structure and emplacement of the alpine type peridotites from beni bousera rif morocco a polyphase tectonic interpretation abstract the beni bousera peridotite antiform in the southern branch of the gibraltar orocline exhibits a lherzolitic core containing thin pyroxenite layers surrounded by harzburgites then by dunites and garnet bearing dunite in the peripheral areas the amount of pyroxenite greatly increases by the development of garnet pyroxenite veins these data indicate a gradient of partial melting increasing upward we conclude that a reversed geothermal gradient was temporarily established in the upper part of the peridotite just under its migmatitic roof kinzigite aureole then sillimanite gneiss study of ductile penetrative structures and discussion of the later cold structures show that the foliation of the tectonites was originally flat with a nw se trending stretching lineation orthopyroxene crystals are used as tectonic markers isostrain zones indicate that simple shear and finite strain increase upward we conclude that a ductile shear zone acted between peridotite and gneiss strain heating along this shear zone probably has been responsible for partial melting of the upper part of the peridotite in our model the peridotite emplacement into the mid continental crust began during crustal extension then as comparison with the ronda massif suggests an intracrustal thrusting stage was followed by high temperature and relatively low pressure metamorphism and by intrusion of acidic dikes the uplift of the mantle slab continued by compressive upthrusting and isostatic doming this complex polyphase evolution probably extended beyond the alpine orogeny as suggested by kornprobst

nd sr isotopic composition of granulites and constraints on the evolution of the lower continental crust in a statistical study of granulite facies rocks we have analysed 32 samples with ages ranging from 2 900 to 200 myr for sm nd rb and sr and 143nd 144nd and 87sr 86sr ratios the sm nd ratio is found to be constant compared with the highly variable rb sr ratio using the model age formalism we show that in many cases internal differentiation of continental crust follows external differentiation with a large time interval

sm nd age of kambalda and kanowna greenstones and heterogeneity in the archaean mantle a precise sm nd age of 2 790 30 myr and an initial ratio corresponding to nd 3 4 0 6 has been determined for the kambalda sequence from the eastern yilgarn block of western australia the high initial 143nd 144nd implies a long lived light rare earth element depleted high sm nd source for the kambalda volcanics and therefore provides definitive evidence for ancient heterogeneities in the archaean mantle

a theory of topographically bound balanced motions and application to atmospheric low level jets abstractthe subject of this study is topographically bound low level jets such as the south american summertime low level jet on the eastern side of the andes and its companion the chilean low level jet on the western side of the andes these jets are interpreted as balanced flows that obey the potential vorticity invertibility principle this invertibility principle is expressed in isentropic coordinates and the mathematical issue of isentropes that intersect the topography is treated by the method of a massless layer in this way the low level jets on the western and eastern sides of the andes can both be attributed to the infinite potential vorticity that lies in the infinitesimally thin massless layer on the topographic feature to obtain a cyclonic flow centered on the topographic feature the mountain crest must have been heated enough to draw down the overlying isentropic surfaces otherwise isentropic surfaces bend upward at the mountain crest and an anticyclonic flow is produced both anticy

wavelengths of earth structures that can be resolved from seismic reflection data the aim of inverting seismic waveforms is to obtain the best earth model the best model is defined as the one producing seismograms that best match usually under a least squares criterion those recorded our approach is nonlinear in the sense that we synthesize seismograms without using any linearization of the elastic wave equation since we use rather complete data sets without any spatial aliasing we do not have the problem of secondary minima tarantola 1986 nevertheless our gradient methods fail to converge if the starting earth model is far from the true earth mora 1987 kolb et al 1986 pica et al 1989

reexamination of the near surface airflow over the antarctic continent and implications on atmospheric circulations at high southern latitudes abstract previous work has shown that winds in the lower atmosphere over the antarctic continent are among the most persistent on earth with directions coupled to the underlying ice topography in 1987 parish and bromwich used a diagnostic model to depict details of the antarctic near surface airflow a radially outward drainage pattern off the highest elevations of the ice sheets was displayed with wind speeds that generally increase from the high interior to the coast these winds are often referred to as katabatic with the implication that they are driven by radiational cooling of near surface air over the sloping ice terrain it has been shown that the antarctic orography constrains the low level wind regime through other forcing mechanisms as well dynamics of the lower atmosphere have been investigated increasingly by the use of numerical models since the observational network over the antarctic remains quite sparse real time numerical weather prediction for the u s antarctic program has been

integrated seismic studies of the baltic shield using data in the gulf of bothnia region in the autumn of 1989 a co operative experiment involving 12 research institutions in northwestern europe collected 2268 km of deep seismic reflection profiles in the gulf of bothnia and the baltic

the polarity of deep seismic reflections from the lithospheric mantle evidence for a relict subduction zone abstract birps offshore deep seismic reflection profiles to the north of scotland have revealed two bright continuous reflectors in the continental lithospheric upper mantle the dipping flannan and sub horizontal w reflectors the polarity of reflections from the flannan has been determined from normal incidence reflection data using the far field source wavelet as a starting model the effective far field wavelet was calculated by adding a receiver ghost and the effects of recording filters attenuation during transmission and sea bottom multiples to the source wavelet reflection polarity was determined in a blind test by comparing the modelled wavelet with stacks of the flannan reflection in eight out of ten stacks the reflection was picked as positive polarity two out of ten were unknown to verify this result the test was repeated for moho reflections in this case nine out of ten stacks were picked as positive the modelling also demonstrated that the flannan reflection is apparently from a simple interface complex interlayering is not required to explain the waveforms and they are not consistent with reflections from a single thin layer seismic data acquired at wide angle across the w mantle reflector show sub crustal high amplitude arrivals which can only be explained as post critical reflections only a high velocity eclogitic layer contained within normal mantle with a sharp upper boundary and a diffuse base can explain all our observations we suggest that the flannan reflector represents the top of a relict oceanic and eclogitic component of a pre caledonian subduction zone within the lithospheric mantle

