the dynamics of southern ocean storm tracks abstractthe mechanisms that initiate and maintain oceanic storm tracks regions of anomalously high eddy kinetic energy are studied in a wind driven isopycnal primitive equation model with idealized bottom topography storm tracks are found downstream of the topography in regions strongly influenced by a large scale stationary meander that is generated by the interaction between the background mean flow and the topography in oceanic storm tracks the length scale of the stationary meander differs from that of the transient eddies a point of distinction from the atmospheric storm tracks when the zonal length and height of the topography are varied the storm track intensity is largely unchanged and the downstream storm track length varies only weakly the dynamics of the storm track in this idealized configuration are investigated using a wave activity flux related to the eliassen palm flux and eddy energy budgets it is found that vertical fluxes of wave activity which correspond to eddy growth

the poleward motion of extratropical cyclones from a potential vorticity tendency analysis abstractthe poleward propagation of midlatitude storms is studied using a potential vorticity pv tendency analysis of cyclone tracking composites in an idealized zonally symmetric moist gcm a detailed pv budget reveals the important role of the upper level pv and diabatic heating associated with latent heat release during the growth stage the classic picture of baroclinic instability emerges with an upper level pv to the west of a low level pv associated with the cyclone this configuration not only promotes intensification but also a poleward tendency that results from the nonlinear advection of the low level anomaly by the upper level pv the separate contributions of the upper and lower level pv as well as the surface temperature anomaly are analyzed using a piecewise pv inversion which shows the importance of the upper level pv anomaly in advecting the cyclone poleward the pv analysis also emphasizes the crucial role played by latent heat release in the poleward motion of the cyclone the l

trans alkaline magmatism in the serrinha pedro velho complex borborema province ne brazil and its correlations with the magmatism in eastern nigeria abstract the sol nea complex 572 8 ma comprises various intrusions that total 200 km 2 it cuts palaeoproterozoic migmatized orthogneisses of the serrinha pedro velho complex which is part of the sao jose do campestre massif north tectonic domain of the borborema province ne brazil it is a post collisional intrusion constituted of coarse grained biotite amphibole monzogranites fine grained leucogranites and diorites major trace and rare earth elements data used to constrain the origin and nature of these rocks indicate that the monzogranites display chemical features of weakly peraluminous ferro potassic trans alkaline granitoids similar to coeval occurrences described in eastern nigerian sm nd data together with the whole rock geochemical data suggest that i the monzogranites were generated by partial melting of tonalitic gneiss from the lower crust leaving behind a granulitic residue ii the diorites originated by partial melting of an enriched lithospheric mantle iii the evolution of the sol nea complex involved magma mixing between the monzogranites and diorites k feldspar and amphibole fractionation appear to have occurred during the evolution of the monzogranites leucogranites constituting an elliptical shaped intrusion 80 km 2 the dona ines pluton are intruded to the north of the sol nea complex they also occur as small intrusions cross cutting the border of the sol nea complex geochemical and isotopic data suggest that the leucogranites were generated by melting of crustal palaeoproterozoic rocks rb sr data gave an age of 544 16 ma trans alkaline magmatism is associated with ductile brittle deformation in the serrinha pedro velho complex of the sao jose do campestre massif this event follows the peak of an extensional related high temperature metamorphism upper amphibolite to granulite facies with estimated age of 578 to 574 ma if a pre gondwana reconstruction is considered this event is seen to extend to the african side of gondwana in eastern nigeria

pan african tectonics in northwestern cameroon implication for the history of western gondwana abstract three main tectonic events related to pan african collision and post collision evolution have been identified in cameroon i crustal thickening ii left lateral wrench movements and iii right lateral wrench movements successively correlation of these events throughout the pan african belt and an integrated synthesis that includes the domains of central africa trans sahara and ne brazil enable us to define an indent type structure in northwestern cameroon this one suggests the existence of a rigid prong in this area which collided with the sao francisco congo craton sfcc active margin at ca 640 ma prior to subsequent widespread remobilization and granitization that partly affected both landmasses between 640 and 580 ma major indent related shear zones were further reactivated during regional clockwise rotation of the newly stabilized domain of northwestern cameroon suggesting that collision with the west african craton wac was still active after 580 ma geodynamic implications of this model are discussed in the context of the pan african global evolution in western gondwana

poleward deflection of storm tracks an analysis of 11 years of european centre for medium range weather forecasts data focuses primarily on the vertically averaged high frequency transients the conclusions are discussed in the context of a the winter storm track b monthly variability and c interannual variability a winter storm track results show that the pattern of the forcing by the high frequency eddies along the storm track is highly correlated with the stationary circulation and the forcing itself is primarily responsible for the location of the trough ridge system associated with the stationary flow the results also clarify the role of wind component covariance terms u9y9 and y9 2 2 u9 2 in the column averaged vorticity forcing the simpler term u9y9 has the well known effect of intensifying the anticyclonic cyclonic tendencies on the southern northern side of the jet thereby producing an increase in the barotropic component of the zonal jet the y9 2 2 u9 2 term displays a quadrupole pattern which is also approximately in phase with the trough ridge system associated with the stationary flow b monthly variability eddy activity has been shown to possess a seasonal life cycle increasing during the early fall and reaching a maximum around the month of november then decaying for most of the winter months month to month variations in eddy activity over the pacific ocean show that energy levels increase up through november decreasing thereafter at the same time the trough ridge circulation pattern is intensifying by december baroclinicity in the western pacific has increased substantially and low level eddies are found to break by the middle of the ocean upper level eddies start to break well before reaching the west coast of north america resulting in a displacement of the maximum in y9 2 2 u9 2 westward from its november position and increasing the trough ridge forcing by the high frequency eddies c interannual variability wintertime eddy kinetic energy is seen to extend further eastward through the pacific ocean during the warm phase but displays an abrupt termination during the cold phase anomalies in the eddy transient forcing tend to be quite similar to that of the pacific north american pattern itself the extension of the storm track during the warm phase resembles that of fall conditions and is present in the winter season because the source of low level baroclinicity

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

an eclogite paragenesis from the aleksod basement central hoggar south algeria abstract eclogite pods and metadolerite dykes occur within the central part of the hoggar precambrian shield a polycyclic domain eburnean event 2000 ma early pan african event 600 750 ma late pan african event 550 ma field relations indicate that the eclogite pods are located within mylonitic zones of early pan african age underlying the contact between metasedimentary and orthogneissic rocks dolerite dykes crosscut only the orthogneissic basement from textural and mineralogical observations of these pods a progressive decrease of the p t regime during a progressive increase of shearing are obvious and correspond first to foliated hydrous eclogite appearance pods core 750 c p min 15 kbar then to foliated amphibolite formation pods margin 650 c p 4 5 kbar late eburnean dykes showing relics of dolerite textures are folded during the pan african event this foliation corresponds to corona forming reactions which transform primary magmatic assemblages into garnet amphibolite parageneses just inside the margin a retrogression towards amphibolite facies conditions is observed so we must consider that the eclogitisation process occurred only inside the zone of shearing because the eclogite pods and metadolerite dykes have not had the same origin and the same evolution during the pan african tangential tectonic event eclogite pods which crystallized at depth have been overthrusted towards supracrustal levels whilst dolerite dykes which cooled near the surface have been buried towards amphibolite facies levels

geophysical and petrological evidence for the presence of an anomalous upper mantle beneath the sahara basins algeria abstract north of 20 n the northwest part of the stable african shield is characterized by an anomalous high heat flow zone 80 110 mw m 2 this east west elongated zone is oblique to major proterozoic structural units and cannot be related to differences in crustal heat production maximum heat flow occurs in the southern part of the sahara basins 100 120 mw m 2 only a mantle heat flow contribution of 60 70 mw m 2 can account for this very high surface heat flow the estimated geotherms suggest that the temperature of upper mantle is anomalously high and partial melting may be present at shallow depths this hypothesis is supported by the existence of a long wavelength negative component of the gravity field that corresponds with the high heat flow axis and with the very low lithospheric s wave velocities 4 2 4 4 km s down to 160 km obtained from surface wave studies in addition the chemistry of the scarce cenozoic melilitite lavas illizi volcanic district geographically associated with the thermal anomaly as well as the mineralogy of their inclusions phlogopitized garnet spinel peridotite and alkali clinopyroxenite show that the upper mantle beneath this region is locally highly metasomatised and partially melted from these preliminary results it appears that the northwest african upper mantle has been modified by recent thermal rejuvenation processes including gas transfer that are likely to be more extensive in the southern part of the sahara basins

