a new occurrence of lazulite from the main central thrust in kumaun himalaya india fluid inclusion epma and raman spectroscopy focusing on lazulite in a highly tectonized zone

climate evolution during the holocene a study with an earth system model of intermediate complexity an earth system model of intermediate complexity mobidic has been used to simulate the transient variations in continental temperature sea surface temperature sst thermohaline circulation thc and sea ice cover over the last 9000 years 9 kyr experiments were designed to determine a the deviation of the climatic system with respect to equilibrium over the last 9 kyr b the individual contributions of oceans and vegetation to climatic changes as well as the potential synergies between these components and c the relative importance of precession obliquity and co2 concentration changes during this period results show a monotonous cooling trend in the northern high latitudes between 9 kyr bp and the present day both over the oceans and the continents north of 60 n this cooling is noticed throughout the year but the largest variations appear in spring and summer up to 6 c over continents along with this cooling the model exhibits a southward shift of the northern treeline by about 600 km most of this shift takes place between 4 and 1 kyr bp during this period reorganisations of the boreal forest introduce a lag of about 200 years in the system with respect to a state in equilibrium with the external forcing sensitivity experiments illustrate the strong impact of this vegetation shift both on the oceans and the continents especially in spring and early summer however the model exhibits a weak synergy between vegetation and ocean throughout the holocene finally a sensitivity study to the forcing components shows the dominant role of the astronomical forcing with respect to co2 as well as the non linear behaviour of climate in response to obliquity and precession

the fluid dynamics of a basaltic magma chamber replenished by influx of hot dense ultrabasic magma this paper describes a fluid dynamical investigation of the influx of hot dense ultrabasic magma into a reservoir containing lighter fractionated basaltic magma this situation is compared with that which develops when hot salty water is introduced under cold fresh water theoretical and empirical models for salt water systems are adapted to develop a model for magmatic systems a feature of the model is that the ultrabasic melt does not immediately mix with the basalt but spreads out over the floor of the chamber forming an independent layer a non turbulent interface forms between this layer and the overlying magma layer across which heat and mass are transferred by the process of molecular diffusion both layers convect vigorously as heat is transferred to the upper layer at a rate which greatly exceeds the heat lost to the surrounding country rock the convection continues until the two layers have almost the same temperature the compositions of the layers remain distinct due to the low diffusivity of mass compared to heat the temperatures of the layers as functions of time and their cooling rate depend on their viscosities their thermal properties the density difference between the layers and their thicknesses for a layer of ultrabasic melt 18 mgo a few tens of metres thick at the base of a basaltic 10 mgo magma chamber a few kilometres thick the temperature of the layers will become nearly identical over a period of between a few months and a few years during this time the turbulent convective velocities in the ultrabasic layer are far larger than the settling velocity of olivines which crystallise within the layer during cooling olivines only settle after the two layers have nearly reached thermal equilibrium at this stage residual basaltic melt segregates as the olivines sediment in the lower layer depending on its density the released basalt can either mix convectively with the overlying basalt layer or can continue as a separate layer the model provides an explanation for large scale cyclic layering in basic and ultrabasic intrusions the model also suggests reasons for the restriction of erupted basaltic liquids to compositions with mgo 10 and the formation of some quench textures in layered igneous rocks

development of asymmetric shale pull aparts in evaporite shear zones abstract antithetically rotated lozenge shaped shale pull aparts are quite common in the shallow domains of the evaporite shear zones of the jura in northern switzerland the pull aparts are formed by synthetic shear fractures related to a transpressive strain field that also induces the antithetic rotation of the pull aparts immediately after their formation a simple analytical model shows that the acute angle of fracture and the antithetic rotation are determined by structural and rheological parameters and the direction of maximum compressive stress the pull aparts are commonly separated by sulphate veins as the elongation resulting from antithetic rotation does not compensate for the bulk elongation despite this some pull aparts are bordered by synthetic drag zones antithetically rotated pull aparts are indicative either of compression normal to shear zone boundaries in decollement type shear zones or in shear zones deformed by simple shear of an initial obliquity of the dismembered layers relative to the shear zone boundary resemblance of the shale pull aparts to type 1 asymmetric pull aparts and book shelf structures both of which rotate synthetically demonstrates that rotated and asymmetric pull aparts may be ambiguous kinematic indicators

inverted metamorphism in the intracontinental ductile shear zone during himalayan collision tectonics collision tectonics in the himalaya ca 50 ma remobilized the proterozoic basement and its cover of the indian plate into the higher himalayan crystalline belt this belt displays an inverted metamorphism progressing from a garnet to a sillimanite k feldspar zone toward higher topographic and structural levels because previously hypothesized mechanisms are not sufficient to explain inverted metamorphism we propose that ductile shear displacements along ubiquitous closely spaced s c shear planes may have caused metamorphic inversion across a 15 20 km thick southwest vergent higher himalayan shear zone

thermotectonic history of the chiplakot crystalline belt in the lesser himalaya kumaon india constraints from apatite fission track thermochronology abstract fission track ages and confined track length distribution of apatite samples separated from the chiplakot crystalline belt ccb of the lesser himalayan crystalline lhc zone located to the south of the main central thrust mct munsiari thrust mt in kumaon india have been determined ages from the ccb along the kali and darma valleys fall in two distinct groups in the northern part of the ccb the ages range from 9 8 0 6 to 7 6 0 6 ma with a weighted mean of 9 6 0 1 ma while in the southern part the ages vary from 17 9 0 9 to 12 9 1 1 ma with a weighted mean of 14 1 0 1 ma the bimodal distribution of track lengths indicates that the ages are mixed ages rather than simple cooling ages the apatite fission track aft ages and already published structural data of the ccb suggest a complex erosional denudation history within the upper 3 4 km of the crust of the ccb the ages further indicate that the ccb was thrust into place earlier than the middle miocene i e at the time of development of the mct since then these rocks have remained within the upper 3 km of crust and were affected by only moderate to slow erosion and exhumation these results have important implications for the tectonic evolution of the lhc zone to the south of the mct mt the exhumation of the lhc zone in different parts of the himalaya was not uniform in the kumaon himalaya it was not controlled as in the himachal himalaya by any major tectonic event since it was thrust over the lesser himalayan meta sedimentary lhms zone and underwent moderate to slow erosion and exhumation

ductile and brittle deformations in the northern snake range nevada abstract the northern snake range east central nevada is one of the metamorphic core complexes of the sevier hinterland within the range a major decollement separates an upper plate composed of brittlely deformed paleozoic sedimentary rocks mostly carbonates from a lower plate composed of metamorphic upper precambrian lower cambrian rocks intruded by gneissic granites a study of the geometry and kinematics of structures and fabrics at outcrop scale and in thin sections indicates that the northern snake range decollement has been a zone of intense non coaxial e vergent shear and transport in a n115 e direction outstanding asymmetric boudinage within the marble sheet capping the lower plate testifies for late ductile shear strains of at least 20 the interface between brittlely and ductilely deformed rocks seems too sharp to represent a regional rheological transition but might result from two distinct phases of deformation ductile deformation in and below the decollement could have occurred before brittle deformation in the upper plate brittle faulting in the upper plate related to basin and range extension reactivated the upper surface of the ductile shear zone high topographic relief on the normal faults bounding the range triggered the slide of olistolites from the upper plate into the adjacent oligo miocene basins

