the junction between pan african mobile belts in namibia its structural history abstract in namibia there is a junction between a pan african age 650 450 m y orogenic system which follows the western rim of africa and an ene trending arm which branches off into eastern namibia and zambia the se verging structures of this eastern arm were superimposed on earlier westward verging folds of the northern coastal belt the eastern arm was a zone of high temperature metamorphism and granite intrusion early in its history and this zone of high heat flow affected the style intensity and orientation of structures of the coastal belt these change from open flexural folds in the north away from the zone of high heat flow to intense recumbent folds associated with high grade metamorphism in the south where they deform rocks of the eastern arm the change in deformation intensity was largely responsible for a change in trend of the folds to produce a pronounced arcuate fold belt at the junction between the coastal and eastern arms

horizontal and vertical copepod distribution and abundance on the mid atlantic ridge in june 2004 abstract copepod composition abundance and distribution were studied in june 2004 on the mid atlantic ridge along a transect extending from iceland 60 n to the azores 41 n the samples were depth stratified from the surface down to a maximum depth of 2500 m the transect covered relatively warm and relatively saline modified north atlantic water mnaw on the reykjanes ridge in the northern part colder and less saline sub arctic intermediate water saiw further south to the charlie gibbs fracture zone cgfz and warm high salinity north atlantic central water nacw in the southern part in total 68 copepod genera and 117 species were identified calanoid copepods dominated the generic richness with 57 genera there was a clear increase in the number of genera southwards from 30 in the northern region to 57 in the south the total copepod number was variable between stations ranging from 45 000 to 178 000 m 2 however the variability did not show any clear north south trend and the lowest abundance was observed close to the sub polar front located near the cgfz cluster analysis divided the transect into three main regions as based on copepod community structure one in the northern part containing mnaw and saiw where calanus finmarchicus and pareuchaea tonsa characterized the region one in the frontal region saiw nacw where c hyperboreus was more abundant than in the other two regions and one southern region containing nacw characterized by several species e g calanus helgolandicus mecynocera clausi and pleuromamma gracialis vertically the copepods mainly occurred in the upper part of the water column in total 53 of the individuals occurred above 100 m and 75 above 500 m depth below 1000 m depth only low numbers were recorded 10 of the total copepod abundance there were large differences in vertical distribution between genera species with m clausi acartia spp and pseudocalanus spp showing the shallowest distribution and oncaea spp pleuromamma spp and microcalanus spp the deepest distribution depth distribution however was also affected by hydrographic differences between stations in the deepest samples there was a tendency for a higher total abundance close to the bottom than at greater distances from bottom

40ar 39ar thermochronology of the pan african damara orogen namibia with implications for tectonothermal and geodynamic evolution abstract forty three 40ar 39ar step heating experiments on white mica biotite hornblende and whole rocks from the damara orogen namibia revise and refine the regional perspective on cooling exhumation and tectonic reactivation across the orogen these data also document post orogenic motion on major shear zones and the thermal effects of late syn to post tectonic granite intrusions these data show regional cooling through temperatures of 300 350 c from 545 to 520 ma in the central kaoko zone 530 510 ma in the eastern kaoko zone and 495 480 ma in the southern zone of the damara belt discordant 40ar 39ar age spectra relate to 1 partial resetting of k ar isotopic system in micas by late stage activity in the major kaoko belt shear zones such as the purros and three palms mylonite zones giving apparent ages of 467 6 481 3 and 492 ma and 2 thermal effects from granitic plutions particularly in the ugab domain of the southern kaoko belt 500 475 ma and in the granite dominated central zone of the damara belt 490 460 ma phyllites of the lower grade flank regions of the orogen record apparent 40ar 39ar mica crystallization ages of 517 ma eastern kaoko zone 526 ma northern zone damara belt and 568 553 ma southern foreland zone damara belt the southern foreland samples however are maximum ages due to variable influence of detrital mica rates of cooling vary between zones within the kaoko belt in the central kaoko belt hornblende biotite and white mica apparent 40ar 39ar ages are similar suggestive of rapid cooling through the 300 550 c temperature range whereas in the orogen core or western kaoko belt hornblende and biotite apparent ages are older than the white mica ages because the white micas record the reactivation of the main structures these data support models of diachronous deformation and metamorphism of the component parts of the orogen and suggest that pan african orogenesis in the damara is broadly bracketed between 580 and 500 ma with final movements through 480 ma and spans through the terminal amalgamation of gondwana in the kuunga orogeny through the initiation of subduction under the gondwana continental margin

diachronic collision slab break off and long term high thermal flux in the brasiliano pan african orogeny implications for the geodynamic evolution of the mantiqueira province abstract the mantiqueira province in se brazil and uruguay is a complex brasiliano pan african orogenic province that has recently been the topic of conflicting geodynamic models focusing the possibility of multiple collision events during the ediacaran ordovician assembly of the western gondwana supercontinent a thorough compilation of recently obtained high resolution u pb geochronology and geochemistry in the mantiqueira province provided the following results a considering the entire orogenic system the pre syn and post orogenic periods range from 670 595 ma 620 550 ma and 560 490 ma respectively and are coeval with the intrusion of 3 distinct magmatic sequences within each geographic segment of this large orogenic province evolving from i calc alkaline arc related magmatism to ii syn orogenic anatectic magmatism coeval with the metamorphic climax to iii calc alkaline alkaline including shoshonitic and a type affinity magmatism that reveal significant lower crust and mantle contribution b progressively younger ages are present from south to north irrespective of the orogenic period considered implying that the different geographic sectors were from south to north diachronically amalgamated into the mantiqueira province results provide important evidence for the existence of a single diachronic collision event at 630 590 ma followed by the thermal climax at 615 560 ma slab break off asthenospheric upwelling and mantle underplating that sustained long term high heat flux conditions until 540 490 ma when post tectonic calc alkaline alkaline granitoids probably admixed with small amounts of slab melts formed at depth intruded the middle low crust magmatic and metamorphic sequences during the final stages of thermal and orogenic collapse of the mantiqueira province belts

key ring structure gradients and sheath folds in the goantagab domain of nw namibia abstract the concept of deformation phases is one of the corner stones of structural geology but despite its simplicity there are situations where the concept breaks down in the goantagab domain of nw namibia structures in an area of complex deformation can be subdivided into at least four sets attributed to four deformation phases on the basis of overprinting relations three of these sets of structures however formed during the same tectonic event under similar metamorphic circumstances but slightly different flow conditions these sets of structures show gradational transitions in space that can be understood by a concept of key ring structure gradients where older d a structures are reoriented and overprinted by new structures d a 1 that have similar orientation and seem to grade into d a structures outside the overprinted area this kind of behaviour may be common in inhomogeneous non coaxial flow in the core of the goantagab domain d 2 structures are thus reoriented and overprinted by local d 2b folds and foliations that have the same orientation and style as d 2 structures outside the domain core

corrigendum strange bedfellows glacial diamictite and cap carbonate from the marinoan 635 ma glaciation in namibia tectonic fold tests conducted in namibia demonstrate that the inclination with respect to bedding of geoplumb palaeovertical tubular structures in the marinoan 635 ma syndeglacial cap dolostone is mainly the result of tectonic strain therefore tubestone inclination data cannot be used to estimate the gradient of the sea floor on the foreslope of the otavi carbonate platform during the marinoan glaciation a gradient steeper than 0 1 slope angle ca 5 7 implying a glacial base level fall 0 5 km is nevertheless supported by boulder size intraclast debrite in the falling stand wedge directly beneath the glacigenic sequence cryogenian oceans lacked skeletal carbonate production raising the carbonate saturation state and persistent deep water anoxia excluded acid producing aerobic respiration facilitating early diagenetic carbonate precipitation lithification and steep submarine slopes

madagascar a continental fragment of the paleo super dharwar craton of india the morphological kinship of madagascar to its immediate neighbors on the west african continent and east indian subcontinent during the early and middle cretaceous has been debated for the past two decades on the basis of available geologic tectonic and paleomagnetic information most of the paleoreconstructions of madagascar have shown its attachment to the east african continent we present magnetic satelite and gravity data and morphological geophysical and geotectonic similarities to hypothesize that in the period before the breakup of gondwana madagascar was a continental fragment of the paleo super dharwar craton of india

crustal evolution in south india constraints from nd isotopes nd isotope analyses from a range of lithologies including charnockites metasediments and granitoids that comprise the high grade terrain of south india define contrasting age provinces north of the palghat cauvery shear zone the karnataka craton and the granulite blocks of nilgiri and madras are characterized by model nd ages from 3 4 to 2 4 ga with significant crustal growth during the late archean south of the palghat cauvery shear zone a pan african granulite facies event resulted in sm nd fractionation as observed in several incipient charnockite localities model nd age calculations that allow for sm nd fractionation at 550 ma yield ages in the range of 2 9 to 1 3 ga this range compares closely with published model nd ages from the high grade terrain of sri lanka mid proterozoic crustal growth 1 1 ga observed in the vijayan complex of sri lanka may have provided source material for metasediments characterized by young model nd ages in southern peninsular india the palghat cauvery shear z

u pb single zircon pb evaporation and sm nd isotopic study of a granulite domain in se madagascar madagascar is a poorly known precambrian domain at the eastern border of the mozambique belt strongly affected by the pan african orogeny this large island represents a key part of gondwanaland and the accurate dating of the major metamorphic event is critical for the reconstruction of that supercontinent in the southeastern zone of the island metamorphism has reached high temperature and low pressure conditions 850 c and 5 kbar with moderate retrogression a u pb conventional multigrain single zircon stepwise pb evaporation and sm nd geochronological study on meta felsic rocks has shown that basement of at least lower proterozoic age was strongly metamorphosed around 570 580 ma this tectometamorphic event nearly obliterated any pre pan african zircon isotopic memory the granulite facies metamorphism is associated with the syn metamorphic emplacement of the anosyan charnock ites and granites a strong channelized fluid contribution at 545 ma has produced the re crystallization of biotite apatite

new zircon ages for precambrian granites gneisses and granulites from central and southern madagascar significance for correlations in east gondwana institiif fiir geozuissenschnffen universitiit mainz 55099 mainz gevniany 2depnrtnienf of geology university of leicesfer leiccsfev le 1 7r h uk 3mnx plnnck lnsfitzit fiiv clzeniie postjnclz 3060 55020 mcriiiz geuniany 4depf of applzed geology ciirfin university of t c c z j i o o pevflz wa 6001 airstvaliii 5dc ppnrfeniei f des sciences nnfirreiles uiiiversitt de toliavn tolinrn mndngascav

