the stabilization of self organised leucogranite networks implications for melt segregation and far field melt transfer in the continental crust abstract net structured leucocratic vein systems are ubiquitous in high grade and migmatite terrains and are interpreted to represent interconnected former magma bearing structures that accommodated the migration of granitic magmas out of their anatectic source regions net structured leucogranite vein systems in high grade gneisses from the damara belt in namibia describe a size based hierarchy and spatial distribution that indicates self organisation of the magma transfer system the internal geometry and connectivity of the net structured leucogranites are controlled by variations in melt pressure whereas regional strains control the orientation of the leucogranites foliation parallel stromatic and shear band hosted granite veins accommodate transport at relatively low melt volumes and account for the metre scale segregation of melt close to the anatectic source during times of higher melt supply the formation of extensional fractures that cross cut stromatic and shear band hosted melt sheets at high angles is promoted extensional fracturing and associated dilatancy create gradients in magmastatic head that cause melt to be drained from pervasive stromatic and shear band hosted vein networks into spaced dilatant fractures this has the potential to initiate buoyancy driven far field melt transfer along suitably orientated extensional fractures as vertical dykes or self propagating hydrofractures our results suggest that melt transfer systems are able to adjust to variations in melt supply by forming conduits that accommodate varying melt volumes and will tend to develop towards a state of self organisation

bed topography of jakobshavn isbrae greenland and byrd glacier antarctica this paper presents the bed topography of jakobshavn isbrae greenland and byrd glacier antarctica derived from sounding these glaciers with high sensitivity radars to understand the processes causing the speed up and retreat of outlet glaciers and to enable the development of next generation ice sheet models we need information on bed topography and basal conditions to this end we performed measurements with the progressively improved multichannel coherent radar depth sounder imager mcords i we processed the data from each antenna array element using synthetic aperture radar algorithms to improve radar sensitivity and reduce along track surface clutter we then applied array and image processing algorithms to extract the weak bed echoes buried in off vertical scatter cross track surface clutter at jakobshavn isbrae we observed 2 7 km thick ice 30 km upstream of the calving front and 850 m thick ice at the calving front we also observed echoes from multiple interfaces near the bed we applied the music algorithm to the data to derive the direction of arrival of the signals this analysis revealed that clutter is dominated by the ice surface at jakobshavn isbrae at byrd glacier we found 3 62 km thick ice as well as a subglacial trench 3 05 km below sea level we used ice thickness information derived from radar data in conjunction with surface elevation data to generate bed maps for these two critical glaciers the performance of current radars must be improved further by 15 db to fully sound the deepest part of byrd glacier unmanned aerial systems equipped with radars that can be flown over lines spaced as close as 5 m apart in the cross track direction to synthesize a two dimensional aperture would be ideal for collecting fine resolution data over glaciers like jakobshavn near their grounding lines

borehole imagery of meteoric and marine ice layers in the amery ice shelf east antarctica a real time video camera probe was deployed in a hot water drilled borehole through the amery ice shelf east antarctica where a total ice thickness of 480 m included at least 200 m of basal marine ice down looking and side looking digital video footage showed a striking transition from white bubbly meteoric ice above to dark marine ice below but the transition was neither microscopically sharp nor flat indicating the uneven nature at centimetre scale of the ice shelf base upstream where the marine ice first started to accrete marine ice features were imaged including platelet structures cell inclusions entrained particles and the interface with sea water at the base the cells are assumed to be entrained sea water and were present throughout the lower 100 150m of the marine ice column becoming larger and more prevalent as the lower surface was approached until near the base they became channels large enough that the camera field of view could not contain them platelets in the marine ice at depth appeared to be as large as 1 2 cm in diameter particles were visible in the borehole meltwater probably marine and mineral particles liberated by the drill but their distribution varied with depth

modeling the formation and deposition of frazil ice beneath filchner ronne ice shelf large areas of the filchner ronne ice shelf are underlain by layers of marine ice which form when supercooled seawater circulating beneath the ice shelf freezes the freezing process initially produces a suspension of disc shaped frazil ice crystals and these are subs quently deposited onto the ice shelf base in areas where the flow of water is slack enough this has been modeled assuming that the freezing takes place within buoyant plumes of ice shelf water ascending the ice shelf base from source regions near the grounding lines of the major inlet glaciers the deposition of the majority of the suspended frazil ice is found to occur in spatially discrete bursts where peak rates of accumulation at the ice shelf base exceed 1 m yr 1 of solid ice there is a good correlation between the location of the zones of crystal deposition and the position of the upstream limits of the marine ice layers the high rates of localized accumulation account for the rapid buildup observed in the layer thickness which then gradually declines as the marine ice is carried downstream with the flow of the ice shelf model results also suggest an origin for the ice platelets observed at depth in the water column near the filchner ice shelf

pre 1 87 ga development of crustal domains overprinted by 1 87 ga transpression in the palaeoproterozoic skellefte district sweden the complex structural evolution within the vms hosting skellefte district sweden has been investigated to provide a solid structural framework for the known mineral deposits in the area the are

microfabrics indicating granulite facies metamorphism in the low pressure central damara orogen namibia abstract the reconstruction of the fluid evolution during progressive metamorphism in the central damara orogen by hoernes and hoffer indicated that fluid present conditions were only periodically attained among these rocks and that the highgrade metamorphism along the west coast of namibia occurred under essentially water absent conditions the characteristic mineral assemblage in the metapelites is plagioclase quartz alkali feldspar biotite cordierite garnet sillimanite apatite zircon ore orthopyroxene bearing parageneses in the metapelites are missing and granulite facies metamorphism may not be detected on the basis of mineral assemblages a detailed study along a 100 km traverse across the migmatite area in the central damara orogen namibia reveals several microfabric features which indicate in analogy to many other well known granulite facies terrains where similar microfabrics have been described that the pan african regional metamorphism in this area culminated at conditions of low pressure granulite facies the observed features are 1 a the activation of basal a and prism c glide systems in quartz under conditions of plastic deformation and the development of equivalent subgrain boundaries parallel prism faces and parallel basal planes 2 b the orientated exsolution of rutile needles in quartz 3 c the exsolution of string perthites mesoperthitic string perthites and antiperthites in feldspars 4 d in addition the homogeneous distribution of major elements in garnet crystals and garnet biotite thermometry suggest granulite facies conditions

active deformation of the continents whereas relatively narrow bands of deformation define oceanic plate boundaries in continental situations deformation is shown to be distributed over much broader zones the complex and heterogeneous nature of boundary regions between major plates is outlined with reference to the mediterranean and the himalayas relatively aseismic regions have traditionally been defined as microplates but although they are presently acting as rigid blocks undergoing slow deformation it is argued that this need not always have been the case and that deformation may simply have moved elsewhere broad zones of deformation surrounding rigid blocks are highlighted as fundamental to the analysis and interpretation of continental tectonics detailed theoretical consideration is given to the kinematics of distributed formation and the dynamics of continental deformation a j barker

in situ stress determination from hydraulic injection test data laboratory and field results have shown that the orientation of a hydraulically propagated fracture is not always perpendicular to that of the minimum principal stress but may be influenced by local weakness planes such as preexisting natural fissures the lower the flow rate the stronger the weakness planes influence this observation has led to a new stress determination method based on the measurement of the normal stress supported by fracture planes with various dips and strikes this new method provides means to evaluate the precision of the stress determination and can be applied to situations in which none of the principal stresses is vertical when all the fracture planes are parallel to the borehole axis the method is still applicable if fast flow rate reopening pressures can be measured and if the principal stress directions are assumed to be independent of depth validity of the method is illustrated by analyzing results from two sets of data one of which has been discussed previously in the literature

the jura mountains an active foreland fold and thrust belt abstract recently completed in situ stress measurements using the borehole slotter at 33 new test sites within the swiss and french jura mountains combined with previously published stress data allow a detailed description of the contemporary state of stress in this fold and thrust belt and the adjacent foreland five stress provinces can be recognized with two different general orientations of maximum horizontal stress sh 1 the central and southwest provinces with a nnw sse to nne ssw orientation 2 the east northwest and southeast provinces with an e w to nw se orientation stress magnitudes are in general higher in the southern than in the northern jura mountains characteristic of the northern jura is an e w trending zone of low stress magnitudes which can be traced from the bresse depression south of besancon to the eastern end of the jura northwest of zurich boundaries of stress provinces show poor agreement with boundaries of tectonic units in the jura in addition stress provinces extend beyond the boundaries of the jura mountains into their foreland furthermore palaeo stress orientations show large discrepancies with respect to the contemporary stress orientation these findings are considered to indicate a termination of jura thin skinned foreland tectonics which probably took place some time between 9 and 4 ma according to palaeontological and tectonic evidence it is suggested that a new style of tectonics has commenced in the jura mountains deforming both the basement directly below the jura fold and thrust belt and the cover rocks in a similar mode

the okahandja lineament and its significance for damaran tectonics in namibia the okahandja lineament is considered to represent the southern margin of a magmatic arc produced by the northerly subduction of the damran ocean between 750 and 520 ma ago sediments of the orogen and granitoid intrusives produced by subduction and crustal melting underwent shear deformation between 675 and 575 ma in a low angle zone of sinistral sense shear

