evidence for a widespread mafic cover sequence and its implications for continental growth in the northeastern superior province abstract archean greenstone belts of the northeastern superior province nesp were emplaced over a 160 ma period 2 88 2 72 ga spanning a major episode of crustal reworking in which early tonalite trondhjemite plutonism evolved to dominant granite granodiorite and pyroxene bearing felsic plutonism the numerous greenstone belts that crop up across the craton contain lavas belonging to three mafic volcanic suites 1 mg tholeiites that have chemical compositions typical of many archean basalts with 4 10 wt mgo 9 15 wt fe2o3t 0 4 1 2 wt tio2 and la sm ratios between 1 and 3 2 fe tholeiites that have similar mgo contents and la sm ratios but markedly higher fe2o3t 11 20 wt tio2 1 0 2 6 wt and gd yb ratios that may reflect derivation from a distinct garnet bearing mantle source 3 light rare earth element lree enriched rocks that have the chemical characteristics typical of calc alkaline mafic magmas with higher sio2 al2o3 and la sm ratios but lower fe2o3t and tio2 than the tholeiitic suites at a given mgo content the presence of unconformities within single greenstone belts numerous inheritance ages in volcanic rocks replicating those of older volcanic plutonic assemblages and the recognition of widespread geographically separate yet coeval 2 78 ga extrusive assemblages containing the distinctive fe tholeiites do not support a plate tectonic model for a large portion of the nesp the correlations in composition and ages of greenstone belts are more consistent with a model in which they represent the remnants of an extensive autochthonous mafic cover sequence nd isotopic evidence for the presence of evolved crust in some syn volcanic felsic plutons and volcaniclastic rocks imply that this mafic cover sequence was erupted onto an older granitoid crust the amalgamation of two isotopically distinct terranes at ca 2 76 2 74 ga created a proto cratonic mass that consisted of a composite tonalite trondhjemite greenstone crust underplating by mafic mantle melts and widespread insulation after 2 75 ga increased temperatures sufficiently to produce extensive melting of this crust to generate pyroxene bearing felsic rocks and granodiorite to granite partial melts the isotopically enriched character of mantle derived magmas after 2 75 ga may reflect more extensive contamination by a felsic crust affected by widespread partial melting furthermore a decrease of nb th ratios in the lree enriched mafic rocks younger than 2 75 ga reflects a change in the nature of the crustal contaminant from tonalite trondhjemite prior to 2 75 ga to granite granodiorite afterwards

modeling near field dispersion from direct injection of carbon dioxide into the ocean 1 in this paper we have predicted the dynamics of double plume formation and dispersion from direct injection of liquid co2 into middle depth ocean water to do so we used a three dimensional two fluid numerical model the model consists of a co2 droplet submodel and a small scale turbulent ocean submodel both of which were calibrated against field observation data with an injection rate of 100 kg s 1 co2 numerical simulations indicated that the injection of 8 mm diameter co2 droplets from fixed ports at 858 m 20 m above the seafloor into a current flowing at 2 5 cm s 1 could create a plume that reaches the bottom and has at most a 2 6 unit decrease in ph the strong interaction between the buoyant rise of the liquid co2 and the fall of the co2 enriched water produced a vertically wavy plume tip at about 190 m above the seafloor the maximum ph decrease however was kept to 1 7 units when the liquid co2 had an initial droplet diameter of 20 mm and it was injected at 1500 m from a towed pipe with a ship speed of 3 0 m s 1 after 70 min the double plume developed into a single phase passive plume with a vertical scale of 450 m and a horizontal scale larger than 150 m this development was attributable to the droplets buoyant rise and dissolution along with ocean turbulence which together diluted the plume and reduced the decrease in ph to less than 0 5 units

a 56 million year rhythm in north american sedimentation during the phanerozoic abstract long term 10 myr fluctuations in climate sea level and sedimentation have been documented in the stratigraphic record but the lack of well constrained data series has made it difficult to rigorously evaluate cyclic periodic or quasi periodic changes at this scale here we utilize a new compilation of the coverage area of sedimentary rocks in north america to investigate the dominant modes orders of stratigraphic variability and to evaluate potential long period cyclic changes in sedimentation on the continent during the phanerozoic our analysis resolves two principal temporal modes of variability 1 a strongly sinusoidal mode with a periodicity of 56 myr 3 myr and 2 a longer term phanerozoic mode the m curve linked to the wilson cycle which is indistinguishable from a stochastic autoregressive process the newly identified 56 myr cycle in sedimentation delineates most of the cratonic sequences that have previously been identified qualitatively in north america but here we propose a quantitative redefinition that includes nine distinct units and two mega sequences the timing of the 56 myr beat in sedimentation is consistent with an orogenic oscillator source or an oscillatory dynamic in mantle convection and its tempo is statistically similar to a known rhythm in number of marine animal genera in the global fossil record thus the identification of a significant periodic signal in the sedimentary record of north america provides new evidence for an important tectonic and or mantle scale cyclic process that links both large scale biological evolution and physical environmental change

distribution and height of methane bubble plumes on the cascadia margin characterized by acoustic imaging submersible investigations of the cascadia accretionary complex have identified localized venting of methane gas bubbles in association with gas hydrate occurrence acoustic profiles of these bubble plumes in the water column in the vicinity of hydrate ridge offshore oregon provide new constraints on the spatial distribution of these gas vents and the fate of the gas in the water column the gas vent sites remained active over the span of two years but varied dramatically on time scales of a few hours all plumes emanated from local topographic highs near the summit of ridge structures the acoustic images of the bubble plumes in the water column disappear at water depths between 500 to 460 m independent of the seafloor depth this coincides with the predicted depth of the gas hydrate stability boundary of 510 to 490 m suggesting that the presence of a hydrate skin on the bubble surface prevents them from rapid dissolution the upper limit of the acoustic bubble plumes at 460 m suggests that dissolution of the residual bubbles is relatively rapid above the hydrate stability zone

dehydration melting and the granulite transition in metapelites from southern namaqualand s africa in a prograde amphibolite granulite transition zone in the namaqualand metamorphic complex metapelites show an interbanding of the amphibolite facies association biotite sillimanite quartz with the granulite facies association garnet cordierite k feldspar relict graded bedding shows that compositional banding is of sedimentary origin the garnet cordierite k feldspar gneisses contain quartzofeldspathic segregations surrounding garnets and have more fe rich bulk compositions than the biotite sillimanite schists

trends cycles and nonstationarities in isotope signals of phanerozoic seawater abstract the new set of 87 sr 86 sr 18 o and 13 c experimental data for phanerozoic seawater the bochum ottawa isotope dataset has been tested by wavelet discontinuity and sliding window correlation dimension analyses for cyclicities and nonstationarities in the isotope signal the tests indicate discontinuities in the strontium isotope signal at 500 340 288 210 65 and 28 ma while for the oxygen and carbon isotopes they are at 500 385 290 210 and 65 ma these discontinuities often coincident with major stage boundaries reflect mostly single likely tectonic events that do not affect the structure of the underlying system the two most pronounced nonstationarities in all isotope systematics are at 65 and 210 ma respectively that is at the k t and norian rhaetian transitions wavelet analysis for all three isotope systems yields a long term quasi periodicity at 94 125 ma best developed during the paleozoic with superimposed intermittent 48 57 and 29 35 ma oscillations all likely a reflection of plate reorganizations within the caledonian hercynian and alpine tectonic cycles

time series analysis of large igneous provinces 3500 ma to present abstract we outline an efficient integrated wavelet a spectral and a cross spectral approach for the time series analysis of geologic data here these techniques are applied to a database of 154 large igneous provinces lips in order to test for cycles trends and abrupt changes in temporal distribution since 3500 ma the average frequency of events is relatively constant and supports an overall model of semicontinuous temporal emplacement of lips however several weak cycles were obtained in the analysis those of longest duration are a 170 m yr cycle from 1600 ma to present and in the late archean a 330 m yr cycle from about 3000 to 1000 ma a cycle decreasing in length from 730 to 550 m yr over the interval 3500 ma to present and a still weaker cycle increasing from 900 to 1000 m yr over the interval 2000 ma to present additional cycles of shorter duration include a 250 m yr cycle in the late archean a 230 m yr cycle in the phanerozoic a 105 m yr cycle in the early proterozoic an

