simulations of two last glacial maximum ocean states two stable thermohaline circulation states were produced in simulations of the last glacial maximum lgm ocean both forced by the same prescribed atmosphere direct application of this atmosphere led to an equilibrium state with strong north atlantic deep water formation northern sinking state nss upon applying a weak freshwater flux anomaly to the north atlantic for a short time a state was entered with little north atlantic but significant southern ocean deep water formation southern sinking state sss a fully dynamic and thermodynamic iceberg trajectory model was developed and icebergs were seeded into both states distinctly different pathways were found particularly in the eastern atlantic where the sss icebergs tend to move south along the european coast while they move north in the nss while neither ocean state is claimed to be a definitive model for the lgm ocean paleoclimate data are more consistent with the characteristics of the nss

sheath fold development with viscosity contrast analogue experiments in bulk simple shear it has been shown experimentally that sheath folds can develop passively by kinematic amplification of precursor non cylindrical deflections however sheath folds are most commonly multilayers where rheological contrast can be expected non passive sheath folding therefore we studied a natural occurrence of sheath folds where layers show rheological contrast and used analogue modelling to investigate sheath fold development with viscosity contrast the initial deflection in the experiments was always hemispherical and planar layering parallel to the shear plane under the chosen experimental conditions the results show that 1 the viscosity contrast between layers should be lower than one order of magnitude 10 to generate a sheath fold in both higher and lower viscosity layers otherwise the higher viscosity layer behaves as effectively rigid 2 the lower viscosity layer develops into a tubular fold dragged by the higher viscosity layer that undergoes mixed strain low and shear plane parallel translation high 3 the ratio between ellipticities of external and internal ellipses r re ri significantly departs from 1 as the viscosity contrast increases e g r 0 6 for 7 5 in the case where the higher viscosity layer caps the lower viscosity layer this is in great contrast with passively developed sheath folds for which r 1 when the precursor deflection is hemispherical from the present experimental study and previous work investigating other variables we conclude that r should be cautiously used and that extraction of kinematical information from sheath fold geometry alone can be erroneous

modelling twentieth century global ocean circulation and iceberg flux at 48 n implications for west greenland iceberg discharge abstract we have used a coupled ocean iceberg model to study the variation in global ocean circulation and north atlantic iceberg flux from 1900 to 2008 the latter component of the study focused particularly on greenland icebergs feeding into the labrador current and past newfoundland the model was forced with daily heat freshwater and wind fluxes from the twentieth century reanalysis the reanalysis heat fluxes were shown to be offset from the shorter ncep reanalysis and a grid point correction was applied to this component of the forcing the model produces a generally realistic ocean circulation although with an enhanced atlantic meridional overturning largely due to the forcing the modelled iceberg flux at 48 n is well correlated with the long term observed flux when using a modelled iceberg discharge that varies in a similar fashion to the highly variable observed flux at 48 n from this model we infer changes in the spatial and temporal variability of iceberg calving from western greenland during the first third of the twentieth century the majority of modelled icebergs reaching 48 n derive from southern greenland while only after 1930 is the traditional perspective of a majority of such icebergs originating from baffin bay consistent with model results decadal scale changes in the dominant regional sources are found with oscillations between western greenland and northern baffin bay the latter origin was modelled to be most important in the last third of the twentieth century although west greenland sources have increased in importance in recent years the model correctly reproduces the pronounced late spring peak in flux at 48 n for southern greenland icebergs but has an approximately six month offset for icebergs from baffin bay most likely due to resolution issues leading to model icebergs not being delayed in shallow coastal waters whereas in reality they may be grounded for some time or trapped in coastal sea ice

tracing marine cryptotephras in the north atlantic during the last glacial period protocols for identification characterisation and evaluating depositional controls abstract tephrochronology is increasingly being utilised as a key tool for improving chronological models and correlating disparate palaeoclimatic sequences for many sedimentary environments however there is an increased recognition that a range of processes may impart a delay in deposition and or rework tephra these processes can affect the integrity of tephra deposits as time synchronous markers therefore it is crucial to assess their isochronous nature especially when cryptotephras are investigated in a dynamic marine environment a methodology for the identification and characterisation of marine cryptotephras alongside a protocol for assessing their integrity is outlined this methodology was applied to a wide network of north atlantic marine sequences covering the last glacial period a diverse range of cryptotephra deposits were identified and based on similarities in physical characteristics e g glass shard concentration profiles and geochemical homogeneity heterogeneity indicative of common modes of tephra delivery and post depositional reworking a deposit type classification scheme was defined the presence and dominance of different deposit types within each core allowed an assessment of spatial and temporal controls on tephra deposition and preservation overall isochronous horizons can be identified across a large portion of the north atlantic due to preferential atmospheric dispersal patterns however the variable influence of ice rafting processes and an interplay between the high eruptive frequency of iceland and relatively lower sedimentation rates can also create complex tephrostratigraphies in this sector sites within a wide sector to the south and east of iceland have the greatest potential to be repositories for isochronous horizons that can facilitate the synchronisation of palaeoclimatic records

sea ice and iceberg dynamic interaction 1 a model of iceberg motion has been implemented in the los alamos sea ice model cice individual bergs are tracked under the influence of winds currents sea surface tilt coriolis and sea ice forcing in turn sea ice is affected by the presence of icebergs primarily as obstacles that cause the sea ice to ridge on the upstream side or create open water on the downstream side of the bergs open water formed near icebergs due to sea ice ridging and blocking of sea ice advection increases level and ridged ice downstream of the bergs through increased frazil ice formation resulting anomalies in sea ice area and thickness compared with a simulation without icebergs are transported with the sea ice flow expanding over time although local changes in the sea ice distribution may be important for smaller scale studies these anomalies are small compared with the total volume of sea ice and their effect on climate scale variables appears to be insignificant

prediction of iceberg trajectories for the north atlantic and arctic oceans icebergs are a well known hazard for shipping their study also provides information about diverse geophysical processes as varied as ocean circulation air sea fluxes calving rates of glaciers or the mass balance of ice sheets as a first step to obtaining this information from iceberg data we have developed a model of iceberg drift driven by ocean and atmospheric forcing derived from general circulation models we have applied the drift model to a distribution of typical icebergs released from the main tidewater glaciers of the north atlantic and arctic oceans we demonstrate that the main driving force of iceberg motion is rooted in the unsteady component of oceanic advection from simulated trajectories we are able to reproduce the observed southwards limit of iceberg penetration and demonstrate sometimes surprising geographical links between iceberg origin and ultimate melting zones

topography and palaeogeographic evolution of a middle miocene foredeep basin plain northern apennines italy abstract the marnoso arenacea basin was a narrow northwest southeast trending foredeep of middle late miocene age bounded to the southwest by the apennine thrust front the basin configuration and evolution were strongly controlled by tectonics geometrical and sedimentological analysis of serravallian turbidites deposited within the marnoso arenacea foredeep combined with palaeocurrent data turbidite flow provenance reflection and deflection identify topographic irregularities in a basin plain setting in the form of confined troughs the more internal mandrioli sub basin and the external s sofia sub basin separated by an intrabasinal structural high this basin configuration was generated by the propagation of a blind thrust striking northwest to southeast parallel to the main trend of the apennines thrust belt ongoing thrust induced sea bed deformation marked by the emplacement of large submarine landslides drove the evolution of the two sub basins in an early stage the growth and lateral propagation of a fault related anticline promoted the development of open foredeep sub basins that were replaced progressively by wedge top or piggy back basins partially or completely isolated from the main foredeep meanwhile the depocenter shifted to a more external position and the sub basins were incorporated within an accretionary thrust belt

experimental decipherment of the soft sediment deformation observed in the upper part of the talchir formation lower permian jharia basin india abstract four types of soft sediment folds of distinct geometry can be recognized in the upper part of the talchir formation lower permian of jharia basin india these folds on systematic examination indicate some events of progressive deformation experimental study reveals that if a layered stack of clay and overlying sand is allowed to flow slowly down a slope differential velocity due to viscosity contrast leads to the deformation of the rheologic interface the sharp planar contact gradually becomes wavy leading to the development of round hinged folds involving sediments adjacent to it with the advancement of the flow these folds gradually become overturned with the rotation of the axial plane in the direction of flow computer simulation suggests that progressive deformation of these folds by simple shearing may lead to the formation of tight isoclinal folds which on dislocation along intrastratal normal faults may lead to the development of rootless isoclinal folds the sheath folds observed in the studied section also indicate accentuation of the curved hinge due to simple shearing the spatial distribution of these fold types in conjunction with the inferred direction of progressive deformation indicate basinward translation of the slump slice if the same stack of sediments rapidly flows down the slope the waveform generated at the interface quickly breaks in the form of roll up recumbent fold due to kelvin helmholtz instability