properties of a marine ice layer under the amery ice shelf east antarctica the amery ice shelf east antarctica undergoes high basal melt rates near the southern limit of its grounding line where 80 of the ice melts within 240 km of becoming afloat a considerable portion of this later refreezes downstream as marine ice this produces a marine ice layer up to 200 m thick in the northwest sector of the ice shelf concentrated in a pair of longitudinal bands that extend some 200 km all the way to the calving front we drilled through the eastern marine ice band at two locations 70 km apart on the same flowline we determine an average accretion rate of marine ice of 1 1 0 2 m a 1 at a reference density of 920 kg m 3 between borehole sites and infer a similar average rate of 1 3 0 2 m a 1 upstream the deeper marine ice was permeable enough that a hydraulic connection was made whilst the drill was still 70 100 m above the ice shelf base below this marine close off depth borehole video imagery showed permeable ice with water filled cavities and individual ice platelets fused together while the upper marine ice was impermeable with small brine cell inclusions we infer that the uppermost portion of the permeable ice becomes impermeable with the passage of time and as more marine ice is accreted on the base of the shelf we estimate an average closure rate of 0 3 m a 1 between the borehole sites upstream the average closure rate is faster at 0 9 m a 1 we estimate an average porosity of the total marine ice layer of 14 20 such that the deeper ice must have even higher values high permeability implies that sea water can move relatively freely through the material and we propose that where such marine ice exists this renders deep parts of the ice shelf particularly vulnerable to changes in ocean properties

basal mass budget of ross and filchner ronne ice shelves antarctica derived from lagrangian analysis of icesat altimetry traditional methods of deriving temporal variability of antarctic ice shelf elevation from satellite altimetry use a fixed eulerian reference frame where the measured changes include advection of ice thickness gradients between measurement epochs we present a new method which removes advection effects by using an independent velocity field to compare elevations in a moving lagrangian reference frame applying the technique to icesat laser altimetry for the period 2003 2009 over the two largest antarctic ice shelves ross and filchner ronne we show that the lagrangian approach reduces the variability of derived elevation changes by about 50 compared to the eulerian approach and reveals clearer spatial patterns of elevation change the method simplifies the process of estimating basal mass budget from the residual of all other processes that contribute to ice shelf elevation changes we use field data and icesat measurements over ice rises and the grounded ice sheet to account for surface accumulation and changes in firn air content and remove the effect of ice flow divergence using surface velocity and ice thickness data the results show highest basal melt rates 5 m a 1 near the deep grounding lines of major ice streams but smaller melt rates 5 m a 1 near the ice shelf fronts are equally important to total meltwater production since they occur over larger areas integrating over the entire ice shelf areas we obtain basal mass budgets of 50 64 gt a 1 for ross and 124 66 gt a 1 for filchner ronne with changes in firn air content as the largest error source

basal crevasses and associated surface crevassing on the larsen c ice shelf antarctica and their role in ice shelf instability we identify a series of basal crevasses along a 31 km transect across the northern sector of the larsen c ice shelf antarctica using in situ ground penetrating radar the basal crevasses propagate from a region of multiple shallow basal fractures to form widely spaced 0 5 2 0 km but deeply incised 70 134 m features surface troughs observed in visible imagery exist above the basal crevasses as the ice vertically shears to reach hydrostatic equilibrium while widespread surface crevassing occurs along the crests and on the flanks of the undulations primarily aligned with the topography we suggest based on the location of the surface crevasses and the along flow evolution of the basal crevasses that the former are induced by a bending stress created by gradients in hydrostatic forces using a linear elastic fracture mechanics model we investigate the sensitivity of basal crevasse propagation to observed trends of ice shelf thinning and acceleration basal crevasses are large scale structural weaknesses that can both control meltwater ponding and induce surface crevassing together these features may represent an important mechanism in both past and future ice shelf disintegration events on the antarctic peninsula

geochronology and petrogenesis of pan african syn tectonic s type and post tectonic a type granite namibia products of melting of crustal sources fractional crystallization and wall rock entrainment abstract the oetmoed granite migmatite complex ogmc central damara orogen namibia consists mainly of 526 to 516 ma garnet and cordierite bearing granite and subordinate 488 to 494 ma hornblende and titanite bearing granite in the form of planar sheets and dykes additionally a slightly elongated granite body occurs in the center of the complex the garnet and cordierite bearing granite has major and trace element characteristics of s type granite but the hornblende and titanite bearing granite has higher hfse and ree contents similar to a type granite whereas the garnet and cordierite bearing granite contains numerous restitic xenoliths the hornblende and titanite bearing granite is xenolith free the country rocks are cordierite sillimanite k feldspar garnet bearing metasedimentary rocks and migmatite cordierite and garnet rich xenoliths in the s type granite do not represent primary restite their depleted chemical composition is best explained by varying and large degrees of partial melting of incorporated country rocks most chemical variations among the garnet and cordierite bearing granite can be explained by processes linked with fractional crystallization of plagioclase biotite and accessory phases mostly monazite and zircon major and trace element data and high 18 o values suggest that the least evolved members of the garnet and cordierite bearing granite were derived from metapelitic rocks at ca 800 c as inferred from monazite and apatite dissolution thermometry higher cao and na2o but lower sio2 contents and lower rb sr ratios as well as lower 18 o values of the hornblende and titanite bearing granite suggest that they are more likely generated by partial melting of non pelitic sources metagranitoids at temperatures in excess of 900 c decreasing tio2 na2o feotot mgo cao total ree content but increasing al2o3 and k2o indicate fractionation of mainly hornblende and titanite in the case of the hornblende and titanite bearing granite the differing compositions of the garnet and cordierite bearing granite and the hornblende and titanite bearing granite are attributed to different source rocks metapelite instead of metagranitoid and different temperatures during melting as inferred from accessory phase dissolution thermometry furthermore significant entrainment of country rock in some samples played a major role during petrogenesis of the garnet and cordierite bearing granite but was not important during the evolution of the hornblende and titanite bearing granite intrusion of such hot felsic magmas close to the inferred peak of metamorphism has probably caused in part the high temperature metamorphism and anatexis of the country rocks at relatively low pressures

magma accumulation and segregation during regional scale folding the holland s dome granite injection complex damara belt namibia abstract the regional scale upright fold of the holland s dome in the damara belt of central namibia contains a kilometre scale network of intrusive highly fractionated uraniferous leucogranites three broadly orthogonal and intersecting sets of leucogranite sheets that intruded parallel and at right angles to the axial plane of the first order fold can be distinguished the granites are internally sheeted and illustrate the growth of the injection complex through the successive addition of thousands of smaller magma batches spatial and timing relationships point to a stepwise evolution of the injection complex early dilatancy driven segregation and accumulation of granitic magmas in the core of the fold above a basal detachment was followed by compaction driven segregation of a melt phase during fold tightening the intersecting leucogranite sets provide a suitably organized permeability structure for melt segregation while the successive injection of magma batches ensures compatibility between regional strain rates during folding and the rates of magma segregation the three dimensional network of melt bearing structures further assisted regional shortening past the lock up of the fold the holland s dome injection complex illustrates the geometric complexity of magma transfer pathways and the significance of regional scale folding for the accumulation segregation and fractionation of granitic magmas in suprasolidus crust

a wideband radar for high resolution mapping of near surface internal layers in glacial ice snow accumulation rate is an important parameter in determining the mass balance of polar ice sheets accumulation rate is currently determined by analyzing ice cores and snow pits inadequate sampling of the spatial variations in the ice sheet accumulation has resulted in accumulation rate uncertainties as large as 24 we designed and developed a 600 900 mhz airborne radar system for high resolution mapping of the near surface internal layers for estimating the accumulation rate of polar ice sheets our radar system can provide improved spatial and temporal coverage by mapping a continuous profile of the isochronous layers in the ice sheet during the 2002 field season in greenland we successfully mapped the near surface layers to a depth of 200 m in the dry snow zone 120 m in the percolation zone and 20 m in the melt zone we determined the water equivalent accumulation rate at the nasa u spl i bar 1 site to be 34 9 spl plusmn 5 1 cm year from 1964 to 1992 this is in close agreement with the ice core derived accumulation rate of 34 6 cm year for the same period

melt segregation and far field melt transfer in the mid crust abstract migmatites in the damara belt in namibia preserve relationships between anatectic leucosomes and intrusive leucogranite sheets illustrating the consecutive stages of partial melting local melt segregation and far field mobilization of melts over hundreds of metres and out of the anatectic region initial melting and localized segregation of melts are controlled by gradients in fluid pressure created by pervasively developed shallowly dipping dilatant fractures subsequent melt transfer out of these initial sites of melting and melt storage occurs along subvertical disc shaped leucogranite sheets that intersect the leucosomes the leucogranite sheets propagate as isolated melt filled hydrofractures driven by the pressure differential along the subvertical fractures the collapse of wall rocks into former melt bearing fractures and the presence of residual peritectic phases of the melting reaction trapped in wall rocks testify to the efficient extraction of melt by the mobile hydrofractures during their ascent this process of melt drainage into and transport by hydrofractures leaves almost no trace of the ascent conduits this study also shows that melt transport and the stability of melt pathways in strongly layered mid crustal levels characterized by low deviatoric stresses are determined by the presence of pre existing anisotropies and the progressively evolving structure in deforming orogens

a damara orogen perspective on the assembly of southwestern gondwana the pan african damara orogenic system records gondwana amalgamation involving serial suturing of the congo sao francisco and ro o de la plata cratons north gondwana from 580 to 550 ma before amalgamation with the kalahari antarctic cratons south gondwana as part of the 530 ma kuunga damara orogeny closure of the adamastor ocean was diachronous from the aracuao belt southwards with peak sinistral transpressional deformation followed by craton overthrusting and foreland basin development at 580 550 ma in the kaoko belt and at 545 530 ma in the gariep belt peak deformation metamorphism in the damara belt was at 530 500 ma with thrusting onto the kalahari craton from 495 ma through to 480 ma coupling of the congo and ro o de la plata cratons occurred before final closure of the mozambique and khomas damara belt oceans with the consequence that the kuunga suture extends into africa as the damara belt and the lufilian arc and zambezi belt of zambia palaeomagnetic data indicate that the gondwana cratonic components were in close proximity by c 550 ma so the last stages of the damara kuunga orogeny were intracratonic and led to eventual out stepping of deformation metamorphism to the ross delamerian orogen c 520 500 ma along the leading edge of the gondwana supercontinental margin

