the role of hybridization in the genesis of hercynian granitodis in the gredos massif spain inferences from sr nd isotopes the gredos massif is one the better exposed granitoid complexes of the iberian massif it is composed mainly of peraluminous granitoids with subordinate basic and ultrabasic complexes the massif also contains mega enclaves of migmatites with which the granitoids show transitional contacts two major magmatic associations have been distinguished in this study 1 one comprises the granitoids with microgranular enclaves the enclaves and basic rocks 2 the other is formed by leucogranites intrusive into the former series and free of microgranular enclaves field relationships and microstructures indicate that the rocks of the first series are related by a dominant hybridization process the sr nd isotopic study reveals that this process is complex relating different end members of mantle and crustal affinities and occurred around 295 ma ago late with respect to the main deformation phases of the hercynian orogeny the granitoids with microgranular enclaves gme are part of an overall mixing trend involving palaeozoic mantle derived magma and melts of older crustal material amphibole bearing gme in general contain greater proportions of the mantle derived component than the cordierite bearing gme the actual mixing processes took place on a variety of scales sometimes between melts which were themselves hybrids on a local scale this hybridization process can be modelled by simple binary mixing as documented in the case of a composite dyke the isotopic signatures of the basic rocks are probably to a large degree the result of interaction with crustal melts though additionally the presence of an enriched mantle source cannot be elmininated microgranular enclaves and their immediate hosts have differing initial sr and nd isotopic signatures indicating that isotopic equilibrium was not attained this suggests that the enclaves did not reside in their final granitic melt for long before cooling of the whole system the enclaves are considered to have been derived from basaltic melts which had fractionated and hybridised to varying degrees late stage peraluminous leucogranites have similar initial nd isotopic compositions to the evolved gme a crustal source with a radically different nd isotopic composition or age does not need to be invoked in their petrogenesis

non newtonian rheology of igneous melts at high stresses and strain rates experimental results for rhyolite andesite basalt and nephelinite the stress strain rate relationships of four silicate melt compositions high silica rhyolite andesite tholeiitic basalt and nephelinite have been studied using the fiber elongation method measurements were conducted in a stress range of 10 400 mpa and a strain rate range of 10 6 to 10 3 s 1 the stress strain rate relationships for all the melts exhibit newtonian behavior at low strain rates but non newtonian nonlinear stress strain rate behavior at higher strain rates with strain rate increasing faster than the applied stress the decrease in calculated shear viscosity with increasing strain rate precedes brittle failure of the fiber as the applied stress approaches the tensile strength of the melt the decrease in viscosity observed at the high strain rates of the present study ranges from 0 25 to 2 54 log10 pa s the shear relaxation times of these melts have been estimated from the low strain rate newtonian shear viscosity using the maxwell relationship s g non newtonian shear viscosity is observed at strain rates time 1 equivalent to time scales that lie 3 log10 units of time above the calculated relaxation time brittle failure of the fibers occurs 2 log10 units of time above the relaxation time this study illustrates that the occurrence of non newtonian viscous flow in geological melts can be predicted to within a log10 unit of strain rate high silica rhyolite melts involved in ash flow eruptions are expected to undergo a non newtonian phase of deformation immediately prior to brittle failure

millennial scale northwest african droughts related to heinrich events and dansgaard oeschger cycles evidence in marine sediments from offshore senegal 1 we present a suite of new high resolution records 0 135 ka representing pulses of aeolian fluvial and biogenic sedimentation along the senegalese continental margin a multiproxy approach based on rock magnetic element and color data was applied on three cores enclosing the present day northern limit of the itcz a strong episodic aeolian contribution driven by stronger winds and dry conditions and characterized by high hematite and goethite input was revealed north of 13 n these millennial scale dust fluxes are synchronous with north atlantic heinrich stadials fluvial clay input driven by the west african monsoon predominates at 12 n and varies at dansgaard oeschger time scales while marine productivity is strongly enhanced during the african humid periods and marine isotope stage 5 from latitudinal signal variations we deduce that the last glacial itcz summer position was located between core positions at 12 26 and 13 40 n furthermore this work also shows that submillennial periods of aridity over northwest africa occurred more frequently and farther south than previously thought

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

age models for the cape blanc debris flow and the mauritania slide complex in the atlantic ocean off nw africa abstract age models for the emplacement time of mass flow deposits from the cape blanc debris flow the mauritania slide complex and a levee of the mauritania canyon were obtained by offshore x ray fluorescence xrf element stratigraphy on five gravity cores allowing the assessment of slope instability in this part of the nw african continental margin with respect to climate related sea level variations during the quaternary the cape blanc debris flow emplaced approximately 155 kyr ago whereas deposition of the mauritania slide complex is linked to the rapid sea level rise at the pleistocene holocene boundary turbidites on a levee of the mauritania canyon close to the mauritania slide complex occurred at stage boundaries these findings agree with other studies which show that the nw african continental margin has been unstable over the last quaternary cycles and that downslope sediment transport is frequently coupled to periods of climatic changes at stage boundaries

