tectono sedimentary evolution of the bowland basin n england during the dinantian the bowland basin is a ne sw trending sedimentary basin lying between the askrigg bowland high to the n and nw and the central pennine high to the se the basin was initiated in late devonian tournaisian times sediment thickness and facies variations within the dinantian succession show marked asymmetry on both a basin wide and intra basin scale as a result of approximate nw se to n s extension across normal faults thickening of the sedimentary sequence from ne to sw over much of the basin resulted from fault controlled extension along the se margin of the basin with downthrow to the nw giving the basin a half graben character extension occurred in two distinct episodes marked by basin wide debris flow and sedimentary slide slump deposits during the late chadian early arundian the simple half graben became modified by a series of ne sw trending antithetic and nw se trending transfer faults associated with local unconformities and marked lateral thickness and facies variations this interval marks the transition from carbonate ramp to slope sedimentation and a temporary halt in carbonate supply to the basin the second episode of extension the late asbian early brigantian marks the break up of the northern basin margin ending major carbonate deposition within the basin reactivation of these structures at the end of the carboniferous created the ribblesdale fold belt as a result of nw se to n s compression transpression application of this research to areas surrounding the bowland basin has led to the recognition of additional shelf basins the orientation of which suggests control by normal and transfer fault systems discussion and

outlet glacier response to forcing over hourly to interannual timescales jakobshavn isbrae greenland the loss of the floating ice tongue on jakobshavn isbrae greenland in the early 2000s has been concurrent with a pattern of thinning retreat and acceleration leading to enhanced contribution to global sea level these changes on decadal timescales have been well documented here we identify how the glacier responds to forcings on shorter timescales such as from variations in surface melt the drainage of supraglacial lakes and seasonal fluctuations in terminus position ice motion and surface melt were monitored intermittently from 2006 to 2008 dual frequency gps were deployed 20 50 km upstream of the terminus along the glacier center line gaps in surface melt measurements were filled using a temperature index model of ablation driven by surface air temperatures recorded during the same time period our results corroborate the premise that the primary factors controlling speeds on jakobshavn isbrae are terminus position and geometry we also observe that surface speeds demonstrate a complex relationship with meltwater input on diurnal timescales velocities closely match changes in water input however on seasonal timescales a longer more intense melt season was observed to effectively reduce the overall ice flow of the glacier for the whole year

petrology and geochemistry of lavas from the sumisu and torishima backarc rifts thirteen dredge hauls from the active sumisu and torishima rift grabens west of the izu bonin arc at about 30 31 n 140 e recovered a suite of tholeiitic basalts to sodic rhyolites volcanism occurs along tensional faults within and bounding the rift grabens and along the transfer zones between adjoining rift segments the sumisu and torishima rift lavas differ significantly from the lavas of the adjacent arc volcanic centers in having loweral2o3 na2o ba zr v ti andba ce and higher abundances of the rare earth elements the rift lavas also have characteristics of backarc basin basalts in that they are enriched in al2o3 and depleted in total iron and tio2 relative to mid ocean ridge basalt characteristics which are consistent with a higher water content in the source thus the model of a progressive change in backarc basin basalt composition from arc like to mid ocean ridge like as a function of evolution of the basin as has been suggested from many backarc regions is not generally applicable the comparison of the sumisu and torishima rift lavas with mariana backarc basin lavas indicates that backarc basin basalts differ in composition from one basin to another the comparison of these backarc basin suites with mid ocean ridge suites from similar axial depths indicates that the overall control over the spectrum of backarc basin basalt compositions may be different extents of melting of the mantle the sumisu and torishima rift lavas formed by a slightly higher extent of melting than the mariana backarc basin basalts a phenomenon which is related to the depth of the ridge segments furthermore the data suggest a systematically higher extent of melting in the arc lavas than in the backarc lavas for both of these arc backarc systems consistent with a greater flux of water beneath the arc than beneath the backarc region

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

paleoproterozoic volcano sedimentary series in the ca 2 27 1 96 ga birimian orogen of the southeastern west african craton abstract the ca 2270 1960 ma accretionary collisional birimian orogen in the southern west african craton swac contain a number of volcano sedimentary series which were deposited in basins that developed during the regional eburnean collisional orogeny ca 2160 1960 ma understanding their spatiotemporal distribution is thus crucial for constraining the tectonic evolution of the orogenic system this study presents new la icpms data on detrital zircon from volcano sedimentary series in southeastern swac which indicate maximum depositional ages between ca 2160 2130 ma the detrital zircon population is predominantly composed of grains with ages compatible with derivation from local birimian crust but older paleoproterozoic and archean components are also present these may have been derived from crustal domains now present in south american cratons and terranes and which would have been located to the present east or southeast of the swac the deposition of the volcano sedimentary series also coincided with the onset of the eburnean collisional orogeny in eastern swac which suggest a dynamic coupling between compression on one hand and extension on the other where the latter controlled the establishment of the depocentres in which the series were deposited the data in this study support previous observations that sedimentation across swac was diachronous during the course of the eburnean orogeny and that an early pulse occurred primarily in the east between ca 2165 2125 ma while a later pulse occurred in the west around ca 2115 2080 ma followed by limited volcanism and sedimentation at ca 2065 ma this diachroneity is also reflected in the lithostratigraphy with areally extensive shallow water deposits being more frequent in the east whereas carbonates are only present in the younger successions to the west this may reflect a hinterland foreland style relationship between eastern and western swac respectively where sedimentation occurred progressively during convergence related to the regional and diachronous eburnean orogeny

a coal depositional model for the durham coal measures of ne england the upper carboniferous westphalian coal measures of the durham coalfield were deposited on a broad flat deltaic plain in early westphalian a times a lower delta plain setting prevailed evolving through southward deltaic progradation into an upper delta plain by mid westphalian a detailed facies analysis of large opencast exposures supported by extensive subsurface data has shown that a hierarchy of depositional controls operated across the durham area during westphalian times i on the large scale depositional geometry was controlled by patterns of major delta progradation and switching ii on the medium scale by a combination of structurally and compactionally induced subsidence and iii on the small scale by local sedimentary processes and subsidence patterns these observations form the basis of a coal depositional model for the durham coal measures which predicts that the thickest coals are to be found in upper delta plain domains controlled by compactional subsidence and not underlain by substantial channel sandstones this model may also be applicable to other coalfields of northern england and to ancient delta plain sequences elsewhere in the world

overview of the influence of syn sedimentary tectonics and palaeo fluvial systems on coal seam and sand body characteristics in the westphalian c strata campine basin belgium