increased flow speed on a large east antarctic outlet glacier caused by subglacial floods large ice streams and outlet glaciers drain greenland and antarctica an observed acceleration of ice velocity in one of these outlet glaciers byrd glacier east antarctica coincides with a large water discharge from two subglacial lakes allowing direct attribution of the change in glacier dynamics to the water drainage network beneath the ice

marine ice formation in a suture zone on the larsen c ice shelf and its influence on ice shelf dynamics 1 antarctic ice shelves are fed primarily by the glaciers flowing into them downstream of promontories separating these glaciers supercooled water can rise and freeze into suture zones leading to the accretion of marine ice marine ice bodies have been found in several antarctic ice shelves but little is known about their detailed geometry rate of accretion or influence on ice dynamics in this study we investigate marine ice in a suture zone downstream of the joerg peninsula in the southern part of the larsen c ice shelf antarctic peninsula we present ground penetrating radar data from which we infer the base of the meteoric ice and in combination with gps data and assuming hydrostatic equilibrium estimate marine ice thickness within a suture zone we show that the joerg peninsula suture zone contains marine ice layer which is increasing in thickness along flow from 140 m to 180 m over 20 km implying an average basal accretion rate of 0 5 m a 1 in our study area we examined the impact of this inferred marine ice on ice shelf dynamics by modeling the suture zone within an ice flow model the results which replicate observed surface velocities and strain rates show that the warmer and thus softer ice of the suture zone serves to channel shear deformation this enables decoupling of neighboring flow units with different flow velocities while maintaining the structural integrity of the ice shelf

kinematics topography shortening and extrusion in the india eurasia collision we examine the problem of partitioning between shortening and extrusion in the india asia collision since 45 ma we compute the amount of shortening expected from the kinematics of india s motion with respect to eurasia using the reconstruction at collision time to put bounds on the possible amounts of surface loss within greater india and within eurasia we then compute the amounts of surface loss corresponding to the thickened crust of tibet and of the himalayas assuming conservation of continental crust the spatial distribution of the topography reveals a large systematic deficit of crustal thickening distributed rather uniformly west of the eastern syntaxis but an excess of shortening east of it this distribution indicates an important eastward crustal mass transfer however the excess mass east of the eastern syntaxis does not account for more than one third to one half of the deficit west of the eastern syntaxis the deficit may be accounted either by loss of lower crust into the mantle for example through massive eclogitization or by lateral extrusion of nonthickened crust a mass budget of the crust of the himalayas indicates that lower crust has not been conserved there but the deficit is so large that local loss in the mantle is unlikely to be the unique cause of the deficit alternatively following zhao and morgan 1985 lower crust may have been transferred below the tibetan crust we conclude that a combination of possible transfer of lower crust to the mantle by eclogitization and lateral extrusion has to account for a minimum of one third and a maximum of one half of the total amount of shortening between india and asia since 45 ma this conclusion leaves open the possibility that the partitioning between extrusion and loss of lower crust into the mantle on the one hand and shortening on the other hand has significantly changed during the 45 m y history of the collision

lithospheric structure of an archean craton and adjacent mobile belt revealed from 2 d and 3 d inversion of magnetotelluric data example from southern congo craton in northern namibia 1 archean cratons and the stitching proterozoic orogenic belts on their flanks form an integral part of the southern africa tectonic landscape of these virtually nothing is known of the position and thickness of the southern boundary of the composite congo craton and the neoproterozoic pan african orogenic belt due to thick sedimentary cover we present the first lithospheric scale geophysical study of that cryptic boundary and define its geometry at depth our results are derived from two dimensional 2 d and three dimensional 3 d inversion of magnetotelluric data acquired along four semiparallel profiles crossing the kalahari craton across the damara ghanzi chobe belts dgc and extending into the congo craton two dimensional and three dimensional electrical resistivity models show significant lateral variation in the crust and upper mantle across strike from the younger dgc orogen to the older adjacent cratons we find damara belt lithosphere to be more conductive and significantly thinner than that of the adjacent congo craton the congo craton is characterized by very thick to depths of 250 km and resistive i e cold lithosphere resistive upper crustal features are interpreted as caused by igneous intrusions emplaced during pan african magmatism graphite bearing calcite marbles and sulfides are widespread in the damara belt and account for the high crustal conductivity in the central zone the resistivity models provide new constraints on the southern extent of the greater congo craton and suggest that the current boundary drawn on geological maps needs revision and that the craton should be extended further south

trace element and isotopic sr nd pb o arguments for a mid crustal origin of pan african garnet bearing s type granites from the damara orogen namibia geochronological data major and trace element abundances nd and sr isotope ratios 18o whole rock values and pb isotope ratios from leached feldspars are presented for garnet bearing granites locality at oetmoed and outcrop 10 km north of omaruru from the damara belt namibia for the granites from outcrop 10 km n omaruru reversely discordant u pb monazite data give 207pb 235u ages of 511 2 ma and 517 2 ma similar to previously published estimates for the time of regional high grade metamorphism in the central zone based on textural and compositional variations garnets from these granites are inferred to be refractory residues from partial melting in the deep crust because p t estimates from these xenocrystic garnets are significantly higher 800 c 9 10 kbar than regional estimates 700 c 5 kbar the monazite ages are interpreted to date the peak of regional metamorphism in the source of the granites sm nd garnet whole rock ages are between 500 and 490 ma indicating the age of extraction of the granites from their deep crustal sources for the granites from oetmoed both sm nd and pb pb ages obtained on igneous garnets range from 500 to 490 ma these ages are interpreted as emplacement ages and are significantly younger than the previously proposed age of 520 ma for these granites based on rb sr whole rock age determinations major and trace element compositions indicate that the granites are moderately to strongly peraluminous s type granites high initial 87sr 86sr ratios 0 716 high 18o values of 13 8 negative initial end values between 4 and 7 and evolved pb isotope ratios indicate formation of the granites by anatexis of mid crustal rocks similar to the exposed metapelites into which they intruded the large range of pb isotope ratios and the lack of correlation between pb isotope ratios and nd and sr isotope ratios indicate heterogeneity of the involved crustal rocks evidence for the involvement of isotopically highly evolved lower crust is scarce and the influence of a depleted mantle component is unlikely the crustal heating events that produced these granites might have been caused by crustal thickening and thrusting of crustal sheets enriched in heat producing elements very limited fluxing of volatiles from underthrust low to medium grade metasedimentary rocks may have also been a factor in promoting partial melting furthermore delamination of the lithospheric mantle and uprise of hot mantle could have caused localized high t regions the presence of coeval a type granites at oetmoed that have been derived at least in part from a mantle source supports this model

role of faults and layer interfaces on the spatial variation of stress regimes in basins inferences from numerical modelling abstract the spatial variation of the computed state of stress is studied in a series of numerically solved boundary value problems of basin scale deformation these numerical experiments illustrate simple situations of decollement tectonism under compression the study focuses on the local variation of the state of stress in a sedimentary overburden where major discontinuities such as large scale faults and layer interfaces are present the states of stress are computed using either the distinct element method or the finite element method classic elasto plasticity laws are used for the behaviour of the continuum and the discontinuities are modelled as coulomb contact surfaces a geologic interpretation of the numerical experiments is proposed to highlight the role played by major structural discontinuities in the local variations of the stress regime the models are two dimensional in space and assume plane strain but the full three dimensional stress strain constitutive relation is used therefore the stress regime variations can be represented by constructing the contour map of the wallace bott stress ratio this simple technique together with the construction of diagrams of principal stress magnitude across the model enables us to rapidly examine the stress field pattern and further discuss the deformation modes of faulting and fracturing which may take place at a small scale we have found that the local variations of the stress tensor are in good agreement with common geologic interpretation of brittle deformations in foreland and thrust systems we demonstrate how the presence of layer interface and the difference in mechanical properties of formation lithology may strongly affect the variation of stress in space this modelling approach has important issues when interpreting well tests to gain access to in situ stresses

origin of mineralizing fluids of the sediment hosted navachab gold mine namibia constraints from stable o h c s isotopes the navachab gold mine in the damara belt of central namibia is characterized by a polymetallic au bi as cu ag ore assemblage including pyrrhotite chalcopyrite sphalerite arsenopyrite bismuth gold bismuthinite and bismuth tellurides gold is hosted by quartz sulfide veins and semimassive sulfide lenses that are developed in a near vertical sequence of shelf type metasedimentary rocks including marble calcsilicate rock and biotite schist the sequence has been intruded by abundant syntectonic lamprophyre aplite and pegmatite dikes documenting widespread igneous activity coeval with mineralization the majority of quartz from the veins has 18o values of 14 to 15 per mil v smow the total variations in 18o values of the biotite schist and calcsilicate rock are relatively small 12 14 whereas the marble records steep gradients in 18o values 17 21 the lowest values being recorded at the vein margins despite this there is no correlation between 18o and 13c values and the carbonate content of the rocks indicating that fluid rock interaction alone cannot explain the isotopic gradients in addition the marble records increased 13c values at the contact to the veins possibly related to a change in the physicochemical conditions during fluid rock interaction gold is interpreted to have precipitated in equilibrium with metamorphic fluid 18o 12 14 d 40 to 60 at peak metamorphic conditions of ca 550 c and 2 kbars consistent with isotopic fractionations between coexisting calcite garnet and clinopyroxene in the alteration halos the most likely source of the mineralizing fluid was a midcrustal fluid in equilibrium with the damaran metapelites that underwent prograde metamorphism at amphibolite to granulite facies grades although there is no isotopic evidence for the contribution of magmatic fluids they may have been important in contributing to the overall hydraulic regime and high apparent geothermal gradients ca 80 c km 1 in the mine area