progressive development of structures in a ductile shear zone in the singhbhum shear zone of eastern india successive generations of folds grew in response to a progressive ductile shearing during this deformation a mylonitic foliation was initiated and was repeatedly transposed the majority of fold hinges were formed in an arcuate manner at low angles to the y axis in an e w trending subhorizontal position and major segments of the fold hinges were then rotated towards the down dip northerly plunging x axis the striping and intersection lineations were rotated in the same manner the down dip mylonitic lineation is a composite structure represented by rotated early lineations and newly superimposed stretching lineations the consistent asymmetry of the folds the angular relations between c and s surfaces and the evidence of two dimensional boudinage indicate that the deformation was non coaxial but with a flattening type of strain with 1 2 the degree of non coaxiality varied both in space and time from the progressive development of mesoscopic structures it is concluded that the 2 3 km wide belt of ductile shear gave rise to successive anastomosing shear zones of mesoscopic scale when a new set of shear lenses was superimposed on already sheared rocks the preexisting foliation generally lay at a low angle to the lenses no new folds developed where the acute angle was sympathetic to the sense of shear displacements where the acute angle was counter to the sense of shear the pre existing foliation lying in the instantaneous shortening field was deformed into a set of asymmetric folds

flow variation in layered rocks subjected to bulk flow of various kinematic vorticities theory and geological implications abstract flow variation is studied analytically in rocks with layers of varied competence and varied orientation with respect to constant bulk flow fields of given kinematic vorticity 0 w k 1 the results explicitly demonstrate that flow varies significantly from layer to layer and with time in any individual layer even though the bulk flow is constant any flow regime can occur and the instantaneous stretching axes isa spin all the time reverse sense of non coaxiality can occur in the same layer at different times and in different layers at the same time this study further reveals the significance of heterogeneity and non steadiness in natural flows the assumption of a time independent flow history for natural deformation is highly unrealistic the spinning of the instantaneous stretching axes isa and the variation of flow regimes have bearings on the progressive development of fabrics and structures such as folded boudins refolded folds in shear zones and double rotation of porphyroclasts that are often interpreted as being due to multi deformational events

structural analysis of sheath folds in the sylacauga marble group talladega slate belt southern appalachians remnant blocks of marble from the moretti harrah dimension stone quarry provide excellent exposure of meter scale sheath folds tubular structures with elliptical cross sections 4 ryz 5 are the most common expression of the folds the tubes are elongate subparallel to stretching lineation and are defined by centimeter scale layers of schist eccentrically nested elliptical patterns and opposing asymmetry of folds s and z are consistent with the sheath fold interpretation sheath folds are locally numerous in the moretti harrah quarry but are not widely distributed in the sylacauga marble group reconnaissance in neighboring quarries provided no additional observations the presence of sheath folds in part of the talladega slate belt indicates a local history of plastic non coaxial deformation such a history of deformation is substantiated by petrographic study of an extracted hinge from the moretti harrah quarry the sheath folds are modeled as due to passive amplification of initial structures during simple shear using both analytic geometry and graphic simulation as indicated by these models relatively large shear strains y 9 and longitudinal initial structures are required the shear strain presumably relates to nw directed displacement of overlying crystalline rocks during late paleozoic orogeny

an analytical model of iceberg drift abstractthe fate of icebergs in the polar oceans plays an important role in earth s climate system yet a detailed understanding of iceberg dynamics has remained elusive here the central physical processes that determine iceberg motion are investigated this is done through the development and analysis of an idealized model of iceberg drift the model is forced with high resolution surface velocity and temperature data from an observational state estimate it retains much of the most salient physics while remaining sufficiently simple to allow insight into the details of how icebergs drift an analytical solution of the model is derived which highlights how iceberg drift patterns depend on iceberg size ocean current velocity and wind velocity a long standing rule of thumb for arctic icebergs estimates their drift velocity to be 2 of the wind velocity relative to the ocean current here this relationship is derived from first principles and it is shown that the relationship holds in the limit of

mid to late cretaceous ductile deformation and thermal evolution of the crust in the northern dome rock mountains arizona abstract mid to late cretaceous basement involved deformation in the maria tectonic belt of southeastern california and west central arizona varies in style from low angle overthrusts to km scale fold nappes to establish the relationship between tectonite fabrics and thermal evolution of the rocks structural and petrographic analysis was conducted in the northern dome rock mountains where the two different styles converge a penetrative northeast dipping foliation forms the most conspicuous fabric in an 5 km wide zone of distributed ductile deformation within the broader zone of ductile deformation northeast dipping shear zones contain evidence for both southwest and northeast directed deformation textural analysis of mylonitic rocks reveals that southwest directed deformation occurred under prograde conditions whereas northeast directed deformation occurred under retrograde conditions southwest directed deformation resulted in tubular extreme sheath fold development on a kilometer scale in the footwall of a reverse sense shear zone northeast directed deformation cuts the map scale tubular fold and is inferred to have developed as a manifestation of gravitational collapse of rocks mechanically weakened by fluid infiltration and upper greenschist to lower amphibolite facies metamorphism we propose that the mode and degree of fluid infiltration rather than temperature or structural level alone controlled the style of deformation in the maria belt

scale invariant sheath folds in salt sediments and shear zones abstract sheath folds are developed in a broad spectrum of geological environments in which material flow occurs including gravity driven surficial deformation in ignimbrites unconsolidated sediments and salt together with deeper level ductile shear zones in metamorphic rocks this study represents the first geometric comparison of sheath folds in these different settings across a wide range of scales elliptical closures defining eye folds represent y z cross sections through highly curvilinear sheath folds our analysis of the published literature coupled with field observations reveals remarkably similar ellipticities r yz for sheath folds in metamorphic shear zones r yz 4 23 salt r yz 4 29 sediment slumps r yz 4 34 glaciotectonites r yz 4 48 and ignimbrites r yz 4 34 nested eye folds across this range of materials n 1800 reveal distinct and consistent differences in ellipticity from the outer r yz to the inner most r y z elliptical rings of individual sheath folds the variation in ratios from outer to inner rings r r yz r y z in gravity driven surficial flows typically displays a relative increase in ellipticity to define cats eye folds r r yz that are remarkably constant r 2 0 99 across 9 orders of magnitude sheath y axes range from 0 1 mm to 75 km our findings lead us to conclude that the geometric properties of sheath folds are scale invariant and primarily controlled by the type and amount of strain with r also reflecting the rheological significance of layering associated with original buckle fold mechanisms the scaling pattern of sheath folds reflects the length scales of the precursor buckle folds and width of deformation zones across a broad range of materials and environments with continued deformation the layering marking the original folds may become increasingly passive to define sheath folds these empirical relationships suggest sheath folding is a fundamental mode of viscous response across a broad spectrum of materials strain rates and scales encompassing a variety of deformation settings

source to sink statistical identification of regional variations in the mineralogy of surface sediments in the western nordic seas 58 n 75 n 10 w 40 w the non clay and clay mineralogies of the 2 mm sediment fraction from 157 seafloor samples in the western nordic seas is calculated by quantitative x ray diffraction analysis our sample coverage includes the iceland shelf the ice impacted east greenland margin and the greenland iceland and irminger seas the bedrock geology of source areas includes precambrian basement rocks devonian red beds and younger marine and fluvial sediments and large outcrops of early tertiary to recent flood basalts and other volcanic facies sediment erosion and transport in the area are associated with ice sea ice and icebergs and glacial meltwater and fluvial sediment plumes we use principal component analysis fuzzy k means clustering and discriminant function analysis to reduce the original 31 minerals 157 sites matrix to more manageable 5 regional coherent clusters of sites which are well discriminated by the weight of six minerals namely quartz pyroxene illites magnetite and maghemite amorphous silica e g glass and muscovite the close spatial relationship between bedrock geology and discrete cluster membership stresses the importance of this variable and indicates that far distant sediment transport by sea ice icebergs or other mechanisms although occurring does not radically change the mineral composition of present day sea floor sediments our improved knowledge of the regional mineral assemblages allows for tracking distinct eastern greenland and icelandic sources

planar non planar and refolded sheath folds in the phulad shear zone rajasthan india abstract progressive ductile shearing in the phulad shear zone of rajasthan india has produced a complex history of folding with development of planar non planar and refolded sheath folds there are three generations of reclined folds f 1 f 2 and f 3 with a striping lineation l 1 parallel to the hinge lines of f 1 the planar sheath folds of f 1 have long subparallel hinge lines at the flanks joining up in hairpin curves at relatively small apices l 1 swerves harmoniously with the curving of f 1 hinge line there is a strong down dip mineral lineation parallel to the striping lineation in most places but intersecting it at apices of first generation sheath folds both the striping and the mineral lineation are deformed in u patterns over the hinges of reclined f 2 and f 3 folding of axial surfaces and hinge lines of earlier reclined folds by later folds was accompanied by very large stretching and led to the development of non planar sheaths the reclined folds of all the three generations were deformed by a group of subhorizontal folds each generation of fold initially grew with the hinge line at a very low angle with the y axis of bulk non coaxial strain and was subsequently rotated towards the down dip direction of maximum stretching the patterns of deformed lineations indicate that the stretching along the x direction was extremely large much in excess of 6000 percent

