extracting time domain green s function estimates from ambient seismic noise 1 it has been demonstrated experimentally and theoretically that an estimate of the green s function between two seismic stations can be obtained from the time derivative of the long time average cross correlation of ambient noise between these two stations this tdgf estimate from just the noise field includes all tensor components of the green s function and these green s function estimates can be used to infer earth structure we have computed cross correlations using 1 to 30 continuous days of ambient noise recorded by over 150 broadband seismic stations located in southern california the data processing yielded thousands of cross correlation pairs for receiver separations from 4 500 km which clearly exhibit coherent broadband propagating dispersive wavetrains across frequency band 0 1 2 hz

geothermobarometry of mafic granulites and metapelite from the palghat gap south india petrological evidence for isothermal uplift and rapid cooling the palghat gap region is located near the centre of the large southern indian granulite terrane at the northern edge of the kodaikanal charnockite massif the dominant rock types in the region are hornblende biotite orthopyroxene gneisses and charnockites along with minor amounts of intercalated mafic granulite metapelite and calc silicate the p t estimates from garnetiferous mafic granulites and metapelite samples are generally in the range 9 10 kbar and 800 900d c using both conventional thermobarometric methods and the tweequ thermobarometry program these p t estimates which should be taken as minimum values are among the highest yet reported for south indian and sri lankan granulites the occurrence of orthopyroxene plagioclase symplectites around embayed garnet grains in the mafic granulites and cordierite rims around garnet grains in metapelite suggest an isothermal decompression type path similarly a core rim p t trajectory indicates c 3 and 7 kbar decompression at high temperature in the mafic granulites and metapelite respectively in both rock types the key to the determination of the retrograde p t path was the recognition of small amounts of second generation plagioclase with a more anorthitic composition than the matrix plagioclase the preservation of high garnet pyroxene temperatures in the mafic granulites despite small garnet grain size suggests rapid cooling of the terrane calculated minimum cooling rates range from 8 to 80d c ma 1 such cooling rates are more rapid than those associated with normal isostatic processes and suggest that the terrane was tectonically exhumed at high temperature

red bed hosted oncolitic manganese ore of the paleoproterozoic soutpansberg group bronkhorstfontein south africa the bronkhorstfontein manganese deposit near tolwe limpopo province south africa constitutes an exceptionally well preserved and unmetamorphosed shallow marine manganese deposit of late paleoproterozoic age it is the oldest known deposit that displays characteristics of economically important shallow marine pisolitic manganese deposits of mesozoic and cenozoic age manganese mineralization at bronkhorstfontein occurs as five laterally continuous up to 2 m thick beds that are hosted by an alluvial red bed succession preserved as part of an erosional outlier of the wyllies poort formation soutpansberg group manganiferous beds mark transgressive surfaces and formed in a storm dominated shallow water environment of a deltaic to litoral setting the massively bedded siliceous ore contains up to 41 wt percent mn at mn fe ratios between 6 and 10 the mineralogy of the ore is simple braunite hematite and quartz are the dominant constituents with minor pyrolusite cryptomelane and chalcedony as products of supergene alteration rare earth element and trace element geochemical indicators suggest that the manganese mineralization originated as hydrogenetic mn4 oxyhydroxide precipitates which were diagenetically transformed into braunite the most striking feature of the manganese ores of the bronkhorstfontein deposit is the beautiful preservation of various types of sedimentary particles including well rounded detrital quartz grains and hematite chert pellets as well as oncoids the oncoids in particular are remarkable because they are very similar in size shape and internal texture to modern biogenic analogues their abundance throughout the ore bearing strata and their absence from the intercalated sediments strongly suggests that oncoid building micro organisms mediated manganese precipitation this observation is used to suggest that microbial mediation may be of greater importance for the origin of shallow marine manganese deposition than is indicated by currently accepted metallogenetic models for shallow marine pisolitic manganese deposits

seismological imaging of ridge arc interaction beneath the eastern lau spreading center from obs ambient noise tomography abstract the lau basin displays large along strike variations in ridge characters with the changing proximity of the adjacent subduction zone the mechanism governing these changes is not well understood but one hypotheses relates them to interaction between the arc and back arc magmatic systems we present a 3d seismic velocity model of the shallow mantle beneath the eastern lau back arc spreading center elsc and the adjacent tofua volcanic arc obtained from ambient noise tomography of ocean bottom seismograph data our seismic images reveal an asymmetric upper mantle low velocity zone lvz beneath the elsc two major trends are present as the ridge to arc distance increases 1 the lvz becomes increasingly offset from the ridge to the north where crust is thinner and the ridge less magmatically active 2 the lvz becomes increasingly connected to a sub arc low velocity zone to the south the separation of the ridge and arc low velocity zones is spatially coincident with the abrupt transition in crustal composition and ridge morphology our results present the first mantle imaging confirmation of a direct connection between crustal properties and uppermost mantle processes at elsc and support the prediction that as elsc migrates away from the arc a changing mantle wedge flow pattern leads to the separation of the arc and ridge melting regions slab derived water is cutoff from the ridge resulting in abrupt changes in crustal lava composition and crustal porosity the larger offset between mantle melt supply and the ridge along the northern elsc may reduce melt extraction efficiency along the ridge further decreasing the melt budget and leading to the observed flat and faulted ridge morphology thinner crust and the lack of an axial melt lens

carbonate silicate inclusions in garnet as evidence for a carbonate bearing source for fluids in leucocratic granitoids associated with granulites of the southern marginal zone limpopo complex south africa abstract we present a study of carbonate bearing polyphase inclusions in garnets from leucocratic granitoids intruding metapelitic granulites of the southern marginal zone smz of the neoarchean limpopo high grade complex south africa during the post peak stage 2710 2650 ma u pb ages for zircons and monazites ternary feldspar thermometry suggests that the granitoid magma cooled from temperatures 800 900 c at a pressure of ca 6 5 kbar abundant co2 fluid inclusions in quartz and t xco2 phase equilibria modeling via perple x imply action of an essentially carbonic fluid in the granitoids cores of almandine rich garnet grains from the granitoids contain polyphase carbonate bearing inclusions with a distinct negative crystal shape the major carbonate in the inclusions is a strongly zoned magnesite siderite variety whereas pyrophyllite is the predominant silicate phase raman spectra of unexposed inclusions revealed a presence of co2 as well as ch4 and h2o the carbonate bearing inclusions coexist with larger polyphase inclusions composed of biotite quartz k feldspar plagioclase sillimanite which are interpreted as relics of granitic melts modeling the mineral assemblage preserved within the carbonate bearing inclusions shows that their present mineral and chemical compositions are a product of interaction of the trapped aqueous carbonic fluid with host garnet during cooling below 400 c despite strong modifications the inclusions bear evidence for initial saturation of the fluid with mg carbonate this is taken as an evidence for an origin of the fluids by devolatilization of the mg rich carbonate bearing ultrabasic greenstone rocks of the kaapvaal craton that were buried under the smz being generated at temperatures between 650 and 700 c the fluid subsequently participated in anatexis and coexisted with the granite magma during exhumation and interaction of the smz granulites with cratonic rocks

meso archaean and palaeo proterozoic sedimentary sequence stratigraphy of the kaapvaal craton abstract the kaapvaal craton hosts a number of precambrian sedimentary successions which were deposited between 3105 ma dominion group and 1700 ma waterberg group although younger precambrian sedimentary sequences outcrop within southern africa they are restricted either to the margins of the kaapvaal craton or are underlain by orogenic belts off the edge of the craton the basins considered in this work are those which host the witwatersrand and pongola ventersdorp transvaal and waterberg strata many of these basins can be considered to have formed as a response to reactivation along lineaments which had initially formed by accretion processes during the amalgamation of the craton during the mid archaean faulting along these lineaments controlled sedimentation either directly by controlling the basin margins or indirectly by controlling the sediment source areas other basins are likely to be more controlled by thermal affects associated with mantle plumes accommodation in all these basins may have been generated primarily by flexural tectonics in the case of the witwatersrand or by a combination of extensional and thermal subsidence in the case of the ventersdorp transvaal and waterberg wheeler diagrams are constructed to demonstrate stratigraphic relationships within these basins at the first and second order levels of cyclicity and can be used to demonstrate the development of accommodation space on the craton through the precambrian