isotopic and geochemical studies of nodules in kimberlite have implications for the lower continental crust lower crustal xenoliths from the calcutteroo kimberlitic pipes in south australia show a surprisingly diverse range in nd and sr isotopic compositions and rare earth element ree abundances mafic granulite and garnet clinopyroxenite xenoliths have measured end 0 values between 6 1 and 7 9 both light l ree enrichments and depletions low rb sr ratios 0 005 0 040 and relatively high 87sr 86sr ratios of from 0 70675 to 0 70919 a felsic xenolith has compositions more typical of upper crustal rocks with end 0 21 4 lree enrichments rb sr 1 47 and 87sr 86sr 0 85987 an approximate correlation also exists between sm nd versus 143nd 144nd and rb sr versus 87sr 86sr indicating major intracrustal differentiation in the lower crust at 2 400 myr this produced extremely low rb sr ratios and both relative lree enrichments low sm nd and depletions high sm nd if these results are typical then they indicate that the lower crust has higher 87sr 86sr and more positive end 0 values than previous estimates1 5 this may require revision of total crustal compositions and crust mantle evolutionary models1 5

petrology of phlogopite sapphirine bearing al mg granulites from ihouhaouene in ouzzal hoggar algeria an example of phlogopite stability at high temperature al mg granulites with cordierite garnet sapphirine orthopyroxene sillimanite spinel phlogopite k feldspar plagioclase and variable quartz from ihouhaouene in ouzzal algeria display a range of decompression textures involving the breakdown of orthopyroxene and sillimanite and of garnet the succession of parageneses suggests that the p t t evolution corresponds to decompression with cooling from peak conditions of about 950 c and 10 kbar this decompression path is obtained from the paragenetic analysis in the fmas system however according to current kfmash grids this p t t evolution should take place outside the stability field of phlogopite quartz yet this assemblage is probably stable during most of the p t evolution notably during peak metamorphism this discrepancy is interpreted as the effect of the high content of f in phlogopite which should shift its stability limit towards higher temperature the consequences of this shift on the phase relationships in the kfemash system are investigated and it is concluded that a topological inversion could exist in the f bearing system

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

monte carlo analysis of seismic reflections from moho and the w reflector near normal incidence reflections have been used to image the moho and the w reflector structure in the lithosphere offshore northern scotland to determine the impedance variations at these reflectors we use a monte carlo technique which allows incorporation of geologically realistic a priori information as well as an extensive exploration of the model space after testing it on a synthetic data set the method is based on bayesian inversion theory the modeled moho consists of a series of layers with a total thickness of 2 4 0 3 km with an overall positive impedance contrast inversion of the w reflector results in a model of five seven layers with a total thickness of about 3 7 0 6 km and mostly nonpositive impedance contrasts the implied fine scale impedance structure of the moho is consistent with the broader velocity structure determined from previous wide angle reflection refraction profiles however the overall nonpositive impedance contrast at the w reflector requires a structure which is overlain or underlain by a broad increase in velocity in order to match amplitudes of reflected phases observed at large offsets interpreted previously to originate at a similar depth

a review of the in ouzzal granulitic terrane tuareg shield algeria its significance within the pan african trans saharan belt the in ouzzal terrane iot or in ouzzal granulite unit iogu is an elongated palaeoproterozoic block within the neoproterozoic pan african belt of north west africa the granulites derive from archaean protoliths that include a large volume of metasediments which were deposited on a 3 2 ga gneissic basement near peak granulite facies conditions between 2 17 and 2 ga were estimated at p 10 kbar and t rising from 800 to 1000 c premetamorphic orthogneisses were intruded at 2 5 ga and followed by the emplacement of syn to late kinematic charnockites syenites and carbonatites at around 2 ga cooling of the granulites occurred till 1800 ma shortly after its exhumation coeval with crustal extension and related alkaline magmatism in adjacent areas the iot was buried beneath late palaeoproterozoic and neoproterozoic cover sequences and then behaved as a rigid block both margins are lithospheric faults as evidenced by the occurrence of shear zone related mafic and felsic plutons pan african tectonothermal events were negligible in the north but granulites in the south were significantly reworked under lower greenschist facies conditions during the northern motion of the block with respect to both the western and the eastern pan african terranes the cambrian molasse associated with widespread alkaline volcanism and subvolcanic granites is horizontal in the north the iot which was part of a larger continental mass including its counterpart in northern mali is interpreted as an exotic terrane which may have docked during pan african plate convergence and lateral collision the unchanged pediplain since c 1 7 ga in the north suggests that the iot is underlain by thick palaeoproterozoic lithospheric mantle whereas its southern part is probably allochthonous and overlies pan african structural units

proterozoic age and cumulate origin for granulite xenoliths lesotho rare earth element ree 143nd 144nd and 87sr 86sr analyses on a range of granulite xenoliths from kimberlite pipes in lesotho demonstrate that useful whole rock ages can be obtained from such lower crustal material the characteristic positive eu anomalies relatively low ree abundances and large variation in sr contents suggest that many of the xenoliths are crystal cumulates rather than residua after partial melting they yield an sm nd whole rock age of 1 400 100 myr and are unrelated to the younger magmatic rocks of the karoo