storm track dynamics this paper reviews the current state of observational theoretical and modeling knowledge of the midlatitude storm tracks of the northern hemisphere cool season observed storm track structures and variations form the first part of the review the climatological storm track structure is described and the seasonal interannual and interdecadal storm track variations are discussed in particular the observation that the pacific storm track exhibits a marked minimum during midwinter when the background baroclinicity is strongest and a new finding that storm tracks exhibit notable variations in their intensity on decadal timescales are highlighted as challenges that any comprehensive storm track theory or model has to be able to address physical processes important to storm track dynamics make up the second part of the review the roles played by baroclinic processes linear instability downstream development barotropic modulation and diabatic heating are discussed understanding of these processes forms the core of our current theoretical knowledge of storm track dynamics and provides a context within which both observational and modeling results can be interpreted the eddy energy budget is presented to show that all of these processes are important in the maintenance of the storm tracks the final part of the review deals with the ability to model storm tracks the success as well as remaining problems in idealized storm track modeling which is based on a linearized dynamical system are discussed perhaps on a more pragmatic side it is pointed out that while the current generation of atmospheric general circulation models faithfully reproduce the climatological storm track structure and to a certain extent the seasonal and enso related interannual variations of storm tracks in depth comparisons between observed and modeled storm track variations are still lacking

acc meanders energy transfer and mixed barotropic baroclinic instability along stream variations in the dynamics of the antarctic circumpolar current acc impact heat and tracer transport regulate interbasin exchange and influence closure of the overturning circulation topography is primarily responsible for generating deviations from zonal mean properties mainly through standing meanders associated with regions of high eddy kinetic energy here an idealized channel model is used to explore the spatial distribution of energy exchange and its relationship to eddy geometry as characterized by both eddy momentum and eddy buoyancy fluxes variations in energy exchange properties occur not only between standing meander and quasi zonal jet regions but throughout the meander itself both barotropic and baroclinic stability properties as well as the magnitude of energy exchange terms undergo abrupt changes along the path of the acc these transitions are captured by diagnosing eddy fluxes of energy and by adopting the eddy geometry framework the latter typically applied to barotropic stability properties is applied here in the depth along stream plane to include information about both barotropic and baroclinic stability properties of the flow these simulations reveal that eddy momentum fluxes and thus barotropic instability play a leading role in the energy budget within a standing meander this result suggests that baroclinic instability alone cannot capture the dynamics of acc standing meanders a challenge for models where eddy fluxes are parameterized

investigating high zircon concentrations in the fine fraction of stream sediments draining the pan african dahomeyan terrane in nigeria sixteen hundred stream sediments 150 m fraction collected during regional geochemical surveys in central and sw nigeria have high median and maximum concentrations of zr that exceed corresponding zr concentrations found in stream sediments collected from elsewhere in the world with similar bedrock geology x ray diffraction studies on a sub set of the analysed stream sediments showed that zr is predominantly found in detrital zircon grains however the main proximal source rocks pan african older granites of nigeria and their proterozoic migmatitic gneiss country rocks are not enriched in zircon or zr nevertheless u pb la icp ms dating with cathodoluminescence imaging on detrital zircons both from stream sediment samples and underlying pan african older granites confirms a local bedrock source for the stream sediment zircons a combination of tropical chemical weathering and continuous physical weathering both by wet season flash flooding and dry season unidirectional winds are interpreted to have effectively broken down bedrock silicate minerals and removed much of the resultant clay phases thereby increasing the zr contents in stream sediments the strong correlation between winnowing index th al and zr concentration across the study area support this interpretation therefore anomalous high values of zr as well as other elements concentrated in resistant heavy minerals in nigeria s streams may not reflect proximal bedrock concentrations of these elements this conclusion has important implications for using stream sediment chemistry as an exploration tool in nigeria for primary metal deposits associated with heavy minerals

mechanisms controlling the downstream poleward deflection of midlatitude storm tracks abstractthe atlantic and pacific storm tracks in the northern hemisphere are characterized by a downstream poleward deflection which has important consequences for the distribution of heat wind and precipitation in the midlatitudes in this study the spatial structure of the storm tracks is examined by tracking transient cyclones in an idealized gcm with a localized ocean heat flux the localized atmospheric response is decomposed in terms of a time and zonal mean background flow a stationary wave and a transient eddy field the lagrangian tracks are used to construct cyclone composites and perform a spatially varying pv budget three distinct mechanisms that contribute to the poleward tilt emerge transient nonlinear advection latent heat release and stationary advection the downstream evolution of the pv composites shows the different role played by the stationary wave in each region in the region where the tilt is maximized all three mechanisms contribute to the poleward propagation of the

geodynamic evolution of the pan african belt in central africa with special reference to cameroon the pan african belt in central africa has benefited from the many petrographic structural and geochronological studies in the recent years that have improved our understanding of the belt however those studies have also produced various and often divergent evolutionary models for the belt some of which do not even involve well defined cratons following a review of the available data in cameroon we propose a model of continent continent collision that involved the congo craton and the north central cameroon active margin showing archean to paleoproterozoic inheritances this model is based among others on i the prominent role of the congo craton as demonstrated by the regional extension of external nappes on its northern edge and the concomitant exhumation of the 620 ma granulitic rocks believed to have formed at the root of the collision zone and ii the late development of a strike slip fault system in central cameroon as the result of horizontal movement following the multistage collision

pan african granitoid emplacement in the adrar des iforas mobile belt mali a rb sr isotope study abstract time and space relationships between pan african intrusives in the iforas belt northeast mali are discussed on the basis of structures lithological composition and plutonism three domains have been defined these are from west to east 1 the tilemsi accretion domain 2 the central domain characterized by an early pan african tectonism with large scale basement nappes and 3 the eastern domain a later pan african trough deposited on continental basement domains 1 and 2 are separated by a huge complex batholith geochronological results confirm the structural distinctions and the existence of two major events as shown by rb sr isochron ages domains 2 and 3 were stabilized 645 ma ago while the multistage evolution of the batholith can be bracketed between 614 and 570 ma rb sr biotite whole rock pairs give very similar ages which indicates a short time lapse between emplacement and cooling the initial ratios in the range 0 705 0 708 are as in many other batholiths slightly higher than the mantle growth curve indicating a complex origin where crust and mantle are involved as granitoids occupy some 60 of the surface area in the iforas belt the pan african epoch corresponds to an important crustal addition in the iforas segment

the effects of orography on midlatitude northern hemisphere dry climates abstract the role of mountains in maintaining extensive midlatitude arid regions in the northern hemisphere was investigated using simulations from the gfdl global climate model with and without orography in the integration with mountains dry climates were simulated over central asia and the interior of north america in good agreement with the observed climate in contrast moist climates were simulated in the same regions in the integration without mountains during all season but summer large amplitude stationary waves occur in response to the tibetan plateau and rocky mountains the midlatitude dry regions are located upstream of the troughs of these waves where general subsidence and relatively infrequent storm development occur and precipitation is thus inhibited in summer this mechanism contributes to the dryness of interior north america as a stationary wave trough remains east of the rockies but is not effective in eurasia due to seasonal changes in the atmospheric circulation the dryness

jets and orography idealized experiments with tip jets and lighthill blocking abstract this paper describes qualitative features of the generation of jetlike concentrated circulations wakes and blocks by simple mountainlike orography both from idealized laboratory experiments and shallow water numerical simulations on a sphere the experiments are unstratified with barotropic lee rossby waves and jets induced by mountain orography a persistent pattern of lee jet formation and lee cyclogenesis owes its origins to arrested topographic rossby waves above the mountain and potential vorticity pv advection through them the wake jet occurs on the equatorward eastern flank of the topography a strong upstream blocking of the westerly flow occurs in a lighthill mode of long rossby wave propagation which depends on a2 u the ratio of rossby wave speed based on the scale of the mountain to zonal advection speed u is the meridional potential vorticity gradient f is the coriolis frequency and a is the diameter of the mountain mountains wider north south than the east west