vorticity determination distribution partitioning and the heterogeneity and non steadiness of natural deformations abstract an investigation on vorticity in general heterogeneous and non steady flows is presented the bulk vorticity of a flow is solely determined by the imposed boundary conditions and it exerts no constraints on local flow the distribution of bulk vorticity in different rheological domains and the partitioning of the distributed vorticity into shear induced vorticity and spin are both time dependent this indicates that heterogeneous flows are inevitably non steady in a quantitative example it is demonstrated that even though the imposed boundary movement is a constant simple shear local flow can be non steady spinning and non coaxial super simple shear flow with internal kinematic vorticity number greater than i occurs but the finite strain does not necessarily pulsate the geological implications of this investigation are discussed the significance of heterogeneity and non steadiness in natural deformations are emphasized it is concluded that vorticity distribution and partitioning is the link between different scale structures the internal kinematic vorticity number is not a criterion of finite strain oscillation

variations in felsic versus mafic sources in the western nordic seas during mis 1 to mis 4 abstract the goal of this paper is to employ quantitative methods to first describe the spatial variations in the quantitative non clay and clay mineral composition of seafloor surface sediments secondly use those data to describe the temporal variations in mineral compositions of nine cores that extend variously between the present day and 85 ka bp and thirdly describe regional variations in the transport of felsic versus mafic rich glacial marine sediments at four sites close to denmark strait k mean clusters of recent seafloor samples n 156 distinguished four mineral facies which characterize a range of mafic to felsic compositions these samples serve as a training set to identify the down core mineral facies in eight cores n 1053 the degree of provenance mixing is measured by fuzzy clustering on an unsupervised cluster analysis classification decision trees validate the mineral facies only 6 misclassified and indicate that variations in the weight of pyroxene are of primary importance in defining cluster membership mineral compositions identify two end member core sites jm07 174gc greenland sea and lo9 18 reykjanes ridge while the cores north and south of the greenland faroes ridge cores ps2644 md99 2274 2280 and 2323 are a mixture of mafic rich and felsic rich mineral facies variations in magnetic susceptibility and quartz wt exhibit coherent runs of values above or below their medians in three of the cores occurred indicating that these cannot be ascribed to random variations p

planar dispersion of folds in ductile shear zones and kinematic interpretation of fold hinge girdles abstract as indicated by theory and multiple case studies hinge lines of minor folds in ductile shear zones are dispersed in a plane that is incompletely rotated toward mylonitic foliation this yields a geometric relationship between the fold hinge girdle and mylonitic foliation that can be used to determine sense of shear statistical study of published data from various shear zones and of new data from the blue ridge thrust complex reveals a small to moderate angle between the best fit plane to hinge lines and mean mylonitic foliation this angle is 4 9 in eight of 12 test cases moderate angles 22 34 are associated with nappes kinematic analyses of 12 fold hinge girdles indicate the sense of shear otherwise determined for host shear zones a theoretical model of contemporary fold rotation in simple shear closely simulates the geometric relationships described above for natural folds using this model large shear strains 10 are implied by the 4 9 angle between the fold hinge girdle and mylonitic foliation

relationship between tidewater glacier calving velocity and water depth at the calving front an analysis of the relationship between iceberg calving rates and water depth has been completed for 22 tidewater glaciers a linear relationship provides reasonable accuracy with a correlation coefficient of 0 85 for all tidewater glaciers examined whether they be polar or temperate the polar glaciers have a slightly lower calving rate for a given water depth this relationship indicates a lower calving rate for water depths over 50 m than determined by brown and others 1982 it is based only on glaciers or ice streams and cannot be applied to ice shelves

a comparison between multiproxy and historical data ad 1990 1840 of drift ice conditions on the east greenland shelf 66 n the reduction in sea ice along the se greenland coast during the last century has severely impacted ice rafting to this area in order to reconstruct ice rafting and oceanographic conditions in the area of denmark strait during the last 150 years we conducted a multiproxy study on three short 20 cm sediment cores from outer kangerdlugssuaq trough 300 m water depth the proxy based data obtained have been compared with historical and instrumental data to gain a better understanding of the ice sheet ocean interactions in the area a robust chronology has been developed based on 210pb and 137cs measurements on core po175gkc 9 66 2 n 32 w and expanded to the two adjacent cores based on correlations between calcite weight percent records our proxy records include sea ice and phytoplankton biomarkers and a variety of mineralogical determinations based on the 2 mm sediment fraction including identification with quantitative x ray diffraction ice rafted debris counts on the 63 150 m sand fraction

geology geochemistry and tectonic setting of archaean greywackes from karnataka nucleus india more than 80 of the area covered by the chitradurga and shimoga schist belts 3 0 2 6 ga of karnataka nucleus india is made up of greywackes feldspathic lithic and quartzose greywackes are deposited respectively along the western northern and the eastern margin of the chitradurga schist belt greywackes of the shimoga schist belt show a large compositional scatter between feldspathic and lithic types compositional differences indicate variable provenance for the different localities tonalite trondhjemite gneisses ttg mafic ultramafic schists basic intermediate acid volcanic rocks sedimentary rocks like banded iron formation quartzites and cherts and k granites in minor quantity were exposed in the source area volcanic debris was also contributed from contemporaneous volcanism in the basin greywackes in this region are invariably interbedded with fine grained biotite rich phyllites and schists marked compositional differences are observed between the greywackes and the interbedded phyllites the greywackes have k2o na2o 1 whereas phyllites show large variation in k2o na2o ratios ranging from 1 5 to 15 0 high cr ni content of the greywackes and phyllites of the chitradurga belt are indicative of an important contribution from ultramafic rocks light ree enrichment and negative eu anomalies are particularly pronounced in the mudpacks of the greywackes represented by phyllite the ree and other chemical data such as la th ti zr and k2 mgo ratios indicate that these greywackes were supplied from a juvenile continental nucleus and were laid down on a simatic and converging active continental margin an archaean miniplate model with low angle subduction and sedimentary basins of moderate size is proposed to explain the geochemical and tectonic features of the schist belts

multi scale analysis of proterozoic shear zones an integrated structural and geophysical study abstract structural mapping of poorly exposed shear zone outcrops is integrated with the analysis of aeromagnetic and bouguer gravity data to develop a multi scale kinematic and relative overprinting chronology for the palaeoproterozoic tallacootra shear zone australia d 2 mylonitic fabrics at outcrop record kimban aged ca 1730 1690 ma n s shortening and correlate with sz 1 movements overprinting d 3 sinistral shear zones record the partitioning of near ideal simple shear and initiated riedel to regional scale sz 2 strike slip on the tallacootra shear zone sz 2 previously undocumented ne sw extension led to the emplacement of aplite dykes into the shear zone and can be correlated to the ca 1595 1575 ma hiltaba magmatic event d 4 dextral transpression during the ca 1470 1450 ma coorabie orogeny reactivated the tallacootra shear zone sz 2 r4 exhuming lower crust of the northwestern fowler domain within a positive flower structure this latest shear zone movement is related to a system of west dipping shear zones that penetrate the crust and sole into a lithospheric detachment indicating wholesale crustal shortening these methods demonstrate the value of integrating multi scale structural analyses for the study of shear zones with limited exposure

geometric analysis and significance of mesoscopic shear zones in the precambrian gneisses around the kolar schist belt south india mesoscopic shear zones are well preserved in the gneisses around the kolar schist belt located in the eastern part of the northern granite greenstone association of the south indian precambrian terrain the majority of these shear zones are of ductile to brittle ductile type with both dextral and sinistral senses of movement the detailed geometry and large scale mapping show that sinistral shear zones strike dominantly nw whereas dextral shear zones strike dominantly ne and form a conjugate pair the bisectors of statistically preferred orientations of these two sets of shear zones indicate that they developed in response to an e w subhorizontal compression which can be correlated to the late stage of the f2 folding seen inside the schist belt displacement vectors along these shear zones are invariably subhorizontal precluding the possibility of overthrusting or underthrusting relationship between the schist belt and surrounding gneissic terrains if the schist belt is a major shear zone the displacement across it can not be more than 15 km