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

cyclonically forced barrier winds along the transantarctic mountains near ross island abstract the effect of the transantarctic mountains on cyclonically forced boundary layer winds in the vicinity of ross island antarctica 77 5 s 167 e is discussed when cyclones are present over the western ross ice shelf and ross sea the low level easterly airflow is toward the mountains a barrier wind regime is set up as the flow is turned northward and becomes parallel to the mountain range it is found that cyclonically forced barrier winds occurred around 5 and 8 of the time during 1984 and 1985 respectively the case histories of two well defined barrier wind events lasting for 24 h are discussed in detail with regional analyses based on satellite photographs and automatic weather station data one case is for a katabatic wind forced mesoscale cyclone forming to the north of ross island and the other is for a synoptic scale cyclone moving through the western ross ice shelf ross sea region a numerical model for the vertically integrated boundary layer flow that calculates two horizontal

the complexity of the crust and moho under the southeastern superior and grenville provinces of the canadian shield from seismic refraction wide angle reflection data the major features of the individual velocity models poisson s ratio values and crustal complexity derived from the interpretation of seismic data sets from four long range seismic refraction wide angle reflection experiments are summarized the experiments were conducted from 1982 92 in the southeastern portion of the canadian shield in the conventional analysis of seismic refraction wide angle reflection data only the onset times and amplitudes of the major arrival phases are used to derive seismic velocity models of the region under study these models are over smoothed have a number of intermediate discontinuities are unable to explain the pg coda and bear very little resemblance to the models derived from the analysis of near vertical seismic reflection data in this paper some of the differences between seismic models derived from near vertical reflection analysis and those from refraction analysis are reconciled from an analysis of the wide angle reflection fields of the crustal coda wav

lead isotopic composition of hercynian granitic k feldspars constrains continental genesis lead isotope compositions have been determined in many k feldspars from granitic rocks emplaced during the variscan hercynian orogeny sensu lato around 300 myr in various parts of europe and the eastern us the ratios show a regular geographical pattern with contours arched around southern france and parallel to the atlantic coast of the us this distribution parallels predriftpalaeogeographical zones during the palaeozoic the lead isotope pattern of variscan granitic rocks is similar to that of some orogenic volcanic suites but is better explained by crustal melting than by an hercynian mantle crust material mixing the continental crust from these areas must be older than 3 000 myr large scale fluid circulation in the crust during orogenic processes apparently explains the observed geographical patterns of lead isotopes

preliminary investigations of 18 o 16 o and d h compositions in rhyo ignimbrites in the in hihaou in zize magmatic center central ahaggar algeria the 620 m y old in hihaou in zize magmatic complex located at the north western boundary of the archaean in ouzzal block western ahaggar is composed of massive alkaline rhyo ignimbrites and rhyolitic domes which are intruded by a granophyric and granitic body the whole is preserved in a cauldron structure extrusive rocks are strongly 18o depleted with values as low as 1 5 smow while granophyres are less depleted minimum 18o value 2 0 smow the granite has values around 6 smow d h compositions are rather low with d 90 to 110 smow isotopic zoning of quartz phenocrysts 18o 16o fractionation among coexisting phases and heterogeneity of the whole rock 18o values suggest that the volcanic rocks have interacted with meteoric water after the eruption several mechanisms of isotopic alteration are discussed the hydrothermal alteration does not seem to have been controlled by the granitic intrusion but rather seems to have followed the deposition of thick pyroclastic deposits on permeable arkosic sandstones and fluvio glacial conglomerates pervasive circulation of water through the cooling volcanic deposits could have produced the observed 18o depletion

early archaean gneisses from the yilgarn block western australia the yilgarn block is an ancient crustal block which extends over 650 000km2 in the south west of western australia and consists of high grade gneiss and granite greenstone terrains1 being one of the largest segments of archaean crust in the world it is important to studies of early crustal evolution we report here a geochronological study of banded gneisses near mount narryer in the north west of the yilgarn block which has given a rb sr whole rock isochron age of 3 348 43 myr with an initial 87sr 86sr ratio of 0 7037 0 0005 the data indicate that the rocks had a prior crustal residence time of 200 myr the validity of this early archaean age is supported by model sm nd ages of 3 510 myr and 3 630 myr for two of the samples taken together these ages represent the oldest evolutionary sequence so far identified in the yilgarn block and are comparable with old ages from other archaean cratons elsewhere in the world

monte carlo bayesian look ahead inversion of walkaway vertical seismic profiles we describe a method to invert a walkaway vertical seismic profile vsp and predict elastic properties p wave velocity s wave velocity and density in a layered model looking ahead of the deepest receiver starting from bayes s rule we define a posterior distribution of layered models that combines prior information on the overall variability of and correlations among the elastic properties observed in well logs with information provided by the vsp data this posterior distribution of layered models is sampled by a monte carlo method the sampled layered models agree with prior information and fit the vsp data and their overall variability defines the uncertainty in the predicted elastic properties we apply this technique first to a zero offset vsp data set and show that uncertainty in the long wavelength p wave velocity structure results in a sizable uncertainty in the predicted elastic properties we then use walkaway vsp data which contain information on the long wavelength p wave velocity in the reflection moveout and on s wave velocity and density contrasts in the change of reflectivity with offset the uncertainty of the look ahead prediction is considerably decreased compared with the zero offset vsp and the predicted elastic properties are in good agreement with well log measurements