the potassic granites of the igbeti area further evidence of the polycyclic evolution of the pan african belt in southwestern nigeria abstract field geochemical and isotopic studies on granitic rocks in the igbeti area of southwestern nigeria indicate that this pan african belt was affected by two episodes of granite plutonism represented by an older augen gneiss and a younger relatively undeformed coarse grained porphyritic granite these rocks have similar major element characteristics but their trace element compositions are sufficiently different to warrant a suggestion that they are probably of different origin rb sr whole rock isotopic analysis give an eburnean age of 1904 18 ma for the augen gneisses and a pan african age of 617 37 ma for the coarse grained porphyritic granite a low initial 87 sr 86 sr ratio 0 7005 0 0025 for the augen gneiss suggests derivation from the mantle or a low rb sr source whereas a value of 0 7120 0 0016 for the coarse grained porphyritic granite indicates a significantly older crustal component in the magma alkaline magmatism may have been an important component of orogenic magmatism during the eburnean event

deformation and related radiochronology in a late pan african mylonitic shear zone adrar des iforas mali the influence of a deformation on the zircon u pb system was studied using the example of the western margin of the iforas granulitic unit mali where undeformed sub alkaline leptynites have evolved to strongly foliated mylonites and then to ultramylonites the deformation episod was late pan african and occurred at low temperature zircons show evidences of a brittle behavior with microcracks development then fracturation of the crystals and dispersion of the fragments in the ductile matrix an equilibrium grain size was reached which probably depended of the stress but was independant of the initial size of the zircons and of the strain

upper proterozoic schist belts in the nigerian sector of the pan african province of west africa abstract schist belts in nigeria occur in a 400 km wide zone which trends nne parallel to the boundary between the pan african province and the west african craton a polycyclic migmatite gneiss granite terrain ranging in age from archaean to lower palaeozoic flanks the schist belt zone to east and west and separates the individual belts within the zone the schist belts fall into two age groups the earlier group contains assemblages of mafic igneous rocks pelitic schists and phyllites banded iron formation and locally coarse grained clastics and carbonate rocks these belts show complex structural styles and are extensively invaded by granitic plutons belonging to the widely distributed pan african magmatic suite schists and gneisses within and adjacent to some of these belts have been dated at 1100 ma and this group is thought to belong to the kibaran orogeny similar kibaran rock associations can be recognised elsewhere in the pan african province in the hoggar and cameroun the kibaran orogeny in this region is interpreted as a largely ensialic event although with possible minor ocean basin formation that led into the more intense and widespread pan african orogeny the later group of schist belts is characterised by coarse to fine grained clastics insignificant mafic igneous rocks a simple straight structural style and an absence of internal granite plutons these schist belts are believed to belong to the pan african orogeny their sediments may have been deposited in a back arc basin related to an e dipping subduction zone at the western margin of the pan african province

on the dynamics of a storm track abstract an idealized primitive equation model is used to determine the factors controlling the dynamics and maintenance of eddy activity in a storm track the results show that localized regions of enhanced baroclinicity do not necessarily lead to localization of eddy activity by studying the energetics of the storm track it is shown that while baroclinic conversion does indeed correlate with the region of maximum baroclinicity it is the downstream radiation of energy through the ageostrophic geopotential fluxes which acts as a trigger for the development and maintenance of eddy activity over less baroclinic regions extending the region of eddy activity much further downstream from the region of high baroclinicity examples of eddy life cycles are given that show that convergence and divergence of ageostrophic fluxes can dominate baroclinic and barotropic conversion especially in regions with weak baroclinicity factors that may limit the zonal extent of a storm track are discussed evidence of down

rare metal pegmatites of wamba central nigeria their formation in relationship to late pan african granites he sn nb ta mineralization of the wamba field central nigeria occurs in muscovite quartz microcline pegmatites which are related to the late orogenic pan african f 550 ma older granites the emplacement of granites and pegmatites was controlled by late pan african shear tectonics the granitoid magmatism was multiphase and has produced peraluminous biotite granite biotite muscovite granite and muscovite granite plutons sodic metasomatism has altered highly evolved granite cupolas and many of the pegmatite dikes the pegmatitic mineralization of predominantly cassiterite is closely associated with albitization chemical data of granites and granitic and pegmatitic muscovites show that rb cs sn nb and ta are enriched during both magmatic and postmagmatic evolution with highest contents of these elements in early muscovites of the albitized and mineralized pegmatites trace element chemistry of the pegmatitic muscovites reveals a chemical zonation of the pegmatite field related to the late orogenic shear system

effect of common bean phaseolus vulgaris on apatite weathering under elevated co2 abstract plants accelerate chemical weathering of minerals through the production of carbonic acid during root respiration root exudation and uptake of ions these plant processes are increased by elevated concentrations of atmospheric co2 and may in turn affect mineral weathering we seek to understand the effect of predicted atmospheric co2 concentrations on plant mediated soil mineral weathering phaseolus vulgaris common bean was grown in flow through microcosms consisting of a mixture of 85 quartz and 15 apatite sands by weight nutrient release from apatite was measured using plants grown under two atmospheric conditions ambient co2 400 ppm and elevated co2 1000 ppm to sustain plant growth a nutrient solution without calcium ca or phosphorous p was supplied to the plants using peristaltic pumps unplanted microcosms receiving the same nutrient solution served as abiotic controls in both co2 treatments using atomic absorption spectroscopy and colorimetry ca and p concentrations of the leachate and plant tissue were measured as proxies for apatite dissolution plants were harvested every 2 weeks during the 8 week experiment to understand how ca and p release in the two co2 treatments changed over time after 8 weeks p vulgaris grown in elevated co2 had a greater root to shoot ratio than plants grown in ambient co2 plants grown in elevated co2 released more p than plants grown in ambient conditions possibly due to greater demands for mineral nutrients p was preferentially released by the plants and there were no significant differences in ca released by plants both elevated and ambient planted microcosms had a lower ph than abiotic controls likely due to root respiration nutrient uptake and exudation of organic acids organic acids were not identified in the leachate likely due to their low concentrations and rapid breakdown plants released as much as 7 times more ca than abiotic controls regardless of co2 concentrations biotic experiments released over 200 times more p than abiotic experiments our results show increased atmospheric co2 can determine rhizosphere processes which in turn affect weathering products thus mineral nutrient fluxes and c sequestration in inorganic pools are likely to increase as co2 concentrations rise

where are the eburnian transamazonian collisional belts the reconstruction of early proterozoic crustal evolution and geodynamic environments in africa and south america is incomplete if cratonic areas alone are studied if the presence of high grade

a tectonic model for the location of palaeozoic ring complexes in air niger west africa the fir region in niger is one of the largest peralkaline granite provinces in the world in addition to the granites some plutons are characterized by an abundance of plagioclase rich cumulates gabbros troctolites and anorthosites with subordinate metaluminous granites and quartz syenites these anorogenic ring complexes were intruded along a 400 km zone near the 9 e meridian the province formed during a major magmatic event newly dated at around 407 8 ma and comprises 28 plutons which range from 0 8 to 65 km in diameter and show no correlation between distribution and age remote sensing over the whole fir massif demonstrates the existence of two main trends of faults or lineaments n50 e n90 e and n120 e n150 e autocorrelation analysis of the ring complexes reveals three large parallel high density strips orientated n20 e based on the structural and geological setting coupled with map analysis a new tectonic model for the location of the complexes is presented we propose a dextral n5 e shear zone model where the emplacement of the ring structures is controlled by n20 e dextral riedel shear r with a secondary n50 e tensional gash t this tectonic model implies a control by lithospheric structures for the emplacement of the ring complexes as well as a relationship between a transtensional tectonic regime and intra plate alkaline magmatism the new geochronological data on the ayir massif allow us to derive a silurian devonian palaeomagnetic pole representative for gondwana which fits data for the other continents better than previous estimates

