the thermal structure of mantle plumes axisymmetric or triple junction summary a mantle plume is probably a complex 3 d thermal structure that possesses approximate axisymmetry as it approaches the base of the lithosphere from below but followed down towards the base of the layer probably consists of a triple junction or quadruple junction of connected hot sheets a relatively weak hot sheet rising only part way through the layer probably connects two neighbouring mantle plumes these conclusions are suggested by numerical experiments on a 3 d constant viscosity plane layer with stress free boundaries which detail the gradational change in the planform of a convecting layer from the top of the layer to its base the planform of a convecting layer is a map in the horizontal plane of the principal thermal anomalies in the layer these anomalies are the main sources of positive for hot fluid or negative for cold fluid buoyancy and therefore they drive the convective flow they may appear in cross section as structures with either axial symmetry columns planar symmetry sheets or some complex asymmetric form when convection is driven at least partially by basal heating the planform near the top of the layer may be described as a network of cold sinking sheets and isolated hot columns while near the base of the layer it appears as a network of hot rising sheets and isolated cold columns the hot columns near the upper surface arise from the vertices or nodes of the network of hot sheets on the lower surface and similarly the cold columns at the base of the layer form below the vertices of the network of cold sheets near the upper surface near the upper surface the apparent planform of this experiment is analogous to that of mantle convection the cold sheets compared to subduction zones and the hot columns compared to mantle plumes the hot plumes impinging on the upper surface produce approximately axisymmetric temperature anomalies surface uplift and extensional stress fields however the relatively minor deviations from axisymmetry of surface observables reflect the deep structure of the mantle plume formed by the junction of three or four hot sheets on the base of the layer it seems likely that the commonly occurring triple junction form of continental rifts may reflect an underlying structure that is implicit in the convective circulation of the mantle beneath

mantle transition zone thickness beneath cameroon evidence for an upper mantle origin for the cameroon volcanic line summary the thickness of the mantle transition zone beneath cameroon has been mapped using data from the 2005 2007 cameroon broadband seismic experiment to evaluate models for the originof the cameroon volcanic line cvl some 2200 receiver functions have been stacked using a 3 d velocity model revealing ps conversions from the mantle transition zone discontinuities at depths of 410 and 660 km results yield a nearly uniform transition zone thickness 251 10 km that is similar to the global average implying that any thermal anomalies in the upper mantle beneath the cvl do not extend as deep as the transition zone this finding when combined with regional p and s velocity models of the mantle supports an explanation attributing the origin of the cvl to upper mantle processes such as an edge flow convection cell in the upper mantle along the northwestern side of the congo craton lithosphere

geomorphological evidence for jokulhlaups from kverkfjoll volcano iceland abstract jokulhlaups glacial outburst floods are known to have drained along the jokulsa a fjollum river in iceland during the holocene however little is known about their number age source and flow characteristics this paper provides detailed geomorphological evidence for jokulhlaups that have routed from the kverkfjoll ice margin and hence into the jokulsa a fjollum erosional evidence of jokulhlaups from kverkfjoll includes gorges cataracts spillways subaerial lava steps and valley wide scoured surfaces depositional evidence includes wash limits boulder bars cataract fill mounds terraces slackwater deposits and outwash fans some of these landforms have been documented previously in association with jokulhlaups however subaerial lava surfaces that have been scoured of the upper clinker gorges within pillow hyaloclastite ridges gorges between pillow hyaloclastite ridges and subaerial lava flows subaerial lava lobe steps cataract fill mounds and boulder run ups are previously undocumented in the literature these landforms may therefore be diagnostic of jokulhlaups within an active volcanic rifting landscape the nature and spatial distribution of these landforms and their stratigraphic association with other landforms suggest that there have been at least two jokulhlaups through kverkfjallarani the biskupsfell eruption occurred between these two jokulhlaups kverkfjallarani jokulhlaups were very strongly influenced by topography geology and interevent processes that together determined the quantity and nature of sediment availability such controls have resulted in jokulhlaups that were probably fluidal turbulent and supercritical over large areas of the anastomosing channel bed kverkfjallarani jokulhlaups would have had highly variable hydraulic properties both spatially and temporally the knowledge of flow characteristics that can be gained from jokulhlaup impacts has implications for recognising jokulhlaups in the rock record and for hazard analysis and mitigation within similar landscapes and upon other glaciated volcanoes

slow but not fast global expansion may explain the surface dichotomy of earth abstract various fast and slow earth expansion hypotheses are reviewed principal objectives are 1 1 to aid geologists to overcome their current mysticism in rejecting accepting or sympathising with earth expansion hypotheses without distinction and 2 2 to stimulate geophysicists into taking full account of the possibility that slow earth expansion by core mantle interaction may have occurred inward migration of the core mantle boundary due to the differentiation and progressive solidification of the liquid outer core might have caused expansion of the earth s radius at a slow mean rate of 0 5 mm a 1 a slowly expanding earth is attractive since it could explain why two distinct types of lithosphere exist oceanic and continental

jokulhlaup deposits in proglacial areas abstract this paper discusses the main causes and characteristics of jokulhlaup glacier burst floods and explores the extent to which they generate depositional landform and sediment assemblages that are distinct from those of normal braided river outwash type i outwash two main jokulhlaup outwash environments are identified type ii outwash produced by sudden drainage of ice dammed lakes and type iii associated with drainage during subglacial geothermal activity and distinguished by deposits resulting from high sediment concentrations and hyperconcentrated flows in fluid flows especially ones yielding type ii outwash the most common deposits are large scale expansion bars and locally eddy and pendant bars and mega ripples or dunes both forms normally composed of large scale gravel cobble cross bedding often capped by an imbricated boulder lag a type b2 lithofacies sequence the armour is absent only where runoff decreased too rapidly to allow surface winnowing other jokulhlaup facies include extensive boulder beds type c inverse normally graded cobble beds type ds ice proximal debris flow deposits and deformed bedding containing diamicton clasts types g and h and slack water sediments type a type iii outwash is dominated by massive homogeneous flood surge granules underlain by pre surge gravels and capped by post surge fluid bedforms reflecting deposition during both the rising and falling limbs of the flood hydrograph type e4 the paper demonstrates that jokulhlaups do generate distinctive assemblages of depositional landforms and sediments and concludes with a model of the dominant lithofacies sequences and associated landforms in proglacial environments subject to jokulhlaup drainage