acoustic green s function extraction from ambient noise in a coastal ocean environment we report on the results of an underwater acoustic field experiment conducted in december 2012 on the continental shelf off the florida keys in water of approximately 100 m depth ambient noise was recorded concurrently on three moored near bottom instruments with horizontal separations of approximately 5 km 10 km and 15 km we focus in this letter on instrument pairs with 5 km and 10 km separations consistent with theoretical predictions coherent sums of many realizations of cross correlations of ambient noise records at two measurement locations are shown to yield approximations to deterministic acoustic green s functions that describe propagation between those two locations the results presented demonstrate the feasibility of extracting information suitable for use in tomographic inversions from measurements of underwater acoustic ambient noise

structural geological and metamorphic significance of the kaapvaal craton limpopo belt contact abstract the hout river shear zone forms the terrane boundary between the granite greenstone terrane of the kaapvaal craton cratonic terrane and the high grade granulite terrane high grade gneiss province of the limpopo belt this structure is superimposed onto a thin skinned northward verging thrust system which exercises a significant control on the distribution of the ene wsw trending greenstone belts the hout river shear zone however dips steeply to the north and flattens out toward the south this structure was responsible for the emplacement of hot granulites against and over the cooler granite greenstone terrane and strongly influenced the metamorphic evolution of the area the shear zone acted as a conduit for infiltrating co2 rich h2o bearing fluids into still hot crust producing a retrograde isograd and associated zone of rehydration in the granulitic hanging wall of the structure retrogressed granulites in the hanging wall are characterized by fine grained kyanite as part of the retrograde assemblages while the greenstones in the footwall are characterized by coarse grained prograde kyanite the terrane boundary appears to be in thermal equlibrium with the lithologies in both the footwall and hanging wall so that tectonically mixed prograde greenstones and retrogressed granulites in and adjacent to the shear zone are apparently at a similar metamorphic grade the granulites appear to have had a hot iron effect on the cooler footwall rocks over which they have been displaced

a half graben setting for the proterozoic soutpansberg group south africa evidence from the blouberg area abstract the ca 1 85 ga soutpanberg group of the kaapvaal craton has in the past been interpreted as a volcano sedimentary sequence deposited either within an aulacogen rift basin or as a more extensive cover sequence preserved only within a post sedimentary graben evidence from the blouberg area of northern province south africa indicates that the soutpansberg strata were laid down within a half graben bound to the south by a northwards dipping normal fault palaeocurrent directions within the wyllie s poort formation of the soutpansberg group suggest that planar bedded ripple marked sandstones were deposited by transverse flowing rivers whilst small to large scale trough and planar cross bedded sandstones indicate transport in large low sinuosity longitudinal trunk rivers flowing approximately parallel to the axis of the half graben the n s oriented syn southpansberg extensional regime may reflect orogenic collapse after the cessation of n s orientated compression developed at ca 2 0 ga by collision or reactivation tectonics in the limpopo belt

structural and p t evolution of a major cross fold in the central zone of the limpopo high grade terrain south africa the central zone cz of the limpopo complex of southern africa is characterized by a complex deformational pattern dominated by two types of fold geometries large sheath folds and cross folds the sheath folds are steeply ssw plunging closed structures whereas the cross folds are north south oriented with near horizontal fold axes in the area south of messina this complexly folded terrain grades continuously towards the south into a crustal scale ene wsw trending ductile shear zone with moderate dip towards the wsw all sheath folds document consistent top to the ne thrust movement of high grade material the timing of this shear deformational event d and thus of the gneissic fabric s is constrained at 2 6 ga by the syntectonic emplacement throughout the cz of precursors to quartzo feldspathic gneisses singelele type gneisses cross folds deform the s fabric and are characterized by a near vertical axial planar cleavage s recent single phase pb pb dating of garnet from a metapelitic gneiss with an s fabric from one of the largest cross folds in the cz constrains the timing of the deformational d and metamorphic m event at 2 0 ga mineral chemistry for metapelites from this cross fold shows a single peak on an n mg histogram for garnet reflecting a single phase of mineral growth metapelites from this cross fold also preserve evidence for only one well developed reaction texture grt sil qtz crd this reaction is accompanied by the simultaneously operating reaction grt fsp h o bt sil qtz both these divariant reactions belong to the univariant kfmash equilibrium crd grt fsp h o bt sil qtz the progress of the two divariant retrograde reactions leads to the consumption of grt and fsp k feldspar or never occurs with both cordierite and garnet microprobe profiling coupled with calculated isopleths for bt grt and crd in divariant equilibria define a decompression cooling p t path that reflects a single m high grade metamorphic event during the evolution of the cross fold this decompression cooling p t path traverses from 780 c 5 7 kbar to 600 c 3 3 kbar

fluids in high to ultrahigh temperature metamorphism along collisional sutures record from fluid inclusions abstract petrographic studies and microthermometric investigations on fluid inclusions associated with high to ultrahigh temperature metamorphic rocks in three major precambrian suture zones on the globe demonstrate the dominant occurrence of co 2 rich fluids these rocks form part of hot orogens developed along collisional plate boundaries the sapphirine quartz bearing mg al rich rock from the palghat cauvery suture zone a trace of the cambrian gondwana suture zone in southern india preserves evidence for a prograde high pressure event and subsequent peak ultrahigh temperature metamorphism along a clockwise path and contains abundant co 2 rich inclusions in corundum garnet and sapphirine most of the fluid inclusions are either primary or secondary and preserve low density co 2 rich fluids 0 569 0 807 g cm 3 similar low density co 2 rich fluid inclusions 0 853 0 953 g cm 3 are also present in pelitic granulites from the limpopo complex of southern africa a neoarchean granulite facies orogen formed by continent continent collision in contrast the garnet orthopyroxene granulite from tonagh island in the neoarchean napier complex in east antarctica contains very high density primary 1 095 1 129 g cm 3 and secondary 0 960 1 179 g cm 3 carbonic inclusions in garnet and quartz the calculated isochores for the fluid inclusions from the palghat cauvery suture zone and the limpopo complex yield significantly lower pressure estimates than those predicted from peak metamorphic conditions we interpret this as a result of significant density decrease due to rapid decompression along a clockwise p t trajectory in contrast the estimated isochores for primary inclusions in garnet orthopyroxene granulites from the napier complex are consistent with the peak p t conditions estimated from mineral phase equilibria for the tonagh island rocks suggesting that most of the fluid inclusions in these rocks did not undergo any marked effect of volume change and density decrease the contrasting fluid densities among the localities investigated in this study are probably related to the nature of the p t trajectory the tonagh island rocks had a near isochoric exhumation history whereas the metamorphic orogens in the other two sutures witnessed rapid decompression our results suggest that whereas the composition of the syn metamorphic fluids are preserved in all cases density reversal occurs within inclusions as a function of the tectonic history and exhumation style

underwater masw to evaluate stiffness of water bottom sediments stiffness measurements are often necessary for geotechnical characterization of an underwater site seismically these measurements can be made through the dispersion analysis of the rayleigh type surface waves successful terrestrial application of this method has been reported by many investigators using spectral analysis of surface waves sasw and more recently using multichannel analysis of surface waves masw the masw method was originally developed as a land survey method to investigate the near surface materials for their elastic properties for example the shear wave velocity vs by recording and analyzing rayleigh type surface waves using a vertical impulsive seismic source and receivers the acquired data are first analyzed for dispersion characteristics and from these the shear wave velocity is estimated using an inversion technique