simulations of northern hemisphere ice sheet retreat sensitivity to physical mechanisms involved during the last deglaciation a 3 dimensional thermomechanical ice sheet model is used to simulate the evolution of the geometry of northern hemisphere ice sheets through the last deglaciation the ice sheet model is forced by a time evolving climatology provided by the linear interpolation through time of climate snapshots simulated by the lmd5 3 atmospheric general circulation model agcm at different periods of the last deglaciation 21 15 9 6 and 0 kyr bp the agcm is driven by insolation atmospheric co2 content ice sheet configuration and sea surface temperatures although our approach is able to produce the complete continental ice retreat our simulated deglaciation presents a phase lag with reconstructions based on observational evidences this suggests that physical mechanisms related to climate forcing and or ice sheet internal dynamics are not properly represented the influence of millennial scale forcing feedback mechanisms between ice sheet elevation and surface mass balance and parameterization of the ice flow is also tested through a set of sensitivity experiments the rapid variability has a strong impact on the evolution of the ice volume because of nonlinear effects in temperature mass balance relationships fennoscandia appears to be strongly sensitive to the small scale ice sheet instability both ice sheets are to some extent sensitive to an increased basal sliding

sedimentary effects of cataclysmic late pleistocene glacial outburst flooding altay mountains siberia rudoy a n and baker v r 1993 sedimentary effects of cataclysmic late pleistocene glacial outburst flooding altay mountains siberia in c r fielding editor current research in fluvial sedimentology sediment geol 85 53 62 pleistocene glacial outburst floods were released from ice dammed lakes of the altay mountains south central siberia the kuray chuja lake system yielded peak floods in excess of 1 106 m 3 s 1 and as great as 18 106 m 3 s i the phenomenally high bed shear stresses and stream powers generated in these flows produced a main channel coarse grained facies of coarse gravel in 1 foreset bedded bars as much as 200 m high and several kilometers long and 2 degradational boulder capped river terraces giant current ripples 50 to 150 m in spacing composed of pebble and cobble gravel are locally abundant the whole sedimentary assemblage is very similar to that of the channeled scabland produced by the pleistocene missoula floods of western north america

spatiotemporal variation in exhumation of the crystallines in the nw himalaya india constraints from fission track dating analysis abstract during himalayan orogeny coeval thrusting along the main central munsiari thrust mct mt and extension along the south tibetan detachment system stds are widely responsible for rapid exhumation of the higher himalayan crystalline hhc zone apatite and zircon fission track data along the kaliganga and darma valleys in the kumaon himalaya serve to document the shallow bedrock exhumation history of the hhc taking into account sample location within the hhc with respect to the mct mt the apatite fission track aft data sets along the darma 1 0 0 1 to 2 8 0 3 ma and kaliganga 1 4 0 2 to 2 4 0 3 ma which are sharing same structural setting and rock types and being separated by 40 km show very similar patterns of exhumation histories since plio quaternary in the kumaon himalaya data sets along darma and kaliganga are very similar to data set of adjacent traverse 50 km away along the goriganga valley studied by patel and carter 2009 whole data sets within the hhc in kumaon himalaya provide clear evidence for plio quaternary tectonic activity along the vaikrita thrust vt precipitation in this region exerts a strong influence on erosional surface processes fluvial erosional unloading along the himalaya is focused on the high mountainous region of the hhc where the orographic barrier forces out the maximum percentage of annual rainfall ft cooling ages reveal coincidence between rapid erosion and exhumation that is focused in a 25 30 km wide sector of the hhc rather than covering the entire orogen similarity of aft age pattern and exhumation rates along all three major traverses goriganga darma and kaliganga indicates that the region has been experiencing constant rate of crustal uplift and erosion since long time comparison of fission track ages from the kumaon himalaya with other segments of the nw himalaya shows spatiotemporal variation in exhumation it is described due to the development of local structures such as dome window in jammu and kashmir and himachal himalayas and quaternary active thrusting along the vt and mct mt in the garhwal kumaon himalaya

response of interior north america to abrupt climate oscillations in the north atlantic region during the last deglaciation several broad scale climatic oscillations during the last deglaciation are well documented in regions around the north atlantic ocean this paper reviews empirical evidence for these deglacial climatic oscillations from non coastal north america and discusses implications for testing climatic simulations and for understanding the cause and transmittal mechanisms paleoclimatic interpretation of oxygen isotope records from several small sites in the eastern great lakes region indicates a classic deglacial climatic sequence that is comparable with records from europe and greenland the climatic events as interpreted from crawford lake oxygen isotopes include the bolling allerod boa warming at 12 700 14c bp a warm boa at 12 500 10 920 14c bp an intra allerod cold period shortly before 11 000 14c bp a cold younger dryas yd climate reversal at 10 920 10 000 14c bp the holocene warming at 10 000 14c bp a brief preboreal oscillation pb at 9650 14c bp and an early holocene cold event at 7500 14c bp 8200 cal bp some of these events were also evident from changes in upland and aquatic vegetation and sediment lithology the pronounced yd climatic reversal has been documented from pollen records along the ecotones at this time and from glacier readvances in the great lakes region along the rocky mountains from alberta to colorado the yd event is indicated by alpine glacier advance and or shift in timberline vegetation in minnesota and upper new york early holocene climatic instability is also suggested by oxygen isotope records and or varve thickness the regional variations in evidence for the yd and other events in north america suggest that climatic oscillations may have different expressions in paleo records depending on geographic location and characteristics of a particular site the extent and magnitude of these climatic oscillations across north america suggest that these oscillations are an expression of climatic change that was probably widespread rather than locally induced by a nearby glacier the location of these sites implies that climatic signals were likely carried over the northern hemisphere through the atmosphere as indicated by general circulation models we hypothesize that the lack of evidence for a cold yd in interior north america west of the great lakes region and east of the rocky mountains was caused by the trapping of cold arctic air mass north of the laurentide ice sheet and by uninterrupted northward expansion of warm caribbean air this strongly contrasted climate also produced non analogous biological assemblages

impact of precipitation seasonality changes on isotopic signals in polar ice cores a multi model analysis abstract for central greenland water isotope analysis indicates a temperature difference of about 10 c since the last glacial maximum lgm however borehole thermometry and gas diffusion thermometry indicate that lgm surface temperatures were about 20 c colder than today two general circulation model studies have shown that changes in the seasonal precipitation timing in central greenland might have caused a warm bias in the lgm water isotope proxy temperatures and that this bias could explain the difference in the estimated paleotemperatures here we present an analysis of a number of atmospheric general circulation model simulations mostly done within the framework of the paleoclimate modeling intercomparison project the models suggest that the seasonal cycle of precipitation and surface mass balance over central greenland at the lgm might have been very different from today this supports the idea that the accuracy of the water isotope thermometry at the lgm in greenland might be compromised as a result of a modified surface mass balance seasonality however the models disagree on the amplitude and sign of the bias for central east antarctica a strong seasonality effect on the lgm isotopic signal is not simulated by any of the analyzed models for the mid holocene 6 kyr bp the models suggest relatively weak isotope paleothermometry biases linked to changes in the surface mass balance seasonality over both ice sheets

analysis of a complex high strain zone at cap de creus spain abstract the structural analysis of a high strain zone developed in medium to high grade metamorphic micaschists from the cap de creus area spain provides an example of the complex relationships between geometry strain and kinematics to be found in deep crustal shear zones this high strain zone is composed of e w trending structural domains characterized by different strain intensities and associated with syntectonic emplacement of pegmatite dykes the main phase of deformation discussed here d2 affects steeply dipping bedding boudinaged quartz veins and s1 developed parallel to bedding d2 deformation of these features and of syn d2 pegmatite dykes gives rise to fold boudin structures in map view a d2 high strain zone coincides with a km scale dextral flexure of s0 s1 and s2 although all small scale structures on outcrop surfaces close to horizontal indicate a prevalent sinistral shear sense in addition a subvertical stretching lineation is present approximately parallel to the axis of the foliation deflection and thus normal to the apparent displacement direction in the high strain zone it is proposed that this high strain zone did not form by horizontal dextral simple shear with a flow plane parallel to the boundaries of high and low strain zones but acted as a zone of highly vortical sinistral non coaxial flow with a strong vertical extension component the dextral flexure of s0 s1 and s2 can be explained by considering that sinistral non coaxial flow in the low strain zones was either slower or earlier that in the high strain core this example stresses the necessity of being careful when interpreting displacement directions from the deflection of older fabric elements in deep seated high strain zones