the magnitude timing and abruptness of changes in north african dust deposition over the last 20 000 yr reconstructions of eolian dust accumulation in northwest african margin sediments provide important continuous records of past changes in atmospheric circulation and aridity in the region existing records indicate dramatic changes in north african dust emissions over the last 20 ka but the limited spatial extent of these records and the lack of high resolution flux data do not allow us to determine whether changes in dust deposition occurred with similar timing magnitude and abruptness throughout northwest africa here we present new records from a meridional transect of cores stretching from 311 nt o 191n along the northwest african margin by combining grain size endmember modeling with 230 th normalized fluxes for the first time we are able to document spatial and temporal changes in dust deposition under the north african dust plume throughout the last 20 ka our results provide quantitative estimates of the magnitude of dust flux changes associated with heinrich stadial 1 the younger dryas and the african humid period ahp 11 7 5 ka offering robust targets for model based estimates of the climatic and biogeochemical impacts of past changes in north african dust emissions our data suggest that dust fluxes between 8 and 6 ka were a factor of 5 lower than average fluxes during the last 2 ka using a simple model to estimate the effects of bioturbation on dust input signals we find that our data are consistent with abrupt synchronous changes in dust fluxes in all cores at the beginning and end of the ahp the mean ages of these transitions are 11 870 2 ka 1s and 4 970 2 ka respectively

grain size control on sr nd isotope provenance studies and impact on paleoclimate reconstructions an example from deep sea sediments offshore nw africa five sediment cores from offshore nw africa were analyzed for strontium and neodymium isotope ratios to reconstruct temporal variations in continental weathering regimes sediments were taken from three time slices with well known and distinctive environmental conditions present day dry and warm similar to 6 ka wet and warm and similar to 12 ka dry and cold terrigenous sediment samples were split into two size fractions to distinguish between the two dominant transport mechanisms offshore nw africa fluvial 0 10 mu m and aeolian 10 40 mu m sr isotope data record evidence of marked grain size control with higher isotopic ratios in the fine fraction in contrast epsilon nd values are largely unaffected by grain size minor variability in nd isotope data at each sampling site indicates near constant sources of terrigenous matter over the last similar to 12 ka variations in sr isotope ratios are interpreted to reflect major changes in the evaporation precipitation balance we suggest that the sr nd isotope data record a latitudinal shift of the northern limit of the african rainbelt and associated wind systems causing changes in the humidity and rate of chemical weathering over nw africa while hyperarid conditions prevailed similar to 12 ka more humid conditions and intensified monsoonal rainfall at similar to 6 ka resulted in greater breakdown of easily weathered k bearing phases and increased sr 87 sr 86 in the detritus in late holocene times the monsoonal circulation diminished resulting in a return to arid conditions our results clearly show that it is of vital importance in paleoenvironmental studies to carry out isotopic analyses on individual sediment fractions that were carried to the studied deposition site by distinct sediment transport mechanisms if isotopic analyses are carried out on bulk sediments the observed variability in isotopic values most likely represents changes in the particle size and mixing proportions of the sediment subpopulations

the african humid period and the record of marine upwelling from excess 230th in ocean drilling program hole 658c using a high resolution 230 th normalized record of sediment flux we document the deglacial and holocene history of north african aridity and coastal upwelling at ocean drilling program hole 658c at both the end of the younger dryas and after the 8 2 ka event there are significant drops in terrigenous accumulation at our site indicating an increase in the monsoon moisture flux over africa at this time at 5 5 ka there is an abrupt end to the african humid period and a return to stronger upwelling conditions for carbonate and opal fluxes the 230 th normalization completely changes the shape of each record based on percentage variations alone this site is a clear example of how variations in one sediment component can obscure changes in the others and it demonstrates the need for radionuclide measurements more generally in paleoceanography by taking our new records and a large amount of previous data from this site we conclude that increases in african moisture are tightly coupled to decreases in coastal upwelling intensity

mid holocene land surface conditions in northern africa and the arabian peninsula a data set for the analysis of biogeophysical feedbacks in the climate system large changes in the extent of northern subtropical arid regions during the holocene are attributed to orbitally forced variations in monsoon strength and have been implicated in the regulation of atmospheric trace gas concentrations on millenial timescales models that omit biogeophysical feedback however are unable to account for the full magnitude of african monsoon amplification and extension during the early to middle holocene 9500 5000 years b p a data set describing land surface conditions 6000 years b p on a 1 1 grid across northern africa and the arabian peninsula has been prepared from published maps and other sources of palaeoenvironmental data with the primary aim of providing a realistic lower boundary condition for atmospheric general circulation model experiments similar to those performed in the palaeoclimate modelling intercomparison project the data set includes information on the percentage of each grid cell occupied by specific vegetation types steppe savanna xerophytic woods scrub tropical deciduous forest and tropical montane evergreen forest open water lakes and wetlands plus information on the flow direction of major drainage channels for use in large scale palaeohydrological modeling the data set is available in digital form by anonymous ftp

submillennial to millennial variability of diatom production off mauritania nw africa during the last glacial cycle 1 the low latitude upwelling regime off the mauritanian coast in the subtropical ne atlantic accounts for a significant part of global export production although productivity variations in coastal upwelling areas are usually attributed to changes in wind stress and upwelling intensity productivity dynamics off mauritania are less straightforward because of the complex atmospheric and hydrographic setting here we integrate micropaleontological diatoms and geochemical bulk biogenic sediment components x ray fluorescence and alkenones proxies to examine on submillennial to millennial changes in diatom production that occurred off mauritania nw africa for the last 25 ka during the last glacial maximum lgm 19 0 23 0 ka b p moderate silicate content of upwelled waters coupled with weakened ne trade winds determined moderate diatom productivity no significant cooling is observed during the lgm suggesting that our alkenone based sst reconstruction represents a local upwelling related signal rather than a global insolation related one extraordinary increases in diatom and opal concentrations during heinrich event 1 h1 15 5 18 0 ka b p and the younger dryas yd 13 5 11 5 ka b p are attributed to enhanced upwelling of silica rich waters and an enlarged upwelling filament due to more intense ne trade winds the synchronous increase of caco3 and k intensity and the decreased opal and diatoms values mark the occurrence of the bolling allerod ba 13 5 15 5 ka b p due to weakened eolian input and more humid conditions on land although the high export of diatoms is inextricably linked to upwelling intensity off mauritania variability in the nutrient content of the thermocline also plays a decisive role

