a numerical study of the mediterranean sea circulation abstract a primitive equation ocean model that makes use of a curvilinear orthogonal grid and a sigma coordinate system was used to simulate the mediterranean sea the model was forced with monthly climatological values of wind stress heat and salinity flux with the help of the curvilinear horizontal grid the larger scales of the entire mediterranean sea are modeled and the topography around the narrow and shallow straits of gibraltar is also reasonably well represented the resulting model inflow and outflow seems to mimic the real mediterranean often in considerable detail levantine intermediate water is formed in the levantine basin and exits through the strait of sicily and the strait of gibraltar deep water formation processes are clearly represented by the model the model results indicate that in the western mediterranean the wind stress is very important in establishing the summer northward shift of the atlantic inflow lateral boundary runoff surface salinity and heat fluxes are necessar

reefs of the deep the biology and geology of cold water coral ecosystems coral reefs are generally associated with shallow tropical seas however recent deep ocean exploration using advanced acoustics and submersibles has revealed unexpectedly widespread and diverse coral ecosystems in deep waters on continental shelves slopes seamounts and ridge systems around the world advances reviewed here include the use of corals as paleoclimatic archives and their biogeological functioning biodiversity and biogeography threats to these fragile long lived and rich ecosystems are mounting the impacts of deep water trawling are already widespread and effects of ocean acidification are potentially devastating

baddeleyite geochronology and geochemistry of mafic cobbles from the dwyka diamictite new insights into the sub kalahari basement south africa u pb baddeleyite geochronology and geochemistry of mafic cobbles from the similar to 300 ma dwyka diamictite on the western boundary of the kalahari basin provide new insights into the sub kalahari basement palaeo ice flow directions indicate that the origin of the dwyka diamictite deposits on the border between south africa and namibia is from beneath the kalahari sands to the east or north east this work shows that the mafic cobbles from three localities near rietfontein come from intrusions related to the umkondo large igneous province lip located in the kheis or rehoboth provinces the lack of banded iron formation bif cobbles in these glacial diamictites indicates that their source region lies west or north of the bif dominated griqualand west basin of the kaapvaal craton the four dated mafic cobbles give u pb ages of 1111 1123 ma coeval with dolerites of the umkondo lip the cobbles experienced greenschist fades metamorphism before the dwyka glaciation but are unfoliated the geochemistry of the cobbles indicates that they are genetically similar and have a within plate setting they show many similarities to the umkondo lip dolerites and the coeval rouxville formation an upper unit of the koras group the most likely sources of the cobbles are the mafic intrusives and lavas along the kalahari line southern equivalents of the similar to 1 11 ga tshane and xade complexes or intrusive and extrusive mafic units related to the umkondo lip now concealed beneath the kalahari sands c 2011 elsevier by all rights reserved

chronostratigraphy of uplifted quaternary hemipelagic deposits from the dodecanese island of rhodes greece an integrated magneto biostratigraphic study based on calcareous nannofossils and foraminifers together with the radiometric dating of a volcaniclastic layer found in several outcrops was carried out on the hemipelagic deposits of the lindos bay formation lbf at six localities on the island of rhodes greece our highly refined chronostratigraphic framework indicates that the lower and upper lithostratigraphic boundaries of the lbf are diachronous associated with the 40ar 39ar age determination of 1 85 0 08 ma for the volcaniclastic layer our data show that among the investigated outcrops the lindos bay type locality section provides the longest record 1 1 ma of the lbf hemipelagic deposition occurred continuously from the late gelasian 1 9 ma to the late calabrian 0 8 ma i e from chrons c2n olduvai to c1r 1r matuyama and from nannofossil zones cnpl7 to cnpl10 this long record together with the hemipelagic nature of the deposits make the lindos bay type locality section a unique element in the eastern mediterranean region allowing future comparisons with other early quaternary deep sea sections available in the central and western mediterranean regions

paleoenvironmental reconstruction of challenger mound initiation in the porcupine seabight ne atlantic the understanding of the paleoenvironment during initiation and early development of deep cold water coral carbonate mounds in the ne atlantic is currently a focus of international research the integrated ocean drilling program iodp expedition 307 drilled the 155 m high challenger mound in the porcupine seabight sw off ireland in order to investigate for the first time sediments from the base of a giant carbonate mound in this study we focus in high resolution on 12 m of sediments from site 1317 encompassing the mound base the mound initiation and start up phase coincide with the intensification of the northern hemisphere glaciation inhg at around 2 7 ma further carbonate mound development seems to be strongly dependent on rapid changes in paleoceanographic and climatic conditions at the pliocene pleistocene boundary especially characterized and caused by the interaction of intermediate water masses the mediterranean outflow water mow the eastern north atlantic water enaw and the influence of southern component water scw this study is based on well established proxies such as 18o and 13c of planktonic globigerina bulloides and benthic foraminifera fontbotia wuellerstorfi discanomalina coronata lobatula lobatula lobatula antarctica and planulina ariminensis as well as grain size parameters to identify the paleoenvironmental and paleoecological setting favourable for the initial coral colonization on the mound stable oxygen and carbon isotope records of benthic foraminiferal species indicate that l lobatula provides a reliable isotopic signature for paleoenvironmental reconstructions in particular 18o values of l lobatula indicate that initial mound growth started in a glacial mode with moderate excursions in 18o values carbon isotope values of d coronata are significantly offset compared to other epibenthic species this offset may be related to vital effects bottom water temperatures calculated using standard equations based on 18o of foraminiferal tests range between 7 and 11 c consistent with the known temperature range conducive for cold water coral growth and development bottom currents transporting intermediate water masses of southern origin mediterranean and bay of biscay enhanced at 2 6 ma supporting first coral settlements with the inhg the benthic 13c and the sortable silt records indicate that the early pleistocene hydrodynamic regime was characterized by weaker current intensities associated with vertical movements of mow or its replacement by scw at intermediate depth after these sluggish phases enhanced mow flow dominated again and led to stronger current intensities and most probably sediment erosion on challenger mound erosion in combination with early diagenetic oxidation processes overprinted the sediment layers as indicated by dissolved coral skeletons the increase in ca content and sediment density minimum 13cplanktonic values as well as the occurrence of gypsum and pyrite implying a careful evaluation of original and overprinted geochemical signals we conclude that the challenger mound development was already influenced by short term variability of water masses from southern origin and possible erosional events comparable to the late pleistocene setting

mass wasting and uplift on crete and karpathos during the early pliocene related to initiation of south aegean left lateral strike slip tectonics reconstruction of the vertical motion history of crete and karpathos southeastern aegean region greece from the messinian to recent revealed a previously poorly documented late messinian phase of strong subsidence with rates of 50 100 cm k y followed by stasis during the fi rst 250 k y of the pliocene and then by uplift of 500 700 m during the late early to early middle pliocene uplift continued up to recent albeit at a slower pace and at different rates in different areas the lower pliocene in crete and karpathos is characterized by widespread occurrences of mass wasting deposits which were emplaced over a period of time spanning the fi rst 1 35 m y of the pliocene the origin of these masswasting deposits has long been enigmatic but is here related to uplift which started in crete as early as ca 5 ma it is suggested that the beginning uplift following strong subsidence of various fault blocks until late in the messinian is related to the onset of south aegean strike slip faulting we postulate that small scale tilting of fault blocks by transtensional strike slip faulting and increased seismic activity generated slope failures and subsequent sliding of poorly cemented lower pliocene and uppermost messinian lago mare sediments overlying the terminal miocene erosional unconformity the absence of mass wasting deposits after 3 98 ma while uplift continued is most likely the result of progressive compaction and cementation of the increasingly deeper buried lago mare and lower pliocene sediments thereby preventing slope failure to a depth of the terminal miocene unconformity hiatuses in some places in crete and on karpathos however indicate that slope failures continued to occur although on a smaller scale and less frequent than before connecting the change from subsidence to uplift in the earliest pliocene with the onset of left lateral strike slip tectonics in the southeastern aegean arc would make this major strike slip system much older by 2 m y than the generally accepted age of middle to late pliocene a recently postulated scenario of subduction transform edge propagator step faulting to explain the south aegean strike slip system predicts rates distribution and amount of uplift as rebound to southwestward retreat of the subducted slab along a transform fault zone that is in line with our fi ndings on crete and karpathos and explains the absence of compressional structures associated with the uplift as well as the ongoing southwestward motion of crete