shear wave velocity in marine sediments on young oceanic crust constraints from dispersion analysis of scholte waves summary an experiment with a newly developed implosive source located about 1 m above the sea oor at 3665 m depth revealed a slow interface wave this wave is attributed to s waves in a soft sediment layer covering the hard rock sub bottom dispersion analysis by means of the multiple lter technique yields the group velocity as a function of frequency model calculations showed that the observed dispersion curve can be reproduced by considering a steep gradient of s velocity within the sediment layer nearly perfect agreement of experimental and model data could be achieved with a sediment layer thickness of 21 5 m an s speed of 225 m s 1 at the water sediment interface and an increase by 23 s 1 within the layer these values are relatively high compared to data from the literature however previous estimates of in situ s wave velocity have been obtained on old oceanic crust in the vicinity of continents or islands while our experiment was carried out on young paci c crust therefore we suggest two mechanisms which could support a relatively high s speed in sediments 1 the input of hydrothermally generated metalliferous sediments from the adjacent spreading axis and 2 post depositional diagenesis which has accelerated the induration of sediments

evaluating hot spot ridge interaction in the atlantic from regional scale seismic observations we probe variations in mantle temperature composition and fabric along hot spot influenced sections of the mid atlantic ridge mar using surface waves from nearby ridge earthquakes recorded on broadband island based seismic stations we invert frequency dependent phase delays from these events to estimate one dimensional mean shear velocity and radial shear anisotropy profiles in the upper 200 km of the mantle within two seafloor age intervals 5 10 ma and 15 20 ma mean shear velocity profiles correlate with apparent hot spot flux lithosphere formed near the low flux ascension hot spot is characterized by high mantle velocities while the mar near the higher flux azores hot spot has lower velocities the impact of the high flux iceland hot spot on mantle velocities along the nearby mar is strongly asymmetric the lithospheric velocities near the kolbeinsey ridge are moderately slow while velocities near the reykjanes ridge estimated in previous studies are much slower within each region the increase in shear velocity with age is consistent with a half space cooling model and the velocity variations observed between ascension the azores and kolbeinsey are consistent with approximately 75 potential temperature variation among these sites in comparison the reykjanes lithosphere is too slow to result purely from half space cooling of a high temperature mantle source we speculate that the anomalously low shear velocities within the lithosphere produced at the reykjanes ridge result from high asthenospheric temperatures of 50 75 k combined with 12 by volume gabbro retained in the mantle due to the imbalance between high hot spot influenced melt production and relatively inefficient melt extraction along the slow spreading reykjanes radial shear anisotropy in the upper 150 km also indicates an apparent hot spot influence mantle fabric near ascension is quite weak consistent with previous models of anisotropy produced by corner flow during slow seafloor spreading the fabric near the azores and the kolbeinsey ridge is stronger suggesting that the hot spot increases mantle deformation beyond that produced by slow seafloor spreading in these regions

tectono metamorphic evolution of the simatic basement of the pietersburg greenstone belt relative to the limpopo orogeny evidence from serpentinite abstract detailed studies of low grade ultramafic rocks are integrated with regional structural field observations in the pietersburg greenstone belt these results show that petrographic investigations of serpentinite can reveal more about the tectonometamorphic history of greenstone belts than has previously been appreciated the ultramafic rocks of the pietersburg greenstone belt were affected by an early deformation event d 1 which is reflected by the presence of a relic magnetite fabric s 1 in serpentinite the s 1 fabric is crosscut by the peak metamorphic assemblage of antigorite tremolite talc chlorite both s 1 and the peak metamorphic assemblage are deformed by a second deformation event d 2 on a macroscale d 2 is manifested as a series of northward verging thrust faults that affected the entire belt published u pb zircon data show that d 2 was active between 2 9 and 2 7 ga prior to uplift and southward thrusting of the limpopo belt lb major carbonatization of the simatic basement postdates peak metamorphism and occurred towards the end of d 2 coeval with the late evolution of the limpopo orogeny the alteration assemblages were subsequently deformed the sequence of events that affected the pietersburg greenstone belt is similar to that established for the high grade southern marginal zone of the limpopo belt but their absolute timing is different the results of this study demonstrate the important contribution that low grade ultramafic rocks can make to the understanding of the tectono metamorphic history of the archaean cratons and terrane accretion

predictive mapping of prospectivity for orogenic gold giyani greenstone belt south africa we present a mineral systems approach to predictive mapping of orogenic gold prospectivity in the giyani greenstone belt ggb by using layers of spatial evidence representing district scale processes that are critical to orogenic gold mineralization namely a source of metals fluids b active pathways c drivers of fluid flow and d metal deposition to demonstrate that the quality of a predictive map of mineral prospectivity is a function of the quality of the maps used as sources of spatial evidence we created two sets of prospectivity maps one using an old lithologic map and another using an updated lithological map as two separate sources of spatial evidence for source of metals fluids drivers of fluid flow and metal deposition we also demonstrate the importance of using spatially coherent or geologically consistent deposit occurrences in data driven predictive mapping of mineral prospectivity the best predictive orogenic gold prospectivity map obtained in this study is the one that made use of spatial evidence from the updated lithological map and spatially coherent orogenic gold occurrences this map predicts 20 of the ggb to be prospective for orogenic gold with 89 goodness of fit between spatially coherent inactive orogenic gold mines and individual layers of spatial evidence and 89 prediction rate against spatially coherent orogenic gold prospects in comparison the predictive gold prospectivity map obtained by using spatial evidence from the old lithological map and all gold occurrences has 80 goodness of fit but only 63 prediction rate these results mean that the prospectivity map based on spatially coherent gold occurrences and spatial evidence from the updated lithological map predicts exploration targets better i e 28 smaller prospective areas with 9 stronger fit to training gold mines and 26 higher prediction rate with respect to validation gold prospects than the prospectivity map based on all known gold occurrences and spatial evidence from the old lithological map

geochronology of the hout river shear zone and the metamorphism in the southern marginal zone of the limpopo belt southern africa abstract in this paper monazite u pb and zircon evaporation dates stepleaching pb pb results on garnet staurolite and kyanite and hornblende ar ar data are presented which constrain the timing of granulite facies metamorphism in the southern marginal zone of the limpopo belt and its thrusting onto the kaapvaal craton the southern marginal zone of the limpopo belt is considered to be a lower crustal equivalent of the northern kaapvaal craton granulite exhumation is associated with southward thrusting along the hout river shear zone which is a set of thrust and strike slip shear zones zircon ages for the matok intrusive complex which was emplaced within the zone during this thrusting charno enderbites 2671 4 ma granodiorites between 2667 and 2664 ma have previously been interpreted as evidence for rapid exhumation of the southern marginal zone within only 7 ma we have obtained a u pb date of 2691 7 ma for monazite from the bandelierkop quarry in the southern marginal zone interpreted as the age of high grade metamorphism a single zircon evaporation pb pb date of 2643 1 ma from a leucosome band at the same locality may indicate longer lasting metamorphism or decompression melting during exhumation anatexis of metapelitic xenoliths within the matok intrusive complex was dated at 2663 4 ma by u pb on monazite indistinguishable from existing zircon ages for this complex pb pb step leaching dates obtained on synkinematically grown garnet 2691 20 ma staurolite 2712 37 ma and kyanite 2672 51 ma from the khavagari hills in the giyani greenstone belt in the immediate footwall of the hout river shear zone indicate that early thrusting was contemporaneous with peak metamorphism in the southern marginal zone ar ar dating and geochemistry on syntectonic hornblende separates from the same shear zone system yielded disturbed spectra and indicated multiple populations probably reflecting repeated or continuous tectonic activity of the hout river shear zone up to about 2600 ma