magma addition and possible decoupling of major and trace element behaviour in the bushveld complex south africa abstract incompatible trace element abundances and ratios in cumulate rocks from the bushveld complex may be used to predict the levels at which new magmas may be injected sudden and sustained changes in absolute abundances suggest new magma addition while gradual increases are consistent with fractionation of a large body of magma the present data suggest input above the merensky reef at the top of the critical zone near the top of the lower and main zones there may be other additions however in the latter case the chemical effect is spread through a large vertical interval and suggests very slow diffusive mixing between new and residual liquids the different behavioral patterns may be related to whether new magma intrudes above or below the residual magma the former situation resulting in delays between timing of injection and significant influence in the chemistry of the cumulates

late to post collisional brittle ductile deformation in the himalayan orogen evidences from structural studies in the lesser himalayan crystallines kumaon himalaya india abstract the chiplakot crystalline belt ccb is a broad shear zone located within the meta sedimentary zone of the lesser himalaya evidence from mesostructures and microstructures reveals development of this shear zone in a compressional ductile tectonic regime during continental collision in the himalaya by top to sw shearing along ne dipping shear surfaces the shear surfaces are parallel to both the upper and basal thrust planes of the ccb a hitherto unrecognized late to post collisional brittle ductile tectonic regime comprised of two discrete phases is established from dynamic analysis of the kink bands extensional shear fractures foliation boudinage competent layer boudinage quartz veins and associated flanking folds and brittle normal and reverse faults in the ccb kink bands foliation parallel slip and general shear were major consequences of this late to post collisional brittle ductile tectonic episode and were associated with the intrusion of a swarm of quartz veins quartz veins associated with flanking folds are developed at high angles with respect to the mylonitic foliation and parallel to the axial plane of the kink bands throughout the ccb the geometry of the flanking folds along the quartz veins is a consequence of the fracturing and not vice versa the precise shape of the folds depends on the nature of general shearing along the wall of quartz veins and various orientations to the shear direction

a sm nd isotopic study of the south harris igneous complex the outer hebrides sm nd geochronology may be used to bracket the age of metamorphism in rocks which are difficult to date by other methods by coupling whole rock sm nd analyses of the principal members of the south harris igneous complex with sm nd mineral isochrons on two anothositic gabbros the age of granulite facios metamorphism has been defined whole rock analyses of three pairs of closely spaced samples of the anorthosite give consistent ages averaging 2 18 0 06 gyr but in general the data from the anorthosite do not define an isochron as a result of variable contamination of the evolving magma chamber whole rock data on the tonalite indicate that it is younger than 2 06 gyr its mean tchur age is 1 86 0 05 gyr garnet pyroxene amphibole plagioclase mineral isochrons on two anorthosite samples give identical 1 87 0 04 gyr ages which date cooling after the high pressure granulite facies metamorphism together with the tonalite whole rock data this defines the age of that metamorphism and confirms dearnley s original assignment of an early laxfordian age

ice age terrestrial carbon changes revisited n shackleton 1977 first proposed that changes in the marine 13 c record 13 c could be used to infer ice age changes in carbon storage on land the previously published best estimate from the marine record is equivalent to about 490 gt 0 32 13 c however adams et al 1990 utilized a pollen database to estimate a 1350 gt change in carbon storage which would cause a 13 c of about 0 90 the nearly trillion ton difference in estimates amounts to almost half of the total carbon stored on land to address the nature of this discrepancy i have reexamined the terrestrial carbon record based on a new pollen database compiled by r webb and the cooperative holocene mapping project cohmap group i estimate about 750 1050 gt glacial interglacial change in terrestrial carbon storage with the range reflecting uncertainties in carbon storage values for different biomes additional uncertainties apply to rainforest and wetland extent and presence of c4 plants which have a significantly different isotopic signature than c3 plants although some scenarios overlap a new estimate of carbon storage based on the oceanic 13 c record revised to 0 40 and 610 gt most estimates seem to fall outside the envelope of uncertainty in the marine record several possible explanations for this gap involve 1 a missing sink may be involved in land sea carbon exchange e g continental slopes 2 the geochemistry of the exchange process is not understood 3 carbon storage by biome may have changed under ice age boundary conditions or 4 the standard interpretation of whole ocean changes in the marine 13 c record requires reevaluation this latter conclusion receives some support from comparison of the 13 c records for 18 o stages 2 and 6 for the stage 6 glacial the 13 c changes are 50 60 larger than for the stage 2 glacial yet implications of increased aridity are not supported by longterm trends in atmospheric dust loading to summarize the above analysis implies that despite the uncertainties remaining in estimates of terrestrial carbon storage changes a case can be made that our understanding of the transfer process is incomplete and that the eventual explanation may require clarification of factors affecting the marine 13 c record

late glacial and holocene vegetational changes on the ulagan high mountain plateau altai mountains southern siberia three well dated pollen diagrams from 1985 m 2050 m and at the tree line at 2150 m asl show the vegetational succession in the central altai mountains since 16 cal ka bp pioneer vegetation after deglaciation was recorded first at the lowest site subsequently dense dry steppe vegetation developed coincident with the change from silt to organic sediments at the two lower sites but silt lasted longer at the highest site indicating the persistence of bare ground there forests of pinus sibirica pinus sylvestris picea obovata larix sibirica abies sibirica and betula pendula started to develop about 12 cal ka bp with the change to a warmer and wetter climate at the beginning of the holocene results indicate that the timberline did not rise above the highest site mesophilous dark coniferous forests were fully developed by 9 5 cal ka bp the role of abies and picea decreased by about 7 5 cal ka bp suggesting cooler climate after which the forests changed little until today the vegetational development in this portion of the central altai mountains is compatible with that described in neighbouring areas of the altai southern siberia mongolia and kazakhstan

ice dammed lakes and rerouting of the drainage of northern eurasia during the last glaciation during the quaternary period ice sheets centred over the barents and kara seas expanded several times onto mainland russia and blocked northflowing rivers such as the yenissei ob pechora and mezen large ice dammed lakes with reversed outlets e g toward the caspian sea formed south of these ice sheets some lakes are reconstructed from shorelines and lacustrine sediments others mainly from ice sheet configuration ice dammed lakes considerably larger than any lake on earth today are reconstructed for the periods 90 80 and 60 50 ka the ages are based on numerous optically stimulated luminescence osl dates during the global last glacial maximum lgm about 20 ka the barents kara ice sheet was too small to block these eastern rivers although in contrast to the 90 80 and 60 50 ka maxima the scandinavian ice sheet grew large enough to divert rivers and meltwater across the drainage divide from the baltic basin to the river volga and that way to the caspian sea climate modelling shows that the lakes caused lower summer temperatures on the continent and on the lower parts of the ice sheet the final drainage of the best mapped lake is modelled and it is concluded that it probably emptied within few months we predict that this catastrophic outburst had considerable impact on sea ice formation in the arctic ocean and on the climate of a much larger area c 2003 elsevier ltd all rights reserved less