carboniferous stratigraphy sedimentation and correlation of well 48 3 3 in the southern north sea basin integrated use of palynology natural gamma sonic logs and carbon sulphur geochemistry a methodology for the correlation of offshore silesian upper carboniferous sequences is developed using an integrative approach based upon biostratigraphy downhole logging and geochemistry we develop the combined use of miospore zonation gamma ray spectrometry formation velocities and carbon sulphur ratios to determine the location of marine horizons and other lithofacies in a thick 1 km largely uncored silesian sequence in the southern north sea gas basin at least nine major marine horizons are recognized within a basin fill sequence of marsdenian yeadonian and langsettian ages namurian b c and westphalian a in the well these horizons help to provide a detailed subdivision of the offshore sequence although the correlations must remain tentative until further deep wells are drilled in the area the implications of the correlations for offshore palaeogeography are that the area of northern quadrant 48 was a deep periodically anoxic basin until infill by successive submarine fan lobes in late marsdenian times yeadonian delta front and coastal plain deposits pass upwards into fluvio deltaic wetland facies of the langsettian

automatic measurement of glacier ice ablation using thermistor strings in this work we tested the suitability of thermistor strings as automatic tools for the continuous measurement of glacier ice ablation experimental data collected in summer 2017 over an italian glacier provided ice ablation readings with accuracy similar to manual measurements with ablation stakes and other automatic systems like the draw wire method and the boggild ablatometer thermistor strings have potential for future applications in remote glacier monitoring thanks to their flexibility simple construction and robustness

late archean mantle composition and crustal growth in the western superior province of canada neodymium and lead isotopic evidence from the wawa quetico and wabigoon subprovinces abstract neodymium and lead isotopes are presented for late archean rocks from the wawa quetico and wabigoon subprovinces sp in the western superior province ontario canada the isotopic compositions were determined on greenstone volcanic sequences pre tectonic 2 73 2 69 ga trondhjemite tonalite granodiorite ttg suites metasedimentary rocks and post tectonic 2 69 2 67 ga dioritic to granitic plutons in order to characterize the mantle and crustal reservoirs involved in the evolution of the southern part of the western superior province although derived from a depleted mantle almost all the greenstone volcanism and pre tectonic ttg suites record contamination by crustal material as old as 3 2 ga neodymium and lead isotopic compositions of the pre tectonic ttg can be modeled as products of the melting of amphibolitic crust e g subducted basalts contaminated by 1 10 of subducted sediments moreover lead isotopes of ttg suites provide evidence that the isotopic compositions of these subducted sediments varied as a result of the addition of new ca 2 7 ga juvenile material in agreement with neodymium isotopic differences recorded by sedimentary units of the quetico sp 50 50 mix of young old crusts and wawa sp 75 25 young old neodymium and lead isotopes of the post tectonic ree enriched archean sanukitoid suites define an isotopic trend which is distinct from that of the ttg suites and which is interpreted to reflect the importance of crustal assimilation in the evolution of these suites furthemore such plutons found in the wawa sp have more juvenile neodymium and lead isotopic compositions and higher nd contents than those intruded into the quetico and wabigoon sp which have more crustal isotopic signatures and lower nd concentrations this geographical difference could be due either to differences in nd contents of initial archean sanukitoid magmas lesser assimilation in the wawa sp or more probably because the wawa crust is on average more juvenile than the quetico and wabigoon crusts in agreement with the fact that the geographical difference mirrors that recorded by metasedimentary rocks deposited in quetico and wawa sp

ree patterns nd sm and u pb ages of the metamorphic rocks of the diagorou darbani greenstone belt liptako sw niger implication for birimian palaeoproterozoic crustal genesis

origin and age of paleoproterozoic conglomerates and sandstones of the tarkwaian group in burkina faso west africa abstract the tarkwaian rocks of burkina faso overlie the birimian supergroup which is considered to be part of a juvenile crust formed during the eburnean orogen they comprise fluviatile pebbles quartz rhyolite chert and schist embedded in an acid volcaniclastic matrix during a sinistral transpressive shearing they underwent a low grade metamorphism characterized by a paragonitic muscovite assemblage formed during isoclinal folding fold axes are nearly horizontal parallel to a stretching lineation and oriented 035 215 a typologic study of zircons in the volcaniclastics coupled with radiometric datings enables the volcanism to be characterized and its age to be determined the typologic study has shown that 1 zircons from the rhyolitic pebbles the matrix of the conglomerates and from the sandstone are basically the same 2 the zircons morphology characterizes a calc alkaline to alkaline evolutionary trend implying three main episodes of volcanism 3 the morphology of this zircon population is very similar to those usually observed in post orogenic metasediments or sediments the tectonic setting is one of intracontinental extension associated with rifting and crustal thinning in the course of a major period of crustal growth around 2 1 ga the continental volcaniclastics accumulated in the rift at the same time as the sediments the radiometric study has shown that there are no reworked archean rocks the ages cover the period 2170 to 2124 ma three main volcanic episodes may be defined 2170 2150 and 2124 ma the last one was probably contemporaneous with the sedimentation deformation of tarkwaian rocks took place between 2124 9 ma the age of the youngest inherited zircon and 1991 12 ma the age of a post tarkwaian granite this period 2170 to 2124 ma fits well with the main birimian crustal growth and the ages reported for the tarkwaian rocks of ghana the same period of accretion is also recognized in south america the tarkwaian conglomeratic rocks of burkina faso do not have gold concentrations similar to those in ghana and guyana with respect to the tarkwaian rocks of ghana where gold is concentrated in the quartz pebbles they also differ because 1 there is a volcaniclastic contribution 2 the zircons from the tarkwaian rocks in burkina faso are euhedral indicating that transport distances were short and 3 they have been deformed by shearing with respect to guyana the rocks are very similar and probably have the same origin the short transport distance may not allow a secondary concentration of gold these differences at least may help explain the lack of economic gold concentration in the tarkwaian metasediments

using a draw wire sensor to continuously monitor glacier melt a draw wire sensor has successfully been used to measure surface lowering due to net ablation of a glacier a thin steel wire attached to a weight is inserted and frozen into a borehole in the ice the draw wire sensor installed on a tetrahedron on the surface retracts the wire as the snow or ice melts relative surface lowering of the melting surface is recorded in a data logger and may be converted to mass loss the instrument continuously logs net ablation over several melt seasons and has the potential to be used extensively in future studies of glacier melt whenever high temporal resolution and precision are required