the structural evolution of sheath folds a case study from cap de creus abstract it has long been recognised that within zones of intense non coaxial deformation fold hinges may rotate progressively towards the transport direction ultimately resulting in highly curvilinear sheath folds however there is a surprising lack of detailed and systematic field analysis of such evolving sheath folds this case study therefore focuses on the sequential development of cm scale curvilinear folds in the greenschist facies el llimac shear zone cap de creus spain this simple shear dominated dextral shear zone displays superb three dimensional exposures of sheath folds defined by mylonitic quartz bands within phyllonite increasing amounts of fold hinge curvature are marked by hinge segments rotating into sub parallelism with the mineral lineation lm whilst the acute angle between the axial planar hinge girdle and foliation also displays a sequential reduction although lm bisects the noses of sheath folds it is also clearly folded and wrapped around the sheath hinges lm typically preserves a larger angle with the fold hinge on the lower limb l compared to the upper u limb l u suggesting that lm failed to achieve a steady orientation on the lower limb adjacent sheath fold hinges forming fold pairs may display the same sense of hinge arcing to define synthetic curvature or alternatively opposing directions of antithetic curvature such patterns reflect original buckle fold geometries coupled with the direction of shearing the ratio of long short fold limbs decreases with increasing hinge curvilinearity indicating sheath folds developed via stretching of the short limb rather than migrating or rolling hinge models this study unequivocally demonstrates that both hinges of fold pairs become curvilinear with sheaths closing in the transport direction recording greater hinge line curvilinearity compared to adjacent return hinges this may provide a useful guide to bulk shear sense

environment and structural controls on the intrusion of the giant rare metal greenbushes pegmatite western australia the greenbushes pegmatite district has been the main center of production of alluvial tin in western australia since the beginning of this century and more recently for the production of tin tantalum and lithium from one of the largest rare metal pegmatite deposits in the world the intrusive rocks in the greenbushes pegmatite district are concentrated along the ancient crustal scale donnybrook bridgetown shear zone analogous to the present san andreas fault system and are characterized by steeply dipping planar mylonitic fabrics with horizontal stretching lineations asymmetric folds asymmetric pressure shadows and shear bands suggesting sinistral strike slip movement the greenbushes pegmatite occurs in a higher temperature and higher pressure metamorphic terraine than would be expected for pegmatites containing rare element mineralization the pegmatite contains the same shear fabrics as its host rocks and has evidence for syntectonic crystallization of minerals such as tourmaline fantalite garnet and cassiterite it is proposed that intrusion of the greenbushes pegmatite magma was controlled by the donnybrook bridgetown shear zone any melts or fluids present during movement along the shear zone would have been channeled into it leading to intrusion of the pegmatite the fluid pressure in the pegmatite magma may then have increased causing further failure zones of structural weakness and further intrusion of pegmatite generally parallel to the mylonitic fabric in the shear zone according to present models and classifications the greenbushes pegmatite group should be unmineralized however it is apparent from this study that giant rare metal pegmatites can occur in higher grade metamorphic terranes and that these types of pegmatites need not have obvious parental granitoids they may contain a variety of mineralization and are likely at least in the archean to be associated with tectonism along crustal scale fault systems

magnetic characterisation of present day deep sea sediments and sources in the north atlantic the north atlantic plays a key role in global climate through formation of north atlantic deep water which drives the thermohaline circulation to understand past and future climate change it is essential to understand the processes occurring within the present day north atlantic but ocean wide studies of present day sediment distribution and sources are relatively sparse here we use magnetic measurements to characterise the surface sediments of the north atlantic and identify the major climatic and oceanographic controls on their magnetic signatures the magnetic data and subsequent cluster analysis identify distinct spatial patterns of sediment sources and transport pathways much of the sedimentary magnetic signal appears to be controlled by detrital inputs especially of windblown dust and ice rafted debris ird with a range of different sources these sediment transport pathways can be validated by making direct comparison of the sediment magnetic properties with source rocks and soils and with iceberg trajectory and observation data the spatial distributions of the ird dominated sediments substantiate those mapped previously using lithological tracers provide additional spatial information on sediment sources and pathways and suggest that deep water currents are less significant than proposed in controlling present day sediment mineralogy and distribution the data form a present day basis for comparison with glacial stage ird and dust distributions and modes of ocean circulation

southern greenland glaciation and western boundary undercurrent evolution recorded on eirik drift during the late pliocene intensification of northern hemisphere glaciation abstract we present new sedimentological and environmental magnetic records spanning 3 2 2 2 ma during the intensification of northern hemisphere glaciation from north atlantic integrated ocean drilling program site u1307 on eirik drift our new datasets and their high fidelity age control demonstrate that while inland glaciers and potentially also at times restricted iceberg calving margins have likely existed on southern greenland since at least 3 2 ma persistent and extensive iceberg calving glacial margins were only established in this region at 2 72 ma 300 kyr later than in northeastern and eastern greenland despite a dramatic increase in greenland sourced ice rafted debris deposition on eirik drift at this time contemporaneous changes in the bulk magnetic properties of site u1307 sediments and a reduction in sediment accumulation rates suggest a decrease in the delivery of greenland sourced glaciofluvial silt to our study site we attribute these changes to a shift in depositional regime from bottom current dominated to glacial ird dominated between 2 9 and 2 7 ma in response to a change in the depth of the flow path of the western boundary undercurrent relative to our study site

anatomy of the mondonedo nappe basal shear zone nw spain this paper analyses a major shear zone from the iberian hercynian belt which forms the basal thrust of the mondonedo nappe the shear zone developed by ductile deformation under amphibolite facies metamorphic conditions and later by brittle ductile deformation in greenschists facies folds in the shear zone are asymmetric very tight 1c or similar class and frequently developing sheath geometries the sheath folds originated by non coaxial flow superimposed on earlier irregularities the fabric of quartzitic rocks in the shear zone changes from bottom to top from ultramylonites through blastomylonitic rocks to non mylonitic tectonites c axis fabrics vary across the shear zone but show a dominant monoclinic symmetry the blastomylonitic rocks include the fabrics representing the highest temperatures the main foliation of the schists results from flattening of an earlier foliation recording occasional microfolds the use of different kinematic criteria has allowed an analysis of their validity as well as an assessment of movement direction towards the foreland of the orogen

a robust multisite holocene history of drift ice off northern iceland implications for north atlantic climate an important indicator of holocene climate change is provided by evidence for variations in the extent of drift ice a proxy for drift ice in iceland waters is provided by the presence of quartz q

the nature of miocene amazonian epicontinental embayment high frequency shifts of the low gradient coastline a sedimentological and ichnological data set that covers the lower upper miocene sedimentary series of western amazonian foreland basin indicates that widespread restricted marine ingressions shaped western amazonia throughout the miocene the late lower early upper miocene sedimentary series pebas formation consists of stacked 3 to 10 m thick tidally influenced brackish to freshwater bay margin sequences the overlying upper miocene post pebas strata bear tidally influenced low salinity channel deposits that are interbedded with continental deposits the data suggest that several tens of high frequency ingressions reached the basin during the miocene the ingressions were shallow and restricted and were interspersed with rapid progradation along with the prograding shorelines the continental environments swamps lagoons floodplains and forests constrained the extent of the marginal marine embayment consequently the miocene marginal marine and continental strata are closely interbedded throughout the basin these results refine the recent depositional models for miocene amazonia and challenge the theory that marine ingressions shaped the area only during one brief time interval late middle early late miocene during the epoch much of recent literature has documented fossils of mangrove pollen brackish euryhaline fish and brackish water ostracods brackish water trace fossil assemblages and tidal deposits from various miocene stratigraphic levels commonly these data sets are collected from the same outcrops as those for which data sets imply freshwater conditions we propose that these seemingly contrasting data sets can be unified if the repetitive nature of the ingressions is considered and all the paleoenvironmental data are presented in a detailed lithological and stratigraphical context

a model of icebergs and sea ice in a joint continuum framework the ice melange a mixture of sea ice and icebergs often present in front of outlet glaciers in greenland or ice shelves in antarctica can have a profound effect on the dynamics of the ice ocean system the current inability to numerically model the ice melange motivates a new modeling approach proposed here a continuum sea ice model is taken as a starting point and icebergs are represented as thick and compact pieces of sea ice held together by large tensile and shear strength selectively introduced into the sea ice rheology in order to modify the rheology correctly an iceberg tracking procedure is implemented within a semi lagrangian time stepping scheme designed to exactly preserve iceberg shape through time with the proposed treatment sea ice and icebergs are considered a single fluid with spatially varying rheological properties mutual interactions are thus automatically included without the need for further parametrization an important advantage of the presented framework for an ice melange model is its potential to be easily included within sea ice components of existing climate models