upper mantle structure below the european continent constraints from surface wave tomography and grace satellite gravity data 1 we here exploit fundamental mode rayleigh and love seismic wave information and the high resolution satellite global gravity model ggm02c to obtain a 1 1 3 d image of a upper mantle isotropic shear wave speeds b densities and c density vscoupling below the european plate 20 n 90 n 40 w 70 e the 3 d image of the density vscoupling provides unprecedented detail of information on the compositional and thermal contributions to density structures the accurate and high resolution crustal model allows us to compute a reliable residual topography to understand the dynamic implications of our models the correlation between residual topography and mantle residual gravity anomalies defines three large scale regions where upper mantle dynamics produce surface expression the east european craton the eastern side of the arabian plate and the mediterranean basin the effects of mantle convection are also clearly visible at 1 the eastern sirt embayment 2 the west african craton northern margins 3 the volcanically active region of the canarian archipelago 4 the northern edge of the central european volcanic province and 5 the northeastern part of the atlantic ocean between greenland and iceland strong connections are observed among areas of weak radial anisotropy and areas where the mantle dynamics show surface expression although both thermal and additional dependencies have been incorporated into the density model convective down welling in the mantle below the east european craton is required to explain the strong correlation between the estimated negative mantle residual anomalies and the negative residual topography

geologic structure and neotectonics of the north african continental margin south of sicily abstract marine geological and geophysical data together with drilling information indicate that the north african passive continental margin has been subjected to extension and wrenching after it collided with the northern part of sicily the area of the tripolitania basin jarrafa trough melita and medina bank and the ragusa malta plateau has formed part of a sinking passive margin since the dispersal of gondwanaland at about 180 my ago as observed from geohistory diagrams a record of rifting in a nw se direction accompanied by dextral shear along the southern troughs is observed in seismic reflection data the rifting started during the neocomian and lasted until the eocene when activity became minor a pre middle miocene period of northward subduction of oceanic crust is inferred from the geology in ne sicily uplift of the northern part of the african margin after collision in the middle miocene is seen in wells in southern sicily after the messinian a rift and dextral shear zone established itself across the african margin from the strait of sicily to the medina ridge in the lonian basin the zone is marked by up to 1 7 km deep grabens narrow active wrench faulted channels volcanic fissures and local uplifted keilhorsts such as malta this zone which varies in width from 100 to 35 km forms the southern boundary of a microplate which includes sicily we speculate that the present motion of this microplate is partly due to the eastward movement of the calabrian arc with the sicilian block over the last remaining oceanic lithosphere in the eastern mediterranean

geomorphic effectiveness sandur development and the pattern of landscape response during jokulhlaups skeiðararsandur southeastern iceland by contrast with other historical outburst floods on skeitararsandur the 1996 jokulhlaup was unprecedented in its magnitude and duration attaining a peak discharge of f53 000 m 3 s in 17 h using a combination of field sampling and remote sensing techniques landsat tm sar interferometry airphotos and laser altimetry we document the sandur wide geomorphic impacts of this event these impacts varied widely across the skeitararsandur and cannot be singularly attributed to jokulhlaup magnitude because pre jokulhlaup glacial dynamics and the extant setting largely conditioned the spatial pattern type and magnitude of these impacts topographic lowering and asymmetric retreat of the ice front during the late twentieth century has decoupled the ice sheet from the moraine sandur complex along the central and western sandur this glacial control in combination with the convex topography of the proximal sandur promoted a shift from a primarily diffuse source braided outwash system to a more point sourced channelized discharge of water and sediment deposition dominated within the proglacial depression with approximately 3 8 10 7 m 3 of sediment and along channel systems that remained connected to subglacial sediment supplies this shift to a laterally dissimilar channelized routing system creates a more varied depositional pattern that is not explicitly controlled by the concave longitudinal profile down sandur laterally contiguous units therefore may vary greatly in age and sediment character suggesting that current facies models inadequately characterize sediment transfers when the ice front is decoupled from its sandur water was routed onto the sandur in a highly organized fashion and this jokulhlaup generated major geomorphic changes including sandur incision in normally aggradational distal settings and eradication of proximal glacial landforms dating to fa d 1892 d 2002 elsevier science b v all rights reserved

lateglacial and early holocene environmental dynamics in northern lithuania a multi proxy record from ginkūnai lake abstract to reconstruct the lateglacial and early holocene palaeoenvironmental dynamics in the northern part of lithuania the sediment record from gink nai lake was studied applying a multi proxy approach involving pollen diatom and plant macrofossil surveys as well as 18 o 13 c 14 c and loss on ignition loi measurements together with geochemical investigations the obtained data suggest the deglaciation of the area started at approximately 16 000 16 500 cal bp followed by the formation of a tree less tundra the obtained palaeobotanical records indicate the scarcity of the vegetation cover and the flourishing of the non arboreal taxa in the local flora suggesting a severe climatic regime after 13 700 cal bp the pinus betula predominating forest progressively expanded although this process was partly reversed during the recurrence of a cold interval recorded from 13 000 to 13 100 cal bp since 12 600 cal bp rapid aridification of the climatic regime reflected in the isotope record was accompanied by the formation of cold adapted vegetation including picea stands which acted as an intensive supply of allochthonous material into the basin thereby decreasing the water level the identified period of physical disturbance lasted until approximately 11 800 cal bp significant changes of the palaeoenvironmental regime subsequently occurred in the area and were manifested in most of the obtained proxies i e changes in the sediment type transformations of the vegetation cover and fluctuations of the sedimentation regime isotope records along with the geochemical data indicate a major climatic turn in terms of the temperature and moisture after 11 500 cal bp the rising productivity of the basin was coincident with the stabilisation of the soil layer and the formation of dense vegetation cover enriched by corylus ca 9800 cal bp and ulmus ca 10 300 cal bp during the early holocene after 10 800 cal bp the investigated part of the basin changed to a peat bog