high grade reworking of central australian granulites part 1 structural evolution abstract four distinct deformational events d2 d5 accompanying a granulite facies metamorphic cycle m2 m5 are shown to structurally overprint rework pre existing granulites m1 of the northeast strangways range in the central arunta block the first metamorphic cycle m1 at 1800 ma involved an anticlockwise p t path peaking at 850 950 c at 8 9 kbar m1 metamorphism involved widespread partial melting that produced stromitic migmatites and map scale concordant granitic gneisses isobaric cooling after the peak and accompanying hydration gave rise to a wide variety of coronitic and symplectitic reaction textures that enclose and replace m1 mineral parageneses no kinematic structuring is associated with m1 and the period encompassing the thermal peak and immediately subsequent are thought to have been absent of deviatoric stress the second metamorphic cycle m2 m5 possibly at 1400 1500 ma involved a clockwise p t path with a maximum p of 9 10 kbar and a thermal peak of approximately 800 c this metamorphic cycle accompanied the major ductile deformations recognized d2 d5 and has been labelled the proterozoic reworking the proterozoic reworking has been divided into two thermo barometric and structurally distinct periods d2 d3 involved regionally inclined ene wsw non coaxial shear of high bulk shear strains giving rise to an intense often mylonitic pervasive fabric s2 l2 and isoclinal and sheath folds on all scales f2 f3 s2 l2 is the first recognized kinematic fabric and does enclose and partially recrystallize m1 mineral parageneses including late m1 metamorphic reaction textures d2 d3 deformation was due to crustal shortening and gave rise to crustal over thickening loading accompanying prograde metamorphism m2 d4 d5 involved upright open and asymmetrical folding f4 and e w trending shear zone development s5 within a regionally extensive system of inclined oblique sinistral transpression both d2 d3 and d4 d5 episodes occurred under the same approximately e w directed compressive stress and are considered sequential episodes in the one tectonothermal cycle f4 folding accompanied significant melt formation and the peak of metamorphism of the m2 m5 metamorphic cycle s5 shear zones accompanied 3 4 kbar of decompression with cooling presumably during uplift in isostatic response to crustal over thickening in d2 d3 progression from inclined non coaxial shear to transpression occurred in response to an increase in the relative buoyancy forces limiting crustal thickening as is typical of many compressional orogens however the proterozoic reworking is atypical of simple linear or arcuate mountain belts because late stage shearing s5 is aligned sub parallel not at a high angle to the transport vector of the earlier ductile deformations d2 d3 consequently this region is modelled in terms of crustal shortening directed along the length of an e w trending orogen confined to the north and south by relatively stable crustal blocks when e w shortening could no longer be accommodated by crustal thickening strain was partitioned by sinistral transpression between the north and south bounding crustal blocks

composition oxygen isotope geochemistry and origin of smectite in the metalliferous sediments of the bauer deep southeast pacific the sediments of the bauer deep an open ocean basin situated on the northwest nazca plate in the southeast pacific constitute a regional metalliferous deposit dominated by authigenic smectite two 2 metre long cores from the bauer deep were examined to investigate the nature and origin of the smectite infra red and mossbauer spectroscopy and wet chemical analysis libo2 fusion of isolated smectite indicate the mineral is a mg rich al rich nontronite oxygen isotopic compositions for isolated smectite are uniform and translate to a non hydrothermal temperature of formation of about 3 c sem observations show an abundance of well preserved biogenic opal in surface and near surface sediment but postburial dissolution and transformation of this phase to smectite is evident at depth smectite formation is the result of interaction between iron oxyhydroxide ponded in the bauer deep following a hydrothermal origin at the adjacent east pacific rise and biogenic opal a reaction mechanism is proposed regional factors control smectite formation in particular formation is inhibited in areas of caco3 accumulation topographic elevations but favoured in areas of oxyhydroxide and opal ponding topographic depressions

passive probing of the sound fixing and ranging channel with hydro acoustic observations from ridge earthquakes the international monitoring system includes a hydro acoustic part to verify the comprehensive nuclear test ban treaty besides explosive signals monitoring stations also detect acoustic waves from earthquakes that travel through the sound fixing and ranging sofar channel the travel times of such detections are listed in the reviewed event bulletin which is statistically evaluated for the stations located in the pacific indian and atlantic oceans the celerities of ridge earthquakes are calculated to build up a homogeneous data set based on similar source mechanisms the celerity is defined as the epicentral distance divided by the travel time the global characteristics of these celerities can be well understood in terms of temperature variations in the sofar channel a detailed velocity profile has been retrieved for the atlantic ocean where large differences 14 m s are found between the southern and northern parts of the basin propagation modeling with normal modes supports these findings which shows that the celerity gives an estimate of the sound speed in the sofar channel these results compare remarkably well with those from active experiments showing the ability of passively probing the sofar channel with hydro acoustic waves from earthquake sources

on the use of the norwegian geotechnical institute s prototype seabed coupled shear wave vibrator for shallow soil characterization i acquisition and processing of multimodal surface waves summary pure shear wave data are only very rarely acquired for offshore site investigations and exploration here we present details of a novel seabed coupled shear wave vibrator and field data recorded by a densely populated multicomponent ocean bottom cable to improve shallow soil characterization the prototype shear wave vibrator uses vibroseis technology adopted for marine environments through its instalment on top of a suction anchor assuring seabed coupling in combination with self weight penetration the prototype is depth rated to 1500 m water depth and can be rotated while installed in the seabed the philosophy is to acquire fully complementary seismic data to conventional p and p to s converted waves in particular for 2 d profiling vsp vertical seismic profiling or monitoring purposes thereby exploiting advantages of shear waves over compressional waves for determining for example anisotropy small strain shear modulus and excess pore pressures effective stress the source was primarily designed for reservoir depths however significant energy is emitted as surface waves which provide detailedgeotechnicalinformationthroughmappingofshearwavevelocitiesinpotentiallyhigh resolution of the upper soil units to fully utilize pure shear wave content a proper analysis of surface waves is paramount due to the proximity of surface wave propagation speed with shear wave velocities the experiment was carried out in the northern north sea in 364 m water depth cable dragging was necessary to obtain close receiver spacing 2 5 m effective spacing with total line length of 600 m frequency waveform transforms reveal both scholte and love waves up to six surface wave modes are identified that is fundamental mode and several higher surface wave modes the occurrence of these two different dispersive surface wave types with well resolved higher modes allows for a unique analysis and inversion scheme for highresolution mapping of physical properties in the shallow subsurface as well as anisotropy which is discussed in an accompanying paper the data presented in this paper are thus a unique long and densely populated receiver array allows for multimodal love and scholte surface waves from the marine environment but challenging marine operations marine data set

tectonic and geochronological constraints on late archaean and palaeoproterozoic stratigraphic correlation within and between the kaapvaal and pilbara cratons recent radiometric dating of carbonates and banded iron formations on the kaapvaal craton southern africa has suggested that late archaean and palaeoproterozoic carbonate sedimentation was diachronous across the craton we present new shrimp u pb zircon ages of 2583 5 ma and 2588 7 ma for two samples of a tuff bed 320 km apart in the upper oak tree formation transvaal supergroup these ages in conjunction with published data support previously established sequence stratigraphic correlations that show the temporal equivalence of late archaean carbonates in the region a review of the sequence stratigraphy and geochronology of the mount bruce megasequence set on the pilbara craton western australia and the newly named highveld megasequence set on the kaapvaal craton indicates a remarkably similar geohistory both cratons are characterized by a divergent megacycle hamersley range and vaal megasequences followed by a convergent megacycle chichester range and drakensberg megasequences our data show that the megasequence boundary is marked by drowning of both cratons at approximately the same time c 2590 ma which permits more meaningful geotectonic correlations between them

settling dissolution and burial of biogenic silica in the sediments off somalia northwestern indian ocean abstract particle fluxes of biogenic silica through the water column silica burial fluxes into the sediments and the flux of dissolved silica across the sediment water interface estimated from pore water profiles are used to assess the behaviour of biogenic silica at two stations 80 and 270 km offshore along a transect off the somali coast in the northwestern indian ocean particulate biogenic silica fluxes varied from 0 3 mmol m 2 day 1 in the non upwelling season to 6 mmol m 2 day 1 during upwelling on the somali slope fluxes were lower in the somali basin from 0 2 to 2 3 mmol m 2 day 1 evaluation of the dissolution curves derived by wet chemical leaching in sediment trap and sediment samples shows that the km values the apparent reactivity rates in alkaline medium are higher for the shallow sediment traps than for deep trap and boxcore sediments modelling of pore water profiles shows that in the sediment most dissolution occurs in the top halfcentimetre and pore water effluxes are in close agreement with those from in situ benthic incubations our results show that less than 10 of the biogenic silica arriving on the somali margin is buried in the sediments giving a burial efficiency lower than the approximately 20 reported from the open arabian sea

deep crystal response to continental collision the limpopo belt of southern africa the limpopo belt is an example of an archean 2700 ma continental collisional zone in which the thickened crust 70 km has virtually attained equilibrium with the adjacent non thickened plates by a process of upward movement and lateral spreading of the underthrust high grade metamorphosed plates