configuration of a southern ocean storm track abstractdiagnostics of ocean variability that reflect and influence local transport properties of heat and chemical species vary by an order of magnitude along the southern ocean s antarctic circumpolar current acc topographic hotspots are important regions of localized transport anomalies this study uses a primitive equation channel model to investigate the structure of eddy kinetic energy eke one measure of variability in an oceanic regime a storm track approach emphasizes the importance of stationary eddies which result from flow interactions with topography on setting eke distributions the influence of these interactions extends far downstream of the topography and impacts eke patterns through localized convergence and divergence of heat unlike for zonal averages local contributions to the stationary fluxes from terms that integrate to zero in a zonal average are important the simulations show a strong sensitivity of the zonal structure as well as the distribution and amplitude of sta

u pb pan african ages of two charnockite granite associations from southwestern nigeria u pb dating of zircons from two separate charnockite granite associations from s w nigeria previously considered to be of kibaran or archean age yield panafrican emplacement ages of 631 18 ma and 586 5 ma respectively when all the data points of each association are regressed together these u pb data on zircons do not permit a confirmation of the field established sequence of rock emplacement within each charnockite granite association thus synchronous ages of 620 20 ma charnockite nig 4 634 21 ma charnockite nig 7 and 621 10 ma granite nig 12 are obtained for the three analyzed rocks of the akure ikerre ado ekiti association while those of the idanre association yield younger but synchronous ages of 580 10 ma charnockite nig 2 593 13 ma charnockite nig 3 and 587 10 ma granite nig 11 the ages obtained for the rocks of the akure ikerre ado ekiti association very well fit the existing data on the tectonic evolution of the basement complex of nigeria whereas those from the idanre area are difficult to reconcile with the current interpretation of tectonic data in this area

crust forming ages and proterozoic crustal evoluton in nigeria a reappraisal of current interpretations abstract radiometric ages in the nigerian basement cluster around 3 5 3 1 3 0 2 7 2 5 2 1 1 8 and 0 6 ga however not all these ages correspond to times of crust formation there are significant discrepancies between u pb and rb sr ages and nd and sr model ages with a definite trend for the orthogneisses and granitoids analysed results show good agreement between model ages tnd tsr and emplacement ages for early archaean and imply that these model ages date the time of crust mantle differentiation the archaean early proterozoic boundary was most probably a major crust forming period in sw nigeria as indicated by u pb zircon ages that are in agreement with nd model ages in that segment of the nigerian basement on the other hand late proterozoic and younger rocks which include migmatized gneisses pan african granitoids and jurassic younger granites have model ages in excess of their crystallization ages such model ages that lie between pan african or jurassic and well known older geologic ages do not correspond to any specific crust forming event rather they are mean crust residence ages reflecting contributions from juvenile material and remelted older archaean crustal components the overall scenario is consistent with widespread archaean crust and its involvement in the genesis of late proterozoic and younger crust in the nigerian basement this is in agreement with an intra cratonic setting far removed from the suture to the west

the role of stationary eddies in shaping midlatitude storm tracks transient and stationary eddies shape the extratropical climate through their transport of heat moisture and momentum in the zonal mean the transports by transient eddies dominate over those by stationary eddies but this is not necessarily the case locally in particular in storm track entrance and exit regions during winter stationary eddies and their interactions with the mean flow dominate the atmospheric energy transport here it is shown that stationary eddies can shape storm tracks and control where they terminate by modifying local baroclinicity simulations with an idealized aquaplanet gcm show that zonally localized surface heating alone e g ocean heat flux convergence gives rise to storm tracks which have a well defined length scale that is similar to that of earth s storm tracks the storm tracks terminate downstream of the surface heating even in the absence of continents at a distance controlled by the stationary rossby wavelength scale stationary eddies play a dual role within about half a rossby wavelength downstream of the heating region stationary eddy energy fluxes increase the baroclinicity and therefore contribute to energizing the storm track farther downstream enhanced poleward and upward energy transport by stationary eddies reduces the baroclinicity by reducing the meridional temperature gradients and enhancing the static stability transports both of sensible and latent heat water vapor play important roles in determining where storm tracks terminate

the tuareg shield terranes revisited and extended towards the northern gondwana margin magnetic and gravimetric constraints abstract the trans saharan belt is one of the most important orogenic systems constitutive of the pan african cycle which at the end of the neoproterozoic led to the formation of the gondwana supercontinent it is marked by the opening and closing of oceanic domains collision of continental blocks and the deformation of thick synorogenic sedimentary basins it extends from north to south over a distance of 3000 km in africa including the nigerian shield and the tuareg shield as well as their counterparts beneath the phanerozoic oil rich north and south saharan sedimentary basins in this study we take advantage of potential field methods magnetism and gravity to analyze the crustal scale structures of the tuareg shield terranes and to track these pan african structures below the sedimentary basins offering a new 1000 km extent the map interpretations are based on the classical potential field transforms and two dimensional forward modeling we have identified geophysical units and first order bounding lineaments essentially defined owing to magnetic and gravimetric anomaly signatures in particular we are able to highlight curved terminations which in the trans saharan context have been still poorly documented we provide for the first time a rheological map showing a categorization of contrasted basement units from the south of the tuareg shield up to the atlas belt these units highlight the contrasted rheological behavior of the tuareg tectonostratigraphic terranes during i the northerly pan african tectonic escape characteristic of the trans saharan belt and ii the north sahara basin development especially during intraplate reworking tied to the variscan event the discovery of a relatively rigid e w oriented unit to the south of the atlas system and on which the escaping pan african terranes were blocked offers a new perspective on the structural framework of the north gondwana margin it will help to understand how occurred the rendezvous of the n s oriented pan african terranes and the e w oriented cadomian peri gondwanan terranes

datation u pb sur zircons de l eclogite de la borie haut allier france et consequences sur l evolution ante hercynienne de l europe occidentale a crystallization age of 432 10 20 ma has been determined for the la borie eclogite haut allier france by u pb dating on zircons this age is yielded by the upper intercept with the concordia curve in agreement with recent dates obtained in the southern part of the massif central in brittany and in vendee these data indicate that the high pressure high temperature metamorphic phase representative of the pre variscan evolution of the western europe took place at the silurian ordovician boundary this high pressure high temperature metamorphism could be significant of subduction processes more or less synchronous preceding the main continental collision responsible of the variscan belt s s

structure and u pb geochronology of central hoggar algeria a reappraisal of its pan african evolution discovery of large scale deep seated thrusts in central hoggar with a plurifacial evolution ranging from lower amphibolite facies to upper greenschist facies conditions and linked to a regional refoliation has led us to reconsider the pan african tectonic and metamorphic history in that region two areas are described and a review of other thrusts leads to an interpretative cross section of a large portion of reactivated continental crust the age and kinematics of this structural reworking have been approached using u pb zircon dating in the tamanrasset region despite the difficulty of estimating the age of the initiation of the assumed intracontinental a type subduction the results provide a time span of 30 40 m y between the climax of granitoid emplacement and a late retrogressive offset along the thrust planes some key ages were determined 1 2075 30 ma is the age given by granulite facies remnants which escaped from the refoliation the corresponding lower intercept at 530 70 ma confirms the pan african imprint 2 615 5 ma reflecting the age of syntectonic to late tectonic granitoids emplaced in reworked gneisses and in preserved granulites 3 580 ma the concordant age of sphenes and monazites from the same granitoids which is interpreted as corresponding to the end of medium grade conditions no evidence has been found of a 1000 ma age a kibaran event does not appear to exist in central hoggar the age similarity between the observed deep intracontinental evolution of central hoggar and the collision related tectonics of western hoggar and iforas suggests a common origin for both phenomena

metamorphic zircons from north cameroon implications for the pan african evolution of central africa morphoscopic studies of zircons including optical microscopic and sem backscattered electron mode from the roniers valley west poli northern cameroon indicate an important crystallization of zircon during a medium grade garnet kyanite metamorphic event this was associated with a deformation d1 and with the emplacement of a basic and intermediate plutonic suite bip u pb dating of these zircon gave almost concordant pan african ages 630 ma which contribute to a solution of a major regional uncertainty about the age of the regional d1 event which in the mobile belt of central africa has been considered previously either as pan african or as lower to middle proterozoic a chronological framework is proposed for the evolution of northern cameroon