absolute reconstructions of the paleozoic oceans abstract the orientation of three intraplate magmatic belts the mongolian from 130 to 280 m y south siberian from 320 to 400 m y and baltic from 365 to 400 m y belts which can be considered as hot spot tracks are used together with apparent paleomagnetic pole wander paths to reconstruct true motions of continents during paleozoic times the reconstructions obtained show that the old late precambrian pangea continued to exist in the early paleozoic with an inner arrangement of constituent continents strongly different from that of the late paleozoic pangea the continents were constantly located in the eastern hemisphere only the western hemisphere was occupied by the paleo pacific oceans four oceans existed in paleozoic times between the continents in the eastern hemisphere 1 the iapetus which originated in the late precambrian and closed before devonian times 400 m y 2 the asiatic paleo ocean which originated in the late precambrian and closed 450 m y ago 3 the uralian paleo ocean which opened around 500 m y ago and closed in the latest permian 240 230 m y 4 the paleo tethys which opened in the ordovician 480 450 m y and became the mesozoic tethys life duration of the oceans was 200 400 m y the main trend in the earth s evolution during the paleozoic was a break up of the old precambrian supercontinent and construction of the new late paleozoic pangea

palaeohydrology of the fazzan basin and surrounding regions the last 7 million years abstract we have integrated information on topography geology and geomorphology with the results of targeted fieldwork in order to develop a chronology for the development of lake megafazzan a giant lake that has periodically existed in the fazzan basin since the late miocene the development of the basin can be best understood by considering the main geological and geomorphological events that occurred thought libya during this period and thus an overview of the palaeohydrology of all libya is also presented the origin of the fazzan basin appears to lie in the late miocene at this time libya was dominated by two large rivers systems that flowed into the mediterranean sea the sahabi river draining central and eastern libya and the wadi nashu river draining much of western libya as the miocene progressed the region become increasingly affected by volcanic activity on its northern and eastern margin that appears to have blocked the river nashu in late miocene or early messinian times forming a sizeable closed basin in the fazzan within which proto lake megafazzan would have developed during humid periods the fall in base level associated with the messinian desiccation of the mediterranean sea promoted down cutting and extension of river systems throughout much of libya to the south of the proto fazzan basin the sahabi river tributary know as wadi barjuj appears to have expanded its headwaters westwards the channel now terminates at al haruj al aswad we interpret this as a suggestion that wadi barjuj was blocked by the progressive development of al haruj al aswad k ar dating of lava flows suggests that this occurred between 4 and 2 ma this event would have increased the size of the closed basin in the fazzan by about half producing a catchment close to its current size 350 000 km2 the fazzan basin contains a wealth of pleistocene to recent palaeolake sediment outcrops and shorelines dating of these features demonstrates evidence of lacustrine conditions during numerous interglacials spanning a period greater than 420 ka the middle to late pleistocene interglacials were humid enough to produce a giant lake of about 135 000 km2 that we have called lake megafazzan later lake phases were smaller the interglacials less humid developing lakes of a few thousand square kilometres in parallel with these palaeohydrological developments in the fazzan basin change was occurring in other parts of libya the lower pliocene sea level rise caused sediments to infill much of the messinian channel system as this was occurring subsidence in the al kufrah basin caused expansion of the al kufrah river system at the expense of the river sahabi by the pleistocene the al kufrah river dominated the palaeohydrology of eastern libya and had developed a very large inland delta in its northern reaches that exhibited a complex distributary channel network which at times fed substantial lakes in the sirt basin at this time libya was a veritable lake district during humid periods with about 10 of the country underwater

tectonics and subsidence evolution of the sirt basin libya a backstripping analysis of 225 wells located within the sirt basin libya provides new constraints on the development of the sirt basin four tectonic phases are identified from late jurassic to present the presentation of contour maps of subsidence and crustal stretching allows spatial and temporal variations in stretching to be visualized a close match is observed with stretching phases documented for other african basins consistent with discrete phases in the opening of the tethys and atlantic rifting and reactivation appear to be primarily controlled by the orientation of the basin and the underlying basement structure with respect to stress directions the tectonic subsidence curves have also been forward modeled with an automated modeling technique to quantify the variation in timing and the magnitude of rifting the tectonic subsidence history of the sirt basin is characterized by periods of stretching alternating with periods of relative tectonic quiescence and thermal subsidence stretching started at the centers of the troughs and migrated toward the platform crests