granitic magma transport by fracture propagation granitic magmas commonly ascend tens of kilometres from their source terranes to upper crustal emplacement levels or to the earth s surface apart from its obvious bearing on the interpretation of the geology and geochemistry of felsic igneous rocks the magma ascent mechanism critically affects any modelling of the metamorphic evolution of the upper lithosphere as well as its rheology we propose that in general granitic magmas ascend via propagating fractures as dykes in extremely short time periods long distance diapiric transport of granitoid magmas through crustal sections is not viable on thermal or mechanical grounds and there is an apparent total absence of field evidence for diapiric rise of such magmas even in supposed type localities neither the shapes nor internal or external characteristics of granitic plutons necessarily reveal anything about the transport of their precursor magmas these are purely arrival phenomena dictated by local structure kinematics and stress states based on existing numerical treatments of the problem we show that granitic magmas are apparently sufficiently inviscid to travel through fractures to high crustal levels without suffering thermal death by freezing for example a 2000 km3 granitic batholith can be inflated by a 1 km 3m 20 km deep dyke system in less than 900 years the model proposed has numerous far reaching petrological and rheological consequences some of which are outlined

prediction of faulting from the theories of elasticity and plasticity what are the limits elasticity rigid plasticity and elasto plasticity are the simplest constitutive models used to describe the initiation and evolution of faulting however in practice the limits of their application are not always clear in this paper we test the behaviour of these different models using as examples tectonic problems of indentation of a die compression with basal shear bending of a plate and normal faulting around a dike by comparing the results of these tests we formulate some guidelines that may be useful for the selection of an appropriate constitutive model of faulting the theory of elasticity reasonably predicts the initiation of the fault pattern but gives erroneous results for large strains the theory of rigid plasticity is more appropriate for large deformations where the geometry of faults can be found by the method of characteristics this method works well for zones of failure that are not severely constrained by elastic material outside e g when faults are connected to the free surface a viscous substratum or a zone of weakness non associated elasto plasticity is the most complete theory among those considered in this paper it describes the evolution of faults from the initiation of localized deformations to the formation of a complicated fault network

rheology of granitic magmas during ascent and emplacement flow abstract considerable progress has been made over the past decade in under standing the static rheological properties of granitic magmas in the continental crust changes in h2o content co2 content and oxidation state of the interstitial melt phase have been identified as important compositional factors governing the rheodynamic behavior of the solid fluid mixture although the strengths of granitic magmas over the crystallization interval are still poorly constrained theoretical investigations suggest that during magma ascent yield strengths of the order of 9 kpa are required to com pletely retard the upward flow in meter wide conduits in low bagnold number magma suspensions with moderate crystal contents solidosities 0 1 0 3 viscous fluc tuations may lead to flow differentiation by shear enhanced diffusion ams and mi crostructural studies support the idea that granite plutons are intruded as crystal poor liquids 50 with fabric and foliation development restricted to the final stages of emplacement if so then these fabrics contain no information on the ascent verti cal transport history of the magma deformation of a magmatic mush during pluton emplacement can enhance significantly the pressure gradient in the melt resulting in a range of local macroscopic flow structures including layering crystal alignment and other mechanical instabilities such as shear zones as the suspension viscosity varies with stress rate it is not clear how the timing of proposed rheological transitions formulated from simple equations for static magma suspensions applies to mixtures undergoing shear new theories of magmas as multiphase flows are required if the full complexity of granitic magma rheology is to be resolved

relative rheological evolution of chemically contrasted coeval magmas example of the tichka plutonic complex morocco the tichka plutonic complex presents many unusual structures related to physical interaction between mafic and felsic coeval magmas in this paper a rheological approach is used to attempt to understand the various observed structures after a general presentation of rheological laws and a discussion about the present state of our understanding of the rheology of suspensions we analyse the main steps of rheological evolution of magmas during crystallization taking into account the effect of cooling increase of silica content and increase of crystallinity on viscosity during thermal evolution of magma a general rheological diagram is presented two main rheological thresholds at about 35 crystallinity th1 and about 65 70 th2 allow us to distinguish three major steps in the rheological behaviour of magmas during cooling comparative study of evolution curves for viscosity of coeval mafic m and felsic f magmas shows that an inversion temperature ti exists at which viscosities of both magmas are the same this point has major petrologic significance because the viscosity ratio m f inverses during cooling when the temperature of magma falls below the ti point a model for the evolution of the tichka magmas during cooling has been derived from the knowledge of the chemical and mineralogical composition of the different rocks representing the original m and f magmas as well as some hybrid and differentiated samples viscosities and crystal fractions of magmas as functions of temperature have been calculated from chemical and mineralogical data curves representing the evolution of apparent viscosity of the three initial magmas of the complex are presented from these vicosity temperature curves and from the calculted temperatures corresponding to rheological thresholds th1 and th2 for the different coeval magmas five main evolution stages are defined the different structures observed at the contracts between m and f magmas are developed throughout the entire cooling history of the pluton each structure being related to one of the main evolution stages