crustal structure and dynamics of botswana we present the first nationwide crustal thickness and vp vs maps for botswana based on the analysis of new p wave receiver functions of nars botswana network integrated with previous receiver function results in botswana using h k analysis we found crustal thickness values ranging from 34 km for the okavango rift zone to 49 km at the border between the magondi belt and the zimbabwe craton for stations with significant sediment cover a sediment correction was applied based on sequential h k stacking we observed distinct differences for the kaapvaal craton the eastern part has a high vp vs ratio typical of a predominantly mafic composition suggesting lateral extension of the bushveld mafic complex on the other hand the western part with a vp vs ratio of 1 67 is felsic probably as a result of delamination caused by proterozoic rifting processes further to the west of the kaapvaal craton we found a crustal thickness of 42 km and a vp vs ratio of 1 76 for the nosop basin these values are similar to other cratonic regions in botswana suggesting the presence of a buried craton as proposed by previous studies we confirm a relatively thin crust 34 39 km compared to the rest of botswana and high vp vs ratio 1 84 underneath the okavango rift zone found by previous receiver function studies notably we also found a relatively thin crust 37 km and high vp vs ratio 1 84 in central botswana underneath the passarge basin

changes in vegetation and marine environments in the eastern mediterranean rhodes greece during the early and middle pleistocene pollen records marine faunal associations and stable isotope compositions of sediments from rhodes greece have been determined to track environmental changes in the eastern mediterranean during the early and middle pleistocene a detailed chronostratigraphic curve based on magnetostratigraphic data was obtained by correlating pollen spectra with the mediterranean oxygen isotopic curve of ocean drilling program site 975 five sedimentary sequences that correspond to marine oxygen isotope stage mis 31 27 and to mis 20 17 have been investigated in the confined tsampika microbasin high amplitude pinus variations confirm glacio eustatic changes deduced from changes in marine faunal associations and sedimentary depositional environments reflecting climatic cycles identified in the marine carbonate oxygen isotope record eight vegetation successions characterized by the dominance first of mesothermic elements then of mid and high altitude elements with pinus and ending with maxima in herb and steppe elements have been documented most of them were probably driven by changes in insolation occurring in high northern latitudes obliquity impact during the late early pleistocene and early middle pleistocene

u pb zircon geochronology and tempo of the end permian mass extinction the mass extinction at the end of the permian was the most profound in the history of life fundamental to understanding its cause is determining the tempo and duration of the extinction uranium lead zircon data from late permian and early triassic rocks from south china place the permian triassic boundary at 251 4 0 3 million years ago biostratigraphic controls from strata intercalated with ash beds below the boundary indicate that the changhsingian pulse of the end permian extinction corresponding to the disappearance of about 85 percent of marine species lasted less than 1 million years at meishan a negative excursion in 13 c at the boundary had a duration of 165 000 years or less suggesting a catastrophic addition of light carbon

changes in bryozoan assemblages and bathymetric variations examples from the messinian of northwest algeria abstract studies of recent bryozoan faunas have shown that there is a correlation between colonial growth forms and various physical factors of the environment especially bathymetry each given assemblage of zoarial forms can thus be related to a certain depth interval in marine neogene deposits numerous extant bryozoan species occur and some of them have more or less restricted depth ranges they are used here to complement the indications given by the analysis of growth forms in the messinian uppermost miocene of northwestern algeria a general feature is the occurrence of well developed diatomaceous deposits interbedded with marl beds where benthic invertebrate remains are often abundant in the five sections under study bryozoans are represented by 87 species belonging to nine different colonial growth forms four different assemblages of these zoarial forms characterised by presence absence and number of fragments are distinguished corresponding to four depth intervals the first and deepest assemblage upper bathyal to lower circalittoral occurs in the bottom part of the sections where colonies of a conescharelliniform species strongly predominate the second assemblage lower circalittoral is represented by several species belonging to various zoarial forms conescharelliniform cellariiform vinculariiform adeoniform and reteporiform the third assemblage upper circalittoral is characterised by the absence of conescharelliniforms but membraniporiforms and celleporiforms are abundantly represented together with numerous erect colony fragments in the fourth assemblage infralittoral membraniporiforms and celleporiforms predominate although a number of erect zoarial forms are sometimes associated in various proportions the presence of several extant bryozoan species associated with each of these four assemblages is used as a confirmation for the suggested depth intervals a gradual shallowing is thus indicated by the study of the bryozoan assemblages through the five messinian sections the discussion shows that modern data on zoarial forms and species bathymetric distributions are clearly applicable to cenozoic faunas but that caution must be exercised when using results obtained from different oceanographic regions

geochemical and physical constraints for the occurrence of living cold water corals cold water coral communities cover a wide range of possible habitats in terms of latitude ocean basins and depth with ongoing studies continually expanding occurrences in various regions of the global ocean a range of factors determines the formation of cold water coral reefs such as physical hydrochemical and biological e g food supply factors recently more and more modeling studies have emerged using a variety of mathematical approaches have emerged including environmental niche factor analysis enfa and predictive habitat suitability models however only few studies have attempted to characterize the underlying suite of hydro biogeochemical and physical constraints of cold water coral reefs and to differentiate between pristine reef growth vs sites with reduced or no coral occurrences this study concentrates on new data and a compilation of existing data sets on the physical and chemical properties in the ne atlantic and the mediterranean it explores the influence of ambient bottom waters and its characteristics on living cold water reefs and mounds formed by lophelia pertusa several questions are addressed 1 what are the physical and geochemical boundary conditions of living cold water corals 2 do these geochemical parameters correlate with proposed physical prerequisites 3 is there a general difference in the signature of living and dead coral sites

late proterozoic outer shelf manganese and iron deposits at otjosondu namibia related to the damaran oceanic opening the otjosondu manganese mining area is located at the eastern exposed extent of the inland branch of the late proterozoic damara orogen in spite of intense deformation and a high grade metamorphic overprint the sedimentological interpretation of the manganeserich rocks and their associated lithologies has led to a paleoenvironmental model which gives insight into ore genesis at least two manganese silicate ore horizons manganese formations are associated with banded iron formations this sequence is sandwiched between supermature metaquartzarenites interpreted as a shoreface deposit derived from a cratonic basement and deposited under high energy conditions manganese ore varieties and their country rocks have been classified using their texture mineralogy and geochemistry and have thus been interpreted in sedimentological and facies terms the evaluation of the vertical facies sequences their lateral distribution and their contact relationships renders a spatial reconstruction of the environmental setting for the manganese ores possible the development of facies from a near shore clastic regime to manganese bearing hemipelagic sediments and ultimately to pelagic manganostones and ironstones reflects a zonation on a marine shelf the facies evolution indicates transgressive conditions on a large scale onto a tectonically stable but subsiding shelf starting with the transgressive quartzarenites overlying an unconformity to the pre damara basement followed by chemical sediments of the ore bearing horizon and overlying carbonates and ending up with pelitic schists on a smaller scale both transgressive and regressive conditions prevailed during manganese precipitation small scale sedimentary cycles are interpreted as due to glaciogene influenced fluctuations of the sea level the source of manganese and iron is proposed to have been situated in the deeper oceanic parts of the associated basin initial spreading in the khomas sea and the formation of oceanic crust is documented by tholeiitic volcanics and associated base metal orebodies representing more proximal manifestations of the exhalative activity these base metal sulfide deposits are associated with pyrite bearing graphitic pelagics and this was the first site to partition iron and other base metals from manganese upwelling along the shelf edge was involved and additional separation of mn and fe was due to a change in eh conditions associated with a redox interface intersecting with the outer shelf environment the evolution of the khomas sea provided the ore forming solutions and also initiated the subsidence of the passive shelf margin fluctuations in sea level were due to the basin development on one hand and glacial interglacial cycles on the other the latter fluctuations controlled the amount of terrigenous input into an otherwise pelagic regime