the age of carbonatites kimberlites and lamprophyres from southern west greenland recurrent alkaline magmatism during 2500 million years abstract alkaline rocks were intruded into the precambrian basement in southern west greenland during at least five separate episodes the tupertalik carbonatite intrusion was emplaced around or before 2650 m y it is now metamorphosed in granulite facies but is recognisable by its trace element content lamprophyre dykes were intruded at ca 1800 m y and again at ca 1200 m y at ca 600 m y the sarfartoq carbonatite intrusion and extensive kimberlite dykes were emplaced in the holsteiborg stromfjord region at 225 115 m y the qaqarssuk carbonatite complex was emplaced in the sukkertoppen region and numerous lamprophyre and kimberlite dykes were emplaced between fishenasset and ivigtut all these episodes are correlatable with contemporaneous alkaline activity in canada and scandinavia and substantiate the periodicity of carbonatite emplacement the mesozoic suite of intrusions is related to rifting prior to continental break up and the formation of the north atlantic ocean the eocambrian suite of 600 m y is thought to have formed under similar conditions during continental break up and the formation of the iapetus ocean

a look at the surface based temperature inversion on the antarctic plateau data from radiosondes towers and a thermistor string are used to characterize the temperature inversion at two stations the amundsen scott station at the south pole and the somewhat higher and colder dome c station at a lower latitude ten years of temperature data from a 22 m tower at the south pole are analyzed the data include 2 and 22 m temperatures for the entire period and 13 m temperatures for the last 2 yr statistics of the individual temperatures and the differences among the three levels are presented for summer december and january and winter april september the relationships of temperature and inversion strength in the lowest 22 m with wind speed and downward longwave flux are examined for the winter months two preferred regimes one warming and one cooling are found in the temperature versus longwave flux data but the physical causes of these regimes have not been determined the minimum temperatures and the maximum inversions tend to occur not with calm winds but with winds of 3 5 m s 1 likely due to the inversion wind this inversion wind also explains why the near surface winds at south pole blow almost exclusively from the northeast quadrant temperature data from the surface to 2 m above the surface from south pole in the winter of 2001 are presented showing that the steepest temperature gradient in the entire atmosphere is in the lowest 20 cm the median difference between the temperatures at 2 m and the surface is over 1 0 k in winter under clear skies this difference increases to about 1 3 k monthly mean temperature profiles of the lowest 30 km of the atmosphere over south pole are presented based on 10 yr of radiosonde data these profiles show large variations in lower stratospheric temperatures and in the strength and depth of the surface based inversion the near destruction of a strong inversion at south pole durin g7ho n 8september 1992 is examined using a thermal conductivity model of the snowpack driven by the measured downward longwave flux the downward flux increased when a cloud moved over the station and it seems that this increase in radiation alone can explain the magnitude and timing of the warming near the surface temperature data from the 2003 04 and 2004 05 summers at dome c station are presented to show the effects of the diurnal cycle of solar elevation over the antarctic plateau these data include surface temperature and 2 and 30 m air temperatures as well as radiosonde air temperatures they show that strong inversions averaging 10 k between the surface and 30 m develop quickly at night when the sun is low in the sky but are destroyed during the middle of the day the diurnal temperature range at the surface was 13 k but only 3 k at 30 m

apatite liquid partition coefficients for the rare earth elements and strontium abstract sixteen sets of apatite liquid partition coefficients dap liq for the rare earth elements ree la sm dy lu and six values for sr were experimentally determined in natural systems ranging from basanite to granite the apatite melt glass assemblages were obtained from starting glasses artificially enriched in ree sr and fluorapatite components these were run under dry and hydrous conditions of 7 5 20 kbar and 950 1120 c in a solid media piston cylinder apparatus an sem equipped electron microprobe was used for subsequent measurement of ree and sr concentrations in coexisting apatites and quenched glasses the resulting partition coefficient patterns resemble previously determined apatite phenocryst groundmass concentration ratios in the following respects 1 the rare earth patterns are uniformly concave downward i e the middle ree are more compatible in apatite than the light and heavy ree 2 dreeap liq is much higher for silicic melts than for basic ones and 3 strontium and therefore eu2 is less concentrated by apatite than are the trivalent ree the effects of both temperature and melt composition on dreeap liq are systematic and pronounced at 950 c for example a change in melt sio2 content from 50 to 68 wt causes the average ree partition coefficient to increase from 7 to 30 a 130 c increase in temperature on the other hand results in a two fold decrease in dreeap liq partitioning of sr is insenstitive to changes in melt composition and temperature and neither the sr nor the ree partition coefficients appear to be affected by variations in pressure or h2o content of the melt the experimentally determined partition coefficients can be used not only in trace element modelling but also to distinguish apatite phenocrysts from xenocrysts in rocks reported apatite megacryst host basalt ree concentration ratios 12 for example are considerably higher than the equilibrium partition coefficients which suggest that in this particular case the apatite is actually xenocrystic a reversal experiment incorporated in our study yielded diffusion profiles of ree in apatite from which we extracted a ree ca interdiffusion coefficient of 2 4 10 14 cm2 s at 1120 c extrapolated downward to crustal temperatures this low value suggests that complete ree equilibrium between felsic partial melts and residual apatite is rarely established