multi stage modification of paleoarchean crust beneath the anabar tectonic province siberian craton abstract according to present views the crustal terranes of the anabar province of the siberian craton were initially independent blocks separated from the convecting mantle at 3 1 daldyn terrane 2 9 magan terrane and 2 5 ga markha terrane rosen 2003 2004 rosen et al 1994 2005 2009 previous studies of zircons in a suite of crustal xenoliths from kimberlite pipes of the markha terrane concluded that the evolution of the crust of the markha terrane is very similar to that of the daldyn terrane to test this conclusion we present results of u pb and hf isotope studies on zircons in crustal xenoliths from the zapolyarnaya kimberlite pipe upper muna kimberlite field located within the daldyn terrane and the botuobinskaya pipe nakyn kimberlite field in the center of the markha terrane the data on xenoliths from the botuobinskaya kimberlite pipe record tectonothermal events at 2 94 2 8 2 7 and 2 ga the event at 2 ga caused pb loss in zircons from a mafic granulite u pb dating of zircons from the zapolyarnaya pipe gives an age of 2 7 ga all zircons from the studied crustal xenoliths have archean hf model ages ranging from 3 65 to 3 11 ga this relatively narrow range suggests that reworking of the ancient crust beneath the nakyn and upper muna kimberlite fields was minor compared with the daldyn and alakit markha fields shatsky et al 2016 this study when combined with dating of detrital zircons implies that tectonic thermal events at 2 9 2 85 2 75 2 7 and 2 0 1 95 ga occurred everywhere on the anabar tectonic province and could reflect the upwelling of superplumes at 2 9 2 7 and 2 ga the presence of the same tectonic thermal events in the daldyn and markha terranes rosen et al 2006a b supports the conclusion that the identification of the markha terrane as a separate unit is not valid

episodicity of mesozoic terrane accretion along the pacific margin of gondwana implications for superplume plate interactions a review of evidence for deformation and terrane accretion on the late triassic early jurassic margins of pangea and the mid cretaceous margins of the palaeo pacific ocean shows that deformation was global and synchronous with probable superplume events late triassic early jurassic deformation appears to be concentrated in the period 202 197 ma and was coeval with eruption of the central atlantic magmatic province onset of pangaea break up a period of extended normal magnetic polarity and a major mass extinction event all possible expressions of a superplume event mid cretaceous deformation occurred in two brief periods the first from approximately 116 to 110 ma in the west palaeo pacific and the second from roughly 105 to 99 ma in the east palaeo pacific with both events possibly represented in northeast siberia this deformation was coeval with eruption of major oceanic plateaus core complex formation and rifting of new zealand from gondwana the cretaceous normal polarity epoch and a major radiation of flowering plants and several animal groups all linked with the mid cretaceous superplume event a simple unifying mechanism is presented suggesting that large continental or oceanic plates when impacted by a superplume tend to break up reorganize associated with gravitational spreading away from a broad thermally generated topographic high and with a resulting short lived pulse of plate marginal deformation and terrane accretion

hydrological response of a dryland ephemeral river to southern african climatic variability during the last millennium a long term flood record from the buffels river the largest ephemeral river of nw south africa 9250 km2 was reconstructed based on interpretation of palaeoflood documentary and instrumental rainfall data palaeoflood data were obtained at three study reaches with preserved sedimentary evidence indicating at least 25 large floods during the last 700 yr geochronological control for the palaeoflood record was provided by radiocarbon and optically stimulated luminescence osl dating annual resolution was obtained since the 19th century using the overlapping documentary and instrumental records large floods coincided in the past within three main hydroclimatic settings 1 periods of regular large flood occurrence 1 large flood 30 yr under wetter and cooler prevailing climatic conditions ad 1600 1800 2 decreasing occurrence of large floods 1 large flood 100 yr during warmer conditions e g ad 1425 1600 and after 1925 and 3 periods of high frequency of large floods 4 5 large floods in 20 30 yr coinciding with wetter conditions of decadal duration namely at ad 1390 1425 1800 1825 and 1915 1925 these decadal scale periods of the highest flood frequency seem to correspond in time with changes in atmospheric circulation patterns as inferred when comparing their onset and distribution with temperature proxies in southern africa

the 1988 orange river flood upington region northwestern cape province rsa this paper describes aspects of the 1988 orange river flood the orange river is a perennial bedrock controlled river which experienced severe flooding flood peak 8300 m3 s 1 during february april of 1988 this flood resulted in the loss of life and damage in excess of 126 million negligible bank erosion took place in the main channel margins due to the stabilizing effect of tree lined banks in contrast areas removed from the channel margin were characterized by erosion and deposition within the confines of the natural flood channels macroturbulence gave rise to scour holes which sometimes amalgamated into large erosion fields acres in extent in many cases scour holes were present but macroturbulence originating obstacles were not and it is suggested that these were removed by flood erosion processes in some instances amalgamation of scour hole trails orientated down current led to the formation of steep sided flood channels deposition took the form of large sandsheets constructed from fluvial dune fields and braid bar accumulations post flood winds reworked some of the deposited sand into aeolian dunes

paleoflood hydrology origin progress prospects from an origin in diverse studies of flood geomorphology and quaternary geology paleoflood hydrology emerged as a geophysical and an applied hydrological science during the 1970s and 1980s since acquiring its formal name in 1982 the most productive approach in paleoflood hydrology has become energy based inverse hydraulic modeling of discrete paleoflood events recorded in appropriate settings as slackwater deposits and other paleostage indicators swd psi or as various threshold indicators of non exceedence technological advances particularly in hydraulic modeling and geochronology were instrumental in moving the discipline to its present status the most recent advances include 1 new techniques for the accurate geochronology of flood sediments notably tams radiocarbon analyses and osl dating and 2 the phenomenal increase in computer power that allows complex hydraulic calculations to become feasible for routine studies from its initial demonstration in the southwestern united states swd psi paleoflood hydrology proved its widespread applicability to various landscape environments particularly important studies have been accomplished in australia china india israel south africa spain and thailand paleoflood hydrology has also generated its share of controversy in part because of the differing viewpoints and attitudes of the two scientific traditions from which it emerged quaternary geology geomorphology versus applied hydrologic hydraulic engineering nevertheless the future growth of the discipline is assured given the rapid pace of discoveries that it engenders indeed so many international studies exist that it is appropriate to pursue global syntheses to address interesting and timely questions of extreme flood phenomena in relation to global climatic change

metamorphism and geochronology of granulites and migmatitic granulites from the magondi mobile belt zimbabwe the magondi mobile belt which crops out to the west of the zimbabwe archaean craton is an early to mid proterozoic feature within which sediments of the magondi supergroup were deformed and metamorphosed metamorphic grade increases northwards along the strike of the belt from greenschist facies in the south to granulite facies in the north the granulite facies rocks were partially melted during metamorphism at pressures of 5 7 kbar and temperatures of about 700 750 c with fluids buffered to low values of pfl late stages of partial melting were driven by vapour poor dehydration melting reactions an age for the metamorphism is indicated by rb sr ages of 1890 260 ma from enderbitic granulites at the rukomeshe river causeway east of makuti and of 1780 280 ma from garnet bearing and granitic granulites from the nyaodza region just east of lake kariba these ages confirm an early proterozoic age for the magondi orogeny which can be correlated with deformation in the kheis belt occupying a similar position on the western margin of the kaapvaal craton later brittle deformation and cataclasis distributed throughout the granulites but locally concentrated in n s striking shear zones are the result of deformation in the footwall of the south verging pan african thrust sheets