back arc basin origin for the east sulawesi ophiolite eastern indonesia the east sulawesi ophiolite is one of the three largest ophiolites in the world it displays all the components of a typical sequence from residual mantle peridotites to cumulate gabbros sheeted dolerites and lavas of normal mid oceanic ridge basalt morb composition trace element data on the lavas and dolerites and particularly their depletion in nb compared to neighboring incompatible elements suggest a subduction zone environment for their origin the chemical similarity between the east sulawesi ophiolite lavas and those from the eocene celebes sea back arc basin crust together with their identical age strongly suggest a back arc tectonic environment for this ophiolite which represents a fragment of the eurasian plate obducted onto the east sulawesi basement of australian origin

petrogenesis of juvenile type birimian paleoproterozoic granitoids in central cote d ivoire west africa geochemistry and geochronology abstract the birimian paleoproterozoic granitoids of central cote d ivoire are associated either with greenstone belts or with basin sedimentary rocks field data indicate two stages of granitoid emplacement which according to singlezircon and monazite 207 pb 206 pb age determinations occurred within a short period of time the first generation of granitoids 2123 2108 ma intruded and metamorphosed the greenstone belts whereas the second generation 2108 2097 ma intruded and metamorphosed the sedimentary pile and subordinate andesitic volcanic rocks of the bandama basin unconformably overlying the greenstone belts geochemical and isotopic data show that the various granitoids belong to four magmatic groups 1 sodic calc alkaline nacg 2 potassic calc alkaline kcg 3 peraluminous alg and 4 alkaline alkg all the granitoids show juvenile features with mantle like low initial sr isotopic compositions 0 700 0 702 and positive epsilon nd values 1 5 2 7 these data indicate that a the nacg may have formed by partial melting of a garnet amphibolite crust b the kcg and alkg derived from partial melting of crustal mafic to intermediate metaigneous rocks and c the alg epsilon nd 1 5 represent infracrustal melting of young sialic igneous materials during a regional crustal shortening in a transcurrent tectonic context sm nd model ages 2 25 2 4 ga preclude the existence of an archean crust reworked in the birimian magmas an underplating model is discussed

warming glacier melt and surface energy budget from weather station observations in the melville bay region of northwest greenland the glaciers in the melville bay region of northwest greenland have shown a mean retreat since the earliest observations at the beginning of the 20th century the largest steenstrup gletscher has retreated 20 km partly during the period of atmospheric cooling 1940 80 melville bay air temperature observations starting in 1981 indicate a regional change of 0 208 ca 1 this exceeds the warming on the east coast of greenland confirming the west coast to be a region of relatively large change also in a global perspective the largest temperature increase is observed in the winter months 0 3 0 48 ca 1 results from a 4 year 2004 08 net ablation record obtained by an automatic weather station aws near the calving front of steenstrup gletscher show an ablation rate that is relatively low for a low elevation position on the greenland ice sheet 2 4 m ice equivalent per year a first order estimate from positive degree day totals suggests that net ablation has roughly doubled since the 1980s a surface energy and mass balance model is applied to the steenstrup aws data to quantify the energy flux contributions to surface melt solar radiation is the main source for melt energy but due to shortwave radiation penetration into the ice only one third of the melt takes place at the glacier surface nearly two thirds occurs within the upper ice layers

flotation and retreat of a lake calving terminus mendenhall glacier southeast alaska usa mendenhall glacier is a lake calving glacier in southeastern alaska usa that is experiencing substantial thinning and increasingly rapid recession long term mass wastage linked to climatic trends is responsible for thinning of the lower glacier and leaving the terminus vulnerable to buoyancy driven calving and accelerated retreat bedrock topography has played a major role in stabilizing the terminus between periods of rapid calving and retreat lake terminating glaciers form a population distinct from both tidewater glaciers and polar ice tongues with some similarities to both groups lacustrine termini experience fewer perturbations e g tidal flexure high subaqueous melt rates and are therefore inherently more stable than tidewater termini at mendenhall rapid thinning and simultaneous retreat into a deeper basin led to flotation conditions along approximately 50 of the calving front this unstable terminus geometry lasted for approximately 2 years and culminated in large scale calving and terminus collapse during summer 2004 buoyancy driven calving events and terminus break up can result from small rapidly applied perturbations in lake level

uk onshore oil exploration 1930 1964 abstract the earliest attempts to find oil in britain dating from 1918 1922 based on anticlines flanking the pennines were largely unsuccessful renewal of exploration in the 1930s aimed at a broader range of prospects met with much criticism but went ahead after nationalization of the unknown and undiscovered oil resources 1934 the south of england ranked as first priority but the initial drilling of the new campaign carried out in 1935 1937 yielded only minor quantities of oil and gas attention was transferred in 1938 to upper palaeozoic prospects in the midlands and north resulting in small gas and oil discoveries in scotland and yorkshire and discovery of a series of commerical oilfields in the upper carboniferous of nottinghamshire in the 1950s the first commercial discovery in the jurassic was made at kimmeridge in dorset further carboniferous discoveries were made in nottinghamshire and lincolnshire and a series of fresh objectives defined by seismic reflection were drilled in the mesozoic basin of southern england this phase of exploration was terminated at the end of 1964 by adverse fiscal changes continuation of effective exploration operations remained uneconomic until the oil price rise in 1973

late yeadonian upper sandstone group incised valley supply and depositional systems in the south wales peripheral foreland basin implications for the evolution of the culm basin and for the silesian hydrocarbon plays of onshore and offshore uk abstract the upper sandstone group of south wales has previously been interpreted as a simple nnw sse trending incised valley that was filled during the late yeadonian namurian g1b data presented in this study indicate a far more complex scenario the group is well developed in the pembrokeshire sub basin but it is absent from the main part of the south wales basin located to the east of the penlan axis the principal source area for sediment flux was the forebulge of the basin located to the north in lower palaeozoic foreland strata variscan flexural uplift of this source area and an accompanying rapid high magnitude fall in sea level produced a very high sediment budget thus during the late yeadonian lowstand of sea level massive amounts of sediment were being generated and transported via an incised valley system across the western part of the south wales peripheral foreland basin the location and orientation of the incised supply system and its related interfluves were controlled by intrabasinal synsedimentary variscan structures many of which were reactivated caledonian extensional faults during the succeeding sea level rise the incised valley was filled initially by fluvio deltaic sands and later by estuarine argillaceous sediments the implications of this combined tectonic sedimentological high resolution sequence stratigraphic model for the evolution of the late namurian crackington formation turbidites of the culm basin are discussed examples of silesian incised valley fill sandstone reservoirs developed in the east midlands oilfield and the southern gas basin are briefly summarised and their spatial and temporal distributions are appraised data generated from these case studies are assessed in terms of their use for the modelling of incised valley fill sequences in foreland basins and other tectonic settings