a dynamical model of lithosphere extension and sedimentary basin formation convection in the upper mantle can give rise to normal stresses on the base of the continental lithosphere that may cause localized elevation of its surface and extensional deviatoric stresses this possibility is investigated assuming homogeneous biaxial strain in a continental lithosphere whose strength is governed by a combination of brittle failure and power law creep in the crust and by brittle power law or dorn law behavior in the mantle for a given geotherm and for rheologies based on current laboratory data the lithosphere responds to convection induced uplift in one of three ways depending primarily on the magnitude of the driving stresses these regimes are as follows 1 when the driving stress exceeds a threshold value the resulting strain rates are high enough greater than about 3 10 16 s 1 that the lithosphere strains almost isothermally its effective rheology is strain thinning and it undergoes accelerating extension the magnitude of the dynamic uplift required to produce the threshold value of the driving stress depends on the geotherm characterized here by the moho temperature and varies from about 1 km for a moho temperature of 700 c to around 3 km for a moho temperature of 550 c an uncertainty of at least 100 c is attached to these temperatures owing to the inaccuracies in extrapolating laboratory data to geological strain rates 2 at stress levels due to less than about half of the critical uplift extensional strain is negligible 3 at intermediate stress levels moderate uplift produces extension that is bounded by strength increases due to cooling of the ductile portion of the lithosphere during extension

early tertiary subsidence and sedimentary facies northern sirte basin libya the subsidence curves and subsidence rate curves for the sirte basin constructed from the stratigraphic record show that subsidence was continuous throughout late cretaceous and tertiary times reaching a maximum during the paleocene and eocene when a major reactivation of faults occurred shales and carbonates were deposited during all of the late cretaceous and tertiary abrupt lateral facies changes occur from the platform areas toward the deeper troughs along with steep downdip thickening these conditions were probably assisted by contemporaneous faulting along structurally weak hinge lines where the dominant structural elements are normal step faults the absence of upper paleozoic and lower mesozoic sediments suggests that the area was domed faulted and eroded during the late mesozoic the widespread distribution of the maastrichtian carbonate facies over much of the platform and basinal areas suggests that tectonics were relatively inactive during the maastrichtian as a result of crustal extension during the paleocene a marked lithologic and structural change occurred the heira shale succeeded the kalash limestone in the marada trough reactivation of the earlier faults accompanied by an increase in the sediment supply from the south caused these lower paleocene shales to cover the entire area with the exception of the old highs where carbonate deposition continued an intercalation of shales and carbonates provides a sensitive indicator of change of depth and sediment type commonly this intercalation has been referred to paleocene transgressive and regressive cycles the thickness of the paleocene facies corresponds closely to the structure with the thickening in the troughs and thinning on the highs

impact of structural setting on diagenesis of fluvial and tidal sandstones the bahi formation upper cretaceous nw sirt basin north central libya abstract this petrographic mineral chemical fluid inclusion and stable isotopic study shows that the distribution of diagenetic modifications and their influence on reservoir quality and heterogeneity in tidal and fluvial sandstones of the upper cretaceous bahi sandstones in the rift sirt basin nw libya varies systematically along a series of closely spaced dominantly normal faults between the basin margin and more basinward located areas shallow eogenetic modifications resulting from the percolation of meteoric waters include infiltration of grain coating clays kaolinitization of detrital silicates and cementation by dolomite and k feldspar overgrowths mesogenetic alterations 70 c 2 km include feldspar albitization illitization of infiltrated clay and kaolinite conversion of kaolinite into dickite and cementation by quartz overgrowths t h 112 c 134 c barite t h 145 c 158 c and fe carbonates the restriction of barite and fe carbonate cements to the basinward located sandstones suggests formation by hydrothermal fluids along the faults extensive feldspar dissolution and formation of moldic pores in sandstones from the basin margin were probably caused by deep percolation of meteoric waters results from this study regarding the structural control on the spatial distribution of diagenetic alterations have implications for constraining the flux of pore fluids and by extension reservoir quality in analogous epicratonic rift basins

crustal radial anisotropy beneath cameroon from ambient noise tomography abstract to increase the understanding of crustal deformation and crustal flow patterns due to tectonic processes in cameroon we study the lateral variability of the crustal isotropic velocity and radial anisotropy estimated using ambient noise tomography ant rayleigh and love wave noise correlation functions ncfs were retrieved from the cross correlation of seismic ambient noise data recorded in cameroon and phase velocities at periods of 8 to 30 s were measured to perform surface wave tomography joint inversion of rayleigh and love wave data for isotropic velocity models could not fit the observed dispersions simultaneously we attribute the love rayleigh discrepancy to the presence of radial anisotropy in the crust and estimated its magnitude our 3 d radial anisotropic model reveals the spatial variation of strong to weak positive vsh vsv and negative vsv vsh radial anisotropy in the crust we observe negative radial anisotropy in the upper crust that is associated mainly with the location of a previously reported mantle plume the anisotropy could be attributed to the vertical alignment of fossil microcracks or metamorphic foliations due to the upwelling of plume material a strong positive radial anisotropy is centered at the location of an inferred boundary between the congo craton and the oubanguides belt that might be related to the preferred orientation of crustal anisotropic minerals associated with shearing in this fault zone the middle crust is characterized by a widespread negative radial anisotropy that is likely caused by the flow induced alignment of anisotropic minerals that crystallized during magma intrusion the magnitude of the radial anisotropy varies systematically from predominantly negative in the middle crust to positive in the lower crust the imaged patterns of the isotropic velocity and radial anisotropy are consistent with previous studies and agree with regional tectonics

crust mantle coupling mechanism in cameroon west africa revealed by 3d s wave velocity and azimuthal anisotropy abstract to understand the depth variation of deformation beneath cameroon west africa we developed a new 3d model of s wave isotropic velocity and azimuthal anisotropy from joint analysis of ambient seismic noise and earthquake surface wave dispersion we found that the cameroon volcanic line cvl is well delineated by slow phase velocities in contrast with the neighboring congo craton in agreement with previous studies apart from the congo craton and the oubanguides belt the uppermost mantle revealed a relatively slow velocity indicating a thinned or thermally altered lithosphere the direction of fast axis in the upper crust is mostly ne sw but trending approximately n s around mt oku and the southern cvl the observed crustal azimuthal anisotropy is attributed to alignment of cracks and crustal deformation related to magmatic activities a widespread zone of weak to zero azimuthal anisotropy in the mid lower crust shows evidence for vertical mantle flow or isotropic mid lower crust in the uppermost mantle the fast axis direction changed from ne sw to nw se around mt oku and northern cameroon this suggests a layered mechanism of deformation and revealed that the mantle lithosphere has been deformed ne sw fast azimuths are observed beneath the congo craton and are consistent with the absolute motion of the african plate suggesting a mantle origin for the observed azimuthal anisotropy our tomographically derived fast directions are consistent with the local sks splitting results in some locations and depths enabling us to constrain the origin of the observed splitting the different feature of azimuthal anisotropy in the upper crust and the uppermost mantle implies decoupling between deformation of crust and mantle in cameroon