uncertainty analysis for the integration of seismic and controlled source electro magnetic data abstract we study the appraisal problem for the joint inversion of seismic and controlled source electro magnetic csem data and utilize rock physics models to integrate these two disparate data sets the appraisal problem is solved by adopting a bayesian model and we incorporate four representative sources of uncertainty these are uncertainties in 1 seismic wave velocity 2 electric conductivity 3 seismic data and 4 csem data the uncertainties in porosity and water saturation are quantified by a posterior random sampling in the model space of porosity and water saturation in a marine one dimensional structure we study the relative contributions from the four individual sources of uncertainty by performing several statistical experiments the uncertainties in the seismic wave velocity and electric conductivity play a more significant role on the variation of posterior uncertainty than do the seismic and csem data noise the numerical simulations also show that the uncertainty in porosity is most affected by the uncertainty in the seismic wave velocity and that the uncertainty in water saturation is most influenced by the uncertainty in electric conductivity the framework of the uncertainty analysis presented in this study can be utilized to effectively reduce the uncertainty of the porosity and water saturation derived from the integration of seismic and csem data

evidence of the contrasted fission track clock behavior of the apatites from in ouzzal carbonatites northwest hoggar the low temperature thermal history of an archean basement this fission track study is performed on the apatites from the in ouzzal northwest hoggar algeria carbonatite complex and permits us to present a complete thermal history of this archean core two types of apatites the pink ones early carbonatites and the green ones late carbonatites have been dated the temperature during the pan african orogeny would have reached 100 120 c in the in ouzzal core as both types of apatites from the carbonatites have recorded this event 500 630 m y a mild permian thermal event reset only the fission track clock of the green late apatites this permian thermal event is of quite a large extent in the hoggar shield it has also been detected in the pharusian belt and in central hoggar this event is related to the opening of the atlantic ocean the very high u th content of some of these apatites and consequently the amount of radiation damage in the mineral lattice has been shown to possibly influence the fission track clock behavior of these minerals the f cl content of the apatites does not show any correlation with the degree of age reductions

inversion of traveltime data under a statistical model for seismic velocities and layer interfaces we invert large aperture seismic reflection and refraction data from a geologically complex area on the northeast atlantic margin to jointly estimate seismic velocities and depths of major interfaces our approach combines this geophysical data information with prior information on seismic compressional velocities and the structural interpretation of seismic sections we constrain expected seismic velocities in the prior model using information from well logs from a nearby area the layered structure and prior positions of the interfaces follow information from the seismic section obtained by processing the short offsets instead of using a conventional regularization technique to smooth the interface velocity model we describe the spatial correlation of interfaces and velocities with a geostatistical model using a multivariate gaussian probability density function we impose a covariance function on the velocity field in each layer and on each interface in the model to control the smoothness of the solution the inversion is performed by minimizing an objective function with two terms one term measuring traveltime residuals and the other measuring the fit to the statistical model we calculate the posterior uncertainties and evaluate the relative influence of data and the prior model on estimated interface depths and seismic velocities the method results in the estimation of velocity and interface geometry beneath a basaltic sill system down to 7 km depth this method aims to enhance the interpretation process by combining multidisciplinary information in a quantitative model based approach

lispb revisited a new look under the caledonides of northern britain summary the lithospheric seismic profile in britain lispb shot in 1974 included a 700 km long north south profile of the crust and upper mantle of northern britain the wide angle seismic data from lispb was of exceptional quality and coverage making a reinterpretation using current modelling programs worthwhile this reinterpretation shows that the original crustal model published by bamford et al 1978 satisfies the observed first arrival traveltimes but that further detail of crustal velocity structure may be obtained by model shows strong variations in crustal velocity structure that appear to be correlated with geological and tectonic structures mapped at the surface the new lithospheric model is compatible with the observed gravity field when converted to appropriate densities and the variations in loading along the base of the density model show that the lithosphere is close to local isostatic equilibrium profiles of the variation of velocity and density with depth from the new model are compared with measurements of lewisian rocks made at confining pressures up to 8 kbar allowing the seismic model to be interpreted geologically a synthetic normal incidence profile of the new model is superimposed on the nearby nec deep seismic reflection profile and the features imaged by the two techniques are compared

lithoprobe new perspectives on crustal evolution lithoprobe is canada s national collaborative multidisciplinary earth science research program directed toward an enhanced understanding of how the north american continent evolved research in i

lithospheric structure north of scotland ii poisson s ratios and waveform modelling summary a 200 by 50 km grid of near normal incidence seismic reflection data and a 350 km wide angle seismic line have been acquired across the flannan and w mantle reflectors north of scotland a number of geological models have been published to explain the origin of these reflections we examine these models by considering the physical properties of the lithosphere in this region we present p and s wave velocity models for the lithosphere obtained using ray based inversion schemes we show full wavefield synthetic p wave data generated using a 2 d finite difference code to confirm that our velocity model can reproduce the amplitude behaviour of the observed mantle arrivals p and s wave velocities beneath the moho give a poisson s ratio of 0 25t0 01 which together with evidence from local mantle xenoliths suggests a normal peridotitic upper mantle we observe both normal incidence and wide angle reflections from the mantle reflectors modelling and inversion of these reflection events show that the flannan and w reflectors represent at least a 20 per cent increase in acoustic impedance within the mantle and that the region of elevated acoustic impedance has a thickness of at least 3 km a mafic eclogite is the only common mantle rock type that can have an acoustic impedance significantly higher than peridotite the physical properties of the upper mantle north of scotland thus suggest that the flannan and w reflectors represent reflection events from the tops of slabs of eclogite a weak event is interpreted as a wideangle reflection from the base of one of these slabs a slab thickness of 6 0t1 0 km with a poisson s ratio of 0 29t0 02 was used to model this phase the high poisson s ratio within the slab gives further support to the interpretation that these slabs are eclogitic the lateral extent of these mantle reflection events and the thickness of the slab are most readily explained as oceanic crust that was metamorphosed to eclogite when subducted beneath the existing continent we conclude that the eclogitic slabs are relicts of oceanic crust that remained within the lithosphere when subduction halted