greenland surface mass balance observations from the ice sheet ablation area and local glaciers glacier surface mass balance measurements on greenland started more than a century ago but no compilation exists of the observations from the ablation area of the ice sheet and local glaciers such data could be used in the evaluation of modelled surface mass balance or to document changes in glacier melt independently from model output here we present a comprehensive database of greenland glacier surface mass balance observations from the ablation area of the ice sheet and local glaciers the database spans the 123 a from 1892 to 2015 contains a total of 3000 measurements from 46 sites and is openly accessible through the promice web portal http www promice dk for each measurement we provide x y and z coordinates starting and ending dates as well as quality flags we give sources for each entry and for all metadata two thirds of the data were collected from grey literature and unpublished archive documents roughly 60 of the measurements were performed by the geological survey of denmark and greenland geus previously ggu the data cover all regions of greenland except for the southernmost part of the east coast but also emphasize the importance of long term time series of which there are only two exceeding 20 a we use the data to analyse uncertainties in point measurements of surface mass balance as well as to estimate surface mass balance profiles for most regions of greenland

the structure and evolution of the northumberland trough from new seismic reflection data and its bearing on modes of continental extension an 18 km long n s seismic reflection profile has been acquired across the faulted boundary between the alston block and the northumberland trough a nw se geological cross section has been constructed across the trough by integrating this line with other data and a tricentrol seismic profile confirms the suitability of this model the trough has a markedly asymmetric form with a thickness of more than 4 2 km of dinantian strata adjacent to its faulted southern margin the present day surface faults stublick ninety fathom are related to variscan and later inversion and transpression and do not everywhere correspond precisely to the earlier syn depositional normal faults the basin is believed to have formed in response to reactivation of the iapetus suture in a dominantly n s extensional stress field basin evolution has been analysed by fault restorations and subsidence modelling the fault restorations indicate an upper crustal extension factor of 1 15 to 1 19 subsidence modelling indicates a whole crustal extension factor of 1 30 with similar sub crustal lithospheric extension it is possible that the difference between the inferred extension factors is due to non uniform increasing downwards lithospheric extension but the uncertainties inherent in both methods are such that this cannot yet be confirmed

in situ u pb zircon ages for early ordovician magmatism in the eastern pyrenees france the canigou orthogneisses u pb age determinations with an ion probe on zircons within orthogneisses from the canigou massif eastern pyrenees give an emplacement age for their granitic protolith of 475 10 ma arenigian this invalidates the previous interpretation of basement cover relationships between the orthogneisses and the overlying lower cambrian canaveilles metasedimentary series and corroborates the intrusive character of the orthogneisses which is further supported by field evidence the data also define two groups of inherited zircons paleo meso proterozoic and cadomian in addition to partial lead loss during the hercynian late carboniferous high temperature low pressure metamorphic event these results are in agreement with ages reported from other orthogneissic bodies across the entire variscan chain of western europe and indicate the existence of a widespread cambro ordovician magmatic event

ice ablation and meteorological conditions on the debris covered area of baltoro glacier karakoram pakistan during the recent italian expedition k2 2004 50 years later june july 2004 on baltoro glacier karakoram pakistan glaciological field experiments were carried out on the debris covered area of this high elevation glacier the aim was to investigate the ice ablation and its relations with debris thermal properties and meteorological conditions ablation measurements along the glacier up to about 5000 m and within a dedicated test field were combined with meteorological data from two automatic weather stations located at urdukas 4022 m a s l and at k2 base camp 5033 m a s l in addition temperature measurements of the debris cover at different depth levels along the glacier allowed the calculation of debris surface temperature and of the debris thermal resistance r using the air temperature the local mean lapse rate 0 00758 cm 1 and the measured ablation the degree day factors k at different locations on the glacier were calculated the ice ablation rates were related to debris thickness and elevation they are typically on the order of 4 cm d 1 during the observation period however it was found that the surface topography slope aspect has an influence on the total ablation similar to that of the debris thickness thermal resistance of the debris cover and its distribution over the glacier were estimated finally a best guess estimate of the total meltwater production was calculated from available climate data

reassessment of proterozoic granitoid ages in ghana on the basis of u pb zircon and monazite dating abstract the west african craton in ghana is of early proterozoic age and forms alternating volcanic belts and sedimentary basins made up of rocks of the birimian supergroup as well as younger conglomerates and quartzites of the tarkwaian group and extensive granitoid plutons no consensus exists on absolute and relative ages of plutons within belts and basins u pb geochronology of four different granitoid complexes indicates that contrary to long held views belt granitoid plutons ashanti belt 2172 2 ma sefwi belt 2179 2 ma are 60 to 90 ma older than basin plutons kumasi basin 2116 1 ma belt granitoid are approximately coeval with volcanic rocks of the birimian supergroup while basin plutons are late kinematic and postdate tarkwaian sedimentation the first high precision geochronological data for rocks from ghana i e the eastern part of the birimian domain on the west african craton presented here show that magmatism within this region occurred over a time span of at least 90 ma and provide further evidence that the period between 2 1 and 2 2 ga was one of important magmatism in west africa

mechanisms of pluton emplacement and structural evolution of a 2 1 ga juvenile continental crust the birimian of southwestern niger abstract the 2 1 ga birimian terranes of southwestern niger consist of a granite greenstone association the structures of the greenstone belts result from the interference between pluton emplacement and regional transcurrent deformation sinistral n s strike slip faults ne sw trending schistosity the regional cleavage trend is observed to change in the vicinity of elliptical plutons cleavage trajectories bend around their contacts resulting in triple junctions at the pluton extremities and narrow aureoles of intense penetrative schistosity and high grade amphibolitefacies metamorphism are developed around them structural features of plutons suggest syntectonic emplacement and age data indicate that all plutons were emplaced between 2 16 and 2 10 ga the plutons are assembled in two distinct types of batholith elongated either parallel or perpendicular to the inferred regional shortening direction a batholith parallel to the inferred regional shortening direction dolbel batholith consists of km sized subcircular alkali granite plutons except for the southernmost one which is of a larger size and exhibits an elliptical shape with a nw se short axis parallel to the shortening direction crosscutting relationships indicate a progressively northward emplacement trend for those plutons internal fabrics generally correspond to layering and magmatic flow fabrics and accessorily to schistosity and shear zones which represent late stage plastic deformation restricted to the pluton margin batholiths which are perpendicular to the inferred regional shortening direction are composed of large elliptical plutons 500 to 5000 km 2 in area consisting mainly of granodiorite and covering 70 of the region the plutons never show crosscutting relationships but are moulded against each other their internal fabric corresponds to a solid state foliation it either dips towards the centre of the pluton funnel or teardrop shaped intrusions or is subhorizontal in the centre and vertical at the margin dome shaped intrusions the foliation and the geometry of the largest pluton tera pluton were acquired during pluton ascent and inflation whereas the geometry of later shear zones indicates that deformation in the pluton was then passively acquired at the final emplacement level due to regional deformation the contrast in both the orientation and structural features of the batholiths reflects distinct emplacement mechanisms the small alkali granite plutons resulted from magma injection within nw se fractures parallel to the inferred regional shortening direction whereas the granodiorite plutons are assumed to be emplaced as diapirs forceful intrusions with long axes perpendicular to the shortening direction these two intrusion mechanisms for plutons emplaced during the same deformation event and at the same crustal level are considered to reflect a local change in the rheological behaviour of the crustal segment studied in connection with the thermal effect induced by the large granodioritic plutons this vertical tectonic regime appears to be typical of palaeoproterozoic juvenile continental crust areas and related to an interplay between emplacement of tremendous volumes of granodioritic magma and a regional transcurrent deformation