origin of rare element mineralized damara belt pegmatites a geochemical and light stable isotope study abstract neoproterozoic li cs ta lct pegmatites and pegmatitic sheeted leucogranites slgs of the damara belt namibia although coeval are variably mineralized and have intruded multiple tectono stratigraphic units major and trace element data together with oxygen and hydrogen isotopes were used to identify possible sources for these pegmatite melts two groups of pegmatites can be identified on the basis of o isotopes group a pegmatites which include u enriched pegmatitic slgs with a nb y f nyf pegmatite affinity kinnaird and nex 2007 and li be rich lct pegmatites with 18o 11 2 12 9 are metaluminous to peraluminous suggesting that they evolved from a partial melt of a mixed metasedimentary and igneous source group b pegmatites which contain sn li sn and tourmaline mineralization have higher 18o values 14 16 consistent with their derivation from a purely s type source with possible incorporation of or exchange with carbonates crystallization temperatures calculated using ti in quartz and o isotope thermometry are in agreement with each other and range from 420 to 575 c for lct pegmatites and 650 c for pegmatitic slgs the juxtaposition of coeval post tectonic lct pegmatites and pegmatitic slgs with an nyf affinity in the damara belt precludes tectonic setting as the only source of compositional variability in lct and nyf pegmatite melts

analytical and numerical solutions for alternative overpressuring processes application to the callovo oxfordian sedimentary sequence in the paris basin france 1 previous studies that made use of basin models have shown that the normal geological evolution of the paris basin does not generate the observed albeit weak excess pressures in some shale layers of the basin other processes that may have created the overpressures currently neglected in such models are investigated here terms accounting for osmotic effects and tectonic stress changes are successively added to the diffusivity equation the effect of changes in outcrop boundary conditions is also calculated with a pseudo two dimensional analytical solution these solutions are applied to the callovo oxfordian shale formation in the eastern part of the paris basin france it is shown that a long term transient osmotic effect starting in the tertiary could explain in part the observed excess pressures in the callovo oxfordian shale assuming effective diffusion coefficients of 1 5 10 12 m2 s 1 in line with the measurements and a pore radius b around 20 a for the shales however because of the uncertainty on the value of the shale pore radius additional head measurements and osmotic experiments on samples should be made to fully establish the possibility of an osmotic process our study also shows that recent changes in hydrodynamic boundary conditions could also explain excess pressure distribution in this shale layer it is plausible that a combination of the two processes could best explain the distribution and intensity of the overpressures tectonic stress changes do not appear to be important it is shown that for such processes to maintain high pressures strong and recent increase in tectonic compressive stress would be required

continental plate collision unstable vs stable slab dynamics we study possible scenarios for the evolution of continental collision zones by using a dynamic thermomechanical model that includes brittle elastic ductile rheology surface erosion and explicit metamorphic changes this paper focuses primarily on the influence of four key parameters 1 geotherm or thermotectonic age which controls the rheological profile 2 lower crustal composition weak or strong rheology 3 convergence rate and 4 metamorphic changes in the downgoing crust the experiments suggest that de pending on these parameters plate convergence is accommodated by four distinct mech anisms stable subduction shortening by pure shear thickening or folding and rayleigh taylor instabilities it appears that stable oceanic type subduction can only occur in the case of cold lithospheres moho temperature tmoho 550 8c and basically needs high convergence rates 4 5 cm yr depending on the lower crustal rheology strong or weak either the whole upper and lower crust or only the lower crust can be involved in subduction it appears that in the case of weak metamorphic rheologies phase changes only slightly improve chances for stable subduction lithospheric shortening becomes a dominant mechanism when tmoho 550 8c or convergence rates are 4 5 cm yr pure shear thickening becomes important in all cases of hot lithospheres tmoho 650 8c large scale folding is favored in the case of tmoho 5 500 650 8c and is more effective in the case of mechanical coupling between crust and mantle e g strong lower crust grav itational rayleigh taylor instabilities overcome other mechanisms for very high values of tmoho 800 8c and may lead to development of subvertical cold spots

stress field determinations in france by hydraulic tests in boreholes the stress field has been determined at eight different sites in france four in crystalline or metamorphic rocks four in sedimentary formations by hydraulic tests in boreholes for seven of these sites the complete stress field has been determined using the hydraulic tests on pre existing fractures htpf inversion method validity of the results is demonstrated by the good fit between a priori and a posteriori values for the data and by the low values of the a posteriori standard deviation on the unknowns for four sites results have been compared with those derived according to the hydraulic fracturing theory the two methods yield comparable results for the orientation of the maximum horizontal principal stress h a generally satisfactory fit for the magnitude of the minimum horizontal principal stress h but a very poor agreement for the magnitude of h this latter misfit has been attributed to the effect of fluid percolation prior to the actual opening of the fractures below a depth of 500 m a homogeneous h direction n150 e has been determined at the crystalline sites auriat echassieres le mayet de montagne chassoles all located in the northern massif central for three of these sites the vertical stress is significantly lower than the weight of overburden as computed from the rock density and the depth of the corresponding measurement at auriat and echassieres the stress field is consistent with a mostly strike slip faulting regime at le mayet de montagne and chassoles the maximum stress is vertical but nearly equal to h the stress field has been found to be much more heterogeneous at two of the four sites in sedimentary rocks because of the large variability in mechanical rock properties in such heterogeneous formations inversion with the htpf method must be limited to those data which pertain to the same rock horizon however because of its structure this heterogeneity does not restrict the possibility of defining large scale uniform principal stress directions all the results are consistent with the local seismotectonics

bedrock topography modeling and calcrete uranium prospectivity analysis of central erongo region namibia abstract the tertiary palaeodrainage system in the namibian calcrete uranium province is deeply buried under cenozoic sediments such as fluvial alluvial and desert flash flood terrace sediments duricrusts and or aeolian and fluvially reworked aeolian sands and has no obvious surface expression because of which this province remains under explored in spite of hosting the world s largest calcrete uranium deposit at langer heinrich this paper presents first pass bedrock geology and bedrock topography models for the namibian calcrete uranium province generated using air borne magnetic data which are further used to delineate subsurface tertiary palaeochannels in the province conjunctive interpretations of surface geology bedrock geology and bedrock topography indicated that the major underlying controls on the spatial distribution and configuration of the tertiary palaeochannels in the namibian province are both lithological and structural however the basement structural fabric that formed during the damaran orogeny possibly exerts a fundamental control on both palaeodrainage as well as present drainage the analysis also indicated that the carnotite deposition in the langer heinrich palaeochannel is possibly a consequence of westward reorientation of the course of the proto swakop river finally the paper presents a manual prospectivity analysis of the province for calcrete uranium deposits which resulted in the demarcation of five prospective sections of palaeochannels in the tumas basin swakop river and khan river terraces and in the gawib flats west of the langer heinrich deposit

determination of in situ stress orientation from borehole guided waves we have analyzed the horizontal particle motion of borehole guided waves from three component vertical seismic profile data recorded from three experiments the polarization directions of the guided waves are consistent with the direction of horizontal principal stress determined independently we suggest that stress induced velocity anisotropy in the rock adjacent to the borehole appears to cause antisymmetric flexural guided modes to be polarized in the direction of the maximum horizontal principal stress thus analysis of the horizontal particle motion of borehole guided waves appears to represent a new method for determining the orientation of horizontal principal stresses

glacier acceleration and thinning after ice shelf collapse in the larsen b embayment antarctica 1 ice velocities derived from five landsat 7 images acquired between january 2000 and february 2003 show a two to six fold increase in centerline speed of four glaciers flowing into the now collapsed section of the larsen b ice shelf satellite laser altimetry from icesat indicates the surface of hektoria glacier lowered by up to 38 6 m in a six month period beginning one year after the break up in march 2002 smaller elevation losses are observed for crane and jorum glaciers over a later 5 month period two glaciers south of the collapse area flask and leppard show little change in speed or elevation seasonal variations in speed preceding the large post collapse velocity increases suggest that both summer melt percolation and changes in the stress field due to shelf removal play a major role in glacier dynamics

intraplate seismicity new seismotectonic data in western europe abstract fault plane solutions for 128 earthquakes which occurred in france and neighbouring areas have been determined stress field information given by these data are discussed in relation to previous tectonic knowledge the western alps are characterized by a compressional regime with a fan shaped distribution of p axes which might be related to the interaction mechanism between europe and the adriatic plate the pyrenean region is mainly submitted to a south north compression tectonic activity in this area seems to be determined by the presence of two subduction zones with reverse dips in the south bay of biscay and in the central pyrenees the transition zone between these two active belts is characterized by significant seismicity with a transpression regime