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

exploring the frequency size relationships of pipe collapses in different morphoclimatic regions abstract piping is an important land degradation process that occurs in almost all climatic zones this study analyses for the first time the frequency size distribution of different types of pipe collapses pcs sinkholes and compound depressions related to the coalescence of sinkholes compound sinkholes and elongated blind gullies exploring the frequency size relationships of pcs provides critical morphometric information about this hazardous erosion and ground stability process as well as its impact on hillslope denudation hillslope channel coupling and landscape development the analysis has been performed with field based inventories of pcs from five catchments located in two different morphoclimatic environments one in the semi arid ebro depression ne spain valpalmas catchment and four in the humid bieszczady mts se poland in total 724 pcs were mapped among which 335 pcs in valpalmas and 389 pcs in the bieszczady mts the morphometric analyses were based on the major axis the maximum length or width and area of the pcs despite differences between study areas from ne spain and se poland related to climate conditions rainfall regime soil water storage capacity spatial pattern of pipe networks vegetation cover and land use empirical cumulative frequency curves of pcs show similar patterns in all study sites indicating similar overall effects of piping erosion the size distribution of the inventoried pcs can be modelled satisfactorily with logarithmic functions with high goodness of fit values r2 0 93 for each type of pc and the combined set of pcs sinkholes and compound sinkholes blind gullies mapped in each area and in the five catchments together the pcs mapped in valpalmas are displaced towards larger dimensions attributable to local factors the applicability and limitations of frequency size relationships of pcs for geomorphological studies and hazard assessment are discussed

paleohydrologic bounds and extreme flood frequency of the upper arkansas river colorado usa article i nfo article history the upper arkansas river basin has experienced notable large floods including the event of 2 6 june 1921 that devastated the city of pueblo colorado we investigated flood and paleoflood hydrology at strategic sites to determine the frequency and geographic extent of extreme floods within the basin for a dam safety application streamgage historical and paleoflood data were utilized to develop frequency curves at sites near salida cotopaxi parkdale and pueblo soil stratigraphic descriptions radiocarbon dating and hydraulic modeling were used to estimate paleoflood nonexceedance bounds at the four sites which ranged from 400 to 2200 ybp for late holocene surfaces to late pleistocene surfaces near cotopaxi peak flow data are from lower magnitude snowmelt runoff in may and june in the upper basin and from high magnitude rainfall runoff from june to august in the lower basin flood frequency curves reflect this transition near parkdale from snowmelt to extreme rainfall runoff for similar return periods paleoflood peak discharges increase from about 480 m 3 s upstream at loma linda to about 4250 m 3 s downstream near pueblo this increase is attributed to the larger rainfall component derived from lower elevations between loma linda and pueblo return periods for design floods at pueblo dam exceeded 10 000 years based on paleoflood frequency curve extrapolations

paleohydrology of extraordinary floods along the swakop river at the margin of the namib desert and their paleoclimate implications abstract the swakop river is a major ephemeral river drainage basin area about 30 000 km 2 crossing the western namib desert its hydrology and limited water resources depend on storms and floods therefore the hydrology and flow regime in the swakop river basin determines the availability of water for human use in this study we present a millennia long record of extreme floods using paleoflood hydrological methods and optically stimulated luminescence osl ages the record includes 44 large floods ranging in date from the late pleistocene to the present the oldest six to eight floods are dated from the late pleistocene to the mid holocene between 14 900 1700 and 5100 620 years ago with calculated peak discharges of 550 1280 m 3 s 1 eighteen floods eleven of which were between 1000 and 1600 m 3 s 1 occurred between 740 90 and 190 10 years ago additional 17 later floods occurred between about ad 1850 and 1963 with peak discharges between 100 and 1000 m 3 s 1 the historical record which partly parallels the latter period recorded 17 large floods during the period 1893 1963 flood frequency analysis ffa of the combined systematically gaged record 1963 2003 and the paleoflood record since ad 1270 740 90 years the most complete record was applied using the maximum likelihood method a two component extreme value distribution tcev estimates the 1 annual exceedance probability magnitude using only the systematic gaged record at 520 m 3 s 1 the combined paleoflood historical and gage records resulted in a much larger magnitudes for the 1 annual exceedance probability flood magnitude of 1320 1350 m 3 s 1 the 0 1 annual exceedance probability magnitude is estimated at 770 m 3 s 1 and 2100 2150 m 3 s 1 respectively the hydroclimatic interpretation of this paleoflood record is complex due to the discontinuity of the record during the late holocene our flood data show three intervals differing in their flood regime 1 1280 80 to 740 90 years bp with one extraordinary flood 200 m 3 s 1 about every 180 years 2 740 90 to 200 45 14 c years bp with one large flood per 80 years and 3 200 45 14 c years bp to ad1963 with one large flood per 7 years interval 2 represents large floods probably related to the transition from a drier climate to the colder and probably wetter climate of the little ice age lia the third period relates to the end of the lia and to a wetter period of the 19th and early 20th centuries in which more rainfall in the kalahari and namib deserts and many historical floods were documented the presence of three distinct calcretes rich in calcified rhizoliths at one of the sites two of which were dated by osl to 13 2 1 9 ka and 8 8 1 0 ka indicates at least locally higher moisture availability than in the current semiarid climate the presence of denser vegetation can be associated with increased groundwater seepage and wetter climatic conditions

plio pleistocene time evolution of the 100 kyr cycle in marine paleoclimate records to constrain theories for the dynamic evolution of global ice mass through the late neogene it is important to determine whether major changes in the record were gradual or rapid of particular interest is the evolution of the near 100 kyr ice age cycle in the middle pleistocene we describe and apply a new technique based on multiple taper spectral analysis which allows us to model the time evolution of quasi periodic signals this technique uses both phase and amplitude information and enables us to address the question of abrupt versus gradual onset of the 100 kyr periodicity in the middle pleistocene we analyzed three long 2 6 m y time series from deep sea drilling project dsdp site 607 midlatitude atlantic and ocean drilling program odp site 677 equatorial pacific the published time scales for these sites differ significantly influencing the interpretation of the mid pleistocene climate transition when the data series are expressed on the same time scale there is evidence for a coherent 18o signal at both sites in the eccentricity and obliquity frequency bands consistent with variations in global ice volume as the causative factor if the shaddeton et al 1990 time scale for odp 677 is accepted the amplitude match between the 18o obliquity cycle and the 65 n insolation derived from the recent astronomical solution of laskar 1988 1990 is excellent for times t 2 3 ma if the ruddiman raymo time scale for dsdp 607 is accepted the 41 kyr 18o cycle has enhanced amplitude between 1 0 and 1 5 ma relative to the late pleistocene t 1 0 ma implying a nonlinear earth system response to obliquity insolation cycles we do not find compelling evidence for an abrupt change in the 100 kyr 18o signal although we cannot rule it out at this time if the shackleton et al 1990 time scale for odp 677 is accepted our three 18o records are consistent with a low amplitude 100 kyr cycle between 1 2 and 2 6 ma whose local period of oscillation alternates between the 95 and 124 kyr eccentricity periods and a gradual increase in the 100 kyr 18o signal between 0 5 amd 1 0 ma the dsdp 607 time scale is more favorable to an abrupt jump in amplitude for the 95 kyr 18o envelope but not in the 124 kyr envelope rather long period 18o fluctuations appear phase locked with the 124 kyr eccentricity cycle some 300 400 kyr prior to its growth in amplitude and phase lock with the 95 kyr eccentricity cycle in the late pleistocene

analyses of the magnitude and frequency of a 400 year flood record in the fish river basin namibia abstract ephemeral rivers in dryland regions exhibit a high interannual variability of streamflow regime mainly dominated by floods in these environments floods are a water resource and a potential hazard with important socioeconomic implications the fish river 86 600 km2 is the largest ephemeral stream in namibia and recently also the focus of new development plans including construction of the largest dam in namibia the hydrological analysis to support decisions implies large uncertainties owing to spatial limited and short hydrological records since 1962 here we investigate the current and past patterns of extreme floods combining instrumental historical and palaeoflood records palaeoflood studies were performed at two reaches with preserved sedimentary evidence at the upper sector vogelkranz upstream of hardap dam 13 catchment area and in the lower part of the river in the fish river canyon national park 70 catchment area in the hardap reach the palaeoflood record identified at least eight large floods during the last 350 years with the largest flood reaching a minimum discharge of 4800 m3 s 1 150 year return period in the fish river canyon reach the sedimentary record shows at least 12 large floods over the last 400 years the largest with an estimated minimum discharge of 8700 m3 s 1 the elevation of alluvial surfaces without flood evidence provided an upper bound for flood stages associated with discharges of 6400 m3 s 1 and 16 140 m3 s 1 for the upper and mid lower fish river respectively in the upper reach the flood frequency analysis ffa combining systematic and palaeoflood data provided lower discharges 25 of the flood quantiles than the ones using only systematic data sets in the fish river canyon reach incorporation of the palaeoflood data into the ffa results in slightly higher values in the magnitude of the higher flood quantiles 4 7 the ffa analysis using an upper limited lognormal distribution function ln4 fitted with palaeoflood data shows a good performance with a slow behaviour approaching the upper limit our flood frequency results suggest that the hardap dam should increase the spillway capacity and safety check flood of the original design in order to satisfy the dam safety criteria however the projected reevaluation figures calculated from conventional hydrological methods results in an overestimation of the safety floods according to our estimations