stratigraphic position of the cretaceous tertiary boundary in the northern sirt basin libya abstract biostratigraphic analysis reveals that the contact of the kalash limestone with the hagfa shale does not coincide with the cretaceous tertiary boundary as previously used in the subsurface geology of the sirt basin in well a1a nc29a in the northern sirt basin the cretaceous tertiary boundary lies within the uppermost part of the kalash limestone this is based on a study of the foraminiferal assemblages below and above this boundary key species of both planktonic and benthic foraminifera are used to illustrate this conclusion the local eoglobigerina minutula zone is here proposed for the earliest palaeocene biostratigraphic interval which lies between the cretaceous tertiary boundary and the subbotina pseudobulloides zone

freshwater outbursts to the oceans from glacial lake agassiz and their role in climate change during the last deglaciation abstract lake agassiz was the largest lake in north america during the last deglaciation as the laurentide ice sheet lis retreated large volumes of water stored in this proglacial lake were episodically released into the oceans these waters were variably routed to the gulf of mexico arctic ocean north atlantic ocean and hudson bay during this period the three largest cooling events in the northern hemisphere closely followed 4 of the 5 largest outbursts from lake agassiz 1 the younger dryas which was preceded by a release of 9500 km 3 2 the preboreal oscillation preceded by releases of 9300 km 3 and 5900 km 3 and 3 the 8 2 ka cold event preceded by a 163 000 km 3 outburst these are respectively fluxes of 0 30 sv 0 29 sv 0 19 sv and 5 2 sv if released in 1 year because the influx of freshwater reaching the north atlantic ocean can inhibit thermohaline circulation partly depending on whether the ocean was in a glacial interglacial or transitional mode of circulation we believe that at least these large outbursts from lake agassiz may have provided the triggers for changes in ocean circulation and in turn for widespread climate change

reconstruction of the largest holocene jokulhlaup within jokulsa a fjollum ne iceland abstract glacial outburst floods jokulhlaups have a significant role for landscape evolution in ne iceland a number of jokulhlaups have routed from the northern margin of vatnajokull during the holocene in this study reconstruction of the largest holocene jokulhlaup along jokulsa a fjollum ne iceland was undertaken using the hec ras hydraulic modelling and hec georas flood mapping techniques with a digital elevation model dem derived from ers insar data and field based wash limit evidence the largest jokulhlaup produced extensive erosional and depositional landforms across an inundated area of 1390 km2 and is calculated to have had a peak discharge of 0 9 106 m3 s 1 power per unit area within this jokulhlaup varied from 6 to 46 000 w m 2 jokulhlaup hydraulics are related to geomorphogical evidence at three key sites in va alda upptyppingar and mo rudalur sub areas in order to explain the abrupt spatial variation of the flood characteristics on a regional scale and to relate erosional and depositional features to spatial variations in jokulhlaup hydraulics these process form relationships of the largest jokulhlaup along the jokulsa a fjollum are compared with large outburst floods elsewhere the largest jokulsa a fjollum jokulhlaup had a factor of 20 times smaller discharge and a factor of 20 times lower power per unit area than altai palaeoflood the largest known flood on the earth

petrology of the bimodal cenozoic volcanism of the kapsiki plateau northernmost cameroon central africa the kapsiki plateau is the northernmost volcanic zone of the cameroon line the volcanism 27 35 ma is of alkaline type and has a typical bimodal lava series diversity with basalts and hawaiite as mafic lavas and phonolites trachytes and rhyolites as felsic lavas mg rich olivine phenocrysts occur only in basalts the hawaiite contains andesine olivine and ca rich pyroxene phenocrysts and sanidine and quartz xenocrysts the phonolites contain alkali feldspar and na rich clinopyroxene phenocrysts two types of trachytes occur peralkaline trachytes with ti na and f rich aegirine augite richterite arfvedsonite phenocrysts and non peralkaline trachytes with an aenigmatite type undetermined mineral ti rich biotite and zircon phenocrysts similarly two types of rhyolites occur peralkaline with quartz and arfvedsonite phenocrysts and non peralkaline with quartz and biotite phenocrysts differentiation indices thornton and tuttle 1960 of the lavas range from 22 to 97 with a large gap in the range 34 82 some basalts are primitive 530 ppm ni 1100 ppm cr in basaltic lavas phonolites and non peralkaline trachytes and rhyolites zr and nb covary with approximately constant ratios o3 1 zr nb 5 3u however peralkaline trachytes and rhyolites have high concentrations in zr up to 2180 ppm and nb up to 780 ppm with correlative higher zr nb ratios 6 3 8 3 some rhyolites have abnormal ree patterns with kinks depleted in light ree probably resulting from stability of na ree f complexes under hydrothermal conditions despite a large gap between basaltic and felsic lavas major and trace element distributions indicate co magmatism for both the basaltic and felsic lavas the differentiation of the lava series is dominated by crystal fractionation the role of fluids in rhyolite genesis being of minor effect as evidenced by constant values of y ho and zr hf throughout the series the kapsiki plateau basalts are similar in their chemical and isotopic data character to other basalts from both the continental and oceanic sectors of the cameroon line the continental crust appears to have no significant role in their genesis the hawaiite mineralogical and geochemical characteristics are consistent with an origin by mixing of basaltic and felsic phonolitic magmas the kapsiki plateau basaltic magmas may have originated from an infra asthenospheric reservoir similarly to other basaltic magmas generated throughout the cameroon line the sr isotope variations observed in trachytes and rhyolites point to some contamination of the magmas by crustal materials while the nd isotopic composition is only slightly affected q 2000 elsevier science b v all rights reserved