thermal energy conservation in icebergs and tracking by temperature calculations using a two dimensional numerical model which simulates the heat balance and temperature distribution of icebergs show that the temperature in the central region of an iceberg is almost unaffected by the thermal conditions imposed on its boundary hence the original temperature of the iceberg at the time of calving is retained in its core owing to the insulating quality of the ice when correlated with glacioclimatic conditions this temperature conservation may be used to backtrack an iceberg to its parent ice cliff nine different icebergs were manually temperature profiled in the barents sea during late winter 1988 1990 a scenario is considered wherein one of these icebergs under the forces of winds and currents is assumed to set off from franz josef land in late autumn 1987 the numerical heat balance model is used to compute the temperature distribution heat balance and ablation of this iceberg as it drifts through the barents sea the calculations show that the heat balance of the part of the iceberg exposed to air has marginal influence on the ablation and thus on the expected lifetime of the iceberg ablation of the submerged body also occurs during winter when the water temperature is well below the melting point of glacial ice this is caused by freezing point depression leading to an ablation of about 0 5 m per month of the underside and sidewalls of the submerged body for water temperatures above 0 6 c the ablation results in a steep temperature gradient reaching the core temperature 3 m into the ice with a tolerance of 0 4 c the temperature is virtually free of any thermal boundary influence 12 m into the ice the simulated and measured temperature profiles of the iceberg are compared with measurements of the thermal regime of austfonna nordaustlandet and the churlenis ice dome on franz josef land the model calculations indicate that the ice cliffs on svalbard can be excluded as parent ice cliffs for these icebergs and that the core temperature of the bergs appears to be a conservative tracer

the sizes frequencies and freeboards of east greenland icebergs observed using ship radar and sextant the scoresby sund fjord system east greenland contains the most productive fast flowing outlet glaciers draining east from the greenland ice sheet calving 18 km3 a 1 of icebergs the sizes frequencies and freeboards of 1900 icebergs were measured from f s polarstern using ship x band radar and sextant radar beam spreading exaggerates iceberg width by 60 m per nautical mile of range beyond the first mile data sets on iceberg size e g that collated for antarctic icebergs collected using ship radars which do not take this effect into account will overestimate iceberg dimensions significantly the location and concentration of icebergs within the fjord complex can be explained by 1 the locations of the principal source glaciers and 2 fjord topography and bathymetry iceberg concentration maximum 0 6 icebergs km 2 declines with distance from the major iceberg sources we found that 69 of icebergs within the fjord system are 1 km in length the largest is 2 7 km long icebergs become spread more evenly over the range of size classes in the outer fjord and shelf modal iceberg keel depth calculated from freeboard measurements is 4 500 m in the inner fjords shifting to lower values in the outer fjords reflecting shallower bathymetry radar measurements of iceberg width cannot be used to infer keel depths accurately because width and keel depth are only weakly correlated comparison between freeboards and keel depths for icebergs from east greenland and the barents sea indicates that the iceberg source i e floating or grounded exerts a fundamental control on iceberg dimensions the drift pattern of icebergs is from the head to the mouth of the fjord system although fjord bifurcations bathymetry and currents provide additional complications the trends in observed iceberg size and frequency and in inferred keel depth in the scoresby sund region are likely to be applicable to other fjords and shelves around greenland

sheath folds as discriminators of bulk strain type abstract nested elliptical closures defining eye folds represent classic y z cross sections through highly curvilinear sheath folds generated during intense ductile deformation in metamorphic rocks systematic analysis of 1425 such eye folds based on our own field observations and examples from the published literature reveals distinct and consistent differences in ellipticites measured from the outer r yz to the inner most r y z elliptical rings of individual sheaths the variation in overall aspect ratios from outer to inner rings is defined as r where r r yz r y z and may display a relative increase or decrease in ellipticity to define cats eye r bulls eye r 1 fold patterns respectively layer thicknesses may also be measured along the y axis parallel to the axial surface t y and at 90 to this along the z axis t z to define the ratio of t yz sheath folds generated during broadly simple shear deformation k 1 display mean r yz 4 61 t yz 3 31 and cats eye folds r 0 69 sheath folds generated during general shear also display cats eye folds with identical mean r 0 69 values but greater thickness variations and elliptical ratios t yz 4 35 r yz 5 76 thus within both simple and general shear dominated deformations the overall variation in layer thickness t yz and ellipticity of eye folds r yz increases with increasing deformation and a greater component of pure shear whilst the r value remains constant and reflects original fold patterns sheath folds formed during constrictional k 1 deformation display markedly lower aspect ratios r yz 2 42 and thickness variations t yz 2 94 together with distinctive bulls eye folds r 1 23 these empirical relationships suggest fundamental and universal constraints on curvilinear fold generation across this broad spectrum of deformation types and allow sheath folds to act as both effective 95 consistent and robust discriminators of bulk strain type

the three dimensional shape and localisation of deformation within multilayer sheath folds abstract sheath folds are widely believed to develop by the passive geometric amplification of folds in which layering has no mechanical influence during non coaxial deformation where layering becomes rheologically significant then active sheath folds may form in which the inner nose of the sheath decouples along detachments and undergoes a translation relative to the outer folded layers that surround and envelope the nose we present a systematic 3 d analysis of multiple folded layer geometries in a serially sectioned natural sheath fold weak layers which are reactivated as detachments define different shaped sheath folds relative to other layers with the sense of cut off along detachments reversing across the axial surface as the inner fold nose has protruded into the surrounding sheath envelope detachment layers are more tightly folded meaning that such active sheath folds are non similar shapes the obliquely oriented bifurcating en echelon nature of the fold hinges developed in adjacent layers suggest that pre cursor folds formed with pronounced 3 d obliquity relative to the subsequent shear plane mineral lineations folded around sheath closures display asymmetric star burst patterns consistent with recrystallisation during active folding and hinge rotation we show that the eye fold shapes exposed in any 2 d y z slice can be used to predict the geometry of marker horizons back along the x axis in the third dimension this self similarity may be of value when tracing stratiform mineralised horizons in large scale sheath folds

ocean circulation at the last glacial maximum a combined modeling and magnetic proxy based study 1 formation of north atlantic deep water nadw is an important component of the ocean thermohaline circulation but debate exists over the ocean circulation state during glacial stages some geological and modeling studies suggest decreased nadw and increased formation of southern ocean deep water during the last glacial maximum lgm others indicate similar or higher rates of nadw advection here we test three different potential lgm ocean states by comparing the modeled iceberg trajectories each produces with magnetically mapped patterns and sources of lgm ice rafted debris ird the three lgm states are characterized by vigorous nadw formation deepwater production in the southern ocean and a third intermediate state with southern ocean deepwater formation but also some north atlantic intermediate water formation cluster analysis of sediment magnetic properties was used to characterize north atlantic ird patterns and sources which match most closely iceberg trajectories arising from some combination of the southern sinking and intermediate ocean circulation states the magnetic data indicate two major ird sources fennoscandia and greenland iceland and one minor source the st lawrence region the model and magnetic data suggest that the lgm north atlantic circulation was dominated by a cyclonic central north atlantic gyre separated from the north atlantic current which was displaced south of 42 n

inverted fold hinge an end member of hinge rotation by superposed buckle folding in the precambrian terrain of western india abstract plunge reversal by hinge rotation due to superposed buckle folding ends when one of the hinge segments rotates to a reclined sensu stricto geometry rotation beyond this limit called the critical stage of hinge rotation inverts the rotating hinge segment from an antiform to a synform or vice versa in the sawar bajta area in the precambrian terrain of western india a segment of a synformal early fold has been inverted to an antiform producing an antiform synform fold pair with a common hinge and a common axial surface later compression folded the limbs and axial surface of the early synform and rotated its two adjacent hinge segments in opposite directions unequal rotation inverted one of the segments and bowed the common hinge upward to produce a mega sheath fold with 46 hinge angle the sheath folded basement cover rock contact delineates an eye fold with more than 30 km long perimeter on map profile the inverted segment rotated more than 17 beyond the critical position to what is probably the end member of hinge rotation the interlimb angle of the inverted segment increased indicating progressive opening up of the fold which inhibits further rotation beyond the limit set by the end member

shear zones between rock units with no relative movement abstract shear zones are normally viewed as relatively narrow deformation zones that accommodate relative displacement between two blocks that have moved past each other in opposite directions this study reports localized zones of shear between adjacent blocks that have not moved past each other such deformation zones which we call wakes form due to the movement of exotic blocks within a viscous medium denser blocks sinking within a salt structure the paths between separated boudins melt in partially molten surroundings melt movement during migmatisation or solid blocks sinking through a partially molten magma body stoping from the fluid dynamics perspective these shear zones can be regarded as low reynolds number deformation zones within the wake of a body moving through a viscous medium while compact moving bodies aspect ratio 1 1 1 generate axial symmetric cone like shear zones or wakes elongated bodies vertical plates or horizontal rod like bodies produce tabular shear zones or wakes unlike conventional shear zones across which shear indicators usually display consistent symmetries shear indicators on either side of the shear zone or wake reported here show reverse kinematics thus profiles exhibit shear zones with opposed senses of movement across their center lines or planes we have used field observations and results from analytical and numerical models to suggest that examples of wakes are the transit paths that develop where denser blocks sink within salt structures bodies of melt rise through migmatites between boudins separated by progressive extension and perhaps where slabs of subducted oceanic lithosphere delaminate from the continental crust and sink into the asthenosphere we also argue that such shear zones may be more common than they have been given credit for and may be responsible for some reverse kinematics reported in shear zones