rossby wave packets on the midlatitude waveguide a review abstractrossby wave packets rwps are rossby waves for which the amplitude has a local maximum and decays to smaller values at larger distances this review focuses on upper tropospheric transient rwps along the midlatitude jet stream their central characteristic is the propagation in the zonal direction as well as the transfer of wave energy from one individual trough or ridge to its downstream neighbor a process called downstream development these rwps sometimes act as long range precursors to extreme weather and presumably have an influence on the predictability of midlatitude weather systems the paper reviews research progress in this area with an emphasis on developments during the last 15 years the current state of knowledge is summarized including a discussion of the rwp life cycle as well as rossby waveguides recent progress in the dynamical understanding of rwps has been based in part on the development of diagnostic methods these methods include algorithms to identify and track rwps

downstream self destruction of storm tracks the northern hemisphere storm tracks have maximum intensity over the pacific and atlantic basins their intensity is reduced over the continents downstream here simulations with an idealized aquaplanet general circulation model are used to demonstrate that even without continents storm tracks have a self determined longitudinal length scale their length is controlled primarily by the planetary rotation rate and is similar to thatofearth sstormtracksforearth srotationrate downstream stormtracksself destruct thedownstream eddy kinetic energy is lower than it would be without the zonal asymmetries that cause localized storm tracks likely involved in the downstreamself destruction ofstormtracks arethe energyfluxesassociatedwiththem the zonalasymmetries that causelocalized stormtracksenhancetheenergytransport throughthe generation of stationary eddies and this leads to a reduced baroclinicity that persists far downstream of the eddy kinetic energy maxima

impact of variable resolution meshes on midlatitude baroclinic eddies using cam mpas a abstractthe effects of a variable resolution mesh on simulated midlatitude baroclinic eddies in idealized settings are examined both aquaplanet and held suarez experiments are performed using the model for prediction across scales atmosphere mpas a hydrostatic dynamical core implemented within the national science foundation department of energy nsf doe community atmosphere model cam mpas a in the real world midlatitude eddy activity is organized by orography land sea contrasts and sea surface temperature anomalies in these zonally symmetric idealized settings transients should have an equal probability of occurring at any longitude however the use of a variable resolution mesh with a circular high resolution region centered at 30 n results in a maximum in eddy kinetic energy on the eastern side and downstream of this high resolution region in both aquaplanet and held suarez cam mpas a simulations the presence of a geographically confined maximum in both simulations suggests this response

petrology and u pb geochronology of the telohat migmatites aleksod central hoggar algeria the aleksod region is composed of metasedimentary rocks and large areas of biotite and hornblende bearing migmatites anatexis associated with the main deformation stages occurred under high pressure and temperature conditions estimated at 13 2 kbar and 750 50 c the bulk mineralogical composition of the telohat migmatites shows that their protolith was granodioritic internal structures of zircons and u pb data suggest a polyphased evolution with a 2131 12 ma age for the protolith and a 609 17 ma age for the pan african tectono metamorphic evolution thus precluding any kibaran event in the aleksod area leucosomes are richer in sr and display lower rb zr nb y th u and ree contents than melanosomes wherein accessory phases are stored eu contents are also lower in the leucosomes but in lesser proportion than the other rare earth s leading to a significant positive anomaly petrogenetic modelling accounting for accessory mineral phases clearly shows that the trace element contents of leucosomes and melanosomes follow a distribution law consistent neither with equilibrium nor fractional melting their trace element patterns are best explained by the model of disequilibrium melting with mixing of a few residual phases the present results and previous sr isotopic data as well raise the question of disequilibrium melting in anatexis of crustal material

an example of the relationship between rift and dome recent geodynamic evolution of the hoggar swell and of its nearby regions central sahara southern algeria and eastern niger abstract from essential gravimetrical and petrological data we propose a new model of geodynamic evolution for the volcanic hoggar swell and its nearby regions the eastern hoggar the tenere troughs and the north hoggar troughs correspond to a single distensive structure which developed during the late mesozoic by reactivation of n s and nw se trending pan african faults in response to the stress field probably associated with the central atlantic opening contemporaneously as suggested by the xenoliths entrained by the cenozoic alkali basaltic eruptions of the eastern hoggar an extensive lower crustal basic magmatism of tholeiitic affinity and probably associated with crustal stretching may have developed beneath the hoggarian part of this distensive structure on the other hand at the hoggar level a transverse lineament oriented ne sw oued amded cross cuts the distensive structure this lineament subsequently controlled the localization of the mio plio quaternary volcanic districts and possibly upper mantle modifications through reheating partial melting metasomatism and magmatic veining produced by astenospheric material transfer gas fluid kimberlitic to carbonatitic magmas resulting in a density reduction on the other hand the high deformation of the peridotitic inclusions of the cenozoic alkali basalts emplaced along the oued amded lineament may indicate that this lineament has also controlled the emplacement of small sized asthenospheric diapirs into the upper mantle thus in the hoggar it appears that doming was probably preceded by a rifting phase and that doming and cenozoic volcanism were possibly controlled by the intersection of this rift by a transverse wrench fault

u pb dating of the end of the pan african orogeny in the tuareg shield the post collisional syn shear tioueine pluton western hoggar algeria abstract the tioueine pluton intrudes the neoproterozoic series of the iskel terrane located in the tuareg shield western hoggar the consistency of the internal structures as well as the nature and organization of the associated microstructures demonstrate that the tioueine pluton was emplaced syn kinematically while n s strike slip shear zones were active the syn tectonic emplacement of the tioueine massif implies that this pluton although belatedly crystallized entirely belongs to the concept of post collisional magmatism in order to date precisely the late pan african tectono metamorphic event in the studied area an u pb age of 523 1 ma was obtained from abraded zircons of a late quartz syenite from the tioueine pluton this early cambrian age is younger than the other plutons of the tuareg shield which were mainly emplaced between 630 ma and 580 ma this dating also shows that the tuareg shield was not a single coherent block at 525 ma but rather an amalgam of active terranes moving each other along major shear zones finally the tioueine massif represents probably the final welding of the tuareg shield assembly of terranes and consequently the end of the post collisional orogenic episode in the whole pan african belt

charnockitic and monzonitic pan african series from north central nigeria trace element and nd sr pb isotope constraints on their petrogenesis abstract major and trace element studies indicate that the evolution of the calc alkaline charnockitic and sub alkaline monzonitic pan african rocks in north central nigeria cannot be explained in terms of a unique or classical process of fractional crystallisation at toro the crystallisation history of the more differentiated members appears more complex with the participation of accessory phases the quartz fayalite monzonites show evidence of a cumulative phase during their differentiation the fayalite bearing quartzmonzonites are not common rocks and their occurrence in the nigerian basement remains the only reported exposure in the pan african belt of west africa like the charnockitic suite at toro they were emplaced during the pan african and at deep structural levels unlike the adjacent high level jurassic peralkaline granites of jos plateau the application of multiple radiogenic tracers sr nd and pb indicates the involvement of pre existing crust in the formation of these rocks sr i 30 to 86 nd i 15 5 to 5 5 206 pb 204 pb i 16 34 17 71 and 207 pb 204 pb i 15 48 15 58 for toro charnockitic complex and sr i 19 to 51 and nd i 8 0 to 4 0 for the quartz fayalite monzonites of bauchi long crustal residence ages tnd of between 2 0 and 1 5 ga preclude their complete derivation from the mantle the bulk of evidence is consistent with the partial melting of a u poor source typical of the lower crust during the pan african orogeny and in particular the involvement of older precambrian crust

the dynamics of idealized convection schemes and their effect on the zonally averaged tropical circulation in this paper the effect of a simple convection scheme on the zonally averaged tropical general circulation is examined within an idealized moist gcm to obtain broad classifications of the influence of convection on the tropics this is accomplished with a simplified convection scheme in the style of betts and miller the scheme is utilized in a moist gcm with simplified physical parameterizations gray radiation with zonally symmetric slab mixed layer ocean boundary conditions comparisons are made with simulations without a convection scheme i e with large scale condensation lsc only with the moist convective adjustment mca parameterization and with various formulations and parameter sets with a simplified betts miller sbm scheme with the control run using the sbm scheme the tropics become quieter and less dependent on horizontal resolution as compared with the lsc or mca simulations the hadley circulation mass transport is significantly reduced with the sbm scheme as is the itcz precipitation an important factor determining this behavior is the parameterization of shallow convection without shallow convection the convection scheme is largely ineffective at preventing convection from occurring at the grid scale the sensitivities to convection scheme parameters are also examined the simulations are remarkably insensitive to the convective relaxation time and only mildly sensitive to the relative humidity of the reference profile provided significant large scale condensation is not allowed to occur the changes in the zonally averaged tropical circulation that occur in all the simulations are understood based on the convective criteria of the schemes and the gross moist stability of the atmosphere