erratum to contrasting compositions of saharan dust in the eastern atlantic ocean during the last deglaciation and african humid period earth planet sci lett 278 2009 257 266

the greening of the mcgill paleoclimate model part ii simulation of holocene millennial scale natural climate changes various proxy data reveal that in many regions of the northern hemisphere nh the middle holocene 6 kyr bp was warmer than the early holocene 8 kyr bp as well as the later holocene up to the end of the pre industrial period 1800 ad this pattern of warming and then cooling in the nh represents the response of the climate system to changes in orbital forcing vegetation cover and the laurentide ice sheet lis during the holocene in an attempt to better understand these changes in the climate system the mcgill paleoclimate model mpm has been coupled to the dynamic global vegetation model known as vecode see part i of this two part paper and a number of sensitivity experiments have been performed with the green mpm the model results illustrate the following 1 the orbital forcing together with the vegetation albedo feedback result in the gradual cooling of global sat from about 6 kyr bp to the end of the pre industrial period 2 the disappearance of the lis over the period 8 6 kyr bp associated with vegetation albedo feedback allows the global sat to increase and reach its maximum at around 6 kyr bp 3 the northern limit of the boreal forest moves northward during the period 8 6 4 kyr bp due to the lis retreat 4 during the period 6 4 0 kyr bp the northern limit of the boreal forest moves southward about 120 km in response to the decreasing summer insolation in the nh and 5 the desertification of northern africa during the period 8 2 6 kyr bp is mainly explained by the decreasing summer monsoon precipitation

late pleistocene and holocene dune activity and wind regimes in the western sahara desert of mauritania the western sahara desert in mauritania is dominated by extensive sand seas consisting largely of linear dunes analyses of landsat images geomorphic and stratigraphic studies and optically stimulated luminescence dating of dunes in the azefal agneitir and akchar sand seas provide evidence that three main generations of dunes were formed during the periods 25 15 ka centered around the last glacial maximum 10 13 ka spanning the younger dryas event and after 5 ka the wind regimes that occurred during each of these periods were significantly different leading to the formation of dunes on three distinct superimposed trends northeast north northeast and north and the development of the sand seas as composite geomorphic features

assessing the collection efficiency of ross sea sediment traps using 230th and 231pa abstract annual fluxes of particulate 230th and 231pa collected by sediment traps deployed at two locations in the ross sea are compared to expected fluxes derived by combining the seasonal depletion of 230th and 231pa in surface waters with the steady state production of each nuclide by radioactive decay of its respective dissolved uranium parent conservative minimum estimates of the expected 230th flux are greater than the flux collected by sediment traps deployed at 200 m by factors of 3 and 6 at the two sites the actual discrepancy may be much larger the discrepancies between expected and measured fluxes of 231pa are slightly smaller but comparable in magnitude to those for 230th the discrepancies between expected and measured fluxes for these natural radionuclides are similar in magnitude to those determined previously using the same sediment traps for particulate organic carbon which suggests that the sediment traps undercollected the actual flux of sinking particles

fabric evidence for granodiorite emplacement with extensional shear zones in the variscan gredos massif spanish central system abstract three granitoid bodies in the central part of the gredos massif spanish central system batholith are tabular around 1 km in thickness and intruded into a migmatitic middle crust during the d3 deformation phase of the variscan orogeny petrologically they are composed of bt granodiorite and crd monzogranite and they show varying abundance of large cm scale feldspar megacrysts a detailed study of the shape preferred orientation spo magmatic fabric defined by these megacrysts together with a kinematic analysis of the structures due to interactions between them and the measurement of quartz c axis fabrics in migmatites and granitoids suggests that granitic magma and country rocks were mechanically coupled during deformation the emplacement took place along large scale extensional shear zones active during the first stages of the d3 phase the shape of the spo ellipsoids varies from constrictional at the centre of the granitic bodies to flattening or even oblate at their external contacts with the migmatitic host rocks the favoured interpretation of this spatial fabric variation is the overprinting of the emplacement fabrics by a constrictional tectonic regime associated with the growth of tabular magma chambers along extensional detachments followed by shear zone development commonly at the top of the granitic bodies the entire structure was later folded during the last stages of the d3 phase

a new greenland ice core chronology for the last glacial termination 1 we present a new common stratigraphic timescale for the north greenland ice core project ngrip and grip ice cores the timescale covers the period 7 9 14 8 kyr before present and includes the bolling allerod younger dryas and early holocene periods we use a combination of new and previously published data the most prominent being new high resolution continuous flow analysis cfa impurity records from the ngrip ice core several investigators have identified and counted annual layers using a multiparameter approach and the maximum counting error is estimated to be up to 2 in the holocene part and about 3 for the older parts these counting error estimates reflect the number of annual layers that were hard to interpret but not a possible bias in the set of rules used for annual layer identification as the grip and ngrip ice cores are not optimal for annual layer counting in the middle and late holocene the timescale is tied to a prominent volcanic event inside the 8 2 kyr cold event recently dated in the dye 3 ice core to 8236 years before a d 2000 b2k with a maximum counting error of 47 years the new timescale dates the younger dryas preboreal transition to 11 703 b2k which is 100 150 years older than according to the present grip and ngrip timescales the age of the transition matches the gisp2 timescale within a few years but viewed over the entire 7 9 14 8 kyr section there are significant differences between the new timescale and the gisp2 timescale the transition from the glacial into the bolling interstadial is dated to 14 692 b2k the presented timescale is a part of a new greenland ice core chronology common to the dye 3 grip and ngrip ice cores named the greenland ice core chronology 2005 gicc05 the annual layer thicknesses are observed to be log normally distributed with good approximation and compared to the early holocene the mean accumulation rates in the younger dryas and bolling periods are found to be 47 2 and 88 2 respectively

