow direction during dyke emplacement the n s striking nlds intruded the n s trending 700 km long lebombo faulted monocline between 182 and 178 ma the lebombo faulted monocline which forms the eastern most sub province of the karoo large igneous province lip is essentially a volcanic rifted margin with a protracted magmatic and tectonic history related to the break up of southern gondwana it also forms the southern limb of the karoo triple rift system which has been hypothesised to have formed by a mantle plume centred on the triple junction locus the other two arms of the rift which converge on the triple junction are the wnw trending okavango dyke swarm ods and ne trending save limpopo dyke swarm slds in the nlds it is found that the magnetic fabric which is of magmatic origin in 50 of the samples is carried predominantly by stoichiometric magnetite there is remarkable agreement between the orientations of the ams and spo fabrics and at least two dykes show coaxial ams and mineral spo fabric consistent with lateral magma flow from the north this direction is supported by the imbrication of the foliations of the well defined bulk ams and plagioclase spo fabric when viewed in a dyke co ordinate system although steeply plunging maximum axes occur in the spo fabric their possible use as magma flow vectors is questioned because they do not necessarily mimic the elongation of plagioclase grains visible in orientated thin sections the magnetic and petrofabric of the nlds is interpreted to have developed during lateral magma flow from the locus of the karoo triple junction

development of symmetrical and asymmetrical fabrics in sheet like igneous bodies the role of magma flow and wall rock displacements in theoretical and natural cases abstract two orientations of fabrics are recognised in vertical dykes a symmetric orientation associated with magma injected into non deforming wall rocks and an asymmetric orientation usually associated with external non coaxial shearing or magma injected into an active fault the notion of two fabric arrangements stems from two planes of symmetry the dyke symmetry plane dsp and the fabric symmetry plane fsp this paper uses conceptual models to simulate all possible orientations of magma flow indicators that might be produced within a vertical dyke emplaced under coeval internal and external stresses the effect of magmatic buoyancy related stresses are portrayed in terms of magma flow velocities mfv and the external non coaxial tectonic related stresses in terms of wall displacement velocities divided by two wmv 2 the tectonic setting is that of a sinistral transcurrent fault using these assumptions five cases are proposed mfv wdv 2 mfv wdv 2 mfv wdv 2 mfv

flow dynamics in mid jurassic dikes and sills of the ferrar large igneous province and implications for long distance magma transport abstract magma flow paths in sill fed dikes of the ferrar large igneous province lip contrast with those predicted by classic models of dike transport in lips and magmatic rift settings we examine anisotropy of magnetic susceptibility ams flow paths in dike networks at terra cotta mountain and mt gran which intruded at paleodepths of 2 5 and 1 5 km these intrusions up to 30 m thick exhibit irregular interconnected dike sill geometries and adjoin larger sills 200 300 m thick at different stratigraphic levels both shallowly dipping and sub vertical magma flow components are interpreted from ams measurements across individual intrusions and often match macroscopic flow indicators and variations in dike attitudes flow paths suggest that intrusive patterns and magma flow directions depended on varying stress concentrations and rotations during dike and sill propagation whereas a regional extensional tectonic control was negligible or absent unlike giant dike swarms in lips elsewhere e g 1270 ma mackenzie lip dikes of the ferrar lip show no regionally consistent vertical or lateral flow patterns suggesting these intrusion were not responsible for long distance transport in the province in the absence of regionally significant colinear dike swarms or observed intrusions at crustal depths 4 km we suggest that long distance magma transport occurred in sills within beacon supergroup sedimentary rocks this interpretation is consistent with existing geochemical data and thermal constraints which support lateral magma flow for 3 500 km across the gondwana supercontinent before freezing

results from the ice sheet model intercomparison project heinrich event intercomparison ismip heino results from the heinrich event intercomparison heino topic of the ice sheet model intercomparison project ismip are presented ismip heino was designed to explore internal large scale ice sheet instabilities in different contemporary ice sheet models these instabilities are of interest because they are a possible cause of heinrich events a simplified geometry experiment reproduces the main characteristics of the laurentide ice sheet including the sedimented region over hudson bay and hudson strait the model experiments include a standard run plus seven variations nine dynamic thermodynamic ice sheet models were investigated one of these models contains a combination of the shallow shelf ssa and shallow ice approximation sia while the remaining eight models are of sia type only seven models including the sia ssa model exhibit oscillatory surges with a period of 1000 years for a broad range of parameters while two models remain in a permanent state of streaming for most parameter settings in a number of models the oscillations disappear for high surface temperatures strong snowfall and small sediment sliding parameters in turn low surface temperatures and low snowfall are favourable for the ice surge cycles we conclude that further improvement of ice sheet models is crucial for adequate robust simulations of cyclic large scale instabilities

discerning magmatic flow patterns in shallow level basaltic dykes from the ne rift zone of tenerife spain using the anisotropy of magnetic susceptibility ams technique discerning magmatic flow patterns in shallow level basaltic dykes from the north east rift zone of tenerife spain using the anisotropy of magnetic susceptibility ams technique