potential vorticity modeling of the itcz and the hadley circulation abstract a simple zonally symmetric balanced model of the hadley circulation is presented the model is based on potential vorticity arguments and consists of a predictive equation for the potential pseudodensity and an invertibility principle to diagnose the associated balanced wind and mass fields when the theory is formulated in the potential latitude coordinate the meridional advection is implicit in the coordinate transformation which makes the prediction equation for potential pseudodensity analytically solvable for convective heating patterns that simulate the itcz the model produces upper and lower tropospheric potential vorticity anomalies of opposite sign the associated winds are easterly at low levels and westerly aloft except between the equator and the itcz where there are low level westerlies and upper level easterlies since the potential vorticity anomalies develop within a background state that has potential vorticity increasing to the north reversed poleward gradients of potenti

old model nd ages in namibian pan african rocks the nd isotope analytical technique is a powerful tool for studying many geological processes particularly the evolution of the upper mantle as seen through the isotope and trace element geochemistry of mantle derived volcanic rocks1 3 and magmatic processes along destructive plate margins4 5 however previous studies of continental crust6 8 have been largely restricted to archaean examples and concerned primarily with dating their time of derivation from the upper mantle in the present study we investigated the nd and sr isotope characteristics of rocks involved in a relatively young 800 450 myr orogenic event such events represent critical stages in the evolution of most of the earth s continental crust new material is added from the mantle and pre existing crust is remobilized by erosion and sedimentation deformation and magmatic activity nd and sr isotopes were used to outline the range in age and geochemical characteristics of rock sequences and provinces within both the upper mantle and the pre existing crust which were sampled during the orogeny and hence provided the major components of what is now a stable segment of continental crust

evolution of the precambrian in ouzzal block central sahara algeria abstract new structural and geochronological data are used to constrain the evolution of the precambrian in ouzzal block situated on the eastern side of the west african craton the results of this study can be summarized as follows 1 i the block can be subdivided into two series one is mainly composed of archaean granitic gneisses 3 2 2 5 ga old while the other contains metasedimentary and basic to ultrabasic rocks which can be compared with those described in archaean supracrustal terrains 2 ii the entire block is affected by very high temperature granulite facies metamorphism 3 iii the metamorphism is associated with two main deformational phases the first of which is responsible for a regional foliation foliation refolded during the second phase 4 iv the major tectono metamorphic event in the area is related to a 2 0 ga old eburnian orogeny which ended with carbonatite emplacement nearly all evidence of the earliest crustal evolution events i e the initial emplacement and deformation of the granitic and supracrustal materials has been erased by the eburnian event

2 46 ga kalsilite and nepheline syenites from the awsard pluton reguibat rise of the west african craton morocco generation of extremely k rich magmas at the archean proterozoic transition abstract the awsard pluton in the moroccan part of the reguibat rise west african craton contains the oldest kalsilite bearing rocks discovered to date with a shrimp zircon u pb age of 2 46 0 01 ga the pluton is composed of nepheline syenites kalsilite syenites and minor silica saturated syenites all with the same primitive sr and nd initial isotope compositions and nd model age that cluster around 87 sr 86 sr 2 46ga 0 7029 nd 2 46ga 1 4 and t dm 2 75 ga silica saturated syenites are in fact contact fenites that grade into nepheline syenites but the two feldspathoidal syenites are true magmatic rocks that crystallized from two coeval highly fractionated k rich magmas with sharply different chemical compositions chemical isotopic and textural evidence suggests that the two magmas originated by liquid immiscibility within an already fractionated alkaline potassic magma of asthenospheric origin that split in two phases a nephelinitic melt rich in hfse ree and a kalsilitic melt poor in hfse ree the awsard pluton spatially associated to carbonatites and other alkaline rocks does not mark a late archean fossil subduction zone but represents the first manifestation of a paleoproterozoic alkaline province located in the reguibat rise the full extent and importance of which is yet to be determined

seismic profiling the continental lithosphere deep seismic reflection profiling is successfully being used worldwide to study the structure of the continental crust1 7 the moist8 profile recorded by birps british institutions reflection profiling syndicate was the first to show that the structure of the upper mantle could also be imaged given a powerful source and long recording times birps has recently acquired an ultra deep seismic reflection line drum deep reflections from the upper mantle the drum line was designed specifically to investigate the reflectivity of the lower continental lithosphere and was therefore recorded to a two way time twt of 30 s giving a depth of penetration into the lithosphere of 110 km this profile has now revealed strong reflections which are unequivocally from the lower lithosphere well below the base of the crust and which are as far as we are aware the deepest and most continuous structures imaged in the upper mantle by multi channel seismic reflection profiling