ice fracturing during jokulhlaups implications for englacial floodwater routing and outlet development theoretical studies of glacial outburst floods jokulhlaups assume that i intraglacial floodwater is transported efficiently in isolated conduits ii intraglacial conduit enlargement operates proportionally to increasing discharge iii floodwater exits glaciers through pre existing ice marginal outlets and iv the morphology and positioning of outlets remains fixed during flooding direct field observations together with historical jokulhlaup accounts confirm that these theoretical assumptions are not always correct this paper presents new evidence for spatial and temporal changes in intraglacial floodwater routing during jokulhlaups secondly it identifies and explains the mechanisms controlling the position and morphology of supraglacial jokulhlaup outlets and finally it presents a conceptual model of the controls on supraglacial outbursts field observations are presented from two icelandic glaciers skei ararjokull and solheimajokull video footage and aerial photographs taken before during and after the skei ararjokull jokulhlaup and immediately after the solheimajokull jokulhlaup reveal changes in floodwater routing and the positioning and morphology of outlets field observations confirm that glaciers cannot transmit floodwater as efficiently as previously assumed rapid increases in jokulhlaup discharge generate basal hydraulic pressures in excess of ice overburden under these circumstances floodwater can be forced through the surface of glaciers leading to the development of a range of supraglacial outlets the rate of increase in hydraulic pressure strongly influences the type of supraglacial outlet that can develop steady increases in basal hydraulic pressure can retro feed pre existing englacial drainage whereas transient increases in pressure can generate hydraulic fracturing the position and morphology of supraglacial outlets provide important controls on the spatial and temporal impact of flooding the development of supraglacial jokulhlaup outlets provides a new mechanism for rapid englacial debris entrainment

hydrologic and geomorphic effects of temporary ice dammed lake formation during jokulhlaups glacial outburst oods jokulhlaups occur frequently in glaciated environments and the resultant ooding causes geomorphic change and in some instances damage to local infrastructure during some jokulhlaups oodwater is stored temporarily in ice marginal locations in july 1999 a linearly rising jokulhlaup burst from solheimajokull iceland during this remarkable event subglacial oodwater pooled transiently in two relict ice dammed lake basins before draining suddenly back into solheimajokull the signi cance of such rapid formation and attendant drainage of temporary ice dammed lakes during jokulhlaups has not been addressed consequently this paper i assesses the hydrologic and geomorphic effects of temporary ice dammed lake formation caused by lake basin retro lling and ii discusses the impact and signi cance of transient retro lling under jokulhlaup conditions pre and post ood eldwork at solheimajokull enabled the impact and signi cance of lake basin retro lling to be assessed field evidence demonstrates that the july 1999 jokulhlaup had an unusually rapid rise to peak discharge resulting in subglacial oodwater being purged to ice marginal locations the propensity for temporary retro lling was controlled by rapid expulsion of oodwater from solheimajokull coincident with locations suitable for oodwater storage floodwater inundated both ice marginal lake basins permitting signi cant volumes of sediment deposition coarse grained deltas prograding from the ice margin and boulders perched on scoured bedrock provide geomorphic records of sudden retro lling the depositional characteristics of lake basin deposits at solheimajokull are similar to jokulhlaup sediments documented in proglacial settings elsewhere however their depositional setting and association with ice marginal landforms is distinctive findings suggest that temporary ice dammed lake formation and drainage has the capacity to alter the shape of the ood hydrograph especially if drainage of a temporary lake is superimposed on the original jokulhlaup deposits associated with lake basin retro lling have a long term preservation potential that could help to identify temporary ice dammed lake formation in modern and ancient glacial environments copyright 2003 john wiley sons ltd

upper mantle structure beneath cameroon from body wave tomography and the origin of the cameroon volcanic line the origin of the cameroon volcanic line cvl a 1600 km long linear volcanic chain without age progression that crosses the ocean continent boundary in west central africa is investigated using body wave tomography relative arrival times from teleseismic p and s waves recorded on 32 temporary seismic stations over a 2 year period were obtained using a multichannel cross correlation technique and then inverted for mantle velocity perturbations the p and s wave models show a tabular low velocity anomaly directly beneath the cvl extending to at least 300 km depth with perturbations of 1 0 to 2 0 for p and 2 0 to 3 0 for s the s wave velocity variation can be attributed to a 280 k or possibly higher thermal perturbation if composition and other effects on seismic velocity are negligible the near vertical sides of the anomaly and its depth extent are not easily explained by models for the origin of the cvl that invoke plumes or decompression melting under reactivated shear zones but are possibly consistent with a model invoking edge flow convection along the northern boundary of the congo craton lithosphere if edge flow convection in the sublithospheric upper mantle is combined with lateral flow channeled along a fracture zone beneath the oceanic sector of the cvl then the oceanic sector can also be explained by flow in the upper mantle deriving from variations in lithospheric thickness