40ar39ar thermochronological constraints on the structural evolution of the mesoproterozoic natal metamorphic province se africa abstract ten 40 ar 39 ar age spectra are presented for hornblende grains separated from mylonitized and weakly deformed amphibolites from the mesoproterozoic 1 2 1 0 ga natal metamorphic province nmp since the proposal of the sweat hypothesis the nmp has become a crucial area in which to study grenville aged locally termed namaquan accretion in this part of rodinia gondwana unlike many segments of the world s 1 1 ga belts the nmp is particularly suited to such studies since it is devoid of a high temperature pan african 500 ma overprint in this study we have dated hornblendes from within the earliest d1 ne directed thrust dominated structural domains of the belt the d1 domains are characterized by ne directed recumbent fold and thrust nappes and a pervasive generally s to sw dipping metamorphic foliation with down dip stretching lineations the new data constrain the minimum age of this early event which includes obduction of the northernmost tugela terrane onto the kaapvaal craton to 1135 9 ma in the d1 domains of the mzumbe terrane to the south cooling to below 550 c had only been attained by 1005 ma we also dated amphibolites from the later d2 structural domains which are characterized by sub vertical sinistral mylonite belts with sub horizontal to oblique stretching lineations our data show that d2 oblique shearing commenced at 1050 1035 ma in the mzumbe terrane and only terminated at 980 ma the youngest movements also included reactivation of major d1 structures such as the melville thrust mzumbe margate terrane boundary at 990 ma finally indications of minor pan african resetting was detected during the first 5 of the ar ar spectra from mylonites of the lilani matigulu shear zone tugela mzumbe terrane boundary indicating the polyphase nature of this major structure it has been suggested that the d1 event records the early ne sw directed arc continent collision history of the belt whilst the d2 event represents a continuation of essentially the same convergence vectors after extensive crustal thickening the new data place important time constraints on the major tectonic events in the nmp which appear comparable in many ways to other parts of the global grenvillian orogen

pleistocene calabrian deep water corals and associated biodiversity in the eastern mediterranean karpathos island greece diversified deep water corals and associated communities occur in early pleistocene early calabrian 1 7 1 6 ma calcareous crusts discovered on the south western coast of karpathos island eastern greece apart from abundant and conspicuous solitary and colonial corals 12 species the accompanying fauna comprises mostly bivalves 10 species serpulid worms 4 species and bryozoans 40 species the growth and deposition of the studied organisms occurred in an upper bathyal environment at water depths around 400 600 m the calculated mean uplift rate after deposition of the crusts indicates that karpathos was drowned during the earliest calabrian and experienced severe uplifts by the late calabrian a comparison between these communities and those of the pleistocene to recent mediterranean and north east atlantic shows that a common stock of taxa exist ed among them two azooxanthellate colonial scleractinians lophelia pertusa and madrepora oculata are predominant

environmental boundary conditions of cold water coral mound growth over the last 3 million years in the porcupine seabight northeast atlantic iodp expedition 307 made it for the first time possible to investigate the entire body of a cold water coral carbonate mound here we provide new insights into the long term history of challenger mound on the european continental margin off ireland this study is based on age determinations 230th u 87sr 86sr and geochemical signals mg li and ba ca measured in the scleractinian cold water coral lophelia pertusa from iodp site 1317 in the porcupine seabight the paleoceanographic reconstructions reveal that coral growth in the porcupine seabight was restricted to specific oceanographic conditions such as enhanced export of primary production and bottom water temperatures bwt between 8 10 c related to the water mass stratification of the mediterranean outflow water mow and eastern north atlantic water enaw the geochemical signals from the coral skeletons can be explained by the close interaction between cold water coral growth sea surface productivity and the surrounding water masses the boundary layer between mow and enaw enhanced sea surface productivity and the build up of a stable water mass stratification between enaw and mow caused enhanced nutrient supply at intermediate water depths and facilitated a steady mound growth between 3 0 2 1 ma with the decrease in sea surface productivity and related reduced export productivity the food supply was insufficient for rapid coral mound growth between 1 7 1 ma during the late pleistocene over the last 0 5 myr mound growth was restricted to interglacial periods during glacials the water mass boundary between enaw mow probably was below the mound summit and hence food supply was not sufficient for corals to grow

tectonic setting of the matchless belt pyritic copper deposits namibia massive sulfide cu zn ag au deposits are sporadically developed along the 350 km strike length of the late proterozoic matchless amphibolite belt in the damara orogen in central namibia the amphibolites represent metamorphosed tholeiitic basaltic rocks formed in an extensional tectonic setting samples from the eastern part of the belt generally show affinities to ocean ridge basalts whereas samples from the western part are more highly fractionated and show characteristic features of within plate basalts during emplacement the western part of the matchless belt was floored by thinned and stretched continental crust in the eastern part newly formed oceanic crust is indicated apparently in a graben or pull apart subbasin the massive sulfide deposits thus formed in a tectonic setting of advanced continental rifting and initial ocean floor spreading analogous tectonic settings basaltic rocks and sulfide deposits are found in the present day red sea and gulf of california the similarities in geology and geochemistry between the matchless deposits and the besshi type deposits in the sanbagawa belt of japan suggest that the latter also formed in an extensional tectonic setting i e in an advanced rift environment

tectonic motion in oblique subduction forearcs insights from the revisited middle and upper pleistocene deposits of rhodes greece the sedimentary model of coastal deposits in eastern rhodes over the last 2 ma is refined and improved in accuracy new field investigations and u th dating of spondylus bivalve shells combined with micropalaeontological and sedimentological data allow the recognition of four synthems separated by major erosional surfaces we present here evidence for two of these erosional surfaces this new model allows the identification and quantification of the vertical movements recorded by the studied exposures the history of these vertical motions is characterized by two periods of uplift and two periods of subsidence such an evolution is unique at the regional scale in the eastern hellenic forearc we interpret these results as reflecting the individualization of rhodes as a single tectonic block during increasing trench bending this trench bending is accommodated by an increase in the curvature of the forearc during the last 2 ma

an audio magnetotelluric investigation of the otjiwarongo and katima mulilo regions namibia abstractas an additional opportunistic component to the southern african magnetotelluric experiment natural source audio magnetotelluric amt data were acquired during phase iv to investigate the local scale electric conductivity subsurface structure in the otjiwarongo and katima mulilo regions namibia as an aid in locating the installation points for high voltage direct current earth electrodes the study showed that the shallow subsurface of areas containing one measurement site in the otjiwarongo region and three sites in the katima mulilo region have appropriate high conductivities for the optimal placement of the earth electrodes both of the amt surveys are situated close to the edge of the orogenic neo proterozoic damara mobile belt dmb previous studies all suggest the existence of a highly conductive midcrustal zone which correlates well with the spatial location of the dmb two dimensional inverse modeling of the otjiwarongo amt data confirms the existence of the high conductive zone at m

plio pleistocene climate evolution trends and transitions in glacial cycle dynamics abstract we describe the evolution of climate system dynamics by examining the climate response to changes in obliquity and precession over the last 5 3 myr in particular we examine changes in the shape of glacial cycles and the power of obliquity and precession response in benthic 18 o when the exponential trend in 18 o variance is removed its spectral power exhibits strong proportional responses to amplitude modulations in orbital forcing over most of the plio pleistocene precession responses correlate with modulations in forcing for the last 5 myr but 41 kyr response is sensitive to obliquity modulation only before 1 4 myr where responses are sensitive to modulations in forcing we demonstrate that glacial cycles are orbitally forced rather than being self sustained or paced by orbital changes the shapes of glacial cycles have several nonlinear properties which may be indicative of glacial interglacial differences in climate sensitivity or response time the saw tooth asymmetry of glacial cycles first appears shortly after the onset of major northern hemisphere glaciation and the relative duration of interglacial stages decreases at 1 4 myr collectively trends in the shape of glacial cycles and the sensitivity of 18 o to obliquity and precession are suggestive of major transitions in climate dynamics at approximately 2 5 and 1 4 myr but show no significant change associated with the appearance of strong 100 kyr cycles during the mid pleistocene transition