fundamental and higher mode rayleigh wave characteristics of ambient seismic noise in new zealand 1 in order to use ambient seismic noise for mapping earth s structure it is important to understand the spatiotemporal characteristics of the noise field this study uses data collected during four austral winter months of 2002 to investigate new zealand s ambient seismic noise field in the double ocean wave frequency range 0 1 0 3 hz it is shown via beamforming analysis that there are two distinct dispersive waves in the data these waves can be separated their estimated phase velocities 2 5 2 and 4 3 km s in the frequency range 0 14 0 25 hz match well with fundamental and higher mode rayleigh dispersion curves studies of double wave frequency microseisms elsewhere generally show the rayleigh noise fields to be dominated by fundamental mode waves the reason why higher mode signals are observed here may reflect a combination of long ocean wave periods large waveheights the direct deep water approach to narrow continental margins and the proximity of the seismograph array to the source regions citation brooks l a j townend p gerstoft s bannister and l carter 2009 fundamental and higher mode rayleigh wave characteristics of ambient seismic noise in new zealand geophys res lett 36 l23303 doi 10 1029 2009gl040434

on the coherent components of low frequency ambient noise in the indian ocean this letter demonstrates that the dominant coherent component of low frequency 1 hz f 20 hz ambient noise propagating between hydrophone pairs of the same hydroacoustic station deployed in the deep sound channel of the indian ocean is directional and mainly originates from antarctica however the amplitude of the peak coherent noise arrivals obtained using a 4 month long averaging interval was relatively low given the small hydrophones spacing hydrophones 2 km hence extracting similar coherent arrivals between two distinct hydroacoustic stations separated instead by thousands of kilometers for noise based acoustic thermometry purposes seems unlikely even using a year long averaging

an isotopic and geochemical study of the northern kaapvaal craton and the southern marginal zone of the limpopo belt are they juxtaposed terranes abstract to test existing lateral accretion models for the archean kaapvaal craton in south africa metapelites and leucocratic granitoids from two different provinces the granulite facies southern marginal zone of the limpopo belt and the northern part of the adjacent greenschist to amphibolite facies kaapvaal craton have been studied both provinces resemble each other in terms of isotope and trace element characteristics they show a general depletion of incompatible elements independent from the grade of metamorphism this observation conflicts with theories involving mobilisation during granulite facies metamorphism a further common fingerprint is the unusually high cr and ni concentrations in the metapelites in addition they define a very narrow range of nd model ages 2 9 3 05 ga constraining the average crustal age of their cratonic source region the granitoid gneisses show somewhat larger variations of nd model ages indicating some age heterogenity within the crust however the data from both provinces completely overlap the southern marginal zone and the kaapvaal craton also show uniform low 207 pb 204 pb relative to their 206 pb 204 pb ratios that differ clearly from other southern african provinces these results support the view that the southern marginal zone represents a part of the kaapvaal craton exposed at a deeper crustal level it rules out a hypothesis that both provinces are separate amalgamated terranes furthermore there is very little difference between granitoids and metapelites which indicates that the latter largely derived from the former the exposure of the granulite facies metapelites of the southern marginal zone at the same level as the lower grade metamorphic rocks of the kaapvaal craton indicates uplift along a sharp tectonic boundary

crustal and uppermost mantle shear velocity structure adjacent to the juan de fuca ridge from ambient seismic noise 1 based on 6 months of obs data from the cascadia initiative experiment near the juan de fuca ridge we obtain rayleigh wave group and phase speed curves from 6 s to about 20 s period from ambient noise cross correlations among all station pairs we confirm the hypothesis that the dispersion data can be fit by a simple age dependent formula which we invert using a bayesian monte carlo formalism for an age dependent shear wave speed model of the crust and uppermost mantle between crustal ages of 0 5 and 3 5 ma igneous crustal structure is age invariant with a thickness of 7 km water depth varies in a prescribed way and sedimentary thickness and mantle shear wave speeds are found to increase systematically with crustal age the mantle model possesses a shallow low shear velocity zone lvz with a velocity minimum at about 20 km depth at 0 5 ma with lithosphere thickening monotonically with age minimum mantle shear velocities at young ages are lower than predicted from a half space conductively cooling model hscm and the lithosphere thickens with age faster than the hscm providing evidence for nonconductive cooling in the young lithosphere the shallow lvz is consistent with expectations for a largely dehydrated depleted harzburgite mantle with a small retained near ridge partial melt fraction probably less than 1 with melt extending to a lithospheric age of approximately 1 ma i e 30 km from the ridge

offshore rayleigh group velocity observations of the south island new zealand from ambient noise data we thank captain and crew of the r v thomas g thompson cruise tn229 the r v roger revelle cruise rr1002 and the sio obsip facility for expert assistance at sea the obs instruments used in this field program were provided by the u s national ocean bottom seismic instrumentation pool http www obsip org moana seismic data are archived at the iris data management center the collection of ocean bottom seismic data was funded by the national science foundation under grants ear 0409564 ear 0409609 and ear 0409835 new zealand onshore seismic data made available by the geonet project sponsored by eqc gns science and linz j stachnik acknowledges support from the cires postdoctoral fellowship program f c lin was supported by nsf grant cybersees 1442665 and the king abdullah university of science and technology kaust under award ocrf 2014 crg3 2300 we thank martha savage dan zietlow justin ball craig jones bill fry and peter molnar for valuable comments and discussion and john collins for leading the moana obs experiment we thank two anonymous reviewers and editor mike ritzwoller for their constructive comments

the vredefort structure a perspective with regard to new tectonic data from adjoining terranes abstract the deformational history of the area surrounding the vredefort structure in south africa is long and complex involving several periods of contraction and extension a general chronology of events covering a time span of 1000 ma is established the vredefort deformation is identified as a separate structural event which is distinct from the earlier deformations it is essential therefore that deformational structures related to the pre vredefort event should be identified and subtracted from the total spectrum of tectonism so that only those structures which can be related with certainty to the formation of the vredefort structure are defined this approach could influence interpretations of the origin of the vredefort structure which attribute most structural and geological associations to the deposition of sediments onto a granitoid basement followed by a vredefort deformational event

early diagenetic processes affecting nutrients in the pore waters of central indian ocean cores abstract pore water nutrients nitrite nitrate phosphate silicate ph and solid phase organic carbon were analysed for one core from the arabian sea and three cores from the manganese nodule area in the central indian ocean basin possible denitrification and unoxidized organic carbon are observed in the arabian sea core while nitrification with intermediate low nitrate and high organic carbon are found in the mottled zones of the pelagic cores mobilization of iron host oxyhydroxides and peaked profiles of organic carbon are related to a gradual increase in the phosphate profiles the asymptotic silica values are less than the 100 saturation levels of amorphous silica and they fall outside the stability field of sepiolite

a terrane interpretation of the archaean limpopo belt the limpopo belt is a zone of thickened archaean crust whose origin is currently explained by a late archaean continent continent collision between the kaapvaal and zimbabwe cratons this review shows that the two cratons have fundamentally different geological histories and that the zimbabwe craton was unlikely to have behaved as a stable cratonic block at the time of the limpopo belt collision the geological histories of the zimbabwe craton the north marginal central and south marginal zones of the limpopo belt and the kaapvaal craton are shown to be sufficiently different from one another to warrant their consideration as discrete terranes the boundaries between the five units outlined above are all major shear zones further supporting a terrane model for the limpopo belt the five units were all intruded by late to syn tectonic granites c 2 6 ga constraining the accretion event to c 2 6 ga