lithostratigraphy and composition of 2 1 ga greenstone belts of the west african craton and their bearing on crustal evolution and the archean proterozoic boundary we have determined the lithostratigraphy of three early proterozoic 2 1 ga birimian greenstone belts in the west african craton and measured the concentrations of major and trace elements in samples of volcanic rocks from those belts the nangodi belt largely consists of tholeiitic basalt flows with mg numbers of 37 to 69 and flat to slightly lree depleted chondrite normalized ree patterns overlain by sodic na 2 o k 2 o 3 7 29 calc alkaline dacitic tuffs and lavas that are hree depleted la yb 4 5 8 9 times chondrites yb 3 9 8 7 times chondrites the volcanic sequence is capped by calc alkaline andesitic crystal tuffs with high mg numbers 66 75 and a high la yb ratio 10 5 times chondrites similar to archean and modern sanukitoids the dixcove belt largely consists of tholeiitic hyaloclastites and pillow lavas mg numbers 52 61 that grade upward into high mg mg numbers 59 calc alkaline andesitic tuffs and flows the winneba belt largely consists of massive tholeiitic h

the vibroseis system a high frequency tool exploration methods are extended to their limits as we continue the search for energy resources successful application of high frequency seismic method requires evaluating each element in the seismic acquisition system and ensuring that each part of the system contributes to the success of the method this extends from seismic signal generation through data processing where good equipment performance and correct parameter selections are required the vibroseis system depends upon the ability of vibrators to generate synchronous repeatable sweeps over the frequency range of interest to support our high frequency seismic efforts conoco inc has developed and built a new high frequency vibrator this paper includes some of the consideration used in building the vibrator along with typical baseplate responses showing excellent drive levels to the design goal of 200 hz with an excellent source available correct application is essential to assure retention of high frequency data recording offsets ar

seismic facies analysis of westphalian sequences of the southern north sea abstract differing seismic facies are observed from within the westphalian sequences of the southern north sea a review of published synthetic seismograms and seismic data from known coal bearing basins establishes seismic facies types and their relationships similar reflection character is recognized from westphalian sequences in the southern north sea and lithologies confirmed by borehole information probable sand silt or clastic prone upper coal measures westphalian c d stages and coal prone lower and middle coal measures westphalian a b and part c stages can be defined and mapped providing information on the regional distribution of source and reservoir rocks and stratigraphic plays it is suggested that sand prone channel complexes or clastic dominated areas within the mainly a b stages can be recognized and mapped providing information on migration pathways and stratigraphic plays

short paper evidence of negative structural inversion beneath central england from new seismic reflection data a major seismic reflection transect acquired by the british geological survey between 1984 and 1986 has elucidated the deep geology and structure of some well known surface features of central england the worcester basin a mesozoic extensional graben system is bounded to west and east by major sub planar normal faults which formed by the extensional reactivation of much older structures respectively the malvern axis and a similar though concealed structure forming its mirror image to the east within the worcester basin a thick permo triassic sequence rests directly upon precambrian basement rocks to the west and east permo triassic rocks are thin or absent but relatively thick palaeozoic sequences are preserved above the precambrian these strikingly different successions together with the fault geometry suggest that the worcester basin lies within a major inverted oblique slip fault system or flower structure in palaeozoic times transpressivt stresses led to reverse faulting along the malvern axis and its complementary structure to the east with consequent regional uplift of the intervening tract and the formation of a positive flower structure in permian and early mesozoic times transtensional stresses caused collapse and negative inversion of the flower structure and consequent subsidence of the worcester basin

increased ablation of wisconsin ice in eastern north greenland observations and modelling in kronprins christian land at 80 n in eastern north greenland it has been observed that the surface of the wisconsin ice is significantly darker than the holocene ice found immediately upstream from a transition located 710 m from the ice margin delta 18o analysis has shown the dark surface is of wisconsin origin deep ice cores from the greenland ice sheet all indicate that the wisconsin ice contains orders of magnitude more micro particles which could be the reason for the dark appearance of the wisconsin surface photographic documentation spectral surface albedo measurements and satellite image analysis all indicate a reduced albedo of wisconsin ice the effect of this reduced albedo is not confirmed by the ablation measurements however measurements of ablation variability within small test sites has documented that large errors will arise if only one stake per measuring point is used energy balance calculations show ablation rates should be 10 70 i e 1 8 8 4 mm d 1 respectively less than experienced additionally a satellite image analysis shows even higher albedo contrasts 20 30 km to the south of our transect locality immediately after the termination of the ice age most of the surface in the ablation zone consisted of this low reflectance ice so in the early holocene the dark ice of wisconsin origin is likely to have resulted in higher ablation rates than previously considered this may probably partly explain the fast rate of retreat disintegration of the ice sheets in the northern hemisphere after the termination of the wisconsin ice age