petrogenesis of early syn tectonic monzonite granodiorite complexes crustal reprocessing versus crustal growth abstract the 563 7 6 1 ma old early syntectonic mon repos complex is a predominantly metaluminous magnesian calc alkalic granodioritic to granitic intrusion major and trace element variations imply that the rocks evolved through fractional crystallization processes involving amphibole biotite fe ti oxides zircon and apatite rocks less evolved than granodiorite show evidence of hornblende accumulation initial sr 87sr 86sr 0 7090 0 7111 and nd end 5 3 to 12 3 isotopic compositions of the granodiorites and granites are highly heterogeneous and vary with sio2 contents indicating that assimilation of older crustal components occurred initial 206pb 204pb 16 65 17 65 and 207pb 204pb ratios 15 52 15 60 are similar to other mafic intermediate complexes from the damara belt one monzodiorite and two quartz monzonites are k2o lile and hfse enriched and have similar sio2 contents as the granodiorites these samples are similar to post collisional mafic magmas from elsewhere in the world their e nd values 3 8 to 4 3 87sr 86sr ratios 0 7051 0 7073 and trace element characteristics imply that their unexposed parental melts are derived from a lithospheric mantle source that was contaminated and metasomatized by crustal material during ancient subduction processes the data are explained by assuming a flat subduction model where melting predominantly involves ancient continental mantle lithosphere and crust with limited if any melting of the underlying asthenospheric mantle during flat subduction a sliver consisting of buoyant oceanic lithosphere was amalgamated with the base of the ancient continental lithosphere of the overriding plate the oceanic mantle lithosphere and the oceanic crust dehydrated but did not melt and these fluids lowered the solidus of the overlying continental mantle lithosphere and crust this scenario can explain the occurrence of rare k2o and lile enriched monzodiorites and quartz monzonites with crustal like isotopic compositions observed in this study as well as some more alkaline rocks in the belt because the metasomatized continental lithospheric mantle has lower solidus temperatures than peridotitic mantle it is very likely that such metasomatized rocks may melt early during flat subduction thus k2o enriched monzodiorites to monzogranites are not restricted to late stages of the evolution of an orogen but may already form at the onset of an orogeny

the brine zone in the mcmurdo ice shelf antarctica abstract observations of a 4 4 m high brine step in the mcmurdo ice shelf antarctica show that it has migrated about 1 2 km in 4 years the present brine wave is overriding an older brine soaked layer this migration is proof of the dynamic nature of the step which is the leading edge of a brine wave that originated at the shelf edge after a major break out of the mcmurdo ice shelf the inland boundary of brine penetration is characterized by a series of descending steps that are believed to represent terminal positions of separate intrusions of brine of similar origin the inland boundary of brine percolation is probably controlled largely by the depth at which brine encounters the firn ice transition 43 m however this boundary is not fixed by permeability considerations alone since measurable movement of brine is still occurring at the inland boundary freeze fractionation of the seawater as it migrates throught the ice shelf preferentially precipitates virtually all sodium sulfate and concomitant removal of water by freezing in the pore spaces of the infiltrated firm produces residual brines approximately six times more concentrated than the original seawater

widespread rifting and retreat of ice shelf margins in the eastern amundsen sea embayment between 1972 and 2011 the major outlet glaciers that drain the eastern sector of the amundsen sea embayment smith haynes thwaites and pine island are among the largest fastest flowing and fastest thinning glaciers in west antarctica their recent ice flow acceleration is linked to ocean induced ice shelf thinning but may also arise from additional losses of ice shelf buttressing that are not well understood here we present a comprehensive history of coastal change in the eastern amundsen sea embayment between 1972 and 2011 derived mostly from landsat imagery the termini of all four major outlet glaciers have retreated but retreat is most rapid along the ice shelf margins where progressive rifting has occurred this pattern of retreat coincides with the recent acceleration of grounded ice and contributed to loss of ice shelf buttressing the observed pattern of margin led gradual ice shelf disintegration appears to be common in accelerating ocean terminating outlet glaciers we hypothesize that this pattern is part of a positive feedback between glacier acceleration and rift growth that could drive further buttressing loss in the eastern amundsen sea embayment

integrated stress determination by joint inversion of hydraulic tests and focal mechanisms an inversion method based on a genetic algorithm is proposed for interpreting jointly various kinds of stress data in order to overcome the limitation in number and quality of each data set the method has been applied to results from hydraulic tests in boreholes and to focal mechanisms of induced seismicity observed within the same depth interval the regional stress field is described by two symmetrical tensors the first one represents the stress at a given depth and the second one the vertical stress gradient results indicate that one of the principal directions is vertical they are consistent with all but one of the hydraulic tests considered in the inversion and with about 70 of the focal mechanisms this inversion confirms previous results suggesting that for the scale of these induced microseismic events the regional stress field cannot be determined solely from an inversion of the fault plane solutions

record of the damaran khomas hochland accretionary prism in central namibia refutation of an ensialic origin of alate proterozoic orogenic belt sedimentation patterns in multiply deformed and highly metamorphosed sedimentary rocks of the khomas trough indicate deposition on an elongate submarine fan system that developed in a late proterozoic depository forming part of the inland branch of the damara orogen structural patterns exhibit mainly compressional features and indicate that the damara sequence metasedimentary rocks and interposed amphibolites metagabbros and ultramafic rocks lie within several imbricate thrust slices floored by relatively thick pelitic units the highly deformed matchless amphibolite and associated pelagic sedimentary rocks are interpreted as the upper parts of an oceanic crustal sequence these features are explained in a model that interprets the sediments as accumulating in a late proterozoic accretionary prism developed during convergence of the congo and kalahari cratons preservation of original sedimentary structures and coherent structural profiles allow the documentation of what may be the best preserved precambrian accretionary prism yet described our interpretation helps to resolve some earlier speculation concerning an ensialic origin for this fold and thrust belt

three dimensional geometry and kinematic evolution of the pine mountain thrust system southern appalachians the pine mountain thrust system is the westernmost major structure in the southern appalachian valley and ridge in virginia tennessee and kentucky surface subsurface and seismic reflection data have been integrated to construct a series of balanced cross sections and interpret the three dimensional geometry and kinematic evolution of the structure the structure is subdivided into three main areas on the basis of structural style in the southwestern and northeastern areas the structure consists of a simple ramp related anticline the powell valley anticline related to a ramp in the pine mountain thrust from the cambrian rome to the devonian chattanooga formation in the central oil producing area the pine mountain thrust is exposed in several erosional fensters as a result of folding by one or more subthrust imbricates here the pine mountain thrust develops a double ramp geometry with an intermediate flat in the cambrian maynardville formation the structure varies from a system of overlapping anticlines in the brooks and martin creek fenster sections to two anticlines separated by an intermediate syncline in the bethel and sulphur springs fenster sections to a partially overlapping duplex imbricate system in the big stone gap section the total shortening associated with the pine mountain thrust system decreases from about 70 000 ft 21 3 km at the southwest end to less than 10 000 ft 3 0 km at the northeast end in the central fenster area the total shortening is distributed between the pine mountain thrust and its subthrust imbricates the structural elevations in both the pine mountain and bales i sheets are greatest in the central fenster area the greatest structural elevations in the bales i sheet are in the rose hill and ben hur oil fields palinspastic restorations of the thrust system and the construction of detailed fault maps indicate that variations in structural geometry and shortening along strike are accommodated by interactions between thrust faults and oblique ramps or tear faults the variations in three dimensional geometry can be explained in terms of combinations of three main mechanisms unequal hanging wall displacements over a single footwall ramp equal displacements over a footwall ramp displaced along an oblique ramp and displacement of part of a footwall ramp due to movement on one or more frontal thrusts

roles of marine ice rheology and fracture in the flow and stability of the brunt stancomb wills ice shelf marine ice sometimes as part of an ice melange significantly affects ice shelf flow and ice fracture the highly heterogeneous structure of the brunt stancomb wills ice shelf bsw system in the east weddell sea offers a rare setting for uncovering the difference in rheology between meteoric and marine ice here we use data assimilation to infer the rheology of the brunt stancomb wills ice shelf by an inverse control method that combines interferometric synthetic aperture radar measurements with numerical modeling we then apply the inferred rheology to support the hypothesis attributing the observed 1970s ice shelf flow acceleration to a change in the stiffness of the ice melange area connecting brunt proper with stancomb wills and to examine the consequences of frontal rift propagation we conclude that while the brunt stancomb wills system is currently not susceptible to extreme fragmentation similar to that of the larsen b ice shelf in 2002 our inverse and forward modeling results emphasize its vulnerability to destabilization by relatively rapid changes in the ice melange properties resulting from the interaction of its marine ice component with ocean water or by the further propagation of a frontal rift copyright 2009 by the american geophysical union

accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf acceleration exceeds 27 km 3 per year and ice is thinning at rates of tens of meters per year we attribute this abrupt evolution of the glaciers to the removal of the buttressing ice shelf the magnitude of the glacier changes illustrates the importance of ice shelves on ice sheet mass balance and contribution to sea level change index terms 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 3349 meteorology and atmospheric dynamics polar meteorology 6924 radio science interferometry 9310 information related to geographic region antarctica citation rignot e g casassa p gogineni w krabill a rivera and r thomas 2004 accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf geophys res lett 31 l18401 doi 10 1029 2004gl020697