northern winter stationary waves theory and modeling a review is provided of stationary wave theory the theory for the deviations from zonal symmetry of the climate to help focus the discussion the authors concentrate exclusively on northern winter several theoretical issues including the external rossby wave dispersion relation and vertical structure critical latitude absorption the nonlinear response to orography and the interaction of forced wave trains with preexisting zonal asymmetries are chosen for discussion while simultaneously presenting a decomposition of the wintertime stationary wave field using a nonlinear steady state model

metacraton nature genesis and behavior article i nfo in this paper we show with examples that cratons involved in intercontinental collisions in a lower plate position are often affected by orogenic events leading to the transformation of their margins in some cases craton interiors can also be shaped by intense collisional processes leading to the generation of intra cratonic orogenic belts we propose to call these events metacratonization and the resulting lithospheric tract metacraton metacratons can appear similar to typical orogenic belts i e active margin transformed by collisional processes but are actually sharply different their main distinctive characteristics not all are present in each metacraton are 1 absence of pre collisional events 2 absence of lithospheric thickening high pressure metamorphism being generated by subduction leading to high gradient in strain and meta morphic intensity 3 preservation of allochthonous pre collisional oceanic terranes 4 abundant post collisional magmatism associated with shear zones but not with lithospheric thickening 5 presence of high temperature low pressure metamorphism associated with post collisional magmatism 6 intraconti nental orogenic belts unrelated to subduction and oceanic basin closures reactivation of the rigid but fractured metacratonic lithosphere will cause doming asthenospheric volcanism emplacement and mineral izations due to repetitive mineral enrichments this paper provides several geological cases exemplifying these different metacratonic features in scandinavia sahara central africa and elsewhere a special focus is given to the saharan metacraton because it is where the term metacraton originated and it is a vastly ex panded tract of continental crust 5 000 000 km 2

monazite in situ 207pb 206pb geochronology using a small geometry high resolution ion probe application to archaean and proterozoic rocks this paper reports the application of secondary ion mass spectrometry sims using a small geometry cameca ims4f ion probe to provide reliable in situ 207pb 206pb ages on monazite populations of archaean and proterozoic age the reliability of the sims technique has been assessed on two samples previously dated by the conventional id tims method at 2661 1 ma for monazites extracted from a pelitic schist from the jimperding metamorphic belt yilgarn craton western australia and 1083 3 ma for monazites from a high grade paragneiss from the northampton metamorphic complex pinjarra orogen western australia sims results provide 207pb 206pb weighted mean ages of 2659 3 ma n 28 and 1086 6 ma n 21 in close agreement with id tims reference values for the main monazite growth event monazites from the northampton complex document a complex history the spatial resolution of about 30 m and the precision achieved successfully identify within grain heterogeneities and indicate that monazite growth and recrystallisation occurred during several events this includes detection of one inherited grain dated at ca 1360 ma which is identical to the age of the youngest group of detrital zircons in the paragneiss younger ages at ca 1120 ma are tentatively interpreted as dating a growth event during the prograde stages of metamorphism these results demonstrate that the closure temperature for lead diffusion in monazite can be as high as 800 c at last ages down to ca 990 ma are coeval with late pegmatitic activity and may reflect either lead losses or partial recrystallisation during the waning stages of metamorphism a third unknown sample was analysed to test the capability of the in situ method to date younger monazite populations the sample a pelitic metatexite from northwestern hoggar algeria contains rounded metamorphic monazites that provide a 207pb 206pb weighted mean age of 603 11 ma n 20 this age is interpreted as recording emplacement of a gabbronoritic body during amphibolite facies regional metamorphism and is representative of the late pulse of the pan african tectonometamorphic evolution in the western part of the tuareg shield in situ sims analyses using a widely available small geometry ion probe can thus be successfully used to accurately determine ages for complex precambrian monazite populations

late proterozoic schist belts and plutonism in nw nigeria the deformed low grade metasedimentary volcanic schist belts of nw nigeria and the voluminous granitoid plutons which invaded them are expressions of late proterozoic early phanerozoic activity in the terrain separating the w african and congo cratons recent interpretations of nigerian geotectonic evolution have invoked two generations of schist belt one a product of kibaran c 1100 ma ensialic processes the other due to pan african 700 450 ma marginal basin development the detailed histories of the anka belt pan african maru belt kibaran and birnin gwari belt unknown age and the plutons emplaced in them are documented here on the basis of new field chemical and isotopic data each belt represents a dominantly quiet water sedimentary environment but volumetrically minor lithologies reveal important differences between the belts the volcanic rocks and early minor intrusions have strong affinities with those of destructive plate margins the schist belts were deformed congruently and simply before c 750 ma following the development of flat lying possibly thrust related structures in the maru and birnin gwari belts subsequent plutonism beginning c 750 ma ago has the calcalkaline i type characteristics of subduction zones but younger plutons have mildly peralkaline compositions due to thickening crust

midwinter suppression of storm tracks in an idealized zonally symmetric setting abstractthe midwinter suppression of eddy activity in the north pacific storm track is a phenomenon that has resisted reproduction in idealized models that are initialized independently of the obse

winter cold of eastern continental boundaries induced by warm ocean waters in winter northeastern north america and northeastern asia are both colder than other regions at similar latitudes this has been attributed to the effects of stationary weather systems set by elevated terrain orography and to a lack of maritime influences from the prevailing westerly winds however the differences in extent and orography between the two continents suggest that further mechanisms are involved here we show that this anomalous winter cold can result in part from westward radiation of large scale atmospheric waves nearly stationary rossby waves generated by heating of the atmosphere over warm ocean waters we demonstrate this mechanism using simulations with an idealized general circulation model with which we show that the extent of the cold region is controlled by properties of rossby waves such as their group velocity and its dependence on the planetary rotation rate our results show that warm ocean waters contribute to the contrast in mid latitude winter temperatures between eastern and western continental boundaries not only by warming western boundaries but also by cooling eastern boundaries

nutrient cycling in a tropical montane rainforest under a supply limited weathering regime traced by elemental mass balances and mg stable isotopes abstract forest ecosystem nutrition and rock weathering are intimately linked through the supply of mineral nutrients and recycling re utilisation of nutrients released from plant litter during mineralisation here we report on novel applications of mg stable isotopes in combination with mass balance calculations to quantify nutrient supply from bedrock atmospheric wet and dry deposition and recycling in a tropical ecosystem characterised by a supply limited weathering regime in the central highlands of sri lanka we measured mg stable isotopes radiogenic sr isotope ratios and element concentrations in the different compartments of the earth s critical zone bedrock saprolite soil soil pore water stream water and vegetation as tracers for element mass fluxes and pathways we find that in the clay rich saprolite mg isotopes are fractionated towards isotopically heavy mg during secondary mineral formation soil pore water and stream water are the complementary isotopically light reservoirs the mg isotope composition of above ground vegetation is on average identical to soil pore water based on these observations we suggest that a tight near surface mineral nutrient loop between plants and soil sustains the tropical forest ecosystem this loop is disconnected from the rock weathering nutrient source in the saprolite beneath soil external atmospheric wet and dry inputs mainly mg and ca strongly contribute to the plant available nutrient budget that the nutrient depleted regolith does not supply these findings based on mg stable isotopes agree with independently obtained information on bio element p k ca mg si recycling this nutrient supply setting contrasts with that reported for the southern sierra nevada usa where a kinetically limited weathering regime prevails our study illustrates the application of new flux estimators based on elemental and metal stable isotope measurements across a range of climate and weathering regimes