single zircon ages for granitoid gneisses in the central zone of the limpopo belt southern africa and geodynamic significance abstract ion microprobe evaporation and vapour digestion single zircon emplacement ages for granitoid rocks throughout the central zone cz of the limpopo mobile belt south africa define three broad age groups namely at 3188 3314 2637 2734 and ca 2500 ma respectively all these granitoids including the youngest group display polydeformational features suggesting that much of the tectonic history of the cz occurred in the late archaean and early proterozoic the oldest group of predominantly tonalitic trondhjemitic granodioritic gneisses was found in relatively small domains from the extreme w to the e of the cz and probably represents remnants of a once more widespread early archaean terrain the most abundant granitoid gneisses belong to the 2 6 2 7 ga age group and were emplaced into already ductilely deformed gneisses of the oldest group several of these rocks were derived from intracrustal melting rare metamorphic zircons with ages of 2560 2575 ma reflect a high grade metamorphic event in the cz that has also been postulated by other workers a strongly deformed orthogneiss with a protolith emplacement age of 2510 ma proves beyond doubt that at least part of the structural history in the cz occurred in the early proterozoic an anatectic granite in the extreme w of the cz intruded at 2022 ma and is probably related to the peak of granulite facies metamorphism previously dated at 2027 6 ma in the messina area the second period of granitoid magmatism is broadly contemporaneous with charnockite and enderbite emplacement and granulite facies metamorphism in the northern and southern marginal zones of the limpopo belt broad contemporaneity of extensive granitoid magmatism in all three zones raises doubts about the exotic nature of the cz and suggests a revised evolutionary model for the limpopo belt

the geological evolution of the magondi mobile belt zimbabwe abstract the zimbabwe archaean craton is flanked to the nw by the magondi mobile belt within this belt magondi supergroup volcanics and sediments were deposited during the early proterozoic before being deformed and metamorphosed 1850 ma ago in the south the belt is a typical thin skinned thrust belt with magondi supergroup rocks thrust se onto the archaean craton northwards the character of the belt changes with the structural style changing to a more thick skinned type metamorphic grade increasing from greenschist to granulite facies and with increasing amounts of early proterozoic basement gneisses imbricated within the sedimentary sequence metamorphism in the northern part of the belt is explained by thermal relaxation after the overthrusting of hot early proterozoic rocks from the west effectively sandwiching the magondi rocks between archaean basement below and allochthonous early proterozoic basement above flat lying isograds that date from this event were subsequently folded along s plunging axes during continued orogenic squeezing the rocks within the magondian belt are essentially unaffected by subsequent pan african deformation and metamorphism the evolution of the belt seems to fit the emerging pattern of cratonisation of the african plate within which pattern the magondian mobile belt may be correlated with other eburnian cycle belts in south africa but not with any of the tectonothermal provinces to the north of the pan african zambezi damara belt

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

evidence from dwyka tillite cobbles of archaean basement beneath the kalahari sands of southern africa we have found a new source of information about what lies beneath the kalahari sands the kheis and rehoboth provinces of southern africa were thought to be underlain by either an similar to 1800 ma orogenic belt or a northern branch of the similar to 1200 ma namaqua natal province glacial diamictites of the permocarboniferous dwyka group exposed at rietfontein west of the kalahari sands carry cobbles plucked from the bedrock by the ice sheet which covered the gondwana supercontinent about 300 ma ago despite altered mineralogy the cobbles are not severely altered geochemically their normative mineral compositions give classifications as trondhjemites and granites supported by rare earth element diagrams microbeam u pb zircon dating of the granitic cobbles shows that they contain no evidence of crustal growth or orogeny at either 1800 or 1200 ma rather they testify to the presence of 2500 to 2900 ma archaean trondhjemitic and granitic crust beneath the kalahari with a lesser similar to 2050 ma granite component the pebble assemblages from the diamictites we sampled lack the diagnostic banded iron formation bif stromatolitic limestone and other supracrustal pebbles which characterise diamictites derived from the kaapvaal craton thus we envisage shorter transport distances and derivation from the region now beneath the kalahari sands three of the archaean granite cobbles have unusual less than mantle zircon oxygen isotope values around 3 delta 18 o vsmow which may reflect interaction of their source with high temperature originally meteoric water before melting to produce the granites the mafic cobbles described in a companion paper are much younger and are related to intrusions of the 1 1 ga umkondo large igneous province probably located on the kalahari line or rehoboth province five trondhjemitic granites from the westernmost outcrops of the kaapvaal craton were dated the oldest being 3061 9 ma and four others between 2882 7 ma and 2854 7 ma reflecting the cratonisation of the kimberley terrane four of the archaean dwyka cobbles we dated are younger than the 2 7 ga kaapvaal cover sequence and are thus too young to be derived from the craton all the dwyka cobbles described here are most likely derived from either the rehoboth province or the kalahari line with origins from the kheis province kaapvaal craton or further afield considered unlikely we envisage the rehoboth province to consist of an archaean core supplemented by palaeoproterozoic granitoids which was joined to the kaapvaal craton at an early stage of crustal development and played an important role during later tectonic events this has important implications not only for the tectonic framework and assembly of southern africa but also for exploration for diamonds and other ore deposits c 2011 elsevier b v all rights reserved

a review of calcareous nannofossil astrobiochronology encompassing the past 25 million years this paper presents a review of astrobiochronological calibration of recent to late oligocene calcareous nannofossil datum events biohorizons included in the paper are those of the widely used standard nannofossil zonations of martini e 1971 standard tertiary and quaternary calcareous nannoplankton zonation in farinacci a ed proceedings of the second international conference on planktonic microfossils roma rome ed tecnosci vol 2 pp 739 785 and okada h and bukry d 1980 supplementary modification and introduction of code numbers to the low latitude coccolith biostratigraphy zonation bukry 1973 1975 marine micropaleontology 51 321 325 as well as supplementary biohorizons proposed in the literature the biohorizons have been selected on the basis of the unambiguous taxonomy of the index taxa and their biostratigraphic usefulness we emphasise the application of rigorous methodology in nannofossil studies which permits an evaluation of biohorizons in terms of reliability and calibrates their potential correlatability astrochronological age estimates rely on the geologic timescale developed by the ics in 2004 with some new calibrations included we provide an overview of the relative position of biohorizons versus the astronomically calibrated ages of magnetic reversals and reference isotope stratigraphies surprisingly there are still few high resolution quantitative biostratigraphic studies of astrochronologically tuned sections in spite of the central role of such studies in addressing fundamental problems such as the tempo and mode of plankton evolution

geochemistry and tectonic significance of the mesoproterozoic kgwebe metavolcanic rocks in northwest botswana implications for the evolution of the kibaran namaqua natal belt the c 1 1 ga kgwebe metavolcanic rocks exposed in the northwest of botswana are late kibaran rocks they represent a bimodal suite of within plate low titanium phosphorus ltp continental tholeiites and post orogenic within plate high k rhyolites the chemical compositions of the kgwebe mafic rocks are characterized by low values of ce pb 10 and high la nb ratios average c 2 maximum 4 mid ocean ridge basalts morb normalized spidergrams show marked enrichment in mobile elements sr k rb ba and negative anomalies in nb these features suggest they may have originated in a mantle enriched during a previous subduction event the kgwebe metarhyolites are marked by y 60 ppm sr y 20 and high k contents they cannot therefore be the product of melting of sediments or a subducting slab it is inferred that they represent felsic magmas resulting from melting of mesoproterozoic kibaran calcalkaline rocks underplated in the middle and or lower crust the kgwebe bimodal metavolcanic rocks pre date the neoproterozoic ghanzi group rocks which are correlated with the lower part of the damara sequence the chemical composition and field relations suggest that these metavolcanic rocks were emplaced during a late orogenic collision associated extensional collapse this collapse affected a crust thickened during the kibaran orogeny in the namaqua natal belt of southwest africa