the geochronology of large igneous provinces terrestrial impact craters and their relationship to mass extinctions on earth the relationship between sudden catastrophic events and sudden mass extinctions on earth has attracted a great deal of research effort including measurements of the absolute and relative ages of these events the result has been a rapid improvement in the precision and accuracy of radiometric dates for large igneous provinces lips for which there is now a consensus on the peak eruption times over the last 250 ma in parallel there has been a more gradual improvement in the radiometric dates for craters formed by hypervelocity impacts hvis of asteroids and comets with earth the absolute ages of the largest craters are known to within a few million years but the ages of many smaller craters are poorly constrained in addition whereas relatively few lips have formed in the last 250 ma at least one hvi crater larger than 20 km diameter has formed on earth every million years only 25 such craters have been discovered so coincidence between the ages of hvi crater formation and ages of mass extinctions is related as much to crater preservation as to coincidence of the events the difference in quality of geochronology between hvis and lips is the result of differing availability of samples lips cover large areas and present an array of basalt samples suitable for dating whereas hvi craters are often poorly preserved and present few datable samples the radiometric data for preserved hvis and lips are now sufficiently precise and accurate for us to be confident that there is no one to one correlation between either lips or hvis and global mass extinctions of life on earth however there is a close association between a significant number of lips and sudden environmental change and a very close association between at least one massive hvi crater formation and a mass extinction further there is no apparent correlation between hvis and lips and thus no reason to suggest a causative relationship finally the radiometric dating of hvi craters indicates several clusters in the geological record but the correlation with sudden environmental change is no better than for large craters

from dust to varnish geochemical constraints on rock varnish formation in the negev desert israel abstract chemical compositions of rock varnish from the negev desert of israel and local settled dust were used to constrain the mechanisms of varnish formation and patterns of mn enrichment and accumulation in the varnish rock varnish was sampled from coeval undisturbed prehistoric flint artifacts along a south north climatic transect 30 120 mm yr of rain our analyses indicate that mn ba and pb in the varnish are significantly enriched 100 in respect to the local settling dust and that mn content systematically fluctuates with depth in the varnish the varnish and settled dust data combined with basic thermodynamic and kinetic reasoning are used to constrain the following geochemical model of rock varnish formation dust accumulates in micro basins on exposed rock surfaces under ph 8 common negev value and during wetting by dew and rain mn in the dust is mobilized and leached to a depth of 5 m under the varnish surface where hollandite mn oxides precipitate and are adsorbed onto and between the porous clay minerals that comprise most of the varnish during its mobile phase mn oxide is negatively charged and adsorbs rare earth elements once the solution dries abrasion removes the upper weakly cemented dust sediment which contains mainly si al and fe which are not mobile at ph 8 ca is also removed in large quantities mn ba pb and the ree are deposited at a depth and thus protected from erosion reoccurrences of these processes result in a noticeable accumulation of these elements but not of si al or fe the alternating mn rich and mn poor laminas form as a result of a competition between the leaching rate of mn and the adhesion rate of the clay minerals when moisture is high low lamina with high low mn clay mineral ratio forms

fe mg mixing in cordierite constraints from natural data and implications for cordierite garnet geothermometry in granulites from chemicaldata on 108 pairs of naturally occurring garnet and cordierite the interchange energy of fe mg mixing in cordierite wo and the free energy change of the exchange equilibrium feralrsiro t t zmgraiosiro y mgralrsiro yzferalosiro in the temperature range of 700 850 c have been extracted using standard methods of linear regression analysis the input thermochemical parameters for multicomponent camg fe mn garnet solid solutions are based on the updated and revised values after ganguly and saxena 1984 e g wr 2500 cal w w 3000 cal calculated on the basis of one third formula unit on the basis of the derived values at 710 760 800 and 850 c fe mg mixing in cordierite has been approximated to a symmetric regular solution the fe mg substitution shows positive deviation from ideality in cordierite with tzi 1 9 34 4 54kj mol of divalent cation the values derived for the free energy of mixing at xi d xi ds 0 5 are 3 26 kj at 700 c and 4 39 kl at 900 c the ag value in joules for the above equilibrium can be expressed as 15079 8 552 incorporating these parameters the geothermometric expression for the exchange equilibrium is

earliest paleoproterozoic high grade metamorphism and orogenesis in the gawler craton south australia the southern cousin in the rae family neoarchean earliest paleoproterozoic rocks of the gawler craton preserved within the mulgathing complex south australia record metamorphism and deformation during an earliest paleoproterozoic tectonothermal event known as the sleafordian orogeny we present metamorphic data from pelitic gneisses of the mulgathing complex from three locations challenger mt christie and blackfellow hill in order to constrain the pressure temperature time p t t evolution of this orogeny in situ u pb dating via laser ablation inductively coupled plasma mass spectrometry shows that monazites within garnet porphyroblasts have ages ca 2470 ma providing an estimate of the timing peak p t conditions monazite crystallisation from melt occurred at ca 2460 ma in the presence of peritectic garnet either late on the prograde path during biotite dehydration melting or immediately following peak t lobate and patchy rims in leucosome hosted monazite suggest a dissolution reprecipitation process occurred during cooling across the elevated dry solidus at ca 2440 ma monazite younger than ca 2430 2420 ma that is located at grain boundaries between matrix minerals likely reflects monazite crystallisation during cooling across the dry solidus and or subsolidus mineral breakdown during the waning stages of the thermal event phase diagram modelling of pelitic gneisses suggests peak metamorphic conditions at the three localities overlap at 7 kbar and 850 c with apparent thermal gradients in the order of 125 135 c kbar 35 45 c km metamorphism involved a tight anticlockwise p t t path based on mineral assemblage paragenesis and indicates a slow shallow decompressive cooling path sub parallel to the apparent thermal gradient in the order of few degrees per myr a protracted magmatic history involving bimodal magmatism associated with a thinned and underplated extensional basin in the mulgathing complex was likely a thermal precursor to the high thermal gradient metamorphism and convergent transpressive deformation a significant period of post 2 4 ga quiescence in the gawler craton subsequent to basin inversion suggests that this orogenesis may have occurred interior to a plate margin the similarity in timing and style between the sleafordian orogeny in the gawler craton and the 2 5 2 28 ga arrowsmith orogeny of the rae craton supports the notion that these terranes together with the broader gawler adelie craton of the mawson continent the north australia craton elements of the north china sino korean and dharwar cratons formed part of a family of neoarchean terranes described as the rae family of cratons with a distinctive tectono metamorphic record as compared with other archean cratons we suggest the mawson continent is the southern member of this family dominated as it is by terranes now present in the northern hemisphere

increased subaerial volcanism and the rise of atmospheric oxygen 2 5 billion years ago the hypothesis that the establishment of a permanently oxygenated atmosphere at the archaean proterozoic transition approximately 2 5 billion years ago occurred when oxygen producing cyanobacteria evolved is contradicted by biomarker evidence for their presence in rocks 200 million years older to sustain vanishingly low oxygen levels despite near modern rates of oxygen production from approximately 2 7 2 5 billion years ago thus requires that oxygen sinks must have been much larger than they are now here we propose that the rise of atmospheric oxygen occurred because the predominant sink for oxygen in the archaean era enhanced submarine volcanism was abruptly and permanently diminished during the archaean proterozoic transition observations are consistent with the corollary that subaerial volcanism only became widespread after a major tectonic episode of continental stabilization at the beginning of the proterozoic submarine volcanoes are more reducing than subaerial volcanoes so a shift from predominantly submarine to a mix of subaerial and submarine volcanism more similar to that observed today would have reduced the overall sink for oxygen and led to the rise of atmospheric oxygen