organic geochemistry and petrology of lower cretaceous black shales in the qiangtang basin tibet implications for hydrocarbon potential abstract lower cretaceous black shales have recently been discovered in the qiangtang basin especially in its northern basin tibet implying a new interval with hydrocarbon resource potential in the region this potential however has not been investigated to date and is addressed in this paper based on organic petrology and geochemistry data from the representative shengli river outcrop organic petrology including optical and scanning electron microscope observations indicates that the organic matter om within these shale sequences has precursors from abundant benthic algae some bacteria and amorphous om and a few land plants suggesting that the black shales were deposited in a generally reducing platform lagoonal environment favorable for om preservation the shales have high total organic carbon toc contents 1 74 7 71 wt type ii kerogen tending to type iii and high thermal maturity vitrinite reflectance equivalent of ca 1 3 ro biomarkers in the shales suggest deposition under reducing brackish or saline water with aquatic benthic organisms bacteria and amorphous om as the dominant input thus the results of organic petrology organic geochemistry and biomarker geochemistry are generally consistent and imply that the shengli river black shale samples have significant hydrocarbon resource potential and most likely produced gas due to relatively high maturity and gas prone kerogen the neighboring black shales within the same stratigraphic interval as the shengli river shales if having moderate om maturity and oil prone kerogen can be expected to generate oil the presence of well developed micropores and fractures and abundant brittle minerals within the black shales suggests that they may have unconventional hydrocarbon resource potential especially in intervals with high toc 4 0 wt these results provide new data and understanding for regional hydrocarbon exploration

paleohydrology of late pleistocene superflooding altay mountains siberia cataclysmic flooding is a geomorphological process of planetary significance landforms of flood origin resulted from late pleistocene ice dammed lake failures in the altay mountains of south central siberia peak paleoflows which exceeded 18 x 10 6 cubic meters per second are comparable to the largest known terrestrial discharges of freshwater and show a hydrological scaling relation to floods generated by catastrophic dam failures these seem to have been earth9s greatest floods based on a variety of reconstructed paleohydraulic parameters

sirte basin north central libya cretaceous rifting above a fixed mantle hotspot the complex pattern of horsts and grabens in the sirte basin may have developed when mesozoic drift of the african plate put north central libya over a fixed mantle hotspot in early cretaceous time 140 to 100 m y ago significant change in the motion of the plate during the prolonged residence above a hypothetical cameroon plume may have produced stress that fragmented thinned and weakened lithosphere successive uplift and subsidence along a reconstructed track of the plume as well as in the sirte basin are compatible with predicted effects of the drift of northern africa over a fixed mantle hotspot this speculation suggests a plausible alternative to the possibility that rifting throughout northern africa in early cretaceous time may have been produced along a wide zone of extension between two african plates when they were at rest relative to underlying plumes

facies patterns and stratal geometries clues to the nature of the platform margin during the paleocene northeast sirte basin libya abstract the sirte basin of libya is a mesozoic tertiary rift basin comprising a series of horsts and grabens which were formed as a result of collapse of a n s trending arch in late cretaceous time the paleocene section is made up of two shallowing upward cycles where each cycle is under and overlain by deeper pelagic facies a gradual shallowing trend from basin to shallow platform is seen in both bio and lithofacies these are interpreted as representing prograding ramps with no evidence of a significant break in slope between the basin and the adjacent platform the fact that most facies are dominated by micrites suggests that deposition was in a low energy setting possibly on a leeward margin these ramps were initially foram dominated platforms setting the style for later tertiary ramps large forams filling the role of inner mid ramp carbonate producers after the demise of the rudist coral assemblage at the end of the cretaceous based on local and regional information a number of episodes of relative sea level change can be recognized the two shallowing upward ramp cycles are interpreted as representing two highstand system tracts the precipitation of evaporites in the lower cycle appears to be associated with the earlier highstand system tract culminating during a sea level lowstand a drop in sea level during the late paleocene is also invoked to explain the interpreted influx of meteoric waters following the later highstand this event resulted in the creation of good mouldic and vuggy porosities thus making these porous inner mid ramp facies potential targets for hydrocarbon exploration on a basin wide scale this event is also very important because all the paleocene reservoirs at least in the eastern sirte basin seem to be related to this possible subaerial exposure event

the opening of sirte basin result of slab avalanching abstract north africa s sirte basin opening is an enigmatic feature in the complex meso cenozoic rearrangement of mediterranean tectonics new borehole data inversion constrains its deformation history showing a stretching event starting 70 ma and terminating in a further abrupt increase at 50 ma rapidly fading afterwards the timing of this event hardly reconciles with the mesozoic major plates reorganization following the spreading of the atlantic and the neogene central mediterranean tectonics active at different times here we propose that sirte rifting could have been driven by the pull exerted by the hellenic subduction reconstructions of hellenic convergence and slab deep subduction as constrained by plate kinematics and tomography show that large slab mass accumulates in the upper mantle by late cretaceous paleogene eventually forcing further sinking in the lower mantle coeval to the growing strain evolution recorded in sirte furthermore the 20 m y phase of large pull here recorded the consequent rapid growth over 10 m y and following decay are compatible with the dynamics of slab avalanche as revealed by numerical models showing that the sirte basin opened in response to the large pull force developed during the mass flush and transferred from deep slab to surface

lithospheric instability and the source of the cameroon volcanic line evidence from rayleigh wave phase velocity tomography the cameroon volcanic line cvl is a 1800 km long volcanic chain extending sw ne from the gulf of guinea into central africa that lacks the typical age progression exhibited by hot spot related volcanic tracks this study investigates the upper mantle seismic structure beneath the cvl and surrounding regions to constrain the origin of volcanic lines that are poorly described by the classic plume model rayleigh wave phase velocities are measured at periods from 20 to 182 s following the two plane wave methodology using data from the cameroon seismic experiment which consists of 32 broadband stations deployed between 2005 and 2007 these phase velocities are then inverted to build a model of shear wave velocity structure in the upper mantle beneath the cvl results show that phase velocities beneath the cvl are reduced at all periods with average velocities beneath the cvl deviating more than 2 from the regional average and 4 beneath the congo craton this distinction is observed for all periods but is less pronounced for the longest periods measured inversion for shear wave velocity structure indicates a tabular low velocity anomaly directly beneath the cvl at depths of 50 to at least 200 km and a sharp vertical boundary with faster velocities beneath the congo craton these observations demonstrate widespread infiltration or erosion of the continental lithosphere beneath the cvl most likely caused by mantle upwelling associated with edge flow convection driven by the congo craton or by lithospheric instabilities that develop due to the nearby edge of the african continent