the role of katabatic winds on the antarctic surface wind regime abstract antarctica is known for its strong and persistent surface winds that are directed along topographic pathways surface winds are especially strong during the winter period the high directional constancy of the wind and the close relationship of the wind direction to the underlying terrain can be interpreted as evidence of katabatic wind activity observations show that the directional constancy of the antarctic surface wind displays little seasonal variation summertime winds cannot be expected to contain a significant katabatic component owing to enhanced solar heating of the ice slopes observations also show that the coastal environs are subjected to wide variation in atmospheric pressure associated with frequent cyclone activity the robust unidirectional nature of the antarctic surface wind throughout the year implies that significant topographic influences other than those from katabatic forcing must be acting idealized numerical simulations have been performed to illustrate the potential

exposed cross sections through the continental crust implications for crustal structure petrology and evolution abstract the continental crust is exposed in cross section at numerous sites on the earth s surface these exposures which appear to have formed by obduction along great faults during continental collision may be recognized by exposures of deep crustal rocks exhibiting asymmetric patterns of metamorphic grade and age across the faults and by distinctive bouguer anomaly patterns reflecting dipping basement structure and an anomalously deep mantle from an examination of five complexes which meet these criteria it is concluded that the most prominent layering in the crust is not compositional but metamorphic the lower crust consists of granulite facies rocks of mafic to intermediate composition while the intermediate and shallow levels consist predominantly of amphibolite facies gneisses and greenschist facies supracrustal rocks respectively post metamorphic granitic intrusions are common at intermediate to shallow levels position of discontinuities in refraction velocity where present commonly correspond to changes in composition or metamorphic grade with depth the continental crust is characterized by lateral and vertical heterogeneities of varying scale which are the apparent cause of the complex seismic reflections recorded by cocorp field observations coupled with geochemical data indicate a complex evolution of the lower crust which can include anatexis multiple deformation polymetamorphism and reworking of older crustal material the complexity of the crust is thus the result of continuous evolution by recycling and metamorphism through time in a variety of tectonic environments

the pleasant bay layered gabbro diorite coastal maine ponding and crystallization of basaltic injections into a silicic magma chamber

the effects of out of plane seismic energy on reflections in crustal scale 2d seismic sections abstract crustal scale seismic surveys mostly collect data along single profiles and the data processing has an underlying assumption that the data have imaged two dimension 2d structure striking at right angles to the seismic profile however even small amounts of out of plane topography on a reflector can result in reflections that do not map the reflector shape accurately out of plane energy will migrate within the plane of the section to an apparent depth represented as two way time twt that is greater than the depth of the reflection point out of the plane of the section it will fall within the plane of the section at depths less than equal to or greater than the intersection of the reflector with the plane of the section depending on both the amount of out of plane topographic relief on the reflector and the offset of the topographic relief from the plane of the section reflectors that are a single surface can therefore be manifested in the seismic section as a band of several reflections rather than a single reflection more complex reflectors such as shear zones that have a finite thickness because they are made up of several to many anastomosing layers of altered and anisotropic rock embedded in protolith will appear as laterally short reflections within a laterally continuous reflection band other examples of such reflectors would be the moho in some places and rock with compositional layering with increasing out of plane topographic relief on the reflector the top of the reflection band for both single and multi layer reflectors will be a poor indicator of the top of the reflector in the earth the bottom of the reflection band will always be a poor indicator of the bottom of the reflector because out of plane energy can arrive at twts that are different from those of the reflector in the plane of the section out of plane energy has the potential to interfere constructively or destructively with the in plane energy in synthetic data calculated for a simple model assuming one layer and topographic relief of 250 m over wavelengths of 4 5 km similar to that imaged in a real sub horizontal detachment amplitudes ranged up to 2 6 times the expected amplitude for the layer a model with anastomosing layers built to resemble a thick shear zone rather than a discrete fault surface allowed tuning between layers the effects of out of plane energy when combined with the effects of tuning caused amplitudes up to 3 1 times those expected larger amplitudes could be achieved if a suitable model was contrived the results indicate that care must be taken when calculating impedance contrasts using real data the highest amplitude reflections are likely to yield overestimates of the true impedance contrast

a dynamical explanation of the topographically bound easterly low level jet surrounding antarctica this study investigates the topographically bound easterly low level jet surrounding antarctica this jet is modeled as a balanced flow that satisfies the potential vorticity invertibility principle based on local linear balance in spherical coordinates and expressed in isentropic coordinates in this way this easterly low level jet is shown to be the balanced flow associated with the topography of the antarctic plateau moderated by a shallow potential vorticity anomaly atop the plateau produced by radiative cooling the dynamical connection of the jet with katabatic winds can be understood through the meridional circulation equation model results based on the simple theoretical arguments developed here are found to be consistent with high resolution reanalysis data from the european centre for medium range weather forecasts ecmwf for the 2008 2010 period

new carbonatite complexes in the archaean in ouzzal nucleus ahaggar algeria mineralogical and geochemical data several massifs of very old carbonatites have been discovered in the archaean granulitic block of in ouzzal western ahaggar algeria these carbonatites are original since they are associated with silica saturated syenitic magmatism and present in the late stages of evolution a very uncommon mineralogy with silicate minerals especially wollastonite allanite and quartz the mineralogy c and o isotopes and r e e distributions indicate that the late stages of crystallization occurred under high sio2 activities and produced the uncommon mineralogy and extremely high r e e concentrations in phosphate minerals apatite and britholite interaction with continental crust is a possible mechanism to explain the original features of these carbonatite complexes