the 7 13 march 2006 major saharan outbreak multiproxy characterization of mineral dust deposited on the west african margin mineral dust deposits were collected at mbour senegal throughout the spring of 2006 and especially during the well documented march 7 13 large saharan dust outbreak during this 7 day period significant changes in mass flux grain size clay mineralogy and sr and nd isotopic compositions were recorded indicating distinct provenances for the dust transported and deposited during and outside the event all these terrigenous proxies as well as freshwater diatom taxa also showed significant temporal variations during the outbreak implying contributions from at least two different provenance regions tri dimensional back trajectories and satellite imaging enabled us to link those distinct signatures to regions increasingly to the southeast within a large area covering mauritania mali and southern algeria identified by the total ozone mapping spectrometer toms as the main source of the prominent winter spring plume over the tropical atlantic the multiproxy characterization of the march 7 13 dust fall therefore enables us to typify the terrigenous signature of two different regions supplying dust off west africa and provide valuable clues for the interpretation of northeastern tropical atlantic ocean dust sedimentary records in terms of changes in provenance regions and transport systems additionally because dust deposition data are scarce flux and grain size data obtained in this study among other parameters such as clay assemblages provide important constraints for atmospheric transport models and dust deposition budget estimates in this area

a three year time series of mineral dust deposits on the west african margin sedimentological and geochemical signatures and implications for interpretation of marine paleo dust records keywords saharan dust deposition flux grain size clay mineralogy sr and nd isotopes amma program a b s t r a c t mineral dust deposits in the northeastern tropical atlantic ocean netao are an important contribution for reconstructing paleoenvironments and paleoclimates of west africa however the interpretation of the changes in the sedimentological and geochemical characteristics of the dust deposits recorded in the marine sediments from the netao remains incomplete in order to improve our understanding of dust proxies in particular its mineralogical and geochemical provenance tracers present day dust deposition has been monitored at mbour 80 km south of dakar on the senegalese margin here we report a multi proxy investigation of a unique three year continuous time series of mineral dust deposits collected at a weekly or better temporal resolution over the march 2006 march 2009 period mass deposition flux and mean modal grain size display marked but reverse seasonal features with higher flux during the winter spring seasons and coarser grain size mode in summer when flux is minimal reflecting contrasting transport patterns throughout the year similarly clay mineralogy the illite kaolinite ratio in particular shows seasonal fluctuations manifesting the latitudinal displacement of the contributing domains of provenance in response to the seasonal migration of the itcz position and the associated wind systems our three year record also reveals the occurrence of major deposition events superimposed on the seasonal pattern generally during the winter spring dry seasons and most frequently during the month of march our study shows that these major events which contribute a large fraction of the total annual deposition flux all originate from the western sahara sahel a major area of emission in the region beside the bodele depression stretching from the mauritanian and western saharan coasts to the hoggar mountains combined with air mass tri dimensional back trajectories and satellite images the mineralogical and sr nd isotopic compositions of these large dust events enable us to identify several mineralogically and geochemically distinct provenance sectors within this vast western area unlike the background dust deposits the 87 sr 86 sr and the e nd isotopic signatures of the major dry events closely match that of the netao late holocene sediments supporting the hypothesis that these events account for most of the aeolian terrigenous supply reaching the ocean floor although this database needs to be expanded our results already provide useful constraints for the interpretation of the dust proxies variations in marine sedimentary archives off mauritania senegal in terms of changes in wind regimes and aridity over west africa

correlation frequency of emplacement and source directions of megaturbidites on the madeira abyssal plain abstract this paper describes the sediments of the whole madeira abyssal plain including the madcap area to the south previous studies have centred on the central sub basin which occupies most of the plain and which has become one of the most studied areas of seafloor the plain is made up of a series of large turbidites separated by thin pelagic layers which can be used to provide a high resolution stratigraphy back to about 750 000 years many of the turbidites were deposited close to oxygen isotope stage boundaries suggesting a strong relationship to sea level climate change although the exact initiation mechanism for the flows is not known data from the northern and southern extensions of the plain shows the effects of different entry points for the four main turbidite groups volcanic rich turbidites enter from the ne and e calcareous turbidites from the w one group of organic rich turbidites enters from the se and one group from the ne and e different turbidite sources operate for limited periods before switching to one of the other source areas it is very rare for two sources to provide turbidites at the same isotope stage boundary