paleoflood evidence for a natural upper bound to flood magnitudes in the colorado river basin the existence of an upper limit to the magnitude of floods in a region is a long standing and controversial hypothesis in flood hydrology regional envelope curves encompassing maximum flood magnitudes stabilize with progressive increases in the areal coverage and period of observation wolman and costa 1984 however the short lengths of conventional gaging records limit substantial advances in testing whether this stabilization is evidence of an upper limit in the colorado river basin there are 32 120 station years of gage data but the average period at a gaging station is only 20 years with most stations having less than 70 years of observation paleoflood magnitudes derived from sediments of large prehistoric floods from 25 sites on rivers in arizona and utah provide additional data to extend the records of the largest floods the paleoflood data identify the maximum flood discharges that have occurred on individual rivers over the last several hundred to several thousand years even with this increase in the observational period the largest paleoflood discharges do not exceed the upper bound of maximum peak discharges delineated by the envelope curve derived from the available gaged and historical records this result accords with the hypothesis of an upper physical limit for flood magnitudes and suggests that for the colorado river basin the upper limit can be approximated by existing systematic and historical data for large floods similar relationships also hold when paleofloods and gaged records are presented for the subregion of southern arizona

three dimensional acoustic monitoring and modeling of a deep sea co2 droplet cloud 1 we show that release of 5 liters of liquid co2 at 1000 m depth can be readily detected acoustically and tracked for over 30 minutes and 150 m of ascent with both surface ship 38 khz and rov 675 khz sonars the released liquid broke up into droplets covered with a hydrate film the remarkably sensitive acoustic response of the droplets may be attributed to the high sound speed contrast between co2 300 m sec and sea water 1500 m sec the near spherical shape of the droplets created by the hydrate shell and the high compressibility of the liquid the observed cloud conformed closely to models of co2 disposal allowing for reasonable predictions of larger scale processes this offers a remarkably sensitive technique for examination in real time of engineered releases of co2 volcanic sea floor liquid co2 plumes or leakage from geologic co2 storage

an 60 million year periodicity is common to marine 87sr 86sr fossil biodiversity and large scale sedimentation what does the periodicity reflect abstractwe find that the marine 87sr 86sr record shows a significant periodicity of m yr the 87sr 86sr record is out of phase with a 62 3 m yr periodicity previously reported in the record of marine animal diversity these periodicities are close to 58 4 m yr cycles found for the number of gap bounded sedimentary carbonate packages of north america we propose that these periodicities reflect the operation of a periodic pulse of the earth in large scale earth processes these may be linked to mantle or plate tectonic events possibly uplift which affects the earth s climate and oceans and thus the geochemistry sedimentation and biodiversity of the marine realm

paleoflood hydrology and extraordinary flood events paleoflood hydrology is the study of past or ancient flood events the most accurate technique involves the analysis of slackwater deposits and paleostage indicators swd psi slackwater deposits are sand and silt emplaced from suspension in exceptionally deep high velocity floods that characterize narrow deep canyons in resistant geologic materials these and other paleostage indicators are used to establish hydraulic grade lines for the generating flow events favorable sites for slackwater deposition include channel margin areas where stresses and velocities are reduced below critical values necessary to maintain fine grained bedload material sand and silt in suspension computerized procedures for hydraulic flow modeling are used to tie the elevations of the highest slackwater deposits to surveyed river cross sections the flood discharges calculated by this method can be calibrated through the study of modern floods on gaged rivers correlation of multiple swd psi sites along a river reach is used to identify the maximum paleostage achieved by a given flood advances in the dating of flood deposits permit estimates of flood frequency to be made extending over a data base of thousands of years the major geochronologic tool is radiocarbon dating of various kinds of organic matter intercalated with the slackwater deposits an important development is the use of the tandem accelerator mass spectrometer for direct measurement of 14c tiny blebs of charcoal seeds and other organics can be analyzed in order to date ancient flood deposits of hydrologic significance since swd psi studies yield very accurate determinations of paleoflood ages and magnitudes there is a pressing need for new statistical procedures that make optimum use of the information content in paleoflood records for flood frequency analysis nevertheless swd psi paleoflood hydrology has moved beyond the research phase its use should be encouraged in evaluating past experience of extraordinary floods at appropriate hazardous sites

singularity analysis of global zircon u pb age series and implication of continental crust evolution abstract recently compiled global databases of igneous and detrital zircon u pb ages have been integrated with other types of isotope data e g neodymium hafnium and oxygen to characterize the episodic growth of the continental crust and the development of supercontinents however to what extent do the age peaks reflect the rate of continent crust growth or the differential preservation due to supercontinent settings is a long standing question instead of analyzing amplitudes and periodicity of the age series here the analysis focuses on shapes of the individual age peaks described by a power law model for measuring the scale invariant fractality and singularity of time series records the results indicate that zircon age distributions around peaks follow power law distribution regardless of the bin size used to measure the age distribution based on the commonly accepted mechanisms phase transition self organized criticality and multiplicative cascade processes for generation of power law distributions one can relate the nonlinearity of the age peaks to short spurts of accelerated magmatic activities due to avalanches superplumes slab breakoff etc occurred during episodic convection of the mantle the exponents of the power law age distributions estimated from the age peaks can be used as an index to quantify the intensity of a singularity the values of exponents calculated at all major age peaks from five global databases exhibit an episodic nature with a mean duration of approximately 600 800 myr both intensity of the zircon episodes and their duration for the interval from 3 ga to 0 5 ga depict a descending trend which may signify mantle cooling

superplumes plates and mantle magmatism in two dimensional numerical models 1 two dimensional numerical models are systematically presented for magmatism in a convecting mantle internally heated by incompatible radioactive elements plate tectonics is self consistently modeled as a part of convection of newtonian fluid with temperature and stress history dependent viscosity driven by thermal and compositional buoyancy while magmatism is modeled as an upward migration of basaltic magma generated by decompression melting ridge magmatism actively takes place to chemically differentiate the mantle and a superplume is well developed in the lowermost mantle as a broad hot region enriched in basaltic component provided that the lithosphere is mechanically strong enough to inhibit spontaneous initiation of subduction by its own weight and that the internal heating rate is sufficiently high hot narrow plumes frequently rise up from the superplume to induce deep magma generation and hot spot magmatism which in turn frequently induces new plate margins the frequent plate margin formation keeps the activity of plate tectonics rather steady the thermal and chemical state of the mantle remains rather steady too as the internal heating rate becomes lower however the narrow plumes from superplume become sporadic and significantly colder deep magma generation becomes rare and new plate margin formation becomes less frequent as a consequence the activity of plate tectonics and the thermal and chemical state of the mantle become fluctuating much with time the mantle structure and plume behavior modeled here match with many observations for those of the earth from the archean to the present

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

earth s middle age this research was funded through natural environment research council grant ne j021822 1 and the apc was paid through the rcuk oa block grant

thermobarometry and fluid evolution of enderbites within the magondi mobile belt northern zimbabwe abstract metamorphic conditions within arenaceous calcareous and argillaceous supracrustal rocks of the magondi mobile belt zimbabwe range from greenschist to granulite facies within the high grade segment basement gneisses of early proterozoic age and argillaceous rocks of the mid proterozoic piriwiri group are intruded by charnockites and enderbites metamorphic mineral assemblages and thermobarometric data for enderbitic granulites of nyaodza show temperatures of 700 800 c and pressures of 5 7 kbar for the peak of granulite facies metamorphism microthermometry and raman microspectroscopy reveal that co 2 associated with minor n 2 has been the dominant fluid phase during granulite facies metamorphism the chronology of the co 2 inclusions and the development of microtextures and mineral assemblages in the enderbites indicates that isolated negative crystal shaped co 2 inclusions in quartz and plagioclase porphyroclasts entrap syn metamorphic fluids of medium high densities 0 88 0 90 g cm 3 lower density 0 71 0 77 g cm 3 co 2 inclusions in trails and clusters within the same minerals were formed from local re equilibration and re entrapment of the former near peak granulitic co 2 inclusions as in many other granulites syn metamorphic co 2 is associated with intrusives emplaced near the peak of metamorphism