applications of single grain zircon evaporation analyses to detrital grain studies and age discrimination in igneous suites abstract 207 pb 206 pb ages determined by evaporation of single zircon grains from rocks of the new england appalachians in the usa permit an assessment of the applicability of this method to rocks that have been strongly disturbed by orogenesis tests conducted on highly metamorphosed and deformed orthogneisses of known protolith age whose u pb upper intercept zircon ages obtained through isotope dilution methods show profound post crystallization discordance demonstrate that the evaporation method is successful in recovering protolith ages uncertainties are 10 15 my for rocks of paleozoic age or older preliminary results suggest that the method is particularly useful for provenance studies of sedimentary rocks and in the discrimination among groups of rocks that are known to differ in age but are otherwise difficult to separate on the basis of field or petrographic characteristics 207 pb 206 pb ages of detrital zircon grains from two metasedimentary units from opposite sides of the new england appalachians reveal the distinct provenances of these rocks the missisquoi formation in southeastern vermont usa is comprised in part of detritus derived from the 1 0 1 2 ga grenvillian province of laurentia whereas this component is conspicuously absent from the plainfield formation in easternmost connecticut the detrital zircon age data suggest that the missisquoi formation accumulated peripheral to the laurentian craton during the early paleozoic whereas the plainfield formation detritus was supplied by non laurentian ossibly west african sources during the late proterozoic or early paleozoic in southern vermont the evaporation method successfully distinguishes between two groups of augen gneisses with ages of 960 ma and 1120 ma which were first recognized through isotope dilution methods collectively these results indicate that the dating of single zircon grains by the evaporation method can be widely exploited in the study of complexly deformed and strongly metamorphosed rocks

early and late pan african orogenies in the air assembly of terranes tuareg shield niger our study of the 60 000 km 2 air massif which comprises the southeastern extension of the tuareg shield in the niger republic has identified three major displaced terranes aouzegueur barghot and assod6 these terranes were all affected differentially by the pan african orogeny during an early major phase 750 660 ma and a late follow up phase 650 580 ma the greenschist facies aouzegueur terrane comprises an ophiolite and is intruded by a late kinematic mediumk calc alkaline tonalite trondhjemite granodiorite ttg suite 730 ma the amphibolite facies barghot terrane made up of a basement and a supracrustal sequence is intruded by late to post kinematic high k calcalkaline granitoids 715 665 ma both terranes form a nne to e verging belt thrust 680 670 ma upon an unknown eastern rigid continent and are covered unconformably by an early pan african molasse in contrast the amphibolite facies assod6 terrane composed of a basement and several supracrustal sequences collided with the eastern rigid block without being thrust upon it tibetan type crustal thickening occurred and ht lp metamorphism proceeded to regional anatexis enhanced by widespread emplacement of anatectic granites of lower crustal origin 670 ma this thermal event is attributed to the delamination of the continental lithospheric mantle and consequent rise of the asthenosphere the late pan african phase affecting mainly the assod6 terrane is marked by the intrusion of high k calc alkaline n s elongated batholiths 645 580 ma and by right lateral displacement along the n s raghane mega shear zone located on the edge of the eastern rigid continent this movement along the northern extent of the rigid continent in eastern hoggar 8 30 e created the tiririne intracontinental linear fold belt made up of early panafrican molasse in the model proposed we estimate that the assod6 terrane driven by an oblique spreading ridge moved to the north over a distance of 1000 km during 60 ma along the lithospheric raghane shear zone the latter was the subsequent locus of the silurian devonian alkaline ring complexes the calc alkaline granitoids are regarded as subduction related mantle products contaminated by lower and medium level crust the model and mechanisms proposed can be applied to the entire tuareg shield

deep structure of the hoggar domal uplift central sahara south algeria from gravity thermal and petrological data abstract initial heat flow determinations show that no significant regional thermal disturbance is associated with the volcanic hoggar swell however the large scale negative gravity anomaly associated with this swell strongly suggests the occurrence of a low density upper mantle beneath the hoggar this is inferred from the occurrence of the depleted metasomatized and intensely recrystallized and veined peridotitic upper mantle revealed by the study of xenoliths from the cenozoic basaltic eruptions we suggest that the low density of the upper mantle is due to magmatic events of asthenospheric origin from various observations including petrographie gravimetric and thermal constraints the age of these events is estimated to be late mesozoic to early cenozoic the alkali cenozoic volcanism cannot be regarded as the cause of the swelling the occurrence of an altered lithosphere now cooled and presently marked by low density magmatic materials is consequently proposed as a possible model for the hoggar

storm track organization and variability in a simplified atmospheric global circulation model a storm track is simulated in a coarse resolution multilevel primitive equation model with linear surface friction and heating terms a restoration temperature distribution consisting of a dipole embedded in a zonally symmetric profile forces the model to simulate the surface heating three simulations each with a different dipole orientation are performed to test the sensitivity of storm track organization to the external forcing field the climatological time mean circulation and the transient disturbances of the reference simulation agree well with observations of northern hemisphere wintertime storm tracks local energetics show that baroclinic instability is responsible for the enhanced eddy kinetic energy downstream of the jet and the downstream end of the storm track results from the barotropic conversions of eddy kinetic energy to the time mean flow low frequency fluctuations with a period of about 50 days associated with a retrograding large scale wave pattern are identified by a complex empirical orthogonal function analysis of the vertically averaged stream function a composite life cycle of the low frequency variability reveals the growth and decay of a blocking anticyclone downstream of the storm track a cyclogenesis initiated by an eastward propagating wave train is observed a few days before the amplitude of the low frequency anomaly attains its peak the net forcing of the high frequency eddies contributes to the growth and decay of the blocking anticyclone the results suggest that the occurrence of the blocking like event is part of the storm track dynamics

equilibration of the antarctic circumpolar current by standing meanders the insensitivity of the antarctic circumpolar current acc s prominent isopycnal slope to changes in wind stress is thought to stem from the action of mesoscale eddies that counterbalance the wind driven ekman overturning a framework verified in zonally symmetric circumpolar flows substantial zonal variations in eddy characteristics suggest that local dynamics may modify this balance along the path of the acc analysis of an eddy resolving ocean gcm shows that the acc can be broken into broad regions of weak eddy activity where surface winds steepen isopycnals and a small number of standing meanders across which the isopycnals relax meanders are coincident with sites of i strong eddy induced modification of the mean flow and its vertical structure as measured by the divergence of the eliassen palm flux and ii enhancement of deep eddy kinetic energy by up to two orders of magnitude over surrounding regions within meanders the vorticity budget shows a balance between the advection of relative vorticity and horizontal divergence providing a mechanism for the generation of strong vertical velocities and rapid changes in stratification temporal fluctuations in these diagnostics are correlated with variability in both the eliassen palm flux and bottom speed implying a link to dissipative processes at the ocean floor at larger scales bottom pressure torque is spatially correlated with the barotropic advection of planetary vorticity which links to variations in meander structure from these results it is proposed that the flexing of standing meanders provides an alternative mechanism for reducing the sensitivity of the acc s baroclinicity to changes in forcing separate from an acc wide change in transient eddy characteristics

u pb dating of an early paleozoic bimodal magmatism in the french massif central and of its further metamorphic evolution in the marvejols area southern french massif central the gneissic marvejols supergroup is overthrust on the metasedimentary serie du lot deposited in part prior to 540 ma the allochtonous terranes are characterized by the occurrence of a leptyno amphibolitic group a complex association of mafic and felsic rocks of igneous and sedimentary derivation a 480 10 ma age has been obtained by u pb dating of zircons for the crystallization of both mafic and felsic meta igneous rocks these rocks were emplaced during an important extensional tectonics relics of eclogites pyrigarnites coronite gabbros and hp trondhjemites are clear evidence for a further hp ht event dated at 415 6 ma on zircons from a hp trondhjemite subsequently the marvejols supergroup underwent an amphibolite facies metamorphism with incipient mobilization dated at 345 10 ma