seismicity of the cameroon volcanic line 1982 1990 abstract this paper is dedicated to soba djallo director of irgm who died in 1991 results of seismic monitoring of the continental segment of the cameroon volcanic line for the period 1982 to 1990 are reported mount cameroon continues to be seismically the most active area with a well defined seismicity pattern characterized by single and swarm events with duration magnitudes between 2 and 3 occurring at depths down to 55 km on average the earthquakes here occur at the rate of about 2 events every 3 days further north the seismicity is low except for isolated felt events located along the foumban shear zone which probably controls the location of the earthquakes as well as the volcanic centres making up the volcanic line during this eight year period two fatal carbon dioxide gas emissions from crater lakes occurred in 1984 l monoun and 1986 l nyos causing 37 and 1700 deaths respectively seismic monitoring indicates that no seismic activity was associated with these lakes thus favouring an aseismic cause to the disasters these gas emissions the increased number of felt earthquakes and a recent natural explosion on mount cameroon suggest either a more active phase of the cameroon volcanic line and or a greater awareness by the local population of the phenomena associated with this volcanic province

global tectonics since the breakup of pangea 180 million years ago evolution maps and lithospheric budget abstract pangea the earth s youngest single supercontinent broke up 180 million years ago tectonic plates were subsequently formed by dispersal of the continental fragments and accretion of new oceanic lithosphere the configurations of all the major lithospheric plates at 0 20 65 95 140 180 ma bp are reconstructed on six globes of the earth each with a radius of 10 cm it appears that plate boundaries maintain a remarkably close fit on model globes of constant radii if the reconstructions include the recovery of subducted spreading patterns this is illustrated with maps in equatorial orthographic oblique orthographic and transverse hammer aitoff projections the snug fit of the tectonic plates at every tested time since the breakup of pangea 180 ma bp is consistent with the theory of plate tectonics on a non expanding earth and contradicts rapidly expanding earth models the areas of oceanic lithosphere produced and consumed during the past 180 ma bp are estimated from surface measurements of the globes reconstructed on the basis of particular assumptions these measurements suggest a consistent increase in the production rate of oceanic lithosphere during the past 140 ma it was decided to revise the assumptions and see if alternative reconstructions of the ancient spreading patterns on the floors of the tethys and eo pacific oceans could avoid implying an increase of lithospheric production rates with time this appeared to be possible the revised maps suggest that ophiolites older than 180 ma bp may have been obducted in cenozoic collision zones of the himalayas andes rockies and the western part of the banda arc timor new guinea estimates of the ocean floor production and consumption budget appear to be quite similar for both map series and only the possible ranges are summarized here time averaged for the past 180 ma world wide production and consumption of oceanic lithosphere appears to have varied between 2 6 and 3 5 km2 a 1 at most the mean of world wide spreading velocities ranges between 2 4 and 3 5 cm a 1 and world wide means of subduction velocities lie between 4 2 and 6 3 cm a 1 more accurate estimates are possible on the basis of particular assumptions and have been specified for the various time intervals distinguished i e the neogene 0 20 ma bp palaeogene 20 65 ma bp upper cretaceous 65 95 ma bp lower cretaceous 95 140 ma bp and middle and upper jurassic 140 180 ma bp

did rifting on australia s southern margin result from tectonic uplift subsidence analyses from australia s southern margin are used to investigate the idea that rifting on the margin was caused by an extensional lithospheric stress field induced by a hot thermal plume impinging on the base of the lithosphere from beneath the continental lithosphere is modelled as a thin sheet with depth dependent rheology the model does not explicitly include elastic flexural or thermal stresses the excess buoyancy of a thermal plume pushes the lithosphere upward by as much as 1 km thereby increasing its potential energy if the horizontal components of lithospheric stress are fixed at some distant plate boundary the lithosphere is a stress guide the lithospheric stress field above the plume will produce horizontal extension that may ultimately be followed by continental separation and seafloor spreading the uplift is modelled by a time dependent function which increases smoothly from zero to its peak value and then decreases back to zero using a plausible mantle rheology the amount of extension inferred at well sites on the margin typically 20 to 30 extension is explained by uplift of the lithosphere to approximately 900 m with a subsequent decay of the uplift over a period of approximately 60 m y the rise time of the uplift function is poorly constrained in the uplift model extension occurs in the subaerial environment during the late jurassic early cretaceous the initial stage of rapid subsidence observed at most well sites on the margin is due in large part to decrease of the vertical stress anomaly caused by the decay of the thermal plume or by the lithosphere moving away from above it the rapid subsidence is followed by very gradual continued subsidence attributed to cooling of the extended lithosphere as in previous studies

freshwater forcing of abrupt climate change during the last glaciation large millennial scale fluctuations of the southern margin of the north american laurentide ice sheet occurred during the last deglaciation when the margin was located between about 43 and 49 n fluctuations of the ice margin triggered episodic increases in the flux of freshwater to the north atlantic by rerouting continental runoff from the mississippi river drainage to the hudson or st lawrence rivers we found that periods of increased freshwater flow to the north atlantic occurred at the same time as reductions in the formation of north atlantic deep water thus providing a mechanism for observed climate variability that may be generally characteristic of times of intermediate global ice volume