a record of rapid holocene climate change preserved in hyrax middens from southwestern africa the discovery of sensitive paleoenvironmental proxies contained within fossilized rock hyrax middens from the margin of the central namib desert africa is providing unprecedented insight into the region s environmental history high resolution stable carbon and nitrogen isotope records spanning 0 11 700 cal calibrated yr b p indicate phases of relatively humid conditions from 8700 7500 6900 6700 5600 4900 and 4200 3500 cal yr b p with a period of marked aridity occurring from 3500 until ca 300 cal yr b p transitions between these phases appear to have occurred very rapidly often within 200 years of particular importance are 1 the observed relationship between regional aridifi cation and the decline in northern hemisphere insolation across the holocene and 2 the signifi cance of suborbital scale variations in climate that covary strongly with fl uctuations in solar forcing together these elements call for a fundamental reexamination of the role of orbital forcing on tropical african systems and a reconsideration of what factors drive climate change in the region the quality and resolution of these data far surpass any other evidence available from the region and the continued development of this unique archive promises to revolutionize paleoenvironmental studies in southern africa

correlation chart of the proterozoic assembly of the northeastern canadian greenland shield chart 1 correlation chart of the proterozoic assembly of the northeastern canadian greenland shield including a location map and reference list

thermohaline fine structure in an oceanographic front from seismic reflection profiling we present acoustic images of oceanic thermohaline structure created from marine seismic reflection profiles across the major oceanographic front between the labrador current and the north atlantic current the images show that distinct water masses can be mapped and their internal structure imaged using low frequency acoustic reflections from sound speed contrasts at interfaces across which temperature changes the warm cold front is characterized by east dipping reflections generated by thermohaline intrusions in the uppermost 1000 meters of the ocean our results imply that marine seismic reflection techniques can provide excellent spatial resolution of important oceanic phenomena including thermohaline intrusions internal waves and eddies

the last millennium largest floods in the hyperarid kuiseb river basin namib desert the largest kuiseb river floods initiate at the basin s semi arid headwater downstream along the hyperarid namib desert these floodwaters are feeding shallow alluvial aquifers the only available water for human activity and for the natural ecology here we characterize the largest floods and their changing frequency and magnitudes using palaeohydrological methods along 120 km of the rivers canyon 35 palaeoflood deposit sites were identified at five of these sites we conducted stratigraphic and geochronological analyses and flood discharge estimations the upper bound of the largest flood over the late holocene is 1475 m 3 s 1 over the last 1300 years more than ten floods have exceeded 1250 m 3 s 1 an additional 33 floods exceeded 400 m 3 s 1 the last millennium was characterized by one large flood every 30 40 years during the periods 1250 1335 1355 1565 and 1715 ad to the present a slight increase in flood frequency not magnitude occurred during 1565 1715 ad one large flood in 20 years and during two short episodes 1185 1205 and 1335 1355 ad seven and four floods in 20 years respectively these episodes of increased flood frequency are associated with other proxy records of higher water availability in the namib copyright 2013 john wiley sons ltd

chronology of crustal growth and recycling in the paleoproterozoic amisk collage flin flon belt trans hudson orogen canada u pb zircon ages have been compiled for magmatic and sedimentary rocks from the low metamorphic grade portion of the flin flon greenstone belt now recognized as a paleoproterozoic tectonic collage

correlation chart of the evolution of the trans hudson orogen manitoba saskatchewan segment a correlation chart showing the evolution of the manitoba saskatchewan segment of the paleoproterozoic trans hudson orogen tho has been constructed based on available geochronological and isotopic data geological petrological geochemical and structural data have also been incorporated into the correlation chart to provide finer detail and to allow comparison of specific tectonic events the chart also emphasizes that the bounding archean cratons superior hearne sask have been significantly affected by paleoproterozoic events and it is hoped that this chart will also facilitate comparison with other paleoproterozoic orogens in north america and beyond

major episodes of geologic change correlations time structure and possible causes abstract published data sets of major geologic events of the past 250myr extinction events sea level lows continental flood basalt eruptions mountain building events abrupt changes in sea floor spreading ocean anoxic and blackshale events and the largest evaporite deposits have been synthesized with estimated errors these events show evidence for a statistically significant periodic component with an underlying periodicity formally equal to 26 6 myr and a recent maximum close to the present time the cycle may not be strictly periodic but a periodicity of 30myr is robust to probable errors in dating of the geologic events the intervals of geologic change seem to involve jumps in sea floor spreading associated with episodic continental rifting volcanism enhanced orogeny global sea level changes and fluctuations in climate the period may represent a purely internal earth pulsation but evidence of planetesimal impacts at several extinction boundaries and a possible underlying cycle of 28 36 myr in crater ages suggests that highly energetic impacts may be affecting global tectonics a cyclic increase in the flux of planetesimals might result from the passage of the solar system through the central plane of the milky way galaxy an event with a periodicity and mean phasing similar to that detected in the geologic changes

archaean crustal growth processes in southern west greenland and the southern superior province geodynamic and magmatic constraints abstract eo to mesoarchaean greenstone belts e g 3800 3700 ma isua c 3075 ma ivisaartoq 3071 ma qussuk occur within orthogneisses of the southern west greenland craton greenstone belts are composed mainly of metavolcanic rocks with minor ultramafic and sedimentary schists compositionally volcanic rocks are dominantly tholeiitic basalts boninites and picrites with minor intermediate to felsic volcanic rocks these greenstone belts appear to have formed in convergent margin geodynamic settings detailed field observations contrasting ages and metamorphic and structural histories suggest that this craton was assembled in several accretionary tectonothermal events involving accretion of arcs back arcs forearcs and continental fragments by horizontal tectonics the superior province of canada was also built by the amalgamation of oceanic and continental fragments ranging in age from 3700 to 2650 ma during five discrete tectonothermal events over 40 ma between 2720 and 2680 ma the neoarchaean 2750 2670 ma wawa greenstone belts are composed of tectonically juxtaposed fragments of oceanic plateaux oceanic island arcs back arcs and siliciclastic trench turbidites following juxtaposition these diverse lithologies were collectively intruded by syn to post kinematic granitoids with subduction zone geochemical signatures oceanic island arc lavas are easily distinguished from oceanic plateau counterparts because they possess positively fractionated rare earth element la sm cn 1 and gd yb cn 1 and high field strength element depleted nb th pm pm c 1000 km scale subduction accretion complex that formed along an intra oceanic convergent plate margin during trenchward migration of the magmatic arc axis several first order geological observations on archaean greenstone belts of sw greenland and the superior province suggest that phanerozoic style plate tectonic models can provide an elegant explanation for their structural lithological metamorphic and geochemical characteristics

sedimentology of high stage flood deposits of the tagus river central spain abstract this paper details the sedimentology of high stage flood deposits with the definition of sedimentary environments and their characteristic sequences along two bedrock reaches of the tagus river central spain high stage flood deposits accumulated in bedrock canyons include slackwater flood deposits swd and other types of deposits located at flow separation zones and associated with slow moving flow 1 these flood deposits are common indirect indicators of flood stages used in palaeoflood studies for estimating the discharges associated with quaternary floods depositional environments of flood deposits include 1 channel widening 2 canyon expansion 3 bedrock obstacles and 4 backflooded areas along tributary streams these flood deposits can be found associated with other non fluvial environments namely aeolian reworked and slope washflow facies channel widening due to flood stage variations comprises internal and external zones of the channel margins and their characteristic sequences contain similar facies to those of alluvial floodplains canyon expansion environments favour vertical accretion of slackwater units and the development of flood deposit benches which contain four sequences related to bench elevation and distance from the channel s main thread of flow at the lee side of bedrock obstacles characteristic sedimentary sequences are dominated by reverse flow structures e g climbing ripples migrating upstream due to eddies with a high sand concentration flood deposits located within tributary mouths contain typical sequences of reworked floodplain deposits backflooding of tributaries during flood stages produces deposition from suspension of sand silt and clay within three sequences characterised by non structure or parallel lamination and intense bioturbation a better understanding of the flood deposit sequences may contribute to the characterisation of flood magnitudes and flood hydraulics and can also be applied to some ancient depositional environments

archean komatiite volcanism controlled by the evolution of early continents the generation and evolution of earth s continental crust has played a fundamental role in the development of the planet its formation modified the composition of the mantle contributed to the establishment of the atmosphere and led to the creation of ecological niches important for early life here we show that in the archean the formation and stabilization of continents also controlled the location geochemistry and volcanology of the hottest preserved lavas on earth komatiites these magmas typically represent 50 30 partial melting of the mantle and subsequently record important information on the thermal and chemical evolution of the archean proterozoic earth as a result it is vital to constrain and understand the processes that govern their localization and emplacement here we combined lu hf isotopes and u pb geochronology to map the four dimensional evolution of the yilgarn craton western australia and reveal the progressive development of an archean microcontinent our results show that in the early earth relatively small crustal blocks analogous to modern microplates progressively amalgamated to form larger continental masses and eventually the first cratons this cratonization process drove the hottest and most voluminous komatiite eruptions to the edge of established continental blocks the dynamic evolution of the early continents thus directly influenced the addition of deep mantle material to the archean crust oceans and atmosphere while also providing a fundamental control on the distribution of major magmatic ore deposits