regional aeromagnetic and stratigraphic correlations of the kalahari copperbelt in namibia and botswana abstract the late mesoproterozoic to neoproterozoic kalahari copperbelt kcb in namibia and botswana is widely covered by kalahari sand which precludes direct correlations between known stratabound sediment hosted cu ag districts we use a combination of review of literature data and newly processed and interpreted high resolution aeromagnetic maps in both countries to provide a new correlative cross border interpretation lithostratigraphic control on the aeromagnetic response allows detailed indirect mapping of the kalahari copperbelt lithotectonic domains below the sand cover this enabled us to redefine the width and lateral extent of the kcb as two continuous magnetic domains the rehoboth and ghanzi chobe domains extending from central namibia to northern botswana and helped in resolving problems of stratigraphic correlations across the international border the rehoboth magnetic domain in the western part of the kcb in namibia records continental arc magmatism at 1200 ma during orogenic events along the northwestern edge of the kalahari craton this was followed at 1110 1090 ma by widespread magmatism identified within the entire kcb and related to the 1112 1106 ma umkondo large igneous province the basal parts of the tsumis group in namibia and ghanzi group in botswana were deposited in shallow water environments after a period of erosion and peneplanation subsequently and prior to the sturtian glaciation the host rocks of the cu ag deposits formed by the deposition of chemically reduced shales and siltstones that formed in deeper water and overlie chemically oxidised shallow water sandstones this regional interface which is both a permeability barrier and redox boundary played a critical role in the formation of the stratabound sediment hosted cu ag deposits of the kalahari copperbelt and the interface with its strong magnetic contrast can be followed through the entire ghanzi chobe magnetic domain of the copperbelt the whole kcb was affected by the damara orogeny during early cambrian times which resulted in the formation of a ne sw trending 250 km wide fold and thrust belt

unraveling the record of successive high grade events in the central zone of the limpopo belt using pb single phase dating of metamorphic minerals abstract dating of relic metamorphic assemblages can provide important information about the timing and character metamorphic grade and or pt evolution of early high grade episodes in polymetamorphic provinces using pb stepwise leaching of metamorphic silicates we have dated multiple granulite facies metamorphic episodes in the central zone cz of the limpopo belt ages of 2 52 ga were obtained from sillimanite and cogenetic garnet and ages of about 2 01 ga from titanite garnet and clinopyroxene together with new and published conventional age data from accessory phases and in the context of combined petrological and structural data these results lead us to a reinterpretation of the tectono metamorphic history of the cz three distinct high grade events at about 3 2 3 1 ga 2 65 2 52 ga and 2 0 0 05 ga are recognized each of these is suggested to correspond to a tectonic episode of distinct character a for the 3 2 ga event magmatic activity can mainly be identified best represented for example by the sand river gneisses or the messina layered intrusion the field relationships concerning the tectonometamorphic history of this early archean event are largely erased by at least two high grade metamorphic overprints b late archean 2 6 2 52 ga low pressure granulite facies metamorphism was associated with voluminous granitic and charnockitic plutonism the anticlockwise p t evolution of these granulites probably reflects deep crustal processes associated with magmatic underplating or in plating contemporaneous with vertical crustal growth of the zimbabwe craton around 2 6 ga c during the proterozoic event 2 05 1 95 ga tectonic thickening was caused by the collision of the kaapvaal and zimbabwe cratons the cz was squeezed between these two cratons and as a consequence underwent high pressure granulite facies metamorphism with a clockwise p t evolution the structural metamorphic and geochronological data can be best explained with a tectonic model that describes this final event as a dextral transpressive orogeny

detrital magmatic muscovite from the lower carboniferous of southwest ireland buried granites uncovered lower carboniferous limestones from the kilmaclenine area co cork contain magmatic muscovites individual granitic minerals and granitic fragments these constitute primary evidence for the presence of buried granites to explain various geological and geophysical observations in southwest ireland

proliferation and demise of deep sea corals in the mediterranean during the younger dryas abstract uranium series and radiocarbon ages are reported for deep sea corals madrepora oculata desmophyllum dianthus lophelia pertusa and caryophyllia smithii from the mediterranean sea u series dating indicates that deep sea corals have persisted in the mediterranean for over 480 000 years especially during cool interstadial periods the most prolific period of growth however appears to have occurred within the younger dryas yd period from 12 900 to 11 700 years bp followed by a short 330 years phase of post yd coral growth from 11 230 to 10 900 years bp this indicates that deep sea corals were prolific in the mediterranean not only during the return to the more glacial like conditions of the yd but also following the rapid deglaciation and transition to warmer conditions that followed the end of the yd surprisingly there is a paucity last glacial maximum lgm coral ages implying they were largely absent during this period when cold water conditions were more prevalent radiocarbon ages show that the intermediate depth waters of the mediterranean generally had 14 c compositions similar to surface waters indicating that these waters were extremely well ventilated the only exception is a narrow period in the yd 12 500 100 years bp when several samples of lophelia pertusa from the ionian sea had 14 c values falling significantly below the marine curve using a refined approach isolation ages isol of 300 years to 500 years are estimated for these intermediate 800 1000 m depth waters relative to surface marine waters indicating a reduction or absence of deep water formation in the ionian and adjacent adriatic seas during the yd contrary to previous findings we find no evidence for widespread intrusion of low 14 c atlantic waters into the mediterranean prolific growth of deep sea corals in the mediterranean ended abruptly at 10 900 years bp with many of the coral bearing mounds on the continental slopes being draped in a thin veneer of mud their demise is attributed to a number of factors including the direct loss of habitat due to high sedimentation that accompanied glacial meltwater pulses together with rising temperatures that would have finally pervaded the deeper water of the mediterranean following the onset of holocene warming

paleomagnetism of the 1 1 ga umkondo large igneous province in southern africa 1 the umkondo dolerites are present over a wide area in the kalahari craton southern africa thirty nine sampling sites in botswana and south africa yielded tightly grouped paleomagnetic directions due south with shallow inclinations and three sites of opposite polarity the dolerites have u pb single crystal baddeleyite or zircon crystallization ages of 1112 0 5 to 1108 0 9 ma these results can be combined with published data from 39 additional umkondo sites and 33 sites in the grunehogna province of antarctica after restoring east antarctica to its position next to southern africa grouping the sites geographically yields 10 site mean poles with mean at 64 0 n 38 8 e a95 3 7 this umkondo pole can be correlated with keweenawan poles from laurentia because both sets of poles are precisely of the same age as well as predominantly of one polarity the relative orientation of the two cratons within the rodinia supercontinent is fixed this implies that the namaqua natal maud belt which rims the southern part of the kalahari craton faced away from laurentia the umkondo pole combined with published poles suggest that the kalahari craton remained distinctly south of the laurentian craton between 1 1 and 1 0 ga making it highly unlikely that the two cratons collided

the koras sinclair ghanzi rift in southern africa volcanism sedimentation age relationships and geophysical signature of a late middle proterozoic rift system abstract a number of relatively undeformed late middle proterozoic volcano sedimentary basins are aligned along the western and northern margins of the kalahari craton although the assemblages of these basins have been compared and lithologically correlated by many authors radiometric ages partly contradict these interpretations attempts to incorporate all of these basins into a regional structural interpretation have been rare watters magmatic arc interpretation is contradicted by the overall geochemistry of the volcanic rocks the coarse continental red bed sediments and the structural style of the basins the depositional troughs were narrow fault bounded continental rift grabens with a structural style and sedimentation pattern controlled by strong vertical tectonism the development of the basins along two branches of a propagating continental rift system the koras sinclair ghanzi rift ksg rift is proposed radiometric ages reveal a distinct younging along the rift from the southern toward the northeastern branch the geophysical signature of the rift is dominated by positive bouguer gravity anomalies flanked by gravity lows which follow a linear trend the basins contain distinctly bimodal volcanic rocks with some intermediate volcanics in the sinclair basin contemporaneous with the extrusion of the volcanics granitic high level intrusions were emplaced along the rift the possible existence of a northern coastal branch of the ksg rift and the location of a triple junction is discussed the rift process possibly continued and led to the development of the early damara rift in swa namibia the african plate kalahari and congo cratons migrated southward between 1050 and 950 ma ago a stationary mantle plume probably caused the development of both the ksg and damara rift a comparison with rift migration in kenya is drawn the regional trends and structures of the ksg rift were reactivated several times undergoing both extension and compression the most prominent of these events was the development of the damara orogen which followed the regional trend of the late middle proterozoic rift but fault structures have been reactivated up to recent times