late quaternary ice sheet history of northern eurasia the maximum limits of the eurasian ice sheets during four glaciations have been reconstructed 1 the late saalian 140 ka 2 the early weichselian 100 80 ka 3 the middle weichselian 60 50 ka and 4 the late weichselian 25 15 ka the reconstructed ice limits are based on satellite data and aerial photographs combined with geological field investigations in russia and siberia and with marine seismic and sediment core data the barents kara ice sheet got progressively smaller during each glaciation whereas the dimensions of the scandinavian ice sheet increased during the last ice age the barents kara ice sheet attained its maximum size as early as 90 80 000 years ago when the ice front reached far onto the continent a regrowth of the ice sheets occurred during the early middle weichselian culminating about 60 50 000 years ago during the late weichselian the barents kara ice sheet did not reach the mainland east of the kanin peninsula with the exception of the nw fringe of taimyr a numerical ice sheet model forced by global sea level and solar changes was run through the full weichselian glacial cycle the modeling results are roughly compatible with the geological record of ice growth but the model underpredicts the glaciations in the eurasian arctic during the early and middle weichselian one reason for this is that the climate in the eurasian arctic was not as dry then as during the late weichselian glacial maximum

the periglacial climate and environment in northern eurasia during the last glaciation this paper summarizes the results of studies of the late weichselian periglacial environments carried out in key areas of northern eurasia by several queen teams european science foundation esf programme quaternary environment of the eurasian north the palaeoglaciological boundary conditions are defined by geological data on timing and extent of the last glaciation obtained in the course of the eu funded project eurasian ice sheets these data prove beyond any doubt that with the exception of the northwestern fringe of the taymyr peninsula the rest of the eurasian mainland and severnaya zemlya were not affected by the barents kara sea ice sheet during the last glacial maximum lgm inversed modelling based on these results shows that a progressive cooling which started around 30 ka bp caused ice growth in scandinavia and the northwestern areas of the barents kara sea shelf due to a maritime climate with relatively high precipitation along the western flank of the developing ice sheets in the rest of the eurasian arctic extremely low precipitation rates less than 50 mm yr 1 did not allow ice sheet growth in spite of the very cold temperatures palaeoclimatic and palaeoenvironmental conditions for the time prior to during and after the lgm have been reconstructed for the non glaciated areas around the lgm ice sheet with the use of faunal and vegetation records permafrost eolian sediments alluvial deposits and other evidences the changing environment from interstadial conditions around 30 ka bp to a much colder and drier environment at the culmination of the lgm at 20 15 ka bp and the beginning of warming around 15 ka bp have been elaborated from the field data which fits well with the modelling results

review of flanking structures in meso and micro scales a variety of host fabric elements he cut by cross cutting elements ce in rocks defines flanking structures fs on mesoscopic and microscopic scales there has been renewed interest in studying and classifying the fs for their morphologies useful as shear sense indicators and geneses existing non genetic morphologic parameters for the fs are reviewed and two new classification schemes are presented one of these is based on the nature of the ce and whether he penetrates it the other scheme takes account of all the potential combinations of drag no drag and slip no slip of the he deciphering the shear sense of the rock body from fs is complicated because the angular relationship between the ce and the primary shear planes might be opposite to what is found between s and c ductile shear fabrics further single ces can curve and several similar fs occur in reverse forms as with mineral fish the shape asymmetries of microscopic ces indicate the shear sense conjugate fs with non parallel ces with interfering perturbation fields around the ces are more reliable shear sense indicators than fs with single ce during low but increasing bulk strains fs may evolve from one type to another e g from a to s type at high strain fs can resemble intrafolial or sheath fold whether the drag is normal or reverse depends fundamentally on the initial angle between the he and the ce and the relative magnitudes of throw and vertical separation

huge ice age lakes in russia during an early phase of the last ice age weichselian valdaian about 90 000 yr ago an ice sheet formed over the shallow barents and kara seas the ice front advanced on to mainland russia and blocked the north flowing rivers yenissei ob pechora dvina and others that supply most of the freshwater to the arctic ocean the result was that large ice dammed lakes were formed between the ice sheet in the north and the continental water divides to the south here we present reconstructions and calculations of the areas and volumes of these lakes the lake on the west siberian plain was nearly twice as large as the largest lake on earth today the well mapped lake komi in northeast europe and a postulated lake in the white sea basin would also rank before the present day third largest lake the lakes overflowed towards the south and thus the drainage of much of the eurasian continent was reversed the result was a major change in the water balance on the continent decreased freshwater supply to the arctic ocean and increased freshwater flow to the aral caspian black and baltic seas a sudden outburst of the lakes water to the arctic ocean when the ice sheet thinned is postulated copyright 2001 john wiley sons ltd

late acadian alleghenian transpressional deformation evidence from asymmetric boudinage in the casco bay area coastal maine abstract asymmetric competent layer and foliation boudinage in the casco bay area of coastal maine formed during regional f 2 hinge parallel elongation and distributed dextral strike slip movement it consists of backward rotated oblique shear fracture boudinage and forward rotated extension fracture boudinage initially symmetric quartz filled extension fractures were intermittently developed as boudin partings orthogonal to the prominent ductile stretching lineation due to fault related fluctuations in fluid pressure and tectonic stresses modification by concurrent non coaxial deformation related to dextral strike slip movement resulted in the clockwise rotation sinistral slip and vein parallel elongation reflected in the forward rotated asymmetric boudinage discordant planar pegmatite veins and variably oriented boudinaged pegmatite sheets common in these exposures underwent a similar evolution the development of this asymmetric boudinage is attributed to progressive brittle ductile non coaxial deformation related to orogen parallel dextral transpression along the norumbega nonesuch river strike slip fault system during the later stages of the acadian orogeny and subsequent alleghenian deformation

the kalka layered intrusion central australia the kalka intrusion central australia has a 5000 m thick layered sequence comprising pyroxenite norite and anorthosite zones an olivine gabbro zone is enclosed by and has a facies relationship with the norite zone contamination is evidenced by high initial 87sr 86sr ratios 0 708 in the lower four fifths of the intrusion and resulted in pyroxenite rather than peridotite as a basal crystal accumulate after an early phase of erratic buildup in contamination due to assimilation of ragged granulite wall rock armouring of the walls and mixing produced an homogeneous norite zone threshold magma body crystallising opx cpx plag within the succeeding 3500 m of section plagioclase an values have a general decline up sequence an74 60 feldspar threshold with superimposed short term digressions to more calcic compositions initial 87sr 86sr ratios also fall very gradually 0 7081 0 7078 isotopic threshold with transient fluctuations to distinctly lower values maxima in plagioclase an contents and 87sr 86sr minima may be correlated with the spasmodic appearance of olivine and pronounced lithological variation such features are explained by the ponding of fresh bodies of uncontaminated magma on the floor of the chamber these formed a hybrid with threshold magma that temporarily dominated crystallisation thereby perturbing plagioclase and isotopic compositions eventually mixing of the hybrid into the overlying threshold magma returned crystallisation to its initial state the facies controlled olivine gabbro zone is the physical expression of ponded basal hybrid magmas the onset of the anorthosite zone is marked by a pronounced decline in 87sr 86sr ratios to around 0 705 registering a major influx of new magma in this instance the new magma dominated the system and a change to plagioclase as the dominant liquidus phase indicates a drastic change in input magma composition the evolution of kalka was determined by an interplay of crystal fractionation fresh magma pulses and magma mixing with the latter effects producing both increases and decreases in plagioclase an contents the complete process can only be revealed by combined mineral composition and isotopic data

factors affecting the kinematic interpretation of asymmetric boudinage in shear zones abstract based on work along a major mylonite zone in the northern appalachians and scale model studies a new mechanism for the origin of asymmetric boudins in shear zones is proposed along the honey hill fault in southern connecticut granitic sills intruded into calc silicate gneisses and schists were oblique to the boundaries of the mylonite zone and experienced the following sequence in the production of sigmoidal boudins 1 as the calc silicate schists experienced mylonitization and flow the more competent coarse grained granitic sills deformed by extensional fracturing and quartz veining 2 continued extension of granitic sills was accommodated by normal shear on early formed quartz veins 3 continuing extension of sills and ductile modification of the corners of boudins resulted in granitic fish with tails which stream from the top of the boudin in the down dip direction and from the bottom in the up dip direction based on a variety of kinematic indicators the sense of asymmetry of the tails is identical to that expected for recrystallization tails on sheared augen structure models composed of silicone putty and plasticine were created to investigate the effect of pre shearing geometry on boudin evolution and were deformed in a simple shear device the models reproduce the kinematics deduced from field relations and suggest that one of the primary factors in controlling the amount of extension is the angle which early veins make with the shear zone boundary varying the angular relationships in the models suggests that other pre shearing geometric factors may affect the geometry of boudins formed in this way boudins formed through this mechanism appear very similar to hanmer s type ii asymmetric boudins because the pre shearing geometry can exert a control on boudin asymmetry caution should be used when attempting to deduce shear sense or shear strain values from asymmetric boudinage