the pan amazonian ucayali peneplain late neogene sedimentation in amazonia and the birth of the modern amazon river system abstract we review the neogene geologic history of lowland amazonia in an attempt to focus attention on areas of agreement as well as areas in dispute in this research arena we reinterpret pre existing hypotheses present new data and discuss new insights intended to support a unified synthesis of the amazon basin as a single sedimentary basin albeit on a vast scale during the late miocene to middle late pliocene we document the ucayali peneplain as an isochronous pan amazonian geologic feature that formed following the early to mid miocene quechua i orogenic phase of andean tectonism peneplanation began possibly as early as 15 ma and terminated abruptly near the beginning of the late miocene quechua ii orogenic event at 9 5 9 0 ma subsequently a thin cover of sediments comprising the madre de dios formation began blanketing most of lowland amazonia excepting only the eastern subandean fold and thrust belt and isolated highlands within the basin the buried peneplain is readily observed in river cutbanks throughout amazonia as the marked often angular ucayali unconformity that separates eroded older often folded faulted and weathered moderately to well consolidated tertiary formations from unconsolidated near horizontal upper neogene deposits the dates of formation of major unconformities and subsequent depositional events at widely separated areas within the andes of bolivia ecuador and peru are coincident with that of the ucayali unconformity and deposition of the madre de dios formation and suggest that the events are linked to a common cause which is interpreted to be the still on going collision between the south american and nazca tectonic plates the madre de dios formation has three members the oldest of which documents a short lived high energy depositional environment followed by a moderate energy depositional environment both occurring at a time when drainage from the basin was unobstructed the upper two members record fluctuations between moderate and low energy continental depositional environments during a period when drainage from the basin was obstructed disorganized and took place over long distances with extremely low gradients the sedimentology of the madre de dios formation particularly the thick massive beds of clay and the widespread presence of paleodeltas and associated geomorphic features on the amazonian planato are consistent with the hypothesis that much of the upper two members formed as lacustrine and deltaic deposits within a gigantic lake lago amazonas or more probably within a complex series of interconnected mega lakes that occasionally united to cover most or all of lowland amazonia to a shallow depth from the latest miocene until 2 5 ma the presence of the ucayali unconformity and the relatively uniform lithostratigraphy basin wide of the fluvial fluviolacustrine and lacustrine sediments of the upper neogene madre de dios formation are consistent with the hypothesis that the amazon basin acted as a single undivided sedimentary basin in the late neogene the biostratigraphic correlation across important modern drainage divides of both micro and macro sized late miocene fossil vertebrates recovered from basal conglomerates of the madre de dios formation and the absence therefrom of fossil vertebrates of any other age is also consistent with this hypothesis two 40ar 39ar dates on ash deposits within the madre de dios formation corroborate the upper miocene age of the basal horizons of that formation indicated by fossil vertebrates and support an upper pliocene age for the youngest sediments of the formation the modern amazon river drainage system was established in the late pliocene at 2 5 ma by the breaching of the eastern rim of the sedimentary basin as a result of the basin being overfilled or by headward erosion of the lower amazon river or both data on cenozoic mass accumulation rates and the chemistry of terrigenous sediments in the atlantic ocean obtained by ocean drilling project leg 154 ceara rise support the postulated timing of the establishment of the modern amazon river drainage system at 2 5 ma rather than the long held view that this event occurred in the late miocene we discuss the important role of ocean currents and sea level fluctuations on terrigenous mass accumulation rates on the ceara rise the postulated time of formation of the modern amazon river is nearly coincident with the onset of the plio pleistocene glacial climatic regime and the lowest sea level stands since the latest middle miocene this analysis indicates that modern amazonia is a product of terrain development within an erosional regime since 2 5 ma

successive development of plane noncylindrical folds in progressive deformation in the history of superposed deformations of the iron formations at the western border of the kolar gold field in s india an important event was the successive growth of broadly coaxial plane noncylindrical folds in course of a progressive deformation concomitant with development of ductile mesoscopic shear zones the noncylindrical folds were initiated as active folds by the creation of a buckling instability at successive stages on newly developed foliation surfaces the nucleation of noncylindrical folds and the subsequent axial plane folding of the tightened mature folds are explained by the mechanical inhomogeneity of the rocks and the heterogeneous character of strain the correlation between increasing tightness and increasing noncylindricity of the folds indicates that the initial curvatures of hinge lines were accentuated by an extension parallel to the subhorizontal stretching lineation from the patterns of deformed lineations over folds of varying tightnesses it is concluded that the passive accentuation of hinge line curvatures was mostly achieved when the folds had already become isoclinal or very tight

quartz fabric and strain partitioning in sheath folds an example from the voltri group western alps italy abstract a mesoscopic sheath fold defined by an almost pure quartzitic level in quartz mica schists of the voltri group southeastern border of the western alps was sampled and studied in detail microstructural analysis was carried out on thin sections from different structural domains in the quartzitic layer with the emphasis to analyse quartz shape fabrics and the crystallographic preferred orientation of the c axes of this mineral quartz was dynamically recrystallized but its degree of preferred orientation is not as high as it would be expected in a high shear strain regime required for the development of sheath folds this may suggest strain partitioning between the quartzitic level and the micaceous matrix crystallographic preferred orientation of c axes and grain shape fabric asymmetry indicate the same sense of shear on both fold limbs in the xz plane this is in agreement with a model of sheath fold development by the passive rotation of material lines clustering of the c axis poles shows a configuration indicative of a different strain state in different domains of the sheath fold and suggests an evolution of the fold characterized by combined simple shear and coaxial progressive deformation

tubular folds and sheath folds definitions and conceptual models for their development with examples from the grapesvare area northern sweden abstract two parameters are used in the classification of highly non cylindrical folds the hinge line angle and the ratio of the length of the cone axis to that of the long axis of the cross section of the structure at the position where the hinge angle was measured if in a non cylindrical fold a cross section can be selected so that the hinge angle w measured where the cross section cuts the cone is less than 90 and the length of the cone axis x is more than a quarter of the length of the long axis y of the cross section the fold is a sheath fold a tight sheath fold with w x y 1 can be termed a tubular fold a tubular fold is thus an extreme type of sheath fold while sheath folds with w 20 but x y 0 25 but tubular folds from the grapesvare area comply to the definition given above and they occur in a structural setting that is consistent with the models described

analysis of transpressional deformation from geometrical evolution of mesoscopic structures from phulad shear zone rajasthan india abstract transpressional deformation in ductile shear zones may be analyzed by simultaneous wall parallel simple shear with strain rate xy along with three dimensional coaxial deformation with strain rates x y and z several types of transpression may be distinguished on the basis of relative values of strain rate ratios x a y b and z c it is possible to identify the nature of transpressional deformation from a detailed analysis of shear zone structures in the phulad shear zone of rajasthan india evidence of general flattening a thrusting sense of shear occurrence of transport parallel stretching lineation frequent occurrence of sheath folds with apical direction parallel to the stretching lineation and occurrence of u shaped lineation patterns indicate that the deformation was transpressional with b a ratio ranging between 1 and 2 the bulk deformation was not dominated by simple shear but involved both simple and pure shear with extrusion along the transport direction much greater than along the vorticity vector rotation of long tectonic clasts caused reorientation of monoclinic rolling structures so that strongly monoclinic structures with opposite senses of asymmetry in different domains appear on subhorizontal outcrops parallel to the vorticity vector although the component of simple shear is zero on these surfaces