control of saharan mineral dust transport to barbados in winter by the intertropical convergence zone over west africa 1 the reasons for the inter annual variability of dust transport from the sahara across the atlantic are not well understood here we address this issue by defining three new climate indices that capture the position and intensity of the zone of near surface convergence over west africa a part of the global intertropical convergence zone itcz we then relate these indices to a 38 year record of mineral dust concentrations at barbados focusing on the winter season the results show that the latitudinal displacement of the itcz over west africa and the dust load in barbados are statistically significantly correlated with a correlation coefficient of r 0 69 a southward movement of the itcz corresponds to an increased dust load at barbados this correlation represents an improvement upon previous results which focused on traditional teleconnection indices such as the north atlantic oscillation or the el nino southern oscillation from analyzing composites of wind and precipitation we conclude that for the winter season the inter annual variability of the barbados dust load is related to changes in near surface northeasterly winds in semi arid regions in north africa coincident with the movement of the itcz changes in precipitation appear to only play a minor role citation doherty o m n riemer and s hameed 2012 control of saharan mineral dust transport to barbados in winter by the intertropical convergence zone over west africa j geophys res 117 d19117 doi 10 1029 2012jd017767

h type hybrid granitoids a proposed revision of the granite type classification and nomenclature abstract recent studies on calc alkaline plutonic rocks reveal field and petrographic relationships with strong implications for the processes involved in their genesis the presence of magmatic inclusions and magma mingling zones in these rocks supports an origin by magma mixing compositional variations and isotopic anomalies are a good test for such petrogenetic models most of these calc alkaline plutonic rocks which characterize both collision related and active plate margin environments have been classically identified as i igneous type granitoids following chappell and white s 1974 classification and then interpreted as derived from partial melting of older igneous rocks restite model if the magma mixing origin of a type of granitoids can be demonstrated from field and petrographic evidence and supported by chemical and isotopic variations the nomenclature must be changed in this sense the new category of h hybrid type is tentatively introduced it includes most of the so called i type granitoids and some of the s type as deduced from field relationships of magma mingling zones a mixing model is suggested that can operate on a large scale to explain the petrogenesis of the hercynian calc alkaline granodiorites of iberia finally a modified classification of common plutonic rocks in orogenic environments is proposed

a review of criteria for the identification of magmatic and tectonic foliations in granitoids abstract foliations in granitoids can form by magmatic flow submagmatic flow high temperature solid state deformation and moderate to low temperature solid state deformation a review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent diapiric emplacement and expansion emplacement during regional deformation or regional deformation post dating emplacement however a magmatic origin is favoured for foliations defined by the alignment of igneous commonly euhedral minerals particularly where the foliation is parallel to internal or external pluton contacts foliations formed during expansion or ballooning of diapirs may be strictly magmatic in origin although some studies suggest that solid state deformation also may occur if so we would hope to find evidence of deformation of crystal melt systems and that the solid state deformation occurred at high temperatures the inference of syntectonic foliations is most convincing where magmatic and high temperature solid state foliations are subparallel these foliations are continuous with regionally developed foliations in the wall rocks synkinematic porphyroblasts are present in the wallrocks and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage a strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals no alignment of igneous minerals occurs the foliation is locally at high angles to pluton wallrock contacts and the foliation is continuous with a regionally developed cleavage

submagmatic microfractures in granites microstructures in hercynian granite plutons of the pyrenees indicate fracturing of plagioclase crystals in the presence of melt this is represented mainly by quartz and or feldspar that fills fractures electron microprobe analyses demonstrate that in compositional agreement with the rims of the zoned plagioclase the feldspar infillings are more sodic with decreasing melt fraction fracturing resulted from concentration of stress at contacts between grains while the granite was submagmatic i e when the melt was below the critical fraction for magmatic flow material compositionally far from eutectic is petrologically the best candictate to develop submagmatic fractures because during cooling the volume fraction of melt remains low for a large temperature range a strong tendency for microfractures to be oblique to flow foliation may constitute a new criterion for shear sense in magmas

petrology and geochemistry of the orbicular granitoid of caldera northern chile models and hypotheses on the formation of radial orbicular textures abstract the orbicular granitoid of caldera located at the northern part of the chilean coastal range is a spectacular example of radial textures in orbicular structures the orbicular body crops out as a 375 m 2 tabular to lensoidal intrusive sheet emplaced in the lower jurassic relincho pluton the orbicular structures are 3 7 cm in diameter ellipsoids hosted in a porphyritic matrix the orbicules are comprised by a qtz dioritic core 3 5 cm in diameter composed by pl hbl qtz bt kfs with equiaxial textures and a gabbroic shell 2 3 cm in diameter characterized by feathery and radiate textures with a plagioclase hornblende paragenesis the radial shell crystals are rooted and orthogonally disposed in the irregular contact with the core the radial shell called here inner shell is in contact with the granodioritic equiaxial interorbicular matrix through a 2 3 mm wide poikilitic band around the orbicule outer shell the outer shell and the matrix surrounding the orbicules are characterized by the presence of large hornblende and biotite oikocrystals that include fine grained rounded plagioclase and magnetite the oikocrystals of both the outer shell and the matrix have a circumferential arrangement around the orbicule i e orthogonal to the radial inner shell the coarse grained granodioritic interorbicular matrix present pegmatitic domains with large acicular hornblende and k feldspar megacrysts this work presents a review of the textural characteristics of the orbicules and a complete new mineral and whole rock geochemical study of the different parts of the orbicular granitoid together with thermobarometric and crystallographic data and theoretical modeling of the crystallization and element partitioning processes we propose a model for the formation of the orbicular radial textures consisting of several processes that are suggested to occur fast and consecutively superheating volatile exsolution undercooling geochemical fractionation and columnar and equiaxial crystallization according to the obtained results the formation of the orbicular granitoid of caldera may have initiated 1 during the generation of a magmatic fracture in the crystallization front of the relincho pluton where the water released by the host crystal mush was dissolved in the new batch of dioritic magma 2 the high influx of water rich liquids induced superheating conditions in the newly intruding magma that became a depolymerized liquid where the only solid particules were the small irregular fragments of the host mush dragged from the fracture walls 3 volatile exsolution promoted crystallization under undercooling conditions 4 undercooling and nucleation around the core cold germs involved the physical and geochemical fractionation between two sub systems a gabbroic sub system that comprises the solid paragenesis with a residual water rich liquid and a granodioritic sub system 5 the orbicules including core and inner shell behaved as viscous bodies crystals residual liquid floating in the granodioritic magma 6 higher undercooling rates occurred at the starting stage close to the liquidus promoting columnar crystallization around the cores and formation of the shells conversely in the granodioritic matrix sub system equiaxial crystallization was promoted by low relative crystallization rates 7 the rest of the crystallization process evolved later in the outer shell and the matrix as suggested by the poikilitic textures observed in both sides of the orbicule contact and under conditions close to the solidus of both sub systems shell and matrix the water rich residual liquid expelled during the orbicular shell crystallization was mingled with the partially crystallized matrix magma generating the pegmatitic domains with large kfs megacrysts