chironomid record of late quaternary climatic and environmental changes from two sites in central asia tuva republic russia local regional or global causes abstract sediment cores from two mountain lakes lake grusha at 2413 m a s l and ak khol at 2204 m a s l situated in the tuva republic southern siberia russia just north of mongolia were studied for chironomid fossils in order to infer post glacial climatic changes and to investigate responses of the lake ecosystems to these changes the results show that chironomids are responding both to temperature and to changing lake depth which is regarded as a sensitive proxy of regional effective moisture the post glacial history of this mountain region in central asia can be divided into seven successive climatic phases the progressive warming during the last glacial interglacial transition ca 15 8 14 6 cal kyr bp the warm and moist bolling allerod like interval ca 14 6 13 1 cal kyr bp the cool and dry younger dryas like event ca 13 1 12 1 cal kyr bp warmer and wetter conditions during ca 12 1 8 5 cal kyr bp a warm and dry phase ca 8 5 5 9 cal kyr bp cold and wet conditions during ca 5 9 1 8 cal kyr bp as well as cold and dry climate within the last 1800 years the chironomid records reveal patterns of climatic variability during the late glacial and holocene which can be correlated with abrupt climatic events in the north atlantic and the asian monsoon dominated regimes apparently the water balance of the studied lakes is controlled by the interrelation between the dominant westerly system and the changing influence of the summer monsoon as well as the influence of alpine glacier meltwater supply it is possible that monsoon tracks could have reached the southwest tuva resulting in an increase in precipitation at ca 14 6 13 1 and ca 12 1 8 5 cal kyr bp whereas cyclonic westerlies from the north atlantic were likely responsible for considerable moisture transport accompanying the global neoglacial cooling at ca 5 9 1 8 cal kyr bp these events suggest the changes of the regional pattern of atmospheric circulation which could be in turn induced by the global climatic shifts some discrepancies compared with other reconstructions from central asia may be associated with regional spatial differences between the changing predominant circulation mechanisms and with local differences in uplift and descent of air masses within the complicated mountain landscape in this paper we also discuss the possibilities and perspectives for using chironomids in reconstructions of past temperatures and climate induced changes in water depth of lakes in central asia

structural and thermochronological studies of the almora klippe kumaun nw india implications for crustal thickening and exhumation of the nw himalaya

ductile thrusting in the himalayas shear sense criteria and stretching lineations the most useful criteria used for the determination of the sense of shear can be found in the huge himalayan ductile shear zone known as m c t they allow the determination of the sense of shear in relation with an unequivocal southward directed intracontinental thrust the various criteria analyzed at different scales of observation show the permanence of the shear flow regime throughout the metamorphic evolution however most of the observed shear criteria are due to late nappe emplacement postdating the climax of the metamorphism the most striking feature is the stretching lineation which is roughly normal to the thrust trace all along the belt it parallels everywhere the shear direction and thus indicates the transport direction of the nappes the observed radial pattern of stretch trajectories implies that the local motion does not correspond to the average plate convergence

the association of folds and veins in shear zones abstract paired hook shaped asymmetric folds with shared limbs cut by axial planar veins dikes or fractures are common in glaciers and rocks the sense of offset across the vein or fracture is characteristically opposite to that expected from the sense of fold asymmetry the kinematics of development of the folds and the relationship of the folds to the fractures or veins can be understood by considering the nature of glacial flow and by utilizing simple clay experiments in zones of simple shear dilational fractures may form at about 45 to the shear zone boundaries the perturbation of strain around such a fracture during its development and subsequent rotation and closure under continued deformation causes layers or pre existing foliation cut by the fracture to become folded thus folding is a consequence of the fracturing and not vice versa the precise shape of the fold depends on the nature of the perturbation and particularly whether or not the fracture remains for a time open or fluid filled and thus has walls unable to sustain a shear stress during vein rotation paired hook folds provide evidence for non coaxial strain and are good sense of shear indicators however not all folds with veins parallel to their axial surfaces form in the way described in this paper in some cases vein emplacement occurs late during fold development or post dates folding in these cases offsets along the veins are consistent with fold asymmetry

sensitivity of the late saalian 140 kyrs bp and lgm 21 kyrs bp eurasian ice sheet surface mass balance to vegetation feedbacks this work uses an atmospheric general circulation model agcm asynchronously coupled to an equilibrium vegetation model to investigate whether vegetation feedbacks could be one of the reasons why

monoclinic model shear zones abstract although many ductile shear zones are supposed to have developed by approximately simple shear flow some must have formed under different conditions a few types of such non simple shear zones have been proposed in the literature such as transpression transtension and stretching shear zones this paper presents a full three dimensional kinematic model of shear zones with monoclinic flow geometry monoclinic shear zone types can be classified according to flow parameters and according to the geometry and orientation of accumulating finite strain modelling of finite strain accumulation shows that a number of unusual and potentially problematic structural features may develop in monoclinic shear zones these include stretching lineations normal to the displacement direction foliations normal to the shear zone boundary switching of the position of principal finite strain axes x y and z during progressive deformation and strain gradients along shear zones that may give rise to obliqueness of fabric elements of the same age in adjacent rock volumes

large scale foliation boudinage in gneisses abstract large scale extensional structures in gneisses symmetric and asymmetric foliation boudinage c type shears are common in crustal scale shear zones and may constitute shear criteria as well as being indicators of high strain

possible role of atmosphere biosphere interactions in triggering the last glaciation we coupled a global biome model iteratively with an atmospheric general circulation model to study the possible role of vegetation in the climate system at the time of glacial inception 115 000 years ago orbital forcing alone was not sufficient to initiate glaciation when other components of the climate system were kept as present atmospheric composition oceans biosphere and cryosphere summers were however cold enough to induce major vegetation shifts in high northern latitudes southward migration of the boreal forest tundra limit helped to create favourable conditions for continental ice sheet growth with increasing snow depth and duration in labrador arctic canada and northern western fennoscandia these results support a role for biogeophysical feedback in initiating glaciations

contrasts in the quaternary of mid north america and mid eurasia notes on quaternary landscapes of western siberia the west siberian plain was formed by marine deposits that extended from the mediterranean basin to the arctic tectonic action later produced a striking series of long straight ne sw grabens in the southern part of the plain pleistocene advance of the kara ice sheet onto the continent resulted in blockage of the ob and yenisey rivers to form huge proglacial lakes that drained through these grabens south via the turgay pass and the aral caspian black and mediterranean seas to the north atlantic ocean but during the last glacial maximum late weichselian isotope stage 2 the kara ice sheet did not advance onto the continent in northwestern siberia the altai mountains which bound the west siberian plain on the south contained large deep intermontane ice dammed lakes which drained catastrophically when the ice dams broke forming giant ripples on the basin floors pollen studies of glacial lakes indicate that the lateglacial steppe vegetation and dry climatic conditions continued into the early holocene as summer insolation maintained high levels permafrost development on a drained lake floor in the western altai mountains resulted in the formation of groups of small pingos in north america the growth and wastage of the huge laurentide ice sheet had an indirect role in the climatic history of western siberia during the glacial and lateglacial periods after which the climate was more affected directly by insolation changes whereas in north america in the early holocene the insolation factor was coupled with the climatic effects of the slow wastage of the ice sheet and the time of maximum dryness was postponed until the mid holocene copyright 2005 john wiley sons ltd