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

directly measured mid depth circulation in the northeastern north atlantic ocean the circulation of water masses in the northeastern north atlantic ocean has a strong influence on global climate owing to the northward transport of warm subtropical water to high latitudes but the ocean circulation at depths below the reach of satellite observations is difficult to measure and only recently have comprehensive direct observations of whole ocean basins been possible here we present quantitative maps of the absolute velocities at two levels in the northeastern north atlantic as obtained from acoustically tracked floats we find that most of the mean flow transported northward by the gulf stream system at the thermocline level about 600 m depth remains within the subpolar region and only relatively little enters the rockall trough or the nordic seas contrary to previous work our data indicate that warm saline water from the mediterranean sea reaches the high latitudes through a combination of narrow slope currents and mixing processes at both depths under investigation currents cross the mid atlantic ridge preferentially over deep gaps in the ridge demonstrating that sea floor topography can constrain even upper ocean circulation patterns

petrogenesis of cogenetic nepheline and quartz syenites and granites northern damara orogen namibia enriched mantle versus crustal contamination abstract geochemical and nd sr o whole rock isotope data and pb isotope data from leached feldspars for a suite of synorogenic ca 540 ma syenites and granites from the northern part of the proterozoic damara orogen namibia constrain their sources and petrogenesis major rock types are mildly potassic nepheline syenites and quartz syenites that were primarily generated by fractional crystallization from a mantle derived alkaline magma even the most primitive samples show pronounced depletion in nb ti sr and p on a primitive mantle normalized diagram indicating the involvement of a recycled crustal component in the source extrapolation of the sr nd pb o isotope composition of the syenites back to a hypothetical parental melt with 8 wt 10 wt mgo suggests derivation from a moderately enriched lithospheric upper mantle documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage t

the transpressional connection between dom feliciano and kaoko belts at 580 550 ma a new u pb shrimp age of 551 4 ma on a mylonitic porphyry that intruded into the sierra ballena shear zone southernmost dom feliciano belt uruguay and a review of relevant published data make possible a more refined correlation and reconstruction of brasiliano pan african transpressional events paleogeographic reconstruction kinematics and timing of events indicate a connection between the shear systems of the dom feliciano and kaoko belts at 580 550 ma sinistral transpression recorded in shear zones accommodates deformation subsequent to collision between the congo and rio de la plata cratons the correlation is strengthened by the similarity of magmatic and metamorphic ages in the coastal terrane of the kaoko belt and the punta del este terrane of the dom feliciano belt this post collisional sinistral transpression brought these units near to their final position in gondwana and explains the different evolution at 550 530 ma while in the kaoko belt an extensional episode resulted in exhumation as a consequence of collision in the damara belt in the dom feliciano belt sinistral transpression occurred associated with the closure of the southern adamastor ocean due to kalahari rio de la plata collision

complex vein systems as a data source in tectonics an example from the ugab valley nw namibia abstract neoproterozoic metaturbidites in the lower ugab domain namibia contain a complex network of four sets of quartz calcite veins overprinted by km scale folds associated with four regional foliations the veins formed by fluid overpressure predating the main deformation deformation structures developed at the junction of two mobile belts during the assembly of gondwana the ns kaoko belt and the ew trending damara belt km scale ns trending folds were initiated during ew constriction in the kaoko belt while their further development and all subsequent events are related to constriction in the ew damara belt with coeval sinistral strike slip in the kaoko belt deformation of the veins and development of four orthogonal foliations are due to gradual changes in the bulk tectonic framework rather than separate orogenic events the veins are deformed in a complex manner allowing a full 3d reconstruction of regional sequence of events the local complex tectonics could be reconstructed because of the perfect local exposure and the multitude of veins it illustrates the potential complexity of tectonic events and structural evolution in apparently simple slate belts

530 ma syntectonic syenites and granites in nw namibia their relation with collision along the junction of the damara and kaoko belts abstract the lower ugab and goantagab structural domains are located at the junction between the n s trending kaoko and the e w trending damara belts nw namibia where neoproterozoic metavolcano sedimentary sequences were intruded by several syenitic granitic plutons we present here new u pb ages on zircon grains from the voetspoor and doros plutons together with petrological geochemical and structural data we evaluate the timing of the deformation and relation to the geodynamics during the final stage of gondwana amalgamation the plutons are composed of three main rock types hornblende quartz syenite syenodiorite and biotite granite the two former are predominant and show genetic correlation such as magma mingling structures and similar geochemical signatures the biotite granite occurs in the sw parts of the intrusions and clearly cuts the syenitic rocks although the plutons are mainly isotropic the structures around them demonstrate that their intrusion occurred during a second deformation phase d2 with a component of sinistral solid state rotation with respect to the wall rocks in response to d2 transpression four samples were dated using u pb shrimp methodology in single zircon grains a hornblende monzodiorite from the voetspoor pluton yielded an age of 534 4 5 ma a hornblende monzonite from the doros pluton produced an age of 528 5 ma the biotite granite facies was sampled in the doros intrusion and yielded an age of 530 4 5 ma in addition a granitic vein folded by d2 close to the northeastern contact of the doros pluton with the encasing phyllites amis river formation was also dated yielding an age of 533 6 ma the data show that all granite syenite from doros and voetspoor intrusions are contemporaneous and crystallized in the period between 539 and 522 ma within the errors d1 d2 deformational phases took place under greenschist facies biotite zone conditions and during d3 the metamorphic grade was slightly lower we interpret that the plutons are coeval to peak metamorphism of the region 530 520 ma and that d2 and d3 sinistral transpressional phases are due to collision in the damara belt the e w compressional event and second metamorphic episode in the kaoko belt occurred between 580 and 560 ma and are apparently unrelated to the thermo tectonic evolution described here although d1 might be partially related to this event the sinistral transpressional d2 phase resulted probably from the position of the area considered at the junction between the belts and not in the frontal damara collision further to the east this new interpretation is consistent with the ar ar ages for the region about 500 ma interpreted to reflect cooling of the orogen the enrichment in lree k rb ba and sr and depletion in nb of these basic to intermediate alkalic rocks could indicate that they partially derived from melting of a subcontinental lithospheric mantle that was affected by subduction and the granitic rock types represent lower crust contamination we interpret that they could be related to heating in the mantle caused by asthenosphere influx in a zone of slab breakoff during collision between kalahari and congo cratons

structure and strain variation at mid crustal levels in a transpressional orogen a review of kaoko belt structure and the character of west gondwana amalgamation and dispersal abstract the neoproterozoic cambrian kaoko belt is an orogen scale 800 180 km transpressional system important in the amalgamation of west gondwana mid crustal transpression at amphibolite to granulite facies conditions is dominated by two major 400 km exposed strike slip shear zones bounding a 20 40 km wide high grade orogen core to the east a deeply buried nappe dominated escape zone has inverted metamorphic sequence and verges outwards onto a platformal foreland to the west an arc like neoproterozoic coastal terrane was amalgamated and variably reworked during transpression the major purros and three palms mylonite zones have calculated shear displacements on the order of 120 180 km these shear zones are moderately to steeply dipping mylonite zones of 1 5 km width are arcuate and curvilinear in map view and show along strike variation in slip kinematics also highly curved in vertical section the shear zones define a flower to half flower geometry for the orogen core an oblique network of mylonitic shear zones akin to riedel shears links the major shear zones and defines regional scale shear lozenges internally deformed by tight upright folding and shear fabrics these shear zones create domains in the orogen core with varying dominance of pure shear in shear lozenges and simple shear in shear zones however absence of dip slip domains and the smoothly continuous traces of sub horizontal to shallow and acute oblique stretching lineations across all parts of the belt preclude marked kinematic partitioning and the internal part of the belt resembles large scale triclinic shear clast aspect ratios boudin train extension sheath fold aspect geometry degree of rotation of planes producing flanking folds composite s c foliations pressure fringes on pyrite and garnet porphyroclasts provide a semi quantitative measure of strain intensity average strain ratios are x z 40 1 for the major shear zones x z 12 1 for the orogen core x z 8 1 for the escape zone and x z 3 1 for the coastal terrane a more continuous pattern of strain intensity across the whole belt is mapped using a qualitative foliation intensity index foliation traces have a sigmoidal pattern in the orogen core swinging from sub parallel to the boundary shear zones to higher acute angles in the internal parts deformation character also varies from upright open folding in amphibolite facies domains in the north upright tight chevron folding in a low grade central domain to a high grade domain of tight to isoclinal inter folded basement and cover with inclination decreasing towards the south the kaoko belt is a well exposed sector of an extensive 3000 km long broad 400 km arcuate orogenic system adamastor orogen that amalgamated west gondwana bringing the south american sao francisco and rio de la plata cratons and african kalahari and congo cratons components together though a complex system most sectors involved oblique collision and accretion of magmatic arcs of 660 610 ma age followed by peak metamorphism and main phase transpressional orogenesis between 585 and 560 ma with shear zones remaining active until 530 ma this e w amalgamation immediately pre dates the final n s amalgamation of gondwana along the kuunga orogen between 535 and 510 ma the large scale adamastor orogen consisting of kaoko dom feliciano ribeira aracuai and west congo mobile belts also shows broadly similar and symmetric structural architecture throughout the high grade thermally softened core partitioned intense wrench dominated strains and networks of transcurrent shear zones that dip inwards with listric form either side of the internal zone containing amalgamated arcs and high grade core are nappe fold and thrust belts that rework attenuated passive margin basement and adamastor ocean sediments and structures verge outward at moderate to high angles onto both foreland margins the kaoko belt well illustrates the highly partitioned nature of transpressional systems in general and patterns in common throughout the greater adamastor orogen such as metamorphic zonation and heterogeneous distribution of deformation style flow regime and highly variable degrees of reworking strain and recrystallization this highly partitioned and steep structural grain localized lithospheric extension and rifting 415 ma later during breakup and dispersal of gondwana