3d seismic evidence of buried iceberg ploughmarks from the mid norwegian continental margin reveals largely persistent north atlantic current through the quaternary over 7500 buried linear and curvilinear depressions interpreted as iceberg ploughmarks were identified within the quaternary naust formation from an extensive three dimensional seismic dataset that covers 40 000 km2 of the mid norwegian continental margin the morphology and net orientation of ploughmarks were mapped and analysed these features are up to 28 km long 700 m wide and are incised up to 31 m deep on average ploughmarks are incised 5 m deep with median width of 185 m and median lengths ranging from 1 2 to 2 7 km for individual palaeo surfaces width to depth ratio ranges from 8 1 to 400 1 and is on average 36 1 the presence of ploughmarks buried deeply within some palaeo slope surfaces implies the occasional presence of very large icebergs since the middle quaternary suggesting that thick ice sheet margins with fast flowing ice streams were present in order to calve icebergs of such dimensions into the norwegian sea the wide geographical distribution of ploughmarks suggests unrestricted iceberg drift and an open norwegian sea during the periods of iceberg calving since the early quaternary ploughmark trajectory analysis demonstrates that the ocean current circulation now dominated by the northeasterly flowing norwegian atlantic current nwac has largely persisted throughout the quaternary despite the overall strikingly consistent pattern of iceberg drift ploughmark mapping also shows evidence for short lived nwac reductions possibly related to major phases of iceberg discharge and or meltwater pulses from the fennoscandian ice sheet during the middle and late quaternary

structural pattern in the dharwar craton high grade and low grade supracrustal belts sargur and dharwar set in a matrix of gneisses dotted with granitic plutons of different ages make up the dharwar craton the overall structural pattern is one of subparallel linear belts and stringers whose regional trend changes from nw se in the northern part to n s in the south both the high grade and low grade belts show three principal episodes of folding however the correlation between the deformation episodes in the sargur and the dharwar rocks is controversial and the structural relations between them are not unequivocally established parallelism of the first phase structures in the sargur and the dharwar rocks can be interpreted either as synchronous development or rotation of the earlier sargur structures into parallelism with the younger dharwar structures during later deformation the extension of the supracrustal belts is parallel to the f 2 axial traces of the second deformation granite diapirism and basement uplift further modified

deformation progressive or multiphase abstract complexly deformed rocks that exhibit refolded folds and overprinting planar fabrics are often interpreted as the result of polyphase deformation and described in a scheme of deformation phases d 1 d 2 d n at the same time a number of models have been presented over the last decades that explain how complex and overprinting structures can form by single phase progressive deformation as strain accumulates strain generally accumulates heterogeneously in a deforming region depending on strain intensity and geometry flow parameters rheology and lithology the result is regions of different structural complexity fabrics and fabric orientations that can be sorted into different structural facies such structural pattern should be mapped and presented before defining a scheme of deformation phases and attempts should be made to explain them by a single structural kinematic model such as partitioned transpression only if this is not possible or if independent geochronologic data reveals a more punctuated history should we define an additional tectonic regime and deformation phase multiphase and progressive deformation histories are not mutually exclusive and it is important that both possibilities are considered when describing the deformation history of any complexly deformed region when the concept of deformation phases is employed the number of phases should be minimized and be related to major tectonic event of external character uncritically defining deformation phases can easily generate a complicated discrete deformation history with no link to tectonic reality

quantifying flow regimes in a greenland glacial fjord using iceberg drifters funded by the national science foundation grant numbers oce 1130008 arc 0909274 and the university of oregon

surface sea floor and near surface box cores sediment mineralogy in baffin bay as a key to sediment provenance and ice sheet variations to better understand the glacial history of the ice sheets surrounding baffin bay and to provide information on sediment pathways samples from 82 seafloor grabs and core tops and from seven box cores were subjected to quantitative x ray diffraction weight percent wt analysis of the 2000 m all show an abrupt drop in calcite wt post 5 cal ka bp following a major peak in detrital carbonate mainly dolomite this dolomite rich detrital carbonate dc event in jr175bc06 is possibly coeval with the younger dryas cold event four possible glacial sourced end mem bers were employed in a compositional unmixing algorithm to gain insight into down core changes in sediment provenance at the deep central basin estimates of the rates of sediment accumulation in the central basin are only in the range of 2 to 4 cm cal ka surprisingly low given the glaciated nature of the surrounding land

laurentide ice sheet dynamics in the bay of fundy canada revealed through multibeam sonar mapping of glacial landsystems abstract recent multibeam sonar data collected in the bay of fundy canada interpreted in conjunction with geophysical profiling and sediment sampling reveal in unprecedented detail a suite of glacial landforms associated with the southwest margin of the laurentide ice sheet these landforms constitute four glacial landsystems 1 subglacial landsystem i in southwestern bay of fundy the elongated grand manan basin contains ice contact sediments of possible mid wisconsinan age overlain by late wisconsinan ice contact sediments strongly imprinted by iceberg furrows and pits in places possible mid wisconsinan glaciomarine sediments have been eroded by late wisconsinan ice creating streamlined landforms eroded bedrock and megafluted ice contact sediment on the flanks of grand manan basin indicate the southwest direction of topographically steered ice 2 subglacial landsystem ii along the southern margin of the bay of fundy an array of drumlins with superimposed esker complexes was formed by glacial ice that emanated northwest from the interior of nova scotia and was deflected to the southwest by the ice flowing out of the bay of fundy to the gulf of maine the esker complexes formed later when the nova scotia ice sheet stagnated and meltwater escaped northwest via topographic gaps 3 ice marginal landsystem i in northern bay of fundy both small de geer moraines and larger basin bounding moraines were created when retreating late wisconsinan ice became grounded in relatively shallow water new radiocarbon ages show that the owen basin moraine in this landsystem was abandoned prior to c 14 600 14 c yr bp cal bp 17 015 17 270 0 7 17 286 17 405 0 3 4 ice marginal landsystem ii this distinctive landsystem consists of numerous arcuate moraines commonly superimposed on one another this landsystem was formed by thin 170 m lightly grounded ice that retreated northeast into the bay of fundy the splayed pattern of the ice margin was a response to a large accommodation space and velocity variations within the ice the isle haute moraine is a large lobate moraine its age is poorly constrained by shells from glaciomarine sediments 13 750 60 14 c yr bp cal bp 15 970 15 996 0 03 16 047 16 665 0 97 but the moraine may be coeval with the onset of a terrestrial ice margin established at the head of the bay of fundy by c 14 300 14 c yr bp cal bp 16 824 17 013

development of highly non cylindrical folds around rigid ellipsoidal inclusions in bulk simple shear regimes natural examples and experimental modelling abstract the three dimensional development of strongly non cylindrical folds around competent ellipsoidal inclusions in bulk simple shear regimes is studied in experiments and natural examples examples from the continental allochthonous terrane of the braganca nappe complex ne portugal show rim folds and sheath folds associated with different parts of rigid ellipsoidal boudins experimental work has been carried out with models made from analogue materials silicone putty and plasticine and deformed in a simple shear machine we have considered three different models to simulate natural examples and the results show that fold morphology depends on the shape of the inclusion and position around the inclusion although the bulk deformation regime is layer parallel homogeneous simple shear we can distinguish local deformation regimes responsible for the folding associated with different parts of the rigid body flow and strain patterns must therefore be complex around inclusions or populations of inclusions prolate to oblate strain ellipsoids can be expected in different positions close to the rigid inclusion in our experiments the competent inclusions do not rotate synthetically with the applied bulk simple shear e g clockwise rotation of inclusion in dextral shear instead they back rotate early in the shearing history and keep this position throughout the experiment this is the result of the size relationship between the inclusion and the finite width of the shear zone the fold pattern around rigid inclusions may be used as a shear sense criterion

internal ice sheet variability as source for the multi century and millennial scale iceberg events during the holocene a model study abstract the climate of the holocene the current interglacial covering the past 11 700 years has been relatively stable compared to previous periods nevertheless repeating occurrence of rapid natural climate changes that challenged human society are seen in proxy reconstructions ocean sediment cores for example display prominent peaks of enhanced ice rafted debris ird during the holocene with a multi decadal to millennial scale periodicity different mechanisms were proposed that caused these enhanced ird events for example variations in the incoming total solar irradiance tsi volcanic eruptions and the combination of internal climate variability and external forcings we investigate the probable mechanisms causing the occurrence of ird events over the past 6000 years using a fully coupled climate ice sheet iceberg model i loveclim we performed 19 experiments that differ in the applied forcings tsi volcanic and the initial atmospheric conditions to explore internal ice sheet variability one further experiment was done with fixed climate conditions all the model runs displayed prominent peaks of enhanced iceberg melt flux imf independent of the chosen experimental set up the spectral analysis of the experiments with the ice sheet climate model coupled displays significant peaks at 2000 1000 years in all the experiments and at 500 years in most runs the experiment with fixed climate conditions displays one significant peak of about 1500 years related to internal ice sheet variability this frequency is modulated to 2000 and 1000 years in all the experiments with a coupled climate ice sheet due to interactions between the climate components we further investigate the impact of minimum tsi events on the timing and occurrence of enhanced imf in the experimental set up that was forced with idealized sinusoidal tsi variations 4 wm 2 we find a significant occurrence of an increased iceberg melt flux about 60 years after the minimum tsi value yet we also see a significant time lag of 80 years between reconstructed tsi minima and the simulated enhanced iceberg melt flux in some of the experiments without tsi forcing the fact that also model runs that are not forced with tsi variations display an 80 year time lag indicates that the relationship between tsi and imf is due to internal dynamics of the coupled system from our experiments we conclude that internal ice sheet variability seems to be the source of the multi century and millennial scale iceberg events during the holocene