relationships between marginal thrusting and movement on major internal shear zones in the northern highland caledonides scotland abstract the development of the syn metamorphic sgurr beag slide zone a major ductile shear zone of initially low dip caused at least 50 km north western thrust displacement of part of the internal metamorphic complex of the northern highland caledonides of scotland initiation of the zone and movements upon it were earlier than formation of the marginal moine thrust zone movement on the zone followed but overlapped the peak caledonian metamorphism and the mid to high amphibolite facies mineral assemblages fabrics and structures produced during the development of the slide zone and those surviving from earlier events were reworked under greenschist facies conditions during mylonitization associated with initiation of the moine thrust zone displacements on the slide zone and thrust movements were separated by emplacement of a regional suite of pegmatites and a considerable change of metamorphic grade thus they may not constitute members of a progressive sequence of caledonian thrusts formed over a short time interval rather preliminary isotopic data may imply an interval of c 25 ma between movement on the slide zone and final ductile translation along the moine thrust zone

strontium isotopic constraints on the origin of proterozoic anorthosites abstract the proterozoic giles complex central australia contains an almost complete range of anorthosite types from minor or major layers in gabbronorite intrusions to large anorthosite troctolite bodies to small orthopyroxene anorthosite massifs each type has a distinctive sr isotopic signature anorthosite dominated masses have a regular relationship between ferromagnesian mineralogy initial 87 sr 86 sr and anorthite contents in plagioclase anorthosite troctolite bodies have significant olivine relatively low initial 87 sr 86 sr 0 7038 0 7043 and an 50 69 orthopyroxene dominant anorthosites have relatively high 87 sr 86 sr 0 7045 0 7063 and an 45 60 the pattern is found worldwide detailed study of one intrusion demonstrates that contamination by wall rock granulite produces the higher 87 sr 86 sr values anti correlation between 87 sr 86 sr and an and determines olivine orthopyroxene proportions olivine bearing anorthosites form from a primary aluminous tholeiite magma with plagioclase dominating the liquidus progressive contamination of this parent magma produces a gradation to orthopyroxene anorthosites

deformational and strain patterns of an intracontinental collision ductile shear zone an example from the higher garhwal himalaya the higher himalayan metamorphic belt of garhwal appears to have evolved in a major 15 20 km wide ne dipping ductile shear zone of the overthrust type due to intracontinental crustal shortening during cenozoic continental collision within the indian plate out of the four major distinct deformation phases the most widespread d2 deformation phase is marked by a prominent s2 foliation axial planar to the reclined and recumbent f2 folds a coaxial ne plunging l2 stretching lineation and syntectonic growth of index metamorphic minerals these deformational structures are developed during the sw directed d2 ductile shear deformation irrespective of the orientation of the s2 foliation and thrust zones regional strain patterns in quartzite of the lesser himalayan garhwal group beneath the main central thrust mct and the central crystalline zone using rf data on quartz clasts and aggregates reveal that the maximum values of finite strain es are attained within narrow mylonitic shear zones within the broad ductile shear zone during its progressive translation towards higher structural levels in late d2 and d3 deformation phases the shortening was accommodated by development of thrust sheets with the mct and jutogh thrust surfaces coinciding with zones of mylonite and maximum strain at higher levels ductile shear zones became compressional brittle ductile zones which resulted in the development of sw verging f3a and f3b folds during the southward migration of the thrust sheets in the lesser himalaya

secondary cleavages in ductile shear zones abstract secondary cleavages developed at late stages in ductile shear zones show several features that are inconsistent with progressive simple shear in the zone these are the orientation of a single secondary cleavage oblique to the shear zone boundaries conjugate sets with opposite senses of shear and multiple sets with the same sense of shear these features can be explained if the bulk flow is partitioned into slip along discrete failure planes parallel to the primary foliation s coaxial stretching along the foliation and spin

sense and non sense of shear in flanking structures abstract the deformation of marker horizons across slip surfaces can lead to the development of several types of flanking structures the development of such structures was investigated using a numerical model to simulate the flow around a slip surface in a viscous medium the modelled structures can be classified on the basis of three criteria 1 the extensional or contractional offset of markers 2 the co or counter shearing sense along the slip surfaces and 3 the normal or reverse drag of markers relative to the shear sense along the slip surfaces as a function of the kinematic vorticity of the flow and the initial orientation of the slip surface with respect to the shear zone boundaries three main types of structures can develop 1 shear bands developed along co shearing extensional slip surfaces 2 a type flanking folds developed along counter shearing slip surfaces and 3 s type flanking folds developed along co shearing contractional slip surfaces all these structures can occur with either normal or reverse drag effects shear bands and a type flanking folds recording opposite shear senses can be geometrically very similar and consequently lead to kinematic misinterpretations however correctly identified flanking structures can provide quantitative information about the kinematic vorticity of the flow

miocene to holocene exhumation of metamorphic crustal wedges in the nw himalaya evidence for tectonic extrusion coupled to fluvial erosion the himalayan crystalline core zone exposed along the sutlej valley india is composed of two high grade metamorphic gneiss sheets that were successively underthrusted and tectonically extruded as a consequence of the foreland directed propagation of crustal deformation in the indian plate margin the high himalayan crystalline sequence hhcs is composed of amphibolite facies to migmatitic paragneisses metamorphosed at temperatures up to 750 c at 30 km depth between eocene and early miocene during early miocene combined thrusting along the main central thrust mct and extension along the sangla detachment induced the rapid exhumation and cooling of the hhcs whereas exhumation was mainly controlled by erosion since middle miocene the lesser himalayan crystalline sequence lhcs is composed of amphibolite facies para and orthogneisses metamorphosed at temperatures up to 700 c during underthrusting down to 30 km depth beneath the mct the lhcs cooled very rapidly since late miocene as a consequence of exhumation controlled by thrusting along the munsiari thrust and extension in the mct hanging wall this renewed phase of tectonic extrusion at the himalayan front is still active as indicated by the present day regional seismicity and by hydrothermal circulation linked to elevated near surface geothermal gradients in the lhcs as recently evidenced in the himalayan syntaxes active exhumation of deep crustal rocks along the sutlej valley is spatially correlated with the high erosional potential of this major trans himalayan river this correlation supports the emerging view of a positive feedback during continental collision between crustal scale tectono thermal reworking and efficient erosion along major river systems

in memoriam herbert e wright jr 1917 2015 herbert e wright jr was one of the foremost quaternary scientists of the last century he made wide ranging contributions to our understanding of the late quaternary of north and south america europe asia and africa this was based largely on reconstructing palaeoenvironments from lake sediments and included the important implications for glacial vegetational fire and climatic history geoarchaeology and conservation many of his inter disciplinary research projects involved fieldwork with his graduate students and co workers from the university of minnesota where he created and led the renowned limnological research center perhaps his most outstanding contribution was as an instigator of the co operative holocene mapping project cohmap this triggered a paradigm shift in holocene climatic research involving the comparison of climate model simulations of past climates with palaeoclimatic data

quantitative kinematic flow analysis from the main central thrust zone nw himalaya india implications for a decelerating strain path and the extrusion of orogenic wedges quantitative kinematic indicators from the main central thrust zone mctz in the nw himalaya have been used to characterize the type of flow during deformation different generations of tension gashes have been rotated by variable angles with respect to the mylonitic foliation forming associated fringe folds these record the late stage brittle ductile flow and reveal that a strong pure shear component of deformation occurred throughout the mctz to characterize earlier deformation increments fabrics from highly deformed quartz ribbons were analyzed well developed shape and lattice preferred orientation patterns show a systematic change of the glide systems and suggest inverted palaeotemperatures within the mctz investigations of the c axes patterns reveal a strong asymmetry at the top of the mctz whereas the samples from the base of the mctz show almost perfectly symmetrical type i crossed girdles deformation within the mctz probably started close to simple shear flow at higher temperatures which progressively became a more general shear during cooling and ended in a pure shear dominated flow during the final stages of brittle ductile deformation i e a decelerating strain path using the higher himalaya crystalline as an example a kinematic model for the extrusion of crustal wedges above major thrust zones is suggested