the application of single zircon evaporation and model nd ages to the interpretation of polymetamorphic terrains an example from the proterozoic mobile belt of south india the technique of single zircon dating from the thermal evaporation of 207pb 206pb kober 1986 1987 provides a means of dating successive periods of growth and nucleation of zircons in polymetamorphic assemblages in contrast nd model ages may provide a measure of the period of crustal residency for the sample or its protolith these two techniques have been combined to elucidate the tectonic history of the proterozoic mobile belt of southern india exposed south of the palghat cauvery shear zone that marks the southern boundary of the archaean craton of karnataka the two main tectonic units of this mobile belt comprise the madurai and trivandrum blocks both of which are characterised by massive charnockite uplands and low lying polymetamorphic metasedimentary belts that have undergone a complex tectonic history throughout the proterozoic evidence for early palaeoproterozoic magmatism is restricted to the madurai block where single zircon evaporation ages from a metagranite 2436 4 ma are similar to model nd ages from a range of lithologies suggesting crustal growth at that time the trivandrum block to the south of the achankovil shear zone is comprised of the kerala khondalite belt the nagercoil charnockites and the achankovil metasediments single zircon evaporation ages together with conventional zircon and garnet chronometry suggest that all three units underwent upper amphibolite facies metamorphism at 1800 ma an event unrecorded in the metagranite from the madurai block this implies that the madurai and trivandrum blocks represent distinct terrains throughout the palaeoproterozoic model nd ages from the achankovil metasediments are much younger 1500 1200 ma than those from the adjacent kerala khondalite belt and madurai blocks 3000 2100 ma but there is no evidence for zircon growth in these metasediments during the mesoproterozoic hence the comparatively young model nd ages of the metasediments are indicative of a mixed provenance rather than a discrete period of crustal growth zircon overgrowths from the madurai block 547 17 ma and achankovil metasediments 530 21 ma suggest that all tectonic units of the proterozoic mobile belt of south india shared the same metamorphic history from the early palaeozoic this event has been recognised in the basement lithologies of sri lanka and east antarctica confirming that the constituent terrains of east gondwana had assembled by this time

lower crustal melting and the role of open system processes in the genesis of syn orogenic quartz diorite granite leucogranite associations constraints from sr nd o isotopes from the bandombaai complex namibia abstract the bandombaai complex southern kaoko belt namibia consists of three main intrusive rock types including metaluminous hornblende and sphene bearing quartz diorites allanite bearing granodiorites and granites and peraluminous garnet and muscovite bearing leucogranites intrusion of the quartz diorites is constrained by a u pb zircon age of 540 3 ma quartz diorites granodiorites and granites display heterogeneous initial nd and o isotope compositions end 540 ma 6 3 to 19 8 18o 9 0 11 6 but rather low and uniform initial sr isotope compositions 87sr 86srinitial 0 70794 0 70982 two leucogranites and one aplite have higher initial 87sr 86sr ratios 0 70828 0 71559 but similar initial end 11 9 to 15 8 and oxygen isotope values 10 5 12 9 the geochemical and isotopic characteristics of the bandombaai complex are distinct from other granitoids of the kaoko belt and the central zone of the damara orogen our study suggests that the quartz diorites of the bandombaai complex are generated by melting of heterogeneous mafic lower crust based on a comparison with results from amphibolite dehydration melting experiments a lower crustal garnet and amphibole bearing metabasalt probably enriched in k2o is a likely source rock for the quartz diorites the granodiorites granites show low rb sr 1 and are most likely generated by biotite dehydration melting of heterogeneous felsic lower crust all segments of the lower crust underwent partial melting during the pan african orogeny at a time 540 ma when the middle crust of the central damara orogen also underwent high t medium p regional metamorphism and melting geochemical and isotope data from the bandombaai complex suggest that the pan african orogeny in this part of the orogen was not a major crust forming episode instead even the most primitive rock types of the region the quartz diorites represent recycled lower crustal material

cetacean distributions relative to ocean processes in the northern california current system abstract associations between cetacean distributions oceanographic features and bioacoustic backscatter were examined during two process cruises in the northern california current system ccs during late spring and summer 2000 line transect surveys of cetaceans were conducted across the shelf and slope out to 150 km offshore from newport oregon 44 6 n to crescent city california 41 9 n in conjunction with multidisciplinary mesoscale and fine scale surveys of ocean and ecosystem structure occurrence patterns presence absence of cetaceans were compared with hydrographic and ecological variables e g sea surface salinity sea surface temperature thermocline depth halocline depth chlorophyll maximum distance to the center of the equatorward jet distance to the shoreward edge of the upwelling front and acoustic backscatter at 38 120 200 and 420 khz derived from a towed undulating array and a bioacoustic system using a multiple logistic regression model 60 2 and 94 4 of the variation in occurrence patterns of humpback whales megaptera novaeangliae during late spring and summer respectively were explained sea surface temperature depth and distance to the alongshore upwelling front were the most important environmental variables during june when humpbacks occurred over the slope 200 2000 m during august when humpbacks concentrated over a submarine bank heceta bank and off cape blanco sea surface salinity was the most important variable followed by latitude and depth humpbacks did not occur in the lowest salinity water of the columbia river plume for harbor porpoise phocoena phocoena the model explained 79 2 and 70 1 of the variation in their occurrence patterns during june and august respectively during spring latitude sea surface salinity and thermocline gradient were the most important predictors during summer latitude and distance to the inshore edge of the upwelling front were the most important variables typically a coastal species harbor porpoises extended their distribution farther offshore at heceta bank and at cape blanco where they were associated with the higher chlorophyll concentrations in these regions pacific white sided dolphin lagenorhynchus obliquidens was the most numerous small cetacean in early june but was rare during august the model explained 44 5 of the variation in their occurrence pattern which was best described by distance to the upwelling front and acoustic backscatter at 38 khz the model of the occurrence pattern of dall s porpoise phocoenoides dalli was more successful when mesoscale variability in the ccs was higher during summer thus the responses of cetaceans to biophysical features and upwelling processes in the northern ccs were both seasonally and spatially specific heceta bank and associated flow topography interactions were very important to a cascade of trophic dynamics that ultimately influenced the distribution of foraging cetaceans the higher productivity associated with upwelling near cape blanco also had a strong influence on the distribution of cetaceans

partial melting of diverse crustal sources constraints from sr nd o isotope compositions of quartz diorite granodiorite leucogranite associations kaoko belt namibia the torrabaai koigabmond complex southern kaoko belt namibia consists of three main intrusive rock types including metaluminous hornblende and titanite bearing quartz diorites metaluminous hornblende and biotite bearing granodiorites and peraluminous garnet and muscovite bearing leucogranites uranium pb zircon data obtained on the granodiorites and leucogranites indicate concordia upper intercept ages of 553 40 ma although pb 207 pb 206 ages of ca 650 ma in zircon from the granodiorites suggest some inheritance of older material uranium pb monazite data obtained on the leucogranites give concordant ages of 550 ma 3 ma these ages are similar to the rb 87 sr 86 whole rock age of 584 35 ma obtained on the granodiorites and leucogranites although the rb sr age seems to be biased towards older ages due to limited assimilation of older material in contrast to other plutonic complexes from the kaoko belt the quartz diorites granodiorites and granites show a restricted range in their initial nd sr and o isotope compositions quartz diorites epsilon nd init 5 4 to 6 7 delta o 18 8 3 9 4 parts per thousand sr 87 sr 86 0 7081 0 7098 granodiorites epsilon nd init 6 1 to 7 7 delta o 18 9 9 10 9 parts per thousand sr 87 sr 86 0 7071 0 7105 leucogranites epsilon nd init 4 9 to 8 7 delta o 18 9 8 11 3 parts per thousand sr 87 sr 86 0 7060 0 7125 enclaves are found in the granodiorites and leucogranites but not in the quartz diorites they have a granodioritic composition with quartz plagioclase k feldspar and hornblende and some have additional garnet relative to the country rock gneisses the so called nk formation enclaves are depleted in sio2 na2o k2o sr ba and enriched in cao feo total mgo tio2 sc v cr ni rb and y rare garnet bearing enclaves are additionally depleted in lree and enriched in hree relative to the granodiorites these features are qualitatively consistent with the hypothesis that these enclaves may represent moderately depleted melting residues of nk formation gneisses in comparison with experimentally derived melts and based on low al2o3 feo mgo tio2 ratios and high al2o3 feo mgo tio2 values it is suggested that the quartz diorites are generated by dehydration melting of a mafic amphibole and plagioclase bearing lower crustal source of pan african age the granodiorites likely represent fractionation products of the quartz diorites however it is also possible that the granodiorites represent partial melting products of a mafic to intermediate lower crustal source but experienced likely slightly lower degrees of melting probably at water present conditions the leucogranites display higher al2o3 feo mgo tio2 ratios but lower al2o3 feo mgo tio2 values and are most likely generated by biotite dehydration melting of felsic crustal sources major and trace element and isotope variations indicate that fractional crystallization with only limited crustal contamination was the major rock forming mechanism it is suggested that most of the isotope variation reflects pre existing heterogeneities of the sources consequently interpretation of geochemicaland isotope data from the complex suggests that the pan african igneous activity in this part of the damara kaoko belt was not a major crust forming episode and all rock types represent reprocessed crustal material c 2008 elsevier b v all rights reserved less