lithospheric structure north of scothland i p wave modelling deep reflection profiles and gravity the w reflector and the flannan reflector are two enigmatic planar structures in the lithospheric mantle north of scotland mapped in three dimensions using deep reflection profiling the flannan reflector dips eastwards from the moho to a depth of 80 km at an angle of 30 whilst the w reflector is subhorizontal lying 10 20 km beneath the moho and terminating westwards against the flannan both reflectors have a strong impedance contrast and are laterally coherent over tens of kilometres wide angle seismic reflection and refraction data are used here to constrain the p wave velocity structure of the lithosphere in this region and to investigate the physical properties and geometry of these reflective structures we report the results from an integrated seismic survey in which a series of explosive shots fired at sea were recorded by ocean bottom seismographs land stations and a streamer towed by a second ship in an expanding spread configuration a well resolved p wave velocity structure for the lithosphere above the mantle reflectors is derived by inverting the seismic traveltimes to optimize the fit of the model to the data and by generating synthetic seismograms from the model to match the relative amplitudes of the observed seismic phases a high amplitude post critical reflection indicates the existence of a discrete reflecting interface dipping gently westwards between 40 and 50 km depth in the lithospheric mantle the p wave velocity model is converted to two way reflection time and correlated with normal incidence reflection data the moho structure agrees well and the reflecting interface defined by the wide angle data is coincident with the subhorizontal w reflector less distinct later arrivals observed in the data are consistent with modelled reflections from the flannan reflector in the mantle and from the extrapolated position of the outer isles fault in the lower crust modelling of the relative amplitudes and critical distances of the wide angle mantle reflection shows that the reflecting layer must be at least 3 km thick and that it has considerably higher velocity 8 5 km s 1 and density 3 5 g cm 3 than normal mantle further modelling tests the vertical structure within the layer and constrains the sharpness of the upper interface and whether the reflection amplitude could have been enhanced by internal layering gravity data measured over the region are consistent with the velocity model and with the high density layer in the mantle which is constrained by the gravity modelling to be less than 10 km thick published laboratory measurements of the physical properties of mantle rocks indicate that mafic eclogite with high velocity and density satisfies all the observations from the mantle layer our preferred hypothesis for the origin for such a layer is that it is a fragment of oceanic crust that has been subducted and metamorphosed to eclogite facies

the surface windfield over the antarctic ice sheets the intense radiative cooling of air over the ice slopes of antarctica generates a surface wind regime that is strongly controlled by topography and plays a key role in determining the behaviour of the atmosphere and ocean in high southern latitudes1 7 resultant surface winds are intimately linked to the orientation of the ice terrain fig 1 and display the highest degree of persistence found on earth the close coupling between wind and topography allows estimation of the former if the latter is known with some precision here we report on time averaged near surface airflow over the antarctic continent during winter diagnosed from a recent accurate synthesis of terrain slopes and from estimates of the lower atmospheric temperature structure the simulated drainage pat tern exhibits strong spatial variability with the airflow concentrated into several zones near the coastal margin these confluence regions are responsible for strong persistent katabatic winds over downstream coastal stretches and are indicative of zones of greatest katabatic potential

contrasting shrimp u pb zircon ages of two carbonatite complexes from the peri cratonic terranes of the reguibat shield implications for the lateral extension of the west african craton abstract the oulad dlim massif of the western reguibat shield contains several carbonatite complexes of previously unknown age the largest and best studied are gleibat lafhouda composed of magnesiocarbonatites and twihinate composed of calciocarbonatites gleibat lafhouda is hosted by archean gneisses and schists it has a shrimp u th pb zircon crystallization age of 1 85 0 03 ga a nd model age of t cr 1 89 0 03 ga and a sm nd age of 1 85 0 39 ga it forms part of the west reguibat alkaline province twihinate on the other hand is much younger it is hosted by late silurian to early devonian deformed granites and has a zircon crystallization age of 104 4 ma which is within error of the age of the carbonatites of the famous richat structure in the southwest reguibat shield like these the twihinate carbonatites are part of the mid cretaceous peri atlantic alkaline pulse the twihinate carbonatites contain abundant inherited zircons with ages that peak at ca 420 ma 620 ma 2050 ma 2466 ma and 2830 ma this indicates that their substratum has west african rather than as previously suggested avalonian affinities it has however a paleoproterozoic component that is not found in the neighboring western reguibat shield the 421 ma to 410 ma gneissic granites hosting twihinate are epidote biotite ca rich garnet deformed i type to a type granites derived from magmas of deep origin compatible therefore with being generated in a subduction environment these granites form a body of unknown dimensions and petrogenesis the study of which will be of key importance for understanding the geology and crustal architecture of this region

chemical and textural relations of britholite and apatite group minerals from hydrothermal ree mineralization at the rodeo de los molles deposit central argentina

a nd isotope investigation of sediments related to crustal development in the british isles the times of original fractionation of the sm and nd component of clastic sediments from a mantle source crustal residence age have been estimated from sm nd model ages calculated relative to a depleted mantle evolution in this way the provenance and evolution of selected precambrian and phanerozoic sediments and metasediments from the british isles have been examined whereas some archaean and early proterozoic sediments have sm nd model ages that are close to their stratigraphic age the phanerozoic sediments analysed have model ages as much as 2 0 ga in excess of their stratigraphic age a more detailed study of lower palaeozoic sediments deposited on the northern margin of the iapetus ocean provides evidence for a marked change of provenance in the ordovician after the deposition of the dalradian supergroup a component with comparatively high143nd 144nd and sm nd ratio presumably basaltic is present in the sediments throughout the accretionary prism crustal residence age estimates average about 1 5 ga for both these lower palaeozoic sediments and modern pelagic clays and collectively fail to provide any evidence for significant continental growth during the phanerozoic