diabatic and orographic forcing of northern winter stationary waves and storm tracks abstract in this study a dry global circulation model is used to examine the contributions made by orographic and diabatic forcings in shaping the zonal asymmetries in the earth s northern hemisphere nh winter climate by design the model mean flow is forced to bear a close resemblance to the observed zonal mean and stationary waves the model also provides a decent simulation of the storm tracks in particular the maxima over the pacific and atlantic and minima over asia and north america are fairly well simulated the model also successfully simulates the observation that the atlantic storm track is stronger than the pacific storm track despite stronger baroclinicity over the pacific sensitivity experiments are performed by imposing and removing various parts of the total forcings in terms of the nh winter stationary waves in the upper troposphere results of this study are largely consistent with previous studies diabatic forcings explain most of the modeled stationary waves with orographic

experimental study of igneous and sedimentary apatite dissolution control of ph distance from equilibrium and temperature on dissolution rates apatite dissolution experiments were conducted using both a fluidized bed and stirred tank reactor over a range of ph temperature solution saturation state and on non carbonated and carbonated apatite compositions igneous fluorapatite fap and sedimentary carbonate fluorapatite cfa respectively from 2 8 5 were not possible due to detection limits of the analytical techniques used in this study and the high insolubility of fap for the cfa compositions studied the dissolution rate decreased with increasing ph from 4 ph 7 during early stages of the dissolution reaction for both fap and cfa mineral components were released in non stoichiometric ratios with reacted solution ratios of dissolved ca p and ca f being greater than mineral stoichiometric ratios suggesting that ca was preferentially released compared to p and f from the mineral structure during the early stages of dissolution an increase in reacted solution ph accompanies this early elevated release of ca as the dissolution reaction proceeded to steady state dissolution became congruent when normalized to bet measured surface area fap dissolved faster from 4 ph 7 compared to cfa the apparent arrhenius activation energy ea of fap dissolution over the temperature range of 25 55 c at ph 3 0 i 0 1 and pco2 0 is 8 3 0 2 kcal mol 1 both the apparent exchange of solution h for solid bound ca at low ph in the early stage of dissolution and the ea of dissolution suggest a surface and not a diffusion controlled dissolution reaction for fap and cfa the degree of undersaturation of the solution gr with respect to fap was important in determining the dissolution rate at ph 3 0 i 0 1 and pco2 0 the dissolution rate of fap was 5 greater in the far from equilibrium region compared to the near equilibrium slope region a simple apatite weathering model incorporating the experimental results from this study was constructed and numerical calculations suggest that during the phanerozoic both the surface area of igneous rock available for weathering and the average global temperature were important factors in determining the p weathering flux from apatite dissolution it is possible that elevated global temperatures coupled with relatively high surface area of igneous rock during the early to mid paleozoic resulted in elevated p weathering fluxes which along with climatic evolutionary pressures of the neoproterozoic facilitated the radiation of multicellular organisms large scale phosphorite deposition and abundance of calcium phosphate shelled organisms during the early cambrian

alkali calcic and alkaline post orogenic po granite magmatism petrologic constraints and geodynamic settings abstract the end of an orogenic wilson cycle corresponds to amalgamation of terranes into a pangaea and is marked by widespread magmatism dominated by granitoids the post collision event starts with magmatic processes still influenced by subducted crustal materials the dominantly calc alkaline suites show a shift from normal to high k to very high k associations source regions are composed of depleted and later enriched orogenic subcontinental lithospheric mantle affected by dehydration melting and generating more and more k and lile rich magmas in the vicinity of intra crustal magma chambers anatexis by incongruent melting of hydrous minerals may generate peraluminous granitoids bearing mafic enclaves the post collision event ends with emplacement of bimodal post orogenic po suites along transcurrent fault zones two suites are defined i the alkali calcic monzonite monzogranite syenogranite alkali feldspar granite association characterised by biotite plagioclase fractionation and moderate lile hfse enrichments and ii the alkaline monzonite syenite alkali feldspar granite association characterised by amphibole alkali feldspar fractionation and displaying two evolutionary trends one peralkaline with sodic mafic mineralogy and higher enrichments in hfse than in lile and the other aluminous biotite bearing marked by hfse depletion relative to lile due to accessory mineral precipitation alkali calcic and alkaline suites differ essentially in the amounts of water present within intra crustal magma chambers promoting crystallisation of various mineral assemblages the ultimate enriched and not depleted mantle source is identical for the two po suites the more primitive lile and hfse rich source rapidly replaces the older orogenic mantle source during lithosphere delamination and becomes progressively the thermal boundary layer of the new lithosphere present rock compositions are a mixture of major mantle contribution and various crustal components carried by f rich aqueous fluids circulating within convective cells created around magma chambers in favourable areas po suites pre date a new orogenic wilson cycle

on the existence of storm tracks abstract given that middle latitude weather systems transport heat in a manner such as to weaken the baroclinicity that is thought to be crucial to their growth it is perhaps surprising that concentrated regions of such eddy activity i e storm tracks are found in the northern hemisphere winter the existence and possible self maintenance of storm tracks is investigated using a linear stationary wave model with storm track region forcings taken from data averaged over a number of winters it is found that the direct thermal effect of the eddies does indeed act against the existence of the storm track their vorticity fluxes lead to some reduction of this effect it is argued that the mean diabatic heating in the storm track region is an indirect eddy effect this heating is found to maintain the mean maximum in baroclinicity in the region further the mean low level flow induced by the eddy effects is such as to enhance the warm western oceanic boundary currents that are crucial to the existence of t

repeated granitoid intrusions during the neoproterozoic along the western boundary of the saharan metacraton eastern hoggar tuareg shield algeria an ams and u pb zircon age study article i nfo the n s oriented raghane shear zone 8 30 delineates the western boundary of the saharan metacraton and is with the 4 50 shear zone the most important shear zone in the tuareg shield it can be followed on 1000 km in the basement from southern air niger to ne hoggar algeria large subhorizontal movements have occurred during the pan african orogeny and several groups of granitoids intruded during the neoproterozoic we report u pb zircon datings laser icp ms showing that three magmatic suites of granitoids emplaced close to the raghane shear zone at c 790 ma c 590 and c 550 ma a comprehensive and detailed 158 sites more than 1000 cores magnetic fabric study was performed on 8 plutons belonging to the three magmatic suites and distributed on 200 km along the raghane shear zone the main minerals in all the target plutons do not show visible preferential magmatic orientation except in narrow shear zones the ams study shows that all plutons have a magnetic lineation and foliation compatible with the deformed zones that are zones deformed lately in post solidus conditions these structures are related to the nearby mega shear zones the raghane shear zone for most of them the old c 793 ma touffok granite preserved locally its original structures the magnetic structures of the c 593 ma ohergehem pluton intruded in the aouzegueur terrane are related to thrust structures generated by the raghane shear zone while it is not the case of the contemporaneous plutons in the assode issalane terrane whose structures are only related to the subvertical shear zones finally the c 550 ma granite group has magnetic structure related to the n s oriented raghane shear zone and its associated nne ssw structures when close to them but nw se oriented when further these nw se oriented structures appear to be characteristic of the late neoproterozoic evolution of the saharan metacraton and are in relation to the convergence with the murzuq craton this evolution reflects the rheological contrast existing along the raghane shear zone marking the western boundary of the saharan metacraton

atmospheric responses to north atlantic sst anomalies in idealized experiments part ii north american precipitation abstractthis is the second part of a two part paper that addresses deterministic roles of the sea surface temperature sst anomalies associated with the atlantic multidecadal oscillation amo in variations of atmospheric circulation and precipitation in the northern hemisphere using a sequence of idealized model runs at the spring equinox conditions this part focuses on the effect of the sst anomalies on north american precipitation major results show that in the model setting closest to the real world situation a warm sst anomaly in the north atlantic ocean causes suppressed precipitation in central western and northern north america but more precipitation in the southeast a nearly reversed pattern of precipitation anomalies develops in response to the cold sst anomaly further examinations of these solutions reveal that the response to the cold sst anomaly is less stable than that to the warm sst anomaly the former is dynamically charged in the sense that positive eddy kinetic energy eke

fabric asymmetry and shear sense in movement zones asymmetry between lattice preferred orientation and shape fabrics is commonly observed in microstructural studies of many deformed mineral aggregates e g ice carbonate rocks peridotites and quartzites theoretical predictions are limited by the nature of the models available and more can be gained by discussion of the mechanisms which give rise to such asymmetry in specific natural and experimental situations