the timescale and mechanism of granulite formation at kurunegala sri lanka incipient charnockite formation at kurunegala in sri lanka is characterized by the growth of orthopyroxene at the expense of amphibole and biotite in an originally homogeneous gneiss mineral equilibria in the charnockite assemblage record pressure temperature p t conditions of 738 60 c and 6 9 1 2 kbar at 17 0 1 2 log fo2 and ah2o 0 18 0 16 wholerock trace element and isotopic measurements show that charnockite formation was accompanied by a systematic depletion of sm rb pb u sr nd with a fractionation of rb sr sm nd and th u ratios and crystallization of the charnockite assemblage at 535 5 ma major element fe mg ca and sm nd equilibration between minerals occurred at 524 9 ma whereas pb and rb sr underwent continued exchange to 501 5 ma and 486 1 ma respectively trace element data for both amphibolite and charnockite minerals show that depletion on a whole rock scale can be accounted for either by changes in mineral modes or trace element abundances within the immediate area of dehydration the fractionation of sm nd on a whole rock scale is controlled by the breakdown of amphibole without the growth of a major new host phase for sm in the charnockite rubidium and sr are dependent on the relative behaviour of biotite plagioclase and alkali feldspar modelling of dehydration melting involving the breakdown of amphibole biotite and alkali feldspar reproduces the observed sm nd and rb sr fractionation and indicates the loss of small melt fractions on a cm scale from the charnockite these observations suggest that partial melting is the most plausible means of effecting both the dehydration and depletion that accompanies charnockite formation

the kaoko belt namibia part of a late neoproterozoic continental scale strike slip system the neoproterozoic kaoko belt of northwestern namibia to the west of the congo craton consists of basement gneisses and a sequence of highly deformed metasedimentary and metavolcanic rocks the metasedimentary rocks are interpreted as the infill of a narrow north trending basin floored by attenuated continental crust similar basins are represented by the western continuation of the kaoko belt the dom feliciano and ribeira belts of eastern brazil on the basis of consideration of data from these belts the kaoko belt rocks indicate the absence of a large neoproterozoic oceanic basin between the congo and southwestern american cratons deformation structures in the kaoko belt show transpressional kinematics with strain partitioning at around 550 ma north northwest south southeast sinistral transcurrent shear in the west passes eastward into east southeast directed thrusting the transcurrent shear component dominates temporally and in strain intensity dextral transcurrent motion at 550 ma is recorded in the east northeast trending schlesien mwembeshi shear zone bordering the congo craton to the south accordingly the congo craton extruded northeastward along the kaoko belt and the schlesien mwembeshi shear zone relative to the kalahari craton in the southeast and the south american cratons to the west this extrusion probably resulted from a final phase of convergence between east and west proto gondwana

the north atlantic current and surrounding waters at the crossroads the north atlantic current is a well defined western boundary current that flows north along the east side of the grand banks from 40 to 51 n where it turns sharply to the east and begins its journey across the ocean the current is unique in transporting warm tropical waters to much higher latitudes than any other western boundary current and thus plays a crucial role in ameliorating the climate of the european subcontinent the north atlantic current originates in the gulf stream when the latter curves north around the southeast newfoundland rise a major submarine ridge that stretches se from the grand banks a well defined front delineates the path of the current as long as it flows north as a western boundary current after the current turns east in the north it broadens into a widening band of eastward drift without a sharp or permanent front in the sense of the eastward flowing gulf stream after it separates from cape hatteras the north atlantic current transports more than 40 sv 1 sv 106 m s 1 in the south and about 20 sv by the time it flows east across the mid atlantic ridge the currents along the northward flowing front are quite swift with typical maximum average speeds in the upper 300 m near 1 m s 1 2 knots the current meanders almost as wildly as a snaking river but unlike steep meanders in the gulf stream these meanders appear to be stable and with one exception have not been observed to break off to form pools of warm and or cold waters as frequently occurs in the gulf stream the meanders appear to be induced by major topographic features along the path of the current namely the southeast newfoundland rise the newfoundland seamounts and flemish cap strong recirculations develop on the concave side of the meanders one of these the mann eddy at the first meander crest of the north atlantic current should be regarded as a permanent feature of the north atlantic circulation other meanders also contain recirculations that can persist for months under certain conditions these can merge together to form an extended sw flow recirculation just east of the north atlantic current

the sierra ballena shear zone in the southernmost dom feliciano belt uruguay evolution kinematics and deformation conditions the sierra ballena shear zone sbsz is part of a high strain transcurrent system that divides the neoproterozoic dom feliciano belt of south america into two different domains the basement on both sides of the sbsz shows a deformation stage preceding that of the transcurrent deformation recognized as a high temperature mylonitic foliation associated with migmatization grain boundary migration and fluid assisted grain boundary diffusion enhanced by partial melting were the main deformation mechanisms associated with this foliation age estimate of this episode is 658 ma the second stage corresponds to the start of transpressional deformation and the nucleation and development of the sbsz during this stage pure shear dominates the deformation and is characterized by the development of conjugate dextral and sinistral shear zones and the emplacement of syntectonic granites this event dates to 658 600 ma based on the age of these intrusions the third stage was a second transpressional event at about 586 to 560 ma that was associated with the emplacement of porphyry dikes and granites that show evidence of flattening deformation in the sbsz took place during the late stages under regional low grade conditions as indicated by the metamorphic paragenesis in the supracrustals of the country rocks granitic mylonites show plastic deformation of quartz and brittle behavior of feldspar a transition from magmatic to solid state microstructures is also frequently observed in syntectonic granites mylonitic porphyries and quartz mylonites resulted from the deformation of alkaline porphyries and quartz veins emplaced in the shear zone quartz veins reflect the release of silica associated with the breakdown of feldspar to white mica during the evolution of the granitic mylonites to phyllonites which resulted in shear zone weakening quartz microstructures characteristic of the transition between regime 2 and regime 3 grain boundary migration and incipient recrystallization in feldspar indicate deformation under lower amphibolite to upper greenschist conditions 550 400 c on the other hand the mylonitic porphyries display evidence of feldspar recrystallization suggesting magmatic or high t solid state deformation during cooling of the dikes

fingerprints of late neoproterozoic ridge subduction in the pan african damara belt namibia subduction of mid ocean ridges is a common feature in recent convergent margins but is rarely documented in proterozoic to paleozoic orogenic belts here we describe evidence for ridge trench interaction in the deeply eroded late neoproterozoic damara orogenic belt central namibia the earliest interaction is indicated by primary intrusive contacts between amphibolite facies mid ocean ridge metabasalts and trench metasediments u pb zircon ages of 550 540 ma from syntectonic granites in the forearc indicate the timing of partial melting and mafic underplating of the prism in response to ridge subduction the thermal peak in the damara belt associated widespread granitic and alkalic plutonism and hydrothermal activity coincide with the waning stages of tectonism at 530 520 ma and are interpreted to indicate slab window widening and slab delamination we suggest that the proposed two stage thermal evolution of the damara belt comprising latest neoproterozoic ridge subduction and early cambrian slab delamination represents a fingerprint of ridge subduction in ancient orogens

small scale spatial variability of sperm and sei whales in relation to oceanographic and topographic features along the mid atlantic ridge abstract the 2004 mid atlantic ridge mar eco expedition on the r v g o sars provided the first opportunity to correlate oceanic distributions of cetaceans with synoptic acoustic adcp to 700 m depth multi beam echosounders measurements of high resolution three dimensional 3d potential habitat spatial scale

shear zone evolution and timing of deformation in the neoproterozoic transpressional dom feliciano belt uruguay abstract new structural microstructural and geochronological u pb la icp ms ar ar k ar rb sr data were obtained for the dom feliciano belt in uruguay the main phase of crustal shortening metamorphism and associated exhumation is recorded between 630 and 600 ma this stage is related to the collision of the rio de la plata and congo cratons at ca 630 ma which also involved crustal reworking of minor crustal blocks such as the nico perez terrane and voluminous post collisional magmatism subsequent orogen parallel sinistral shearing gave rise to further deformation up to ca 584 ma and resulted from the onset of the convergence of the kalahari craton and the rio de la plata congo cratons sinistral shear zones underwent progressive strain localization and retrograde conditions of deformation during crustal exhumation dextral ene striking shear zones were subsequently active at ca 550 ma coeval with further sinistral shearing along n to nne striking shear zones the tectonothermal evolution of the dom feliciano belt thus recorded the collision of the rio de la plata and congo cratons which comprised one of the first amalgamated nuclei of gondwana and the subsequent incorporation of the kalahari craton into western gondwana