causes of pre collapse changes of the larsen b ice shelf numerical modelling and assimilation of satellite observations satellite observations revealed that beside a rapid thinning the larsen b ice shelf lbis was undergoing a significant acceleration before its collapse in 2002 this paper investigates the ice shelf acceleration between 1995 and 1999 using a combination of data assimilation and numerical modelling based on a flow model adjusted to the 1995 insar velocities perturbation experiments are performed such as ice front retreat thinning increase in tributary flow and rheological weakening furthermore an inversion for ice shelf rheology and tributary flow velocity is performed for both the 1995 and the 1999 insar velocities the perturbation experiments together with the inversion strongly suggest that the acceleration cannot solely be explained by the retreat of the ice shelf front but relies on a further significant rheological weakening of the already weak shear zones within the lbis minor tributary acceleration is found to be an effect rather than a cause of the ice shelf acceleration furthermore the observed acceleration cannot be explained by the observed recent thinning we conclude that for smaller ice shelves such as the lbis such weak shear margins play a crucial role in controlling their dynamics and are the key to understand changes in the future finally we compare the dynamic thinning likely to be associated with the observed acceleration with the observed thinning for the ice shelf as a whole this thinning accounts for 20 of the observed value which implies that factors such as enhanced basal melt were the primary cause of the observed thinning

subglacial melt channels and fracture in the floating part of pine island glacier antarctica 1 a dense grid of ice penetrating radar sections acquired over pine island glacier west antarctica has revealed a network of sinuous subglacial channels typically 500 m to 3 km wide and up to 200 m high in the ice shelf base these subglacial channels develop while the ice is floating and result from melting at the base of the ice shelf above the apex of most channels the radar shows isolated reflections from within the ice shelf comparison of the radar data with acoustic data obtained using an autonomous submersible confirms that these echoes arise from open basal crevasses 50 100 m wide aligned with the subglacial channels and penetrating up to 1 3 of the ice thickness analogous sets of surface crevasses appear on the ridges between the basal channels we suggest that both sets of crevasses were formed during the melting of the subglacial channels as a response to vertical flexing of the ice shelf toward the hydrostatic condition finite element modeling of stresses produced after the formation of idealized basal channels indicates that the stresses generated have the correct pattern and if the channels were formed sufficiently rapidly would have sufficient magnitude to explain the formation of the observed basal and surface crevasse sets we conclude that ice shelf basal melting plays a role in determining patterns of surface and basal crevassing increased delivery of warm ocean water into the sub ice shelf cavity may therefore cause not only thinning but also structural weakening of the ice shelf perhaps as a prelude to eventual collapse

rapid bottom melting widespread near antarctic ice sheet grounding lines as continental ice from antarctica reaches the grounding line and begins to float its underside melts into the ocean results obtained with satellite radar interferometry reveal that bottom melt rates experienced by large outlet glaciers near their grounding lines are far higher than generally assumed the melting rate is positively correlated with thermal forcing increasing by 1 meter per year for each 0 1 c rise in ocean temperature where deep water has direct access to grounding lines glaciers and ice shelves are vulnerable to ongoing increases in ocean temperature

regional dome evolution and its control on ore grade distribution insights from 3d implicit modelling of the navachab gold deposit namibia abstract we introduce a novel approach to analyse and assess the structural framework of ore deposits that fully integrates 3d implicit modelling in data rich environments with field observations we apply this approach to the early palaeozoic navachab gold deposit which is located in the damara orogenic belt namibia compared to traditional modelling methods 3d implicit modelling reduces user based modelling bias by generating open or closed surfaces from geochemical lithological or structural data without manual digitisation and linkage of sections or level plans instead a mathematically defined spatial interpolation is used to generate 3d models that show trends and patterns that are embedded in large drillhole datasets in our 3d implicit model of the navachab gold deposit distinctive high grade mineralisation trends were identified and directly related to structures observed in the field the 3d implicit model and field data suggest that auriferous semi massive sulphide ore shoots formed near the inflection line of the steep limb of a regional scale dome where shear strain reached peak values during fold amplification this setting generated efficient conduits and traps for hydrothermal fluids and associated mineralisation that led to the formation of the main ore shoots in the deposit both bedding parallel and highly discordant sets of auriferous quartz sulphide veins are interpreted to have formed during the later lock up stage of the regional scale dome additionally pegmatite dykes crosscut and remobilise gold mineralisation at the deposit scale and appear to be related to a younger joint set we propose that kilometre scale active folding is an important deformation mechanism that influences the spatial distribution and orientation of mineralisation in ore deposits by forming structures traps and pathways for fluids at different preferred sites and orientations we also propose that areas that experience high shear strain located along the inflection lines of folds can act as preferred sites for syn deformational hydrothermal mineralisation and should be targeted for regional scale exploration in fold and thrust belts our research also suggests that examination of existing drillhole datasets using 3d implicit modelling is a powerful tool for spatial analysis of mineralisation patterns when combined with fieldwork this approach has the potential to improve structural understanding of a variety of ore deposits

crevasse initiation and history within the mcmurdo shear zone antarctica while large scale observations of intensified fracture and rifting can be observed through remote sensing observations understanding crevasse initiation may best be achieved with small scale observations in which crevasses can be directly observed here we investigate the kinematic drivers of crevasse initiation in the mcmurdo shear zone msz antarctica we delineated 420 crevasses from 95 km of 400 mhz frequency ground penetrating radar data and compared these data with kinematic outputs derived from remotely sensed ice surface velocities to develop a statistical method to estimate crevasse initiation threshold strain rate values we found the msz to be dominated by simple shear and that surface shear strain rates proved best for predicting crevasse features with regions of higher shear strain rate more likely to have a greater number of crevasses in the surveyed portion of our study region values of shear strain rate and vorticity rate derived from the measures2 velocity dataset range between 0 005 0 020 and 0 006 0 022 a 1 respectively with crevasses located at 0 011 and 0 013 a 1 while threshold values from this study cannot be directly applied to other glacial environments the method described here should allow for the study of shear margin evolution and assessment of localized damage and weakening processes in other locations where in situ data are available

ice shelf dynamics near the front of the filchner ronne ice shelf antarctica revealed by sar interferometry fifteen synthetic aperture radar sar images of the ronne ice shelf also referred to as the filchner ronne ice shelf antarctica obtained by the european remote sensing satellites ers 1 and 2 are used to study ice shelf dynamics near two ends of the iceberg calving front interferograms constructed from these sar images are used to resolve the ice shelf displacement along several directions in response to both ocean tide and long term creep flow tidal motion is separated from creep flow using differential interferometry i e two or more interferograms in which fringe patterns common to all are predominantly associated with creep flow creep flow velocities thus determined compare well with prior ice shelf velocity surveys using these data we studied the influence of large scale rifts ice rises and coastal separation on the ice shelf flow many of the large rifts that appear to form the boundaries where tabular icebergs may eventually detach from the ice shelf are filled with a melange of sea ice ice shelf debris and wind blown snow the interferograms show that this melange tends to deform coherently in response to the ice shelf flow and has sufficient strength to trap large tabular ice shelf fragments for several decades before the fragments eventually become icebergs in many instances the motion of the tabular fragments is a rigid body rotation about a vertical axis that is driven by velocity shear within the melange the mechanical role of the rift filling melange may be to bind tabular ice shelf fragments to the main ice shelf before they calve this suggests two possible mechanisms by which climate could influence tabular iceberg calving first spatial gradients in oceanic and atmospheric temperature may determine where the melange melts and thus the location of the iceberg calving margin second melting or weakening of ice melange as a consequence of climate change could trigger a sudden or widespread release of tabular icebergs and lead to rapid ice shelf disintegration

kinematic evolution of the aracuai west congo orogen in brazil and africa nutcracker tectonics during the neoproterozoic assembly of gondwana abstract the neoproterozoic aracuai west congo a wc orogen is one of many brasiliano pan african orogens that developed during the assembly of west gondwana this orogen was split apart in mesozoic time due to opening of the south atlantic the aracuai orogen now underlies eastern brazil whereas the west congo belt fringes central africa s atlantic coast significantly at the time it formed the a wc orogen was bounded on the west north and east by the sao francisco congo craton a crustal block that had the shape of a lopsided upside down u thus the orogen was partially confined during tectonism in that it occupied an enclave surrounded on three sides by cratonic crust formation of the a wc orogen resulted in kinematically complex deformation substantial crustal shortening and production of a large volume of magma how such features could develop in this particular setting has long been a mystery our field studies in the aracuai orogen together with published data on the west congo belt characterize the kinematic picture of the a wc orogen and lead to a tectonic model that addresses its evolution in our model the a wc orogen formed in response to closure of the macaubas basin this basin was underlain by oceanic crust in the south but tapered northward into a continental rift which terminated against the cratonic bridge linking the eastern and western arms of the sao francisco congo craton closure occurred when the western arm now the sao francisco craton rotated counterclockwise towards the eastern arm now the congo craton this closure may have been driven by collision of the paranapanema amazonian and kalahari cratons against the external margins of the sao francisco congo craton rather than by slab pull associated with subduction of the macaubas basin s floor thus the process of forming the a wc orogen resembled the process of crushing of a nut between two arms of a nutcracker such nutcracker tectonics led to a series of kinematically distinct deformation stages initially internal portions of the orogen flowed northwards then substantial crustal thickening occurred in the orogen s interior and the deformation front migrated outwards producing thrust belts that overlapped the internal margins of the craton with continued closure space in the enclave became restricted and the orogen s interior underwent lateral escape to the south late stage extensional collapse triggered both production of late to post collisional granites and exhumation of high grade rocks from mid crustal levels