timing and nature of magmatic fabrics from structural relations around stoped blocks abstract structural relations around several large 100 m stoped blocks in the granodiorite of castle creek sierra nevada california suggest that magmatic fabrics formed as or after the space making phase of pluton emplacement had ended the stoped granite blocks are approximately 360 m below the exposed pluton roof magmatic foliations in granodiorite surrounding the blocks commonly follow km scale orientation trends to within 1 m of block contacts where they become deflected parallel to the block boundaries locally undeflected foliations occur within a few centimeters of block contacts mafic enclave fabric ellipsoid intensities 0 2 we draw the following conclusions concerning magmatic fabrics in the granodiorite of castle creek 1 magmatic fabrics formed during and or after the latest stages of pluton emplacement and thus contain little or no information about space making processes 2 magmatic fabrics had poor strain memory and 3 during magmatic fabric formation pluton strains were partially decoupled from host rock strains published data from other plutons indicate that these relationships may be common particularly in the brittle upper crust it follows from these conclusions that even in the best case magmatic fabrics contain only qualitative information about pluton emplacement mechanisms regional tectonics and internal magma chamber processes interpretations of magmatic fabrics must be corroborated by evaluating causes kinematics and magnitudes of syn emplacement host rock deformation in the worst case the causes of magmatic fabrics cannot be uniquely determined

hydrological impact of heinrich events in the subtropical northeast atlantic reconstructing the impact of heinrich events outside the main belt of ice rafting is crucial to understanding the underlying causes of these abrupt climatic events a high resolution study of a marine sediment core from the iberian margin demonstrates that this midlatitude area was strongly affected both by cooling and advection of low salinity arctic water masses during the last three heinrich events these paleoclimatic time series reveal the internal complexity of each of the last three heinrich events and illustrate the value of parallel studies of the organic and inorganic fractions of the sediments

the tropical rainbelt and productivity changes off northwest africa a 31 000 year high resolution record abstract benthic foraminiferal assemblage compositions and sedimentary geochemical parameters were analyzed in two radiocarbon dated sediment cores from the upwelling area off nw africa at 12 n to reconstruct productivity changes during the last 31 kyr high latitude cold events and variations in low latitude summer insolation influenced humidity wind systems and the position of the tropical rain belt over this time period this in turn caused changes in intensity and seasonality of primary productivity off the southern northwest african continental margin high accumulation rates of benthic foraminifera carbonate and organic carbon during times of north atlantic melt water events heinrich 2 25 4 to 24 3 kyr bp and 1 16 8 to 15 8 kyr bp indicate high productivity dominance of infaunal benthic foraminiferal species and high numbers of deep infaunal specimens during that time indicate a strong and sustained supply of refractory organic matter reworked from the upper slope and shelf a more southerly position of the tropical rainbelt and the northeast trade wind belt during heinrich 2 and 1 may have enhanced wind intensity and almost permanent upwelling driving this scenario a phytodetritus related benthic fauna indicates seasonally pulsed input of labile organic matter but generally low year round productivity during the last glacial maximum 23 to 18 kyr bp the tropical rainbelt is more expanded to the north than during heinrich events and relatively weak ne trade winds resulted in seasonal and weak upwelling thus lower productivity high productivity characterized by a seasonally high input of labile organic matter is indicated for times of orbital forced warming such as the african humid period 9 8 to 7 kyr bp an intensified african monsoon during boreal summer and the northernmost position of the tropical rainbelt within the last 31 kyr resulted in enhanced river discharge from the northward extended drainage area initiating intense phytoplankton blooms in the late holocene 4 to 0 kyr bp strong carbonate dissolution may have been caused by even more enhanced organic matter fluxes to the sea floor increasing aridity on the continent and stronger ne trade winds induced intensive seasonal coastal upwelling