upper palaeozoic basins of the british isles caledonide inheritance versus hercynian plate margin processes hypotheses to explain the origin and location of upper palaeozoic sedimentary basins are presented group a basins were those old red sandstone basins whose location and subsidence were controlled by caledonide features including active and remnant fore arc basins midland valley and major transcurrent movements orcadian basin newer granite plutonic magmas in the southern uplands subduction complex post dated subduction and resulted from post collision cratonization processes as yet little understood group b basins were those whose initial positions in space were controlled by palaeo plate factors in particular the location of granitoid plutons but whose subsidence was controlled by active hercynian neoplate processes some group a basins were regenerated by this process midland valley a new hypothesis is presented to account for the origin of a carboniferous rift to sag trend similar to that proven in modern continental margin basins such as the north sea the hypothesis is an adaptation of mckenzie9s lithospheric stretching model some of whose major features are well shown in the carboniferous history of the north and central british isles group b processes were controlled by ligurian bretonic plate consumption in the s brittany massif central vosges areas group c basins in the rheno hercynian zone also owe their location and origin to back arc rifting and continental crustal oceanization processes caused by liguro bretonic subduction

simple kinetic models of petroleum formation part i oil and gas generation from kerogen modern basin analysis uses kinetic models to predict the extent of petroleum generation within potential source rocks the global kinetic model presented here assigns kinetic parameters based on gross depositional environment and stratigraphic age this is useful in areas of low geochemical knowledge including exploration frontiers five kerogen kinetic organofacies each characterized by a specific organic matter input and early diagenetic overprint can broadly be related to sedimentary facies age associations even using seismic sequence stratigraphy alone a aquatic marine siliceous or carbonate evaporite any age b aquatic marine siliciclastic any age c aquatic non marine lacustrine phanerozoic d e terrigenous non marine ever wet coastal mesozoic and younger and f terrigenous non marine coastal late palaeozoic and younger routine pyrolysis data allow partitioning of organic carbon in the immature source rock into four initial components oil oil generative gas generative and inert separate kinetic parameters for the oil and gas generative fractions allow computation of the evolving concentration and composition of the products it was assumed that the activation energy distributions for each fraction were normally distributed non linear regression of large combined laboratory and field data sets varying by up to 12 orders of magnitude in heating rate established optimum values for the required ten 5 2 kinetic parameter sets a emean e differences in kinetic parameters can be reconciled with known chemical properties although our knowledge base is still inadequate to construct kinetic models from first principles mean activation energies governing oil generation increase systematically in the order a f causing a corresponding increase in generation temperature at a reference heating rate 2 c ma 1 the oil generation window 10 90 oil generative kerogen degraded increases from ca 95 135 c to 145 175 c organofacies c has the tightest energy distribution and narrowest oil generation window the gas generation window ranges from ca 105 155 c to 175 220 c thermal stress results from a combination of temperature and time an order of magnitude increase decrease in heating rate elevates depresses reaction temperatures by ca 15 c heating rates in subsiding sedimentary basins can vary by two orders of magnitude dramatic differences in generation temperature can result if the heating rate and organofacies effects compound thus global oil and gas generation temperature thresholds constitute blunt instruments with which to screen the petroleum potential of sedimentary basins because unexpelled oil provides a potential feedstock for oil to gas cracking in the source rock quantitative models of petroleum generation become really useful only when coupled with models of oil gas cracking and expulsion for example the frequently observed association between gas provinces and coals cannot be explained solely by their generation behaviour

automatic glacier ablation measurements using pressure transducers an instrument is described that automatically records ice ablation while eliminating the need for ablation stakes a pressure transducer placed at the bottom of a hole drilled into the ice is connected by a hose to a bladder lying on the surface ice ablation is detected as a reduction in the hydrostatic pressure measured by the transducer

geochronological evidence for reworking of archean terrains during the early proterozoic 2 1 ga in the western coˆte d ivoire man rise west african craton abstract a period of accretion of continental crust is recognized during the archean at ca 3 2 3 3 ga on the basis of wholerock nd model ages and pb ages on detrital zircons from various gneisses of the man area in co te d ivoire there is no significant evidence of any older event such as the 3 5 3 6 ga accretion episode recognized in the archean of the reguibat rise in mauritania the earliest magmatic events recorded in the northern domain of the man area correspond to tonalitic grey gneisses of a minimum age ca 3 05 ga leonian event a subsequent major phase of igneous and metamorphic activity around 2 8 ga liberian event is recorded in the entire archean terrain without any evidence of new accretion this second event began at 2830 7 ma its climax is indicated by charnockitic intrusions followed by granodioritic magmas at ca 2800 10 pb pb and u pb ages on monazite and zircon cooling after the liberian event occurred ca 2 75 ga ago k feldspar whole rock garnet sm nd ages paleoproterozoic reworking is mainly found in the southern part of the archean terrain of the man area zircon monazite sphene and garnet geochronometers all indicate an important thermal effect during the birimian event i e 2 1 ga the earliest influence of this event is recorded in a range of 2250 40 70 ma with a major event ca 2 1 ga partial to total resetting of u pb and sm nd geochronometers indicate that high temperature conditions were reached during the birimian event this metamorphic overprint and the diapiric emplacement of coeval paleoproterozoic juvenile magmas at 2 1 ga in the southern domain toule pleu massif suggest that the 2 1 ga overprint could be related to major underplating processes under the archean basement the thermal flow related to the differentiation and the accretion of these juvenile birimian magmas may account for the reworking of the surrounding archean domain which had been possibly thinned during underplating these results suggest close proximity between the archean continents and an accretion zone associated with juvenile magmatism 2 1 ga ago

rapid thinning of lake calving yakutat glacier and the collapse of the yakutat icefield southeast alaska usa both lake calving yakutat glacier 337 km 2 alaska usa and its parent icefield 810 km 2 are experiencing strong thinning and under current climate conditions will eventually disappear comparison of digital elevation models shows that yakutat glacier thinned at area averaged rates of 4 76 0 06 m w e a 1 2000 07 and 3 66 0 03 m w e a 1 2007 10 simultaneously adjacent yakutat icefield land terminating glaciers thinned at lower but still substantial rates 3 79 and 2 94 m w e a 1 respectively for the same time periods indicating lake calving dynamics helps drive increased mass loss yakutat glacier terminates into harlequin lake and for over a decade sustained a 3 km long floating tongue which started to disintegrate into large tabular icebergs in 2010 such floating tongues are rarely seen on temperate tidewater glaciers we hypothesize that this difference is likely due to the lack of submarine melting in the case of lake calving glaciers floating tongue ice losses were evaluated in terms of overall mass balance and contribution to sea level rise the post little ice age collapse of yakutat icefield was driven in part by tidewater calving retreats of adjacent glaciers the lake calving retreat of yakutat glacier a warming climate and by the positive feedback mechanisms through surface lowering