electron microprobe dating of monazites from high grade gneisses and pegmatites of the kerala khondalite belt southern india monazites of five samples one leptynitic garnet biotite gneiss one khondalite one augen gneiss and two pegmatites from the central and northern part ponmudi unit of the kerala khondalite belt kkb in southern india were analyzed with the electron microprobe dating technique monazites in the augen gneiss and the pegmatites yield grain sizes between 200 800 m th abundances are rather low 10 wt and the distribution of th u and pb within single grains is fairly homogeneous contrasting to this monazites in the leptynitic gneiss and the khondalite are small 150 m they often display very complex th u pb patterns and contain high th concentrations up to 20 wt the statistical treatment of individual ages from the investigated samples revealed three populations of lower proterozoic 1 9 ga upper proterozoic 580 ma and ordovician age 470 ma as well as mid proterozoic ages between 0 8 1 7 ga which are not regarded to be of geological significance lower proterozoic ages are preserved in the cores of monazites from leptynitic gneisses and khondalites they fairly agree with sm nd model ages for similar rocks of the kkb and give a minimum age for first monazite growth or complete homogenization the prominent pan african population with mean values between 540 and 580 ma is present in the leptynitic gneiss the khondalite and the augen gneiss and in line with published isotope ages for the kkb the ordovician population finally marks the emplacement of granitic pegmatites subsequent to the pan african high grade metamorphic event there is an obvious discrepancy between khondalites and leptynitic gneisses on the one hand and augen gneisses on the other concerning the presence of lower proterozoic ages while these are abundant in the former often rimmed by upper proterozoic ages they are completely absent in the latter it appears unlikely that lower proterozoic ages were completely reset during a pan african event exclusively in the augen gneisses while they were preserved in leptynitic gneisses and khondalites it is further concluded that the augen gneisses are of magmatic origin and were derived from porphyritic granites thus the upper proterozoic age of 605 37 ma calculated for the investigated sample approximates the time of magma emplacement which slightly precedes the peak stage of pan african high grade metamorphism in the kkb and of monazite crystallization from the granitic melt a characteristic feature of the investigated monazites is the resetting of lower proterozoic and pan african ages to significantly younger values due to partial pb loss monazites not shielded by other minerals e g garnet suffered selective mobilization of pb along fractures or at their rims while th and u concentrations remained almost unchanged the results presented in this study indicate that this was mainly due to fluid rock interaction it is concluded that thermal diffusion of pb even at the suggested temperatures of 900 c only had minor influence on the th u pb composition in monazite and that the closure temperature for this system must be significantly higher than previously assumed 750 c

geochemical character and petrogenesis of pan african amspoort suite of the boundary igneous complex in the kaoko belt nw namibia abstract in the kaoko belt of northwestern namibia syn tectonic granitoids of the boundary igneous complex intruded a nnw sse trending suture between the neoproterozoic coastal terrane and archaean neoproterozoic congo craton derived foreland typical representatives of this syn collision igneous association are ca 550 ma old k feldspar phyric bt cam granites granodiorites of the amspoort suite with minor cpx gabbro and rare two pyroxene dolerite bodies the scarce opx cpx bt dolerites could come from a chur like mantle source 87sr 86sr550 0 7045 e550nd 0 the rest of the amspoort suite is interpreted as a product of ht anatexis of a heterogeneous lower crust built mainly by immature metapsammites rich in arc derived detritus with minor metabasite and intermediate metaigneous bodies the most likely source appears to be the anatectic coastal terrane gneisses yet partial melting of so far little constrained congo craton cover if formed by immature and youthful detritus unrelated to the basement cannot be discounted in any case the rather primitive sr nd isotopic compositions of the amspoort suite apart from dolerites e550nd 3 4 to 5 3 87sr 86sr550 0 7063 0 711 rule out any major role for the congo craton basement derived material which is in sharp contrast to other more southerly pan african granitoid complexes of the kaoko belt

alkali granites with pan african affinities from kerala s india in the kerala region of the south indian shield alkali granite plutons ranging in age from 550 to 740 ma intrude precambrian charnockites and migmatites all of them are spatially related to regional fault lineaments their geochemistry is similar to pan african post kinematic plutons from the afro arabian shield they are of granitic to adamellitic composition with high sio 2 k 2 o up to 12 and na 2 o and low cao feo and mgo their major element composition together with a low 87 sr 86 sr i 0 703 from one pluton denote a type affinities however local charnockites have such low rb and radiogenic 87 sr that the granites could have formed by crustal melting their overall trace element characteristics are typical of alkaline granites in general but the details show that most resemble more recent volcanic arc granites and one pluton is typical of a within plate setting the presence of co 2 rich fluid inclusions supports the notion that the enrichment of rare earth an

dextral pan african shear along the southwestern edge of the achankovil shear belt south india constraints on gondwana reconstructions the achankovil shear belt of the southern indian peninsular shield is prominent on landsat images it coincides with the boundary between charnockites of the madurai block to the north and khondalites of the trivandrum block to the south and with a major change in the aeromagnetic pattern that can be traced across southern india field investigations reveal a major shear zone along the southwestern edge of the achankovil shear belt the tenmala shear zone rocks in the tenmala shear zone include charnockite garnet biotite gneiss garnet sillimanite gneiss cordierite gneiss and granite kinematic indicators include stretched and asymmetric garnet feldspar and quartz porphyroclasts shear bands asymmetric folds extensional and contractional composite structures hook folds on rotated and deformed gash veins and lineations kinematic analysis of these features along 60 km of the shear zone indicates primarily dextral shear with a minor component of reverse shear textures as well as mineral assembl

archean proterozoic evolution of indian charnockites isotopic and geochemical evidence from granulites of the eastern ghats belt pyroxene granulites and charnockites are associated with metasedimentary khondalite garnet sillimanite gneiss and migmatites in the eastern ghats mobile belt various stages of in situ granulitization similar to those observed in southern india and sri lanka are present geochemically the granulite suite around visakhapatnam is bimodal with acid and mafic variants there is an increase in the concentration and fractionation of the rare earth elements ree from pyroxene granulites to charnockites and six sm nd models ages t dm range from 2 86 to 2 35 ga with 2 1 ga for a younger granite the charnockites a pyroxene granulite and granite define a pb pb isochron age of 1176 pm 201 ma u pb ages of zircons and monazites from a charnockite near phulbani orissa yield near concordant data with ages of 985 pm 5 ma and 965 pm 7 ma respectively the new isotopic data and the available ages from the northeastern part of the eastern ghats in orissa suggest a major charnockite formati

proterozoic events in the eastern ghats granulite belt india evidence from rb sr sm nd systematics and shrimp dating metamorphic and protolith ages of five rock types mafic granulite orthopyroxene granulite leptynite sillimanite granite and metapelite from rayagada in the north central part of the eastern ghats granulite belt eggb india were determined from rb sr and sm nd whole rock and mineral isochrons in combination with shrimp u pb zircon data most of the whole rock isochron ages in both sm nd and rb sr systems point to either 1450 or 1000 ma and the mineral isochron ages are 1000 800 and 550 ma shrimp u pb zircon ages of 940 ma were obtained from metapelite which are in close agreement with the sm nd and rb sr isochron ages from all these data four age clusters 1450 1000 800 and 550 ma have been noted the 1450 ma ages are interpreted to represent igneous protolith formation of mafic granulite and leptynite the 1000 ma age cluster is regarded as the intrusion ages of sillimanite granite and charnockite and associated granulite facies metamorphism two other age clusters 800 a

the physical oceanographic conditions along the mid atlantic ridge north of the azores in june july 2004 abstract during the mar eco expedition in june and july 2004 39 deep ctd stations were occupied with uneven spacing along the mid atlantic ridge mar between 41 n and 61 n the ctd stations coincided with the biological sampling stations and thus gave a detailed vertical description of the water properties at these locations many different water masses were identified however using the predominant water mass in the upper 500 m four different hydrographic regions were identified stations in the vicinity of the reykjanes ridge north of 57 n were dominated by modified north atlantic water mnaw stations south of 56 30 n and north of the sub polar front spf were dominated by sub arctic intermediate water saiw ctd data combined with along track continuous surface temperature and salinity measurements satellite sea level anomaly data and sea surface temperature sst and along track current data adcp data showed that the position of the spf was at about 52 n at the southern edge of the charlie gibbs fracture zone cgfz south of the spf a frontal region with both north atlantic central water nacw and saiw present was observed and south of about 50 n the predominant water mass in the upper 500 m was nacw a ship mounted rdi 75 khz adcp was operated during the cruise and yielded high quality data during the first leg the currents showed large variations over short distances and were strongly affected by topography the adcp data was be used to describe the mesoscale eddy field along the mar and to detect three jets in the north atlantic current nac data from a current meter mooring left for 10 months after the cruise at a seamount just to the south of cgfz showed the dominance of the tidal currents in the area typical spring tidal currents were about 0 15 m s 1 and the mean flow an order of magnitude smaller about 0 03 m s 1 a short term mooring on a seamount farther south showed tidal currents of 0 3 m s 1 binned 8 day 4 km sst satellite imagery were collected and processed for 2004 the sst data were analysed and showed characteristic frontal structures in the mar eco region

pan african rifting and orogenesis in southern to equatorial africa and eastern brazil abstract the upper proterozoic pan african belts of africa and brasiliano belts of south america are assumed to have evolved from an elaborate system of continental rifts which formed on the west gondwana continent 1100 1000 ma ago reviews of the damara belt kaoko belt gariep belt saldania belt west congolian belt lufilian arc and zambezi belt of southern and equatorial africa and the ribeira mantiqueira belt of brazil show the applicability of the continental rifting model to all these belts strict application of the rifting model describing the formation and further evolution of rift structures in the continental crust leads to the conclusion that the west congolian belt as traditionally described is incomplete and has to be supplemented by the mayumbian belt and a rift structure situated farther to the west the lufilian arc is assumed to have evolved from two rift structures separated from each other by the later domes region the golfe du katanga which branches off northeastwards from the lufilian arc is interpreted as an aulacogen shaba aulacogen the pan african brasiliano rift system is comparable to the mesozoic arctic north atlantic rift system with regard to size and distribution of rifts ocean floor spreading and opening of a proto south atlantic ocean most probably occurred along a main line of rifting situated between the west coast of africa and eastern brazil the wedge like ocean may have terminated at the sao francisco congo cratonic bridge between northeastern brazil and gabon cameroon there is no evidence of ocean floor spreading in the zambezi belt the lufilian arc and the traditional west congolian belt which are arranged along another major line of rifting closure of the pan african brasiliano rift system took place during two successive orogenic episodes the zambezi belt lufilian arc and west congolian belt underwent their main orogenic deformation with prevailing ene and wsw oriented tectonic transportation directions during the katangan episode at 900 750 ma the transcontinental sinistral mwembeshi shear zone adjusted opposite movement directions in the zambezi belt and lufilian arc and at its western prolongation concurrently supported opening of the khomas trough in the southern damara belt the proto south atlantic ocean still opening during the katangan episode was gradually closed during the damaran episode at 750 500 ma closure proceeded from north to south and was accompanied by northwest directed subduction underneath the brazilian plate and southeast directed tectonic transportation in the kaoko damara and gariep belts in the previously formed west congolian belt lufilian arc and zambezi belt a second deformation phase characterized by strike slip faulting and local emplacement of nappes occurred during this episode