late hercynian extensional and transcurrent tectonics in central iberia abstract at the end of the hercynian orogeny the central iberian crust which had been thickened by nappe tectonics and heated weakened overthickened by massive granitic intrusions was subjected to extensional tectonics through lithospherescale low angle detachment systems thus releasing the gravitational potential inherent in thickened crust these detachment faults evolved by tectonic denudation isostatic rebound and upward arching to define an extensionaltardi hercynian eth basin and range type province with upper plate listric high angle normal faults delimiting tilted blocks and asymmetric troughs occasionally filled with permian sediments and volcanics n s oriented transfer faults and synform antiform pairs with denuded metamorphic core complexes in the antiforms and local compressional features in the synforms postdating the gravitational collapse of the thickened crust the residual late hercynian e w oriented compressional stresses triggered a ductile transcurrent tardi hercynian dtth event this is expressed by two major shear zones and by the extrusion and deformation of porphyry dyke swarms along the previously thinned and weakened antiformal trends finally interplate stresses related to a continental scale shear zone gave rise to a brittle transcurrent tardi hercynian btth event with different sets of brittle faults indicating a n s oriented compression the present geological features are the result of these events overprinted by the alpine reactivation of the entire complex

sahel megadroughts triggered by glacial slowdowns of atlantic meridional overturning 1 the influence of the large scale ocean circulation on sahel rainfall is elusive because of the shortness of the observational record we reconstructed the history of eolian and fluvial sedimentation on the continental slope off senegal during the past 57 000 years our data show that abrupt onsets of arid conditions in the west african sahel were linked to cold north atlantic sea surface temperatures during times of reduced meridional overturning circulation associated with heinrich stadials climate modeling suggests that this drying is induced by a southward shift of the west african monsoon trough in conjunction with an intensification and southward expansion of the midtropospheric african easterly jet

late magmatic healed fractures in granitoids and their influence on subsequent solid state deformation abstract late magmatic fractures developed during final stages of magma crystallization are recorded in many granitoid plutons by planar trails of magmatic biotite and or leucocratic veins but have not been previously described such biotite rich trails with lengths of a few centimetres to several tens of centimetres widths of a few millimetres and a variable orientation are ubiquitous in meta granitoids of the neves area tauern window eastern alps italy these pre alpine granitoids were metamorphosed to upper amphibolite facies and deformed during the alpine orogeny allowing the effect of planar healed magmatic fractures on the nucleation of subsequent solid state ductile shear zones to be directly assessed numerical models considering power law viscous materials predict that planar arrays of a weaker mineral e g biotite oriented at around 45 to the shortening direction should nucleate and localize viscous deformation however no discernible localization on the natural small scale biotite trails is observed even when they delineate planes that were well oriented for shear reactivation in contrast heterogeneous shear zones nucleated on larger scale 1 10 m long planar compositional or structural boundaries such as joints dykes quartz veins alteration layers surrounding veins and zones of magma mingling outlined by densely packed clusters of basic enclaves irrespective of their orientation relative to the imposed shortening direction clearly some crucial aspect is missing from purely viscous numerical models we propose that this missing aspect is the observed interplay between fracture and flow with new fractures developing and localizing shear at bulk strains too low for discernible localization to occur on the pre existing healed magmatic fractures natural granitoids have a truly elasto plasto viscous rheology even at low differential stress as in the neves example and for high grade metamorphic conditions considered as typical of wet ductile middle crust

dike transport of granitoid magmas thermal and fluid dynamical analyses suggest that for viscosities and density contrasts spanning the range considered typical for many calc alkalic granitoids dike ascent is a viable mechanism for the transport of large volumes of granitoid melt through the continental crust we present calculations showing that a granitoid melt with calculated viscosity of the order of 10 6 pa s and a density contrast between magma and crust of 200 kg m 3 can be transported 30 km through the crust in 1 month corresponding to a mean ascent velocity of 1 cm s using analysis modified from numerical studies of the flow of basaltic magmas in dikes we also present an expression that allows the calculation of the critical minimum dike or fault width required for granitic magma to ascend without freezing for all reasonable estimates of cordilleran granitoid viscosity and density contrast the critical dike width is determined to be between 2 and 7 m calculated peak batholith filling rates are orders of magnitude greater than mean cavity opening rates based on estimated fault slippage which is consistent with chemical evidence for intermittent supply of magma pulses

extremely fluid behavior of hydrous peralkaline rhyolites the viscosities of a series of water bearing peralkaline rhyolitic melts have been experimentally determined the dry melt compositions are composed of a series of additions of na2o to a metaluminous base composition the melts initially hydrated at high pressures and quenched isobarically have been prepared by cutting and polishing then reheating across the glass transition at 1 atm where they are annealed to a relaxed metastable state and then investigated dilatometrically using micropenetration methods the measurements have been performed in the viscosity range of 108 5 1011 5 pa s which corresponds to temperatures in the range of 675 220 c for these compositions despite the relatively low viscosities of dry peralkaline melts in comparison with metaluminous melts of similar sio2 content the viscosities of peralkaline rhyolitic melts also decrease strongly and non linearly with the addition of water the resulting viscosity temperature relationships for water bearing peralkaline rhyolitic melts are shifted to much lower temperatures such that glass transition temperatures for moderate cooling rates correspond to extraordinarily low temperatures a model is presented for the calculation of melt viscosities in the range of 108 5 1011 5 pa s for peralkaline rhyolites with up to 7 wt h2o the very fluid nature of these peralkaline rhyolites over a wide range of water contents may facilitate a very efficient degassing history of glassy peralkaline rhyolites in nature efficient degassing might explain the apparent contradiction of the presence of common water rich melt inclusions in phenocryst phases hosted in water free glassy rhyolites versus the absence of vesicular layers or textural evidence for a vesicular past for the glassy rocks