scholte wave tomography for shallow water marine sediments summary we determine the 3 d in situ shear wave velocities of shallow water marine sediments by extending the method of surface wave tomography to scholte wave records acquired in shallow waters scholte waves are excited by air gun shots in the water column and recorded at the seafloor by ocean bottom seismometers as well as buried geophones our new method comprises three steps 1 we determine local phase slowness values from slowness frequency spectra calculated by a local wavefield transformation of common receiver gathers areal phase slowness maps for each frequency used as reference in the following step are obtained by interpolating the values derived from the local spectra 2 we infer slowness residuals to those reference slowness maps by a tomographic inversion of the phase traveltimes of fundamental scholte wave mode 3 the phase slowness maps together with the residuals at different frequencies define a local dispersion curve at every location of the investigation area from those dispersion curves we determine a model of the depth dependency of shear wave velocities for every location we apply this method to a 1 km2 investigation area in the baltic sea northern germany the phase slowness maps obtained in step 2 show lateral variation of up to 150 per cent the shear wave velocity models derived in the third step typically have very low values 60 80 m s 1 in the top four meters where fine muddy sands can be observed and values exceeding 170 m s 1 for the silts and sands below that level the upper edge of glacial till with shear wave velocities of 300 400 m s 1 is situated approximately 20 m below sea bottom a sensitivity analysis reveals a maximum penetration depth of about 40 m below sea bottom and that density may be an important parameter best resolvable with multimode inversion

the crustal architecture of the kaapvaal crustal block south africa between 3 5 and 2 0 ga following terrane amalgamation of early oceanic lithosphere the southern and central parts of the kaapvaal craton were a coherent unit by 3 1 ga juxta position of the northern and western granitoid greenstone terranes including the murchison island arc was the result of terrane accretion that started at 3 1 ga the culmination of these events was the collision of the kaapvaal craton the pre cratonic zimbabwe block and the central zone to generate the limpopo granulite gneiss terrane coeval with these orogenic events the central kaapvaal craton underwent extension to accommodate the development of the dominion witwatersrand pongola and ventersdorp basins the craton scale thabazimbi murchison lineament development during the 3 1 ga accretion event and continued to influence the tectonic evolution of the kaapvaal block throughout the period under review as indicated by the syn sedimentary tectonics of the 2 64 ga wolkberg group overlying black reef formation and the transvaal sequence the transvaal and griqualand west basins developed in the late archaean 2 55 ga with basin dynamics influenced by far field stresses related to the limpopo orogeny during this period the thabazimbi murchison lineament lay close to the northern margin of the depository reactivation of the lineament between 2 4 and 2 2 ga resulted in inversion of the transvaal basin and formation of the northward verging mhlapitsi fold and thrust belt the half graben setting envisaged for the deposition of the pretoria group was influenced by the thabazimbi murchison lineament as was the emplacement and subsequent deformation of the bushveld complex

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 significance of rb sr ages of biotite and phlogopite for the thermal history of the central and southern marginal zones of the limpopo belt of southern africa and the adjacent portions of the kaapvaal craton abstract generally rb sr biotite and phlogopite ages range from 2050 ma to 1950 ma between the murchison thabazimbi and triangle tuli sabi shear zones of southern africa in the rocks comprising the central and southern marginal zones of the limpopo belt and adjacent portions of the kaapvaal craton in some shear zones however similar ages ocurr as young as 800 ma in contrast within the kaapvaal craton south of the murchison thabazimbi shear zone rb sr biotite ages range between 3150 ma and 2100 ma and show no regular distribution in all cases the ages are interpreted to indicate the time elapsed since the biotite last cooled below its closure temperature approximately 320 c the regular pattern of 2050 ma to 1950 ma rb sr biotite ages north of the murchison thabazimbi shear zone suggests that the present erosional surface approximates a paleo isothermal surface this surface was exposed by uplift and erosion which was completed by 1900 ma to 1800 ma ago when the sedimentary and volcanic rocks of the soutpansberg group were deposited the regional uplift was accommodated by motion on the murchison thabazimbi and triangle tuli sabi shear zones and probably was related to tectonism associated with the emplacement of the anorogenic bushveld and phalaborwa complexes other anorogenic granitic magmatism accompanied uplift the depth represented by the paleo isothermal surface and the magnitude of the uplift that exhumed it cannot be constrained precisely it is suggested however that the majority of the 10 to 15 km by which crust in this area is thinner than that of the adjacent cratons is the manifestation of this uplift

extracting coherent wave fronts from acoustic ambient noise in the ocean a method to obtain coherent acoustic wave fronts by measuring the space time correlation function of ocean noise between two hydrophones is experimentally demonstrated though the sources of ocean noise are uncorrelated the time averaged noise correlation function exhibits deterministic waveguide arrival structure embedded in the time domain green s function a theoretical approach is derived for both volume and surface noise sources shipping noise is also investigated and simulated results are presented in deep or shallow water configurations the data of opportunity used to demonstrate the extraction of wave fronts from ocean noise were taken from the synchronized vertical receive arrays used in the frame of the north pacific laboratory npal during time intervals when no source was transmitting

silica distribution in interstitial waters and sediments from the southeastern pacific abstract analyses for silica in the interstitial water of five cores from the southeast pacific are presented silica is enriched in these interstitial waters resulting in a vertical flux of silica of between 10 and 50 mol cm 2 yr 1 from the sediment into the overlaying seawater this flux is generated by the dissolution of biogenic silica the dissolution of which is increased in areas of bottom water turbulence the si al and calculated opal leinen 1977 contents of the bulk sediment of these cores are also presented small scale variations over depth intervals of tens of centimetres are present as a result of chaning conditions of sedimentation

three dimensional structure of the upper mantle beneath the atlantic ocean inferred from long period rayleigh waves 1 group and phase velocity distributions group and phase velocity distributions of rayleigh wave fundamental modes are determined in the atlantic ocean using a method of regionalization without a priori constraints for a period range from 50 to 200 s eighty six direct source station paths are studied and 38 out of the best data from weidner s 1974 phase velocities are added to our data set for periods lower than 100 s the importance of shallow layer corrections is described in detail the general features of the distributions agree with global models and smaller scale heterogeneities are revealed a spatial resolution of 1500 km is needed to detect fine lateral heterogeneities in the atlantic the small lateral extent of young oceanic regions and strong lateral velocity gradients do not allow to quantify velocity versus age relationships using our data set on the other hand the regionalization without a priori constraints of phase velocities confirms the presence of a high velocity anomaly beneath the central part of the atlantic for periods greater than 100 s

geochronological problems related to polymetamorphism in the limpopo complex south africa abstract the integration of new and published geochronologic data with structural magmatic anatectic and pressure temperature p t process information allow the recognition of high grade polymetamorphic granulites and associated high grade shear zones in the central zone cz of the limpopo high grade terrain in south africa together these two important features reflect a major high grade d 3 m 3 event at 2 02 ga that overprinted the 2 63 ga high grade neoarchaean d 2 m 2 event characterized by sw plunging sheath folds these major d 2 m 2 folds developed before 2 63 ga based on u pb zircon age data for precursors to leucocratic anatectic gneisses that cut the high grade gneissic fabric the d 3 m 3 shear event is accurately dated by u pb monazite 2017 1 2 8 ma and pbsl garnet 2023 11 ma age data obtained from syntectonic anatectic material and from sheared metapelitic gneisses that were completely reworked during the high grade shear event the shear event was preceded by isobaric heating p 6 kbar and t 670 780 c which resulted in the widespread formation of polymetamorphic granulites many efforts to date high grade gneisses from the cz using pbsl garnet dating resulted in a large spread of ages 2 0 2 6 ga that reflect the polymetamorphic nature of these complexly deformed high grade rocks

green s functions extraction and surface wave tomography from microseisms in southern california we use crosscorrelations of seismic noise data from 151 stations in southern california to extract the group velocities of surface waves between the station pairs for the purpose of determining the surface wave velocity structure we developed an automated procedure for estimating the green s functions and subsequent tomographic inversion from the 11 325 station pairs based on the characteristics of the noise field we eliminate specific events by a procedure that does not introduce any spurious spectral distortion in the band of interest 0 05 0 2 hz further we only used the emerging arrival structure above a threshold signal to noise ratio the result is that mostly station pairs with their axes oriented toward the sea are used consistent with the noise having a microseism origin finally it is the time derivative of the correlation function that is actually related to the green s function the emergence of the time domain green s function is proportional to the square root of the recording time and inversely proportional to the square root of the distance between stations the tomographic inversion yields a surface wave velocity map that compares favorably with more conventional and elaborate experimental procedures