depositional ages and provenance of the neoproterozoic damara supergroup northwest namibia implications for the angola congo and kalahari cratons connection abstract the damara orogen is composed of the damara kaoko and gariep belts developed during the neoproterozoic pan african orogeny the damara belt contains neoproterozoic siliciclastic and carbonate successions of the damara supergroup that record rift to proto ocean depositional phases during the rodinia supercontinent break up there are two conflicting interpretations of the geotectonic framework of the damara supergroup basin i as one major basin composed of the outjo and khomas basins related to rifting in the angola congo kalahari paleocontinent or ii as two independent passive margin basins one related to the angola congo and the other to the kalahari proto cratons detrital zircon provenance studies linked to field geology were used to solve this controversy u pb zircon age data were analyzed in order to characterize depositional ages and provenance of the sediments and evolution of the succession in the northern part of the outjo basin the basal nabis formation nosib group and the base of the chuos formation were deposited between ca 870 ma and 760 ma the upper chuos berg aukas gauss auros and lower brak river formations formed between ca 760 ma and 635 ma it also includes the time span recorded by the unconformity between the auros and lower brak river formations the ghaub upper brak river karibib and kuiseb formations were deposited between 663 ma and 590 ma the geochronological data indicate that the main source areas are related to i the angola congo craton ii rift related intrabasinal igneous rocks of the naauwpoort formation iii an intrabasinal basement structural high abbabis high and iv the coastal terrane of the kaoko belt the kalahari craton units apparently did not constitute a main source area for the studied succession this is possibly due to the position of the succession in the northern part of the outjo basin at the southern margin of the congo craton comparison of the obtained geochronological data with those from the literature shows that the abbabis high forms part of the kalahari proto craton and that angola congo and kalahari cratons were part of the same paleocontinent in rodinia times

trace element fractionation during high grade metamorphism and crustal melting constraints from ion microprobe data of metapelitic migmatitic and igneous garnets and implications for sm nd garnet chronology abstract rare earth element ree and other trace element y sr ti cr v na abundances in garnet from a garnet bearing metapelite a pelitic migmatite a syn tectonic granite and a post tectonic leucogranite were measured by secondary ion mass spectrometry sims in order to identify the effective variables on the trace element distribution between garnet and the host rock garnet from the garnet bearing metapelite the pelitic migmatite and the syn tectonic granite is zoned with respect to ree the cores are enriched by a factor of 2 3 relative to the rims for the garnets from the garnet bearing metapelite equilibrium distribution following a simple rayleigh fractionation is responsible for the decreasing concentrations in ree from core to rim garnet from the pelitic migmatite shows a more complex trace element pattern following distinct enrichment and depletion patterns for ti v cr and ree from core to rim these features suggest disequilibrium between garnet and the associated melt in which the enrichment of trace elements probably correspond to a period of open system behaviour in these rocks at a time when the garnet originally nucleated in the metamorphic environment was incorporated into the melt the garnet from the syn tectonic granite shows stepwise decreasing concentrations in ree from core to rim a ree rich core can be distinguished from a broad ree depleted rim notably from core to rim an inflection of the yb er and yb dy ratios is visible whereas the decrease of hree abundance in the core region of the garnet from the syn tectonic granite may arise from equilibrium partitioning during garnet growth the inflection can be interpreted as a result of partial melting garnet cores with high yb er and yb dy 1 nucleated in the metamorphic environment without the presence of a melt whereas the rims with lower yb er and yb dy

intrusion mechanisms in a turbidite sequence the voetspoor and doros plutons in nw namibia abstract two syntectonic plutons of cambrian age intruded neoproterozoic metaturbidites in namibia at the junction of the ns trending kaoko and ew trending damara belts sinistral transpression in the kaoko belt produced km scale upright d1 folds overprinted by minor d2 folds d3 is associated with n s shortening in the damara belt the plutons show two main pulses of intrusion hornblende syenite intruded late during d1 or during d2 and biotite granite during d3 each tectonic event produced a strain shadow defined by the shape of folds and the foliation trend around the plutons the internal igneous fabric and the arrangement of wall rock xenoliths that locally make up 50 of the intrusion mass suggest that the plutons have a disk or wedge shape a marginal shear zone indicates that one of the syenite intrusions descended during emplacement with respect to the wall rock emplacement is therefore inferred to be at least partly accommodated by descent of the intrusion floor the biotite granite intruded into mega strain shadows and tension gashes alongside and in the syenite during d3 the plutons show evidence of sinistral solid state rotation with respect to the wall rocks in response to d1 d2 transpression

distribution of sei whales balaenoptera borealis in the subarctic subtropical transition area of the western north pacific in relation to oceanic fronts the subarctic subtropical transition area of the western north pacific is an important summer feeding grounds of sei whales the oceanographic structure and circulation of this area are largely determined by strong oceanic fronts and associated geostrophic currents namely the polar front pf subarctic front saf and kuroshio extension front kef the relationship between the distribution of sei whales and oceanographic fronts was investigated using a generalized additive model gam and the cetacean sighting survey data and oceanographic observations in july from 2000 to 2007 were used in the analysis the number of individual sei whales was used as the response variable while the distances from the pf saf and kef to the whales were used as explanatory variables along with the longitude values sei whales were concentrated north and south of the saf and the areas from 250 to 300 km north and from 100 to 200 km south of the saf were estimated as high density areas of sei whales the entire inter frontal zone between the pf and saf featured an elevated concentration of sei whales and the area south of the pf and along the saf was identified as an important feeding ground of sei whales in july from 2000 to 2007

precambrian tectonics and crustal evolution in south india about 3 4 ga ago voluminous calc alkaline magmas represented by the granitoid gneisses of southern india were emplaced into a now poorly preserved non continental crust unstable ensialic basins initiated at about 3 0 ga were filled with volcanic and sedimentary rocks up to about 2 6 ga this basement cover sequence was deformed at the close of the archaean first by northward accretion and thickening of several crustal slabs formerly separated by prisms of stable shelf sediments structures produced by this episode were refolded and dislocated by large n s strike slip shear belts to impart intense steep planar fabrics to large volumes of the crust fluids rich in co possibly derived from sedimentary material driven beneath the crustal slabs by the thickening mechanism purged the deep crust of h o to form granulites upward migration of hot h o rich fluids encouraged partial melting at intermediate crustal levels these late archaean tectonic and thermal events began soon after the close of basin f

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

smnd isotopic systematics of enderby land granulites and evidence for the redistribution of sm and nd during metamorphism measurements of143nd 144nd and147sm 144nd are reported for whole rocks and mineral separates from granulites of the napier complex at fyfe hills charnockites leuconorites and gabbros yield a whole rock smnd isochron age of3060 160m y and an initial143nd 144nd ratio of0 50776 10 nd 3060m y 2 0 1 8 the negative nd value and the presence of geologically induced dispersion in the data suggest that the isochron age does not represent the time of primary crystallization of the complex but instead indicates a time of later redistribution of sm and nd and partial re equilibration of143nd 144nd ratios this probably occurred during the upper granulite facies metamorphism which has also been dated at 3100m y by rbsr and upb zircon studies 1 coexisting clinopyroxene apatite and total rock fractions in two adjacent samples define an approximately linear array corresponding to an age of 2300 300 m y this array indicates that redistribution of sm and nd and re equilibration of143nd 144nd ratios occurred on an intermineral scale during the upper amphibolite to lower granulite facies metamorphism at 2450m y due to the resetting of the smnd system on both whole rock and mineral scales the primary crystallization age of the igneous protolith is not well constrained by the present data although it is clearly3100m y if it is assumed that the complex was derived initially from a depleted mantle reservoir nd t 2 evolution of the negative nd value of 2 0 with the observed sm nd ratios requires a prehistory of 380m y this implies a primary age of 3480m y however substantially older primary ages can be inferred if the source reservoirs had nd t 2 and or substantial reductions in the sm nd ratio occurred in whole rocks during the granulite facies metamorphism at 3100 m y such an inferred reduction in the sm nd ratio may have been the result of preferential loss of sm relative to nd or introduction of a low sm nd fluid with nd 0 during granulite facies metamorphism

the metamorphic architecture of a transpressional orogen the kaoko belt namibia s 64 61 goscombe b hand m 2001b metamorphic model for the inland branch of the damara orogen a paired metamorphic mountain belt pmmb geological society of australia specialist group in tectonics and structural geology conference ulverstone