metamorphic response and crustal architecture in a classic collisional orogen the damara belt namibia abstract the damara belt is a well exposed mid crustal section through a collisional orogen of cambrian age that closed the khomas ocean basin between passive margins on the congo and kalahari cratons collision resulted in a bi vergent orogen with a distinct paired metamorphic pattern of foreland vergent high p low t orogenic margins and a broad high grade low p high t orogen core spatial and temporal patterns of the metamorphic response to collision have been characterized for all parts of the belt using a large dataset n 240 of internally consistent quantitative pt determinations evolution of mineral parageneses and semi quantitative p t paths metamorphic mapping and quantitative metamorphic field gradients integration with deformation history structural profiles metamorphic chronology magmatic history and stratigraphy constrains a dynamic model of crustal architecture during peak metamorphic events the pattern of zonal metamorphic response is demarcated by three major metamorphic discontinuities md with steep pressure gradients inferring crustal scale structures that accommodated lateral exhumation of crustal wedges discontinuities are confirmed by deformation features in the field and metamorphic mineral growth indicates that vertical flattening at the peak of metamorphism progressing through ductile to brittle extensional structures crustal wedges along the orogenic margins experienced steep clockwise p t paths with peak pt conditions terminated by isothermal decompression during rapid exhumation in isostatic response to deep burial metamorphic chronology and over printing metamorphic fabrics constrain a sequence of foreland propagating out wedging of crustal thrust wedges that resulted in telescoping of the orogenic margins peak metamorphism at a geothermal gradient of 20 25 c km and 8 9 kb in the southern zone wedge i was attained between 517 530 ma followed by south directed out wedging at the uis pass suture accommodated by relative extension md1 at high structural levels near the boundary with the high grade orogen core out wedging of the southern zone further buried the southern margin zone wedge ii in the footwall below the uis pass suture peak metamorphism at 17 c km and 9 5 11 5 kb in the southern margin zone was attained at 517 4 ma and followed by out wedging on basal thrusts accommodated by vertical flattening and extensional reactivation of the uis pass suture md2 peak metamorphism at 17 c km and 10 5 kb in a high p low t crustal wedge in the northern margin wedge iii occurred at 510 4 ma peak metamorphism in this wedge was terminated by isothermal decompression during north directed thrusting accommodated by extension at higher structural levels near the high grade orogen core md3 granulite facies metamorphism in the high grade orogen core which formed the upper plate to the deeper thrust wedges occurred at significantly lower pressures 4 3 6 0 kb higher t depth ratios 38 47 c km and low p t clockwise p t paths indicating only moderate burial and protracted high grade conditions metamorphic chronology confirms high grade conditions 540 505 ma persisted beyond isostatic adjustment of the high p low t orogenic margins high heat flow conditions and long thermal lag are best explained by lithospheric breakoff during collision in contrast to the high p low t margins that experienced lateral exhumation in a convergent system the high grade orogen core was exhumed vertically as a broad core complex during extensional collapse of the orogen peak metamorphic conditions in the central part of the orogen core record higher pressures than marginal lower grade zones to both the north and south indicating vertical exhumation as a broad massif this post peak vertical exhumation of the central core occurred after 505 ma was accommodated by brittle extensional detachments at high stratigraphic levels with km scale omission of crustal section

ground penetrating radar profiles of the mcmurdo shear zone antarctica acquired with an unmanned rover interpretation of crevasses fractures and folds within firn and marine ice abstractthe crevassed firn of the mcmurdo shear zone sz within the ross ice shelf may also contain crevasses deep within its meteoric and marine ice but the surface crevassing prevents ordinary vehicle access to investigate its structure geophysically we used a lightweight robotic vehicle to tow 200 and 400 mhz ground penetrating radar antennas simultaneously along 100 parallel transects over a 28 km2 grid spanning the sz width transects were generally orthogonal to the ice flow total firn and meteoric ice thickness was approximately 160 m firn crevasses profiled at 400 mhz were up to 16 m wide under snow bridges up to 10 m thick and with strikes near 35 40 to the transect direction from the top down 200 mhz profiles revealed firn diffractions originating to a depth of approximately 40 m no discernible structure within the meteoric ice a discontinuous transitional horizon and at least 20 m of stratified marine ice 28 31 m of freeboard found more marine ice exists based on 10 consecutiv

evidence for basal marine ice in the filchner ronne ice shelf the filchner ronne ice shelf which drains most of the marine based portions of the west antarctic ice sheet is the largest ice shelf on earth by volume the origin and properties of the ice that constitutes this shelf are poorly understood because a strong reflecting interface within the ice and the diffuse nature of the ice ocean interface make seismic and radio echo sounding data difficult to interpret1 2 ice in the upper part of the shelf is of meteoric origin but it has been proposed2 5 that a basal layer of saline ice accumulates from below here we present the results of an analysis of the physical and chemical characteristics of an ice core drilled almost to the bottom of the ronne ice shelf we observe a change in ice properties at about 150 m depth which we ascribe to a change from meteoric ice to basal marine ice the basal ice is very different from sea ice formed at the ocean surface and we propose a formation mechanism in which ice platelets in the water column accrete to the bottom of the ice shelf

oblique lineations in orthogneisses and supracrustal rocks vertical partitioning of strain in a hot crust eastern borborema province ne brazil detailed structural work conducted at the eastern area of the neoproterozoic brasiliano zpan african borborema province northeastern brazil has shown two orientations of stretching lineations with ese trend in supracrustal rocks and ne trend in underlying orthogneisses in the metasedimentary sequence numerous kinematic indicators showing a top to the northwest sense of shear denote a welldeveloped non coaxial deformation in the orthogneisses lineations formed dominantly during coaxial deformation although a component of ne directed shear is locally observed the two lineations were produced under similar high temperature metamorphic conditions and are interpreted as the result of a protracted nw se contractional strain regime where i subhorizontal non coaxial shear with superimposed flattening led to an initial phase of nw directed thrusting ii flattening strains mainly accumulated in the orthogneisses with progressive deformation leading to a lineation oblique to the transport direction iii the subhorizontal fabric in basement and cover rocks was refolded by overturned folds and then iv cross cut by conjugate ene striking dextral and nne striking sinistral shear zones contemporaneous with ne trending upright folds it is proposed that vertical partitioning of strain between basement and cover may explain the presence of oblique lineations in this orogenic belt that did not go through a final stage of extensional collapse q 2005 elsevier ltd all rights reserved

petrogenesis of the granitic donkerhuk batholith in the damara belt of namibia protracted syntectonic short range crustal magma transfer the areally extensive 5000 km2 syn tectonic ca 520 ma mainly s type donkerhuk batholith was constructed through injection of thousands of mainly sheet like magma pulses over 20 25 myr it intruded schists of the southern zone accretionary prism in the damara belt of namibia each magma pulse had at least partly crystallised prior to the arrival of the following batch however much of the batholith may have remained partially molten for long periods close to the h2o saturated granite solidus the batholith shows extreme variation in chemistry while having limited mineralogical variation and seems to be the world s most heterogeneous granitic mass the nd model ages of 2 ga suggest that eburnean rocks of the former magmatic arc structurally overlain by the accretionary wedge are the most probable magma sources crustal melting was initiated by mantle heat flux probably introduced by thermal diffusion rather than magma advection the granitic magmas were transferred from source to sink with minimal intermediate storage the whole process having occurred in the middle crust resulting in feeble crustal differentiation despite the huge volume of silicic magma generated source heterogeneity controlled variation in the magmas and neither mixing nor fractionation was prominent however due to the transpressional emplacement regime local filter pressing formed highly silicic liquids as well as felsic cumulate rocks the case of the donkerhuk batholith demonstrates that emplacement level tectonics can significantly influence compositional evolution of very large syn tectonic magma bodies

petrology of basement dominated terranes i regional metamorphic t t path from u pb monazite and sm nd garnet geochronology central damara orogen namibia abstract mineral chemistry combined with sm nd garnet whole rock and u pb monazite ages provides important constraints on the geological history of metapelites migmatites and granites from the high grade basement dominated part of the damara orogen namibia based on thermobarometric data the peak of regional metamorphism reached c 700 750 c and c 5 kbar u pb monazite and sm nd garnet mineral ages obtained from unmigmatized garnet and cordierite bearing metapelites indicate that these peak metamorphic conditions were reached between 525 2 and 504 3 ma some migmatites with restitic melanosomes record sm nd garnet ages of 518 3 ma indicating that high grade regional metamorphism also culminated in partial melting some other migmatites and granites give older sm nd garnet whole rock ages between 530 3 and 540 4 ma different initial nd isotope compositions of granites leucosomes and melanosomes indicate that in this case migmatization is intrusion related crust derived granites intruded at 515 2 512 3 and 509 5 ma and are probably related to the second peak of regional metamorphism the youngest granites intruded at 469 3 ma indicating that intrusion of melts is superimposed upon the regional metamorphic history age constraints for the period between c 540 and 510 ma provide substantial evidence for crustal thickening together with igneous activity whereas the high temperature event which occurred at c 470 ma must be related to extensional tectonics the concordance between the monazite and garnet ages suggest that monazite grew at high grade metamorphic conditions it is further demonstrated that combined monazite and garnet chronometry are useful in identifying the timing and nature of particular tectonometamorphic and intrusive events in polymetamorphic high grade terranes