a collision related pressure temperature time path for prieska copper mine namaqua natal tectonic province south africa abstract prieska copper mine lies in highly metamorphosed gneisses of the 1100 ma namaqua natal tectonic province of south africa close to its boundary with the kaapvaal craton the complex stratigraphy of the mining district is thought to have formed in a 1300 ma arc environment while the ore is regarded as a volcanogenic massive sulphide deposit petrographic study of key peraluminous and metabasic rock types led to the recognition of four principal metamorphic events m 1 4 which together with three deformation phases constitute an orogenic cycle staurolite garnet kyanite m 1 parageneses reflect medium pressure metamorphism but are largely obscured by the dominant m 2 muscovite free cordierite garnetsillimanite assemblages the latter reflect the metamorphic peak at about 730 c and 4 1 kbar with partial melting suppressed by dry conditions m 2 peak temperatures at different localities within the 30 km long mining district vary between 550 and 800 c suggesting a localized heat source m 3 sillimanite phlogopite grain boundary reactions represent a brief hydration event in p t conditions close to those of m 2 m 4 parageneses which feature retrograde kyanite staurolite garnet muscovite and chlorite are developed only in f 3 shear zones and represent a near isobaric return to a geotherm close to that which prevailed during m1 p t data for the metamorphic cycle is integrated with comprehensive geochronology to develop a model ptt path for the prieska mine region the model envisages an orogenic cycle at the end of a 1300 900 ma wilson cycle characterised by collision crustal thickening followed by a long period of uplift and erosion the peak metamorphic conditions are ascribed to two short lived essentially contact metamorphic events due to the intrusion of the plat sjambok anorthosite suite the 400 ma wilson cycle 300 ma orogeny and 0 05 mm yr uplift rate envisaged for prieska mine are long and slow compared with phanerozoic examples however similar parameters seem to characterize more than one proterozoic event

large deep water coral banks in the porcupine basin southwest of ireland the porcupine basin southwest of ireland was one of the earliest sites from where the deep water corals lophelia sp and madrepora sp were recovered these deep water corals have since been found all along the atlantic margins of europe in water depths ranging from 50 to more than 2000 m recent geophysical studies have demonstrated the mound building potential of deep water corals available data indicate that three major provinces of coral bank occurrences can be identified in the porcupine basin 1 high relief surface mounds which have a dimension of 1 by 5 km and a height up to 200 m hovland mounds flanked to the north by 2 a swarm of buried mounds somewhat smaller up to 90 m and with more irregular shapes than those recognised in area 1 magellan mounds and 3 outcropping or buried conical mounds single or in elongated clusters up to 150 m high occurring on the southeastern slope of the basin belgica mounds as far as can be inferred from shallow cores the surface lithology predominantly consists of an upper layer rich in foraminiferal sand and terrigenous silty clay with intercalations of biogenic rubble the banks host a remarkable number of colonies of living and dead lophelia pertusa and madrepora oculata the living and dead assemblages are underlain by a significant layer of coral debris in a muddy matrix deep water coral debris together with a living association of the same species covers the surface of the belgica and hovland mounds which may suggest that these corals have played a significant role in the development of the mound structures the capacity for mound formation by scleractinian corals in the aphotic zone has been known for some time examples are found at different locations along the shelves and the continental margins of the north atlantic the role of the corals in these deep water build ups is still a point of debate though the genesis and initial control of mound settings in this basin might be related to hydrocarbon seeps it appears that the major development of the porcupine coral banks in recent geological times has most likely been controlled by oceanic circulation and dynamics in water masses and nutrient supply

thermobarometric and40ar 39ar geochronologic constraints on eohimalayan metamorphism in the dinggye area southern tibet mineral assemblages in the dinggye area of southern tibet 28 n 88 e provide new insights regarding the poorly understood eohimalayan metamorphic event in the eastern himalayan orogen major element partitioning thermobarometry of pelitic rocks indicates temperatures of 750 830 k at depths of 14 3 km consistent with the presence of kyanite sillimanite and andalusite schists in the area laser and resistance furnace40ar 39ar analyses of hornblendes from intercalated amphibolites yield closure ages of 25 ma overlap between the probable range of ar closure temperatures for these hornblendes and the metamorphic temperatures estimated through thermobarometry suggests that eohimalayan metamorphism in the dinggye area occurred in late oligocene time no more than about 10 million years before the main or neohimalayan phase of metamorphism in early to middle miocene time muscovite biotite and k feldspar40ar 39ar ages indicate an important episode of rapid cooling between 16 and 13 ma which is interpreted as a signature of tectonic denudation related to movement on n dipping extensional structures of the south tibetan detachment system

biogenic corrosion caused by bats in drotsky s cave the gcwihaba hills nw botswana abstract located in north west botswana drotsky s cave is a vast cavity hosted within dolomite known for the discovery of archaeological strata dating back to the late stone age it is home to a colony of bats comprising several ten thousands of individuals who are responsible for the accumulation of a thick deposit of guano on which we have conducted research in some of the deeper parts of the cavity a detailed study of the morphologies of the dolomite walls and roofs demonstrates that part of the cave s speleogenesis is probably due to biogenic corrosion this type of weathering and corrosion process is still poorly described in underground environments however it is now accepted that the role played by bats and their droppings on the condition of walls their weathering and the enlargement of galleries and chambers by condensation corrosion processes should be taken into account when considering the way deep networks evolve in this article we discuss the mechanisms of this biogenic corrosion in drotsky s cave as well as their morphogenic impacts the dissolution and recession of walls the formation of lateral notches arches bell holes and ceiling spherical cupolas and the consequences for the wall retreat of conduits finally we propose a new typological classification the consequences of this biogenic corrosion are important for archaeological questions related to the taphonomy of archaeological remains in the buried deposits and the conservation of cave art because of the important impacts of guano on buried biological archaeological remains such a population would have encouraged accelerated weathering corrosion and dissolution of the walls potentially contributing to the total or partial disappearance of any trace of human activity on the surrounding rocks

iron formations related to mafic volcanism and ensialic rifting in the southern margin zone of the damara orogen namibia abstract iron formations occur within the metasedimentary metavolcanic sequence of the chuos formation in the southern margin zone of the upper proterozoic damara orogen of south west africa namibia the iron rich lithosome comprises magnetite quartz rock haematite quartz rock and magnetite quartz silicate carbonate rock on the basis of their host rock associations iron formation in diamictite sequences and in amphibolite are distinguished iron formation in amphibolite is developed both as a chemical sediment and as a composite rock consisting of chemical iron rich sediment with admixture of varying amounts of volcanic detritus iron formation in diamictite represents a pure chemical sediment both of the iron rich rocks contain abundant apatite and their manganese contents are low an exhalative origin for the iron formations of the southern margin zone related to mafic volcanism and ensialic rifting is suggested