palaeoproterozoic sedimentation on the singhbhum craton global context and comparison with kaapvaal the singhbhum craton has a limited palaeoproterozoic supracrustal record which suggests a three part history comprising a long period of high freeboard and palaeosol formation on granitoids subsequent rift related mafic ultramafic volcanism and subordinate sedimentation c 2 25 2 1 ga dhanjori and jagannathpur basin fills possibly also simlipal malangtoli and ongarbira basin fills which overlapped locally with mafic soil formation and a major regression at around2 0 ga followinga longhiatus theapproximately 1 6 gadhalbhum dalmasuccession was laid down probably under continental conditions this rather truncated record stands in con trast to the chronologically and geographically much more widespread supracrustal basin fills of the kaapvaal craton and there appears to be an overall poor comparison between these two early precambrian crustal blocks however on kaapvaal three analogous events are identified widespread approximately 2 2 ga mafic volcanism followed by a well developed palaeosol and a major transgression prior to 2 05 ga the three shared events between the two cratons are com patible with the postulate of a global approximate 2 45 2 2 ga shutdown of magmatic and tec tonic geodynamics with the origin of the triumvirate directly reflecting its resumption again after about 2 2 ga we recognize here that a large diversity of views on singhbhum s geodynamic history exists predicated on a lack of precise geochronology and commonly poor outcrops and the current hypotheses are presented with these factors in mind

partial melting and the formation of granulite facies assemblages in namaqualand south africa abstract dehydration melting reactions in which water from a hydrous phase enters the melt leaving an anhydrous solid assemblage are the dominant mechanism of partial melting of high grade rocks in the absence of externally derived vapour equilibria involving melt and solid phases are effective buffers of ah2 o the element partitioning observed in natural rocks suggests that dehydration melting occurs over a temperature interval during which for most cases ah2o is driven to lower values the mass balance of dehydration melting in typical biotite gneiss and metapelite shows that the proportion of melt in the product assemblage at t 850 c is relatively small 10 20 and probably insufficient to mobilize a partially melted rock body granulite facies metapelite biotite gneiss and metabasic gneiss in namaqualand contain coarse grained discordant unfoliated anhydrous segregations surrounded by a finer grained foliated matrix that commonly includes hydrous minerals the segregations have modes consistent with the hypothesis that they are the solid and liquid products of the dehydration melting reactions bt sil qtz pi grt crd kfs l metapelite bt qtz pl opx kfs l biotite gneiss and hbl qtz opx cpx pl l metabasic gneiss the size shape distribution and modes of segregations suggest only limited migration and extraction of melt growth of anhydrous poikiloblasts in matrix regions development of anhydrous haloes around segregations and formation of dehydrated margins on metabasic layers enclosed in migmatitic metapelites all imply local gradients in water activity also they suggest that individual segregations and bodies of partially melted rock acted as sinks for soluble volatiles the preservation of anhydrous assemblages and the restricted distribution of late hydrous minerals suggest that retrograde reaction between hydrous melt and solids did not occur and that h2o in the melt was released as vapour on crystallization this model combined with the natural observations suggests that it is possible to form granulite facies assemblages without participation of external fluid and without major extraction of silicate melt

late cretaceous magmatism in madagascar palaeomagnetic evidence for a stationary marion hotspot late cretaceous basaltic volcanics in the morondava basin sw madagascar possess high quality and pre fold palaeomagnetic data declination 353 5 inclination 54 8 95 2 4 the palaeomagnetic data are all of normal magnetic polarity and remanence acquisition is linked to the terminal stages of the cretaceous normal superchron 83 ma this is sustained by an 40ar 39ar age of 83 6 1 6 ma from one of the tested basaltic flows a precise u pb zircon baddeleyite age from northeast madagascar demonstrates magmatism at least back to 91 6 0 3 ma thus reliable isotope ages for the madagascar cretaceous igneous province span a range of 8 million years late cretaceous palaeomagnetic data obtained from volcanics and dolerites all over madagascar are directionally concordant and the combined palaeomagnetic pole latitude 68 5 n longitude 230 3 e a95 5 5 n 8 studies sampling age range ca 84 90 ma represents one of the best late cretaceous poles for the former gondwanan elements the collective palaeomagnetic data yield a palaeolatitude of 45 3 s 5 3 4 7 for the proposed focal point of the marion plume volcan de l androy southeast madagascar during the late cretaceous this is in perfect agreement with hotspot controlled reconstructions that place the marion hotspot 46 0 s beneath southeast madagascar during the late cretaceous setting true polar wander aside hotspot movements in the indian ocean do not appear to exceed ca 0 75 cm yr and the marion hotspot appears stationary within the resolution power of palaeomagnetic data

bayesian flood frequency analysis with paleohydrologic bound data 1 it is valuable to construct likelihood functions that rigorously incorporate measurement errors and annual peak discharge historical and paleohydrologic bound information in bayesian flood frequency analyses estimates of primary posterior modes for common three parameter frequency distributions are constructed using simulated annealing and the simplex method parameter and flood frequency probability intervals are calculated directly by systematic parameter space integration bayesian flood frequency analyses with annual peak discharge historical and paleohydrologic bound data for the santa ynez river california and the big lost river idaho demonstrate that paleohydrologic bounds reduce quantile biases by placing large observed peak discharges in their proper long term contexts and substantially narrow peak discharge confidence intervals when estimating floods with low exceedance probabilities

two neoarchean supercontinents revisited the case for a rae family of cratons abstract a variety of early paleoproterozoic supercontinents or supercratons have been proposed including kenorland superia and vaalbara improved geologic knowledge of the churchill province canadian shield suggests that the classical kenorland configuration with linked rae and hearne cratons is invalid and represents a younger ca 1 9 ga nuna configuration we review the early paleoproterozoic tectonic history of the rae craton and propose that the ca 2 55 2 50 ga macquoid and 2 50 2 28 ga arrowsmith orogens represent critical piercing points for supercontinent reconstruction utilizing the stratdb and dateview databases we have identified nearly two dozen regions worldwide that show effects of these orogenic events including the rae gawler mawson peninsular india west africa amazonia north china and sask crustal blocks we propose that these cratonic blocks form a distinct rae family lineage utilizing new time space plots for the rae family and components of the superia and vaalbara supercratons we demonstrate that the rae family lacks the distinctive early paleoproterozoic depositional and glaciogenic record characteristic of superia and vaalbara but has an important contemporaneous record of juvenile crust formation and contractional orogenesis we propose the collective history of the rae family is best reconciled if it represents the now dispersed blocks of a third earliest paleoproterozoic supercraton that we term nunavutia in this model the magmatic and tectonometamorphic record of these cratons includes an early 2 55 2 45 ga period of interior collisional orogenesis in which the supercraton amalgamated and a later phase of long lived peripheral orogenesis 2 45 2 28 ga prior to its partial break up at ca 2 05 ga forming within an otherwise globally quiescent period the history of the rae family is important for assessment of constrasting models of early paleoproterozoic geodynamic activity

paleofloods and a dam failure flood on the virgin river utah and arizona abstract flood deposits on the virgin river in southern utah and northern arizona preserve a record of the magnitude and frequency of the largest natural floods that have occurred on the river since ad 900 in the virgin river gorge several of the natural paleoflood discharges were comparable with or even greater than the flood from a catastrophic dam failure in 1989 that produced an estimated peak discharge of 1730 m3 s 1 in this reach of the virgin river to estimate the paleoflood discharges the heights of slackwater flood deposits in the lower bedrock gorge of the virgin river were compared with water surface elevations calculated with the hec 2 step backwater hydraulic model the timing of the paleofloods was determined by the ages of layers rich in human artifacts that are interbedded with the flood deposits the estimates of both the 1989 flood and the paleofloods were carried out in the virgin river gorge about 60 km downstream of the dam break site no natural floods of this magnitude have occurred within the relatively short period of the gaged record this study illustrates how incorporating rare extreme events from the long term paleoflood record improves the database for flood frequency analysis the 1989 flood serves as an analog to the effects of rare natural floods with similar magnitudes along the lower virgin river