thrust related dome structures in the karibib district and the origin of orthogonal fabric domains in the south central zone of the pan african damara belt namibia abstract the high grade metamorphic south central zone scz exposes an oblique crustal section through the internal parts of the pan african damara belt in namibia the structural pattern of the south central zone is characterized by kilometer scale northeast trending dome structures for which a number of different origins have been proposed detailed structural mapping of the karibib and usakos domes in the karibib district indicates an origin of the two domes as large tip line folds located above blind thrusts the two domes are overridden from the southeast by a crystalline thrust sheet along the mon repos thrust zone and the thrusts and associated folds form part of a deeply eroded foreland northwest vergent fold and thrust belt intrusive relationships of syn and post tectonic granitoids constrain the timing of thrusting to between 550 and 540 ma corresponding to the main phase of collisional tectonics in the damara belt on a regional scale the south central zone is underlain by two orthogonal fabric domains northwest vergent tectonic transport documented in this study dominates the lower amphibolite facies northeastern parts of the south central zone in contrast a southwest directed orogen parallel extrusion is widely documented for the lower structural levels of the upper amphibolite to granulite facies southwestern parts of the south central zone that contain voluminous synkinematic granites and widespread migmatization fabric development in the two orthogonal domains can be shown to be contemporaneous and related to the main collision between the congo and kalahari cratons at ca 540 550 ma we suggest that the formation of the two orthogonal fabric domains represents the different crustal response of the rheologically layered crust to the late proterozoic crustal shortening at shallower crustal levels shortening of sufficiently strong crust in the karibib district resulted in vertical thickening and orogen normal folding and thrust transport shortening of the deeper levels of the scz resulted in pure shear deformation of the rheologically weakened and partially molten crust and the lateral orogen parallel displacement of rocks

role of crustal contribution in the early stage of the damara orogen namibia new constraints from combined u pb and lu hf isotopes from the goas magmatic complex abstract the tholeiitic to calc alkaline goas complex represents the earliest magmatic activity of the inland branch of the damara orogen and reflects the pan african convergence between the congo and kalahari cratons new results from laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb geochronology coupled with single zircon lu hf isotopic data constrain the age of emplacement and allow the identification of a crustal contribution to the magma sources no temporal gap is registered between the first tholeiitic intrusion at ca 575 ma and the main diorite to granite episodes invariably sub chondritic ehf t values 3 8 to 34 4 attest to a significant crustal residence time with long term reworking of multiple and mixed crustal components no significant juvenile magma was involved in the magmatogenesis hf model ages coupled with an exhaustive dataset of u pb geochronological data suggest that the crustal components which fed the goas sources might have formed during specific orogenic events with a major contribution of material derived from the central western african paleoproterozoic eburnean orogen geochemistry u pb geochronology and hf isotope systematics however do not unequivocally clarify the long debate regarding whether or not the pan african orogenic cycle included ocean closure and subduction as the involvement of crustal melting processes during the early stage of the damaran event seems to have played a major role

crust mantle interaction during syn collisional magmatism evidence from the oamikaub diorite and neikhoes metagabbro damara orogen namibia abstract the early syntectonic 563 7 6 1 ma old oamikaub diorite damara orogen namibia consists of metaluminous magnesian calc alkalic to calcic diorites granodiorites and granites associated gabbro diorites and gabbros belong to the neikhoes metagabbro linear major and trace element variations imply that the rock suite evolved through fractional crystallization processes involving amphibole biotite fe ti oxides zircon and apatite initial sr 87sr 86sr 0 7058 0 7123 and nd e nd 2 1 to 18 8 isotopic compositions are highly variable and negatively correlated indicating that assimilation of crustal components occurred unradiogenic initial 206pb 204pb 16 23 17 23 and 207pb 204pb ratios 15 50 15 57 suggest derivation from or interaction with ancient crust with low u pb two gabbro diorites have mgo ni and cr abundances that are compatible with derivation of these rocks from upper mantle lithologies their initial e nd values 2 1 and 7 4 and 87sr 86sr ratios 0 7058 and 0 7076 imply derivation from an aged metasomatized lithospheric mantle other mafic samples have mgo abundances and compatible element concentrations that exceed the values commonly accepted for primary mafic melts implying some accumulation of clinopyroxene and amphibole the granodiorites form a homogenous group in which the isotope data initial e nd 12 4 to 14 1 initial 87sr 86sr 0 7083 0 7096 imply a lower crustal source the granites are also magnesian and calc alkaline but two of them are strongly peraluminous their isotope data initial e nd 13 2 to 18 8 initial 87sr 86sr 0 7099 to 0 7123 imply derivation from more ancient sources alternatively these samples gained their isotope systematics through extensive afc processes from parental granodiorites a common subduction zone environment as suggested from negative nb ta anomalies in multi element diagrams seems unlikely for all samples because of a lack of isotopically depleted signatures the data from the oamikaub diorite and other mafic complexes are better explained by a flat subduction model involving mainly continental mantle lithosphere and crust with limited if any melting of asthenospheric mantle

field studies of bottom crevasses in the ross ice shelf antarctica analyse des crevasses observees par sondages radar aeriens et terrestres localisation de leurs points de formation

the relationship between the structural orientation and the gold mineralisation of quartz sulphide veins in the navachab gold deposit namibia abstract this study uses a representative dataset of more than 500 vein samples from the navachab gold deposit containing information on the gold concentration structural orientation vein thickness alteration and host rock type to investigate small scale controls on the grade distribution in the veins in order to obtain a better understanding of the architecture of the navachab ore bodies we calculated mean gold concentrations for each vein set in different parts of the mine we then examine the preferential mineralisation of certain vein sets by setting this grade information into context to other geological parameters such as the structural geology of that particular part of the mine as well as the above mentioned factors structural orientation of the vein vein thickness alteration and host rock type our main findings are that vein sets oriented favourably to the stress field and at high angles to the bedding are better mineralised than veins sets oriented at low angles to the bedding other factors do not exert a great influence on the gold concentration in the veins we propose that the gold grade in the veins is influenced by the effectiveness of fluid flow into the veins and by time rates during vein propagation and fluid flow

calving fluxes and basal melt rates of antarctic ice shelves iceberg calving has been assumed to be the dominant cause of mass loss for the antarctic ice sheet with previous estimates of the calving flux exceeding 2 000 gigatonnes per year1 2 more recently the importance of melting by the ocean has been demonstrated close to the grounding line and near the calving front3 4 5 so far however no study has reliably quantified the calving flux and the basal mass balance the balance between accretion and ablation at the ice shelf base for the whole of antarctica the distribution of fresh water in the southern ocean and its partitioning between the liquid and solid phases is therefore poorly constrained here we estimate the mass balance components for all ice shelves in antarctica using satellite measurements of calving flux and grounding line flux modelled ice shelf snow accumulation rates6 and a regional scaling that accounts for unsurveyed areas we obtain a total calving flux of 1 321 144 gigatonnes per year and a total basal mass balance of 1 454 174 gigatonnes per year this means that about half of the ice sheet surface mass gain is lost through oceanic erosion before reaching the ice front and the calving flux is about 34 per cent less than previous estimates derived from iceberg tracking1 2 7 in addition the fraction of mass loss due to basal processes varies from about 10 to 90 per cent between ice shelves we find a significant positive correlation between basal mass loss and surface elevation change for ice shelves experiencing surface lowering8 and enhanced discharge9 we suggest that basal mass loss is a valuable metric for predicting future ice shelf vulnerability to oceanic forcing

gpr reflection profiles of clark and commonwealth glaciers dry valleys antarctica englacial horizons deeper than 100 m are absent within 100 mhz ground penetrating radar gpr surface profiles we recorded on clark and commonwealth glaciers in the antarctic dry valleys region both glaciers show continuous bottom horizons to 280 m with bottom signal to noise ratios near 30 db density horizons should fade below 50 m depth because impermeable ice occurred by 36 m folding within commonwealth glacier could preclude radar strata beneath about 80 m depth but there is no significant folding within clark glacier strong sulfate concentrations and contrasts exist in our shallow ice core however it appears that high background concentration levels and possible decreased concentration contrasts with depth placed their corresponding reflection coefficients at the limit of or below our system sensitivity by about 77 m depth further verification of this conclusion awaits processing of our deep core chemistry profiles

recent and future trends in paleopiezometry in the diagenetic domain insights into the tectonic paleostress and burial depth history of fold and thrust belts and sedimentary basins paleopiezometry provides an access to the past stress magnitude key to better understand the behaviour of the earth s crust over long period of time this contribution presents a review of some paleopiezometric techniques that can be used in the diagenetic domain in fold and thrust belts and sedimentary basins calcite twinning and stylolite roughness techniques have been selected and are presented through a critical description of their methodologies along with approaches to further reconstruct the complete effective stress tensor major geological lessons learned over the past decades from published studies are summarized and discussed along with a way forward to potential breakthroughs

the provenance of northern kalahari basin sediments and growth history of the southern congo craton reconstructed by u pb ages of zircons from recent river sands the southern congo craton is widely overlain by meso to cenozoic sediments of the northern kalahari basin which hamper any correlation of basement units the latter are represented by the archaean angola and kasai blocks while the southern cratonic margin is framed by several meso to neoproterozoic orogenic belts for provenance analysis of the sedimentary cover and reconstruction of the main zircon forming events we studied zircons from recent sediments of the largest rivers at the southern margin of the congo craton u pb zircon ages suggest a major amount of the sediments to originate from e lufilian and kibaran belts while input from the s damara belt seems to increase to the w ages related to the angola block were only noticed in the westernmost parts of the working area which is not in accordance with the se trending drainage pattern proposed to have been onset during the cretaceous thus it is assumed that the meso to cenozoic sedimentary cover extended further west than today prior to the mesozoic to neogene uplift of the angola block and that subsequent erosion exhumed the basement stepwise from west to east a recurrent destabilisation of the southern margin of the congo craton at 2 7 1 9 1 0 and 0 6 ga is supposed to be represented by major peaks in the age distribution pattern of the total amount of concordant zircons this is in accordance with similar studies in adjacent areas additionally the obtained data fit well to several hypothesised major events during the supercontinent cycle