juxtaposition of india and madagascar a perspective abstract proterozoic terrains in south india and madagascar provide important clues in understanding the gondwanaland tectonics especially the assembly of this mega continent during the pan african period the archaean terrains in both madagascar and india are characterized by n s trending greenstone belts occurring within gneissose granitic rocks in the northern part extensive development of k rich granitic rocks of ca 2 5 ga is also characteristic in both areas such a broad age zonation of younger dharwar ca 2 6 3 0 ga in the north and the older sargur ca 3 0 3 4 ga in the south as in south india remains to be identified in future studies from madagascar the occurrence of greenschist facies rocks in the northeastern part and higher grade rocks in most of other parts in the north central terrain of madagascar is comparable with the general tendency of increasing metamorphic grade from northwestern to southern areas ranging from greenschist to granulite facies in south india the proterozoic crystalline rocks in both continents show pronounced lithological similarity with the wide occurrence of graphite bearing khondalite in association with charnockitic rocks while the archaean proterozoic boundary is well defined in southern india by the palghat cauvery or the kkpt shear zones as recently identified this boundary is ill defined in madagascar due to extensive pan african overprinting as well as the development of the proterozoic cover sequence the itremo group there is also a possible general correlation between the mesoproterozoic cover sequences in madagascar and india such as between the itremo group of west central madagascar and the kaladgi and cuddapah sequences of south india the pan african granulite facies metamorphism of ca 0 5 ga extensively developed in both india and madagascar is generally comparable in intensity and extent p t conditions and p t t paths also appear comparable with the general range of ca 700 1000 c and 6 9 kb and near isothermal decompressional paths a type granite plutons and alkaline rocks including anorthosites and mafic plutonic rocks of ca 500 800 ma develop in both terrains provide strong basis for the correlation of both terrains and define a pan african igneous province within east gondwanaland major shear zones in both continents are expected to play a critical role in the correlation albeit are still poorly constrained detailed elucidation of the tectonic history of the shear zones and the timing of various events along the shear zones would provide important constraints on the correlation of the two continental fragments

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

the position of the amazonian craton in supercontinents abstract this paper examines the extensive regions of proterozoic accretionary belts that either formed most of the amazonian craton or are marginal to its southeastern border their overall geodynamic significance is considered taking into account the paleogeographic reconstruction of columbia rodinia and gondwana amazonia would be part of columbia together with laurentia north china and baltica forming a continuous continental landmass linked by the paleo to mesoproterozoic mobile belts that constitute large portions of it the rodinia supercontinent was formed in the mesoproterozoic by the agglutination of the existing cratonic fragments such as laurentia and amazonia during contemporary continental collisions worldwide the available paleomagnetic data suggest that laurentia and amazonia remained attached until at least 600 ma since all other cratonic units surrounding laurentia have already rifted away by that time the separation between amazonia and laurentia marks the final break up of rodinia with the opening of the lapetus ocean

african linkage of precambrian sri lanka new age and isotopic data show that the high grade basement rocks of sri lanka were not linked to the archaean granulite domain of southern india but experienced their main structural and metamorphic development during the pan african event some 950 to 550 ma ago this occurred when west gondwana and east gondwana collided to form one of the longest collisional structures in the supercontinent the mozambique belt that extends from mozambique to ethiopia and sudan a major tectonic boundary interpreted as a thrust zone divides the highland southwestern complex in the central part of sri lanka from the vijayan complex in the e and se the former is interpreted to represent the remnant of a once extensive passive margin extending west in a gondwana reconstruction via madagasgar to tanzania and mozambique the vijayan complex may have been part of a separate continental margin plutonic assemblage and its collision with the highland southwestern complex marks the final amalgamation of east and west gondwana into a supercontinent some 550 ma ago the sri lankan granulites cannot be correlated with the distinctly older granulites of the eastern ghats belt of india and this suggests that sri lanka was situated close to the se coast of madagascar in a gondwana reconstruction

neoproterozoic igneous complex emplaced along major tectonic boundary in the kaoko belt nw namibia ion probe and la icp ms dating of magmatic and metamorphic zircons granitoid intrusions of the boundary igneous complex separate segments with different ages of high grade metamorphism in the kaoko belt nw namibia two granitoids of this complex were dated at 575 10 ma secondary ionization mass spectrometry sims or 571 9 ma laser ablation inductively coupled plasma mass spectrometry la icp ms and 562 11 ma sims or 572 4 ma la icp ms respectively the age of granulite facies metamorphism in the eastern part of the western kaoko zone was established at 549 5 ma sims by analysing metamorphic overgrowths of older c 1850 1000 ma zircons from melt segregations in amphibolites the coastal part of the western kaoko zone consists of horizons of migmatitic metasedimentary rocks that are intercalated with fine grained orthogneisses and amphibolites resembling metamorphosed sequences of bimodal volcanic rocks zircons from felsic members of two bimodal suites have sims ages of 805 4 ma and 810 840 ma respectively that are interpreted as dating their respective igneous protoliths melt segregations in the mafic member of the lower bimodal suite contain two populations of zircon dated at 650 5 ma sims or 645 5 ma la icp ms and 629 6 ma sims or 630 5 ma la icp ms respectively the later age is indistinguishable from the age of 630 4 ma sims or 625 10 ma la icp ms obtained from melt patches present in overlying metagreywackes the available age data suggest that the boundary igneous complex masks the suture between the coastal terrane and the rest of the kaoko belt ages of granitoid intrusions in this igneous complex are indicative of magmatic activity between 580 and 550 ma

closing the clymene ocean and bending a brasiliano belt evidence for the cambrian formation of gondwana southeast amazon craton we report new paleomagnetic and geochronological data from ediacaran rift drift carbonates in the paraguai belt at the southern end of the suture zone between the amazon craton and the sao francisco and rio de plata cratons south america early thrusting resulted in remagnetization ca 528 36 ma or later the mean age is established by 40 ar 39 ar encapsulation dating of mixed authigenic and detrital illite from remagnetized carbonates from the unmetamorphosed fold thrust belt this remagnetization overlaps with a 525 ma gondwana reference pole metamorphic illite from the slate belt yields 40 ar 39 ar ages of 496 484 ma the timing of peak regional metamorphism oroclinal bending of the paraguai belt was caused by a 90 clockwise rotation of the east west limb after ca 528 ma probably refl ecting the irregular margin of the southeast amazon craton the age of the paraguai belt overlaps with that of the pampean orogeny farther south along the western margin of the rio de plata craton suggesting a coeval closure for the clymene ocean separating the amazon craton from the sao francisco and rio de plata cratons

variable late neogene exhumation of the central european alps low temperature thermochronology from the aar massif switzerland and the lepontine dome italy 1 several recent studies proposed an important increase in exhumation rate in the western european alps since circa 5 4 ma in order to assess potential spatial differences in exhumation histories we present new apatite fission track aft and apatite u th he ahe ages from the central aar massif guttannen area switzerland and the western lepontine dome formazza area italy internal u th zoning in apatites explains alpha ejection corrected ahe ages that are older than the corresponding aft ages in this study a qualitative interpretation of aft and ahe age elevation relationships suggests a two phase 9 7 and 5 3 ma exhumation scenario affecting the central alps with a stronger expression of the pliocene signal in the formazza area however a quantitative evaluation of exhumation scenarios using the 3 d heat equation solver pecube highlights the existence of several other likely scenarios casting doubt on the validity of a qualitative interpretation of the age elevation relationships in formazza scenarios suggested by quantitative modeling include continuous denudation at a rate of 750 m ma and a one step exhumation rate change from 300 to 1000 m ma at 5 ma in guttannen they include continuous denudation at a rate of 400 m ma with valley deepening and two periods of higher exhumation rate increasing from 300 to 700 m ma repeatedly at 9 7 and at 5 3 ma contingent upon further flexural isostatic modeling the magnitude of exhumation recorded in the axial region of the alps since circa 5 ma does not appear sufficient to solely explain the denudation recorded in the north alpine foreland basin

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

age and geotectonic setting of late neoproterozoic juvenile mafic gneisses and associated paragneisses from the ribeira belt se brazil based on geochemistry and sm nd data implications on gondwana assembly abstract mafic gneisses and associated paragneisses from the cabo frio tectonic domain in the southeastern part of the ribeira belt along the coast of rio de janeiro state in southeast brazil were subjected to a geochemical and sm nd isotope study four lithotypes are distinguished aluminous paragneisses mainly sillimanite kyanite garnet biotite gneiss calcsilicate lenses quartzo feldspathic metasedimentary gneisses and mafic ultramafic lenses the whole rock major and trace including rare earth element distributions in the mafic ultramafic intercalations indicate that derivation from subalkaline basalt gabbro of tholeiitic affinity with e morb signature from a non subduction environment these mafic rocks have positive end t and tdm of 1 1 ga the metasedimentary rocks have negative end t and tdm of 1 7 ga a sm nd whole rock isochron of mafic rocks yielded an age of 604 38 ma for the crystallization this matches with the age of some detrital zircon grains from the paragneisses the depositional basin named buzios palmital was active at least until 620 ma age of the youngest detrital zircon and was subsequently deformed and metamorphosed at ca 525 ma age of metamorphic zircons during the buzios orogeny it is interpreted as a back arc basin with relation to the 630 ma magmatic arc of the oriental terrane in the ribeira belt to the nw however after 600 ma the buzios palmital basin changed to an active margin setting because the arc collided with the continental margin and the subduction shifted to the back arc environment by 610 ma most of the brasiliano belts registered collisional events related to multiple convergent blocks the stress fields and paleocontinent shapes would have allowed the occurrence of extensional areas with not only sedimentary deposition but also ocean floor spreading its remnants are preserved in this brazilian coastal region as an ancient suture reworked intensively during the mesozoic rifting events the reconnaissance of late neoproterozoic basins in the brasiliano pan african belts is of major importance to partially unravel the final amalgamation events of sw gondwana considering that the buzios palmital basin rock units are mostly covered by the marginal atlantic basins it is possible that other evidence could be preserved in the coastal regions of sw africa and se south america