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

quaternary glaciation of gurla mandhata naimon anyi abstract the quaternary glaciation of gurla mandhata naimona nyi an impressive isolated dome shaped massif situated in a remote region of southern tibet was examined using glacial geomorphic methods and dated using 10 be terrestrial cosmogenic nuclides the oldest moraines representing an expanded ice cap that stretched 5 km into the foreland of the gurla mandhata massif likely formed during marine isotope stage mis 10 or an earlier glacial cycle another glacial expansion of coalescing piedmont type occurred during the early part of the last glacial cycle or the penultimate glacial cycle after which glaciers became restricted to entrenched valley types impressive latero frontal moraines at the mouths of most valleys date to the early part of the last glacial cycle and mis 3 a succession of six sets of moraines within the valleys and up to the contemporary glaciers show that glaciers advanced during the lateglacial early holocene neoglacial and possibly little ice age the change of style of glaciation throughout the latter part of the quaternary from expanded ice caps to deeply entrenched valley glaciers might reflect 1 climatic controls with reduced moisture supply to the region over time and or 2 tectonic controls reflecting increase basin subsidence due to detachment faulting and enhanced valley incision

composite ferromagnetic fabrics magnetite greigite measured by ams and partial aarm in weakly strained sandstones from western makran iran summary we present examples of composite magnetic fabrics ams and partial aarm in five sites from the western makran accretionnary prism these rocks are weakly strained sandstones of mio pliocene age rock magnetic investigations show a large occurrence of magnetite and also some greigite at two sites partial anhysteretic remanent magnetization coercivity spectra reveal that grain size is not homogeneous suggesting several grain fractions ams foliations are close to bedding except at one greigite bearing site where ams foliation is almost perpendicular to bedding soft aarm 0 50 mt foliations are slightly oblique both to bedding and ams foliation surprisingly hard aarm 50 100 mt foliations are perpendicular to bedding comparison of two limbs of one fold suggests that ams foliations are pre tilting while hardaarm foliations are post tilting corrected degree of anisotropy parameters display similar trends for all sites p 1 04 p 1 11 p hardaarm 1 19 we suggest that ams soft aarm and hard aarm reflect distinct magnetic grain populations however a possible relationship between hard aarm and a component of natural remanent magnetization is possible our study shows that two orthogonal fabrics exist in these weakly strained rocks coarse grains are parallel to bedding while fine grains are aligned perpendicular to bedding

late quaternary landscape evolution in the kunlun mountains and qaidam basin northern tibet a framework for examining the links between glaciation lake level changes and alluvial fan formation abstract the qaidam basin in northern tibet is one of the largest hyper arid intermontane basins on earth alluvial fans pediment surfaces shorelines and a thick succession of sediments within the basin coupled with moraines and associated landforms in the adjacent high mountain catchments of the kunlun mountains record a complex history of late quaternary paleoenvironmental change and landscape evolution the region provides an ideal natural laboratory to examine the interaction between tectonics and climate within a continent continent collision zone and to quantify rates of landscape evolution as controlled by climate and the associated glacial and hydrological changes in hyper arid and adjacent high altitude environments geomorphic mapping analysis of landforms and sediments and terrestrial cosmogenic radionuclide surface exposure and optically stimulated luminescence dating serve to define the timing of formation of late quaternary landforms along the southern and northwestern margins of the qaidam basin and in the burhan budai shan of the kunlun mountains adjacent to the basin on the south these dates provide a framework that suggests links between climatic amelioration deglaciation lake desiccation and alluvial fan evolution at least three glacial advances are defined in the burham budai shan of the kunlun mountains on the northern side of this range these occurred in the penultimate glacial cycle or early in the last glacial cycle during the last glacial maximum lgm lateglacial and during the holocene on the south side of the range advances occurred during the penultimate glacial cycle mis 3 and possibly the lgm lateglacial or holocene several distinct phases of alluvial fan sedimentation are likewise defined alluvial fans formed on the southern side of the kunlun mountains prior to 200 ka ice contact alluvial fans formed during the penultimate glacial and during mis 3 extensive incised alluvial fans that form the main valley fills north of the burham budai and extend into the qaidam basin are dated to 30 ka these ages suggest that there was a period of alluvial fan aggradation and valley filling that persisted until desiccation of the large lakes in the qaidam basin post 30 ka led to base level lowering and active incision of streams into the valley fills the continued lateglacial and holocene desiccation likely led to further degradation of the valley fills ice wedge casts in the qaidam basin date to 15 ka indicating significant lateglacial climatic amelioration while holocene loess deposits north of the burham bdudai suggest that aridity has increased in the region since the early holocene from these observations we infer that the major landscape changes within high glaciated mountains and their adjacent hyper arid intermontane basins such as the kunlun mountains and qaidam basin occur rapidly over millennial timescales during periods of climatic instability

geophysical evidence for the crustal variation and distribution of magmatism along the central coast of mozambique abstract for our understanding of the timing and geometry of the initial gondwana break up still a consistent image of the crustal composition of the conjugated margins of central mozambique and antarctica and the location of their continent ocean boundaries is missing in this regard a main objective is the explanation for the source of the different magnetic signature of the conjugate margins based on a revised investigation of wide angle seismic data along two profiles across the mozambican margin by means of an amplitude modelling this study presents the crustal composition across and along the continental margin of central mozambique supported by 2d magnetic modelling the results are compared to the conjugate margin in antarctica and allow new conclusions about their joined tectonic evolution an observed crustal diversity between the north eastern and south western parts of the central mozambican margin testifies to the complex break up history of this area conspicuous is the equal spatial extent of the hvlcb along the margin of 190 215 km the onset of oceanic crust at the central mozambican margin is refined to chron m38n 2n 164 1 ma magnetic modelling supports the presence of reversed polarized sdrs in the continent ocean transition that were mainly emplaced between 168 5 and 166 8 ma m42 m40 inferred sdrs in the riiser larsen sea might be emplaced sometime between 166 8 and 164 1 ma m39 m38 but got overprinted by normal polarized intrusions of a late stage of rift volcanism causing the opposite magnetic signature of the conjugate margins the distribution of the magmatic material along the central coast of mozambique clearly indicates the eastern extension of the north eastern branch of the karoo triple rift along the entire margin the main magmatic phase affecting this area lasted for at least 12 myr between 169 and 157 ma followed by the cease of the magmatism perhaps due to the relative southwards motion of the magmatic centre