the mechanisms of petrogenesis of the archaean continental crust comparison with modern processes the petrographic and chemical composition of magmatic rocks generated during the archaean appears to be different from that of post archaean rocks komatiites are widespread before 2 5 ga and rarely occur afterwards in addition the archaean continental crust is primarily ttg tonalitic trondhjemitic and granodioritic in composition exhibiting typical trondhjemitic differentiation trends whereas modern equivalents are granodioritic to granitic following classical calc alkaline differentiation trends this distinction becomes more prominent when rare earth elements ree are taken into account archaean ttg are yb poor ybn 8 5 and have high layb ratios 5 layb n 150 in comparison the post 2 5 ga granitoids emplaced in subduction zone geodynamic environments have high yb content 4 5 ybn 20 with very low layb n ratios 20 theoretical calculations and experimental petrology have shown that the ttg can be produced by partial melting of an archaean tholeiite transformed into garnet bearing amphibolite consequently the low heavy ree content of the ttg is explained by the influence of both residual garnet and hornblende in their source after 2 5 ga the role of these minerals in calc alkaline magma genesis becomes progressively less important which is interpreted in terms of a cooling earth model in modern subduction zone environments the subducted oceanic slab is relatively old and cold and the geothermal gradient along the benioff plane in low ca 10 c km consequently the down going lithosphere undergoes dehydration before partial melting is able to occur the liberated fluids are light ree and lile enriched and ascend into the overlying mantle wedge where they induced partial fusion the produced magmas separate from their mantle source region leaving a residue mainly composed of olivine and pyroxenes mantle derived magmas typically exhibit high yb contents due to low kdyb values for olivine and pyroxenes during the archaean the subducted lithosphere was relatively young and hot providing high geothermal gradients along the benioff zone thus partial melting of the subducted slab was possible at lower temperatures before dehydration would take place garnet and hornblende are the main residual phases accounting for the low yb contents of the archaean ttg this model can be tested using a modern analogue of archaean like subduction processes in south chile an oceanic ridge has subducted and all thermodynamic calculations indicate that this creates locally high geothermal gradients along the benioff zone thus in very small areas archaean like environments may be simulated in modern subduction zones the modern andesites produced in this environment show archaean geochemical characteristics with low ybn 5 whereas the majority of andesites along the andean arc have modern patterns with ybn ranging from 8 to more than 17 this conclusion was generalised to all young subducted lithospheres all over the world in conclusion it appears that since the archaean there has been a change in the site of continental crust genesis the location of calc alkaline magma source in subduction zone environments has migrated through time from the subducted slab to the mantle wedge this is a direct consequence of the progressive cooling of the earth

shallow seismic reflection investigations of coal in the sydney basin surface reflection profiling with the mini sosie technique successfully mapped shallow coal seam structure in the western sydney basin new south wales several minor faults and zones of fracturing were detected in regions of thick triassic sandstone cover data quality was poor and unsuitable for geologic interpretation synthetic seismograms based on nearby borehole and petrophysical control show excellent agreement with the mini sosie sections and illustrate the deleterious filtering effects of coal seams and sequences to establish a phenomenological basis for seismic wave propagation in shallow coal measures two vertical seismic profiles vsps which used small explosive charges were recorded with high spatial and temporal sampling numerous multiple reflections were observed in the downgoing wave display the isolated upgoing waves were migrated to yield blurred images of the main coal seams the subsurface velocity function also deduced from the vsp shows broad correlation with the geologic log

age constraints on gold mineralization and paleoproterozoic crustal evolution in the ashanti belt of southern ghana abstract u pb isotopic dating in the region of the ashanti belt in ghana has been carried out to improve understanding of crustal evolution and gold mineralization in this paleoproterozoic terrane a u pb zircon age of ca 2174 2 ma for a belt type granitoid from sekondi accords with previous ages on belt type plutons in ghana but metamorphic titanite within this granitoid gives an age of 2092 3 ma coeval with intrusion of the late basin type granitoids the within belt granitoid at banso village has an age close to 2097 3 ma which falls into the age range typical for basin type granitoids 2116 2088 ma these results indicate that widespread plutonism in the birimian sedimentary basins also produced magmatic and metamorphic effects within the volcanic belts most dated detrital zircons from birimian metasediment samples at the ashanti gold mine agree in age at 2155 2 ma with others that are slightly older this gives a maximum estimate for the depositional age of the sediments and for gold mineralization at the ashanti gold mine hydrothermally altered and auriferous granitoid plutons along the western flank of the ashanti belt at anyankyerim nhyiaso yaomensakrom and esuajah ayanfuri all have intrusion ages within error of 2105 2 ma this is also the age of metamorphic titanite in an unmineralized basin type granitoid at kwapia rutiles from the hydrothermally altered yaomensakrom and the mineralized esuajah ayanfuri granitoid were dated at 2098 7 and 2086 4 ma respectively and probably provide minimum ages on gold mineralization these data indicate a close temporal association among auriferous granitoid plutons near the western margin of the ashanti belt at 2105 ma gold mineralization may be only slightly younger than magmatic emplacement of the plutons granitic plutonism in the sedimentary basins metamorphism and hydrothermal gold mineralization in southern ghana are all part of a major crust formation event the eburnean orogeny that can be constrained to the time range 2120 2080 ma

glaciological characteristics of the ablation zone of baltoro glacier karakoram pakistan baltoro glacier in the karakoram pakistan is one of the world s largest valley glaciers it drains an area of about 1500 km 2 and is 60km long in 2004 an italian german expedition carried out a glaciological field program on the ablation zone of the glacier focusing on the ablation conditions and the dynamic state of the glacier as baltoro glacier is a debris covered glacier ice ablation also depends on the debris properties stake measurements of ice ablation and debris cover in combination with meteorological data from automatic weather stations close by have been used to determine the local melt conditions results from these calculations have been combined with an analysis of different classes of surface cover and information about precipitation using remote sensing techniques in order to calculate mass fluxes for the upper part of baltoro glacier the dynamic state of the glacier has been investigated by gps based surface velocity measurements along the stake network a comparison of these short term measurements during the melt season with surface velocities computed from feature tracking of satellite images shows a high seasonal variability of the ice motion we have shown that this variability is up to 100 of the annual mean velocity on the basis of these investigations the mass fluxes at the concordia cross section have been quantified this approach can now be used together with the ablation calculations to reconstruct the variability of glacier extent and volume in the past using available climate data from the central karakoram from the comparison of historical measurements and photographs it is shown that the snout of baltoro glacier is oscillating back and forth a couple of hundred metres otherwise it seems not to react with the same magnitude as other glaciers to the climatic change elevation changes at concordia are a few tens of metres at the most