variation in metamorphic style along the northern margin of the damara orogen namibia the northern margin of the inland branch of the pan african damara orogen in namibia shows dramatic along strike variation in metamorphic character during convergence between the congo and kalahari cratons m3 metamorphic cycle low p contact metamorphism with anticlockwise p t paths dominates in the western domains ugab zone and western northern zone and high p barrovian metamorphism with a clockwise p t path is documented from the easternmost domain eastern northern zone the sequence of m3 mineral growth in contact aureoles shows early growth of cordierite porphyroblasts that were pseudomorphed to biotite chlorite muscovite at the same time as an andalusite biotite muscovite transposed foliation was developed in the matrix the peak t metamorphic assemblages and fabrics were overprinted by crenulations and retrograde chlorite muscovite the kfmash p t pseudosection for metapelites in the ugab zone and western northern zone contact aureoles indicates tight anticlockwise p t loops through peak metamorphic conditions of 540 570 c and 2 5 3 2 kbar these semi quantitative p t loops are consistent with average p t calculations using thermocalc which give a pooled mean of 556 26 c and 3 2 0 6 kbar indicating a high average thermal gradient of 50 c km in contrast the eastern northern zone experienced deep burial high p moderate t barrovian m3 metamorphism with an average thermal gradient of 21 c km and peak metamorphic conditions of c 635 c and 8 7 kbar the calculated p t pseudosection and garnet compositional isopleths in kfmash appropriate for the metapelite sample from this region document a clockwisep t path early plagioclase kyanite biotite parageneses evolved by plagioclase consumption and the growth of garnet to increasing xfe xmg and xca and decreasing xmn compositions indicating steep burial with heating the developed kyanite garnet biotite peak metamorphic parageneses were followed by the resorption of garnet and formation of plagioclase moats indicating decompression which was followed by retrogressive cooling and chlorite muscovite growth the clockwise p t loop is consistent with the foreland vergent fold thrust belt geometry in this part of the northern margin earlier formed 580 570ma pervasive matrix foliations m2 were overprinted by contact metamorphic parageneses m3 in the aureoles of 530 3ma granites in the ugab zone and 553 514ma granites in the western northern zone available geochronological data suggest that convergence between the congo and kalahari cratons was essentially coeval in all parts of the northern margin with similar ages of 535 530ma for the main phase of deformation in the eastern northern zone and northern platform and 538 505ma highgrade metamorphism of the central zone immediately to the south consequently nne ssw directed convergent deformation and associated m3 metamorphism of contrasting styles are interpreted to be broadly contemporaneous along the length of the northern margin of the inland branch in the west heat transfer was dominated by conduction and externally driven by granites whereas in the east heat transfer was dominated by advection and internally driven radiogenic heat production the ultimate cause was along orogen variation in crustal architecture including thickness of the passive margin lithosphere and thickness of the overlying sedimentary succession

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

tracking a northern fulmar from a scottish nesting site to the charlie gibbs fracture zone evidence of linkage between coastal breeding seabirds and mid atlantic ridge feeding sites the seas above mid ocean ridges are biodiversity hotspots in an otherwise largely oligotrophic environment but the nature and extent of linkage between these offshore regimes and coastal ecosystems remains uncertain using a combination of gps and geolocation tracking data we show that a male fulmar breeding on the scottish coast foraged over areas of persistent thermal fronts along the charlie gibbs fracture zone cgfz of the mid atlantic ridge during the incubation period the bird travelled over 6200 km in 14 9 days first passage time analysis identified seven areas of restricted search four on the shelf and three in the vicinity of the mid atlantic ridge previous studies of incubation foraging trip durations at this site suggest that a trip of this duration is unusual and further work is required to assess the extent to which different individuals use these offshore resources nevertheless these data highlight the potential importance of high sea areas beyond the limits of national jurisdiction when considering the management and conservation of seabirds breeding in nw europe and raises the potential for even greater linkage between the cgfz and seabirds breeding colonies in other regions

the petrogenesis of some migmatites and granites central damara orogen namibia evidence for disequilibrium melting wall rock contamination and crystal fractionation

dextral pan african shear along the southwestern edge of the achankovil shear belt south india constraints on gondwana reconstructions a discussion achankovil shear zone acsz figure 1a intering charnockites garnet biotite gneiss biotite gneiss calc gneiss and granite as revealed in the preted as a major nw trending strike slip shear zone of probable proterozoic age drury et al 1984 lithological mapping carried out as part of this study figure 2 a preliminary structural assesshas prominently figured in gondwana correlative studies e g windley et al 1994 although it is ment reveals the presence of a distinctive assemblage of asymmetric structures in the gneissic exclear that there is a major lithologic change across acsz figure 1b detailed studies indicating the posures indicating a consistent sinistral sense of shear gneissosity forming the secondary compoprobable sense of shear are lacking across this 10 20 km wide and 100 km long shear zone resitional banding strikes wnw ese to nw se dipping moderately toward sw with relatively rare cently sacks et al 1997 reported field observations bearing on the probable sense of shear of the ne dip and is conformable in all the rock types of the area in conformity with the general nw se acsz after combining field criteria and kinematic analysis of asymmetric features along the sw trend of the acsz structurally the terrain has undergone four phases of deformation summarized in edge of the acsz known as the tenmala shear zone they concluded that the acsz indicated prirajesh and santosh 1997 open to isoclinal intrafolial folds encountered in marily dextral shear with a minor component of reverse shear the authors illustrate their discusthe study area are invariably asymmetric fold limbs are zones of transposed gneissosity that consion with examples of kinematic indicators including shear bands asymmetric folds stretched and tain indicators of a probable sinistral sense of shear and well developed mineral elongation lineation asymmetric garnet feldspar and quartz porphyroclasts hook folds and lineations thus they appardefined by quartz ribbons trails of xenoblastic garnet grains and the alignment of the long axes of kently question the correlation of the dextral achankovil shear zone rather tenmala shear zone with feldspar augen pinch and swell structures and boudins commonly show rotation with respect to the sinistral bongolava ranotsara shear zone brsz in the india madagascar gondwana enfoliation tight s asymmetric f2 folds and synkinematic pegmatite veins are seen in the western semble we focus the present discussion on the probable part of the study area the development of a refold structure by superposed buckling is illustrated in sense of shear of acsz in the light of new data our study area falls around punalur and its adjoinfigure 3a there are smaller asymmetric sand zfolds congruent with the larger f1 with one limb ing areas within the quilon district southern kerala south india and is located between longitudes striking e w while the other strikes n50 e showing negative extension during the development of 76 55 and 76 59 e and latitudes 9 1 and 9 3 n figure 2 represented in the survey of india topo f2 on the traces of preserved axial planar cleavage the simple shear is dextral on the left limb and sisheet no 58c 16 outcrops within the study area expose a strongly deformed suite of rocks comprisnistral on the right limb asymmetrically folded veins figure 3b the bulk asymmetry of which represents a sinistral sense of shear have been found 1 manuscript received may 14 1997 in a few locations elongate quartz lenticles and 2 centre for earth science studies pb 7250 trivandrum 695 031 india garnet and biotite aggregates define penetrative lin

closure of the clymene ocean and formation of west gondwana in the cambrian evidence from the sierras australes of the southernmost rio de la plata craton argentina abstract the formation of gondwana took place across a series of brasiliano pan african suture zones that record late neoproterozoic to earliest paleozoic collisions between precambrian cratons in south america an internal suture zone marks the disappearance of the clymene ocean that separated the amazon craton from the sao francisco and rio de la plata cratons new geochronological data from the southern end of this massive collision zone in the sierras australes of central eastern argentina document paleoproterozoic crust and suggest an ediacaran age for the oldest sedimentary rocks these two observations extend the known limit of the rio de la plata craton at least 200 km sw of previous estimates new data also confirm the occurrence of late ediacaran to late cambrian magmatism in the sierras australes the age of these hypabyssal to volcanic rocks corresponds to igneous events in the pampean belt along the western margin of the rio de la plata craton although the shallow level magma emplacement in the sierra da ventana study area contrasts with the deeply exhumed rocks of the pampean orogeny type locality these new age data are compared with a broad compilation of geochronological age clymene collision belts to the north the paraguai and araguaia belts the close overlap of the timing of orogenesis indicates the age of clymene ocean closure in its northern reaches in the south the pampean belt was unconfined allowing continued tectonic activity and crustal accretion throughout the paleozoic

distribution identity and possible processes sustaining meso and bathypelagic scattering layers on the northern mid atlantic ridge abstract as an element of comprehensive exploratory studies of the poorly known pelagic ecosystem associated with the mid atlantic ridge acoustic data collected by r v g o sars during the mar eco expedition in june july 2004 were used to describe vertical and geographical distributions of meso and bathypelagic scattering layers scattering layers were observed in the entire study area and at all depths to 3000 m with a split beam echosounder using the 18 khz drop keel mounted transducer the spatial variation in density of surface to 1500 m area backscattering strength and that of individually identifiable vertical layers is described in relation to bathymetry and large scale hydrographical features such as the sub polar front the inhabitants of scattering layers were identified by exploratory midwater trawling a major pattern was the elevated backscattering associated with waters around the charlie gibbs fracture zone cgfz and the associated sub polar front at and south of 52 n in this section of the mid atlantic ridge the interaction between topographical features circulation and primary production appears to create favorable conditions for many taxa across all trophic levels by comparison the density was lower both to the south of the cgfz towards the azores and in particular northwards towards iceland along the north south gradient a peak in near surface algal biomass was also found near the cgfz with lower values further north and south we hypothesize that fish productivity is bottom up regulated i e limited by primary production within the ridge associated system supplemented by advection of allochthonous material especially in the south

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