enhanced ice sheet growth in eurasia owing to adjacent ice dammed lakes large proglacial lakes cool regional summer climate because of their large heat capacity and have been shown to modify precipitation through mesoscale atmospheric feedbacks as in the case of lake agassiz 1 several large ice dammed lakes with a combined area twice that of the caspian sea were formed in northern eurasia about 90 000 years ago during the last glacial period when an ice sheet centred over the barents and kara seas 2 blocked the large northbound russian rivers 3 here we present high resolution simulations with an atmospheric general circulation model that explicitly simulates the surface mass balance of the ice sheet we show that the main influence of the eurasian proglacial lakes was a significant reduction of ice sheet melting at the southern margin of the barents kara ice sheet through strong regional summer cooling over large parts of russia in our simulations the summer melt reduction clearly outweighs lake induced decreases in moisture and hence snowfall such as has been reported earlier for lake agassiz1 we conclude that the summer cooling mechanism from proglacial lakes accelerated ice sheet growth and delayed ice sheet decay in eurasia and probably also in north america

strontium isotopic resolution of magma dynamics in a layered intrusion kalka is one of about 14 major layered mafic ultramafic intrusions forming the giles complex in central australia1 the stratigraphical section of the kalka intrusion passes from a basal pyroxenite zone 450 m through norite and olivine gabbro zones 3 500 m to an uppermost anorthosite zone 800 m 2 further resolution into 21 cyclic units is derived from repeated mineral crystallization sequences and cyclical variation in plagioclase and olivine compositions a conventional interpretation would have a basaltic magma progressively fractionating as crystallization proceeded from mafic base to leucocratic top with periodic resetting to less evolved states by fresh incursions of primary magma or by convectional overturn within a sealed magma chamber however the reconnaissance isotopic data reported here indicate development of a more complex open system high initial 87sr 86sr ratios in conjunction with normal 143nd 144nd ratios demonstrate massive contamination by country rock within the main part of the intrusion3 furthermore great variation in sr initial ratios from 0 7049 to 0 7088 suggests substantial changes in magma composition beyond those induced by fractionation similar isotopic heterogeneity in the newer gabbros of scotland4 and the bushveld complex5 has been attributed to variable contamination or the emplacement of magma batches of differing composition

structures associated with superimposed inhomogenous shearing of basic dykes from finnmark norway abstract metadolerites within the olderfjord nappe of the kalak nappe complex of north norway show a wide range of structural relationships during d2 simple shear rotated the dykes from an initially orthogonal relationship towards parallelism with the sedimentary layering and boudinaged those dykes that rotated to within 25 of the sedimentary layering during d3 dykes which had been rotated to within 5 of the layering were folded those lying between 5 and 25 were boudinaged whilst those at angles between 25 and 75 had folds developed against their margins at higher angles of discordance little evidence of d2 or d3 deformation has been observed in other thrust sheets within the kalak nappe complex metabasites have generally been sheared into parallelism with the compositional banding by comparison with the olderfjord nappe the average d2 shear strain within the kalak nappe complex ranges between 5 and 10 the anomalous preservation of high angle discordant relationships in the olderfjord nappe reflects the preservation of a large scale low strain block mega boudin within the nappe samples taken from the mega boudin are more suitable for geochemical analysis for tectonic setting discrimination

simulating the amplification of orbital forcing by ocean feedbacks in the last glaciation according to milankovitch theory the lower summer insolation at high latitudes about 115 000 years ago allowed winter snow to persist throughout summer leading to ice sheet build up and glaciation1 but attempts to simulate the last glaciation using global atmospheric models have failed to produce this outcome when forced by insolation changes only2 3 4 5 these results point towards the importance of feedback effects for example through changes in vegetation or the ocean circulation for the amplification of solar forcing6 7 8 9 here we present a fully coupled ocean atmosphere model of the last glaciation that produces a build up of perennial snow cover at known locations of ice sheets during this period we show that ocean feedbacks lead to a cooling of the high northern latitudes along with an increase in atmospheric moisture transport from the equator to the poles these changes agree with available geological data10 11 12 13 14 15 and together they lead to an increased delivery of snow to high northern latitudes the mechanism we present explains the onset of glaciation which would be amplified by changes in vegetation in response to weak orbital forcing

the development of flanking folds during simple shear and their use as kinematic indicators abstract we used a numerical finite element simulation to model the formation of flanking folds around rotating planar structures e g veins faults or dykes in a non linear viscous medium during ideal simple shear if the planar structure is much more viscous than its host it does not deform and flanking folds with no displacement along the structure develop their vergence is consistent with the overall sense of shear however if the planar structure is much less viscous than its host strain is concentrated within the structure and a secondary shear zone is developed in which slip is opposite to the overall sense of shear then flanking folds develop that have a vergence which is incompatible with the drag on structure if the deflection of markers is not clearly preserved then such flanking folds can be easily misinterpreted as shear bands indicating a wrong shear sense the deflection of the foliation in flanking folds is very similar to deformed asymmetric pull aparts and can therefore help to interpret these otherwise ambiguous shear sense criteria because our model ideal simple shear boundary conditions fails to describe back rotation of planar structures in rocks we speculate that shear band geometries are indicative for general shear

simulated influences of lake agassiz on the climate of central north america 11 000 years ago eleven thousand years ago large lakes existed in central and eastern north america along the margin of the laurentide ice sheet the large scale north american climate at this time has been simulated with atmospheric general circulation models1 2 but these relatively coarse global models do not resolve potentially important features of the mesoscale circulation that arise from interactions among the atmosphere ice sheet and proglacial lakes here we present simulations of the climate of central and eastern north america 11 000 years ago with a high resolution regional climate model nested within a general circulation model the simulated climate is in general agreement with that inferred from palaeoecological evidence our experiments indicate that through mesoscale atmospheric feedbacks the annual delivery of moisture to the laurentide ice sheet was diminished at times of a large cold lake agassiz relative to periods of lower lake stands the resulting changes in the mass balance of the ice sheet may have contributed to fluctuations of the ice margin thus affecting the routing of fresh water to the north atlantic ocean a retreating ice margin during periods of high lake level may have opened an outlet for discharge of lake agassiz into the north atlantic a subsequent advance of the ice margin due to greater moisture delivery associated with a low lake level could have dammed the outlet thereby reducing discharge to the north atlantic these variations may have been decisive in causing the younger dryas cold event3 4

a precautionary note on shear bands as kinematic indicators the orientation of shear bands relative to foliations defined by elongated mineral aggregates is often used to determine the large scale sense of displacement in ductile faults data from contact strain zones at the bases of large overthrust complexes in the eastern alps and the betic cordilleras demonstrate that there is not always a simple geometrical relation between shear band orientation and sense of vorticity in bulk non coaxial flow in addition to single shear band sets that show displacements synthetic with the large scale displacement there are single sets with antithetic displacements and conjugate sets the last two observations are at variance with published data and interpretations and cast doubts on the general applicability of shear bands as indicators of large scale flow kinematics