simulating heinrich event 1 with interactive icebergs during the last glacial major abrupt climate events known as heinrich events left distinct fingerprints of ice rafted detritus and are thus associated with iceberg armadas the release of many icebergs into the north atlantic ocean we simulated the impact of a large armada of icebergs on glacial climate in a coupled atmosphere ocean model in our model dynamic thermodynamic icebergs influence the climate through two direct effects first melting of the icebergs causes freshening of the upper ocean and second the latent heat used in the phase transition of ice to water results in cooling of the iceberg surroundings this cooling effect of icebergs is generally neglected in models we investigated the role of the latent heat by performing a sensitivity experiment in which the cooling effect is switched off at the peak of the simulated heinrich event icebergs lacking the latent heat flux are much less efficient in shutting down the meridional overturning circulation than icebergs that include both the freshening and the cooling effects the cause of this intriguing result must be sought in the involvement of a secondary mechanism facilitation of sea ice formation which can disturb deep water production at key convection sites with consequences for the thermohaline circulation we performed additional sensitivity experiments designed to explore the effect of the more plausible distribution of the dynamic icebergs melting fluxes compared to a classic hosing approach with homogeneous spreading of the melt fluxes over a section in the mid latitude north atlantic na ocean the early response of the climate system is much stronger in the iceberg experiments than in the hosing experiments which must be a distribution effect the dynamically distributed icebergs quickly affect western nadw formation which synergizes with direct sea ice facilitation causing an earlier sea ice expansion and climatic response furthermore compared to dynamic thermodynamic icebergs a homogeneous hosing overestimates the fresh water flux in the eastern ruddiman belt causing a fresh anomaly in the eastern north atlantic leading to a delayed recovery of the circulation after the event

folds and the strain ellipsoid a general model abstract a new general model is proposed to account for non orthogonal relationships of fold axes and axial surfaces to the axes of the finite strain ellipsoid descriptions in the literature of such relationships e g plunge variations and transecting cleavage are reviewed and the development of the model is outlined the model is described in which plane strain k 1 or slight flattening strain k 0 71 or slight constrictional strain k 1 85 are imposed on a suite of layers with oblique orientations to the bulk strain axes x y z folds will initiate perpendicular to the direction of shortening in the layer and develop as non material lines moving through the layer by angular migration at geologically realistic finite strains the folds will be transected by xy cleavage in slight flattening strain folds are likely to be cylindroidal with axes not measurably oblique to xy in slight constrictional strain en echelon periclines are predicted with marked transection by cleavage in some examples the interference of two directions of folding will give rise to severe plunge variations and even downward facing although strain markers frequently indicate finite strains in the flattening field this may result from the superimposition of slightly constrictional tectonic strain on compactional flattening

structural evolution of gavilgarh tan shear zone central india a possible case of partitioned transpression during mesoproterozoic oblique collision within central indian tectonic zone abstract gavilgarh tan shear zone gtsz a map scale ductile shear zone occurring in the southern part of the central indian tectonic zone citz contains many important clues to the tectonics of this important proterozoic mobile belt of central india which is yet controversial and hotly debated data from structural mapping microstructural study and kinematic analyses of the gtsz have been presented in this paper which clearly implies that gtsz is mainly represented by a wrench dominated transpressional shear zone with sinistral shear sense analysis of lineation patterns shows that while horizontal stretching dominated within the ductile shear zone rocks on either side north and south of it stretched vertically with a transition zone of simultaneous horizontal and vertical stretching in between these two domains a general transpressional strain has thus been partitioned into wrench and pure shear dominated domains at large scale further strain partitioning also took place at the outcrop scale geochemistry of the granitoids occupying the shear zone and its surrounding area indicates continental collision set up which supports the structural model on the basis of earlier proposed tectonic models and limited geochronological data available of the southern part of citz it is suggested here that an oblique collision and final suturing between the northern bundelkhand craton and southern bastar craton terrains within the citz around 1100 ma have been partly accommodated by partitioned transpression along gtsz and its surrounding gneiss granitoid terrain more intensive geochronological constraint of the collision event from both northern and southern side are however lacking at present

polyphase deformation in the western mount isa inlier australia episodic or continuous deformation abstract the polyphase deformation history of the mount novit ranges western mount isa inlier australia involved the development of four generations of n s to nw plunging folds development of which alternated with movement on n s striking faults shear zones with west over east displacement the spatial relationships between successive structures and the distinct similarities in the kinematics style and asymmetry are all consistent with continuous deformation during e w to ne sw shortening additional support for this hypothesis is based on the development of the three major fold generations and the intervening faults and shear zones during low p high t metamorphism although the duration of this metamorphic event cannot be constrained recent studies have linked low p metamorphism to heat advection by migrating melts thermal modelling indicates that such metamorphic events will be short lived in comparison to those associated with thermal relaxation of over thickened crust low p metamorphism is unlikely to persist through two or more separate tectonic events therefore syn metamorphic polyphase deformation in the mount novit ranges apparently formed during a single continuous tectonic episode this structural history differs from both the regional scheme and that of previous workers in this area the main differences are i no evidence was found for early thrust faulting prior to n s folding ii there are four generations of n s to nw se folds as opposed to two and iii this study favors a single consistent tectonic regime in contrast to the interpretation of three discrete isolated tectonic events

holocene ice rafting and sediment transport from the glaciated margin of east greenland 67 70 n to the n iceland shelves detecting and modelling changing sediment sources abstract we examine variations in the ice rafted sources for sediments in the iceland east greenland offshore marine archives by utilizing a sediment unmixing model and link the results to a coupled iceberg ocean model surface samples from around iceland and along the e ne greenland shelf are used to define potential sediment sources and these are examined within the context of the down core variations in mineralogy in the

the geometry and topology of natural sheath folds a new tool for structural analysis abstract curvilinear sheath folds are classically depicted as displaying symmetrical geometries about two orthogonal mirror planes centred along the x y axial surface and the x z medial culmination depression surface which bisects the fold nose however 10 000 geometric analyses of minor folds and fabrics formed during ductile thrusting in the caledonides of northern scotland reveals that major dome and basin sheath folds can display distinct and predictable asymmetries across both axial and medial surfaces the strain is typically heterogeneous so that structural fabrics and younging evidence are preserved within sheath folds at varying stages of development this allows an analysis of the evolution of such structures from tongue folds to more extreme tubular forms geometric relationships between measured orientations of fold hinges axial planes extension lineations and foliations are compared on fabric topology plots ftps which provide an effective tool for monitoring planar and linear fabric rotations with increasing progressive non coaxial deformation they consistently display systematic variation from regions of lower to higher strain on passing from upper to lower fold limbs across major axial surfaces and on crossing medial surfaces from short to long hinge line segments axial and medial surfaces effectively therefore divide major sheath folds into quadrants with different amounts senses and combinations of planar and linear fabric rotation within each domain such heterogeneous deformation implies that models of intense non coaxial deformation uniformly affecting pre existing folds may overestimate bulk displacement and shear strain variable fold hinge line rotation about medial surfaces also provides an effective mechanism for the closure of major sheaths which may otherwise project for unfeasible distances in the x direction bedding cleavage intersections are developed at greater angles to the transport direction than fold hinges which they transect in a consistent and predictable sense thereby confirming the direction of fold rotation even in areas which lack information on fold facing in cross section asymmetric tear drop eyes indicate the sense of fold hinge line vergence whilst fold limb and hinge line asymmetry may be combined on 3 d vergence ranking and rotation grids which allow location and relative strain states of minor structures to be accurately predicted within the overall sheath framework systematic fabric analysis on ftps may be applied to the investigation of ductile deformation across a broad range of scenarios

shear zone related folds folds in ductile shear zones are common structures that have a variety of origins these can be pre existing folds that become modified or folds developed during the shearing event among the syn shearing folds a second subdivision is based on the relative age of the folded surface which can be pre existing or newly formed during the shearing event in each of the three categories final fold geometry and orientation show complex relationships with the kinematic frame the final fold geometry depends on the vorticity within the shear zone the rheology and the initial orientation of the folded surface relative to the kinematic framework it follows that folds are complex structures difficult to use as kinematic indicators however in shear zones where undeformed wall rocks with pre shear structures are accessible and where kinematics can be well established folds can provide a valuable natural means to understand the initiation and evolution of structures under non coaxial regimes we point to the need of discriminating among different plausible categories based on the nature of the folded surface and on the inherent structural features of each category q 2004 elsevier ltd all rights reserved