upper carboniferous and lower permian tectonostratigraphy on the southern margin of the central north sea hydrocarbon exploration in uk licence blocks 31 26 and 39 2 on the southern margin of the central north sea in 1996 and 1997 proved the presence of a palaeozoic sedimentary succession previously undescribed in the area biostratigraphic data from the sediments and radiometric dating of interbedded basaltic lavas indicate that the succession spans the upper carboniferous westphalian b to lower permian asselian interval this has been informally split into three sedimentary units the lower flora sandstone is of westphalian b c age this clastic interval is overlain by the first of two main lavas which is in turn overlain by the westphalian c d to stephanian upper flora sandstone the flora sandstones are interpreted to be lithostratigraphic equivalents to the schooner formation of the southern north sea the flora sandstones are separated from the grensen formation by an angular unconformity possibly related to variscan asturic inversion the grensen formation is in turn overlain by a second basaltic lava flow both basaltic units are assigned to the inge volcanics formation rotliegend group petrographic data indicate that the grensen formation also forms part of the rotliegend group a major extensional event of permian age post dates all of these strata extensional faults are planed by a regional unconformity saalian unconformity that is overstepped by permian to cretaceous strata

fluvial channel and overbank deposits from the westphalian of the durham coalfield ne england the westphalian upper carboniferous coal measures of the durham coalfield in ne england were deposited in lower and upper delta plain environments distributary channels crossed the plain and were separated by shallow interdistributary lakes and bays detailed observation of three dimensional 3 d opencast surface mine exposures in collaboration with subsurface borehole analysis has revealed the existence of five varieties of channel deposits and two associated overbank facies within the durham coal measures major distributary channels were the major avenues of sediment transport across the coal measures plain were variably sinuous mostly 1 2 km wide and deposited elongate belts of sand mostly up to 5 km wide proximal major crevasse splay channels formed by the breaching of major channel banks during flood events were straight sand filled and up to 400 m wide minor distributary channels formed by the sustained operation of such crevasses varied from straight to highly sinuous and deposited shoestring sand mud belts up to a few hundred metres wide minor crevasse channels generally straight and up to 50 m wide were formed through bank breaching of minor and in a few cases major distributaries distal feeder channels formed down palaeocurrent extensions of minor distributaries which supplied interdistributary minor delta subsystems were generally straight and up to 200 m wide of the two types of channel overbank levee deposits recognized one comprising thinly interbedded fine grained sandstone and siltstone claystone is mostly though not exclusively associated with major distributary channels the other consisting of massive siltstones with regularly spaced thin claystone bands is uniquely developed at the margins of minor distributary channels the lower part of the westphalian a succession in the northern pennines records a change in the depositional environment upwards from a lower to upper delta plain through this transition major channel deposits show evidence of having evolved from being of dominantly low sinuosity to being more variable in morphology channel sedimentation was profoundly influenced by regular possibly seasonal variations in flow stage and sediment load

aspects of seismic character and extent of upper carboniferous coal measures gulf of st lawrence and sydney basins abstract a seismic facies related to upper carboniferous coal measures has been mapped in the gulf of st lawrence sydney basin region of atlantic canada the inferred original extent of this area of coal deposition is over 100 000 km 2 which is two to three orders of magnitude larger than the fault bounded terrestrial coal basins on the surrounding landmass the depositional continuity of this seismic sequence is on the scale of cyclothem deposition in the illinois basin and mid continent basin region of the united states and suggests an analogous stable cratonic shelf setting for the gulf of st lawrence sydney basin region in paleogeographic reconstructions for the late carboniferous cyclothems in a paleoequatorial setting in the mid continent basin of euramerica have been correlated with cyclothems in a similar setting on the russian platform the gulf of st lawrence and sydney basins lie midway between these two regions it will be an interesting test of these correlations to determine the exact age and environment of the coal measures seismic facies

calculation of sensible heat flux over a melting ice surface using simple climate data and daily measurements of ablation sensible heat flux is obviously important for glacier ablation but is difficult to measure routinely sensible heat flux can be estimated from wind speed and temperature data using a dimensionless heat transfer coefficient values of the heat transfer coefficient are evaluated for six sites by correlating measured melt energy with a wind temperature variable product of daily mean wind speed temperature and mean atmospheric pressure for the altitude in question data are available for short periods from two sites in arctic canada and two sites in north greenland and for hundreds of days of record at nordbogletscher and qamanarssup sermia in south and west greenland respectively average transfer coefficients for four out of the six sites are close to 0 003 which is in reasonable agreement with values reported elsewhere while larger values of 0 0047 and 0 0057 are found at the other two sites heat transfer coefficients are also estimated on a monthly basis for the two long records and substantial variations are found suggesting that the method should not be used for 20 30 days of data the present study is based on manually observed ablation and climate data but the approach could be updated to use data from automatic recording stations using modern sensors

can thin coals be resolved in seismic data an example of the challenges for carboniferous visean to westphalian source rock definition across the elbow spit platform netherlands and the mid north sea high uk through well synthetic modelling

late eburnean granitization and tectonics along the western and northwestern margin of the archean kenema man domain guinea west african craton abstract recent mapping of the eastern part of guinea has revealed a vast plutonic belt that rims the margin of the archean kenema man craton and that is made up of a variety of granitic rocks granodiorite biotite granite monzogranite two mica granite granodiorite with common clinopyroxene being the most abundant single zircon dating by stepwise pb evaporation carried out on characteristic rock types granodiorite monzogranite two mica granite reveals their age to be late eburnean c 2090 2070 ma and indicates that their emplacement occurred within a relatively short time interval 20 ma the granodiorite biotite granite and monzogranite of the plutonic belt all present a calc alkaline and highly potassic nature and show numerous analogies with the neogene calc alkaline rocks of the central andes there is evidence to suggest strong contamination from the archean crust these analogies make it possible to envisage the emplacement of the plutonic belt directly above a subduction zone the occurrence of peraluminous two mica granite at the northern extension of the plutonic belt in the siguiri basin may suggest that the convergence here was associated with the local melting of metasedimentary rocks in the deep part of the basin contemporaneously with its emplacement the late eburnean plutonic belt accommodated regional convergence by major wnw ese sinistral movements along the northwestern margin of the archean block horizontal shortening towards the wsw is envisaged at the scale of eastern guinea in relation with these kinematics the late eburnean plutonic belt of eastern guinea is a key element of the archean proterozoic transition zone in the southern part of west africa evidence along the western and northwestern border of the archean kenema man domain leads us to interpret this transition zone as an active margin along which strike slip tectonism accommodated regional convergence towards the end of the eburnean cycle