petrographic and melt inclusion constraints on the petrogenesis of a magmaclast from the venetia kimberlite cluster south africa abstract kimberlitic magmaclasts are discrete ovoid magmatic fragments that formed prior to emplacement from disrupted kimberlite magma to provide new constraints on the origin and evolution of the kimberlite melts we document the mineralogy and petrography of a magmaclast recovered from one of the ca 520 ma venetia kimberlites south africa the sample bi9883 has a sub spherical shape and consists of a 10 mm diameter central olivine macrocryst surrounded by porphyritic kimberlite the kimberlitic material consists of concentrically aligned altered olivine phenocrysts set in a crystalline groundmass of calcite chromite perovskite phlogopite apatite ilmenite titanite sulphides rutile and magnetite along with abundant alteration phases i e serpentine talc and secondary calcite these features are typical of archetypal hypabyssal kimberlites we examined primary fluid melt inclusions in chromite perovskite and apatite containing a diversity of daughter phases chromite and perovskite host polycrystalline inclusions containing abundant alkali carbonates i e enriched in k na ba sr phosphates na k chlorides sulphides and equal to lesser quantities of olivine phlogopite and pleonaste in contrast apatite hosts polycrystalline assemblages with abundant alkali carbonates and na k chlorides and lesser amounts of olivine monticellite and phlogopite numerous solid inclusions of shortite na 2 ca 2 co 3 3 na sr carbonates and apatite occur in groundmass calcite along with fluid inclusions containing daughter crystals of na carbonates and na chlorides the primary inclusions in chromite perovskite and apatite are considered to represent remnants of fluid s melt s trapped during crystallisation of the host minerals whereas the fluid inclusions in calcite are probably secondary in origin the component proportions of these primary fluid melt inclusions were estimated in an effort to constrain the composition of the evolving kimberlite melt these estimates suggest melt evolution from a silicate carbonate kimberlite melt that became increasingly enriched in carbonates phosphates alkalis and chlorides in response to the fractional crystallisation of constituent minerals i e olivine to apatite the concentric alignment of crystals around the olivine kernel and ovoid shape of the magmaclast can be ascribed to the low viscosity of the kimberlite melt and rapid rotation whilst in a liquid or partial crystalline state or to progressive layer by layer growth of the magmaclast although the mineralogy of our sample is similar to hypabyssal kimberlites worldwide it differs from hypabyssal kimberlite units in the main venetia pipes which contain monticellite phlogopite rich assemblages and segregationary matrix textures therefore magmaclast bi9883 probably originated from a batch of magma distinct from those that produced known hypabyssal units within the venetia kimberlite cluster

structures in granitoid gneisses and associated migmatites close to the granulite boundary of the limpopo belt south africa abstract river platform exposures of quartzofeldspathic gneiss and associated migmatites occur close to the boundary of the southern marginal zone of the limpopo belt with the adjacent low grade granite greenstone terrane of the kaapvaal craton the complex gneisses and migmatites are described and are considered to represent the tectono thermally reworked equivalents of the pietersburg sutherland greenstone belts and early tonalitic and trondhjemitic intrusive granitoids structural features associated with the hout river shear zone and the limpopo orogeny 2700 2650 ma old and which include subhorizontal overthrust fault slices produced as a result of lateral extrusion of rock material following transpressional displacement along a strike slip fault or thrust zone support structural metamorphic and geophysical evidence suggesting southwestwards overthrusting of the high grade southern marginal zone rocks onto lower grade rocks of the kaapvaal craton

a theoretical study of diagenetic concentration fields near manganese nodules at the sediment water interface the effects of manganese nodules on concentration fields in adjacent pore waters and sediments are investigated theoretically with a three dimensional axisymmetric diffusion reaction model the nodules are idealized as impermeable and poorly permeable disks and hemispheres the first part of this paper examines the organic matter and coupled pore water oxygen fields in order to evaluate the local redox conditions under an isolated nodule oxygen is allowed to disappear on a free surface that is calculated as an output of the model in sediments that become suboxic like those at manop site c the model predicts that pore waters neighboring a nodule are markedly more depleted in oxygen than in its absence although these reduced conditions may remobilize metals that would be available for nodule growth these same conditions would appear to favor nodule dissolution typically the presence of a nodule raises the local position of the zero oxygen surface but it is deeper than expected for large nodules on sediments containing only highly reactive organic matter the redox changes calculated for an isolated nodule on a hypothetical but representative oxidized abyssal red clay appear inadequate to raise the redox boundaries enough to facilitate metal transport from reduced regions deeper in the sediment in the later part of the paper the model is used to evaluate the influence of a nodule field on the outward flux of dissolved silica from a siliceous ooze although the flux attenuation is generally modest some dense packings observed in the pacific should produce substantial reductions 35 which should promote opal preservation

modeling explosion generated scholte waves in sandy sediments with power law dependent shear wave speed experimental measurements of scholte waves from underwater explosions collected off the coast of virginia beach va in shallow water are presented it is shown here that the dispersion of these explosion generated scholte waves traveling in the sandy seabed can be modeled using a power law dependent shear wave speed profile and an empirical source model that determines the pressure time series at 1 m from the source as a function of tnt equivalent charge weight

deep ocean sound speed characteristics passively derived from the ambient acoustic noise field the propagation of acoustic waves in the ocean strongly depends on the temperature lowfrequency acoustic waves can penetrate the ocean down to depths where few in situ measurements are available it is therefore attractive to obtain a measure of the deep ocean temperature from acoustic waves the latter is especially true if the ambient acoustic noise field can be used instead of deterministic transient signals in this study the acoustic velocity and hence the temperature is derived in an interferometric approach from hydrophone array recordings the arrays were separated by over 125 km near ascension island in the atlantic ocean at a depth of 850 m furthermore the dispersive characteristics of the deep ocean sound channel are resolved based on the retrieved lag times for different modes in addition it is shown how the resolution of the interferometric approach can be increased by cross correlating array beams rather than recordings from single sensor pairs the observed acoustic lag times between the arrays corresponds well to modelled values based on full wave modelling through best known oceanic models

tectonic model for the evolution of the limpopo belt abstract constraints on which any model for the tectonic evolution of the limpopo belt must be based include the following data crustal thickening to at least 65 km between 2700 and 2650 ma ago was responsible for the formation of the granulite terrane exposed in the limpopo belt today this crustal thickening probably resulted from the thrusting of the kaapvaal craton over the zimbabwe craton along the south dipping triangle tuli sabi shear zone other northwardverging thrusts related to this event occur in the kaapvaal craton south of the limpopo belt in the central and southern marginal zones and the kaapvaal craton this shortening is directed to the southwest while in the northern marginal zone it was directed to the north northwest peak metamorphism was superimposed on the thickened crust and then nearly isothermal decompression of several kbars occurred throughout the belt during this decompression rocks moved upward and spread outward onto the adjacent cratons from the zone of thickened crust along several inward directed shear zones creating a regional pop up high grade rocks were thrust over low grade rocks on the cratons producing along the terrane boundary retrograde metamorphism in the limpopo belt and prograde metamorphism on the cratons

asymptotic forms and solutions of the model for silica opal diagenesis in bioturbated sediments early diagenesis of the coupled dissolved silica opal system in bioturbated sediments may be explained by one of three possible models of increasing mathematical complexity i e the simple but unused constant opal model abbreviated c o the schink et al 1975 model abbreviated sg f for which wong and grosch 1978 have supplied an analytical solution designated as w g and the improved but much more sophisticated model proposed by schink and guinasso 1980 that must be solved numerically s g scaling analysis and computational comparisons show that the c o model and the sg f model as calculated via the w g solution are asymptotically valid forms of more complete s g model for the limits of large and small opal concentrations respectively specifically the c o model is found to provide an excellent approximation to the vastly more complicated s g model if the amount of opal preserved at depth in the sediment b satisfies the inequality b 0 1 1 db ds cs cw 1 b 1 1 where is the porosity db is the mixing coefficient ds is the tortuosity corrected molecular diffusivity of silica cs is the solubility of opal cw is the silica concentration in the overlying waters b is the intrinsic density of opal and is a unit conversion constant if b and c are in different units schink and guinasso 1980 have criticized wong and grosch 1978 for utilizing their solution of the earlier sg f model to describe opal accumulation a situation for which they believed the sg f model was invalid this study has found however that for conditions characteristic of the deep sea the sg f model and so the w g solution remain reasonably accurate even if small amounts of opal escape dissolution and collect but that radical divergence from the s g model can be expected if the flux of silica is sufficient to create a siliceous ooze this reflects the minor role played by the divergence of the advective flux in the balance of terms in the diagenetic conservation equations when little opal is preserved these findings should resolve any uncertainty and controversy over the use of the w g solution opal diagenesis in bioturbated shelf like sediments appears to be adequately described by the c o model alone the c o model coupled where necessary to the w g solution constitutes an attractive alternative to the s g model because relatively simple analytical methods of solution may be employed rather than advanced numerical techniques