the kibaran of southern africa tectonic evolution and metallogeny abstract the tectono magmatic and metallogenic evolution of the 1100 ma late kibaran high grade namaqua natal metamorphic belt is reviewed and compared with that of the grenville province and the kibaran belts of central and eastern africa in namaqualand 1300 1600 ma supracrustal rocks are recognised including arc related sequences in the east and extensional foreland basin sequences in the west these were laid down either on a heterogeneous basement of eburnian 2000 ma age or on juvenile oceanic crust prior to late kibaran deformation metamorphism and multiple plutonic intrusion the pre namaqua basement infrastructure has been recognised largely in the eastern and nw parts of the orogen adjacent to areas of exposed eburnian crust and probably only the areachap terrane in the east comprises juvenile kibaran crust in natal 1400 ma predominantly metavolcanic supracrustal gneisses were extensively intruded by voluminous plutonic batholiths during the 1050 ma tectonothermal event the northern terrane is interpreted as an ophiolitic assemblage obducted to the ne onto the kaapvaal craton whereas the southern domains are predominantly arc related no older basement is recognised and the entire belt comprises juvenile crustal material none of which is apparently older than 1400 ma the kinematic evolution of the entire namaqua natal belt is comparable in both areas early sw and ne d directed thrusting and nappe emplacement during collision were followed by pervasive steep transcurrent ductile shearing in natal the later shearing event gave rise to major wnw trending oblique sinistral transcurrent steep belts whereas the same event in namaqualand is manifested as major subvertical nw trending dextral transcurrent shear zones the difference in dominant sense of movement in the two regions can be attributed to changes in orientation of the adjacent archaean craton margin within a progressive consistently oriented ne sw directed zone of plate convergence in eastern namaqualand and namibia the transcurrent shearing gave rise to a north propagating stricke slip rift system in which the volcano sedimentary koras sinclair ghansi sequences were deposited mineral cooling ages of 950 ma reported from throughout the belt are considered to signal the end of the kibaran cycle in southern africa the metallogenic evolution is discussed in the light of the tectono magmatic model presented economically important stratiform volcanic and sedimentary exhalative base metal occurrences are extensively associated with early supracrustal sequences throughout the belt other metal deposits and occurrences are predominantly related to the voluminous granitoid and more restricted metabasic suites which characterise the younger rocks in both natal and namaqualand the metallogenesis of the dominantly convergent collisional namaqua natal belt is similar to that seen in the grenville province but distinct from and younger than the lower grade strike slip basin related kibaran belt of central and eastern africa

lithospheric structures and precambrian terrane boundaries in northeastern botswana revealed through magnetotelluric profiling as part of the southern african magnetotelluric experiment 1 within the framework of the southern african magnetotelluric experiment a focused study was undertaken to gain improved knowledge of the lithospheric geometries and structures of the westerly extension of the zimbabwe craton zim into botswana with the overarching aim of increasing our understanding of southern african tectonics the area of interest is located in northeastern botswana where kalahari sands cover most of the geological terranes and very little is known about lithospheric structures and thicknesses some of the regional scale terrane boundary locations defined based on potential field data are not sufficiently accurate for local scale studies investigation of the nnw sse orientated 600 km long zim line profile crossing the zimbabwe craton magondi mobile belt and ghanzi chobe belt showed that the zimbabwe craton is characterized by thick 220 km resistive lithosphere consistent with geochemical and geothermal estimates from kimberlite samples of the nearby orapa and letlhakane pipes 175 km west of the profile the lithospheric mantle of the ghanzi chobe belt is resistive but its lithosphere is only about 180 km thick at crustal depths a northward dipping boundary between the ghanzi chobe and the magondi belts is identified and two middle to lower crustal conductors are discovered in the magondi belt the crustal terrane boundary between the magondi and ghanzi chobe belts is found to be located further to the north and the southwestern boundary of the zimbabwe craton might be further to the west than previously inferred from the regional potential field data

the geology and regional correlation of the proterozoic okwa inlier western botswana abstract the small okwa inlier exposes four unconformity bounded proterozoic lithostratigraphic assemblages in the kalahari region of central southern africa where the precambrian geology is otherwise largely obscured by phanerozoic strata the inlier provides the only exposures in the okwa basement a structural block between the archaean kaapvaal zimbabwe craton and the late proterozoic damara orogen felsites and sericitic quartzites of the okwa basement complex were intruded by varied granites between 2 1 and 2 0 ga ago deformation of the granites to form gneisses at 1 8 ga d1 was accompanied by metamorphism of pre tectonic mafic dykes in the epidote amphibolite facies in the mid proterozoic felsic tuffs and red sandstones of the lower okwa group were intruded by dolerite sills and a contemporary northeasterly dyke swarm intruded the basement granitoids a fracture cleavage was imposed on the lower okwa group during folding and greenschist low amphibolite facies metamorphism d2 at 1 15 ga the red bed sediments of the upper okwa group suffered only very mild deformation d3 the youngest assemblage is assigned to the late proterozoic nama group it mainly consists of gently dipping fine grained sandstones the okwa basement complex is confirmed as a segment of eburnian aged crust which was accreted to the archaean craton at about 1 8 ga probably adjacent to epicratonic sediments allied to the magondi supergroup of zimbabwe or to the olifantshoek sequence of south africa although such sediments are nowhere exposed in the okwa sector the lower okwa group is thought to be equivalent to volcano sedimentary sequences such as the sinclair group of namibia the upper okwa group can be correlated either with immediately pre damaran units such as the auborus formation of the sinclair area or with the nosib and ghanzi groups in the early damara supergroup the occurrence of nama sediments overlying the okwa basement seems to confirm their suspected presence beneath the karoo supergroup in the ncojane nosop basin of western botswana

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

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

geochemical evidence for magma source heterogeneity and activity of a mantle plume during advanced rifting in the southern damara orogen namibia abstract activity of a mantle plume is indicated by magma source heterogeneity documented in the chemical compositions of metabasaltic rocks in the upper part of the swakop group in the southern damara orogen in contrast metabasaltic rocks in the lower part of the swakop group have homogeneous compositions and activity of a mantle plume during earlier crustal extension is chemically not recorded the basaltic rocks are considered to have been erupted between 750 and 650 ma during continental rifting lower part of the swakop group tholeiitic basaltic magmatism related to normal faulting occurred perpendicular to the main direction of extension the basaltic rocks have been derived from a homogeneous magmatic source and variations in their chemical compositions are related mainly to fractionation processes continuous rifting created subsidence and the formation of a major depositional basin underlain by thinned and stretched continental crust as well as newly formed oceanic crust upper part of the swakop group tholeiitic basaltic magmatism occurred along the axis of the basin over a strike length of at least 400 km the basaltic rocks have been derived by melting of a heterogeneous magmatic source the magma source heterogeneity is considered to indicate the activity of a mantle plume at this late rifting stage the gradual decrease of the degree of magmatic heterogeneity from west to east along strike of the intrabasinal basaltic rocks indicates that the mantle plume was active to the west of the investigated area

u pb zircon ages for granitoid gneisses in northern namibia and their significance for proterozoic crustal evolution of southwestern africa abstract the pre damara 900 ma basement gneisses and granitoids of northern namibia and southern angola constitute the southern margin of the congo craton and are poorly known geochronologically we report u pb zircon concordia intercept ages for an augengneiss of the epupa complex 1800 ma a banded quartzofeldspathic gneiss of the huab complex 1810 ma a post huab granitic gneiss 1750 ma and for inherited zircons 2120 ma from a damaran syenite that has a true emplacement age of 800 840 ma these ages suggest that the epupa and huab complexes are broadly coeval and their formation together with the emplacement of the widespread fransfontein granitoid suite constitutes a significant and regionally extensive crust forming event in northern namibia and southern angola the huabian episode that corresponds to the eburnian event in western central and eastern africa older crust in excess of 2120 ma is likely to be present in the region but has not been documented geochronologically similarities in age rock association and broad geologic evolution for pre damara basement complexes in northern central and southern namibia suggest that the entire territory may have been part of the congo craton in mid proterozoic times whose southwestern portion was largely reworked during the late precambrian namaqua and pan african events

re os dating and sulfur isotope composition of molybdenite from tungsten deposits in western namaqualand south africa implications for ore genesis and the timing of metamorphism both stratiform stratabound and granite related models have been used to explain the genesis of w mo deposits in the okiep copper district in western namaqualand south africa apparently stratabound mineralization fe rich wolframite with accessory molybdenite occurs in foliation parallel quartz veins in high grade 750 c 5 6 kbar metapelites of the wolfram formation and less commonly in small bodies of silicified leucogranites and pegmatites six re os ages for molybdenites from four deposits nababeep tungsten far west kliphoog narrap tweedam range between 1000 4 and 1026 5 ma these molybdenites define a well constrained 187re 187os isochron with an age of 1019 6 ma which is interpreted as the age of w mo mineralization this age is significantly younger than proterozoic protolith ages for supracrustal rocks and the emplacement ages for the main intrusive suites but geologic evidence requires overlap with a period of high grade metamorphism we suggest that w mo mineralization is genetically linked to intra crustal magmatic processes at 1020 ma thereby precluding the 1060 ma concordia granite as the source for mineralizing fluids a narrow range of positive 34s compositions 3 6 to 4 5 for eight molybdenites from five w mo mines is consistent with a so2 rich fluid and a granite related genetic model post peak metamorphic deformation and metamorphism of w mo ores is most likely related to the retrograde stage of the namaquan orogeny which overlaps emplacement of late orogenic evolved granites and pegmatites and the formation of w mo deposits in western namaqualand therefore the effects of retrograde namaquan metamorphism extend at least to 1020 ma or alternatively these w mo veins were affected by a poorly constrained later event e g early pan african