radiogenic heat production in the lithosphere of sulu ultrahigh pressure metamorphic belt abstract the chinese continental scientific drilling ccsd project is located at the sulu ultrahigh pressure metamorphic uhpm belt it offers a unique opportunity for studying the radiogenic heat production of both shallower and deeper rocks based on the concentrations of radiogenic elements u th and k on 349 samples from main hole of ccsd ccsd mh pilot holes and exposures we determined radiogenic heat productions of all major rock types in the sulu uhpm belt results show the mean values of orthogneiss and paragneiss are respectively 1 65 0 81 and 1 24 0 61 w m 3 due to different composition and grade of retrogressive metamorphism the eclogites display significant scatter in radiogenic heat production ranging from 0 01 to 2 85 w m 3 with a mean of 0 44 0 55 w m 3 the radiogenic heat production in ultramafic rocks also varies within a large range of 0 02 to 1 76 w m 3 and the average turns out to be 0 18 0 31 w m 3 based on the measurements and crustal petrologic model the vertical distribution model of heat production in sulu crust is established the resulting mean heat production 0 76 w m 3 contributes 24 mw m 2 to the surface heat flow 1 d thermal model indicates that the temperature at the moho reaches above 750 c and the thermal thickness of the lithosphere is 75 km in good agreement with the geophysical results the high teat flow 75 mw m 2 together with thin lithosphere presents strong support for the extension events during the late cretaceous and cenozoic

calculation and application of clinopyroxene garnet plagioclase quartz geobarometers recently published thermodynamic and experimental data in a variety of chemical systems have been evaluated to derive gibbs free energies for hedenbergite and pyrope these were used to calculate the geobarometric equilibria

fractionation between inorganic and organic carbon during the lomagundi 2 22 2 1 ga carbon isotope excursion article i nfo the lomagundi 2 22 2 1 ga positive carbon isotope excursion in shallow marine sedimentary carbonates has been associated with the rise in atmospheric oxygen but subsequent studies have demonstrated that the carbon isotope excursion was preceded by the rise in atmospheric oxygen the amount of oxygen released to the exosphere during the lomagundi excursion is constrained by the average global fractionation between inorganic and organic carbon which is poorly characterized because dissolved inorganic and organic carbon reservoirs were arguably larger in the paleoproterozoic ocean at a time of lower solar luminosity and lower ocean redox state decoupling between these two variables might be expected we determined carbon isotope values of carbonate and organic matter in carbonates and shales of the silverton formation south africa and in the correlative sengoma argillite formation near the border in botswana these units were deposited between 2 22 and 2 06 ga along the margin of the kaapvaal craton in an open marine deltaic setting and experienced lower greenschist facies metamorphism the prodelta to offshore marine shales are overlain by a subtidal carbonate sequence carbonates exhibit elevated 13 c values ranging from 8 3 to 11 2 vs vpdb consistent with deposition during the lomagundi positive excursion the total organic carbon toc contents range from 0 01 to 0 6 and 13 c values range from 24 8 to 13 9 thus the isotopic fractionation between organic and carbonate carbon was on average 30 3 2 8 n 32 in the shallow marine environment the underlying sengoma shales have highly variable toc contents 0 14 to 21 94 and 13 c values 33 7 to 20 8 with an average of 27 0 3 0 n 50 considering that the shales were also deposited during the lomagundi excursion and taking 13 c values of the overlying carbonates as representative of the 13 c value of dissolved inorganic carbon during shale deposition a carbon isotope fractionation as large as 37 appears to characterize the production of bulk organic matter in the deeper part of the pretoria basin at that time this enhanced fractionation relative to that observed in shallow water environments likely reflects heterotrophic secondary and chemotrophic productivity at and below a pronounced redoxcline consistent with the euxinic conditions inferred from independent evidence for the deeper part of the pretoria basin greater variability in organic carbon vs carbonate carbon isotopic values on the shallow marine carbonate platform suggests that the carbon cycling was dominated by a large dissolved inorganic carbon reservoir during the lomagundi excursion our study suggests that in contrast to the late neoproterozoic and phanerozoic when carbon isotope fractionation between carbonate and organic carbon in the open ocean was mostly controlled by primary producers in the paleoproterozoic redox stratified ocean heterotrophic and chemotrophic productivity overprinted a signal of primary productivity below the redoxcline this strong imprint of heterotrophic and chemotrophic productivity on organic carbon isotope records complicates the reconstruction of spatial patterns and secular trends in the 13 c values of dissolved inorganic carbon in the paleoproterozoic seawater

superplumes and ridge interactions between ontong java and manihiki plateaus and the nova canton trough the initial pulse of volcanism on the ontong java and manihiki plateaus began prior to 123 124 ma and had largely ceased by about 122 ma whereas the intervening pacific phoenix spreading ridge probably was disrupted between 120 and 115 ma by formation of the nova canton trough rift system plateau formation may have resulted from a large plume head that rose beneath each of the two plateau sites during the plateau eruption phase these two plumes spread beyond the plateaus and within the asthenosphere they eventually coalesced beneath the intervening spreading ridge where the nova canton trough rift system is now located at about the time volcanism ceased on the plateaus the two plumes combined to form the nova canton trough rift system by reheating the nascent lithosphere near the ridge crest and widening the plate boundary from the usual 1 10 km to a much broader region the present rift system is a maximum of 500 km wide and much of this area may have formed by normal faulting and crustal accretion across the broadened plate boundary as plate divergence continued eventually coherent spreading resumed somewhere south of the nova canton trough rift system

carbon and u pb evidence for a palaeoproterozoic crustal component in the central zone of the limpopo belt south africa supposedly mid archaean c 3 3 ga granulite grade marbles from the gumbu group central zone limpopo belt south africa have anomalously high 13 c values 4 6 to 7 interpreted as minimum values for their limestone protoliths such elevated 13 c values are unknown in any archaean carbonate worldwide but occur in palaeoproterozoic c 2 4 2 1 ga carbonates for example on the adjacent zimbabwe and kaapvaal cratons this suggests that the central zone contains a previously unrecognized palaeoproterozoic cover sequence contrary to the prevailing view this conclusion is supported by zircon u pb sensitive high resolution ion microprobe ages from two metasediments which in addition to c 2 02 ga metamorphic zircon overgrowths contain detrital zircons no older than c 2 68 ga and some possibly as young as c 2 2 ga the recognition of post 3 3 ga metasediments in the central zone requires a major revision of models of the tectonic evolution of the limpopo belt

re evaluating the geochronology of the permian tarim magmatic province implications for temporal evolution of magmatism the permian tarim magmatic province has 118 published ages ranging from 358 to 205 ma but the timing of mafic magmatism is not well constrained we report two new secondary ion mass spectrometry u pb zircon dates on the halahatang trachydacite and wajilitag olivine clinopyroxenite which are 287 2 2 0 ma and 283 2 2 0 ma respectively the trachydacite overlies the uppermost basalt and constrains the latest eruption age of basalt in northern tarim the latter is the first high resolution date for the wajilitag mafic layered intrusion by screening all published ages we identified 22 robust ages ranging from 290 9 4 1 to 261 7 1 8 ma the robust ages together with our new data reveal a protracted period of mafic magmatism at c 283 and c 267 ma silicic magmatism occurred from 291 to 272 ma although the current known volume of tarim basalt is too small to qualify as a large igneous province the eroded and intrusive components as well as pyroclastic deposits and silicic lavas may increase the estimated volume further work is required to refine the duration of magmatism and the volume estimate of the province supplementary material a list of all the published dates raw data of published 40 ar 39 ar dates raw data of published u pb dates published geochemistry data used for calculation and complete details of the data evaluation are available at http www geolsoc org uk sup18862

proterozoic tectonic provinces of southern africa abstract recent investigations in southwestern africa i e the nw cape province and namibia require a consistent hierarchy of tectonic provinces subprovinces belts and terranes in order to provide an adaptable framework table i and fig 1 for further progress in crustal evolution studies the proterozoic continental crust of southwestern africa originated during three major orogenic cycles known elsewhere in africa as the eburnean kibaran and pan african episodes