role of orbitally induced changes in tundra area in the onset of glaciation the link between glacial interglacial cycles and changes in insolation due to variations in the earth s orbital parameters is well established1 4 but of the attempts to simulate incipient glaciation using three dimensional general circulation models gcms driven by orbital forcing alone5 10 only one10 has been successful gcm experiments7 11 show that reduced summer insolation 115 000 years ago during an interglacial to glacial climate shift produces sufficient high latitude cooling to cause expansion of tundra at the expense of boreal forest11 which in turn can induce more cooling11 14 here we show using a global climate model that the increase in surface albedo under snow covered conditions that results from a biome model estimate11 of tundra expansion 115 000 years ago is sufficient to induce glaciation over extreme northeastern canada if the additional cooling from this estimated tundra expansion induces further expansion then widespread glaciation occurs at latitudes above 65 n these results suggest that the climate feedback from high latitude tundra expansion might have contributed to the onset of the most recent glaciation

asymmetrical pull aparts and foliation fish as kinematic indicators abstract from field observations asymmetrical boudins pinch and swell structures and ellipsoidal volumes of oblique foliation segments in anisotropic rocks lacking competent layers are potentially useful kinematic indicators in shear zones they are derived by the modification of initially symmetrical pull aparts and or layering by superposed layer parallel shearing the response of boudins to such shear is a synthetic flow of those corner volumes lying within the extensional quadrants of the kinematic framework of the deformation into the inter boudin gaps type 1 asymmetrical pull aparts the response of pinch and swell structure is a back rotation of the swells to form type 2 asymmetrical pull aparts discrete extensional shears in the pinches may be either absent type 2a or present type 2b analogues of types 2a and 2b asymmetrical pull aparts occur in anisotropic materials with no compositionally distinct competent members where ellipsoidal volumes contain back rotated foliation segments foliation fish the field based kinematic interpretation was successfully tested by laboratory experiment wherein initially symmetrical trains of pre formed competent internally anisotropic boudins and pinch and swell structures were deformed in bulk simple shear a mechanical model is presented in terms of two material properties of the inclusions competence viscosity and internal rheological anisotropy in an attempt to account for the observed flow in the experiments and the deduced flow and kinematic significance of the natural examples in the presence of angular corners and associated geometrical stress concentrations competence viscosity controls the response to shear in the boudins in the absence of angular corners internal anisotropy controls the response to shear in the pinch and swell structures and in the foliation fish

upb and rbsr geochronology and geological evolution of the harts range ruby mine area of the arunta inlier central australia abstract supracrustal and meta igneous rock units from the ruby mine area of the harts range eastern arunta inlier central australia have been dated by the zircon u pb and rb sr total rock tr and mica methods a well defined zircon discordia for a weakly deformed specimen of the bruna granite gneiss yields an age of emplacement of 1748 4 5 ma thereby constraining the minimum age of the irindina supracrustal assemblage metapelitic gneiss within the supracrustals and a meta igneous ultramafic boudin from the associated harts range meta igneous complex yield highly discordant zircon data revealing a strong early palaeozoic overprint rb sr tr data from anorthositic gneisses associated with the ultramafic boudin are highly disturbed also apparently during the lower palaeozoic however rb sr model age calculations and the zircon u pb data suggest a maximum age of about 2000 ma for the supracrustal and meta igneous rocks and argue for new proterozoic crust formation zircon u pb data from a deformed pegmatite emplaced in the meta igneous complex yield an emplacement age of 520 4 5 ma further pointing to lower palaeozoic magmatism and deformation correlations of u content and calculated 206 pb 238 u age for the ultramafic boudin zircons suggest that new growth of low u zircons occurred during retrogression associated with this event the sr isotope systematics of the anorthositic gneisses can also be interpreted in terms of introduction of palaeozoic sr our data suggest lower palaeozoic possibly delamerian tectonothermal activity to be more important in the evolution of the harts range area than previously recognised on the other hand rb sr mica ages for deformed and undeformed pegmatites and tr isochrons for the latter show that pervasive tectonothernal activity had ceased by about 315 ma and that regional cooling occurred between about 345 and 325 ma local shear zone biotite resetting may have persisted to about 300 ma consistent with the previously recognised alice springs orogeny possible dilational pb loss in the bruna zircons occurred at about 103 ma

late brittle tectonics in a precambrian ductile belt evidence from brittle structures in the singhbhum shear zone eastern india abstract the singhbhum shear zone a 200 km long tectonic lineament passes close to the boundary between the archaean singhbhum craton and the proterozoic singhbhum mobile belt evidence from meso structures reveals development of this shear zone in a ductile tectonic regime 1 6 2 2 ga by thrusting of the northern block over the southern block along the n dipping shear surface a hitherto unrecognized late brittle tectonic regime 1 6 ga comprising two discrete phases is established from dynamic analysis of the fractures and veins in the shear zone episodic extension parallel and normal to the shear zone was a major consequence of this middle proterozoic 1 6 ga brittle tectonic episode and was associated with the intrusion of a swarm of basic dykes

superlakes megafloods and abrupt climate change about 8200 years ago the climate of much of the northern hemisphere cooled abruptly for a period of about 200 years in their perspective clarke et al examine the most likely culprit for this cooling an outburst of fresh water from a vast ice dammed glacial lake in north america the superlake had formed when the kilometers thick ice sheet covering much of north america disintegrated when the ice dam became unstable fresh water flooded from the lake into the north atlantic it remains unclear how this fresh water affected ocean circulation or whether the outburst occurred in more than one stage but the timing points strongly to the outburst flood as the trigger of the 8200 year climate event

hercynian crustal scale extensional shear zone in the pyrenees detailed structural analysis of metasedimentary rocks and gneisses in the hospitalet massif pyrenees has revealed that upright structures in low grade rocks are in a structurally higher position and older than gently inclined to flat lying structures in higher grade rocks the steep structures were sub vertically flattened during formation of the flat lying structures indicating that the higher grade rocks formed as part of a shear zone system a similar conclusion can be drawn from the relation between deformation and metamorphism in the higher grade rocks we suggest that the shear zone may have accommodated crustal extension

late glacial and holocene vegetational history of the altai mountains southwestern tuva republic siberia two lakes of glacial origin grusha at 2413 m and akkol at 2204 m are located on a broad high mountain plateau on the dry eastern end of the russian altai mountains in the republic of tuva just north of mongolia the present vegetation of the area is an open high mountain tundra steppe mosaic with patches of larix and pinus sibirica forests on north facing slopes 30 km northwest of the lakes alpine meadows occur near streams and snow patches pollen diagrams and radiocarbon dates 16 for grusha and 12 for akkol were used to reconstruct the vegetational history since the last glaciation the core from grusha contains about 50 cm of late glacial sediment of relatively low organic content mostly correlated with the younger dryas episode because of the high percentages of artemisia gramineae and chenopodiaceae along with a diversity of non arboreal pollen types indicating tundra steppe and meadow steppe in the earliest holocene starting with increased organic matter at about 195 cm 12 000 cal yr bp the artemisia gramineae chenopodiaceae pollen assemblage continued along with an increase in shrub betula but without the diversity of minor non arboreal types the interpretation of dry and warm climate at this time is supported by the unconformity at akkol implying a dried lake then after about 1000 years the increase of pinus sibirica p sylvestris picea obovata and abies sibirica suggests that forests developed in response to increased humidity first at the higher elevation grusha and then at akkol after about 6000 cal yr bp the role of forests decreased sharply in the area the picea and abies components almost disappeared as they did also in more westerly areas of the altai mountains steppe elements increased along with alpine herbs especially after 2000 cal yr bp as a result of cooling or increased aridity as well as probable human activity correlation of the vegetation sequence for the tuva sites with those to the west in the central altai mountains indicates that the late glacial open landscapes persisted in the earliest holocene but then were invaded by coniferous forests as a result of increased moisture and temperature associated with atlantic storm systems as well as with insolation enhanced asian monsoon cooling and drying after about 6000 cal yr bp brought the decline of abies and picea and the expansion of tundra steppe