geochronology of basement rocks in the kalahari desert botswana and implications for regional proterozoic tectonics abstract u pb 40 ar 39 ar and sm nd geochronological results from isolated basement exposures and drill core samples in the kalahari desert botswana constrain the distribution and temporal evolution of proterozoic crustal provinces in this part of africa there is evidence that a buried northeast trending mesoproterozoic orogen lies outboard of older crustal provinces in eastern botswana the subsurface part of this orogen includes a granitic gneissic terrain in northern botswana that records plutonism and amphibolite grade deformation between 1 2 and 1 1 ga limited exposures of bimodal volcanic rocks in the kalahari desert are interpreted to represent extensional magmatism superimposed on the buried mesoproterozoic belt previously published geochronological data and new u pb zircon crystallization ages presented here indicate that these volcanic rocks and associated subvolcanic intrusions were emplaced at 1 1 ga as part of the widespread umkondo within plate igneous province geophysical data sparse outcrops drill core samples and reconnaissance 40 ar 39 ar geochronology show the buried mesoproterozoic orogen to be separated from the congo craton by a continuation of the neoproterozoic lower paleozoic damara belt exposed in namibia limited outcrops of 2 05 ga augen gneiss within a largely buried basement massif in northwestern botswana are inferred to be part of a paleoproterozoic crustal province developed along the southern margin of the congo craton the massif was intruded by granites at 1 02 1 00 ga but the extent and tectonic significance of this plutonism are uncertain deformed granite in the subsurface extension of the damara belt has a protolith crystallization age of 0 72 ga complex structural patterns in northwestern botswana are inferred to reflect heterogeneous damaran deformation around a promontory extending from the southern margin of the congo craton

reconnaissance geochronology tectonothermal evolution and regional significance of the middle proterozoic choma kalomo block southern zambia abstract new u pb zircon 40 ar 39 ar rb sr and sm nd data provide the first numerical chronology for the tectonothermal evolution of the choma kalomo block an extensive middle proterozoic terrane in southern zambia to the south of the pan african zambezi belt the choma kalomo batholith a large plutonic body in the central part of the block is shown to be a composite intrusion emplaced from c 1345 to 1200 ma based on u pb zircon ages and detailed mapping of intrusive relations early phases of the batholith were intruded as syntectonic to late syntectonic plutons during development of a penetrative s 1 fabric in the metasedimentary country rocks of the choma kalomo block d 2 folding produced the regional ne sw structural trends in the block and possibly occurred at c 1230 ma based on an rb sr isochron from an early phase of the batholith containing a penetrative s 2 foliation an u pb zircon age of c 1200 ma from post tectonic granite provides a younger limit to the timing of d 2 40 ar 39 ar mineral ages from the posttectonic granite and sm nd and 40 ar 39 ar mineral ages from amphibolites in country rock paragneisses range from c 1200 to 500 ma recording post d 2 cooling followed by subsequent low grade pan african overprinting along the southern margin of the zambezi belt d 1 in the choma kalomo block at c 1345 ma is substantially younger than early proterozoic orogenesis in adjacent terranes to the east in zimbabwe indicating that a major precambrian discontinuity beneath the mid zambezi valley separates these regions d 1 in the block is similar in age to plutonic and deformational events dated at c 1350 1400 ma in the irumide belt of central and northern zambia suggesting that these two terranes on each side of the younger cross cutting zambezi belt are parts of a single originally continuous middle proterozoic mobile belt continuity between the choma kalomo block and broadly coeval terranes to the southwest in namibia and south africa also appears likely and all of these areas are considered to form parts of a middle proterozoic orogenic province lying west of the kaapvaal and zimbabwe cratons

megafauna of vulnerable marine ecosystems in french mediterranean submarine canyons spatial distribution and anthropogenic impacts abstract vulnerable marine ecosystems vme in the deep mediterranean sea have been identified by the general fisheries commission for the mediterranean as consisting of communities of scleractinia lophelia pertusa and madrepora oculata pennatulacea funiculina quadrangularis and alcyonacea isidella elongata this paper deals with video data recorded in the heads of french mediterranean canyons quantitative observations were extracted from 101 video films recorded during the medseacan cruise in 2009 aamp comex qualitative information was extracted from four other cruises two marum comex cruises in 2009 and 2011 and two ifremer cruises in 1995 and 2010 to support the previous observations in the cassidaigne and lacaze duthiers canyons all the species fishing impacts and litter recognized in the video films recorded from 180 to 700 m depth were mapped using gis the abundances and distributions of benthic fishing resources marketable fishes aristeidae octopodidae vulnerable marine species trawling scars and litter of 17 canyons were calculated and compared as was the open slope between the stoechades and toulon canyons funiculina quadrangularis was rarely observed being confined for the most part to the marti canyon and i elongata was abundant in three canyons bourcart marti petit rhone these two cnidarians were encountered in relatively low abundances and it may be that they have been swept away by repeated trawling the lacaze duthiers and cassidaigne canyons comprised the highest densities and largest colony sizes of scleractinian cold water corals whose distribution was mapped in detail these colonies were often seen to be entangled in fishing lines the alcyonacean callogorgia verticillata was observed to be highly abundant in the bourcart canyon and less abundant in several other canyons this alcyonacean was also severely affected by bottom fishing gears and is proposed as a vulnerable marine species our studies on anthropogenic impacts show that seafloor disturbance by benthic fishing is mainly attributable to trawling in the gulf of lion and to long lines where rocky substrates are present the bauxite residue red mud expelled in the cassidaigne canyon was seen to prevent fauna from settling at the bottom of the canyon and it covered much of the flanks litter was present in all of the canyons and especially in considerable quantities in the ligurian sea where the heads of the canyons are closer to the coast three marine protected areas and one fishing area with restricted access have recently been established and should permit the preservation of these deep ecosystems

dating the death of mediterranean deep sea scleractinian corals abstract several c 14 datings of deep sea scleractinians indicate that the major decline of this coral population in the mediterranean sea took place around 30 000 15 000 yrs b p it is probable that the most important factor causing such an event was the rise of temperature that either directly affected the physiology of the corals or more likely triggered unfavorable hydrological conditions

timing and sources of pre collisional neoproterozoic sedimentation along the sw margin of the congo craton kaoko belt nw namibia abstract isotopic dating of detrital zircon populations from metasediments and of magmatic zircon from intercalated metavolcanic layers in the medium to high grade part of the kaoko belt in namibia provides first robust constraints on the age of neoproterozoic sedimentation along the southwestern margin of the congo craton dating of detrital zircons from metasediments directly overlying the cratonic basement shows maximum sedimentary ages of c 1 00 and c 1 45 ga and age populations comparable with known protolith ages from the gneisses of the congo kalahari cratons dating of zircon from associated metavolcanics constrains the age of the earliest preserved sediments at c 740 710 ma detrital zircon populations from the samples collected from the upper parts of the metasedimentary succession contain only small proportion of grains with ages similar to those from the congo craton these samples show dominance of c 1 00 ga c 750 ma and c 650 ma old zircon grains that are probably derived from the punta del este coastal terrane dom feliciano and kaoko belts that acted as an arc back arc domain at c 650 630 ma neodymium model ages for the samples of metasedimentary cover of the cratonic basement provide another evidence that the youngest preserved sediments could not have been derived from the congo craton recognition of the punta del este coastal terrane crust as a source region for the youngest pre collisional sediments of the kaoko belt suggests that this terrane must have been in close proximity to the congo craton passive margin already some time prior to their mutual collision at c 570 550 ma this is in accord with an interpretation that the c 650 630 ma arc back arc punta del este coastal terrane has developed directly on top of or very close to the attenuated congo craton passive margin

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