three dimensional crustal structure of ascension island from active source seismic tomography we present a crustal velocity and moho depth model of the ascension volcanic edifice using one of the most densely sampled 3 d wide angle data sets for volcanic islands currently available we invert traveltimes of first and second arrivals by seeking a layer interface minimum structure model and then test the resulting velocity model by gravity modelling and a 3 d checkerboard resolution test within the shallow extrusive part of the crust two main high velocity regions coincide with the highest topography on land and the gravity maximum off the west coast of the island respectively these features are connected with a high velocity intrusive core that is created either within or on the top of oceanic layer 3 and is interpreted as a possible relic magma chamber the thickness of the surface low velocity region is similar to that observed at hawaii jasper seamount and great meteor seamount suggesting a similar process of edifice construction the mean density of the volcanic edifice is significantly less than that of normal oceanic crust and than load densities typically assumed in studies of flexure as a result of seamount loading there is no simple flexural model that explains the shape of the moho beneath the island perhaps because of the long lived volcanism and the proximity of the island to the mid atlantic ridge mar and the ascension fracture zone there is no evidence for magmatic underplating beneath ascension island

seismic studies in the limpopo belt abstract the structure of the archaean limpopo belt of southern africa has been investigated by means of a deep seismic reflection profile and the recording of the traveltimes of mining induced tremors at regional distances interpretation of the traveltimes suggests that a seismic low velocity zone exists in the upper mantle of the northern kaapvaal craton and limpopo belt beginning at a depth of about 42 km the seismic reflection profile provides evidence for a northward dip of the contact between the kaapvaal craton and the southern marginal zone of the limpopo belt no mid crustal decollement zone is detected the tectonic model of van reenen et al 1990 is in accord with these observations the lower crust is essentially seismically transparent and no reflection moho is observed

corundum quartz and mg staurolite bearing granulite from the limpopo belt southern africa implications for a p t path abstract a new occurrence of the rare corundum quartz assemblage and magnesian staurolite has been found in a gedrite garnet rock from the central zone of the neoarchean limpopo belt in zimbabwe poikiloblastic garnet in the sample contains numerous inclusions of corundum quartz sillimanite magnesian staurolite sapphirine orthopyroxene and sapphirine sillimanite assemblages as well as monophase inclusions corundum often containing subhedral to rounded quartz occurs as subhedral to euhedral inclusions in the garnet quartz and corundum occur in direct grain contact with no evidence of a reaction texture the textures and fe mg ratios of staurolite inclusions and the host garnet suggest a prograde dehydration reaction of st grt crn qtz h2o to give the corundum quartz assemblage peak conditions of 890 930 c at 9 10 kbar are obtained from orthopyroxene sapphirine and garnet staurolite assemblages a clockwise p t path is inferred with peak conditions being followed by retrograde conditions of 4 6 kbar and 500 570 c the presence of unusually magnesian staurolite mg fe mg 0 47 0 53 and corundum garnet assemblages provides evidence for early high pressure metamorphism in the central zone possibly close to eclogite facies the prograde high pressure event followed by high to ultrahigh temperature metamorphism and rapid uplifting of the limpopo belt could have occurred as a result of neoarchean collisional orogeny involving the zimbabwe and kaapvaal cratons

the role of acids in electrical conduction through ice 1 electrical conduction through meteoric polar ice is controlled by soluble impurities that originate mostly from sea salt biomass burning and volcanic eruptions the strongest conductivity response is to acids yet the mechanism causing this response has been unclear here we elucidate conduction mechanisms in ice using broadband dielectric spectroscopy of meteoric polar ice cores we find that conduction through polycrystalline polar ice is consistent with jaccard theory for migration of charged protonic point defects through single ice crystals except that bulk dc conduction is impeded by grain boundaries neither our observations nor modeling using archie s law support the hypothesis that grain boundary networks of unfrozen acids cause significant electrolytic conduction common electrical logs of ice cores by electrical conductivity measurement ecm or dielectric profiling dep and the attenuation of radio waves in ice sheets thus respond to protonic point defects only this response implies that joint interpretation of electrical and chemical logs can determine impurity partitioning between the lattice and grain boundaries or inclusions for example in the greenland ice core project grip ice core from central greenland on average more than half of the available lattice soluble impurities h cl nh4 create defects understanding this partitioning could help further resolve the nature of past changes in atmospheric chemistry

evanescent wave coupling in a geophysical system airborne acoustic signals from the mw 8 1 macquarie ridge earthquake atmospheric low frequency sound i e infrasound from underwater events has not been considered thus far due to the high impedance contrast of the water air interface making it almost fully reflective here we report for the first time on atmospheric infrasound from a large underwater earthquake mw 8 1 near the macquarie ridge which was recorded at 1325 km from the epicenter seismic waves coupled to hydroacoustic waves at the ocean floor after which the energy entered the sound fixing and ranging channel and was detected on a hydrophone array the energy was diffracted by a seamount and an oceanic ridge which acted as a secondary source into the water column followed by coupling into the atmosphere the latter results from evanescent wave coupling and the attendant anomalous transparency of the sea surface for very low frequency acoustic waves

long range correlations of microseism band pressure fluctuations in the ocean we investigate the spatial coherence of underwater ambient noise using a yearlong time series measured off ascension island qualitative agreement with observed cross correlations is achieved using a simple range dependent model constrained by earlier active tomographic studies in the area in particular the model correctly predicts the existence of two weakly dispersive normal modes in the microseism frequency range with the group speed of one of the normal modes being smaller than the sound speed in water the agreement justifies our interpretation of the peaks of the measured cross correlation function of ambient noise as modal arrivals with dispersion that is sensitive to crustal velocity structure our observations are consistent with scholte to moho head wave coupled propagation with double mode conversion occurring due to the bathymetric variations between receivers we thus demonstrate the feasibility of interrogating crustal properties using noise interferometry of moored hydrophone data at ranges in excess of 120 km

chert occurrences in carbonate turbidites examples from the upper jurassic of the betic mountains southern spain in upper jurassic carbonate turbidites of the betic mountains southern spain chert occurs in three morphologies bedded chert nodular chert and mottled chert the last refers to a weak dispersed and selective silification which gives a speckled appearance to the rock the three types of chert are formed by replacement of limestones and are associated with different calcareous facies turbidite packstones of saccocoma and peloids and turbidite lime mudstones of pelagic material contain bedded and nodular cherts the silicification textures are mainly micro and cryptocrystalline quartz with local chalcedonic quartz both length fast and length slow which is more common in the packstones only mottled chert is produced where calcareous breccia beds are silicified mottled chert consists of micro and cryptocrystalline quartz length slow chalcedonic quartz and mosaics or individual crystals of euhedral megaquartz beds and nodules are the result of early diagenetic silicification with silica derived from the calcitization and dissolution of radiolarians and subordinately sponge spicules whereas mottled chert is the consequence of later silicification in a probably mg rich environment early silicification is mainly confined to turbidite beds and only rarely occurs in the interbedded pelagic limestones turbidite sedimentation favours silicification because rapid burial of the transported siliceous tests prevents silica from the dissolution of tests passing into overlying sea water a silica rich interstitial fluid develops in the turbidite layer and this migrates to more permeable zones giving rise to bedded and nodular chert